Wavelength-Controlled Laser Nanostructuring of Nickel Electrodes for Enhanced Hydrogen Evolution

Herein, we report a chemistry-free approach to enhance nickel HER performance via wavelength-controlled femtosecond laser-induced periodic surface structuring (LIPSS). Planar nickel sheets were nanostructured using ultrashort laser pulses at 515 and 1030 nm under identical irradiation intensity, yielding highly ordered low-spatial-frequency ripples with excellent long-range uniformity. Morphological analysis reveals well-defined, polarization-oriented ripple patterns with wavelength-dependent periodicity, increasing from ~400 nm at 515 nm to ~800 nm at 1030 nm. Complementary AFM measurements demonstrate a pronounced increase in vertical modulation and surface roughness at the longer wavelength, with ripple peak-to-valley heights increasing from ~100 nm to ~240 nm, reflecting deeper subsurface energy deposition and enhanced melt modulation. Electrochemical evaluation in 1.0 M KOH shows a strong correlation between ripple geometry and HER activity. The 1030 nm-structured electrode exhibits the most favorable kinetics, achieving a Tafel slope of ~85 mV dec ^-1 , a more than fourfold increase in electrochemically active surface area compared with untreated Ni, and the lowest charge-transfer resistance. ECSA-normalized analysis confirms that activity enhancement is governed primarily by morphology-induced charge-transfer processes Excellent stability at 100 mA cm ^-2 highlights the robustness of wavelength-tuned laser nanostructures for alkaline HER.