Construction Of Low-loading Noble Metal-based Composites And Hydrogen Electrocatalytic Performance

The ongoing consumption of fossil fuels has caused serious environmental issues and energy shortage problems.Therefore,it is a compelling need to find green energy sources to address these challenges.Hydrogen energy has received extensive attention due to its high combustion energy density and non-polluting water as the combustion product.However,the traditional hydrogen production method is prepared by methane steam reforming,which produces CO₂,CO and other by-products,which seriously pollute the environment.Therefore,extensive effort has been devoted to developing efficient and clean energy conversion and storage devices based on hydrogen and water.Electrochemical energy conversion and storage through hydrogen has revolutionized sustainable energy systems using fuel cells and electrolyzers.Its electrochemical conversion is achieved through two fundamental and crucial reactions of hydrogen evolution reaction（HER）and hydrogen oxidation reaction（HOR）.Much less fundamental work has been directed towards understanding the HER and the HOR in alkaline solutions,although these two processes are of paramount importance for the development of alkaline electrolysers and alkaline fuel cell systems.However,lack of means probing the interface during reactions forbids clear understandings of reaction mechanisms,which in turn impedes rational design of catalysts.Recently,Pt-based materials are used widely as the electrocatalysts for HOR,but their HOR activity are approximately two orders of magnitude drop when switching from acidic to alkali electrolytes.At the same time,the wide applications are inevitably plagued by expensive Pt-based catalysts.Therefore,the development of composite catalysts with low loading of noble metals,high activity and good stability is the key to realize the commercial application of fuel cells and electrolyzers.Based on the above analysis,we designed and synthesized a series of noble metal based electrocatalysts by support-induced strategy.We regulated the calcination temperature of the support and loading of noble metals.The relationship between structure and performance,and the catalytic mechanism were systematically explored.The detailed experimental researches are as follows:1.Pt/Ni₃N-Mo₂C catalyst:We developed a highly effective bifunctional catalyst of Pt/Ni₃N-Mo₂C via facile self-assemble,nitriding and wet-chemical strategies and investigated its electrocatalytic performance for HOR/HER.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）test analysis showed the catalyst presented a uniform flower-like morphology.X-ray powder diffraction（XRD）,Raman spectroscopy（Raman）,X-ray photoelectron spectroscopy（XPS）confirmed the composition of the catalyst and the valence state of the metal.Electrochemical tests showed that the optimized Pt/Ni₃N-Mo₂C catalyst exhibits exceptional HOR activity in 0.1 M KOH in terms of high mass activity at 0.05 V（vs.RHE）and exchange current density of 1.28 m Amg_Pt^-1and 2.83 m A cm^-2,which are 5.1 and 1.5 times that of commercial Pt/C,respectively.Besides,the Pt/Ni₃N-Mo₂C can deliver an HER current density of 10 with a small overpotential of 11m V in 1.0 M KOH,which is lower than that of Pt/C and most reported electrocatalysts.Density functional theory（DFT）calculations combined with experimental results reveal that Pt/Ni₃N-Mo₂C possesses an optimal suitable Pt-H_ad and Ni₃N-OH_ad strengths,as well as Mo₂C preferentially adsorbed and activated H₂O molecules for dissociation and accessible active sites supplied by micro/nanostructure of Pt/Ni₃N-Mo₂C,which contribute to the excellent HOR/HER performance.Thus,this work may guide bifunctional HOR/HER catalyst design in the conversion and transport of energy.2.Ru/Ni-NiO@C catalyst:We developed a metal-organic framework-assisted replacement-pyrolysis approach to obtain highly dispersed Ru nanoparticles anchored on the Ni-NiO@C hollow microsphere（Ru/Ni-NiO@C）.SEM and TEM characterizations confirmed that the catalyst presented typical microspherical hollow morphology.XRD,Raman spectroscopy,specific surface area（BET）,electron paramagnetic resonance spectroscopy（EPR）,XPS and inductively coupled plasma atomic emission spectrometer（ICP-AES）elucidated the crystal structure,morphological characteristics,chemical valence state and actual content of metals of the catalyst.The annealing treatment at 400°C introduces nickel and oxygen vacancies on the surface of Ru/Ni-NiO@C.Nickel/Oxygen vacancies promote the electrical conductivity and charge transport,and at the same time,offer a strong metal–support interaction.Based on the advantages mentioned above,the optimal Ru/Ni-NiO@C exhibits up to 7.75-and 1.14-fold hydrogen oxidation reaction（HOR）improvement in mass activity(2.79 m Amg_Ru^-1)and exchange current density(2.65 m A cm^-2),compared with the commercial Pt/C counterpart.Experimental results and density functional theory（DFT）calculations reveal that an electronic effect between Ru and Ni-NiO@C support,as well as nickel/oxygen vacancies in the NiO species,enable optimization of hydrogen binding energy（HBE）and hydroxide binding energy（OHBE）,thus improving HOR performance.Besides,the Ru/Ni-NiO@C with a unique hierarchical hollow micro/nanostructure favors the inner surface-active sites contact with reactants to accelerate HOR kinetics.The present findings may open an avenue for designing cost-effective precious-metal-based catalysts applied in various hydrogen-based renewable energy technologies.3.Ru/NC@WOC catalyst:We developed a series of different loadings of Ru clusters evenly distributed on NC@WO₃-W₂C（Ru/NC@WOC）via co-precipitation and polyol reduction in this work.XRD,TEM,BET,XPS,and ICP-AES were used to determine the composition,morphological characteristics and chemical valence state of the catalyst.The optimized Ru/NC@WOC catalyst exhibit exceptional HOR activity in 0.1 M KOH in terms of high mass activity at 0.05 V（vs.RHE）and exchange current density of 1.96 m Amg^-（16）and2.4 m A cm^-2,superior to commercial Pt/C and Ru/C.Systematic investigations further demonstrated that WO₃ offers OH adsorption sites and Ru provides adsorption sites for H,H_addesorption on Ru sites increases the reactivity of OH_ads desorption to form H₂O.Besides,the role of W₂C is not negligible,providing high electrical conductivity.The encouraging HOR performance in alkaline electrolyte could arise from the following aspects:1)strong electronic interaction between Ru and NC@WOC substrate;2)high surface area;and 3)highly conductivity.