1. Phenomenological Model of Electrochemically Active Surface Area Loss Mechanisms in the Cathode Catalyst Layer Steven G. Rinaldo, a,b Wendy 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. 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 decreasing influence on ECSA loss under normal conditions dissolution redeposition agglomeration detachment 2. General Degradation Model (illustrative) particle radius distribution (PRD) in the cathode catalyst layer degradation stressor 1  PRD temporal evolution (imaging)  ECSA temporal evolution(voltammetry)  decoupling mechanisms is difficult 3. General Degradation Model (mathematical) spatially invariant PRD continuity equation (PRD temporal evolution) 2,3,4 Pt mass balance (closed CCL) and ECSA temporal evolution  particles interact via mean-field concentration 4. Simplifications (minor support corrosion) (i) PRD/ECSA temporal evolution determined by dissolution and redeposition (ii) particle radius change controlled by Gibbs-Thomson interfacial kinetics (iii) dimensionless governing equations 5. Main Effective Parameters 6. Dissolution Limit (Potentiostatic) 5 7. Dissolution Limit (Potential Cycling) 6  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 8. Redeposition (Parametric Study) 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 (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 ( ) 3 3 2 2 3 3 main factors: particle size electrode potential oxide growth/reduction agglomeration dissolution/redeposition detachment main factors: temperature support properties particle movement main factors: support corrosion 2 2 1 1 1 1 1 1 2 2 3 3 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). interrelated mechanisms  small particles dissolve faster  effective dissolution rate  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  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 transition dissolution redeposition  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