Self-improving Electrocatalysts For Gas Evolution Reactions

Abstract

In some embodiments, the present disclosure pertains to methods of mediating a gas evolution reaction by exposing a gas precursor to an electrocatalyst that comprises a plurality of layers with catalytic sites. The exposing results in electrocatalytic conversion of the gas precursor to a gas. Thereafter, the generated gas enhances the electrocatalytic activity of the electrocatalyst by enhancing the accessibility of the catalytic sites to the gas precursor. In some embodiments, the electrocatalyst is associated with an electrically conductive surface (e.g., an electrode) that provides electrical current. In some embodiments, the electrocatalyst is a hydrogen production electrocatalyst that converts H.sup.+ to H.sub.2. In some embodiments, the electrocatalyst includes a transition metal dichalcogenide. Further embodiments of the present disclosure pertain to the aforementioned electrocatalysts for mediating gas evolution reactions.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/084,415, filed on Nov. 25, 2014. The entirety of the aforementioned application is incorporated herein by reference.

BACKGROUND

[0003] Current electrocatalysts for mediating gas evolution reactions have limitations in terms of efficiency, long-term use, and affordability. The present disclosure addresses these limitations.

BRIEF SUMMARY

[0004] In some embodiments, the present disclosure pertains to self-improving methods of mediating a gas evolution reaction. In some embodiments, the methods include a step of exposing a gas precursor to an electrocatalyst that includes a plurality of layers with catalytic sites. In some embodiments, the exposing results in electrocatalytic conversion of the gas precursor to a gas between the layers. In some embodiments, the generated gas enhances the electrocatalytic activity of the electrocatalyst by enhancing the accessibility of the catalytic sites to the gas precursor. In some embodiments, the generated gas enhances the accessibility of the catalytic sites to the gas precursor by increasing distances between the layers, thereby making the catalytic sites more accessible to the gas precursor. In some embodiments, the produced gas enhances the electrocatalytic activity of the electrocatalyst with time.

[0005] In some embodiments, the electrocatalyst is associated with an electrically conductive surface that provides electrical current. In some embodiments, the electrically conductive surface is an electrode. In some embodiments, the electrocatalyst is a hydrogen production electrocatalyst that converts H.sup.+ to H.sub.2.

[0006] In some embodiments, the electrocatalyst includes a transition metal dichalcogenide. In some embodiments, the transition metal dichalcogenide includes a group V transition metal dichalcogenide. In some embodiments, the transition metal dichalcogenide includes the following formula: MX.sub.2, where M is a transition metal (e.g., Ti, Hf, Zr, Mo, W, Ta, Nb, V, Tc, Re, Sn and combinations thereof); and where X is a chalcogen (e.g., S, Se, O, Te, and combinations thereof). In some embodiments, the electrocatalyst includes, without limitation, TaS.sub.2, NbS.sub.2, VS.sub.2, and combinations thereof.

[0007] In some embodiments, the electrocatalyst layers are in the form of crystal plates. In some embodiments, the electrocatalyst layers are separated by a distance ranging from about 0.1 nm to about 1 nm. In some embodiments, the electrocatalyst layers are porous.

[0008] Further embodiments of the present disclosure pertain to the aforementioned electrocatalysts for mediating gas evolution reactions. In some embodiments, the electrocatalysts of the present disclosure may be associated with fuel cells. In some embodiments, the methods and electrocatalysts of the present disclosure may be utilized for generating gases (e.g., hydrogen) directly for fuel cells within a contained system. In some embodiments, the electrocatalysts of the present disclosure may be associated with solar cells. In some embodiments, the methods and electrocatalysts of the present disclosure may be utilized for generating gases (e.g., hydrogen) from electricity provided by solar cells.

BRIEF DESCRIPTION OF THE FIGURES

[0009] FIG. 1 provides a scheme of a method of mediating a gas evolution reaction in accordance with the methods of the present disclosure.

[0010] FIG. 2 provides data relating to the self-improving performance of H--TaS.sub.2 catalysts for hydrogen evolution reactions (HER). FIG. 2A is a schematic illustration of HER at the surface sites of stacked layers. Spheres represent produced H.sub.2 bubbles formed from H.sup.+. FIGS. 2B-E show atomic force microscopy (AFM) (FIG. 2B), low-magnification transmission electron microscopy (TEM) (FIG. 2C), high resolution TEM (HRTEM) (FIG. 2D) images, and Raman spectra (FIG. 2E) for H--TaS.sub.2 before cycling, respectively. The inset in FIG. 2B shows the statistical distribution of thickness for total of 50 platelets. FIGS. 2E-H show analogous data for the H--TaS.sub.2 after cycling.

[0011] FIG. 3 provides data relating to the physical characterization of as-grown H--TaS.sub.2 catalysts. FIG. 3A shows a scanning electron microscopy (SEM) image of the catalysts. FIG. 3B shows a TEM image of the catalyst, showing the layer space of .about.0.7 nm. FIG. 3C shows Raman spectroscopy of the catalysts. FIG. 3D shows the x-ray diffraction (XRD) pattern of the catalysts, showing (001) orientation due to preferred TaS.sub.2 growth on a SiO.sub.2/Si substrate.

[0012] FIG. 4 shows data relating to the catalytic activity of H--TaS.sub.2. FIG. 4A shows polarization curves for TaS.sub.2 covered electrodes for various cycling numbers. FIG. 4B shows current density vs. initial current density with cycling for group V transition metal dichalcogenides (TMDCs). Current density grows with cycling to a saturation point and eventually degrades. These non-optimized samples show the effect of self-nanostructuring.

[0013] FIG. 5 shows a complex impedance plot indicating a sharp decrease in resistance with cycling as direct evidence for surface area increase.

[0014] FIG. 6 shows SEM images of H--TaS.sub.2. FIG. 6A shows H--TaS.sub.2 on electrode before HER. FIG. 6B shows H--TaS.sub.2 after HER with 5000 cycles between 0.2.about.-0.6 V vs. RHE at 5 mV s.sup.-1. FIGS. 6A-B have different scale bars.

[0015] FIG. 7 shows HRTEM of as-synthesized (FIG. 7A) and post-cycled (FIG. 7B) H--TaS.sub.2 for 5000 cycles between 0.2.about.-0.6 V vs. RHE at 5 mV s.sup.-1. The post-cycled sample retains the H hexagonal structure with a cell parameter of a=3.3 .ANG., indicating that the change in sample is not chemical but purely an increase in available surface area and closer electrical contact of catalytic sites.

[0016] FIG. 8 provides a states-filling approach based descriptor. FIG. 8A provides a schematic of MX.sub.2-catalyzed HER. FIG. 8B provides DOS of pristine and H-adsorbed TiS.sub.2 (coverage of H:Ti=1:16), with the energy levels with respect to vacuum. The right inset shows the charge density isosurface for states within the energy range of Fermi level to -0.025 eV below. FIG. 8C provides the same information as FIG. 8B but for MoS.sub.2 (M: blue; X: yellow; H: black; charge density isosurface: red). FIG. 8D provides a correlation between .epsilon.LUS and the surface adsorption energy Ea (Eq. 1) of H on a series of MX.sub.2 candidates. TaS.sub.2 (filled circles) are experimentally synthesized and tested later.

[0017] FIG. 9 provides a prediction of group 5 MX.sub.2 as surface-active HER catalysts. FIG. 9A provides computed .epsilon.LUS for all MX.sub.2 candidates, with the target screening range indicated by dotted lines. Row 4/5/6 elements are shown in green/red/blue, with the different chalcogens separated into columns within each group. FIG. 9B provides coverage dependence of G.sub.tot (from Eq. 2) for H adsorption on the group 5 MX.sub.2. FIG. 9C provides G.sub.diff (from Eq. 3) for low adsorbate coverage on the group 5 MX.sub.2 (H:M=1:16), with values for Pt and Ni surfaces (from (3)) and for active MoS.sub.2 edges shown for comparison.

[0018] FIG. 10 provides data relating to the electrocatalysis of HER on TaS.sub.2 and MoS.sub.2. FIG. 10A provides polarization curves for H--TaS.sub.2, H--MoS.sub.2, T-MoS.sub.2 and T-TaS.sub.2, measured in 0.5 M H.sub.2SO.sub.4 with a scan rate of 5 mV s.sup.-1. The H--TaS.sub.2 and H--MoS.sub.2 were first cycled for 5000 cycles between 0.2 and -0.6 V vs. RHE at 100 mV s.sup.-1 before the polarization curve measurement. FIG. 10B shows corresponding Tafel plots for catalysts in FIG. 10A. The numbers indicate the Tafel slopes. FIG. 10C shows the exchange current density obtained by fitting the Tafel plots. FIG. 10D shows the change of current density (recorded at -0.5 V) during cycling.

[0019] FIG. 11 provides structures of various phases of MX.sub.2. The red arrow indicates the unit cell.

[0020] FIG. 12 provides diagrams of adsorption energy E.sub.a (Eq. 1) as a function of the charge on the X atom (left) and the d-band center of bulk M (right).

[0021] FIG. 13 provides data relating to the physical characterization of as-grown H--TaS.sub.2. An SEM image (FIG. 13A) and a TEM image (FIG. 13B) show the layer space of .about.0.7 nm. A Raman spectroscopy (FIG. 13C), and an XRD pattern (FIG. 13D) shows (001) orientation due to preferred TaS.sub.2 grown on SiO.sub.2/Si substrate.

[0022] FIG. 14 shows data relating to the physical characterization of commercial H--MoS.sub.2, including SEM images (FIG. 14A), Raman spectroscopy (FIG. 14B), and XRD pattern (FIG. 14C).

[0023] FIG. 15 shows data relating to the physical characterization of T-MoS.sub.2 chemically exfoliated from H--MoS.sub.2, including SEM images (FIG. 15A), AFM image and corresponding thickness scan (FIG. 15B), and Raman spectroscopy (FIG. 15C). The T-MoS.sub.2 has a typical thickness of around 100 nm, which is comparable to the TaS.sub.2 after 5000 cycles. The emergence of new Raman shifts at 198, 225, and 284 cm.sup.-1 associated with the phonon modes of T-MoS.sub.2, clearly confirmed the formation of T-MoS.sub.2 from exfoliation treatment of commercial H--MoS.sub.2 plates.

[0024] FIG. 16 provides data relating to the physical characterization of exfoliated T-TaS.sub.2, including SEM images (FIG. 16A), AFM image and corresponding thickness scan (FIG. 16B), and Raman spectroscopy (FIG. 16C).

[0025] FIG. 17 provides polarization curves recorded during potential cycling for chemically exfoliated H--MoS.sub.2. The activity approaches to stability after the first 4 cycles and has little loss after 1,000 cycles.

[0026] FIG. 18 provides polarization curves of the first three scans for chemically exfoliated T-TaS.sub.2. The activity was observed to be decreasing with potential cycling.

[0027] FIG. 19 provides repeated measurement of HER catalytic activity. Polarization curves of as-synthesized H--TaS.sub.2 and commercial H--MoS.sub.2 were measured after 3000 cycles between 0.2.about.-0.6 V vs. RHE at a different rate of 5 mV s.sup.-1. The activity follows the same order as in FIG. 10A.

[0028] FIG. 20 provides the polarization curves recorded periodically during potential cycling at 5 mV s.sup.-1 for as-grown H--TaS.sub.2 (FIG. 20A) and commercial H--MoS.sub.2 (FIG. 20B).

[0029] FIG. 21 provides electrochemical impedance spectroscopy recorded periodically during potential cycling at 5 mV s.sup.-1. For H--TaS.sub.2 sample, Nyquist plots show the decreasing of charge transfer resistance with cycling (FIGS. 21A-B). FIG. 21C shows Bode plots demonstrating frequency response. FIGS. 21D-F show data analogous to FIGS. 21A-C for H--MoS.sub.2.

[0030] FIG. 22 shows SEM images of bulk H--TaS.sub.2 after 5000 cycling between 0.2.about.-0.6 V vs. RHE at 5 mV s.sup.-1. FIG. 22A shows low-magnification image of H--TaS.sub.2 crystals on electrode before HER. FIG. 22B shows high-magnification image of smaller and thinner H--TaS.sub.2 after HER.

[0031] FIG. 23 shows HRTEM of as-synthesized (FIG. 23A) and post-cycled H--TaS.sub.2 (FIG. 23B) for 5000 cycles between 0.2.about.-0.6 V vs. RHE at 5 mV s.sup.-1. The post-cycled sample remains the H hexagonal structure with the cell parameter a=3.3 .ANG..

[0032] FIG. 24 shows a comparison between fresh H--MoS.sub.2 and post-cycled H--MoS.sub.2 for 5000 cycles between 0.2.about.-0.6 V vs. RHE at 5 mV s.sup.-1. FIGS. 24A-B show SEM and TEM of the fresh H--MoS.sub.2 transferred on the GC electrodes, respectively. FIGS. 24C-D are analogous to FIGS. 24A-B for post-cycled H--MoS.sub.2. No dramatic thinning of thickness and breaking of size was observed for H--MoS.sub.2 after cycling. No pore on the surface was seen either.

DETAILED DESCRIPTION

[0033] It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory, and are not restrictive of the subject matter, as claimed. In this application, the use of the singular includes the plural, the word "a" or "an" means "at least one", and the use of "or" means "and/or", unless specifically stated otherwise. Furthermore, the use of the term "including", as well as other forms, such as "includes" and "included", is not limiting. Also, terms such as "element" or "component" encompass both elements or components comprising one unit and elements or components that comprise more than one unit unless specifically stated otherwise.

[0034] The section headings used herein are for organizational purposes and are not to be construed as limiting the subject matter described. All documents, or portions of documents, cited in this application, including, but not limited to, patents, patent applications, articles, books, and treatises, are hereby expressly incorporated herein by reference in their entirety for any purpose. In the event that one or more of the incorporated literature and similar materials defines a term in a manner that contradicts the definition of that term in this application, this application controls.

[0035] The generation of gases finds applications in many fields. For instance, hydrogen is an ideal energy carrier and an important agent for many industrial chemical processes. One popular method for generating hydrogen sustainably is electrolysis via the hydrogen evolution reaction (HER), in which aqueous protons are electrochemically reduced with the aid of an appropriate catalyst--traditionally, an expensive noble-metal.

[0036] Recently, layered molybdenum (Mo) and tungsten (W) transition-metal dichalcogenides (MX.sub.2) have attracted substantial interest as earth-abundant, inexpensive replacements for precious-metal HER catalysts. Unfortunately, their performance is limited by the low density of catalytically active sites, which are mainly located at the edges. [T. Jaramillo et al., Science Vol. 317 no. 5834 pp. 100-102 (2007)]. As such, a need exists for more effective electrocatalysts to mediate various gas evolution reactions, including HER.

[0037] In some embodiments, the present disclosure provides self-improving methods of mediating gas evolution reactions. In some embodiments that are illustrated in FIG. 1, the methods of the present disclosure include a step of exposing a gas precursor to an electrocatalyst (step 10). In some embodiments, the exposing results in electrocatalytic conversion of the gas precursor to a gas (step 12). In some embodiments, the generated gas enhances the electrocatalytic activity of the electrocatalyst (step 14). Thereafter, the electrocatalyst may be utilized to mediate additional electrocatalytic reactions. Additional embodiments of the present disclosure pertain to the self-improving electrocatalysts for mediating gas evolution reactions.

[0038] As set forth in more detail herein, the methods of the present disclosure may utilize various types of electrocatalysts. Various methods may also be utilized to expose the electrocatalysts of the present disclosure to various gas precursors to result in the generation of various gases. Moreover, the generated gases may enhance the electrocatalytic activity of the electrocatalysts of the present disclosure by various mechanisms.

[0039] Electrocatalysts

[0040] The electrocatalysts of the present disclosure generally include a plurality of layers. In some embodiments, the layers include catalytic sites for mediating gas evolution reactions.

[0041] The electrocatalysts of the present disclosure can have various compositions. In some embodiments, the electrocatalysts of the present disclosure exclude noble metals, such as Pt and Pd. In some embodiments, the electrocatalysts of the present disclosure exclude Pt and Pd. In some embodiments, the electrocatalysts of the present disclosure include transition metal dichalcogenides. In some embodiments, the transition metal dichalcogenides of the electrocatalysts of the present disclosure include group V transition metal dichalcogenides. In some embodiments, the transition metal dichalcogenides of the electrocatalysts of the present disclosure include group V transition metal disulfides. In some embodiments, the electrocatalysts of the present disclosure include, without limitation, TaS.sub.2, NbS.sub.2, VS.sub.2, and combinations thereof. In some embodiments, the electrocatalysts of the present disclosure include TaS.sub.2.

[0042] In some embodiments, the electrocatalysts of the present disclosure include transition metal dichalcogenides with the following formula:

MX.sub.2.

[0043] In the above formula, M is a transition metal and X is a chalcogen. In some embodiments, M includes, without limitation, Ti, Hf, Zr, Mo, W, Ta, Nb, V, Tc, Re, Sn, and combinations thereof. In some embodiments, M excludes noble metals, such as at least one of Pt and Pd. In some embodiments, M excludes Pt and Pd. In some embodiments, X includes, without limitation, S, Se, O, Te, and combinations thereof. In some embodiments, X is S.

[0044] In some embodiments, the transition metal dichalcogenides are in the H phase. In some embodiments, the transition metal dichalcogenides are in the T phase. In some embodiments, the transition metal dichalcogenides are in the H phase and the T phase.

[0045] The electrocatalysts of the present disclosure may have various types of layers. In some embodiments, the layers are in the form of plates, such as crystal plates. In some embodiments, the layers are porous. In some embodiments, the layers are in the form of porous membranes. In some embodiments, the layers are dispersed within a proton exchange membrane, such as Nafion. In some embodiments, the layers are associated with one another through van der Waals interactions. In some embodiments, the layers are stacked.

[0046] In some embodiments, catalytic sites are located on the surfaces of the layers. In some embodiments, catalytic sites are located on the edges of the layers. In some embodiments, the catalytic sites are located on the surfaces and edges of the layers. In some embodiments, the catalytic sites are uniformly dispersed throughout the surfaces of the layers. In some embodiments, the catalytic sites have the same compositions as the electrocatalysts of the present disclosure. In some embodiments, the location of the catalytic sites causes the self-optimizing performance.

[0047] In some embodiments, the layers (e.g., stacks of layers) have thicknesses ranging from about 50 nm to about 1000 nm. In some embodiments, the layers (e.g., stacks of layers) have thicknesses ranging from about 100 nm to about 600 nm. In some embodiments, the layers are separated by a distance ranging from about 0.1 nm to about 1 nm. In some embodiments, the layers are separated by a distance of about 0.7 nm. In some embodiments, the layers have surface areas that range from about 0.1 nm.sup.2 to about 1 m.sup.2.

[0048] In some embodiments, the layers (e.g., stacks of layers) have widths ranging from about 1 .mu.m to about 10 mm. In some embodiments, the layers (e.g., stacks of layers) have widths ranging from about 1 .mu.m to about 100 .mu.m. In some embodiments, the layers have widths ranging from about 1 .mu.m to about 50 .mu.m. In some embodiments, the layers have widths ranging from about 1 .mu.m to about 20 .mu.m. In some embodiments, the layers have widths ranging from about 1 .mu.m to about 10 .mu.m.

[0049] The electrocatalysts of the present disclosure may also be associated with various surfaces. For instance, in some embodiments, the electrocatalysts of the present disclosure are associated with an electrically conductive surface. In some embodiments, the electrically conductive surface provides electrical current that mediates the gas evolution reaction.

[0050] In some embodiments, the electrically conductive surface is an electrode. In some embodiments, the electrically conductive surface is a cathodic electrode (e.g., glassy carbon). In some embodiments, the electrically conductive surface is an anodic electrode.

[0051] Gas Evolution Reactions

[0052] The methods and electrocatalysts of the present disclosure may be utilized to mediate various types of electrocatalytic reactions. Gas evolution reactions are mediated by exposing a gas precursor to an electrocatalyst. In some embodiments, the gas precursor is a proton (i.e., H.sup.+), such as an aqueous proton.

[0053] In some embodiments, the electrocatalysts of the present disclosure are hydrogen production electrocatalysts that mediate hydrogen evolution reactions (e.g., conversion of H.sup.+ to H.sub.2). In such embodiments, the gas precursor is H.sup.+, and the generated gas is H.sub.2. In some embodiments, the hydrogen evolution reaction can be mediated in accordance with the following formula:

2H.sup.++2e.sup.-.fwdarw.H.sub.2

[0054] Enhancement of Electrocatalytic Activity

[0055] The methods of the present disclosure may enhance the electrocatalytic activity of electrocatalysts by various mechanisms. For instance, in some embodiments, the produced gas from a gas evolution reaction enhances the electrocatalytic activity of the electrocatalyst by enhancing the accessibility of the catalytic sites to the gas precursor. In some embodiments, the gas enhances the accessibility of the catalytic sites to the gas precursor by increasing distances between the electrocatalyst layers, thereby making the catalytic sites more accessible to the gas precursor. In some embodiments, gas generation causes separation of layers, thereby increasing the effective surface area of the electrocatalyst.

[0056] In some embodiments, gas generation results in a mechanical self-structuring (e.g., self-nanostructuring) of the electrocatalyst, which in turn leads to the enhancement of the electrocatalyst's electrocatalytic activity. In some embodiments, the mechanical self-structuring includes the optimization of the electrocatalyst morphology for mediating gas evolution reactions. In some embodiments, the optimization of the morphology results in enhanced basal-plane electrocatalytic activity within electrocatalyst layers. In some embodiments, the optimization of the morphology results in enhanced charge transfer between the layers (e.g., by shortening the electron-transfer pathways between the layers). In some embodiments, the optimization of the morphology results in an increase in the accessibility of the catalytic sites to the gas precursor.

[0057] In some embodiments, the produced gas enhances the electrocatalytic activity of the electrocatalyst with time. In some embodiments, the electrocatalytic activity of the electrocatalyst increases with use. In some embodiments, fresh unused catalytic sites are made accessible over time. In some embodiments, an increase in available catalytic sites improves the activity of the electrocatalysts and prolongs the life of the electrocatalyst itself. In some embodiments, electrocatalytic activity of the electrocatalysts increases over tens of thousands of cycles of use.

[0058] In some embodiments, the electrocatalysts of the present disclosure have an exchange current density ranging from about 2.times.10.sup.-4 A/cm.sup.2 to about 10.times.10.sup.-4 A/cm.sup.2. In some embodiments, the electrocatalysts of the present disclosure have a catalyst loading that ranges from about 10 .mu.g/cm.sup.2 to about 100 .mu.g/cm.sup.2. In some embodiments, the electrocatalysts of the present disclosure have a Tafel slope ranging from about of 25 mV/decade to about 100 mV/decade. In some embodiments, the electrocatalysts of the present disclosure have a current density ranging from about 5 mA/cm.sup.2 to about 50 mA/cm.sup.2.

[0059] Applications and Advantages

[0060] The methods and electrocatalysts of the present disclosure provide a first demonstration where an electrocatalyst enhances its electrocatalytic performance with use (i.e., self-improvement) to form complex nanostructures with high surface areas. Moreover, the electrocatalysts of the present disclosure can be fabricated from affordable materials in bulk quantities.

[0061] As such, the methods and electrocatalysts of the present disclosure could be used as low cost alternatives for producing various types of gases (including hydrogen by HER) without the use of rare or expensive catalysts, such as platinum and nanostructured MoS.sub.2.

[0062] Moreover, the methods and electrocatalysts of the present disclosure may be utilized for various applications, including use in energy storage devices and energy conversion devices. In some embodiments, the methods and electrocatalysts of the present disclosure may be utilized for generating gases (e.g., hydrogen) directly for fuel cells within a contained system. In some embodiments, the methods and electrocatalysts of the present disclosure may be utilized for generating gases (e.g., hydrogen) from electricity provided by solar cells.

[0063] Accordingly, the electrocatalysts of the present disclosure may be associated with various types of energy storage and energy conversion devices. In some embodiments, the electrocatalysts of the present disclosure may be associated with fuel cells. In some embodiments, the electrocatalysts of the present disclosure may be associated with solar cells.

Additional Embodiments

[0064] Reference will now be made to more specific embodiments of the present disclosure and experimental results that provide support for such embodiments. However, Applicants note that the disclosure below is for illustrative purposes only and is not intended to limit the scope of the claimed subject matter in any way.

Example 1

Self-Nanostructuring and Self-Improving Catalysts for Hydrogen Evolution Reactions

[0065] In this Example, Applicants provide a device and its application in which surface catalytic activity on layers within a material causes self-nanostructuring of the catalyst during use for increase in effective surface area. This Example focuses on the use of Group V transition metal dichalcogenides as catalysts. However, this effect is expected for any layered material that is surface catalytic for gas (e.g., H.sub.2) production.

[0066] As illustrated in FIG. 2A, a new class of hydrogen production catalysts is described in this Example, where mechanical self-nanostructuring is forced by the expansion of hydrogen gas produced. Since the hydrogen gas is produced locally at active sites, which include basal surface sites of 2H-TaS.sub.2 (and Group V TMDCs in general) within the material, pressure is created between layers. Thereafter, it is envisioned that the surface-activity-caused nanostructuring of 2H-TaS.sub.2 platelets increases the electron transfer (FIG. 5) and improves accessibility of basal surface sites, contributing to the observed increase in catalytic activity.

Example 1.1

CVD Synthesis Method

[0067] NbS.sub.2 and TaS.sub.2 crystal platelets are grown by a chemical vapor deposition on SiO.sub.2/Si substrates from sulfur and tantalum chloride powders and gaseous hydrogen precursors in a 3-stage furnace via the following reactions where M=Nb or Ta:

H 2 + ( 1 8 ) S 8 .fwdarw. H 2 S ##EQU00001## M Cl 5 + H 2 S + ( 1 2 ) H 2 .fwdarw. M S 2 + ( 5 ) H Cl ##EQU00001.2##

[0068] In one growth example, sulfur, transition metal chloride, and growth substrate regions are held respectively at .about.250.degree. C., .about.300.degree. C., and .about.750.degree. C. for a 10 minute growth period with a 20 sccm flow of Ar/H.sub.2 (85:15). Different temperatures yield different crystal phases which can have varying catalytic activity. VS.sub.2 platelets are similarly grown from VO.sub.x precursors in a sulfur atmosphere.

Example 1.2

Direct Synthesis Method

[0069] Stoichiometric quantities of pure metal and sulfur are sealed in evacuated and Argon backfilled quartz tubes, heated to and held at 750-1000 degrees and cooled depending on a desired phase.

Example 1.3

Hydrogen Evolution Reaction (HER) Setup

[0070] Powders and plates are inherently catalytic for HER, but should preferably be in electrical contact with an electrode in the hydrogen production setup. However, other methods in which a catalyst is in contact with an electrode can also be utilized.

Example 1.4

Electrochemical Studies

[0071] Electrochemical measurements were performed in a three-electrode electrochemical cell using a Autolab PGSTAT302N potentiostat. All measurements were performed in 50 mL of 0.5 M H.sub.2SO.sub.4 (aq) electrolyte (pH=0.16) prepared using 18 M.OMEGA. deionized water purged with Ar gas (99.999%). The glassy carbon electrode (CH Instruments, Dia. 3 mm) casted by the samples was employed as the working electrode while a graphite rod and a saturated calomel electrode (SCE) (CH Instruments) was used as a counter and a reference electrode, respectively. A glassy carbon plate loaded with H--TaS.sub.2 samples was also employed as a working electrode in order to monitor the morphology change during long-time potential cycling.

[0072] The reversible hydrogen electrode (RHE) was calibrated in the high purity H.sub.2 saturated electrolyte using platinum as both working and counter electrodes. Cyclic voltammetry (CVs) was run at a scan rate of 1 mV s.sup.-1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. In 0.5 M H.sub.2SO.sub.4, E (RHE)=E (SCE)+0.254 V.

[0073] The hydrogen evolution reaction (HER) was measured using linear sweep voltammetry between +0.10.about.-0.50 V vs. RHE with a scan rate of 5 mV s.sup.-1. The stability was evaluated by the potential cycling performed using CVs initiating at +0.2 V and ending at -0.6 V vs. RHE at either 100 mV s.sup.-1 or 5 mVs.sup.-1. All data are corrected for a small ohmic drop using electrochemical impedance spectroscopy (EIS). EIS was performed at a biased potential of -0.4 V vs. RHE while sweeping the frequency from 1 MHz to 10 mHz with a 5 mV AC amplitude. The catalytic performance curves have been iR corrected in the measurements.

Example 1.5

Electrode Preparation

[0074] Electrodes were prepared by a mechanical transfer technique. A Bic rubber eraser was lightly rubbed against the sample is one directional strokes and then the eraser was gently tapped against the glassy carbon electrode. The electrode is weighed before and after in order to calculate the loading density. The loading density of all catalyst materials compared was 15 .mu.m/cm.sup.2.

Example 1.6

Characterizations

[0075] Scanning electron microscopy (SEM) images were recorded on an FEI Quanta 400 microscope. Atomic force microscopy (AFM) measurements were taken using an Agilent Picoscan 5500 AFM equipped with a silicon tapping mode tip (AppNano). In the case of comparing the morphology before and after potential cycling, SEM and AFM images were taken on the samples loaded onto the glassy carbon plate. Transmission electron microscopy (TEM) images were collected on a JEOL 2100F TEM. Samples were prepared by drop-drying a diluted suspension in isoproponal onto copper grids covered with lacy carbon films. X-ray diffraction (XRD) was carried out on a Rigaku D/Max Ultima II Powder XRD. Raman spectra were carried out at an excitation wave length of 514 nm.

Example 1.7

Catalyst Performance

[0076] Applicants measured an increase in current with use due an increase in number of available catalytic sites as a result of separation of atomic layers during hydrogen production. An additional implication is that newly exposed catalytic sites have high activity, providing a continuous source of new catalysts. This is in stark contrast to other catalysts where performance decreases with use as number of active sites is fixed and individual sites degrade in efficacy over time (Table 1).

TABLE-US-00001 TABLE 1 Comparison of hydrogen evolution reaction (HER) activities of previously reported catalysts and the catalysts in this Example. Catalyst loading j.sub.0 Tafel slope j@-0.15 V vs. Sample (.mu.g/cm.sup.2) (A/cm.sup.2) (mV/decade) RHE (mA/cm.sup.2) Ref Nanoparticulate MoS.sub.2 N/A 1.3-3.7 .times. 10.sup.-7 55-60 0.2 (12) Particulate MoS.sub.2 4 4.6 .times. 10.sup.-6 120 0.5 (13) Double gyroid MoS.sub.2 60 6.9 .times. 10.sup.-7 50 1 (14) Edge exposed MoS.sub.2 film 8.5 2.2 .times. 10.sup.-6 105-120 0.06 (15) Edge exposed MoS.sub.2 film 22 1.71-3.40 .times. 10.sup.-6 115-123 0.1 (16) 30 nm MoS.sub.2 3400-3900 5.0 .times. 10.sup.-5 66 10.3 (17) T- MoS.sub.2 50 N/A 40 1 (18) T- MoS.sub.2 N/A N/A 43 2 (19) MoS.sub.2/RGO 285 5.1 .times. 10.sup.-6 41 8 (20) Defect-Rich MoS.sub.2 285 8.9 .times. 10.sup.-6 50 3 (21) Electrodeposited MoS.sub.2 N/A N/A 106 2 (22) MoS.sub.2/CNT-graphene 650 2.91 .times. 10.sup.-5 100 2 (23) NanoflakesWS.sub.2 350 N/A 48 1 (24) WS.sub.2/RGO 400 N/A 58 2 (25) T-WS.sub.2 6.5 .sup. 1 .times. 10.sup.-6 60 1 (26) H-TaS.sub.2 50 9.2 .times. 10.sup.-4 99 19 Self nanostructuring catalyst

REFERENCES

[0077] 1. P. E. Blochl, Projector augmented-wave method. Phys. Rev. B 50, 17953 (1994). [0078] 2. G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758 (1999). [0079] 3. J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77, 3865 (1996). [0080] 4. G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169 (1996). [0081] 5. G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558 (1993). [0082] 6. H. J. Monkhorst, J. D. Pack, Special points for Brillouin-zone integrations. Phys. Rev. B 13, 5188 (1976). [0083] 7. B. Hinnemann et al., Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 127, 5308 (2005 Apr. 1, 2005). [0084] 8. K. Mathew, R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, R. G. Hennig, Implicit solvation model for density-functional study of nanocrystal surfaces and reaction pathways. J. Chem. Phys. 140, (2014). [0085] 9. S. A. Petrosyan, A. A. Rigos, T. A. Arias, Joint Density-Functional Theory: Ab Initio Study of Cr2O3 Surface Chemistry in Solution. J. Phys. Chem. B 109, 15436 (2005 Aug. 1, 2005). [0086] 10. M. S. Faber, S. Jin, Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ. Sci., (2014). [0087] 11. B. Hammer, J. K. Norskov, in Advances in Catalysis, H. K. Bruce C. Gates, Ed. (Academic Press, 2000), vol. Volume 45, pp. 71-129. [0088] 12. T. F. Jaramillo et al., Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 317, 100 (Jul. 6, 2007, 2007). [0089] 13. J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Norskov, I. Chorkendorff, Hydrogen evolution on nano-particulate transition metal sulfides. Faraday Discussions 140, 219 (2009). [0090] 14. J. Kibsgaard, Z. Chen, B. N. Reinecke, T. F. Jaramillo, Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nature Mate. 11, 963 (November, 2012). [0091] 15. D. Kong et al., Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 13, 1341 (2013 Mar. 13, 2013). [0092] 16. H. T. Wang et al., Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction. Proceedings of the National Academy of Sciences of the United States of America 110, 19701 (December, 2013). [0093] 17. H. Wang et al., Electrochemical Tuning of MoS2 Nanoparticles on Three-Dimensional Substrate for Efficient Hydrogen Evolution. ACS Nano 8, 4940 (2014 May 27, 2014). [0094] 18. D. Voiry et al., Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction. Nano Lett. 13, 6222 (2013 Dec. 11, 2013). [0095] 19. M. A. Lukowski et al., Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 135, 10274 (2013 Jul. 17, 2013). [0096] 20. Y. Li et al., MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 133, 7296 (May 18, 2011). [0097] 21. J. Xie et al., Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Advanced Materials 25, 5807 (2013). [0098] 22. S. Murugesan et al., Room Temperature Electrodeposition of Molybdenum Sulfide for Catalytic and Photoluminescence Applications. ACS Nano 7, 8199 (2013 Sep. 24, 2013). [0099] 23. D. H. Youn et al., Highly Active and Stable Hydrogen Evolution Electrocatalysts Based on Molybdenum Compounds on Carbon Nanotube-Graphene Hybrid Support. ACS Nano 8, 5164 (2014 May 27, 2014). [0100] 24. L. Cheng et al., Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angewandte Chemie International Edition, n/a (2014). [0101] 25. J. Yang et al., Two-Dimensional Hybrid Nanosheets of Tungsten Disulfide and Reduced Graphene Oxide as Catalysts for Enhanced Hydrogen Evolution. Angewandte Chemie International Edition 52, 13751 (2013). [0102] 26. D. Voiry et al., Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution. Nature Mate. 12, 850 (September, 2013).

Example 2

Surface-Active Metal Dichalcogenide Electrocatalysts with Self-Improving Performance for Hydrogen Evolution

[0103] Efficient electrochemical production of hydrogen (H.sub.2) without the use of expensive precious-metal catalysts has attracted intense interest. Layered transition-metal dichalcogenides (MX.sub.2) based on molybdenum and tungsten are promising catalysts. However, their performance is limited by the availability of active catalytic sites, located mainly at the edges. With the aim of finding higher-performance surface-active MX.sub.2 catalysts, Applicants apply first-principles methods in this Example to reveal simple underlying factors in the electronic structure that ultimately determine catalytic performance. Using these factors as a descriptor for catalyst screening, Applicants predict particularly high intrinsic surface-site activity for group 5 transition metal sulfides (VS.sub.2, NbS.sub.2, TaS.sub.2). This prediction is directly verified experimentally by tests on TaS.sub.2, whose high surface activity leads to overall performance exceeding that of the best edge-active MX.sub.2 competitors. Moreover, the performance of TaS.sub.2 improves upon cycling, as a result of its surface activity. In this Example, Applicants illustrate how theory-motivated rational design guidelines can be formulated and applied for materials screening and discovery.

[0104] In this Example, Applicants use first-principles calculations to unravel the underlying electronic factors that correlate with surface catalytic activity on MX.sub.2. Such insights directly lead to the prediction and experimental demonstration of extraordinary HER activity for group 5 metal sulfides.

[0105] The HER proceeds via two steps, as illustrated in FIG. 8A: (i) H first adsorbs on the catalyst by H.sup.++e.sup.-+*.fwdarw.H*(Volmer reaction), where * denotes a catalytic site; (ii) then an H.sub.2 molecule is formed and desorbed by either 2H*.fwdarw.H.sub.2+2*(Tafel reaction) or H.sup.++e.sup.-+H*.fwdarw.H.sub.2+*(Heyrovsky reaction). An ideal catalyst should provide an optimal balance between adsorption and desorption--a behavior known as the Sabatier principle, typified by the "volcano plot". If the substrate interaction is too weak, then the Volmer reaction is inhibited; if it is too strong, then the Tafel/Heyrovksy reaction cannot proceed. The relative adsorption free energy of the H* intermediate therefore acts as an indicator of the catalytic activity.

[0106] Computing the adsorption free energy on all possible MX.sub.2 combinations depends on the loading density and must be done even for dilute concentrations, which require large unit cells. Instead, Applicants searched for a descriptor based on the intrinsic substrate electronic structure that can readily predict adsorption without the need for explicit calculation, permitting rapid primary screening of MX.sub.2 catalysts. To test possible descriptors, Applicants initially target successful prediction of the dilute H adsorption energy, which Applicants assume captures the dominant contributions to the adsorption free energy. Applicants define this adsorption energy in accordance with the following formula:

Ea=E(H+MX.sub.2)-E(MX.sub.2)-E(H.sub.2)/2.

[0107] In the above formula, E(H+MX.sub.2), E(MX.sub.2) and E(H.sub.2) are the energy of H-adsorbed MX.sub.2, pure MX.sub.2, and an H.sub.2 molecule, respectively. All quantities are calculated using density functional theory (DFT).

[0108] Although binding-energy descriptors based on the metal d-band center have demonstrated success for transition-metal systems (17-19), they cannot be applied to surface binding on MX.sub.2, given that H attaches to the X atom rather than the M atom (FIG. 12). The interaction may be dominated by local electrostatics, in which case the charge on X at the adsorption site might be an appropriate descriptor. However, no such correlation was observed (FIG. 12). Instead, a new descriptor was utilized.

[0109] To design a proper descriptor with broad applicability, Applicants first examine changes in the underlying electronic structure of two representative MX.sub.2 materials, metallic (TiS.sub.2) and semiconducting (MoS.sub.2), upon adsorption. Applicants use a single adsorbate in a 4.times.4 unit cell to approximate dilute adsorption. For metallic TiS.sub.2 (FIG. 8B), H adsorption does not change the overall profile of the electronic density of states (DOS), but rather shifts the Fermi level (.epsilon.F) to a slightly higher energy (i.e., occupies previously empty states). This shift in .epsilon.F corresponds to 1e per H adsorbate, indicating complete charge transfer to TiS.sub.2. The charge density distribution shows that the transferred electrons are delocalized throughout the M layer. For semiconducting MoS.sub.2 (FIG. 8C), the DOS profile also remains largely intact, with the exception of a new narrow band immediately below the conduction band minimum (.epsilon.CBM) that is occupied by the transferred electrons. In other words, H behaves like a shallow n-type dopant. The charge density distribution shows that this state is quasi-localized in space. Applicants can extrapolate the behavior of both materials to the dilute adsorption limit, where the number of transferred electrons becomes negligible with respect to the total DOS. In this case, the Fermi level of the metallic system (TiS.sub.2) would remain unchanged by adsorption, whereas the Fermi level of the semiconducting system (MoS.sub.2) would shift to the newly created localized state, which is pinned close to .epsilon.CBM. In addition, the DOS profile of each would be retained.

[0110] Notably, this behavior is consistent with a model based on the `states-filling work`, which was recently proposed as an appropriate descriptor for predicting charge-transfer binding on sp.sup.2-carbon substrates (20). It is based on a rigid-band approximation, which assumes the underlying substrate DOS profile is unaffected by the adsorbate. Moreover, when operating at the dilute adsorption limit, the states-filling work converges to the energy of the lowest unoccupied state (LUS) .epsilon.LUS, equal to .epsilon.F for metals or .epsilon.CBM for semiconductors (20), which agrees well with the two representative cases in the dilute limit.

[0111] The suitability of .epsilon.LUS as a descriptor is confirmed in FIG. 8D. Applicants calculate this quantity for a set of known MX.sub.2 species (based on the substrate alone without an adsorbate) and compare the result to the dilute H adsorption energy, evaluated explicitly using Eq. 1. The two values correlate linearly with a slope of near unity (FIG. 8D). This implies that differences in Ea amongst the various substrates originate almost exclusively from differences in .epsilon.LUS, and that the key to adjusting the H adsorption energy lies in the vacuum-referenced placement of the substrate LUS level.

[0112] Having established .epsilon.LUS as a viable descriptor for Ea on MX.sub.2 surfaces, Applicants proceed to select its target value that will give a reasonable range of surface adsorption strengths for primary catalyst screening. A target estimate for .epsilon.LUS is obtained by examining results for the H phases of MoX.sub.2 and WX.sub.2. According to FIG. 8D, these surfaces have a comparatively high .epsilon.LUS (>-4.5 eV), which leads to weak surface adsorption (Ea>2.0 eV/H). This prevents the Volmer reaction from taking place and inhibits surface activity (3, 4). In contrast, the active edges of these materials have much stronger Ea (calculated as .about.-0.4 eV/H for MoS.sub.2 edge (see Examples 2.1-2.5), which is apparently more appropriate for effective catalysis. Substituting Ea=-0.4 eV/H into the linear trend in FIG. 8D, Applicants conclude that materials with .epsilon.LUS.about.-6.3 eV would have adsorption strengths competitive with MoX.sub.2 and WX.sub.2 edges. Applicants broaden this criterion for viable candidates to -0.5 eV/H<Ea<+0.5 eV/H, corresponding to -6.4 eV<.epsilon.LUS<-5.5 eV. As additional validation, Applicants point out the T' phases of MoS.sub.2 and WS.sub.2 have values of .epsilon.LUS within this range (-5.7 and -5.6 eV, respectively), and each has recently demonstrated correspondingly enhanced HER activity with respect to the ordinary H phases.

[0113] Applying the .epsilon.LUS criterion to all MX.sub.2 substrates in their most stable phases (H for group 5 and 6, T for group 4 and 10, T' for group 7; structures described in Examples 2.1-2.5), Applicants narrow the list of viable surface-active HER catalysts to a small handful of candidates (FIG. 9A). Two general features are observed. (1) For a given M, .epsilon.LUS increases in the following order: S<Se<Te. Hence, Ea increases in the following order: S<Se<Te. (2) Metallic MX.sub.2 candidates (from groups 4 and 5) have lower .epsilon.LUS and hence stronger Ea than semiconducting MX.sub.2 candidates (from groups 6, 7, and 10). The group 5 metal disulfides (VS.sub.2, NbS.sub.2, and TaS.sub.2) show particular promise, having a relatively low .epsilon.LUS (<-5.8 eV) and a correspondingly strong Ea.

[0114] Next, Applicants performed a more accurate assessment of the group 5 metal disulfides by computing the concentration-dependent free energy of surface H adsorption, including entropic contributions and explicit calculation of Ea. For the HER at pH=0 and at zero potential relative to the standard hydrogen electrode, the free energy of H.sup.++e.sup.- is by definition the same as that of 1/2H.sub.2 at standard conditions. Sabatier's principle implies that on an optimal catalyst, the free energy of the reaction intermediate--in this case, adsorbed H--should be close to this value, which Applicants define to be zero (3, 10, 15, 16). In examining concentration dependence, it is preferable to distinguish between the total (Gtot) and differential (Gdiff) free energies:

Gtot=(Ea+.DELTA.EZP-T.DELTA.S)*nH

Gdiff=.differential.Gtot/.differential.nH (3)

[0115] Here, .DELTA.EZP (the zero-point energy difference between adsorbed H and 1/2H.sub.2) together with T.DELTA.S (the entropy correction) amounts to 0.29 eV, at room temperature. According to FIG. 9B, G.sub.tot increases monotonously with the H coverage on the surface of group 5 disulfides. The behavior indicates that at zero potential, dilute H adsorption is thermodynamically favored over dense adsorption. This further justifies Applicants' choice to focus on the low-coverage limit when considering .epsilon.LUS as a descriptor. On the other hand, Gdiff at the equilibrium H coverage represents the free energy cost to adsorb/desorb H on/from the catalyst, which in turn reflects the kinetics of catalysis near equilibrium (3, 10, 15, 16). FIG. 9C shows that at low surface coverage, each of the group 5 disulfides has a low G.sub.diff (<0.4 eV/H at the coverage H:M=1:16), supporting Applicants' initial supposition that these materials are promising candidates for surface-active HER catalysis.

[0116] Moreover, the shallow slopes of the curves in FIG. 9B below .about.25% surface coverage indicate that low G.sub.diff will be retained even at somewhat higher coverages. In addition, each of the group 5 disulfides is metallic, unlike the semiconductors MoX.sub.2 and WX.sub.2. Their higher intrinsic electronic conductivity should further benefit their operation as electrocatalysts.

[0117] One interesting consequence of surface activity is that for van der Waals layered material like MX.sub.2, the H.sub.2 produced at surface sites and trapped between layers could lead to peeling off of the layers (FIG. 2A), analogous to chemical exfoliation by the lithium intercalation and reaction with water (7, 21). Thinner samples would improve the H accessibility of surface sites, and increases the electron transfer across the layers.

[0118] Applicants tested the surface catalytic activity of group 5 MX.sub.2 on one of the representative H phase TaS.sub.2 platelets (H--TaS.sub.2, see Examples 2.1-2.5 for details). Polarization curve of H--TaS.sub.2 for HER electrocatalysis is measured versus the reversible hydrogen electrode (RHE), and compared to those of commercial H--MoS.sub.2 platelets with similar dimensions (FIG. 10A). The as-synthesized H--TaS.sub.2 platelets have lateral sizes up to 20 .mu.m and thicknesses of 100-600 nm (FIG. 2B and FIG. 13), and show high crystallinity according to X-ray diffraction (FIG. 13), Raman spectroscopy (FIG. 2E), and high-resolution transmission electron microscopy (HRTEM) (FIG. 2D and FIG. 23).

[0119] H--TaS.sub.2 exhibits a nearly zero onset overpotential after 5000 potential cycles (see Examples 2.1-2.5), similar to Pt and far superior to H--MoS.sub.2 under identical cycling conditions. The current density is also much higher, reaching 15 mA/cm.sup.2 at 150 mV, compared with 0.1 mA/cm.sup.2 for H--MoS.sub.2. In FIG. 10A, Applicants also benchmark the performance of H--TaS.sub.2 against the T phases of both MoS.sub.2 and TaS.sub.2 (T are higher in energy compared with H for both materials (see Examples 2.1-2.5 for preparation methods).

[0120] Recent reports indicated that the T phase has higher HER activity than the H phase in the case of MoS.sub.2 (confirmed here) (7, 14). In contrast, the performance of H--TaS.sub.2 far exceeds that of the T phase samples. This can be expected from their .epsilon.LUS values: T-TaS.sub.2 has a larger .epsilon.LUS and therefore weaker Ea than those of H--TaS.sub.2 (FIG. 8D).

[0121] Tafel plots extracted from the polarization curves (FIG. 10B) allow for quantification of the exchange current density for each of the systems. Applicants find that H--TaS.sub.2 has an exchange current density (9.2.times.10.sup.-4 A cm.sup.-2, FIG. 10C) that is more than 200 times higher than that of H--MoS.sub.2 and T-MoS2, and more than 10,000 times higher than that of T-TaS.sub.2. In addition, the Tafel slope of H--TaS.sub.2 is lower than that of H--MoS.sub.2, implying a different rate-determining HER step. This would be expected for a shift in the catalytically active site from a stronger-adsorption edge site (H--MoS.sub.2) to a weaker-adsorption surface site (H--TaS.sub.2). Notably, the H--TaS.sub.2 has the best HER activity over all the reported MX.sub.2 materials (Table 2) in terms of onset overpotential, exchange current density, and current density observed at 150 mV. Collectively, the electrochemical tests for H--TaS.sub.2 are fully consistent with an efficient surface-active HER electrocatalyst, as predicted by Applicants' theoretical investigation.

[0122] The H--TaS.sub.2 multilayer platelets show self-improving performance with cycling (.about.3500-fold increase in cathodic current density after 5000 cycles, FIG. 10D and FIG. 20), as predicted above. Raman spectra and HRTEM confirm the retaining of H phase of TaS.sub.2 after cycling (FIGS. 2E and H). Atomic force microscope (AFM) (FIGS. 2B and F) and TEM (FIGS. 2C and G) show the platelets indeed become thinner, and the electron transfer charge resistance drops by .about.600 times (FIG. 21), as a result of exfoliation induced by surface activity. In contrast, the performance of surface-inactive H--MoS.sub.2 retains similar performance after cycling. Applicants point out that the self-improving behavior requires H intercalation into the weakly coupled interlayers, a unique property for vdW solids.

[0123] In summary, the abundance of active sites, combined with properly tuned adsorption thermodynamics and high intrinsic electrical conductivity, establish H phases of the group 5 metal disulfides as promising surface-active HER electrocatalysts.

Example 2.1

Computational Details

[0124] Spin-polarized DFT calculations were performed using Projector Augmented Wave (PAW) pseudopotentials and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, as implemented in VASP. All structures are relaxed until the force on each atom is less than 0.01 eV/.ANG.. Monkhorst-Pack (MP) k-points sampling is used, with >15 .ANG. vacuum space in the non-periodic direction.

[0125] For MX.sub.2 with H or T phase, E.sub.a is calculated by using a 4.times.4 cell. For MX.sub.2 with T' phase, E.sub.a is calculated by using a 4.times.2 3 cell. .epsilon..sub.LUS is calculated from the unit cell. The edge is modeled by using a nanoribbon with 4.times. 3 cell width.

Example 2.2

Synthesis of TaS.sub.2 Crystal Platelets

[0126] H--TaS.sub.2 crystal platelets are grown by a chemical vapor deposition on SiO.sub.2/Si substrates from sulfur and tantalum chloride powders and gaseous hydrogen precursors in a 3-stage furnace via the following reactions:

H 2 + ( 1 8 ) S 8 .fwdarw. H 2 S ( 1 ) M Cl 5 + H 2 S + ( 1 2 ) H 2 .fwdarw. M S 2 + ( 5 ) H Cl ( 2 ) ##EQU00002##

[0127] The sulfur, tantalum chloride, and growth substrate regions are held respectively at .about.250.degree. C., .about.300.degree. C., and .about.750.degree. C. for a 10 minute growth period with a 20 sccm flow of Ar/H.sub.2 (85:15). 2H-TaS.sub.2 platelets can be converted to the T phase by heating in a sulfur and argon atmosphere at 900.degree. C. for 1 hour and then rapidly quenching.

Example 2.3

n-Butyl Lithium Exfoliation Treatment

[0128] The thinner T-TaS.sub.2 nanosheets were obtained by soaking the as-synthesized T-TaS.sub.2 platelets in 3 ml n-butyllithium solution (1.6 M, Sigma-Aldrich) at room temperature for 48 hours in a sealed vial inside an argon-filled glove box. The excess n-butyl lithium was removed by centrifuging at 4000 rpm following rinsing with hexane. Next, excess deionized water was added in to react with the intercalated lithium, which generated H.sub.2 gas and separated the 2D platelets. After exfoliation, the samples were centrifuged at 10,000 rpm to remove unreacted precipitant and get the suspending solution for future tests. The T-MoS.sub.2 nanosheets were prepared following the similar lithium intercalation and water reaction procedures, but from the commercial H--MoS.sub.2 plates (Sigma-Aldrich).

Example 2.4

Electrochemical Studies

[0129] Electrochemical measurements were performed in a three-electrode electrochemical cell using a Autolab PGSTAT302N potentiostat. All measurements were performed in 50 mL of 0.5 M H.sub.2SO.sub.4 (aq) electrolyte (pH=0.16) prepared using 18 M.OMEGA. deionized water purged with Ar gas (99.999%). The glassy carbon electrode (CH Instruments, Dia. 3 mm) casted by the samples was employed as the working electrode while a graphite rod and a saturated calomel electrode (SCE) (CH Instruments) was used as a counter and a reference electrode, respectively. A glassy carbon plate loaded with H--TaS.sub.2 samples was also employed as a working electrode in order to monitor the morphology change during long-time potential cycling.

[0130] The reversible hydrogen electrode (RHE) was calibrated in the high purity H.sub.2 saturated electrolyte using platinum as both working and counter electrode. Cyclic voltammetry (CVs) was run at a scan rate of 1 mV s.sup.-1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. In 0.5 M H.sub.2SO.sub.4, E (RHE)=E (SCE)+0.254 V.

[0131] The hydrogen evolution reaction (HER) was measured using linear sweep voltammetry between +0.10.about.-0.50 V vs. RHE with a scan rate of 5 mV s.sup.-1. The stability was evaluated by the potential cycling performed using CVs initiating at +0.2 V and ending at -0.6 V vs. RHE at either 100 mV s.sup.-1 or 5 mV s.sup.-1. All data are corrected for a small ohmic drop using electrochemical impedance spectroscopy (EIS). EIS was performed at a biased potential of -0.4 V vs. RHE while sweeping the frequency from 1 MHz to 10 mHz with a 5 mV AC amplitude.

Example 2.5

Characterizations

[0132] Scanning electron microscopy (SEM) images were recorded on an FEI Quanta 400 microscope. Atomic force microscopy (AFM) measurements were taken using an Agilent Picoscan5500 AFM equipped with a silicon tapping mode tip (AppNano). In the case of comparing the morphology before and after potential cycling, SEM and AFM images were taken on the samples loaded onto the glassy carbon plate. Transmission electron microscopy (TEM) images were collected on a JEOL 2100F TEM. Samples were prepared by drop-drying a diluted suspension in isoproponal onto copper grids covered with lacy carbon films. X-ray diffraction (XRD) was carried out on a Rigaku D/Max Ultima II Powder XRD. Raman spectra were carried out at an excitation wave length of 514 nm.

TABLE-US-00002 TABLE 2 HER activity on previous reports and this Example. Catalyst loading j.sub.0 Tafel slope j@-0.15 V vs. Sample (.mu.g/cm.sup.2) (A/cm.sup.2) (mV/decade) RHE (mA/cm.sup.2) Ref Nanoparticulate MoS.sub.2 N/A 1.3-3.7 .times. 10.sup.-7 55-60 0.2 (8) Particulate MoS.sub.2 4 4.6 .times. 10.sup.-6 120 0.5 (9) Double gyroid MoS.sub.2 60 6.9 .times. 10.sup.-7 50 1 (10) Edge exposed MoS.sub.2 film 8.5 2.2 .times. 10.sup.-6 105-120 0.06 (11) Edge exposed MoS.sub.2 film 22 1.71-3.40 .times. 10.sup.-6 115-123 0.1 (12) 30 nm MoS.sub.2 3400-3900 5.0 .times. 10.sup.-5 66 10.3 (13) T- MoS.sub.2 50 N/A 40 1 (14) T- MoS.sub.2 N/A N/A 43 2 (15) MoS.sub.2/RGO 285 5.1 .times. 10.sup.-6 41 8 (16) Defect-Rich MoS.sub.2 285 8.9 .times. 10.sup.-6 50 3 (17) Electrodeposited MoS.sub.2 N/A N/A 106 2 (18) MoS.sub.2/CNT-graphene 650 2.91 .times. 10.sup.-5 100 2 (19) NanoflakesWS.sub.2 350 N/A 48 1 (20) WS.sub.2/RGO 400 N/A 58 2 (21) T-WS.sub.2 6.5 .sup. 1 .times. 10.sup.-6 60 1 (22) H-MoS.sub.2 100 3.4 .times. 10.sup.-6 120 0.08 This T-MoS.sub.2 50 2.5 .times. 10.sup.-6 78 0.2 Example T-TaS.sub.2 80 6.4 .times. 10.sup.-8 92 0.02 H-TaS.sub.2 50 9.2 .times. 10.sup.-4 99 19

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[0154] Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present disclosure to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the embodiments have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein.

Claims

1. A method of mediating a gas evolution reaction, wherein the method comprises: exposing a gas precursor to an electrocatalyst comprising a plurality of layers, wherein the layers comprise catalytic sites; wherein the exposing results in electrocatalytic conversion of the gas precursor to a gas between the layers; and wherein the gas enhances the electrocatalytic activity of the electrocatalyst.

2. The method of claim 1, wherein the electrocatalyst is associated with an electrically conductive surface, wherein the electrically conductive surface provides electrical current.

3. The method of claim 2, wherein the electrically conductive surface is an electrode.

4. The method of claim 1, wherein the gas precursor is H.sup.+, wherein the gas is H.sub.2, and wherein the gas evolution reaction is a hydrogen evolution reaction that converts H.sup.+ to H.sub.2.

5. The method of claim 1, wherein the electrocatalyst is a hydrogen production electrocatalyst that converts H.sup.+ to H.sub.2.

6. The method of claim 1, wherein the electrocatalyst comprises a transition metal dichalcogenide.

7. The method of claim 6, wherein the transition metal dichalcogenide comprises a group V transition metal dichalcogenide.

8. The method of claim 6, wherein the transition metal dichalcogenide comprises the following formula: MX.sub.2, wherein M is a transition metal, and wherein X is a chalcogen.

9. The method of claim 8, wherein the transition metal is selected from the group consisting of Ti, Hf, Zr, Mo, W, Ta, Nb, V, Tc, Re, Sn and combinations thereof.

10. The method of claim 8, wherein the chalcogen is selected from the group consisting of S, Se, O, Te, and combinations thereof.

11. The method of claim 8, wherein X is S.

12. The method of claim 1, wherein the electrocatalyst is selected from the group consisting of TaS.sub.2, NbS.sub.2, VS.sub.2, and combinations thereof.

13. The method of claim 1, wherein the layers are in the form of crystal plates.

14. The method of claim 1, wherein the layers are separated by a distance ranging from about 0.1 nm to about 1 nm.

15. The method of claim 1, wherein the layers are porous.

16. The method of claim 1, wherein the catalytic sites are on surfaces of the layers.

17. The method of claim 1, wherein the produced gas enhances the electrocatalytic activity of the electrocatalyst by enhancing the accessibility of the catalytic sites to the gas precursor.

18. The method of claim 17, wherein the gas enhances the accessibility of the catalytic sites to the gas precursor by increasing distances between the layers, thereby making the catalytic sites more accessible to the gas precursor.

19. The method of claim 17, wherein the produced gas enhances the electrocatalytic activity of the electrocatalyst with time.

20. The method of claim 1, wherein the electrocatalyst has an exchange current density ranging from about 2.times.10.sup.-4 A/cm.sup.2 to about 10.times.10.sup.-4 A/cm.sup.2.

21. The method of claim 1, wherein the electrocatalyst has a catalyst loading that ranges from about 10 .mu.g/cm.sup.2 to about 100 .mu.g/cm.sup.2.

22. The method of claim 1, wherein the electrocatalyst has a Tafel slope ranging from about of 25 mV/decade to about 100 mV/decade.

23. The method of claim 1, wherein the electrocatalyst has a current density ranging from about of 5 mA/cm.sup.2 to about 50 mA/cm.sup.2.

24. An electrocatalyst for mediating a gas evolution reaction, wherein the electrocatalyst comprises a plurality of layers, and wherein the layers comprise catalytic sites.

25. The electrocatalyst of claim 24, wherein the electrocatalyst is associated with an electrically conductive surface, wherein the electrically conductive surface provides electrical current.

26. The electrocatalyst of claim 25, wherein the electrically conductive surface is an electrode.

27. The electrocatalyst of claim 24, wherein the electrocatalyst is a hydrogen production electrocatalyst that converts H.sup.+ to H.sub.2.

28. The electrocatalyst of claim 24, wherein the electrocatalyst comprises a transition metal dichalcogenide.

29. The electrocatalyst of claim 28, wherein the transition metal dichalcogenide comprises a group V transition metal dichalcogenide.

30. The electrocatalyst of claim 28, wherein the transition metal dichalcogenide comprises the following formula: MX.sub.2, wherein M is a transition metal, and wherein X is a chalcogen.

31. The electrocatalyst of claim 30, wherein the transition metal is selected from the group consisting of Ti, Hf, Zr, Mo, W, Ta, Nb, V, Tc, Re, Sn and combinations thereof.

32. The electrocatalyst of claim 30, wherein the chalcogen is selected from the group consisting of S, Se, O, Te, and combinations thereof.

33. The electrocatalyst of claim 30, wherein X is S.

34. The electrocatalyst of claim 24, wherein the electrocatalyst is selected from the group consisting of TaS.sub.2, NbS.sub.2, VS.sub.2, and combinations thereof.

35. The electrocatalyst of claim 24, wherein the layers are in the form of crystal plates.

36. The electrocatalyst of claim 24, wherein the layers are separated by a distance ranging from about 0.1 nm to about 1 nm.

37. The electrocatalyst of claim 24, wherein the layers are porous.

38. The electrocatalyst of claim 24, wherein the catalytic sites are on surfaces of the layers.

39. The electrocatalyst of claim 24, wherein the electrocatalyst has an exchange current density ranging from about 2.times.10.sup.-4 A/cm.sup.2 to about 10.times.10.sup.-4 A/cm.sup.2.

40. The electrocatalyst of claim 24, wherein the electrocatalyst has a catalyst loading that ranges from about 10 .mu.g/cm.sup.2 to about 100 .mu.g/cm.sup.2.

41. The electrocatalyst of claim 24, wherein the electrocatalyst has a Tafel slope ranging from about of 25 mV/decade to about 100 mV/decade.

42. The electrocatalyst of claim 24, wherein the electrocatalyst has a current density ranging from about of 5 mA/cm.sup.2 to about 50 mA/cm.sup.2.