Rational Design And Device Application Of High-Efficiency Electrocatalysts For Hydrogen/Oxygen Evolution Reaction Based On Membrane Electrode Assembly

With high energy density,hydrogen is considered to have great potential for large-scale electrochemical energy storage.The strategy of"carbon neutrality"is the main driving force to promote the development of hydrogen energy,and hydrogen production from water electrolysis has also been developed continuously with the breakthrough and large-scale application of hydrogen energy technology.Water electrolysis for hydrogen production presents advantages,including a simple process,non-pollution and high purity of hydrogen product.The water electrolyzers assembled with solid electrolytes include anion exchange membrane（AEM）water electrolyzers and proton exchange membrane（PEM）water electrolyzers.AEM water electrolyzer technology can use non-noble metal catalysts,significantly reducing the cost of hydrogen production.However,there is no clear understanding of the drastic evolution of the catalysts in the anodes of the AEM water electrolyzers,and clarifying their catalytic activity centers is beneficial for improving the performance of the AEM water electrolyzer.Perfluorosulfonic acid-based polymer exchange membranes with good chemical stability and proton conductivity are selected as solid electrolytes for hydrogen production via PEM water electrolysis.The commonly used catalyst for hydrogen evolution reaction（HER）is precious metal Pt,which has a low utilization rate and high usage,resulting in high hydrogen production costs for PEM water electrolyzers.To address this issue,the highly dispersed noble metal catalytic sites were constructed to reduce the use of Pt,which is helpful to reduce the cost of the PEM water electrolyzer.The choice of anode catalysts for long-life and stable PEM water electrolyzers is the noble metal-based Ir O₂,but its OER activity is generally limited,which restricts the performance improvement of the device under working conditions.As for the activity of Ir-based catalysts in the anodes,enhancing the intrinsic activity of the catalyst by regulating the electronic structure and coordination structure can effectively promote the development of hydrogen production via the PEM water electrolyzer.This thesis focuses on the origin of active centers in AEM anode catalysts,the problem of high Pt loading on the cathodes and low intrinsic activity of the anodes of the PEM water electrolyzers,and conducts basic research on them.This thesis has developed water splitting catalysts with high activity and stability through rational design.The main research contents are as follows:（1）Co Se₂ultra-thin nanosheets were prepared with a high specific surface area through the hydrothermal method.Herein,to prove the pure effect of electrochemistry（excluding the influence of electrolytes）on the Co Se₂catalyst,the Co Se₂nanosheet was tested under the non-electrochemical and the electrochemical operation for comparison.The electrochemical operation was found benignly for building the high-valence cobalt on the surface and enhancing the degree of the weakened-crystalline structure due to the selenium leaching.On this basis,the ligand field splitting leads to the distortion of local symmetry between e_gand t_2gband of O 1s and thus causes the resulting defect sites.The anodized Co Se₂（A-Co Se₂）compound as obtained with the above structures was mainly linked to high intrinsic activity(η=254m V@10 m A cm^-2)and durability（120 h）.Co Se₂had good activity(2 V@1 A cm^-2)and durability(0.5～1 A cm^-2,30 h),which was one of the most stable Co-based catalysts in the AEM water electrolyzer.The outstanding performance of Co Se₂unravels that the increasingly weakly crystalline structure was benign for the dynamical retainment of the metallic Co-defect structure and high-valence active sites.This study was of great significance for understanding the real structure-activity relationship of materials,which promote the development and application of non-precious metal catalysts.（2）A high-stability alloyed Pt single-atom catalyst（SAC）was demonstrated through a plasma-assisted alloying way and assembled the catalyst in a PEM water electrolyzer.The Pt single atoms were strongly anchored onto the surface of the Ru substrate through a steady M-M bond strength,which was verified with morphology and electronic structure.Alloyed Pt SAC exhibits ultra-low overpotential and good stability for hydrogen evolution in a three-electrode system.Otherwise,the alloyed Pt SAC was assembled in a PEM water electrolyzer to reach a cell voltage as low as 1.8 V at 1 A cm^-2.Notably,the electrocatalyst could work over 1000 h hardly any decay.And the electrocatalyst existed in the form of Pt single atoms.This would be the first attempt at Pt-based SACs in the long-term operation of the PEM water electrolyzer.This work paves the way for PEM water electrolyzer to design durable SACs in the actual working conditions.（3）The highly efficient and durable Ir O_xcatalyst was developed by hydrothermal method and applied to the PEM water electrolyzer.Firstly,by optimizing and screening the model catalyst,it was found that the regulation of the Ir-O coordination number of the catalyst had a significant impact on the OER performance.The reduction of the Ir-O coordination numbers calculated by DFT can reduce the reaction energy barrier of the rate-determination step and improve its performance in acid OER.Therefore,Ir O_xwas synthesized with a low coordination number of Ir-O through a hydrothermal method and assembled in a PEM water electrolyzer.The Ir O_xcatalyst was reasonably optimized in the coordination environment and confirmed through XAS testing and corresponding fitting results.In the three-electrode system,the Ir O_xcatalyst showed an overpotential as low as 231 m V at a current density of 10 m A cm^-2.The chronopotentiometry test exhibited long stability of 100 h(@10 m A cm^-2)toward oxygen evolution in 0.5 M H₂SO₄.Impressively,the optimized Ir O_xcatalyst exhibits excellent performance in the PEM water electrolyzer,which attaches a voltage as low as 1.72 V at a current density of 1 A cm^-2and the durability exceeds 1200 h(1 A cm^-2)without obvious decay.The Ir O_xcatalyst with a low coordination number of Ir-O had extremely high efficiency in the PEM water electrolyzer,providing a good design strategy for anodic catalysts in the PEM water electrolyzer under working conditions.（4）A small amount of Mn had been doped into the lattice of Ir O₂ by the salt sealing method to adjust the catalyst crystal structure and electronic structure.The exposure of the dominant crystal facets of Ir O₂increased gradually,with the increasing amount of Mn doping.Otherwise,the shortening of the Ir-O bond length could accelerate electron transfer for the oxygen species as electrophilic centers.In 0.5 M H₂SO₄solution,the optimized Mn_0.1Ir_0.9O₂electrocatalyst exhibited excellent OER performance,requiring an overpotential of 269 m V to reach a current density of 10 m A cm^-2and good intrinsic activity.The Mn_0.1Ir_0.9O₂electrocatalyst also exhibited good stability,which was significantly better than that of the Ir O₂catalyst without Mn doping.In addition,the Mn_0.1Ir_0.9O₂electrocatalyst was used in a PEM water electrolyzer device under working conditions.The cell voltage was only 1.79 V at a current density of 1 A cm^-2,and the stability exceeded 1200 h at the same current density.The negligible decay reflected its excellent performance in PEM water electrolyzers.DFT calculations had shown that the Mn_0.1Ir_0.9O₂catalyst had a lower energy barrier in the rate-determination step,effectively promoting the OER process.The crystal and electronic structure of the Ir O₂catalyst were adjusted by a transition metal,to reduce the reaction energy barrier of the rate-determination step toward oxygen evolution.This doping method significantly improved the efficiency of the catalyst and reduced the costs of the PEM water electrolyzer for practical application.