Carbon black (CB), which has been widely used as a catalyst support, has been treated by various activation processes in order to increase the surface area. High-surface-area CB has a high pore volume in the primary particles. The degree of ionomer impregnation in the nanosized pores is able to be evaluated by the hysteresis volume, which is calculated from N2-adsorption analysis. We investigated the effect of distributions of both Pt and ionomer on the surface of CB support nanopores on the cell performance and durability of cathode catalyst layers with catalysts using CB with various nanopore volumes for polymer electrolyte fuel cells. The optimum ionomer content is affected by the nanopore volume, and the excess ionomer was found to block the entrances of the nanopores. For maximizing the cell performance and durability, we found that a high-surface-area CB was beneficial for achieving decreased Pt particle size, and the ionomer content was optimized to prevent the blocking of the nanopores. Improvement of the transport of oxygen and protons to Pt in the nanopores and increase in the interparticle distance led to both increase in the electrochemical surface area and suppression of Ostwald ripening and coarsening.

Achieving high performance and durability at low Pt loads is an important challenge for polymer electrolyte fuel cells (PEFCs). We investigated the effect of catalyst Pt loading percentage (wt % Pt) on the performance and durability of an ultrahigh surface area carbon black (CB) with a large nanopore volume using morphological observations, nitrogen adsorption, and electrochemical performance measurements. The ratio of the surface areas of Pt on the interior and exterior surfaces of the CB affects the penetration of the ionomer into the nanosized pores. When the exterior Pt surface area is larger than that of the interior, the oxygen diffusion resistance in the ionomer increases and the performance deteriorates due to the thick covering of the ionomer on the exterior Pt. Based on durability testing that combines startup, shutdown, and galvanostatic load cycling, the main deterioration factors are dependent upon the Pt interparticle distance and the thickness of the catalyst layer, which vary with the wt % Pt. The advanced characterization and optimization of the various wt % Pt on an ultrahigh surface area CB, combined with the extensive performance and durability testing, have provided an unprecedented understanding of the reaction sites, mass transport characteristics, and stability, which are crucial for their practical application in PEFCs.

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