Electrocatalytic Oxygen Evolution with an ... - ACS Publications

Research Article pubs.acs.org/acscatalysis

Electrocatalytic Oxygen Evolution with an Atomically Precise Nickel Catalyst Douglas R. Kauffman,† Dominic Alfonso,*,† De Nyago Tafen,†,‡ Jonathan Lekse,†,‡ Congjun Wang,†,‡ Xingyi Deng,†,‡ Junseok Lee,†,‡ Hoyoung Jang,§ Jun-sik Lee,§ Santosh Kumar,† and Christopher Matranga*,† †

National Energy Technology Laboratory (NETL), United States Department of Energy, Pittsburgh, Pennsylvania United States AECOM, Pittsburgh, Pennsylvania United States § Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡

S Supporting Information *

ABSTRACT: The electrochemical oxygen evolution reaction (OER) is an important anodic process in water splitting and CO2 reduction applications. Precious metals including Ir, Ru. and Pt are traditional OER catalysts, but recent emphasis has been placed on finding less expensive, earth-abundant materials with high OER activity. Ni-based materials are promising next-generation OER catalysts because they show high reaction rates and good long-term stability. Unfortunately, most catalyst samples contain heterogeneous particle sizes and surface structures that produce a range of reaction rates and rate-determining steps. Here we use a combination of experimental and computational techniques to study the OER at a supported organometallic nickel complex with a precisely known crystal structure. The Ni6(PET)12 (PET = phenylethyl thiol) complex out performed bulk NiO and Pt and showed OER activity comparable to Ir. Density functional theory (DFT) analysis of electrochemical OER at a realistic Ni6(SCH3)12 model determined the Gibbs free energy change (ΔG) associated with each mechanistic step. This allowed computational prediction of potential determining steps and OER onset potentials that were in excellent agreement with experimentally determined values. Moreover, DFT found that small changes in adsorbate binding configuration can shift the potential determining step within the OER mechanism and drastically change onset potentials. Our work shows that atomically precise nanocatalysts like Ni6(PET)12 facilitate joint experimental and computational studies because experimentalists and theorists can study nearly identical systems. These types of efforts can identify atomic-level structure−property relationships that would be difficult to obtain with traditional heterogeneous catalyst samples. KEYWORDS: electrocatalysis, oxygen evolution reaction, water splitting, density functional theory, atomically precise catalyst, nickel, organometallic

1. INTRODUCTION The electrochemical oxygen evolution reaction (OER: 2H2O → O2 + 4H+ + 4e−) represents the anodic half reaction in both H2 evolution and CO2 conversion systems. These two reactions are considered promising for renewable fuels development, but the high energy requirements of the OER produce challenges for large-scale deployment. For example, the OER formal potential (E0) is 1.23 V vs the reversible hydrogen electrode (RHE), whereas H2 evolution and CO2 → CO conversion occur at E0 = 0.0 and −0.1 V vs RHE, respectively. This makes the OER expensive from an energy standpoint because it accounts for 75−85% of a system’s electricity requirements if one assumes moderate overpotentials (η = Eapplied − E0) of 300−500 mV at both the anode and cathode. Traditional OER catalysts also introduce significant cost barriers to larger-scale applications because they typically contain precious metals such as Pt, Ir, or Ru, and less expensive, higher activity catalysts must © XXXX American Chemical Society

be identified before this technology can be deployed on industrially relevant scales. Renewed interest in alkaline OER technology has shown that transition metal oxides,1,2 bimetallic catalysts,3−9 and nanoparticulate metal sulfides10 can rival or exceed the activity of precious metals in basic electrolytes. Nickel-based materials are among the most promising OER candidates because they show low OER overpotentials, high reaction rates, and good stability. However, the literature contains large variations in OER rates, onset potentials, and rate-determining steps (RDS)4−6,11−13 that point to difficulty in accurately describing the reactive surface structure and number of active sites in heterogeneous catalyst samples. Recent ab initio-based approaches have Received: November 20, 2015 Revised: January 6, 2016

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DOI: 10.1021/acscatal.5b02633 ACS Catal. 2016, 6, 1225−1234

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electrode. A Hydrofelx reversible hydrogen electrode (RHE) was used as a reference electrode, and a Pt wire served as a counter electrode. Data was collected on a Biologic SP150 potentiostat and a BASi Epsilon bipotentiostat. Electrochemical potentials were corrected for uncompensated resistance between the working and reference electrode by E−iR, and the average uncompensated resistance was determined for each sample with impedance spectroscopy at 1.0, 1.5, and 2.0 V vs RHE. Catalyst inks were prepared by dissolving a small amount of the Ni6(PET)12 complex in 100 μL acetone, mixing it with 100 μL XC-72R graphitized carbon black suspended in methanol (1 mg CB/mL MeOH) and 45 uL Nafion solution, and then drop casting 12 μL of the mixture onto the GC electrode in 3 μL aliquots. OER at Ir was conducted by adhering pieces of Ir wire (99.7%; Strem Chemicals) to the GC disk of the RRDE with Nafion solution or conductive carbon cement (SPI supplies). OER at Pt was conducted using the Pt ring of the RRDE. Nonaqueous spectroelectrochemical experiments are described in the Supporting Information. Turnover Frequency (TOF) Determination. The number of electrochemically accessible active sites were determined by dividing the integrated Ni3+/2+ or Hupd (Ir and Pt) peak area by Faraday’s constant (96485 C/mol e−) and the number of electrons associated with each process (1 e−). TOFs were calculated by dividing the OER current by Faraday’s constant, the number of electrons involved in the OER (4 e−) and the number of electrochemically active surface sites. We also report the OER current density as milliamps per squared centimeter of metal. Electrochemical surface areas were estimated from the integrated Ni3+/2+ and Hupd (Ir and Pt) peak areas by assuming 210 μC cm−2. However, we acknowledge current density comparisons between different metals may contain uncertainty. 2.2. Computational Methods. General. We used the generalized gradient approximation (GGA) formulation of Perdew, Burke and Enzerhoff (PBE).33 The electron−ion interaction was described by the projector-augmented wave (PAW) method.34 The Kohn−Sham one electron valence eigenstates were expanded in terms of plane-wave basis sets with cutoff energy of 500 eV. Pt calculations were done on the Pt(111) surface represented by a five-layer slab with a (3 × 3) surface unit cell. Periodic boundary conditions were imposed in the two directions parallel to the surface. Adsorption was allowed on one side of the slab and in order to ensure the decoupling of the consecutive slabs, a 15 Å thick vacuum region is employed. The lattice constant of the Pt(111) slab was fixed to the value obtained from optimizing with DFT this constant for the bulk metal. The k-point sampling of the twodimensional electronic Brillouin zone of the periodic supercells was performed using the Monkhorst−Pack 3 × 3 × 1 k-point mesh.35 The two bottom layers of the slab were fixed while the remaining layers including adsorbates were fully relaxed. A periodically repeating six-layer slab was chosen for the most stable rutile (110) surface of IrO2. A vacuum region of 15 Å was used to separate the slab from its periodic image. A 4 × 4 × 1 Monkhorst−Pack k-point grid was employed. The two bottom layers of the slab are fixed to their optimized bulk lattice constant while the rest including the adsorbates are fully relaxed. Ni6(SCH3)12 was placed in a cubic box with dimensions of 30 Å sides for spin-polarized Γ-point calculations and all atoms, including possible adsorbates, were relaxed. The vibrational frequencies of the gas phase molecule were calculated by DFT. For the adsorbed species, this was done by fixing the metal substrate in their relaxed positions and

successfully developed atomic-level models to describe electrochemical reactions,14−21 but the absence of well-defined catalysts often necessitates approximating the experimental systems with model surfaces. The ability to computationally describe realistic working catalysts is important because it will facilitate the design of higher activity, earth-abundant catalysts by identifying accurate structure−property relationships. Performing electrocatalytic reactions with atomically precise nanoclusters22−25 or small organometallic complexes26−28 can help identify structure activity relationships because experimental and computational studies can analyze nearly identical systems. Recent reports have shown that organometallic nickel complexes efficiently catalyze photocatalytic29,30 and electrocatalytic28,31 H2 evolution reactions. These reports prompted us to explore the OER activity of a nickel complex containing phenylethyl thiol (−SC2H4Ph; PET) ligands. We show the Ni6(PET)12 complex has OER activity that exceeds Pt and rivals Ir. The benefit of Ni6(PET)12 and similarly well-defined catalysts is their precisely known crystal structures allow computational studies on realistic catalysts structures. This type of combined experimental and computational approach is difficult with traditional heterogeneous catalyst samples, and the use of atomically precise catalysts help develop the structure−property relationships that are needed to advance OER catalyst design. Our results showed excellent agreement between experimentally and computationally identified OER mechanistic steps and onset potentials. For example, both experiment and computation identified the formation of an Oads intermediate (OHads → Oads + H+ + e−) to be a rate and potential determining step. We also found that subtle changes in the adsorption configuration of reaction intermediates can shift the predicted reaction energetics and drastically change the reaction onset potential. Gaining a detailed understanding of OER mechanisms and intermediate adsorption configurations is important, and continued work with atomically precise OER catalysts will help establish the atomic-level reactivity details needed to guide catalyst design.

2. METHODS SECTION 2.1. Experimental Details. Ni6(PET)12 synthesis, singlecrystal X-ray diffraction, and matrix assisted laser desorption ionization (MALDI) mass spectrometry details are presented in the Supporting Information. Spectroscopy. UV−vis absorbance spectroscopy was conducted on an Agilent 8453 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI 5600ci spectrometer using kα Al radiation (1486.6 eV). Ni6(PET)12 samples were dissolved in acetone, mixed with graphitized XC72R carbon black in methanol, and deposited onto doublesided carbon tape for X-ray spectroscopy. All binding energies were calibrated to the C 1s of adventitious carbon at 284.6 eV. X-ray absorption spectroscopy (XAS) was collected in the total electron yield (TEY) mode at the Stanford Synchrotron Radiation Lightsource (SSRL) beamline 8−2.32 The spectra were normalized to the incident photon flux and the reported spectra represent one scan with 0.1 eV/step and 0.5 s/step. All XAS energies were calibrated by setting the L3 transition of a simultaneously measured NiO reference standard to 853 eV. Electrochemistry. All aqueous electrochemical measurements were performed in N2 purged 0.1 M KOH prepared with ≥18 MΩ millipure water. Voltammetry was conducted at 2500 rpm with Pine Instruments rotating ring disk electrode (RRDE) containing a glassy carbon disk and a Pt ring 1226


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ACS Catalysis computing the normal-mode frequencies of the adsorbate atoms vibrating harmonically about their binding sites. Oxygen Coverage. We evaluated O binding energies by considering the free energy change of the following reaction: H2O(l) + * ↔ O* + 2H+ + 2e−.36−38 The surface was in equilibrium with both protons and liquid water so that oxygen may be exchanged between the surface and the electrolyte. The free energy of O at any given coverage can be calculated as a function of potential U and pH from ΔGO = θO(GO* + GH2 − GH2O + − 2eU + 2kT ln aH+) where θO is the oxygen coverage. O atoms were progressively added to both Ni6(SCH3)12 and Pt(111) and the free energy for different coverages and their dependence on potential at a given temperature and pH was then plotted. The most stable phase corresponding to a given potential is the point with the lowest free energy at that potential. The predicted free energy dependence on the potential for O covered Ni6(SCH3)12 and Pt(111) at T = 298 K and pH = 0 are discussed in the main text and Supporting Information. OER was assumed to occur on an oxidized Ir surface, and calculations were conducted on IrO2 using a periodically repeating six-layer slab to represent the most stable rutile (110) surface. Computational Potentials. The electrochemical potential in SHE was considered such that the reaction H+ + e− ↔ 1/2 H2(g) was at equilibrium with H2 gas at p = 101325 Pa, T = 298 K, pH = 0, and U = 0 V vs SHE, and the sum of the chemical potential of H+ + e− is given by G(H+ + e−) = 1/2G(H2(g)). The effect of electrode potential (U ≠ 0 V vs SHE) and deviation from pH = 0 are accounted for via shift in energy by −eU and kT ln aH+, respectively. For example, the reaction energy of the first oxidation step (I) is calculated by ΔG = GOH* − GH2O − eU + kT ln aH+. The free energy G is determined by adding zero point energy (ZPE) and an entropic term (TS) to the electronic energy (E) from DFT calculations: G = E + ZPE − TS. For adsorbates, only the vibrational component contributes to the entropic term. An important parameter which can be extracted from the free energy diagram is the onset potential, which identifies the potential at which all reaction steps become downhill or exothermic in free energy. We defined this as Uonset = max((ΔGI, ΔGII, ΔGIII, ΔGIV)/ e),15,18,19 where ΔGI is the free energy for reaction step I at U = 0 V, and e is the charge of an electron. The lower limit of the overpotential (η) was approximated as the difference between the onset potentail and the calculated equilibrium water oxidation potential.

Figure 1. (a) Simplified presentation of the Ni6(PET)12 atomic framework. The organic component of the phenylethyl thiol (PET) ligands are not shown. (b) Optical absorbance spectra collected in dichloromethane. Ni6(PET)12 retained characteristic optical spectra when mixed with the carbon black (CB) catalyst support and after 3 h of OER electrolysis at +1.75 V vs RHE in N2 purged 0.1 M KOH. The retention of characteristic absorbance spectra indicate catalyst stability. The spectra were normalized at the main absorbance peak around 337 nm and offset for clarity. A linear background was subtracted from the “with CB” curve to remove the background scattering contribution from CB.

delocalization throughout ring structure.45,46 However, reports on the catalytic applications of Ni6(SR)12 remain scarce despite their rich synthetic history and predicted stability.29,30 The optical absorbance spectra of Ni6(PET)12 dissolved in dichloromethane are presented in Figure 1b. The Ni6(PET)12 absorbance spectrum contains several peaks throughout the UV−vis spectral region that correspond to charge transfer transitions between orbitals centered on S (ligand) and Ni atoms.29,47,48 The peaks at approximately 337 and 416 nm represent S pσ → Ni d transitions, while the 550 nm peak represents a S pπ → Ni d transition.47 Optical spectroscopy can be used to monitor Ni6(PET)12 stability because cleavage of Ni−Ni or Ni−S bonds, loss of ligands, or the formation of other species like NiO, Ni(OH)2, or NiOOH would necessarily alter the complex’s characteristic absorbance spectrum.29 Figure 1b shows consistent Ni6(PET)12 optical absorbance spectra when the catalyst was mixed with CB support and after a 3 h OER electrolysis experiment. We further confirmed Ni6(PET)12 stability by recrystallizing the catalyst after 3 h OER electrolysis and comparing the unit cells and space groups of pre- and postreaction single crystals. These observations show the Ni6(PET)12 structure remained intact during catalyst preparation, deposition onto the electrode, and application of anodic

3. RESULTS AND DISCUSSION 3.1. Structure and Optical Spectroscopy. MALDI mass spectrometry (Figure S1) and single crystal X-ray diffraction confirmed the identity of the Ni6(PET)12 complex.29 Figure 1a presents the Ni6(PET)12 structural framework. The complex contains six Ni atoms and 12 bridging S atoms arranged in a roughly hexagonal configuration. The organothiol PET ligands (not shown) extend from the S atoms to complete the Ni6(PET)12 structure. The general Ni6(SR)12 structure (SR = organothiol ligand) was first reported in 1965, 39 and subsequent synthetic routes have produced Ni6(SR)12 complexes containing a variety of different ligands.29,30,40−44 The persistence of this particular complex through different synthetic procedures and with various organic ligands indicates a robust atomic arrangement, and DFT studies have predicted Ni6(PET)12 to be a very stable complex due to electron 1227


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configuration of NiO (Ni2+).52 The Ni6(PET)12 photoelectron spectrum closely resembles that of NiO, but it contains an upshifted 2p3/2 shoulder, more pronounced peak splitting, and apparent variations in peak-to-satellite intensities that suggest differences in the local chemical environment of S- and Ocoordinated Ni2+ species. We point out that Ni(OH)2 and NiOOH spectra do not contain 2p3/2 doublets,50 and our photoemission data indicates the Ni6(PET)12 complex contains Ni2+ species that closely resemble those in NiO. We also used synchrotron-based XAS measurements to analyze the Ni L3 (2p3/2 → 3d) and L2 (2p1/2 → 3d) transitions of Ni6(PET)12, NiO, and metallic Ni (Figure 2b). Linear preedge backgrounds were subtracted from the Ni and NiO spectra, while the Ni6(PET)12 background was fit with a single exponential decay and subtracted from the spectrum (Figure S3). The metallic Ni absorption spectrum contains distinct L3 and L2 peaks at 852.5 and 869.8 eV, respectively, and a weak L3 satellite around 858.4 eV that corresponds to hybridization between valence d states and unoccupied sp states.53−55 Strong interactions between core holes and 3d orbitals in Ni2+ species are expected to upshift the L3 transition energy, produce a small L3 shoulder approximately 1.8 eV above the main transition, and induce splitting in the L2 transition.56−58 Both Ni6(PET)12 and NiO showed characteristic Ni 2+ spectra, but the Ni6(PET)12 L3 shoulder occurred at ∼0.3 eV higher energy compared with NiO. The upshifted L3 shoulder mirrors our XPS data and qualitatively agrees with previous comparisons between NiO and bulk NiS2.59 Shifts in the L3 (XAS) and 2p3/2 (XPS) shoulders likely stem from differences in final state effects (core hole screening), coordinating ligands (O vs. S), and/or Ni2+ bonding geometries between Ni6(PET)12 and NiO.60,61 Taken together, the X-ray spectroscopy data confirm the presence of Ni2+ in the Ni6(PET)12 complex. However, subtle spectral variations between NiO and Ni(PET)12 point to differences in the electronic structure, chemical environment, and bonding geometry of oxygen- and sulfur- coordinated Ni2+ species. 3.3. Electrocatalytic Activity vs Catalyst Loading. Figure 3a contains voltammograms of Ni6(PET)12 recorded in N2 purged 0.1 M KOH. The presence of a characteristic Ni2+/3+ redox couple between 1.4 and 1.5 V confirms that Ni atoms within the Ni6(PET)12 complex were electrochemically accessible (Figure 3a inset), and we used the integrated Ni3+/2+ reduction peak area to estimate the number of active sites (see the Methods section). RRDE experiments confirmed the increased current beyond ∼1.5 V represents O2 evolution from the catalyst surface (Figure S4). The dashed black curve in Figure 3a shows the background current produced by the CB support in the absence of catalyst, and the small OER current associated with catalyst-free CB identifies Ni6(PET)12 as the OER reaction center. Initial activity vs loading experiments produced an expected increase in OER current with increasing Ni6(PET)12. However, Figure 3b reveals an inverse relationship between catalyst loading and TOF. The background-subtracted voltammograms in Figure 3b were calculated by first subtracting the CB contribution from the measured OER current, and then determining TOFs from the integrated Ni3+/2+ peak area. The inset of Figure 3b summarizes the activity vs loading trend for Ni6(PET)12, and maximum OER TOFs occurred at loadings below ∼3 × 10−11 mol active sites. Previous work has shown that closely spaced, electroactive reaction centers can form overlapping diffusion regions.62 This phenomenon can negatively impact catalytic activity because it

electrochemical potentials. On the other hand, we used in situ spectroelectrochemistry to track the optical changes associated with intentional Ni6(PET)12 degradation at extremely anodic potentials. Figure S2 shows that intentional Ni6(PET)12 degradation induced a complete loss of characteristic UV−vis absorbance peaks. Taken together, the data in Figure 1b and Supporting Information Figure S2 show that optical spectroscopy is a convenient method for monitoring Ni6(PET)12 stability. 3.2. X-ray Spectroscopy Characterization. We further characterized the electronic structure of Ni6(PET)12 using Xray photoelectron and X-ray absorption spectroscopies (XPS, XAS). Figure 2a compares the Ni 2p photoelectron spectra of

Figure 2. (a) X-ray photoelectron (XPS) and (b) X-ray absorption spectroscopy (XAS) of Ni6(PET)12, NiO, and a metallic Ni foil.

Ni6(PET)12, NiO, and a metallic Ni foil. The metallic Ni spectrum shows 2p3/2 and 2p1/2 peaks at binding energies of 852.9 and 870.1 eV, respectively.49 Characteristic spectral changes can provide information on the oxidation state and chemical environment of different Ni species.50,51 In comparison with metallic Ni, the NiO spectrum contains upshifted Ni 2p peaks, higher-energy 2p satellites, and splitting in the Ni 2p3/2 peak that represent a valence state change from the predominantly d9 configuration of metallic Ni into the d8 1228


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Figure 3. (a) Cyclic voltammograms of carbon black (CB)-supported Ni6(PET)12 at different loadings in N2 purged 0.1 M KOH. Higher Ni6(PET)12 loadings produced larger oxygen evolution reaction (OER) current beyond 1.5 V. The number of active sites were calculated by dividing the integrated Ni2+/3+ reduction peak area by Faraday’s constant. The dashed black curve represents the catalyst-free CB support. (b) Background subtracted OER voltammograms of CB-supported Ni6(PET)12 at different loadings. (inset) Summary of the observed variation in TOF with catalyst loading.

electrocatalyst comparisons because they may impact apparent reaction rates. 3.4. Comparison with NiO, Ir, and Pt. Figure 4a compares the OER voltammograms of Ni6(PET)12, NiO, Ir, and Pt. Here we report OER activity as both TOF (left axis) and current density (right axis). The solid curves in Figure 4a represent the average of at least three independent measurements and the shaded regions represents the standard deviation. The Ni6(PET)12 voltammogram represents the average from six independent experiments across two different synthetic batches (three measurements from each batch), which highlights the consistent electrochemical performance of Ni6(PET)12. Table 1 summarizes several important figures of merit for the various catalyst materials. Ir produced the smallest OER onset potential of 1.493 ± 0.009 V. Ni6(PET)12 produced an onset potential of 1.544 ± 0.011 V that was independent of

reduces the local reactant concentration between closely spaced catalysts and creates competition for the diffusion limited influx of reactant molecules. Along these lines, our group63,64 and others65,66 have noticed loading-dependent activity trends for electrochemical CO2 reduction and CO oxidation reactions at nanoparticle-modified electrodes. For example, high loadings can produce poorly dispersed catalyst aggregates and/or catalyst multilayers on the CB support. Closely spaced catalysts are still electrochemically accessible, but competition for incoming reactants between neighboring catalysts lowers the system’s reactivity on a per active site basis. On the other hand, low loadings should produce well-dispersed, spatially separated catalysts. This scenario reduces competition between neighboring catalysts and the system shows higher TOFs. We observed a much weaker TOF vs loading trend for NiO (Figure S5), and activity vs loading trends should be considered when making 1229


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ACS Catalysis OHads ↔ Oads + H+ + e−

Table 1. Electrocatalytic Data from Voltammetry and Steady State OER Electrolysis

material Ni6(PET)12 Ir NiO Pt CB

OER onset (V vs RHE)a

Tafel slope (mV dec−1)a

± ± ± ± ±

69 ± 12 54 ± 1 70 ± 5 60 ± 11 255 ± 18

1.544 1.493 1.575 1.541 1.676

0.011 0.009 0.015 0.009 0.006

η @ 10 vs RHE)

1.700 ± 0.018 1.730 ± 0.019 1.812 ± 0.048 n/ac n/ac

3hη@ 10 s−1 (V vs RHE)b 1.68 1.77 2.3 2.5 n/ac

From voltammetry data. η = overpotential. bFrom electrolysis data. Did not occur within the considered iR-corrected potential window.

a c

catalyst loading (Figure S6) and statistically equivalent to Pt. NiO showed a slightly higher OER onset at 1.575 ± 0.015 V, and the catalyst-free CB support produced an OER onset of 1.676 ± 0.006 V. The OER mechanism is commonly described with four sequential one electron oxidation steps (eqs I−IV). H 2O(l) + * ↔ OHads + H+ + e−

Oads + H 2O(l) ↔ OOHads + H+ + e−

(III)

OOHads ↔ * + O2 (g) + H+ + e−

(IV)

The initial OER step involves OHads formation/adsorption at a free metal site (denoted as *). Subsequent oxidation of OHads into Oads (step II) is followed by the combination of Oads with a second water molecule to produce OOHads (step III). The final oxidation step releases O2 gas and leaves a free metal site available for additional catalytic cycles (step IV). Tafel analysis can help identify rate-determining steps (RDS) within the OER mechanism. For example, Tafel slopes around 40 mV dec−1 indicate OOHads formation (step III) as the RDS.11,67,68 Tafel slopes close to 60 mV dec−1 indicate a RDS involving the deprotonation of adsorbed OH into Oads (step II),67−70 and slopes larger than 120 mV dec−1 suggest the initial OHads formation (step 1) as an RDS.68,70 Literature values for Nibased materials range from 30 mV dec−1 to over 200 mV dec−1.4−6,11−13 In the present case, Ni6(PET)12 produced an average Tafel slope of 69 ± 12 mV dec−1 that was consistent over a 10-fold variation in catalyst loading (Table 1 and Figure S6). Consistent onset potentials and Tafel slopes over such a large range of catalyst-to-support ratios indicates the CB support did not influence the inherent OER activity of the Ni6(PET)12 catalyst. NiO, Ir, and Pt also produced Tafel slopes close to 60 mV dec−1, and the data indicates Oads formation (step II) as a common RDS for the catalysts considered here. On the other hand, the catalyst-free CB support produced Tafel slopes around 250 mV dec−1. The potential required to obtain a particular reaction rate is another important metric, and we chose to compare the potential at which each catalyst produced TOF = 10 s−1. The voltammograms in Figure 4a and data in Table 1, show that Ni6(PET)12 and Ir required equivalent potentials to obtain TOF = 10 s−1. A similar metric is the potential required to reach a particular steady-state TOF, which provides insight into the catalyst stability during extended OER electrolysis. Figure 4b compares the potential required to sustain TOF = 10 s−1. Ni6(PET)12 required a potential of 1.7 V to sustain TOF = 10 s−1 after 3 h of OER electrolysis. This steady state potential is consistent with the Ni6(PET)12 voltammogram in Figure 4a and it indicates good stability during OER electrolysis. Ir also showed good stability with only a small increase in steady-state OER potential after 3 h. On the other hand, Pt and NiO required larger potentials to sustain 10 s−1, and the increased overpotential vs time curves indicate some degree of catalyst instability. The abrupt increase in NiO potential was reproducible from run-to-run, and it was similar to the behavior reported by Jaramillo and co-workers for NiO and other metal oxides.2 We successfully recrystallized Ni6(PET)12 after a 3 h OER electrolysis experiment and confirmed the retention of its characteristic unit cell and space group. These results, combined with the identical coloration and morphology of the pre- and postreaction single crystals, indicate that no structural changes occurred during OER electrolysis. XAS also confirmed Ni6(PET)12 stability with nearly identical pre- and postreaction spectra (Figure 5). The retention of peak position and spectral shape strongly indicate Ni6(PET)12 did not convert into other Ni species such as NiS, NiO, Ni(OH)2, NiOOH, etc., during OER electrolysis.59,71 The postreaction

Figure 4. (a) Average OER voltammograms of Ni6(PET), NiO, Ir, and Pt. The solid curve represents the average of six Ni6(PET)12 samples across two synthetic batches and the shaded region represents the standard deviation of the average. Three or four NiO were averaged for NiO, Ir, and freshly polished Pt electrodes. (b) Steady-state potential vs time measurements of Ni6(PET)12, NiO, Ir, and Pt at a reaction TOF of 10 s−1. The abrupt increase in the NiO potential vs time curve was consistent from run to run and indicates NiO instability under alkaline OER conditions.

s−1 (V a

(II)

(I) 1230


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Figure 5. X-ray absorption spectra (XAS) of Ni6(PET)12 before and after 3 h OER electrolysis.

crystallographic, XAS, and UV−vis (Figure 1b) results all show Ni6(PET)12 stability during OER electrolysis. This molecular stability allowed us to confidently model the OER mechanism at Ni6(PET)12 using emerging DFT methodologies. The ability to study electrochemical reactions at catalysts with precisely known crystal structures is a unique benefit to atomically precise nanoclusters and molecular catalysts, and it allows atomic level-descriptions of reaction processes that are not easily accessed with traditional heterogeneous catalyst samples. 3.5. Computational Analysis of OER Mechanism. We used DFT to analyze reaction energetics and potential determining steps within the OER mechanism at a model Ni6(SCH3)12 structure. Employing smaller, less computationally demanding organic moieties allows one to accurately model ligand-capped catalysts without significantly impacting their electronic structure or overall reactivity.72−74 In the present case, DFT calculations of the Ni6(SCH3)12 model provided a structure with only 2−6% geometric variation compared with the experimentally determined crystal structure (Figure S7 and Table S1). We adopted a previously developed linear free thermochemical method15,18,19 to construct OER free energy diagrams based on OER steps I−IV. We considered the OER at Ni6(SCH3)12 structure by including an implicit solvation model to describe the electrostatics, cavitation, and dispersion interactions between the solute and solvent.75,76 Reaction energies were calculated using DFT as implemented in the Vienna Ab Initio Simulation Package (VASP) code,77,78 and the chemical potentials of protons and electrons in steps I−IV were referenced to H2 gas using the standard hydrogen electrode (SHE). The OER occurs at potentials that are more anodic than the Ni2+/3+ oxidation wave, and we expect the reactive surface to become oxygen-rich at catalytically relevant potentials. We modeled the Ni6(SCH3)12 structure under anodic potentials by adopting a previously developed scheme to mimic the oxidized system with adsorbed oxygen.17 The most thermodynamically favorable surface structures at OER potentials were determined by calculating the relative stability of Ni6(SCH3)12 at different O coverages, as presented in Figure 6a. An analogous stability diagram for O-covered Pt(111) is presented in Figure S8. Figure 6b contains the predicted structure of Ni6(SCH3)12O5. One Ni atom is left vacant to represent the OER active site as

Figure 6. (a) Stability diagram of O-covered Ni6(SCH3)12 at varying electrode potentials as a function of oxygen coverage; T = 298 K and pH = 0. (b) Predicted structure of oxygen-covered Ni6(SCH3)12O5. The OER active site is an O-free Ni atom that is marked with a pink asterisk, and the organic portion of the ligands has been omitted for clarity.

explained below (vacant site denoted as *). Oxygen preferentially adopted a 2-fold binding configuration between Ni and S atoms, and one-fold oxygen adsorption at Ni was less energetically favorable by 1.68 eV per O atom. Oxygen adsorption only induced minor structural distortions in Ni6(SCH3)12−O5, including a 0.35 Å elongation of the closest Ni−Ni distance that expanded the diameter of the Ni ring by ∼0.7 Å compared with O-free Ni6(SCH3)12 (Table S1). Figure 7a presents a free energy diagram for OER at Ocovered Ni6(PET)12O5. Our calculations were based on the mechanism presented in eqs I−IV. This four proton/electron OER pathway has gained popularity in the field of computational electrochemistry because it has successfully captured the reactivity trends of several OER catalysts.16,18,19,21 This mechanistic picture is characterized by production of O2 through a surface *OOH intermediate. An alternative OER pathway can be proposed where O2 is formed through the direct recombination of adsorbed O atoms. However, this alternative mechanism contains a much larger activation barrier for O2 formation compared with the four proton/electron pathway,79 and it has not been widely adopted in the literature. Therefore, we restrict our computational analysis to the pathway described in eqs I−IV. The initial surface state for OER is generally considered to include free H2O molecules and a vacant metal site (denoted as *).16,18,19,21 A vacant surface site is typically chosen as the starting point because O2 evolution from the catalyst surface leaves a bare metal atom to continue additional catalytic cycles. 1231


Research Article

ACS Catalysis

Figure 7. (a) Free energy diagram for OER on O-covered Ni6(SCH3)12O5 at T = 298 K, pH = 0, and zero applied potential. The starting point consists of a free metal site (denoted as *) at ΔG = 0, and each step corresponds to a one electron oxidation process. (b) Predicted structures of OER intermediates on O-covered Ni6(SCH3)12O5. The organic ligands are included in this presentation, and the vacant Ni site is marked with a pink asterisk. Enlarged views of the OER reaction intermediates are presented in Figure S9 of the Supporting Information. Atom colors: green Ni; yellow S, red O, gray C, white H, magenta: oxygen atoms in adsorbed OER intermediates (OHads, Oads, OOHads).

In the present case, * corresponds to an oxygen-free Ni atom and the initial condition (* + 2H2O) is defined as ΔG = 0. Table 2 contains the ΔG for each reaction step, and the

formation) the potential determining step. This reaction mechanism should produce an exceptionally large onset potential and a Tafel slope close to 40 mV dec−1. These results are inconsistent with experimental data, and a one-fold coordinated Oads species is the likely intermediate in our Ni6(PET)12 system. Identical potential-determining steps were identified in the absence of the implicit solvation model, but the magnitude of ΔG for each step differed in vacuum. Please see the text associated with Table S2 for more details. Figure 7b also shows that Ni6(SCH3)12 retained its structure throughout the OER process. The lack of significant structural changes or bond scission further highlights the robust atomic arrangement of Ni6(PET)12, and the predicted stability is consistent with the experimentally observed steady-state OER performance, our postreaction crystal structure analysis, and our postreaction spectroscopic data. We also modeled the OER on Pt and IrO2 surfaces to spot-check our computational methods, and the free energy changes associated with OER at O-covered Pt(111) and IrO2 (110) are summarized in Table S2 of the Supporting Information. DFT predicted onset potentials and potential determining steps at these surfaces were also in excellent agreement with experimentally determined values, which provides additional support for our computational approach. Our data indicates that the geometry of the Oads intermediate can influence the rate-determining step. For example, the RDS in Ni6(PET)12-catalyzed OER will be step II (OHads → Oads) if the Oads intermediate binds in a one-fold coordination with Ni. On the other hand, step III will be the RDS (Oads → OOHads) if the Oads intermediate binds in a 2-fold coordination with a Ni−S pair. This change in adsorbate binding geometry should produce a Tafel slope around 40 mV dec−1 and increase the OER onset potential. It is extremely difficult to obtain such atomic-level insight with traditional heterogeneous catalysts because they contain a large variety of particle sizes, surface structures and crystallographic faces. Atomically precise nanocatalysts facilitate mechanistic studies because they have welldefined crystal structures, and their use has facilitated atomiclevel understanding of CO2 reduction63 and H2 evolution systems.28−31,80 Recent joint experimental-computational work has identified the active sites responsible for OER activity in

Table 2. Gibbs Free Energy Change (ΔG) for Each OER Reaction Step O-Covered Ni6(PET)12O5a

a

step

process

ΔG per step (eV)

I II III IV total ΔG

H2O (l) → OHads OHads → Oads Oads → OOHads OOHads → O2(g) 2H2O(l) → O2(g)

1.63 1.68 1.44 0.21 4.96

All ΔG values are in electronvolts.

predicted structures for the adsorbed intermediates are presented in Figure 7b. Enlarged views of the bound intermediates are presented in Figure S9 of the Supporting Information. The cumulative free energy change for the overall reaction was ΔG = 4.96 eV, and the resulting theoretical potential of 1.24 V (4.96 eV/4e−) compares very well with the expected OER formal potential of E0 = 1.23 V. Calculations conducted in the absence of the implicit solvent model (i.e., vacuum) predicted an OER formal potential of E0 = 1.09 V that was inconsistent with the accepted value. The onset potential and corresponding potential determining step can be extracted from OER free energy diagrams by identifying ΔGmax.15,18,19 The onset potential is simply determined by dividing ΔGmax by the number of electrons involved in the step, which is 1 e− for each OER step. Our results indicate the formation of a one-fold coordinated Oads intermediate in step II as a potential determining step (ΔGII = 1.68 eV), and all electrochemical reaction steps become downhill in free energy with applied potentials larger than 1.68 V. The theoretical onset potential is within ∼0.14 V of our experimentally determined value for Ni6(PET)12, and the proposed potential determining step agrees with our experimental Tafel slope analysis. It is notable that Oads forms in a one-fold coordination during the OER mechanism. We found that formation of a 2-fold coordinated Oads increased the predicted onset potential to 3.04 V and made step III (OOHads 1232


ACS Catalysis

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bimetallic Ni−Fe thin-film OER catalysts,81 and we expect that continued interest in atomically precise catalysts will help advance the OER field by identifying atomic-level structure− activity relationships. A combination of computationally aided active site design and emerging synthetic strategies should yield atomically precise nanocatalysts with low overpotentials, high TOFs, and exceptional long-term stability.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02633. Contains additional data, analysis, and references (PDF)

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REFERENCES

(1) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977−16987. (2) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347−4357. (3) Fominykh, K.; Chernev, P.; Zaharieva, I.; Sicklinger, J.; Stefanic, G.; Döblinger, M.; Müller, A.; Pokharel, A.; Böcklein, S.; Scheu, C.; Bein, T.; Fattakhova-Rohlfing, D. ACS Nano 2015, 9, 5180−5188. (4) Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. ACS Nano 2014, 8, 9518−9523. (5) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. J. Am. Chem. Soc. 2013, 135, 8452− 8455. (6) Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Trudel, S.; Berlinguette, C. P. J. Am. Chem. Soc. 2013, 135, 11580−11586. (7) Hunter, B. M.; Blakemore, J. D.; Deimund, M.; Gray, H. B.; Winkler, J. R.; Müller, A. M. J. Am. Chem. Soc. 2014, 136, 13118− 13121. (8) Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P. Chem. Sci. 2014, 5, 2955−2963. (9) Gong, M.; Dai, H. Nano Res. 2015, 8, 23−39. (10) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; Ye, B.; Xie, Y. J. Am. Chem. Soc. 2014, 136, 15670−15675. (11) Louie, M. W.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 12329− 12337. (12) Qiu, Y.; Xin, L.; Li, W. Langmuir 2014, 30, 7893−7901. (13) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. J. Am. Chem. Soc. 2014, 136, 7077−7084. (14) Mom, R. V.; Cheng, J.; Koper, M. T. M.; Sprik, M. J. Phys. Chem. C 2014, 118, 4095−4102. (15) Bajdich, M.; García-Mota, M.; Vojvodic, A.; Nørskov, J. K.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 13521−13530. (16) Liao, P.; Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 13296−13309. (17) Rossmeisl, J.; Logadottir, A.; Nørskov, J. K. Chem. Phys. 2005, 319, 178−184. (18) Rossmeisl, J.; Qu, Z.-W.; Zhu, H.; Kroes, G.-J.; Nørskov, J. K. J. Electroanal. Chem. 2007, 607, 83−89. (19) Valdés, Á .; Qu, Z.-W.; Kroes, G.-J.; et al. J. Phys. Chem. C 2008, 112, 9872−9879. (20) Su, H.-Y.; Gorlin, Y.; Man, I. C.; Calle-Vallejo, F.; Nørskov, J. K.; Jaramillo, T. F.; Rossmeisl, J. Phys. Chem. Chem. Phys. 2012, 14, 14010−14022. (21) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. ChemCatChem 2011, 3, 1159−1165. (22) Kauffman, D. R.; Alfonso, D.; Matranga, C.; Qian, H.; Jin, R. J. Am. Chem. Soc. 2012, 134, 10237−10243. (23) Chen, W.; Chen, S. Angew. Chem., Int. Ed. 2009, 48, 4386−4389. (24) Li, G.; Jin, R. Acc. Chem. Res. 2013, 46, 1749−1758. (25) Anderson, R. M.; Yancey, D. F.; Zhang, L.; Chill, S. T.; Henkelman, G.; Crooks, R. M. Acc. Chem. Res. 2015, 48, 1351−1357. (26) Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988−8998. (27) Sun, Y.; Bigi, J. P.; Piro, N. A.; Tang, M. L.; Long, J. R.; Chang, C. J. J. Am. Chem. Soc. 2011, 133, 9212−9215. (28) Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. Science 2011, 333, 863−866. (29) Kagalwala, H. N.; Gottlieb, E.; Li, G.; Li, T.; Jin, R.; Bernhard, S. Inorg. Chem. 2013, 52, 9094−9101. (30) Zhang, W.; Hong, J.; Zheng, J.; Huang, Z.; Zhou, J.; Xu, R. J. Am. Chem. Soc. 2011, 133, 20680−20683. (31) Luca, O. R.; Blakemore, J. D.; Konezny, S. J.; Praetorius, J. M.; Schmeier, T. J.; Hunsinger, G. B.; Batista, V. S.; Brudvig, G. W.; Hazari, N.; Crabtree, R. H. Inorg. Chem. 2012, 51, 8704−8709. (32) Tirsell, K. G.; Karpenko, V. P. Nucl. Instrum. Methods Phys. Res., Sect. A 1990, 291, 511−517.

4. CONCLUSIONS We have shown stable OER operation with an organometallic Ni6(PET)12 complex. We show TOFs approaching 70 s−1 at an applied potential of 2 V, and a steady state operating voltage of ∼1.7 V at 10 s−1. These performance metrics are comparable to other state-of-the-art OER catalysts and they exceed those of NiO and Pt. The benefit of Ni6(PET)12 and similar atomically precise nanocatalysts is that they contain a precisely known crystal structure. This strategy allows atomic-level modeling of realistic catalyst structures and accurate descriptions of reaction mechanisms. Mechanistic analysis showed that subtle changes in intermediate binding geometries can impact the predicted potential determining step and OER onset potential. Continued development of atomically precise OER catalysts will help establish detailed structure activity relationships for the controlled synthesis of next generation OER catalysts. We envision that this type of combined experimental and computational approach can also be extended to other catalyst systems that employ ligand-protected, atomically precise nanoclusters with known crystal structures, such as Au25(PET)18 and related materials.22−25

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.M.). *E-mail: [email protected] (D.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Portions of this work were performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. Use of the Stanford Synchrotron Radiation Lightsource (BL 8-2), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. J.-S.L. acknowledges support by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. 1233


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ACS Catalysis (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (34) Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (35) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (36) Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2008, 10, 3722−3730. (37) Tripković, V.; Skúlason, E.; Siahrostami, S.; Nørskov, J. K.; Rossmeisl, J. Electrochim. Acta 2010, 55, 7975−7981. (38) Tripković, V.; Cerri, I.; Bligaard, T.; Rossmeisl, J. Catal. Lett. 2014, 144, 380−388. (39) Woodward, P.; Dahl, L. F.; Abel, E. W.; Crosse, B. C. J. Am. Chem. Soc. 1965, 87, 5251−5253. (40) Jian, F.-F.; Jiao, K.; Li, Y.; Zhao, P.-S.; Lu, L.-D. Angew. Chem., Int. Ed. 2003, 42, 5722−5724. (41) Xiao, H. L.; Jian, F. F.; Zhang, K. J. Bull. Korean Chem. Soc. 2009, 30, 846−848. (42) Tan, C.; Jin, M.; Ma, X.; Zhu, Q.; Huang, Y.; Wang, Y.; Hu, S.; Sheng, T.; Wu, X. Dalton Trans. 2012, 41, 8472−8476. (43) Capdevila, M.; González-Duarte, P.; Sola, J.; et al. Polyhedron 1989, 8, 1253−1259. (44) Sletten, J.; Kovacs, J. A.; et al. Acta Chem. Scand. 1994, 48, 929− 932. (45) Datta, A.; John, N. S.; Kulkarni, G. U.; Pati, S. K. J. Phys. Chem. A 2005, 109, 11647−11649. (46) Muñoz-Castro, A. J. Phys. Chem. A 2011, 115, 10789−10794. (47) Gorelsky, S. I.; Basumallick, L.; Vura-Weis, J.; Sarangi, R.; Hodgson, K. O.; Hedman, B.; Fujisawa, K.; Solomon, E. I. Inorg. Chem. 2005, 44, 4947−4960. (48) John, N. S.; Kulkarni, G. U.; Datta, A.; Pati, S. K.; Komori, F.; Kavitha, G.; Narayana, C.; Sanyal, M. K. J. Phys. Chem. C 2007, 111, 1868−1870. (49) Biesinger, M. C.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Phys. Chem. Chem. Phys. 2012, 14, 2434−2442. (50) Biesinger, M. C.; Payne, B. P.; Lau, L. W. M.; Gerson, A.; Smart, R. S. C. Surf. Interface Anal. 2009, 41, 324−332. (51) Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S. Surf. Sci. 2006, 600, 1771−1779. (52) Hagelin-Weaver, H. A. E.; Weaver, J. F.; Hoflund, G. B.; Salaita, G. N. J. Electron Spectrosc. Relat. Phenom. 2004, 134, 139−171. (53) Gu, W.; Wang, H.; Wang, K. Dalton Trans. 2014, 43, 6406− 6413. (54) Ufuktepe, Y.; Akgül, G.; Aksoy, F.; Nordlund, D. X-Ray Spectrom. 2011, 40, 427−431. (55) Chen, C. T.; Sette, F.; Ma, Y.; Modesti, S. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 42, 7262−7265. (56) Wang, H.; Patil, D. S.; Gu, W.; Jacquamet, L.; Friedrich, S.; Funk, T.; Cramer, S. P. J. Electron Spectrosc. Relat. Phenom. 2001, 114− 116, 855−863. (57) Wang, H.; Ralston, C. Y.; Patil, D. S.; Jones, R. M.; Gu, W.; Verhagen, M.; Adams, M.; Ge, P.; Riordan, C.; Marganian, C. A.; Mascharak, P.; Kovacs, J.; Miller, C. G.; Collins, T. J.; Brooker, S.; Croucher, P. D.; Wang, K.; Stiefel, E. I.; Cramer, S. P. J. Am. Chem. Soc. 2000, 122, 10544−10552. (58) Wang, H.; Butorin, S. M.; Young, A. T.; Guo, J. J. Phys. Chem. C 2013, 117, 24767−24772. (59) Lussier, A.; Sofie, S.; Dvorak, J.; Idzerda, Y. U. Int. J. Hydrogen Energy 2008, 33, 3945−3951. (60) Preda, I.; Mossanek, R. J. O.; Abbate, M.; Alvarez, L.; Méndez, J.; Gutiérrez, A.; Soriano, L. Surf. Sci. 2012, 606, 1426−1430. (61) Tomellini, M. J. Electron Spectrosc. Relat. Phenom. 1992, 58, 75− 78. (62) Streeter, I.; Baron, R.; Compton, R. G. J. Phys. Chem. C 2007, 111, 17008−17014. (63) Kauffman, D. R.; Alfonso, D.; Matranga, C.; Ohodnicki, P.; Deng, X.; Siva, R. C.; Zeng, C.; Jin, R. Chem. Sci. 2014, 5, 3151−3157. (64) Kauffman, D. R.; Thakkar, J.; Siva, R.; Matranga, C.; Ohodnicki, P. R.; Zeng, C.; Jin, R. ACS Appl. Mater. Interfaces 2015, 7, 15626− 15632.

(65) Manthiram, K.; Beberwyck, B. J.; Alivisatos, A. P. J. Am. Chem. Soc. 2014, 136, 13319−13325. (66) Ma, S.; Lan, Y.; Perez, G. M. J.; Moniri, S.; Kenis, P. J. A. ChemSusChem 2014, 7, 866−874. (67) Lyons, M. E. G.; Doyle, R. L. Int. J. Electrochem. Sci. 2012, 7, 9488−9501. (68) Doyle, R. L.; Godwin, I. J.; Brandon, M. P.; Lyons, M. E. G. Phys. Chem. Chem. Phys. 2013, 15, 13737−13783. (69) Birss, V. I.; Damjanovic, A.; Hudson, P. G. J. Electrochem. Soc. 1986, 133, 1621−1625. (70) Fang, Y.-H.; Liu, Z.-P. J. Am. Chem. Soc. 2010, 132, 18214− 18222. (71) Nakamura, M.; Fujimori, A.; Sacchi, M.; Fuggle, J. C.; Misu, A.; Mamori, T.; Tamura, H.; Matoba, M.; Anzai, S. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 16942−16947. (72) Hartmann, M. J.; Häkkinen, H.; Millstone, J. E.; Lambrecht, D. S. J. Phys. Chem. C 2015, 119, 8290−8298. (73) Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Häkkinen, H. Nat. Chem. 2010, 2, 329−334. (74) Negishi, Y.; Kurashige, W.; Niihori, Y.; Iwasa, T.; Nobusada, K. Phys. Chem. Chem. Phys. 2010, 12, 6219−6225. (75) Mathew, K.; Sundararaman, R.; Letchworth-Weaver, K.; Arias, T. A.; Hennig, R. G. J. Chem. Phys. 2014, 140, 084106−1−084106−8. (76) Fishman, M.; Zhuang, H. L.; Mathew, K.; Dirschka, W.; Hennig, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 245402− 245402. (77) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15−50. (78) Kresse, G.; Hafner, J. J. Phys.: Condens. Matter 1994, 6, 8245− 8257. (79) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; et al. J. Phys. Chem. B 2004, 108, 17886−17892. (80) Kibsgaard, J.; Jaramillo, T. F.; Besenbacher, F. Nat. Chem. 2014, 6, 248−253. (81) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. J. Am. Chem. Soc. 2015, 137, 1305−1313.

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