Suppression of Oxygen Reduction Reaction on Pt-Based Electrocatalysts from Ionomer Interaction

Nafion ionomer is incorporated into cathode catalyst layers of PEMFCs to provide protonic pathways for the ORR reaction.  Although it possible to probe the effect of I/C ration in cathode catalyst layers of PEMFCs, the ionomer cannot be completely eliminated.  In contrast, when catalysts are evaluated as thin films fabricated on RDE tips and studied in a non-adsorbing electrolyte such as perchloric acid, ionomer is not essential for protonic conduction and can be eliminated.

   Recently, the effect of Nafion on oxygen reduction reaction (ORR) activity of catalysts has been studied using rotating disk electrode (RDE) methods on single crystalline, polycrystalline Pt surfaces and nanostructured thin film (3M NSTF) in 0.1 M HClO₄ [1-5].  In these studies, thin films (nm to 10’s of nm) were applied to the electrode surfaces by capping with ionomer.  A positive shift on the onset potential for OH and other oxide species formation and a suppression of the ORR activity is typically observed after capping.  Subbaraman et al. [1] attributed the features observed on cyclic voltammograms under inert atmosphere to sulfonate anion adsorption on Pt surfaces using a CO displacement method.  Moreover, the activity trend: Pt (111) < (100) < (110) is identical for Nafion-free surfaces in sulfuric acid and Nafion-capped surfaces in perchloric acid solution that led them to conclude that sulfonate anion adsorption was the main cause for lowered ORR activity.  In contrast to these detailed kinetics studies on well-defined Pt surfaces, little work has been reported on carbon supported nanoparticle platinum (Pt/C) catalysts [6].  The lack of studies is a consequence of following barriers: (i) ionomer inclusion in a catalyst ink often helps to increase ink dispersion (ii) elimination of ionomer can result in poor catalyst film quality (iii) ionomer is often used for adhesion of catalysts films on substrate surfaces [7].  However, we have recently discovered that the use of extremely thin and uniform films is indispensable to the kinetics study of ORR catalysts without hindrance from O₂ diffusion limitations within the catalyst layer and ionomer is not essential to obtain well-dispersed stable thin catalyst films [8].

    In this work, we present results on the effect of Nafion ionomer on the ORR activity as well as CVs under inert atmosphere for several commercial Pt/C and Pt Alloy/C catalysts by employing thin-uniform catalyst films obtained by advanced film fabrication techniques [8].  Nafion capped catalyst films were obtained by impregnating diluted Nafion solution onto pre-formed Nafion-free ultra-thin films.  The sensitivity of the specific activity of various catalysts including Pt/HSC, Pt/Vulcan, Pt_xCo_1-x/C at 0.9 V vs. RHE in 0.1 M HClO₄ to the ionomer to carbon (I/C) ratio will be discussed.

References

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[2] R. Subbaraman, D. Strmcnik, A. P. Paulikas, V. R. Stamenkovic, and N. M. Markovic, ChemPhysChem, 11 (2010) 2825.

[3] A. M. Gómez-Marín, A. Berná, and J. M. Feliu, J. Phys. Chem. C, 114 (2010) 20130.

[4] M. Ahmed, D. Morgan, G. A. Attard, E. Wright, D. Thompsett, and J. Sharman, J. Phys. Chem. C, 115 (2011) 17020.

[5] K. Kodama, A. Shinohara, N. Hasegawa, K. Shinozaki, R. Jinnouchi, T. Suzuki, T. Hatanaka, and Y. Morimoto, J. Electrochem. Soc., 161 (2014) F649.

[6] S. S. Kocha, J. W. Zack, S. M. Alia, K. C. Neyerlin, B. S. Pivovar, ECS Trans., 50 (2) (2012) 1475.

[7] T. J. Schmidt, H. A. Gasteiger, O. D. Staeb, P. M. Urban, D. M. Kolb, R. J. Behm, J. Electrochem. Soc., 145 (1998) 2354.

[8] K. Shinozaki, B. S. Pivovar, S. S. Kocha, ECS Trans., 58 (1) (2013) 15.