Particle Radius Distribution Model-Based Analysis of PEFC Cathode Catalyst Degradation Mechanisms
There are three mechanisms that could be responsible to changes in the particle radius distribution: 1. Ostwald ripening, i.e. dissolution of especially the smaller, less stable particles and redeposition predominantly on larger particles; 2. Coagulation of particles, i.e. merging of two smaller particles forming a bigger particle and thus decreasing the overall surface energy; 3. Inactivation of particles, e.g. by detachment from the carbon substrate, loss of electronic or ionic connection or by passivating with impurities. These degradation mechanisms lead to a decrease of ECSA and thus to a loss in specific activity of the catalyst.
The current work attempts to link experimentally observed ECSA loss to the proposed degradation mechanisms. The presented model [3-7] combines the Lifshitz- Slyozov-Wagner theory for dissolution and redeposition [4,5] with Smoluchowski’s coagulation theory  and includes a simple deactivation term . The model is able to describe temporal changes in the particle radius distribution and its moments, i.e. the number of active particles, mean particle radius, electrochemically active surface area and the active Pt mass. Properly parametrized, this model is able to give valuable insight into the structural changes in the catalyst layer that occur during PEFC degradation.
In order to parametrize the model, it was fitted to experimental results of accelerated stress tests (AST, see Fig. 1)  that were performed under different conditions (temperature, pH, upper potential limit and waveform of the potential). It was attempted to identify the prevailing mechanism(s) and determine the kinetic parameters of the degradation process under these conditions. However, ambiguities in the fits were found, i.e. different sets of parameters corresponding to different physical scenarios could fit the experimental data. Those scenarios as well as their origins, likelihood and implications under different conditions will be discussed systematically in this talk.
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