Study And Applications Of Transition Metal And Carbon Nanotubes Nanocomposites In Electrochemical Water Splitting

Hydrogen,as an efficient and green renewable energy,holds great promise in the application prospect.Electrochemical water splitting has attracted more and more attention due to its high efficiency,low cost,simple equipment and mature development.Therefore,electrochemical water splitting will become the most promising means of hydrogen production in the future.Transition metals have attracted researchers’attention because of their high catalytic activity.Carbon nanotubes have good electrical conductivity and are environment-friendly catalysts,while their electrocatalytic activity is low.Based on the advantages of transition metals and carbon nanotubes,transition metal-carbon nanotubes nanocomposites were synthesized and applied to the electrochemical water splitting owing to their unique physical,optical properties and electronic structure.（1）To develop low cost and high efficient electrocatalysts towards water oxidation is critical for the sustainable energy system.Herein,CoP layer was successfully deposited onto poly（diallyldimethylammonium chloride）functionalized carbon nanotubes（CoP/PDDA@CNTs）as high effective oxygen evolution reaction（OER）electrocatalyst in alkaline solution.The experiments showed that PDDA played an important role during the synthesis process of electrocatalyst CoP/PDDA@CNTs.On one hand,PDDA could prevent the aggregation of CoP;on the other hand,it could induce the charge transfer between CoP and CNTs.Hence,CoP/PDDA@CNTs showed high OER preformance with the overpotential of 370 m V and the tafel slope of 64 m V·dec^-1.The introduction of PDDA would be a promising strategy for the synthesis of high efficient electrocatalyst.（2）Transition metals,especially noble metals such as Pt,are highly efficient electrocatalysts.However,the applications of noble metals are severely limited due to their high cost.Therefore,polyelectrolyte and ethylene glycol are introduced to improve the catalytic activity of Pt/CNTs.Ethylene glycol（EG）-Pt nanoparticles/poly dimethyl diallyl ammonium chloride（PDDA）modified carbon nanotubes denoted as EG-Pt/p CNTs were synthesized through solvent thermal method.At the Pt loading of 2.3 wt.%,EG-Pt/p CNTs exhibited significant electrochemical hydrogen evolution reaction（HER）performance with the tafel slope of 137 m V·dec^-1 in alkalin solution and 31 m V·dec^-1 in acid solution,respectively.As control experiments,X-ray photoelectron spectroscopy（XPS）and Raman spectra showed,the charge transferred from PDDA to Pt which were more favorable for adsorbing H^*species.Density functional theory calculations further confirmed the intermolecular charge transfer and charge rearrangement of Pt atoms.Moreover,EG could enrich the hydrogen intermediates of Pt.Ultimately,the hydrogen evolving reaction kinetic of EG-Pt/p CNTs dramaticly improved compared with Pt/CNTs and EG-Pt/p CNTs.This strategy could transform the inert CNTs into highly efficient HER electrocatalysts with low noble metal loading.（3）Transition metal sulfides are widely used in the electrochemical water splitting due to their high electrocatalytic activity and high chemical stability.Meanwhile,it has been reported that ethylene glycol can significantly improve the catalytic kinetics of electrocatalyst.MoS₂,as a promising alternative to Pt-based electrocatalysts for HER electrocatalyst aroused more and more interests owing to its low-cost and Pt-like acivity.However,its practical application is severely restricted due to the insufficient active sites and poor conductivity of MoS₂.Herein,ethylene glycol modified MoS₂ grown on carbon nanotubes（EG-MoS₂@CNTs）were synthesized via a solvent thermal method.At a Moloading of 4.64 wt.%,EG-MoS₂@CNTs exhibited dramatic electrochemical HER performance compared with MoS₂ and MoS₂@CNTs in acid solution.The overpotential and Tafel slope of EG-MoS₂@CNTs were 188 m V and 87m V·dec^-1,respectively.As X-ray photoelectron spectroscopy,Raman spectra and Zeta potential showed EG could not only increase the basal-edge-rich defects and the defects between Moand S atoms,but also induce the charge transferred from MoS₂@CNTs to EG which are more favorable of adsorbing H^*.In addition,EG could enrich the proton concentration around MoS₂.As a result,the HER kinetics dramatically improved for EG-MoS₂@CNTs compared with MoS₂@CNTs under the same condition.The introduction of EG could significantly increased the HER kinetics of MoS₂@CNTs and transform inert CNTs into highly active HER electrocatalysts with low Moloading.（4）Engineering the surface and the electronic structure of electrocatalysts is a typical and effective strategy to enhance their catalytic performance.It has been reported that polyelectrolytes can greatly enhance the catalytic activity of electrocatalysts.Herein,MoS₂grown on carbon nanotubes（MoS₂/CNTs）was synthesized through a hydrothermal process,then certain polyelectrolytes were absorbed onto the surface,including poly（sodium-p-styrenesulfonate）（PSS）,polyacrylic acid（PAA）,sodium polyacrylate（PAAS）and polyethyleneimine（PEI）.The results of Raman spectra and XPS spectra confirmed the strong electronic coupling interaction between PSS and MoS₂/CNTs,which increased the electron density around the surface and enlarged the electrochemically active surface area.Besides,the negatively charged groups could also enrich the surrounding H⁺in the acid electrolyte via electrostatic interaction.What’s more,the introduction of carbon nanotubes and PSS improved the conductivity of the electrocatalyst.Therefore,PSS-MoS₂/CNTs displayed the best hydrogen evolution reaction（HER）performance in acid solution.It only required 114 m V to obtain the current density of 10 m A·cm^-2 with a smallest Tafel slope of46.05 m V·dec^-1.Different polyelectrolytes were used to modify MoS₂/CNTs to verify the factors to affect the performance for HER catalytic activity of MoS₂/CNTs.This strategy provides a practicable direction of the synthesis of efficient,low-cost and environmentally-friendly electrocatalysts for HER.