A Novel Sol–Gel Synthesis Strategy of Co-Based Sillenite Composites for Enhanced Electrocatalysis of Water Splitting

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 (Bi_24-<i>x</i> Co _<i>x</i> )-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.