Ni-Based Composite Electrode Material Electrodeposited Under Supergravity Field For Hydrogen Evolution Reaction

The aggravated energy crisis and increased environmental concerns have given rise to a vigorous exploration of clean and renewable alternative energy. Hydrogen, as a clean and sustainable chemical fuel, is recognized as the most promising substitute for fossil fuel in the future energy infrastructure. Among various technologies for hydrogen evolution reaction(HER), water electrolysis is a renewable and secure way to create hydrogen of high purity with zero pollutants emission. However, this technology is restrained because of the high energy consumption caused by the large overvoltage for HER. Thus, it is of great importance to develop active and economical electrode materials to reduce HER overpotential. Up to now, Ni and Ni-based electrode materials have drawn much attention due to their high activity and good stability for HER in alkaline solutions.In this thesis, Ni-CeO2, Ni-carbon nanotubes(Ni-CNTs) and Ni-reduced graphene oxide(Ni-rGO) composite cathodes were prepared by electrodeposition under supergravity fields and characterized by XRD, SEM, EDS and electrochemical analysis methods. The effects of the supergravity field and the composite phase concentration on the morphology,microstructure and electrochemical activity of the composite cathodes were systematically studied.Ni-CeO2 composite cathodes were electrodeposited under supergravity fields. With the help of supergravity field, the mass transfer process was markedly improved, which contributed to the formation of finer grains and the increase of nano-sized CeO2 embedded in coatings. The electrochemical measurements show that the catalytic activities of composite coatings are improved prominently compared with that prepared under normal gravity condition. The coating prepared at rotating speed 2350 rpm from the optimal CeO2 concentration 7 g dm-3 exhibits the highest exchange current density and surface roughness Rf towards HER, the value of exchange current density is about 72 times higher than the value of cathode prepared under normal gravity field. The enhancement of electrocatalytic activity should be radically attributed to the increase of CeO2 particles in films, the reduction of grain size and the enhancement of surface area.CNTs were used as composite phase to synthesis Ni-CNTs composite cathodes under supergravity fields. The pristine CNTs were cut off and oxygen-functionalized by a threestep treatment to effectively improve their dispersion and suspension stabilities. The composite cathode exhibits a unique wool-ball-like structure with large surface area,which is benefited from the marked enhancement of mass transfer process under supergravity field. The electrochemical measurements show that the highest electrocatalytic activity is achieved on the sample prepared at rotating speed 3000 rpm from the optimal CNTs concentration 1 g dm-3, which exhibits the smallest Tafel slope and the largest value of exchange current density among the studied samples. The measured HER activity is about 17 times and 254 times higher than that on Ni-CNTs cathode prepared under normal gravity condition and pure Ni coating respectively. The enhancement of electrocatalytic activity should be attributed to the increased active surface area derived from the wool-ball-like structure feature of hybrid particles, and the interaction between the Ni matrix and the CNTs.Ni-rGO composite cathodes were also electrodeposited under supergravity fields. The composite cathode presents unique multilayer sandwich-like structure coupled with large numbers of nano-sized Ni particles. Such structure feature provides high active surface area for HER. The electrochemical measurements show that the sample prepared at rotating speed 3000 rpm from the optimal GO concentration 0.7 g dm-3 achieves the highest exchange current density for HER, which is about 117 times and 5 times higher than that on pure Ni electrode and Ni-r GO cathode prepared under normal gravity condition respectively. During the long-term HER test, the composite cathode exhibits excellent stability with almost unchanged morphology feature, which indicates that the incorporation of r GO into Ni matrix can not only enhance the electrocatalytic activity for HER but also improve the durability and corrosion resistance of the cathodes.