Patent Application
20240309522
This research was supported by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum and Minerals (KFUPM) under the project DF191048.
The present disclosure is directed to an electrode, particularly a vanadium oxide (VOx)-based electrode for electrochemical water splitting and a method of preparation thereof.
The “background” description provided herein is to present the context of the disclosure generally. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that 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.
Excessive burning of fossil fuels and the subsequent adverse impact on our environment are quite evident almost daily. Therefore, the energy transition toward greener or safer alternatives is discussed and strategized in many countries [Kovač, A.; Paranos, M.; Marciuš, D., Hydrogen in energy transition: A review. International Journal of Hydrogen Energy 2021, 46 (16), 10016-10035 and Capurso, T.; Stefanizzi, M.; Torresi, M.; Camporeale, S., Perspective of the role of hydrogen in the 21st-century energy transition. Energy Conversion and Management 2022, 251, 114898]. The energy transition aims to gradually shift the energy vectors from conventional fossil fuels resources to renewable energy sources, such as wind, solar, and geothermal, etc [Shojaeddini, E.; Naimoli, S.; Ladislaw, S.; Bazilian, M., Oil and gas company strategies regarding the energy transition. Progress in Energy 2019, 1 (1), 012001]. However, most of these renewable resources suffer from intermittency and unsustainability issues. In this regard, hydrogen is considered a renewable and sustainable energy carrier if it is produced in green form i.e., electrocatalytic water splitting yielding net zero CO2 emission, and it is environmentally benign [Abdin, Z.; Zafaranloo, A.; Rafiee, A.; Mérida, W.; Lipiński, W.; Khalilpour, K. R., Hydrogen as an energy vector. Renewable and sustainable energy reviews 2020, 120, 109620 and Yu, M.; Wang, K.; Vredenburg, H., Insights into low-carbon hydrogen production methods: Green, blue and aqua hydrogen. International Journal of Hydrogen Energy 2021, 46 (41), 21261-21273].
Conventionally, electrocatalytic water splitting technology is sensitive to the capacity of an electrocatalyst, and hence, the energy conversion efficiency of the process largely depends on the fundamental improvements in the activity of the electrocatalyst being studied [You, B.; Sun, Y., Innovative strategies for electrocatalytic water splitting. Accounts of chemical research 2018, 51 (7), 1571-1580]. This leads to efforts in designing and subsequent fabrication of new catalytic materials that are efficient, scalable, and stable [Charles, V.; Anumah, A. O.; Adegoke, K. A.; Adesina, M. O.; Ebuka, I. P.; Gaya, N. A.; Ogwuche, S.; Yakubu, M. O., Progress and challenges pertaining to the earthly-abundant electrocatalytic materials for oxygen evolution reaction. Sustainable Materials and Technologies 2021, 28, e00252 and Li, Y.; Zhou, L.; Guo, S., Noble metal-free electrocatalytic materials for water splitting in alkaline electrolyte EnergyChem 2021, 3 (2), 100053].
Yet conventionally, electrochemical water splitting includes two steps, an oxygen evolution reaction (OER) at an anode and a hydrogen evolution reaction (HER) at a cathode. Hydrogen-based fuel production via the electrochemical water splitting is mainly hindered at the industrial scale due to the sluggish four-electron transfer process involved in the OER [Chandrasekaran, S.; Ma, D.; Ge, Y.; Deng, L.; Bowen, C.; Roscow, J.; Zhang, Y.; Lin, Z.; Misra, R.; Li, J., Electronic structure engineering on two-dimensional (2D) electrocatalytic materials for oxygen reduction, oxygen evolution, and hydrogen evolution reactions. Nano Energy 2020, 77, 105080 and Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M., Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chemical Society Reviews 2017, 46 (2), 337-365]. The hindered results put forward the ultimate desire to develop inexpensive OER catalysts that may reduce the over-potential and increase the efficiency of the overall system [Gonçalves, J. M.; Matias, T. A.; Toledo, K. C.; Araki, K., Electrocatalytic materials design for oxygen evolution reaction. In Advances in Inorganic Chemistry, Elsevier: 2019; Vol. 74, pp 241-303]. Conventional Ruthenium (Ru) and Iridium (Ir) based electrocatalysts possess stable OER performance under acidic and alkaline conditions. However, these costly and scarce elements of the catalysts limit their potential for large-scale deployment [Wang, F.; Tian, F.; Deng, Y.; Yang, L.; Zhang, H.; Zhao, D.; Li, B.; Zhang, X.; Fan, L., Cluster-based multifunctional copper (II) organic framework as a photocatalyst in the degradation of organic dye and as an electrocatalyst for overall water splitting. Crystal Growth & Design 2021, 21 (7), 4242-4248]. Therefore, there is a critical need to develop low cost, earth-abundant transition metal-based effective electrocatalytic water splitting.
Active and durable transition metal-based catalytic systems have been reported [Wang, Y.; Zheng, X.; Wang, D., Design concept for electrocatalysts. Nano Research 2021, 1-23 and Linnemann, J.; Kanokkanchana, K.; Tschulik, K., Design strategies for electrocatalysts from an electrochemist's perspective. ACS Catalysis 2021, 11 (9), 5318-5346]. However, these catalysts showed inferior performance relative to the noble metal-based catalysts. Therefore, it is clear that more effort is still required to improve the catalytic performance, which may be achieved by advanced preparation methods, inducing certain features in the catalyst, etc., while keeping an effective cost.
Different monometallic, bimetallic, and trimetallic-based catalysts, non-precious transition metals, such as nickel (Ni), cobalt (Co), and iron (Fe), etc., and their compounds have been reported as active catalysts in the OER process [Liang, H.; Xu, M.; Asselin, E., Corrosion of monometallic iron- and nickel-based electrocatalysts for the alkaline oxygen evolution reaction: A review. Journal of Power Sources 2021, 510, 230387; Jiang, J.; Zhang, Y. J.; Zhu, X. J.; Lu, S.; Long, L. L.; Chen, J. J., Nanostructured metallic FeNi2S4 with reconstruction to generate FeNi-based oxide as a highly-efficient oxygen evolution electrocatalyst. Nano Energy 2021, 81, 105619 and Jiang, S.; Zhu, L.; Yang, Z.; Wang, Y., Self-supported hierarchical porous FeNiCo-based amorphous alloys as high-efficiency bifunctional electrocatalysts toward overall water splitting. International Journal of Hydrogen Energy 2021, 46 (74), 36731-36741].
Vanadium (V) is one of the transition metals with multiple oxidation states, from (+2) to (+5), thus producing various crystalline phases [Shen, T. F. R.; Lai, M. H.; Yang, T. C. K.; Fu, I. P.; Liang, N. Y.; Chen, W. T., Photocatalytic production of hydrogen by vanadium oxides under visible light irradiation. Journal of the Taiwan Institute of Chemical Engineers 2012, 43 (1), 95-101]. Vanadium oxides (VOx) are studied as a cathode in lithium batteries due to their electrochemical properties, high specific capacity, and energy density [Shen, T. F. R.; Lai, M. H.; Yang, T. C. K.; Fu, I. P.; Liang, N. Y.; Chen, W. T., Photocatalytic production of hydrogen by vanadium oxides under visible light irradiation. Journal of the Taiwan Institute of Chemical Engineers 2012, 43 (1), 95-101]. The vanadium-based materials have the capability to develop ionic and molecular interactions, producing better electrocatalytic and/or photocatalytic materials for both the OER/HER [Liardet, L.; Hu, X., Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution. ACS catalysis 2018, 8 (1), 644-650; Merle, G.; Abrahams, I.; Barralet, J., Powerful amorphous mixed metal catalyst for efficient water-oxidation. Materials today energy 2018, 9, 247-253 and Akhoondi, A.; Feleni, U.; Bethi, B.; Idris, A. O.; Hojjati-Najafabadi, A., Advances in metal-based vanadate compound photocatalysts: synthesis, properties and applications. Synthesis and Sintering 2021, 1 (3), 151-168].
There has been a growing trend to develop composite catalyst systems that provide improved surface area, electric conductivity, and better performance. A similar approach may be taken to improve the OER activity of VOx-based electrocatalysts through synergetic interactions with other materials. By taking advantage of the synergistic and additive properties of mixed oxides that exhibit superior electrocatalytic behavior. Several metal oxides (nickel, iron, and cobalt) were incorporated with VOx to obtain bimetallic and trimetallic oxides. The incorporation of vanadium resulted in a positive synergistic effect that significantly increased their OER activity [Liardet, L.; Hu, X., Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution. ACS catalysis 2018, 8 (1), 644-650; Merle, G.; Ehsan, M. A.; Hakeem, A. S.; Sharif, M.; Rehman, A., Direct deposition of amorphous cobalt-vanadium mixed oxide films for electrocatalytic water oxidation. ACS omega 2019, 4 (7), 12671-12679 and Babar, N. U. A.; Hakeem, A. S.; Ehsan, M. A., Direct Fabrication of Nanoscale NiVOx Electrocatalysts over Nickel Foam for a High-Performance Oxygen Evolution Reaction. ACS Applied Energy Materials 2022, 5 (4), 4318-4328].
A systematic approach toward a better electrode design that may have nanoscale features, and with many electroactive sites may reduce the contact resistance among nanoparticles within the catalytic films. Aerosol-assisted chemical vapor (AACVD) deposition process can be used to make catalytic thin films. However, current techniques do not offer a sustainable and cost-effective method for making a catalyst for hydrogen production via electrocatalytic water splitting that allows control of surface morphology and thickness. Therefore, there is an unmet need for catalysts and methods of making catalysts that provide oxygen evolution reaction (OER) performance at high rates.
In an exemplary embodiment, an electrode is described. The electrode includes a metallic substrate. The electrode further includes a layer of particles of a vanadium oxide (VOx) composite at least partially covering a surface of the metallic substrate. In some embodiments, the particles of the VOx composite are in the form of nanobeads having an average particle size of 50 to 400 nanometers (nm).
In some embodiments, the metallic substrate is at least one metal foam selected from the group including an aluminum foam, a nickel foam, a titanium foam, a titanium alloy foam, an aluminum alloy foam, a magnesium alloy foam, a nickel alloy foam, and a steel foam.
In some embodiments, the vanadium oxide composite includes vanadium monoxide (VO), vanadium trioxide (V2O3), vanadium dioxide (VO2), and vanadium pentoxide (V2O5), and the metallic substrate is a nickel foam (NiF).
In some embodiments, the vanadium oxide composite has a particle size in a range of 100 to 200 nm.
In some embodiments, the metallic substrate is a nickel foam. In some embodiments, a combination of the vanadium oxide composite and the nickel foam has a synergistic effect, resulting in improved electrocatalytic performance of the electrode.
In some embodiments, the electrode has a current density of 800 to 1200 milliamperes per square centimeters (mA/cm2) at a potential of 1.7 volts reversible hydrogen electrode (VRHE).
In some embodiments, the electrode has a Tafel slope of 50 to 90 millivolts per decade (mV/decade).
In some embodiments, a method of making the electrode includes mixing and dissolving the vanadium oxide precursor in a solvent to form a solution. The method further includes aerosolizing the solution to form an aerosol containing the vanadium oxide precursor. The method also includes placing the metallic substrate in a heating chamber, and passing the aerosol through the heating chamber with the aid of a carrier gas. In some embodiments, the carrier gas includes nitrogen. In some embodiments, the metallic substrate is in direct contact with the aerosol. Additionally, the method includes heating the metallic substrate in the heating chamber to form the electrode having the layer of the vanadium oxide composite at least partially covered on the surface of the metallic substrate.
In some embodiments, the vanadium oxide precursor is at least one selected from the group consisting of ammonium vanadate, vanadyl oxalate, vanadium pentoxide, vanadium monoethanolamine, vanadium chloride, vanadium trichloride oxide, vanadyl sulfate, vanadium antimonate, antimony vanadate, vanadium oxyacetylacetonate, vanadium oxyacetate, vanadium oxyhalide, and vanadium oxytriisopropoxide.
In some embodiments, the vanadium oxide precursor is present in the solution at a concentration of 0.01 to 0.1 Molarity (M).
In some embodiments, the solvent is at least one selected from the group consisting of a ketone solvent, an ester solvent, an alcohol solvent, an amide solvent, and an ether solvent.
In some embodiments, the solvent is the alcohol solvent. In some embodiments, the alcohol solvent is at least one selected from the group consisting of methanol, ethanol, n-propanol, and isopropanol.
In some embodiments, the aerosol is passed through the heating chamber at a rate of 80 to 120 cubic centimeters per minute (cm3/min) with the aid of the carrier gas.
In some embodiments, the aerosolizing is performed on the aerosol generator including a fluid chamber having a housing inlet, a housing outlet, and a vent. The aerosol generator also includes a vibrating element operably coupled to the support plate for generating the aerosol. In some embodiments, the solution is introduced into the fluid chamber via the housing inlet. In some embodiments, the fluid chamber is in fluid communication with the heating chamber via the housing outlet. In some embodiments, the carrier gas is introduced into the fluid chamber via the vent, thereby carrying the aerosol into the heating chamber.
In some embodiments, the heating is performed at a temperature of 400 to 600 degrees centigrade (° C.) for an appropriate amount of time.
In some embodiments, at least a portion of the vanadium oxide precursor is decomposed to generate the vanadium oxide composite during the heating.
In some embodiments, the metallic substrate is a nickel foam (NiF) having a porous structure.
In another exemplary embodiment, an electrochemical cell is described. The electrochemical cell includes the electrode, a counter electrode, and an electrolyte in contact with both electrodes.
In some embodiments, the electrolyte includes an aqueous solution of a base at a concentration of 0.1 to 3 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), and calcium hydroxide (Ca(OH)2).
In yet another embodiment, a method for an electrochemical water splitting is described. The method includes applying a potential between the electrodes in the electrochemical cell to form hydrogen and oxygen and separately collecting H2-enriched gas and O2-enriched gas.
The foregoing general description of the illustrative present disclosure and the following The detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.
A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In the drawings, reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words “a,” “an” and the like generally carry a meaning of “one or more,” unless stated otherwise.
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.
As used herein, the term “electrode” refers to an electrical conductor used to contact a non-metallic part of a circuit e.g., a semiconductor, an electrolyte, a vacuum, or air.
As used herein, the term “substrate” refers to an underlying layer that supports the primary layer.
As used herein, the term “nanobeads” refers to composites of nanoparticles. Nanoparticles are particles that have at least one dimension in the range less than 100 nanometers (nm), while nanobeads are usually around 50 to 200 nanometers in diameter.
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 “aerosolizing” refers to a process of intentionally oxidatively converting solution for the purpose of delivering the oxidized aerosols to the heating chamber.
As used herein, the term “aerosol” refers to extremely small solid particles, or very small liquid droplets, suspended in the atmosphere.
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 “water splitting” refers to the chemical reaction in which water is broken down into oxygen and hydrogen.
2H2O→2H2+O2
Embodiments of the present disclosure are directed to a vanadium oxide (VOx)-based electrocatalysts for electrochemical water splitting reactions.
According to an aspect of the present disclosure, an electrode is described. The electrode includes a metallic substrate onto which is disposed, at least partially, a layer of particles of vanadium oxide composite covering a surface of the metallic substrate. In some embodiments, at least 50% of the surface of the metallic substrate is covered by the vanadium oxide composite based on a total surface area of the metallic substrate, preferably at least 70%, preferably at least 90%, or even more preferably at least 99%, based on the total surface area of the metallic substrate. In another embodiment, only one side of the metallic substrate is covered with the vanadium oxide composite.
In some embodiments, the metallic substrate is at least one metal foam selected from the group consisting of an aluminum foam, a nickel foam, a titanium foam, a titanium alloy foam, an aluminum alloy foam, a magnesium alloy foam, a nickel alloy foam, and a steel foam. Optionally, the substrate may include other particles of Au, Ag, Pt, Pd, Co, Rh, Ru, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti, Al, and/or alloys thereof. In a preferred embodiment, the metallic substrate may be a nickel foam (NiF) and a nickel alloy foam. The NiF substrate may optionally include metals in addition to nickel, such as iron, aluminum, or alloys thereof. In some embodiments, the NiF substrate is a porous material. In an embodiment, the average pore size of the NiF substrate is about 50 to 500 micrometers (μm), preferably 100 to 450 μm, preferably 150 to 400 μm, preferably 200 to 350 μm, preferably 250 to 300 μm, or even more preferably about 300 μm. Other ranges are also possible. In some further embodiments, the NiF substrate may be of any shapes, such as cubical, conical, cuboidal, pyramidical, or cylindrical. In an embodiment, the pores of the NiF substrate have a spherical shape. In some embodiments, the substrate may be a glass substrate.
The electrode further includes a vanadium oxide composite that at least partially covers the surface of the metallic substrate. It is preferred that the vanadium oxide composite forms a uniform layer that completely covers the surface of the substrate. In some embodiments, the vanadium nanocomposite includes particles in the form of nanobeads. In some embodiments, the vanadium nanocomposite may exist in any other morphological form such as nanorods, nanotubes, nanospheres, nanosheets, and/or combinations thereof. In some further embodiments, the nanobeads may have an average particle size of 50 to 400 nm, preferably between 100 to 300 nm, preferably 150 to 250 nm, and most preferably between 100 to 200 nm. Other ranges are also possible. In some preferred embodiments, the nanoscale vanadium oxide particles may accumulate on the surface of the metallic substrate to form micro-sized aggregates in any irregular shape, as depicted in
In some embodiments, the vanadium oxide composite may include vanadium in several oxidation states, such as +2, +3, +4, and +5. The vanadium oxide composite may include various oxides of vanadium such as vanadium monoxide (VO), vanadium trioxide (V2O3), vanadium dioxide (VO2), and vanadium pentoxide (V2O5). In some further embodiments, the vanadium oxide composite may be VO and VO2 phases. Other states may also exist. The existence of such states is dependent on the choice of the precursor used to prepare the vanadium oxide composite. For example, the use of vanadium(III) acetylacetonate (V(acac)3) as a vanadium oxide composite produces vanadium oxide in the form of V2O3.
In one embodiment, the metallic substrate is a nickel foam. In a preferred embodiment, a combination of the vanadium oxide composite and the nickel foam has a synergistic effect, resulting in improved electrocatalytic performance of the electrode.
The electrode of the present disclosure has a current density of 800 to 1200 mA/cm2 preferably about 800 to 1000 mA/cm2, or about 900 mA/cm2, at a potential of 1.7 VRHE. Other ranges are also possible. One of the indicators that access the OER kinetics is the Tafel slope. The Tafel slope shows how efficiently an electrode can produce current in response to a change in applied potential. Therefore, a low Tafel slope may indicate that less overpotential is required to get a high current. In some embodiments, the electrode has a Tafel slope of 50 to 90 mV/decade, preferably 60-80 mv/decade, and most preferably 68-70 mV/decade, which is lower than the metallic substrate without the vanadium oxide composite layer, therefore the electrode of the present disclosure may facilitate the energy-intensive step during the OER process.
In an exemplary embodiment, a method of making the electrode is described. Referring to
At step 52, the method 50 includes mixing and dissolving a vanadium oxide precursor in a solvent to form a solution. In some embodiments, the vanadium oxide precursor is at least one selected from the group consisting of ammonium vanadate (NH4VO3), vanadyl oxalate (C2H2O5V), vanadium pentoxide (V2O5), vanadium monoethanolamine (C2H7NOV), vanadium chloride (VCl3), vanadium trichloride oxide (VOCl3), vanadyl sulfate (VOSO4), vanadium antimonate (O20Sb5V3-15), antimony vanadate (NH4VO3), vanadium oxyacetylacetonate (C10H16O5V), vanadium oxyacetate (C4H8O5V), vanadium oxyhalide (X2OV; where X=chlorine (Cl), fluorine (F) bromine (Br) and iodine (I)), and vanadium oxytriisopropoxide (C9H24O4V). In a preferred embodiment, the vanadium oxide precursor is vanadium(IV) oxyacetylacetonate (VO(acac)2). Optionally, a combination of vanadium oxide precursors may be dissolved in a solvent to form the solution at an appropriate concentration.
The solvent is at least one selected from the group consisting of a ketone solvent, an ester solvent, an alcohol solvent, an amide solvent, and an ether solvent. In some embodiments, the solvent is the alcohol solvent. In some embodiment, the alcohol solvent is at least one selected from the group consisting of methanol, ethanol, n-propanol, and isopropanol. The dissolution may be conducted manually or with the help of a stirrer. It is conducted till the vanadium oxide precursor is fully dissolved in the solvent, e.g., methanol, to form a solution. In some embodiments, the vanadium oxide precursor is present in the solution at a concentration of 0.01 to 0.1 M. In some further embodiments, the vanadium oxide precursor is present in the solution at a concentration of higher than 0.1 M. Other ranges are also possible.
At step 54, the method 50 includes aerosolizing the solution to form an aerosol containing the vanadium oxide precursor. The aerosolizing is achieved by AACVD process. The AACVD process involves atomizing a precursor solution into fine, sub-micrometer-sized aerosol droplets, which are delivered to a heated reaction zone and undergo evaporation, decomposition, and homogeneous and/or heterogeneous chemical reactions to form the desired products. Using the AACVD route, a batch of films with different thicknesses and morphologies can be fabricated. This can be achieved by controlling parameters, such as deposition temperature, deposition time, gas carrier flow rate, precursor, and concentration of the precursor solution, etc. In some embodiments, the aerosolizing may be conducted at a temperature of 300 to 600° C., preferably 350 to 550° C., preferably about 400 to 500° C., or even more preferably about 470° C., for a period of time in a range of 5 to 180 minutes, preferably 10 to 150 minutes, preferably 15 to 120 minutes, preferably 20 to 90 minutes, preferably about 25 to 60 minutes, or even more preferably about 30 to 60 minutes. Other ranges are also possible. The aerosolizing process may be performed on an aerosol generator. Many different types of aerosol generators are known and may be used depending on the film desired.
In some embodiments, the aerosol generator includes a fluid chamber and a heating chamber. The fluid chamber includes a plurality of openings to allow for the entry and exit of the fluids. In an embodiment, the fluid chamber includes one or more housing inlets. The solution to be aerosolized is introduced into the fluid chamber via the housing inlet. In some embodiments, the solution can be introduced through one inlet, or through a plurality of inlets, where each inlet from among the plurality of inlets leads to the fluid chamber. The fluid chamber is fluidly connected to a vibrating element operably coupled to a support plate. In an embodiment, the vibrating element is an ultrasonic device, such as an ultrasonic humidifier or an ultrasonic atomizer. The ultrasonic humidifier is configured to generate an aerosol from the solution containing the vanadium oxide precursor. The fluid chamber further includes a vent through which is introduced a carrier gas. Suitable examples of carrier gas include H2, Ar, N2, or a combination thereof. In a preferred embodiment, the carrier gas is nitrogen. The fluid chamber further includes a housing outlet that fluidly connects the fluid chamber and a heating chamber.
At step 56, the method 50 includes placing a metallic substrate in a heating chamber and passing the aerosol through the heating chamber with the aid of a carrier gas. In some embodiments, the carrier gas transports the aerosol from the fluid chamber to the heating chamber via the housing outlet. In some further embodiments, the carrier gas may be used to transport the aerosol over long distances (tens to hundreds of meters), which causes some aerosol loss and a change in its size distribution. In some more embodiments, the aerosol is passed through the heating chamber at a rate of 80 to 120 cm3/min, preferably 90-110 cm3/min, and most preferably 100 cm3/min. Other ranges are also possible.
At step 58, the method 50 includes heating the metallic substrate in the heating chamber to form the electrode having the layer of the vanadium oxide composite at least partially covered on a surface of the metallic substrate. The heating chamber is maintained at a temperature range of 400 to 600° C., preferably 420-580° C., preferably 450 to 550° C., preferably 400-500° C., more preferably to about 470° C. Other ranges are also possible. The heating chamber is configured such that the metallic substrate is in direct contact with the aerosol. At this temperature, the solvent from the aerosol evaporates, leaving behind the precursor. At least a portion of the vanadium oxide precursor is decomposed to generate the vanadium oxide composite. In some embodiments, at least 50% of the vanadium oxide precursor is decomposed to generate the vanadium oxide composite based on an initial number of the vanadium oxide precursor molecules introduced into the heating chamber, preferably at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90%, or even more preferably at least 99% based on the initial number of the vanadium oxide precursor molecules introduced into the heating chamber. Other ranges are also possible. The vanadium oxide composite is further deposited on the substrate to obtain the electrode. The thickness of the vanadium oxide composite on the substrate depends on how long the deposition process takes place. In an embodiment, the deposition process is conducted for a period of 0.25-2 hours, preferably 0.5-1 hour, to obtain the electrode. Other ranges are also possible.
In one embodiment, the vanadium oxide nanocomposite was deposited on the nickel foam substrate to form the electrode referred to as VOx0.30, under AACVD conditions having a temperature of about 470° C., a deposition time of 30 minutes, using a solution containing methanol and vanadium(IV) oxyacetylacetonate (VO(acac)2), and the carrier gas is N2 gas. In one preferred embodiment, the vanadium oxide nanocomposite was deposited on the nickel foam substrate to form the electrode referred to as VOx0.60, under AACVD conditions having a temperature of about 470° C., a deposition time of 60 minutes, using a solution containing methanol and vanadium(IV) oxyacetylacetonate (VO(acac)2), and the carrier gas is N2 gas. In some embodiments, the electrodes VOx0.30, and VOx0.60 were evaluated in alkaline media, respectively. In some embodiments, the developed VOx catalyst deposited for 30 minutes (min) requires an overpotential of about 250 to 330 millivolts (mV), preferably about 290 mV, to reach a current decade of about 5 to 15 milliamperes per square centimeters (mAcm−2), preferably about 10 mAcm−2 and reaches to a current density of 800 to 1200 mAcm−2, preferably about 1000 mAcm−2 at an overpotential of about 470 mV. The VOx0.30 electrode has an OER with a current density of 1000 mAcm−2 at 1.7 V (η=470 mV) Vs. a reversible hydrogen electrode (RHE), as depicted in
The crystalline structures of the vanadium oxide nanocomposite may be characterized by X-ray diffraction (XRD), respectively. In some embodiments, the XRD patterns are collected in a Rigaku diffractometer equipped with a Cu-Kα radiation source (λ=0.15406 nm) for a 20 range extending between 5 and 90°, preferably 15 and 80°, further preferably 30 and 60° at an angular rate of 0.005 to 0.04° s−1, preferably 0.01 to 0.03° s−1, or even preferably 0.02° s−1.
An X-ray diffraction (XRD) pattern of the vanadium oxide nanocomposite (VOx) deposited on a non-crystalline plain glass substrate is illustrated in
The electrode thus prepared by the method of the present disclosure forms the working electrode in an electrochemical cell. The electrochemical cell further includes a reference electrode and a counter electrode. In a preferred embodiment, the reference electrode is Ag/AgCl, and the counter electrode is a Pt wire.
Referring to
At step 152, the method 150 includes applying a potential between the electrodes in the electrochemical cell to form hydrogen and oxygen. During the electrochemical process, a potential of 0.0 to 1.0 V is applied to the working electrode, and the counter electrode that are immersed in an aqueous electrolyte solution. The counter electrode forms the auxiliary electrode. The outer surface of the counter electrode includes an inert, electrically conducting chemical substance, such as platinum, gold, or carbon. The carbon may be in the form of graphite or glassy carbon. In a preferred embodiment, the counter electrode is a Pt wire. In one embodiment, the counter electrode may be a wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, a woven mesh, a perforated sheet, or a brush. 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 preferably should not leach out any chemical substance that interferes with the electrochemical reaction or might lead to undesirable. The aqueous electrolyte solution includes an aqueous solution of a base (base and an aqueous solution). The base selected from the group consisting of an alkaline earth metal hydroxide such as beryllium hydroxide (Be(OH)2), magnesium hydroxide (Mg(OH)2), strontium hydroxide (Sr(OH)2), and calcium hydroxide (Ca(OH)2) and an alkali metal hydroxide such as lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH) and rubidium hydroxide (RbOH), and cesium hydroxide (CsOH). In a preferred embodiment, the base is potassium hydroxide. The concentration of the base may lie in a range of about 0.1 molar (M) to 3 M, more preferably 0.5-2.5 M, and yet more preferably of about 1.0 M. Other ranges are also possible. In some embodiments, the electrolyte solution was deoxygenated aqueous electrolyte solution at room temperature (RT).
In some embodiments, the working electrode and the counter-electrode are connected to each other by way of electrical interconnects that allow for the passage of current between the electrodes, when a potential is applied between them. In a preferred embodiment, the electrocatalyst (which forms the working electrode) and the counter electrode are at least partially submerged in the water and are not in physical contact with each other. In an embodiment, the working electrode and the counter-electrode can have the same or different dimensions. In certain embodiments, the working electrode has a cross-section diameter of about 1.68 mm, and the counter-electrode as a cross-section diameter of about 0.2 mm. The working electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art.
At step 154, the method 150 includes the electrochemical process resulting in formation of hydrogen and oxygen. The H2-enriched gas is collected at the cathode, and the O2-enriched gas is collected at the anode.
The following examples demonstrate the catalytic activity of an electrocatalyst, including a vanadium oxide (VOx) layering on an electrode, using the AACVD deposition for electrochemical water splitting, as described herein. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.
All chemicals, vanadium(IV) oxyacetylacetonate (VO(acac)2, 98%), methanol anhydrous (99.8%), and material, nickel foam (NiF), were obtained from Sigma-Aldrich.
A self-designed AACVD setup was employed for thin film deposition. AACVD is a modified version of chemical vapor deposition (CVD), which requires a fully dissolved precursor in solution. Here, the required amount of VO(acac)2 was dissolved in 15 milliliters (mL) of methanol and was employed in AACVD to deposit a VOx thin film on a porous NiF substrate (dimension=1×2 cm2). The deposition was conducted for two different periods—30 and 60 min and at a fixed temperature of 470° C. for each case. The AACVD process involves the generation of precursor aerosol mist using an ultrasonic humidifier and driven towards a horizontal tube furnace with help of carrier gas (industrial nitrogen (N2, 99.9%)) at a flow rate of 100 cm3/min. The temperature of a tube furnace was set at 470° C., and the NiF substrates were placed inside the tube to receive the aerosol mist directly over it, where the decomposition of precursor occurs to generate the thin film. The process of AACVD was continued for 30 and 60 minutes and deposited samples were labeled as VOx/NiF-30 and VOx/NiF-60, respectively.
The crystalline patterns of VOx thin films were studied by an X-ray diffractometer (XRD) using a benchtop Rigaku MiniFlex. Surface morphologies of the VOx films were analyzed using a scanning electron microscope (SEM) JEOL JSM-6460. An X-ray photoelectron spectroscopy (XPS) was used to study the oxidation states of the major elements of the thin film, i.e., vanadium (V) and oxygen (O) using a Thermo Scientific Escalab 250Xi spectrometer. Obtained Spectra were corrected with reference to the adventitious C 1 s peak at 284.8 electroVolts (eV).
In a typical setup, a three-electrode cell (VOx/NiF working, silver (Ag)/silver chloride (AgCl) reference electrode and platinum (Pt) wire counter electrode) was used to study the electrochemical oxidation of water molecules using aqueous 1.0 Molarity (M) potassium hydroxide (KOH) as electrolyte at room temperature (RT). Potentiostat INTERFACE 1010 E was used to record the data. The electrolyte solution was deoxygenated aqueous electrolyte solution at room temperature (RT). All electrochemical data were presented with 10% infrared (IR) correction. The polarization curves were recorded by linear sweep voltammetry (LSV) at a scan rate of 5 millivolts per second (mV/s).
According to the Nernst equation, all the potentials here are converted into reversible hydrogen electrodes (RHE).
Overpotential is calculated according to the following equation
Tafel slopes are calculated from the linear region of the polarization curve where the Tafel region begins using a Tafel equation.
η=b log j+a
Crystalline VOx thin films were deposited on the NiF using the AACVD process at atmospheric pressure. XRD investigated the crystalline structure of the VOx film. The XRD pattern of VOx was hindered due to the higher crystallinity of the NiF substrate, and nickel peaks were obtained in the XRD patterns. Therefore, VOx films were prepared on a non-crystalline plain glass substrate under identical AACVD conditions to examine the real deposited phase and crystalline structure of these samples.
Further, the chemical states of V and O elements within the VOx-30 thin film sample were analyzed by XPS, and the results of this study are shown in
VOx/NiF thin film electrocatalyst was directly employed to study the electrochemical oxidation of water molecules in a 1.0 M aqueous KOH electrolyte. The catalyst samples were cycled between 1.0 to 1.7 volts in a CV-based pre-activation process, and the results of this study are depicted in
After the initial activation of the catalysts, LSV was performed. It was observed that the VOx electrocatalyst derived the water oxidation reaction at low over potential and achieved high current densities under the small potential window, as shown in
Tafel slope, which might be obtained from overpotential, and corresponding current density, is an important indicator to access the intrinsic catalytic kinetics of OER [Anantharaj, S.; Noda, S.; Driess, M.; Menezes, P. W., The Pitfalls of Using Potentiodynamic Polarization Curves for Tafel Analysis in Electrocatalytic Water Splitting. ACS Energy Letters 2021, 6 (4), 1607-1611, incorporated herein by reference in its entirety]. In
VOx catalytic films were deposited on the NiF substrate using the AACVD process and evaluated for the electrochemical water oxidation reaction in a 1.0 M KOH electrolyte. The nanoscale morphology of the VOx developed over conducting surface of the NiF helped accelerate the rate of oxidation reaction, and a current density of 10 mAcm−2 was obtained at 1.52 V, while a high current density of 1000 Acm−2 was achieved merely at 1.7 V vs. RHE. Moreover, the catalyst remained stable during nineteen (19) hours of continuous oxidation reaction monitoring without any apparent mechanical or chemical instability. This OER activity is improved compared to the existing OER representative catalysts, such as IrO2, NiO, and CoO catalysts. It was further illustrated that the fabrication and successful exploitation of a simple transition metal-based electrocatalyst with outstanding OER performances would bring new promises for low-cost hydrogen production.
Numerous modifications and variations of the present disclosure 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.
