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Direct estimation of the reaction environment, e.g., local pH at the anode side of a membrane electrode assembly (MEA) of zero gap electrolyzer, is essential to understand possible key factors, which are influencing the sustainable operation of industrial electrolyzers. Herein, we demonstrate a scanning electrochemical microscopy-based strategy to measure the local pH in the close vicinity of an operating MEA. Local proton concentration changes during the oxygen evolution reaction were monitored in the nonzero gap electrolyzer and MEA systems. The measurements constitute a methodology to evaluate the ion transport efficiency of the MEA. The strategy was extended to investigate the effect of an activation process, buffering of the electrolyte, and poisoning effect on the change in proton transport efficiency. This novel strategy enables the estimation of the actual pH of the MEA system during operation and is of great relevance in understanding the process conditions during sustainable fuel production., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

For the anodic H 2 O 2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H 2 O 2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H 2 O 2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO 3 - , at potentials where H 2 O 2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO 3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm -2 , and 24 h measurements demonstrated no loss of current density of 1 A cm -2 ., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Solar water oxidation is considered as a promising method for efficient utilization of solar energy and bismuth vanadate (BiVO 4 ) is a potential photoanode. Catalyst loading on BiVO 4 is often used to tackle the limitations of charge recombination and sluggish kinetics. In this study, amorphous nickel oxide (NiO x ) is loaded onto Mo-doped BiVO 4 by photochemical metal-organic deposition method. The resulting NiO x /Mo:BiVO 4 photoanodes demonstrate a two-fold improvement in photocurrent density (2.44 mA cm -2 ) at 1.23 V versus reversible hydrogen electrode (RHE) compared with the uncatalyzed samples. After NiO x modification the charge-separation and charge-transfer efficiencies improve significantly across the entire potential range. It is further elucidated by open-circuit photovoltage (OCP), time-resolved-microwave conductivity (TRMC), and rapid-scan voltammetry (RSV) measurements that NiO x modification induces larger band bending and promotes efficient charge transfer on the surface of BiVO 4 . This work provides insight into designing BiVO 4 -catalyst assemblies by using a simple surface-modification route for efficient solar water oxidation., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this paper, the synergistic effect of porosity and gradient of Mo doping in BiVO 4 photoanodes for improving charge separation and solar water oxidation performance is reported. A simple solution-based, three-step fabrication route was adopted using a layer-by-layer assembling technique. A water oxidation photocurrent of ∼1.73 mA cm -2 at 1.23 V vs reversible hydrogen electrode in neutral pH was achieved without using any sacrificial agent or electrocatalyst. The gradient Mo doping was found to enhance charge separation efficiency, which was verified through a shift in the water oxidation onset potential cathodically to ∼200 mV. In addition, these results were further confirmed by a higher open-circuit photovoltage and flat band potential investigations. This was attributed to the surface energetics played by gradient Mo doping that served as the driving force in reducing the onset potential for water oxidation. The coupled effect of enhanced light absorption and charge separation was revealed by monitoring the difference in decoupling the water oxidation efficiencies of porous and planar Mo:BiVO 4 photoanodes. This study demonstrated an improvement in the catalytic and charge separation efficiency of Mo:BiVO 4 photoanodes due to the introduction of porous structured homojunctions in a gradient manner. The simple synthesis approach adopted in the present study can be utilized and scaled up in making efficient photoanodes for competent solar water oxidation cells., Competing Interests: The authors declare no competing financial interest.

The two important factors that affect sunlight assisted water splitting ability of TiO 2 are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO 2 nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO 2 nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO 2 nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti 3+ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO 2 nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO 2 samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO 2 nanotubes are found to be 12 times efficient compared to pristine triphase TiO 2 , for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO 2 with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.

Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g -1 as the specific capacitance at a current density of 1 A g -1 , the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g -1 at a current density of 1 A g -1 , with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g -1 at 1 A g -1 and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg -1 at a high power density of 750 W kg -1 ., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Sunlight-responsive anatase N-doped TiO2 thin films undergoing reversible and switchable hydrophobic to superhydrophilic transition were synthesized by ultrasonic spray pyrolysis in a single step. Film thickness, roughness, surface morphology, crystal structure, composition, band gap, and wetting properties were studied using surface profilometry, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectroscopy, and water contact angle measurements, respectively. Surface X-ray photoelectron spectroscopy before and after sunlight irradiation revealed the major physicochemical process responsible for the hydrophobic → hydrophilic transition as surface hydroxylation and that responsible for hydrophilic → superhydrophilic transition as destruction of surface adsorbed organic species. The kinetic rates of the hydrophobic → superhydrophilic transition under sunlight and superhydrophilic → hydrophobic transition when kept under dark are found to be 0.215 min(-1) and 2.03 × 10(-4) min(-1), respectively.