UC Riverside

Green hydrogen production demands electrocatalysts that are efficient, durable, and composed of earth-abundant elements, particularly under device-relevant current densities (≥10²–10³ mA cm⁻²). This dissertation advances transition-metal borides (TMBs) as a tunable, platinum-free catalyst family by developing synthesis–structure–property relationships at the nanoparticle scale. First, I introduce an Sn/SnCl₂ redox–flux route that leverages Sn–B immiscibility to access phase-controlled, crystalline nanoparticles with clean surfaces and well-defined chemistries. Using this platform, I demonstrate vanadium-stabilized molybdenum monoboride (V₀.₃Mo₀.₇B) nanoparticles (30–60 nm) that outperform commercial 20 % Pt/C at industrially relevant current density: η₁₀₀₀ = 0.452 V for V₀.₃Mo₀.₇B vs 0.837 V for Pt/C in 0.5 M H₂SO₄, with 97 % activity retention after ~28 h at ~1000 mA cm⁻². Kinetic and transport analyses (Tafel, C_dl-derived ECSA, and EIS) reveal higher accessible active-site density and lower charge-transfer resistance for V₀.₃Mo₀.₇B relative to MoB composites. Density-functional theory connects these trends to hydrogen-binding energetics: on the dominant {112} facet, ΔG_H shifts toward thermoneutrality at high H coverage (≈100 %), rationalizing the measured superiority at high current density. Building on this, I establish “phase and morphology engineering” rules in the Mo–B system: by tuning Sn and B feed ratios, I obtain single-phase α-MoB2 (MoB2-SP) without sacrificial excess boron, as well as two composite regimes (MoB2/MoB and MoB2/B). Despite larger particles and lower geometric surface area, MoB2-SP exhibits strong intrinsic HER activity, underscoring the primacy of phase/electronic structure over surface area alone. Collectively, this work delivers scalable syntheses, mechanistic understanding, and design guidelines that elevate TMB nanoparticles to the current-density and durability thresholds required for practical water electrolysis.