HFR Relaxation

Another interesting phenomenon observed so far is that after purge the cell HFR gradually decreases in a time scale of hours, which is called HFR relaxation after purge. Typical results of FIFR relaxation are shown in Fig. 20. When the 60-s purge is completed, the valves at the inlet and outlet of the cell for both anode and cathode gas lines are closed and the cell temperatures are maintained constant at the purge cell temperature during the whole relaxation process. 

Fundamental mechanisms of this relaxation phenomenon remain unknown and need future investigation. However, correlating the T-compensated HFR after relaxation with that after purge is of practical interest, because the initial membrane water content critically important for PEFC cold-start performance corresponds to the HFR after relaxation (during cool down), not the FIFR immediately after purge. For this reason, an empirical correlation is attempted between the HFR after purge and that after relaxation, as shown in Fig. 21. It can be seen that a reasonable correlation exists over a 

The correlation shown in Fig. 21 provides a practical means to estimate the HFR or membrane water content as the important input to evaluate cold-start performance. That is, one can estimate the HFR after purge from Eq. (10) based on the purge conditions, and subsequently correct for HFR relaxation using Fig. 21. Based on the HFR value after relaxation or prior to cold start, one can use the analytical models and performance data developed in previous work to estimate the cold-start performance. 

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Figure 21. Relation between HFR after relaxation and HFR after 60-s purge with purge gas RH greater than 40%. (reproduced with permission from Tajiri et at.33)...

This suggests a very simple way to include a substantial part of the correlations. One may simply perform a Hartree-Fock calculation for the ground and for the excited states, calculate the length and velocity forms of the cross section and take the geometric mean of the two. This is referred to as the HFU approximation if the continuum states are unrelaxed, and as the HFR if the Hartree-Fock geometric mean is calculated using relaxed final states. 

Ba. HFU (Hartree-Fock unrelaxed) and HFR (Hartree-Fock relaxed) curves calculated by Kelly et al., ref. 59. RPAE calculated by Fliflet et al., ref. 64. RPAER calculation by Amusia et al, ref. 63. Expt. by Hecht and Lindau, ref. 65. 

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