METHODS OF IMPROVING ELECTRODES FOR LIQUID ALKALINE WATER ELECTROLYZERS

Description

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 63/721,296, filed 15 Nov. 2024, which is hereby incorporated by reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in this invention.

BACKGROUND

Liquid alkaline water electrolysis is a long-standing commercial technology for production of hydrogen from water feed and electrical power input. This technology is established at scale, and does not utilize expensive platinum-group metals as catalysts. Ni is a preferred catalyst material, due to its low cost and chemical resistance to alkaline media. Classic alkaline electrolyzers utilize a spacer between the porous diaphragm and the Ni-based electrodes, to reduce gas crossover especially at high operating pressure. Recent interest in efficient generation of green hydrogen from renewable electricity has prompted a re-examination of the liquid alkaline water electrolyzer (LAWE) design. The goal is to develop a high-performance, durable next-generation LAWE design that is optimized for low-cost production of hydrogen from green electrons. An approach taken by the Hydrogen from Next-Generation Electrolyzers of Water (H2NEW) consortium is to utilize a zero-or near-zero gap design with the electrodes contacting the membrane to reduce ohmic losses, and to examine aspects of the electrode structure, catalyst composition, separator, operating protocols, and other aspects of the LAWE technology informed by modeling.

The Ni catalyst is typically structured as a macro-porous 3D material such as a perforated sheet, foam, mesh, or felt (or Ni coated on a steel substrate with these shapes). It forms the electrochemically-active electrocatalyst region near the separator, and provides electrical connection between this region and the current collector or flowfield. Further, in the case of zero-gap cell designs, the pores in the structure support transport of liquid to the active region, and removal of hydrogen or oxygen gas through the bulk of the electrode. The structure of the Ni electrode is important for electrochemical performance of the LAWE device.

In particular, low electrochemical surface area (ECSA) of the Ni electrode limits performance. Many previous studies have employed various approaches and techniques to enhance the Ni ECSA. Raney Ni is a well-known high-surface area catalyst formed by dissolving Al or Zn from a Ni alloy, and has been widely applied to LAWEs. Raney Ni catalyst powder was deposited as an ink directly on the separator diaphragm, resulting in a 270 mV cell potential improvement at 0.3 A cm⁻². Porous 3D Ni structures and planar electrodes have also been enhanced by various techniques. Ni foam was cleaned in three solutions, electroplated with NiZn, heat treated in hydrogen at 600° C., and etched in HCl to obtain sub-micron pores that provided a 25-fold improvement in exchange current density. Ni sheet was laser-structured to create sub-micron trenches and nano-scale protrusions that enhanced surface area by ˜15 times and improved cell potential by 120 mV at 0.3 A cm⁻². DC/RF sputtering of Ni onto Ni foam created a 10 μm thick surface layer with nano-scale trenches, improving cell potential by ˜200 mV at 0.8 A cm⁻². Ni mesh was hot-dip galvanized and leached in KOH, producing angular micron-scale roughness that remarkably improved cell potential by 470 mV at 0.5 A cm⁻². While these techniques to increase ECSA are generally effective at improving electrochemical activity, they are relatively complicated and resource-intensive to implement.

SUMMARY

Two methods for enhancing liquid alkaline water electrolyzer performance are described herein. Both improve three dimensional Ni electrodes by introducing a micron-scale rough structure throughout the bulk of the electrode. An oxidation/reduction method relies on a thermal treatment cycle to create surface roughening through the volumetric expansion during nickel oxide (NiO) formation and contraction during reduction back to Ni metal. A catalyst infiltration method introduces a washcoat of additional metal particles throughout the electrode by flooding the electrode with catalyst precursor and converting the catalyst precursor to micron-scale particles via a reducing thermal treatment. For both methods, the observed electrode surface structure and performance is dependent on the thermal treatment temperature.

Details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an SEM image of as-received Ni foam.

FIGS. 2A-2F show the oxidation/reduction processing of Ni foam. FIG. 2A shows the weight gain of Ni foam after oxidizing in air for 2 hours at various temperatures. The 27% weight gain expected for complete oxidation of Ni to NiO is indicated as a dashed line. SEM images of Ni foam (FIG. 2B) as-received and after oxidation at (FIG. 2C) 700° C. or (FIG. 2D) 1000° C. followed by reduction at 600° C. Comparison of as-received and roughened Ni foams using (FIG. 2E) 3-electrode cell CV results with double-layer capacitance (mF cm⁻²), and (FIG. 2F) full LAWE cell polarization curves at 80° C.

FIGS. 3A-3C show SEM images of catalyst-coated Ni foams. SEM images of Ni foam coated with Ni-3x via infiltration, and then reduced to Ni metal at (FIG. 3A) 500° C., (FIG. 3B) 700° C., or (FIG. 3C) 900° C.

FIGS. 4A-4D show the performance of catalyst-coated Ni foams, comparing as-received and catalyst-coated Ni foams using full LAWE cell polarization curves at 80° C. Impact of (FIG. 4A) reducing firing temperature for Ni-3x, (FIG. 4B) reducing firing temperature for NiFe-3x, (FIG. 4C) Ni catalyst loading (all fired at 600° C.), and (FIG. 4D) catalyst composition (all 3x, and fired at 600° C.).

FIG. 5 shows an example of a flow diagram illustrating a method for increasing the active surface area of a Ni electrode.

FIG. 6 shows an example of a flow diagram illustrating a method for increasing the active surface area of a Ni electrode.

DETAILED DESCRIPTION

Reference will now be made in detail to some specific examples of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Particular example embodiments of the present invention may be implemented without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention will sometimes be described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless noted otherwise.

The terms “about” or “approximate” and the like are synonymous and are used to indicate that the value modified by the term has an understood range associated with it, where the range can be ±20%, ±15%, ±10%, ±5%, or ±1%. The terms “substantially” and the like are used to indicate that a value is close to a targeted value, where close can mean, for example, the value is within 80% of the targeted value, within 85% of the targeted value, within 90% of the targeted value, within 95% of the targeted value, or within 99% of the targeted value.

Described herein are methods for increasing the performance of Ni electrodes. Both rely on increasing the active surface area of the catalyst. The first, oxidation/reduction, utilizes thermal treatment in air to oxidize the Ni and impart micron-scale roughness on its surface. Reducing the NiO back to Ni retains the roughness while further introducing micron-scale pores. The second, catalyst infiltration, deposits additional Ni, Fe, Co, Mo, Cr, or mixtures thereof throughout the Ni structure, which is then sintered and bonded to the Ni structure through a reducing thermal treatment.

FIG. 5 shows an example of a flow diagram illustrating a method for increasing the active surface area of a Ni electrode. Starting at block 505 of the method 500 shown in FIG. 5, a nickel electrode is provided. In some embodiments, the nickel electrode is an electrode from a group a nickel foam, a nickel felt, nickel mesh, nickel particles that are sintered together, and a perforated nickel sheet.

At block 510, the nickel electrode is heat treated (a first heat treatment) at about 500° C. to 1000° C. in air to oxidize the nickel electrode. In some embodiments, the heat treatment includes holding the nickel electrode at about 500° C. to 1000° C. in air for about 2 hours. In some embodiments, the heat treatment is at about 600° C., at about 700° C., or at about 900° C.

In some embodiments, the heat treatment generates nickel oxide on surfaces of the nickel electrode. In some embodiments, the heat treatment completely oxidizes the nickel electrode. In some embodiments, a weight gain of the nickel electrode after the heat treatment is about 15% or greater.

At block 515, the nickel electrode is heat treated (a second heat treatment) at about 500° C. to 900° C. in a reducing gas. to reduce the oxidized nickel. In some embodiments, the heat treatment is at about 600° C. In some embodiments, the reducing gas is hydrogen or a hydrogen-containing gas.

In some embodiments, the nickel electrode is fully reduced to nickel metal after the second heat treatment. In some embodiments, there is no weight gain of the nickel electrode after the heat treatment at block 510 and the heat treatment at block 515.

In some embodiments, the method 500 further includes incorporating the nickel electrode in an alkaline water electrolyzer. In some embodiments, the method 500 further includes incorporating the nickel electrode as an anode in an alkaline water electrolyzer. In some embodiments, the method 500 further includes incorporating the nickel electrode as a cathode in an alkaline water electrolyzer. In some embodiments, the method 500 further includes incorporating a first nickel electrode as an anode and a second nickel electrode as a cathode in an alkaline water electrolyzer.

FIG. 6 shows an example of a flow diagram illustrating a method for increasing the active surface area of a Ni electrode. Starting at block 605 of the method 600 shown in FIG. 6, a nickel electrode is provided. In some embodiments, the nickel electrode is an electrode from a group a nickel foam, a nickel felt, nickel mesh, nickel particles that are sintered, and a perforated nickel sheet.

At block 610, the nickel electrode is immersed in an aqueous solution including a metal compound. In some embodiments, the metal compound is a metal salt or a water-soluble metal salt. In some embodiments, the metal compound is a metal nitrate, a metal chloride, a metal acetate, or a metal ammonium salt. The metal compound is a nickel compound, an iron compound, a cobalt compound, a molybdenum compound, a chromium compound, or mixtures thereof. In some embodiments, the nickel compound comprises nickel nitrate hexahydrate. In some embodiments, the iron compound comprises iron nitrate nonahydrate. In some embodiments, the cobalt compound comprises cobalt nitrate hexahydrate. In some embodiments, the molybdenum compound comprises ammonium molybdate tetrahydrate. In some embodiments, the chromium compound comprises chromium nitrate nonahydrate. In some embodiments, the mixture of the nickel compound and the iron compound is about 50 mol % nickel and about 50 mol % iron.

In some embodiments, the aqueous solution includes a chelating agent. In some embodiments, the chelating agent is a chelating agent from a group glycine, citric acid, ethylenediaminetetraacetic acid (EDTA), and malic acid.

At block 615, after the immersing, the nickel electrode is heat treated (a first heat treatment) at about 300° C. to 500° C. in air to convert the metal compound to a metal oxide. The metal oxide is nickel oxide, iron oxide, cobalt oxide, molybdenum oxide, chromium oxide, or mixtures thereof. In some embodiments, the heat treatment includes holding the nickel electrode at about 300° C. to 500° C. in air for about 3 minutes to about 1 hour. In some embodiments, the heat treatment includes holding the nickel electrode at about 400° C. In some embodiments, the first heat treatment includes holding the nickel electrode at about 400° C. in air for about 1 hour.

At block 620, after the heat treatment at block 615, the nickel electrode is heat treated (a second heat treatment) at about 500° C. to 900° C. in a reducing gas to reduce the metal oxide to a metal. The metal is nickel, iron, cobalt, molybdenum oxide, chromium oxide, or mixtures thereof. In some embodiments, the heat treatment includes holding the nickel electrode at about 500° C. to 900° C. in a reducing gas, or at about 600° C. in a reducing gas. In some embodiments, the heat treatment is for about 2 hours. In some embodiments, the reducing gas is hydrogen or a hydrogen-containing gas.

In some embodiments, the immersing at block 610 followed by the heat treatment at block 615 are repeated one or more times (e.g., block 610, block 615, block 610, block 615, and so on) to increase an amount of the metal oxide disposed on the nickel electrode. In some embodiments, the immersing at block 610 followed by the heat treatment at block 615 are repeated three times to increase an amount of the metal oxide disposed on the nickel electrode.

In some embodiments, the method 600 further includes incorporating the nickel electrode in an alkaline water electrolyzer. In some embodiments, the method 600 further includes incorporating the nickel electrode as an anode in an alkaline water electrolyzer. In some embodiments, the method 600 further includes incorporating the nickel electrode as a cathode in an alkaline water electrolyzer. In some embodiments, the method 600 further includes incorporating a first nickel electrode as an anode and a second nickel electrode as a cathode in an alkaline water electrolyzer.

Described below are the results of performing the two methods for increasing the performance of Ni foam electrodes. The roughened electrodes were characterized with scanning electron microscopy (SEM) imaging, and tested as the anode in full cell LAWE devices. The anode was selected to demonstrate the improvement because anode kinetics are moderately more sluggish than cathode kinetics, and the anode is therefore more limiting for full cell performance. It was shown that both of the described methods lead to significant improvement in LAWE performance.

The following examples are intended to be examples of the embodiments disclosed herein, and are not intended to be limiting. Ni foam with 0.3 mm thickness, 110 pores per inch, and 97% porosity was used as the baseline as-received electrode material. FIG. 1 shows an SEM image of as-received Ni foam.

Example—Sample Preparation—Oxidation/Reduction

Controlled oxidation was carried out in ambient air in a box furnace. Ni foam samples were placed horizontally on an alumina plate in the hot zone of the furnace at 400-1000° C. and held for 2 hours. The ramping rate for heating and cooling was 10° C. min⁻¹. The extent of oxidation from Ni to NiO was determined by weighing the samples. It was found that the extent of oxidation was much more sensitive to oxidation temperature than to hold time, so temperature was selected as the variable for optimization. After oxidation, the samples were reduced to Ni in a tube furnace with metal flange caps and alumina tube, and a mineral oil bubbler to prevent backflow of air into the tube. The sample temperature (calibrated at the sample location) was held at 600° C. for 2 hours, with 5° C. min⁻¹ heating and cooling rates. The tube was flushed with 100 sccm flowrate of 2% H₂/98% Ar using a mass flow controller. After reduction, complete conversion back to Ni metal was confirmed by weighing the samples and comparing to the starting weight.

Example—Sample Preparation—Infiltrated Catalyst Coating

Catalysts were coated throughout the bulk of the Ni foam using an infiltration technique. Ni, Fe, or NiFe (50/50 mol ratio) were deposited into the Ni foam from an aqueous nitrate salt solution. A surfactant (Triton-X-100, Sigma Aldrich, Burlington, MA) was dissolved in deionized water (1.2:10 wt) by stirring overnight with a magnetic stir bar. This solution was then mixed with nitrate salts and glycine chelating agent on a shaker table until the salts were completely dissolved. Typical mixing ratios were (Ni) 4.4 g Ni nitrate hexahydrate, 0.26 g glycine, with 1 g solution, (Fe) 3.9 g Fe nitrate nonahydrate, 0.17 g glycine, with 1 g solution, or (NiFe) 1.4 g Ni nitrate hexahydrate and 1.9 g Fe nitrate nonahydrate, 0.17 g glycine, with 1 g solution. Ni foam was then soaked in these catalyst precursor solutions, excess solution was drained off, and the samples were transferred into the box furnace described above.

The samples were held at 90° C. for 1 h to dry the solution, and then fired at 400° C. for 1 hour with 10° C. min⁻¹ heating and cooling rate to convert the nitrate salts to metal oxides. This process was completed 3 or 10 times to adjust the total catalyst loading, represented in the text below as for example “Ni-3x”. The metal oxides were then reduced to metals using the same reducing tube furnace setup described above, with 2 hours hold at various temperatures from 500 to 900° C. Lower reducing temperatures were not tested, as it was found that the catalyst particles were not bonded well at lower temperatures.

Example—Electrochemical Testing

A cell with two 5 cm² Ni serpentine flow fields for both anode and cathode were used for LAWE single cell construction. Ni electrodes were used on the cathode and anode sides accordingly. A commercial Zirfon (Perl UTP 500, Agfa, Mortsel, Belgium) was used as the separator membrane. When assembling, ethylene tetrafluoroethylene (ETFE) gaskets with different thickness were added around the electrodes and separator used to seal the cell. Hot 7 M KOH (80° C., diluted from 85%) at 20 ml/min was fed to both anode and cathode when the cell was heated to 80° C.

All the electrochemical testing was performed with a potentiostat. Polarization curves were collected through chronopotentiometric technique with each constant current held for 20 seconds, and the average potential over the last 5 seconds of each current was used for the steady-state potential. Galvanostatic electrochemical impedance spectroscopy (GEIS) was conducted at each current to determine the ohmic resistance and charge transfer characteristic. The testing was done at a frequency range of 1 Hz-100 kHz with the signal amplitude of 5% of the applied current. The high frequency resistance (HFR) was determined through GEIS fitting with an equivalent circuit including a series combination of a resistor and two RCPEs. The electrochemical active specific area (ECSA) estimation was conducted in a three-electrode system in 1 M KOH solution, with the Ni electrodes as working electrode, graphite rod as reference electrode and Ag/AgCl reference electrode. The ECSA of electrodes were qualitatively estimated by the electrochemical double layer capacitance (C_dl) estimation from the cyclic voltammogram (CV) curves in the non-Faradic region at various scan rates. The C_dl was calculated according to C_dl=i/v, where i is the double layer current densities collected from CV curves at the same potential, and v is the corresponding scan rate.

Example—Results of Oxidation/Reduction of Ni Foam

Ni foam was oxidized and then reduced to create a roughened surface, FIGS. 2A2F. This treatment dud not impact the macroscopic structure of the foam, but did affect the Ni surface throughout the depth of the foam. The extent of oxidation was controlled by the oxidation temperature in air, FIG. 2A, and was consistent with previous studies of Ni oxidation kinetics. At 400° C., minimal oxidation occurred. As the temperature increased to 700° C., the oxidation weight gain slowly increased to about 5%. The oxidation increased dramatically from 700 to 800° C., then increased more slowly up to 1000° C. Full oxidation of Ni to NiO resulted in a theoretical 27% weight gain, which was achieved at 1000° C. The rapid increase in weight gain from 700 to 800° C. was accompanied by a transition from partial to complete oxidation of the Ni surface. At 700° C. and below, partial roughening of the surface was observed (FIG. 2C), with the grain boundaries showing preferential roughening. At 800° C. and above, the entire surface was roughened (FIG. 2D). A distinctly roughened and pitted surface was observed, with feature size around 0.5-1 μm. In contrast, the as-received Ni foam surface was characterized by grains of 2-10 μm size, and was relatively smooth (FIG. 2B).

The surface became rough as it oxidized, due to NiO crystallization and the volume expansion associated with the Ni to NiO transition. When the NiO was reduced back to Ni metal, the volume reduction introduces pores and pits in the surface and the roughness was retained. This reduction step and the higher oxidation temperatures introduced additional surface area compared to the 600° C. oxidation-only process reported previously. Based on these observations, foams oxidized at 600, 700, and 900° C. were chosen to represent minimally oxidized, partially oxidized, and completely oxidized Ni surface, respectively.

The enhanced surface roughness observed after oxidation/reduction processing yielded higher electrochemical activity, FIGS. 2E and 2F. Qualitatively speaking, the double layer capacitance at the electrode/electrolyte interfaces could represent the electrode ECSA for OER. A 3-electrode setup was used to characterize the double layer capacitance, FIG. 2E. The slope increases dramatically from the 0.27 mF cm⁻² observed for baseline as-received Ni foam to 0.7 mF cm⁻² for the foam oxidized at 900° C. The full cell LAWE performance was significantly improved by the enhanced anode surface area, FIG. 2F. The as-received Ni foam achieved 2.56 V at 1.8 A cm⁻². The foams with partial roughening (600 and 700° C.) improved cell potential by 95 mV at 1.8 A cm⁻². The foam with full roughening (900° C.) yielded a 157 mV improvement, the largest improvement observed for any of the pure Ni samples tested in this study. This improvement is within the range observed for other electrode roughening techniques described in the Background.

Example —Results of Ni Foam Coated with Catalyst via Infiltration

Micron-scale Ni, Fe, and NiFe catalyst particles were deposited throughout the bulk of Ni foams using an infiltration technique. Aqueous nitrate salt precursors were flooded into the Ni foam, dried, fired in air, and then reduced to metal particles at various temperatures in the range 500-900° C. Lower temperature was not considered, as minimal bonding between the particles and Ni foam would likely compromise mechanical integrity of the catalyst coatings. For the case of Ni catalyst, particles coated the entire foam surface, FIGS. 3A-3C. After 3 cycles of infiltration followed by reduction, the Ni catalyst loading was approximately 5 wt % based on the original foam weight. A nearly continuous layer of Ni particles was observed, and increasing the infiltration cycles to 10 did not visibly increase the surface roughness or coverage. Similar particulate coatings were observed for Fe and NiFe catalysts. After firing at 500° C., the Ni particles were approximately 1 μm particle size, and appeared to be well-bonded to each other and to the Ni foam, FIG. 3A. After firing at 700° C., the catalyst particles were noticeably coarsened, reducing the total surface area, FIG. 3B. Upon firing at higher temperatures, the surface particles were mostly (800° C.) or completely (900° C., FIG. 3C) consumed into the foam structure due to sintering diffusion, leaving a smooth surface that looked similar to the as-received foam. Sintering is driven by surface energy, so the Ni particles are incorporated into the main body of the foam through densification and grain growth, similar to standard densification of a Ni powder pellet.

The full cell LAWE performance was significantly improved by the added catalyst coatings, FIGS. 4A-4D. For Ni-3x, the best performance was achieved for the 600° C. reducing temperature, FIG. 4A. At lower temperature, the minimal extent of sintering may limit electronic connectivity and mechanical integrity of the deposited catalyst. As temperature increases above 600° C., over-sintering reduced the catalyst area and the performance decreased. At 900° C., where no catalyst particles remained and the surface was smooth (FIG. 3C), the performance was almost identical to the as-received Ni foam (2.56 V at 1.8 A cm⁻²). A similar trend with increasing temperature was observed for NiFe-3x, FIG. 4B.

Consistent with the SEM observations, adding more Ni by increasing the number of infiltration cycles did not further improve performance, FIG. 4C. Presumably, once the Ni foam surface is completely covered with additional Ni particles (Ni-3x), adding more Ni particles (Ni-10x) does not further increase the electrochemically active surface area. This may be because only the outermost layer of catalyst particles participates in the reaction, or because subsequent infiltration cycles fill in the pores, and densify the catalyst coating. Based on these results, Ni-3x reduced at 600° C. was chosen as the optimum processing conditions for infiltrated Ni catalyst. The full cell voltage at 1.8 A cm⁻² was improved by 106 mV. This was slightly less beneficial than the oxidation/reduction processing described above.

Adding Fe to the catalyst coating (NiFe-3x), or replacing Ni with Fe (Fe-3x) further improves performance, FIG. 4D. NiFe-3x provides the best performance of all the samples studied here, achieving 211 mV improvement at 1.8 A cm⁻². This is consistent with previous reports that Fe enhances catalytic activity in LAWE, however there is uncertainty about Fe stability during long-term operation. The presence of Fe also enables SEM/EDX analysis to show that the catalyst was well distributed throughout the foam.

CONCLUSION

Further detail regarding the embodiments described herein can be found in Guanzhi Wang et al., “Simple processing via thermal treatment and catalyst infiltration to enhance nickel electrode performance for liquid alkaline water electrolyzers,” International Journal of Hydrogen Energy, Vol. 77, 5 Aug. 2024, Pages 844-850, which is hereby incorporated by reference.

In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention.