Multicomponent Synergistic Mechanism In Low-content Platinum Based Oxygen Reduction Catalysts For PEMFC

Proton exchange membrane fuel cells(PEMFCs)exhibit the main triple reaction characteristics of harsh corrosiveness,high discharge current density and fast mass transfer in the service state due to the acidic electrochemical environment.In order to meet the requirements of high performance,the loading of noble metals contained in membrane electrode assemblies is generally high.The contradiction between the high cost and the limited reserves of precious metals has led researchers to pay special attention to the development of high-performance with low-platinum loading electrodes.Aiming at the key practical problems such as low activity and poor durability of low platinum catalysts in the low current density domain(LCD),and slow mass transfer in the high current density domain(HCD),this research has proposed and designed scheme for effectively matching synergistic catalysis and an implementation route for the controlled preparation of catalysts.A multi-component synergistic low platinum oxygen reduction(ORR)catalytic system has been mainly developed.More importantly,we adopt a technical route combining theoretical guidance-simulation predictionexperimental detection-quantitative calculation,relying on advanced characterization methods to deeply understand the catalytic mechanism,and clarify the synergistic mechanism among multiple components.More specific research contents are summarized as follows:1.The mechanism of rare earth metals doping to enhance the activity and stability of Pt-based alloy catalysts.(1)Ultra-stable Pt5La intermetallic compound towards highly efficient oxygen reduction reaction.Designing feasible electrocatalysts towards oxygen reduction reaction(ORR)requires advancement in both activity and stability,where attaining high stability is of extreme importance as the catalysts are expected to work efficiently under frequent start-up/shut down circumstances for at least several thousand hours.Alloying platinum with early transition metals(i.e.,Pt-La alloy)is revealed as efficient catalysts construction strategy to potentially satisfying these demands.Here we report a Pt5La intermetallic compound synthesized by a novel and facile strategy.Due to the strong electronic interactions between Pt and La,the resultant Pt5La alloy catalyst exhibits enhanced activity with half wave of 0.92 V and mass activity of 0.49 A mgPt-1,which strictly follows the 4e-transfer pathway.More importantly,the catalyst performs superior stability during 30,000 cycles of accelerated stressed test(AST)with mass activity retention of 93.9%.This study provides new opportunities for future applications of Pt-rare earth metal alloy with excellent electrocatalytic properties.(2)Intrinsic Spin Shielding Effect in Platinum-Rare Earth Alloy Boosts Oxygen Reduction Activity.Oxygen reduction reaction involves multi-step proton-coupled electron process accompanied with the apodictic spin configuration conversion.Accurately understanding the role of the spin configuration of metal on the adsorption and desorption of the oxygen intermediate species has pivotal significance for the design of efficient catalysts for ORR.Herein we introduce the rare earth-platinum alloy catalyst,i.e.,Pt2Gd,which exhibits the unique intrinsic spin reconfiguration via the interactions between Gd-4f orbitals and Pt-5d orbitals,to reveal the role of spin configuration.Through modification of spin symmetry and the electronic structures,the adsorption and desorption of oxygen species has been optimized towards highly efficient ORR.The Pt2Gd alloy has exhibited a half-wave potential of 0.95 V and superior mass activity of 1.5 A·mgPt-1 in 0.1 M HClO4 electrolyte,as well as credible durability compared with conventional Pt/C catalysts.Impressively,theoretical calculations have proved that the spin shielding effect of Gd pairs increases the spin symmetry of Pt-5d orbitals and the adsorption preferences with spin-polarized intermediate to facilitate the ORR.This work explains the modulations of local highspin 4f orbital electrons from rare earth metal on the intrinsic spin state of Pt to benefit the ORR process,supplying a fundamental contribution to the expansion of new descriptors that control ORR activity.(3)Rare earth-oxygen ion pair promotes the electrocatalytic activity and stability of Pt-based alloys.Alloying Pt with the second metal can effectively reduce the usage of Pt metal,but the main obstacle with alloying with early transition metals is poor stability.In this work,rare earth metals were introduced into the lattice of Pt3M alloys,which effectively changed the oxygen adsorption behavior of Pt3M alloys.In the acidic system,the Nd-Pt3Ni catalyst achieves a half-wave potential of 0.97 V and a power density of 2.5 W cm-2(the highest level reported at present).More importantly,the catalyst withstood 200,000 cycles of AST test without performance degradation.This doping strategy was extended and validated in 3d transition metal alloys(Pt3M,M=Sc,Ti,V,Cr,Mn,Fe,Co).2.Stabilized Pt Cluster Based Catalysts Used as Low-loading Cathode in PEMFC.The synergistic effect facilitates the high-efficiency expression of ultra-low platinum PEMFC on the cluster catalyst.Lowering the Pt catalyst loading in fuel cell cathode without sacrificing performance remains topical.However,achieving such goal is highly challenging,since lowering the Pt loading not only reduces the overall kinetics of oxygen reduction reaction(ORR),but also causes serious mass transfer issue in high current density domain(HCD).Herein we overcome this difficulty by obtaining a highly active and stable Pt cluster based catalyst,where the decrease in loading is completely compensated by the extraordinarily high electrochemical specific area and high dispersion of the platinum clusters.The Pt clusters,with average size of 1.3 ± 0.4 nm and atomic utilization rate up to 32.81%,are highly stabilized due to the strong anchoring effect of the N,P doped carbon nanosheets.The final Pt-9.3@NPC catalyst outcompetes commercial Pt/C catalyst in terms of activity and stability during potential cycling.Even further,the cell assembled by Pt-9.3@NPC as cathode(0.05 mgPt cm-2)conveys much higher performance(1071 mW cm-2)in H2/Air mode than the counterpart commercial catalysts(853 mW cm-2,0.1mgPt cm-2)and much lower voltage loss at HCD,clearly evidencing the success in surmounting the mass transfer problem.