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

## TL;DR

A new method to make cobalt-based composites improves their ability to split water for hydrogen production.

## Contribution

A novel sol–gel synthesis and thermal treatment strategy for Co-based sillenite composites with enhanced electrocatalytic performance for water splitting.

## Key findings

- BCO samples calcined at 700 °C showed lowest overpotentials for OER and HER.
- Tafel slopes of 62 and 23 mV dec–1 for OER and HER, respectively, indicate efficient catalytic activity.
- Synergy between Co2+ and Bi3+ species contributes to superior electrocatalytic performance.

## 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–x
Co
x
)­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.