Thermochemically-driven exchange reactions for the rapid synthesis of crystalline metal borides and carbon-supported composites and their investigation as hydrogen evolution electrocatalysts

Transition-metal borides have a long history as refractory materials that often show chemical and thermal stability and high electrical conductivity. These materials have recently seen increased interest as robust electrocatalytic materials for water splitting, such as hydrogen evolution reactions (HER). Most metal boride routes are multi-step processes involving precipitation and calcination or involve exotic processing such as arc melting. This presention describes a synthetic method involving rapid reactions of two or more solids that are highly exothermic enabling thermochemically designed solid-state metathesis (SSM) reactions. This presentation will highlight reactions of anhydrous metal halides intimately mixed with Mg and B powders that produce exothermic short-lived reactions where metal salts are reduced and crystalline metal borides grow in a transient molten MgCl₂ flux (~1400 °C). This rapid synthetic method produces single metal and solid-solution borides in the 3d MB (M=Fe, Co, Ni) series and has been extended to carbon black supported metal borides. Recently successful strategies for one-pot SSM reactions directly yielding carbon-supported crystalline metal borides will be described. The metal borides and composite structures are examined and compared as HER electrocatalysts. Many of the SSM produced metal borides show good chemical robustness in acidic and basic electrocatalytic reduction experiments. The use of a high surface area carbon black additive in these metal boride SSM reactions enables production of diluted catalytic boride solids that have HER activities similar to pure metal borides. Trends in HER activities will be described with respect metal boride composite properties and boron content.