High-Performance Pt Catalysts Supported on Polybenzimidazole-Grafted XC72 for Pemfcs

The performance of a polymer electrolyte membrane fuel cell (PEMFC) is limited by the sluggish oxygen reduction reaction (ORR) and the poor durability of the catalyst. Typically, most common catalysts are chemically deposited Pt nanoparticles on the surface of carbon nanoparticles (XC-72). A functionalized carbon support allows better dispersion of the catalyst nanoparticles, which can reduce the aggregation of metal nanoparticles and, consequently, improve the catalyst performance.¹ Polybenzimidazole (PBI) is one kind of conducting polymer with excellent thermal stability and proton conductivity upon acid doping.² Therefore, polybenzimidazole has been widely studied as proton conducting membrane in high-temperature fuel cells.³Recently, several studies have been reported the use of physically-adsorbed polybenzimidazole on carbon as the stable Pt catalyst support. In this work, covalently-functionalized carbon black XC-72 with polybenzimidazole was prepared as the catalyst support. The Pt nanoparticles were uniformed deposited onto PBI/XC-72 due to a strong interaction between PBI and Pt. Such Pt catalyst supported on the PBI grafted XC-72 will be expected to possess better proton conductivity, higher stability and better mass transport. The synthesized catalysts were characterized using rotating disk electrode (RDE) as well as membrane electrode assembly (MEA).

 The Cyclic voltammetry (CV) curves of PBI-grafted XC-72 supported Pt catalyst at a scan rate of 20 mV/s in N₂-saturated 0.1 M HClO₄ is shown in Figure 1. The electrochemical active surface area (ECSA) was calculated to be about 74 m²_Pt/g_Pt, which is much higher with that of commercial E-TEK Pt/C catalyst (58 m²_Pt/g_Pt). This high ECSA value suggests the uniform distribution of Pt on the surface of PBI/XC-72. Accelerated degradation tests (ADT) were also evaluated by monitoring the changes in electrochemical surface area during voltage cycling between 0.6 V and 1.2 V at a scan rate of 20 mV/s in N₂-saturated 0.1 M HClO₄. After 1000 cycles, a relatively surface area loss of more than 34% was observed in commercial catalyst. In comparison, the relative surface area loss of graphene/PBI supported Pt catalyst was only 14%. The increase in the stability of catalyst may result from the strong binding of Pt nanoparticles on PBI and the electrochemical stability of PBI-grafted XC-72. This catalyst was further compared with commercial Pt/C catalyst in MEA. It is shown that the peak power density of MEA made with was about 1400 mW/cm², much higher than that of the commercial Pt/C catalyst (970 mW/cm²). Besides, the IR-drop of Pt/PBI-grafted XC-72 was lower than Pt/XC-72 mainly due to an enhanced proton transport. It is known that the MEA performance at high current densities is limited by the proton and mass transport. It is believed that the proton transport can be signifantly improved at the triphase interface (PBI-grafted XC-72 surfaces, Pt nanoparticles, and Nafion). Proton conducitivity tests are current being carried out in our group to elucidate our assumptions. In conclusion, the excellent electrochemical performance and better durability of PBI-grafted XC72 supported Pt catalyst illustrates the advantage of polybenzimidazole-grafted XC-72 as a catalyst support in PEMFCs.

Figure 1. (a) CV curves of PBI/XC72 supported Pt catalyst between 0 and 1 V vs RHE at a scan rate of 20 mV/s in N₂-saturated 0.1 M HClO₄; (b) ECSA retention during ADT; and (c) MEA performance.

Reference

(1) Xu, F.; Wang, M.-x.; Sun, L.; Liu, Q.; Sun, H.-f.; Stach, E. A.; Xie, J. Electrochim. Acta 2013, 94, 172-181.

(2) He, R.; Li, Q.; Xiao, G.; Bjerrum, N. J. Journal of Membrane Science 2003, 226, 169-184.

(3) Xiao, L.; Zhang, H.; Scanlon, E.; Ramanathan, L. S.; Choe, E.-W.; Rogers, D.; Apple, T.; Benicewicz, B. C. Chem. Mater. 2005, 17, 5328-5333.