Construction And Activity Analysis Of Pt-based Acid Oxygen Reduction Reaction (ORR) Catalysts

For the sake of environment protection and sustainable development,a series of electrochemical reactions are essential for sustainable energy conversion and storage.For example,using renewable energy sources generated electrical energy to reduce water through hydrogen evolution reaction（HER）is a sustainable route of producing hydrogen as an energy carrier.Electrocatalytic oxygen reduction reaction（ORR）and hydrogen oxidation reaction（HOR）are critical in energy conversion systems such as proton exchange membrane fuel cells.Precious metals,such as platinum and its alloys,usually have good catalytic activities for some of these reactions.However,their large-scale commercialization is restricted by scarcity of these precious metals,exorbitant cost,and poor durability.As sustainable precious-metal-free catalysts or precious metal alloy,many types of materials with ultralow or even none noble metal contents have been explored as promising alternative electrocatalysts,including core-shell Pt alloys,transition metal compounds,carbon-based materials,single-atom catalysts and a wide range of their derivatives.Unfortunately,most of the heterogeneous catalysts synthesized through current methods to reduce precious metal loading suffer from low intrinsic activity and durability.It is therefore valuable to develop a green and cost-effective process that can synthesize heterogeneous catalysts with high electrocatalytic activities.1)Proton-exchange membrane fuel cells have been reported as one of the most promising substitutes for fossil fuel and sluggish kinetics of oxygen reduction reaction（ORR）on the cathode still remain the main bottleneck for commercialization.We report a size-controllable synthesis of monodisperse Pt P₂nanoparticles（NPs）on nitrogen-and phosphorus-doped carbon（NPC）via a combined templates and pyrolysis methods,which demonstrates an initial ORR mass and specific activity of 0.365A?mg Pt^-1 and0.86 m A cm^-2 at 0.9?V in 0.1M HCl O₄ solution.The activity increases to 0.685 A?mg Pt^-1and 1.35 m A/cm²after 3000 potential cycles and is stable within further 20,000 cycles,exhibiting an obvious improvement over those of commercial Pt benchmark(0.24m A/cm²and 0.089 m A/mg _Pt at 0.9 V).The surface of synthesized Pt P₂NPs was converted to Pt during the circulation of ORR tests,thus forming core/shell Pt/Pt P₂ NPs with a thin（≈0.5 nm）Pt shell.Facilitated ORR activity of Pt P₂@NPC cross incipient circles of stability test is due to the formation of Pt shell and consequential geometric and strain effects of Pt skin,contributing to both robustness and catalytic efficiency of the catalysts.Meanwhile,four-electron pathway towards ORR also indicates the high selectivity of our catalyst.Combined computational analysis indicates that the core-shell structure intensifies ORR activity by a more feasible rate-determining step and lower d-band-center value.2)Nanoparticles of Ni（OH）₂ surrounded with ultra-low Pt content and supported on functionalized carbon were prepared by a scalable synthesis method and investigated as electrocatalysts for the oxygen reduction reaction（ORR）in acidic media.The effect of altering the Pt surface composition on the Ni（OH）₂ nanoparticle core was investigated as a route to simultaneously increase the ORR activity and stability.Modifying the Pt surface composition resulted in both structural and electronic changes.Decreasing the Pt surface composition resulted in stronger Pt-Pt compressive strain and decrease in the occupancy of d-band vacancies per atom.The correlation of strain and d-vacancies with ORR activity and stability showed a Volcano-type tendency,with the 6wt%Pt sample showing the highest activity and stability.The electrochemical results obtained using rotating disk electrode（RDE）tests showed an enhancement of about six times higher surface and mass-normalized activity as well as improved durability compared to commercial Pt/C.These results show that new electrocatalysts with higher activity and stability can be obtained through precise control of the atomic-level catalyst structure.