Effect of Organic Model Compound Contaminants on Platinum Catalysts

Polymer electrolyte membrane fuel cells (PEMFCs) hold great promise for providing clean energy without the use of fossil fuels.  A major obstacle that must be overcome before PEMFC’s can successfully compete in a commercial market is durability [1]. The oxygen reduction reaction (ORR) occurring at the cathode is the limiting process in PEM fuel cell performance due to its high activation energy and slower kinetics compared to the hydrogen oxidation reaction at the anode [2]. The catalyst layer facilitating the reaction is typically composed of carbon supported platinum or platinum alloy and is highly susceptible to poisoning from contaminants either entering through the fuel or air streams or being introduced internally through system component degradation [3]. Thus, recognizing potential catalyst contaminants, understanding their adsorption characteristics and mitigating their subsequent performance effects are crucial to improving durability and enabling commercialization of fuel cells.

Research has shown through studies involving commercial membranes and model compounds that when perfluorinated sulfonic acid (PFSA) membranes are exposed to hydroxyl radicals during fuel cell operating conditions, severe chemical decomposition products can be generated [4]. The main membrane degradation compounds of Nafion and 3M commercial membranes were identified by Zhou et al. [4]. Along with losses in membrane conductivity and structural integrity, such degradation products may also adsorb onto the Pt based electrocatalyst, possibly leading to a loss in catalyst electrochemical surface area (ECA), ORR activity, or both. To date, little effort has been reported on the impact PFSA degradation compounds have on catalyst performance.

This work investigates adsorption characteristics and effects from several model compounds, representing PFSA membrane chemical degradation species, on ECA and ORR activity for platinum based electrocatalysts including polycrystalline Pt, high surface area carbon supported Pt, and extended surface Pt. Adsorption properties due to carboxylate and sulfonate functional groups, fluorocarbon chain length, and model compound concentration were investigated using a variety of electroanalytical techniques including cyclic and linear sweep voltammetry and electrochemical quartz crystal microbalance (EQCMB) will be reported.

A rotating disk electrode electrochemical setup utilizing 0.1 M perchloric acid electrolyte (diluted from 70% HClO₄ double distilled veritas grade, GFS Chemical) was used to study the effects of the several model compounds on the ORR activity of polycrystalline Pt as well as commercially available carbon supported Pt electrocatalysts.  After injecting the model compound into the electrochemical cell and determining the effects on ECA and ORR activity, the electrode was then analyzed in clean electrolyte in order to monitor the recoverability of the activity loss for the Pt catalyst. EQCMB was used to measure the change in mass of the electrode as a function of potential, after the organic compound was added to the perchloric acid electrolyte.

This work on the effect of membrane decomposition products on Pt ECA and ORR activity is part of a larger DOE-funded system contaminants project. To better understand the potential adverse effects that system contaminants may have on the fuel cell performance and durability and to lower the cost of balance of plant (BOP) components, the larger project studies families of BOP materials including structural materials, hoses, elastomers for seals and (sub)gaskets, and assembly aids (adhesives, lubricants). For more information, please see http://www.nrel.gov/hydrogen/contaminants.html.

The authors would like to acknowledge funding from the U.S. Department of Energy EERE Fuel Cell Technologies Office, under Contract No. AC36-08GO28308 with the National Renewable Energy Laboratory and collaborations with colleagues at 3M and GM. Membrane degradation products for this study were provided by 3M.

References:

[1]   Zamel, N.; Li, X., Progress in Energy and Combustion Science 2011, 37 (3), 292-329.

[2]   Mayrhofer, K. J. J.; Strmcnik, D.; Blizanac, B. B.; Stamenkovic, V.; Arenz, M.; Markovic, N. M., Electrochimica Acta 2008, 53 (7), 3181-3188.T. Okada, F.N. Büchi et al. (eds.), Polymer Electrolyte Fuel Cell Durability Springer Science Business Media, LLC 2009.

[3]   Okada, T. Polymer Electrolyte Fuel Cell Durability 2009, Springer Science Business Media, LLC.

[4]   Zhou, C.; Guerra, M. A.; Qiu, Z. M.; Zawodzinski, T. A.; Schiraldi, D. A., Macromolecules 2007, 40 (24), 8695-8707T. Xie, C.A. Hayden. Polymer 48 (2007) 5497e5506.