A decade ago, two-dimensional microscopic flow visualization proved the theoretically predicted existence of electroconvection roles as well as their decisive role in destabilizing the concentration polarization layer at ion-selective fluid/membrane interfaces. Electroconvection induces chaotic flow vortices injecting volume having bulk concentration into the ion-depleted di↵usion layer at the interface. Experimental quantification of these important flow patterns have so far only been carried out in 2D. Numerical direct simulations suggest 3D features, yet experimental proof is lacking. 3D simulations are also limited in covering extended spacial and temporal scales.This study presents a new comprehensive experimental method for the time-resolved recording of the 3D electroconvective velocity field near a cationexchange membrane. For the first time, the spatio-temporal velocity field can be visualized in 3D at multiples of the overlimiting current density. In contrast to today's simulations, these experiments cover length and time scales typical for actual electrodialytic membrane processes.We visualize coherent vortex structures and reveal the changes in the velocity field and its statistics during the transition from vortex rolls to vortex ⇤

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in this field is mainly focused on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the micro-environment of the catalyst, i.e. local pH and $$\hbox {CO}_2$$ CO 2 concentration values, play an equivalently important role in the selectivity of product formation. In this study, we present an in operando$$^{13}\hbox {C}$$ 13 C nuclear magnetic resonance technique that enables the simultaneous measurement of pH and $$\hbox {CO}_2$$ CO 2 concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is emphasised.

Abstract. In operando nuclear magnetic resonance (NMR) spectroscopy is one method for the online investigation of electrochemical systems and reactions. It allows for real-time observations of the formation of products and intermediates, and it grants insights into the interactions of substrates and catalysts. An in operando NMR setup for the investigation of the electrolytic reduction of CO2 at silver electrodes has been developed. The electrolysis cell consists of a three-electrode setup using a working electrode of pristine silver, a chlorinated silver wire as the reference electrode, and a graphite counter electrode. The setup can be adjusted for the use of different electrode materials and fits inside a 5 mm NMR tube. Additionally, a shielding setup was employed to minimize noise caused by interference of external radio frequency (RF) waves with the conductive components of the setup. The electrochemical performance of the in operando electrolysis setup is compared with a standard CO2 electrolysis cell. The small cell geometry impedes the release of gaseous products, and thus it is primarily suited for current densities below 1 mA cm−2. The effect of conductive components on 13C NMR experiments was studied using a CO2-saturated solution of aqueous bicarbonate electrolyte. Despite the B0 field distortions caused by the electrodes, a proper shimming could be attained, and line widths of ca. 1 Hz were achieved. This enables investigations in the sub-Hertz range by NMR spectroscopy. High-resolution 13C NMR and relaxation time measurements proved to be sensitive to changes in the sample. It was found that the dynamics of the bicarbonate electrolyte varies not only due to interactions with the silver electrode, which leads to the formation of an electrical double layer and catalyzes the exchange reaction between CO2 and HCO3-, but also due to interactions with the electrochemical setup. This highlights the necessity of a step-by-step experiment design for a mechanistic understanding of processes occurring during electrochemical CO2 reduction.