Ni(II)–Aniline Complex-Modified TiO2 as a Bifunctional Electrocatalyst for Enhanced Hydrogen and Oxygen Evolution in Alkaline Water Splitting

Developing cost-effective bifunctional electrocatalysts for alkaline water splitting is essential for sustainable hydrogen production. Herein, Ni(II)–aniline complex-modified TiO2 (Ni–TiO2) nanoparticles were synthesized via a simple surface-modification strategy and evaluated for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH. X-ray diffraction and Raman spectroscopy confirmed the retention of the anatase TiO2 structure without detectable crystalline nickel-containing secondary phases. Surface characterization of Ni–TiO2 nanoparticles before and after OER testing was performed using FE-SEM and EDX. X-ray photoelectron spectroscopy revealed predominantly Ti4+ and surface-associated Ni2+ species, while optical and photoluminescence analyses indicated modifications to the optical and interfacial properties of TiO2. Ni–TiO2 exhibited improved HER activity, requiring overpotentials of 267.2 and 373.1 mV at current densities of 10 and 50 mA cm−2, respectively, compared with 317.3 and 462.2 mV for pristine TiO2. Its lower Tafel slope (126.5 versus 163.79 mV dec−1) and higher electrochemically active surface area (142.25 versus 5.87 cm2) further indicated improved reaction kinetics and surface accessibility. Ni–TiO2 retained 92.55% of its initial HER activity after 12 h at 100 mA cm−2. For OER, it achieved 50 mA cm−2 at an overpotential of 381.3 mV, compared with 500.6 mV for pristine TiO2, and exhibited a lower Tafel slope (86.7 versus 143.4 mV dec−1). The catalyst maintained 88.2% of its initial OER current after 12 h at 50 mA cm−2. These findings demonstrate that Ni(II)–aniline modification improves the electrocatalytic activity and operational stability of TiO2 toward both HER and OER, highlighting its potential as a coordination-chemistry-assisted strategy for alkaline water splitting.