燃料电池用低铂族金属催化剂的稳定性挑战与材料解决方案

Low－PGM and PGM－Free Catalysts for Proton Exchange Membrane Fuel Cells： Stability Challenges and Material Solutions

Lei Du

Venkateshkumar Prabhakaran

Xiaohong Xie

Sehkyu Park

Yong Wang

Yuyan Shao

Abstract

Fuel cells as an attractive clean energy technology have recently regained popularity in academia， government， and industry． In a mainstream proton exchange membrane （PEM） fuel cell， platinum－group－metal （PGM）－based catalysts account for ≈50％ of the projected total cost for large－scale production． To lower the cost， two materials－based strategies have been pursued： 1） to decrease PGM catalyst usage （so－called low－PGM catalysts）， and 2） to develop alternative PGM－free catalysts． Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly （MEA）—the power generation unit of a PEM fuel cell—or when PGM－free catalysts are integrated into an MEA． More importantly， there is a significant knowledge gap between materials innovation and device integration． For example， high－performance electrocatalysts usually demonstrate undesired quick degradation in MEAs． This issue significantly limits the development of PEM fuel cells． Herein， recent progress in understanding the degradation of low－PGM and PGM－free catalysts in fuel cell MEAs and materials－based solutions to address these issues are reviewed． The key factors that degrade the MEA performance are highlighted． Innovative， emerging material concepts and development of low－PGM and PGM－free catalysts are discussed．

Figure 1． Stability challenges for fuel cells． A） The gaps between the state－of－the－art MEAs and U．S． DOE ultimate targets B） Polarization curves of MEAs with two different PGM loadings during the stability test using the square wave catalyst AST protocol． The schematics of degradation mechanisms for low－PGM catalysts （D）

Degradation Mechanisms：

1Metal Dissolution of PGM Nanoparticles

2Increased Mass Transfer Resistance of Low－Loading PGM Catalysts

3Carbon Support Corrosion

4Catalyst Stability in Medium／Heavy－Duty Fuel Cells

这里单独为中重载车辆用的催化剂设立一个小节

The working conditions of medium／heavy－duty fuel cells are
usually different from the light－duty fuel cells， for example，
long haul， long idle time， long service time are needed． These
conditions require high fuel cell efficiency， e．g．， 68–72％ in peak
efficiency is targeted for the Class 8 Long－Haul Tractor－Trailers，
and thus low power densities （high voltages） and high working
temperatures．

Figure 2． Metal dissolution of PGM particles． A–F） Electron tomography results for a PtCo spongy particle from cathode layer in the beginning－of－life （BOL） （A–C） and end－of－life （EOL） （D–F） MEA： A，D） HAADF－STEM image， B，E） 3D reconstructed volume， and C，F） transparent surface render and surface cut in half； G） the PtCo particle size versus composition for individual particles in BOL and EOL MEAs． Bulk CCL compositional averages shown by dashed lines for BOL （blue） and EOL MEAs （red）． Transparent ellipses denote size versus composition regimes where BOL spongy nanoparticles （blue） and EOL hollow sphere nanoparticles （red） were observed．

Figure 3． Carbon oxidation of low－PGM fuel cells． A） The cross－section SEM images of cathode CLs before and after AST； B） the PtCo particle size distribution and C） the pore size distribution of CLs before and after AST．

Figure 4． Intermetallic compounds． A） Schematic of L10－CoPt／Pt NPs with 2–3 atomic layers of Pt shell， where the silver－colored atom is Pt and the blue－colored atom is Co； B） STEM image of L10－CoPt／Pt NPs， showing the 2–3 atomic layers of Pt shell （indicated by yellow arrows） and the L10－CoPt core， Pt is colored in red and Co is colored in blue； C） Mass activities of L10－CoPt／Pt by measuring the current at 0．9 V （iR－free） in 150 kPaabs H2／O2（80 °C， 100％ RH， 500／1000 sccm） with correction for measured H2 crossover． The horizontal lines indicate the DOE 2020 target requirement for mass activity at BOL and EOL （30 000 AST cycles）， respectively．

Materials－Based Solutions for Low－PGM Catalysts

Stable PGM Catalysts

Advanced Carbon and Noncarbon Support Materials

Rational Design of Catalyst Layer Architecture