Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from

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Redox Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework Hyunho Noh, Chung-Wei Kung, Ken-ichi Otake, Aaron W. Peters, Zhanyong Li, Yijun Liao, Xinyi Gong, Omar K. Farha, and Joseph T. Hupp ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02921 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Redox Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework Hyunho Noh,† Chung-Wei Kung,† Ken-ichi Otake,† Aaron W. Peters,† Zhanyong Li,† Yijun Liao,† Xinyi Gong,† Omar K. Farha†‡* and Joseph T. Hupp†* †

Department of Chemistry, and ‡Department of Chemical and Biomolecular Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ABSTRACT: The Zr6-based metal–organic framework, NU-1000, was successfully functionalized with candidate catalysts – MoSx units – via SIM (solvothermal deposition in MOFs) of molybdenum(VI), followed by reaction with H2S gas. The structure of the material, named MoSx-SIM, was characterized spectroscopically and through a single crystal X-ray diffraction measurement. These measurements and others established that the catalyst is monometallic, with mixed oxygen and sulfur coordination, and that it forms from a MOF-node-supported molybdenum-based cluster featuring only oxy ligands. Notably, the formal potential for the MOFgrafted complex, like that for the metal-sulfur active-site of hydrogenase, is nearly coincident with the formal potential for the hydrogen couple. Its effective concentration within the mesoporous MOF is several hundred millimolar, and its porous-frameworkbased immobilization/heterogenization obviates the need for aqueous solubility as a condition for use as a well-defined catalyst. MoSx-SIM was evaluated as an electrocatalyst for evolution of molecular hydrogen from aqueous acid. Although the MoS xfunctionalized framework exhibits catalytic behavior, the highly insulating nature of the support inhibits high electrocatalytic performance. Introduction of an archetypal redox mediator (RM), methyl viologen (MV2+), resulted in more than 20-fold enhancement in its catalytic performance on a turnover frequency (TOF) basis, implying efficient RM-assisted electron transfer (ET) to otherwise electrochemically non-addressable MoSx moieties. Electrochemical kinetic studies with additional viologens as mediators reveal an unexpected square-root dependence of overall reaction rate on mediator concentration, as well as sensitivity to the strength of RM+ as a reductant. These observations, together with observations of potential-dependent H/D isotope effects and potential-dependent pH effects, provide useful insights into the catalysis mechanism and help to explain how the MOF-affixed monometallic catalyst can effectively catalyze a two-electron reduction reaction, i.e. hydrogen evolution from acidified water.

KEYWORDS Hydrogen evolution reaction, electrocatalysis, redox mediator, metal–organic framework, molybdenum sulfide, site-isolated catalyst

INTRODUCTION Molecular hydrogen is used in enormous quantity both for ammonia synthesis1-2 and for syngas-based (i.e., FischerTropsch based) production of hydrocarbons.3-4 It also finds limited, but growing, commercial use in electricity-producing fuels cells, including cells that power vehicles.5-6 H2 is the ultimate clean-fuel as its direct combustion (or its indirect, but much more efficient, electrochemical consumption in fuel cells) produces only water as a chemical product. Most molecular hydrogen is derived from fossil fuels – for example, from methane via steam reforming or from coal via gasification.7 In principle, a much more sustainable, and carbon-neutral, route would be electrochemical or photoelectrochemical water splitting (the reverse of fuel-cell chemistry) with solar photons as the energy input.8-10 Either variant requires electrocatalysts, both for oxygen evolution and for the hydrogen evolution reaction (HER).10-14 While state-of-the-art electrocatalysts for hydrogen evolution generally contain precious and expensive metals, most often platinum,8,15-17 earth abundant and inexpensive molybdenum sulfide (MoSx)-based catalysts have, at least at the laboratory scale, exhibited prom-

isingly high electrocatalytic activity.18-25 Experimental21-22,26-28 and computational29-30 studies suggest that unsaturated Mo centers, present on MoSx edge sites, are primarily responsible for the catalysis. Hence, amorphous MoSx materials that present high surface densities of catalytically active edge sites often outperform their crystalline counterparts.18-20,31 Unfortunately, amorphization both complicates identification and impedes structural characterization of catalytic sites. To resolve structural ambiguity, attain site-uniformity, and thereby facilitate the experimental and computational interrogation of heterogeneous catalysts and their atomic-scale activity, chemists have begun exploring metal–organic frameworks (MOFs) as catalyst supports. The nodes of many MOFs consist of oxy-metal clusters that can be viewed as structurally well-defined analogues of conventional metal-oxide supports (e.g., alumina, zirconia, or ceria powders). The most useful of these clusters present readily accessible catalyst-grafting points, defined by reactive or displaceable –OH and/or –OH2 ligands.32-40 The porous, crystalline nature of MOFs facilitates: a) exact assessments of the proton topology of candidate grafting points,32-33,40-41 b) sterically uniform access to these points and to installed catalysts, and c) uniformity of Lewis and

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Brønsted acidities and/or basicities, for given structural types (e.g., bridging oxo/hydroxo, or terminal hydroxo/aqua sites).42 The uniformity or near-uniformity and periodicity of supporting nodes, as well as installed catalysts, facilitates experimental reproducibility and greatly enhances the practical value of computational modeling. Indeed, these kinds of systems lend themselves well to predictive reactivity and to mechanistically instructive experimental assessment of structureactivity relationships.32-37 Notably, the three-dimensional nature of hierarchically porous MOFs typically yields large internal surface areas and large densities of reactant-accessible catalysts.43 At the same time, the organic spacers constituting linkers between nodes can be remarkably effective for inhibiting or altogether preventing sintering or agglomeration of installed catalysts.44-45 These properties can be used to advantage to obtain structurally characterizable and catalytically active metallic, metal-oxide or -chalcogenide clusters, as well as monometallic species.46-47 Our preferred approach to system assembly has been postsynthetic installation of catalysts, either through vapor-phase atomic layer deposition in MOFs (AIM),35-37,48 or through solution-phase solvothermal deposition in MOFs (SIM).3235,39,49 The resulting arrays have proven catalytically competent for gas-phase chemical and for condensed-phase chemical, photochemical, and electrochemical transformations. Herein, we present the functionalization of a Zr6-based MOF, NU-1000 (Figure 1), with MoSx through SIM (the material here onwards named MoSx-SIM, with the precise structure of the resting state of the catalyst active-site being established by X-ray methods). The MOF consists of eight-connected Zr6(µ3– nodes and 1,3,5,8-(pO)4(µ3–OH)4(H2O)4(OH)4 benzoate)pyrene linkers (TBAPy4-).40-41 The framework offers multiple catalyst-grafting points41 and high hydrothermal stability,50 making it a potentially attractive heterogeneous support for electrocatalytic HER. Notably, the MOF can be readily physically integrated, in thin-film form, with conductive glass or other electrode materials – either before or after catalyst installation. The highly insulating nature of this MOF,51 however, prevents the vast majority of installed candidate electrocatalysts from being probed electrochemically or contributing to desired electrochemical transformations.14,52-53

Figure 1. Non-functionalized metal–organic framework, NU1000 with corresponding structures of the inorganic nodes and the organic linkers. Three of the twelve reactive hydroxo/aqua groups on the node are highlighted.

To overcome this problem, and enable delivery of reducing equivalents from the electrode to locations throughout the

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catalyst-functionalized MOF, we have introduced and examined three molecular-scale redox mediators (RMs, Scheme 1). Consistent with the hierarchical porosity of NU-1000 (Figure 1),40 we find that dissolved RMs can readily diffuse to the MOF-affixed MoSx catalysts and substantially boost overall catalytic rates. As described and discussed below, we additionally find that by changing the concentrations of RMs, by varying their formal potential (Ef), and by altering the amount of substrate (here via changes in pH), substantial mechanistic insight, specifically for aqueous electrocatalytic generation of H2 by heterogenized MoSx species, can be obtained.

Scheme 1. Proposed Redox Mediator-Assisted Electrocatalytic System with Various Redox Mediators

RESULTS AND DISCUSSION Characterization of MoSx-SIM: The MoSx-SIM synthesis is detailed in the Methods section. N2 adsorption-desorption isotherms reveal small decreases in Brunauer–Emmett–Teller (BET) surface area (from 2100 to 1700 m2/g) upon installation of molybdenum oxide clusters, the material named MoOxSIM synthesized according to the reported procedure,34 and to 1400 m2/g following sulfidation to MoSx-SIM (Figure 2a). (The differences in BET surface area are considerably lessened if account is taken of increases in sample mass upon introduction of molybdenum and upon replacement of selected oxygen-containing ligands by their sulfur-containing analogues and areas are expressed in volumetric (m2/cm3) rather than gravimetric (m2/g) form; see Table S1.) The preservation of the type IVc feature in the isotherm of MoSx-SIM implies that the MOF’s mesoporosity is retained. Indeed, density functional theory (DFT)-calculated pore size distributions (based on experimental N2 isotherm measurements) indicate only a modest decrease in mesopore size, i.e. from 30 to 27 Å (Figure S1). MoSx installation decreases the total pore volume by about 20% – more-or-less what one would anticipate based on scaling of pore volume with the square of the channel diameter; see Table S1. Thermogravimetric analysis (TGA) profiles of MoOx-SIM and MoSx-SIM, assessed at 600 °C, revealed additional residual masses of 27 and 32 wt %, respectively, relative to NU-1000. The added masses are attributed to the deposited MoOx or MoSx species that remain within the decomposed framework (Figure S2). The 5 wt % difference between MoOx-SIM and MoSx-SIM suggests that after treatment with H2S, partial replacement of oxo, hydroxo, and/or aqua ligands by sulfur-containing ligands (e.g. sulfide, disul-


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Figure 2. a) N2 isotherms at 77 K of NU-1000, MoOx-SIM, and MoSx-SIM. Filled and unfilled data points represent adsorption and desorption, respectively. The persistent step near P/P0 = 0.25 points to retention of the characteristic mesoporosity of NU-1000. b) The MoSx-anchored Zr6 node of the representative structure of MoSx-SIM showing all the crystallographically distinct MoSx units (Mo1-03) and the framework viewed along the c) c-axis (the minor structure Mo3 is omitted for clarity) and the d) a-axis. ure S5b).21 The ligation of sulfur(2-) by the Mo center is also fide, and/or HS–) has occurred. Scanning electron microscopy (SEM) images show that no changes in MOF crystallite morevident from the extended X-ray absorption fine structure phology40 accompany MoSx installation, and energy dispersive (EXAFS) spectrum (Figure S6). Namely, the phaseuncorrected peaks at 1.4 and 1.9 Å are attributable to Mo–O spectroscopy (EDS) line scanning shows a uniform distribution of Mo after H2S treatment (Fig ure S3a). As the Mo Land Mo–S single scattering events, respectively. The relatively low intensity of the features at 2.2–3 Å, attributed to a combiedge and the S K-edge energies significantly overlap, inductively-coupled plasma optical emission spectroscopy (ICPnation of multiple scattering events (e.g. Mo–O–Zr, Mo–O– OES) was used to determine the average Mo and S loadings, Mo, and/or Mo–S–Mo), suggests the presence of mononuclear MoSx or perhaps the MoSx clusters composed of only a few with the results being 2.6 ± 0.2 Mo/Zr6 and 2.5 ± 0.2 S/Mo; Mo atoms within the framework, similar to what has been the Mo loading is in agreement with the previously reported observed for MoOx-SIM.34 value for MoOx-SIM.34 As evidenced by EDS line scans (Figure S3d), we find that H2S exposure of unfunctionalized NUTo further elucidate the structure, we turned to single-crystal 1000 results in no detectable sulfur incorporation. X-ray diffraction measurements. (See the SI for measurement and refinement details.) While almost all of the MoOx units in Diffuse-reflectance infrared Fourier transform (DRIFT) specMoOx-SIM reside, in monometallic form, within the ~10 Å tra reveal that the intensities of peaks associated with the micropore between the two nodes parallel to the crystalMOF-node-based terminal/bridging –OH stretches at 3674 cm1 lographic c-axis (here onwards named "c-pores"; ca. 90% of for both MoOx-SIM and MoSx-SIM are much weaker than MoOx is estimated to reside within the c-pores; see Figures that of NU-1000 (Figure S4).40 This observation suggests that S7-10 and Table S2),55 treatment with H2S results in migration Mo remains grafted to the oxy-Zr6 node even after treatment of roughly two-thirds of the installed Mo to oxy ligands diwith H2S. Mo 3d X-ray photoelectron spectroscopy (XPS, rected toward the 31 Å mesopore, i.e., one bridging oxo and Figure S5a) revealed a 2.4 eV negative shift in binding energy one bridging hydroxo per isolated molybdenum ion, as shown with H2S treatment, implying molybdenum reduction from in Figure 2. The Mo sites directed toward mesopores are disoxidation-state VI to IV, and replacement of at least some oxo ordered over three positions, two of which are crystallographor hydroxo ligands with electron-rich sulfur-containing ligands. ically independent (labeled Mo1 and Mo3 in Figure 2b and in We note the similar shift in binding energy observed upon the SI). In contrast, the fraction Mo ions remaining in the ctreating thin layers of MoO3 with H2S.54 The 3d5/2 binding pores are disordered between two equivalent positions (Mo2 energy of MoSx-SIM is ~1 eV more positive to that of the 20 in Figure 2d). Like those in the mesopores, however, the mobulk MoS2, which may be attributed to coordination of Mo to lybdenum units are sited as monometallic complexes, rather the highly electron-deficient Zr6 node. S 2p XPS shows that than multi-molybdenum clusters. nearly all sulfur in the structure is in oxidation state 2– (Fig-


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Electrocatalytic HER: The monomeric MoIV(SH)2 units structurally resemble edge sites of MoS2, i.e. sites considered primarily responsible for molybdenum disulfide’s electrocatalytic activity.21-22,26-28 Thus, the electrocatalytic HER performance of MoSx-SIM was examined; for experimental details, see the Methods section. As illustrated in Figure S11, cyclic voltammograms (CVs) of all samples in aqueous H2SO4 at pH = 1.2 show at the scan reversal point of –520 mV vs. RHE (reversible hydrogen electrode),56 current densities (j) that are small and that are “more similar than different.” Brief bulk electrolysis measurements (5 min. at –420 mV vs. RHE) yielded no detectable H2 with MOF films of either MoOx-SIM or NU-1000 during a bulk electrolysis experiment. In contrast, MoSx-SIM produced small, but detectable, amounts of H2, corresponding to a turnover number (TON) of 1.8 ± 0.5 and a turnover frequency (TOF) of 0.36 ± 0.09 min-1, if all molybdenum atoms are included in the calculations. It is tempting to attribute the poor catalytic performance to the aforementioned insulating character of the supporting framework,51 and resulting electrochemical inaccessibility of all but a small fraction of MoSx sites, i.e. those sites located proximal to the underlying conductive-glass electrode (fluorine-doped tin oxide (FTO)). This explanation is consistent with the comparatively small sizes of the voltammetric peaks assignable in Figure S11 to MoOx-SIM and MoSx-SIM. Indeed, integration of the peaks implies electrochemical accessibility of only those molybdenum ions lying within about 20 nm of the underlying electrode, corresponding to only 0.3 % of the total amount of species (see Figure S12 for the film thickness measurement via SEM that was used to calculate the aforementioned values). The voltammograms point to very similar or identical formal potentials (Ef) for framework-supported MoOx and MoSx i.e. values of roughly −20 mV vs. RHE – more-or-less ideal if the ultimate goal is a reversible, heterogeneous, single-metal-atom electrocatalyst. With these baseline results in hand, we turned to methyl viologen (MV2+) as a candidate redox mediator; see Scheme 1. The Ef for the MV2+/+ couple (–360 mV vs. RHE), suggested that electrochemically generated MV+ would have sufficient reducing power to drive the putative HER catalyst, Mo(SH)2. The small size of the mediator relative to the pores of the MOF, together with the high aqueous solubility of the chloride salt of MV2+, suggested to us that it would effectively permeate NU-1000 and shuttle electrons to otherwise nonaddressable Mo(SH)2 units. The voltammograms in Figure 3 show that the viologen couple: a) is chemically reversible in aq. H2SO4, b) can readily reach the FTO electrode and accept electrons, but c) is ineffective on its own, or in combination with Nafion, NU-1000, or MoOx-SIM, in catalyzing hydrogen evolution. In striking contrast, in the presence of MoSx-SIM the mediator yields a classic electrocatalytic wave, i.e. a sigmoidal voltammogram lacking hysteresis and plateauing well above the peak current for reduction of the mediator in the absence of catalyst (Figure 3). The onset of the catalytic wave, relative to the Ef of the mediator, contains information about the rate of reaction of the mediator with the catalyst (node-attached MoS2) and/or the rate of the reaction of the catalyst with H+. The height of the wave is a measure of the maximum catalytic rate for a given set of conditions and is typically limited by the rate of one of the component reactions shown or implied in Scheme 1, but, under some condition, can be limited by the rate at which

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the mediator can diffuse between the electrode and catalyst. The overall shape of the wave can, to a first approximation, be understood in terms of the fraction of mediator present in the catalytically useful reduced form at the electrode/solution (porous MOF) interface at a given applied potential. Returning to Figure 3, the introduction of 10 mM MV2+ increases the catalytic current at –500 mV vs. RHE (i.e. near the top of the catalytic wave) nearly 60-fold, and decreases the apparent Tafel slope (slope of plot of applied potential vs. log (catalytic current)) from –200 mV/decade to –86 mV/decade (Figure S13a). Electrolysis for five minutes at a fixed potential of –420 mV vs. RHE yielded a TON of 38 ± 5 hydrogen molecules per MoS2 site (based on measurement of the amount of H2 generated), and a TOF of 8 ± 1 min-1; for all chronoamperometric curves recorded during the bulk electrolysis, see Figure S14).57

Figure 3. CVs of the bare FTO, Nafion-coated FTO (labeled "Nafion"), NU-1000, MoOx-SIM, and MoSx-SIM measured in pH 1.2 H2SO4 solution with 10 mM of MV2+. A CV of MoSxSIM without MV2+ is included for comparison.

Powder X-ray diffraction (PXRD) patterns of MoSx-SIM after H2S treatment, after drop casting, and after electrolysis all show that the framework has stayed intact (Figure S16). SEM images of the MoSx-SIM electrode before and after electrolysis show no morphological differences and the EDS line scans show that Mo remains uniformly distributed within the MOF (Figure S3b and c). Transmission electron microscopy (TEM) images of the sample after catalysis (Figure S17) offer no indication of nanoparticle formation (for example, via agglomeration of the Mo(SH)2 units). ICP-OES measurements of a digested sample of MoSx-SIM, following use as a catalyst for bulk electrolysis indicated loss of a small fraction of catalyst (0.6 ± 0.1 Mo/Zr6). Notably, bulk electrolysis of the solution into which Mo had leached generated no detectable H2. Mediator Formal Potential Effects on the Electrocatalysis: We reasoned that if the rate of electrocatalysis is limited by the rate of electron transfer (ET) from the mediator to the catalyst, the system response would be sensitive to the Ef of the RM2+/+, as a more negative Ef would correspond to a greater thermodynamic driving force (ΔGRMET) for mediator-tocatalyst ET. We selected for screening diaquat dibromide (DQ2+; Ef = −289 mV vs. RHE) and triquat dibromide (TQ2+; Ef = −531 mV vs. RHE); their structures are shown in Scheme 1. (See Figure S18 for CVs of bare FTO, nafion-coated FTO, NU-1000, and MoOx-SIM in the presence of 10 mM DQ2+ or


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Figure 4. CVs of MoSx-SIM in pH 1.2 H2SO4 solution with either b) 10 or c) 2 mM of various redox mediators. d) Faradaic efficiency (FE) plotted against the concentration of redox mediators. e) log(TOF) vs. log([RM2+]) plots, of which the slopes indicate the reaction order.

TQ2+.) Shown in Figure 4a are slow-scan-rate voltammograms of MoSx-SIM (on FTO) without and with 10 mM of each of the three candidate mediators. (Similar plots are shown in Figure 4b for RM concentrations of 2 mM.) Although the measurements are constrained (especially for TQ2+) by scan reversal at –520 mV, it is clear that plateau currents for electrocatalytic waves increase in the order DQ2+ < MV2+ < TQ2+. This trend is consistent with the notion that the rate of electrocatalysis is governed, at least in part, by the driving force and the rate of reaction for ET from the mediator to the catalyst. Figure 4c presents Faradaic efficiencies (FE values) for H2 formation via electrolysis at a fixed potential of –420 mV vs. RHE. Ideally, the FE values should approach 100%, provided only negligible amounts of RM+ escape from the MOF film. Particularly for DQ2+ at concentrations of 5 and 10 mM, however, the FE values fall well below 100%. They can be largely recovered, however, by changing the potential for electrolysis to – 349 mV (= Ef(DQ2+/+) – 60 mV). We have been unable to identify cause for the diminished FE. Given the sensitivity to electrolysis potential and to mediator concentration, however, we speculate that at sufficiently high concentrations, DQ+ dimerizes58 and that the dimer is only marginally competent, if at all, for delivery of reducing equivalents to catalytic Mo(SH)2 sites. As shown in the SI, a further complication is the chemical instability of TQ+ in acidic solutions. As such,

further investigation of triquat as a redox mediator was not pursued. By varying the RM2+ concentration and measuring the TOF through bulk electrolysis – again at –420 mV vs. RHE – a reaction order of ~0.5 was observed for both MV2+ and DQ2+. (See Figure 4d; for CVs with various concentrations, see Figure S21.) The unexpected fractional reaction order rules out many mechanisms for mediated electrocatalysis. A potential mechanism that does yield a reaction order of 0.5 for RM2+ is summarized by Equations 1–6. The required conditions are that: 1) the first or second ET to MoSx participates in, or precedes, the rate-determining step (RDS),59 2) electrogenerated RM+ exists in a steady-state, and 3) due to electrochemical reduction, the concentration of RM2+ within the pores of MoSx-SIM at any given time is significantly lower than the concentration in bulk solution. (For the mathematical derivations, see the SI.) Note that steps 3 and 4, in some form, conceivably are interchangeable (i.e. the initial reduction step occurs first, followed by proton uptake, and then reduction by a second electron); if so, the square-root dependence of overall catalytic rate on RM2+ concentration would still hold (note that this simultaneously introduces complex pH dependence, see the SI).


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Figure 5. a) CVs of MoSx-SIM with pH or pD = 1.2 electrolyte solutions with 10 mM MV2+, which was used to derive b) KIE between –300 to –520 mV vs. RHE. c) CVs of MoSx-SIM with various pH electrolytes with 10 mM MV2+. d) log(TOF) vs. –pH plot of which the slope indicates the reaction order relative to H+. k1 catalytic rate (Figure 5d). Namely, when the pH is below 1.5 (1) 2RM 2RM2+ + 2eand the potential of the working electrode is fixed at –300 mV k-1 vs. RHE, a zero-order dependence on [H+] is observed, sugk2 gesting that RM-assisted ET alone defines the RDS, in agree(2) RM + (Mo IV(SH)2)2+ RM2+ + (Mo III(SH)2)+ ment with the kinetic isotope results. When the pH is ink-2 k3 creased from 1.6 to 2 and 2.5, the reaction order in protons RM + (Mo III(SH)2)+ (3) RM2+ + MoII(SH)2 changes to ~1. At pH 1.6, the rates for ET and proton transfer k-3 are evidently equal, or nearly so. k4 II + IV + Extending the applied potential to –450 mV vs. RHE, and (4) Mo (SH)2 + H (Mo (Hydride)(SH)2) k-4 therefore generating greater amounts of MV+ (and boosting k5 the rates of ET steps 2 and 3), we find that the dependence of (5) (Mo IV(Hydride)(SH)2)+ (Mo IV(S)(SH))+ + H2 the overall reaction rate on [H+] persists down to pH 1 (the k6 lowest pH examined). The relevant data points in Figure 5d (6) (Mo IV(S)(SH))+ + H+ (Mo IV(SH)2)2+ admittedly are somewhat scattered (R2 = 0.95); nevertheless, a reaction order in protons of close to 1 (i.e. ~0.77) is clearly As the aforementioned applied potential (−420 mV vs. RHE) evident. We conclude that proton transfer defines, or particiis 60 mV negative of the Ef(MV2+/+), the applied potential for pates in, the overall reaction rate. (See the SI for the rate exthe DQ2+-assisted system was also set to 60 mV negative of pression in the two aforementioned scenarios.) Equation 4, the mediator’s Ef (i.e. −349 mV vs. RHE). (Note that at this the formation of a molybdenum-bound hydride from H+ and a potential, Faradaic efficiencies are reasonably close to 100%; reduced catalyst, is one possibility for the overall mixed rate see Figure 4c.) Figure 4d shows that while TOF values are control.60 lower at –349 mV than at –420 mV, the apparent reaction order is unaffected. This observation implies that the first or Foot-of-the-wave Analysis: The preceding discussion is fosecond ET, i.e., the reaction in Equation 2 or 3, still particicused on identifying rate-defining steps, but leaves unquantipates in, or precedes, the RDS.53 fied the magnitudes of rate constants and thus, intrinsic catalytic activity. The foot-of-the-wave analysis method (FOWA) pH Effects on the Electrocatalysis: Despite the observed developed by Savéant and co-workers61 has been widely apsquare-root dependence of the overall rate of electrocatalysis plied to both homogeneous and heterogeneous electrocatalytic on mediator concentration, it is conceivable that subsequent systems in order to understand the intrinsic catalytic perforproton-dependent steps function as the reaction bottleneck (see mance of a system, where the performance is typically quantithe SI for details). Comparison of electrocatalytic waves for fied in terms of a potential-independent second-order rate conMoSx-SIM in H2SO4/H2O versus D2SO4/D2O suggests particistant (k) and overpotential (η) dependent TOFs.62 To our pation of H+ in the overall reaction rate (Figure 5a), but only knowledge, the FOWA approach has yet to be applied to meat potentials more negative than about –300 mV vs. RHE. As diated electrocatalysis of hydrogen evolution in aqueous solushown in Figure 5b, the H/D kinetic isotope effect (KIE) vartions. One potential roadblock to using this analysis in an ies from 1 at –300 mV (implying lack of participation H+ in aqueous system is that cathodic peak current of the catalyst the RDS) to greater than 2 at –500 mV vs. RHE (now imply(ip0) in the absence of the substrate (i.e., H+ or H2O) cannot be ing H+ participation). Support for this conclusion is provided trivially determined.63 However, in the present system, at sufby CVs in Figure 5c, where the pH of the electrolyte is varied 2+ ficiently low pH and sufficiently low applied potential, profrom 1 to 2.5 while holding the concentration of MV in solutons are enlisted only after the RDS. Under these special contion constant. Significant differences in catalytic current, at ditions, the FOWA can be implemented by defining RM+ and different pH values, are evident only at applied potentials reduced MoSx as the catalyst and substrate, respectively. To more negative than –300 mV vs. RHE. this end, we define ip0 to be the peak current for the reduction Bulk electrolysis experiments at fixed applied potentials of – of RM2+ in the absence of MoSx, i.e., when a non-catalytic 300 or −450 mV vs. RHE, in a series of solutions ranging in MoOx-SIM-coated FTO is used as the electrode (see the SI pH from 1 to 2.5, also show that the applied potential and the for detailed calculations). Values of the second-order rate consolution proton activity act together to determine the signifistant, kRM, reflective of the kinetics of ET (i.e. Equations 2 cance (or not) of proton uptake in defining the overall electro-


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Figure 6. The foot-of-the-wave analysis of MoSx-SIM with 10 mM of MV2+ or DQ2+ in the pH 1.2 H2SO4 electrolyte. a) !!

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! ! !!

vs.

! 1 + exp 𝐸 − 𝐸!" plots, of which the slopes indicate the value of the second order rate constant (k), and b) !!/∙! !" log(TOF)-η plots. achieve with a homogeneous monometallic catalyst or with a and 3), determined from the FOWA, are shown in Figure 6a non-porous heterogeneous catalyst. Notably, the Ef for the and Table 1. The larger value for kRM for the MV2+/+ is consistent with the aforementioned order of the reducing power. MOF-grafted catalyst, like that for the metal-sulfur active-site Further using kRM yields log(TOF) versus η plot (where η is of hydrogenase, is nearly coincident with the formal potential for the hydrogen couple. In electrode-supported thin-film form, the overpotential for HER, Figure 6b). Intriguingly, the ordering of intrinsic TOF values at η = 0 mV (denoted as TOF0) the functionalized MOF is weakly electrocatalytic for hydrogen evolution from aqueous acid. The initially marginal elecis the opposite of that for kRM. Namely, the TOF0 in the trocatalytic activity of the material denoted MoSx-SIM, is DQ2+/+-mediated system is more than an order of magnitude greater than that in the MV2+/+-mediated system (Table 1), largely due to the insulating nature of the electrode-supported thin-film MOF–which leaves the overwhelming majority of behavior that is also reflected in the less negative onset potential for mediated electrocatalysis (Figures 4a-b). At low or the potentially catalytic sites electrochemically isolated from zero overpotential for the HER itself, the fraction of mediator the underlying electrode. The introduction of viologen-type redox mediators in the solution phase is followed by their present in reduced form (i.e., reactive form, RM+) is considerpermeation of the functionalized MOF and diffusion through ably less for methyl viologen than for diquat – enoughless to more than offset the greater driving force and larger kRM value the framework to an underlying electrode poised to reduce the mediators. The reduced mediators then deliver electrons in for reaction of MV+ versus with DQ+ with the MOF-noderepetitive fashion to otherwise isolated catalytic sites, thereby supported molybdenum catalyst. engendering sizeable catalytic currents and readily detectable Table 1. Second-order rate constants (kRM) and intrinsic production of molecular hydrogen. Mechanistic studies show TOF at zero overpotential (TOF0) of methyl viologen and that overall control of the rate of catalysis can be defined by diquat-assisted electrocatalysis estimated through the footmediator-to-catalyst ET, solution-to-catalyst proton transfer, of-the-wave analysis or both. We suggest that RM-assisted electrocatalysis can be profitably extended to other reactions and other catalysts supk TOF RM 0 Ef vs. RHE RM ported on highly porous, but problematically electrically insu(mV) (M–1 s–1) (×10–3 min–1) lating MOFs, and thus can further contribute to atomically precise understandings of mechanisms and of structureDQ2+ –289 16 ± 1 24 ± 2 activity relationships for other electrocatalytic systems. 2+

MV

–360

28 ± 2

1.9 ± 0.2

METHODS CONCLUSIONS The water-stable Zr6-based MOF, NU-1000, is amenable to functionalization with molybdenum and sulfur, elements that yield a catalyst that is functional for hydrogen evolution from acidified water. Single-crystal X-ray diffraction and related measurements show the catalyst to be a monometallic complex of molybdenum grafted to the hexa-zirconium(IV) node of the MOF by a bridging oxo ligand and a bridging hydroxo ligand, with the molybdenum center also coordinating, in its resting state, a pair of non-bridging SH– ligands. Siting the singlemetal-atom catalyst on the MOF permits it to be presented to the infilling aqueous solution at a concentration of more than 100 mM, in a zone just a few microns thick, exclusively proximal to the electrode – a desirable but challenging scenario to

MoSx-SIM Synthesis: MoOx-SIM was synthesized according to the reported procedure.34,64 Approximately 50 mg of the freshly activated MoOx-SIM was mounted onto a custom-made stainless steel sample holder, further mounted onto a sample holder chamber in an ALD reactor (Savannah S100, Cambridge Nanotech Inc.). The sample chamber was set to 125 °C at all times. Prior to any H2S pulsing, the sample was left in a N2 flow at approximately 0.6 Torr for 30 min. Subsequently, the stop-valve was closed and H2S was pulsed for 0.015 s followed by a 120 s exposure time to allow diffusion of the H2S molecules throughout the MoOx-SIM crystallites. The stop-valve was then opened and the sample chamber was purged under vacuum for 120 s afterwards to remove excess H2S. This process was repeated for 150 cycles (or 3 cycles/mg) and the resulting sample was stored in an argon-filled glove box. Electrocatalytic HER: All MOF-based electrodes were synthesized through simple drop casting of the sample suspended in a Nafion/ethanol solution. Briefly, 0.3 mL of Nafion solution was added to


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2.7 mL of ethanol and the mixture was dried using MgSO4. After filtration of the MgSO4 powder, 0.5 mL of the solution was used to disperse 6 mg of a MOF sample. In the case of MoSx-SIM, this was conducted using a N2-connected standard Schlenk line and a 2–5 mL Biotage microwave vial to minimize the exposure to oxygen. The mixture was sonicated for 5–10 min to form a suspension, and 6 µL of the suspension was dropcasted onto a FTO conducting glass with an exposed area of 0.25 cm2. The electrochemically active exposed area was controlled with insulating polyamide tape. The electrodes were left to dry and were used as the working electrode in a standard H-cell setup with a septum seal for collecting H2 gas. For MoSx-SIM-based electrodes, the drop casting and electrochemical cell fabrication were all done in an argon-filled glove box. Prior to any electrochemical measurements, the electrolyte (with or without RMs; for the triquat synthesis and characterization, see the SI) was purged with N2 gas for 20 min. CVs were taken with a 25 mV/s scan rate with a platinum coil and a Ag/AgCl electrode (3M) as the counter and reference electrodes, respectively (see the SI for details). Bulk electrolysis experiments were performed for 5 min and 200 µL of the headspace were injected into a GC for analysis of headspace composition. TOF was estimated by determining the total Mo loading via a UV-visible spectrum of a decomposed MoSx-SIM (for details in GC or TOF calculation, see the SI).

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no financial interest.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at ACS Publication Website. Detailed Material Synthesis and Characterization (PDF) Crystallographic information file of MoOx-SIM (cif) Crystallographic information file of MoSx-SIM (cif)

ACKNOWLEDGMENTS This work was supported by the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001059. H. N. gratefully acknowledges support from the Ryan Fellowship program of the Northwestern University International Institute of Nanotechnology. C.-W. K. acknowledges support from the Postdoctoral Research Abroad Program (105-2917-I-564-046) sponsored by the Ministry of Science and Technology (Taiwan). We thank Prof. Michael R. Wasielewski and his lab for the use of a gas chromatograph. This work made use of the IMSERC, J. B. Cohen X-Ray Diffraction, EPIC, and KECK II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. Materials Research Collaborative Access Team (MRCAT, Sectors 10ID)

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operations are supported by the Department of Energy and the MRCAT member institutions.

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(59) Here, we are loosely using the term, rate-determining step. As the later discussion reveals, multiple chemical steps can lead to overall reaction bottleneck, depending on the applied potential. (60) We note that sulfhydryl and hydride moieties in both amorphous and crystalline MoSx have been detected. See ref. 20, 2628. (61) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M., Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis, J. Am. Chem. Soc. 2012, 134, 11235-11242. (62) Hod, I.; Sampson, M. D.; Deria, P.; Kubiak, C. P.; Farha, O. K.; Hupp, J. T., Fe-Porphyrin-Based Metal–Organic Framework Films as High-Surface Concentration, Heterogeneous Catalysts for Electrochemical Reduction of CO2, ACS Catal. 2015, 5, 6302-6309. (63) Elgrishi, N.; Chambers, M. B.; Fontecave, M., Turning It Off! Disfavouring Hydrogen Evolution to Enhance Selectivity for Co Production During Homogeneous CO2 Reduction by CobaltTerpyridine Complexes, Chem. Sci. 2015, 6, 2522-2531. (64) On the basis of several studies (most of which have yet to be published), we find that single-crystal X-ray structures for metalion-functionalized MOFs can be more easily obtained by installing metals slowly in the presence of an inert solvent such as dry heptane (i.e. SIM), than by installing them via AIM, a comparatively rapid, vapor-phase method. As a rule, high-porosity, solvent-evacuated MOFs are poor thermal conductors and AIM/SIM of metal ions is appreciably exothermic. As such, thermal disorder at the metalinstallation stage is much more likely to accompany AIM of a vacuum-filled MOF than slow SIM of a solvent-filled MOF – where even a poorly thermally conductive solvent can function as a heat sink, thereby moderating the temperature increases accompanying MOF modification. Additionally, although not explored here, we find SIM to be more easily scalable than AIM, at least for AIM with our commercial ALD reactors.


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Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from

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