Rapid and Energetic Solid-State Metathesis Reactions for Iron, Cobalt, and Nickel Boride Formation and Their Investigation as Bifunctional Water Splitting Electrocatalysts

Abstract

Metal borides have long-standing uses due to their desirable chemical and physical properties such as high melting points, hardness, electrical conductivity, and chemical stability. Typical metal boride preparations utilize high-energy and/or slow thermal heating processes. This report details a facile, solvent-free single-step synthesis of several crystalline metal monoborides containing earth-abundant transition metals. Rapid and exothermic self-propagating solid-state metathesis (SSM) reactions between metal halides and MgB 2 form crystalline FeB, CoB, and NiB in seconds without sustained external heating and with high isolated product yields (∼80%). The metal borides are formed using a well-studied MgB 2 precursor and compared to reactions using separate Mg and B reactants, which also produce self-propagating reactions and form crystalline metal borides. These SSM reactions are sufficiently exothermic to theoretically raise reaction temperatures to the boiling point of the MgCl 2 byproduct (1412 °C). The chemically robust monoborides were examined for their ability to perform electrocatalytic water oxidation and reduction. Crystalline CoB and NiB embedded on carbon wax electrodes exhibit moderate and stable bifunctional electrocatalytic water splitting activity, while FeB only shows appreciable hydrogen evolution activity. Analysis of catalyst particles after extended electrocatalytic experiments shows that the bulk crystalline metal borides remain intact during electrochemical water-splitting reactions though surface oxygen species may impact electrocatalytic activity.

Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

“…The relative electrocatalytic HER performance of crystalline nickel-rich to phosphorus-rich phases synthesized in this work was evaluated in a three-electrode system in H 2 -saturated 0.5 M H 2 SO 4 with powders adhered to sticky conducting carbon-wax working electrodes (50% graphite: 50% paraffin wax by mass) analogous to our recent work (Figure S1). ,, Representative negative applied potential linear sweep voltammetry (LSV) curves for each Ni–P sample are shown in Figure . LSV data were obtained without iR compensation, and higher relative HER activity was indicated by increased current flow at lower negative applied potentials.…”

“…The nickel phosphides and Pt/C powder reference catalyst were evaluated using a graphite powder/paraffin wax composite electrode similar to that described in our recent work. ,, Solid conducting graphite/carbon wax electrodes have been used in electrochemical analysis and are stable and inert with excellent reproducibility and negligible background current. − Relative to our prior work, slight modifications were made to the working electrode’s graphite powder/paraffin wax composition, tip connections, and process of catalyst particle deposition (Figure S1). A 50% graphite/50% paraffin wax composition (by mass) for working electrode tips was identified as having better conductivity and retaining good adhesion for catalyst particles.…”

“…The 50% C wax electrode with the catalyst sample embedded on its surface leads to ∼40–50 Ω cell resistance values. Details of the experimental setup are similar to those described in our recent work. ,, …”

“…Details of the experimental setup are similar to those described in our recent work. 38,59,60 Hydrogen evolution reaction (HER) polarization curves were obtained using linear sweep voltammetry (LSV) at 5 mV s −1 in acidic 0.5 M H 2 SO 4 (pH = 0.3) aqueous solutions. The electrolyte solutions were continually purged with H 2 (ultrahigh purity 99.999%, Praxair) to maintain a reversible (H + /H 2 ) Nernstian potential in the solution for HER reactions.…”

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

“…9 This study was restricted to the synthesis of Group 4-6 metal diborides, in which metal chloride precursors were reacted with MgB 2 at 1123 K. This method was recently extended by Gillan et al to late transition metal borides. 10 The syntheses of hafnium diboride HfB 2 and tungsten borides WB x were performed by the reaction of a resistive heated metal wire (Hf or W) with boron bromide instead of using a metal halide precursor. 11,12 This method required a sophisticated apparatus to deal with the liquid boron bromide source and the Hf or W metal wire was only partially converted into boride, thus preserving a metallic core.…”