| Due to the fast consumption of fossil fuels and the ever growing environmental pollution issues,people's demand for clean and sustainable alternative energy is becoming more and more agent.Hydrogen is widely considered as the most ideal new energy carrier because of its high energy density and clean output At present,electrocatalytic water splitting,including hydrogen evolution reaction(HER)at the cathode and oxygen evolution reaction(OER)at the anode,is the most efficient and environmental friendly method to produce H2.Noble metal based catalysts,including Ir?Ru and their oxides(IrO2 and RuO2)are the state-of-the-art OER catalysts and Pt based catalysts exhibite the most excellent catalytic performance for HER.However,precious metals are expensive and have scarce reservation in the crust.Thus,research has been foucused on how to improve utilization rate of nobel metals and optimize catalytic activity of nobel metal based catalysts.It is generally believed that the effective ways to solve these problems are as follows:1.By tuning the size and shape of the catalysts,we can increase surface area of the prepared nanomaterials and control its surface atomic arrangement to enhance its catalytic performance.2.We can modify chemical composition of nanomaterial by doping relatively cheap metals 3d transition metals or non-metals.This method can not only reduce usage of nobel metals but also improve catalytic performance and stability owing to synergistic effect and electronic effect between metals or metal and non-metal.3.We can load nobel nanomaterial onto supports(such as graphene?carbon nanotubes?TiO2?TiC and so on)to reduce the use amount of noble metals as well as improve the utilization rate of noble metals.Ir-based nanocatalysts with different morphologies and compositions were designed and prepared by one-step hydrothermal method.These catalysts are applied as OER,HER or bifunctional catalysts for overall water splitting process.The main research contents are as follws:1.Poly(allylamine hydrochloride)(PAH)was used as morphology control agent,coordination agent and protecting agent.After adding reducing agent,ultrafine Ir nanowires(Ir NWs)of abundant nano pores were manufactured by one step hydrothermal method.The synthesized Ir NWs with an average diameter of 12 nm are made up of interconnected 2.5 nm Ir nanoparticles.Nano pores of 2 nm are filled between the nanoparticles.Porous structure provided this catalyst with abundant surface active sites and large specific surface area,which can increase mass transfer rate.Interconnected structure can effectively reduce the dissolution,migration,aggregation and Ostwald ripening processes of noble metal nanoparticles to increase catalytic stability.Electrochemical measurements showed that electrochemical active area(ECSA)of porous Ir NWs was 787.6 cm2/mgir.Compared with RuO2 catalyst,Ir NWs has a smaller overpotential for OER,it needs an overpotential of merely 248 mV to reach current density of 10 mA/cm2,which is much smaller than commercial RuO2 catalyst(351 mV).2.With the help of Poly(diallyldimethylammonium chloride)(PDDA)as morphological regulator,palladium salt and iridium salt as metal precursors,two-dimensional ultra-thin self-supporting PdIr porous nanosheets that made up of PdIr alloy nanoparticles were prepared by one-step hydrothermal method.The unique physical and chemical characteristics of this ultra-thin two-dimensional nanosheet material(several nanometers thick)make it have bigger specific surface area?more exposed atoms on the surface?better electrical conductivity?larger electrode-electrolyte interface and shorter ion and electron conduction paths,which is very beneficial for the catalytic activities of the OER and HER.In addition,its 2D sheet structure will also do good to the the stability of the prepared catalyst.The porous structure provided the material with even bigger surface area and interface area,which facilitates its contact with the solution and release of H2 and O2,and also shortens the electron/mass transfer distance.This catalyst exhibits outstanding performance towards HER in alkaline environment.It has an Tafel slope of 57.1 mV/dec and needs an overpotential of 91 mV to reach a current density of 10 mA/cm2.In addition,this catalyst also exhibited excellent catalytic performance for OER under alkaline condition.It has an OER Tafel slope of 84.2 mV/dec,which is small than that of RuO2 and it needs an overpotential of 276 mV to reach a current density of 10 mA/cm2.It can be seen that,two-dimensional ultra-thin PdIr porous nanosheets are excellent bifunctional catalysts for OER and HER.3.We used poly(allylamine hydrochloride)(PAH)as nitrogen source,IrClb as metal precursor,graphene oxide as support material to directly grow highly dispersed ultrafine Ir nanoparticles on nitrogen-doped reduced graphene though one pot hydrothermal method.Graphene has a large surface area and can effectively prevent nanoparticles from aggregation.Conductivity and stability of the catalyst can both be increased by chemical interaction and electron coupling between graphene and Ir nanoparticles.Moreover,the addition of graphene can reduce the use amount of precious metals to reduce cost.The supported ultrafine Ir nanoparticles we prepared were uniformly dispersed on the single-layered graphene and the average particle size of the nano-Ir particles was 2.1 nm.Follwing electrochemical tests indicate that Ir-N-rGO catalyst has Pt/C-like HER catalytic property.It's HER Tafel slope is 52.6 mV/dec,and needs an overpotential of 76 mV to reach current density of 10 mA/cm2.Moreover,it's OER catalytic overpotential is much smaller than 20%Pt/C and RuO2.Ir-N-rGO catalyst was used as both anode catalyst and cathode catalyst in a electrolyzer to test its electrolytic performance toward full water splitting in 1 M KOH electrolyte.It turns out that,the catalytic activity and stability of Ir-N-rGO ? Ir-N-rGO electrolyzer was better than that of Pt/C ? RuO2 electrolyzer.It's current density has no obvious attenuation after a i-t test of 100000s. |
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