The hydrogen evolution reaction (HER) is often considered
parasitic
to numerous cathodic electro-transformations of high technological
interest, including but not limited to metal plating (e.g., for semiconductor
processing), the CO2reduction reaction (CO2RR), the dinitrogen → ammonia conversion (N2RR),
and the nitrate reduction reaction (NO3–RR). Herein, we introduce a porous
Cu foam material electrodeposited onto a mesh support through the
dynamic hydrogen bubble template method as an efficient catalyst for
electrochemical nitrate → ammonia conversion. To take advantage
of the intrinsically high surface area of this spongy foam material,
effective mass transport of the nitrate reactants from the bulk electrolyte
solution into its three-dimensional porous structure is critical.
At high reaction rates, NO3–RR becomes, however, readily mass transport
limited because of the slow nitrate diffusion into the three-dimensional
porous catalyst. Herein, we demonstrate that the gas-evolving HER
can mitigate the depletion of reactants inside the 3D foam catalyst
through opening an additional convective nitrate mass transport pathway
provided the NO3–RR becomes already mass transport limited prior to the HER onset.
This pathway is achieved through the formation and release of hydrogen
bubbles facilitating electrolyte replenishment inside the foam during
water/nitrate co-electrolysis. This HER-mediated transport effect
“boosts” the effective limiting current of nitrate reduction,
as evidenced by potentiostatic electrolyses combined with an operando
video inspection of the Cu-foam@mesh catalysts under operating NO3–RR conditions.
Depending on the solution pH and the nitrate concentration, NO3–RR partial
current densities beyond 1 A cm–2were achieved.