| Proton exchange membrane fuel cells (PEMFCs) have been acknowledged as one of themost promising alternative power sources due to their advantages, such as high power density,zero or low emission and quick startup at low temperature etc. As the key component ofPEMFC, membrane electrode assembly (MEA) has a great influence on both performanceand cost of fuel cell. At present, state-of-art carbon supported platinum is still the widely usedelectrocatalyst in MEA, which accounts for a large portion of PEMFC cost. To realize thecommercialization of PEMFC, lowering the platinum loading without loss of performance ofmembrane electrode assemblies has been one of the hottest topics in fuel cell field. In thisthesis, several novel techniques has been explored to be used for the preparation of MEAswith low Pt loading, and the performance of these MEAs, as well as the effect of preparationparameters on the performance have been extensively investigated.Firstly, we prepared a high performance MEA with low Pt loading by using an atomiclayer deposition (ALD) technique. A cell performance of637mA/cm~2at0.7V was achievedwhen the platinum loading was lowered to0.26mg/cm~2. The morphology and distribution ofPt nanoparticles on the surface of the ALD-electrode was observed using a high-resolutiontransmission electron microscope (HR-TEM). The results revealed that although the activecomponent Pt on the ALD-electrode had a big particle size of8-10nm due to the limit ofALD technique, the electrode still showed superior activity to the electrode prepared withcommercial catalyst and conventional preparation method; The mass activity could be high upto3.34kW/gPt, which is1.76times higher than that of later. This improved mass activity canbe attributed to the well-dispersed Pt particles on the outer surface of the ALD-electrode; incontrast, most of the Pt nanoparticles in the electrode prepared with commercial catalyst andconventional preparation method were dispersed on the interior surface of the electrode,resulting in low Pt utilization.Secondly, a high performance low platinum loadings MEA with carbon nanotubes(CNTs) instead of XC-72R was prepared by using ALD technique. It is well recognized thatCNTs has much better anti-corrosion properties than carbon black, using CNT as support canprevent the corrosion of support and the migration of platinum, resulting better performanceand durability. TEM images showed the Pt nanopaticles were highly dispersed on the CNTsbased ALD-electrode and had a particle size range of3-4nm, rather than8-10nm of theXC-72R based ALD-electrode. As the platinum loading was0.26mg/cm~2, the current density of the CNTs based ALD-MEA can reach as high as667mA/cm~2at0.7V of cell potential. Themass activity can reach4.80kW/gPt. In addition, this MEA showed excellent durability, after100h long term testing, no obvious performance change can be observed. The voltage of theCNTs based ALD-MEA reached0.65V, higher than that of XC-72R based ALD-MEA, and9.5%voltage loss compared with16.2%voltage loss of conventional MEA, confirming thehigh durability in ALD-MEA, and the superiority of CNTs substrate; This improvedperformance can be attributed to the higher order degree and better oxidation resistance ofCNTs. EIS and cyclic voltammetry (CV) test also reveal the good electrochemical stability ofCNTs based ALD-MEA.To further investigate the effect of substrates supports on the performance of ALD-MEA,we prepare another ALD-MEA with by depositing Pt catalyst on the substrate layer of XC-72,It is found that the XC-72carbon black based MEA showed better performance comparedwith XC-72R and CNTs based MEAs. TEM images showed the Pt nanopaticles were highlydispersed on the XC-72based ALD-electrode and had a particle size range of3-4nm. Themass activity reached4.80kW/gPt with Pt loading of0.18mg/cm~2, which is2.53times higherthan that of the MEA prepared with commercial catalyst and conventional method, and,1.43times and1.63times higher than those of XC-72R based ALD-MEA and CNT basedALD-MEA, respectively. In100h of durability testing, the XC-72based ALD-MEAexhibited excellent durability,1.8%voltage loss when the MEA was discharged at a currentdensity of400mA/cm~2compared with7.5%,16.2%and9.5%voltage loss of conventionalMEA, XC-72R based ALD-MEA and CNT based ALD-MEA, respectively. Furthermore, thegood electrochemical stability is also confirmed by CV testing results. This article was not yetgiven a reasonable explanation of why XC-72R based ALD-MEA had better performance andfurther exploration and research were expected.Another new technique we used for the preparation of MEA is pulse electrodeposition(PED) method; the MEA was prepared by electrodepositing Pt on the substrate prepared withRu/C catalyst, to form a Ru@Pt/C catalyst on the surface of the electrode. The effects of ratioof Ru/C to Nafion, deposition current density, and deposited Pt loading on the cellperformance were investigated. The optimal ratio of Ru/C to Nafion is ca.2:1, optimal pulsecurrent density is0.5mA/cm~2. The MEA prepared at optimal conditions and with Pt loadingof0.045mg/cm~2shows a performance of336mA/cm~2at0.7V, with the maximum outputpower of0.56W/cm~2in the H2/air fuel cell; The conventional MEA had a maximum outputpower of0.40W/cm~2with the same Pt loading. The particle size of Ru@Pt nanoparticle is~4.5nm by the TEM results. Microwave synthesis technique has also been used for the preparation of MEA with lowPt loading. TEM images showed the Pt particles were highly dispersed in the electrode andthe particle size is in the range of2-3nm. The effects of loading amounts of carbon black andTeflon in the substrate layer on the cell performance were investigated. The optimal carbonblack loading is0.37mg/cm~2, and optimal Teflon loading is ca.5wt.%. The MEA preparedat optimal conditions and with Pt loading of0.12mg/cm~2at anode was prepared, in H2/O2single cell, it achieved current density of538mA/cm~2at0.7V with maximum power densityof0.89W/cm~2. Moreover, EIS and CV test provided strong clues for the goodelectrochemical stability of this low Pt loading MEA.In summary, three types of new techniques has been explored to use for the preparationof low Pt loading MEA, all of them showed excellent activity and good stability, as well asgood durability. Especially, ALD and PED techniques may be the promising techniques forthe preparation of MEAs for PEM fuel cell applications. |
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