Mesoscopic Study On Membrane Electrode Assemble Of Low Pt Loading Fuel Cell

With the rapid improvement of exhaustion of energy,and the deteriorating environmental problems have forced us to change the conversion ways of traditional fossil energy,and develop an efficient and clean energy conversion way.Proton exchange membrane fuel cells（PEMFCs）generate electricity from hydrogen,powering a range of applications while emitting nothing but water,especially application in transportation field.Therefore,the PEMFCs are regarded as an environmentally friendly alternative to internal combustion engines in the future.Nevertheless,the high cost and scarce of platinum（Pt）source prevent the widespread adoption of fuel cells.With the development of fuel cell manufacturing technology,the current Pt utilization is largely increased to a relatively high level of 0.2 g _Pt k W^-1 in PEMFC.However,according to the PGM market report from Johnson Matthey（2020）,the current Pt utilization in fuel cell is still too high to meet the need of large-scale application of automobiles,unless the Pt utilization further decrease to a ultra-low level(0.01 g _Pt/k W).Therefore,a higher Pt mass activity and a higher Pt utilization must be achieved in membrane electrode assemble（MEA）with ultra-low Pt loadings to reduce the required Pt usage.Many factors of key primary components affect the performance of MEA,for electrocatalysts,the super-high activity of the developed Pt catalyst can hardly be realized in the real fuel cell,because of lack of fundamental understandings of reaction interface structure and mass transfer properties in real cells.In the multiphase reactions of fuel cell,the mesoscale formation of phase,interface structure,and heterogeneous structure between nanoparticles and MEA plays a vital role in performance of fuel cell.Thus,the paper focuses on the construction of MEA with excellent mass transfer,exploring the mechanism of enhanced performance,and discussing the relationship between MEAs and catalysts at the mesoscale:①Based on the Mesoscale problems of the interface between ionomers and nanoparticles in the catalytic layer:Aiming at the higher proton transfer resistance,flooding,and poor stability in the porous catalyst layer,we reported a low Pt loading membrane electrode assembly（MEA）featured by three-dimensional carbon framework（3DF）with embedded Pt Zn intermetallic nanoparticles（i NPs）and vacuum aspirated Nafion ionomers.A well-established catalyst layer guaranteeing sufficient connection of the reaction species（protons,O₂ and water）to the active sites（Pt Zn i NPs）in the inner pores is difficult to be constructed.As a result,the fuel cell of MEA fabricated by using the same Pt Zn@3DF catalyst via a traditional Nafion coating method exhibits a low output power density of 510 m W cm^-2 and a serious decay up to 25%after 24 hours continuous operation.The maximum power output reaches 826 m W cm^-2 at a loading of60μg _Pt cm^-2 for the PEMFC fabricated with the as-prepared MEA with vacuum aspirated treatment（Pt Zn@3DF-E）,and a high Pt utilization of 144 mg _Pt k W^-1 is realized which is 1.6 times greater than that of commercial Pt/C catalyst.Moreover,excellent stability is achieved at low Pt loading(60μg _Ptcm^-2)with no decay during 300hours continuous operation.②Based on the mesoscale problem of nanoparticle dispersion and mass transfer in catalytic layer:As for the conventional catalytic layer structure,it is a random stack of catalyst particles,usually 30～50 nm carbon particles banded with each other by ionomers.The random distributed catalytic layer brings lots of narrow channels and dead ends for gas transportation,leading to inefficient mass transport and low Pt utilization.The rational design of catalytic layer in the membrane-electrode assembly is the key to achieve high performances from proton exchange membrane fuel cells（PEMFCs）.Herein,inspired by neural-network structure of brain,we constructed a bionic catalytic network for oxygen reduction reaction（ORR）,via setting up Pt-organic ligands-Co²⁺-organic ligands-Pt connections and then thermally transforming them into a metal-organic-framework（MOF）-like matrix in which 3～5 nm hollow Pt Co alloy nanoparticles（NPs）are bridged together by carbon nanotubes（Pt Co@CNTs-MOF）.The bionic catalytic network endows highly efficient linkages of various species-transport channels to active sites;as a result,an order of magnitude improvement is achieved in mass transfer efficiency as compared to the traditional Pt/C catalytic layer.Besides,the hollow Pt Co alloy derived from Pt NPs shows a high initial mass activity of 852 m A mg^-1_Pt@0.90 V and an undetectable decay in an accelerate aged test.Accordingly,a remarkable Pt utilization efficiency of 58 mg _Pt k W^-1 in fuel cell cathode and 98 mg _Pt k W^-1 in both anode and cathode,were eventually achieved,respectively.The latter is almost 3 times higher than that from the traditional catalytic layer.Moreover,no decay was detected during continuous operation at 1 A cm^-2 for 130hours from the bionic catalytic network based fuel cell.This strategy offers a new concept for designing ultra-low Pt loading yet highly active and durable catalytic layer for fuel cell applications and beyond.③Based on mesoscale problem of dynamic current and drainage space in catalyst layer:Considering the output（current or voltage）of fuel cell is vibratory under dynamic conditions,which largely affects the quality of power and the cost of fuel cell system.Here,we reported that a template-assisted epitaxial assembly strategy to obtain the Pt Co catalyst with adjustable space for water draining.The anti-flooding electrode not only realized quality power output with a current vibration less 25 m A cm^-2,but also enhances the mass transfer and depresses the water flooding within its 3D carbon frameworks.As a result,a maximum specific power density of 11.69 W mg _Pt^-1 was also achieved,and no decay was detected in fuel cell continuous operation at 0.6 V over 100h.In addition,the anti-flooding electrode shows very high application value.④Based on the mesoscale problems of novel confined effects and nanoparticle dispersion:Aiming at the low current density output,the higher resistance of gas-liquid,and the low Pt utilization in the working range of high voltage.Herein,we developed a confined Pt-based catalyst.The high Pt utilization and high current density output at the high voltage in the fuel cell fabricated by confined catalyst with low Pt loading.In the H₂/O₂ fuel cell test,the higher mass activity at 0.9 V after i R correction(MA_{0.9V-i R-free})could be obtained by confined catalyst(2.9 A/mg _Pt),which is 6.59 times than that of2025 DOE target.Also after 30000 cycles based on DOE accelerated stress tests（ASTs）protocols,the decay ratio of MA is 17.40%,which has achieved the target of 2025 DOE（<40%）.The results imply that the confined catalyst exhibits excellent performance in the fuel cell,making it among the best published catalysts.