Towards Estimating the Effect of SO3- Adsorption on the ORR in Pt (111)
Authors: Salvado Miguel Barreiros , Schott Pascal , Guetaz Laure , Gerard Mathias , Bultel Yann 
Published: May 2020

Optimizing proton exchange membrane fuel cells (PEMFC) performance is crucial in order to attain sustainable commercialization. A major part of the issue involves understanding the role of Nafion in the catalyst layer. Nafion’s behavior on bulk mode has been extensively reported in the literature, though on ultra-thin layer mode (<20nm) its structural arrangement which dictates the transport properties highly depends on the type of substrate interfacing [1]. In PEMFC catalyst layers, Nafion covers carbon-supported platinum aggregates on ultra-thin layer mode. In these composite layers, Nafion works as a binder for the aggregates, at the same time as it is meant to ensure pathways for protons and let arrive to the catalyst surface.
At high current densities, a steep performance drop is usually observed in PEMFC operation. A part of this drop is reported to be due to activation losses as a doubling of the Tafel slope is observed [2]. The origin of this Tafel slope doubling has been subject of multiple studies. Understanding the causes requires understanding the phenomena occurring at the electrode/electrolyte interface. A technique which is commonly employed is cyclic voltammetry (CV) as it can provide insights regarding these interactions.
Comparative CV studies on monocrystalline platinum Pt (111) in a PFSI solution and solution exhibit distinct features, where oxide formation appears to be inhibited at the beginning of this range for PFSI [3]. These CV studies coupled with electrochemical quartz crystal microbalance (EQCM) measurements showed that when extending the potential range to 1.4V and holding the potential at 1.1V (place-exchanged oxide formation range) a large mass gain at 0.5V emerges. The origin of these features is found to be due to strongly adsorbed sulfonate groups on the platinum surface. Other works validate this behavior [4], where sulfonate groups are found to be adsorbed in both the double layer region (0.4-0.5V) and in the hydroxyl adsorption region (0.6-0.85V).
In this work, the continuum model proposed by Huang et al. [5] is coupled with a reaction framework comprising multistep mechanism [6]. The adsorption of SO3- is expected to follow a Langmuirian behavior [7], indicating that the oxygen reduction reaction (ORR) is hindered through site blocking. For the adsorption of on Pt(111) a single electron transfer mechanism is assumed [8]. Kinetic parameters are obtained by fitting the model’s response to experimental data as in [6].
The simulations allow quantifying the adsorption of sulfonate groups on the platinum surface and estimating its impact on the ORR.
References:
1. A. Kusoglu, A.Z. Weber, Chemical Reviews, 117 : 987-1104 (2017).
2. P. Subramanian, T.A. Greszler, J. Zhang, W. Gu, R. Makharia, Journal of The Electrochemical Society, 159(5) : B531-B540 (2012).
3. T. Masuda, F. Sonsudin, P.R. Singh, H. Naohara, K. Uosaki, The Journal of Physical Chemistry C, 117 : 15704-15709 (2013).
4. K. Kodama, R. Jinnouchi, T. Suzuki, H. Murata, T. Halanka, Y. Morimoto, Electrochemistry Communications, 36 : 26-28 (2013).
5. J. Huang, A. Malek, J. Zhang, M.H. Eikerling, The Journal of Physical Chemistry C, 120: 13587-13595 (2016).
6. B. Jayasankar, K. Karan, Electrochimica Acta, 273 : 367-378 (2018).
7. S.M. Andersen, Applied Catalysis B : Environmental, 181: 146-155 (2016).
8. K. Kodama, K. Motobayashi, A. Shinohara, N. Hasegawa, K. Kudo, R. Jinnouchi, M. Osawa, Y. Morimoto, ACS Catalysis, 8 : 694-700 (2018).