Directional Felt–Mesh Stainless-Steel Anodes for Zero-Gap Alkaline Water Electrolysis: Bubble-Size Gradients, In Situ NiFe (Oxy)hydroxide Activation, and the Convergence of Architecture and Surface Chemistry

Oxygen bubbles, not catalysis, limit the anode of zero-gap alkaline water electrolysis (AWE) at industrial current densities. Here we compare stainless-steel (SS) mesh (M), felt (F), and a bonded felt–mesh bilayer mounted with the felt facing the Zirfon separator (FM) or reversed (MF) in a zero-gap cell (6 M KOH, 80 °C), bare and Ni-plated. On bare SS, architecture dominates: the felt generates fine bubbles (54 μm) and the mesh coarse ones (109 μm), and the correctly oriented bilayer exploits this contrast—fine generation at the separator, coarse evacuation (95 μm) through the mesh—to deliver 1.85 V at 1.0 A cm−2, whereas the reversed stack is the worst electrode tested, showing voltage fluctuations symptomatic of interfacial gas blanketing. Ni plating improves every architecture (bilayer: 391 → 325 mV OER overpotential; 1.85 → 1.80 V) yet compresses the differences between them. Voltammetry and X-ray photoelectron spectroscopy show why: OER cycling converts the plated Ni into an Fe-incorporated NiFe (oxy)hydroxide—Fe appearing despite a nominally Fe-free bath—whose hydrophilicity shrinks bubbles on every architecture. Architecture and surface chemistry are thus complementary attacks on the same gas-management problem, and stacking orientation is a free design variable that no catalyst can replace. Operando impedance at 1.8 V and an overpotential decomposition price the penalty: flipping the electrode nearly triples the gas-transport overpotential at 1.0 A cm−2, from ≈140 to ≈402 mV, while ohmic and kinetic terms remain nearly untouched.