Study of the oxygen reduction reaction (ORR) at Pt interfaced with phosphoric acid doped polybenzimidazole at elevated temperature and low relative humidity
Introduction
In recent years, there has been intense research interest in the development of high temperature (120 °C and above) polymer electrolyte membrane (PEM) fuel cells. Operating a PEM fuel cell at elevated temperature will significantly improve tolerance of the Pt/C electrode to carbon monoxide (>1% at 150 °C [1]), and simplify the cooling system for ease of heat dissipation.
One such system, the phosphoric acid doped polybenzimidazole (PBI-H3PO4) phosphoric acid system was introduced by Savinell and Litt [2], [3], [4] and has received a great deal of attention. This membrane system was operated in fuel cells at elevated temperatures (100–200 °C) and low relative humidity. The proton conduction in this membrane system does not depend on water, and it has been shown that the phosphoric acid doped PBI exhibits good proton conductivity at elevated temperature [5], i.e. ∼0.02 to 0.04 S cm−1 under the experimental conditions discussed in the this paper. Other prior studies in our group have shown that phosphoric acid doped PBI has low gas permeability and water electro-osmotic drag [6], good thermal stability [7], and adequate mechanical properties at elevated temperature [8].
Although the elevated temperatures give great advantages to performance, the oxygen reduction reaction still limits the fuel cell efficiency. An understanding of kinetic rates and mechanism of the oxygen reduction reaction (ORR, O2 + 4H+ + 4e− → 2H2O) at the Pt/PBI-H3PO4 interface is fundamentally important, but little work has been done to determine the mechanism of ORR under these elevated temperatures and low water environments.
Zecevic et al. [9] studied the oxygen reduction at Pt/PBI-H3PO4 interface by rotating disk covered with a thin PBI film. The work of Zecevic et al. has shown that the reaction path and kinetics of the oxygen reduction are the same in 0.1 M aqueous solutions of H3PO4, H2SO4 and HClO4 either with or without the presence of the recast polymer films. Their work has also shown that the PBI does not influence the mechanism of the oxygen reduction reaction. The rigid, long PBI chain mainly acts as a matrix to hold the phosphoric acid. However, the experimental condition in their research is substantially different from the Pt/PBI-H3PO4 interface under fuel cell operating conditions since liquid electrolyte is required by rotating disk technique.
The micro electrode technique has also been used to study the oxygen reduction at Pt/polymer electrolyte interface [10], [11], [12]. However, that technique often suffers from the large, non-uniform mass transport resistances. Consequently, the Tafel plot obtained by the technique is often limited to a relative small potential range.
We have reported on a micro band electrode technique to investigate the oxygen reduction reaction at a Pt/PBI-H3PO4 interface at elevated temperature and low relative humidity [13]. The advantages of the micro band electrode design include: a well-defined electrode area; a negligible or tiny IR loss because of micro-order current generated at the working electrode; small and uniform mass transport resistance across the coated polymer electrolyte film, which enables a large well-defined limiting current density; extended range of kinetic current density; well-defined reference electrode; etc. The micro band electrode cell design, the cell design validation, the electrode current distribution, and the criteria for the polymer electrolyte coating thickness have been reported in that paper [13]. In this paper, we apply that experimental technique to acquire data on the effect of different H3PO4 doping level and relative humidity on oxygen reduction performance and mechanism at a platinum surface, and the oxygen transport in PBI-H3PO4 polymer electrolyte.
Section snippets
Experimental
Fig. 1 shows the detail of the micro band electrode (MBE) design and the dimensions. The Pt micro band electrode is fabricated by sputtering platinum on the surface of a smooth Pyrex substrate. The Pt/PBI-H3PO4 interface is formed by coating a thin film (typical thickness ∼ 10 μm) of the polymer electrolyte on top of the platinum micro band electrode. The end of the substrate is then dip coated in the same solution of PBI-H3PO4 (acid and polymer in solvent of trifluoroacetic acid (TFA, Aldrich,
Cell design validation
To study electrochemical kinetics, the experiment setup must have a uniform current distribution on working electrode, negligible or controllable ohmic and mass transport resistances, and a well-defined reference electrode for accurate voltage measurement. A detailed discussion of the cell design validation can be found elsewhere [13]. The current distribution is a tertiary distribution, i.e. ohmic, kinetic, and mass transport resistances affect the current distribution. The uniformity of the
Conclusion
The oxygen reduction reaction on platinum interfaced with phosphoric acid doped PBI at elevated temperature and low relative humidities has been investigated by using micro band electrode technique. Both the kinetic and the mass transport parameters in the Pt/PBI-H3PO4 system are comparable to those of the Pt/H3PO4 system under similar conditions. The study suggests that it is the amorphous H3PO4 that functions as the electrolyte. The comparison of Tafel slopes obtained from different H3PO4
Acknowledgements
Prepared through collaborative participation in the Power and Energy Consortium sponsored by the U.S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement DAAD19-01-2-0010. The U.S. Government is authorized to reproduce and distribute reprints for Government purpose notwithstanding any copyright notation thereon.
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