As a remedy to the increasing concentration of greenhouse gases and depleting fossil resources, the electrochemical CO 2 reduction closes the carbon cycle and provides an alternative carbon feedstock to the chemical and energy industry. While most contemporary research focuses on the catalyst activity, we emphasize the importance of the reactor design for an energetic efficient (EE) conversion. A design strategy for an electrochemical membrane reactor reducing CO 2 to hydrogen, carbon monoxide (CO) and ethylene (C 2 H 4 ) is developed.We present the stepwise development from an H-cell like setup using full-metal electrodes to a cell with gas diffusion electrodes (GDE) towards high current efficiencies (CE) at high current densities (CD). At 300 mA cm =2 a CO-CE of 56% for a Ag GDE and a C 2 H 4 -CE of 94% for a Cu GDE are measured. The incorporation of the developed GDEs into a zero-gap assembly eliminates ohmic losses and maximizes EE, however the acidic environment of the ion exchange membrane inhibits CO 2 reduction. As a compromise a thin liquid buffer layer between cathode and membrane is a prerequisite for a highly active conversion. We demonstrate that industrial relevant CDs with high CEs and EEs can only be achieved by moving beyond today's research form catalyst development only to an integrated reactor design, which allows to exploit the viable potential of electrochemical CO 2 reduction catalysts.

We present a combination of comprehensive experimental and theoretical evidence to unravel the mechanism of two-electron oxygen reduction reaction (ORR) on a catalyst composed of mildly reduced graphene oxide supported on P50 carbon paper (mrGO/P50). This catalyst is unique in that it shows >99% selectivity toward H2O2, the highest mass activity to date, and essentially zero overpotential in base. Furthermore, the mrGO catalytically active site is unambiguously identified and presents a unique opportunity to investigate mechanisms of carbon-based catalysis in atomistic detail. A wide range of experiments at varying pH are reported: ORR onset potential, Tafel slopes, H/D kinetic isotope effects, and O2 reaction order. With DFT reaction energies and known thermodynamic parameters, we calculate the potential and pH-dependent free energies of all possible intermediates in this ORR and propose simple kinetic models that give semiquantitative agreement with all experiments. Our results show that mrGO is semiconducting and cannot support the conventional mechanism of coherently coupled proton–electron transfers. The conducting P50 provides electrons for initiating the ORR via outer sphere electron transfer to O2(aq), while the semiconducting mrGO provides the active catalytic sites for adsorption of O2 –(aq) or HO2(aq), depending upon electrolyte pH. Due to this unique synergistic effect, we describe the mrGO/P50 as a co-catalyst. This concept implies departure from the traditional picture of predicting catalytic activity trends based on a single descriptor, and the co-catalyst design strategy may generally enable other semiconductors to function as electrocatalysts as well.

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State-of-the-art all-vanadium redox flow batteries employ porous carbonaceous materials as electrodes. The battery cells possess non-scalable fixed electrodes inserted into a cell stack. In contrast, a conductive particle network dispersed in the electrolyte, known as slurry electrode, may be beneficial for a scalable redox flow battery. In this work, slurry electrodes are successfully introduced to an all-vanadium redox flow battery. Activated carbon and graphite powder particles are dispersed up to 20 wt.% in the vanadium electrolyte and charge-discharge behavior is inspected via polarization studies. Graphite powder slurry is superior over activated carbon with a polarization behavior closer to the standard graphite felt electrodes.3D-printed conductive static mixers introduced to the slurry channel im-prove the charge transfer via intensified slurry mixing and increased surface area. Consequently, a significant increase in the coulombic efficiency up to 80 % and energy efficiency up to 50 % is obtained. Our results show that slurry electrodes supported by conductive static mixers can be competitive to state-of-the-art electrodes yielding an additional degree of freedom in battery design. Research into carbon properties (particle size, internal surface area, pore size distribution) tailored to the electrolyte system and optimization of the mixer geometry may yield even better battery properties.

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