Understanding Performance Fading during CO Electrolysis in Zero Gap Electrolyzers

Abstract

Electrochemical CO reduction (ECOR) can act as a potential bridge between CO2-to-CO technologies and renewable production of C2+ chemicals. Copper has been the most widely studied cathode catalyst for ECOR because of its unique ability to produce multicarbon products. Iridium and nickel are the most-widely used anode materials for acidic and alkaline electrolysis, respectively. However, recent reports on the instability of Ir in alkaline conditions and Ni in near neutral conditions has made it imperative to understand the anodic processes for achieving stable long term operation at high current densities for CO2/CO electrolysers.In this work, our aim was to investigate anode catalysts for ECOR at high current densities in zero gap electrolyser. Commercial Cu nanoparticles (25 nm) was used as the cathode catalysts with Ir black anode in alkaline conditions. Initially, CORR was studied in a hybrid cell with a catalyst coated anion exchange membrane and recirculated catholyte and anolyte. In this cell configuration we observe stable production of ethylene, acetate and ethanol for a total current density of upto 500 mA cm-2. However, in the zero gap electrolyser the catalytic activity decays rapidly (2-3 minutes) and leads to predominance of hydrogen evolution reaction (HER).Analysis of the cathode and anolyte after electrolysis reveals the dissolution of Ir and subsequent deposition at the cathode. This rapid decay is counter intuitive since the dissolution of Ir in alkaline solutions is very slow and should take a few hours to affect the catalytic activity. More importantly this dissolution of Ir does not happen when Ar is circulated at the cathode instead of CO, which indicates that the Ir dissolution is not entirely due to the alkaline environment. NMR analysis of the anolyte shows the presence of CO reduction products acetate and ethanol. Therefore, it is possible that the presence of CO reduction products could aggravate the Ir dissolution. This study is currently underway to analyse the possible Ir dissolution mechanisms. This is an important observations since Ir is still the most widely used anode catalyst for CO electrolysis. This also stresses on the need to explore alternative anode catalysts and/or design strategies that can circumvent the migration of reduction products to the anode.