One of the major challenges in the commercialization of direct methanol fuel cell (DMFC) is the effect of metal cation contaminants on its performance¹. The metallic cations such as Fe³⁺, Cr³⁺ and Ni²⁺ may leach as a result of the degradation of catalyst layers and bipolar plates, which can further influence different parts of fuel cells². While many studies have focused on the effect of contaminants on the membrane and cathode part of fuel cells³, the effect of metal cations on the anode part of DMFCs is not well understood. In a previous work by authors, the effect of various metal cations (i.e., Co²⁺, Ni²⁺, Zn²⁺) on polycrystalline Pt during methanol electro-oxidation reaction (MOR) was studied. The results showed that MOR peak current density decreased with increasing the concentration of metal solutions. The purpose of current study is to understand how metal cation can affect the mechanism of MOR on the anode electro-catalyst of DMFC. The MOR at the DMFC anode occurs at the interface of the electro-catalyst layer composed of PtRu supported on carbon (PtRu/C) and ionomer. In order to differentiate the effect of metal contaminants on Nafion-ionomer from the Pt surface during MOR, the influence of Ni²⁺on Nafion-coated Pt and bare Pt electrodes is studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Nafion-coated Pt electrode was prepared by applying 10μL Nafion (5wt.%) on an electrochemically cleaned polycrystalline Pt disk embedded in Teflon rod. The electrode was dried for a minimum of 24 hours prior to performing the experiments. The Nafion-coated Pt and bare Pt electrodes were electrochemically cleaned using cyclic voltammetry prior to exposure to 0.2 M NiSO₄ solutions at open circuit potential for 24 hours. CV and EIS techniques were performed to examine the mechanism of methanol oxidation reaction on these electrodes in the absence and presence of Ni²⁺ solutions.

The cyclic voltammograms of these electrodes in 0.5 M H₂SO₄ and 0.2 M NiSO₄ solutions showed that Ni²⁺ has lower effect on Nafion-coated Pt compared to the bare Pt electrode under conditions used in the study. Similarly, the CV results of MOR in the presence of Ni²⁺ solutions show that the MOR peak current may drop significantly on bare Pt, as compared to the Nafion-coated Pt. To further characterize the effect of Ni²⁺ on the methanol electro-oxidation reaction, EIS was performed at different potentials, E= 0.35-0.75 V vs reversible hydrogen electrode. Analyzing the EIS results show that at low potential, the Nafion-coated and bare Pt electrodes have similar behaviors in the presence and absence of Ni²⁺. Therefore, it can be suggested that Ni²⁺ has a negligible effect on CH₃OH dissociation and dehydrogenation at low potentials. However, at more positive potentials, the EIS results of Nafion-coated Pt show smaller impedance compared to the bare Pt in the presence of Ni²⁺ solutions. Therefore, these results suggest that Nafion plays a role as an inhibiting factor, and might block Ni²⁺ effect on the Pt active sites, resulting in higher MOR current density and lower impedance than bare Pt in the presence of Ni²⁺solution.

The recovery tests were performed by transferring the contaminated electrodes to non-contaminated solution. Results show a higher recovery potential for Nafion-coated Pt as compared to bare Pt electrodes.

The results of this study can assist in better understanding of anode performance in the presence of metal contaminants to improve the performance of fuel cell.

Acknowledgement

This work was supported by the Institute of Energy and Climate Research (IEK), Jülich.

References

1.         S. Chen, H. A. Gasteiger, K. Hayakawa, T. Tada, and Y. Shao-Horn, J. Electrochem. Soc., 157, A82 (2010).

2.         A. Pozio, Silva, R.F.,  De Francesco, M.,  Giorgi, L.  , Electrochim. Acta, 48, 1543 (2003).

3.         H. Li, J. Gazzarri, K. Tsay, S. Wu, H. Wang, J. Zhang, S. Wessel, R. Abouatallah, N. Joos and J. Schrooten, Electrochim. Acta, 55, 5823 (2010)