Crystallographic Facet Dependence of the Hydrogen Evolution

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Crystallographic Facet Dependence of the Hydrogen Evolution Reaction on CoPS: Theory and Experiments Tao Wu, Michael L Stone, Melinda J. Shearer, Matthew J. Stolt, Ilia A. Guzei, Robert J Hamers, Ruifeng Lu, Kaiming Deng, Song Jin, and Jordan R. Schmidt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03167 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Crystallographic Facet Dependence of the Hydrogen Evolution Reaction on CoPS: Theory and Experiments Tao Wu,†,‡,∥ Michael L. Stone,†,∥ Melinda J. Shearer,† Matthew J. Stolt,† Ilia A. Guzei,† Robert J. Hamers,† Ruifeng Lu,‡ Kaiming Deng,‡ Song Jin,*,† J. R. Schmidt*,† †

‡

Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, Wisconsin 53706, USA.

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, P. R. China

Abstract Cobalt phosphosulfide (CoPS) has recently emerged as a promising earth-abundant electrocatalyst for the hydrogen evolution reaction (HER). Nonetheless, the influence of crystallographic surface on the HER activity of CoPS and other non-metallic electrocatalysts remains an important open question in the design of high-performance catalysts. Herein, the HER activities of the (100) and (111) facets of CoPS single crystals were studied using complementary experimental and computational approaches. Natural (111) and polished (100) facets of CoPS single crystals were selectively exposed to reveal that the HER behaviors on these two facets are quite different, with current density-potential curves crossing near 0.35 V vs. RHE. Computational analysis can explain this phenomenon in terms of strongly differing H atom adsorption free energies and H-H recombination barriers on the facets, in conjunction with a simple kinetic model. At low potential (0 - 0.35 V), H adsorption (Volmer step) is rate limiting due to the endergonic adsorption on the (111) facet vs. exergonic adsorption on the (100) facet, yielding a faster HER rate for the latter. However, at high potential (> 0.35 V), H2 recombination/desorption becomes limiting and thus the (111) facet, with lower associated barriers, shows better HER activity. Explicit consideration of both steps and their interplay 1 ACS Paragon Plus Environment

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allows for a comprehensive description of the overpotential-dependence of the HER activity. This integrated study yields additional insight into the factors which govern the facet-dependence of catalytic activity on non-metallic electrocatalysts and can further improve the design of advanced nanostructured HER catalysts.

Keywords: hydrogen evolution reaction (HER), electrocatalyst, CoPS, electrocatalytic properties, crystallographic facet dependence, DFT calculations, theoretical insights

Introduction Molecular hydrogen (H2) has the potential to serve as an environmentally friendly energy carrier to help meet growing future global energy demands.1,2 Unfortunately, hydrogen production via electrochemical or photoelectrochemical water splitting is currently quite inefficient due to the substantial overpotentials required to obtain reasonable reaction kinetics. As such, there is a crucial need for highly efficient and robust catalysts to reduce the required overpotential and minimize the energy losses inherent in the energy conversion process.3,4 Platinum (Pt) and other noble metals or alloys are by far the most active electrocatalysts for hydrogen evolution reaction (HER), but their large-scale applications are greatly limited by their high cost and low reserves.5 Discovery of high-performance and earth-abundant electrocatalysts are thus urgently needed. Motivated by this challenge, many electrocatalysts have been developed in recent years,6-10 including: non-noble transition metals, chalcogenides, nitrides, phosphides, carbides, boride, and ternary compound.11-21 In particular, pyrite-type ternary compound cobalt phosphosulfide (CoPS) has recently been demonstrated as a highly efficient earth-abundant non-noble catalyst for robust electrochemical and photoelectrochemical hydrogen production in acidic environments.19 2 ACS Paragon Plus Environment

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Although more and more non-noble HER electrocatalysts have been developed, the influence of the exposed crystallographic facet(s) on HER catalytic activity is not well studied or understood. A notable exception may be catalysts with layered structures, such as MoS2, for which the question about basal plane vs. edge of the layers was unavoidable and investigated early on.22,23 Such facet-dependent activity is fundamentally important in understanding the electrocatalytic activity and mechanism of the HER,24-26 particularly for complex (non-metallic) materials that may expose many distinct types of surfaces, and practically useful for further designing advanced high performance nanostructured electrocatalysts. The lack of understanding on facet dependence is more acute for recently discovered earth-abundant metal compound electrocatalysts,6-10 as high surface area nanostructures with complex morphologies and various facets were often synthesized and directly evaluated for their HER performance. Even among noble metal electrocatalysts that were thoroughly studied before the recent surge of earthabundant electrocatalysts, discussion of facet-dependent HER activity is rare. The limited studies concluded that the HER activities of Pt and Au polycrystals are slightly better than that of the corresponding (111) single crystals,27-30 while the HER activity of Ag polycrystals is worse than that of its (111) single crystal.27,31 In addition, Rej et al. found the HER activity of Au-Pd nanocrystals depends strongly on the surface planes and the Au-Pd with tetrahexahedral shape exposing (730) facets showed the highest turnover frequency for HER.32 In comparison, the facet-dependent catalytic activity by metals and bimetallic compounds for oxygen reduction reaction (ORR) is well documented and has had a significant impact on the development of advanced nanostructured electrocatalysts.33,34 These prior reports emphasize the general expectation that electrocatalytic activity can and often does exhibit a strong facet dependence, although little systematic work has examined or explained these trends for HER catalysis, 3 ACS Paragon Plus Environment

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especially for the new earth-abundant binary or ternary compound electrocatalysts that often exhibit complex surface atomic structures. Theoretical calculation provides a promising way to understand these surface effects. To date, two main mechanisms of HER in an acidic environment have been proposed: the VolmerTafel and the Volmer-Heyrovsky mechanisms. The Volmer reaction refers to the initial adsorption of protons from the acid solution and proton-coupled electron transfer to form adsorbed H (H+ + e- → Had).35 The Heyrovsky reaction refers to the reaction of a proton and electron with an adsorbed surface hydrogen, yielding molecular H2 (Had + H+ + e- → H2),36 while in the Tafel reaction, two adsorbed surface hydrogen atoms recombine on the surface to generate H2 (Had + Had → H2).37 Via either mechanism, a good HER electrocatalyst should attract protons from solution and efficiently desorb H2. Nørskov and coworkers used the bonding energies of hydrogen to understand the trends in the exchange current for hydrogen evolution,38 and a volcano curve was obtained by plotting measured exchange current densities as a function of H adsorption energy. In conjunction with a kinetic model, their results suggest that the ideal H atom adsorption free energy of a HER electrocatalyst should be near zero. This general approach has been followed by most of the theoretical works on predicting and understanding earthabundant HER electrocatalysts.19,23-26, 39-41 Motivated by these prior works, we present here a combined experimental/computational investigation of the influence of CoPS surface faceting on electrocatalytic activity toward HER. We sought to synthesize large single crystals of CoPS and selectively expose the (100) and (111) facets to measure their corresponding HER activities. The HER activities of the two facets are substantially different at lower overpotentials but their electrocatalytic activity is observed to cross with increasing overpotential. We explain these observations based on our computational 4 ACS Paragon Plus Environment

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results, finding that the different HER behaviors of the CoPS facets arise from substantial differences in the surfaces’ H atom adsorption energies and barriers to H2 desorption. These differences give rise to a change in the rate-determining step from adsorption of H to the desorption of H2 with increasing overpotential. This work provides an interesting case study into the facet dependence of HER electrocatalytic activity, as well as unique mechanistic insights into the HER processes on compound electrocatalysts. Results and Discussion Synthesis of CoPS single crystals with exposed (111) and (100) facets Large (~2 mm) and well-faceted single crystals of CoPS were grown through chemical vapor transport by following an earlier report.42 Pure CoPS powder along with the transport agent, cobalt chloride, were sealed in a quartz tube under vacuum and heated at a source zone set to 900-1000°C and transferred to the growth zone set to be 50 °C lower than the source zone for about 1 week (see Materials and Methods for details). As shown in a photograph of a crystal in Figure 1a, the as-grown CoPS crystals are shiny with a silver-gold hue. These crystals are often octahedral in shape and only have triangular (111) facets. To obtain a (100) facet, a CoPS single crystal was cut along a suitable direction and its surface was polished (see Materials and Methods for more information).

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Figure 1. (a) A photograph of a representative CoPS single crystal. (b) XRD patterns for the asgrown single crystal of CoPS with the (111) facet (top), a single crystal with a cut and polished (100) facet (middle), in comparison with the standard PXRD pattern of the cubic pyrite phase CoPS (simulated from ICSD #62414). (c) and (d) optical images of the as-grown (111) and cut and polished (100) facets, respectively. (e) EDS spectra for the as-grown (111) and cut and polished (100) facets. Note, all data shown in b-e were taken on the same two single crystals.

Structure characterization of CoPS single crystals with exposed (100) and (111) facets The growth of these large CoPS single crystals enabled us to collect a high quality singlecrystal X-ray diffraction data set and refine the crystal structure. Table 1 summarizes the basic crystallographic information of CoPS and the data collection and the resulting crystallographic information file (CIF) is included in the Supporting Information. The solved crystal structure is in agreement with what was previously reported.42,43 Powder X-ray diffraction (XRD) patterns (Figure 1b) confirm the formation of CoPS single crystals with exposed (111) and (100) facets, respectively. The as-grown CoPS single crystal shows only one main peak with 2θ close to 28° (top trace), consistent with the (111) peak of the standard XRD pattern of cubic CoPS (bottom trace). This extreme preferential orientation of the (111) diffraction peak, together with the observed triangular facets of the crystal, supports that these exposed crystal facets are the (111) plane. Similarly, the XRD for the crystal cut/polished from the as-grown CoPS crystals reveals a 6 ACS Paragon Plus Environment

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predominant (200) peak at 33° (Figure 1b middle trace), which is consistent with the (100) facet. The signal-to-noise ratio of this XRD pattern is much worse than that from naturally grown crystals because the polished crystal was mounted in wax/glass holders (as shown in Figure S2S4 in the Supporting Information) to only expose the presumed (100) facet and it is very difficult to achieve the perfect surface orientation for the cut and polished crystals which are needed for obtaining a strong XRD pattern. The surface of the (111) facet of CoPS single crystals is very smooth (Figure 1c), while the (100) facet displays several small polishing scratches (Figure 1d). However, the percentage of the area occupied by these scratches and imperfection is quite small relative to the smooth (100) surface. Energy dispersive X-ray spectroscopy (EDS) spectra (Figure 1e) for the (111) as-grown and (100) cut and polished facets revealed the presence of Co, P and S in similar elemental ratios. The Raman spectra taken on these two facets (Figure S1 in the Supporting Information) do not show the distinguishable difference and are similar to what was previously reported.19 These results are all consistent with CoPS single crystals exposing (100) and (111) facets. Table 1. Crystal data and structure refinement for CoPS single crystal. Formula weight Temperature/K Crystal system Space group a/Å Volume/Å3 Z ρcalcg/cm3 Crystal size/mm3 Radiation Independent reflections Data/restraints/parameters Goodness-of-fit on F2

121.77 300.01 Cubic Pa3 5.4251(11) 159.67(10) 4 5.065 0.348 × 0.318 × 0.286 MoKα (λ = 0.71073) 306 [Rint = 0.0486, Rsigma = 0.0101] 306/0/8 1.440 7 ACS Paragon Plus Environment

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Final R indexes [I>=2σ (I)] Final R indexes [all data] Largest diff. peak/hole / e Å-3

R1 = 0.0289, wR2 = 0.0604 R1 = 0.0289, wR2 = 0.0604 0.95/-1.32

Electrocatalytic properties of the (100) and (111) facets of CoPS The HER catalytic properties of the CoPS (100) and CoPS (111) facets were evaluated on a rotating disk electrode (RDE) in 0.5 M H2SO4 solution with graphite electrode as the counter electrode. The single crystals were mounted directly onto the glassy carbon tip of the electrode using silver paint and the facet of interest was isolated using nail polish to carefully cover the rest of the crystal and electrode (see Materials and Methods for details and Figure S2-S4 in the Supporting Information for illustration of the crystal mounting process). To test if nail polish could effectively isolate the surface, an entire crystal was painted over and measured as a control experiment and it was found that the HER activity was completely eliminated. From the linear sweeping voltammograms (LSVs) shown in Figure 2a, the apparent HER performance on the surfaces of single crystal CoPS are generally much lower than the CoPS films, CoPS nanowires and CoPS nanoparticles that we have previously reported,19 due to the much smaller surface area of the electrode. However, with well-defined single crystal samples, we can accurately measure the exact surface area of the CoPS electrocatalyst (see an example in Figure S5 in the Supporting Information) and use such values as the real surface area of the catalyst in the calculation of the current density (J). In contrast to most recent reports of electrocatalytic performance, the geometric electrode area is used in the current density calculation, which is much smaller than the actual surface area of the high surface area nanostructured electrocatalysts. [The additional contribution of surface area from the scratches and other imperfections on the polished (100) facet is not very significant as compared to the area of the smooth (100) facet.] Note that the 8 ACS Paragon Plus Environment

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increased noise in the data at high overpotentials is due to the formation of bubbles which are more prone to stick to the flat surface of the crystals.13 Moreover, with well defined surface facets and accurate surface areas that can be measured directly from single crystals, the catalytic turnover frequency (TOF) values can be calculated with much more confidence for these single crystal samples compared to high surface area nanostructures with poorly defined facets. The only remaining assumption is the active site densities on different facets (the details of the TOF calculations are presented in Materials and Methods). The TOF for both (111) and (100) CoPS facets calculated here are quite high, which range from 1 to over 100 s-1 for 0 V to 0.40 V overpotentials. For example, at an overpotential of 0.10 V, the TOF values for (111) and (100) facets are 1.47 s-1 and 3.38 s-1, respectively. These values are higher than those reported for other highly active earth-abundant HER catalysts at comparable overpotentials, such as MoS2 (including edge or basal planes, 2H- or 1T-phase) and metal phosphosphides.22,44,45 Such high TOF values clearly confirm that CoPS is highly catalytically active for HER.

Figure 2. Comparison of the electrochemical performance of the as-grown (111) facet and cut/polished (100) facet of CoPS towards HER. (a) Current density vs. potential vs. RHE, (b) Tafel plots, and (c) TOF vs. potential. The current density values presented here are based on the real electrode surface area of the exposed CoPS single crystal facets.

After accounting for the surface area differences, it becomes clear that the electrocatalytic activity of the (100) facet is better than that of the (111) facet at low overpotential (Figure 2a). In 9 ACS Paragon Plus Environment

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addition, their J-V curves actually intersect as the overpotential increases over 0.35 V, so that the (111) facet becomes slightly more active at overpotentials above 0.35 V. These differences are also reflected in the exchange current density (j0) and Tafel slope (β) values that are extracted from the Tafel plots shown in Figure 2b and summarized in Table 2. The higher exchange current density (j0) of CoPS (100) facet as compared to CoPS (111) facet reflects its faster kinetics toward the redox reaction. However, CoPS (111) displays a lower Tafel slope (86 mV/decade) than the (100) facet (109 mV/decade). These suggest that the crossover of the J-V curve is not coincidental. In terms of the TOF values, we find that the two facets become extremely close but do not cross with the increase of overpotential (Figure 2c), when we use the Co as the active site to calculate the TOF of both facets. It should be noted that the Tafel slopes of CoPS (111) and (100) measured here are higher than the film (57 mV/decade), nanowires (48 mV/decade) and nanoplates (56 mV/decade) of CoPS from prior work.19 This observation suggests that the Tafel slope values apparently observed for nanostructured samples calculated using the geometric surface area of the electrode (which is the common practice) are likely influenced by many factors, including crystal orientations, surface sites, and the high surface area of the nanostructures. Single crystal studies, like the case here, are thus necessary to eliminate these additional factors and to probe the underlying facet dependence of the HER activity on complex materials such as CoPS and other compound electrocatalysts.

Table 2. Summary of the Tafel slope and exchange current density (j0) of CoPS (100) and CoPS (111) facets (left) and the theoretical data of the surface energies of CoPS (100), CoPS-1 (111) and CoPS-2 (111) (right). Electrochemical data Tafel Slope (mV/decade)

j0 (µA/cm2)

Theoretical data: surface energy (eV/Å2) Surface- Surface- Surface- Surface- Surface- Surface1 2 3 4 5 6 10 ACS Paragon Plus Environment

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CoPS (100) CoPS (111)

109

38.7

86

4.7

CoPS 0.142 (100) CoPS-1 0.241 (111) CoPS-2 0.218 (111)

0.169

0.210

0.141

-

-

0.220

0.193

0.205

0.250

0.240

0.198

0.175

0.205

0.244

0.218

We also carried out optical imaging, Raman, EDS and XPS analysis of the CoPS samples with different facets before and after the HER electrocatalysis test (see Figure S6-S12 in the Supporting Information), and they show that there are minimal changes to the CoPS samples after the HER electrocatalysis. These results further confirmed the chemical stability of CoPS in acidic HER conditions. Surface structures of the (100) and (111) facets of CoPS To model the overpotential-dependent HER electrocatalytic activity of the various CoPS facets, models of the stable (111) and (100) facets are required. On both facets, there exist many possible symmetry-unique terminations, some of which are related to exchange of S/P atoms; the symmetry unique terminations are depicted by red lines in Figure 3. For (100) facet, there are six unique terminations of the surface unit cell, which are denoted with numbers 1-6 (see Figure 3a), with surfaces 1-3 essentially symmetry equivalent with surfaces 4-6. The (111) facet can be divided into two classes of surfaces related via P / S exchange, denoted as CoPS-1 (111) and CoPS-2 (111). Both structures have 15 different terminations of the surface unit cell, with three essentially identical groups of surfaces: 1-5, 6-10, 11-15 (Figure 3b, c). As such, we focus primarily on the stability of surfaces 1-3 of the (100) facet and the surfaces 1-5 of the two (111) facets. To determinate the most stable surface structure(s), we calculated and compared the surface energies of various potential terminations, 11 ACS Paragon Plus Environment

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=

 −    

where Eslab is the total energy of the surface slab, Ebulk is the total energy of the bulk atoms (eV per atom), N is the number of atoms in the slab and A is the surface area. As shown in Table 2, surface 1 is the most stable (0.142 eV/Å2) among surfaces 1-3 of CoPS (100). As expected, surface 4 (0.141 eV/Å2) has a very similar surface energy to surface 1, indicating that both surfaces are essentially identical. For simplicity, we utilize surface 1 of CoPS (100) as a representative of CoPS (100) for all subsequent calculations. After examining possible CoPS (111) terminations, we find surface 3 of CoPS-1 (111) [0.193 eV/Å2] and CoPS-2 (111) [0.175 eV/Å2] to be the most stable, and they are denoted as S-terminated and P-terminated CoPS, respectively, in the subsequent discussion. For both S-terminated CoPS and P-terminated CoPS surfaces, the top atom P or S is exposed with P-S dumbbells broken. We also considered possible reconstructions (see Figure S13) that restore the dumbbell at the expense of a surface vacancy. However, we find that the alternative reconstruction is much less stable, likely because it breaks three Co-P or Co-S bonds to reform the P-P or S-S dumbbell.

Figure 3. (a) The (100) surface of CoPS with different surface terminations. (b) and (c) show the two types of (111) surface with different arrangements, denoted CoPS-1 (111) and CoPS-2 (111). CoPS-2 (111) is obtained by exchange of P and S atoms. The different atomic layers are depicted with red lines and labeled by the numbers on the right side. 12 ACS Paragon Plus Environment

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Facet-dependent H atom adsorption energy Given the most stable CoPS (100) and (111) surfaces (Figure 4a, c and e), the binding free energy of the first and second adsorbed H atoms were calculated, examining all symmetryunique binding sites; the most stable, minimum (free) energy binding sites are depicted in Figure 4b, d and f. For the (100) surface, the first H atom is preferentially adsorbed on the P and subsequently, the second H atom is adsorbed on the Co-1 site, consistent with our prior work.19 In contrast, for the (111) facet, the first H atom is preferentially adsorbed on the Co-4 site for both P-terminated and S-terminated surfaces. The second H atom is then adsorbed on the P site or Co-3 site, for P- or S-terminated (111) surface, respectively. Adsorption of H on one site of the (111) facet does not typically significantly affect the co-adsorption strength of an additional H atom at other sites (Figure S14). In contrast, for the (100) surface, the H adsorbed energy (0.24 eV) at the Co site significantly weakens (-0.07 eV) when an H atom is co-adsorbed on the P site (see Figure S15) due to the proximity of the neighboring Co and P atoms on the (100) surface. Many of these H atom adsorption trends can be understood in terms of simple chemical arguments. The CoPS (100) surface contains intact PS3- dimer dumbbells, which are electron rich as compared to the S22- dumbbells in CoS2 due to the lower electronegativity of P versus S. This electron-rich P site provides a strong H binding site and is reflected in a small peak near the Fermi level in the surface PDOS (see Figure S16a). Adsorption of H onto P induces a reduction of Co3+ to Co2+, opening a new site for subsequent facile H adsorption.19 In contrast, both CoPS (111) surfaces lack intact PS3- dumbbells. Rather both the P-terminated and S-terminated CoPS (111) surfaces expose four symmetry-unique Co atoms, three 4-coordinated (Co-1, Co-2, and Co-3) and one 3-coordinated (Co-4). The electron-rich 3-coordinated site preferentially adsorbs

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H atom compared to other Co sites, which is consistent with its higher surface PDOS on both Sterminated CoPS (111) and P-terminated CoPS (111) (see Figure S16b and c).

Figure 4. Top view of (a) the CoPS (100) and (b) its adsorption state with two hydrogen atoms. Top view of (c) the S-terminated CoPS (111) and (d) its adsorption state with two hydrogen atoms. Top view of (e) the P-terminated CoPS (111) and (f) its adsorption state with two hydrogen atoms.

Setting the reference potential to that of the standard hydrogen electrode (SHE), the change in free energy of H+ + e- → ½ H2 can be defined as zero, defining a “computational hydrogen electrode”.23,38 In conjunction with the computed H atom adsorption energies, the free energy profile for HER on (100) and (111) surfaces of CoPS was computed. The thermodynamics of the two sequential proton-coupled electron transfer (PCET) steps at an applied potential of U=0 vs. SHE were computed. Results for the various CoPS surfaces are summarized in Figure 5, which displays the relative free energy changes for HER: sequential adsorption of two hydrogen atoms, followed by the desorption of H2 into gas phase. We find that adsorption of the first H atom on the (111) facet is modestly endergonic, while adsorption on the (100) facet is exergonic. The 14 ACS Paragon Plus Environment

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trend continues for the second adsorbed H atom, yielding free energies for two adsorbed H atoms on the P-terminated (111) surface (0.25 eV) comparable in magnitude (but opposite in sign) with that of adsorbed H atoms on the (100) facet (-0.23 eV). Using such a free energy landscape, the overpotential is often identified computationally via the most “uphill” PCET step, suggesting that the (100) surface may exhibit superior electrocatalytic activity at low overpotentials. Note the final H-H recombination (the final step in Figure 5) is not an electrochemical PCET step, but rather a simple on-surface thermal reaction. However, these thermodynamic results also suggest the likelihood that the recombination / desorption may also exhibit significant kinetic barriers on the (100) surface, potentially limiting its HER performance. A more detailed analysis is thus required.

Figure 5. The calculated free energy changes (U = 0 vs. SHE) of the lowest energy HER pathways on CoPS (100), S-terminated CoPS (111) and P-terminated CoPS (111) surfaces. Reaction path of the H adsorbed on the (100) and P-terminated (111) surfaces of CoPS The energetics of the on-surface H-H recombination, i.e. the Tafel mechanism, was examined in Figure 6, using the H-H bond distance as the reaction coordinate. Similar consideration of the 15 ACS Paragon Plus Environment

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energetics and barriers of the Tafel step have been previously considered by several authors in the case of MoS2.24,26 During the course of Tafel step, the H-H bond distance decreases to the gas-phase equilibrium bond distance of ~0.74 Å. For the (100) surface, the reaction proceeds over a barrier of ~0.9 eV with an endothermicity of 0.65 eV (Figure 6a). The corresponding reaction on CoPS (100) proceeds via a concerted recombination/desorption in which two H atoms initially adsorbed on the P and Co-1 sites on the (100) surface approach one another and move away from the surface, forming H2 gas without a stable, molecularly-adsorbed H2 intermediate. Due to the substantial entropy difference between the initial (H* + H*) and final (H2(g)) reactants/products, we estimated the relative free energy ( ∆ ) curve of the H-H recombination on the (100) surface; although the free energy differences of the initial/final states can be easily calculated, the exact free energy difference along the reaction path cannot be trivially estimated and is illustrated schematically by the green dashed line in Figure 6, along with a range of possible free energy barrier (0.52-0.92 eV). The ∆ between the initial adsorbed state and final gas phase state on (100) surface is 0.25 eV, consistent with the relative free energy on (100) surface in Figure 5. Consideration of entropic effects thus likely substantially reduces the estimated free barriers and suggests that the Tafel step is energetically accessible, in contrast to the case of MoS2, where the corresponding barriers have been estimated to be as high as 1.5 eV.26

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Figure 6. Reaction profile for H-H recombination / desorption. The H-H bond length is plotted vs. the ZPE-corrected reaction energy (∆ ) [black] and estimated free energy (∆ ) [green] for (a) the CoPS (100) and (b) the P-terminated-CoPS (111) surfaces. The free energy of gas phase hydrogen and initial adsorbed state with two hydrogen atoms are shown with a red circle and purple star, respectively. The free energy, ZPE and entropy of gas phase hydrogen are set to be zero.

For the (111) surface, we focus on the P-terminated (111) surface due to the enhanced stability of adsorbed H atoms as compared to the S-terminated (111) surface. In contrast to the (100) face, the desorption of the H atoms on P-terminated (111) proceeds via a stably adsorbed molecular hydrogen intermediate, with the H2 intermediate adsorbed on the Co-1 site (H-H bond distance of ~1 Å). H2 then desorbs over a modest energy barrier (see Figure 6b). As in the case of the (100) surface, entropic effects are also important. In this case, we assume that entropy change contributes predominately to the final desorption step and neglect entropic perturbations to the H-H recombination. As such, the ∆ and ∆ curves are similar; the only difference occurs in the final desorption process. The ∆ between the initial adsorbed state and final gas phase state on the (111) surface is -0.23 eV, consistent with the relative free energy on the (111) surface in Figure 5. The free energy barrier of the H-H recombination on the (111) surface is estimated from 0-0.31 eV, significantly less than that of the (100) surface (0.52 -0.92 eV). These 17 ACS Paragon Plus Environment

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results suggest that the Tafel step on the (111) surface will be much more facile as compared to the (100) surface, but (in isolation) are insufficient to provide a complete description of HER kinetics. Kinetic discussion A more detailed kinetic model for the HER is required to fully explain the facet-dependent electrocatalytic behavior as a function of potential; one such model is shown in Figure 7a. This simplified model considers a single effective electrochemical PCET “adsorption” step (Volmer, with rate  and free energy change ∆ ), followed by subsequent recombination and desorption (Tafel, with rate  and free energy change ∆ ) . The generation of the adsorbed H intermediates (Volmer reaction) is often the rate-limiting step for HER in many catalysts. Indeed, the measured Tafel slopes of both CoPS facets in the low-overpotential regime range from 90110 mV/decade, consistent with the expected value for a rate-limiting Volmer step. In such a regime, the rate is dictated principally by  , and, consistent with Marcus theory,46 lower activation free energies,  ǂ , (and higher rates) associated wither exergonic PCET (∆ < 0),  ∝ 



∆ǂ !"

∆ ǂ =

#$% ∆&' )( )$

Thus, at low overpotential, the (100) surface would be expected to display more facile Volmer kinetics as compared to the (111) facet, consistent with the observed HER activity in this regime.

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Figure 7. (a) The reaction scheme for the H adsorption and desorption processes. The rate constants of this reversible reaction at equilibrium can be represented by k1, k-1, k2 and k-2, respectively. (b) The scheme for Gibbs free energy activation ∆ǂ as the function of free reaction energy ∆ #Ƞ), where ∆ #Ƞ) = ∆ + Ƞ. The slope of Gibbs free energy for CoPS (100) and (111) facets are also shown in the figure.

A more detailed analysis can be conducted at equilibrium (zero overpotential, Ƞ = 0). In this case, the rates of the forward and backward reactions (e.g. exchange current) are equal and entirely determined by the rate of proton reduction. Given the significantly more favorable PCET energetics on the (100) facet, one would expect a significantly higher exchange current for the facet, consistent with experimental observations (see Figure 2b). However, the (111) facet exhibits a significantly larger overpotential-dependence of the HER activity (i.e. smaller Tafel slope). This observation is also consistent with this simple model and Marcus theory predictions: considering the condition |∆ | ≪ λ (i.e. outside of the Marcus inverted regime), the Gibbs free activation energy with the function of overpotential is given by ∆ ǂ =

#$ % ∆&' % Ƞ)( )$

.

As the overpotential, Ƞ, decreases, the rate constant k1 would be expected to increase for both (111) and (100) in accordance with Marcus theory. However, the rate of change is expected to be faster for the (111) facet, due to its initially higher free energy barrier (see Figure 7b). 19 ACS Paragon Plus Environment

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As the rates of the Volmer step accelerate with increasing overpotential on both facets and H coverage increases, the recombination/desorption (Tafel) step must become kinetically relevant and likely rate determining. Due to the higher free energy barrier of the (100) facet for the desorption process, a crossover in HER activity would be expected, as was observed experimentally. Thus, in the regime of high overpotential, we conclude that the (111) surface exhibits higher currents/activity due to its favorable Tafel kinetics. Note that the Tafel slopes of both facets increase significantly at high overpotentials, clearly indicating a change in the rate determining step / mechanism, but not clearly consistent with expected slopes for any known step (Volmer, Tafel, Heyrovsky), perhaps due to the complex surface structure of CoPS as compared to noble metal catalysts. Alternatively, it is possible that the (100) surface circumvent the Tafel bottleneck via a change in mechanism, potentially transitioning to a Volmer-Heyrovsky mechanism (i.e. direct reaction of H+ with adsorbed H) at higher overpotential. Such a Heyrovsky mechanism has been speculated for MoS2, which displays significantly higher Tafel barriers (~1.5 eV) than calculated for CoPS.26 Nonetheless, a complete model of the CoPS HER kinetics requires accounting for both proton reduction and recombination to describe the facetand overpotential-dependent behaviors. Conclusion CoPS single crystals with exposed (100) and (111) facets were successfully synthesized or prepared, exhibiting differing HER behaviors as a function of overpotential. In the low potential region (0 - 0.35 V), the HER catalytic activity of the (100) facet is superior to that of (111) facet, a situation that reverses for the high potential region (> 0.35 V). Our results suggest that the differing HER behaviors arise from pronounced differences in both the energetics of H atom adsorption and barriers to H-H recombination on the two CoPS facets. In particular, 20 ACS Paragon Plus Environment

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adsorption of H on the (111) facet is endergonic while that on the (100) facet is exergonic, yielding significant differences in the H adsorption rate and its potential dependence. At the low potential, the H adsorption process is rate limiting, but H2 recombination/desorption becomes significant with increasing potential. As such, the (111) facet, with lower desorption barriers, exhibits higher HER catalytic activity as compared to the (100) facet in this regime. Explicit modeling of both Volmer and Tafel steps, and their interplay, within the computational modeling allows for a comprehensive description of the overpotential-dependence of the HER activity. This combined experimental and theoretical study thus provides an interesting case study into the facet dependence of HER electrocatalytic activity, as well as unique mechanistic insights into the HER processes, on new non-noble metal electrocatalysts, and thus crucial insights into the design of advanced nanostructured HER catalysts.

Materials and Methods Synthesis of CoPS Single Crystals CoPS powder was prepared by combining stoichiometric amounts of high purity sulfur, phosphorus, and cobalt; and grinding to a fine powder with a mortar and pestle in an argon atmosphere, which was then was loaded into a quartz ampule and sealed under vacuum. The sealed ampule was heated to 650 °C at a rate of 1°C/hr and held for 6 days. The resulting product was removed from the ampule and ground to a fine powder. The composition was confirmed using powder XRD. Single crystals of CoPS were grown using chemical vapor transport with cobalt chloride as a transport agent. Specifically, 0.25 to 1 g of CoPS powder was loaded into a quartz tube (o.d. ½” inch) along with 10 to 25 mg of cobalt chloride in an argon atmosphere and sealed under vacuum into a quartz tube at a length of about 6 ¾ inches. Then the sealed tube was placed in a custom-built horizontal two zone tube furnace with the powder kept in the source 21 ACS Paragon Plus Environment

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zone set to a temperature between 900 and 1000 °C, and the growth zone was set to a temperature 50 °C lower than the source zone. The furnace was quickly ramped to the set temperature. After 1 week, the furnace was allowed to cool naturally and the tubes were broken to remove the crystals. These as-grown single crystals of CoPS are shiny with a silver-gold hue. As large as 2 mm, they have octahedral shape and naturally have triangular (111) facets. The (100) facets were developed by cutting a (111) facet crystal using a circular rotating diamond saw and then manually polishing the cut surface using progressively finer grit SiC sandpapers. Structural Characterizations CoPS single crystals were characterized using a LEO SUPRA 55 VP field-emission scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) capabilities, and a Bruker D8 ADVANCE powder X-ray diffractometer (PXRD) using Cu Kα radiation. Raman spectra were taken using a Thermo Scientific DXR confocal Raman microscope using a 532 nm excitation laser. High-resolution XPS measurements were taken using a custom-built XPS system (Phi Electronics, Eden Prairie, MN) with a model 10-610 Al Kα X-ray source at 1486.6 eV photon energy and a model 10-420 toroidal monochromator. A model 10-360 hemispherical analyzer with a 16-channel detector array was used, which under effective operating conditions had an analyzer resolution of 0.4 eV. All X-ray photoelectron spectra were shifted so that the adventitious carbon 1s peak was at 284.8 eV. S 2p peaks were fitted using doublets with a 1:0.5 (3/2p:1/2p) area ratio, 1.18 eV apart, and with the same full width at half maximum (FWHM). P 2p were fitted using doublets with a 1:0.5 (3/2p:1/2p) area ratio, 0.87 eV apart, and with the same FWHM. The single crystals were examined using the same techniques again after the electrochemical tests.

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Electrochemical Measurements The electrodes were prepared by attaching a CoPS single crystal with the chosen (111) or (100) facets directly to the glassy carbon tip of the rotating disk electrode (RDE) apparatus. As illustrated in Figure S2 in the Supporting Information, the crystals were attached to the glassy carbon tip using silver paint, and everything except for the facet of interest was carefully covered with nail polish to prevent any contribution to the observed HER activity. The electrochemical characteristics were measured in 0.5 M H2SO4 with bubbling hydrogen, using a rotating disk electrode with the CoPS single crystal mounted spinning at a rate of 2000 rpm with a graphite counter electrode and a Hg/HgSO4 reference electrode. The turnover frequency (TOF) was calculated by assuming that the cobalt atoms are the active sites and calculating the number of cobalt atoms per surface area of the specific crystal facets: 2 ./ 0/12 111 4560 1 /1 ∗ ∗ 111 4560 25.46 /1 #1 ∗ 10? ) 61 = 7.9 ∗ 10) ./ 0/12 B> 61 /< 111 4560 2 ./ 0/12 100 4560 1 /1 ∗ ∗ 100 4560 29.40 /1 #1 ∗ 10? ) 61 = 6.8 ∗ 10) ./ 0/12 B> 61 /< 100 4560 Then the total number of active sites on the surface could be calculated using the exact surface area of the facet, which could be determined based on the geometry using measurements of the crystal facets taken on an optical microscope (see an example in Figure S5). For the specific (111) facet shown in Figure 1b, the total surface area was calculated to be 0.0147 cm2 and for the specific (100) facet shown in figure 1c the total surface area was calculated to be 0.0418 cm2. 23 ACS Paragon Plus Environment

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The TOF was then calculated at every point of the J-V curve using the current density and the fact that two electrons are transferred in HER. .D>>J56 F∗ ∗ ∗ 2 1 1. 2   K/05L # 60NO IN02 = H =50P B> 560NO 2N0 B> 26/

J + J + #< − 1)H) + #H )) 2 where E(surf + nH) is the total DFT energy for the CoPS system with n hydrogen atoms adsorbed on the surface, E(surf + (n-1)H) is the total DFT energy for (n-1) hydrogen atoms adsorbed on the surface and E(H2) is the DFT energy for a hydrogen molecule in the gas phase. As the DFT binding energies do not contain contributions from zero-point energies and entropy, thus we add these contributions separately in order to obtain the Gibbs free energy: ∆S = ∆E + ∆TUV − K∆I where ∆TUV is the zero-point energy (ZPE) difference which can be obtained from vibrational frequencies derived from Hessians calculated from analytic gradients on single molecule in vacuum or adsorbates on CoPS slab models. In our calculation, all the vibrational frequencies of the slab have been considered. The entropic contribution for gaseous molecule H2 is taken from standard thermodynamics table52 and the −K∆I term can be set to 0.20 eV, one half of the entropic contribution for molecular H2 (entropic contributions from the slab are omitted).53,54 For (111) surface, the calculated ∆TUV is close to 0.04 eV in different adsorbed sites, leading to ∆S = ∆E + 0.24 W. However, for (100) surface, the ∆TUV is close to 0.09 eV in different adsorbed sites, leading to ∆S = ∆E + 0.29 W. The reaction paths were calculated by limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) algorithm55 implemented in atomic simulation environment (ASE)56 with the energy and force calculated from VASP calculator. In the simulation process, the bond length of two hydrogen atoms was fixed in every image until the maximum energy difference and residual 25 ACS Paragon Plus Environment

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force on atoms converged. The initial state was set to be the favorable adsorption states of two hydrogen atoms and the final state was set to be the two hydrogen atoms on the surface with the same bond length as gas-phase hydrogen (0.74 Å). Note that the ZPE is also considered in the reaction paths calculation, and the ZPE will change with decreasing of H-H bond length. Associated Content Supporting Information Available: Diagrams of crystal mounting procedures for different characterization methods, and the obtained SEM images, optical images, EDS data, Raman spectra, XPS data of the CoPS single crystals before and after HER test, and projected densities of states of CoPS. The CIF file of crystallographic data, which is also available from the Cambridge Structural Database with code CCDC 1564316. The structures with reconstruction and free energy diagram for H adsorbed on different surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Authors *[email protected] *[email protected] Author Contributions ||

T.W. and M.L.S. contributed equally

Notes The authors declare no competing financial interest

Acknowledgments: T.W. and J.R.S. acknowledge support by the National Science Foundation Grant No. CHE-1362136 for the computational work presented in this study. Computational 26 ACS Paragon Plus Environment

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work utilized resources, in part, from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number TGCHE120088. M.L.S., M. J. Shearer, R.J.H. and S.J. thank support by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-09ER46664 for materials synthesis and electrochemical study. M. L. S. thanks the support of a UW-Madison Hilldale Research Fellowship. M. J. Shearer and M. J. Stolt thank NSF Graduate Research Fellowship for support. Crystallographic instrumentation at UW– Madison is supported by a generous gift from the Paul J. Bender fund.

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