Topotactic BI3-assisted borodization: synthesis and electrocatalysis applications of transition metal borides

Abstract

A facile and universal route for synthesizing transition metal borides has been developed by reaction of boron triiodide (BI₃) with elemental transition metals. This method employs relatively low synthesis temperatures to afford single-phase samples of various binary and ternary metal borides, such as Fe₂B, Co₂B, Ni₃B, TiB₂, VB₂, CrB₂, and Ni₂CoB. This synthesis protocol can be utilized for the topotactic transformation of metal shapes into their respective borides, as exemplified by transformation of Ni foam to Ni₃B foam. In situ powder X-ray diffraction studies of the Ni–BI₃ system showed that the crystalline nickel borides, Ni₄B₃ and Ni₂B, start to form at temperatures as low as 700 K and 877 K, respectively, which is significantly lower than the typical synthesis temperatures required to produce these borides. Ni₃B synthesized by this method was tested as a supporting material for oxygen evolution reaction (OER) in acidic media. Composite electrocatalysts of IrO₂/Ni₃B with only 50% of IrO₂ exhibit current densities and stability similar to pure IrO₂ at mass loadings lower than 0.5 mg cm⁻², indicating Ni₃B could be a promising supporting material for acidic OER.