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.

Electroreduction of CO 2 to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage.However,the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO 2 conversion to C 2+ products at industrially relevant current densities.T he B-Cu GDE exhibited ah igh Faradaic efficiency of 79 %f or C 2+ products formation at ac urrent density of À200 mA cm À2 and apotential of À0.45 Vvs. RHE. The long-term stability for C 2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu + species under CO 2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, whichlead to the excellent conversion of CO 2 to C 2+ products on B-Cu-Zn GDEs.

Developing highly efficient and selective electrocatalysts for the CO 2 reduction reaction to produce valueadded chemicals has been intensively pursued. We report as eries of Cu x O y C z nanostructured electrocatalysts derived from aC u-based MOF as porous self-sacrificial template. Blending catalysts with polytetrafluoroethylene (PTFE) on gas diffusion electrodes (GDEs) suppressed the competitive hydrogen evolution reaction. 25 to 50 wt %t eflonized GDEs exhibited aF aradaic efficiency of % 54 %f or C 2+ products at À80 mA cm À2 .The local OH À ions activity of PTFE-modified GDEs was assessed by means of closely positioning aP tnanoelectrode.Asubstantial increase in the OH À /H 2 Oactivity ratio due to the locally generated OH À ions at increasing current densities was determined irrespective of the PTFE amount.

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH− and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO2RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH− and H2O activity that in turn can possibly affect activity, stability, and selectivity of the CO2RR. We determine the local OH− and H2O activity in close proximity to a CO2‐converting Ag‐based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear‐force‐based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt‐tip nanosensor. We show that high turnover HER/CO2RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.

The selectivity of the chlorine evolution reaction over the oxygen evolution reaction during the electrolysis of aqueous NaCl is, despite being very high, still insufficient to prevent expensive separation of the formed Cl2 and O2 by means of liquefaction. We hypothesize that, by decreasing the local activity of H2O near the anode surface by substantially increasing the ionic strength of the electrolyte, the oxygen evolution reaction would be suppressed, leading concomitantly to a higher selectivity of Cl2 over O2 formation. Hence, the influence of the ionic strength on the competition between electrochemical evolution of O2 and Cl2 at dimensionally stable anodes (DSAs) was investigated. Addition of a high concentration of NaNO3, an inert electrolyte additive, increases the selectivity for chlorine at high current density, as determined by means of online electrochemical mass spectrometry and UV‐vis spectroscopy. We propose conditions in which free water is suppressed, owing to under‐coordination of the solvation shells of ions, as a general concept to modulate the selectivity of competing electrochemical reactions.