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Metal (oxy)­hydroxides (MO_<i>x</i>H_<i>y</i>, M = Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry. In particular, MO_<i>x</i>H_<i>y</i> are the fastest known catalysts for the oxygen evolution reaction (OER) in alkaline media. While key descriptors such as overpotentials and activity have been thoroughly characterized, the nanostructure and its dynamics under electrochemical conditions are not yet fully understood. Here, we report on the structural evolution of Ni_1−δCo_δO_<i>x</i>H_<i>y</i> nanosheets with varying ratios of Ni to Co, in operando using atomic force microscopy during electrochemical cycling. We found that the addition of Co to NiO_<i>x</i>H_<i>y</i> nanosheets results in a higher porosity of the as-synthesized nanosheets, apparently reducing mechanical stress associated with redox cycling and hence enhancing stability under electrochemical conditions. As opposed to nanosheets composed of pure NiO_<i>x</i>H_<i>y</i>, which dramatically reorganize under electrochemical conditions to form nanoparticle assemblies, restructuring is not found for Ni_1−δCo_δO_<i>x</i>H_<i>y</i> with a high Co content. Ni_0.8Fe_0.2O_<i>x</i>H_<i>y</i> nanosheets show high roughness as-synthesized which increases during electrochemical cycling while the integrity of the nanosheet shape is maintained. These findings enhance the fundamental understanding of MO_<i>x</i>H_<i>y</i> materials and provide insight into how nanostructure and composition affect structural dynamics at the nanoscale

Morphology Dynamics of Single-Layered Ni(OH)₂/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by <i>in Situ</i> Electrochemical Atomic Force Microscopy

Nickel (oxy)­hydroxide-based (NiO_<i>x</i>H_<i>y</i>) materials are widely used for energy storage and conversion devices. Understanding dynamic processes at the solid–liquid interface of nickel (oxy)­hydroxide is important to improve reaction kinetics and efficiencies. In this study, <i>in situ</i> electrochemical atomic force microscopy (EC-AFM) was used to directly investigate dynamic changes of single-layered Ni­(OH)₂ nanosheets during electrochemistry measurements. Reconstruction of Ni­(OH)₂ nanosheets, along with insertion of ions from the electrolyte, results in an increase of the volume by 56% and redox capacity by 300%. We also directly observe Fe cations adsorb and integrate heterogeneously into or onto the nanosheets as a function of applied potential, further increasing apparent volume. Our findings are important for the fundamental understanding of NiO_<i>x</i>H_<i>y</i>-based supercapacitors and oxygen-evolution catalysts, illustrating the dynamic nature of Ni-based nanostructures under electrochemical conditions

Direct in Situ Measurement of Charge Transfer Processes During Photoelectrochemical Water Oxidation on Catalyzed Hematite

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-Fe₂O₃, in contact with one of the fastest known water oxidation catalysts, Ni_0.8Fe_0.2O_<i>x</i>, by directly measuring/controlling the current and/or voltage at the Ni_0.8Fe_0.2O_<i>x</i> catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe₂O₃ directly transfer to the catalyst film over a wide range of conditions and that the Ni_0.8Fe_0.2O_<i>x</i> is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe₂O₃. The Ni_0.8Fe_0.2O_<i>x</i> therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-Fe₂O₃|Ni_0.8Fe_0.2O_<i>x</i> interface is significantly decreased by the oxidation of Ni²⁺ to Ni³⁺ and the associated increase in the Ni_0.8Fe_0.2O_<i>x</i> electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement

Catalyst Deposition on Photoanodes: The Roles of Intrinsic Catalytic Activity, Catalyst Electrical Conductivity, and Semiconductor Morphology

Semiconducting oxide photoanodes are used to drive the oxygen evolution reaction (OER) in water-splitting systems. The highest-performing systems use nanostructured semiconductors coated with water-oxidation catalysts. Despite much work, the design principles governing the integration of catalysts with semiconductors are poorly understood. Using hematite as a model system, we show how semiconductor morphology and electrical conductivity of the catalyst affect the system photoresponse. Electrically conductive catalysts can introduce substantial “shunt” recombination currents if they contact both the semiconductor surface and the underlying conducting-glass substrate, leading to poor performance. This recombination can be largely eliminated by using pinhole-free semiconductors, using selective photoassisted electrodeposition of thin catalyst layers on the semiconductor surface, using electrically insulating catalyst layers, or adding an intermediate insulating oxide layer. The results of this study are used to clarify the mechanisms behind several important results reported in the literature