ELECTROCATALYST FOR HYDROGEN EVOLUTION REACTION

Abstract

The electrocatalyst for hydrogen evolution reaction includes nanosheets of molybdenum disulfide (MoS2) deposited on a carbon fiber substrate. The catalyst is formed in stepwise fashion by chemical vapor deposition of nanosheets of MoO3 onto the substrate, then reducing the MoO3 to nanosheets of MoO2 using sublimed sulfur, then by reaction of sulfur vapor with the MoO2 to form nanosheets of MoS2 on the carbon fiber substrate. The catalyst is multifaceted, having a large density of edges providing catalytically active sites for the hydrogen evolution reaction. The activity of the catalyst is enhanced by coating the catalyst with spherical fullerenes (nC60).

Description

TECHNICAL FIELD

The disclosure of the present patent application relates to an electrocatalyst for hydrogen evolution reaction, and particularly to an electrocatalyst for hydrogen evolution reaction that includes nanosheets of molybdenum disulfide (MoS₂) deposited on a carbon fiber substrate.

BACKGROUND ART

Sunlight shining on earth is intermittent. As such, a fundamental impediment to meaningful utilization of sunlight is the effective storage of solar energy. Water splitting, or the disproportionation of H₂O into H₂ and O₂, represents a promising strategy but is non-trivial because it requires the concerted transfer of four electrons and four protons. In nature, the complex biological machinery for photosynthesis couples multiple platforms wherein the light harvesting, water oxidation, and proton reduction steps are each performed by discrete components. Consequently, there is much interest in hybrid structures wherein discrete components perform each of the individual reactions required for photocatalysis. A viable photocatalytic cycle can be constituted by coupling photocatalytic water oxidation with electrocatalytic hydrogen evolution. The latter hydrogen evolution reaction (HER), however, is beset by a distinctive set of challenges.

The Pt group metals are excellent catalysts for HER and evolve hydrogen at near-zero overpotentials in acidic media but are cost prohibitive and amongst the least abundant elements available to mankind. There has been a strong push to develop alternatives and some success was achieved with MoS₂ as well as transition metal phosphides. In those strategies, however, the electrocatalytic activity of MoS₂ was mainly derived from catalytically active edge sites. The basal planes were thought to be catalytically inert with some exceptions. Furthermore, the low charge carrier mobility of MoS₂ was an impediment to its use as an electrocatalyst. Also, those materials generally tend to evolve H₂ at high overpotentials in comparison to Pt.

Chemical vapor deposition is ubiquitously used to prepare well-crystallized MoS₂ architectures, typically using molybdenum oxide or chloride precursors. A major drawback of this method as applied to the growth of MoS₂ is that it necessitates the operation of several concurrent reactions. Consequently, previously obtained MoS₂ electrocatalyst samples are often plagued by poor size and shape homogeneity, with sparse substrate coverage.

Thus, an electrocatalyst for hydrogen evolution reaction solving the aforementioned problems are desired.

DISCLOSURE OF INVENTION

The electrocatalyst for hydrogen evolution reaction includes homogeneously sized nanosheets of molybdenum disulfide (MoS₂) integrated on a carbon fiber paper substrate, with exposed catalytically active edge sites. The nanosheets are well distributed and vertically oriented. The electrocatalyst is prepared by the stepwise reduction and sulfidation of MoO₃. The direct integration of edge-exposed MoS₂ nanosheets onto CFP yields a 3D architecture with a high surface-to-volume ratio desirable for electrocatalytic applications. The inherent HER activity of the edge-sites of MoS₂ can be enhanced significantly by interfacing with nC₆₀ nanoclusters, as a result of the enhancement of the conductivity of MoS₂ owing to charge transfer.

These and other features of the present disclosure will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram illustrating the steps for preparing the electrocatalyst for hydrogen evolution reaction.

FIG. 2A is a FESEM image showing homogeneous distribution of MoS₂ nanosheets grown on textured CFP substrate.

FIG. 2B is a high-magnification SEM image of an individual MoS₂ nanosheet.

FIG. 2C is a low-magnification TEM image of a nanosheet depicting locations of “clean” well-faceted and “collapsed” edges.

FIG. 2D is an HRTEM image of a “clean” edge.

FIG. 2E is an HRTEM image of a discontinuous “collapsed” edge.

FIG. 3A is a FESEM image of MoO₃ harvested from a flat Si(100) substrate.

FIG. 3B is a FESEM image of MoO₂ harvested from a flat Si(100) substrate.

FIG. 3C is a FESEM image of MoS₂ harvested from a flat Si(100) substrate.

FIG. 3D is a FESEM image of MoS₂ nanosheets harvested from a flat Si(100) substrate.

FIG. 3E is a FESEM image of MoS₂ nanosheets harvested from a flat Si(100) substrate.

FIG. 3F is a FESEM image of MoS₂ nanosheets harvested from a flat Si(100) substrate, showing additional edge sites created on the basal plane of the nanosheets.

FIG. 4A shows XRD patterns of MoO₃ nanosheets, MoO₂ nanodisks, and MoS₂ nanosheets prepared on CFP.

FIG. 4B shows Raman spectra (514.5 nm laser excitation) of MoO₃ nanosheets, MoO₂ nanodisks, and MoS₂ nanosheets prepared on CFP.

FIG. 4C shows XPS spectra indicating Mo 3d binding energies.

FIG. 4D shows XPS spectra indicating O 1s binding energies

FIG. 4E shows XPS spectra indicating S 2p binding energies

FIG. 5 shows Raman spectra of the nC₆₀ cluster and hybrid nC₆₀/MoS₂ architectures.

FIG. 6 shows the mass spectrum of negatively charged ions emitted from the surface of the clusters deposited on CFP.

FIG. 7A shows polarization curves of various concentrations of nC₆₀ clusters deposited directly onto CFP.

FIG. 7B shows polarization curves of various concentrations of nC₆₀ clusters interfaced with 3D MoS₂ nanosheets on CFP.

FIG. 8A shows a cyclic voltammogram acquired in the range between 0.10-0.30 V vs. RHE for neat nC₆₀ clusters deposited on CFP from solutions of C₆₀ concentration of 0.1 mg/mL.

FIG. 8B shows a cyclic voltammogram acquired in the range between 0.10-0.30 V vs. RHE for neat nC₆₀ clusters deposited on CFP from solutions of C₆₀ concentration of 0.5 mg/mL.

FIG. 8C shows a cyclic voltammogram acquired in the range between 0.10-0.30 V vs. RHE for neat nC₆₀ clusters deposited on CFP from solutions of C₆₀ concentration of 2.0 mg/mL.

FIG. 8D is a graph showing differences in current density Δj=j_a−j_c at 0.20 V versus RHE, plotted as a function of scan rate with each plot fitted to a straight line to determine the C_dl values.

FIG. 9A shows a voltammogram acquired in the range of 0.15-0.30 V versus RHE for as-prepared 3D MoS₂.

FIG. 9B shows a voltammogram acquired in the range of 0.15-0.30 V versus RHE for hybrid nC₆₀ (0.1 mg/mL)/MoS₂.

FIG. 9C shows a voltammogram acquired in the range of 0.15-0.30 V versus RHE for for hybrid nC₆₀ (0.5 mg/mL)/MoS₂.

FIG. 9D shows a voltammogram acquired in the range of 0.15-0.30 V versus RHE for for hybrid nC₆₀ (2.0 mg/mL)/MoS₂.

FIG. 9E is a graph showing differences in current density (Δj=j_a−j_c) at 0.23 V versus RHE are plotted as a function of the scan rate (the C_dl values are extrapolated from a linear fit to the plot).

FIG. 10A shows the Nyquist plots of as-prepared 3D MoS₂ nanosheets and hybrid nC₆₀/MoS₂ architectures prepared on CFP measured at −150 mV vs. RHE.

FIG. 10B shows R_ct values plotted as a function of potential for nC₆₀, as-prepared 3D MoS₂ nanosheets, and hybrid nC₆₀/MoS₂ architectures prepared on CFP.

Similar reference characters denote corresponding features consistently throughout the attached drawings.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

The electrocatalyst for hydrogen evolution reaction includes high-edge-density MoS₂ nanosheets directly integrated with conductive carbon fiber paper (CFP). The nanosheets can be vertically aligned. The nanosheets can be 3-dimensional, polycrystalline nanosheets. The nanosheets are homogeneously dispersed across centimeter scales and have a high density of exposed edge sites. The nanosheets can include a mixture of faceted as well as discontinuous collapsed edges within the basal planes, induced by volume expansion accompanying topochemical sulfidation, as described herein. The MoS₂ nanosheets on the CFP can exhibit an overpotential 110 value of about 245 mV at 10 mA/cm², a Tafel slope of about 81 mV/dec, and a turnover frequency (TOF) of about 1.28 H₂/s per active site at −0.2 V versus RHE in a 0.5 M acidic solution.

The catalyst can further include Buckminsterfullerenes or spherical fullerenes (nC₆₀). For example, the 3D MoS₂ nanosheets can be interfaced with nC₆₀ clusters by a facile solution-deposition method. The hybrid structures show greatly enhanced HER activity with an overpotential η₁₀ value of about 172 mV and a Tafel slope of about 60 mV/dec when the deposition concentration of C₆₀ is about 0.5 mg/mL. This condition corresponds to about a 2% coverage of the MoS₂ nanosheets by nC₆₀ clusters. The improved activity of the hybrid catalysts is believed to derive from the interfacial charge transfer at nC₆₀/MoS₂ p-n heterojunctions. An optimal coverage of nC₆₀ with a homogeneous distribution can facilitate such interfacial doping.

The catalyst can be formed in stepwise fashion by chemical vapor deposition of nanosheets of MoO₃ onto a carbon fiber substrate, reducing the MoO₃ to nanosheets of MoO₂ using sublimed sulfur, then reacting sulfur vapor with the MoO₂ to form nanosheets of MoS₂ on the carbon fiber substrate. The resulting catalyst is multifaceted, having a large density of edges providing catalytically active sites for hydrogen evolution reaction.

The stepwise vapor transport, reduction, and sublimation steps used to prepare edge-exposed MoS₂ nanosheets on CFP are schematically illustrated in FIG. 1. In the first step, MoO₃ nanosheets that are about 1-2 μm in lateral dimensions are deposited onto CFP by the vapor transport of MoO₃ powder heated to 850° C.

In the next step, reaction with sublimed sulfur at 400° C. as per:

2MoO₃(s)+S(g)→2MoO₂(s)+SO₂(g)  (1)

yields MoO₂ nanosheets with retention of the vertical growth orientation, although the edges are slightly rounded. Finally, the topochemical sulfidation of MoO₂ at 850° C. as per:

MoO₂(s)+3S(g)→MoS₂(s)+SO₂(g)  (2)

yields faceted MoS₂ nanosheets that are uniformly dispersed and vertically oriented across a large area (about 2 cm²) of the CFP (FIG. 2A). FIGS. 2B-2E illustrate “clean,” well-faceted, and “collapsed” edges of an individual MoS₂ nanosheet.

FIGS. 3A-3F illustrate the morphologies of vapor transported MoO₃ collected on a Si (100) substrate before and after stepwise reduction and sulfidation. These images further enable visualization of the vertical growth direction and high density of edge sites. Notably, the vertical growth orientation is achieved without mediation of a catalyst. Interestingly, the faceted MoO₃ nanosheets are transformed to thicker rounded MoO₂ discs upon reduction and finally converted to faceted MoS₂ nanosheets during sulfidation. The edge geometries in large measure reflect the intrinsic crystal structures of the phases. Orthorhombic α-MoO₃ crystallizes in a layered structure and thus faceted nanosheets are obtained comprising stacked layers. Reduction to monoclinic MoO₂ yields rounded edges, whereas topochemical transformation to 2H—MoS₂ again yields faceted structures reflecting the layered stacking of MoS₂ sheets. The considerable lattice mismatch between MoO₂ and MoS₂ results in a substantial volume change, which creates a distinctive discontinuous motif characterized by faceted “clean” and discontinuous “collapsed” domains along the MoS₂ basal planes. The latter is important as it allows for exposure of an increased density of catalytically active edge-sites.

FIGS. 4A-4E corroborate the phase identification of the prepared materials based on X-ray diffraction (XRD) and Raman microprobe analysis. The XRD patterns acquired on CFP are dominated by the (002) reflections of the graphitic substrate. However, reflections corresponding to the deposited materials are discernible and are indexed to orthorhombic α-MoO₃ (Joint Committee on Powder Diffraction Standards (JCPDS) 76-1003), monoclinic MoO₂ (JCPDS 86-0135), and hexagonal 2H—MoS₂ (JCPDS 87-2416) as shown in FIG. 4A.

Clearer phase assignment is enabled from the Raman spectra shown in FIG. 4B since the graphitic D and G bands from the substrate are only observed above 1300 cm. The Raman bands of the nanosheets formed in the first step are well matched with the Raman active modes of orthorhombic α-MoO₃ reported in the literature. The detailed Raman band assignments of the prepared α-MoO₃ nanosheets are listed in Table 1.

TABLE 1
Photon Mode Assignments for Raman bands
measured for α-MoO3 nanosheets
α-MoO3 nanosheet
(produced by the 1st process)
Bands
(cm−1)	Raman modes	Assignments
995	Ag	νas O═Mo stretch
819	Ag	νs O═Mo═O stretch
665	B2g/B3g	νas Mo—O—Mo stretch
472	Ag	νas Mo—O—Mo stretch and bend
378	B1g	δ O═Mo═O scissoring
364	Ag	δ O═Mo═O scissoring
337	Ag/B1g	δ Mo—O—Mo bend
290	B3g	δ O═Mo═O wagging
282	B2g	δ O═Mo═O wagging
245	B3g	τ O═Mo═O twist
216	Ag	rotational rigid MoO4-chain mode, Rc
196	B2g	τ O═Mo═O twist
157	Ag/B1g	translational rigid MoO4-chain mode, Tb
127	B3g	translational rigid MoO4-chain mode, Tc
114	B2g	translational rigid MoO4-chain mode, Tc

The Raman spectra of the nanodiscs formed by the reduction of the α-MoO₃ nanosheet using sulfur are an excellent match for phonon modes of monoclinic MoO₂ as reported previously in the literature. The sulfide structures on CFP show clear Raman signatures of 2-H MoS₂ including Raman bands at 282, 377, and 404 cm⁻¹, which can be ascribed to modes of E_1g, E_2g¹, and A_1g symmetry, respectively. X-ray photoelectron spectroscopy (XPS) analysis was performed by acquiring Mo 3d, O 1s, and S 2p core level spectra of each product, to investigate the evolution of the chemical composition (FIGS. 4C-4E). The Mo 3d core level spectra are characterized by a distinctive doublet at 233.20 and 236.35 eV ascribed to the binding energies of Mo 3d_5/2 and 3d_3/2 states, respectively; these binding energies are characteristic of hexavalent molybdenum, verifying stabilization of the MoO₃ phase. The O is singlet at 530.95 eV is further in good agreement with the value expected for an inorganic oxide.

Upon reduction with sulfur, the XPS spectra for the nanodiscs shows a substantial alteration of the Mo 3d binding energies to 229.80 and 233.05 eV for the Mo 3d_5/2 and 3d_3/2 states, suggesting the stabilization of a tetravalent oxide of molybdenum. A remnant shoulder at 236.35 eV attributable to the binding energy of Mo 3d_3/2 for hexavalent molybdenum indicates incomplete reduction. Corresponding features in the O is core level spectra at 530.75 and 531.85 eV, can be attributed to MoO₂ and MoO₃ respectively. Furthermore, a distinctive doublet is discernible in S 2p core level spectra at 162.75 and 163.80 eV and can be ascribed to S 2p_3/2 and S 2p_1/2 binding energies, respectively, revealing surface sulfidation forms some MoS₂ even at a temperature of 400° C. The nanosheets after sulfidation at 850° C. show Mo 3d core level spectra at 229.70 and 232.85 eV attributable to binding energies for Mo 3d_5/2 and Mo 3d_3/2, respectively; these values are characteristic of MoS₂. The small shoulder at 226.95 eV is attributed to S 2s. A much more pronounced doublet is observed in S core level spectra at 162.65 and 163.75 eV assigned to S 2p_3/2 and S 2p_1/2 binding energies, respectively. These values verify the sulfidation of MoO₂. A broad O is spectrum with a peak at 532.60 eV is attributed to surface-adsorbed oxygen species.

The enthalpy of hydrogen adsorption on MoS₂ edges has been estimated to be endothermic by ca. 0.08 eV and the extent of H-coverage is limited to one in four atoms at the edges of MoS₂. Reducing the overpotential and increasing catalytic efficiency requires a further decrease of the hydrogen adsorption enthalpy and an increase of the extent of H-coverage. One approach involves polarizing Mo—S bonds at the edges via electronic coupling with electron-donating or withdrawing moieties, ideally other semiconductors. Here, the faceted MoS₂ nanosheets were interfaced with nC₆₀ clusters deposited from solution to prepare hybrid architectures. Upon solution deposition from chlorobenzene solution (nC₆₀ of 0.5 mg/mL), nC₆₀ clusters that are ca. 7 μm in diameter are deposited onto the fibers of CFP. Similar morphologies of nC₆₀ were grown on the MoS₂ nanosheets. Energy dispersive X-ray spectroscopy (EDS) maps acquired at C, Mo, and S elemental edges verified the co-localization of the C₆₀ clusters atop the MoS₂ nanosheets. EDS line profiles further verified the co-localization of nC₆₀ clusters on the MoS₂ basal planes. The Raman spectra of the nC₆₀ cluster and hybrid nC₆₀/MoS₂ architectures are shown in FIG. 5. Distinctive Raman modes of C₆₀ are evidence in both spectra with bands assigned to phonons of A_g(1,2) and H_g(1-8) symmetry. Both MoS₂ and C₆₀ modes are discernible in the hybrid architecture. The coverage of nC₆₀ clusters strongly depends on the concentration of the precursor solution. The size of the nC₆₀ clusters increases with increasing concentration from 0.1 to 2.0 mg/mL. Notably, at a concentration of 0.1 mg/mL, the relatively small nC₆₀ clusters are homogenously distributed throughout the sample; however, upon increasing the concentration to 0.5 mg/mL, the homogeneity is somewhat reduced. Upon increasing the precursor concentration to 2.0 mg/mL, the nC₆₀ clusters are mostly present as large agglomerations that are rather sparsely distributed across the surface. In the concentration range examined, the clusters do not appear to form a continuous percolative network. In order to evaluate quantitatively the coverage of nC₆₀ clusters for a precursor concentration of 0.5 mg/mL, the sample deposited onto CFP has been examined by secondary ion mass spectrometry (SIMS) using 50 keV C₆₀²⁺ ions as the source. The mass spectrum of negatively charged ions emitted from the surface of the clusters deposited on CFP is shown in FIG. 6. The ions emitted were carbon, hydrocarbon clusters, and intact molecular cluster ions of C₆₀ with a yield (number of secondary ions detected per single projectile impact) of 0.08%. The C₆₀ clusters are ca. 7 μm in diameter and cover ca. 2% of the total surface area of the carbon fibers of CFP. Indeed, the SEM and SIMS measurements indicate that optimal concentrations of the C₆₀ solution are necessary to maximize interfacial interactions and prevent crystallization into larger nC₆₀ clusters.

The electrocatalytic HER performance of CFP based samples with nC₆₀ clusters alone, as-prepared 3D MoS₂ nanosheets, and hybrid nC₆₀/MoS₂ architectures were investigated in a 0.5 M aqueous solution of H₂SO₄, using a conventional three-electrode setup. Bare CFP was contrasted as a control and was essentially catalytically inert towards HER. In contrast, nC₆₀ (0.5 mg/mL) clusters on CFP exhibited a finite cathodic current density with an overpotential of 353 mV, reaching a current density of 10 mA/cm² (η₁₀) and a Tafel slope of 169 mV/dec. The 3D faceted MoS₂ nanosheets on CFP showed HER activity with a η10 value of 245 mV and a Tafel slope of 81 mV/dec. Remarkably, interfacing the MoS₂ nanosheets with nC₆₀ resulted in a much lower overpotential. Hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structures had a η10 value of 172 mV and a Tafel slope of 60 mV/dec. These results clearly indicate the synergistic enhancement of HER activity as a result of coupling between nC₆₀ and MoS₂.

A high Tafel slope value of pristine nC₆₀ on CFP (>120 mV/dec) indicated that HER proceeds through the Volmer mechanism, wherein proton reduction yielding hydrogen ad-atoms bound to the active sites represents the rate determining step. In contrast, low Tafel slope values measured for as-prepared 3D MoS₂ and hybrid nC₆₀/MoS₂ (60 and 80 mV/dec) suggest the operation of the Volmer-Heyrovsky mechanism wherein the rate-determining steps involve both proton reduction and hydrogen desorption. It is noteworthy that the HER performance of the 3D array of MoS₂ nanosheets with a high density of exposed edge-sites and their hybrid structures interfaced with nC₆₀ are either higher or comparable to previously reported values for bulk or nanostructured MoS₂. In addition, the hybrid materials reported here possess the advantages of well-defined architectures, conductive substrates, and scalability to centimeter-sized dimensions.

FIGS. 7A-7B contrast the polarization curves of various concentrations of nC₆₀ clusters either deposited directly onto CFP or interfaced with 3D MoS₂ nanosheets on CFP. The cathodic current density of the neat nC₆₀ cluster formed on CFP measured at −0.4 V versus RHE is gradually decreased from 22.6 to 18.5 to 14.6 mA/cm² as the concentration of C₆₀ deposition solution is increased from 0.1 to 0.5 to 2.0 mg/mL. With increasing concentration of C₆₀ solution, the overpotential η₁₀ is also increased from 331 to 353 to 363 mV. As noted above, the hybrid nC₆₀/MoS₂ electrocatalyst prepared using 0.5 mg/mL C₆₀ deposition solution shows the best HER performance with the highest current density (J_0.2V=18.0 mA/cm² at 0.2 V vs. RHE), lowest η₁₀ value (172 mV), and the lowest Tafel slope (60 mV/dec). The nC₆₀ (0.1 mg/mL)/MoS₂ sample (J_0.2V=5.2 mA/cm², η₁₀=245 mV, and Tafel slope=74 mV/dec) exhibits substantially worse performance that is analogous to the 3D MoS₂ nanosheets without C₆₀ hybridization (J_0.2V=5.0 mA/cm², η₁₀=245 mV, and Tafel slope=81 mV/dec). At such low concentrations, the limited nC₆₀ coverage likely limits the extent to which the edge reactivity is modulated. At substantially higher solution concentrations of C₆₀, the HER performance is diminished as well. The nC₆₀ (2.0 mg/mL)/MoS₂ sample is characterized by values of J_0.2V=3.9 mA/cm², η0=273 mV, and a Tafel slope=80 mV/dec. The lack of synergistic enhancement in the latter case can be attributed to the sparse and heterogeneous distribution of nC₆₀ (2.0 mg/mL). The large agglomerations observed suggest that the buckministerfullerene clusters are not effectively interfaced with MoS₂, which likely perturbs the electronic coupling necessary for improved HER performance as described below.

In order to examine the mechanistic basis for the observed modulation of electrocatalytic properties upon interfacing with nC₆₀, electrochemically active surface areas (ECSA) of all the samples were estimated by measuring the double-layer capacitance (C_dl) from cyclic voltammetry (CV) data across a potential range with no Faradaic current. The voltammograms were collected at various scan rates (20-100 mV/s) in the potential range of 0.10-0.30 V versus RHE, where the current is preponderantly due to the charging of the double layer (and not due to proton reduction). FIGS. 8A-8D and 9A-9E depict CV curves acquired for nC₆₀, 3D nanosheets of MoS₂, and hybrid nC₆₀/MoS₂ architectures with various C₆₀ concentrations. The differences (Δj) of anodic and cathodic current densities at 0.20 and 0.23 V versus RHE for each CV plot is shown as a function of the scan rate in FIG. 8D and FIG. 9E. The slope of each Δj versus scan rate plot is equal to a value of 2C_dl. The ECSA were obtained from the ratio of the measured C_dl with respect to the specific capacitance of flat crystalline MoS₂ (ca. 66.7 μF/cm²). The resulting C_dl and ECSA values are displayed as a function of C₆₀ concentration in FIG. 8C. Significantly, the C_dl and ECSA of nC₆₀ on CFP (9A) and hybrid nC₆₀/MoS₂ on CFP (9B and 9C) are respectively lower and higher than those of 3D MoS₂ nanosheets on CFP (9D), and are decreased with increasing C₆₀ concentration. From these results, it can be inferred that the nC₆₀ clusters formed on CFP or MoS₂/CFP are increasingly agglomerated and crystallized with increasing C₆₀ concentration in solution, which is consistent with the morphologies observed by SEM. Furthermore, the C_dl and ECSA of hybrid nC₆₀/MoS₂ appear to be the sum of those of nC₆₀ and 3D MoS₂ nanosheets on CFP. However, an increased concentration of electrochemically active sites does not necessarily translate to increased HER activity since the nC₆₀ clusters alone are much less active as compared to the 3D MoS₂ architectures. The decrease of C_dl and ECSA with increasing C₆₀ concentration of the precursor solution leads only to a slight deterioration of the cathodic current density for nC₆₀/CFP and the Tafel slope is mostly preserved, indicating that the changes in C_dl, ECSA, and the resulting number of active sites do not fundamentally alter the HER mechanism (i.e., Volmer reaction in the neat C₆₀) and rate (FIG. 9E). These two sets of observations suggest that the improved HER performance observed for the hybrid nC₆₀ (0.5 mg/mL)/MoS₂ electrocatalyst is derived from an intrinsic enhancement of the inherent catalytic activity of MoS₂ for HER rather than an increase in the number of active sites upon C₆₀ deposition.

The turnover frequency (TOF), defined as the number of H₂ molecules evolved per active site per unit time, is an essential parameter to contrast the inherent catalytic activity of different systems. The TOF can be calculated using the expression TOF=JN_A/2Fn(ECSA), where J is the current density, N_A is Avogadro's number, 2 represents the stoichiometric number of electrons consumed at the electrode during HER, F is Faraday's constant, n is the number of active sites (1.164×10¹⁵ cm²) on a flat surface of crystalline MoS₂, and ECSA is the electrochemically active surface area of the electrode. FIG. 7D plots the TOF (per active site) of the 3D MoS₂/CFP and hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structure prepared on CFP in the applied potential range of −0.1 to −0.3 V versus RHE; in this regime, the HER is controlled by electrode kinetics with minimal influence from other effects. The measured TOF of the hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structure at −0.2 V (2.33 H₂/s per active site) is nearly twice as high as that of 3D MoS₂ nanosheets (1.28 H₂/s per active site) on CFP. These results highlight the synergistic enhancement of the inherent catalytic activity of the edge sites of the MoS₂ nanosheets upon nC₆₀ hybridization. The 3D hybrid architectures constructed on mesoporous CFP clearly represent viable electrocatalysts.

In order to further investigate a possible origin of the enhanced HER performance observed for the hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structure, electrochemical impedance measurements were performed at various potentials between 10 and −250 mV by sweeping the frequency from 200 kHz to 100 mHz with an AC amplitude of 10 mV. FIG. 10A shows the Nyquist plots of as-prepared 3D MoS₂ nanosheets and hybrid nC₆₀/MoS₂ architectures prepared on CFP measured at −150 mV vs. RHE. The Nyquist plots are fitted to an equivalent circuit model (inset of FIG. 10A) comprising the following elements: an ohmic resistance (R_s), a charge-transfer resistance (R_ct), constant phase element (Q), and a Warburg constant (W). The obtained R_ct values are plotted as a function of potential in FIG. 10B for nC₆₀, as-prepared 3D MoS₂ nanosheets, and hybrid nC₆₀/MoS₂ architectures prepared on CFP. The kinetics of electrocatalytic HER on the different electrode samples can be evaluated based on their respective R_ct values with a lower R_ct value corresponding to a faster reaction rate. The resulting R_ct values reveal a similar trend as the η₁₀ and Tafel slope values deduced from the cathodic current density in polarization curves (FIGS. 7A-7B and 8A-8B); specifically, the R_ct values of 3D MoS₂/CFP and hybrid nC₆₀/MoS₂ prepared on CFP are nearly two orders of magnitude lower than those of nC₆₀/CFP. Furthermore, the lowest R_ct values are obtained for the hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structure. Taken together, these results suggest that the enhanced HER performance observed upon interfacing with nC₆₀ derive in large measure from the increased conductance of the hybrid constructs when C₆₀ is appropriately interfaced with MoS₂.

Indeed, recent ab initio density functional theory calculations of C₆₀/MoS₂ constructs are particularly instructive in understanding the nature of the interface formed between these two semiconductors. Prior studies have determined that the lowest energy configuration for these heterostructures corresponds to the hexagonal rings of C₆₀ situating directly above S sites on the basal planes of MoS₂ resulting in buckministerfullerene molecules being able to rotate freely on the surface. This configuration yields a Type-II interface with charge depletion from C₆₀ and charge accumulation on MoS₂ estimated to be ca. 0.055 e⁻ per C₆₀ unit. This directional charge transfer is thought to be key to the reduced resistance of the hybrid constructs. Indeed, the Type-II alignment has been further verified by recent theoretical and experimental studies of C₆₀/MoS₂ hybrids. Studies have predicted that the valence band edge of MoS₂ (−4.5 eV) resides lower than that of C₆₀ (−3.8 eV), resulting in charge transfer and electron accumulation on MoS₂ when the two semiconductors are interfaced. Upon application of an electric field, the steadily increasing electron density in MoS₂ reduces the junction-barrier height, further allowing facile electron tunneling and transport and giving rise to conductive pathways along the interfaces of the resulting C₆₀/MoS₂ p-n heterojunctions. Therefore, based on the measured TOF, deduced resistance values, and charge transfer resistance values extrapolated from EIS data, the enhanced HER performance of the hybrid nC₆₀ (0.5 mg/mL)/MoS₂ structure likely derives from a charge transfer mechanism. The nC₆₀ clusters donate electron density to MoS₂ and give rise to a conductive interfacial layer that is much more effective at charge transport as compared to the relatively insulating basal planes of MoS₂. Such charge transfer may also polarize the Mo—S bonds reducing the enthalpy of hydrogen adsorption. Notably, this mechanism, essentially invoking interfacial doping of MoS₂, is quite distinct from hybrid MoS₂/carbon nanotube and MoS₂/graphene heterostructures wherein the latter components actually form conductive pathways for electron transport between the CFP electrodes and the catalytically active edge sites, thereby mitigating the poor transport characteristics of the basal planes of 2H—MoS₂.

The role of interfacial doping is further underscored by the dependence of HER performance on the concentration of the C₆₀ precursor solution and the morphology of the nC₆₀ clusters. Agglomerated C₆₀ clusters that are homogeneously dispersed across the MoS₂ basal planes will be ineffective at modulating the electronic structure of MoS₂ through electron transfer. To assess the long-term stability of nC₆₀ (0.5 mg/mL)/CFP, 3D MoS₂/CFP, and hybrid nC₆₀ (0.5 mg/mL)/MoS₂ on CFP as electrocatalysts for HER, CV sweeps have been performed for 1000 cycles in a 0.5 M aqueous solution of H₂SO₄ in the range between −0.2 and 0.2 V versus RHE at a scan rate of 100 mV/s. The polarization curve for the 3D MoS₂ on CFP is almost exactly superimposable upon the initial data suggesting no degradation in performance (FIG. 7B). In contrast, after 1000 cycles, the hybrid nC₆₀/MoS₂ catalysts show a slight increase of the overpotential η₁₀ to 181 mV and the Tafel slope is changed to 65 mV/dec. The observed changes are suggestive of the partial loss of C₆₀ clusters upon prolonged electrocatalytic cycling, which likely disrupts some of the interfacial charge transfer and thereby disrupts charge transport between the CFP substrate and active catalytic edges.

The electrocatalyst for hydrogen evolution reaction will now be illustrated by the following examples, which do not limit the scope defined by the appended claims.

Example 1

Preparation of MoS₂ Nanosheets, C₆₀ Clusters, and their Hybrid Structures

The CVD processes were performed using a 1-inch-diameter horizontal cold-wall quartz tube furnace equipped with gas flow controls. In the first step to prepare MoO₃ nanosheets, 15.0 mg of MoO₃ powder (Sigma-Aldrich, purity >99.5%) was placed within an alumina boat, which was placed at the center of tube. A bare CFP substrate (Toray Paper 120) with dimensions of 7 cm×1 cm size was placed downstream from the MoO₃ source at a distance of 15 cm from the alumina boat. After an initial Ar purge for 30 min, the MoO₃ powder was heated to 850° C. at a ramp rate of 20° C./min and transported under a 68.3 sccm Ar flow at 1 atm. After holding at 850° C. for 10 min, the furnace was allowed to cool naturally to room temperature. Subsequently, MoO₃ nanosheets integrated onto ca. 2 cm² areas of the CFP were recovered. Such nanosheets were reproducibly formed at a distance of ca. 18-20 cm from the alumina boat. The MoO₃-deposited CFP was cut to dimensions of 4 cm×1 cm thereby preserving margins on all sides. This substrate was then placed at the center of the tube furnace but downstream at a distance of 20 cm from an alumina boat containing 100 mg of elemental sulfur powder (Alfa Aesar, 99.5% purity). Next, after purging with Ar, the reactor was heated to a temperature of 400° C. at a ramp rate of 20° C./min under an Ar flow of 100 sccm at 1 atm to facilitate the reaction of sublimed sulfur with the MoO₃ nanosheets. After holding at 400° C. for 20 min, the furnace was then naturally cooled to room temperature. Subsequently, a final CVD step was performed by replacing the spent sulfur in the alumina boat with an additional 100 mg of fresh elemental sulfur. The reactor was heated to 850° C. at a ramp rate of 20° C./min under a 100 sccm flow of Ar at 1 atm for 20 min after which the furnace was allowed to naturally cool to room temperature. The CFP paper was then removed from the center of the furnace for characterization and electrocatalytic evaluation.

In order to prepare hybrid nC₆₀/MoS₂ structures on CFP, C₆₀ powder (Strem Chemicals Inc., 99.9% purity) was dissolved in chlorobenzene at concentrations of 0.1, 0.5, and 2.0 mg/mL, respectively. The MoS₂/CFP (as well as bare CFP as a control) were immersed within the chlorobenzene solutions for 1 min and then removed. Subsequently, the samples were annealed at 160° C. for 10 min under a flowing Ar atmosphere.

Example 2

Structural Characterization

The morphology of the prepared materials was examined by field-emission scanning electron microscopy using a JEOL JSM-7500F instrument. The edge-sites of MoS₂ flakes harvested from the MoS₂/CFP sample by ultrasonication for 1 h in toluene were examined by high-resolution transmission electron microscopy using a JEOL JEM-2010 instrument operated at an accelerating voltage of 200 keV. Phase assignment was performed with the help of X-ray diffraction using a Bruker D8-Advance instrument equipped with a Cu Kα source (λ=1.5418 Å) as well as by Raman microprobe analysis using a Jobin-Yvon HORIBA LabRAM HR800 instrument coupled to an Olympus BX41 microscope. Raman spectra were collected with excitation from the 514.5 nm line of an Ar-ion laser; the laser power was kept below 10 mW to minimize photooxidation. The chemical composition and oxidation states of MoO₃, MoO₂, and MoS₂ prepared on CFP were investigated by X-ray photoelectron spectroscopy (XPS, Omicron XPS) with Mg Kα radiation (1253.6 eV). Energy calibration was achieved by setting the C1s line from adventitious hydrocarbons to 284.8 eV. The elemental composition of the C₆₀ clusters deposited on CFP and MoS₂-deposited CFP was examined by energy-dispersive X-ray spectroscopy (EDS) coupled to the FE-SEM system. The coverage of C₆₀ (0.5 mg/mL) clusters deposited on CFP was measured on a custom-made secondary ion mass spectrometer (SIMS) using C₆₀²⁺ projectiles with an energy of 50 keV as the source.

Example 3

Electrochemical Characterization

The HER performance of the prepared materials was evaluated using a three-electrode cell with the help of a Bio-Logic potentiostat (SP-200). All of the measurements were performed in a 0.5 M aqueous solution of H₂SO₄ purged with N₂ gas. MoS₂/CFP, C₆₀/CFP, and the hybrid structures prepared on CFP were individually used as the working electrodes. A saturated calomel electrode (SCE) and a Pt plate were used as reference and counter electrodes, respectively. The potential versus SCE (E_SCE) was converted to the potential versus the reversible hydrogen electrode (RHE) (E_RHE) using the relation E_RHE=E_SCE+0.279 V. Polarization curves for HER were measured using linear sweep voltammetry (LSV) in the range between 0.1 and −0.4 V versus RHE at a scan rate of 8 mV/s. The polarization curves were corrected for the ohmic potential drop (iR) losses, where R is the series resistance of the electrochemical cell as determined by electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were performed in the range between 200 kHz and 50 mHz using an AC amplitude of 25 mV. The EIS measurements for obtaining the charge-transfer resistance (R_ct) values were performed at various potentials between 10 and −250 mV by sweeping the frequency from 200 kHz to 100 mHz using an AC amplitude of 10 mV. In order to estimate the electrochemically active surface area (ECSA) of the samples, the double-layer capacitance (C_dl) of the samples was determined by cyclic voltammetry (CV) in the potential range of 0.10-0.30 V versus RHE at scan rates between 20-100 mV/s.

It is to be understood that the electrocatalyst for hydrogen evolution reaction is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed electrocatalyst for hydrogen evolution reaction.

Claims

1. An electrocatalyst for hydrogen evolution reaction, comprising:

a carbon fiber paper substrate; and

a plurality of nanosheets of MoS2 on the carbon fiber substrate, the nanosheets having a plurality of catalytically active edge sites along basal planes thereof.

2. The electrocatalyst for hydrogen evolution reaction according to claim 1, wherein the basal planes comprise faceted edges.

3. The electrocatalyst for hydrogen evolution reaction according to claim 2, wherein the basal planes comprise collapsed edges.

4. The electrocatalyst for hydrogen evolution reaction according to claim 1, wherein the nanosheets further comprise spherical fullerene nanoclusters.

5. The electrocatalyst for hydrogen evolution reaction according to claim 4, wherein the spherical fullerene nanoclusters are about 7 μm in diameter.

6. The electrocatalyst for hydrogen evolution reaction according to claim 5, wherein the spherical fullerene nanoclusters cover about 2% of the total surface area of carbon fibers of the carbon fiber paper substrate.

7. The electrocatalyst for hydrogen evolution reaction according to claim 1, wherein the nanosheets are dispersed across an area of about 2 cm2 of the carbon fiber paper substrate.

8. A method of making an electrocatalyst for hydrogen evolution reaction, comprising the steps of:

depositing nanosheets of MoO3 onto a carbon fiber paper by chemical vapor deposition;

reducing the nanosheets of MoO3 to nanosheets of MoO2 by reaction with sublimed sulfur; and

sulfiding the nanosheets of MoO2 to form nanosheets of MoS2 integrated with the carbon fiber paper substrate, the nanosheets of MoS2 integrated with the carbon fiber paper substrate providing the electrocatalyst for hydrogen evolution reaction.

9. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 8, wherein the nanosheets of MoO3 deposited onto the carbon fiber paper are about 1-2 μm in lateral dimensions.

10. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 8, wherein the reaction with sublimed sulfur occurs at a temperature of about 400° C.

11. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 8, wherein the sulfiding of the nanosheets of MoO2 occurs at a temperature of about 850° C.

12. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 8, wherein the nanosheets of MoS2 comprise a plurality of catalytically active edge sites along basal planes thereof.

13. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 8, further comprising:

immersing the nanosheets of MoS2 integrated with the carbon fiber paper substrate in a solution of spherical fullerenes (nC60); and

annealing the nanosheets of MoS2 integrated with the carbon fiber paper substrate after immersion in the fullerenes.

14. An electrocatalyst for hydrogen evolution reaction prepared according to the method of claim 8.

15. A method of making an electrocatalyst for hydrogen evolution reaction, comprising the steps of:

depositing nanosheets of MoO3 onto carbon fiber paper by chemical vapor deposition;

reducing the nanosheets of MoO3 to nanosheets of MoO2 by reaction with sublimed sulfur;

sulfiding the nanosheets of MoO2 to form nanosheets of MoS2 integrated with the carbon fiber paper substrate, the nanosheets of MoS2 integrated with the carbon fiber paper substrate providing the electrocatalyst for hydrogen evolution reaction;

immersing the nanosheets of MoS2 integrated with the carbon fiber paper substrate in a solution of spherical fullerenes (nC60); and

annealing the nanosheets of MoS2 integrated with the carbon fiber paper substrate after immersion in the fullerenes.

16. The method of making an electrocatalyst for hydrogen evolution reaction, according to claim 15, wherein the nanosheets of MoO3 deposited onto the carbon fiber paper are about 1-2 μm in lateral dimensions.

17. The method of making an electrocatalyst for hydrogen evolution reaction, according to claim 15, wherein the reaction with sublimed sulfur occurs at a temperature of about 400° C.

18. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 15, wherein the sulfiding of the nanosheets of MoO2 occurs at a temperature of about 850° C.

19. The method of making an electrocatalyst for hydrogen evolution reaction according to claim 15, wherein the nanosheets of MoS2 comprise a plurality of catalytically active edge sites along basal planes thereof.

20. An electrocatalyst for hydrogen evolution reaction prepared according to the method of claim 15.