Ultra-Low Pt Loading Catalyst Layers for PEMFC Using Reactive Spray Deposition Technology

Reactive spray deposition technology (RSDT) has proven to be a cost-effective method for producing high performance PEMFC membrane electrode assemblies (MEAs) for both low temperature (<100 ^o C) [1,2] and high temperature (>160 ^o C) operations [3,4]. RSDT is a flame-based method that combines catalyst synthesis and processing in a single step while having the flexibility to optimize the catalyst composition, making gradient and other complex structures possible. This has allowed our team to reduce the cathode Pt loading below 0.1 mg cm^-2 with enhanced electrochemical performance. The RSDT-derived Pt/Vulcan catalyst displayed twice the mass-activity with 25% Pt loading compared to commercial catalyst and, in particular, the Pt/Ketjen Black system surpassed the 2017 DOE target with a mass activity of 0.51 A mg^-1at 0.9V and oxygen pressure of 180 kPa [2].  (Figure 1a).

At ultra-low Pt loading, catalyst layer structure plays a more important role than catalyst loading. Therefore each catalyst layer formulation needs to be carefully controlled. The effect of ionomer content in the catalyst layer deposited through RSDT has been investigated and the optimum ionomer content for cathode was found to be 16 wt% for Vulcan XC-72R [2], approximately half of the optimum content, 33 wt%, reported in most literatures [2]. Ketjen Black carbon required more ionomer to penetrate into the porous network due to its high surface area and microporosity. The best performance was obtained at about 40 wt% ionomer in the catalyst layer. (Figure 1b). Since mass transport effects were significantly reduced by elevated back-pressure, the ionic conductivity became the dominant factor in determining performance and the highest performance was achieved by the highest ionomer content, ~55 wt%. (Figure 1b)

We also used the RSDT process to deposit Pt supported on multi-wall carbon nanotube (MWCNT) (Figure 1c) and reduced-graphene oxide (rGo) (Figure 1d). The high-surface-area and porous nature of the nanotube networks make them ideal support materials for electrocatalytic processes. The advantage of RSDT is that we can deposit uniformly distributed particles (Figure 1 c and d) that do not require additional surface treatment [5] or element doping [6] for the carbon support to enhance Pt adhesion.

In this study, mass transport loss correlated to ECSA reduction, varying ionomer content, and support microstructures will be presented. Impedance spectroscopy and model simulation will be carried out to investigate the reactant transport properties and water management for different pore characteristics. In the meantime, catalyst layer degradation upon accelerated stress test are also being conducted with Pt loading at 0.05, 0.1 and 0.2 mg cm^-2. 

Figure 1. a) comparison of mass activity vs. Pt or PGM loading with state-of-the-art Pt and Pt alloy catalysts, commercial Pt/C, and DoE target, adapted from ref [2]; b) comparison of H₂/O₂ I-V polarization curves of RSDT-deposited Pt/Ketjen Black CCMs with varying I/C weight ratios at both ambient and 180kPa back pressure, 80^oC, 100RH%. Surface morphology of RSDT-deposited Pt/MWCNT and Pt/rGo are shown in c) and d), respectively.

Acknowledgement

The authors gratefully acknowledge the National Science Foundation (award number CMMI-1265893) for financial support.

References:

[1] J.M. Roller, M.J. Arellano-Jimenez, H. Yu, R. Jain, C.B. Carter, R. Maric, Electrochim. Acta 107 (2013) 632-655

[2] H. Yu, J.M. Roller, W.E. Mustain, R. Maric J Power Sources 283 (2015) 84-94.

[3] H. Yu, J.M. Roller, S. Kim, Y. Wang, D. Kwak, R. Maric, J. Electrochem. Soc. 161 (2014) F622-F627.

[4] S. Kim, T.D. Myles, H.R. Kunz, D. Kwak, Y. Wang, R. Maric, Electrochim. Acta (2015) in press.

[5] Yu, R.; Chen, L.; Liu, Q.; Lin, J.; Tan, K.-L.; Ng, S. C.; Chan, H. S. O.; Xu, G.-Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718 −722

[6] Chen, Y.; Wang, J.; Liu, H.; Banis, M. N.; Li, R.; Sun, X.; Sham, T. K.; Ye, S.; Knights, S.J. Phys. Chem. C 2011, 115, 3769–3776