Photoelectrochemical CO₂-to-Formic Acid Conversions: Advances in Photoelectrode Designs and Scale-Up Strategies

Rapid decarbonization requires renewable technologies that convert carbon dioxide (CO₂) into energy-dense, carbon-neutral fuels. Among those, photoelectrochemical CO₂ conversion systems offer a direct and efficient pathway by coupling light-harvesting and electrocatalytic components within a single device. Among CO₂-derived by-products, formic acid remains significant owing to its high volumetric energy density, liquid-phase storability, and transportable hydrogen carrier. This review outlines fundamental light-driven and catalytic processes of CO₂-to-formic acid conversion and demonstrates its key performance merits. Device configuration of various photoelectrochemical CO₂-to-formic acid conversion systems is analyzed with their recent advancements and bottlenecks. Despite significant progress of these systems, studies confirm that practical deployment remains limited by insufficient power output from photoelectrodes that limits bias-free operation, sluggish multi-electron kinetics that suppress conversion rates, and complex device architecture that hinders long-term and scale-up operation. Engineering and operational limitations that prevent photoelectrodes from bias-free operation, long-term stability, and efficient solar-to-fuel conversion efficiency are then investigated, and strategies to overcome these limitations are outlined. Furthermore, engineering strategies of compact electrolyzers are discussed to perform CO₂-to-formic acid conversion under high light-intensity. Key considerations to overcome mass transport limitations and address downstream formic acid separation challenges are discussed to bridge gap between laboratory-scale demonstrations and real-world applications.