Local Chemical Environment Governs Anode Processes in CO₂ Electrolyzers

Abstract

A major goal within the CO 2 electrolysis community is to replace the generally used Ir anode catalyst with a more abundant material, which is stable and active for water oxidation under process conditions. Ni is widely applied in alkaline water electrolysis, and it has been considered as a potential anode catalyst in CO 2 electrolysis. Here we compare the operation of electrolyzer cells with Ir and Ni anodes and demonstrate that, while Ir is stable under process conditions, the degradation of Ni leads to a rapid cell failure. This is caused by two parallel mechanisms: (i) a pH decrease of the anolyte to a near neutral value and (ii) the local chemical environment developing at the anode (i.e., high carbonate concentration). The latter is detrimental for zero-gap electrolyzer cells only, but the first mechanism is universal, occurring in any kind of CO 2 electrolyzer after prolonged operation with recirculated anolyte.

Anode Catalysts in CO2 Electrolysis: Challenges and Untapped Opportunities

“…In almost half of the cases (47%), Ir was employed as the anode catalyst, 10 , 12 , 13 , 16 , 18 , 29 , 32 , 34 , 36 − 87 Another 30% and 14% account for Ni 7 − 10 , 17 , 36 , 39 , 41 − 44 , 88 − 117 and Pt, 32 , 114 , 118 − 133 respectively, while only 9% is related to other metals, metal alloys, or carbon 11 , 45 , 60 , 117 , 134 − 140 ( Figure 3 A). The dominance of Ir and Ni is rooted in the fact that these are the generally used catalysts in acidic and alkaline water electrolyzers, respectively.…”

“…Interestingly, Ir is the second most frequently applied electrocatalyst, despite its known slow dissolution in highly alkaline solutions. 18 This might be because of the short duration of the experiments or because of the gradual decrease of the anolyte pH to a near-neutral value during continuous operation, as detailed above. About one-third of the studies employed near-neutral electrolytes at the anode.…”

“…This is not surprising on the basis of the dissolution of Ni in neutral solutions, while Ir is stable (and active) during neutral pH OER. 18 , 144 Finally, only a handful (five) of the entries can be found under acidic conditions. 10 , 32 , 61 …”

“…It is worth noting that these observations explain why Ir is a robust anode catalyst in CO 2 electrolyzer cells, even though it is not stable in alkaline solutions. 18 …”

Sequential vs integrated CO2 capture and electrochemical conversion: An energy comparison

“…[22][23][24] Such CO2 evolution should occur at the cost of increasing anode overpotentials. 34 When the carbonate requires regeneration into CO2, an energy penalty of at least 230 kJ/molCO2-converted is associated with it in the case of 50% CO2 utilisation efficiency. Our analysis then takes this energy penalty into account.…”