Preparation Of Nano-structured Cobalt-based Catalyst And Its Application In Water Electrolysis

Increased concern over energy depletions and increasingly serious environment pollution caused by great usage of traditional fossil energy have triggered intense research in the exploration of renewable clean energy sources and carriers to replace fossil fuels.Nowadays,hydrogen energy is one of the main clean energy sources,because of good combustion performance,high calorific value,non-toxic and no pollution of it.At present,electrochemical splitting of water for producing clean hydrogen with high purity have been deemed to one of the approaches to solve the problems above-mentioned.Oxygen evolution reaction?OER?and hydrogen evolution reaction?HER?are fundamental electrochemical reactions for preparation of hydrogen energy.Actually,OER suffer from thermodynamic and kinetic sluggishness as well as the high overpotential,which has been considered a major bottleneck in water splitting owing to its four-electron-transfer pathway,so it is essential to develop the catalyst to reduce the overpotential and accelerate the reaction rate of water splitting.Generally,IrO₂ and RuO₂ was considered as the most efficient electrocatalysts towards oxygen evolution reaction,while platinum was considered as the most efficient electrocatalysts towards hydrogen evolution reaction.Nevertheless,the scarcity and high cost of such catalysts significantly hamper their widespread applications in electrochemical splitting of water for hydrogen production.As such,it is highly desired to explore and develop non-precious metal catalysts that are cost-effective,highly-efficient,stable and environmentally friendly for hydrogen production and energy storage.Recently,cobalt have been designed and developed as variety of high-performance catalysts for HER and OER,because of its the high storage and high conductivity.In this thesis,the transition metal composites with different nanostructures have been successfully prepared and used as efficient catalyst for water splitting.1.Herein,nickel cobalt sulfide?NiCo₂S₄?nanoflakes grown on 3D nickel foam?NF?was prepared by a reverse microemulsion-assisted method appearing benign water-splitting activity and stability.The nanoflakes exposes more active sites,which could largely enhance catalytic activity.The binder-free,self-supported Ni Co₂S₄/NF electrode delivers a current density of 100 mA cm^–2 at an overpotential of 319 mV for the oxygen evolution reaction and a current density of 10 mA cm^–2 at 169 mV for the hydrogen evolution reaction in 1 m KOH.When using it as a cathode and an anode in a two-electrode water electrolysis system,it demonstrates a remarkable activity for overall water splitting to deliver 10 mA cm^–2 at a cell voltage of 1.61 V,also exhibits a long-term stability with negligible degradation over 70 h,thus offering an attractive cost-effective earth abundant electrocatalyst toward water splitting.2.MoS₂ nanoflowers grown on Co₉S₈ hollow spheres?Co₉S₈/MoS₂?was prepared by two step-hydrothemal process.Co₉S₈/MoS₂ exhibited the higher HER performance in comparison with Co₉S₈ and MoS₂.It delivered a current density of 10 mA cm^–2 at an overpotential of 157 mV in 1.0 M KOH,also exhibited a long-term stability with negligible degradation over 35 h,when the potential was 0.89 V.The superior HER performance was attributed to the interface effect between MoS₂ nanoflowers and the Co₉S₈ hollow spheres?Co-Mo-S phase?,which can not only accelerate charge transfer but also synergistically increase structural and electronic modulations between MoS₂ and Co₉S₈.Thus,it is an attractive cost-effective earth abundant electrocatalyst toward hydrogen evolution reaction.