The Rate Limiting Step (RLS) for the Oxygen Reduction Reaction at the Cathode of Polymer Electrolyte Membrane Fuel Cell

Proton exchange membrane fuel cells (PEMFCs) can be operated by hydrogen and pure oxygen (not air) by in-situ photo-catalytic splitting of water (gasoline of the future). Recently, several auto makers have introduced H2-fed fuel cell vehicles on the road. However, some concerns, i.e., catalyst and membrane’s cost and durability, catalyst activity, membrane proton conductivity, and water management, still exist. In order to make the fuel cell vehicle affordable and durable, further work needs to be done to improve each of the above mentioned aspects [1]. In a recent review by Ramaswamy and Mukerjee [2] have shown that the mechanism proposed by Damjanovic et al. describes the rate RLS. The purpose of this publication is to show that the rate limiting step for the ORR at the cathode of PEMFC is the rate of dissolution of gases oxygen in the aqueous media. It is well known that the Henry constant for oxygen in aqueous solutions is very low at range of temperature between 60 to 80 ̊ C (figure 1). The second purpose is to derive an equation that shows the strong relationship between the mass transfer coefficient and oxygen solubility. Strong supports for such a mechanism are:

1-In 2010 Erlebacher and co-workers [3] discovered a new ionic liquid that has strong affinity to absorb oxygen and were used to improve the rate of ORR.  2- The enhancement of the rate of ORR after humidification of the cathodic region (humid air contains dissolved oxygen) while humidification of the anodic region (can humidify the membrane and increase its conductivity) has no effect on the rate of ORR [4-6]

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2-Ramaswamy N., and Mukerjee S. Fundamental Advances in Physical Chemistry Article ID 491604, (2012).

3-Snyder et al. Nature Materials 9, 904, (2010).

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5-Xu H., Song Y., Kunz H.R., and Fenton J.M. , 205th meeting of the Electrochemical Society, San Antonio, Texas, (2004).

6-Zhang J., Tang Y., Song C., et al.  Electrochimica Acta 53, 5315–5321 (2008).

7- Jeon D.H., Kim K.N., Baek S.M.,Nam J.H. In. J.of Hydrogen Energy, 36,19, 12499-12511 (2011).