(Invited) Trials and Travails with Two New Catalyst Layer Architectures
Authors: Karan Kunal 
Published: February 2020

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:

Rod L. Borup et al. (2018),PEM Fuel Cell Catalyst Layer Architectures
, ECS Meeting Abstract MA2018-01 1591.

Karan (2019) Interesting Facets of Surface, Interfacial, and Bulk Characteristics of Perfluorinated Ionomer Films, Langmuir (Invited Feature Article), doi: 10.1021/acs.langmuir.8b03721

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.

K Karan (2017) PEFC catalyst layer: Recent advances in materials, microstructural characterization, and modeling, Current Opinion in Electrochemistry 5 (1), 27-35

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.

Figure 1