Transport Model for Non-PGM Cathodes in Proton-Exchange Membrane Fuel Cells
Authors: Leonard Nathaniel , Serov Alexey , Workman Michael J , Atanassov Plamen , Calabrese Barton Scott 
Published: February 2020

Expensive oxygen reduction (ORR) catalysts have impeded commercialization of low temperature fuel cells for transportation applications, and driven significant efforts to develop non-precious-metal alternatives to platinum. One challenge for these less expensive catalysts is that they generally require higher loadings, leading to transport limitations. A second challenge is minimal knowledge of the active site structure, making quantification of site density difficult. In the present work the transport limitations of Metal/nitrogen/carbon (MNC) electrocatalysts are considered using a two-phase porous transport model with ionic and electronic conductivity, and a simple kinetic model assuming catalytic activity exists evenly at boundaries between ionomer, solid, and gas phases.
A number of phenomena have been observed specifically with regards to MNC catalysts in fuel cells. A relationship between mesoporosity and activity showing that increased mesoporosity increases catalytic performance [1-3] is likely related to transport to and from micropores, but could also relate to ionomer permeation, because ionomer particle size (~40 nm) [4] is mesoscale. Therefore, increased mesoporosity allows a larger interface between ionomer and solid catalyst phases.
Hydrophobicity is generally tied to catalyst flooding and has also been observed in precious metal catalysts using carbon supports. The high required ionomer content is also explored [5].
To analyze these mechanisms, the following parameters will be used to model catalyst function: exposed Surface Area (Aexp) a summation of macro- and mesoporous surface area. This parameter is dominated by mesoporous area (>90%). Micropores are excluded because ionomer particle size is around 40 nm [4], indicating micropores have little to no contact with the electrolyte. Hydrophobicity as well and porosity are also used to establish active surface area. For example, the exposed area, A
exp, may be modified by ionomer intrusion, f
e, and saturation, S, to calculate an active surface area A
act:

A
act = A
exp
f
e (1-S) [1]
This allows ionomer intrusion, a function of carbon pore size, and hydrophobicity to impact active sites access. In this new way electrochemically active surface area can be calculated without it being necessary to identify active sites.
These ideas show in the sensitivity of current to various catalyst parameters. The significant increase with hydrophobic pore fraction and catalyst exposed area indicate that catalyst activity is limited primarily by flooding of active sites and exposed catalyst sites. These observations are typical of catalysts with low turn-over and large transport limitations because low turn-over requires a high quantity of active sites to attain high activity and large water transport limitations will cause high capillary pressures increasing flooding.
ACKNOWLEDGEMENT
We gratefully acknowledge the partial financial support from the U.S. Department of Energy (EERE), under a Non PGM Catalyst development effort (Contract no EE 0000459) lead by Northeastern University (Dr. Sanjeev Mukerjee, P.I.).
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Figure 1
. Sensitivity of current density at 0.8 V to various parameters showing how increased exposed surface area and hydrophobicity have the largest impact on activity.