(Invited) Effects of the Local Chemical Environment on the Anode and Cathode Processes of CO2 Electrolyzers

Abstract

Continuous-flow electrolyzers allow CO2 reduction at industrially relevant rates, but long-term operation is still challenging. In this talk, I am going to present some interesting findings on the role of different ions, crossing the anion exchange membrane in zero-gap electrolyzer cells, contributing to unstable operation.In the first part of my talk, I will show that while precipitate formation in the cathode gas diffusion electrode is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-containing solutions. [1] This enables deionized water-fed electrolyzers to operate at a CO2 reduction rate matching that of those using alkaline electrolytes (CO partial current density of 420 ± 50 mA cm−2 for over 200 hours). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different commercial membranes. Finally, we demonstrate the scalability of this approach on a multi-cell electrolyzer stack, with a 100 cm2 / cell active area.In the second part of the presentation, I will discuss the role of anode catalyst in CO2R cells. The urge to substitute Ir is driven by its high- and steeply rising market price as well as its limited stability in alkaline media. Although Ni is a ~ten thousand times cheaper, active, and stable oxygen evolution reaction (OER) catalyst in alkaline media, I will demonstrate that there are factors, which hinder its application in CO2 electrolysis. While Ni is a suitable OER catalyst in short experiments, the cell voltage increases, and the measured total Faradaic efficiency decreases continuously during prolonged electrolysis. This is caused by the local acidic pH at the anode surface, the crossing CO3 2- ions and by the gradual change in the anolyte composition, leading to Ni dissolution. The catalyst loss is only a minor part of the problem; the dissolving metal ions also penetrate into the anion exchange membrane, where precipitate forms due to the high local carbonate ion concentration, inducing cell failure.Finally, I will present a complex machine learning based approach, through which we aim to find the optimal operating conditions of such CO2 electrolyzer cells.