Application Of Nanoconfinement Effect In Nanoporous Metal Electrode In Hydrogen Fuel Cell

The overuse of fossil energy sources such as coal,oil and natural gas has led to the global climate crisis and energy crisis.Fuel cell is an efficient power generation device with high energy conversion efficiency as there is no combustion process in the power generation process,and thus it is not limited by the Carnot cycle.The presence of ionomer in the fuel cell makes Pt toxic,which leads to low performance,poor stability and high resistance to oxygen transfer.The design of ionomer-free ultra-thin catalyst layer can meet the high performance of fuel cell and reduce the Pt loading at the same time.Meanwhile,the nanoconfinement effect can increase the frequency of reactant molecules colliding with the charged surface,thus increasing the reaction rate;it can also affect the electron interaction between the catalyst and the inner wall of the nano-channel,which can hinder the migration and sintering of the catalyst;meanwhile,the modulation of the catalytic reaction by the sub-surface structure and the"interfacial binding"effect of the catalyst can make the ions transport in the bulk.The low-cost and flexible nanoporous gold prepared by dealloying method shows unique structural advantages,which is especially suitable for the construction of ultra-thin catalytic layers for fuel cell membrane electrodes.In this thesis,the effect of pore size on the performance and stability of nanoporous metal electrodes in the nanoconfinement effect is systematically investigated using a combination of experimental means and theoretical simulations.In addition to this,the plasmonic conduction mechanism in the ultrathin catalytic layer without ionomer addition is investigated in depth as follows.(1)First,the effect of pore size on the performance of nanoporous metal electrodes for fuel cells in the nanoconfinement effect was investigated.It was shown that the pore size distribution of NPG-Pt became more and more dispersed with increasing pore size,corresponding to an increase and then a decrease in O₂ permeability.Subsequently,fuel cell single cell tests and electrochemical impedance tests were performed on NPG-Pt with different pore sizes,both of which showed that the performance of each component improved and then decreased with increasing pore size of NPG-Pt.The most important reason for the improved performance is the modulation of the pore gap,which promotes the nanoconfinement effect,thus increasing the reaction rate in the activation polarization zone and improving the problem of water flooding in the mass transfer polarization zone.In which,NPG-Pt₆₅ has the best performance with the MA of 1.21A·mg_Pt^-1,the power density of 1.33 W·cm^-2 at 0.6 V and the peak power density of 2.48W·cm^-2 at 1.5 bar back pressure.It is derived from finite element analysis and fuel cell simulation that with the gradual increase of pore size,the fluid permeability gradually increases with the gradual increase of pore size,while the catalyst layer with large permeability improves the fuel cell performance more significantly.This experiment demonstrates that the nanoconfinement effect requires suitable pore size and pore gap distribution,and expands the application of nanoporous metal electrodes for PEMFCs catalysts.(2)Secondly,a combination of Ex-situ and IL-TEM was used to explore the ECSA and ORR changes and morphological structural changes of NPG-Pt under different stability procedures in a half-cell.The ECSA of NPG-Pt decreased by 23.1%after30,000 turns of square-wave cycling;TEM images indicated that Oswald ripening of Pt nanowhisker on the NPG surface occurred.After 5,000 turns of delta wave cycle,ECSA increased and then decreased,and the ORR performance change was consistent with the ECSA change pattern;transmission electron microscope images showed that Pt nanowhisker on the surface of NPG-Pt migrated and exfoliated,and its own pore structure did not change,and some areas of carrier NPG were corroded and formed hollow NP-Pt,so the increase of active sites led to ECSA increase.It indicates that the nanoconfinement effect can effectively hinder the migration and sintering of the catalyst.After that,an IL-TEM system was built to further demonstrate the structural stability of the nanoporous metal electrode at the atomic level.Finally,fuel cell stability tests were performed for NPG-Pt₆₅and Pt/C.At 0.5 bar back pressure,ECSA decayed 27.0%and MA decayed 32.9%during the cycle,and the voltage decreased 70.9 m V at a current density of 1.5 A·cm^-2.This chapter will lay the foundation for closing the testing gap between RDE and MEA.(3)Finally,the bulk proton conduction mechanism in PEMFC without ionomer catalytic layer is reported for the first time.Using first-principles calculations,a new fuel cell proton conduction mechanism is proposed:H⁺can conduct in the bulk of the continuous Pt ligaments of NPG-Pt.Electrochemical impedance showed that the proton conduction resistance of the NPG-Pt catalytic layer increased with the increase of relative humidity and decreased with the increase of Pt loading.This differs from the proton-conducting properties of conventional ionomer-doped catalytic layers.It shows that the NPG-Pt bicontinuous channel produces the nanoconfinement effect,which enables H⁺to conduct proton conduction in the bulk.Finally,the electron microscope characterization of NPG-Pt before and after electrochemical treatment shows that the bulk H in the Pt lattice can be observed,which confirms the conjecture in the experiment.In this chapter,it is confirmed that the nanoconfinement effect promotes the proton conduction of ionomer-free PEMFC in the bulk of nanoporous metal electrodes,which expands the application of nanoconfinement effect in fuel cells,and provides new insights into the theory of proton conduction.