Efficient and Safe Membrane‐Free Flow Electrolyzer for Formate Synthesis and Direct Fuel Cell Integration

Abstract

ABSTRACT Membrane‐free electrocatalysis represents a promising alternative to conventional systems, yet the potential H 2 /O 2 intermixing stands as the foremost barrier to practical implementation. Here, we report an efficient and safe membrane‐free flow electrolyzer that kinetically matches methanol oxidation and CO 2 reduction for symmetric formate production. To suppress H 2 /O 2 generation, we developed low‐cost, separate catalysts that can operate compatibly in a single electrolyte. When stabilized by lattice‐matched FeOOH, Ni 5 (II)O(OH) 8 , a new material synthesized for the first time, addresses the common issue of Ni‐based catalysts being oxidized to NiOOH. This feature effectively suppresses competing O 2 evolution, enabling ∼100% methanol‐to‐formate conversion within an expanded potential window. In parallel, SO 4 2− incorporated Bi 2 O 2 CO 3 shields methanol from the electrolyte while promoting highly exclusive CO 2 reduction to formate at evaluated current densities. Thus, the system achieves >195% overall formate Faradaic efficiency over a record‐wide current density range (2.0 to 424.6 mA cm −2 ) with minor H 2 /O 2 production rates (e.g., H 2 : 1.6 mL h −1 , O 2 : 0.5 mL h −1 , at 200 mA cm −2 ), demonstrating excellent production efficiency and safety under fluctuating renewable energy input. The produced formate‐rich solution can be further utilized in a high‐performance fuel cell. This work establishes a low‐cost and safe CO 2 ‐to‐power loop route for sustainable energy conversion.