Abstract:

As an emerging technology for achieving carbon neutrality, the electrocatalytic CO₂ reduction reaction (CO₂RR) converts CO₂ and water to valuable fuels and chemicals with power supply from renewable energy. Currently, the practical application of the CO₂RR suffers from insufficient electrocatalytic performance in terms of selectivity, reaction rate, energy efficiency, and long-term stability. Electrolytes are considered as equally critical as catalysts for enhancing the CO₂RR performance. From a catalysis perspective, electrolytes significantly affect the reaction microenvironments around catalytically active sites. From an electrochemical perspective, electrolytes determine the electric double layer structure. The electrocatalytic electrode/electrolyte interface where the CO₂RR takes place is strongly influenced by electrolyte composition and identity. Thus, beyond catalyst design, rational electrolyte design is an alternative strategy for advancing the CO₂RR towards industrial applications. This review presents important electrolyte effects in the aqueous CO₂RR using the most recent studies, with an emphasis on those conducted under industrially-relevant reaction conditions. The effects of (local) pH, cations, and anions in aqueous inorganic electrolytes and their coupled effects with solid polymer electrolytes on tuning the activity and selectivity of the CO₂RR are summarized. Although their influences on CO₂RR performance are interconnected, pH effects, cation effects, and anion effects as well as electrolyte effects in membrane electrode assembly (MEA) electrolyzers are discussed separately considering the leading role of each factor. The experimentally observed performance dependence on the electrolyte composition and identity as well as the underlying reaction mechanism are discussed. The unique role of vibrational spectroscopies, such as surface-enhanced infrared absorption and Raman, in the characterization of electrode/electrolyte interfaces and the elucidation of electrolyte effects is highlighted. An innovative experimental strategy that involves the validation of specific adsorption of alkali-metal cations on the surface of an electrode by precisely monitoring the vibrational spectroscopic characteristics of probe molecules is highly recommended. The review focuses on the pH-dependent electrocatalytic performance and reaction pathways, the uncertain mechanisms of cation effects, and the roles of typical anions, such as bicarbonate and halides. Particularly, emphasis is given to the transport of key species, such as anions, cations, water, and products in ion exchange membranes, as well as their dynamic behaviors at the electrode/electrolyte interface in MEA CO₂ electrolyzers. Although it has some drawbacks, anion exchange membranes are currently the most promising polymer electrolytes for practical application of the CO₂RR. However, some emerging strategies based on cation exchange and bipolar membranes as well as tandem electrolysis processes are in progress. In all cases, a detailed understanding of electrolyte effects in the complex environments of MEA electrolyzers is indispensable for achieving performance enhancement. In conclusion, the remaining challenges and research opportunities in terms of the experimental and theoretical investigation of the electrolyte effects in the CO₂RR process are proposed. This review provides novel insights into rational electrolyte design and useful guidelines for researchers in the field.

Key words: Electrocatalytic CO₂ electroreduction, Electrolyte effect, Electric double layer, pH effect, Cation effect, Anion effect, Solid polymer electrolyte, Membrane electrode assembly