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

Membrane-free electrocatalysis represents a promising alternative to conventional systems, yet the potential H2/O2 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 CO2 reduction for symmetric formate production. To suppress H2/O2 generation, we developed low-cost, separate catalysts that can operate compatibly in a single electrolyte. When stabilized by lattice-matched FeOOH, Ni5(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 O2 evolution, enabling ∼100% methanol-to-formate conversion within an expanded potential window. In parallel, SO4 2- incorporated Bi2O2CO3 shields methanol from the electrolyte while promoting highly exclusive CO2 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 H2/O2 production rates (e.g., H2: 1.6 mL h-1, O2: 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 CO2-to-power loop route for sustainable energy conversion.