Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2 , and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes.

We report metallic NiPS 3 @NiOOH core−shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS 3 nanosheets are obtained by exfoliation of bulk NiPS 3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS 3 @ NiOOH core−shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS 3 @NiOOH core−shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS 3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS 3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS 3 @ NiOOH core−shell heterostructure, where the presence of P and S modulates the surface electronic structure of Ni in NiPS 3 , thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS 3 @ NiOOH core−shell electrode materials for the electrochemical water oxidation.

“Single entity” measurements are central for an improved understanding of the function of nanoparticle‐based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF‐67)‐derived Co−N‐doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non‐interfering Os complex as free‐diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s−1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects.

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH ions and consumes O and H O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH ions and H O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH ions and H O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.

The reproducible fabrication of nanometre-sized carbon electrodes poses great challenges. Especially, the field of single entity electrochemistry has strict requirements regarding the geometry of these electrochemical probes. Herein, an automated setup for the fabrication of carbon nanoelectrodes based on the pyrolysis of a propane/butane gas mixture within pulled quartz capillaries by means of a moving heating coil is presented. It is shown that mere electrochemical characterisation with conventional redox mediators does not allow for a reliable assessment of the electrode's geometry and quality. Therefore, highthroughput transmission electron microscopy is used in parallel to evaluate and optimise preparation parameters. Control of the latter gives access to three different electrode types: nanopipettes, nanosamplers and nanodisks.