Ice Formation and Mechanical Stress in the PEFC during Cold Start

Self-startup of polymer electrolyte fuel cell (PEFC) without external heating is advantageous for the system. It is generally recognized that the produced ice accumulating in the cathode catalyst layer (CL) reduce the electrochemically active area (ECA) for oxygen reduction reaction (ORR) and the open pore volume for oxygen diffusion. Visualization results obtained from a transparent PEFC shows the importance of the catalyst layer design and gas purge to freeze-starts.^1,2 A cyclic voltammetry (CV) technique was developed to investigate the effect of ice formation in the cathode CL on electrochemically active Pt area during and post subzero startup of a PEFC.³Mechanical stress built in the cell during cold start may limit its lifetime. To our knowledge, no work has been done about mechanical stress during cold start in PEFC.

A cyclic voltammetry (CV) technique is developed to further investigate the effect of membrane thickness and startup current density on cumulative product water, rate of ice formation in electrode and rate of water absorption by ionomer and membrane in PEFC during cold start. Figure 1a shows typical CVs of the cathode prior to and post cold start. Obviously, the electrochemically active Pt surface area of the cathode CL post cold start is lower than that prior to. It is found that thick membrane offers much more capacity for water to be absorbed by the membrane at −10^oC and −20^oC. The product water almost does not increase with membrane thickness for starting from −30^oC because water absorption by the membrane is negligible. The rate of ice formation in cathode catalyst layer increases with current density but decreases with the startup temperature. The rate of water absorption by the membrane increases with the startup temperature, current density and membrane thickness. 

A mathematical model is developed to investigate mechanical stress during cold start in PEFC. Ice produced in the catalyst layer forces the catalyst layer to expand. Figure 1b shows the effect of cold-start temperature on stress during cold start. Cold starting from low temperature brings high compress mechanical stress. Cold starting from low temperature also bring high strain in CL, but low strain in the membrane. The membrane is compressed during cold start from -30^oC. That is because the amount of water absorbed by membrane is negligible but CL expands a lot. This work demonstrates the effect of thickness of gas diffusion layer (GDL), micro-porous layer (MPL) and CL on the mechanical stress built in the membrane. Membrane constraint is considered in the model.⁴The simulation result shows that mechanical stressed during purge, cool down and cost start operation do not vary serious due to elastic and porous MPL and GDL. The model is to help elucidate the fundamental physics of ice formation in the PEFC during cold start.

Figure 1. (a) Cyclic voltammograms of the cathode prior to and post cold start at startup temperature of −10^oC and −30^oC. (CCM-25 mm. m_Pt=0.4 mg/cm². Startup conditions: i_start=0.05 A/cm², x_H2=14.35, x_air=12.06, RH_H2=RH_air=0%, p_H2=p_air=1 atm. CV conditions: T_CV=T_start, n=50 mV/s, F_N2=F_H2=40 mL/min, RH_N2=RH_H2=0%.) and (b) Effect of cold-start temperature on stress during cold start (l_Initial= 6.0).

References

1. S. Ge and C. Y. Wang, Electrochem. Solid-State Lett., 9 (2006) A499.

2. S. Ge and C. Y. Wang, Electrochim. Acta, 52 (2007) 4825.

3. S. Ge and C. Y. Wang, J. Electrochem. Soc.154 (2007) B1399.

4. A. Z. Weber and J. Newman, AIChE J., 50 (2004) 3215.