Surface Modification Effects on Electrochemical Synthesis of NaBH4 for Hydrogen Storage

Abstract

Sodium borohydride, owing to its high hydrogen content and chemical stability, is a promising solid hydrogen storage material. However, its commercialization is limited by the high cost of conventional thermochemical synthesis. This study investigated the feasibility of sodium borohydride synthesis via electrochemical reduction of metaborate, with particular emphasis on the role of electrode surface properties in suppressing the hydrogen evolution reaction and enhancing selectivity. Boron-doped diamond electrodes were employed as both working and counter electrodes, with surface terminations controlled electrochemically and characterized by contact angle, X-ray photoelectron spectroscopy, and linear sweep voltammetry. Under neutral conditions, the boron-doped diamond electrode used as the counter electrode produced a maximum sodium borohydride yield of 1.8%, demonstrating that hydrogen evolution reaction suppression enables selective metaborate reduction. O-terminated boron-doped diamond electrode surfaces promoted metaborate ion adsorption and suppressed the hydrogen evolution reaction, but their surface instability reduced reproducibility. This study provides design guidelines for next-generation electrodes, highlighting that electrode engineering is essential for cost-effective sodium borohydride electrosynthesis.