Water electrolyzers that produce green hydrogen have shown great potential in reducing carbon emissions and promoting a cleaner energy future. However, their widespread adoption has been limited due to the high cost of production, which makes them less competitive compared to hydrogen obtained through fossil fuels. For example, the cost of Catalyst-Coated Membranes (CCMs) is a significant factor contributing to the overall cost of a Proton Exchange Membrane (PEM) electrolyzer stacks. To address this bottleneck, the use of earth-abundant electrocatalysts based on transitional metal carbides, oxides, nitrides and sulphides is being explored to reduce the cost of CCMs and improve the affordability and accessibility of PEM electrolyzers. Recent research has shown that transition metal nitrides have demonstrated promise as earth-abundant electrocatalysts for Hydrogen Evolution Reaction (HER). By utilizing transition metal nitrides instead of platinum, the cost of manufacturing CCMs can be reduced significantly, which would have a positive impact on the overall cost of PEM electrolyzers. In this context, theoretical studies have shown that among various transitions metal nitrides, Ta-N phases have ideal adsorption kinetics for hydrogen evolution reaction (HER) in PEM electrolyzers. However, most of the literature on Ta-N as an electrocatalyst is focused on its use as photocatalysts and photo electrocatalysts, and its application in PEM electrolyzers has not been extensively explored. Strategies to enhance HER Kinetics in Ta-N phases through nanostructuring are also rather limited. In this thesis, an attempt to improve the HER kinetics further in Ta-N compositional space through strategies such as nano structuring into fibers and self-supported architectures is described. To begin with, nitrogen rich phase in Ta-N phase diagram, i.e -Ta3N5 was engineered into a fibrous network through electrospinning. The ideal processing conditions based on the ammonolysis temperature and its effect on hydrogen evolution kinetics was brought out. With a fiber diameter less than 200 nm, Ta3N5 fibers exhibited 5-fold improvement in onset potential and higher current density compared to the existing literature on Ta3N5 phases. Further, even under the highly acidic conditions of a PEM water electrolyser, Ta3N5 fibers demonstrated a stability of 6 h. To further improve the processing times and elucidate the role of TaON layer that is formed always on Ta3N5 surface, new synthesis strategy to obtain TaON fibers faster and safer was executed. An-inhouse developed centrifugal spinning setup was developed which could generate fibers 1000X faster than the conventional electrospinning technique. Optimization studies to obtain bead free fibers focusing on rotation speed, collector distance, viscosity of the initial solution is also presented.
The effect of pH on the overall HER Kinetics was also understood in this study. Interestingly, TaON demonstrated significant HER current densities only in acidic medium. DFT studies were used to understand this disparity which was attributed to a weakly passivating layer on TaON in 1 M KOH (basic medium) leading to surface oxidation. In addition to this, the thesis also describes novel synthesis strategies to obtain self-supported electrocatalysts based on ammonia free processing and single source precursor synthesis. Self-supported electrocatalysts offer superior adhesion, are free from expensive binders with no compromise on HER kinetics. Vacuum assisted annealing approach resulted in the formation of TaCxNy phases on Ta foil. This work is the first report wherein carbonitrides of tantalum are reported for HER without the use of any corrosive NH3. Using oxygen free, nitrogen containing precursors like melamine, and thiourea, TaC-TaN were grown on Tantalum foil. Such an approach demonstrated superior stability for about 6 h with more than 90 % retention in current densities of the order of 15 mA/cm2. The work culminates with an effort to use commercially existing single source precursors with supple Ta-N bonds to directly grow the electrocatalyst on nickel foam without the use of ammonia. Interestingly, the lowest onset potential of about 50 mV was obtained when an amorphous Ta-N (O) was grown on nickel foam using chemical vapor deposition (CVD). These results demonstrate significant improvements in HER kinetics and stability, suggesting that the use of earth-abundant electrocatalysts may become more feasible in the future.