Novel Catalyst-Layer Structures with Rationally Designed Catalyst/Ionomer Interfaces and Pore Structures Aided By Catalyst Functionalization

Abstract

The cost and durability of the electro-catalysts used in the proton exchange membrane fuel cells (PEMFCs) are two critical factors affecting the early market penetration for both transportation and stationary applications. Although progress has been made in the rational designs of highly active ORR electro-catalysts with excellent performance characterized by rotating disc electrode (RDE), [1, 2] there still remains a huge challenge to implement these RDE findings into an actual membrane electrode assembly (MEA) for PEMFC systems. Because the ORR performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain 100% ionomer coverage for maximizing catalyst utilization. In addition, the thickness of ionomer film over the catalyst nanoparticles should be optimal to facilitate gas diffusion and water balance without sacrificing its protonic conductivity. Additionally, the appropriate pore structure in the catalyst layer is also necessary for providing transport paths for both reactants (O2 and H2) to reach reaction sites as well as allow water mobility throughout the catalyst layer. In some of the more conventional MEA fabrication processes, the ionomer coverage of the catalyst particles can be partial or non-uniform. This is partly due to the lack of control of ionomer deposition onto the catalyst surface. As a consequence, some of the catalyst becomes electrochemically inactive. Furthermore, insufficient ionomer coverage suppresses the amount of H+ transport, thus impacting the oxygen reduction reaction (ORR). On the other hand, the aggregation of ionomer with increasing ionomer content (to increase catalyst coverage) in a catalyst layer leads to a thicker ionomer film, which results in an increased gas and water diffusional barrier. We will present a novel approach to construct an innovative ionomer/catalyst interface with high ionomer coverage and a thin ionomer layer. This is done by an electrostatic charge attraction of a negatively charged “-SO3 -” on the surface of ionomer particles and a positively charged “-NH3 +” on the surface of catalyst particles (catalyst surface charge is realized by chemically grafting different functional groups via diazonium reaction). In our approach, the improved ionomer/catalyst interface is formed during the ink preparation. Using a unique method of combined ultra-small angle x-ray scattering (USAXS) and cryo-TEM, we observed a significant increase on carbon aggregate size after the NH3 +- functionalized carbon black (CB) particles were mixed with Nafion ionomer, whereas only a negligible size change was observed when SO3 - functionalized CB was used. These results suggest that the coulombic attraction facilitates the mixing of catalyst and ionomer. The ionomer/catalyst interface was characterized by Scanning Transmission Electron Microscopy (STEM) combined with Energy-dispersive X-ray Spectroscopy (EDS) mapping. Figure 1(b) shows the EDS mapping of F distribution for both NH3 +-CB and SO3 --CB. We will report on the performance of MEAs using this approach. Reference [1] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces, Science, 343 (2014) 1339-1343. [2] X. Huang, Z. Zhao, L. Cao, Y. Chen, E. Zhu, Z. Lin, M. Li, A. Yan, A. Zettl, Y.M. Wang, X. Duan, T. Mueller, Y. Huang, High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction, Science, 348 (2015) 1230-1234. Figure 1