Structural modulation of low-valent iron in LDH-derived Ni3Se4 nanosheets: a breakthrough electrocatalyst for the overall water splitting reaction

Abstract

Transition metal-based layered double hydroxides (LDHs) have attracted significant attention in recent research in electrochemistry and hold promising capacity in a wide range of applications. Owing to their diverse chemical structure, improved mass- and charge-transfer kinetics, and fascinating 2D layered structure, LDHs have emerged to be promising electrocatalysts for water electrolysis. Yet, some limitations, including their low stability and poor conductivity, remain as major obstacles towards the application of LDHs in the field of electrocatalysis. As a result, various LDH-derived nanomaterials, like selenides, sulphides, phosphides, etc., have been employed as effective electrocatalysts towards water splitting. Here in this study, a solvothermal reaction strategy was employed to derive a selenide from NiFe-LDH and the successful selenization resulted in the formation of Fe-doped Ni₃Se₄ (Fe@Ni₃Se₄) with a 2D microporous structure. The Fe@Ni₃Se₄ showed excellent electrocatalytic performance for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline conditions, requiring just 185 and 33 mV overpotentials to drive 10 mA cm⁻² current density in the OER and HER, respectively, which outperformed many recently reported benchmark electrocatalysts. Also, a 10-fold increase in turnover frequency (TOF) was noticed in Fe@Ni₃Se₄ compared to pristine NiFe-LDH, revealing an enormous improvement in specific activity as a result of the selenization. The presence of Fe with a lower valency provided an added advantage over the redox activity and stable structural outcomes, which was confirmed from operando impedance analysis and various other characterizations. First-principles DFT calculations revealed that doping low-valent Fe ions largely increased the 3d-2p repulsion in the Ni–O bonds, thereby facilitating the O₂ desorption process as compared to pristine Ni₃Se₄, as a result of the electronic exchange. Moreover, the downward shifting of the d-band centre with respect to the Fermi level modulated the binding strengths of the Fe-doped substrate towards H* compared to the pristine substrates.