Current state of the art PEMFC cathode catalyst layer can be described as a complex, heterogeneous porous system with multiple components across various length scales at which both the electrochemical reaction and mass transport of reactants/products occur. The functional components in the catalyst layer comprises of the catalyst nanoparticles (few nm in diameter) for electrochemical reaction, carbon support (few tens of nm to µm) for electron conduction, ionomer chains (few nm to µm) for proton transport, and pore structure (few nm to few hundred nm) for gas transport. Mass transport of reactants and products are supported by the intrinsic pore structure in the carbon support and the secondary pore structure induced by the ionomer chains in the catalyst ink formulation stage. [1]

There is a strong impetus to decrease the catalyst loading to minimize Pt material cost and also, ease the supply-demand scenario. While extensive research into the development of Pt-alloy nanoparticle catalysts has led to improvements in the low current density kinetic region, the current state of the art cathode electrodes suffer from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low PGM loadings. [2]

Recent studies have pointed out to the presence of a local transport resistance in the cathode catalyst layer that affects HCD performance via i) decrease in O₂ permeability through the ionomer thin film covering the Pt/C agglomerates, and/or ii) O₂ transport within the Pt/C agglomerates. The latter resistance arises due to the necessity of O₂to be transported within the tortuous pore structure in the Pt/C agglomerates to reach the active site. [2, 3] Perhaps, the most complicating aspect of the catalyst layer microstructure arises from i) the complex pore structure of the carbon support upon which the Pt-alloy nanoparticles are deposited, and ii) the extensive aggregation of both the porous carbon support & the colloidal ionomer particles.

A well optimized cathode catalyst layer could be envisioned as one where the ionomer is distributed around the Pt/C agglomerates with an optimal coverage in such a fashion that there is no direct Pt-ionomer interface formation (to minimize sulfate poisoning), but with Pt nanoparticles present at vantage locations within the agglomerates that are more open, less tortuous and easily accessible by O₂. Such a structure would essentially maximize the mass activity in the kinetic regime and minimize the local-O₂transport resistance in the mass transport regime.

Current state-of-the-art carbon supports for cathode catalyst applications involve moderate to high surface area materials that feature either a disordered-porous (e.g. Ketjen Black) or non-porous (e.g. Vulcan) structure. While the porous carbon supports host the Pt nanoparticles within its micro-meso pores thereby preventing it from coming into direct contact with the sulfate functional groups in the ionomer, the disorder in these pores leads to high tortuous pathways for O₂transport that affects the HCD performance. Contrarily, the non-porous carbon supports host Pt nanoparticles on the surface in direct contact with the ionomer leading to a poor low mass activity.

In this study, we demonstrate the use of ordered mesoporous carbon (OMC) supports for cathode catalyst applications to mitigate the HCD performance losses. As shown in Figure 1, OMC support features a unique pore structure with the majority of surface area in the mesoporous region along with the presence well-ordered, open, continuous pores for reactant transport. [4] High mass activity is achieved since some or all of the Pt nanoparticles are deposited inside the pores so that there is no direct contact with the ionomer but at the same time the pores have an open, continuous and ordered structure that enables better O₂ transport at high current densities. The merits and challenges that arise from the use of OMC supports for cathode catalyst applications will be discussed. 

References

1) S. Holdcroft, Fuel Cell Catalyst Layers: A Polymer Science Perspective, Chem. Mater. 2014, 26 (1), pp 381-393

2) A. Kongkanand, M. F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 2016, 7 (7), pp 1127–1137

3) S. Jomori et al., An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC, J. Electrochem. Soc. 2013, 160 (9), pp F1067-F1073

4) R. Ryoo et al., Ordered Mesoporous Carbon, Adv. Mater. 2001, 13 (9), pp 677-681