MoS2 has attracted significant attention as a non-platinum group electrocatalyst for the hydrogen evolution reaction (HER). There have been extensive efforts demonstrating that by doping MoS2 with various transition metals, such as Co, the HER activity of the catalyst is enhanced. In particular, this work has shown that various cobalt sulfide phases can act as a co-catalyst with MoS2. Here, we report on the electrodeposition of a c-CoSx/MoS2 heterostructure catalyst for the HER reaction in both acidic and alkaline conditions. Using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning electron microscopy, it is demonstrated that depending on the precursor concentrations, various morphologies, grain size, and c-CoSx phases can be achieved, all of which have an impact on the activity and stability of the c-CoSx/MoS2 catalysts. The most promising catalyst composition demonstrated excellent stability in both acidic and alkaline conditions with low overpotentials to reach 10 mA cm−2 of 112 mV and 60 mV and with Tafel slopes of 113 mV dec−1 and 81 mV dec−1, respectively. This report demonstrates that the c-CoSx/MoS2 heterostructure is one of the most catalytically active materials for HER, especially in alkaline conditions.
As the global demand for energy consumption increases, there will be a need to transition away from conventional energy sources. To meet this demand while mitigating further contributions to irreversible climate change, there is a need for sustainable and renewable energy sources. 1 One such technology is the generation of hydrogen via water splitting as it is a sustainable, secure, and clean alternative energy carrier. 2 However, to facilitate and fully realize this technology, catalysts with sufficient catalytic activity, efficiency, low-cost, and environmentally friendly materials must be developed. 3–5 Among the most active and best catalysts are noble metals, such as platinum, however their use is limited due to their scarcity and cost. 3,6
In this context, alternative catalysts based on naturally abundant materials, such as transition metal sulfides, phosphides, nitrides, and carbides are being strongly pursued. 7–19 One prominent example is MoS2, an electrocatalyst that features a low Gibbs adsorption energy for H+, ∆GH 0,ads, which correlates with the ability to facilitate the hydrogen evolution reaction (HER). 20–25 However, only along the edge planes of the crystalline MoS2 is this low ∆GH 0,ads observed, thereby limiting the density of catalytically active sites. Recent efforts have attempted to increase the number of active sites in MoS2 and other transition metal dichalcogenides via several approaches such as: 3D substrates, 26 vertically aligned synthesis, 27,28 the creation of sulfur defects, 29–33 increasing interlayer spacing, 24,25 or heteroatom doping. 25,34,35
In recent years, great attention has been given to other transition metal sulfides, such as Co, Ni, and Fe. Among the most successful have been those of Co sulfides, showing strong electrocatalytic activity towards HER in both acidic and alkaline conditions. 36–39 Bonde et al. demonstrated that doping Co-atoms on the edges of MoS2 greatly enhanced the HER activity, owed to the decrease of adsorption energy of hydrogen. 40 Another example showed the synergistic effect between Co9S8 and MoS2 towards HER in alkaline environments. 41 Finally it has been shown that the interface between cobalt sulfides and molybdenum sulfide is critical to the HER performance because the formation of nano-interfaces facilitates enhanced charge transfer between Co and Mo through S-linkage. 42
Many catalysts for HER require a polymer binder to anchor the catalyst to the working electrode, increasing resistance and hindering active sites or causing long-term stability issues. 43–45 Therefore, direct growth of the catalyst on the current collector can increase the HER performance of the catalyst towards HER. Carbon paper exhibits high conductivity, good mechanical strength, inexpensive, and is flexible, making it an excellent candidate for the substrate. In addition, carbon paper is a porous material with a three-dimensional morphology, allowing more active sites per unit area. Additionally, it has been shown that MoS2 on carbon paper provides higher catalytic activity when compared to a traditional flat substrate. 46–48
In this work, we demonstrated a simple electrochemical deposition and annealing method to fabricate and control the phase of various CoSx structures alloyed with MoS2 on carbon cloth for use as an HER catalyst. Several compositions were tested to elucidate the optimal conditions, as well as determine the underlying factors that influence the overall HER properties. By controlling the electrolyte bath parameters, such as the cobalt concentration, various morphologies, such as platelets, clusters, and nanostrips, and crystal structures were measured and exhibited drastically different HER properties. It was observed that while all samples were stable and exhibited promising HER properties in acidic conditions, only the equimolar 75–75 sample was stable in an alkaline environment, however, it exhibited extremely high activity. This performance was explored with a variety of techniques and together, these results highlight the synergistic effects among various cobalt sulfide species and MoS2 towards HER.
Experimental
Electrolyte preparation
The Co-Mo sulfide catalysts were electrodeposited from an electrolyte containing precursors of Co, Mo, and S. The electrolyte was prepared from cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O), sodium molybdate dihydrate (Na2MoO4 · 2H2O), and thiourea (CH4N2S). The sodium molybdate dihydrate concentration was held at 75 mM and the thiourea concentration was held at 0.2 M while the cobalt nitrate hexahydrate concentration was varied from 5 mM up to 100 mM. The sample sets can be seen in Table I. Samples will be referred by their Co–Mo precursor concentrations in mM, e.g., 25–75 will be in reference to 25 mM Co(NO3)2 · 6 H2O–75 mM Na2MoO4 · 2H2O.
Electrodeposition
Electrochemical deposition was carried out using a G&G PAR Model 273 A potentiostat in a standard three-electrode setup containing a graphite rod as a counter electrode and saturated Hg/Hg2SO4 (MSE, +0.64 V vs RHE) as the reference electrode. 49 The substrate was binder free carbon paper and acted as the working electrode. The carbon paper substrates were washed with distilled water prior to deposition. The films were deposited under potentiostatic conditions at −1.5 V vs MSE with varying Co precursor concentrations for 10 min. A reference sample of CoSx was deposited in the same manner, but without Mo precursor in the electrolyte.
Post-deposition processing
To crystallize the samples, annealing was carried out in an MTI OTF-1200X tube furnace. Annealing was performed by heating the sample to 450 °C at a ramp rate of 14 °C min−1 where it was held for 60 min before being naturally cooled back to ambient temperature. All anneals took place in the presence of flowing forming gas (5% H2/95% Ar) with 3 grams of elemental sulfur upstream of the substrate. Prior to the anneal, the chamber was purged with excess forming gas for 45 min to minimize the presence of any impurities within the furnace. Natural cooling was carried out under the forming gas prior to the samples being removed from the furnace.
Electrochemical characterization
Similar to our previous study, electrochemical characterization of the prepared films for HER was performed by a three-electrode configuration using a BioLogic SP-150 potentiostat. 49 The c-CoSx-MoS2, graphite rod, and Ag/AgCl were used as the working, counter, and reference electrode, respectively. All experiments were carried out at room temperature and in fresh 0.5 M H2SO4 for the acidic conditions and fresh 1 M KOH for the alkaline conditions. The electrochemical potential difference between the reversible hydrogen electrode (RHE) and the reference electrode is given by E(RHE) = E(Ag/AgCl) + 0.197 V + 0.059*pH. All potentials were iR-compensated to 85% with the built-in program. The polarization curves of HER were measured using linear sweep voltammetry (LSV) at a starting potential of open circuit potential and swept at a rate of 10 mV s−1 with a cutoff current density of 40 mA cm−2. The shown LSV data is after 10 sweeps unless otherwise stated.
Materials characterization
Raman spectroscopy was collected using a Renishaw Raman instrument with a 514 nm laser excitation, 1800 grating, 20x objective magnification. X-ray diffraction was collected with a Malvern-Panalytical Empyrean diffractometer with a Cu anode (1.54 Å wavelength) in a Bragg-Brentano geometry with a reflection-transmission spinner sample holder. X-ray photoelectron spectroscopy (XPS) spectra were collected from a Scienta Omicron R3000 using monochromatic Al Kα emission with an excitation energy of 1486.7 eV and a pass energy of 50 eV. The features of the Mo 3d were deconvoluted with the doublet of 3d5/2 and 3d3/2 with the area ration of 3:2 and a binding energy difference of 3.13 eV. The spectrum of the S 2p was deconvoluted with the doublet of 2p3/2 and 2p1/2 with an area ratio of 2:1 and a binding energy difference of 1.18 eV. The Co 2p3/2 was deconvoluted without the Co 2p1/2. SEM was taken on a FEI Quanta 650 Scanning Electron Microscope in secondary electron mode with an accelerating voltage of 15.00 kV and a spot size of 3.0.
Results and Discussion
The CoSx component was grown on carbon paper using a facile electrodeposition with the species pertaining to the CoSx deposition being Co(NO3)2 and CH4N2S. During the deposition, the OH− ions generated from reduced NO3 − ions react with CH4N2S to form S2− ions. The following reactions can describe the process
Based on the charges of the Co and S ions, one would expect to form CoS, as found in Liu et al. study. 50 However, a cyclic voltammetry deposition method was utilized in their study, whereas in this study a potentiostatic deposition method and annealing was utilized, resulting in a different stoichiometry. In regards to MoS2, the primary formation occurs during the annealing step, as the MoO4 2− ion has a high energy of formation (−1354 kJ mol) it cannot be easily separated into Mo ions to react with the S ions via electrodeposition as it far exceeds the cathodic potential required for the discharge of hydrogen. 51 Therefore, the formation of MoS2 occurs during the annealing step when the excess sulfur reacts with the deposited molybdenum oxide.
Morphology and structure
To understand the structural and morphological features of the catalysts, SEM images were collected. In Fig. S1, a low magnification image shows uniform coverage of the carbon paper substrate. The lower Co:Mo ratio samples showed less coverage of the carbon paper, notable 5–75, due to the difficulty in depositing only Molybdenum. 52 The high magnification SEM images revealed that the morphology drastically changed depending on the concentration ratio of the Co and Mo precursors as seen in Fig. 1. In the sample with the lowest Co concentration, 5–75, thin sheets with small clusters are observed along the edges. When the Co was increased to 15–75, the sheets are no longer observed, and instead large plate-like structures are formed showing clustering on the surface. In the 25–75 sample, the plates grew, however the surface was largely featureless and little clustering was observed. The morphology continued to change in 50–75 as the large plate like structures are no longer observed and instead a much rougher, scaled surface is formed with clusters of sheets on the surface. The 75–75 sample lacked the scaled surface and instead exhibited round, textured structures with long, flat, and wide features protruding perpendicular to the surface. Finally, the 100–75 sample is even more textured than 75–75 and lacks the flat sheets protruding from the surface.
XRD patterns shown in Fig. 2 provide crystallographic information on the catalysts. The Bragg diffraction hump located at 2θ ≈ 14.5° was indexed to the (002) plane of hexagonal MoS2. 53 The peaks located at 2θ ≈ 29.5°, 39.4°, and 47.5° are indexed as the (311), (331), and (333) planes of cubic Co9S8 respectively. 54 The diffraction peaks associated with the (200) and (210) planes of cubic CoS2 were observed at 2θ ≈ 32.4, and 36.3° respectively. 55 It is noticed that the MoS2 (002) diffraction peak increases in intensity as the Co concentration increases. This increase in MoS2 is being attributed to the co-deposition of Mo with Co, in which the Co induces more Mo to be deposited during the electrodeposition process. 52 This is the case as the redox potential of Mo metal is quite negative, preventing Mo from depositing without the co-deposition process. 52 Additionally, the diffraction peaks decrease in intensity and sharpness as the Co concentration increases, indicating that the crystallite size is decreasing. As the Co precursor concentration increases, the diffraction peaks associated with Co9S8 decrease in relative intensity while the diffraction peaks associated with CoS2 increase in relative intensity.
To further elucidate the crystalline structure of the grown materials, Raman spectroscopy was performed on all materials as shown in Fig. 3. The peaks at roughly 384 cm−1 and 407 cm−1 are attributed to the in-plane E2g mode and the out-of-plane A1g mode, respectively. 56 The peaks at 289 cm−1, 318 cm−1, and 393 cm−1 are ascribed to the Eg, Tg(1), and Ag modes of CoS2 with the shoulder at roughly 418 cm−1 the Tg(2) mode. 57 The peak found at 680 cm−1 is attributed to the primary vibration mode of Co9S8. 58 The primary peaks for the different species are labelled with a dashed line in Fig. 3. In the CoSx sample, both peaks associated with CoS2 and Co9S8 are present. However, when in the presence of Mo species, the Co9S8 vibration modes are extinguished and are not seen at any Co/Mo ratio. At low Co/Mo ratio, as in 5–75 and 15–75, the MoS2 features dominate the spectra. As the Co/Mo ratio increases to 25–75 and beyond, the CoS2 features dominate the spectra as can be seen in more detail in Fig. S5.
Table I.List of Sample ID and respective precursor concentrations.
| Sample ID | Co(NO3)2 precursor concentration | Na2MoO4 precursor concentration |
|---|---|---|
| 5–75 | 5 mM | 75 mM |
| 15–75 | 15 mM | 75 mM |
| 25–75 | 25 mM | 75 mM |
| 50–75 | 50 mM | 75 mM |
| 75–75 | 75 mM | 75 mM |
| 100–75 | 100 mM | 75 mM |
Chemical analysis
To determine elemental and chemical composition of the 75–75 catalyst, XPS was conducted. However, depending on where the sample was scanned, different spectra would appear as can be seen in Figs. S6, S7, and S8. While different chemical composition ratios are present, the same chemical species were present in all samples. The most distinct one was selected for deconvolution, as seen in Fig. 4. The Mo 3d spectra can be deconvoluted into distinctive peaks of a single double (3d5/2 and 3d3/2) of Mo4+ and singlets of the various overlapping S 2 s states that occur in the same energy range. 49,59,60 The Mo 3d peaks were deconvoluted with a binding energy difference of 3.13 eV between the 3d5/2 and 3d3/2 and with a fixed intensity ratio of 3:2. In the sulfur spectrum, the presence of S2− and S2 2− was detected; this is consistent with previous literature. 59,61 The sulfur spectrum was deconvoluted with two doublets, with the constraints of an energy difference of 1.18 eV between the 2p3/2 and 2p1/2 and with an intensity ratio of 2:1. The cobalt 2p3/2 spectrum was deconvoluted with two primary features at 777.96 eV, 778.6 eV corresponding to Co3+ and Co2+ respectively, and with a large satellite peak at higher binding energy. 39,62
Electrochemical properties
The impact of the Co/Mo precursor ratio on the HER properties of the catalysts in acidic conditions, polarization experiments were performed, shown in Fig. 5. The overpotential required to reach the cathodic current density of 10 mA cm−2 (η10) was used to compare the catalytic activity of the HER catalysts and these are shown in the subset of Fig. 5. As seen in Fig. 5, the 75–75 sample performed the best by exhibiting the lowest overpotential (112 mV) to reach 10 mA cm−2. This performance was greater than that of either CoSx or MoS2 and equal or superior than that of similar cobalt sulfide-based catalysts as seen Table SI. 63 The other values of the catalysts η10 can be seen in the Fig. 5 subset plotted against their Co/(Co+Mo) as well as in Table II.
Table II.Electrochemical values for the various catalysts, (—) indicates the sample was unstable in indicated conditions.
| Sample | Co/(Co+Mo) | Acidic η10 (mV) | Acidic b (mV/dec) | Alkaline η10 (mV) | Alkaline b (mV/dec) |
|---|---|---|---|---|---|
| 5–75 | 0.282 | 190 | 157 | — | — |
| 15–75 | 0.482 | 159 | 106 | — | — |
| 25–75 | 0.597 | 149 | 84 | — | — |
| 50–75 | 0.626 | 123 | 118 | — | — |
| 75–75 | 0.635 | 112 | 113 | 60 | 81 |
| 100–75 | 0.653 | 121 | 114 | — | — |
| MoS2 | 0 | 232 | 123 | — | — |
| CoSx | 1 | 175 | 87 | — | — |
HER in alkaline medium is also of high interest due to the widespread usage of alkaline water electrolysis in industry, as well reducing the corrosion of electrolytic cells found in acidic cells. Polarizations in 1 M KOH were carried out to investigate the Co-Mo sulfide catalysts affinity towards HER in basic condition. The sample 75–75 showed good stability, as can be seen in Fig. 6 Additionally, 75–75 showed an extremely low η10 value of 60 mV, one that competes with state-of-the-art catalysts in alkaline medium. 64 This overpotential is among the best reported in current literature as seen in Table SII. All but the 75–75 sample degraded quickly per scan within the alkaline medium. The subset of Fig. 6 shows the degradation of the 25–75 sample as an example of the instability found in other samples. The origin of this lack of stability will be discussed in later sections.
The Tafel slope (b), i.e., the slope of the linear, low overpotential regime in the Tafel plot, is the overpotential required to increase the current density by one order of magnitude. As such, it is optimal to have a catalyst with a lower Tafel slope for HER applications. The overall Tafel slope is dependent on the mechanistic pathway where the HER mechanism involves three known elementary reaction steps in acidic media:
and three known elementary reaction steps in alkaline media:
where the reactions are strongly dependent on the inherent surface chemistry and electronic structure of the catalyst. In both cases, the Volmer reaction will be followed by either the Tafel or Heyrovsky reaction; therefore, there are two possible reaction pathways; Volmer-Tafel and Volmer-Heyrovsky. If the Volmer adsorption reaction is the rate limiting step, a Tafel slope of 120 mV decade−1 would be expected. If the Tafel or Heyrovsky recombination's are rate limiting, the Tafel slope would be between 30–40 mV decade−1. The Tafel slopes for the Co-Mo sulfide catalysts are listed in Table II. and suggest a Volmer-Heyrovsky mechanism, consistent with literature. 62,65,66
Discussion
As the Raman data demonstrates, the samples with more CoS2 like structures had the highest performance in acidic conditions compared to those with a more MoS2 like spectra. This is to be expected as previous literature has demonstrated that cobalt sulfide based materials superior catalytic performance over crystalline MoS2. 18,21,24,63,65–67 However, while the samples 25–75 through 100–75 have similar Raman spectra, they have very different catalytic activities in acidic conditions and stability in alkaline conditions.
Diffraction patterns of various catalysts obtained via XRD can help elucidate the origin of these performances as the XRD between the samples are quite different. At high cobalt fraction, the crystallite size of the cobalt sulfide phases decreases, allowing an increase in the concentration of linking disulfide anions, the catalytically active sites, to be exposed to the electrolyte. 68 Also, as smaller grains allow for more of these nano-interfaces, a higher density of active sites which led to an improvement of the electrocatalytic activity was observered. 42,62,69 Additionally, the increase in the MoS2 phase with the higher Co/Mo ratios likely contributes to this increase in overall catalytic activity of the catalyst due to their synergistic properties, such as better charge transfer and facilitation of H− containing species adsorption-desorption at the active sites. 62,70,71
However, this does not directly explain the difference in stability and overall performance of 75–75 compared to the rest of the synthesized catalysts in alkaline conditions. As the only major difference in the high Co/Mo ratio catalysts is the morphology, we suggest that this stability is related to the morphology and crystallography of the 75–75 catalyst. As seen in Fig. 1, the 75–75 sample is the only catalyst to exhibit the wide, flat, and protruding features. In an effort to determine the composition of the strands, SEM and EDS were carried out as shown in Fig. S4. However, due to the large penetration depth with the used accelerating voltage, the sample looks hom*ogenously distributed in Co, Mo, and S. We suggest that at low Co concentration, the stabilizing CoSx in alkaline conditions show low concentration, as per the SEM EDS, and at high Co concentration the grains are too small and are more easily broken down due to the more accessible linking disulfide anions. 68
Conclusions
This study developed a facile and scalable electrodeposition technique and annealing methods for catalytically active CoSx/MoS2. These catalysts are extremely active for HER in acidic conditions and one of the best performances in alkaline conditions, with overpotentials to reach 10 mA cm−2 of 112 mV and 60 mV and with Tafel slopes of 113 mV dec−1 and 81 mV dec−1, respectively. The 75–75 exhibited excellent stability in both acidic and alkaline conditions and performed superior to all other samples. This has been attributed to the synergistic properties evolving through the intimate growth of alloys between Co and Mo sulfides, such as better charge transfer and facilitating the adsorption-desorption of hydrogen at the interface of the electrode/electrolyte, as well as increased nano-interfaces between the CoSx and MoS2 features. In addition, an unusual morphology was found in the 75–75 catalysts that has not been reported on before. Overall, these scalable catalysts are some of the highest performing HER catalysts while remaining noble-metal free.
Acknowledgments
This work was supported by the University of Virginia's Nanoscale Materials Characterization Facility and was partially funded by the Virginia Space Grant Consortium Graduate Research STEM Fellowship. The authors would like to acknowledge Zachary Piontkowski and Thomas Beechem for their assistance in Raman interpretation. Author Contributions, The manuscript was written through contributions of all authors.
