A Solid-State Electrolyte Facilitates Acidic CO2 Electrolysis without Alkali Metal Cations by Regulating Proton Transport

Electrochemical CO₂ reduction (CO₂R) in acidic media provides a pathway to curtail CO₂ losses by suppressing the formation of (bi)carbonates. In such systems, a high concentration of alkali metal cations is necessary for mitigating the proton-rich environment and suppressing the competing hydrogen evolution reaction. However, a high cation concentration also promotes salt precipitation within the gas diffusion layer, resulting in poor system durability. Here, we resolve this conundrum by replacing the liquid catholyte with a solid-state proton conductor to regulate H⁺ transport. This is postulated to allow for a locally alkaline environment at the cathode, enabling selective CO₂R even without alkali metal cations. We show that this strategy is effective over a broad range of catalyst systems. For instance, we achieve an 87% CO faradaic efficiency (FE) at 300 mA/cm² using a composite nanoporous Au and single-atom Ni catalyst, with 0.25 M H₂SO₄ as the anolyte. Stable operation over 110 h and a high single-pass carbon efficiency of 82.8% were also successfully demonstrated. Importantly, we find that this solid-state system is also particularly effective at converting dilute feedstock (5% CO₂) with a CO FE of 47.7%, a factor of 16.4 times higher than a conventional system. Our results introduce a simple yet effective design approach for developing efficient acidic CO₂R electrolyzers.