Ambient CO<sub>2</sub> Capture and Valorization Enabled by Tandem Electrolysis Using Solid-state Electrolyte Reactor

Electrocatalytic carbon dioxide reduction is a promising technology for addressing global energy and environmental crises. However, its practical application faces two critical challenges: the complex and energy-intensive process of separating mixed reduction products and the economic viability of the carbon sources (reactants) used. To tackle these challenges simultaneously, solid-state electrolyte (SSE) reactors are emerging as a promising solution. In this review, we focus on the feasibility of applying SSE for tandem electrochemical CO2 capture and conversion. The configurations and fundamental principles of SSE reactors are first discussed, followed by an introduction to its applications in these two specific areas, along with case studies on the implementation of tandem electrolysis. In comparison to conventional H-type cell, flow cell and membrane electrode assembly cell reactors, SSE reactors incorporate gas diffusion electrodes and utilize a solid electrolyte layer positioned between an anion exchange membrane (AEM) and a cation exchange membrane (CEM). A key innovation of this design is the sandwiched SSE layer, which enhances efficient ion transport and facilitates continuous product extraction through a stream of deionized water or humidified nitrogen, effectively separating ion conduction from product collection. During electrolysis, driven by an electric field and concentration gradient, electrochemically generated ions (e.g., HCOO– and CH3COO–) migrate through the AEM into the SSE layer, while protons produced from water oxidation at the anode traverse the CEM into the central chamber to maintain charge balance. Targeted products like HCOOH can form in the middle layer through ionic recombination and are efficiently carried away by the flowing medium through the porous SSE layer, in the absence of electrolyte salt impurities. As CO2RR can generate a series of liquid products, advancements in catalyst discovery over the past several years have facilitated the industrial application of SSE for more efficient chemicals production. Also noteworthy, the cathode reduction reaction can readily consume protons from water, creating a highly alkaline local environment. SSE reactors are thereby employed to capture acidic CO2, forming CO3 2– from various gas sources including flue gases. Driven by the electric field, the formed CO3 2– can traverse through the AEM and react with protons originating from the anode, thereby regenerating CO2. This CO2 can then be collected and utilized as a low-cost feedstock for downstream CO2 electrolysis. Based on this principle, several cell configurations have been proposed to enhance CO2 capture from diverse gas sources. Through the collaboration of two SSE units, tandem electrochemical CO2