Unlocking the potential of NiCo2O4 nanocomposites: morphology modification based on urea concentration and hydrothermal and calcination temperature

Abstract

Oxygen evolution reaction (OER) electrocatalysts are critical in minimizing energy loss during the anodic four-electron transfer process that is required for water oxidation. Improving and selecting optimal non-noble OER electrocatalysts are key strategies for elevating their overall performance and efficiency of energy storage and conversion. Eight NiCo₂O₄ electrocatalysts were synthesized using the hydrothermal method by changing the amount of urea as a nucleation agent and hydrothermal and calcination temperature to achieve an outstanding catalyst in terms of morphology and electrochemical activity. For examining the physicochemical properties of the electrocatalyst, analyses such as XRD, SEM, TEM, and EDS were conducted. Although XRD analysis revealed the formation of pure NiCo₂O₄ for all eight samples, SEM and TEM analysis unraveled the best electrocatalyst in terms of morphology to be NiCo-S3 (urea: 10 times higher, T_hydrothermal: 120 °C, and T_Calcination: 350 °C) and NiCo-S4 (urea: 10 times higher, T_hydrothermal: 120 °C, and T_Calcination: 400 °C) with a mum-flower-like shape and particle dimension between 20 and 45 nm. NiCo-S4 displayed robust electrochemical activity, primarily in the OER, with an overpotential of 327 mV at 10 mA cm⁻² in 1.0 M aqueous KOH solution. The OER performance was enhanced as demonstrated by the exceptional durability of >24 hours and a Tafel slope of 79.7 mV dec⁻¹. Electrochemical impedance spectroscopy (EIS) revealed a low resistance of 1.03 Ω and a double-layer capacitance of 2.43 mF cm⁻², substantiating the outstanding OER performance of NiCo-S4.