Transport Matters: The Critical Role of the Hydrogen Evolution Reaction (HER) in Accelerating Electrochemical Nitrate to Ammonia Conversion

A porous Co-based metal-oxide foam catalyst is fabricated via the dynamic hydrogen bubble template electrodeposition method followed by calcination (6 h at 300 °C thermal treatment). Electrolysis results demonstrate excellent performance of this catalyst in the electrochemical nitrate reduction reaction ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NO</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> <mml:mi>RR</mml:mi></mml:mrow> <mml:annotation>${\mathrm{NO}}_3^ - {\mathrm{RR}}$</mml:annotation></mml:semantics> </mml:math> ), attaining near-unity Faradaic efficiency (97.8% ± 3.6% at j_NH3,lim = -59.5 ± 2.3 mA cm^-2) at a low (over)potential of -0.2 V vs RHE, which represents maximum achievable performance in 0.1 mol L^-1 nitrate solutions (pH 13.7) under transport-limiting conditions in the absence of extra convection. Digital simulations show that, without forced convection, the catalyst's electrochemically active surface area changes dynamically due to rapid nitrate depletion inside the 3D foam. Electrolyte replenishment, triggered by vigorous hydrogen evolution, is shown to restore the active surface in the foam interior. This self-convection enables high ammonia partial current densities exceeding hundreds of mA cm^-2 (e.g., j_NH3 = -220 ± 18 mA cm^-2 at -0.6 V vs RHE, with FE_NH3 = 80.2% ± 2.2%). Operando XAS, XRD, Raman spectroscopy, and electrochemical analysis reveal the in situ evolution of a "tandem" composite catalyst during electrolysis, where β-Co(OH)₂ and metallic Co function both as the active phases for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>NO</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> <mml:mi>RR</mml:mi></mml:mrow> <mml:annotation>${\mathrm{NO}}_3^ - {\mathrm{RR}}$</mml:annotation></mml:semantics> </mml:math> , with β-Co(OH)₂ remaining kinetically stabilized under the cathodic operating conditions.