Using Complex Hydrides for Hydrogen Storage and Direct Borohydride Fuel Cells for Electricity Production

Hydrogen storage and utilization are the technologies to achieve carbon-neutral energy systems with renewable energy sources. Among the various materials that have been investigated, complex hydrides are a material exhibiting high gravimetric hydrogen density and operate with hydrogen in a reasonable temperature ranges (Room temperature to 200 °C). To establish a hydrogen-based clean energy system, it is necessary to develop both an electricity production technology using hydrogen and a hydrogen storage technology. Therefore, the objectives of this thesis are to understand the mechanism of the catalyzed hydrogen desorption of complex hydrides and to develop platinum group metal (PGM)-free catalysts for direct borohydride fuel cells (DBFCs). The specific objectives are as follows : (1) decreasing hydrogen release temperature and preventing material expansion on alanate complex hydrides using nickel with porous carbon sheets; (2) replacing palladium catalyst with nickel catalyst on DBFC anode; (3) replacing platinum catalyst with atomically dispersed transition metal with nitrogen-doped carbon materials; and (4) H2O2 production via 2-electron oxygen reduction reaction (ORR) and identification of the active site of atomically dispersed and nanoparticle cobalt.Alanate materials, one of the complex hydrides, were investigated as a hydrogen storage material. The combination of nickel-containing porous carbon sheets (Ni-PCS) with various alanates decreased the hydrogen release temperature and prevented volume expansion upon decomposition. It turned out, that the Ni-PCS prevented volume expansion by allowing hydrogen to escape from the liquid layer. The catalytic activity of Ni-PCS is proportional to the electronegativity of the cation in M (Li, Na, or Mg) in M(AlH4).The nickel catalyst for the DBFC anode was synthesized by precipitation and growth from solution on nickel foam. The nickel catalyst can replace costly palladium catalysts due to its high selectivity and fuel utilization efficiency. Different types of ionomers regulate the local pH conditions. The selective catalytic activities of the nickel catalyst for the borohydride oxidation reaction (BOR) and the hydrogen oxidation reaction (HOR) are the most significant performance-determining factors. The PGM-based catalysts were completely replaced with transition metal catalysts in DBFC. The M-N-C materials are used in the DBFC cathode for hydrogen peroxide reduction reaction (PRR). The Fe-N-C has higher activity on PRR than Co-N-C. Co-N-C is however more stable than Fe-N-C under DBFC operating conditions.Electrochemical hydrogen peroxide production via 2-electron ORR is demonstrated with Co-N-C catalysts. The atomically dispersed CoN4 site is known as high selective H2O2 production. The small amount of cation can influence the selectivity of H2O2 production and can differentiate the active site on nanoparticle Co and atomically dispersed Co. The selectivity of H2O2 production is enhanced on nanoparticle Co meanwhile, is decreased on atomically dispersed Co by the cation shielding effect.This thesis promoted hydrogen storage and electricity production using complex hydrides and various catalysts. This knowledge has the potential to be implemented in small devices that generate decentralized electricity, heat energy, and chemical feedstock. This thesis contributes to a fundamental understanding of catalyst preparation for cost-effective hydrogen storage and utilization.