A novel sol-gel synthesis strategy of co-based sillenite composites for enhanced electrocatalysis of water splitting

Abstract

This work reports a new synthetic strategy and investigates the effect of thermal treatment on Bi and Co oxide composites (BCO), comprising Co-based sillenite (Bi24−xCox)Co2O40, Co3O4, and Bi2O3, as well as their application as electrocatalysts in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The materials were synthesized by a modified Pechini sol−gel method using citric acid as a complexing agent and ethylene glycol as a polymerization agent and were calcined in the range between 500 and 1000 °C. Rietveld refinement of the XRD data confirmed the predominance of the sillenite and Co3O4 crystalline phases, both with cubic structures, with minor quantities of α-Bi2O3 (monoclinic) at 500 and 600 °C. A higher sillenite content (>60 wt %) was observed at 700 and 800 °C, while at 900 °C the sillenite phase decreased, disappearing completely at 1000 °C, where α-Bi2O3 became dominant. The BCO sample calcined at 700 °C exhibited the lowest overpotentials for both the OER and HER in 1 mol L−1 KOH (pH 14), 475 and 307 mV at j ± 10 mA cm−2 , respectively. The corresponding Tafel slopes for the OER and HER of 62 and 23 mV dec−1 , respectively, also exhibited the lowest values. The superior catalytic performance of BCO-T700 can be related to the synergy between Co2+ and Bi3+ species shown by X-ray photoelectron spectroscopy. These findings reinforce the potential of sillenite-based composites as efficient, low-cost, bifunctional electrocatalysts for water splitting and hydrogen production. Notably, no prior electrochemical studies of cobalt sillenites synthesized via the adapted Pechini method were found, emphasizing the relevance of this work.