Electrochemical regeneration of NaBH₄ from the hydrolysis reaction of NaBH₄ with the Ni– Co/Hydroxyapatite catalyst as a part of the hydrogen release cycle

Sodium borohydride (NaBH4) is a high-hydrogen-content chemical hydride, but its practical use requires an efficient regeneration route from the hydrolysis by‑product (NaBO2·2H2O). This work investigates a two-step NaBH₄–H₂ cycle comprising catalytic hydrogen release and electrochemical regeneration of NaBH₄. A Ni– Co/hydroxyapatite (HAp) catalyst was synthesized by an electrochemical one-step deposition route (160 mA cm⁻2, 90 min). The catalyst shows nano-sized primary particles (≈10–100 nm) agglomerated into 1–5 μm secondary clusters. The results of catalyst characterization with FTIR confirmed the presence of HAp with additional peaks indicating surface carbonaceous species. XRD of the catalyst identifies a composite of HAp with oxidized Ni/Co phases (NiCo₂O₄ and CoO), evidencing successful incorporation of Ni–Co active phases on the HAp support. Mechanistically, the Ni/Co oxide surface is expected to provide hydrogen-activation (H* formation) and borate adsorption sites, while HAp enhances dispersion and interfacial hydroxyl/phosphate functionality—together facilitating the hydrogenation of NaBO₂·2H₂O toward NaBH₄ when H₂ is supplied electrochemically. In this work, NaBH4 was first hydrolyzed in water using an electrochemically synthesized Ni–Co/hydroxyapatite catalyst to release H2 and form borate, and the spent solution was then regenerated electrochemically in a two‑chamber cell separated by a bipolar membrane. In‑situ hydrogen generated via cathodic water reduction was utilized to convert NaBO2·2H2O back to NaBH4. Regeneration was evaluated at current densities of 0.10–0.20 A cm−2 for up to 120 min. NaBH4 concentration increased with both electrolysis time and current density, reaching ~0.020 mol L−1 at 0.20 A cm−2 after 120 min (compared with ~0.017 mol L−1 at 0.10 A cm−2). These results demonstrate the feasibility of coupling hydrogen release and electrochemical regeneration in a closed.