Enhancement Of Pt/C Electrocatalysts Durability In Pem Fuel Cell Through Covering Thin Layer Of Oxide On The Surface Of Pt Nanoparticles

Proton exchange membrane fuel cells(PEMFCs),with the advantages of high energy conversion efficiency and environmental friendliness,have broad application prospects in the military and civilian fields such as transportation,emergency / backup power,aviation / aerospace,and underwater submarines.However,it still faces severe challenges of high cost and insufficient durability in front of large-scale commercial application.A large number of studies have shown that the degradation of catalyst performance caused by the corrosion of support and the agglomeration / migration / dissolution of Pt nanoparticles in carbon-supported platinum-based catalyst is one of the most important reasons for the insufficient durability of fuel cell.Although a lot of research work has been carried out at home and abroad,the problem has not been solved well.In this paper,we attempted to use a thin oxide layer to coat Pt/C catalyst to improve the stability of fuel cell catalyst.A thin layer of multi-component oxides covering the catalyst surface greatly improves the durability of catalyst and membrane-electrode-assembly(MEA).The main research works are described as bellow:Firstly,the thin layer of silica coating on the surface of Pt/C catalyst particles was realized by using organosilicate as precursors,and through a controlling hydrolysis-high-temperature annealing process.It is revealed by TEM images that the thin layer of silica not only covered on the surface of the carbon carrier,but also covered on the surface of Pt nanoparticles with a thickness of about 2-3nm.To evaluate the influence of coating treatment on the activity and durability of the catalyst,the membrane-electrode-assemblies were prepared with silica coated and uncoated Pt/C catalyst as cathode,and assessed with an accelerated stress test(AST)method suggested by the U.S.department of energy.It is found that that the activity of catalyst remained basically unchanged and the durability of catalyst was greatly improved after coating with silica thin layer.After 10,000 cycles of AST,the current density of the membraneelectrode-assembly with silica thin layer coated Pt/C as cathode was reduced by only 4%(at 0.6V),and the decay of the electrochemical activity specific surface area(ECSA)is by only 45%;compared with 27% and 80% for the MEA with uncoated Pt/C catalyst as cathode.Furthermore,the influence of coating conditions on the performance of the catalyst was studied,and it was found that the optimal coating amount of silica is 10 wt%,and the optimal annealing temperature is around 700?.Due to the precise anchoring of silicon oxide on platinum particles,the migration,agglomeration of Pt nanoparticles and the migration of dissolved Pt ions are effectively suppressed,thereby slowing down the growth of platinum particles and the attenuation of performance,and finally resulting to the greatly improved stability of the catalyst.Further,XPS analysis revealed that the binding energy of platinum increased after the thin coating of silicon oxide,indicating the existence of a possible interaction between platinum and the coated silicon oxide.Secondly,we further investigated the effect of co-coating of PTFE on the silica coated Pt/C catalyst,it is found that the addition of PTFE brings no negative effect towards the durability of the catalyst,but significantly improves the water management ability of the catalyst layer and MEA.It is shown by TEM images that a thin hybride oxide layer of 2-3nm was formed on the surface of platinum nanoparticles,and no obvious particles growing can be observed after annealed at 700 ?,indicating that a F doped silica thin layer is well coated on the surface of Pt nanoparticle.The accelerated stress test results indicate that the further addition of PTFE can still maintain the catalyst with better activity and durability.Furthermore,we investigated the effect of humidity on the performance of the catalyst,and measured the contact angle of the membrane electrode.It is found that the MEAs with F-doped silica coated Pt/C catalyst as cathode exhibited much better voltage stability at high current discharging region,and the larger contacting angle compared with the MEA with uncoated Pt/C catalyst as cathode,indicating that the further addition of PTFE is conducive to the water management of the catalytic layer,making it more adaptable to the actual working conditions of humidifying conditions.