Preparation Of Novel Cobalt-based Selenide Electrodes And Their Catalytic Properties Toward Hydrogen Evolution Reaction

The increasing energy crisis,environmental issues caused by fossil fuels have led to the intense search for clean energy.Hydrogen is considered as ideal sustainable alternative to fossil fuels.Water splitting as a promising method for hydrogen production has become a research hotspot.Hydrogen evolution reaction?HER?is an uphill reaction with large overpotential,the practical electrolysis operating voltage is much higher than the theoretical value.The key issue for electrocatalysis and photoelectrocatalysis is how to realize efficient energy conversion from electric energy and solar power to chemical energy.Decorating electrodes with catalysts can promote the kinetics of hydrogen evolution reaction.The state-of-the-art hydrogen-producing catalysts are based on noble metals?such as Pt?,while the scarcity of Pt makes it impractical for global-scale applications.In this thesis,we designed Co-based selenide electrocatalysts by innovative preparation methods and demonstrated their high performance for HER in electrochemisty and photoelectrochemistry?PEC?systems.In this thesis,the preparation methods of Co-based selenides have been studied systematically.For the first time,we reported one step synthesis of pyrite-phase CoSe₂ with a low Tafel slope of?40 mV/decade by reacting drop-casted cobalt precursors coating with Se vapor.To further improve the catalytic performance of CoSe₂,we developed a novel and facile approach of synthesis of polymorphic CoSe₂?p-CoSe₂?with mixed orthorhombic?o-CoSe₂?and cubic phases?c-CoSe₂?by calcining amorphous CoSex fabricated by the electrodeposition method.The structure of polymorphic grain boundaries virtually lack periodicity and the additional degree of atom freedom would lead to more defects than those in the bulk.These grain-boundary defects will serve as active sites to further improve the catalytic performance.The p-CoSe₂ exhibited excellent catalytic activity with a low Tafel slope of?30 mV/decade?close to Pt?.The research works mentioned above open up new strategies to improve the sluggish interface catalysis of bulk crystals.In PEC system,especially for the Si-based photocathodes,few electrocatalysts have been used because of the difficulties to satisfy many harsh requirements,such as Si/catalyst chemical incompatibility,induced interfacial defect states.Here,we designed a novel p-Si/NiCoSex core/shell nanopillar?NP?photocathode by mildly photo-assisted electrodeposition of amorphous NiCoSex film on 3D p-Si NP arrays backbones.The excellent electrocatlytic activity?Tafel?39 mV/decade?of NiCoSex film attributed to the abundant active sites caused by random atoms in amorphous structure.Notably,the semitransparent film is expected to be used in PEC system.A stable photocurrent density of-37.5 mA/cm2 at 0 V?vs.RHE?under simulated 100 mW/cm2 was achieved,which is the highest value reported for p-type Si photocathodes.The enhanced PEC performance of p-Si/NiCoSex core/shell NP array photocathode was attributed to the perfect combination of the high light harvesting of 3D Si NP array and active NiCoSex electrocatalyst with good optical transparency.To obtain efficient electrode materials for water splitting,in this thesis,we systematically investigated the structure-activity relationship of Co-based selenides prepared in different conditions and explored hydrogen evolution kinetics.The designed "Semiconductor-Catalyst" heterostructure greatly improved the PEC-HER performance.The results of our work are of great meaning to prepare other analogous heterogeneous catalysis and will provide new materials and new route for the commercialization of Si-based photoelectrodes in solar-driven hydrogen production.