Catalyst Coated Membrane Electrodes for the gas phase CO2 electroreduction to formate

Abstract

The electrochemical valorisation of captured CO2 is an attractive option to obtain value-added products, and at the same time, to chemically store energy from intermittent renewable sources. Among the different products, formic acid/formate is particularly interesting since it is one of the most promising materials for hydrogen storage and candidate fuel for low-temperature fuel cells. In this work, a process for CO2 electroreduction to formate is studied on a continuous filter-press cell using an innovative electrode: Sn Catalyst Coated Membrane Electrodes (Sn-CCMEs) - comparing with previous approaches based on Sn Gas Diffusion Electrodes (Sn-GDEs), using the same synthesised tin nanoparticles (Sn NPs) and operating conditions. The Sn-CCME is prepared by depositing Sn NPs directly over a Nafion 117 membrane, and it allows working with a gaseous CO2 flow humidified with water as the input of the electrochemical cell, avoiding the use of the liquid catholyte. Sn-CCME operates at lower current densities (45 mA cm-2) than previous Sn-GDEs (200 mA cm-2), which resulted in lower rates of formate production. However, the proposed Sn-CCME, allowed achieving even higher formate concentrations with an energy consumption 50% lower than with the Sn-GDEs. The influence of key variables such as temperature and water input flow on the performance of the process using Sn-CCMEs was also analysed in a controlled experimental set-up specifically designed and built for this goal. Increasing the temperature of the gaseous stream did not improve the performance. The best results were obtained at ambient conditions of temperature (20 °C) and with the amount of water in the CO2 stream at 0.5 g h-1, giving the highest formate concentration (19.2 g L-1) with a Faradaic efficiency close to 50% and an energy consumption of 244 kW h kmol-1. More research is still required to further improve CCME configuration in order to increase formate rate and efficiency without increasing energy consumption.