Effects of Ni doping on the structural, vibrational, and magnetic properties and on the hydrogen evolution reaction in the FeNbO4 compound

In this study, we conducted a comprehensive experimental investigation on nickel-doped iron niobate ceramics (Fe 1-x Ni x NbO 4 with x = 0.0 – 0.25) crystallized in the monoclinic wolframite phase. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, electron paramagnetic resonance (EPR), field-dependent (M-H) and temperature-dependent (M-T) magnetization, and measurements of the hydrogen evolution reaction (HER). XRD and Raman analyses revealed local structural distortions induced by the substitution of Fe 3+ ions (3d 5 , S = 5/2) by Ni 2+ ions (3d 8 , S = 1). Magnetic characterizations, including M-H isotherms, M-T measurements, and EPR spectra, indicated the emergence of ferrimagnetic ordering, evidencing an antiferromagnetic (AFM) to ferrimagnetic (FiM) transition. The origin of ferrimagnetic ordering can be attributed to the increase in oxygen-mediated superexchange interactions between Fe and Ni ions (Ni 2+ -O-Fe 3+ ) and the formation of oxygen vacancies. Electrochemical measurements demonstrated promising activity in the hydrogen evolution reaction (HER), with an overpotential of approximately 617 mV at a current density of 10 mA/cm 2 and a Tafel slope of 184 mV/decade for the composition x = 0.20. Furthermore, a negligible loss of overpotential was observed after 120 consecutive hours of hydrogen evolution. These results suggest that Ni-doped FeNbO 4 possesses considerable potential for applications in sustainable hydrogen production.