The Improved Platinum Utilization Efficiency and Durability in Polybenzimidazole-Wrapped Carbon Black for Polymer Electrolyte Membrane Fuel Cell
Authors: Jayawickrama Samindi Madhubha , Han Ziyi , Fujigaya Tsuyohiko 
Published: February 2020

It is well known that polymer electrolyte membrane fuel cells (PEMFCs) give an ideal energy solution because of their high energy conversion efficiency, zero emission and unlimited renewable source of reactants.1 One of the major technical challenges in PEMFC technology is the development of efficient cathode catalysts, which drive the oxygen reduction reaction (ORR).2 Platinum (Pt) is the most active metal catalyst for ORR.3 The efficient cathode includes both high Pt utilization efficiency and durability which ultimately reduces the cost of PEMFC.
The efficiency of a PEMFC thoroughly depends on the nature of the membrane electrode assembly (MEA), especially the structure of the catalyst layer (CL) which is composed of Pt nanoparticle supported carbon black (CB). Over the past decade there have been numerous studies focusing on optimization of the CL in order to reduce the cost of PEMFC.1 Incorporation of non-precious metals and decreasing the catalyst particle size are common methods to reduce the amount of Pt used in PEMFC. However, these methods involve dissolution of non-precious metals in acidic condition4 and agglomeration of small sized Pt5, resulting in decreased Pt utilization and durability. On the other hand, optimizing the catalyst structure by increasing number of triple phase boundaries is a promising strategy to improve Pt utilization efficiency.6 One of the aspects is to change the ionomer/carbon ratio. However, recent high-resolution transmission electron microscopy studies suggest that the ionomer coverage in the electrode may be rather inhomogeneous.7

In this study, we demonstrate a novel approach to improve Pt utilization efficiency and durability of PEMFC by polymer wrapping onto CB. Polybenzimidazole (PBI) coats carbon materials via π-π and acid-base interactions. Here, PBI is used to support homogeneous coating of Nafion due to the acid base interaction between Nafion and PBI, improving Pt utilization efficiency. Furthermore, PBI layer can provide binding sites through coordination of Pt with PBI, improving durability of Pt particles simultaneously. This preferential binding might prevent Pt migration on CB. Moreover, lone pair electrons of imidazole group will increase the electron density of Pt, preventing Pt oxidation. Therefore, PBI wrapping approach may increase both Pt utilization efficiency and durability. The PBI wrapping onto CB was easily done by addition of CB into PBI dissolved N,N-dimethylacetamide and 1 hr sonication to the mixture to prepare CB/PBI. Then Pt nano-particles were deposited onto CB/PBI to prepare CB/PBI/Pt and compared with non-wrapped CB/Pt. Protonic (H+) resistance in CB/PBI/Pt and CB/Pt CLs were measured by impedance spectra under H2 in anode and N2 in cathode and found that H+ resistance is lowered by 65 % in CB/PBI/Pt CL (0.44 Ω cm2) compared to that of CB/Pt CL (1.26 Ω cm2). This observation suggests that degree of Nafion coverage in Vulcan/PBI/Pt CL is rather homogeneous than that of CB/Pt CL, resulting a higher Pt utilization efficiency. Moreover, we have noticed that CB/Pt catalyst ink easily forms aggregates, whereas Vulcan/PBI/Pt remains a very stable dispersion for at least a few days and which may be advantageous in large scale production of MEAs8. Possible reason is the homogenous Nafion dispersion in CB/PBI/Pt ink which induces surface hydrophilicity due to SO3- groups and improve their dispersion in aqueous solution.9 The durability study of CB/Pt vs CB/PBI/Pt shows that CB/PBI/Pt has higher durability (36%) after potential cycling between 0.6-1.0 V vs RHE at 0.1 M HClO4 solution. Therefore, here we conclude that the PBI wrapping approach can improve both Pt utilization efficiency and durability.

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