Research On Anode Catalyst Materials And Catalyst Layer Structure For Proton Exchange Membrane Electrolyzer

"Hydrogen economic"has been recognized as a promising solution for decarbonization and energy demand in the world in the future.Proton exchange membrane water electrolyzed is widely considered to be able to efficiently couple with intermittent renewable energy（such as water energy,wind energy,solar energy and tidal energy）to prepare"green hydrogen"because of its advantages of low operating temperature,small occupied space,large operating pressure,fast dynamic response and short start-up time.Although the numerous advantages,the high cost of the membrane electrode assembly,especially the use of rare precious metal Ir-based catalyst,has seriously hindered its large-scale commercialization of proton exchange membrane water electrolyzed.Therefore,minimizing Ir content to reduce the cost of the membrane electrode assembly is an effective strategy to solve this problem.However,the catalytic activity and stability of membrane electrode assembly have been greatly challenged with the decrease of Ir content.Aiming at the problem,this study focuses on the optimization and improvement of the key materials and structural design of the anodic catalytic layer,including the study of the influence characteristics of the side-chain structure and content of ionomer on the performance of membrane electrode assembly,the development of high-efficiency supported Ir O₂ composite catalyst with defective supports and the gradient structure design of the anode catalytic layer based on the selection results of ionomer and composite catalyst.Further,the catalytic performance of Ir in the catalytic layer is maximized,and the amplification and verification of key materials and structure in the short stack are completed.（1）Defect engineering assisted support effect:enhances the oxygen evolution performance of Ir O₂ with defective supports.Based on the adjustable structure and composition of defective supports,Ir O₂ nanocomposite catalysts supported on defective materials are constructed.Three kinds of defective carriers with different dimensions（Ti N_x,N-doped Ti O₂（N-Ti O₂）and N defective g-C₃N₄（N-CN））are prepared.Effects of defective supports on the oxygen evolution activity and stability of Ir O₂ have been evaluated.The results showed that the defective supports could effectively disperse Ir O₂nanoparticles and increase the number of active sites.The interface electron interaction between the defective supports and Ir O₂ can not only optimize the surface electronic structure of Ir O₂,reduce the adsorption of oxygen intermediates,and enhance the reaction kinetics of Ir O₂,but also anchor Ir O₂ nanoparticles on the supports to reduce the dissolution or shedding of Ir,thereby enhancing the stability of Ir O₂.Different supports also have a great influence on the catalytic performance of Ir O₂.The test results showed that the current densities at 1.8 V corresponding to Ir O₂/Ti N_x,Ir O₂/N-Ti O₂ and Ir O₂/N-CN are 0.32 A cm^-2 mg_Ir^-1,0.28 A cm^-2 mg_Ir^-1 and 0.15 A cm^-2 mg_Ir^-1,respectively,indicating that Ir O₂/Ti N_x has the optimal mass activity.In addition,the degradation rate of Ir O₂/Ti N_x is only 54μV h^-1 in the 100 h stability test,implying that Ir O₂/Ti N_x has good stability and realizes the application of efficient catalysts in the PEM electrolyzer.（2）Effects of side-chain structure and content of ionomer on the performance of anode catalytic layer.Perfluorosulfonic acid with long side-chain D2020 and short side chain D79 have been selected as the ionomer of the anode catalytic layer.The effects of the side-chain structure and different content（I/Ir ratio 5-30 wt%）of the ionomer in the catalytic layer on the performance of the membrane electrode assembly are analyzed,the D2020 and D79 ionomers exhibit the optimal performance when I/Ir ratio is 10 wt%and 15 wt%respectively,and the corresponding cell voltage is 2.141 V and2.182 V when the current density is 2 A cm^-2.The results showed that the catalyst layer with D79 shows more active sites,faster reaction kinetics and proton conductivity than D2020,while the D79 catalyst layer has low porosity,which hinders the mass transfer in the catalyst layer and leads to high cell voltage at medium and high current densities.Based on the experimental characterization and data analysis,it has been found that the mass transfer in the anode catalytic layer is the key factor affecting the performance of the membrane electrode assembly by the side chain structure of the ionomer.（3）Gradient structure design of anodic catalytic layer improves the electrochemical performance of membrane electrode.To solve the problem of reaction space differentiation and mass transfer balance in the anode catalyst layer,this chapter proposes a gradient hierarchical structure technology for the anode catalyst layer,regulates the composition parameters of the anode catalyst layer ionomer and catalyst,and improves the pore size distribution in the layer by pore-forming to optimize the structure of the anode catalyst layer.It is found that the corresponding cell voltages of conventional MEA-1,MEA-3（the ionomer content gradient decreased away from the membrane）,MEA-4（the catalyst amount gradient increased away from the membrane）and MEA-7（the catalyst amount increased with the gradient away from the membrane and the ionomer content decreased with the gradient away from the membrane）at the current density of 3 A cm^-2are 2.206 V,2.176 V,2.146 V and 2.118 V,respectively,indicating that the structure and performance of the catalytic layer are optimized and improved by adjusting the combination parameters of ionomer and catalyst.This is mainly because the gradient design can improve the utilization rate of the anode catalyst layer,enhance the kinetic reaction and reduce the mass transfer resistance,thereby increasing the electrochemical performance of the membrane electrode assembly.Pore-forming is used to improve the gradient distribution of pore size in the catalytic layer.Hierarchical gradient structure with gradually increasing pore size away from the membrane showed the lowest cell voltage of 2.043 V@3 A cm^-2,which could achieve 88.52%of the water electrolysis efficiency,reaching the advanced level of literature reports.This is mainly because it could enhance mass transfer in the layer,and improve catalyst utilization and activity.（4）Validation of high-performance membrane electrode assembly in proton exchange membrane electrolyzer short stack.The short stack（3 cells,single membrane electrode assembly active area 5 cm×5 cm）is developed.The optimal operating parameters of the short stack assembly clamping force,working temperature and water supply flow rate were explored,and the engineering short reactor verification experiments are carried out.The results show that the performance trend of the short stack is consistent with that of the single electrolyzer cell.At the same time,the stability experiment of photovoltaic power generation under simulated power fluctuation is carried out,a little durability loss is observed in the cyclic test under fluctuating conditions within 100 h,and the decay rate is about 133μV h^-1 for a single membrane electrode assembly.The change of the anode catalyst layer after the dynamic condition test is analyzed.The results show that the developed Ir O₂/Ti N_x composite material and gradient catalyst layer have excellent performance,and shows practicability.