Development and investigation of gas-diffusion electrodes for the electrochemical reduction of CO2

Abstract

An attractive solution to produce carbon-based chemicals is to utilize CO2 as raw material and renewable electricity as energy source for its activation such as in the reduction of CO2 (CO2RR) into formate salts. In the course of this work, carbon-based and PTFE-bound gas-diffusion electrodes (GDE) were developed with highly promising performance characteristics combining a newly developed simple and scalable dry deposition technique with a homogeneous precipitation method to obtain fine dispersion of the tin oxide electrocatalyst on the carbon substrate. Optimizing electrode properties yielded GDEs that allowed for operation at current densities >500 mA/cm² while maintaining FE towards formate >80%. While these are promising values and already in the range of what is targeted for technical application, long-term stability evidenced by a continuously increasing rise of hydrogen evolution over time is far from suitable from a technical point of view. By investigating the effect of the GDE properties - that are the composition (carbon type, PTFE content, electrocatalyst content), texture (adjusted via pore-forming agents and compacting pressure) as well as hydrophobicity (function of carbon substrate and adjusted by its oxidation) - and evaluating the time behavior, it could be shown that working at high current densities is determined by i) reactant and product transport, ii) wettability and iii) catalyst accessibility. Accordingly, an elaborate and systematic investigation of the relationship between said variables and the electrochemical performance was conducted to establish a starting point for a more targeted optimization of the GDE in future work. Ultimately, an analysis on the effect of the operating point on total electrolysis cost was performed to derive performance targets for formic acid production.