PEM fuel cell relative humidity (RH) and its effect on performance at high temperatures
Introduction
As researchers work on increasing the power density, reducing the cost, and improving the reliability/durability of proton exchange membrane fuel cells (PEMFCs), high temperature operation of a PEMFC at >90 °C is considered an effective way to improve performance in terms of reaction kinetics, catalyst tolerance, heat rejection, and water management [1], [2]. However, when the fuel cell is operated at high temperatures using a perfluorosulfonic acid (PFSA) membrane (e.g. Nafion® membrane) as an electrolyte, a high relative humidity or nearly saturated humidity (RH >80%) is still required in order to obtain practical performance because the conductivity of the PFSA membrane depends on its water content. Therefore, relative humidity is one of the important operation conditions potentially affecting fuel cell performance [3], [4], [5], [6].
Recently, many studies have examined the effects of RH on PEMFC performance [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. This research includes modeling [7], performance evaluation [8], [9], [10], [11], [12], [13], [14], [15], [16], flow field design [7], membrane preparation and modification [12], [17], [18], [19], [20], [21], degradation of MEA [22], [23], [24], oxygen reduction reaction (ORR) [25], [26], [27], and catalyst layer design [28]. The results indicate that RH reduction can strongly influence fuel cell performance by increasing the membrane resistance [8], [12], [13], [16], [19], [29], decreasing the proton activity in catalyst layers [8], [12], [19], [29], [30], decreasing the Pt utilization [12], [19], [27], [30], reducing the electrode kinetics [7], [8], [13], [16], [26], [27], and increasing the gas mass transfer resistance [7], [8], [12], [13], [19], [31]. Most of these studies are focused on low-temperature PEM fuel cells, although some of the literature examines the RH effect in high-temperature PEMFCs [8], [12], [24], [26], [32]. Our results show that even in a PEMFC operated at 120 °C, RH can still play a significant role in performance [32].
In order to fundamentally understand and then effectively improve the performance of a high-temperature PEMFC, it is necessary to conduct a systematic investigation of RH distribution inside a fuel cell and its effect on the performance in terms of fuel cell electrode reaction kinetics, mass transfer, and membrane conductivity. In this study, Nafion 112-based membrane electrode assemblies (MEAs) were assembled in an in-house, high-temperature, single-cell hardware for tests and diagnosis [33]. The fuel cells were operated at a typical high temperature of 120 °C, ambient pressure, and various RHs from 25 to 100%.
Section snippets
Experimental
The membrane electrode assembly (MEA), with an active area of 4.4 cm2, was prepared by hot pressing the anode, a Nafion 112 membrane, and the cathode together at 135 °C and 75 kg/cm2 for 2 min. The gas diffusion electrode (GDE) was prepared by spraying a homogeneous catalyst ink – consisting of catalyst, Nafion solution, and iso-propanol – onto a gas diffusion layer (GDL). This GDL was a PTFE and carbon black impregnated carbon paper (Toray, TGP-H-060). E-TEK 20% Pt/Ru/C and 40% Pt/C were used as
Mathematical analysis of the relative humidity (RH) in a fuel cell
In an operating PEM fuel cell, water was introduced into the fuel cell at anode and cathode, together with their respective gases, and reached the catalyst layers through their respective flow fields and gas diffusion layers. At the anodic catalyst layer, part of the water combined with protons and reached the cathodic catalyst layer via the membrane as electro-osmotically dragged water. The remaining water was drained using anodic exhaust gas. At the cathodic catalyst layer, oxygen molecules
Conclusions
The RH-related water balance inside a fuel cell was analysed and several equations were introduced as a function of fuel cell gas-stream inlet and outlet pressures, inlet RHs, temperature, pressure drops across flow channels, and reactant partial pressures. The inlet and outlet RHs are different, due to fuel cell reactions and drops in flow channel pressure. Therefore, an average RH between inlet and outlet RHs was defined to reflect the actual RH at which the fuel cell operates.
The effect of
Acknowledgements
The authors would like to thank the NRC National Fuel Cell Program and the NRC Institute for Fuel Cell Innovation for their financial supports. The authors also wish to acknowledge discussions with Dr. Jing Li, Ms. Keping Wang, Dr. Paul Kozak, and Mr. Scott McDermid from Ballard Power Systems Inc.
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