Preparation Of Pt-based Electrocatalysts And Their Oxygen Reduction Performanc

Proton exchange membrane fuel cells（PEMFCs）has been recognized as an effective energy conversion and storage technology,which can reduce dependence on fossil fuels,alleviate the energy crisis to a large extent,and promote industrial upgrading.However,owing to the bottleneck of slow kinetics of cathode oxygen reduction reaction（ORR）,it is difficult for PEMFCs to realize large-scale application.At present,carbon supported platinum（Pt/C）was considered the best catalyst choice due to its optimal adsorption free energy of oxygen-containing compounds,but it still faces problems of high cost,low activity,low stability,and susceptibility to poisoning.Therefore,the development of efficient and stable Pt based catalysts has become the key to the commercialization of PEMFCs.To solve the problems of the above Pt based catalysts,this paper focuses on the adjustment of the molecular modified surface and electronic structure.From the perspective of reducing cost,improving activity and stability,the main research work developed from the following three aspects:（1）This work introduced different thickness of nitrogen-doped carbon layer（NC）on the surface of commercial Pt/C catalyst by adjusting the feed amount of dopamine,and successfully prepared nitrogen-doped carbon layer modified Pt/C catalyst（Pt/C@NC-X,X represents the added amount of dopamine）.As compared to undecorated Pt/C catalyst,the Pt/C@NC-X catalyst exhibits a particle size is less than3.5 nm even after annealing,which is ascribed to the space-confined effect of the dopamine and Si O₂.Electrochemical testing shows that the as-obtained Pt/C@NC-X catalysts have enhanced activity and durability.Especially,the Pt/C@NC-0.06 sample with moderate thickness of NC shell shows specific activity（SA）of 0.65 m A.cm^-2,which is 2.32 times of Pt/C(0.28m A.cm^-2).Besides,the as-prepared Pt/C@NC-0.06catalyst with moderate dopamine feeding content exhibits a small ORR activity loss（10%）respect to the commercial Pt/C catalyst（35%）after 5000 CV cycles.The catalyst and activity improvement prepared in this paper are attributed to the NC layer which not only contributes to the formation of the active center and allows the reactants to enter without blocking the internal Pt catalytic active center,but also prevents the aggregation of Pt NP and protects the carbon carrier from being directly exposed to the harsh working environment,improving the chemical stability of the obtained catalyst.The perfect structure of this nitrogen doped carbon layer provides a convincing pathway for the preparation of fuel cell nanomaterials.（2）On the basis of the above study ways,this work synthesize a structurally ordered Pt Fe alloy catalyst modified with nitrogen-doped carbon coating layers（Pt Fe@NC-XPDA,X represents the added amount of dopamine）by confined space annealing strategy.The structurally ordered Pt Fe@NC-XPDA catalyst exhibits greater electrocatalytic properties respect to the pristine Pt/C catalyst.Especially,the mass activity（MA）and SA of the synthesized Pt Fe@NC-0.06PDA sample with the optimized thickness of NC shell exhibit 9.95 and 11.53 times higher than that of commercial Pt/C catalyst.In addition,after 20,000 CV cycles,the Pt Fe@NC-0.06PDA sample achieves the minimum activity loss（7%）,which is far less than the activity loss of Pt/C catalyst（35%）.The enhanced ORR activity and stability are attributed to the NC shell formed in situ during the high temperature and the formation of the structurally ordered Pt Fe phase.This work not only shows the benefits of improved stability due to doping,but also further improves the catalytic activity by combining the ordered alloy strategy,which provides an effective strategy for the development of fuel cells.（3）To simplify the synthesis steps and enhance the availability of Pt,this work designed a simple synthesis strategy based on the previous research,which is to directly convert solid Pt nanoparticles into hollow Pt Ni alloy nano cubes through eutectic salt mediated pyrolysis strategy and nano Kirkendall effect.The electrochemical test results show that the as-prepared Pt₁Ni₈@NC sample with moderate feeding contents exhibits impressive SA of 2.88 m A·cm^-2,which is 9.29 times larger than that benchmark Pt/C catalyst(0.31 m A·cm^-2).In addition,the Pt₁Ni₈@NC presents a mass activity decay only 9.9%after 30,000 cycles,while the commercial Pt/C sample exhibits a rapid decrease of 45.5%.Such improvement in ORR properties is mainly owing to the moderate surface compositions,well-designed hollow nanostructure and high surface area.Indeed,this facile surfactant-and template-free synthetic strategy in this work can possibly be extended for fabricating electrocatalysts with a similar hollow cubic structure.