Design and electrochemical study of Pd-based nanostructures for hydrogen economy

Abstract

Hydrogen storage remains one of the most challenging prerequisites to overcome toward the realization of a hydrogen based economy. The use of hydrogen as an energy carrier for fuel cell applications has been limited by the lack of safe and effective hydrogen storage materials. Another major challenge associated with fuel cells is the design of highly efficient electrocatalysts to reduce the high overpotential of the oxygen reduction reaction (ORR). Palladium has a high affinity for hydrogen sorption and has been extensively studied, both in the gas phase and under electrochemical conditions. Palladium has strong potential to play a major role in many aspects of a hydrogen based economy, leading to promising applications, encompassing hydrogen purification, storage and as a cathode to facilitate the oxygen reduction reaction. During my PhD study, Cd@Pd core/shell nanostructured materials were synthesised for enhanced hydrogen sorption and storage. The effect of a capping agent was investigated, showing that it plays a critical role in the formation of the uniform and small size of the Cd@Pd nanostructures. The capacity for hydrogen sorption and storage was strongly contingent on the composition and structure of the formed Pd-based nanomaterials, as well as the applied electrode potential. The Cd@Pd core/shell nanostructure with an optimized composition of 1:2 exhibited the highest capacity for hydrogen storage, where a 340% increase was achieved in contrast to pure Pd nanoparticles. The behaviours of the electrosorption of hydrogen into untreated and annealed Pd thin foils were systematically investigated with a primary effort concentrated on elucidating the effects of annealing on hydrogen uptake capacities. The change of the crystalline structure of Pd during the annealing process was monitored by in situ X-ray diffraction. Scanning electron microscopic images revealed the significant effect of the annealing temperature on the morphology of Pd thin foils. Cyclic voltammetric and chronoamperometric techniques were employed to study the kinetics of hydrogen electrosorption, where α phase, β phase, and their transition were determined with respect to the electrode potential. The morphological and structural changes, as well as lattice parameters, played an important role in hydrogen uptake. A Pd thin foil annealed at 700 °C exhibited an over 19.4-fold increase in hydrogen uptake capacity in comparison to an untreated one. In the next step, Pd nanoparticles and reduced graphene oxide nanocomposites were synthesized using a facile one-step electrochemical approach for promising applications in hydrogen sorption. The prepared nanocomposites exhibited significantly improved performance toward hydrogen sorption and storage in contrast to rGO and pure Pd nanoparticles. The Pd/rGO nanocomposite exhibited an increased hydrogen storage capacity of over 11 fold in the α phase and more than a fivefold enhancement in the β phase when compared to Pd nanoparticles. Finally, to reduce the high overpotential of the oxygen reduction reaction in fuel cells, a facile photoassisted method was employed for the direct deposition of palladium nanoparticles onto graphitic carbon nitride for the efficient reduction of oxygen. The synthesized Pd-g-C3N4 nanocomposite exhibited robust electrocatalytic behavior toward the ORR in 0.1 M KOH solution, where the desirable four electron pathway was achieved. In addition, the developed catalyst demonstrated a significantly improved tolerance against methanol as well as enhanced stability in comparison to the commercial benchmark platinum-loaded carbon catalysts.