Evaluation of the H2 Evolving Activity of Benzenehexathiolate

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Evaluation of the H2 Evolving Activity of Benzenehexathiolate Coordination Frameworks and the Effect of Film Thickness on H2 Production Courtney A. Downes, Andrew J. Clough, Keying Chen, Joseph W. Yoo, and Smaranda C. Marinescu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15969 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Evaluation of the H2 Evolving Activity of Benzenehexathiolate Coordination Frameworks and the Effect of Film Thickness on H2 Production Courtney A. Downes, Andrew J. Clough, Keying Chen, Joseph W. Yoo, and Smaranda C. Marinescu* Department of Chemistry, University of Southern California, Los Angeles, CA, 90089, USA KEYWORDS Metal dithiolenes, metal-organic frameworks, electrocatalysis, coordination polymers, solar energy conversion, hydrogen evolution

ABSTRACT: The design of earth-abundant catalysts for the electrochemical production of H2 from water is important for the realization of a sustainable energy future. Incorporation of molecular catalysts into extended frameworks has emerged as a viable strategy for improving catalytic performance and durability while maintaining a high degree of control over the structure and properties of the catalytic active site. Here, we investigate benzenehexathiolate (BHT) coordination frameworks as electrocatalysts for the hydrogen evolution reaction (HER) in pH 1.3 aqueous solutions. The electrocatalytic HER activity of BHT based coordination frameworks follows the order of CoBHT > NiBHT > FeBHT. CoBHT operates at an overpotential of 185 mV, the lowest observed overpotential of the reported metal dithiolene-based metal organic frameworks and coordination polymers to date. To further understand the properties that dictate electrocatalytic activity, the effect of film thickness on the HER performance of CoBHT, a parameter that has not been extensively explored for electrocatalytic coordination frameworks, was examined. As the thickness was increased to ~1 µm, charge and proton transfer through CoBHT was hindered, the number of electrochemically accessible active sites decreased, and the mechanical robustness of the modified electrode was diminished. The observed thickness-dependent HER activity of CoBHT highlights the importance of practical electrode construction and offers insight into how to optimize proton and electron transfer properties and active site densities within coordination frameworks without reducing the mechanical robustness of the immobilized catalysts.

INTRODUCTION To sustainably meet the rising energy demand associated with rapid global population and economic growth, continued development and deployment of renewable energy resources is paramount.1 The unparalleled abundance of solar energy, while attractive to meet the massive global energy demand, is hindered by its intermittent nature, resulting in a mismatch between energy supply and demand.2 Sustainable hydrogen production from water splitting (2H2O → 2H2 + O2) has emerged as a promising pathway for the storage and conversion of solar energy.2-3 Hydrogen is a valuable energy carrier that can be transformed into electricity using fuel cell technology or used in the production of industrially relevant chemicals such as ammonia and methanol.4-5 Hydrogen is currently generated from steam methane reforming whereby four molecules of H2 are produced per CH4; however, one molecule of CO2 is also produced.6 A sustainable energy future will require the carbon-neutral production of H2 and water splitting is a viable pathway to do so. Electrocatalysts that are costeffective and scalable must be developed for the hydrogen evolution (HER) and oxygen evolution reactions (OER) to facilitate water splitting at high efficiencies with minimal energy input. Earth-abundant homogeneous7-8 and heterogeneous9-11 catalysts have been extensively explored as alternatives to

platinum, the benchmark electrocatalyst for the HER. Inherent problems with homogeneous catalysts such as the lack of solubility and stability in aqueous media and the diffusion (to the electrode surface) dependent activation of the solubilized catalyst limit their practical feasibility. Similarly, heterogeneous catalysts have ill-defined active sites making mechanistic understanding, rational design, and optimization difficult. These short-comings can be overcome by combining the advantageous properties of homogeneous and heterogeneous catalysts. The heterogenization of molecular catalysts has been accomplished through a variety of methods12-13 such as surface adsorption,14-18 electropolymerization,19-21 covalent attachment,2224 and incorporation into extended frameworks.25-31 These systems can display the improved stability and robustness associated with heterogeneous catalysts while the welldefined catalytic active sites reminiscent of molecular systems can be extensively studied and modified to give insight into the mechanism of catalysis. Molecular catalysts incorporated into extended frameworks such as coordination polymers and metal-organic frameworks (MOFs) have been investigated as electrocatalysts for energy converting applications.32-34 The use of MOFs as intrinsic electrocatalysts, however, has been limited because of their traditionally insulating nature. The facilitation of efficient charge and electron transfer is an

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important property for the promotion of high electrocatalytic activity. The ability to deposit thin film coordination frameworks directly onto electrode surfaces has improved their electrocatalytic performance because thin film MOFs exhibit higher conductivities than the bulk materials.35-36 However, most of these systems still exhibit insufficient charge transfer properties inhibiting high electrocatalytic activity. Additionally, the weak coordination bonds in the frameworks lead to structural instability in aqueous conditions. The development of conductive coordination frameworks has been a breakthrough in the application of MOFs for electrocatalytic energy conversion.37 The use of redox active ligands has facilitated charge transport through the frameworks and stronger metal-ligand bonds have improved stability in aqueous acidic and/or alkaline media. The few successful electrocatalytic MOFs displaying high activity and stability for the HER,28-30 oxygen reduction reaction (ORR),31 oxygen evolution reaction (OER),38-41 and CO2 reduction (CO2RR)42-43 all exhibit intrinsic electrical conductivity. The conductive nature of metal dithiolene based coordination frameworks makes these systems attractive candidates for electrocatalytic applications.36, 44-46 We have extensively explored the use of trinucleating and dinucleating dithiolene ligand scaffolds to synthesize 2D and 1D metal dithiolene coordination frameworks28 and polymers,25-26 which perform as robust electrocatalysts for the HER. To further investigate the variables that dictate the HER activity of dithiolene-based coordination frameworks, we examine here the effect of the coordinated metal by performing detailed electrochemical analysis of CoBHT, NiBHT, and FeBHT (BHT = benzenhexathiolate, Figure 1). FeBHT was synthesized through an interfacial reaction previously reported in the literature for the isolation of BHT-based coordination frameworks.28, 44 The synthesis and conductivity of NiBHT was reported by Nishihara and coworkers,44-45 however, the electrocatalytic HER activity was not studied. We have previously explored the electrocatalytic HER performance of CoBHT with the system displaying an overpotential of 340 mV and Faradaic efficiency of 97% in pH 1.3 aqueous solutions and a Tafel slope of 108 mV/dec (pH 2.6).28 A modified synthetic method was utilized here for the isolation of CoBHT and electrochemical analysis was performed using glassy carbon electrodes (GCE). Electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and double-layer capacitance (Cdl) measurements, which provide insight into the charge transfer/HER kinetics and an estimation of the electrochemically active surface area, respectively, were not employed in the previous report on CoBHT. This analysis is used here to improve the fundamental understanding of the parameters that dictate the observed HER activity of CoBHT. Additionally, we explore the effect of film thickness on the HER activity of CoBHT. For layered materials, thicker films correspond to higher bulk catalyst loadings (higher number of available active sites) leading to improved HER performance.47-48 However, it has been observed that a plateau can be reached whereby increases in catalyst loading do not generate higher electrocatalytic current densities.18, 42, 47, 49 Poor diffusion of electrons and protons

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through thick films limits the number of electrochemically accessible active sites even as bulk catalyst loading increases, thus inhibiting the HER activity.47, 49-50 The longterm durability of thicker films can also be problematic. High mass loadings can lead to catalyst modified electrodes exhibiting limited mechanical robustness with cracking and delamination from the electrode surface occurring during electrochemical testing.

Figure 1. Structure of the metal benzenehexathiolate (BHT) coordination frameworks (CoBHT, NiBHT, and FeBHT) studied here.

Previous investigation into the HER performance of monolayer and bulk powders of cobalt dithiolene and mixed dithiolene-diamine coordination frameworks revealed increases in the overpotential to achieve 10 mA/cm2 and the Tafel slope as the method of catalyst deposition was changed from a monolayer (0.8 ± 0.1 nm thickness) to bulk powder (0.12 mg catalyst prepared as an ink).30 These differences are expected as thin film dithiolene- and diamine- based coordination frameworks are more conductive than the bulk materials because of a reduction in the number of grain boundaries, which inhibit charge transfer.35-36, 44-45 Further investigation into the effect of film thickness on the HER activity by increasing the amount of deposited catalyst from a monolayer to multilayer films was not performed. The influence of film thickness on the activity of intrinsic MOF electrocatalysts, although an important parameter for optimizing catalytic performance, has not been extensively explored. To the best of our knowledge, the most comprehensive study on the effect of MOF film thickness on catalytic activity was performed by Yaghi, Yang, and coworkers on a cobalt porphyrin based MOF, Al2(OH)2TCCP-Co (TCPP-H2 = 4,4’,4”,4”(porphyrin5,10,15,20-tetrayl)tetrabenzoate).42 The CO2 reduction performance increased with the number of atomic layer deposition (ALD) cycles before reaching maximum activity at 50 cycles (MOF film thickness of ~30-70 nm). At higher loadings, the performance decreased because of poor charge and mass transport properties. Extensive analysis of the influence of film thickness on the HER performance of coordination frameworks is necessary to facilitate the continued development of these systems as practical catalysts for energy converting devices. By varying the film thickness from ~20-1000 nm, we look to understand how efficiently charges and protons move through CoBHT. We can then determine the optimal


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conditions to maximize the number of accessible active sites and HER activity without increasing the resistance of the system and limiting electron and proton transfer.

RESULTS AND DISCUSSION Electrochemical Analysis of FeBHT and NiBHT. To investigate the electrocatalytic hydrogen evolving activity of CoBHT, NiBHT, and FeBHT, we performed detailed electrochemical measurements in a three-electrode configuration with N2-saturated pH 1.3 H2SO4 electrolyte, where all of the potentials are referenced to the reversible hydrogen electrode (RHE). Multiple electrodes modified with the BHT-based frameworks were prepared and electrochemically tested with the results presented in the Supporting Information. We have recently shown that the deposition method used here, immersion of the electrode through the interfacially grown film, leads to small variances in the bulk catalyst loading even for films of the same thickness.51 These small variances in bulk loading and the morphological differences the deposition method presented here induces for each sample (how the sample adheres to the substrate) can influence the measured Cdl and Rct values. Thus, it is important to establish the trends observed for the electrocatalytic HER activity are valid for multiple modified electrodes. The EIS response for all three systems studied was best described by the two-time constant serial model (2TS), where the high frequency semi-circle is related to the surface porosity of the electrode or the contact between the electrode and the catalyst layer and the low frequency semi-circle is related to the charge transfer kinetics of HER (Figure S1).48, 52-54 NiBHT and FeBHT were readily deposited on GCE following the interfacial synthesis and electrochemically explored as hydrogen evolving catalysts. FeBHT exhibits overpotentials ranging from 473-541 mV (Figure 2a, Table 1, Figure S3, Table S1). Increases in Cdl resulted in reduced HER activity for FeBHT, which was not expected (Figure S3-S5, Table S1). The rise in Cdl was accompanied by an increase in the Rct and Tafel slope. Enhancements in the Cdl (1.4 to 2.6 mF/cm2) corresponded to increases in the Rct from 102.8 to 221.3 Ω (η = 470 mV) and Tafel slope from 112 to 143 mV/dec (Figure S4-S6, Table S1). These results indicate poor charge and proton transfer for the FeBHT samples with higher catalyst loading. The reproducibility of the polarization curves was also limited for FeBHT. Successive scans generated smaller overpotentials and improved charge transfer properties. This could be correlated to the observed removal of FeBHT from the electrode surface during electrochemical testing as we have shown that FeBHT is more active for the HER at lower catalyst loadings. For NiBHT, overpotentials as low as 331 mV were achieved at a Cdl value of 6.29 mF/cm2 (Figure 2, Table 1, Figure S8, Table S2). A broad reduction feature is observed at ~0 V for NiBHT and is assigned to reduction of the framework. This feature has been previously reported for NiBHT and the related nickel benzene-1,2,4,5-tetrathiolate coordination polymer (Figure S9).26, 44 Higher electrocatalytic activity was associated with larger Cdl values and reduced Rct (Table S2-S3). The value of Rct was dependent on the applied overpotential as at more negative potentials

the charge transfer resistance was reduced indicating faster HER kinetics (Figure S10-S11, Table S3). The intensity of the low frequency feature in the Bode plot (Figure S11) was diminished and shifted to higher frequencies at larger overpotentials. The Tafel slope did not change with Cdl, but the exchange current density (j0) increased with more electrochemically accessible active sites (Figure S12).49 Controlled potential electrolysis (CPE) of NiBHT at 0.72 V revealed a steady current response for the first hour followed by a gradual decline in generated current (Figure S13). The current drop-off was a response to the delamination of NiBHT from the electrode surface as the black insoluble catalyst was seen suspended in solution and minimal NiBHT was observed on the GCE surface after 7 h of electrolysis. ICP-OES studies revealed no solubilized nickel species confirming the decline in activity was due to catalyst delamination not dissolution. Analysis of the gaseous products by gas chromatography after 1 h of CPE confirmed H2 production with the Faradaic efficiency (FE) ranging from 65-75%. Most nickel dithiolene based catalysts operate at low to moderate efficiencies (FE ~ 6080%)26, 55-56 in comparison to cobalt dithiolene catalysts, which perform at high efficiencies (FE > 90%).14, 25, 28, 57 Xray photoelectron spectroscopy (XPS) following CPE revealed nickel and sulfur features analogous to those of the as-prepared NiBHT (Figure S14-S15).44

Figure 2. (a) Polarization curves for CoBHT (pink), NiBHT (cyan), and FeBHT (blue); scan rate: 100 mV/s, (b) The current density difference (Δj = ja – jc) at 0.15 V vs RHE plotted against the scan rate (markers); the Cdl values are estimated through linear fitting of the plots (dashed lines). All measurements were carried out in N2-saturated pH 1.3 H2SO4 solutions at room temperature.



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Table 1. Summary of the Electrocatalytic Hydrogen Evolution Properties of Metal Benzenehexathiolate Coordination Frameworks Sample

η @ 10 mA/cm2 (mV)

Cdl (mF/cm2)

Rct (Ω)

Tafel slope (mV/dec)

j0 (A/cm2)

CoBHT

185

27.1

64.4 (η = 270 mV)

88

10-4.9

NiBHT

331

6.29

218.5 (η = 370 mV)

67

10-7.8

FeBHT

473

1.43

102.8 (η = 470 mV)

119

10-6.7

Comparison of the HER Activity of CoBHT, NiBHT, and FeBHT. Polarization curves of CoBHT, NiBHT, and FeBHT are presented in Figure 2a highlighting the influence of the metal center on the HER activity of benzenehexathiolate-based coordination frameworks. CoBHT displayed the lowest overpotential, 185 mV, in comparison to NiBHT (331 mV) and FeBHT (405 mV). The increased activity of CoBHT is correlated to a higher electrochemically accessible surface area (Figure 2b) and lower charge transfer resistance at smaller overpotentials (Table 1, Figure 3a). NiBHT and FeBHT exhibit very large Rct values at -0.27 V vs RHE inhibiting high electrocatalytic HER at low overpotentials (Figure S16-S17, Table S4), however, at larger overpotentials, the Rct values decreased significantly (Figure 2a, Figure S16-S17, Table S4). For NiBHT, Rct decreased from 5,316 (η = 270 mV) to 218.5 Ω (η = 370 mV), while for FeBHT, a comparable Rct to CoBHT and NiBHT was not achieved until η = 470 mV. The Rct for FeBHT decreased from 1.8 × 104 (η = 270 mV) to 102.8 Ω (η = 470 mV). CoBHT is the superior HER catalyst stemming from the significantly enhanced charge transfer through the material at modest overpotentials and greater number of electrochemically accessible active sites. Higher electrocatalytic HER activity was achieved for CoBHT at all film thicknesses investigated (~20-1000 nm) in comparison to NiBHT (131 nm) even as the number of electrochemically accessible active sites decreased for CoBHT (Table 2). The larger j0 of CoBHT at all thicknesses compared to NiBHT highlights the higher intrinsic electrocatalytic HER activity of CoBHT (Table 1, Table 2). To further improve our understanding of how the metal center influences the electrocatalytic HER activity, the conductivity of the BHT-based coordination frameworks is currently under investigation in our laboratory to gain a complete understanding of the origin of the distinct electrochemical responses observed for CoBHT, NiBHT, and FeBHT. Tafel analysis revealed Tafel slopes of 88, 67, and 119 mV/dec for CoBHT, NiBHT, and FeBHT, respectively (Figure 3b, Table 1). As with previously reported metal dithiolene-based coordination frameworks and polymers, the Tafel slopes indicate the Volmer discharge reaction is the rate-limiting step.25-26, 28-30 The Tafel slope of FeBHT, 119 mV/dec, is much larger than that of CoBHT and NiBHT. The electronic structure of the analogous molecular complex, [Fe(bdt)2]2⁻, could provide insight into the origins of the larger Tafel slope and greater barrier for Hads. The Fe 3d orbitals are much higher in energy than the ligand (benzenedithiolate) orbitals resulting in a predominately metal centered singly occupied molecular orbital (SOMO). The

electronic structure is best described as a spin triplet FeII coordinated to two innocent benzenedithiolate ligands, [FeII(bdt2⁻)2]2⁻.58-59

Figure 3. (a) EIS spectra measured at -0.27 V, -0.37 V, and 0.47 V vs RHE presented as Nyquist plots (markers) with respective fits (solid line) for CoBHT (pink), NiBHT (cyan), and FeBHT (blue), respectively. (b) Tafel analysis of CoBHT (pink), NiBHT (cyan), and FeBHT (blue). All measurements were carried out in N2-saturated pH 1.3 H2SO4 solutions at room temperature.

Computational studies propose the molecular complexes, [Co(bdt)2]⁻ and [Ni(bdt)2]⁻ (bdt = benzene-1,2dithiolate), undergo relatively similar HER mechanisms with the thiolate moieties of the ligand scaffold the most likely site for initial protonation.55, 60 The non-innocent nature of these complexes arises because the SOMO of [Co(bdt)2]⁻ contains signiVicant mixed-metal ligand character, while the SOMO for [Ni(bdt)2]⁻ is mostly ligandbased.58-59 The lack of non-innocence in the benzenedithiolate ligands and metal dominant SOMO of [Fe(bdt)2]2⁻


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could give rise to a different mechanism for HER than the computational studies propose for [Co(bdt)2]⁻ and [Ni(bdt)2]⁻. The increase in effective nuclear charge as the metal is changed from Fe to Co to Ni for the benzenedithiolate complexes modulates the metal and ligand character in the molecular orbitals, potentially resulting in different HER mechanisms and rate-limiting steps for the frameworks. More comprehensive theoretical studies are necessary to further understand the differences in the measured Tafel slopes.

Evaluation of the Thickness-Dependent HER Activity of CoBHT. Looking to understand proton and charge transfer in CoBHT to optimize the HER performance, we varied the amount of BHT used in the synthesis of CoBHT, which resulted in films of different thicknesses. The average film thickness ranged from 23 nm to 1 μm (Figure S19S22). It has been shown that hydrogen production can be dependent on the film thickness because of changes to the electron, charge, and proton transfer properties and the number of available and accessible active sites.47-50, 61-62 The effect of film thickness on the electrochemical hydrogen evolving activity of CoBHT was investigated on glassy carbon electrodes. To distinguish the electrochemical results of films of different thicknesses, the electrodes studied are denoted as CoBHT-X where X corresponds to the film thickness in nanometers. Scanning electron microscopy (SEM) images revealed morphological differences for the CoBHT films of different thicknesses (Figure S23-S27). Films within the thickness range of 20-250 nm exhibited a sheet like morphology expected for thin films. Puckering and wrinkling of the film was observed, which can help to expose more active sites for HER. The 1000 nm films displayed an extensively cracked and rough morphology with puckered and overlapping flakes of film. This type of morphology has the potential to expose more active sites and the interior of the thick film, which can induce higher electrocatalytic activity. However, the cracked morphology gives rise to extensive grain boundaries that can potentially inhibit electron transfer from the electrode surface to the exposed active sites, thus reducing the number of electroactive catalytic sites. Polarization curves of CoBHT modified GCE with 5 different film thicknesses, CoBHT-23, CoBHT-57, CoBHT-157, CoBHT-244, and CoBHT-1000 are presented in Figure 4a. As the film thickness was increased from 23 to 244 nm, the overpotential to achieve 10 mA/cm2 decreased from 246 mV to 185 mV (Figure 4a, Table 2). The reduced overpotential was accompanied by an increase in the Cdl from 2.1 to 27.1 mF/cm2. Within the thickness range 23 to 244 nm, an increase in the amount of deposited CoBHT resulted in the expected increase in the number of accessible active sites (Figure 4b, Table 2). As the thickness of the films increased to 1000 nm, reduced HER activity was observed and an overpotential of 213 mV was measured. Concomitantly, the number of electrochemically accessible active sites did not improve as the film thickness was increased from 244 to 1000 nm indicating not all the cobalt sites were electrocatalytically active. The substantial increase in mass loading for CoBHT-1000 did not result in a greater number of electrochemically accessible active sites as compared to films in the thick-

ness range of 20-250 nm, suggesting insufficient electron migration from the electrode surface through the 1000 nm thick film. The inability to electrochemically access the greater number of cobalt active sites associated with the large increase in film thickness from 244 to 1000 nm inhibits the electrocatalytic HER activity of CoBHT-1000.

Figure 4. (a) Polarization curves for CoBHT-23 (purple), CoBHT-57 (blue), CoBHT-157 (orange), CoBHT-244 (red), and CoBHT-1000 (green); scan rate: 100 mV/s, (b) The current density difference (Δj = ja – jc) at 0.15 V vs RHE plotted against the scan rate (markers); the Cdl values are estimated through linear fitting of the plots (dashed lines). All measurements were carried out in N2-saturated pH 1.3 H2SO4 solutions at room temperature.

The charge transfer resistance, extrapolated from EIS experiments, can reveal differences in the ability to move charge through CoBHT as the film thickness is modulated. EIS analysis at -0.27 V vs RHE for CoBHT-23, CoBHT-57, CoBHT-157, CoBHT-244, and CoBHT-1000 is presented in Figure 5a. The Rct decreases from 116 to 64 Ω as the film thickness increases from 23 to 244 nm. The improved charge transfer kinetics for CoBHT-244 facilitates electrocatalytic hydrogen evolution at smaller overpotentials. The intensity of the major feature of the Bode plot was reduced and shifted to lower frequencies for the most active electrode, CoBHT-244, confirming the superior HER kinetics (Figure S33). Further increases in film thickness to 1000 nm (CoBHT-1000) led to diminished charge transfer properties and HER kinetics as evidenced by the larger Rct (144.6 Ω) and the greater intensity of the major feature of the Bode plot (Figure S33). The larger charge transfer resistance as the film thickness reached 1000 nm indicates the charge transfer from the electrode to the CoBHT sur-



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face and finally to the protons in solution is faster for thinner films.50

Table 2. Summary of the Electrochemical Properties of CoBHT with Different Film Thicknesses for the Hydrogen Evolution Reaction Sample

Average Film Thickness (nm)

η @ 10 mA/cm2 (mV)

Cdl (mF/cm2)

Rct (Ω) @ η = 270 mV

Tafel slope (mV/dec)

j0 (A/cm2)

CoBHT-1000

1000

213

15.9

144.6

62

10-6.3

CoBHT-244

244

185

27.1

64.4

88

10-4.9

CoBHT-157

157

192

22.8

78.3

92

10-4.8

CoBHT-57

57

225

3.10

100.8

76

10-5.6

CoBHT-23

23

246

2.11

116.2

77

10-5.6

Figure 5. (a) EIS spectra measured at -0.27 V vs RHE presented as Nyquist plots (markers) with respective fits (solid line) for CoBHT-23 (purple), CoBHT-57 (blue), CoBHT-157 (orange), CoBHT-244 (red), and CoBHT-1000 (green) (b) Comparison of the current density (mA/cm2) versus thickness (nm) at -0.20 V vs RHE (blue), -0.22 V vs RHE (green), -0.25 V vs RHE (pink), and -0.27 V vs RHE (purple). All measurements were carried out in N2-saturated pH 1.3 H2SO4 solutions at room temperature.

CoBHT-1000 exhibited an overpotential superior to CoBHT-23 and CoBHT-57, however, the Rct indicates sluggish charge transfer properties as the HER reaction rate is enhanced at larger overpotentials. Analyzing the electrocatalytic current density versus the film thickness at four

potentials (-0.20, -0.22, -0.25 and -0.27 V vs RHE) revealed the HER activity increased until a thickness of 244 nm (Figure 5b). At 1000 nm, smaller current densities were generated at all potentials compared to CoBHT-157 and CoBHT-244. For CoBHT-1000, the higher catalyst loadings associated with increased film thickness did not result in larger current densities and lower overpotentials as compared to films in the thickness range of ~20-250 nm because of poor charge transfer properties and fewer electrochemically accessible active sites (Table 2). At small overpotentials, CoBHT-1000 was catalytically competent and generated higher current densities than CoBHT-57. However, at larger overpotentials (η = 270 mV), the HER activity of CoBHT-57 (25.1 mA/cm2) surpassed the activity of CoBHT-1000 (23.1 mA/cm2) confirming the results of the EIS experiments performed at this potential (Table 2, Figure 5). Sluggish electron and charge transfer and poor proton permeation through the thick film limited the catalytic performance at larger overpotentials (Figure 5).50 Since variances in bulk catalyst loading, which can lead to deviations in the measured Cdl and Rct values even for films of the same thickness, are expected because of the deposition utilized here (immersion of the electrode through the film), multiple CoBHT-modified electrodes were evaluated to assess the validity of the observed thickness-dependent HER activity. CoBHT-1000 modified electrodes reproducibly exhibited insufficient charge transfer and proton diffusion properties at large overpotentials limiting the HER activity (Figure S34). The larger electrochemically accessible surface areas achieved for CoBHT-1000 modified electrodes in comparison to CoBHT-23 and CoBHT-57 (Table 2, Table S5) initially resulted in higher HER performance (lower overpotentials to achieve 10 mA/cm2), however, this high activity could not be sustained at larger overpotentials as poor charge and proton diffusion rendered CoBHT-1000 catalytically incompetent. Therefore, an optimal film thickness of ~250 nm has been identified for CoBHT whereby proton diffusion, charge transport, and the number of electrochemically accessible active sites are maximized. The Tafel slopes were extrapolated from similar potential windows and ranged from 62-92 mV/dec for CoBHT


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modified GCE (Figure S35, Table 2). The Tafel slopes correspond to a rate-liming Volmer discharge step. The cobalt dithiolene-based coordination frameworks and polymers previously reported as HER catalysts all displayed large Tafel slopes suggesting the reaction was limited by insufficient adsorption of H+.25, 28, 30 Similar Tafel slopes and exchange current densities (Table 2) were measured for samples with relatively analogous film thicknesses (CoBHT-23/CoBHT-57 and CoBHT-157/CoBHT-244). Identifying if different barriers for Hads arise as the thickness is modulated from ~20-1000 nm is currently under investigation. The examination into the role of film thickness on the HER activity for CoBHT revealed an optimal thickness range (~250 nm) where charge transport, proton transfer, and the number of electrochemically accessible active sites were maximized. We have shown that the diminished charge and proton transfer properties and reduced number of electrochemically accessible active sites (even at higher bulk catalyst loadings) led to the poorer than expected HER performance of CoBHT-1000. In addition to the fundamental properties such as charge and proton transfer that dictate the electrocatalytic activity, practical device construction is an important factor to consider when determining the optimal catalyst loading and deposition method. The mechanical robustness of the deposited catalyst was greatly decreased as the film thickness was increased to 1000 nm. Cracking of the film and delamination from the electrode surface during electrochemical testing was common for CoBHT-1000, thereby limiting the practical feasibility of electrodes constructed with such high mass loadings. Therefore, it is important to consider the mechanical robustness of the catalyst-modified electrodes, in addition to electrocatalytic performance, when selecting an appropriate deposition method and catalyst thickness.

CONCLUSIONS In summary, cobalt, nickel, and iron coordination frameworks based on benzenehexathiolate were synthesized via an interfacial reaction and electrochemically investigated as hydrogen evolving catalysts in pH 1.3 aqueous solutions. CoBHT exhibited the lowest overpotential of 185 mV. The ability to operate at lower overpotentials for CoBHT was related to the improved charge transfer properties in comparison to NiBHT and FeBHT. Understanding the effect of film thickness on electrocatalytic activity, an insufficiently explored area in coordination framework electrocatalysis, is vital for determining the scalability and practicality of these systems as catalysts for energy conversion applications. Our exploration into the movement of charge and protons through the CoBHT films revealed thickness dependent HER activity. As the film thickness was increased, the overpotential initially decreased as higher accessible active sites were realized. An optimal thickness was achieved at 244 nm where charge transport, proton diffusion and transfer, and the number of electrochemically accessible active sites were optimized. As the film thickness was increased further to ~1000 nm, the charge transfer and proton permeation through the thick films was insufficient to promote high electrocatalytic activity espe-

cially when the HER kinetics were enhanced at larger overpotentials. Although the cracked and rough morphology of CoBHT1000 allowed for the exposure of more active sites, the introduction of significant grain boundaries diminished the charge transfer properties and hindered the electrochemical accessibility of many of the available active sites. Diffusion of protons and electrons through the thick film limited the overall HER performance canceling out any potential gains associated with increased mass loading. The identification of an optimized thickness (~250 nm) is valuable for informing device construction as the higher mass loading films (~1000 nm) display minimal mechanical robustness limiting their practical viability. The optimized CoBHT modified GCE, presented here, operate at overpotentials ranging from 185-246 mV representing, to the best of our knowledge, the highest performing HER activity of metal dithiolene based coordination frameworks. Understanding the ability to move charge and protons through these systems has allowed for the design and construction of heterogenized molecular catalysts displaying HER activity more reminiscent of traditional heterogeneous catalysts.

ASSOCIATED CONTENT Supporting Information. Cyclic voltammetry data, EIS circuit, EIS fit data, double-layer capacitance measurements, Bode plots, electrolysis results, and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT We are grateful to the University of Southern California (USC) and USC Women in Science and Engineering (WiSE) for funding. We thank Buddhinie Jayathilake and Prof. Sri R. Narayan for assistance with ZSimpWin software. XPS and SEM data were collected at the Center for Electron Microscopy and Microanalysis, USC. AFM measurements were collected using Prof. Mark E. Thompson’s instrument.

ABBREVIATIONS BHT, benzenehexathiolate; GCE, glassy carbon electrode; GR, graphite electrode; CV, cyclic voltammetry; Cdl, double-layer capacitance; FE, faradaic efficiency; SEM, scanning electron microscopy; EIS, electrochemical impedance spectroscopy; XPS, X-ray photoelectron spectroscopy; AFM, atomic force microscopy

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Evaluation of the H2 Evolving Activity of Benzenehexathiolate

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