Investigation of Solid-State Metathesis as a Rapid and Convenient Synthetic Method to the Formation of Cobalt-Iron Boride Solid-Solutions and the Effect of Chemical Composition on Electrocatalytic Activity

Hydrogen has been gaining popularity as a reliable and environmentally friendly energy source for future energy demands. Water is an abundant source of hydrogen and electrochemical water splitting into hydrogen and oxygen gasses is an efficient, effective, and non-greenhouse gas emitting process. The search for cost-effective electrocatalysts for the cathodes and anodes in electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) that are active at low applied potentials is a subject of great interest. Late transition metal (cobalt, nickel, and iron) compounds have been extensively studied as electrocatalysts due to their abundance and low cost. Because of their intriguing chemical and physical properties, late transition metal borides are appealing as ideal electrocatalyst with active, long-lasting, and persistent electrocatalytic properties. Borides of cobalt and iron have been investigated and identified as moderate electrocatalysts, both individually and in combination. Some studies have found that a synergistic effect improves electrocatalytic activity of multi-metal combinations as compared to individual metal boride performance. Understanding how synergy works with crystalline cobalt and iron borides and what extent it applies to bulk cobalt iron borides are important research areas. The current study describes cobalt and iron solid-solution formation in crystalline form from rapid and energetic solid-state metathesis reactions using combinations of MCl₂ (CoCl₂ and FeCl₂) + Mg + B. X-ray diffractograms (XRD) show that solid solutions formed in all cobalt and iron compositions, and chemical compositions of solid-solution samples were confirmed using inductively coupled plasma spectroscopy (ICP) and energy dispersive spectroscopy (EDS). Properties of cobalt-iron solid solutions were characterized using scanning electron microscopy, surface area analyzer, and crystallite sizes (XRD peak width), and the results show that structural properties gradually change from one end of the compositional range to the other. These crystalline borides show no evidence of synergy between cobalt and iron in HER electrocatalysis performed in 1.0 M KOH, instead the relative electrocatalytic activity was highest for cobalt boride (10 mA cm^-2 at -223 mV), lowest for iron boride (10 mA cm^-2 at -342 mV), and intermediate for the different solid-solutions compositions. After 24 hours of chronoamperometry, all crystalline cobalt and iron borides showed bulk stability and maintained HER activity. Post-electrochemistry studies confirmed the retention of metal borides after extensive electrochemistry studies.