Preparation Of Carbon-supported Platinum-based Nano-alloys And Their Electro-catalytic Performance For Oxygen Reduction Reaction

Oxygen reduction reaction(ORR)is an important cathodic reaction,which plays a significant role in developing new energy resources and curbing environmental pollution.However,the ORR process has slow kinetics,high overpotential and complex mechanism.In practical application,catalysts are generally needed to lower the overpotential and promote the reaction.Nowadays,the Pt/C catalyst has been applied in industrial fields.Nevertheless,because of its limited storage,the platinum element has an expensive price,which results in the high costs of fuel cells.Besides,the catalytic stability of the Pt/C catalyst is poor,leading to the unsustainable high activity.These above factors make it more difficult to achieve the commercialization of fuel cells.Therefore,developing the cathodic catalysts with low cost and high performance to replace the commercial Pt/C catalyst is of great importance to realize the commercial application of fuel cells and solve the environmental pollution as soon as possible.This topic focuses on the supported Pt-based alloys(Pt-Co and Pt-Cu),which are served as the ORR catalysts.These materials can not only decrease the costs but also improve the catalytic performance through the synergistic effect between two metals.In this study,a series of Pt-based alloys were firstly prepared by regulating the alloy compositions,structures,particle sizes and the type of carbon supports.Subsequently,these alloy catalysts were tested by various physical and chemical characterizations,which were mainly used to investigate the effects of alloy composition,structure,particle size and carbon support on the ORR performance.Finally,multiple characterization means such as X-ray Photoelectron Spectroscopy(XPS)and X-ray Absorption Spectroscopy(XAS)were used to reveal the possible mechanism for the enhanced ORR performance,which contributes to the in-depth understanding of ORR kinetics and designing new generation catalysts with low cost and high performance.The main research results are as follows.(1)Carbon-supported Pt-Co alloy nanoparticles with different compositions were successfully prepared by a modified polyol method.The optimal pH value of the reaction solution is 10.The nanoparticles with a narrow size range(2?4nm)are uniformly dispersed on carbon support surface.The electrochemical tests indicate that these catalysts exhibit a double volcano-type dependence on alloy compositions in terms of ORR activity.The Pt76Co24 catalyst displays the optimum activity,far exceeding the commercial Pt/C catalyst.XPS measurements show that the enhancement of ORR activity is mainly ascribed to the change in Pt electronic structure caused by the alloying.This study is helpful to understand deeply the relationship between alloy composition and catalytic activity.(2)A series of Pt-Cu alloy nanoparticles with similar particle sizes(2.5?3.5 nm)but different alloy compositions were fabricated by the classical ethylene glycol method.Then,the lattice contraction percentages of these alloys were figured out through XAS characterizations,followed by the transformation between alloy composition and lattice contraction percentage.The electrochemical experiments suggest that the ORR activity shows a well-defined volcano type dependent relationship toward the lattice contraction percentage.This dependent relationship can be explained by the Sabatier principle.The ORR activity reaches the highest value when the lattice contraction percentage is 3.83%.This study not only bridges the electro-catalytic activity and alloy composition at the atomic level,but also provides a reference for understanding the composition-structure-activity relationship of Pt-based alloys.(3)A novel class of Pt-Co secondary solid solution catalysts with different Co solid solubility was prepared through the thermal treatment on the primary solid solution alloys.These secondary solid solution catalysts own a special alloy structure with long-range ordered intermetallic CoPt3 as solvent and Co as solute.The electrochemical tests reveal that the activities of these secondary solid solution catalysts display a volcano-type dependence on the Co solid solubility.The catalyst with 10%Co solid solution has the highest activity.XPS characterizations show that the enhanced ORR activity of the best Co-10%catalyst origins from its unique electronic structure,which can make the catalyst balance the adsorption of active oxygen and the desorption of intermediate products in a better way.The stability tests suggest that the Co-10%catalyst is more stable than the commercial Pt/C catalyst.This is mainly because the solid solubility of Co element in CoPt3 is low.As a result,the long-range ordered structure is not drastically destroyed and the atomic binding force is still strong,which limits the loss of Co atoms during the reaction and then decreases the ORR activity loss.This study proposes a promising idea to synthesize the secondary solid solution alloy as a new catalyst by doping the transition metal into their intermetallic compound,which provides reference value for designing the efficient ORR catalysts with both excellent activity and stability.(4)A simple and effective space-confined method assisted by PDA was adopted to prepare the intermetallic CoPt3/C-S(4.06 nm)catalyst,which has the same alloy structure but smaller particle size compared with the CoPt3/C-L(5.82 nm)prepared by the direct thermal treatment.The electrochemical experiments suggest that CoPt3/C-S intermetallic catalyst with small size exhibits over 7-fold higher ORR activity and comparable stability compared to the large intermetallic nanoparticles(CoPt3/C-L).XPS and XAS results show that the superior ORR performance of the CoPt3/C-S catalyst mainly come from the abundant active sites and unsaturated coordinated bonds.This work proposes a feasible synthesis route to restrict the increase in particle size of the intermetallic nanoparticles,contributing to improving the ORR performance of ordered intermetallic catalyst.(5)The pristine graphene(G)was firstly modified by citric acid(CA),which can introduce abundant oxygenated functional groups on the graphene surface as well as keeping its high electron conductivity without destroying the original graphene structure.Subsequently,Pt-Co alloy nanoparticles were supported on the modified graphene support(CA-G)by the classical ethylene glycol method,in order to prepare the Pt-Co/CA-G catalyst with residual oxygenated functional groups on the surface.These groups can interact with the water molecules during the reaction,forming the hydrogen-bond network to provide a convenient channel for the proton transfer,finally promoting the proton transfer.The electrochemical tests indicate that the Pt-Co/CA-G catalyst exhibits excellent ORR activity and stability,which can be ascribed to its fast proton transfer ability.This work proposes not only a simple approach to modify graphene but also an effective strategy to improve the ORR performance of Pt-based catalysts.