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.