Preparation And Thermal Annealing Of Platinum-Nickel Octahedral Fuel Cell Catalysts

Improving the catalytic activity and stability of the cathode oxygen reduction reaction?ORR?catalyst and reducing the Pt loading in the catalyst are the keys to the commercialization of proton exchange membrane fuel cells.Platinum-nickel?Pt-Ni?octahedral nanoparticles?NPs?with high catalytic active?111?facets are considered to be one of the most promising ORR catalysts.But the prepared Pt-Ni octahedral NPs usually exhibit component segregation,resulting in low catalytic activity and stability.This component segregation is closely related to the growth mechanism of NPs.This paper focuses on the preparation process of Pt-Ni octahedral NPs and the thermal annealing process under different atmospheres.We prepared Pt-Ni octahedral NPs using solvothermal method and organic liquid phase method,and controlled the surface composition of the NPs by thermal annealing to optimize their catalytic performance.The specific work is divided into the following two parts:We studied the effect of benzoic acid?BA?on the growth mechanism and alloy composition of Pt-Ni octahedral NPs prepared by solvothermal method using N,N-dimethylformamide.The results showed that BA had a minor role in selective protecting of the?111?facets,as well as the conventional effect in inhibiting the growth and hence decreased the particle size.In addition,BA slowed down the reduction/deposition rate of Ni by forming a new complex compound with Ni²⁺,leading to a phase separation?i.e.,the coexistence of Pt-rich alloy NPs and pure Ni NPs?in the final product and thereby the lower ORR electrocatalytic activity and stability.This study solved the problem of the contradiction in the literature whether BA should be used as surfactant in preparation of Pt-Ni octahedral NPs.We also systematically studied the effects of thermal annealing at different atmospheres?hydrogen and ammonia?and different temperatures?200?,300?and400??on the properties of Pt-Ni octahedral catalysts prepared by different methods?solvothermal method and organic liquid phase method?.We pointed out that the ammonia atmosphere thermal annealing at 300?was an effective method to improve the catalytic activity of Pt-Ni octahedral NPs.For the Pt-Ni octahedral NPs prepared by organic liquid phase method,the surface Pt content and alloying degree of NPs increased as well as the ORR catalytic stability after annealing at 400?compared with that at300?.Compared with the hydrogen atmosphere thermal annealing at 300?,the surface Pt content of NPs increased from 36.7%to 38.0%after annealing at 400?,and the decrease of catalytic activity decreased from 49.9%to 7.6%after 8000 CV cycles.In addition,compared with the traditional hydrogen atmosphere thermal annealing,the ammonia atmosphere thermal annealing at 400?not only maintained the octahedral morphology of NPs,but also increased the alloying degree of NPs,forming the Pt-Ni alloy octahedral NPs with richer surface Pt content?49.6%?,thus the catalytic activity and stability were significantly improved?the catalytic activity decreased by only 5.8%after 8000 CV cycles?.