| With the advancement of science and technology and the improvement of human society,the depletion of traditional fossil energy and the ecological environment pollution are getting more severe,and the development of environmental protection and renewable energy technology is becoming more urgent.Hydrogen is a future-oriented energy carrier,which has attracted wide attention for its clean and lightweight characteristics with high efficiency.It is considered as a new energy form that could effectively cope with the environmental and energy crisis and achieve the strategic goals of"emission peak"and"carbon neutrality".The popularization and application of hydrogen energy need perfect supporting facilities of production,storage,transportation and utilization,and the technology of electrolysis of water and fuel cell based on electrochemical technology have great advantages in hydrogen production and utilization.Among them,polymer electrolyte membrane（PEM）water electrolysis technology and high temperature polymer electrolyte membrane fuel cell（HT-PEMFC）technology have become the most promising hydrogen production/utilization electrochemical devices with their advantages of compact structure and high efficiency,strong resistance and simplified system,respectively.However,PEM electrolysis of water for hydrogen production is limited by the slow kinetics of anodic oxygen evolution reaction（OER）,which limits its large-scale application.To meet these challenges,the design of low-cost and efficient OER catalyst and the improvement of its actual performance in PEM electrolytic water devices are conducive to the development of this kind of devices.As for another aspect,HT-PEMFC technology also faces several challenges in catalysis,such as low catalyst utilization rate,serious phosphate poisoning,and intensified corrosion of catalysts and supports.It has great significance to improve the performance of HT-PEMFC devices to design catalysts and reasonably construct triple-phase interfaces of catalytic reaction.This thesis mainly focused on the main challenges existing in the catalytic reaction process of PEM electrolytic water device and HT-PEMFC device,aiming at the multi-level construction strategy from the improvement of intrinsic catalytic activity of catalyst to the triple-phase interfaces of catalyst layer.The electrochemical process and catalytic mechanism of reactions in hydrogen electrochemical devices including PEM water electrolysis device and HT-PEMFC were studied in detail.Firstly,defect-rich iridium oxide anchored active ruthenium atoms,the structure symmetry was changed,thus enhanced the intrinsic activity of OER and enhanced the performance of PEM electrolysis water device.Secondly,the triple-phase interfaces in HT-PEMFC catalyst layers were constructed by reasonable control of binder choices,flow rate of gaseous reactants,loading mass of catalysts and thickness of catalyst layers,which effectively improved the performance of membrane electrode.Then,iron phosphide species were introduced into the platinum-based catalyst to tune the adsorption behavior of hydrogen species and phosphate species,and enhance the catalytic activity of hydrogen oxidation reaction（HOR）,achieving a relatively ideal output performance with low loading mass of catalysts.Finally,solid acid materials based on oxide were prepared,and their effects on proton transfer in HT-PEMFC catalyst layers were studied,and the structure-function relationship was verified on another carbon-based solid acid material.The research particulars are as follows:（1）Defective iridium oxide substrates were used to disperse ruthenium atoms onto the oxide substrates by adsorption anchoring method.Various characterization showed that ruthenium was atomically dispersed on the substrates.Electrocatalytic tests showed that the ruthenium-modified catalysts exhibited excellent OER catalytic activity,requiring only 266m V overpotential to reach the current density of 10m A cm-2,and the intrinsic and mass activity of the catalysts were also in the front rank compared with similar catalysts.The catalyst also had excellent electrochemical stability,and the stability in solution was similar to that of commercial catalysts.In addition,the catalyst also showed good performance in actual PEM water electrolysis devices,with a cell voltage of only 1.72V at 1A cm-2current density and excellent performance in100-hour stability tests.The introduction of ruthenium atom destroyed the symmetry of the pristine structure of iridium oxide,weakened the interaction between metal atoms and oxygen atoms,and promoted the formation of catalytic intermediates,thus enhancing the performance of the catalysts.（2）Different binders made great difference in the wettability of the catalytic layer and the transport capacity of gaseous reactants and protons.The performance of HT-PEMFC membrane electrode assemblies（MEA）with PTFE binder had some advantages,mainly because of the effect of the binder,the triple-phase interfaces had the characteristics of hydrophobic and phosphoric-acid-phobic,which effectively promoted the transport of gas and proton in the MEA,thus improving the performance of the fuel cells.Secondly,the flow rate of gaseous reactants had an impact on the reaction at the triple-phase interfaces.Excessive loss of phosphoric acid from high-temperature proton conductors might result in excessive loss of gas flow,which reduced device performance.In addition,the catalyst loading mass in the anode also had an important influence on the triple-phase interface reaction.An extremely low loading mass of catalysts would seriously restrict the progress of anode HOR process,and a certain higher amount of anode loading mass was needed to avoid the restriction on device performance by anode activity.Finally,the thickness of the catalysts layer had great influences on their structures.When the thickness of the catalyst layer was much too thick,it would significantly hinder the proton transfer in the catalyst layer and restrict the increase of the peak power density of HT-PEMFC device.Based on the discussion above,reasonable design strategies of MEA structures and optimization of triple-phase interface microstructures could effectively improve the operating efficiency of electrochemical devices.（3）Iron phosphide was introduced into platinum-based catalyst,and there was obvious electron transfer from platinum nanoparticles to phosphide in the catalyst.In terms of electrocatalytic performance,the catalyst had been proved to have excellent HOR activity in acidic environment（especially phosphate-rich environment）,and the HOR exchange current density in phosphoric acid solution could reach 0.455m A cm-2.According to the Tafel slope,HOR of the catalyst followed Volmer-Tafel reaction pathway,with Volmer step regarded as the rate-determining step.Due to its excellent HOR catalytic activity and long-term stability,the performance of HT-PEMFC device as anode catalyst was further improved.Under the condition of only 0.125mg Ptcm-2anode loading mass,the peak power density of 465m W cm-2could be achieved,and it remained stable in the 100-hour durability test.In addition,the stability of the device under start-up/shut-down conditions was also improved.The introduced phosphide compound reduced the binding energy of hydrogen intermediates and platinum nanoparticles,and it became easier to adsorb phosphoric acid species,which promoted the proton transfer process while optimizing the sorption of hydrogen species,and further accelerated the HOR kinetics.The catalyst achieved considerable power output with a lower loading mass,and expanded the range of HOR catalyst options at the same time,providing a new strategy for rational design of advanced HOR catalysts.（4）The performance of HT-PEMFC MEAs were partly improved by adding several supported solid acids based on oxides into the catalyst layers.Further research showed that fuel cell performance was correlated with acid strength and acidic sites amount of corresponding solid acid.That is,the promoting effect on proton transport was more obvious,and the power output performance of fuel cells were better,when the solid strength was stronger as well as the acid sites were more.In order to verify the above inference and further improve the electrical conductivity problem caused by solid acids based on oxides,a carbon-based solid acid with better conductivity was designed.Experimental results showed that carbon-based solid acid with strong acid strength could also greatly improve the proton transport properties of catalytic layers,and also enhance the fuel cell output performance,and the increase of acid site amounts increased further.This understanding had some significance for improving the proton transfer in catalyst layers and understanding the regularity of proton transfer. |
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