Sol–gel synthesis of magnesium-doped lithium cobalt, nickel, and zinc olivine phosphates, and their electrochemical application

Abstract

Electrochemical water splitting is one of the most promising methods for producing green oxygen and hydrogen fuels to meet concurrent increasing energy demands. Herein, versatile single phase olivines magnesium doped lithium metal phosphates (LiMPO₄) where M = Co, Zn, Ni were synthesized using the non-aqueous sol–gel method. For structural and morphological analysis of LiCoPO₄ (LCP), LiNiPO₄ (LNP), LiZnPO₄ (LZP), and Mg-doped compositions, various characterization techniques like X-ray diffraction (XRD), Fourier transform infrared (FTIR), Scanning electron microscopy (SEM), and energy dispersive X-ray diffraction (EDX) techniques were used. Their electrochemical behavior was examined in various analyte solutions with the addition of facilitators such as glucose and methanol in I M KOH. When the active surface area of the said electrocatalyst was measured using cyclic voltammetry, LNMP was determined to be the best electrocatalyst to have a maximum surface area of 0.0345 cm², the highest current density (J) value of 39.1 mA/cm² with lowest overpotential of 0.217 V, and onset potential of 1.17 V. The water oxidation peak current density values suggest the strong catalytic activity and aided the water evolution reaction (OER). The electrocatalytic activity observed in methanol revealed that LNMP provides the highest heterogeneous rate constant (k^o) value of 8.75 × 10⁻⁵ cm/s, largest diffusion coefficient (D^o) value of 2.77 × 10⁻⁸ cm²/s, and with a low Tafel slope value of 42.3 mV/dec. The findings of the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry measurements agreed that proposed nanomaterials enhanced electron transport, and charge transfer kinetics and are electrochemically more favorable. These key discoveries from the water oxidation reactions (WOR) are the basis for their use in high-energy devices. In summary, LNMPs have been demonstrated to be innovative and effective catalytic platforms for electrochemical water oxidation.