Water Vapor Transport Measurement across PEMs

Polymer electrolyte membrane (PEM) high performance without external humidification is essential to reduce cost, and simplify design for early commercialization and adoption of fuel cell electric vehicles (FCEVs). Water transport property through PEM is one of the key parameters to design low-cost and high-performance fuel cell system. In this work, a simple and inexpensive water transport measurement set-up has been developed to obtain a water vapor rate (WVR) at PEMFCs operating conditions in order to characterize and compare PEMs. PEM polarization curves have been evaluated at differential relative humidity (RH) to understand effect of water transport and humidification of the anode and cathode.

Water transport occurs due to electro-osmotic drag from anode to cathode and concurrent back-diffusion from cathode to anode. This transport depends primarily on thickness, equivalent weight (EW) and operating conditions. New PEMs with low thickness, low EW, and optimal distribution of H⁺conducting functional groups are being developed to improve water transport and conductivity. These improvements will reduce ohmic losses and allow for FCEV operation at low RH using system-generated water at cathode [1]. Therefore, it is important to measure water transport for new membranes characterization. In PEMFCs without external humidification, system-generated water provides hydration to the membrane mostly on cathode side while the anode side suffers from dehydration because of electro-osmotic drag. Additionally, cathode operation at high RH can cause flooding that limits the performance severely at high current density.

Ex-situ water transport measurements, especially at PEMFCs operating condition, are extremely difficult, expensive, and require sophisticated instruments [2-3]. The WVR set-up developed in this work consists of fuel cell test stand, cell hardware, and a humidity sensor near the anode outlet to measure the outlet RH and corresponding dew point. WVR is obtained from the ideal gas law as shown below.

                                                                                    WVR = P_H2O.V /( R.T.A)                       [1]

Here WVR is in μmol.cm^-2.min^-1. P_H2O is the water vapor pressure at anode outlet dew point, V is dry gas (N₂) flow rate at anode outlet, T is system temperature, R is universal gas constant and A is membrane active area. The cathode was supplied with humidified N₂ at a target RH and the anode was supplied with dry N₂to carry permeated water vapor to the humidity sensor. Stabilized RH and dew point at anode outlet were recorded to obtain water vapor pressure.

Fig. 1 shows the WVR results obtained from Eqn. [1] for NR 211 (25μm) and Nafion XL (30μm). As expected, WVR increases by 3 times for cathode RH from 30 to 100% because water transports faster due to increased water activity at cathode. NR 211 showed higher WVR than that of Nafion XL because NR 211 is thinner and non-reinforced. These results suggest that, at same conditions, NR 211 should produce higher IV performance than Nafion XL.

IV performance of a Nafion XL CCM was evaluated under differential RH conditions to understand the importance of anode humidification as shown in Fig. 2. The anode and cathode were maintained at unequal RH. It is clear from Fig. 2 that differential RH operation in which the anode is at 100% RH and the cathode at low RH produces similar performance to the anode and cathode both at 100% RH, and higher performance than with the anode at low RH and cathode at high RH. Fig. 2 also shows that anode at high RH provides a greater advantage over low anode RH with increasing current density. These results suggest that anode humidification is important and may require an external humidifier, whereas a cathode external humidifier can be eliminated as it can self-humidify from system-generated water.

In this study, WVR across variety of reinforced and non-reinforced PFSA PEMs will be presented to compare their water transport behavior. Polarization curves for variety of PEMs will be discussed to emphasize the importance of water transport and differential RH operation that can help eliminate cathode humidification to simplify FCEVs design.

 References

1. W. Liu, ECS Trans., 50(2), 51-64 (2012).

2. Adachi et al., J. Electrochem. Soc., 156(6), B782-B790 (2009).

3. Majsztrik et. al., J. Membrane Sci. 301, 93-106 (2007).

Fig. 1 Preliminary WVR results as a function of cathode RH.

Fig. 2 IV performance of a CCM at differential RH.