From phase-engineered MoAlB to MOB Mbene-MOF nanocomposites: a viable route to high-performance electrocatalysts for hydrogen evolution

The growing global energy crisis and the environmental consequences of fossil fuel consumption have accelerated the search for clean and sustainable energy sources. Among these, hydrogen has emerged as a promising energy carrier due to its high gravimetric energy density and zero-carbon emissions upon use. Electrocatalytic water splitting offers a green and efficient route to hydrogen generation, but its widespread application is hindered by the need for high-performance, earth-abundant electrocatalysts capable of facilitating both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Two-dimensional (2D) transition metal borides, known as MBenes, have recently gained attention as next-generation electrocatalysts owing to their high electrical conductivity, chemical stability, and large surface area. In this thesis, a two-step strategy was developed to design MBene-based nanostructures for efficient HER catalysis. In the first part of this thesis, the synthesis and chemical characterization of Mo2AlB2 were investigated using MoAlB, a member of the ternary transition metal boride (MAB phase) family. The primary objective of this study was to develop a novel approach for obtaining Mo2AlB2, which is considered the most suitable precursor phase for MoB MBene due to its favorable bonding characteristics. However, Mo2AlB2 is metastable and cannot be synthesized directly through conventional solid-state methods. Existing techniques reported in the literature typically require high temperatures, prolonged reaction times, and multiple processing steps. To address these limitations, a more efficient, low-temperature synthesis method was developed. The reaction was carried out at 450 °C for 2 hours using HCl gas, which was generated in situ through the thermal decomposition of NH4Cl. This process enabled the production of high-purity Mo2AlB2 in a single step. Structural and compositional analyses using XRD, SEM, TEM, EDX, ICP-MS and XPS confirmed the successful synthesis of Mo2AlB2 as a viable precursor for MoB MBene. In the second part of the thesis, MoB MBene was synthesized from the MoAlB phase using a Lewis acid molten salt etching method. The as-obtained MBene was then combined with Ni-BDC, a nickel-based metal-organic framework (MOF), and deposited directly onto nickel foam via a solvothermal process at varying loading ratios. The aim was to fabricate a nanocomposite with a well-integrated interface that could lower the overpotential for HER by harnessing the high surface area of Ni-BDC and the excellent electrical conductivity of MoB MBene. At a current density of 10 mA cm-2, the nanocomposite containing 7.5 wt.% MoB MBene exhibited an overpotential of 214 mV. Upon annealing at 400 °C, the formation of nickel nanoparticles further increased the surface area, resulting in a significantly reduced overpotential of 120 mV. Moreover, chronopotentiometric analysis demonstrated remarkable long-term stability, with the optimized catalyst maintaining consistent performance over 50 hours at 50 mA cm-2. This work introduces an efficient synthesis route for MBene-type materials and demonstrates their successful integration into hierarchical nanocomposites for electrocatalytic hydrogen production. The findings not only expand the synthetic toolbox for 2D boride materials but also underscore the promise of MBene-MOF architectures in green energy applications.