Is Hydroxide Just Hydroxide? Unidentical CO₂ Hydration Conditions during Hydrogen Evolution and Carbon Dioxide Reduction in Zero-Gap Gas Diffusion Electrode Reactors

Abstract

The implementation of gas diffusion electrodes is a prerequisite to achieving industrially relevant reaction rates in gasphase electrochemical CO 2 reduction (CO 2 RR). In the state-of-the-art anion exchange membrane flow electrolyzers, however, there is a substantial loss of reactants due to a nonelectrochemical CO 2 consumption at the cathode and the transport of its products to the anode. Our detailed analysis of CO 2 crossover in a zero-gap CO 2 -to-CO flow electrolyzer showed a change in the chemical nature of the transported ionic species through the membrane. With the increasing reaction rate, a continuous shift from HCO 3 − to CO 3 2− conduction was found to be similar to pure carbonate conduction in the high current density region (>100 mA cm −2 ). As competing hydrogen evolution takes over the cathodic reaction in a CO 2 -rich environment, hydroxide conduction becomes more pronounced. This reveals an alteration in the chemical CO 2 consumption, the so-called CO 2 hydration (CO 2 + OH − ↔ HCO 3 − + OH − ↔ CO 3 2− ), implying an unidentical environment for the hydroxide ions generated in CO 2 RR and hydrogen evolution reaction under a CO 2 atmosphere. Our work draws attention to the incomplete description of CO 2 hydration at the confined cathode/ membrane interface in membrane electrode assembly-type zero-gap CO 2 electrolyzers.

Visualisation and quantification of flooding phenomena in gas diffusion electrodes (GDEs) used for electrochemical CO2 reduction: A combined EDX/ICP–MS approach

“…The microporous layer acts as an electrically conductive mechanical support to the catalyst, holding it in direct contact with the membrane. In many electrolysers, the applied catalyst ink contains silver nanoparticles that direct ec-CO 2 RR towards the formation of CO as a dominant reaction product [8,11,15,[20][21][22]. Note here that CO belongs to the most economically viable ec-CO 2 RR products [23] and has large global annual production [3].…”

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.…”

“…When the anode is pressed to the membrane (cell types I and III), however, the carbonate ion flux can detrimentally affect the stability of the anode catalyst. Another necessary consequence of the carbonate transport was revealed during long-term experiments with recirculated anolyte: the pH of the anolyte changes to near-neutral, even if a highly alkaline solution was applied at the beginning of the experiments. , This sets new requirements for the anode catalyst that must therefore be stable and active in OER at near-neutral pH and, in some cases, even in concentrated carbonate solution.…”

“…In almost half of the cases (47%), Ir was employed as the anode catalyst, ,,,,,,,,− Another 30% and 14% account for Ni − ,,,,− ,− and Pt, ,,− …”

“…This provides conditions similar to the case of alkaline water electrolyzers. Recent studies, however, revealed that in the case of recirculating the anolyte it is neutralized during continuous operation. − Furthermore, in electrolyzer cells where the catalysts are directly pressed to a separator (i.e., fuel cell type or zero-gap cells; see Figure ), the surface chemistry might be significantly different from that in the electrolyte bulk, setting new requirements for the anode catalyst for stable operation. 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 …”