Preparation And Hydrogen Evolution Performance Of Cobalt-based Layered Double Hydroxide/rGO Nanocomposites

Exploitation of energy technologies that are both eco-friendly and sustainable has drawn significant attention in recent years under the stress of energy over-consumption and severe environmental issues.Electro-catalytic water-splitting for hydrogen is considered as an alternative solution for low-carbon energy production.However,the high over-potential of hydrogen evolution reaction(HER)strongly limits the development of this technology due to the presence of energy barriers.To overcome this problem,massive efforts have been devoted to increase the electro-catalytic reaction rate and energy efficiency by increasing the number of active sites and improving the intrinsic activity of materials.Cobalt-based nanocomposites have shown great potential in electro-catalysis owing to their variable valence and environmental friendliness.In this thesis,by combining with conductive substrate to improve the conductivity,tuning the morphology to increase the number of edge active sites,and doping to change the electronic structure,we prepared two types of materials by using cobalt-based layered double hydroxide(LDH)as the precursor,and significantly improved the reaction rate and energy efficiency of HER.The main contents are as follows:1.The Co-based LDHs,supported by reduced graphene oxide(rGO),were used as precursors to prepare non-noble metal electro-catalysts through a one-step hydrothermal selenization method.As active site of the composite material,Co conforms to the volcanic type rule and synergizes with Se,which is biased towards metal state,to improve the conductivity of the composites and reduce the energy of the catalytic reaction.Whilst the rGO substrate improves the electrical conductivity of the composites,and prevents the agglomeration of LDHs during the growth process.The final product has been proved as Co0.85Se/rGO using techniques such as IR,Raman,and XRD.The Co-based LDHs are uniformly dispersed on rGO as shown in SEM and HRTEM images.The electronic states of Co atoms have been investigated by XPS.The electrochemical performances show that the Co0.85Se/rGO composite has an over-potential of 117 mV at current of 10 mA/cm2 in 1 M KOH electrolyte,and a Tafel slope of 80 mV/dec.In 0.5 M H2SO4 electrolyte,the Co0.85Se/rGO has over-potential of 107 mV at current of 10 mA/cm2 and a Tafel slope of 68 mV/dec.The synthesized selenide composites inherit the layered structure of LDHs,thus increasing the active specific surface to expose more active sites and accelerate the rate of electron conduction,which all contribute to the enhancement of electro-catalytic performances.2.The CoCoRu-LDH nanosheet/rGO composite was synthesized by a one-step refluxing method.As a platinum group metal,Ru has similar chemical structure and properties with platinum.When serving as a hydrogen evolution catalyst,it can accelerate the adsorption of hydrogen,thus speeding up the hydrolysis process of HER under alkaline condition and reducing the reaction energy barrier.The composite material shows a nanosheet structure and high specific surface area,which are beneficial to exposure of edge active sites.CoCo-LDH/rGO doped with different Ru content was explored by SEM,XRD,IR,Raman,HRTEM,XRF and XPS in order to obtain the morphology and structure.When the ratio of Co and Ru is 1:0.094,CoCoRu-LDH/rGO exhibites excellent hydrogen evolution performance in 1 M KOH electrolyte with.a lower over-potential of 60 mV at current of 10.mA/cm2 and a Tafel slope of 74 mV/dec.