For proton exchange membrane fuel cells (PEMFCs), meeting automotive lifetime requirements during transient operating conditions such as start-up/shut-down (SUSD) remains one of the main challenges to date. During SUSD, a hydrogen/air front moves through the anode, causing oxidation of the cathode carbon support.¹ While changing the support material from conventional to graphitized carbon blacks has a mitigating effect on degradation,² the dependence of SUSD degradation on temperature and relative humidity (RH) has not yet been thoroughly quantified.

In the first part of this work, we study the effect of temperature on SUSD degradation in 50 cm² single cell experiments and compare the results with model predictions based on the carbon oxidation reaction (COR) kinetics for conventional and graphitized carbon supports. Polarization curves on commercially available membrane electrode assemblies (MEAs) with conventional carbon supports exhibit a clear trend of performance degradation, which is more severe at higher temperatures (Figure 1). While SUSD cycles performed at 5 °C hardly cause any activity loss, SUSD cycles at 40 and 80 °C result in a severe performance degradation, illustrated by comparing polarization curves after 100 SUSD cycles.

Further, a one-dimensional steady-state SUSD model, as proposed by Gu et.al,³ is adapted to predict the COR current induced by SUSD cycles. On MEAs with conventional carbon supports, the predicted COR currents vs. SUSD temperature coincide well with measured degradation rates (≈ 10-fold lower degradation rates at 5 °C vs. 80 °C in comparison to ≈ 8‑fold lower COR currents predicted by our model). However, for MEAs with graphitized carbon support, the measured performance degradation decreases by a factor of ≈ 10 when lowering the SUSD temperature from 80 °C to 5 °C, which is in strong contrast to predicted ≈ 39-fold decrease of the COR currents. As we will show, this discrepancy can be explained by a change of the prevalent degradation mechanism from COR to platinum surface area loss, which is not considered in the model.⁴

In the second part of this work, we focus on the impact of RH on SUSD degradation. The model used in the first part of this study is therefore extended by the water activity dependence of the COR kinetics, which is determined experimentally via exhaust gas analysis in half-cell experiments. A predicted factor of ≈ 4 between the COR during SUSD events carried out at low RH (25 %) and full humidification (100 %) at 80 °C on conventional carbon supported electrodes can be calculated, when a COR reaction order of one with respect to RH is applied. Within the accuracy of the model, this agrees well with the experimentally determined factor of ≈ 3 for the decrease in performance.

In summary, in this work we will compare experimental SUSD degradation rates vs. temperature, RH, and carbon-support type with predictions from a simple kinetic model.

Acknowledgments

The authors gratefully acknowledge financial support by Volkswagen AG and supply of graphitized carbon electrode decals by Greenerity GmbH.

References

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[2] P. T. Yu, W. Gu, R. Makharia, F. T. Wagner, and H. A. Gasteiger, ECS Trans., 3, 797 (2006)

[3] W. Gu, T. Y. Paul, R. N. Carter, R. Makharia, and H. A. Gasteiger, in Modern Aspects of Electrochemistry vol. 49, U. Pasaogullari and C.-Y. Wang Editors, p. 45, Springer New York (2010)

[4] T. Mittermeier, A. Weiß, F. Hasché, G. Hübner, and H. A. Gasteiger, J. Electrochem. Soc., 164(2), F127 (2017)

Figure 1: Polarization curves of commercial Gore Primea Mesga MEAs (0.1/0.4 mg_Pt/cm² on anode/cathode) in pristine state and after 100 SUSD cycles at the indicated temperature. Polarization conditions: 80 °C, 66 % RH, s = 1.5/1.8 (H₂ anode / air cathode), p = 70 kPa_gauge. SUSD conditions: 113 nccm H₂ or air (anode) & air (cathode), ≥100 % RH, p = 50 kPa_{gauge, inlet}.