Design, Fabrication And Properties Of Nano Electrocatalysts

Recently,two serious issues that impose restrictions on the development of the society are the increasing energy demands and the worse environment.The usage of fossil fuel can ease the urgent energy requirement for current development,however,it also gives a destructive impact on our ecology.For the long-term sustainable process,it is imperative to find a clean and renewable energy that can replace fossil fuels.Hydrogen energy is considered as one of the best energy sources in the future due to its advantages of high calorific value,no pollution,wide storage and renewable features.Electrolytic water technology,a direct conversion of electrical energy into hydrogen energy,with high efficiency and non-pollution,is widely used for hydrogen generation.Besides,electrolytic water technology can also be used in conjunction with other renewable energy sources such as solar,tidal and wind energy to store these intermittent sources of energy in the form of hydrogen energy for better regulation and use.However,this technology is highly dependent on electrocatalysts.Platinum（Pt）is considered to be the best hydrogen evolution catalyst,but its rare amount and wide dispersion inhibit its practical application.Therefore,searching for low cost and excellent performance of hydrogen evolution catalyst is the hot spots.In this thesis,we focus on the development of high efficient and low cost hydrogen evolution electrocatalysts.Herein,some excellent hydrogen evolution catalysts were designed and the mechanism was also discussed.There are four works in this thesis.First,we introduced rhodium（Rh）on molybdenum disulfide（MoS₂）nanosheets to improve its performance of hydrogen evolution.Then we designed and synthesized osmium/silicon（Os/Si）composite according to the hydrogen adsorption free energy（ΔG_H）.Also,we used the carbon cloth as the carrier and fabricated the multifunctional electrocatalyst CoP@CC,which exhibited a wide range of applications.Finally,we obtained the cobalt selenide（CoSe₂）catalyst,which uniformly dispersed on carbon cloth.The details are as follows:（1）The MoS₂ was chosen in our first work.Although MoS₂ is considered to be the most promising candidate for hydrogen evolution,high overpotential and low efficiency are the two main factors that restrict its practical applications.Herein,Rh-MoS₂nanocomposites with a small amount of Rh（5.2%）exhibited an excellent hydrogen evolution activity with low overpotentials,small Tafel slope(24 mV·dec^-1),and long-term stability.Experimental results revealed that 5.2 wt%Rh-MoS₂ nanocomposite exhibited an excellent HER activity,even exceeded the commercial 20 wt%Pt/C when the overpotential is higher than 0.18 V.It also exhibited an excellent mass activity of13.87 A mg^-1_metal at-0.25 V,four times as large as that of the commercial 20 wt%Pt/C catalyst.The hydrogen yield of 5.2 wt%Rh-MoS₂ nanocomposite is 26.3%larger than that of the commercial 20 wt%Pt/C at the potential of-0.25 V.The dramatically improved electrocatalytic performance of Rh-MoS₂ nanocomposites may be attributed to the hydrogen spillover from Rh to MoS₂.（2）We designed and fabricated the Os/Si composite from the point ofΔG_H.In this work,we also investigated the electrocatalytic performance of M/Si（M=Rh,Pt,Pd,Re,Ru,Au or Ag）nanocomposites for hydrogen evolution reaction.The results showed that Os/Si nanocomposites exhibited the best catalytic efficiency with a negligible onset overpotential（25 mV）,a small Tafel slope of 24 mV·dec^-1 and remarkable long-term stability.The most important thing is that the energy conversion efficiency of the Os/Si nanocomposite is 29.3%higher than that of the commercial 40 wt%Pt/C at a current density of the typical industrial production(-1000 mA·cm^-2).The density functional calculations revealed that such outstanding catalytic activity of the Os/Si catalyst arised from the thermodynamically more favorable hydrogen adsorption free energy（ΔG_H*=-0.03 eV）at the osmium/silicon interfaces.（3）Excellent performance and multifunctional electrocatalysts are of great importance for the development of renewable energy.Here,a multifunctional carbon cloth supported cobalt phosphide（CoP@CC）electrocatalyst was synthesized via a simple electrochemical deposition followed by subsequent phosphorization.This self-standing CoP@CC presents superior HER performance in both acid and alkaline media and excellent OER activity in an alkaline environment.When applied as both anode and cathode catalyst to overall water splitting in alkaline media,CoP@CC only required a low cell voltage of 1.68 V to drive a current density of 10 mA·cm^-2,making this catalyst a potential candidate in industrial application.Moreover,inspired by its satisfying ORR activity,CoP@CC was first used as the air-electrode in zinc-air batteries,which outputted an open-circuit potential of 0.6 V and a power density of～30 mW·cm^-2.（4）We obtained high dispersed CoSe₂/C composite from the view of coordination.Compared with CoSe₂,CoSe₂/C composite exhibited a good hydrogen evolution activity with a lower overpotential and smaller Tafel slope,and also high stability.The excellent performance of CoSe₂/C composite may due to its good conditivity and uniformly dispersed.Herein,an electrocatalyst with a uniformly dispersion and good conductivity was obtained according to the view of coordination,which provides a way for the development of hydrogen evolution electrocatalysts.