D isotope effects reveal factors controlling catalytic activity in Co

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H/D isotope effects reveal factors controlling catalytic activity in Co-based oxides for water oxidation Chiara Pasquini, Ivelina Zaharieva, Diego Gonzalez-Flores, Petko Chernev, Mohammad Reza Mohammadi, Leonardo Guidoni, Rodney D. L. Smith, and Holger Dau J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10002 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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Journal of the American Chemical Society

H/D isotope effects reveal factors controlling catalytic activity in Co-based oxides for water oxidation Chiara Pasquini1, Ivelina Zaharieva1, Diego González-Flores1, Petko Chernev1, Mohammad Reza Mohammadi1,2, Leonardo Guidoni3, Rodney D. L. Smith*1,4, Holger Dau*1 1

Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin (Germany).

2

Department of Physics, University of Sistan and Baluchestan, Zahedan, 98167-45845 (Iran).

3

Dipartimento di Scienze Fisiche e Chimiche, Università degli studi dell’Aquila,Via Vetoio (Coppito), 67100

L’Aquila (Italy). 4

Department of Chemistry, University of Waterloo, 200 University Ave. W, N2L 3G1 Waterloo, ON (Canada).

1 ACS Paragon Plus Environment

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Abstract Understanding the mechanism for electrochemical water-oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In-situ visible and quasi-in-situ Xray absorption spectroscopy reveals that the hydrogen to deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co-oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in literature. Introduction The water oxidation reaction plays a key role in the reduction of carbon dioxide emissions,1-2 because it can be coupled with reactions such as the hydrogen evolution reaction, CO2 reduction or NH3 production to allow storage of renewable energy into carbon-neutral fuels.3-4 Achieving changes in the worldwide energy production system requires easy-to-assemble catalysts based on widely available and cheap materials.3 Amorphous metal oxides based on first row transition metals such as Mn, Fe, Co, Ni are promising catalysts that can be readily prepared by a variety of fabrication techniques, including precipitation,5-6 electrodeposition7-10 or photochemistry.11-12 The electrodeposited Co catalyst studied in this work can be taken as a model for this class of materials.13-14 Research in the field aims to understand the details of the water oxidation mechanism in order to improve catalysts efficiency and stability.15-17 The water oxidation reaction: 2𝐻2𝑂→𝑂2 +4𝐻 + +4𝑒 ―

Eq. 1

entails the progressive removal of 4 protons and 4 electrons. In amorphous metal oxide catalysts, the metal ion accumulates oxidation equivalents by changing its oxidation state.17-21 The electron removal is often coupled with proton removal to avoid charge accumulation, which would increase the required overpotential.22-23 Such protoncoupled electron transfer (PCET) reactions can follow a two-step pathway, where either the electron transfer or the proton transfer can be rate limiting, or a concerted pathway (CPET).22, 24-26 The exchange of hydrogen for deuterium affects both the thermodynamics and the kinetics of proton-coupled electron transfer reactions.27 In this work we separate the two effects and refer to them as equilibrium isotope effect (EIE) and kinetic isotope effect (KIE), respectively.28 An EIE implies a change in the reaction thermodynamics28-30 due to an increase in the vibrational zero-point energy of a bond involving hydrogen by a factor proportional to the bond strength. Isotope exchange alters the protonation-deprotonation equilibrium if the bond strength of proton donor differs from that


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of proton acceptor, thereby shifting the standard reduction potential for the PCET reaction. On the other hand, the observation of a KIE implies a change in the kinetics of the reaction27, 31 and provides proof for proton transfer being involved in the rate-determining step. Amorphous Co-oxide films prepared by electrodeposition in neutral solution (CoCat) were introduced by Kanan and Nocera.7 The catalyst resting state is composed of a CoII-CoIII mixture, with CoIV species appearing at potentials relevant to catalytic water oxidation.20, 32-33 The Co oxidation state change is coupled to release of a proton to the buffer species in solution and represents a pre-equlibrium preceding the turn-over limiting step, which has been proposed to be the formation of an oxygen-oxygen bond.14, 20, 34 The catalyst is formed by molecular fragments containing 14-19 Co atoms,35-36 water oxidation takes place in the bulk of the film14, 37 at the periphery of the CoOx fragments.36, 38 CoCat has been the subject of numerous mechanistic studies.39-40 Bediako et al. studied the dependence of current on overpotential (Tafel plots) with different buffer concentrations and conditions, decoupling the catalytic reaction from the proton-electron hopping inside the film.13 A subsequent work focused on CV analysis to estimate a lower limit for the catalytic rate and proposed that catalysis is limited by the diffusion of the proton accepting base.41 Previous works evidenced the dependence of catalytic activity on the protonation state and the nature of the buffer electrolyte, which works as a proton acceptor.37-38, 42-43 An isotope effect study appears to be the missing information to enable clear distinction of the factors controlling the catalytic rate.13 The effect of hydrogen to deuterium exchange has been previously applied to a molecular model compound made by Co4O4 units that resemble the structure of CoCat.44 Two cases were observed: proton and electron are transferred to the same chemical species with a CPET, or they are transferred each to a distinct chemical species with a PTET. The transferability of this result to the extended oxide has yet to be proven. The electron conductivity inside the CoCat catalyst was studied in deuterated electrolytes, a shift to higher voltages was observed for the potential at which the film becomes conductive.45 Similar catalysts have been examined in alkaline conditions but clear distinction between the thermodynamic and kinetic isotope effects were not reported.42, 46 In this work we investigate the effects of hydrogen-to-deuterium and 16O-to-18O exchange on the water oxidation reaction by CoCat. We observe consistently lower catalytic activity in deuterated buffers across a range of solution pH values. Analysis by visible (in-situ) and X-ray (quasi-in-situ) absorption spectroelectrochemistry enable us to identify an EIE and the absence of a KIE in the precatalytic redox transitions and to formulate an hypothesis on the factors controlling catalytic activity; correlated behavior between the precatalytic and catalytic processes lead us to exclude deprotonation as a component of the rate-determining step. On the basis of these result we proposed a mechanism for water oxidation at neutral pHs.

Results Isotope Effect on the Catalytic Activity The influence of the deprotonation steps on the catalytic activity was investigated for CoCat in 0.1 M potassium phosphate (KPi) solutions in the close-to-neutral pH range (6 ≤ pL ≤ 9) using different hydrogen isotopes. Tafel plot data from galvanostatic measurements show an anodic shift in potential on the Normal Hydrogen Electrode (NHE) scale with pH decrease. The Nernst equation states that the equilibrium potential for a reaction involving transfer of protons and electrons in equal number will vary of 59 mV per pH unit at room temperature. The potential needed to obtain a fixed catalytic current exhibits near-ideal Nernstian behavior (Fig. 1b-c), indicating that the



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catalytic activity involves the coupled release of one proton per one electron transferred. Preparing electrolyte solutions with D2O leads to further anodic shifts (Fig. 1a, full data range in Fig. S1) and to a decrease in the current at parity of potential. This effect is observed in two ways: (i) the current at 1.25 VNHE and pL 7 is decreased by 87% in D2O solutions relative to H2O solutions, or (ii) the potential required to obtain a given current density is shifted by 58 ± 4 mV and 68 ± 8 mV for 30 and 300 μA cm-2 catalytic current, respectively.

Figure 1. Steady state chronopotentiometry measurements (values taken after 3 min) of CoCat at different pL in 0.1 M KPi buffer prepared with H2O or D2O. (a) Tafel plot for selected pH (full series in Fig. S1) in water (closed symbol) and deuterated water (open symbols) solutions. (b) Potential needed to obtain 30 μA cm-2 and (c) 300 μA cm-2, these current values are highlighted with colored bands in panel a. Black lines represents Nernstian behavior (59 mV pH-1). The H-D shift for 6 ≤ pL ≤ 9 corresponds to 58 ± 4 mV (at 30 μA cm-2) and 68 ± 8 mV (at 300 μA cm-2). All solutions were stirred during measurement. Analysis of Kinetic Isotope Effect on Co Redox Transitions Time-resolved in-situ visible absorption spectroscopy during “potential jump” experiments, where the voltage is stepped between a reducing and an oxidizing potential, showed no evidence of a KIE for Co redox transitions. CoCat has a wide absorption band between 400 and 700 nm (Fig. S2) with a molar extinction coefficient that increases linearly with Co oxidation state, as previously proven by comparison with X-ray absorption experiments20 and again in this study (below). UV-vis absorption spectroscopy thus enables the tracking of changes


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in the oxidation state of transition metals during electrochemical experiments20, 47-49 Here, the absorption time traces recorded with 10 ms temporal resolution during potential jumps show no significant differences in kinetics when hydrogen is substituted with deuterium (Fig. 2a).

Figure 2. (a) Time-resolved in-situ visible absorption spectroscopy during step-wise changes between reducing and oxidizing potential, for CoCat in 0.1 M KPi H2O electrolyte (red and violet lines) or D2O electrolyte (blue and cyan lines) with pL 6, 7, 8. The potential jumps were repeated 30 times with a time resolution of 10 ms, the recorded absorption signals were averaged and simulated using a sum of three exponential functions. (b) Halflives of the first two exponentials. All simulation parameters are provided in Table S1. Three different time components are clearly distinguished when the absorbance is mathematically fitted (Table S1) or plotted on a logarithmic time scale (Fig. S3). The fastest phase exhibits half-lives of 0.15 s for reduction and 0.4 s for oxidation and accounts for the major changes in oxidation state. An intermediate phase with half-life of about 1.5 s and a slow phase with half-life in the minute regime are also observed (Fig. S4). During the Cooxidation process the oxidation of neighboring atoms and the structural rearrangements change the environment, the local pH and the effective potential inside the catalyst film. These changes likely result in a different kinetic for the oxidation of further Co atoms. The presence of multiple time constants for Co oxidation state changes is



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therefore believed to arise from a stretched exponential decay, here modeled as three time-phases. Half-life values for the first two phases are presented in Fig. 2b and show no dependence on the isotope used or on the pL. Values for the third phase are reported in Table S1, the third exponential was approximated to a linear decay due to the very long process studied (several minutes) as compared to the experiment time scale (40 s per jump). Potential jump measurements conducted in KPi buffered H2O and D2O at three different pLs and multiple electrode substrates establish confidence in the accuracy of these results. For H2O solutions the reducing potential was chosen on the Reversible Hydrogen Electrode scale (VRHE) to lie well below the first oxidation wave (1.03 VRHE) while the oxidizing potential was in the catalytic region (1.68 VRHE). An additional 50 mV was added when measuring in D2O electrolyte to compensate for the isotopic shift and to ensure consistency of the analyzed processes.50 Control measurements with identical potentials for H2O and D2O buffers (i.e. no 50 mV shift considered; Fig. S5), and on a substrate with lower resistance (glassy carbon; Fig. S6), showed no further differences between the recorded time traces. The comparison of half-lives revealed that the kinetics of Co oxidation state changes are unaffected by pL (in the range 6

D isotope effects reveal factors controlling catalytic activity in Co

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