210th ECS Meeting , Abstract #615, copyright ECS
Interpretation of Low-Frequency Inductive Loops in PEM Fuel Cell Impedance Data Sunil Kumar Roy and Mark E. Orazem
A similar model was developed which accounted for Pt dissolution and associated deactivation of catalytic activity. This model, too, yielded low-frequency inductive behavior. An experimental investigation is underway to distinguish between reactions responsible for the lowfrequency features in the impedance response.
University of Florida, Department of Chemical Engineering PO Box 116005, Gainesville, FL 32611-6005, USA
-0.12
The objective of the present work is to develop models that incorporate processes that may be responsible for the inductive response seen at low frequencies. We have developed a mathematical model for the overall impedance response proposing formation of hydrogen peroxide as an intermediate in two steps oxygen reduction reaction (ORR). Hydrogen peroxide is considered a degrading agent for materials used in the fuel cell components (membrane, electrodes) and its formation in the fuel cell operating conditions is reported by Mittal et al.4 The calculated impedance response, presented in Figure 2, shows inductive features at low frequency. Peroxide formation kinetics dominates in the calculated impedance response at low frequency while the oxygen reduction reaction ORR dominates at high frequency. The calculated result shown in Figure 2 is in quantitative agreement with the experimental results. The experimental results may be influenced by the consumption of reactants in the flow channels, and work is underway to improve the uniformity of the reactant concentration. This work shows that peroxide formation is one of the possible explanations for the low-frequency inductive loop in the impedance response of the fuel cell.
Zj / Ω cm
2
Impedance measurements were conducted with the 850C fuel-cell test station supplied by Scribner Associates. The experiments were conducted in galavanostatic mode in frequency range of 0.001–3000 Hz with 10 mA peak-topeak sinusoidal perturbation. Ten points per frequency decade were recorded at 40 °C and 1 atmosphere using ultra pure hydrogen as anode fuel and compressed air as oxidant. The experimental data shown in Figure 1 was first analyzed with the measurement model developed by Agarwal et al.3 that can be used to filter and thus identify non-stationary (drift) and high frequency (noise) components in experimental data.
0.08 A/cm 2 0.12 A/cm 2 0.18 A/cm 2 0.22 A/cm 2 0.26 A/cm
1.99 Hz
-0.08 -0.06 -0.04 -0.02 0.00 0.02 0.00
0.04
0.08
0.12
0.16
0.20
0.24
2
Zr / Ω cm
Figure 1: Experimental Impedance Responses of PEM Fuel Cells with steady-state current density as a parameter.
0.08
2
0.06
Zj / Ω cm
Circuit analogues are commonly used to interpretation impedance data for fuel cells. Circuit models are not unique and lead to ambiguous explanations of the processes. Very few impedance models have been developed from first principles, and none of them has interpreted low-frequency inductive loops in protonexchange-membrane fuel cells (PEMFCs). Side reactions and intermediates in the fuel cell may be possible reasons for this inductive loop.1 These side reactions and intermediates affect the lifetime and performance of the fuel cell, which is one of the most crucial issues in commercialization of the fuel cell. The inductive loop can also be due to or affected by the non-stationary behavior of the fuel cell. In our previous work, we have demonstrated that the inductive loop was characteristic of the process once the fuel cell operates under stationary conditions.2
2
-0.10
0.04
10 Hz 1.0 Hz
0.02 0.00
100 Hz
0.01 Hz 0.001 Hz
-0.02 0.00
0.04
0.08
0.12
Zr / Ω cm
0.16
0.20
2
Figure 2: Predicted Impedance Response of PEM Fuel Cells. Acknowledgement Financial support under NASA grant NAG3-2930 is gratefully acknowledged. References 1. R. Makharia, M. F. Mathias, and D. R. Baker, J. of the Electrochemical Society, 152 (5) (2005), A970. 2. S. K. Roy and M. E. Orazem, Abstract # 1163, 209th meeting of the Electrochemical Society, Denver, May 2006. 3. P. Agarwal, M. E. Orazem, and L. H. Garcia- Rubio, J. of the Electrochemical Society, 142 (1995), 4159. 4. V. Mittal, R. Kunz, and J. Fenton, Abstract # 37, 209th meeting of the Electrochemical Society, Denver, May 2006.
