D. Kwabena Bediako

Structure–Activity Correlations in a Nickel–Borate Oxygen Evolution Catalyst

An oxygen evolution catalyst that forms as a thin film from Ni(aq)(2+) solutions containing borate electrolyte (Ni-B(i)) has been studied by in situ X-ray absorption spectroscopy. A dramatic increase in catalytic rate, induced by anodic activation of the electrodeposited films, is accompanied by structure and oxidation state changes. Coulometric measurements correlated with X-ray absorption near-edge structure spectra of the active catalyst show that the nickel centers in activated films possess an average oxidation state of +3.6, indicating that a substantial proportion of nickel centers exist in a formal oxidation state of Ni(IV). In contrast, nickel centers in nonactivated films exist predominantly as Ni(III). Extended X-ray absorption fine structure reveals that activated catalyst films comprise bis-oxo/hydroxo-bridged nickel centers organized into sheets of edge-sharing NiO(6) octahedra. Diminished long-range ordering in catalyst films is due to their ostensibly amorphous nature. Nonactivated films display a similar oxidic nature but exhibit a distortion in the local coordination geometry about nickel centers, characteristic of Jahn-Teller distorted Ni(III) centers. Our findings indicate that the increase in catalytic activity of films is accompanied by changes in oxidation state and structure that are reminiscent of those observed for conversion of β-NiOOH to γ-NiOOH and consequently challenge the long-held notion that the β-NiOOH phase is a more efficient oxygen-evolving catalyst.

Mechanistic Studies of the Oxygen Evolution Reaction Mediated by a Nickel–Borate Thin Film Electrocatalyst

A critical determinant of solar-driven water splitting efficiency is the kinetic profile of the O2 evolving catalyst (OEC). We now report the kinetic profiles of water splitting by a self-assembled nickel-borate (NiBi) OEC. Mechanistic studies of anodized films of NiBi exhibit the low Tafel slope of 2.3 × RT/2F (30 mV/decade at 25 °C). This Tafel slope together with an inverse third order rate dependence on H(+) activity establishes NiBi as an ideal catalyst to be used in the construction of photoelectrochemical devices for water splitting. In contrast, nonanodized NiBi films display significantly poorer activity relative to their anodized congeners that we attribute to a more sluggish electron transfer from the catalyst resting state. Borate is shown to play two ostensibly antagonistic roles in OEC activity: as a promulgator of catalyst activity by enabling proton-coupled electron transfer (PCET) and as an inhibitor in its role as an adsorbate of active sites. By defining the nature of the PCET pre-equilibrium that occurs during turnover, trends in catalyst activity may be completely reversed at intermediate pH as compared to those at pH extremes. These results highlight the critical role of PCET pre-equilibria in catalyst self-assembly and turnover, and accordingly suggest a reassessment in how OEC activities of different catalysts are compared and rationalized.

A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid

First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn(3+) disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.

Efficient solar-to-fuels production from a hybrid microbial–water-splitting catalyst system

Significance Renewable-fuels generation has emphasized water splitting to produce hydrogen and oxygen. For accelerated technology adoption, bridging hydrogen to liquid fuels is critical to the translation of solar-driven water splitting to current energy infrastructures. One approach to establishing this connection is to use the hydrogen from water splitting to reduce carbon dioxide to generate liquid fuels via a biocatalyst. We describe the integration of water-splitting catalysts comprised of earth-abundant components to wild-type and engineered Ralstonia eutropha to generate biomass and isopropyl alcohol, respectively. We establish the parameters for bacterial growth conditions at low overpotentials and consequently achieve overall efficiencies that are comparable to or exceed natural systems. Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods of storing the intermittent electricity they produce are required for the large-scale implementation of solar energy. Current solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO2, along with H2 and O2 produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical energy-to-fuels transformations.

Proton-electron transport and transfer in electrocatalytic films. Application to a cobalt-based O2-evolution catalyst.

Solar-driven electrochemical transformations of small molecules, such as water splitting and CO2 reduction, pertinent to modern energy challenges, require the assistance of catalysts preferably deposited on conducting or semiconducting surfaces. Understanding mechanisms and identifying the factors that control the functioning of such systems are required for rational catalyst optimization and improved performance. A methodology is proposed, in the framework of rotating disk electrode voltammetry, to analyze the current responses expected in the case of a semigeneral reaction scheme involving a proton-coupled catalytic reaction associated with proton-coupled electron hopping through the film as rate controlling factors in the case where there is no limitation by substrate diffusion. The predictions concern the current density vs overpotential (Tafel) plots and their dependence on buffer concentration (including absence of buffer), film thickness and rotation rate. The Tafel plots may have a variety of slopes (e.g., F/RT ln 10, F/2RT ln 10, 0) that may even coexist within the overpotential range of a single plot. We show that an optimal film thickness exists beyond which the activity of the film plateaus. Application to water oxidation by films of a cobalt-based oxidic catalyst provides a successful test of the applicability of the proposed methodology, which also provides further insight into the mechanism by which these cobalt-based films catalyze the oxidation of water. The exact nature of the kinetic and thermodynamic characteristics that have been derived from the analysis is discussed as well as their use in catalyst benchmarking.

Intermediate-Range Structure of Self-Assembled Cobalt-Based Oxygen-Evolving Catalyst

Continual improvements in solar-to-fuels catalysis require a genuine understanding of catalyst structure-function relationships, not only with respect to local order, but also intermediate-range structure. We report the X-ray pair distribution function analysis of the nanoscale order of an oxidic cobalt-based water-splitting catalyst and uncover an electrolyte dependence in the intermediate-range structure of catalyst films. Whereas catalyst films formed in borate electrolyte (CoB(i)) exhibit coherent domains consisting of 3-4 nm cobaltate clusters with up to three layers, films deposited in phosphate electrolyte (CoP(i)) comprise significantly smaller clusters that are not coherently stacked. These structural insights are correlated with marked differences in activity between CoP(i) and CoB(i) films.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Water Oxidation Catalysis by Co(II) Impurities in Co(III)[subscript 4]O[subscript 4] Cubanes

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films

Significance Iron-doped nickel oxide films are the most active nonnoble metal oxygen evolution reaction (OER) catalysts in alkaline electrolyte. Since Corrigan’s original discovery of enhanced activity with Fe doping in nickel oxides, the chemical basis for this synergy remains unclear. Recent studies suggest iron to assume a high valent oxidation state, thus promoting OER. We provide evidence for an alternative role of Fe3+ as a Lewis acid in the host nickel oxide. We observe that Fe3+ promotes the formation of Ni4+, which leads to enhanced catalytic activity. This result is consistent with Fe3+ to be one of the strongest Lewis acidic metals by any measure of Lewis acidity, including hard–soft acid base theory, metal ion pKas, and chemical inertness. Iron doping of nickel oxide films results in enhanced activity for promoting the oxygen evolution reaction (OER). Whereas this enhanced activity has been ascribed to a unique iron site within the nickel oxide matrix, we show here that Fe doping influences the Ni valency. The percent of Fe3+ doping promotes the formation of formal Ni4+, which in turn directly correlates with an enhanced activity of the catalyst in promoting OER. The role of Fe3+ is consistent with its behavior as a superior Lewis acid.

Reduction of a Redox-Active Ligand Drives Switching in a Cu(I) Pseudorotaxane by a Bimolecular Mechanism

The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR(+)) into the reduced [3]pseudorotaxane ([3]PR(+)) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR(+) and [3]PR(2+) are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M(+)) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR(2+). The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR(+) to 1088 nm for [3]PR(2+). Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K(1) = 8.9 x 10(8) M(-1) and K(2) = 3.1 x 10(6) M(-1), indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR(+), and [3]PR(2+) shows the one-electron reductions at E(1/2) = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu(+) ion. CVs of [2]PR(+) show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR(0) + [2]PR(+) = [3]PR(+) + Me(2)BPTZ(0) (K(D)*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR(+):Me(2)BPTZ. The driving force (DeltaG(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR(+) over reduced [2]PR(0) by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M(-1)). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.