Metal-based Nanocomposites For Efficient Electrochemical Water Splitting

The demand for energy grows rapidly due to the development of economy and population.However,serious environmental problems have been produced by using fossil fuels.Therefore,the development of renewable and clean energy is currently an important issue.Hydrogen is considered as a promising clean resource,which may replace fossil fuels and can serve as an energy carrier to establish a new green energy system.Sustainable hydrogen generation is the major prerequisite to realize the goal.Electrochemical water splitting can convert electricity into storable hydrogen with no pollution,which is a fascinating and efficient energy conversion technology.The vigorous development of this technology is important for the establishment of an environment-friendly energy system.Although many studies have been carried out in electrochemical water splitting reactions(OER and HER)in recent years and great achievements have been made,there are still many difficulties,such as the large overpotential and instability of catalyst,to realize the efficient applications.It is thus important to develop the low-cost,highly-efficient and stable electrocatalysts for the future applications.The purpose of this thesis is to design catalysts with high efficiency and good stability for oxygen evolution reaction and hydrogen evolution reaction.In this study,the highly active catalysts for oxygen/hydrogen evolution reactions have been prepared through various treatments.Synchrotron radiation X-ray absorption spectroscopy(XAS)has been applied to investigate the electronic structure of catalysts.In order to gain further insights into the catalytic mechanism,we have also used in-situ XAS to reveal the electronic structure changes during the reaction.Simultaneously,theoretical calculations have also been used to simulate the catalytic processes.The research contents and related results are shown as below:1.Ultra-small PtOx nanoparticles with hybrid Pt chemical states on carbon nanotubes have been prepared as highly efficient alkaline HER catalyst,which show a low overpotential of 19.4 mV at 10 mA cm-2,a high mass activity of 5.56 A mgPt-1 at 0.1 V,and extremely stable durability for at least 20 h.The HER performance is better than that of the benchmark 20 wt%Pt/C while the Pt content in the catalyst is only about one-tenth of that in Pt/C.It also represents one of the best catalysts ever reported for HER in an alkaline solution.Synchrotron radiation XAS reveals that the efficient and stable alkaline HER performance can be attributed to the favorable design of hybrid chemical states of Pt on carbon nanotubes,which exhibits abundant Pt-O sites as the active catalytic center and forms stable Pt-C interfacial interaction to both anchor the nanoparticles and enhance the synergistic effect between catalyst and substrate.2.Sulfur-treated Fe-based metal-organic-framework has been prepared as a promising electrocatalyst for OER,which shows a low overpotential of 218 mV at the current density of 10 mA cm-2 and exhibits a very low Tafel slope of 36.2 mV dec-1 at room temperature.It can work on high current densities of 500 and 1000 mA cm-2 at low overpotentials of 298 and 330 mV,respectively,by keeping 97%of its initial activity after 100 h.Notably,itcan achieve 1000 mA cm-2 at 296 mV with good stability at 50℃,fully fitting the requirements for large-scale industrial water electrolysis.XAS and density-functional theory(DFT)calculation reveal that the high catalytic performance can be attributed to the thermocatalytic processes of H+ capture by-SO3 group from*OH or*OOH species,which is cascaded to the electrocatalytic pathway and then significantly reduces the OER overpotentials.3.FeNi-LDH on nickel foam has been modified by a sulfur treatment(S-FeNi/NF).The S-FeNi/NF catalyst exhibits excellent catalytic activities for oxygen/hydrogen evolution reaction,which only requires overpotentials of 220 and 85.5 mV to obtain the current density of 10 mA cm-2 for OER and HER,respectively.Its stability is also very good.By using S-FeNi/NF as both anode and cathode to catalyze overall water splitting,the overpotentials at the 10 and 100 mA cm-2 are 1.53 and 1.81 V,respectively.The corresponding voltages are much lower than those for the Pt/C-RuO2 couple.Combining the XAS and electrochemical data,it is revealed that the surface of S-FeNi/NF is reconstructed and then the Ni2+Fe3+OOH active structure can appear during the activation reaction.More importantly,we find that there are stable Ni-S and Fe-S sites in the subsurface of the sample,even after a long-time reaction.The metal sites(Ni-S and Fe-S site)can act as co-catalysts to cooperate with Ni2+Fe3+OOH to catalyze HER and OER with high efficiency and good stability.4.Under high temperature and high pressure,double precious metal perovskite materials with a type of A2B’B"O6(La2RhRuO6)have been prepared.The material prepared at 1400℃(LRRO-1400)shows a good HER activity,with an overpotential of 20 mV at the current density of 10 mA cm-2.Furthermore,LRRO-1400 is very stable in alkaline solution and can maintain high performance even after 5000 cycles.Synchrotron radiation XAS reveals that LRRO contains a large number of M-O active sites and can maintain its structure after long time reaction.