Proton exchange membrane fuel cells (PEMFCs), with its zero-emission feature, have received much attention as environment-friendly power sources. However, a massive amount of Pt-based catalyst is used due to the slow kinetics of the oxygen reduction reaction at the cathode. For the dissemination of the PEMFCs, many efforts have been made to maximize the mass activity of Pt catalysts, to reduce Pt usage. Alloying Pt with other transition metals or shape control of Pt-based nanoparticles has been reported to show high mass activity. Despite the progress made in the electrocatalytic field, high activity was recorded mostly in a half-cell setup, which does not guarantee excellent performance in a single-cell. Moreover, many catalysts suffer from poor durability in long-term cell operation. Furthermore, the catalyst layer is inevitably thickened to reduce the Pt weight fraction, largely suppressing mass transport phenomena. Therefore, it is urgent to design a catalyst system with high mass activity and durability to overcome the mass transport issue while minimizing Pt usage.

For the practical application of the PEMFCs, the catalyst must show meaningful performance in the actual PEMFC operating condition. The reaction environment of the single cell is different from that of the half-cell. Whereas reaction occurs at the interface between the liquid electrolyte and the catalyst surface in a half-cell, the so-called triple-phase boundary should be optimized in a single cell. The intrinsic activity and the electrical conductivity of the catalyst and mass transport phenomena become essential, especially in the high current density region. In the previous studies, our group reported ultra-low Pt (1wt%) supported on block-copolymer-based carbon particles. Pt-Fe alloy nanoparticles encapsulated by thin carbon shells showed high activity and durability. To enhance high current density region performance, support morphology should be tuned. Pores within the carbon particles and the secondary pore created between the carbon agglomerates affect the mass transport of the oxygen and proton.

Herein, we report oblate or lens-shaped mesoporous carbon particles with ultra-low amounts of Pt (1 wt%) to achieve highly active and durable catalysts for PEMFCs. The lens-shaped carbon particles are designed to have perpendicular channels to maximize the mass transport behavior by providing the shortest pathways for the reactants and products. The membrane emulsification affords highly-elongated, lens-shaped block copolymer particles. Then it was successfully converted into the lens-shaped mesoporous carbon particles with well-ordered channels through hyper-crosslinking and carbonization steps. The single-cell performance enhanced as the aspect ratio (width/height of particle) increased. Especially, the lens-shaped carbon particles with the highest aspect ratio (= 6.2) showed exceptionally high mass transport behaviors, resulting in high power density and durability of 1135 mW cm^-2 and 1039 mW cm^-2 after 30,000 cycles. This outstanding cell performance surpassed commercial Pt/C catalysts, even with 1/20 of the Pt loading. The higher the aspect ratio of the carbon particles, the particles were better aligned on the cathode resulting in a higher packing density. The effect of the aspect ratio on PEMFC performance was further investigated by analyzing the overpotential, which decreased from 0.29 V to 0.18 V as the aspect ratio increased from 2.2 to 6.2. By tuning the aspect ratio of the lens-shaped mesoporous carbon particles, mass transport of the reactants and products were significantly enhanced.