Synthesis And Electrocatalytic Performance Of Transition Metal Selenides And Their Composites

The over-consumption of fossil fuels makes it urgent to develop cheap,green and renewable energy.With the advantages of high-energy density,zero pollution and renewability,hydrogen（H₂）has been recognized as one of the potential alternatives for fossil fuels in the future.Electrocatalytic water splitting is one of the best methods for renewable hydrogen production due to its stable yield,high purity,non-toxicity and without generation of carbon dioxide.At present,for the two half reactions of water splitting,the hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,platinum（Pt）and ruthenium oxide（RuO₂）are the best electrocatalysts,respectively.Unfortunately,the scarcity of these precious metal-based electrocatalysts inevitably impedes their commercial applications.In recent years,transition metal selenides（TMSes）with low cost and abundant reserves have attracted great attention due to their advantages of high intrinsic conductivity and variable chemical valences.Therefore,we designed and prepared a series of TMSes nanomaterials and investigated their electrocatalytic properties.Firstly,the synthesis and HER performance of nickel selenides nanowires and their composites have been studied.Then,the preparation and overall water splitting properties of Co_0.85Se hollow nanospheres have been studied.Finally,the synthesis and and overall water splitting performance of Co_0.85Se/CNT and Co_0.85Se@NG hybrid have been further investigated.The main research contents and results are as follows.1.The NiSe NWs,NiSe₂ NWs and their composites（rGO/Ni NWs,rGO/NiSe NWs,rGO/NiSe₂ NWs）have been synthesized through a facile hydrothermal method and subsequent selenization process.The results show that compared with bare nickel-based nanowires,the HER performance of all composites has been improved significantly.In particular,rGO/NiSe₂ NWs exhibits a low onsetpotential of-153 mV and can achieve a current density of 10 mA cm^-2 at only-190 mV.It also displays a small Tafel slope of 35.6 mV dec^-1 and excellent stability.2.The Co_0.85Se hollow nanospheres and corresponding composite(Co_0.85Se/CNT)have been synthesized by a facile and one-step hydrothermal approach.The Tafel slopes of Co_0.85Se/CNT hybrid for HER and OER in alkali condition decrease down to 73.9mV dec^-1 and 75.9 mV dec^-1,and the electrocatalytic mechanism of HER is promoted from Volmer mechanism to Volmer-Veyrovsky mechanism.When used as a bifunctional catalyst for overall water splitting in alkaline electrolyte,a current density of 10 mA cm^-2 of Co_0.85Se/CNT can be obtained by only applying 1.65 V.The remarkable multifunctional electrocatalytic properties can be attributed to the synergistic effect of Co_0.85Se hollow nanospheres and highly conductive CNT networks,which can not only improve the availability of the reaction region and active sites of Co_0.85Se,but also greatly improve the electrocatalytic surface area and conductivity,and romote the electrocatalytic charge transfer efficiency.3.We present a facile and cost-effective hydrothermal approach to synthesize a novel hybrid electrocatalyst of Co_0.85Se/nitrogen-doped graphene(Co_0.85Se@NG).The results show that the Tafel slopes of the hybrid for HER and OER in alkaline solution decrease to 76.5 mV dec^-1 and 63.3 mV dec^-1.When used as both cathode and anode for overall water splitting,the overpotential of 1.63 V is required to drive a current density of 10 mA cm^-2,and it can work continuously for 23 hours.The excellent electrocatalytic performance of Co_0.85Se@NG can be attributed to its well-designed nanoarchitecture.By anchoring Co_0.85Se hollow nanospheres on highly conductive NG nanosheets,the aggregation of Co_0.85Se nanospheres is well suppressed,thus increasing the specific surface area,producing abundant exposed active sites and improving the structural stability of materials.In addition,the conductivity of material is improved and the charge transfer is greatly facilitated.