The electrolyte matters: Stable systems for high rate electrochemical CO2 reduction

Abstract

Products from electrochemical CO2 reduction have the potential to replace fossil resources in the chemical industry by using renewable energies. However, stable electrolyte systems are an ongoing challenge for long-term electrolysis operation. We report the stability of different electrolyte combinations for two different electrochemical membrane reactor (ecMR) designs. Gas diffusion electrodes (GDEs) with different composition and manufacturing processes were subsequently tested in a stable electrolyte. Electrolytes in cation exchange membrane based ecMRs are only stable, if the anolyte is an acid with only protons as cations. Pure water usage is possible if a zero-gap assembly is used on the anode side. The supporting catholyte can be chosen freely as long as it is not electrochemically active and does not react with CO2. In a stable electrolyte system, pressed and non-pressed Nafion- and PTFE-bonded GDEs with copper and silver as catalyst are compared for current densities up to −300 mA cm−2. The results highlight the importance of the individual optimization of the GDE network in terms of porosity and hydrophobicity. An C2H4 current efficiency of 51% for a copper GDE and a C:H synthesis gas ratio of 2:1 with a silver GDE are measured for −300 mA cm−2. Further, experiments with highly acidic and highly buffered electrolytes, respectively electrolytes with a high ionic strength, show a strong decrease in cathodic overpotential. This work gives a clear guideline for ecMR electrolyte operation for a long-term stable system performance. We highlight the need for a deeper understanding of interfacial phenomena to maximize energetic efficiency.