Hydrogen generation by water electrolysis has been comprehensively studied for several decades, although it still faces many difficulties in commercialization due to low efficiency. Even though IrO₂ and RuO₂ have superior activities to oxygen evolution reaction (OER), such novel metal catalysts still need high overpotential compared to hydrogen evolution reaction. To overcome the limitation of OER, former researches have been studied with various materials which are cheaper and perform better than the novel metal-based electrocatalysts. Among them, nickel is considered as the best catalyst candidate due to hydroxide state transformation between Ni(OH)₂ and NiOOH during the surface reaction.

Apart from the intrinsic properties, the morphology construction to the nanoscale structure is another useful way to enlarge the surface area, which is directly correlated to the overpotential decrease. Nanostructuring can inhibit continuous growth of bubbles on the surface and detach much easily. The bubble blocking effect on the electrode surface has been deeply studied within mechanical category but this effect has been poorly regarded for enhancement of the OER activity of the electrode. Also, the bimetallic system of nickel/iron chosen in this research enables great improvement for OER.

Herein, we synthesized three-dimensional ordered nanoporous mesh-like (3D-NM) nickel electrode by the photolithography method, which outperforms conventional porous nickel electrode. Successfully synthesized ordered nanoporous mesh-like structure provides highly extensive surface area within thickness of only 5 μm. In addition, we investigated that fast bubble formation/detachment from the pore can reduce the bubble blocking effect, which disturbs supply of electrolyte to the catalyst surface. 3D-NM nickel electrode only generates 12 times smaller extraordinary small bubbles compared to the typical bubbles in basic electrolyte. In line with the size of bubbles, impedance during the OER with 3D-NM nickel electrode smoothly undergoes while the nickel foam shows unstable graph.

Owing to the enlarged surface area and the fast bubble formation/detachment effect, overpotential measured at 10 mA cm^-2 was 372 mV and the calculated Tafel slope was 94.4 mV dec-¹. Through the unique nanoporous structure of nickel electrode, outstanding stability for 50 hours was achieved. To further improve the OER activity of as-prepared 3D-NM nickel electrode, the decoration of nickel and iron was conducted. The final 3D-NM NiFe electrode showed 261 mV of overpotential at 10 mA cm^-2 with 43.8 mV dec^-1 of Tafel slope.