Elsevier

Journal of Power Sources

Volume 195, Issue 9, 1 May 2010, Pages 2806-2809
Journal of Power Sources

Short communication
Cell resistances of poly(2,5-benzimidazole)-based high temperature polymer membrane fuel cell membrane electrode assemblies: Time dependence and influence of operating parameters

https://doi.org/10.1016/j.jpowsour.2009.10.100Get rights and content

Abstract

Time-dependent measurements of cell impedance of a HT-PEFC based on ABPBI were performed at constant frequencies close to the high-frequency (h.f.) intercept of the corresponding Nyquist plots with the real axis. The h.f. impedances approximate the ohmic resistance of the cell and they decrease, when current (140 mA cm−2) is switched on. Steady-state values are attained after 10 min. Vice versa, when current is switched off (OCV), the h.f. impedances instantaneously increase but reach steady-state values only after about 1 h. These values rise with increasing gas flow rates. The results are discussed in terms of hydration/dehydration processes, changing the equilibrium between orthophosphoric and pyrophosphoric acid and thus the conductivity of the electrolyte as well as the mobility of molecules and charge carriers. Impedance spectra were recorded after each time-dependent measurement under OCV conditions. The fit of these impedance data based on an equivalent circuit revealed ohmic resistances corrected by h.f. inductances and low frequency impedances associated with the cathode oxygen exchange reaction. The charge transfer resistances deduced from the low frequency impedances strongly depend on both air and hydrogen flow rates.

Introduction

Polymer electrolyte fuel cells (PEFCs) with significantly higher working temperatures than 100 °C do not require high-purity hydrogen as fuel, and therefore they are beneficial for devices that run on hydrogen produced on-site by reforming of hydrocarbon energy carriers, e.g. in the field of mobile and stationary applications where middle distillates (kerosene, diesel, fuel oil) can be used to provide the necessary hydrogen.

Most of today's high temperature cells (HT-PEFCs) rely on membrane electrode assemblies (MEAs) with phosphoric acid absorbed in PBI (=poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole)) as electrolyte [1]. Recently membranes based on the chemically related ABPBI (=poly(2,5-benzimidazole)) have emerged as a promising alternative [2], [3], [4].

Various aspects of the steady-state cell impedance of HT-PEFCs based on PBI as membrane polymer at different operating conditions have already been reported in the literature (e.g. [5], [6], [7]). The influence of water activity on the steady-state conductivity of the electrolyte [8], [9] as well as the distribution of phosphoric acid within the membrane electrode assembly (MEA) [10], [11] are intensively studied at present.

We will not only discuss the influence of several testing parameters on the cell resistance of a MEA based on ABPBI instead of PBI, but we will also turn our attention to its time dependence after changes of the operating conditions.

Section snippets

Experimental

MEAs (A = 14.4 cm2) have been assembled from ABPBI membranes (FuMA-Tech) and gas diffusion electrodes (∼1 mg Pt cm−2) produced in-house which have been impregnated with appropriate amounts of phosphoric acid [4]. Except the experiments with humidified hydrogen, where MEAs with 26 μm thin ABPBI membranes were used, all studies were performed with MEAs including 38 μm thin ABPBI membranes. They have been characterized in a single cell using pure hydrogen and air at ambient pressure as the reactants.

Results and discussion

After changing the operating parameters of a HT-PEFC MEA it takes several minutes up to a few hours before the ohmic resistance reaches a steady state. This is particularly the case when the current density is changed. While at OCV ohmic resistances of 250–300  cm2 have been observed, these values drop down within a few minutes to about 200  cm2 when current is drawn from the cell.

Fig. 1 shows the time-dependent behaviour of the ‘ohmic resistance’, more precisely the high-frequency impedance at

Conclusions

It turns out, that time-dependent measurements of cell impedance at different operating conditions are crucial since they provide a valuable insight into the hydration/dehydration processes of HT-PEFCs. We attribute the fact that high water production by the fuel cell reaction, high water insertion and its low removal by the gas flows (i.e. the water content in the cell) lowers the ohmic resistance to the increasing amount of mobile H3PO4 in the electrolyte. The hydration process is fast

Acknowledgement

We thank Matthias Prawitz for performing the impedance measurements.

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