ELECTROCATALYST FORMED FROM ACTIVATED VACUUM RESIDUE AND METHOD OF ELECTROCATALYTICALLY PRODUCING HYDROGEN PEROXIDE

Description

STATEMENT OF PRIOR DISCLOSURE BY AN INVENTOR

Aspects of the present disclosure are described in Maimuna U. Zarew, Mohamed Javid, and Almaz S. Jalilov; “Porous Carbon from Vacuum Residue as an Effective Electrocatalyst for a 2e⁻ Oxygen Reduction Reaction in Alkaline Media”; Energy and Fuels, 2023, 37, 23, 19166-19175, which is incorporated herein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

The financial support of King Fahd University of Petroleum and Minerals (KFUPM), and the Deanship of Research Oversight and Coordination for funding this work through project DF191019 is gratefully acknowledged.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to an electrocatalyst that includes P-, N-, and S-doped porous carbon from activated petroleum vacuum residue, a method of forming the electrocatalyst, and a method of electrocatalytically producing hydrogen peroxide using the electrocatalyst.

Discussion of the Background

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.

Many industries use hydrogen peroxide as a vital chemical in the production of medical disinfectants, paper bleaching, treatment of wastewater, and the synthesis of fine chemicals. This is because of its effective oxidation properties and eco-friendly nature. [Yamanaka, I., et. al., Angew. Chem., Int. Ed. 2003, 42, 3653-3655; Siahrostami, S., et. al., Nat. Mater. 2013, 12, 1137-1143; Lu, Z., et. al., Nat. Catal. 2018, 1, 156-162; and Fellinger, T. P., et. al., J. Am. Chem. Soc. 2012, 134, 4072-4075]. Even though hydrogen peroxide has wide application and significance, its major source is the industrial anthraquinone process which has a lengthy procedure, consumes high energy, and is not eco-friendly. [Campos-Martin, J. M., et. al., Angew. Chem., Int. Ed. 2006, 45, 6962-6984]. Due to the disadvantages of the industrial process, there is an urgent need to use more eco-friendly methods. The electrochemical process is a very good alternative, it is a very simple method using sustainable sources (water and O₂ from the air) to generate H₂O₂ from a two-electron oxygen reduction reaction, but the process suffers problems of slow kinetics and is not very selective due to high competition with the four-electron mechanism. [Chang, Q., et. al., Nat. Commun. 2020, 11, 2178; Liu, Y., et. al., Angew. Chem., Int. Ed. 2015, 54, 6837-6841; Iglesias, D., et. al., Chem 2018, 4, 106-123; Pang, Y., et. al., ACS Catal. 2020, 10, 7434-7442; Liu, W., et. al., Appl. Catal., B 2022, 310, 121312; Wang, Y., et. al, Adv. Energy Mater. 2021, 11, 2003323; Sa, Y. J., et. al, Angew. Chem., Int. Ed. 2019, 58, 1100; Li, L., et. al., Adv. Energy Mater. 2020, 10, 2000789; and Zhang, C., et. al., ACS Sustainable Chem. Eng. 2021, 9, 9369-9375]. Therefore, there is a high need for very selective and active electrocatalysts. [Chang Z., et. al., J. Am. Chem. Soc. 2023, 145 (21), 11589-11598.]Researchers focus on the use of metal-free carbon-based electrocatalysts because they are very conductive, more economical, durable, and efficient ORR electrocatalysts for the synthesis of H₂O₂. For many years metal-doped-carbon electrocatalysts such as Pt, Hg, Au, Pd, etc. had been used but they are very scarce and non-economical even though they are selective to the two-electron pathway. [Verdaguer-Casadevall, A., et. al, Nano Lett. 2014, 14, 1603-1608; Slanac, D. A., et. al., J. Am. Chem. Soc. 2012, 134, 9812-9819; Jiang, Y., et. al., Adv. Energy Mater. 2018, 8,1801909; and Melchionna, M., Fornasiero, P., Chem 2019, 5, 1927-1928]. Several modifications have been made to the transition metal from a single atom to compounds such as CoSe₂, and Fe₃O₄ for H₂O₂ synthesis, but they still show instability to corrosion. [Jung, E., et. al., Nat. Mater. 2020, 19, 436-442; Barros, W. R. P., et. al., Electrochim. Acta 2015, 162, 263-270; and Sheng, H., et. al., ACS Catal. 2019, 9, 8433-8442]. Therefore, there is a great need to design metal-free carbon-based material with good selectivity and stability. Doping using hetero atoms causes intrinsic defects that improve the electronic properties of carbon-based electrocatalysts thereby making it more selective and active. These heteroatoms (N, P, B, S, etc.) vary in their atomic radii and electronegativity, this variation of doping different hetero-atoms into carbon-based framework can cause the rearrangement of local electronic structure making it more catalytically active. [Xia, Y., et. al., Nat. Commun. 2021, 12, 4225; Melchionna, M., et. al., Adv. Mater. 2019, 31, 1802920; Jiao, Y., et. al., J. Am. Chem. Soc. 2014, 136, 4394-4403; Chen, G., et. al., Nano Res. 2019, 12, 2614-2622; and Zhang, J., et. al., Nano-Micro Lett. 2021, 13, 65].

Accordingly, it is an objective of the present disclosure to provide an electrocatalyst for hydrogen peroxide production that overcomes the limitations described above.

SUMMARY OF THE INVENTION

The present disclosure relates to an electrocatalyst, comprising a microporous network of carbon comprising phosphorus, sulfur, and nitrogen dopant atoms, wherein a portion of the phosphorous is present as isolated phosphorous atoms doped into the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises 70.0 to 78 wt % carbon, 11 to 19 wt % oxygen; 3.50 to 8.0 wt % phosphorus, 2.0 to 5.0 wt % sulfur, and 0.25 to 1.5 wt % nitrogen based on a total weight of electrocatalyst by XPS.

In some embodiments, the microporous network of carbon has a BET surface area of 300 to 4000 m²/g, a pore volume of 0.2 to 2.4 cm³/g, and a micropore volume of 0.1 to 2.3 cm³/g.

In some embodiments, a surface of the microporous network of carbon includes carboxylate functional groups and phosphate functional groups.

In some embodiments, the microporous network of carbon has a ratio of a D band intensity to a G band intensity I_D/I_G of 0.75 to 1.25 by Raman spectroscopy.

The present disclosure also relates to a method of producing the electrocatalyst, the method comprising mixing petroleum vacuum residue and phosphoric acid to form a crude mixture, annealing the crude mixture at 375 to 525° C. in a first inert atmosphere for 1 to 5 hours to form an intermediate product, and heating the intermediate product at 400 to 900° C. in a second inert atmosphere for 1 to 5 hours to form the electrocatalyst.

In some embodiments, the petroleum vacuum residue and phosphoric acid are present in the crude mixture in a ratio of 1.5:1 to 1:1.5 by weight.

In some embodiments, the petroleum vacuum residue comprises 80.0 to 85 wt % carbon, 7 to 9 wt % hydrogen, 3.0 to 5.0 wt % sulfur, and 0.25 to 1.0 wt % nitrogen based on a total weight of petroleum vacuum residue.

In some embodiments, the phosphoric acid has a concentration of 50 to 99% in water.

In some embodiments, the first and second inert atmosphere are flowing nitrogen gas.

In some embodiments, the annealing and heating are performed with a temperature increase rate of 5 to 10° C./min.

In some embodiments, the method does not involve reduction with hydrogen gas.

In some embodiments, the method further comprises washing the electrocatalyst with water and drying at 25 to 100° C.

The present disclosure also relates to a method of producing hydrogen peroxide, the method comprising applying a potential between a counter and a working electrode in an electrochemical cell containing an electrolyte to form hydrogen peroxide and collecting the hydrogen peroxide, wherein the working electrode includes the electrocatalyst; and wherein the electrolyte including an aqueous solution of a base at a concentration of 0.001 to 5 M.

In some embodiments, the method has an onset potential of 0.750 to 0.875 V vs RHE.

In some embodiments, the working electrode has a Tafel slope of 80 to 115 mV/dec.

In some embodiments, the method has an electron transfer number of 1.75 to 3.

In some embodiments, the method has a yield of 80 to 95% OH₂⁻ at a potential of 0.5 to 0.65 V vs RHE.

In some embodiments, the aqueous solution of a base at a concentration of 0.001 to 5 M is 0.1 M KOH is saturated with oxygen.

In some embodiments, the working electrode further comprises glassy carbon and a sulfonated fluoropolymer, and the electrocatalyst is disposed on the surface of the glassy carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram for the preparation of the activated Vacuum Residue (VR-PA).

FIG. 2A shows the XRD pattern for VR-PA.

FIG. 2B shows the comparative Raman spectra for VR-PA and graphene.

FIG. 3 is an SEM image of VR-PA.

FIG. 4A shows a N₂-absorption-desorption isotherm for VR-PA at 77 K.

FIG. 4B shows a plot of the pore-size distribution of VR-PA estimated using the NLDFT method.

FIGS. 5A-5D are XPS spectra of the VR-PA where FIG. 5A is an XPS survey spectrum of the VR-PA, FIG. 5B is a high-resolution XPS spectra of the C1s peaks of the VR-PA, FIG. 5C is a high-resolution XPS spectra of the O1s peaks of the VR-PA, and FIG. 5D is a high-resolution XPS spectra of the P2p peaks of the VR-PA.

FIG. 6A is a plot of CV curves of VR-PA at different scan rates.

FIG. 6B is a plot of the current dependence on scan rate for VR-PA.

FIG. 6C shows Nyquist plots of VR-PA and Pt/C.

FIG. 7A shows a comparison of cyclic voltammograms for VR-PA and Pt/C under argon and oxygen atmosphere in 1 M KOH solution.

FIG. 7B shows LSV curves for VR-PA and Pt/C in O₂-saturated solution of 0.1 M KOH with the 1600 rpm rotation speed.

FIG. 7C shows Tafel plots for VR-PA and Pt/C.

FIG. 7D shows LSVs of VR-PA as a function of rotation speed in the O₂-saturated 0.1 M KOH at 10 mV/s scan rate.

FIG. 7E shows K-L analysis for VR-PA at different potentials.

FIG. 7F shows disc current and the ring current responses for VR-PA at 1600 rpm scan rate and 10 mV/s scan rate in O₂-saturated 0.1 M KOH.

FIG. 7G shows OH₂⁻ percentage yield and corresponding electron transfer number (n) for the VR-PA as ORR catalyst.

FIG. 7H shows a comparison of chronoamperometric plots for VR-PA and Pt/C in O²-saturated 0.1 M KOH with the 1600 rpm rotation speed, methanol tolerance tested at ˜30 min.

FIG. 7I shows LSV curves of VR-PA before and after 1000 cycles in O₂-saturated 0.1 M KOH with the 1600 rpm rotation speed and 10 mV/s scan rate.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, it is understood that other embodiments may be utilized and structural and operational changes may be made without departure from the scope of the present embodiments disclosed herein.

Definitions

When describing the present disclosure, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings wherever applicable, in that some, but not all embodiments of the disclosure are shown.

As used herein the words “a” and “an” and the like carry the meaning of “one or more.”

As used herein, the terms “optional” or “optionally” means that the subsequently described event(s) can or cannot occur or the subsequently described component(s) may or may not be present (e.g., 0 wt. %).

Furthermore, the terms “approximately”, “approximate”, “about”, and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

The use of the terms “include”, “includes”, “including,” “have,” “has,” or “having” should be generally understood as open-ended and non-limiting unless specifically stated otherwise.

As used herein, “nanoparticles” are particles having a particle size of 1 nm to 500 nm within the scope of the present invention.

As used herein, “particle size” and “pore size” may be thought of as the lengths or longest dimensions of a particle and of a pore opening, respectively. The term “mesopore” as used herein refers to a pore having a diameter of 40 to 100 Å. The term “micropore” refers to a pore having a diameter of less than 40 Å. The term “macropore” refers to a pore having a diameter that exceeds 100 Å.

As used herein, the term “room temperature” refers to a temperature range of “25° C.±3° C. in the present disclosure.

As used herein, the term “electrode” refers to an electrical conductor used to contact a non-metallic part of a circuit, such as a semiconductor, an electrolyte, a vacuum, or air.

As used herein, the term “current density” refers to the amount of electric current traveling per unit cross-section area.

As used herein, the term “Tafel slope” refers to the relationship between the overpotential and the logarithmic current density.

As used herein, the term “electrochemical cell” refers to a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions.

As used herein, the term “overpotential” refers to the difference in potential that exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is directly associated with a cell's voltage efficacy. In an electrolytic cell, the occurrence of overpotential implies that the cell needs more energy as compared to that thermodynamically needed to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally measured by determining the potential at which a given current density is reached.

The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.

In addition, the present disclosure is intended to include all isotopes of atoms occurring in the present compounds and complexes. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium. Isotopes of naturally occurring carbon include ¹²C, ¹³C, and ¹⁴C. Isotopes of oxygen include ¹⁶O, ¹⁷O, and ¹⁸O. Isotopes of naturally occurring nitrogen are ¹⁴N and ¹⁵N. Isotopes of naturally occurring sulfur include ³²S, ³³S, ³⁴S, and ³⁶S. Isotopes of naturally occurring phosphorous include ³¹P and ³²P. Isotopically-labeled compounds of the disclosure may generally be prepared by conventional techniques known to those skilled in the art or by processes analogous to those described herein, using an appropriate isotopically-labeled reagent in place of the non-labeled reagent otherwise employed.

In general, a microporous network can have any suitable structure or form. The microporous network has micropores, but may also have other types of pores in addition to the micropores. Such other types of pores may be mesopores and/or macropores. In some embodiments, the microporous network comprises open pores. A network that comprises open pores may be referred to as an “open cell network” or other similar term. Open pores are pores that have solid edges and open faces, and fluid flow is possible to penetrate among them. This type of pore is in contrast to closed pores which form a “closed cell network”. Closed pores are pores that are connected via solid faces with no interconnectivity among them. In some embodiments, the microporous network further comprises closed pores. Open pores may be advantageous for increasing a catalytically active surface area. Open pores may allow for flow of gases and/or liquids into the network such that the gases and/or liquids can contact interior surfaces of interior pores.

In general, the network of pores may be ordered or disordered. An ordered network of pores refers to a network having a regular or periodic arrangement of pores that repeats throughout the network. Ordered networks of pores typically have relatively uniform pore sizes. A disordered network of pores refers to a network lacking a regular or periodic arrangement of pores. In a disordered network of pores, pores may be randomly arranged. Disordered networks of pores may be regular pore sizes, irregular pore sizes, or regions of regular pore sizes and regions of irregular pore sizes. In some embodiments, the microporous network of pores is ordered. In some embodiments, the microporous network of pores is disordered.

Electrocatalyst

According to a first aspect, the present disclosure relates to an electrocatalyst. The electrocatalyst includes a microporous network of carbon comprising phosphorus, sulfur, and nitrogen dopant atoms.

In some embodiments, the microporous network of carbon comprises 70 to 78 wt % carbon, preferably 70.5 to 77.5 wt %, preferably 71.0 to 77.0 wt %, preferably 71.5 to 76.5 wt % carbon, preferably 72.0 to 76.0 wt % carbon, preferably 72.25 to 75.5 wt % carbon, preferably 72.5 to 75.25 wt % carbon, preferably 72.75 to 75.0 wt % carbon, preferably 73.0 to 74.75 wt %, carbon preferably 73.25 to 74.5 wt % carbon, preferably 73.5 to 74.25 wt % carbon, preferably 73.75 to 74.0 wt % carbon, based on a total weight of the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises 11 to 19 wt % oxygen, preferably 11.5 to 18.75 wt % oxygen preferably 12.0 to 18.5 wt % oxygen preferably 12.5 to 18.25 wt % oxygen preferably 13.0 to 18.0 wt % oxygen preferably 13.5 to 17.75 wt % oxygen preferably 14.0 to 17.5 wt % oxygen preferably 14.5 to 17.25 wt % oxygen, preferably 15.0 to 17.0 wt % oxygen, preferably 15.25 to 16.5 wt % oxygen, preferably 15.5 to 16.25 wt % oxygen, preferably 15.75 to 16.0 wt % oxygen, based on a total weight of the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises 3.50 to 8.00 wt % phosphorus, preferably 3.75 to 7.50 wt % phosphorus, preferably 4.00 to 7.00 wt % phosphorus, preferably 4.25 to 6.75 wt % phosphorus, preferably 4.50 to 6.50 wt % phosphorus, preferably 4.75 to 6.75 wt % phosphorus, preferably 5.00 to 6.50 wt % phosphorus, preferably 5.25 to 6.25 wt % phosphorus, preferably 5.50 to 6.00 wt % phosphorus, preferably 5.60 to 5.85 wt % phosphorus, preferably 5.70 to 5.75 wt % phosphorus, based on a total weight of the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises 2.00 to 5.00 wt % sulfur, preferably 2.75 to 4.75 wt % sulfur, preferably 2.50 to 4.50 wt % sulfur, preferably 2.75 to 4.25 wt % sulfur, preferably 3.00 to 4.00 wt % sulfur, preferably 3.25 to 3.85 wt % sulfur, preferably 3.40 to 3.75 wt % sulfur, preferably 3.50 to 3.60 wt % sulfur, based on a total weight of the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises 0.25 to 1.50 wt % nitrogen, preferably 0.35 to 1.35 wt % nitrogen, preferably 0.50 to 1.25 wt % nitrogen, preferably 0.60 to 1.15 wt % nitrogen, preferably 0.75 to 1.00 wt % nitrogen, preferably 0.80 to 0.90 wt % nitrogen, based on a total weight of the microporous network of carbon.

In general, the oxygen, sulfur, and nitrogen may each independently be present in the form of isolated atoms or ions, clusters of atoms or ions, and/or present as complexes or groups comprising the element. For example, oxygen may be present as isolated oxygen dopant atoms in the carbon that makes up the microporous network of carbon, as isolated oxygen ions present in the carbon that makes up the microporous network of carbon, as atomic oxygen or an oxygen-containing gas such as water vapor, carbon monoxide, carbon dioxide, or the like present on a surface of pores or trapped within pores, as a component of an oxygen-containing functional group such as carbonyl, hydroxyl, carboxyl, ether, ester, and the like, or combinations of these. In some embodiments, a portion of the oxygen is present as isolated oxygen dopant atoms in the carbon that makes up the microporous network of carbon and/or as isolated oxygen ions present in the carbon that makes up the microporous network of carbon. Similarly, nitrogen may be present as isolated nitrogen dopant atoms in the carbon that makes up the microporous network of carbon, as isolated nitrogen ions present in the carbon that makes up the microporous network of carbon, as atomic nitrogen or a nitrogen-containing gas such as ammonia, nitric oxide, nitrogen dioxide, and the like present on a surface of pores or trapped within pores, as a component of a nitrogen-containing functional group such as a amide, amidine, amine, imine, imide, azide, cyanate, nitrate, nitrile, nitro, and the like, or combinations of these. In some embodiments, a portion of the nitrogen is present as isolated nitrogen dopant atoms in the carbon that makes up the microporous network of carbon and/or as isolated nitrogen ions present in the carbon that makes up the microporous network of carbon. Sulfur may be present as isolated sulfur dopant atoms in the carbon that makes up the microporous network of carbon, as isolated sulfur ions present in the carbon that makes up the microporous network of carbon, as atomic sulfur or a sulfur-containing gas such as sulfur dioxide, hydrogen sulfide, dimethyl sulfide, and the like present on a surface of pores or trapped within pores, as a component of a sulfur-containing functional group such as thiol, sulfide, disulfide, sulfoxide, sulfone, sulfinic acid, sulfonic acid, thiocyanate, thioketone, thiocarboxylic acid, thioester, and the like, or combinations of these. In some embodiments, a portion of the sulfur is present as isolated sulfur dopant atoms in the carbon that makes up the microporous network of carbon and/or as isolated sulfur ions present in the carbon that makes up the microporous network of carbon.

In some embodiments, the microporous network of carbon can further include an impurity element which is at least one selected from the group consisting of metals or semimetals such as iron, vanadium, aluminum, magnesium, calcium, nickel, copper, zinc chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, arsenic, beryllium, boron, cadmium, and silicon. In some embodiments, the microporous network of carbon is substantially free of an impurity element. In some embodiments, the carbonaceous particles are devoid of an impurity element. That is, the microporous network of carbon does not contain a detectable amount of the impurity element. In some embodiments, the microporous network of carbon is substantially free of iron. In some embodiments, the network of carbon is devoid of iron.

The aforementioned weight percentages of carbon, oxygen, sulfur, phosphorus, nitrogen, and impurity elements such as iron may be determined by elemental analysis techniques such as energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), neutron activation analysis, and a combination thereof.

In some embodiments, the microporous network of carbon exists as particles. In general, the particles of a microporous network of carbon can be any shape known to one of ordinary skill in the art. Examples of suitable shapes the particles of a microporous network of carbon may take include spheres, spheroids, lentoids, ovoids, solid polyhedra such as tetrahedra, cubes, octahedra, icosahedra, dodecahedra, hollow polyhedra (also known as nanocages), stellated polyhedra (both regular and irregular, also known as nanostars), triangular prisms (also known as nanotriangles), hollow spherical shells (also known as nanoshells), tubes (also known as nanotubes), nanosheets, nanoplatelets, nanodisks, rods (also known as nanorods), and mixtures thereof. In the case of nanorods, the rod shape may be defined by a ratio of a rod length to a rod width, the ratio being known as the aspect ratio. For particles of a microporous network of carbon of the current invention, nanorods should have an aspect ratio less than 1000, preferably less than 750, preferably less than 500, preferably less than 250, preferably less than 100, preferably less than 75, preferably less than 50, preferably less than 25. Nanorods having an aspect ratio greater than 1000 are typically referred to as nanowires and are not a shape that the particles of a microporous network of carbon are envisioned as having in any embodiments.

In some embodiments, the particles of a microporous network of carbon have uniform shape. Alternatively, the shape may be non-uniform. As used herein, the term “uniform shape” refers to an average consistent shape that differs by no more than 10%, by no more than 5%, by no more than 4%, by no more than 3%, by no more than 2%, by no more than 1% of the distribution of particles of a microporous network of carbon having a different shape. As used herein, the term “non-uniform shape” refers to an average consistent shape that differs by more than 10% of the distribution of particles of a microporous network of carbon having a different shape. In one embodiment, the shape is uniform and at least 90% of the particles of a microporous network of carbon are spherical or substantially circular, and less than 10% are polygonal. In another embodiment, the shape is non-uniform and less than 90% of the particles of a microporous network of carbon are spherical or substantially circular, and greater than 10% are polygonal.

In some embodiments, the particles of a microporous network of carbon have a mean particle size of 0.1 to 1000 μm, preferably 0.5 to 750 μm, preferably 1.0 to 600 μm, preferably 5 to 500 μm, preferably 10 to 400 μm, preferably 15 to 350 μm, preferably about 20 to 300 μm. In embodiments where the particles of a microporous network of carbon are spherical, the particle size may refer to a particle diameter. In embodiments where the particles of a microporous network of carbon are polyhedral, the particle size may refer to the diameter of a circumsphere. In some embodiments, the particle size refers to a mean distance from a particle surface to particle centroid or center of mass. In alternative embodiments, the particle size refers to a maximum distance from a particle surface to a particle centroid or center of mass. In some embodiments where the particles of a microporous network of carbon have an anisotropic shape such as nanorods, the particle size may refer to a length of the nanorod, a width of the nanorod, an average of the length and width of the nanorod. In some embodiments in which the particles of a microporous network of carbon have non-spherical shapes, the particle size refers to the diameter of a sphere having an equivalent volume as the particle. In some embodiments in which the particles of a microporous network of carbon have non-spherical shapes, the particle size refers to the diameter of a sphere having an equivalent diffusion coefficient as the particle.

In some embodiments, the particles of a microporous network of carbon of the present disclosure are monodisperse, having a coefficient of variation or relative standard deviation, expressed as a percentage and defined as the ratio of the particle size standard deviation (σ) to the particle size mean (μ) multiplied by 100 of less than 25%, preferably less than 10%, preferably less than 8%, preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%. In some embodiments, the particles of a microporous network of carbon of the present disclosure are monodisperse having a particle size distribution ranging from 80% of the average particle size to 120% of the average particle size, preferably 90-110%, preferably 95-105% of the average particle size. In some embodiments, the particles of a microporous network of carbon are not monodisperse.

In general, the particle size may be determined by any suitable method known to one of ordinary skill in the art. In some embodiments, the particle size is determined by powder X-ray diffraction (PXRD). Using PXRD, the particle size may be determined using the Scherrer equation, which relates the full-width at half-maximum (FWHM) of diffraction peaks to the size of regions comprised of a single crystalline domain (known as crystallites) in the sample. In some embodiments, the crystallite size is the same as the particle size. For accurate particle size measurement by PXRD, the particles should be crystalline, comprise only a single crystal, and lack non-crystalline portions. Typically, the crystallite size underestimates particle size compared to other measures due to factors such as amorphous regions of particles, the inclusion of non-crystalline material on the surface of particles such as bulky surface ligands, and particles which may be composed of multiple crystalline domains. In some embodiments, the particle size is determined by dynamic light scattering (DLS). DLS is a technique which uses the time-dependent fluctuations in light scattered by particles in suspension or solution in a solvent, typically water to measure a size distribution of the particles. Due to the details of the DLS setup, the technique measures a hydrodynamic diameter of the particles, which is the diameter of a sphere with an equivalent diffusion coefficient as the particles. The hydrodynamic diameter may include factors not accounted for by other methods such as non-crystalline material on the surface of particles such as bulky surface ligands, amorphous regions of particles, and surface ligand-solvent interactions. Further, the hydrodynamic diameter may not accurately account for non-spherical particle shapes. DLS does have an advantage of being able to account for or more accurately model solution or suspension behavior of the particles compared to other techniques. In some embodiments, the particle size is determined by electron microscopy techniques such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

In some embodiments, the microporous network of carbon has a Brunauer-Emmett-Teller (BET) surface area of 200 to 4000 m²/g, preferably 225 to 2000 m²/g, preferably 250 to 1000 m²/g, preferably 275 to 750 m²/g, preferably 300 to 400 m²/g, preferably 310 to 395 m²/g, preferably 325 to 385 m²/g, preferably 340 to 375 m²/g, preferably 350 to 360 m²/g.

In some embodiments, the microporous network of carbon has a total pore volume of 0.10 to 2.4 cm³/g, preferably 0.125 to 1.75 cm³/g, more preferably 0.15 to 1.00 cm³/g, preferably 0.175 to 0.50 cm³/g, preferably 0.20 to 0.40 cm³/g, preferably 0.225 to 0.375 cm³/g, preferably 0.25 to 0.350 cm³/g, preferably 0.275 to 0.325 cm³/g, preferably 0.285 to 0.300 cm³/g. In some embodiments, the microporous network of carbon has a mean pore size of 10 to 250 Å, preferably 15 to 225 Å, preferably 20 to 200 Å, preferably 25 to 175 Å. In some embodiments, the microporous network of carbon has a micropore volume of 0.01 to 2.4 cm³/g, preferably 0.025 to 1.75 cm³/g, preferably 0.05 to 1.00 cm³/g, preferably 0.075 to 0.75 cm³/g, preferably 0.10 to 0.40 cm³/g, preferably 0.15 to 0.30 cm³/g, preferably 0.175 to 0.225 cm³/g, preferably 0.19 to 0.21 cm³/g, preferably about 0.20 cm³/g.

In some embodiments, the microporous network of carbon has a pore size distribution that is bimodal. In some embodiments, the bimodal pore size distribution has a first mode at 0.75 to 1.25 Å. In some embodiments, the bimodal pore distribution has a second mode at 1.75 to 2.25 Å. In general, pore size may be determined by techniques including, but not limited to, gas adsorption (e.g., N₂ adsorption), mercury intrusion porosimetry, and imaging techniques such as scanning electron microscopy (SEM), and x-ray computed tomography (XRCT).

In some embodiments, a portion of the phosphorous is present as isolated phosphorous atoms doped into the microporous network of carbon. That is, a portion of the phosphorus is present as species other than phosphate, phosphite, phosphonate, phosphinate, or other oxygen-containing phosphorous groups. This portion of the phosphorous can be include as dopant atoms in the carbon that makes up the microporous network of carbon. The phosphorous atoms can be, for example, included in sheets of graphene or graphitic carbon, as organophosphines, as phosphoranes, as phosphonium salts, phosphoalkenes, phosphoalkanes, phospohalkynes, and combinations thereof. In general, the phosphorous can be present in any oxidation state, such as phosphorous (O), phosphorous (I), phosphorous (II), phosphorous (III), phosphorous (V), and combinations of these.

In some embodiments, the microporous network of carbon comprises phosphate or organophosphate functional groups. In some embodiments, such phosphate or organophosphate functional groups are present on a surface of the microporous network of carbon.

In some embodiments, the microporous network of carbon comprises carboxylate functional groups. In some embodiments, such carboxylate functional groups are present on a surface of the microporous network of carbon.

In some embodiments, the electrocatalyst has a Raman spectrum showing an absorbance centered at about 1550 to 1650 cm⁻¹. Such an absorbance may be referred to as a “G band”. In some embodiments, the electrocatalyst has a Raman spectrum showing an absorbance centered at about 1350 to 1475 cm⁻¹. Such an absorbance may be referred to as a “D band”. In some embodiments, the electrocatalyst has a Raman spectrum showing both a D band and a G band. In some embodiments, a ratio of a D band intensity to a G band intensity I_D/I_G is 0.75 to 1.25, preferably 0.80 to 1.20, preferably 0.85 to 1.15, preferably 0.90 to 1.10, preferably 0.95 to 1.05 by Raman spectroscopy. The presence of the G band, D band, or both in the Raman spectrum is typically considered evidence that the electrocatalyst comprises disordered graphitic carbon.

Method of Forming the Electrocatalyst

The present disclosure also relates to a method of producing the electrocatalyst. In some embodiments, the method comprises mixing petroleum vacuum residue and phosphoric acid to form a crude mixture, annealing the crude mixture to form an intermediate product, and heating the intermediate to form the electrocatalyst.

In some embodiments, the crude mixture is annealed at 375 to 525° C., preferably 400 to 500° C., preferably 425 to 475° C., preferably 450° C. In some embodiments, the crude mixture is annealed in a first inert atmosphere. In some embodiments, the crude mixture is annealed for 1 to 5 hours, preferably 1.25 to 4 hours, preferably 1.5 to 3 hours, preferably 1.75 to 2.5 hours, preferably 1.9 to 2.25 hours, preferably about 2 hours. In some embodiments, the annealing is performed with a temperature increase rate of 5 to 10° C./min, preferably 6 to 9° C./min, preferably 7 to 8° C./min preferably 7.25 to 7.75° C./min, preferably 7.5° C./min.

In some embodiments, the intermediate product is heated at 400 to 1000° C., preferably 550 to 950° C., preferably 700 to 900° C., preferably 725 to 875° C., preferably 750 to 850° C., preferably 775 to 825° C., preferably 790 to 810° C., preferably about 800° C. In some embodiments, the intermediate product is heated in a second inert atmosphere. In some embodiments, the crude mixture is annealed for 1 to 5 hours, preferably 1.25 to 4 hours, preferably 1.5 to 3 hours, preferably 1.75 to 2.5 hours, preferably 1.9 to 2.25 hours, preferably about 2 hours. In some embodiments, the heating is performed with a temperature increase rate of 5 to 10° C./min, preferably 6 to 9° C./min, preferably 7 to 8° C./min preferably 7.25 to 7.75° C./min, preferably 7.5° C./min.

Petroleum vacuum residue (also called “VR”) refers to a complex residue from the vacuum distillation of crude oil. Vacuum residue includes a complex hydrocarbon mixture of several thousand different chemical species, typically having a number of carbons greater than 34 (C34 or above) and/or a boiling point of at least about 495° C. In conventional petroleum refining processes, crude oil typically is fractionated by an atmospheric distillation tower, producing fractions with different boiling points, including: gases, light naphtha, heavy naphtha, jet fuel, kerosene, diesel oil, atmospheric gas oil, and atmospheric bottoms (or atmospheric reduced crude). Among these products, gases undergo gas processing that eventually yields products including fuel, butanes, liquefied petroleum gas (LPG), and the like. The most commercially valuable fractions are the lower boiling liquid fractions, which undergo further hydroprocessing, including hydrocracking and hydrotreating, to yield gasoline blending products, jet fuel, kerosene, and diesel oil. The highest boiling fractions, atmospheric bottoms, are further fractionated by a vacuum distillation tower, producing fractions with increasing boiling points including: gas, light vacuum gas oil, heavy vacuum gas oil, vacuum residuum (or vacuum reduced crude), and asphalt. Light vacuum gas oil and heavy vacuum gas oil are further processed to yield gasoline blending products, while vacuum residuum is typically either removed or further processed by a coker, i.e., a system that reforms high boiling heavy oil (typically vacuum residuum) by thermal cracking, forming upgraded hydrocarbons and coke.

In some embodiments, the vacuum residue comprises 80 to 85 wt % carbon, preferably 80.50 to 84.75 wt %, preferably 80.75 to 84.50 wt % carbon, preferably 81.0 to 84.25 wt % carbon, preferably 81.25 to 84.0 wt % carbon, preferably 81.50 to 83.75 wt % carbon, preferably 81.75 to 83.50 wt % carbon, preferably 82.0 to 83.25 wt %, carbon preferably 82.25 to 83.0 wt % carbon, preferably 82.50 to 82.90 wt % carbon, preferably 82.70 to 82.80 wt % carbon, based on a total weight of the vacuum residue.

In some embodiments, the vacuum residue comprises 7.0 to 9.0 wt % hydrogen, preferably 7.25 to 8.75 wt % hydrogen, preferably 7.40 to 8.50 wt % hydrogen, preferably 7.50 to 8.25 wt % hydrogen, preferably 7.75 to 8.15 wt % hydrogen, preferably 7.80 to 8.05 wt % hydrogen, preferably 7.90 to 8.00 wt % hydrogen, based on a total weight of the vacuum residue.

In some embodiments, the vacuum residue comprises 3.00 to 5.00 wt % sulfur, preferably 3.25 to 4.75 wt % sulfur, preferably 3.50 to 4.50 wt % sulfur, preferably 3.75 to 4.25 wt % sulfur, preferably 3.90 to 4.15 wt % sulfur, preferably 4.00 to 4.10 wt % sulfur, based on a total weight of the vacuum residue.

In some embodiments, the vacuum residue comprises 0.25 to 1.00 wt % nitrogen, preferably 0.30 to 0.85 wt % nitrogen, preferably 0.35 to 0.75 wt % nitrogen, preferably 0.40 to 0.60 wt % nitrogen, preferably 0.45 to 0.55 wt % nitrogen, based on a total weight of the vacuum residue.

In some embodiments, the vacuum residue can further include an impurity element which is at least one selected from the group consisting of metals or semimetals such as iron, vanadium, aluminum, magnesium, calcium, nickel, copper, zinc chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, arsenic, beryllium, boron, cadmium, and silicon.

In some embodiments, the petroleum vacuum residue and phosphoric acid are present in the crude mixture in a ratio of 1.5:1 to 1:1.5, preferably 1.4:1 to 1:1.4, preferably 1.3:1 to 1:1.3, preferably 1.25:1 to 1:1.25, preferably 1.20:1 to 1:1.20, preferably 1.15:1 to 1:1.15, preferably 1.10:1 to 1:1.10, preferably 1.05:1 to 1:1.05, preferably 1:1.

In some embodiments, the petroleum vacuum is heated to facilitate mixing with the phosphoric acid. In some embodiments, the petroleum vacuum residue is heated to 100° C. to facilitate such mixing.

In some embodiments, the phosphoric acid has a concentration of 50 to 99% in water, preferably 60 to 95% in water, preferably 75 to 92.5% in water, preferably 80 to 90% in water, preferably 82.5 to 87.5% in water, preferably about 85% in water.

In some embodiments, an acid other than phosphoric acid is used in addition to phosphoric acid. The acid other than phosphoric acid may be an inorganic (mineral) acid or an organic acid. Examples of inorganic acids include but are not limited to nitric acid, sulfuric acid, perchloric acid, hydrobromic acid, hydrochloric acid, hydroiodic acid, hydrofluoric acid, boric acid, and the like. Examples of organic acids include but are not limited to formic acid, acetic acid, propionic acid, butyric acid, valeic acid, caproic acid, oxalic acid, lactic acid, malic acid, citric acid, carbonic acid, benzoic acid, phenol, uric acid, carboxylic acids, sulfonic acid, and the like.

In general, an inert atmosphere may be provided by either a vacuum or by nitrogen, helium, argon, neon, or other suitable inert gas or mixture thereof. The inert atmosphere may be static or may have a flow of one or more gases. In some embodiments, the first inert atmosphere is provided by flowing nitrogen gas. In some embodiments, the second inert atmosphere is provided by flowing nitrogen gas.

In some embodiments, the method does not involve the use of hydrogen gas. Hydrogen gas may be a suitable reducing agent for various carbonaceous materials. However, preferably, the method of the present disclosure does not include a reducing or reduction treatment, such as treatment with hydrogen.

In some embodiments, the method further comprising washing the electrocatalyst with water and drying at 25 to 100° C., preferably 30 to 90° C., preferably 40 to 85° C., preferably 50 to 75° C., preferably 55 to 65° C., preferably 60° C.

Method of Producing Hydrogen Peroxide

The present disclosure also relates to a method of producing hydrogen peroxide. The method includes applying a potential to an electrochemical cell to form hydrogen peroxide and collecting the hydrogen peroxide. In some embodiments, the applied potential is 0.15 to 1.50 volts (V), preferably 0.25 to 1.40 V, preferably 0.30 to 1.25 V, preferably 0.35 to 1.10 V, and preferably 0.40 to 1.00 V, preferably 0.45 to 0.95 V, preferably 0.50 to 0.90 V. In some embodiments, the method has an onset potential of 0.750 to 0.875 V vs RHE, preferably 0.760 to 0.860 V vs RHE, preferably 0.770 to 0.850 V vs RHE, preferably 0.780 to 0.840 V vs RHE, preferably 0.790 to 0.830 V vs RHE, preferably 0.795 to 0.825 V, preferably 0.798 to 0.821 V vs RHE. The electrochemical cell includes a working electrode that includes the electrocatalyst, an electrolyte, and a counter electrode. In some embodiments, the counter electrode is made from a material selected from the group consisting of platinum, gold, and carbon. In some embodiments, the counter electrode may contain an electrically-conductive material such as platinum, platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, and/or some other electrically-conductive material, where an “electrically-conductive material” as defined here is a substance with an electrical resistivity of at most 10^{−6 Ω·}m, preferably at most 10^{−7 Ω·}m, more preferably at most 10^{−8 Ω·}m at a temperature of 20-25° C. The form of the counter electrode may be generally relevant only in that it needs to supply sufficient current to the electrolyte solution to support the current required for the electrochemical reaction of interest. The counter electrode material should thus be sufficiently inert to withstand the chemical conditions in the electrolyte solution, such as acidic or basic pH values, without substantially degrading during the electrochemical reaction. The counter electrode should preferably not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable electrode contamination. In a preferred embodiment, the counter electrode is platinum.

In some embodiments, the electrochemical cell further includes a reference electrode in contact with the electrolyte solution. A reference electrode is an electrode that has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each relevant species of the redox reaction. A reference electrode may enable a potentiostat to deliver a stable voltage to the working electrode or the counter electrode. The reference electrode may be a standard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), a reversible hydrogen electrode (RHE), a saturated calomel electrode (SCE), a copper-copper (II) sulfate electrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a mercury-mercurous sulfate electrode, mercury/mercuric oxide (Hg/HgO) electrode, or some other type of electrode. In a preferred embodiment, a reference electrode is an Hg/HgO electrode. However, in some embodiments, the electrochemical cell does not include a third electrode.

In some embodiments, the electrochemical cell is at least partially submerged in an electrolyte, preferably 50%, preferably 60%, or more preferably at least 70%. In some embodiments, the electrolyte includes an aqueous solution of water and a base. The water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In some embodiments, the electrolyte includes the aqueous solution of the base at a concentration of 0.001 to 5 M, preferably 0.005 to 2.5 M preferably 0.01 to 1 M, preferably 0.05 to 0.5 M, preferably 0.09 to 0.25 M, preferably about 0.1 M. In some embodiments, the base is at least one selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), barium hydroxide (Ba(OH)₂), calcium hydroxide (Ca(OH)₂). In some embodiments, an organic base may be used, such as sodium acetate and potassium acetate. Preferably, to maintain uniform concentrations and/or temperatures of the electrolyte solution, the electrolyte solution may be stirred or agitated during the step of the subjecting. The stirring or agitating may be done intermittently or continuously. This stirring or agitating may be done by a magnetic stir bar, a stirring rod, an impeller, a shaking platform, a pump, a sonicator, a gas bubbler, or some other device. Preferably, the stirring is done by an impeller or a magnetic stir bar. In some embodiments, the electrolyte is saturated with oxygen.

In some embodiments, the working electrode further comprises a glassy carbon substrate. In some embodiments the electrocatalyst is disposed on the surface of the glassy carbon substrate. In some embodiments, the working electrode further comprises a sulfonated fluoropolymer.

In some embodiments, the working electrode has a Tafel slope of 80-115 millivolt/decade (mV dec⁻¹), preferably 82.5 to 110 mV dec⁻¹, preferably 87.5 to 107.5 mV dec^−1, preferably 90 to 105 mV dec⁻¹, preferably 92.5 to 102.5 mV dec⁻¹, preferably 95 to 100 mV dec⁻¹, and preferably 97.5 to 99 mV dec⁻¹. In some embodiments, the working electrode has a charge transfer resistance (RCT) of preferably 550 to 700 Ω, preferably 575 to 675 Ω, preferably 580 to 670 Ω, preferably 590 to 660 Ω, preferably 600 to 650 Ω, preferably 610 to 640 Ω, preferably 615 to 630 Ω, preferably 620 to 625 Ω. In some embodiments, the electrode has a capacitance of 2000 to 2750 μF, preferably 2050 to 2600 μF, preferably 2100 to 2500 μF, preferably 2150 to 2450 μF, preferably 2200 to 2400 μF, preferably 2225 to 2350 μF, preferably 2250 to 2325 μF, preferably 2275 to 2300 μF, preferably 2290 μF.

In some embodiments, the method has an electron transfer number of 1.75 to 3, preferably 1.85 to 2.9, preferably 1.95 to 2.8, preferably 2.00 to 2.75, preferably 2.05 to 2.70, preferably 2.10 to 2.65, preferably 2.2 to 2.6.

In some embodiments, method has a yield of 80 to 95% OH₂⁻, preferably 81 to 94% OH₂⁻, preferably 83 to 93% OH₂⁻, preferably 84 to 92% OH₂⁻, preferably 85 to 91% OH₂⁻, preferably 86 to 90% OH₂⁻, preferably 86.5 to 89.5% OH₂⁻, preferably 87 to 89% OH₂⁻, preferably 87.5 to 88.5% OH₂⁻, preferably 88 OH₂⁻% at a potential of 0.5 to 0.65 V vs RHE, preferably 0.51 to 0.63 V vs RHE, preferably 0.52 to 0.62 V vs RHE, preferably 0.53 to 0.61 V vs RHE, preferably 0.54 to 0.60 V vs RHE, preferably 0.55 to 0.59 V vs RHE, preferably 0.56 to 0.58 V vs RHE, preferably 0.57 V vs RHE.

The examples below are intended to further illustrate protocols for preparing and characterizing the electrocatalyst, forming the working electrode, and performing the method of procuring hydrogen peroxide and are not intended to limit the scope of the claims.

EXAMPLES

Materials and Methods

Vacuum residue (VR) was acquired from the local Ras Tanura refinery in the Eastern Province of Saudi Arabia. The average elemental composition of VR was 82.74 wt % carbon, 7.94 wt % hydrogen, 4.02 wt % sulfur, and 0.50 wt % nitrogen. Orthophosphoric acid (85%) was purchased from Millipore-Sigma and used without further purification. Ultrapure water processed from the Milli-Q (Milford, MA, USA) system.

Activated vacuum residue (VR-PA) was prepared using an orthophosphoric acid as an activating agent. Briefly, the vacuum residue was mixed with H₃PO₄ at a 1:1 mass ratio, the mixture was heated in an oven at 100° C. under an air atmosphere for 5 minutes to enable effective homogeneous mechanical mixing, then the sample was placed in a horizontal tubular furnace under nitrogen flow. The furnace temperature was raised gradually from room temperature to 450° C. at the rate of 7.5° C. min⁻¹, and the sample was annealed at 450° C. for two hours, the temperature was then increased to 800° C. and activated for another two hours. The resulting samples denoted as VR-PA cooled to room temperature under a nitrogen atmosphere, then it was washed with de-ionized water until a neutral pH, the sample was dried overnight at 60° C. in an oven.

Physical Characterization Methods

A Quattro ESEM 400 high-resolution field emission scanning electron microscope (SEM) at 20 keV was used for the SEM images and energy dispersive x-ray analysis (EDX). Brunauer-Emmett-Teller (BET) analysis was performed to investigate the texture of the carbon materials using the Quantachrome Autosorb-3b. Before the N₂ physisorption measurements, the samples were activated at 300° C. for 24 h under a vacuum. XRD spectroscopy was done using a Rigaku Ultima IV X-ray diffractometer with a scan range of 5-80° 20 at a speed of 3°/min, 40 kV, and 40 mA. Raman measurements were carried out on a Horiba LabRAM HR Raman spectrometer (iHR320 with CCD detector) using a 532 nm excitation laser (300 mW, green laser) with a 50×LWD microscope objective.

Electrochemical Measurement Methods

An electrochemical workstation (Biologic Potentiostat) was used in all electrochemical measurements with a three-electrode setup. RDE and RRDE were used with modulated rotor from Pine research (MRS) to evaluate the electrocatalytic activity with a glassy carbon electrode (0.1964 cm2) deposited with VR-PA slurry made from Nafion (5%) solution was used as the working electrode. For the three-electrode system, Pt wire served as the counter electrode, and the saturated Ag/AgCl was the reference electrode. Linear sweep voltammetry was measured at 10 mV/s scan rate and in the potential range of 0.3 V to −0.6 V vs Ag/AgCl in 0.1 M KOH as electrolyte. The potential was converted to a reversible hydrogen electrode (RHE) using the equation (1) given below:

ERHE ⁡ ( V ) = EAg / AgCl ⁡ ( V ) + 0.197 + 0.059 × pH ( 1 )

Synthesis and Physical Characterization of VR-PA

Surface properties of carbon materials in electrocatalytic activity heavily depend on the types of doped heteroatoms, such as nitrogen, oxygen, sulfur, boron, and phosphorous, or combinations of these heteroatoms for dual-doped carbon materials. Synthesis of phosphorous-doped carbon materials has done by activating various carbon precursors with phosphoric acid. However, this results in any phosphorous existing as phosphate-like species in activated carbons. It is therefore necessary to also reduce the phosphate-like species under the H₂ atmosphere to produce a phosphorous-doped carbon materials can be produced. [Wang, Y., et. al., Langmuir 2017, 33, 3112-3122, incorporated herein by reference in its entirety]. In this disclosure, phosphoric acid-activated carbons were prepared without the need for hydrogen reduction from the vacuum residue (VR) as a carbon source and the waste precursor from the local petroleum industry. The elemental composition of VR possessed 82.74 wt % carbon, 7.94 wt % hydrogens, 4.02 wt % sulfur, and 0.50 wt % nitrogen. A schematic diagram for the preparation of activated VR with phosphoric acid (VR-PA) is shown in FIG. 1. Synthesis was done by direct activation of VR in the presence of 1 equivalent of phosphoric acid by mass at 800° C. under a nitrogen atmosphere. The details of the synthetic procedure are described in the Materials and Methods section above.

The resultant VR-derived activated carbon, VR-PA, was characterized by various spectroscopic analyses such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, as well as the N2-physisorption analysis for porous structure. Elemental analysis of VR-PA reveals 50.39 wt % carbon, 0.93 wt % hydrogen, 1.46 wt % sulfur, and 0.90 wt % nitrogen. C, H, and S content decreased, whereas N content increased with respect to the VR. This points to a significant degree of oxidation, which resulted in triply-doped activated carbon with, N, S, and P in the final VR-PA. XRD spectrum was obtained from the sample to determine the structure as shown in FIG. 2A. The peaks observed at 25.4° and 45° matched with the graphic planes of (002) and (101). [Wang, T. H., et. al., J. Power Sources. 2014, 248, 427-433, incorporated herein by reference in its entirety]. The graphitic nature of VR-PA was confirmed by observing similar peaks in graphite Spectra. The change of graphitic structure between graphite and VR-PA was the result of the modification of Vacuum Residue with phosphoric acid. The peak of VR-PA was observed to be lower than that of graphite which indicates that the interplane distance of (002) was larger in VR-PA than in graphite. [Liu, Q., et. al., Langmuir. 2013, 29, 3821-3828; and Ma, Y., et. al., Electrochim. Acta 2014, 133, 391-398, each of which is incorporated herein by reference in its entirety]. XRD broad peak pattern around 15° to 25° showed VR-PA was disordered graphite due to defect. [Yuan, D., et. al., J. Power Sources. 2016, 301, 131-137, incorporated herein by reference in its entirety]. The appearance of D and G bands at 1443 cm⁻¹ and 1601 cm⁻¹ respectively in the Raman shift shown in FIG. 2B confirmed the graphic nature of VR-PA which is in good agreement with the XRD pattern. [Jiao, X., et. al., ACS Appl. Mater. Interfaces. 2019, 11, 30858-30864, incorporated herein by reference in its entirety]. VR-PA had a dominant D band with an ID/IG=0.99 which shows that VR-PA had a structural disorder, where ID and IG correspond to the D and G band peak intensities, respectively. [Li, W. J., et. al., J. Mater. Chem. A. 2016, 4, 505-511; Chen, X., et. al., New J. Chem. 2019, 43, 6197-6204; and Cheng, Z., et. al., Solid State Ionics. 2023, 389, 116098, each of which is incorporated herein by reference in its entirety].

The SEM image of the VR-PA morphology showed the surface is not smooth and with a porous structure. See FIG. 3. The surface area of 357 m²/g, and pore volume of 0.29 cm³/g (at 0.95 P/PO) with the micropore volume of 0.20 cm³/g were estimated for VR-PA from BET analysis, which suggests that the material was predominantly microporous in structure confirming the images obtained from SEM. See FIGS. 4A-4B.

X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface characteristics of the VR-PA. FIG. 5A shows the XPS survey scan of VR-PA showing the surface elemental composition as 73.97 wt % carbon, 15.85 wt % oxygen, 5.71 wt % phosphorous, 3.53 wt % sulfur, and 0.86 wt % nitrogen. The High-resolution XPS spectra for VR-PA are given in FIGS. 5B-5D. Based on the C1s, O1s, and P2p peak analysis, VR-PA exhibited a predominantly graphitic structure with surface functionalization composed of complex structures such as carboxylate, phosphate, and sulfur-doped carbon network.

Electrochemical Characterization of VR-PA

Structural characteristics of VR-PA, in particular, the porous structure and the graphitic content are expected to be crucial for VR-PA in potential applications as electrode materials. The capacitance of VR-PA was measured in 0.1 M KOH electrolyte and a potential range of 200 mV as shown in FIG. 6A. CV curves at varying scan rates showed rectangular CV curves indicating efficient charge transport and ion diffusion. Based on the slope of the non-faradaic current and the scan rate (J=C×dU/dt) the electrode capacitance of VR-PA was estimated to be 2290 μF (see FIG. 6B). The ion diffusion kinetics of VR-PA was also investigated by electrochemical impedance spectroscopy. The Nyquist plot for VR-PA is shown in FIG. 6C. The impedance spectra were fitted by an electric equivalent circuit model with resistance (Rs) in series with a combination of a parallel circuit of charge transfer resistance (RCT) and a constant phase element (Q1) in series with a constant phase element (Q2). The fitting values for VR-PA are estimated as R_s=44 ohms, Q1=2495 μFs^n-1 (n=0.82), R_CT=623 ohms, and Q2 =40 μFs^n-1 (n=0.16). Based on the similarity of the capacitance value estimated from the CV measurements and the Q1, Q1 was attributed to the electrode capacitance and Q2 was attributed to double-layer capacitance. Relatively high capacitance points to the phosphate groups decorated on the activated vacuum residue.

Electrocatalytic Performance

The ORR performance of VR-PA was tested and compared with commercial 5% Pt/C as a benchmark. CV measurements using a three-electrode setup under an inert atmosphere and oxygen presence for both VR-PA and Pt/C are shown in FIG. 7A. CV under 0.1 M KOH as electrolyte shows strong cathodic peaks for both VR-PA and Pt/C. The cathodic peak for VR-PA appeared at 0.718 V vs RHE, and for Pt/C at 0.763 V vs RHE in the oxygen-saturated 1M KOH solution. In contrast, under an argon atmosphere, no peaks were observed for both VR-PA and Pt/C. This points to the ORR activity of both VR-PA and Pt/C. The onset potential for ORR activity was observed at 0.821 V vs RHE for VR-PA, and at 0.887 V vs RHE for Pt/C.

A comparison of ORR activities of VR-PA and Pt/C under hydrodynamic conditions was carried out using the linear sweep voltammetry (LSV) in the oxygen-saturated solution of 0.1 M KOH. FIG. 7B displays the LSV curves for VR-PA and Pt/C at 1600 rpm rotation speed. Under these conditions, the onset potential for VR-PA appeared at 0.798 V vs RHE versus Pt/C at 0.891 V vs RHE, and the diffusion-limiting current appears relatively close to

Pt/C pointing to good ORR activity of VR-PA. Additionally, VR-PA exhibited a higher Tafel slope at 98 mV/dec in comparison to 78 mV/dec for Pt/C. However, VR-PA showed higher half-wave potential at 0.711 V vs RHE as opposed to 0.66 V vs RHE for Pt/C under identical conditions. All in all, it is plausible to conclude preliminarily that VR-PA possesses a different ORR mechanism rather than the ORR activity of Pt/C.

The ORR reaction kinetics and the mechanism of catalytic ORR activity were studied using LSV curves at varying rotation speeds in the range of 400 rpm to 2000 rpm, as shown in FIG. 7D. The faster electron transfer kinetics was evident from the increased current densities with rotation speed. Koutecky-Levich (K-L) analysis was employed to estimate the electron transfer number (FIG. 7E) for VR-PA under hydrodynamic conditions. Based on the K-L theory the relationship of the limiting current (i) to kinetic current (ikin) and the angular frequency ω is given by equation 2:

1 i = 1 i kin + 1 B ⁢ ω 1 / 2 ( 2 )

• • where B is the Levich constant and is given by equation 3:

B = 0.62 AnF [ O 2 ] ⁢ D O ⁢ 2 2 / 3 ⁢ v - 1 / 6 ( 3 )

• • where A=0.1944 cm², n is the electron transfer number, F=96485 C mol⁻¹, [O₂]=1.2×10⁻⁶ mol cm⁻³, D_O2=1.9×10⁻⁵ cm² s⁻¹ and the υ=8.7×10⁻³ cm² s⁻¹.

K-L analysis revealed the average electron transfer number at four different potentials (0.45 V-0.60 V) to be equal to 2.66. The linearity of K-L plots pointed to the accuracy of the K-L analysis and consistency of the estimated electron transfer number. The estimated electron transfer number suggests for two-electron transfer mechanism for VR-PA in contrast to Pt/C which exhibits a four-electron transfer pathway. This observation supports the difference in ORR activity observed from the voltammograms between VR-PA and Pt/C. Further evidence of electrocatalytic ORR for VR-PA was evidenced from the rotating ring disc electrode (RRDE) measurements. The ring currents and the disc currents were recorded with a rotating speed of 1600 rpm, see FIG. 7F. Both electron transfer number (n) and the yield of the % yield of OH₂⁻ were estimated using the equations (4) and (5), respectively

n = 4 ⁢ I D I D + I R / N ( 4 ) OH 2 - = 100 ⁢ % × 2 ⁢ I R / N I D + I R / N ( 5 )

• • where I_D and I_R correspond to the disc current and the ring current, respectively and the N is the collection efficiency which is equal to 24%, which was determined separately for the ring disc electrode using potassium ferricyanide redox couple. FIG. 7G displays the calculated OH₂⁻ (%) and n for ORR of VR-PA. OH₂⁻ (%) yield reached as high as 88% at 0.57 V vs RHE and the electron transfer number reached 2.2 at 0.57 V vs RHE. It is worth noting that the electron transfer number agrees with the estimated value from the K-L analysis. The discrepancy of the ORR activity between Pt/C and the VR-PA is attributed to the differences in the ORR mechanisms where VR-PA is selective towards the two-electron transfer oxygen-reducing catalyst whereas Pt/C is known as an effective four-electron transfer oxygen transfer catalyst.

Metal-free dual heteroatom-doped carbon materials have been shown to have good ORR activity, which includes P co-doped carbon materials with either N or S. While these materials have a demonstrated high onset potential, they all exhibit four-electron reduction of O₂ to H₂O. On the other hand, the intrinsic triple N-doped, S-doped, and P-doped structure of VR-PA along with the porosity and the conductivity induced the electrode reactions to assist in the two-electron reduction of O₂. In alkaline media, O₂ reduction can follow two mechanisms; a) either direct four-electron reduction to OH⁻ or b) stepwise consecutive 2-electron reductions first to OH₂⁻, followed by another 2-electron reduction of OH₂⁻ to OH⁻. It is evident that VR-PA follows the second mechanism with the microporosity and the triple N-doped, S-doped, and P-doped structure of VR-PA to be favorable for kinetics of ion transfer and electrode reactions toward fast 2-electron reduction of O₂ to OH₂⁻. The stability of the VR-PA catalyst was also tested using chronoamperometric studies. FIG. 7H displays the comparison of the chronoamperometric responses of VR-PA and Pt/C over the period of 1 h, which included also the methanol tolerance test at ˜30 min. The addition of 3 M methanol shows a considerable effect on the current response of Pt/C, whereas methanol addition had a significantly smaller effect on the current response of VR-PA. Moreover, the limiting current of the LSV plots increases after 1000 cycles as shown in FIG. 7I. This stability improvement could be due to the availability of well-dispersed and robust active sites upon cycling.

Where a numerical limit or range is stated herein, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.