Ultra-Low Pt Catalyst Supported on Block Copolymer-Based Carbon with Connected Channels for High-Performance PEMFCs

As the demand for carbon neutrality continuously grows, hydrogen is getting attention as a carbon-free energy storage medium that can store surplus energy produced from renewable energy sources. Proton exchange membrane fuel cells (PEMFCs) can utilize hydrogen with high efficiency without emitting pollutants and can be adopted in various applications. Nonetheless, due to the slow kinetics of oxygen reduction reaction (ORR) at the cathode, excessive amount of Pt is needed. The cost of the catalyst layer takes up about 43% of the total stack cost,^[1] which hampers the commercialization of the PEMFCs. Many attempts have been made to reduce Pt usage by shape control, alloying with other metals, and single atomic structure. Alloying Pt with 3d transition metals such as Fe, Ni, Co, Cu, Mn, Zn is commonly known to improve ORR activity by changing the electronic structure of the Pt, which leads to the reduced binding energy of the oxygen species.^[2] However, these transition metals are vulnerable to acidic operating conditions of the PEMFCs.<br/>Our previous work reported carbon-shell-encapsulated PtFe alloy nanoparticles to be highly durable even after the 30,000 cycles of accelerated degradation test (ADT) in the single-cell setup.^[3] Despite the 1/20 of the Pt usage, 1 wt% Pt supported on porous carbon particles showed nearly the same performance as the commercial 20 wt% Pt/C when tested under H₂-O₂ flow. However, single-cell performance under H₂-air flow was not as compatible due to limited access to the reactant. To overcome the mass transport limitation issue occurring at the high-current density region ≥1.5 A cm^-2, especially with the low platinum group metal (PGM) loading of &lt;0.1 mg_PGM cm^-2,^[4] support material structure should be engineered.<br/>In this study, ultra-low loading of 1 wt% Pt was supported on porous carbon derived from the crosslinked block copolymer (BCP) particles. Well-connected channels with pore sizes varying from 13 to 63 nm were fabricated to ensure effective reactant (proton and oxygen) supply and product (water) removal. Carbonization step is necessary to obtain enough electrical conductivity to be used as a support material for the electrocatalyst. Pre- and hyper-crosslinking step was conducted before the carbonization to retain the internal structure of the BCP even after the high-temperature treatment. Pt was deposited on the support by the facile incipient wetness impregnation method followed by the reduction step to obtain the final catalyst powder.<br/>The pore size effect on the mass transport phenomena during the electrochemical reaction was studied. The synthesized catalyst showed impressive mass activity both in half-cell and single-cell setup. Significantly, the catalyst with a large pore showed improved single-cell performance due to facile reactant supply. The origin of improved cell performance upon enlarging the pore size was investigated by separating the overpotential contributions. It was confirmed that proton and mass transport overpotential was reduced as the pore size got larger. Also, due to PtFe encapsulation with the thin graphene layers, the catalyst showed excellent durability. To our knowledge, our synthesized catalyst showed the best beginning-of-life (BOL) and end-of-life (EOL) performance reported to date. This study will give insight into the strategy to design catalyst support for highly active and durable PEMFC catalysts.<br/><br/><b>References</b><br/>[1] S. Satyapal, in <i>FC EXPO</i>, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, <b>2018</b>.<br/>[2] H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, <i>Appl. Catal. B </i><b>2005</b>, <i>56</i>, 9-35.<br/>[3] J. Choi, Y. J. Lee, D. Park, H. Jeong, S. Shin, H. Yun, J. Lim, J. Han, E. J. Kim, S. S. Jeon, Y. Jung, H. Lee, B. J. Kim, <i>Energy Environ. Sci. </i><b>2020</b>, <i>13</i>, 4921-4929.<br/>[4] A. Kongkanand, M. F. Mathias, <i>J. Phys. Chem. Lett. </i><b>2016</b>, <i>7</i>, 1127-1137.