Development of Anodic Materials with Elevated Hydrogen Sulfide Tolerance for HT-PEMFCs Using Reformat

Proton exchange membrane fuel cells (PEMFCs) became attractive for mobile and portable applications. Even though, there are many aspects that should be optimize to have a better performance and durability [1]. One of these is the resilience of the electrode materials to contaminants generally present in the fuel. In this sense, the fuel for the PEMFC is produced by hydrocarbon reforming in a “hydrogen challenged” economy [2]. Therefore, the presence of contaminants such as CO and H₂S in small amounts (ppm) is unavoidable. Unfortunately platinum (considered the best electrocatalytic material for the hydrogen oxidation reaction -hor-, occurring at the anode of the FC) has a large capability to adsorb these species. Consequently, low quantities of contaminant in the fuel can notably diminish the performance of the FC electrodes.

In contrast to the CO poisoning, there is not so much information regarding to the contamination of the PEMFC electrodes by H₂S. Similarly to CO, bimetallic materials represent a good alternative to investigate in order to increase the tolerance of the fuel cell to the presence of H₂S in the hydrogen used as fuel. In the case of the sulfur species the authors suggest the use of Pt-M electrodes, where M could be: Pd, Cu, Ru, Rh, Ir, Co, etc. [3]. Taking this into account, this work is going to focus on the development of anodic materials with high tolerance to H₂S.  For this purpose the electrocatalytic activities of the materials for the hor in presence of H₂S under HT-PEMFC conditions were evaluated.

The working electrodes tested consist of homemade Pt, M_B (base metal) and Pt-M_B supported on carbon and deposited on a GDL H2315 (FREUDENBERG FCCT SE & CO.KG9). A three electrode cell working under HT-PEMFC conditions and connected to a DEMS (to analyze the released gases) was used to evaluate the inhibition of the material by H₂S. H₃PO₄ 85% (Carl Roth) was used as electrolyte. A RHE in the same medium was used as a reference electrode, and a large area Pt electrode as counterelectrode. The measurements were carried out at 145°C and ambient pressure. Phosphoric-acid-doped polybenzimidazole (PBI) membranes were used as separators.

Dependences of the steady state current on the potential for the tested materials are presented in Figure 1. There is appreciable that M_B is electrochemically inactive for the hor. Even though, at lower overpotentials (where the hor occurred) Pt-M_B shows higher electrocatalytic activity than Pt (more considering that in Pt-M_B the Pt active area expose is smaller). Besides that, by DEMS was verified that the ratio of SO₂ to H₂S in the exhaust is higher for Pt-M_B than for Pt, indicating an increased conversion of H₂S. Also it is observed in Figure 1 that at higher potentials (of interest in the cathodic side of the FC), the Pt-M_B shows higher activity relatively close to Pt. Even more, in Figure 2 is shown that the formation of SO₂ as product of reaction is also very high.

Considering all this experimental evidence it is possible to conceive the implementation of this kind of materials as electrodes in the HT-PEMFCs in order to improve the tolerance to H₂S.

References

1. S. Gottesfeld, T.A. Zawodzinski, in: R.C. Alkire, H. Gerischer, D.M. Kolb, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, vol. 5, John Wiley, 1998, p. 195.
2. B.M. Besancon, V. Hasanov, R. Imbault-Lastapis, R. Benesch, M. Barrio, M.J. Mølnvik, Int. J. Hydrogen Energy, 2009, 34, p. 2350.
3. T. Lopes, V. A. Paganin, E. R. Gonzalez, Int. J. of Hydrogen, 2011, 36, p. 13703.