PEMFC membrane dehydration

Water management is one of the most important aspects for the proper operation of PEMFCs. If there is too much water in a PEMFC, it will cause cathode flooding. On the other hand, if there is too little water in a PEMFC, it wiU result in proton exchange membrane dehydration. The cathode... 

Three possible concepts for hydrogen-chlorine fuel cells have been evaluated by Thomassen et al. (2006) (1) an ordinary PEMFC based on a Nafion membrane, (2) a fuel cell based on a combination of circulating hydrochloric acid and a Nafion membrane, and (3) a system based on a phosphoric acid-doped polybenzimidazole (PBI) membrane. None of these systems were able to demonstrate stable operation, due to catalyst corrosion, membrane dehydration, and/or electrode flooding. 

Two major membrane faults, that is, water flooding and membrane dehydration, will be discussed. Several models with these two faults will be overviewed in this section. In Sections 12.4.2 and 12.4.1, their respective causes and consequences on PEMFC performance will be discussed. Finally, two fault-anbedded models will be introduced for instances in Section 12.4.3. 

Gerteisen et al. [28] have summarized the PEMFC modeling development and introduced a ID, two-phase, transient model including GDL, catalyst layer, and membrane, under the assumption that GDL, catalyst layer, and membrane are spatially resolved in ID with an agglomerate approach for the structure of catalyst layer. The faults of water flooding and membrane dehydration are anbedded by the assumption that the saturation due to the continuous capillary pressure and immobile saturation due to the mixed wettability of the GDL structure are discontinued. In order to allow dehydration of the ionomer on the anode side, the water content is not a constant but follows the Cauchy boundary condition. The model is validated by voltammetry experiments, and the simnlated cnrrent responses are compared with the measured ones from chronoamperometric experiments. 

Water flood and membrane dehydration are two main PEM performance limitations. Rama et al. [35] summarized live top events reflecting PEMFC performance degradation (1) activation losses, (2) mass transportation losses, (3) ohmic losses, (4) fuel efficiency losses, and (5) catastrophic cell failnre. Here, the activation, mass transportation, and ohmic losses will be discnssed in details since they are closely related to membrane failures. 

The EOD coefficient, is the ratio of the water flux through the membrane to the proton flux in the absence of a water concentration gradient. As r/d,3g increases with increasing current density during PEMFC operation, the level of dehydration increases at the anode and normally exceeds the ability of the PEM to use back diffusion to the anode to achieve balanced water content in the membrane. In addition, accumulation of water at the cathode leads to flooding and concomitant mass transport losses in the PEMFC due to the reduced diffusion rate of O2 reaching the cathode. 

The actually developed PEMFCs have a Nafion membrane, which partially fulfills these requirements, since its thermal stability is limited to 100 °C and its proton conductivity decreases strongly at higher temperatures because of its dehydration. On the other hand, it is not completely tight to liquid fuels (such as alcohols). This becomes more important as the membrane is thin (a few tens of micrometers). Furthermore, its actual cost is too high (more than 500 m ), so that its use in a PEMFC for an electric car is not cost competitive. 

Another issue for PEMFCs is that of water management. Whereas, dehydration of the membrane leads to a reduction in proton conductivity, an excess of water may lead to the electrode being flooded. 

The conductivity of the membrane depends on the degree of its hydration. However, this is altered by a great many phenomena in the fuel cell. The need to preserve the water content of the electrolyte membrane limits the operating temperature of the PEMFC to below 80°C. Beyond this temperature, the membrane becomes dehydrated, the ionic resistance increases very rapidly and the performances suffer drastically. The thickness of the membrane results from a compromise between low ionic resistance - which requires as thin a membrane as possible - and sufficient mechanical strength. Indeed, it has to be able to withstand pressure variations on both sides of its surface during the operation of the cell. 

The performance of a HT-PEMFC depends mainly on the amount of phosphoric acid in the polymer membrane and in the porous catalyst layer as well as on the temperature. Furthermore, it is well known that phosphoric acid dehydrates at low water vapour partial pressure and rehydrates with increasing partial pressure, effects which can be observed in HT-PEMFCs under operating conditions [1, 2]. The composition change of phosphoric acid results in a variation of the ionic conductivity as well as of the viscosity. 

The main reasons for PEMFC performance degradation at high temperatures are material shortages, memrane dehydration, gas crossover through the membrane, and accelerated material aging. 

Tsushima et al. (2010) developed an MRI system to investigate the effects of relative humidity (RH) and current density on the transverse water content profile in a membrane under fuel cell operation at a practical PEMFC operating temperature. The MRI visualization revealed that in dry conditions (40% RH), the membrane hydration X number was 3, and the water content profile in the membrane was fiat because the diffusion process in the membrane was dominant in the water transport. In a standard condition (80% RH) the water content in the membrane was 8, and a partial dehydration at the anode was observed at a current density of 0.2 A/cm, indicating that electroosmosis was influential. At the higher RH level of 92%, the water content X within the membrane at 0.2 A/cm was around 22, corresponding to the eqnilibrium state of the membrane in liquid water, and the water content profile with the increase in current density became fiat. This indicates that the liquid water generated in the cathode catalyst layer permeated the membrane, where water transport plays a more dominant role. 

PEMFC functions twofold, it acts as the electrolyte that provides ionic communication between the anode and cathode, and also serves as a separator for the two reactant gases. Both optimized proton and water membrane transport properties and proper water management are crucial for efficient fuel cell operation. Dehydration of the membrane reduces proton conductivity, and excess water can flood the electrodes. Both conditions may result in poor cell performance. 

The membrane has two functions. First, it acts as the electrolyte that provides ionic conduction between the anode and the cathode but is an electronic insulator. Second, it serves as a separator for the two-reactant gases. Some sources claim that solid polymer membranes (e.g., sulfonated fluorocarbon acid polymer) used in PEMFC are simpler, more reliable, and easier to maintain than other membrane types. Since the only liquid is water, corrosion is minimal. Pressure balances are not critical. However, proper water management is crucial for efficient fuel cell performance [6]. The fuel cell must operate under conditions in which the by-product water does not evaporate faster than it is produced, because the membrane must be hydrated. Dehydration of the membrane reduces proton conductivity. On the other hand, excess of water can lead to flooding of the electrodes. 

Zirconium phosphate (ZrP) has been widely used as inorganic fillers to modify Nation to get high performances in both PEMFC and DMFC at high tem-peratures. " It was observed that a higher loading of ZrP made the composite membrane less sensitive to the dehydration effect and reduced methanol crossover. 

---