Construction And Performance Of Electrocatalysts And Microporous Layers For Fuel Cells

Proton exchange membrane fuel cell（PEMFC）is supposed to be one of the most promising sustainable power conversion devices for its low operation temperature,high power density,easy scalability and zero emission.However,membrane electrode assembly（MEA）,which acts as the core component of PEMFC,still have some problematic issues need to be overcome.For catalyst,the oxygen reduction reaction（ORR）of cathode is kinetically sluggish,its expensive price and poor stability limit the extensive commercialization of PEMFC.What’s more,the deficient water management during PEMFC operation has a negative impact on the performance of battery as well.As one of another type of fuel cell,anion exchange membrane fuel cell（AEMFC）,also faces challenges in catalyst optimization.Therefore,it’s important to develop low-cost catalysts with high efficiency,stability through catalytic metal selecting and support designing,and present effective microporous layer（MPL）materials and structures to optimize PEMFC water management.In this work,a series of carbon nanofibers（CNFs）were developed as Pt supports,or combined with transition metals as ORR catalysts,or as microporous layer material by electrospinning.The main research contents and conclusions in the work are as following aspects:（1）Porous nanofiber（PCNF）was mixed together with carbon black（CB）as a hybrid support for Pt,and Pt nanoparticles were deposited on the hybrid support as ORR catalyst.PCNF was prepared through the electrospinning,pre-oxidation and carbonization process.And Pt/（PCNF+CB）electrocatalysts were fabricated by the ethylene glycol reduction method based on the hybrid support of PCNF and CB.The results explained that Pt/（PCNF+CB）showed a higher electrochemical activity area due to the pore structure of PCNF.The hybrid network could prevent both PCNF and CB from agglomerating,the mass ratio of PCNF to CB is 3:2 may constitute a superior 3D porous electron network to provide more accessible catalytic sites to improve the electron transfer rate and electrocatalytic activity.Further more,the half-wave potential of Pt/（PCNF+CB）-40%only decreased by 4 mV,which was 86 mV lower than that of Pt/C（90 mV）after 2000 cycles,it revealed better catalytic durability than Pt/C because of the greater interaction between Pt particles and PCNFs as well as the high graphitization of PCNFs,which is conducive to enhancing the resistance of the support to corrosion.（2）TiO₂ doped CNF support was prepared by electrospinning,and Pt nanoparticles were deposited on it as ORR catalyst.Moreover,a hollow CNF contains TiO₂ was developed as hydrophilic MPL to construct a novel hydrophilic and hydrophobic double MPL with CNF for PEMFC.The TiO₂ doped carbon nanofibers（TCNF）with different mass ratio were prepared by electrospinning and following thermal treatment as catalyst support.And Pt nanoparticles were successfully deposited on the support to be employed as ORR catalyst（Pt/TCNF）by ethylene glycol reduction method after that.The results illustrated that Pt ions adsorbed near TiO₂ nanoparticles on the surface of TCNF can be preferentially reduced and formed C-Ti-Pt triple junction structure.When the mass ratio of PAN to TBT is 1:1,Pt particles were well dispersed,and the strong metal-support interaction（SMSI）of Pt Ti alloy improved the ORR performance of catalysts.The ECSA of Pt/TCNF-1 reached 65 m² g^-1,while its half-wave potential approached to 0.731 V.In addition,TCNF-1 support combines the strong interaction between TiO₂ and Pt,high stability of TiO₂ and conductivity of CNF,which enhanced the binding force between Pt and carrier and improved the stability of Pt/TCNF-1.The half-wave potential of Pt/TCNF-1 only decreased by 19 mV,lower than that of Pt/C（90 mV）after 2000 cycles.On the other hand,nanofiber could form a 3D porous structure as MPL materials compared with CB.Based on its hydrophilic characteristic,hollow carbon nanofiber（HTCNF）,which contains TiO₂,was applied as hydrophilic MPL material after coaxial electrospinning.The double MPL constructed with hydrophilic MPL and hydrophobic MPL formed by HTCNF and CNF in different order and loading to achieve a variation of gradient properties.The study explored the relationship between the characteristic and the structure of MPL,investigated the performance of the MEA assembled by it.Attribute to its moisturizing function,the MEA with hydrophilic MPL showed a higher power density than those who only contained with hydrophobic MPL under the low humidity condition（15%RH）.The double MPL presented the highest power density and higher corresponding current density,indicates its low mass transfer polarization.It reveals that the PEM moisture capacity and discharge the generated water timely have a strong impact on improving the performance of MEA under high current density.Nevertheless,the interface of double MPL with different materials will increase the internal resistance and promote the ohmic polarization of MEA.Therefore,it is significant to optimize the interface of MEA inside.（3）A metal-heteroatom-doped porous carbon nanofiber（Co/Zn-N-CNF）was obtained as cathode ORR catalyst to meet the demand of AEMFC.A particle-embedded nanofiber precursor（Co/Zn-ZIF@PAN）based on a dual metal-organic framework（Co/Zn-ZIF）,polyacrylonitrile（PAN）,polymethyl methacrylate（PMMA）blend was fabricated by electrospinning.Then,a metal-heteroatom-doped porous carbon nanofiber（Co/Zn-N-CNF）was obtained as ORR catalyst after pre-oxidation and carbonization.The results revealed that abundant porous and defects were distributed throughout the CNF to promote the electrochemical activity.When the molar ratio of Co/Zn is 1:1,Co/Zn（1:1）-N-CNF catalyst exhibits the optimizing ORR performance.The half-wave potential of Co/Zn（1:1）-N-CNF catalyst reaches 0.830 V,which is approach to that of Pt/C（0.837V）in alkaline electrolyte.Meanwhile,the half-wave potential of Co/Zn（1:1）-N-CNF only decreased by 7 mV after 2000 ADT cycles,while Pt/C catalyst decreased by 25 mV,indicating that Co/Zn（1:1）-N-CNF possesses stronger stability.Remarkably,the Zinc-air batteries assembled by Co/Zn（1:1）-N-CNF shows higher power density(146.9 m W cm^-2)than Pt/C,imply its superior power conversion efficiency as cathode catalyst in AEMFC.