Novel Transition Metal Sulfides-Based 3D Electrodes For Electrocatalytic Hydrogen Evolution Reaction

Hydrogen is an important clean energy source.Among the various strategies for hydrogen production,electrocatalytic hydrogen evolution is a sustainable and pollution-free method and has attracted a large number of attentions in recent years.Platinum-group metals are the most effective electrocatalyst for hydrogen evolution reaction（HER）,but the wide application of these noble metals is hindered by their scarcity and high cost.To realize a prosperous development of HER,exploring non-noble electrode materials that are not only efficient but also earth-abundant to replace Pt-group catalysts is crucial.Transition metal sulfides with abundant reserves and low price show excellent catalytic activity,which makes them promising HER catalysts.In addition,if used practically,conventional powdery catalysts have to be immobilized or shaped into models using organic binder agents and problems such as activity loss and extra cost have to be solved.It is of great significance to explore a 3D free-standing electrode that is highly efficient,stable and easy to use.Therefore,in this thesis,novel,efficient and stable 3D electrodes based on transition metal sulfides were developed.The great promise of the 3D electrodes for electrocatalytic hydrogen evolution was also revealed.First,tungsten disulfide（WS₂）based 3D electrode,denoted as WS₂/GR,was successfully prepared by a simple one-step solvent-thermal method using the highly conductive graphite rods with rich subsurface caves and slits.The HER performance of the WS₂/GR 3D electrode was significantly higher than that of WS₂ membrane electrode,with an overpotential of 282 mV at 10 mA·cm^-2 and a Tafel slope(47.9 mv·dec^-1)close to that of Pt electrode(38.3 mV·dec^-1).Due to the intimate contact between the substrate and in-situ grown WS₂,the charge transfer resistance of the electrode was extremely small（～1Ω）,speeding up the process of the charge transfer in HER and making the electrode exhibit excellent stability.Via a scrutiny on the WS₂ electrode’s morphology,it was found that WS₂ in three different kinds of structures were formed in this electrode:while the hemispheric WS₂ outside the graphite rod was the least important for HER due to the stacking and partial oxidation,the vertically grown WS₂nanosheets as well as WS₂ nanoparticles（30-50 nm）that formed inside the graphite rod due to the confinement effect played crucial roles in the outstanding HER performance because of their abundant exposed active sites and anti-oxidation properties with the protection of the graphite scaffold.At last,to explore the universality of the above methods and further improve the HER activity of the electrode,another 3D free-standing electrode based on molybdenum disulfide（MoS₂）with mixed metallic（1T）and semiconductor phases（2H）,denoted as MoS₂/GR,was prepared by a facile one-step hydrothermal method.It was found that on the surface of the limited area inside the graphite rod,MoS₂ also grew in the form of vertical nanoscale,exposing abundant active sites.Meanwhile,with the increase in the hydrothermal temperature and the concentration of the reaction precursors,the content of MoS₂ grown on electrode increased,and the HER performance improved accordingly.The optimal MoS₂/GR electrode was obtained with the hydrothermal temperature maintained at200°C.To reach the current density of 10 mA·cm^-2 on the optimal eletrode,the overpotential required was only 177 mV,which was 105 mV lower than that of WS₂/GR electrode.The results obtained in this thesis are important to inspire the further development and application of highly efficient and stable 3D electrode for HER.