Preparation,Structure And Properties Of Low Platinum Loading Catalyst Layer For High Oxygen Transportation In Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells?PEMFCs?are very important in new energy vehicles,and have been raised to the country's development strategy by Chinese government.However,the very scarce platinum?Pt?resources lead to high prices.Moreover,the annual Pt extraction cannot meet the demand of PEMFC vehicles.Therefore,the reduction of Pt loading is one of the key issues for the commercialization of PEMFCs.In the catalyst layer?CL?compact ionomer-film will be formed on Pt surface result from the strong adsorbability between sulfonate groups of ionomer and Pt surface.Meanwhile,a large amount of water accumulates around Pt surface to form water-film under high current density.When the Pt loading of CL is reduced to 0.1 mg_Pt/cm²,the ionomer-film and water-film will block the oxygen transport and result in a poor performance.In this work,polytetrafluoroethylene?PTFE?nanoparticles and poly diallyldimethylammonium?PDDA?were doped into CLs to reduce the compactness of ionomer-film and the oxygen transport resistance by the steric effect of the doped polymer.And continuous hydrophobic network structure was built by doped PTFE nanoparticles which could reduce the water-film and improve the oxygen transport resistance.The structure,oxygen transport resistance and the performance of the CLs doped with PTFE and PDDA were studied and the below conclusions can be obtained.1.The oxygen transport resistance and performance of catalyst coated membrane?CCM?can be improved by reducing the compactness of ionomer-film in CL,which is the most critical factor for performance improvment of low Pt loading membrane elecctrode assembly?MEA?.The negative charge from sulfonic groups of ionomer and positive charge from the quaternary ammonium?PDDA?will attract closely to each other Due to the steric effect of PDDA,the compactness of the ionomer-film reduces as well as the oxygen transport resistance of ionomer-film(R_ionomer),then the oxygen transport capability of CL improves.For the CLs with the low Pt loading of 0.07mg_Pt/cm²,R_ionomer of the CL doped with 1B30 PDDA?the mole ratio of charged functional groups in the PDDA and ionomer?decreased 50.2%than that without PDDA.And the performance increases above 200 mV under 2.7 A/cm² current density.However,the compactness of ionomer-film in CL doped with PTFE does not change,thus the R_ionomer of the CL has not been improved,and the performance improves unremarkable.2.The oxygen transport resistance and MEA performance can be improved by constructing continuous hydrophobic network structure.For 40%PTFE-doped CL,the oxygen transport resistance of Knudsen diffusion in CL(R_Knudsen)increases caused by low porosity in CL,and part of Pt surface is covered by PTFE lead to R_ionomer increase.Although these are harmful to oxygen transport in CL,the continuous hydrophobic network structure has been formed in CL,which facilitates the water management of CL and reduces the water-film around the Pt surface at high current densities.Then the oxygen transport capacity of CL increases.At 2.0 A/cm² current density,the performance of 40%PTFE-doped CL is 149 mV higher than that without PTFE.3.Optimizing the pore size distributions in CL,the oxygen transport capacity and performance of high Pt loading MEA can be improved,but cannot be improved significantly for low Pt loading MEA.For 20%PTFE-doped CL,the porosity of primary pore in CL and the proportion of pores at around 100 nm in primary pores increases,together.Which lead to R_knudsen decrease and the oxygen transport capacity of CL improve.For 0.40 mg_Pt/cm² CL,the performance is 70 mV higher than that without PTFE at 2.0 A/cm² current density.However,the R_ionomer of CL does not change,and the continuous hydrophobic network structure in CL has not formed.Therefore,the oxygen transport capacity of MEA has not been effectively improved.For 0.07 mg_Pt/cm² CL doped with 20%PTFE,the performance is only 29 mV higher than that without PTFE at 2.0 A/cm² current density.