Differential Cell Study on Impact of in-plane O2 Diffusivity Under Land on O2 Transport Loss
Authors: Kumaraguru Swami , Arisetty Srikanth , Gu Wenbin 
Published: February 2020

Proton-exchange-membrane fuel cells (PEMFC) have been greatly advanced towards commercially viable hydrogen-powered fuel cell systems for automotive application in recent years. Significant reduction in cost has been achieved by reducing the loading of Pt catalyst in the electrodes. Unfortunately lower Pt catalyst loadings accompany higher losses in cell voltage at high power. To quantify various voltage loss terms and thus identify the opportunity for improvement, mathematical models with parameters extracted from experiments are commonly employed. A differential cell operates at high stoichiometry flow with minimal pressure drop down the channel, making it almost one-dimensional across the cell. Hence the experiments are conducted on it to extract model parameters. The cell performance can be described by:

        Ecell = Erev - iRΩ - ηHOR - ηORR - i(RH+,a +RH+
,c) - ηtx O2
           (1)

 where E
cell is the cell voltage; E
rev is the reversible cell voltage, i is the current density; R
Ω

is the sum of the Ohmic resistances of proton conduction through the membrane and of electron conduction (commonly referred to as high frequency resistance or HFR); η
HOR and η
ORR are the charge transfer overpotentials for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), respectively;  is the effective resistance to proton conduction in an electrode with subscripts a and c denoting anode and cathode, respectively; and lastly, is the oxygen transport loss which consists of two parts: one results from oxygen transport through gas phase, and the other is associated with the oxygen transport resistance local to the Pt surface, namely

 ηtx O2 = ηtx O2 
(gas) + ηtx O2 

(Pt-local)  (2)

 Limiting current densities are commonly used to evaluate gas phase and local oxygen transport resistances. Measurements with oxygen pressure and concentration variation can be used to evaluate oxygen transport resistance through the bulk phase, 1 and limiting current density measured on MEAs with varying Pt loading can be used to estimate local transport resistance.2 While both gas phase and local transport resistances are coupled, models with 1D transport lengths (across the gas diffusion layer (GDL), catalyst layer etc.) are used to de-convolute the resistances. The differential cells used are usually land-channel configuration which also has second diffusion length under the land. Transport resistance due to oxygen under the land across the GDL can dominate the total transport resistance. The pO2
is also largely decreased under the land and in an actual (non-differential) cell its impact on local oxygen transport resistance is enhanced down the channel where oxygen concentrations are lower. Therefore an accurate description of transport resistance under the land is needed to decouple the transport resistances. Differential cell tests with an active area of 5 cm2 were designed to study the effects of O2 diffusivity under the land by varying land/channel dimensions or flow field pitch. Cathode land/channel dimensions (mm/mm) of 0.2/0.2, 0.5/0.5, 0.7/0.7 and 1.0/1.0 were studied. In addition to the flow field dimensions, membrane electrode assemblies (MEAs) with different carbon supports and various Pt loadings were fabricated and tested. Catalyst such as Pt/HSC (high surface area carbon) and Pt/Vulcan at different Pt wt% were chosen to provide wide contrast in local oxygen transport resistance owing to the Pt particle distribution and location.  Electrochemical performance of the electrodes were characterized with electrochemical surface area, mass activity and polarization curve measurements. Additionally, protonic resistance of the catalyst layer were determined with electrochemical impedance spectroscopy in H­2-N2.

 An electrochemical model coupled with transport across the cell was developed to interpret the differential cell data. The oxygen transport resistance local to the Pt surface, hypothesized to originate from ionomer-Pt interaction, is also evaluated by model-data comparison. The model parameters perceived to be critical for accurate voltage-loss and cell performance prediction will be examined by sensitivity analysis to design and operating variations

 References

1) D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc., 156, B991 (2009).

2) T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012).