| Proton exchange membrane fuel cells（PEMFCs）are considered as a clean power source for portable power mobile and stationary power plants,thus having a great potential to solve the increasingly serious environmental and global energy shortage issues.But as a key component of PEMFCs,the catalyst,such as the commercial Pt-based catalysts（Pt/C）,has low stability under the harsh operating environment,because of the electrochemical oxidation of the carbon support leading to the motion,detachment and aggregation of Pt nanoparticles,or partially due to the dissolution of Pt catalyst.As a result,the expected life of the Pt catalyst can not be reached,and then the commercialization of PEMFCs is hindered.Therefore,how to improve the life of the catalyst has become a serious problem.This paper aims at the stability of Pt metal nanoparticles by confinement and anchoring effect which can limit Pt nanoparticles from motion,agglomeration or coalescence etc.,therefore improving the stability of fuel cell catalyst.The research contents are listed as follows:1.Adopting carbon nanospheres to confine Pt nanoparticles by inhibiting Pt nanoparticles from arbitrary movement.Meanwhile,a highly stable gC3N4 is adopted to act as the support.Firstly,Pt nanoparticles are prepared by colloid method,followed by introducing gC3N4 to support Pt nanoparticles,and then carbon nanospheres（30 nm）are added to fabricate the Pt/gC3N4-C catalyst.Electrochemical tests show Pt-gC3N4/C owns a greatly enhanced electrochemical stability compared with commercial Pt/C:after 6,000 cycles of accelerated potential scanning,the retention rate of the electrochemical surface area（ECSA）for Pt-gC3N4/C remains as high as 85.0%(initial value is 61.4 m2 g-1),and the mass activity retention rate is78.7%（Pt/C is 32.3%）.At the same time,the Pt-gC3N4/C catalyst also presents superior activity for methanol oxidation reaction（MOR）compared with Pt/C catalyst.2.An ultra-thin carbon layer（UTCL）is grown on Pt nanoparticles surface using soluble starch,which is as thin as 0.58 nm,approaching the molecular level.The UTCL anchoring Pt nanoparticle catalyst（Pt-UTCL/C）demonstrates a comparable catalytic activity to commercial Pt/C,and most importantly,excellent electrochemical stability:After 10,000 cycles of accelerated potential scanning,the ECSA of Pt-UTCL/C exhibits a retention rate up to 68.4%（Pt/C is 45.2%）,the mass activity shows a value of 60.3%（Pt/C is 31.1%）.The much enhanced stability can be attributed to the UTCL which can firmly anchor Pt nanoparticles on the carbon support,thereby preventing Pt nanoparticles from migration,detachment from the support or coalescence,and may also alleviate the Ostwald ripening process.3.In order to overcome the accumulation of graphene layers,which inhibits mass transfer,pore structure is introduced into the GNS,followed by loading of Pt nanoparticles,and then a high ORR activity catalyst is obtained.The ORR activity is1.5-folds than that of pure GNS supported Pt catalyst.Typically,the oxygen-containing functional groups on GNS surface can effectively adsorbing and anchoring Pt nanoparticles,however,too much oxygen-containing groups can reduce the stability of GNS.Therefore,a H2 heating treatment is introduced to reduce a certain content of oxygen-containing groups on the porous GNS,reaching a balance between the anchoring effect of oxygen-containing groups and the stability of the GNS.The resulting Pt/rPGO-H2 catalyst obtains1.9 times mass activity than the non-treatment one（Pt/rGO）,and meanwhile,the stability is also improved.After a3,000 cycles of accelerated potential scanning,Pt/rPGO-H2 has an ECSA retention rate of 40.7%,higher than Pt/rPGO（19.7%）as well as Pt/rGO catalyst（23.4%）.Thus,the H2 heat treatment porous graphene supported Pt catalyst（Pt/rPGO-H2）owns excellent ORR activity and electrochemical stability.4.Hydrophilic polymer（Nafion）and TiO2 ceramic nano-flakes are used to synergistically stabilize Pt nanoparticles on carbon support,hindering Pt nanoparticles from motion in three dimensions of the space（x,y,z-axis direction）.The stability of the obtained catalyst（Pt-PFSA-TiO2/GNS）is significantly higher han the catalyst solely introducing Nafion（Pt-PFSA/GNS）or Ti O2（Pt-TiO2/GNS）.After6,000 cycles of accelerated potential scanning,the ECSA retention rate is 68.1%,much higher than that of Pt-PFSA/GNS（54.0%）and Pt-TiO2/GNS（50.1%）,and far higher than the Pt/C（39.5%）.The sequence of stability for ORR activity（mass activity retention）is:Pt-PFSA-TiO2/GNS（69.5%）>Pt-PFSA/GNS（51.0%）>Pt-TiO2/GNS（43.2%）>Pt/C（33.9%）.The main reason for the improved stability is that the Nafion polymer can anchor the Pt nanoparticles on the support surface,and at the same time,the TiO2 ceramic nano-flakes distributed around the Pt nanoparticle can inhibit the Pt nanoparticles from movement on the support surface,thereby the Pt nanoparticles are hindered in the three dimensions space. |
PDF Full Text Request