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Synthesis, Characterization and Electrocatalytic Performance of Bulk Transition Metal Borides

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Hydrogen is considered a promising alternative to carbon-based fuels as it is a clean, renewable and abundant energy source. In recent years, hydrogen evolution reaction (HER) via electrochemical water splitting has received a great deal of attention as a clean and efficient technique to produce hydrogen. A few precious metals such as platinum (Pt) have exhibited state-of-the-art electrocatalytic activity for HER with much low overpotential, but their high cost and lack of abundance limit their application industrially. Thus, HER electrocatalysts that are both highly active and economical are in high demand in order to reduce the overpotential while maintaining low cost.

Recently, transition metal borides (TMBs) have been considered as potential alternatives to noble metals as HER electrocatalysts due to their abundance, low cost and excellent activity and stability for HER. Chapter 2 discusses synthesis of four binary bulk molybdenum borides (Mo2B, α-MoB, β-MoB and α-MoB2) by arc-melting. HER activity measured for all four phases in acidic condition increased with the amount of boron. In Chapter 3, tin flux synthesis of β-MoB2, which has the same boron content as α-MoB2 but different structure, is discussed. Two different boron layers, flat (graphene-like) and puckered (phosphorene-like) boron layers, were found in β-MoB2. the two boron layers showed significantly different HER activity, which was supported by the Gibbs free energy calculations of H-adsorption. In Chapter 4, tungsten-based boride HER electrocatalyst was studied experimentally and theoretically for the first time. Tungsten could be successfully substituted (up to 30 at.%) for molybdenum in α-MoB2. The resulting α-Mo1-xWxB2 presented better HER activity than binary phases of both WB2 and MoB2. DFT calculation revealed that graphene-like boron layer was the most active and that tungsten promoted hydrogen generation by facilitating bonding between hydrogen atoms. In addition, importance of working electrode preparation method was investigated. General drop-casting method worked well for nanosized particles, but it was not suitable for bulk samples. New method (i.e. grinding and polishing arc-melting sample to disc shape) has been devised, which led to better HER performance.

In Chapter 5, chromium-based boride HER electrocatalyst was investigated for the first time. Chromium-molybdenum diborides (Cr1-xMoxB2 (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75), AlB2-type) were successfully synthesized using solid solution synthesis. Cr1-xMoxB2 catalysts exhibited much higher HER activity than binary phases of CrB2 and MoB2. Among Cr-Mo-B electrocatalysts, Cr0.4Mo0.6B2 presented the highest HER activity and even outperformed Pt/C HER activity at high current density (about 500 mA/cm2)

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