Research Of Ultra-Low PT Loading Ordered Membrane Electrodes For Proton Exchange Membrane Fuel Cells

H₂-powered proton exchange membrane fuel cells（PEMFC）are one of the most promising power sources for replacing conventional internal combustion engines in transportation applications because of their high energy conversion efficiency,zero emissions and no pollution.However,the large-scale application of PEMFC still faces many challenges,especially in terms of the high cost of Pt-based electrocatalysts and insufficient durability.Currently,commercially available catalysts have Pt loadings of 0.2-0.3 mg cm^-2,which is far from the fuel cell sustainability goal(0.1 mg cm^-2,1 W cm^-2).When the Pt loading of commercial Pt/C powder catalysts is less than 0.1 mg cm^-2,they face problems such as increased oxygen transport resistance,low Pt utilization and poor stability,which are caused by the disordered stacking of powder-based Pt/C catalysts in the membrane electrode assembly（MEA）,and seriously limit the development of ultra-low Pt fuel cells.Therefore,the research and development of a low Pt loading MEA without sacrificing performance is of great importance to effectively reduce the fuel cell cost and promote the commercialization of PEMFC.In this paper,with the objective of achieving ultra-low Pt loading in proton exchange membrane fuel cells,we use plasma-enhanced chemical vapor deposition（PECVD）technology to prepare vertical arrays of carbon nanotubes（VACNTs）carriers instead of conventional powder-based carbon carriers,and via electron beam evaporation and atomic layer deposition techniques to develop highly active and stable Pt catalysts,so as to construct ordered membrane electrodes with ultra-low Pt loading.The main elements are as follows:（1）Precise growth of verticaly aligned carbon nanotube by plama enhanced vapor depostion.Based on each influencing parameter during the growth of VACNTs,we systematically investigated the effects of plasma pretreatment,growth temperature,growth time,growth power,catalyst thickness,and precursor gas flow ratio on the morphology of VACNTs.The results indicate that VACNTs for fuel cell catalyst carriers must be properly plasma pretreated before growth.During the growth of VACNTs,with the increase of growth temperature,growth time and growth power,the height of carbon nanotube array will increase.In addition,the catalyst thickness used to catalyze the formation of carbon nanotube arrays was too thin or too thick,which resulted in sparse carbon nanotube arrays with poor vertical orientation.Interestingly,different vertical carbon nanoarray structures are formed when the precursor gas flow ratio is changed.（2）Fabrication of ultra-low Pt/VACNTs order-structured electrode via electron beam evaporation of Pt.The effect of the ratio of Pt catalyst to the length of VACNTs on cell performance was investigated for the first time.The results show that the cell has the best performance when the length of VACNTs is 4.6μm and the Pt deposition depth is about 600 nm.Peak power densities of 1.61 W cm^-2（H₂/O₂,150 k Pa）and 0.79 W cm^-2（H₂/Air,150 k Pa）were achieved when the Pt loading was 50μg cm^-2,exceeding most of the previously reported performance using other strategies.Even at Pt loadings down to 30μg cm^-2(1.36 W cm^-2),the performance is superior to that of commercial Pt/C at 100μg cm^-2(1.24 W cm^-2)with higher stability.（3）Fabrication of ultra-low Pt/VACNTs order-structured electrode via atomic layer deposition of Pt.The difference in performance between atomic layer deposition and electron beam evaporation deposition on VACNTs was investigated comparatively.The results showed that Pt nanoparticles deposited by ALD achieved a peak power density of 898 m W cm^-2（H₂/Air,150 k Pa）at 38μg cm^-2 Pt loading due to high dispersion,which was much higher than the performance at 50μg cm^-2 Pt loading by electron beam evaporation(789 m W cm^-2).Also,the stability is higher due to the chemical interaction between the Pt particles and the carrier.Since ALD is not affected by a physical barrier,the performance becomes better with increasing Pt loading.At a Pt loading of 80μg cm^-2,the cell has a maximum output power of 1014 W cm^-2 under H₂/Air test conditions.This work provides a viable approach for developing PEMFCs with ultra-low Pt loadings with high power output and high durability at the same time.