Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV-vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s-1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O-O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts.

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(µ-O)]2+ (IrMolecular, pyalc = 2-(2'pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with potential-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations.

Carbon nitride (CNx ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CNx photoelectrodes has remained low. Here, cyanamide-functionalized CNx (NCN CNx ) was co-deposited with ITO nanoparticles on a 1.8 Šthick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the NCN CNx , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al2 O3 |ITO : NCN CNx film displays a benchmark performance for CNx -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CNx in fundamental and applied photoelectrochemical (PEC) studies.

Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes.

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionization of these states. We obtain an activation energy of ∼0.2 eV for this ionization process, with thermally activated electron detrapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionized states, however, function as deep hole traps, with such trapped holes energetically unable to drive water oxidation. These observations help address recent controversies in the literature regarding oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes.

The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.

Rate law analysis of water oxidation on a hematite surface.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar water splitting.

This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoO(x)) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga(2)O(3) overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe(2)O(3) for a given applied bias.

Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging.

Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.

Water-hydroxide trapping in cobalt tungstate for proton exchange membrane water electrolysis.

The oxygen evolution reaction is the bottleneck to energy-efficient water-based electrolysis for the production of hydrogen and other solar fuels. In proton exchange membrane water electrolysis (PEMWE), precious metals have generally been necessary for the stable catalysis of this reaction. In this work, we report that delamination of cobalt tungstate enables high activity and durability through the stabilization of oxide and water-hydroxide networks of the lattice defects in acid. The resulting catalysts achieve lower overpotentials, a current density of 1.8 amperes per square centimeter at 2 volts, and stable operation up to 1 ampere per square centimeter in a PEMWE system at industrial conditions (80°C) at 1.77 volts; a threefold improvement in activity; and stable operation at 1 ampere per square centimeter over the course of 600 hours.

Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring.

WO3/BiVO4 heterojunction photoanodes can be efficiently employed in photoelectrochemical (PEC) cells for the conversion of water into molecular oxygen, the kinetic bottleneck of water splitting. Composite WO3/BiVO4 photoelectrodes possessing a nanoflake-like morphology have been synthesized through a multistep process and their PEC performance was investigated in comparison to that of WO3/BiVO4 photoelectrodes displaying a planar surface morphology and similar absorption properties and thickness. PEC tests, also in the presence of a sacrificial hole scavenger, electrochemical impedance analysis under simulated solar irradiation, and incident photon to current efficiency measurements highlighted that charge transport and charge recombination issues affecting the performance of the planar composite can be successfully overcome by nanostructuring the WO3 underlayer in nanoflake-like WO3/BiVO4 heterojunction electrodes.

A Perovskite Photovoltaic Mini-Module-CsPbBr3 Photoelectrochemical Cell Tandem Device for Solar-Driven Degradation of Organic Compounds.

Recently, halide perovskites have been widely explored for high-efficiency photocatalysis or photoelectrochemical (PEC) cells. Here, in order to make an efficient photoanode electrode for the degradation of pollutants, concretely 2-mercaptobenzothiazole (MBT), nanoscale cesium lead bromide (CsPbBr3) perovskite was directly formed on the surface of mesoporous titanium dioxide (meso-TiO2) film using a two-step spin-coating process. This photoelectrode recorded a photocurrent of up to 3.02 ± 0.03 mA/cm2 under standard AM 1.5G (100 mW/cm2) illumination through an optimization process such as introducing a thin aluminum oxide (Al2O3) coating layer. Furthermore, to supply high voltage for efficient oxidation of MBT without an external bias, we developed a new photovoltaic/PEC tandem system using a methylammonium lead iodide (MAPbI3) based mini-module consisting of three solar cells interconnected in series and confirmed its successful operation. This approach looks very promising due to its applicability to various PEC reactions.

Highly Durable Nanoporous Cu2-xS Films for Efficient Hydrogen Evolution Electrocatalysis under Mild pH Conditions.

Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2-xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2-xS electrodes reach a state-of-the-art performance of ∼400 mA cm-2 at -1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2-xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure-activity relationship. This facile synthesis of highly durable and efficient Cu2-xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions.

The role of crystal facets and disorder on photo-electrosynthesis.

Photoelectrochemistry has the potential to play a crucial role in the storage of solar energy and the realisation of a circular economy. From a chemical viewpoint, achieving high conversion efficiencies requires subtle control of the catalyst surface and its interaction with the electrolyte. Traditionally, such control has been hard to achieve in the complex multinary oxides used in PEC devices and consequently the mechanisms by which surface exposed facets influence light-driven catalysts are poorly understood. Yet, this understanding is critical to further improve conversion yields and fine-tune reaction selectivities. Here, we review the impact that crystal facets and disorder have on photoelectrochemical reactivity. In particular, we discuss how the crystal orientation influences the energetics of the surface, the existence of defects and the transport of reactive charges, ultimately dictating the PEC activity. Moreover, we evaluate how facet stability dictates the tendency of the solid to undergo reconstructions during catalytic processes and highlight the experimental and computational challenges that must be overcome to characterise the role of the exposed facets and disorder in catalytic performance.

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts.

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO4, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4. Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4, which is further affected by the recombination loss at the BiVO4/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts.

Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes.

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.

Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT.

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O-O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.

Publisher Correction: Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts.

Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.

Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface.

Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen.