Patent Application
20200109479
The present invention relates to the electrochemical splitting of water and, more particularly, to an electrocatalyst comprising certain transition metal chalcogenide materials for use as the cathode for the electrochemical hydrogen evolution reaction.
Hydrogen (H2) is a unique energy carrier in that it can be produced via diverse pathways utilizing a variety of domestically available feedstock, including natural gas, biomass, and water (reference 1).
The electrochemical splitting of water (electrolysis) is among the most versatile methods of hydrogen generation that is expected to play a significant role in long-term, high-volume hydrogen gas production (reference 2).
The electrolysis of water is a combination of two space separated electrochemical processes—the hydrogen and oxygen evolution reactions (HER and OER, respectively). During HER, water is reduced to H2 using electrons supplied by the negative electrode (cathode) of an electrochemical cell, while oxygen (O2) is produced by OER at its positive counterpart (anode) (
In an ideal electrochemical cell, operating at 25° C. under atmospheric pressure (760 mmHg or 101325 Pa, or 14.696 psi), oxidation of water occurs at the anode potential of Eoa=1.23V and the hydrogen evolution takes place at the cathode at Eoc=0 V (standard hydrogen electrode).
Anode (OER): H2O(aq)→½O2(gas)+2H+(aq)+2e−
Cathode (HER): 2H+(aq)+2e−→H2(gas)
Thus, the voltage to be applied to an ideal electrolyzer cell is Eocell=Eoc−Eoa=−1.23V. However, in practice, water electrolyzers operate at higher cell voltages due to energy losses by non-ideal electrochemical processes. The fraction of the electrical potential (E), exceeding the theoretical value of −1.23V is defined as overpotential (η) that includes the overpotentials of the anode (ηa) and cathode (ηc) materials as well as other detrimental contributions (ηo) attributed to solution and contact resistance and other factors (reference 3).
E=Eocell+η
η=ηa+ηc+ηo
Among known cathode materials, platinum shows the highest electro-catalytic activity for HER in acidic solutions and the lowest ηc that is below −0.1V at the current density of 10 mA/cm2. Also the Tafel slope, which defines mechanism and efficiency of HER is very low for Pt—only about 30 mV/dec in an acidic environment (references 4, 5).
Several types of water electrolyzers are currently in use (reference 6). They include low-temperature electrochemical systems that are based on the proton exchange membrane technology (PEMEC), which operates in acidic environment, alkaline electrolytic cells (AEC) operating in basic solutions, and high-temperature water-splitting systems that utilize the decomposition of vaporized water on solid oxide electrodes (SOEC). Among the aforementioned technologies, PEMEC emerges as superior contender, demonstrating high working current density (2 Am−2), low operating temperature (82° C. vs. 650-1000° C. for SOEC) and, unlike AEC, an ability to function under high pressures of H2 (about 200 bar). One of the main weakness of PEMECs, impeding their broad practical applications, is high cost and scarcity of platinum (Pt), which is an essential element of their working electrodes.
Layered transition metal dichalcogenides (TMDCs), such as MoS2, WS2, MoSe2, WSe2 and some others (reference 1), represent a less expensive and more abundant alternative to Pt. They are built from covalently bonded metal-chalcogen layers that are held together by weak van der Waals (vdW) forces (
TMDC-based HER electrocatalysts are usually binary or ternary compounds that combine two or three different chemical elements, i.e. transition metals and chalcogens (S, Se, Te), in their makeup. In a number of cases two or more individual (single-phase) TMDCs have been combined in composite materials (reference 10) that can also contain graphite or graphene, increasing their electrical conductivity (references 11, 12).
An embodiment of the present invention provides an electrode material useful for electrolysis of water comprising certain transition metal chalcogenides having four or more chemical elements as single phase, solid solution cathode material.
In an illustrative embodiment, the single phase, solid solution electrode (e.g. cathode) material is represented by the general formula of: (Ma1 Mb2 Mc3 . . . Mn)(X1AX2BX3C), wherein M1, M2, M3 . . . Mn each represents a different transition metal and wherein the sum of a+b+c . . . n is between 1 and 3, wherein X1, X2, X3 each represents a chalcogen element selected from the group consisting of S, Se, and Te and wherein the sum of A+B+C is between 2 and 6, and wherein the pristine material preferably does not contain platinum as a principle (major) component.
Illustrative embodiments of such cathode materials include, but are not limited to, multi-metal and multi-chalcogen materials such as (Mo0.5W0.5)SSe, (Mo0.4W0.4Ta0.2)SSe and (Mo0.475W0.475Ta0.05)SSe as well as multi-metal and single chalcogen materials such as (W0.4Mo0.4Ta0.2)S2, or (W0.475Mo0.475Ta0.2)S2, Wherein the overpotential and Tafel slopes of the these materials are superior to those of similarly treated MoS2 and WSe2.
Certain embodiments of the invention provide the electrode material in the form of particles of a size between 0.6 nm and 1 micron (1000 nm) and/or other particulates of a size in one dimension between 0.6 nm and 1 micron (1000 nm) in the second and/or third dimensions such as represented by nano- or micro-plates, rods, wires, tubes, thin films and similar asymmetrical structures.
Other embodiments of the invention provide the electrode material in the bulk form as monolithic sheets, rods, plates, wires, or tubes whose electric resistivity is less than 20 Ω-cm. Such bulk forms can be made by cold and/or hot pressing the particles or by other fabricating techniques.
Cathode materials pursuant to certain illustrative embodiments of the invention are advantageous in that they are stable in acidic aqueous environments during water electrolysis and exhibit an overpotential below 0.5V vs. the reversible hydrogen electrode (RHE) at 10 mA/cm2 current density during electrolysis of water in acidic environment at room temperature. The cathode material can have a conductive nature itself or the material can be combined with a conductive material to form a composite cathode material.
Furthermore, other embodiments of the invention provide an electrode (e.g. cathode) comprising a transition metal dichalcogenide substrate having platinum (Pt) nanosized islands (localized Pt areas) deposited on the electrode surface. The Pt nanosized islands are less than 50 nm as measured along at least one dimension of each localized island. Such a cathode electrode behaves electrochemically, in water electrolysis, similar to a platinum metal electrode. A certain method embodiment forms such an electrode by repeated (consecutive) volt scans performed on the pristine transition metal dichalcogenide cathode substrate in an aqueous solution in the presence of a platinum counter electrode until the nano-sized islands are deposited on the surface of the electrode cathode substrate.
The present invention envisions a water electrolysis cell and method that use the electrode material described above as the working cathode to positively affect the hydrogen evolution reaction.
The present invention and advantages thereof will be described below in more detail with respect to certain embodiments offered for purposes of illustration and not limitation in relation to the following drawings.
The present invention provides in some embodiments certain multi-principal element, high-entropy transition metal dichalcogenide materials (HE-TMDCs) comprising four and more chemical elements and present as a single phase, solid solutions for use as electrocatalysts for the hydrogen evolution reaction (HER) in the electrolysis of water. The TMDCs described below are referred to as high entropy (HE) materials in that they have unique features of high-entropy materials; namely, a combination of structural irregularities such as random distribution of different atoms in corresponding metal and chalcogen sub-lattices, lattice defects and vacancies and an enhanced stability such as chemical resistivity toward acidic environment.
Certain embodiments of the invention involve a cathode material that is represented by the general formula of: (Ma1 Mb2 Mc3 . . . Mn)(X1AX2BX3C), wherein M1, M2, M3 . . . Mn each represents a different transition metal and wherein the sum of a+b+c . . . n is between 1 and 3, wherein X1, X2, X3 each represents a chalcogen element selected from the group consisting of S, Se, and Te and wherein the sum of A+B+C is between 2 and 6.
Such materials can combine two or more group 4 and/or group 5 transition metals, and/or group 6 transition metal, and/or group 9 transition metal, and/or group 10 transition metal with one or more chalcogens (S, Se, Te; group 16 chalcogen elements) in their structures. These group numbers correspond to those of the Periodic Table of Elements. Moreover, the cathode material does not contain platinum (Pt) as a principal (major) component. The Pt content of the cathode material preferably is less than 10 atomic % to this end.
Certain embodiments provide the cathode material in the form of particles of a size between 0.6 nm and 5 micron (5000 nm) and/or other particulates of a size in one dimension between 0.6 nm and 5 micron (5000 nm) in the second and/or third dimensions such as represented by plates, rods, wires, tubes, thin films, and similar asymmetrical structures. Such particle/particulates themselves each can comprise a single TMDC nano layer or multiple, stacked TMDC layers (
Bulk forms such as bulk monolithic plates, rods, wires, tubes, etc. for cathode bodies can be made by cold and/or hot pressing powder/particulates using known powder metallurgy techniques, or by other fabricating methods.
The cathode particles/particulates can be formed by various techniques into a working cathode of the water electrolysis cell. Such techniques include, but are not limited to electrophoretic deposition of the particles/particulates on a substrate (reference 13), drop-casting, (reference 14), or any other electrode preparation technique.
Such particles/particulates can be mixed with any conductive material in particle, fiber or other particulate form such as graphite, graphene, graphene oxide, reduced graphene oxide, metal, polymer, and/or ceramics to form a composite material for use as a cathode for the HER that shows overpotential below 0.5 V vs the reversible hydrogen electrode (RHE) at 10 mA/cm2 current density during electrolysis of water in acidic environment at room temperature. The mixture then can be placed on a substrate using drop-casting or electrophoretic deposition procedure, or otherwise formed into a working cathode for HER.
Importantly, the working cathode or composite working cathode is stable in an acidic aqueous environment, such as having pH less than 1, and can exhibit an overpotential below 0.5 V vs the reversible hydrogen electrode (RHE) at 10 mA/cm2 current density during electrolysis of water in such acidic environment at room temperature. Preferably, the working cathode can exhibit an overpotential below 0.2 V vs the reversible hydrogen electrode (RHE) at 10 mA/cm2 current density during electrolysis of water in acidic environment at room temperature.
The present invention also provides in still other embodiments an electrode (e.g. cathode) comprising a transition metal dichalcogenide substrate having platinum (Pt) nanosized particles (localized areas) disposed on the cathode surface as described below.
The following examples are offered to further illustrate and not limit embodiments of the present invention.
The following TMDC were used in powder form with particle sizes below 1 μm and tested:
All materials, except for commercially acquired MoS2 and WS2 were made in a powder form by process described in patent application Ser. No. 15/998,266 filed Jul. 26, 2018, US. publication No. 2019/0039913, the disclosure of which is incorporated herein by reference above.
The glassy carbon electrodes, CHI104, were acquired from CH Instruments Inc. (Austin, Tex., USA) and used for the preparation of the working electrodes (i.e. as substrate for the TMDC cathode material). Isopropyl alcohol (IPA, ≥99.5%) was purchased from Fisher Scientific (Fair Lawn, N.J., USA) and alumina slurry (particle size=0.05 μm) and the polishing pad (Texmet, diameter=2⅞″, a part of PK—4 Polishing Kit) were obtained from BASi (West Lafayette, Ind., USA). Concentrated sulfuric acid (H2SO4≥99.9%, Sigma-Aldrich, St. Louis, Mo., USA) was used to prepare the IM H2SO4 electrolyte solutions. Nafion solution (20 wt % in lower aliphatic alcohols and water) was acquired from Sigma-Aldrich (St. Louis, Mo., USA). MoS2 (99%) and WS2 (99.8%) were purchased from Alfa Aesar (Haverhill Mass., USA). Acetone was purchased from Sigma-Aldrich.
Prior to the deposition of a TMDC material on the glassy carbon electrode (CHI104), the electrode was cleaned by polishing its surface with the Texmet alumina pad in the presence of the alumina slurry. The cleaning step was followed by rinsing the electrode with acetone and deionized water.
The TMDC powder, such as MoS2, WS2, (Mo0.5W0.5)S2 (Mo0.5W0.5)SSe, (Mo0.4W0.4 Ta0.2)S2 or (Mo0.475W0.475Ta0.05)S2, was suspended in IPA (5 mg/mL) and sonicated using Branson 3800 ultrasonic cleaner for 2 hours at ambient conditions. An aliquot of about 10 μL of the suspension formed was drop cast onto the glassy carbon electrode and dried for 20 min. at room temperature. This procedure was repeated several times. Subsequently, the electrode was dipped into the Nafion® binder solution for about 5 seconds and dried in air. Thus prepared working electrode was used for the electrochemical experiments.
The electrophoretic deposition was carried out in a two electrode, electrochemical cell.
The glassy carbon electrode (CHI104) was used as substrate for the TMDC deposition and a platinum wire served as the counter electrode.
Specifically, 0.3 g of (Mo0.4W0.4Ta0.2)SSe powder was dispersed in 15 mL of IPA and sonicated for 8 hours under ambient conditions. Prior to the electrophoretic deposition, the glassy carbon electrode (CHI104) was cleaned by sonication for 5 min in ethyl alcohol, then for 5 min in deionized water. A cyclic voltammogram was recorded from the cleaned electrode and, if no peaks were detected, the electrode was deemed clean.
For the electrophoretic deposition, the cleaned glassy carbon electrode and the platinum wire electrode were placed vertically in an electrochemical cell with a separation distance of 1 cm between them. A deposition voltage 10V was applied across the electrochemical cell for 15 min using the CH16051E potentiostat from CH Instruments. Subsequently, the working electrode formed was dried at 60° C. for 30 min and used for the electrochemical experiments.
The electrochemical experiments were performed in an electrochemical cell with a three-electrode configuration using BASi Cell Stand and the Epsilon potentiostat.
The glassy carbon electrode (CHI104) with the deposited TMDC served as working electrode (WE) for HER and a 2 mm-platinum wire was used as counter electrode, where OER took place. A silver chloride electrode (Ag/AgCl in 1 M KCl, CH Instruments), calibrated with respect to the reversible hydrogen electrode (RHE), served as the reference electrode, by using high purity H2 saturated 0.5M H2SO4 electrolyte. The linear sweep voltammetry (LSV) scans were performed at the rate of 2 mV/s in H2 saturated 0.5M H2SO4 aqueous electrolyte. The acidic aqueous electrolyte was stirred using a magnetic stirrer at a speed of 60 rpm. The LSV data for all TMDCs were corrected for electrolyte resistance (iR correction).
In
The overpotential and Tafel slopes of the multi-element TMDCs cathode materials, pursuant to this invention are far superior to the binary MoS2 and WS2 (see Table 1), whereby (Mo0.5W0.5)SSe performs similarly to (Mo0.5W0.5)S2, and all Ta-containing TMDCs are far more active as HER electrocatalysts than other tested materials. After 1000 consecutive LSV scans, the single phase, solid solutions deposited cathode materials represented by (W0.4Mo0.4Ta0.2)S2 and (Mo0.475W0.475Ta0.05)S2 showed electrocatalytic performance exceeding or close to that of the best electrode materials known to date, including Pt (Table 1).
The (W0.4Mo0.4Ta0.2)S2 and (Mo0.475W0.475Ta0.05)S2 materials consisting of four principle elements, exhibit strong improvements of their performance during LSV scanning and reach the steady performance state after 500 consecutive scans.
High electrical conductivity of Ta containing materials is at least partially responsible for their exceptional performance since it reduces an internal resistivity contributions to their overpotentials.
For example (W0.4Mo0.4Ta0.2)S2 showed metallic behavior, while conventional TMDCs are indirect band gap semiconductors and have poor electric conductivity (reference 9). The electrical resistivity of (W0.4Mo0.4Ta0.05)S2 was recorded between 350K and 2K in the absence of magnetic field using a four-terminal alternating current transport setup available in the physical property measurements system from Quantum Design, Inc. The room temperature resistivity of this material is 4.7 mil-cm and it slightly decreases with the temperature decreasing into the cryogenic region (
In general, room temperature electrical resistivities below 1 Ω-cm can be obtained by the TMDC material pursuant to embodiments of the present invention.
The present invention also provides in still other embodiments a cathode comprising a transition metal dichalcogenide substrate having platinum (Pt) nanosized islands or precipitates disposed on the cathode surface as described below. As example,
The X-ray photoelectron spectroscopic (XPS) examination of this (Mo0.4W0.4Ta0.2)S2 electrode confirmed the presence of Pt on its surface as well as the presence of all other elements including Mo, W, Ta, S and fluorine (F) that is an element of the Nafion conductive binder coating used for the electrode preparation (
Deposition of Pt and the performance improvement associated with the presence of Pt nano-islands on the surface of the TMDC electrode has not been observed for the other electrode materials that did not contain Ta. Thus, Ta and, possibly other dopants that improve intrinsic electrical conductivity of TMDCs, can be employed to achieve the observed phenomenon.
Certain illustrative method embodiments of the present invention form such a cathode by applying consecutive voltage scans to a TMDC material that contains Ta in an aqueous solution in the presence of a platinum counter electrode until the Pt nanosized islands comprising platinum are deposited on the material. For example, the cathode can be formed by applying repeated LSV scans to a pristine conductive Ta-containing TMDC cathode substrate in aqueous solution in the presence of a platinum counter electrode until the Pt nanosized islands or precipitates form on the cathode substrate. An exemplary method embodiment forms such a cathode electrode using a 0.5M H2SO4 aqueous electrolyte and a cathode electrode material that contains Ta as described above, a Pt counter electrode, and a Ag/AgCl reference electrode of the type described above and changing the potential (voltage) from 0 V to −0.5V or so. The scans are repeated multiple times until Pt precipitates as nanosized islands on the TMDC cathode electrode.
Alternately, chemical deposition of the nanosized islands comprising platinum on a surface of a TMDC material that contains Ta as described above can be effected by immersing or otherwise contacting the Ta-containing TMDC material in an aqueous solution containing a Pt compound to effect a chemical reduction reaction (without applied electrical current/voltage). The Pt compound can include, but is not limited to, a water-soluble Pt salt represented by MyPtXz, wherein M represents H, Li, Na, K, NH4 or another appropriate cation and X represents at least one of Cl, Br, I, NO3 and an organic substituent such as C2O4, y=1-2, and z=1-6.
Cathode materials pursuant to these illustrative embodiments of the invention are advantageous in that they are stable in acidic aqueous environments, meaning that they do not decay, during water electrolysis and exhibit an overpotential below 0.5 V vs. the reversible hydrogen electrode (RHE) at 10 mA/cm2 current density during electrolysis of water in acidic environment at room temperature.
Although the present invention has been described with respect to certain illustrative embodiments, those skilled in the art will appreciate that changes and modifications can be made therein within the scope of the invention as set forth in the appended claims.
This application claims benefit and priority of U.S. provisional application Ser. No. 62/766,165 filed Oct. 4, 2018, the entire disclosure of which is incorporated herein by reference.
This invention was made with government support under Contract No. DE-AC02-07CH11358 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62766165 | Oct 2018 | US |
