Modeling Impedance Caused by Ohmic Losses in High Surface Area Carbons in Polymer Electrolyte Fuel Cells
Authors: Darling Robert M. , Burlatsky Sergei F. 
Published: December 2020

Platinum alloys catalyze oxygen reduction in conventional polymer-electrolyte fuel cells [1]. The platinum is deposited on carbon, and the nature of the carbon support affects cell polarization. Two commonly used carbon blacks are Vulcan and Ketjenblack. Vulcan (V) has a relatively low surface area of (228 m2/g [2]) and ~95% of the Pt resides on the external surface [3]. Ketjenblack (KB) has a higher surface area, 891 m2/g [2], and ~60% of the Pt resides in micropores in the carbon [3]. Pt/KB displays higher catalytic activity, while Pt/V exhibits better performance at high current densities. Pt inside micropores is not in direct contact with ionomer and, consequently, is not poisoned by adsorbed anions. However, Pt inside KB micropores is relatively inaccessible to oxygen and protons, which detrimentally affects performance at high current densities [4].

The area-specific impedance of catalyst layers is frequently measured by operating cells with nitrogen at the working electrode, to create a blocking electrode, and hydrogen at the counter electrode [5, 6, 7, 8]. The area-specific impedance of the catalyst layer can be determined from the difference between the maximum value of the real part of the impedance at low frequency and the intercept of the impedance with the real axis at high frequency. The area-specific resistance of a catalyst layer determined in this way is R = l⸱(3κ)-1, where l is the thickness of the catalyst layer and k is the effective conductivity of ionomer.

Recently, Harzer et al. deposited platinum on Ketjenblack by different techniques [4]. They fabricated two electrodes with platinum preferentially on the external surface of carbon black, and one electrode with platinum preferentially inside micropores like commercial Pt/KB. They measured higher resistance to oxygen transport, higher values of R, and larger polarization on oxygen at high current densities on catalyst layers having platinum predominantly in micropores. They postulated that higher R was caused by a poorer distribution of ionomer. A different hypothesis is considered in this work: that ohmic losses in micropores contribute significantly to R. Firstly, a formula is developed to describe the impedance of a blocking electrode with substantial resistance inside micropores. Secondly, the model is used to interpret the experiments of Harzer et al. and estimate the conductivity of the pore solution that would fit their results.

Acknowledgements:

This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-EE0007652. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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