The field of CO₂ electrolysis witnessed rapid development in recent years. At the same time, the role of the anodic half-reaction has received considerably less attention. Iridium is almost exclusively used as the anode catalyst in polyelectrolyte membrane water electrolyzers. This practice quickly became a standard in CO₂ electrolysis due to its similarity to water electrolysis (oxygen evolution - OER is the anode process in both cases). An alkaline electrolyte solution is typically recirculated in the anode compartment to ensure a high reaction rate during CO₂ electrolysis. However, iridium is known to slowly but continuously dissolve in alkaline solutions. Additionally, while using iridium as the anode catalyst on a laboratory scale is acceptable, economic reasons urge the exploration of non-noble alternatives.

In my presentation, I am going to discuss alternative anode electrocatalysts that have been tried already in CO₂ electrolyzers. A closer look will be taken at Ni, which should be stable at least under the initial process conditions. We found that while Ni showed high activity (similar cell voltage to that with iridium) at the beginning of our experiments, it instantaneously started to decrease over time. If the anolyte is recirculated (typical scenario), the continuous carbonate transport through the anion exchange membrane decreases the bulk pH of the anolyte. In the case of the zero-gap electrolyzer cells employed in our study, the carbonate ion flux directly hits the anode catalyst layer, causing a high carbonate ion concentration in the close vicinity of the electrode. In parallel, H⁺ ions formed at the anode as the result of OER are not neutralized instantly (unlike in water electrolysis), which leads to an acidic surface pH. These two effects together alter the local chemical environment at the surface of the anode electrocatalyst leading to a gradual dissolution of Ni.[1] Based on our measurements, a set of criteria will be described that have to be fulfilled by an anode catalyst to achieve high performance.

In the second part of my talk, I am going to provide an outlook on replacing OER at the anode with various alternative anode reactions (such as electrocatalytic alcohol oxidation) to deliver high-value products in parallel with increasing the energy efficiency for the whole process.[2] A closer look will be taken at issues regarding the selectivity and stability of these systems along with discussing possible mitigation strategies and future research directions.