Enabling direct CO2 electrolysis by alkali metal cation substituted membranes in a gas diffusion electrode reactor

Abstract

Direct gas phase electrolysis using a membrane electrode assembly (MEA) reactor has been recognized as a promising direction for large scale continuous CO2 conversion. The so called “zero-gap” CO2 reduction was however seldom achieved when proton is the main charge carrier in the system due to the overwhelmed competing hydrogen evolution reaction (HER) taking place at the cathode. In this work, the suppression of HER is achieved by regulating the rate of proton transport through the membrane electrolytes and is realized by the modification of the sulfonic acid side chains with alkali metal cations. The MEA’s CO2 reduction reaction (CO2RR) faradaic efficiency (CO2RR-FE) is significantly improved as the alkali cations are varied sequentially from Li to Cs. Such improvement is attributed to the significantly hindered proton transport in the Cs-MEA, as reflected by the substantially larger high frequency resistance (HFR), which leads to enhanced competitive adsorption of CO2 on the copper catalyst. In addition, the Cs-MEA showed the highest robustness for both the membrane cations and the copper cathode against leaching and corrosion. Moreover, the observed phase transition of copper metal into faceted cuprous oxide crystals exposing low index surfaces also favors CO2RR over HER. The collective desirable properties of the Cs-MEA result in the best CO2RR performance, exhibiting a maximum CO2RR-FE of 0.44% at −1.2 V vs RHE and a space time yield (STY) of 1.06 mmol·h−1·gcat-1 for methane (CH4) and 2.02 mmol·h−1·gcat-1 for carbon monoxide (CO).