Design Of Mo,Co Based Non-noble Metal Catalysts For Water Splitting

Hydrogen economy as a promising clean energy has been extensively discussed.Electrocatalytic water splitting is one of the environmentally friendly and renewable hydrogen production method.The key to water splitting is the design of highly active catalysts to promote the hydrogen production process.In addition,in view of the lack of freshwater resources,the development of overall seawater splitting technology is expected to achieve the sustainable energy development.It is of great value to study the Mo,Co based non-noble metal electrocatalysts due to their low price and special properties of the d-orbital electrons.However,their catalytic performance still needs to be improved and the catalytic mechanism is also unclear.In this thesis,design of non-noble metal electrocatalysts and their water splitting performance were studied.Catalysts with high activity were prepared via various methods.The catalytic process and the active sites were also studied in depth.The application of solar cell-driven overall water splitting was also realized by these bifunctional electrocatalysts.The detailed work and the corresponding conclusions are as follows:(1)A simple one-step molten salt method was developed,which can directly grow preferentially oriented Mo₂C through template-sacrificed strategy by using carbon fiber paper.The lamellar morphology of(1 0 0)plane oriented Mo₂C gave a high specific surface area,which can expose more active sites.Besides,density functional theory calculations showed that the Gibbs free energies of the(1 0 0)plane wre close-to-zero,revealing the high HER catalytic activity with an overpotential of 95.8 m V to reach-10m A cm^-2.In addition,the oxide species on the surface of Mo₂C randomly resolved in acid electrolyte,which can expose more catalytic active sites but decrease the TOF values.While in alkaline condition,molybdenum oxides on the surface were mainly reduced to Mo species with low oxidation states(such as Mo³⁺)by applying cathodic potential.These increased Mo³⁺species acted in the form of Mo³⁺OOH,which facilitated the water dissociation process and the generation of hydrogen intermediates in alkaline solution,thus more easily to initiate the HER process than in acid electrolyte.(2)In order to obtain highly active bifunctional(HER&OER)overall water splitting catalysts,a Ni O(Ni⁰)/Ni Mo O₄ self-supported electrocatalyst was obtained through electrodeposition in ammonia plating system with an overpotential of 36.8 m V to reach-10 m A cm^-2 for HER.The catalytic active sites were demonstrated to be the highly active Mo⁵⁺.In addition,Ni²⁺benefited for the water dissociation process.Their combination contributed to the high HER performance.But the OER performance still needed further improvement.(3)In order to improve the OER performance,another Ni O/Ni Mo O4 self-supported electrocatalyst was obtained through electrodeposition in boric acid plating system with an overpotential of 318 m V to reach 10 m A cm^-2.The catalytic active sites were demonstrated to be the Ni²⁺and the synergistic effect of Mo species.Then OER performance in saline alkaline electrolyte showed a similar catalytic property and high stability,indicating its good chlorine evolution reaction inhibition and high corrosion resistance.The reason can be contributed to the dissolved Mo species in the form of Mo O₄^2-,which can repel the aggressive Cl^-ions and prevent their corrosion.The catalysts obtained from these two electrodeposition systems were used as cathode and anode for overall water-splitting reaction,which showed high catalytic activity and stability.Besides,solar cell-driven overall water splitting can also be realized.(4)In order to reveal the bifunctional(OER/ORR)mechanism of Co-based catalysts,Co-NCNT catalysts were prepared at different sintering temperatures.By comparison of the properties and the chemical composition,it was found that Co ^IIspecies were presumably responsible for the oxygen evolution,while Co ^IIIspecies were predominately associated with the ORR process.In addition,the correlation between the occupancy of e_g orbit and the intrinsic catalytic activity was also established.