Unraveling the Contributions of Laser-Induced Ripples and Nanoparticles to Enhanced HER Performance of Nickel Electrodes.

Herein, we report a chemistry-free strategy to enhance the hydrogen evolution reaction (HER) performance of nickel electrodes through 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 decorated with laser-generated nanoparticle-like structures. The ripples were oriented perpendicular to the laser polarization, with periodicity increasing from ∼400 nm at 515 nm to ∼800 nm at 1030 nm. AFM analysis showed higher ripple amplitudes and surface roughness at 1030 nm, with peak-to-valley heights increasing from ∼100 to ∼240 nm, indicating deeper energy deposition and melt depth. Electrochemical measurements in 1.0 M KOH revealed a strong relationship between surface morphology and HER activity. The 1030 nm-structured electrode exhibited the best performance, with a Tafel slope of ∼85 mV dec-1, over fourfold higher electrochemically active surface area than untreated Ni, the lowest charge-transfer resistance, and excellent stability at 100 mA cm-2. Although the formation of ripples alone accounts for an estimated 23% increase in geometric surface area, the substantially larger enhancement in electrochemically surface area (ESCA) indicates that the superimposed nanoscale features create a hierarchical morphology crucial for the improved HER performance.