Arguably, the most commonly reported catalyst layers for polymer electrolyte fuel cells are derived from colloidal ink of Pt/C and ionomer in a suitable dispersion media [1]. The resulting CL architecture can be described as a nanoporous, nanocomposite layer. In high performance CLs with such an architecture, the constituent material phases Pt/C and ionomer are randomly distributed but form a percolating network of respective phases to allow for electron and ion transport, respectively. The ionomer forms an ultrathin coating on Pt/C whose minimum thickness of 3 nm is not surprisingly comparable to a critical dimension of ionomer aggregate in dispersions [2]. The pores in these CLs are randomly distributed in space and have a wide size distribution but are controlled by the size of carbon particle [3]. Thus, for CLs made with 30 nm spherical carbon particles, the pore size range from roughly one to four times the particle size, i.e. 20-120 nm. Knudsen diffusion plays an important role in the oxygen transport in such pores and contribute to gas-phase transport resistance. Control of pore size is difficult although ionomer:carbon (I:C) ratio and dispersion media can mildly affect the pore size distribution. Low I:C ratio results in a poor connectivity of the ionomer phase whereas high I:C ratio result in small pores.

This prompted us to imagine the possibility of designing new catalyst layer architectures with larger and/or independent control of pore size. At UCalgary, two such architectures are being explored: (i) ordered pores with controlled size independent of the ionomer content, and (ii) nanoporous, nanocomposite layers made with large carbon nanoparticles that result in large pores (100-400 nm). The first architecture arises from the nanoporous, inverse opal structure carbon films developed in Prof. Birss group (Chemistry, UCalgary) with controllable pores ranging 20-85 nm - Figure 1a [4, 5]. The building block of the second CL architecture is depicted in Figure 1b and comprises a N-doped carbon nanoparticle of 135 nm in diameter decorated with 3-4 nm Pt nanoparticles.

The presentation will discuss the difficulties in creating functional CL with such architectures, the successes achieved along the way, and the remaining challenges.

References:

1. Rod L. Borup et al. (2018),PEM Fuel Cell Catalyst Layer Architectures, ECS Meeting Abstract MA2018-01 1591.
2. Karan (2019) Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films, Langmuir (Invited Feature Article), doi: 10.1021/acs.langmuir.8b03721
3. Sabharwal, L.M. Pant, N. Patel, M. Secanell (2019) Computational Analysis of Gas Transport in Fuel Cell Catalyst Layer under Dry and Partially Saturated Conditions,J. Electrochemical Society, 166 (7) F3065-F3080.
4. K Karan (2017) PEFC catalyst layer: Recent advances in materials, microstructural characterization, and modeling, Current Opinion in Electrochemistry 5 (1), 27-35
5. V Birss, LI Xiaoan, K Karan, DW Banham (2017), Fuel cells constructed from self-supporting catalyst layers and/or self-supporting microporous layers, US Patent App. 15/267,876.