Strain Engineering And Performance Enhancement Of Oxygen Reduction Electrocatalytic Materials

As a crucial electrode reaction in various new clean energy conversion devices such as metal-air batteries and fuel cells,oxygen reduction reaction（ORR）has the characteristics of slow kinetics and high overpotential.How to utilize various performance-enhancing strategies such as component designs,structural regulations and strain engineering to develop advanced ORR electrocatalytic materials with ultra-high activity and stability is still a challenging issue.This paper focuses on non-platinum and low-platinum materials for oxygen reduction reaction（ORR）catalysts.Through the rational design and precise construction of their chemical composition,morphology,and structure,nitrogen-doped carbon,as well as d,p,and f block metal-modified platinum-based binary and ternary alloy electrocatalysts,have been successfully prepared.By controlling the microgeometry and electronic structure of the catalyst through strain engineering,we have achieved improvements in the local electronic configuration and intrinsic activity of the active site.We investigated the feasibility of applying catalysts in metal-air batteries and proton exchange membrane fuel cells.The correlation between catalyst composition,structure and performance was studied using various characterization methods and density functional theory calculations.The specific innovative achievements in this thesis are as follows:（1）Based on the highly tunable composition and morphology of nitrogen-doped nanocarbons,polyaniline was selected as the precursor to prepare in-situ derived jagged carbon nanotubes（JCNTs）via facile Zn Cl₂-assisted carbonization and then NH₃-activation.The obtained nitrogen-doped carbon material retains the jagged structure in the precursor molecular chain and features abundant zigzag or armchair edge defects,leading to a high strain level in the JCNTs.The regulation of strain is achieved by regulating material composition,pore structure and defect content through different doses of activating agent Zn Cl₂.The strain-dependent excellent ORR activity exceeds state-of-the-art Pt/C with a half-wave potential reaching 0.88 V in alkaline media.In Zn-air battery test,JCNT demonstrats a high power density of up to142 m W cm^-2 and good stability.DFT calculations verify that moderate compressive strain makes the zigzag edge structure the main active site in JCNTs.Strain can efficiently change the electronic structures of C-C and C-N active sites,which can significantly activate oxygen molecules and lower the reaction energy barrier,facilitating ORR kinetics.Furthermore,the ultra-high specific surface area and abundant hierarchical pore structure of JCNTs make it an ideal carbon support,exhibiting higher intrinsic activity compared to traditional graphitized carbon blacks when loaded with platinum-based alloy catalysts in subsequent studies.（2）Praseodymium（Pr）,a rare-earth element from the f-block,was selected for modifying and enhancing platinum.Carbon-supported Pt Pr binary alloy catalysts（Pt Pr/C）were prepared using a polyol thermal reduction method.Taking advantage of the large atomic size of Pr,it acts as a dilation center and induces atomic-level tensile-compressive coupling strain in the alloy.Pt Pr/C exhibites a half-wave potential of 0.924 V and a mass activity of 0.54 A mg _Pt^-1,which is four times higher than that of Pt/C.It also demonstrates superior stability in accelerated durability testing.In PEMFC test,Pt Pr/C showcase a peak power density of 1.92 W cm^-2 and a mass activity of 0.28 A mg _Pt^-1,showing significant enhancement over Pt/C.Experimental characterization and DFT calculations revealed the mechanisms behind the improved performance of the binary alloy:the local strain induced by dilation center and the ligand effect induced by electrophilicity synergistically modulates the electronic structure,resulting in a moderate negative shift of the d-band center of Pt atoms.Additionally,the spillover effect caused by the oxygen-philic nature of Pr optimizes the adsorption and desorption relay process,thereby enhancing the ORR kinetic process.（3）The d-block element tungsten（W）was used for surface modification of the Pt Co alloy system.We prepared carbon-supported monodisperse Pt Co W alloy catalyst（Pt Co W/C）.The ultrafine alloy particles with an average size of less than 2 nm expose abundant defect sites such as terraces,kinks,edges,and corners on the surface.The doping of trace amounts of W on the alloy surface stabilized these defect structures.Pt Co W/C exhibites significantly enhanced ORR activity(half-wave potential of 0.94 V,mass activity of 0.97 A mg _Pt^-1),durability,and structural stability compared to Pt Co.In the PEMFC test,Pt Co W/C catalyst achieves an ultra-high peak power density of 2.54 W cm^-2,with a mass activity of 0.45 A mg _Pt^-1.The improvement in performance is attributed to the modification of alloy surface defect structures by trace amounts of W,leading to strong compressive strain and electronic state reconstruction.This weakens the adsorption of oxygen intermediates,thereby accelerating the ORR reaction kinetics.（4）By incorporating the p-block element lead（Pb）as the key third component,Pt Co Pb ternary alloy rich in dislocation defects was successfully prepared for the first time.The mechanism of Pb inducing and pinning dislocation defects in the alloy is explored.The density of dislocations in the alloy increases with the increase of Pb content,which induces varying degrees of compressive strain and enables the regulation of strain-dependent catalytic performance.The carbon-supported Pt Co Pb catalyst（D-Pt Co Pb/C）with medium dislocation density exhibites excellent ORR activity and stability in multi-level tests.Under acidic conditions,the half-wave potential reaches 0.944V,and the mass activity is as high as 0.83 A mg _Pt^-1.D-Pt Co Pb/C catalyst achieves a high power density of 2.45 W cm^-2,with a mass activity of 0.58 A mg _Pt^-1 in PEMFC test,surpassing the 2025 target value set by the U.S.Department of Energy（DOE）.The super-high catalytic activity primarily originated from the moderate dislocation density induced by the appropriate addition of Pb,which tuned the strain field.The rational adjustment of dislocation-induced compressive strain and electronic structure improved the adsorption of oxygen molecules and desorption of products,ultimately leading to enhanced ORR activity.