1. Introduction

1. Phenomenological Model of Electrochemically Active Surface Area Loss Mechanisms in the Cathode Catalyst Layer Steven G. Rinaldo,a,bWendy Lee,b Jürgen Stumper,b and Michael Eikerling.a Department of Chemistry, Simon Fraser University,a and the Automotive Fuel Cell Cooperation Corporation,b Burnaby, BC, Canada. agglomeration dissolution redeposition detachment 1. Introduction Durability and lifetime continue to be foremost issues in polymer electrolyte fuel cell (PEFC) development. Performance issues arise due to the instability of functional materials found in the fuel cell. These include, but are not limited to, the catalyst support, polymer electrolyte membrane (PEM) and dispersed nanoparticle catalyst. Platinum, the hitherto irreplaceable catalyst is thermodynamically unstable at high potential and low pH as suggested by the potential-pH diagrams developed by Pourbaix; it is also well known that potential cycling exacerbates this instability. Mechanisms of Pt mass loss and redistribution in the cathode catalyst layer (CCL) are sensitive to materials properties, CCL structure as well as conditions such as pH, temperature and potential. The primary effect of Pt mass loss and redistribution is the loss of electrochemically active surface area. The cartoon below illustrates the relative importance of various ECSA loss mechanisms under normal operating conditions 5. Main Effective Parameters (A) surface tension ( )  strong function of oxide coverage (PtO) (B) extended surface dissolution rate ( )  Pt/PtO dissolution mechanisms (C) redeposition coefficient ( ), Pt mass/electrode volume ( ) 6. Dissolution Limit (Potentiostatic)5 • analytical parametric solution via method of characteristics • parametric study revealed strong influence of surface tension on ECSA loss • dissolution rate dispersion could result in mean radius increases with ECSA loss decreasing influence on ECSA loss under normal conditions 2. General Degradation Model (illustrative) particle radius distribution (PRD) in the cathode catalyst layer 1 dissolution/redeposition main factors: • particle size • electrode potential • oxide growth/reduction • comparison with potentiostatic experiments in high/low potential regions • fitted surface tension values indicate dissolution occurs at a PtO interface • ratio of surface tension (high/low potential) agree with experimental ratios • extended surface dissolution rate trends follow experiment 2 agglomeration interrelated mechanisms 7. Dissolution Limit (Potential Cycling)6 main factors: • temperature • support properties • particle movement 1 2 3 3 detachment main factors: • support corrosion • degradation stressor1 • PRD temporal evolution (imaging) • ECSA temporal evolution(voltammetry) • decoupling mechanisms is difficult  model evaluation via an in-situ transmission electron microscopy study  extracted surface tension and dissolution rate suggest cathodic dissolution  mechanism of dissolution changes; potentiostatic vs. potential cycling 3. General Degradation Model (mathematical) 8. Redeposition (Parametric Study) spatially invariant PRD continuity equation (PRD temporal evolution)2,3,4 3 1 2 Pt mass balance (closed CCL) and ECSA temporal evolution dissolution • parametric study on the effect of redeposition • loss of small particles and growth of large particles •  mean-field concentration approaches asymptotic value • two ECSA loss mechanisms identified • • dissolution induced ECSA loss • • redeposition induced ECSA loss transition  particles interact via mean-field concentration redeposition 4. Simplifications (minor support corrosion) (i) PRD/ECSA temporal evolution determined by dissolution and redeposition 9. Conclusions and Outlook • good agreement with experimental data (potentiostatic and cycling) • predict PRD evolution based on ECSA loss and vice-versa • relate observable phenomena (PRD, ECSA) to material properties (surface tension) • fundamental link between Pt oxide formation/reduction and Pt mass balance • key aspect is the relation between oxide mediated Pt dissolution and ECSA loss outlook • evaluate surface tension of nanoparticles under electrochemical conditions • deconvolution of ECSA loss curves  insight into mechanism(s) • refine sub-model of particle radius change  dissolution mechanism (ii) particle radius change controlled by Gibbs-Thomson interfacial kinetics • small particles dissolve faster • effective dissolution rate (iii) dimensionless governing equations 1. K.J.J. Mayrhofer, Electrochem. Commun., 10, 1144 (2008). 2. M. Smoluchowski, Physik. Zeitschr., 17 (1916), pp. 557–599. 3. I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids., 19, 35 (1961). 4. C. Wagner, Z. Elektrochem., 65, 581 (1961). 5. S.G. Rinaldo, J. Stumper, and M. Eikerling, J. Phys. Chem. C., 114, 5773 (2010). 6. S.G. Rinaldo, W. Lee, J. Stumper and M. Eikerling, ESL, 14, B47 (2011).