Hydrogen Gas Generation System

Description

RELATED APPLICATIONS

The current application is a national stage of PCT Application No. PCT/US2024/021572 entitled “Hydrogen Gas Generation System”, filed Mar. 27, 2024 and published as WO 2024/206363 A1 on Oct. 3, 2024, which claims the benefit of and priority under 35 U.S.C. § 119 (e) to U.S. Provisional Application No. 63/455,396, entitled “Hydrogen Gas Generation System”, filed Mar. 29, 2023, the disclosures of which are incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD

The present disclosure relates to systems for hydrogen gas generation and methods of making and operating the same.

BACKGROUND

The world's demand for clean energy production has been steadily on the rise for decades. One of the promising, non-fossil-based sources of clean energy may be hydrogen. To date, different systems for hydrogen production have been explored, but their success has been limited due to high initial and operating costs, scale-up difficulties, and inefficiency.

SUMMARY

In at least one embodiment, a hydrogen gas production system is disclosed. The system may include a first electrode having an electrocatalyst, a second electrode having an electron donor material including a plurality of active sites, the second electrode being structured to release electrons from the active sites in a predetermined operating potential range lower than an operating potential triggering oxygen evolution reaction. The system may further include a first electrolyte in contact with the first and second electrodes, the electrolyte being a source of hydrogen protons. The system may also have a power source structured to provide the predetermined operating potential range to the system for the release and transfer of the electrons from the second electrode to the first electrode such that the hydrogen protons combine with the electrons to generate hydrogen gas. The system may be a multi-chamber system. The electron donor material may be a non-metallic conductor. The operating potential range may be about 0.25-2.1 V vs SHE at pH of about 0 to 8. The system may be an electrolyzer. The first electrolyte may have an acidic pH. The first electrolyte may be seawater. The system may also include a third electrode and a secondary power source. The system may also include a divider separating the first and second electrodes. The system may also include a second electrolyte different from the first electrolyte. The system may be membrane-free.

In another embodiment, a hydrogen gas production system is disclosed. The system may include a first chamber including a first power source connected to a first electrode and a second electrode. The second electrode may have a plurality of active sites including releasable electrons at a voltage range lower than a voltage required to trigger an oxygen evolution reaction. The first electrode may be structured as a cathode to which electrons from the second electrode flow when the voltage is applied, and a first electrolyte structured as a source of hydrogen protons to be combined with the electrons from the second electrode. The system may further include a second chamber physically divided from the first chamber by a divider and having a second electrolyte, a third electrode in contact with the second electrolyte, and a second power source connected to the second electrode and a third electrode. The second electrolyte may be different from the first electrolyte. The second chamber may also include a redox system. The divider may be an anion exchange membrane. The third electrode may be inactive when the first power source is on. The operating potential range may be about 0.25-2.1 V vs SHE at pH of about 0 to 8. The first electrolyte may have an acidic pH.

In yet another embodiment, a hydrogen gas production system is disclosed. The system may include a first electrode, a second electrode, an electrolyte, and a power source connecting the first and second electrodes and structured to provide operating potential to the electrodes. The system may have a first state of doping the second electrode by applying operating potential in a range lower than voltage required for oxygen evolution reaction to release electrons from the second electrode and transferring the electrons via the first electrode to hydrogen protons in the electrolyte to produce hydrogen gas. The system may also include a second state of de-doping the second electrode while not producing hydrogen gas. The de-doping may include oxidizing a redox species present in a second electrolyte to generate electrons for the de-doping. The second electrode may be active during the first and second states. The operating potential may be about 0.25-2.1 V vs SHE at pH of about 0 to 8. The electrolyte may be acidic. The electrolyte may be seawater and the operating potential may be lower than potential required for chlorine gas production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of a traditional hydrogen production electrolysis;

FIG. 2 shows a schematic depiction of a non-limiting example of a system disclosed herein according to one or more embodiments;

FIGS. 3A and 3B show a schematic depiction of a non-limiting example of a dual-chamber system disclosed herein in a first state (3A) and a second state (3B);

FIGS. 4A and 4B show a schematic depiction of a non-limiting example of a single-chamber system including a divider disclosed herein in a first state (4A) and a second state (4B);

FIGS. 5A and 5B show a schematic depiction of another non-limiting example of a single-chamber system, the system being free of a divider, disclosed herein in a first state (5A) and a second state (5B);

FIGS. 6A and 6B show a schematic depiction of a non-limiting example of a single-chamber system having more than one divider disclosed herein in a first state (6A) and a second state (6B);

FIG. 7 shows cyclic voltammetry (CV) curves of the experimental non-limiting example polypyrrole (PPy) electrodes in a 0.5 M Na₂SO₄ electrolyte obtained at a scan rate of 20 mV/s;

FIG. 8 shows a linear sweep voltammetry of a 1-hour non-limiting example PPy electrode in both 0.5 M Na₂SO₄ and 0.5 M H₂SO₄, scan rate 1 mV/s;

FIGS. 9A-9C are micrographs of a non-limiting example PPY electrode disclosed herein in pristine state (9A), after 2 h of polarization at 0.6 V vs standard hydrogen electrode (SHE) (9B), and 1.1 V vs SHE in 0.5 M H₂SO₄ (9C);

FIG. 10 shows a plot of potential difference between the hydrogen evolution reaction (HER) at the first electrode and the doping process of the second electrode in a non-limiting example system disclosed herein;

FIG. 11 shows the effect of de-doping time on the current measured during H₂ production by the non-limiting example system;

FIG. 12 shows continuous operation of the non-limiting example PPy electrode involving de-doping steps (negative current density) and doping steps (positive current density) of 40 sec each, for a total of 6 h;

FIG. 13 is a graph of energy and economical calculations related to the processes and systems described herein in systems utilizing Na₂SO₄ or H₂SO₄;

FIG. 14 shows CV of a non-limiting example PPy electrode in different electrolytes obtained at scan rate of 20 m V/s;

FIGS. 15A and 15B show potential difference between the Fe²⁺ oxidation and a non-limiting example PPy de-doping process (15A), and the potential difference between the HER at a non-limiting example Pt cathode and the doping process of the non-limiting example PPy anode (15B);

FIG. 16 is a graph of energy and economical calculations related to the processes and systems described herein in systems utilizing seawater;

FIG. 17 is a schematic process flow diagram of a non-limiting example of the system implementing non-limiting example acid-derived electrolyte disclosed herein;

FIG. 18 is a schematic process flow diagram for a non-limiting example seawater electrolysis in conjunction with the acid-derived electrolyte disclosed herein;

FIG. 19A is a plot showing electrode potential profiles measured during the electropolymerization of pyrrole onto carbon fiber (CF) at different conditions of Example 3;

FIG. 19B shows chronoamperometry of the doping step of the different PPy electrodes in 0.5 M H₂SO₄ of Example 3;

FIG. 19C shows electrochemical impedance spectroscopy (EIS) of the different PPy electrodes obtained in 0.5 M H₂SO₄ of Example 3;

FIG. 19D shows chronoamperometry of the doping step of the different PPy electrodes fabricated at the same current density, 0.25 mA cm⁻² at different pH, obtained in 0.5 M H₂SO₄ of Example 3;

FIG. 20 shows doping and de-doping cycles of non-limiting example PPy electrodes fabricated using various anions during the electropolymerization process at 0.25 mA cm⁻² for 3 h of Example 3;

FIGS. 21A-D shows an effect of different parameters on the current measured while operating a PEM electrolyzer of Example 4;

FIG. 22A shows electrochemical impedance spectroscopy analysis on the doping process of the PPy electrode at different temperatures in a 0.5 M H₂SO₄ solution of Example 4;

FIG. 22B is a plot of current density in time at different temperatures for PPy electrode of Example 4;

FIG. 22C is a plot of potential in time for PPy electrode of Example 4;

FIG. 23A shows de-doping/doping cycles of the PPy electrode in 0.5 M H₂SO₄ of Example 5;

FIG. 23B shows energy consumption calculations for different de-doping scenarios of Example 5;

FIG. 24A shows de-doping/doping cycles of the PPy electrode in 0.5 M H₂SO₄ of Example 6;

FIG. 24B shows CV cycles of the PPy electrode at 100 mV/s in 0.5 M H₂SO₄ of Example 6;

FIG. 24C shows de-doping/doping cycles of the PPy electrode in 0.5 M H₂SO₄ of Example 6;

FIG. 24D shows EIS of the different electrode samples in 0.5 M H₂SO₄, (E) FT-IR spectra of the electrodes of Example 6; and

FIG. 24E shows FT-IR spectra of Example 6.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed.

The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

As used herein, the term “substantially,” “generally,” or “about” means that the amount or value in question may be the specific value designated or some other value in its neighborhood. Generally, the term “about” denoting a certain value is intended to denote a range within +/−5% of the value. As one example, the phrase “about 100” denotes a range of 100+/−5, i.e., the range from 95 to 105. Generally, when the term “about” is used, it can be expected that similar results or effects according to the invention can be obtained within a range of +/−

5% of the indicated value. The term “substantially” may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such instances, “substantially” may signify that the value or relative characteristic it modifies is within +0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or 10% of the value or relative characteristic.

It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range 1-10 explicitly includes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to 100 includes 1, 2, 3, 4, . . . 97, 98, 99, 100. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by 10 can be taken as alternative upper or lower limits. For example, if the range is 1.1. to 2.1 the following numbers 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0 can be selected as lower or upper limits. Similarly, whenever listing integers are provided herein, it should also be appreciated that the listing of integers explicitly includes ranges of any two integers within the listing.

In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples.

For all compounds expressed as an empirical chemical formula with a plurality of letters and numeric subscripts (e.g., CH₂O), values of the subscripts can be plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures. For example, if CH₂O is indicated, a compound of formula C (0.8-1.2) H (1.6-2.4) ((0.8-1.2). In a refinement, values of the subscripts can be plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures. In still another refinement, values of the subscripts can be plus or minus 20 percent of the values indicated rounded to or truncated to two significant figures. The terms “compound” and “composition” are used interchangeably.

As used herein, the term “and/or” means that either all or only one of the elements of said group may be present. For example, “A and/or B” means “only A, or only B, or both A and B”. In the case of “only A”, the term also covers the possibility that B is absent, i.e. “only A, but not B”.

It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.

The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

The term “one or more” means “at least one” and the term “at least one” means “one or more.” The terms “one or more” and “at least one” include “plurality” as a subset.

The description of a group or class of materials as suitable for a given purpose in connection with one or more embodiments implies that mixtures of any two or more of the members of the group or class are suitable. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

Hydrogen gas, H₂, is proposed to be a clean alternate energy source to alleviate the critical problem of global warming and energy crisis faced by modern society. Hydrogen as fuel not only gives a superior energy density compared to fossil fuels (specific energy of 143 MJ per kg), but hydrogen gas also falls into the definition of zero emission fuel since its combustion product is only water. Moreover, depending on the synthesis route, hydrogen gas can be considered as either clean or green energy. The electrochemical production of hydrogen is a process that can be classified as a green process if the energy input comes from a renewable source.

The production typically involves breaking up the water molecule using electricity in a process called electrolysis. The traditional electrolytic reaction can be divided into two half-reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). The cathodic half reaction includes reduction of the water molecule into H₂ gas and OH⁻. The anodic half reaction includes oxidation of water to produce O₂ and H⁺.

The traditional water splitting electrolysis requires specific conditions. The redox potential of both half reactions is pH dependent. A cell potential of 1.23 V is needed in the overall process. Moreover, due to the nature of both reactions, involving 2 and 4 electron transfers, an overpotential (n) is typically needed to run the process. The electrode material may influence the overpotential demand. Traditionally, metals with catalytic properties towards HER and OER, such as platinum and iridium oxide respectively, are commonly employed in industrial electrolyzers. Yet it is the cost of these materials, as well as cost of additional components in electrochemical devices, that traditionally render hydrogen production cost prohibitive.

To enable hydrogen gas to become a viable, commercially competitive fuel, which in turn will allow for a more wide-spread utilization of the hydrogen technology, the U.S. Department of Energy set a goal to decrease production cost of clean hydrogen from the current $4 USD/kg to $2 USD/kg by 2026 and to $1 USD/kg by 2031. To meet these goals, there is an evident need for development of an efficient, effective, and economical hydrogen gas production.

Different aspects of hydrogen production have been targeted for improvement. For example, various exchange membranes have been designed, featuring novel materials or structures. Another approach has been to develop coatings protecting vulnerable metal components from the corrosive environment of the electrochemical cells. Focus has also been on the noble metals traditionally present on the electrodes and reduction of their loadings and degeneration. Further focus has been on identification of the most suitable electrode materials with catalytic properties towards OER and HER. For example, earth abundant electrocatalysts, such as Ni, Fe, Co, Mo, etc. have been in the forefront of the studies.

Another approach to reduce the cost of electrochemically produced hydrogen gas has been a hybrid water electrolysis system. The focus of a hybrid water electrolysis approach is the replacement of the OER reaction for another anodic reaction that either requires lower potential or brings an added value to the overall process. Examples of such anodic reactions are oxidation of urea, hydrazine, ethanol, or glucose. In general, new drawbacks arise with the replaced OER reactions, such as an increased risk of crossover contamination due to production of N₂ or CO₂ when hydrazine or urea are used, additional costs associated with introduction or alcohols and glucose, etc. The overall costs and production difficulties associated with the hybrid electrolysis have not been satisfactorily resolved. The cost of electrical input required has not been satisfactorily addressed.

Hence, the overall improvement in the hydrogen gas production systems have been minimal, especially on the large-scale production of hydrogen gas, and has not provided a pathway to reach the challenging summit mapped out by the U.S. Department of Energy.

In one or more embodiments, a hydrogen gas production system overcoming one or more drawbacks described above is disclosed. The system is an electrochemical system, cell, or unit. A non-limiting example of the electrochemical system may be an electrolyzer. The system may form a portion of a small-scale, mid-scale, or large-scale system including commercial and industrial systems.

An example traditional electrolyzer such as a polymer electrolyte membrane (PEM) electrolyzer is shown in FIG. 1. As can be seen in FIG. 1, the electrolyzer 10 includes a container 12 divided into two compartments or sides: a cathode side 13 and an anode side 15. The electrolyzer 10 further includes a cathode 14, an anode 16 placed in the container 12—the cathode 14 in the cathode side 13 and the anode 16 in the anode side 15. The electrolyzer 10 additionally includes an external power circuit 17 connected to the cathode 14 and anode 16, and a PEM 18 dividing the container 12. The PEM 12 acts as a separating barrier between the contents of the anode and cathode compartments 13, 15, allowing electrical contact from the anode side 15 to the cathode side 13, preventing mixing of gasses between the sides, and facilitating transport of hydrogen protons from the anode side 15 to the cathode side 13.

As was explained above, water splitting in a traditional electrolysis includes the OER at the anode 16 and the HER at the cathode 14. Typically, in simplified terms, water is provided to the anode side 15 to be split into oxygen (O₂), hydrogen protons (H⁺), and electrons (e⁻) at the anode 16. The formed protons then travel via the PEM 18 to the cathode side 13. The electrons exit via the external power circuit 16, which provides voltage for the reaction. At the cathode 14, the protons and electrons recombine to produce hydrogen gas (H₂). The traditional production of H₂ through water electrolysis at neutral pH (˜6) thus involves two different half-cell reactions occurring at the anode (1) and cathode (2):

2 ⁢ H 2 ⁢ O ( l ) → O 2 ⁢ ( g ) + 4 ⁢ H + + 4 ⁢ e - ⁢ E 0 = 0.905 V ⁢ vs ⁢ SHE ( 1 ) 4 ⁢ H + + 4 ⁢ e - → 2 ⁢ H 2 ⁢ ( g ) ⁢ E 0 = - 0 .324 V ⁢ vs ⁢ SHE ( 2 )

While the PEM electrolysis has a number of advantages, when compared to other types of electrolysis such as alkaline or solid oxide electrolysis, the PEM electrolysis features a number of drawbacks. The main one is a high cost of components, namely the requirement of catalysts present on the electrodes to catalyze the HER and OER reactions. Noble metals such as platinum (Pt), palladium (Pd), or their combination are usually used at the cathode while iridium (Ir), ruthenium (Ru), and/or their oxides are typically used on the anode to catalyze the OER. Additionally, the presence of the PEM presents a significant expense and limits the lifetime of the electrolyzer. Furthermore, deionized water (DI) is used for the splitting which imposes an operational limit. Additionally, still, the reactions are pH dependent.

At low pH (acidic conditions), the HER occurs at lower potentials, while the OER demands higher potentials. The opposite happens at high pH (basic conditions). At any pH, the minimum full-cell potential needed to achieve both reactions is about 1.23 V. Due to the complex nature of both half-cell reactions, which involve diffusion, chemical reactions, and electrochemical reactions, an overpotential (or extra applied potential) is usually needed to drive both the OER and HER reactions. Therefore, expensive electrode materials with catalytic properties such as Pt, Pd, or iridium oxide (IrO₂) are typically used to decrease the overpotential demands. Hence, regardless of the pH, at least about 2V are required for the system to operate. With all the factors combined, the PEM electrolysis equals currently about 3.7 USD/kg H₂ production.

In contrast to the traditional electrolyzer, the system disclosed herein features a different structure and operating mechanism resulting in numerous advantages discussed herein.

Among the advantages is absence of the OER reaction and/or the traditional water splitting reaction. The system omits or avoids triggering the OER reaction and lacks production of the oxygen molecules, hydrogen protons, and electrons from water. The system may replace the OER with an alternative, energetically less demanding anodic reaction. The system may thus produce hydrogen gas in a non-traditional way by providing an alternative source of electrons than water splitting. The herein-disclosed system is less energetically demanding, thereby having lower cost than the traditional systems utilizing the water splitting reaction.

Therefore, the system disclosed herein features an anode which may be free of noble or precious metals or the OER catalyst. As such, the system presents a less complex system with less overall material and assembly costs.

Additionally, the system may be free of a membrane. The system may be membrane-less. The system may lack or be free of a PEM and/or another membrane. The absence of the membrane results in additional structural simplification as well as component and assembly cost reduction. The estimated overall capital cost of an electrochemical cell utilizing the system disclosed herein may be reduced by at least about 20%.

The different structure and operating mechanism results in additional advantages such as the system not being dependent on pH. The system may operate at any pH level or condition including pH of about 0-14, 1-12, or 2-10. The operating pH, or the pH of the electrolyte, may be about, at least about, or at most about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14. The operating pH, or the pH of the electrolyte, may be negative pH or pH below 0. The operating pH range may be, in a non-limiting example, about 0-8, 0-6, 0-4, or 0-2. Any pH range utilizing the above-mentioned numerals is contemplated. The system may thus employ an electrolyte with acidic or highly acidic pH, including electrolytes with increased concentrations of ions such as Cl⁻, or both.

A further advantage of the system is a lower operating potential requirement than the traditional PEM electrolyzer described above. The system may operate with voltage as low as 0.5V (or lower) in acidic pH or 0.8V in seawater with pH of about 7. The operating voltage may be about 0.25-2.1, 0.4-1.2, or 0.5-0.8 V at pH of about 0-14, 0-8, or 0-6. The operating voltage may be at least about, as low as about, at most about, or about 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.10, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 2.0, 2.05, or 2.1 V at pH range disclosed herein. The operating voltage may include any value range lower than the operating potential required for one or more undesirable reactions such as the OER reaction, chlorine gas generating reaction, irreversible oxidation of the second electrode electron donor material, or a combination thereof.

Non-limiting example cell operating voltages for the systems disclosed herein at various pH values are shown in Table 1.

TABLE 1
Cell voltage at various pH values
during doping and de-doping steps

Parameter/Property		Minimum		Optimal	Maximum

Cell voltage at pH 0

0.25

V

0.45

1.65

V

doping step

Cell voltage at pH 2

0.38

V

0.55

1.78

V

doping step

Cell voltage at pH 4

0.52

V

0.75

1.92

V

doping step

Cell voltage at pH 6

0.64

V

0.95

2.04

V

doping step

Cell voltage at pH 8

0.77

V

1.15

2.07

V

doping step

Cell voltage at pH 0

0.62

V

0.7

V

No maximum

de-doping step

Cell voltage at pH 2

0.67

V

0.7

V

No maximum

de-doping step

Cell voltage at pH 4

0.7

V

0.82

V

No maximum

de-doping step

Cell voltage at pH 6

0.7

V

1

V

No maximum

de-doping step

Cell voltage at pH 8

0.7

V

1.25

V

No maximum

de-doping step

The redox potential (half-cell potential) for the anode reaction (doping) may be about 0.125 to 0.8 vs SHE. The redox potential (half-cell potential) for the anode reaction (doping) may be about, at least about, or at least about 0.125, 0.13, 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.175, or 0.8 vs SHE. The redox potential for the anode reaction (de-doping) may be about −1.25 to 0.15 vs SHE. The redox potential for the anode reaction (de-doping) may be about, at least about, or at least about −1.25, −1.2, −1.15, −1.1, −1.05, −1.0, −0.95, −0.90, −0.85, −0.8, −0.75, −0.7, −0.65, −0.6, −0.55, −0.5, −0.45, −0.4, −0.35, −0.3, −0.25, −0.2, −0.15, −0.1, −0.05, 0, 0.05, 0.1, 0.12, or 0.15 vs SHE.

The current density, at pH 0-8, 22° C. and optimal voltage disclosed herein may be about 4-22 mAm/cm² per one stack of cm² area. The current density may be about, at least about, or at most about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, or 22 mAm/cm² per one stack of cm² area. For example, the current density, at pH 0, 22° C. and optimal voltage disclosed herein may be about 11-22 mAm/cm² per one stack of cm² area. For example, the current density, at pH 2, 22° C. and optimal voltage disclosed herein may be about 10-22 mAm/cm² per one stack of cm² area.

The area represents the projected area of the membrane separating individual chambers. The stack may include, in a non-limiting example, an intrinsic conductive polymer(s) (ICPs) anode and Pt cathode. The current density may increase in a linear fashion with inclusion of additional stacks of CIP anode and Pt cathode.

In another non-limiting example, the stack may include a stack of ICP anodes such that multiple ICP anodes are stacked to work together as an anode with increased amount of ICP material and electroactive area. The size of the anode may be increased without increasing the size of the anion exchange membrane.

The hydrogen generation output may be about 0.0004-0.001 mmol/cm² of H₂ per 40s in a single stack (ICP anode+Pt cathode). The hydrogen generation may be about or at least about 0.0004, 0.0005, 0.0006, 0.0007, 0.0008, 0.0009, 0.001 mmol/cm² of H₂ per 40s in a single stack at pH 0-8, 22° C. and optimal potential disclosed herein.

The overall energy demand of the herein-disclosed system may be thus up to about 50% lower than the energy demand of a traditional electrolysis system. Considering all the factors named above, the estimated overall cost of green hydrogen may be thus reduced by at least about 40% by utilizing the system disclosed herein.

The system disclosed herein includes a plurality of components. The components may include a container, tank, vessel, chamber, cistern, receptacle, can, canister, tub, or the like. The system may be a single-chamber container. The system may be a multi-chamber system. The system may include one or more containers, for example 2, 3, 4, 5, or more containers. The containers may be closed, open, closeable, sealed, sealable, resealable, or a combination thereof. At least some of the containers may include an inlet, outlet, or both. A number of inlets and outlets may be varied. An example outlet may be a hydrogen gas outlet. The container(s) may be configured to accept, release, and hold a feedstock for a certain amount of time. The system may include a first container, a second container, etc.

The system may further include at least two electrodes. The electrodes may be in contact with the one or more containers. The contact may be direct or indirect. The electrodes may be asymmetrical electrodes regarding their composition and structure. Alternatively, the electrodes may be symmetrical, in at least some of the containers or their aspects. For example, the shape, structure, the base or bulk portion may be the same while the chemical composition of the top portion/layer/film or another property may be different.

The electrodes may include a first electrode. The first electrode may be or function as a cathode. The first electrode may be an inert electrode. The first electrode is structured to perform the HER or combination of protons and electrons to form hydrogen gas. The first electrode may be a HER electrode. The first electrode may include a bulk portion and a top layer. The top layer may be directly adjacent to the bulk portion. The top layer may include a loading of an electrocatalyst material. The electrocatalyst material may include one or more precious metals such as platinum (Pt), palladium (Pd), iridium (Ir), the like, or their combination. Alternative or additional catalytic material is also contemplated. In general, the electrocatalyst may be any material having high intrinsic electrocatalytic activity and providing good catalytic performance for the HER.

The bulk portion may include any suitable catalyst support material such as metal oxides and/or carbon. Carbon may be in any suitable form such as carbon paper, cloth, felt, glassy carbon, nanotubes, nanofibers, buckyballs, graphite, graphene, the like, or their combination. The bulk portion and/or the top layer may be modified by thermal, chemical, electrochemical treatment(s), for example to increase surface roughness, doping to introduce functional groups, the like, or a combination thereof. The mentioned materials may be especially suitable for acidic electrolyte environment.

In a neutral or basic electrolyte, the following one or more materials may be included on the first electrode: Pt alloys with nickel (Ni), cobalt (Co), molybdenum (Mo), iron (Fe) and oxides, ruthenium (Ru) alloys with Ni, Co, Mo, Fe and oxides, rhodium (Rh) alloys with Ni, Co, Mo, Fe and oxides, Pt, Pd, Ru, Rh and chromium (Cr) alloys, Pt, Pd, Ru, Rh coated carbon electrodes, molybdenum carbide (Mo₂C), Ni, Mo, Fe, or Cu alloys.

The system may include a second electrode. The second electrode may replace the traditional anode. The second electrode may include an electron donor material in the hydrogen gas production system and during the hydrogen gas production step. The second electrode may be an inert electrode. The second electron may have a plurality of active sites from which electrons may be released under suitable operating potential. The second electrode may be doped and de-doped due to the presence of the active sites. During a doping stage/step/state, the electrons may leave the active sites and be replaced with a plurality of anions. During a de-doping stage/step/state, the anions may leave the active sites and electrons may replenish the active sites.

The second electrode may have such structure and/or composition that the electrode may release electrons into the system. The electron release may be potential dependent. The material may have redox chemistry suitable for production of electrons via anodic reaction. The second electrode may include the electron donor material in a top layer, portions of the top layer, the bulk, portions of the bulk, or a combination thereof.

The electron-donor material may include a single material or a mixture of materials. The electron-donor material may be non-metallic conductor material. The material may include a polymer such as a conductive polymer or a mixture of conductive polymers. The material may have relatively high conductivity, good redox potential, or a combination thereof. The material may have an excellent environmental stability such that the material is corrosion resistant and acid stable in the pH of about 0-14 for an extended period of time. The resistance and stability of the electron-donor material, and the entire second electrode, may be long-term, including at least several days, weeks, months, or years.

The electron-donor material may be flexible, planar, wire-shaped. The material may include one or more nanostructures such as nanotubes, nanofibers, nanorods, nanowires, nanoparticles.

The electron-donor material may include a plurality of electron-donation sites. The amount of donation sites may vary. The electron-donation sites may be finite, predetermined, calculated, estimated, renewable, or a combination thereof. As the operating potential is applied to the system, electrons are released from the sites which are doped with anions. When all of the available sites are doped, the material may be undoped or de-doped (the terms are used interchangeably).

In general, when the second electrode includes the electron-donation material, denoted as Y, the redox chemistry of the material is introduced as a viable anodic reaction to produce electrons via two reactions:

Reaction 3 is the doping or discharging of the Y material of the anode. Reaction 3 includes the release of an electron and the uptake of an anion (A) from the electrolyte. Reaction 3 and the reverse process constitute the intrinsic mechanism through which the material is able to conduct electricity. The reaction potential of Reaction 3 may be dependent on the (a) synthesis conditions of the second electrode, (b) properties of the electrolyte such as the anion size and charge, or both. Reaction 3 may occur in a potential range with values named above. Reaction 4 is the irreversible oxidation of the second electrode. Reaction 4 may occur at potentials higher than about 0.8 V vs SHE in all electrolytes.

The material may include intrinsic conductive polymer(s) (ICPs). ICPs are organic polymers that conduct electricity. ICPs may have metallic conductivity or be semiconductors. IPCs generally have very good processability. Non-limiting examples of ICPs are polyacetylene, polyaniline, polypyrrole, polyphenylene, polyphenylene vinylene, polyphenylene sulfide, polythiophene, polyfuran, and poly(3,4-ethylenedioxythiophene) (PEDOT), or the like.

doping A non-limiting example of the material may be polypyrrole (PPy). When the second electrode includes PPy, the redox chemistry of PPY is introduced as a viable anodic reaction to produce electrons via two reactions:

Reaction 3′ is the doping of the PPy electrode. Reaction 3′ includes the release of an electron and the uptake of an anion (A) from the electrolyte. This reaction and the reverse process constitute the intrinsic mechanism through which PPy is able to conduct electricity. The potential of Reaction 3 seems to be dependent on the synthesis conditions of the PPy electrode, properties of the electrolyte such as the anion size and charge, or both. A non-limiting example of the Reaction 3′ potential range may be about 0.7 to 0.8 V vs SHE in a Na₂SO₄ electrolyte (pH 6) or about 0.6 to 0.8 V vs SHE in H₂SO₄ (pH 0). Reaction 3′ may occur in a different potential range with values named above, for example 0.25-0.79 or 0.5-0.79 V vs SHE. Reaction 4′ is the irreversible oxidation of the PPy electrode. Reaction 4′ may occur at potentials higher than about 0.8 V vs SHE in all electrolytes and is not desirable.

In the system including the first and second electrode described herein, the only half-cell reaction possible at the first electrode is the HER as the cathode-catalyst is an inert material. For the HER to occur, a suitable anodic reaction needs to occur to supply electrons to be consumed during the HER. When using the electron-donation material described herein, the second electrode or anodic reactions which may occur are limited to Reactions 1, 3 (or 3′), and 4 (or 4′). The operating potential may be used to determine which one of the anodic reactions (1, 3, 3′, 4, 4′) should occur. In other words, which of these reactions is active may be determined by the half-cell potential of the anode during the operation of the system. While Reaction 1 (the OER reaction) requires a relatively high potential, it can be avoided by keeping the potential lower than Reaction 1 operating potential requirement. Additionally, since Reaction 3 (3′) occurs at a lower potential than Reaction 1, hydrogen gas may be produced at a lower energy than the traditional electrolysis which utilizes Reaction 1. Thus, by keeping the half-cell potential on the anode in a range of about 0.4-0.7, 0.45-0.69, or 0.5-0.65 V vs SHE, the only anodic reaction active, and thus the only source of electrons, is Reaction 3 or the doping of the second electrode.

The system also includes an electrolyte. The electrolyte may be acidic in nature. The electrolyte may include any acidic aqueous solution. The electrolyte may include an acid. Non-limiting examples of an acid include acidic water, wastewater, steel pickling waste liquid, mining wastewater, spent industrial treatment solution(s), metal-rich water, seawater, brackish water, water with concentration of salts and/or minerals above the levels of drinking or deionized (DI) water. The electrolyte may include a strong acid or a weak acid. The acidic electrolyte may have pH of below 0, 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, or 6.5.

The electrolyte may be neutral such as having pH of about 6 to 7. The electrolyte may have a pH greater than 6 or 7, for example 8, 9, or higher.

The electrolyte may be pretreated to remove heavy metals, noble metals, rare earth metals, other metals, organic matter, sludge, debris, the like, or a combination thereof. The electrolyte may be alkaline. If the acidic electrolyte is sourced from other than pure acid, the electrolyte may provide an additional advantage of an environmental solution and a zero-waste technology.

The electrolyte may include one or more types of ions. Non-limiting examples of the ions include chloride (Cl⁻), sodium (Na⁺), sulfate (SO₄²⁻), magnesium (Mg²⁺), calcium (Ca²⁺), potassium (K⁺), metal ions, the like, or a combination thereof. Non-limiting examples of the electrolyte may include sulfuric acid (H₂SO₄), sodium sulfate (Na₂SO₄), sodium chloride (NaCl), the like, or a combination thereof. Other acidic compounds are contemplated.

The ions may potentially introduce additional anodic reactions to the system. The additional anodic reactions may occur at a higher operating potential than the desirable anodic Reaction 3. In a non-limiting example, the electrolyte may include sea water with Cl⁻.

The presence of Cl⁻ in the solution adds the following possible anodic reaction:

The oxidation of chloride gas would be an undesirable reaction. Reaction 5 occurs at a lower potential than the OER, due to its faster kinetics, thus replacing it. This is one of the main problems that has limited viability of the seawater electrolysis for H₂ production. When using the traditional Pt and IrO₂ electrode configuration, chlorine gas is produced before water splitting occurs which is problematic since chlorine is a highly hazardous gas and its dissolution products (hypochlorous acid) are highly corrosive.

In the herein-disclosed system including the electron-donor material described above at the second electrode and an electrocatalyst such as Pt at the first electrode, reactions 1, 3, 3′, 4, 4′, and 5 are possible anodic reactions to donate electrons to the cathodic reaction 2. The undesirable anodic reactions (1, 4, 4′, and 5) may be avoided by a selection of the operating potential to a value or set of values in a predetermined range. The predetermined range includes values below the values needed to initiate reactions 1, 4, 4′, and/or 5. Reaction 3 occurs at a potential range of about 0.1 to 0.65 V vs SHE in the seawater due to the smaller size and charge of the Cl⁻ anion.

In a non-limiting example, when 1.5 V cell potential is applied to the herein-described system including the first and second herein-described electrodes, and seawater as electrolyte, the half-cell potential of the anode is only about 0.6 V vs SHE, hence, the OER, CI gas formation, and the irreversible oxidation of the second electrode (reactions 1, 4, 4′, and 5) can be avoided. Reaction 3 is thus the only source of electrons to be consumed during H₂ production within the system.

Thus, the system may be structured to favor or trigger release of the electrons from the electron-donation material of the second electrode (anode) as opposed to triggering undesirable anodic reactions such as the OER, unwanted gas generation reactions, etc. This may be achieved by (a) the choice of the electron-donation material of the second electrode, (b) production parameters of the second electrode, (c) the choice of electrolyte, or a combination thereof.

The system may further include one or more additional components. For example, the system may include an external or secondary container, cell, chamber, etc. with a secondary electrolyte, secondary power source, and a third electrode. The power source may supply potential to a secondary electrical circuit which connects the second and third electrodes. The mentioned additional components may be structured to de-dope the electron donation material of the second electrode.

Further, the system may include one or more physical and/or chemical dividers. The barrier may include a polymeric spacer. The dividers may be membranes or salt bridges. The divider(s) may be located between the first cell and the second cell, within the first cell, or both. The divider(s) may be structured to separate different electrodes, electrolytes, containers, from one another, or a combination thereof. In a non-limiting example, a single-chamber system may include a membrane or salt bridge between the first and second electrodes, separating the HER electrode from the electron donor electrode. A divider may separate different electrolytes, for example an acid electrolyte surrounding the first electrode from an inert electrolyte surrounding the second and/or third electrodes. In another non-limiting example, a two-cell system may include a divider between the cells. In yet another non-limiting example, a single-chamber system may include two dividers, separating the first electrode from the second electrode and the second electrode from the third electrode.

The membrane may be included to prevent contact of heavy metal ions with the Pt catalyst which would cause contamination and a parasitic reaction competing against hydrogen production. The membrane may have one or more of the following properties: high selectivity (>98%), low resistance (about 0.2-5 ohm cm²), low thickness (about ≤220 μm), stability in a pH range of about 0 to 9, and stability up to about 60° C. The membrane may be an anion exchange membrane.

To provide additional capital cost reductions of the system, the system may be structured as a single-chamber cell free of a membrane, salt bridge, or both, as is further described below.

If more than one membrane or salt bridge is present, their composition may be the same or different, and at least some of the electrolytes may have the same or different composition and/or pH.

The electrode of the external cell may be referred to as the third electrode. The third electrode may be an inert electrode. The function of the electrode is to enable de-doping of the second electrode. The third electrode may include one or more materials. The type of materials used for the synthesis of the third electrode may be determined by the type of electrolyte used. If the electrolyte is acidic, the electrode may include carbon in any form mentioned above such as cloth, paper, fiber, graphite, glassy carbon, graphene, nanotubes, etc. Alternatively or in addition, the third electrode may include Pt, rhodium (Rh), gold (Au), IrO₂, titanium (Ti), stainless steel, ruthenium oxide (RuO₂), and/or platinum oxide (PtO₂).

In a neutral or basic electrolyte, the third electrode may include Pd oxides, nickel (Ni) oxides such as NiO, Ni₂O₃, NiO₂, Ni—Pd alloys, Ni-cobalt (Co) oxides, Ni-iron (Fe) oxides, Ni-zinc (Zn) alloys, Ni-aluminum (Al) oxides, Ni-phosphides, molybdenum (Mo) oxides, molybdenum disulfide (MoS₂), Mo—Fe oxides, Co—Mo oxides, Co oxides, Co hydroxides, Co phosphide, Co phosphates, Co-selenium (Se) oxides, Co—Se hydroxides, M_xCO_3-xO₄ (where M is Co, Mn, Fe), silver (Ag) oxides, Al oxides, manganese dioxide (MnO₂), manganese oxide (Mn₂O₃), trimanganese tetraoxide (Mn₃O₄), manganese oxide-hydroxide (MnO(OH)), Mn—Co oxides, iron oxide-hydroxide (FeO(OH)), iron phosphate-borate, iron phosphide (FeP), iron oxide (Fe₂O₃), copper (Cu) oxides, copper selenide (Cu₂Se), copper oxide (Cu₂O), Cu—Co alloys, stainless steel, the like, or a combination thereof.

The redox species in the external cell and the membrane or salt bridge may have various configurations non-limiting examples of which are shown in Table 2 below.

TABLE 2
Non-limiting examples of redox systems with corresponding electrolyte
requirements and salt bridge/membrane in the secondary external cell.

Redox couple	Electrolyte	Salt bridge/membrane
Fe2+/Fe3+	Aqueous acid electrolyte	Anion exchange membrane
	pH 0-3	or Salt bridge
ferroxyanide/ferrycyanide	Aqueous electrolyte	Cation exchange
	pH 0-14	membrane or Salt bridge
NAD+/NADH	Aqueous electrolyte	Salt bridge/Ion exchange
	pH 4-8	membrane
Quinone/Hydroquinone	Aqueous electrolyte	Salt bridge/Ion exchange
	pH 0-10	membrane

The redox system may enable high reversibility of the system described herein. The solution may be electrochemically regenerated when one of the species is depleted. When the redox system runs out such as, for example, all the Fe²⁺ is oxidized, a replacement redox system may be introduced or the redox system may be restored, for example chemically by introducing a reducing agent. A non-limiting example may be a Zn bar.

Additionally, the redox system may be replaced by a compound capable of being electrochemically oxidized. If the compound is a pollutant, the system may also serve as a remediation system for a contaminated water stream or another contaminated liquid. Non-limiting examples of the compound may include urea, hydrazine, ethanol, methanol, or formic acid. The compound may be oxidized using an inert electrolyte of basic pH (about 7-10).

The second electrode and other components of the system may thus be implemented in a synergistic seawater and acidic system, where seawater and an acid are used as electrolytes to produce H₂ and de-dope the second electrode, respectively. After complete exhaustion of the acid (such as after all the Fe²⁺ is oxidized to Fe³⁺), the acid may go through the acid treatment process to remove heavy metals, and the resulting pure acid may then be used in the first/second electrode system.

The system may further include one or more controllers programmed to set, control, operate, maintain, monitor, or change operating potential of the half-cell reactions within the system to predetermined or desired levels. For example, the controller(s) may maintain the operating voltage within the range such that the only anodic reaction active, and thus the only source of electrons, is Reaction 3 or the doping of the polymer during hydrogen production. The controller may also start, stop, interrupt, and/or maintain doping or hydrogen production. The controller may initiate, maintain, interrupt, and/or stop de-doping. The system may also include one or more sensors, providing and/or receiving input from/to the controller.

The system may have one or more states such as the first state and the second state. During the first state, the power source is turned on. During the first state, the second electrode may be undergoing doping, releasing electrons from a plurality of active sites and taking up anions from the electrolyte surrounding the second electrode. During the first state, the electrons may be travelling to the first electrode to combine with hydrogen protons to form hydrogen gas. During the first state, the power source may be releasing a relatively low voltage in the range described herein. The operating potential or voltage is below a potential needed to trigger one or more unwanted reactions such as the OER or unwanted gas production reactions. The first state may last until the active sites are all filled with anions and no electrons are available for release. During the first state, the first electrode functions as a cathode and the second electrode functions as an anode.

During the second state, the power source may be turned off and the first electrode may become inactive. During the second state, the second electrode may be de-doped such that anions are released from the active sites and the active sites are replenished with electrons. During the second state, a secondary external power source may be turned on, triggering oxidation of a redox species such that electrons generated by the reaction travel to the active sites of the second electrode. During the second state, the second electrode functions as a cathode and the third electrode functions as an anode. The second state may last until all active sites are de-doped. The second state may be followed by the first state. The second and first states may be repeated one or more times.

A non-limiting example of the system 100 disclosed herein is shown in FIG. 2. The system 100 of FIG. 2. includes a container 102 having a cathode 104, an anode 106, a power circuit 108 with a power source 109, and an electrolyte 110. The cathode 102 includes an electrocatalyst such as Pt, Pd, the like, or a combination. The anode 106 includes an electron-donation material described herein. The power source 109 serves to apply such operating potential that a non-OER anodic reaction is generated, resulting in release of electrons from the material of the anode 106. The electrons from the doping process are then spent at the cathode during the HER to reduce H⁺ and produce hydrogen gas. FIG. 2 also schematically shows the movement of the anions to the anode material. As can be seen in contrast to the system 10 of FIG. 1, no oxygen production occurs at the anode as the OER reaction is not triggered and no membrane is required to separate the electrodes. A controller 150, an optional component, is included in FIG. 2, but may be likewise present in all non-limiting examples described herein.

Another non-limiting example system 200 disclosed herein is shown in FIGS. 3A and 3B. In addition to the configuration 100 of FIG. 2, the system 200 includes an external cell 120, a third electrode 122, a second electrolyte 124, and a secondary external power circuit 126 with a secondary power source 128. The external cell 120 may be connected to the container 102 via a divider such as a salt bridge or membrane 130. In step/stage/state 1, shown in FIG. 3A, the system 200 produces hydrogen gas, as is described herein, until all available electron donation sites in the electrode 106 are doped. During hydrogen production, the secondary external power source 128 is turned off.

To de-dope the electron donation material of the anode/electrode 106, the primary power source 108 is turned off and the secondary external power circuit 126 is activated by turning the secondary power source 128 on. The de-doping mechanism is shown schematically in FIG. 3B. During the de-doping phase or step, hydrogen production is ceased. In a non-limiting example, an acidic electrolyte may be used as the electrolyte 124 in the external cell 120. The electrolyte 124 may be an acid including a redox system. In a non-limiting example shown in FIGS. 3A, 3B, the redox system includes Fe²⁺ and Fe³⁺ ions, respectively, which may be utilized during the de-doping. Under the applied potential, the Fe²⁺ ions may be oxidized to Fe³⁺ ions at the electrode 122 which works as the anode of the secondary power circuit 126. The electrons generated in the oxidation may be used to de-dope the electrode 106, which works as a cathode of the secondary power circuit 126. The influx of electrons at the electrode 106 causes the anions to leave active sites which are subsequently reoccupied by the incoming electrons. Other redox systems are contemplated as the Fe²⁺ and Fe³⁺ redox pair is only a non-limiting example.

In other non-limiting examples, the first, second, and third electrodes 104, 106, 122 may be all included in a single-chamber container 102. Such configuration is shown in FIGS. 4A-6B with different systems 300, 400, and 500.

In the system 300 shown in FIGS. 4A and 4B, a membrane or a salt bridge 130 is included within the container 102 to separate the first electrode 104 from the second 106 and third 122 electrodes. The first electrode 104 is thus isolated from the second 106 and third 122 electrodes. The membrane or salt bridge 130 also separates electrolytes 110 and 124. The electrolytes 110 and 124 may have the same or different pH, ion concentration(s), and/or composition. FIG. 4A depicts the doping step and FIG. 4B depicts the de-doping step with the anodic reaction A_red->A_ox+e⁻ at the third electrode 122 and a cathodic reaction at the second electrode 106.

In the system 400 shown in FIGS. 5A and 5B, the first, second, and third electrodes 104, 106, 122 are in a single-chamber cell 102. The system 400 may include an acid, alkaline, or seawater electrolyte, but not pure DI water. The electrodes' composition, and the redox system, may thus be selected as described above, based on the pH of the electrolyte 110. The redox system should be chosen such that reduction of the active species does not compete with the hydrogen gas production as that would lower efficiency of the system 400. Due to lack of any membrane, salt bridge, or a requirement for a separate container with a second electrolyte, the system presents a low capital cost investment and low manufacturing complexity.

A yet another non-limiting example system is shown in FIGS. 6A and 6B as system 500. As can be seen in the FIGS. 6A, 6B, the system 500 includes a separate compartment for each electrode 104, 106, and 122. Each compartment 110, 124, and 134 may thus include a different electrolyte. The same electrolyte in at least two chambers is also contemplated. The system 500 allows for specific targeting of each chamber solution and electrode to achieve maximum efficiency of the system 500. In a non-limiting example, the chamber 110 containing the first electrode 104 may include an acid, alkaline, seawater, or pure water electrolyte, the compartment 132 with the second electrode 106 may include an acidic electrolyte 134, which may be optimal for the doping and de-doping processes. Additionally, small size anions such as Cl⁻ may be present in the electrolyte 134, facilitating the processes, and resulting in lower potentials to achieve either of the redox reactions of the second electrode 106. Additionally, the redox system may be replaced with a compound capable of being electrochemically oxidized, for example a pollutant.

Additional arrangements utilizing configurations, principles, and systems disclosed herein are shown in FIGS. 17 and 18. Namely, FIG. 17 is a process flow diagram for a system disclosed herein utilizing an acid feed which may be utilized as the source of electrolyte for the herein-disclosed system. As can be observed from FIG. 17, the acid feed may optionally undergo one or more treatments to produce clean acid which may be provided to the system as an electrolyte. The acid may also be utilized to provide the redox system for doping of the second electrode. As a non-limiting example, ions may be Fe²⁺ from the waste acid feed. In a non-limiting example, the acid may be a wastewater acid feed.

In FIG. 18, seawater feed may be used for seawater electrolysis utilizing one of the materials named herein. An acid feed may be used in conjunction with the seawater. The system thus utilizes two sources of water for hydrogen production. The acid may be used for doping in the seawater electrolytic process as well as for doping in the low operating potential hydrogen production. The acid may also be treated and used for the low operating potential hydrogen production disclosed herein. In a non-limiting example, the acid may be a wastewater acid feed.

The electrodes described herein may be combined and/or multiplied as needed to form stacks, to increase current density, to increase hydrogen output, the like, or a combination thereof. For example, multiple ICP electrodes may be bundled to form an anode with increased amount of ICP and electroactive area to be paired with a cathode to form a stack. The bundle may include about 2-20, 4-15, or 6-10 ICP electrodes. A higher number is contemplated.

In another non-limiting example, a stack may be formed by a single ICP electrode and cathode. The stacks may be multiplied and combined to form a hydrogen generation system of a commercial or industrial scale. Multiple cells having one or more components described herein may be utilized together to form a lab, industrial, or commercial scale hydrogen generation system.

The herein-disclosed stack may be a stack of cells with polar or bipolar electrodes. The system may be a flow system with electrolyte flowing through the cells.

The electrodes, cells, units, systems, principles described herein may be used in a single-chamber or multi-chamber systems. The systems may include lab, commercial, industrial systems. The systems may include additional components, systems, subsystems. The systems may include hydrogen gas production systems. The systems may have additional functions such as water remediation.

A method of producing the system is described herein. The method may include preparing the first and second electrodes. The first and second electrodes may be prepared by the methods described in the experimental section. Other types of synthesis or manufacturing methods are contemplated.

A method of producing hydrogen gas is disclosed herein. The method may include providing a system having one or more components described above including one or more cells, one or more chambers, the first, second, and/or third electrode, one or more electrolytes, one or more membranes and/or salt bridges, and/or one or more controllers and sensors. The method may include keeping the system free of a membrane and/or salt bridge. The method may include separating the first, second, and/or third electrodes from one another, as was described above in the non-limiting examples. Alternatively, the method may include providing the first, second, and/or third electrode in a single chamber, which is or is not membrane free.

The method may include introducing electrolyte(s) into the chamber(s) such that the electrolyte(s) are in contact with the electrode(s). The method may include initiating, maintaining, and/or stopping doping of the second electrode and/or the hydrogen production. The method may include turning the power source on in a predetermined potential range such that the electrons from the second electrode travel to the first electrode, anions from the electrolyte fill active sites of the second electrode, and the electrons combine with protons at the first electrode to generate hydrogen gas. The method may include continuing hydrogen production until all active sites of the second electrode are filled, for a predetermined amount of time, or both.

The method may include regenerating or de-doping the second electrode. The method may include turning the first power source off. The method may include turning the second power source on, oxidizing a redox species to release electrons from the third electrode such that the electrons travel to the second electrode and reoccupy the active sites, and releasing anions form the second electrode active sites. The method may include continuing the de-doping process until all active sites of the second electrode are filled, for a predetermined amount of time, or both.

The method may include reinitiating hydrogen production as was described above. The method may include switching from the doping to de-doping processes in a predetermined manner, at a predetermined interval, upon input from one or more sensors or another source of information. At least some or all the steps may be done automatically, led by the controller(s).

The hydrogen gas production systems and principles described herein may form a portion of a small-scale, mid-scale, and/or large-scale systems and operations including numerous containers, chambers, membranes, and other portions. The hydrogen gas production system and principles described herein may thus form a portion of an industrial system such as a flow system, for example an existing commercial electrolyzer. Other industrial and commercial systems, incorporating the herein-disclosed hydrogen gas production system, are contemplated.

EXPERIMENTAL SECTION

Examples 1—a System with a PPY Electrode in 0.5 M Na₂SO₄ or H₂SO₄ Electrolyte

Materials

H₂SO₄ was purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Na₂SO₄, FeCl₃, FeSO₄ were purchased from Sigma-Aldrich and used as received. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric from Rock West Composites was used as electrode.

PPy Anode Synthesis

PPy films were synthetized over a carbon fiber (CF) bulk portion by electrochemical polymerization of the monomer at a constant potential of 1097 mV vs SHE, using a 1M NaCl solution that contained pyrrole in a concentration of 0.1 M. The electropolymerization time was studied by varying it in a range from 15 min to 6 h, the cyclic voltammetry of each electrode, and the bare CF control electrode, is shown in FIG. 7. As can be seen from FIG. 7, the CV showed different current values and redox process depending on the electropolymerization time or duration. For example, the CV curves of the electrodes made by 15 and 30 min of electropolymerization time showed similar current values; however, they showed a peak associated with a redox (oxidation) process at 0.6 and 0.8 V vs SHE, respectively. This redox process corresponds to the p-doping mechanism of PPy, resulting in the extraction of an electron of the x-bond, leaving a charge imbalance that is later compensated by the introduction of an anion in the polymer matrix. See Reaction 3′ described above.

No redox process related to the de-doping step was observed in the potential range explored during the CV. After 1 hour of electropolymerization, the peak related to the p-doping was still present at around 0.8 V vs SHE; this electrode showed the highest current values. Longer polymerization times, i.e., 3 to 6 h, resulted in a CV response with no peaks associated with redox process and a low current, which can be indicative of the overoxidation of the polymer.

Characterization of the Electrode

The PPy electrode was characterized by FT-IR using a Thermo Scientific Nicolet 6700 Ft-IR. The spectra were obtained using an ATR module and a compilation of 64 scans at a resolution of 16 cm⁻¹. X-ray photoelectronic spectroscopy (XPS) experiments were collected in a PHI 5000 VersaProbe II system from Physical Electronics, using monochromatic Al—Ka (1486.6 eV) radiation and a charge neutralizing system. Spectra of dried samples were recorded using a 100 mm beam size, operating at 25 W and 15 KV. The high-definition spectra were obtained using a pass energy of 23.5 eV and a 0.2 eV step size for 50 sweeps. SEM analyses were performed using a Nova 600 NanoLab DualBeam™-SEM/FIB, from FEI Company, Hillsboro, Oregon, USA.

Electrochemical Methods

The PPy was characterized using cyclic voltammetry (CV) and chronoamperometry (CA). A three-electrode configuration was used, where the PPy on CF was used as the working electrode (or second electrode), a Pt coil as counter electrode (or the first electrode), and a [Ag/AgCl/NaCl sat] system as reference electrode. A Biologic SP-50e Potentiostat, controlled by the software EC-Lab, was employed for the measurements. Either Na₂SO₄ or H₂SO₄ were used as electrolytes, both at a concentration of 0.5 M.

Results

The synthesized PPy electrodes and the CF control were used as anodes in an electrochemical cell with a Pt coil as cathode and 0.5 M Na₂SO₄ as electrolyte. The system was polarized at different cell potentials, and the current and half-cell potential of the anode were registered after 1 min of polarization. The results are shown in Table 3.

TABLE 3
Operation parameters of PPy electrodes with Pt as cathode and 0.5M Na2SO4 electrolyte

	Cell potential (V)	Anode potential (V vs SHE)	Current (mA cm−2)

6 h Electropolymerization	1.5	0.68 ± 0.01	0.65 ± 0.19
	1.8	0.93 ± 0.01	1.84 ± 0.37
	2	1.06 ± 0.01	2.76 ± 0.19
	2.2	1.18 ± 0.01	4.21 ± 0.74
	2.5	1.36 ± 0.04	6.84 ± 1.12
3 h Electropolymerization	1.5	0.66 ± 0.03	1.05 ± 0.37
	1.8	0.90 ± 0.03	2.10 ± 0
	2	1.04 ± 0.04	2.89 ± 0.37
	2.2	1.15 ± 0.04	4.07 ± 0.56
	2.5	1.32 ± 0.06	6.97 ± 1.30
1 h Electropolymerization	1.5	0.67 ± 0.01	1.71 ± 0.19
	1.8	0.89 ± 0	3.81 ± 0.93
	2	1.01 ± 0.03	5.26 ± 0.37
	2.2	1.09 ± 0.05	7.36 ± 0.74
	2.5	1.34 ± 0.07	11.18 ± 0.25
30 min Electropolymerization	1.5	0.66 ± 0	1.18 ± 0.19
	1.8	0.90 ± 0	2.63 ± 0
	2	1.01 ± 0	4.07 ± 0.19
	2.2	1.10 ± 0	6.57 ± 0.74
	2.5	1.22 ± 0	10.26 ± 1.12
15 min Electropolymerization	1.5	0.66 ± 0.01	0.39 ± 0.19
	1.8	0.90 ± 0.01	1.97 ± 0.19
	2	1.0 ± 0.01	3.15 ± 0.37
	2.2	1.09 ± 0.01	5.65 ± 0.19
	2.5	1.51 ± 0.03	2.76 ± 0.56
CF electrode	1.5	0.72 ± 0	0.26 ± 0
	1.8	0.99 ± 0	0.52 ± 0.1
	2	1.19 ± 0.01	0.65 ± 0.19
	2.2	1.36 ± 0.01	0.92 ± 0.19
	2.5	1.64 ± 0.01	1.84 ± 0.37

Stable values of current (compared to the control) were obtained when the anode reached a potential of 0.66 V vs SHE or higher. Differences in the current registered were observed for the different polymerization times studied. For example, the 1 h electrode showed the higher current at 0.66 V vs SHE, followed by the 30 min and the 3 h electrode. Interestingly, the 15 min electrode was outperformed by the 6 h electrode. The 1 h electrode outperforming all the other electrodes was expected and in concordance to the CV curves in FIG. 7. The performance of the 30 and 15 min electrodes may be explained as longer polymerization times should result in higher amounts of polymer on the electrode, and, since the doping process (i.e., electrons released from the polymer) is directly related to the amount of polymer, a thin film such as those obtained at 30 and 15 min might not have enough mass to compete against a PPy film obtained using longer polymerization times, despite of the latter being overoxidized. According to these results, the 1 h electrode was the electrode that showed the best performance and, hence, it was selected for all future experiments.

The anodic potential window, depicted in FIG. 8, showed only two peaks related to redox processes of the polymer and the potential window cut-off related to the OER. The first anodic process (ca. 0.45 V vs SHE) is the doping of the polymer according to reaction (3′). Meanwhile the anodic process observed at potentials higher than 1.1 V vs SHE is related to the irreversible oxidation of the polymer.

It was proposed that the irreversible oxidation of PPy happens simultaneously to water splitting at the anode to give out oxygen or OH, which are responsible for the introduction of oxygen in the polymer chains. See Reaction 4′. The introduction of oxygen moieties, pyrrolinone unit, interrupts the conjugation of the polymeric chains and increases the spacing between them, affecting the doping and de-doping capabilities of the material, and leads to the degradation of the polymer. This was confirmed by polarizing the PPy electrode at two different half-cell potentials, 0.6 and 1.1 V vs SHE and measuring the solution pH and total organic carbon (TOC) after the polarization, the results are summarized in Table 4.

TABLE 4
Solution pH and TOC before and after continuous operation at different potentials

		Applied Half Cell
Solution	Electrode	Voltage (V vs SHE)	Cell Voltage (V)	Time	Anode pH	Cathode pH	Feed solution pH	Anode TOC (ppm)

0.5M Na2SO4	1 h PPY	0.6	1.5	2 hours	4.4 ± 0.2	11 ± 0.3	5.2	2.8
0.5M Na2SO4	1 h PPY	1.1	2.2	2 hours	3.1 ± 0.2	11.2 ± 0.2	5.4	82.8
0.5M H2SO4	1 h PPY	0.6	0.6	2 hours	0.7 ± 0.1	0.7 ± 0.1	0.7	2.8
0.5M H2SO4	1 h PPY	1.1	1.5	2 hours	0.9 ± 0.1	0.7 ± 0.2	0.8	147.5
0.5M H2SO4	CF	0.66		2 hours	0.9 ± 0.1	0.9 ± 0.2	0.9	—
0.5M H2SO4	CF	1.14		2 hours	1 ± 0.1	0.9 ± 0.1	0.9	—

0.5M Na2SO4	CF	Adsorption Blank, NO potential	2 hours	3.8 ± 0.1	5.2	5.2	2.9
0.5M Na2SO4	1 h PPY	Adsorption Blank, NO potential	2 hours	4.4 ± 0.2	5.2	5.2	2.9

The results showed that applying a half-cell voltage of 0.6 V vs SHE does not cause the irreversible oxidation of the polymer, since the TOC of the electrolyte remained unchanged compared to a blank experiment where no potential was applied (adsorption blank). However, after a 2-hour polarization at 1.1 V vs SHE, the TOC in solution reached 82.8 and 147.5 ppm, in Na₂SO₄ and H₂SO₄ respectively. It is a clear indicator of the overoxidation and degradation of the polymer. After a few minutes of being polarized, a reddish/brownish sludge appeared around the electrode, indicating that the parts of the film were dissolving in the electrolyte.

Additionally, SEM micrographs (FIGS. 9A-9C) showed that after polarization at high potentials, the polymer film started to show cracks in its surface which could be related to the observed dissolution process that increased the TOC of the solution.

Table 4 above also shows the pH of the solution after the polarization of the electrode. A decrease in the pH, compared to the adsorption blank, was only observed when a potential of 1.1 V vs SHE was applied. At 0.6 V vs SHE the pH of the anodic chamber was the same as in the blank where no potential was applied, thus the decrease in pH does not indicate water splitting. The decrease in pH is usually observed during adsorption processes of cations on the surface of carbon materials, which usually involve a complexation or ion-exchange with the oxygenated groups present in these materials, mainly carboxylic acids, where the proton of the acid is exchanged with the metal ion (Na⁺) and thus the pH of the solution decreases. The pH decreased to a value close to the pKa of the carboxylic acid in carbon materials, which are present at a certain concentration in commercial carbon fiber. Additionally, the pH decrease was more pronounced when the bare CF was immersed in the electrolyte for 2 h (no potential applied). Thus, as long as a low potential is applied (0.6 V vs SHE), both water splitting at the anode and the overoxidation of the polymer can be avoided.

The anodic potential window of a PPy electrode and the cathodic potential window of a Pt electrode, both in 0.5 M H₂SO₄, is shown in FIG. 10. It was observed that when the reactions happening at both electrodes are combined, a cell potential of only 0.541 V is needed to achieve the production of H₂. When working with a cell configuration including a PPy anode and a Pt cathode, doping of the second electrode (Reaction 3′), overoxidation of the second electrode (4′), and the OER (1) are the only possible anodic reactions that could occur at the PPy electrode (second electrode). The HER is the only possible cathodic reaction at the Pt electrode (first electrode). A selection of the operation potential determines which anodic reaction takes place. Since reaction (3′) happens at a much lower potential than reaction (4′) and the OER (1), reactions 4′ and 1 may be avoided by choosing a lower operating potential. As a result, both reactions 4′ and the OER (1) can be avoided while producing H₂ at a lower energy than a standard electrolysis process involving both the OER and the HER. When the PPy electrode (second electrode) and low cell potentials were used, the electrons released during reaction (3′), at the anode, were consumed at the cathode (first electrode) to produce H₂.

A cell potential of 0.51 V was applied to the electrochemical cell and the current was registered. A rapid decrease in the current (until reaching the same current as in a carbon fiber blank experiment) was observed in a little more than 100 seconds, as is shown in FIG. 11. The effect was observed due to the complete doping of the polymer such that no more electrons were available for the production of H₂. After reaching complete doping, the PPy electrode was de-doped at −0.1 V vs SHE (half-cell potential). FIG. 11 shows the current obtained during the doping process (hydrogen production) after de-doping the PPy electrode for different times. It was observed that 2 min was enough for a complete de-doping of the electrode if the PPy electrode was completely doped before the de-doping process. Yet because complete doping of the electrode is not useful for the anode of a water electrolysis cell, 40 seconds was used as both doping and de-doping time.

Stability of the PPy 1-hour electrode was tested by cycling between doped and de-doped states for over 6 h. FIG. 12 shows doping and de-doping cycles of 40 seconds each for a continuous operation of 6 h. No important changes in performance were observed other than a decrease in current during the first 2 h of continuous operation that recovered to initial values of current. Symmetry between the positive (doping) and negative (de-doping) current densities was observed. Both observations indicate that the PPy electrode is suitable for long and continuous process and that the totality of electrons introduced to the polymer during the de-doping step are being consumed at the doping step to produce hydrogen.

The data in FIG. 12 was integrated to obtain the total charge, Q, of both steps to calculate moles of H₂ per kWh, kWh per m³ of H₂, and cost (USD)/kg of H₂. During the doping step 37 moles of H₂ were produced per kWh which accounted for almost 5-times the H₂ compared to the bare CF using a cell potential of 3.2 V (cell voltage). But during the de-doping process no H₂ was produced, therefore when considering this step into the calculations (whole process), there was only a slight improvement (1.4 times increase) in the overall efficiency (moles of H₂ per kWh) of the process (denoted as Approach 1 in FIG. 13) compared to the bare CF. To avoid this limitation, a de-doping approach using a redox system such as Fe^2+/3+ system was considered (Approach 2 in FIG. 13).

In Approach 2, the system included a separate cell that was connected to the anodic chamber (PPy electrode chamber) of the original electrochemical cell by a salt bridge, as is schematically shown in FIGS. 3A, 3B. The cell contained a CF electrode in a 0.5 M H₂SO₄ solution with 0.04 M of Fe²⁺.

During the de-doping step, the separate cell was activated, and the CF electrode worked as an anode (thus, the first electrode was deactivated) while the PPy electrode in the original cell worked as a cathode. The oxidation of Fe²⁺ to Fe³⁺ was achieved at 0.65 V vs SHE which translated in a cell potential of 0.75 V for the de-doping. The system was able to perform at 15.8 moles of H₂ per kWh which accounted for a 2.6-times increase in efficiency compared to the bare CF at 3.2 V. Using de-doping, the cost/kg of H₂ was as low as $2.2 USD (calculated using the actual retail price of electricity for the industrial sector of 7.2 cents per kWh) which falls close to the targeted goal of $2 USD/kg of H₂ set by the U.S. DOE for 2026.

Example 2—a System with a 1 Hour PPY Electrode in 0.1 M NaCl Solution

Materials

NaCl and FeSO₄ were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric from Rock West Composites was used as electrode.

PPy Anode Synthesis

The PPy film was synthetized over the CF by electrochemical polymerization of the monomer at a constant potential of 1097 m V vs SHE for 1 hour using a 1M NaCl solution that contained pyrrole in a concentration of 0.1 M.

Electrochemical Methods

The PPy was characterized using cyclic voltammetry (CV) and chronoamperometry (CA). A three-electrode configuration was used, where the PPy on CF was used as working electrode (second electrode), a Pt coil as counter electrode (first electrode), and a [Ag/AgCl/NaCl sat] system as reference electrode. All the half-cell potentials are reported vs SHE. A Biologic SP-50e Potentiostat, controlled by the software EC-Lab, was employed during these experiments. A 0.6 M NaCl solution was used as electrolyte to simulate seawater.

Results

The PPy electrode showed a slightly different behavior in 0.5 M NaCl than was observed in both Na₂SO₄ and H₂SO₄ electrolytes. FIG. 14 shows the CV of the PPy electrode in both Na₂SO₄ and NaCl solutions. It was observed that the peak related to the doping process of PPy appeared at lower potential compared to the doping process in Na₂SO₄. This was confirmed by the low scan rate exploration. The doping process of PPy happened at 0.1 and 0.5 V vs SHE for NaCl and Na₂SO₄ electrolytes, respectively. The de-doping process also shifted to more positive potentials, from −0.65 V vs SHE in Na₂SO₄ to −0.5 V vs SHE in NaCl. This could be related to the lower charge and smaller size of Cl⁻ compared to SO₄²⁻, thus making it easier to introduce it and release it from the polymeric matrix.

FIG. 15A shows the cathodic potential window of the PPy electrode in the 0.6 M NaCl solution and the anodic potential window of a CF inside the de-doping Fe^2+/3+ system. When working with Cl⁻ as anion, the cell potential needed for the de-doping of the PPy electrode was only 1.24 V. When doing seawater electrolysis, Fe^2+/3+ system was implemented to avoid both the OER and production of chlorine gas.

FIG. 15B shows the anodic potential window of the PPy electrode in the 0.6 M NaCl solution and the cathodic potential window of a Pt electrode in the same solution, showing that only 0.81 V of cell voltage was needed to achieve the production of H₂.

The PPy electrode and other components of the system may thus be implemented in a synergistic seawater and acidic wastewater system, where seawater and acidic wastewater feeds are used to produce H₂ and de-dope the PPy electrode, respectively. After complete exhaustion of the acidic wastewater (i.e., after all the Fe²⁺ is oxidized to Fe³⁺), the acidic wastewater may go through the acid treatment process to remove heavy metals, and the resulting pure acid may then be used in the acid PPy/Pt system.

Several doping and de-doping steps of 40 seconds were performed until 4 h of continuous operation were reached. The maximum current density obtained by the system was 25 mA/cm² and it decreased constantly over the 40 seconds of operation until reaching 1 mA/cm². FIG. 16 shows the moles of H₂ per kWh, the kWh per m³ of H₂, and the cost per kg H₂ obtained from the system and compared to a CF/Pt system in 0.6 M NaCl.

Example 3—Optimization of the PPy Anode

Materials

NaCl, sodium oxalate (Na₂C₂O₄), oxalic acid (C₂H₂O₄), dodecyl benzene sulfonate (DBS salt), and dodecyl benzene acid (DBS acid) were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric (CF) from Rock West Composites was used as electrode.

PPy Anode Synthesis

Different parameters of the PPy anode synthesis process, such as electropolymerization at constant potential or constant current, time, pH, and anion size were investigated. First, the effect of using either constant potential or current during the synthesis was investigated. PPy anodes were fabricated applying constant 0.9 V or constant current densities of 0.025, 0.25 and 2.5 mA cm⁻²; all of them synthesized using a 1 M NaCl solution that contained 0.1 M Pyrrole. The effect of pH of the monomer solution was investigated by adjusting pH from 5 to 8 using either NaOH or HCl. The PPy film was synthetized over the CF by electrochemical polymerization of the monomer using 1 M solutions of either NaCl, Na₂C₂O₄, C₂H₂O₄, DBS salt or DBS acid, that contained pyrrole in a concentration of 0.1 M.

Results

Previous research used a PPy anode produced using a solution of 0.1 M PPy and 1 M NaCl (pH=2.5) and an electropolymerization process at 1.1 V vs SHE. The electropolymerization was varied between 15 min to 6 h. The results showed that 1 h electropolymerization resulted in the most redox active PPy film, described by the high current and low potential of the redox process observed by chronoamperometry and cyclic voltammetry, respectively. Higher times resulted in a more oxidized PPy film and, thus, hindered redox activity.

The effect of current density was investigated by using values of 0.25 and 0.025 mA cm⁻² during the electropolymerization process instead of a fixed potential of 1.1V vs SHE. When the polymerization was carried out using 0.25 and 0.025 mA cm⁻², the potential of the CF electrode stayed at a constant 0.75 V vs SHE. High positive potentials, close to the water splitting potential (1.1 V vs SHE), generally produce films with a higher degree of oxidation, due to the production of OH· radicals, which can react with the pyrrole units in the polymeric chains and oxidize them into different pyrroline units (Eq. 4′). The latter eventually results in the loss of conjugation, increase in the spacing between the chains, its hydrophilicity, and, if the oxidation continues, hydrolysis of the chain. Therefore, the introduction of oxygen moieties has implications in the doping and de-doping capabilities of the film since the doping process of the polymer replaces the OER, thus, maximizing the available doping sites for a given mass of PPy.

FIG. 19A shows electrode potential profiles measured during the electropolymerization of pyrrole onto CF at different conditions. FIG. 19B shows chronoamperometry of the doping step of the different PPy electrodes in 0.5 M H₂SO₄. FIG. 19C depicts electrochemical impedance spectroscopy (EIS) of the different PPy electrodes obtained in 0.5 M H₂SO₄. FIG. 19D shows chronoamperometry of the doping step of the different PPy electrodes fabricated at the same current density, 0.25 mA cm⁻² at different pH, obtained in 0.5 M H₂SO₄.

The electrodes produced by polymerization at a constant potential of 1.1 V (equivalent to 2.5 mA cm⁻², labeled HI-PPy) and 0.25 mA cm⁻² (low current PPy, labeled LI-PPy) were polarized at the doping potential of 0.5 V vs. SHE, FIG. 6B. LI-PPy reached higher peak currents than HI-PPy; however, for LI-PPy the current decreased sharply until reaching values close to 0, while HI-PPy showed a slower current decline. This decrease of current corresponds to the exhaustion of all the accessible doping sites in the film; results suggest this behavior was related to the amount of polymer deposited on the CF substrate due to the different electro-polymerization rates. The surface morphology was neglected as a factor since all the films showed a similar nodular morphology. The weights of PPy deposited on CF were 2.64 and 0.82 mg cm⁻² at 2 and 0.25 mA cm⁻², respectively. The polymerization at 0.25 mA cm⁻² was extended to 3.3 h (3hLI-PPy) to match the mass of PPy on the HI-PPy electrode. The anode showed a steadier decrease in current, similar to the HI-PPy electrode. Moreover, the charge transfer resistance, described by the diameter of the semicircle in the EIS (FIG. 19C) measurements, was 2 to 3 times higher for the HI-PPy electrode, compared to the 3hLI-PPy, showing that the latter has higher structural order, thus, more doping sites are available, and this conferred lower resistance.

The effect of pH during the electropolymerization process was also investigated (the current density was fixed at 0.25 mA cm⁻²) and is depicted in FIG. 19D.

The effect of anion size during the polymerization reaction was investigated and is shown in FIG. 20. Different performance was obtained based on the anion used, the performance followed the order DBS salt<DBS acid<Oxalate salt, Cl—≤Oxalate acid. Oxalic acid performed better than chloride in the first doping/de-doping cycles, but performance of oxalic acid started to drop until both films had a similar performance. The results were the opposite of what was observed in the chemical polymerization of pyrrole, in that case, using a sterically large anion such as dodecyl benzene sulfonate (DBS) resulted in a PPy film of higher conductivity/performance.

Overall, the best performance anode was achieved when synthesized by an electrochemical polymerization at a constant density current of 0.25 mA cm⁻² in either NaCl (pH 7) or oxalic acid electrolyte for a total of 3.5 of polymerization. However, the anode can be synthesized in a wide range of conditions: different supporting electrolyte, pH, current, potential applied, the like, or a combination thereof.

Example 4—Performance of the PPy Electrode at Different Temperatures

Materials

NaCl and HCl were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric (CF) from Rock West Composites was used as electrode.

PPy Anode Synthesis

PPy anodes were fabricated applying constant current density of 0.25 mA cm⁻² to a CF piece immersed in a 1 M NaCl solution that contained 0.1 M Pyrrole for 3.3 h.

Evaluation

The performance of the PPy cell (PPy as anode, Pt as cathode) was evaluated at different temperatures, 35, 45, 55° C. A lab unit PEM electrolyzer from Ecscell (Ezer_025) having a commercial Membrane Electrode Assembly (MEA) with IrO₂ and Pt, each coated on a side of a Nafion® membrane, was also evaluated for different key performance indicators (KPIs), such as voltage, feed solution flow rate, feed solution composition and temperature.

Results

The KPIs of the PEM electrolyzer are shown in FIGS. 21A-D which provide insight to the operation conditions of the PPy electrolyzer.

These results allowed identification of differences between a PEM and the PPY electrolyzer. For example, when operated at below 2 V of cell voltage, the PEM electrolyzer produced almost no current at all (FIG. 21A), despite the electrolyte used. Moreover, the PEM electrolyzer performs better with DI or acid (pH 0) as feed, while its performance was completely diminished when Na₂SO₄ was fed into it (FIG. 21D). Without limiting the disclosure to a single theory, it is believed that Nafion® performs the best when only protons are responsible for moving the charge through the membrane and that the performance of the membrane is significantly diminished when other charged species are present on the media. On the other hand, the PPy electrolyzer, due to the fact that the OER is being replaced by the doping process of the polymer, was able to operate at voltages as low as 0.5 V and 1.2 V in acid and neutral electrolyte, respectively.

The experiments also showed that the flow rate of the solution running through the electrolyzer did not have a significant effect on its performance (FIG. 21B). It is believed that probably due to the slow kinetics of water splitting, flow rate is not a limiting factor. A similar observation was made in a lab scale PPy electrolyzer. There was an increase in performance as the temperature of the feed solution increased (FIG. 21C), where the PEM electrolyzer showed a 20% increase in performance when operated at 45° C. in comparison to operation at 22° C.

The effect of temperature on the PPy electrode doping process was investigated by electrochemical impedance spectroscopy (EIS), shown in FIG. 22A. The effect on the charge transfer resistance (R_CT), which can be interpreted as the resistance of the PPy anode for donating an electron during the doping process, was found to be minimally affected by temperature. The R_CT values were 0.304, 0.284, and 0.25 (2 at 35, 45 and 55° C., respectively (for comparison the R_CT at room temperature was 0.299Ω). A small increase in performance during the doping and de-doping cycles was observed until 45° C.; no significant improvement was seen when the temperature was raised to 55° C., shown in FIG. 22B. This indicated that the PPy electrolyzer could be operated at temperatures similar to a PEM electrolyzer. Experiments revealed that the doping of the PPy electrode could be a spontaneous process, therefore not affected by the temperature. For example, a PPy electrode was completely de-doped by applying a potential of −0.5 V vs SHE for 6 min, then the electrode potential was monitored while it was immersed in acid, FIG. 22C. The potential of the PPy electrode slowly increased until reaching a constant value of around 0.45 V vs SHE. Doping of the electrode happened spontaneously when it was immersed in acid and, to obtain the maximum performance from the electrode, the de-doping of the electrode needed to be followed by its polarization at the doping potential, otherwise its charge was lost. The de-doping step of the anode, and the redox processes at the cathode, were improved by a temperature increase, due to the endothermic nature of the reactions; therefore, the performance of the PPy electrolyzer could also be improved by working at high temperatures, just as fora PEM electrolyzer.

Example 5-De-Doping Strategies

Materials

NaCl, HCl, H₂SO₄, FeSO₄, and Na₂S₂O₅ were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric (CF) from Rock West Composites was used as electrode.

PPy Anode Synthesis

PPy anodes were fabricated applying constant current density of 0.25 mA cm⁻² to a CF piece immersed in a 1 M NaCl solution that contained 0.1 M Pyrrole for 3.3 h.

Electrochemical Evaluation

A flow through cell with two compartments was implemented as a lab scale electrolyzer. One compartment of the cell contained the PPy anode and a Pt-coated Ti mesh, both electrodes were stacked (simulating a sandwich or stack configuration) only separated by a plastic spacer. The compartment was filled with 0.5 M H₂SO₄ at a flow rate of 1.5 mL/min. A second compartment of the cell contained a CF electrode; the second compartment was filled with 1 M Fe₂SO₄ and 0.5 M H₂SO₄ solution, simulating the concentration of pickle liquor; the solution was also fed at a rate of 1.5 mL/min. An anion exchange membrane (Membranes International Inc.) was used to separate the two compartments. An anion exchange membrane is preferred to avoid Fe ions reaching the PPy/Pt compartment. The system was cycled between doping and de-doping states of the PPy electrode. During the doping step, a power supply was used to apply 0.55 V between the PPy anode and the Pt-coated Ti mesh. During the de-doping step, the power supply applied 0.77 V of cell voltage between the PPy, working as a cathode, and the CF electrode immersed in the solution containing Fe⁺².

Results

The system represented a burst to burst or cyclic approach to produce hydrogen, where only one of the steps (doping step) was producing hydrogen, while the other was consuming Fe²⁺ and energy to regenerate the electrode. De-doping/doping cycles were performed to calculate the energy required to produce a kg of H₂, shown in FIGS. 23A and B. As can be observed from FIG. 23B, most of the energy was spent in de de-doping cycles, even after the implementation of the Fe²⁺ solution to de-dope the electrode. The results indicated that the energy spent (kWh) on de-doping was higher than for the doping step, 61 and 39%, respectively. The system required only 33 kWh per kg of H₂, which was a considerable decrease compared to using OER as anodic reaction to de-dope the PPy electrode. The reactions of this approach were as follows:

Cathodic and anodic reactions for the H₂ production step (doping step):

Cathodic and anodic reactions for the de-doping step:

A chemical approach to de-dope the polymer was also provided to decrease the energy and time of this step and potentially transform the system into a continuous production of H₂. The approach included using sodium bisulfite as a reducing agent, which may reduce the PPy electrode:

Sodium bisulfite may also be used to regenerate the exhausted Fe²⁺ solution:

By extrapolation of the experimental data, it was calculated that the PPy would need to donate 6.02×10²⁶ electrons to achieve a 1 kg H₂ production. Since doping and de-doping are almost symmetrical processes, at least the same number of electrons would be needed to be introduced into the PPy by the chemical de-doping step.

The electrochemical de-doping of the polymer consuming Fe²⁺, or the like, is a viable option to regenerate the electrode. Any other ion or compound able to be oxidated may be used to regenerate the electrode, such as an organic pollutant. This situates the PPy electrolyzer as an alternative for water remediation and simultaneous H₂ production as long as the particular feed contains a chemical available to be oxidated (especially if its oxidation potential is below OER).

Example 6—PPy Electrode Stability Experiments

Materials

NaCl, HCl, H₂SO₄, FeSO₄ were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric (CF) from Rock West Composites was used as electrode.

PPy Anode Synthesis

PPy anodes were fabricated applying constant current density of 0.25 mA cm⁻² to a CF piece immersed in a 1 M NaCl solution that contained 0.1 M Pyrrole for 3.3 h.

Electrochemical Evaluation of Electrode Stability

The stability of the electrode was assessed by two different approaches: (1) operating the system described in Example 5 continuously for 6 h and (2) cycling the electrode using a potential scan from the de-doping to the doping potentials at 100 m Vs⁻¹ for 1,500 cycles, simulating 100 h of operation. A three-electrode cell with the PPy as the working electrode, a Pt coil the counter electrode, and a Ag/AgCl/NaCl (sat) as reference electrode were used. The 3-electrode cell was filled with 0.5 M H₂SO₄.

Results

The 6-hr continuous operation included some fluctuations on the performance of the PPy anode, i.e., a slight decrease in current density during the first 30 min of operation, followed by a steady increase in current density until reaching the initial performance at ˜180 min, as is illustrated in FIG. 24A. The variations could be due to morphological changes of the polymer matrix during doping and de-doping cycles, since the FT-IR spectra showed no significant changes after the process, as evidenced by FIG. 24E. Specifically, no increase in the C═O stretching vibration (at ˜1710 cm⁻¹) was observed, verifying that the fluctuations in performance were not caused by irreversible oxidation. The C/O ratios obtained from the XPS spectra were 1.32 and 1.34 for the pristine PPy and the electrode after 6 hours of operation, confirming that there was no irreversible oxidation of the electrode after long operation times.

The electrode also showed high stability through the 1,500 cycles, as is shown in FIG. 24B. After the simulated 100 hr of continuous cycling, the de-doping/doping profiles of the electrode showed similar peak current values to the pristine electrode, as is shown in FIG. 24C. The charge transfer resistance of the electrode was 0.28 and 0.30 ohms before and after 1,500 cycles, respectively, indicating no significative change in performance caused by irreversible oxidation, shown in FIG. 24D. The results pointed to high stability on the intrinsic redox process of the PPy anode and suitability of the material's life expectancy for an electrolyzer application, as long as the potential of the PPy electrode is kept below its over oxidation potential, 1.05 V vs SHE.

Example 7—Improving the Performance of the PPy Electrolyzer

Materials

NaCl, HCl, H₂SO₄, FeSO₄, and Na₂S₂O₅ were purchased from Sigma-Aldrich and used as received. Pyrrole from Alfa Aesar was kept in a N₂ atmosphere and refrigerated at 4° C. before use. Toray T300 Fiber, 3K Plain Weave Carbon Fiber Fabric (CF) from Rock West Composites was used as electrode.

PPy Anode Synthesis

PPy anodes were fabricated applying constant current density of 0.25 mA cm⁻² to a CF piece immersed in a 1 M NaCl solution that contained 0.1 M Pyrrole for 3.3 h.

Performance Increase by Stacking Multiple PPy Anodes

Multiple PPy/CF pieces were stacked to work together as an anode with increased amount of PPy and electroactive area. This approach allowed to increase the size of the anode without increasing the size of the anion exchange membrane (separating the clean acid and the Fe₂SO₄ compartment) and the Pt-coated mesh. The area of the anion exchange membrane was kept at 20 cm². The performance of the system was evaluated by doping and de-doping cycles.

Results

The doping/de-doping cycles of 5 different PPy pieces was observed. The performance of each individual piece was the same, showing the efficiency of the anode fabrication. Once put together into a stack, the 5 PPy pieces showed a 2× increase in performance. A linear relationship between the number of PPy pieces stacked and the performance was observed.

The processes, methods, or algorithms disclosed herein may be deliverable to or implemented by a processing device, controller, or computer, which may include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms may be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, or algorithms may also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software, and firmware components.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.