Development of high-performance SnO2 gas diffusion electrodes for CO2 reduction to formic acid in a flow cell

CO2 electrochemical reduction reaction (CO2RR) to formic acid (HCOOH) is an attractive strategy to address both environmental and energy issues such as mitigate the CO2 emission and generates a key intermediate renewable chemical feedstock as well as a potential fuel for direct formic acid fuel cell. However, CO2 presents a high thermodynamic stability and low solubility in aqueous electrolyte, then the productivity is often limited by CO2 diffusion through a liquid electrolyte to the surface of the catalyst. Here, we developed a SnO2-based gas diffusion electrode by spray-coating to be applied in an alkaline flow-cell, where the CO2 can be fed directly through the electrode in the interface electrode/electrolyte to overcome the CO2 solubility issue. Carbon black and Sustainion® ionomer developed a key role in the GDE’s conductivity and SnO2 particles distribution, respectively. The SnO2 loading and thickness presented a great impact in the materials performance. The increase at KOH concentration up to 1.0 M improved both current density and selectivity for HCOOH formation, however a further increase at KOH concentration to 2.0 M resulted in a worst selectivity for HCOOH. The optimized GDE and CO2RR conditions exhibited a current density higher than 160 mA.cm-2 and HCOOH selectivity higher than 93%, this performance is much better than when applied SnO2 as electrode in a classical H-cell. The GDE prepared in the commercial carbon paper presented a poor stability under CO2 electrolysis conditions. On the other hand, when the GDE was prepared using PTFE membrane as substrate a stable electrochemical cell performance for more than 24 h was experimentally demonstrated at a current density higher than 100 mA cm−2 at -1.2 V vs RHE.