A Solid-State\nElectrolyte Facilitates Acidic CO<sub>2</sub> Electrolysis without\nAlkali Metal Cations by Regulating Proton\nTransport

Electrochemical CO<sub>2</sub> reduction\n(CO<sub>2</sub>R) in acidic\nmedia provides a pathway to curtail CO<sub>2</sub> losses by suppressing\nthe formation of (bi)carbonates. In such systems, a high concentration\nof alkali metal cations is necessary for mitigating the proton-rich\nenvironment and suppressing the competing hydrogen evolution reaction.\nHowever, a high cation concentration also promotes salt precipitation\nwithin the gas diffusion layer, resulting in poor system durability.\nHere, we resolve this conundrum by replacing the liquid catholyte\nwith a solid-state proton conductor to regulate H<sup>+</sup> transport.\nThis is postulated to allow for a locally alkaline environment at\nthe cathode, enabling selective CO<sub>2</sub>R even without alkali\nmetal cations. We show that this strategy is effective over a broad\nrange of catalyst systems. For instance, we achieve an 87% CO faradaic\nefficiency (FE) at 300 mA/cm<sup>2</sup> using a composite nanoporous\nAu and single-atom Ni catalyst, with 0.25 M H<sub>2</sub>SO<sub>4</sub> as the anolyte. Stable operation over 110 h and a high single-pass\ncarbon efficiency of 82.8% were also successfully demonstrated. Importantly,\nwe find that this solid-state system is also particularly effective\nat converting dilute feedstock (5% CO<sub>2</sub>) with a CO FE of\n47.7%, a factor of 16.4 times higher than a conventional system. Our\nresults introduce a simple yet effective design approach for developing\nefficient acidic CO<sub>2</sub>R electrolyzers.