  We present an electron tomography method that allows for the identification
of hundreds of electrocatalyst nanoparticles with one-to-one correspondence
before and after electrochemical aging. This method allows us to track, in
three-dimensions (3-D), the trajectories and morphologies of each Pt-Co
nanocatalyst on a fuel cell carbon support. The use of atomic-scale electron
energy loss spectroscopic imaging enables the correlation of performance
degradation of the catalyst with changes in particle/inter-particle
morphologies, particle-support interactions and the near-surface chemical
composition. We found that, aging of the catalysts under normal fuel cell
operating conditions (potential scans from +0.6 V to +1.0 V for 30,000 cycles)
gives rise to coarsening of the nanoparticles, mainly through coalescence,
which in turn leads to the loss of performance. The observed coalescence events
were found to be the result of nanoparticle migration on the carbon support
during potential cycling. This method provides detailed insights into how
nanocatalyst degradation occurs in proton exchange membrane fuel cells
(PEMFCs), and suggests that minimization of particle movement can potentially
slow down the coarsening of the particles, and the corresponding performance
degradation.
