Patent Application
20240247387
Support provided by the Interdisciplinary Research Center for Hydrogen and Energy Storage (IRC-HES) at King Fahd University of Petroleum and Minerals (KFUPM) is gratefully acknowledged.
The present disclosure is directed to an electrocatalyst, and particularly to an electrocatalyst including a layer of metallic nickel particles on a nickel foam substrate for electrolytic oxygen evolution.
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.
Hydrogen is an efficient energy carrier with great potential for clean and sustainable energy applications. However, the sustainable and renewable character of hydrogen mainly depends upon its production strategy. Currently, hydrogen production is dominated by methane steam reforming and coal gasification processes, which are associated with significant CO2 emissions. To address such challenges, electrocatalytic water splitting represents a promising alternative to achieve clean and pure hydrogen and oxygen through two half-cell reactions called hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).
The water splitting process mainly relies on OER as it provides the necessary protons and electrons to achieve the target reduction. However, the slow kinetics of OER is the bottleneck of water electrolysis. It is crucial to improve the efficiency of the entire electrochemical conversion process. To accelerate the OER rate, highly active and conductive platinum group (PGM) metals such as Pt, Ir, and Ru are used as electrocatalysts. Although PGM electrocatalysts are inherently capable of water oxidation under acidic/neutral environments, their price and scarcity have prompted the search for proficient electrocatalysts made from cheap and readily available elements as sustainable and economically viable alternatives.
Electrocatalyst materials based on 3d transition elements (Mn, Fe, Co, and Ni) have emerged as favorable choices to PGMs for water oxidation processes. In particular, Ni-based electrocatalysts have become increasingly important due to their electrical conductivity, high stability, and corrosion resistance characteristics under alkaline conditions. Therefore, Ni-based anodes are preferred for large-scale commercial liquid alkaline (LA) electrolyzes. Several general synthetic methods can be used to fabricate Ni-based anodes, including sol-gel, pulsed laser deposition, electrodeposition, sputtering, and chemical vapor deposition, and have shown high activity in OER catalysis.
For example, Berlinguette et al. studied the OER electrocatalysis of an amorphous mixed-metal NiOx thin-film electrode fabricated by a photochemical metal-organic deposition approach [Smith R D, Prévot MS, Fagan R D, Zhang Z, Sedach P A, Siu M K J, et al. Photochemical route for accessing amorphous metal oxide materials for water oxidation catalysis. Science. 2013; 340:60-3]. Similarly, the production of amorphous Ni/Fe(oxy)hydroxide [(Ni, Fe)OOH] thin films by mechanical stirring treatment to achieve remarkable activity and resistance to OER was demonstrated [Zhou H, Yu F, Zhu Q, Sun J, Qin F, Yu L, et al. Water splitting by electrolysis at high current densities under 1.6 volts. Energy & Environmental Science. 2018; 11:2858-64]. The non-stoichiometric synthesis of NiOx nanocrystals, which exhibit unusual activity and stability and combine with oxygen atoms to form NiOOH species has been reported [Duan H, Chen Z, Xu N, Qiao S, Chen G, Li D, et al. Non-stoichiometric NiOx nanocrystals for highly efficient electrocatalytic oxygen evolution reaction. Journal of Electroanalytical Chemistry. 2021; 885:114966]. Protocols for synthesizing porous 3D Ni/NiOx bifunctional oxygen electrocatalyst from freeze-dried Ni(OH)2. It was observed that stacking 2D layers into 3D volumes seems to play a compelling function following the dual functionality of freeze-dried Ni/NiOx have been disclosed [Shudo Y, Fukuda M, Islam M S, Kuroiwa K, Sekine Y, Karim M R, et al. 3D porous Ni/NiOx as a bifunctional oxygen electrocatalyst derived from freeze-dried Ni(OH)2. Nanoscale. 2021; 13:5530-5].
The development of a facile solution-processed in-situ synthesis of organic-inorganic nanocomposite from NiOx films embedded in acetate-based organic matrices, offering a competitive, low-cost, and highly scalable approach to electrocatalysts for water oxidation was disclosed [Noguera-Gómez J, Garcia-Tecedor M, Sanchez-Royo J F, Valencia Linan L M, de la Mata M, Herrera-Collado M, et al. Solution-processed Ni-based nanocomposite electrocatalysts: an approach to highly efficient electrochemical water splitting. ACS Applied Energy Materials. 2021; 4:5255-64].
Although OER catalytic activity has been observed with Ni-based catalysts, most of these catalysts suffer from drawbacks such as, multiple process steps and long processing, with severe deficiencies in reproducibility and homogeneity, making them unattractive for large-scale industrial manufacturing. Hence, there exists a need for an electrocatalyst that can overcome the limitations of the art. It is one object of the present disclosure to provide an electrocatalyst including nickel. It is another object of the present disclosure to provide an electrocatalyst including nickel with a high activity for OER.
In an exemplary embodiment, an electrocatalyst is described. The electrocatalyst includes a nickel foam substrate and a layer of metallic nickel particles on the nickel foam substrate. The metallic nickel particles are spherical and have an average diameter of 100-500 nanometers (nm). The metallic nickel particles are aggregated with aggregates having an average size of 0.5 to 5 micrometers (μm).
In some embodiments, the aggregates of the metallic nickel particles have a popcorn shape.
In some embodiments, the metallic nickel particles have a cubic crystal structure.
In some embodiments, the metallic nickel particles include Ni0.
In some embodiments, at least 90% of an outer surface area of the nickel foam substrate is covered with the layer of metallic nickel particles.
In some embodiments, the metallic nickel particles form a continuous layer on the nickel foam substrate.
In some embodiments, the layer of the metallic nickel particles on the nickel foam substrate has a thickness of 0.01 μm to 50 μm.
In some embodiments, the nickel foam substrate is porous and has an average pore size of 50 to 500 μm.
In some embodiments, the pores have a spherical shape.
In another exemplary embodiment, a method of oxidizing water is described. The method includes contacting the electrocatalyst and a counter electrode with the water. The method includes applying a potential to the electrocatalyst. The electrocatalyst and the counter electrode are at least partially submerged in the water and are not in physical contact with each other.
In some embodiments, the water is an aqueous electrolyte solution with a base selected from the group consisting of an alkaline earth metal hydroxide and an alkali metal hydroxide.
In some embodiments, the base is potassium hydroxide.
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 electrocatalyst has a water oxidation overpotential of 280-305 millivolts (mV) at 10 milliampere per square centimeter (mA cm−2).
In some embodiments, the water oxidation overpotential does not vary by more than 5% after the potential is applied for 10-50 hours.
In some embodiments, the electrocatalyst has an electrochemically active surface area (ECSA) of 250-300-centimeter square (cm−2).
In some embodiments, the electrocatalyst has a current density of at least 1,000 mA cm−2 at 1.6V.
The foregoing general description of the illustrative present disclosure and the following 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, like 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 there between.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other, and inclusive of all intermediate values of the ranges. Thus, ranges articulated within this disclosure, e.g., numerics/values, shall include the individual points within the range, sub-ranges, and combinations thereof.
As used herein, ‘working electrode’ refers to the electrode in an electrochemical cell/device/sensor on which the electrochemical reaction of interest is occurring.
As used herein, ‘counter-electrode’, is an electrode used in an electrochemical cell for voltametric analysis or other reactions in which an electric current is expected to flow.
As used herein, the term ‘glassy carbon’ refers to a non-graphitizing carbon which combines glassy and ceramic properties with those of graphite.
Aspects of the present disclosure are directed to a low-cost electrocatalyst (otherwise referred to as the catalyst) for oxygen evolution reaction (OER). The electrocatalyst includes nanostructured nickel (Ni) thin films deposited on porous nickel foam (NF) substrate, formed via an aerosol-assisted chemical vapor deposition (AACVD), which shows a favorable impact on the OER performance. The electrocatalyst is thoroughly analyzed via various analytical techniques. Scanning electron microscopy (SEM) analysis shows the conformal coating of the nanostructured nickel (Ni) thin films over the substrates. The catalyst demonstrated OER activity in the electrocatalytic layer in an alkaline medium. The OER performance is attributed to the synergic effect generated between nanostructured Ni and porous NF substrate, which enhances the electrical conductivity of the catalyst. This low manufacturing cost, robustness, and durability make this catalyst viable in solar energy conversion and storage applications.
According to an aspect of the present disclosure, an electrocatalyst, also referred to as a catalyst, is described. The electrocatalyst includes a substrate onto which is dispersed a layer of metallic particles. In an embodiment, the substrate is made from at least one metal selected from the group consisting of Mn, Fe, Co, and Ni. In an embodiment, the substrate is in the form of a foam, a mesh, or a solid metal sheet. As used herein a metal foam is a cellular structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. In a preferred embodiment, the substrate is nickel foam (NF).
In an embodiment, the NF substrate may optionally include metals in addition to nickel, such as nickel, aluminum, or alloys thereof. The NF substrate is a porous material. In an embodiment, the average pore size, or largest diameter, of the NF substrate is about 50 to 500 micrometers (μm), preferably 100 to 400 μm, or 200 to 300 μm. In an embodiment, the substrate has a thickness of 0.1 to 10 mm, preferably 0.5 to 8 mm, 1 to 5 mm or 2-3 mm. In an embodiment, the pores NF substrate has a shape such as cubical, conical, cuboidal, pyramidical, or cylindrical. The pores have a spherical shape.
The electrocatalyst further includes a layer of metallic nickel particles on the nickel foam substrate. The metallic nickel particles include metallic nickel particles in zero oxidation state, Ni0. In some embodiments, the metallic nickel particles may have traces of nickel particles of other oxidation states as well, such as +2, +3, and in +4 forms (represented by Ni2, Ni3′, and Ni4 forms). In an embodiment, the metallic nickel particles have a cubic crystal structure. In some other embodiment, the metallic nickel particles may have a body-centered cubic structure (BCC) or a face-centered cubic structure (FCC) or both. In some embodiments, the metallic nickel particles have an X-ray diffraction peak for (111) from 43 to 48°, preferably 44 to 47°, or 45 to 46°, a (200) peak from 50 to 55°, preferably 51 to 54°, or 52 to 53°, and a (220) peak from 75 to 80°, preferably 76 to 79°, or 77 to 78°. In some embodiments, the electrocatalyst may further include other particles of Au, Ag, Pt, Pd, Co, Rh, Ru, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti, Al particles, or alloys thereof, in addition to the metallic nickel particles.
In an embodiment, the metallic nickel is in the form of particles having any shape known to one of ordinary skill in the art. Examples of suitable shapes may take include spheres, spheroids, lentoids, ovoids, solid polyhedra such as tetrahedra, cubes, octahedra, icosahedra, dodecahedra, hollow polyhedral (also known as nanocages), stellated polyhedral (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. 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 nanoparticles 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 nanoparticles having a different shape.
In an embodiment, the metallic nickel is in the form of particles having a substantially spherical shape. That is, the particles are preferably not jagged or irregular in shape. In an embodiment, an average diameter of the particles 100 to 500 nanometers (nm), more preferably 200 to 400 nm, and yet more preferably 250 to 350 nm.
Following deposition on the substrate, the metallic nickel particles are aggregated to provide aggregates having an average size of 0.5 to 5 μm, preferably 1 to 4 μm, or 2 to 3 μm. In some embodiments, the aggregates of the metallic nickel particles have a popcorn shape. As referred to herein, popcorn shape refers to a single center particle with multiple other particles random dispersed on its surface, such as that depicted in
The particles can be deposited using various techniques, including direct-write deposition, and can form thin layers with a high packing density. In a preferred embodiment, the deposition/dispersion of the metallic nickel particles over the NF substrate is performed by an aerosol assisted chemical vapor deposition (AACVD) process. As used herein, the term ‘AACVD’ refers to the process which involves the atomization of 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 in a very short time. The AACVD deposition method is more controllable where physicochemical properties of film material such as morphology, and thickness can be tuned easily by simply varying deposition time, temperature, and precursor solvent, reproducible and versatile than other approaches. Depositing by AACVD prevents the formation of aggreagtes and leaching of the metal during catalysis. In some embodiments, the carrier gas during AACVD is nitrogen or argon. In a preferred embodiment, the metallic nickel particles were deposited on the nickel foam substrate, under AACVD conditions at a temperature of 300 to 800° C., preferably 400 to 700° C., or 500 to 600° C., a deposition time of 1 to 300 minutes, preferably 50 to 250 mins, 100 to 200 mins or approximately 150 mins.
In an embodiment, the substrate is deposited partially or wholly with at least one layer of the metallic nickel particles in a uniform and continuous manner. In a preferred embodiment, the metallic nickel particles form a continuous layer on the nickel foam substrate. In other words, the nickel particles are closely packed along the surface and are not isolated in islands. In an embodiment, particles of the metallic nickel particles form a monolayer on the nickel foam substrate. In another embodiment, particles of the metallic nickel particles may include more than a single layer on the nickel foam substrate. While not wishing to be bound to a single theory, the electrocatalytic performance of the electrocatalyst is attributed to the uniform and continuous spherical morphology of metallic nickel particles, which provides many active catalytic centers.
In a preferred embodiment, at least 90% of the outer surface area of the nickel foam substrate is covered with a layer of metallic nickel particles, preferably 92%, 94%, 96%, 98%, or 100%. The layer of the metallic nickel particles on the nickel foam substrate has a thickness of 0.01 μm to 50 μm, more preferably 10 to 45 μm, and yet more preferably 25 to 35 μm.
A method of oxidizing water is described. The method includes contacting the electrocatalyst and a counter electrode with the water, and further applying a potential of 0.1 to 1.0 V, preferably 0.2 to 0.9 V, 0.3 to 0.8 V, 0.4 to 0.7 V, or 0.5 to 0.6 V, to the electrocatalyst and a counter electrode immersed in the aqueous electrolyte solution. The electrocatalyst forms the working electrode, while 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 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 contamination of either electrode.
The aqueous electrolyte solution includes water and an inorganic base. 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 1-2.5 M, and yet more preferably 1.5-2.5 M.
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 1.68 mm, and the counter-electrode as a cross-section diameter of 0.2 mm. The working electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art.
The electrocatalyst of the present disclosure has a water oxidation overpotential of 280-305 mV at 10 mA cm−2, preferably 285-300 mV, or 290-295 mV. The water oxidation overpotential does not vary by more than 5%, preferably 4%, 3%, 2%, or 1% after the potential is applied for 10-50 hours. In other words, the electrocatalyst is stable for at least 10-50 hours. As used herein, the term ‘overpotential’ is referred to as the difference between the equilibrium potential for a given reaction (also called the thermodynamic potential) and the potential at which a catalyst operates at a specific current under specific conditions. The electrocatalyst has an electrochemically active surface area (ECSA) of 250-300-centimeter square (cm−2), preferably 260 to 290 cm−2, or 270 to 280 cm−2. As used herein, the term ‘ECSA’ refers to the property for catalysis where the rate of an electrocatalytic reaction is directly proportional to the active surface area. The electrocatalyst has a current density of at least 1,000 mA cm−2 at 1.6 V, preferably 1,100 mA cm-2, 1,200 mA cm−2, 1,300 mA cm−2 or 1,400 mA cm−2. As used herein, the term ‘current density’ refers to the amount of current travelling per unit cross-section area. While not wishing to be bound to a single theory, the deposition of the metallic nickel particles on the nickel foam substrate results in generating a synergic effect between nanostructured Ni in the metallic nickel particles and the porous NF substrate, which enhances the electrical conductivity of the catalyst, thereby resulting in a higher electrocatalytic performance.
The electrocatalyst of the present disclosure may be used in water-splitting reactions. In some embodiments, the electrocatalyst may also be used in the field of batteries, fuel cells, photochemical cells, water splitting cells, electronics, water purification, hydrogen sensors, semiconductors (such as field-effect transistors), magnetic semiconductors, capacitors, data storage devices, biosensors (such as redox protein sensors), photovoltaics, liquid crystal screens, plasma screens, touch screens, OLEDs, antistatic deposits, optical coatings, reflective coverings, anti-reflection coatings, and/or reaction catalysis.
The following examples describe and demonstrate exemplary embodiments of the electrocatalyst described herein. The examples are provided solely for the purpose of 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.
Ni films were prepared using the AACVD equipment and method mentioned in the previous work [Ehsan M A, Rehman A. Facile and scalable fabrication of nanostructured nickel thin film electrodes for electrochemical detection of formaldehyde. Analytical Methods. 2020; 12:4028-36], incorporated herein by reference in its entirety. Compared to previous work, two deposition parameters were changed—namely, porous nickel foam (NF) was used as the substrate material, and the deposition time was extended for a longer time (˜2 hours). Other parameters such as precursor nickel (II) acetylacetonate (Ni(acac)2), deposition solvent (methanol), deposition temperature (500° C.), and carrier gas/flow rate (N2 gas (150 cm3 min−1) remained the same. Films growth time intervals were 30, 60, and 120 min, and the thin film electrodes were named Ni@NF-30, Ni@NF-60 and Ni@NF-120, respectively (referring to Ni films deposited on the nickel foam for 30, 60, and 120 minutes respectively). The electrode may be interchangeably referred to as the electrocatalyst throughout the draft.
Electrocatalytic Ni thin films were investigated through various characterizations. X-ray diffraction (XRD) was measured in a Rigaku MiniFlex X-ray diffractometer (Japan) using a Cu Kα1 source (γ=0.15416 nm). SEM JEOL JSM-6610LV (Japan) was used for the morphological characterization of the samples. The specific oxidation and chemical state of deposited nickel were determined by X-ray photoelectron spectroscopy (XPS) performed on a Thermo Scientific Escalab 250Xi spectrometer equipped with a monochromatic Al Kα (1486.6 eV) X-ray source of 0.5 eV resolution. For charge correction, the data were calibrated against C (is) random carbon (284.8 eV), and peaks were modeled with Casa XPS software.
Electrochemical water oxidation tests were performed in 1.0 M KOH electrolyte solution at pH 14. Typical cell with three electrodes, consisting of Ni@NF catalyst (working electrode), Ag/AgCI (reference electrode), and Pt wire (counter electrode), connected to a computer-controlled potentiostat INTERFACE 1010 E. All electrochemical measurements were performed in a de-oxygenated electrolyte aqueous solution at room temperature. Polarization curves were recorded using linear sweep voltammetry at a scan rate of 5 millivolts per second (mV s−1). All potentials here are converted to reversible hydrogen electrodes (ERHE) according to the Nernst equation (1):
The overpotential (i) is calculated according to the following formula:
The Tafel slope is calculated from the linear region of the polarization curve (where the Tafel region begins) using the Tafel equation (2):
where ‘b’ defines Tafel slope while ‘a’ is constant.
Metallic nickel (Ni) thin films deposited by AACVD on Ni-foam (NF) substrates were characterized by XRD. Coincidentally, the crystalline peaks of AACVD-deposited Ni merged into the XRD peaks of Ni from NF. Three high-intensity peaks were observed at the 20 positions of 44.7°, 51.8°, and 76.6° on the reflective surfaces (111), (200), and (220), respectively, hence the existence of a cubic-structured ‘metallic nickel’ (PDF #01-077-8341). AACVD experiments were repeated on a simple amorphous glass substrate to verify this observation.
The surface morphology of nickel deposited by AACVD on NF substrates after 30-120 minutes deposition process was analyzed by SEM (
One of the defining features of the AACVD process is the change in morphology in such a short time, providing a variety of nanostructure designs and thicknesses, which determine the catalytic performance of each material towards better performance. Furthermore, the deposited species are strongly bound to the NF, and the resulting thin-film electrodes are used for electrochemical demonstration without further thermal or mechanical treatments.
XPS was used to determine the specific oxidation and chemical state of nickel deposited by AACVD. For this purpose, studies were conducted using a nickel film fabricated on a simple glass substrate. Survey scans (
The electrocatalytic OER performance of nanostructured Ni films deposited on Ni-foam substrates (Ni@NF) was evaluated in a 1.0 M KOH electrolyte medium. Cyclic voltammetry (CV) testing was primarily performed to observe the I-V behavior of thin-film electrodes over a range of applied potential (0.0-0.8 V vs. normal hydrogen electrode (NHE). Initial CV experiments provide insight into the chemical and mechanical stability of electrocatalysts under applied electrochemical conditions. The continuous CV scans develop high oxidative electroactive species on the surface of Ni based catalyst, which expedites the kinetics of water oxidation reaction.
Therefore, all three Ni electrocatalysts were initially activated by conducting 50 CVs at the rate of 50 mV s−1. Bare nickel foam (NF) was also tested in a similar environment to observe its contribution to the activation process.
Following CV experiments, linear sweep voltammetry (LSV) or polarization curves were recorded for all electroactive catalysts in 1.0 M KOH and at a scan rate of 5 mV s−1.
All electrocatalysts showed peak currents between 1.35-1.40 V vs. RHE due to the oxidation of Ni2+ to Ni3+ for the generation of NiOOH, which may be active intermediates for OER (
As shown in
For Ni-based OER catalysis, Tafel slope values above 60 mV dec−1 indicate the dominance of the electroactive spice NiOOH, while higher slopes of 120 mV dec−1 indicate that the adsorbed Ni—O is the catalytic surface predominant [Shinagawa T, Garcia-Esparza A T, Takanabe K. Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion. Scientific reports. 2015; 5:1-21]. In this example, the nanostructured Ni indicates the existence of kinetic conditions dominated by adsorbed NiOOH, leading to higher current densities.
Further, electrochemical impedance spectroscopy (EIS) measurements were performed to reveal the interfacial behavior and the intrinsic OER activity of the as-synthesized Ni@NF electrodes.
Further, Ni@NF-60 manifests a smaller charge transfer resistance (Rct) value of 120 ohms (Q) whereas, Ni@NF-30, Ni@NF-120, and bare NF show Rct of 320 Ω, 350Ω and 470Ω respectively (
Next, turnover frequency (TOF) was calculated to evaluate the intrinsic catalytic activity of these electrocatalysts. The TOF value is derived from the following formula:
In the above relation, j represents the current density measured at an overpotential of 350 mV in A cm−2). A is the surface area of the NF substrate (1 cm−2), whereas F is the Faraday constant (96,485 C mol−1) and m is the number of moles of catalyst deposited on the NF substrate. The mass of as-synthesized electrocatalysts Ni@NF-30, Ni@NF-60, and Ni@NF-120 are 0.08, 0.23, and 0.46 mg, respectively. The TOF values of Ni@NF electrocatalysts at different overpotentials are directly calculated from the polarization curves, as shown in
Since Ni@NF-60 exhibited OER activity and kinetics, the long-term stability of the catalyst was further tested to verify its suitability for large-scale applications. Long-term electrolysis was performed by chronopotentiometry by applying current densities of 10 and 20 mA cm−2, respectively, for 20 h (
Robustness is a property that demonstrates the in-situ activation of the catalyst and the creation of an increasing number of electroactive sites on the catalyst surface during long-term OER testing under alkaline conditions. The results also showed that the overvoltage remained almost constant at the two distinctive applied current densities, and no drop or drop-in activity was observed. Likewise, no signs of deterioration or detachment of the electrodes were observed in the long-term stability test. This is one of the advantages of thin film electrocatalysts over powder electrocatalysts, in which the catalyst does not separate during electrochemical studies.
After a long-term chronopotentiometry test, the polarization curve of the Ni@NF-60 electrode was re-evaluated and compared with the response before the stability test, as shown in
Moreover, the electrochemically active surface area (ECSA) of the best-performing catalyst, i.e. Ni@NF-60, was calculated using the following relationship:
Here, Cs implies the specific capacitance of the metal electrodes, which is reported to be 0.04 cm−2 for the alkaline electrolyte [McCrory C C, Jung S, Peters J C, Jaramillo T F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. Journal of the American Chemical Society. 2013; 135:16977-87, incorporated herein by reference in its entirety].
To measure the double-layer capacitance (CDL), continuous CVs of varying scan rates from 10-60 mV s−1 were performed in non-Faradaic regions, as shown in
The thin film catalysts prepared in 60 minutes of the AACVD process demonstrate low overpotential to obtain 10 mA cm−2 and can approach a higher current density of 1000 mA cm−2, which the other catalysts do not have. The high OER activity is attributed to the nanoscale features of Ni obtained on the surface of porous nickel foam. Additionally, the synergy between highly conductive nickel foam and nanostructured nickel catalyst favors fast electronic transportation between electrode/electrocatalyst interfaces. The simple and straightforward fabrication strategy is attractive for the up scaling of the electrocatalyst. Thus, the low-cost nickel film obtained through inexpensive AACVD using a simple chemical precursor is more feasible than the other conventionally used synthetic methods where reproducibility and performance was a challenge.
The present disclosure provides a simple one-step chemical vapor deposition process to fabricate nanostructured nickel-based thin-film electrocatalysts with convincing water oxidation propensity. The catalyst shows oxygen evolution activity by exhibiting a low overpotential of 285 mV at 10 mA cm−2 and a high current density of 1000 mA cm−2 at 1.6 V (overpotential of 423 mV). The developed nano electrocatalyst remained stable for at least 20 hours. The electrode exhibits a small Tafel slope value of 70 mV dec−1, higher TOF and large electrochemically active surface area, better charge transport performance, and long-term stability over 20 hours of continuous operation without significant loss. The OER performance is attributed to the synergic effect generated between nanostructured Ni and porous NF substrate, enhancing the electrical conductivity of the prepared catalysts. The low manufacturing cost, robustness, and durability make the electrocatalyst viable in solar energy conversion and storage applications.
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.
