Oxygenated iron boride (FeBOx) as a bifunctional electrocatalyst for overall water splitting in prototype electrolyzer

Abstract

We report a scalable synthesis of novel oxygenated iron boride (FeBO_x) thin films using the successive ionic layer adsorption and reaction (SILAR) method. The influence of deposition cycle numbers on the structural, morphological, and electrocatalytic properties is systematically examined for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The FeBO_x catalyst deposited over 100 SILAR cycles exhibits excellent bifunctional activity, requiring overpotentials of only 96 mV for HER and 300 mV for OER at 10 mA cm⁻². Tafel analysis reveals a transition in HER kinetics from a Volmer to a Heyrovsky limited mechanism with increasing deposition cycles, while OER primarily followed the adsorbate evolution mechanism (AEM). Enhanced redox reversibility, larger electrochemically active surface area (ECSA), and improved catalytic site accessibility have been confirmed through cyclic voltammetry. The optimized FeBO_x catalyst demonstrates remarkable stability over 100 h at 10 mA cm⁻², maintaining a stable cell voltage of ∼1.87 V. In a prototype alkaline electrolyzer, the FeBO_x (100 cycles) catalyst sustains 100 h of continuous operation, generating ∼245 mL h⁻¹ of O₂ with a 2:1 H₂:O₂ ratio. These results establish FeBO_x as a durable, cost-effective, and (platinum) Pt-free catalyst for efficient and cost-effective alkaline water splitting.