Investigation of PEMFC Design and Operation

PEMFC performance is strongly dominated by operating temperature, inlet relative humidity (RH) and flow pattern etc. Proper operating temperature leads to high reaction rate and low activation and ohmic losses. Appropriate RH prevents water clogging effect, moisturizes the membrane and increase the electrical conductivity of membrane. Besides, the performance of PEMFC is also depended on the distributions of reactant species, and therefore, the primary goal of flow pattern design is to increase reactant species uniformity, which can lead to uniform distributions of current density and temperature. Thus, performance would be improved, the lifetime of PEMFC would be extended, and the water flooding effect would be alleviated.

Hontañón et al. [13] tested various widths of flow channels and ribs to determine the effect of reactant species’ consumption. According to this study, optimal rib width is 1mm, and optimal channel widths are 3 and 4 mm, instead of not 1 mm. In addition, the optimal consumption rate is 45.7%. Scholta et al.

[14] investigated the mass transport and electrical conductivity of a flow pattern by varying the width of channels and ribs. Their analytical results showed that an optimum value of 0.7-1 mm applies to channel and rib widths. For wide dimensions of width, the influence on mass transport or lateral electrical conductivity is significant. For very small dimensions of widths, the

manufacturing effort becomes excessively high and the probability of channel clogging via the formation of water droplets increases. The influence of aspect ratio (AR) channel height to width—was tested by Chiang and Chu [15].

According to their simulation results, membrane electrical conductivity increases when AR decreases. Shimpalee and Van Zee [16] investigated how serpentine flow fields with different channel/rib cross-sectional areas affect performance and species distributions for both automotive and stationary conditions. Their simulation results revealed that for a stationary condition, a narrow channel with wide rib spacing improves performance; however, the opposite occurs when the automotive condition is applied.

In order to study the effect of humidity on the performance, Fell et al. [17]

assessed the performance of experimental flow-field designs under different levels of humidity by utilizing a single-phase isothermal model of PEMFC segments. Shimpalee [18] studied the humidification effect numerically and experimentally. The results indicated that dry steams on either anode or cathode cause a low membrane electrical conductivity and performance. On the other hand, super-saturation streams results in a higher current density such that improves the performance. A few years later, Matamoros and Brüggemann [19]

adopted steady and three-dimensional models to investigate the influence of geometrical parameters on the performance under different hydrating conditions.

According to their results, anode and cathode liquid water saturation affects species transport and the polymer electrolyte water content. Thus, one must simultaneously calculate both water absorption and desorption through the polymer electrolyte and liquid water saturation in anode and cathode porous media to acquire an actual view of ohmic and concentration losses in PEMFC performance. The performance of PEMFC is considerably improved by

applying 100% RH at inlet flow in comparison with 50% RH, and such better performance is achieved especially at high current densities. Zhang et al. [20]

studied the effect of RH on the performance as well. The fuel cells were performed at a typical high temperature 120°C, ambient pressure and various RHs from 25% to 100%. The experimental results indicated that the membrane resistance at 25% RH is about five times higher than that at 100%.

At high RHs, the membrane adsorbs more water than at low RHs which enable more ionic clusters are filled with water, and therefore, protons can transport easily as free ions through membrane, results in low membrane resistance. As expected, the PEMFC performance increases dramatically with increasing RH.

Since temperature plays an important role in PEMFC operation, the investigations for the effect of temperature on PEMFC is necessary. Coppo et al. [21] proposed a three-dimensional model that accounts for water transport in the liquid, gas and dissolved phases to study the effect of temperature-dependent parameter variations on cell performance. The results showed that in the activation regime of the polarization curve, the performance of PEMFC is mainly improved by high temperature that leads to high values of both exchange current density and charge transfer coefficient. In ohmic region of the polarization curve, benefits of running the cell at high temperature can be explained with the high membrane ionic conductivity and an increase of water diffusivity as well as water content, which results in a decrease of electrical resistance to ion transport. Al-Baghdadi [22] numerically investigated the effect of temperature distribution on material deformation. According to the results, the non-uniform distribution of stresses, caused by the temperature gradient and moisture change in the cell, induces localized bending stresses and deformation, which can contribute to delaminating between membrane, GDL

and bipolar plates, especially in the cathode side. Also, the results indicated that the maximum von Mises stress exists on the corner of flow channels and the interface between membrane and GDL.

The effects of the permeability of the electrodes and the type of gas flow distributors on the PEMFC performance were investigated by Soler et al. [23].

The results of simulation showed that the effect of permeability is not notable in the case of grooved plates, but it is rather significant in case of the solid plates.

The results also revealed that the performance with solid plates declines when the permeability of the electrodes decreases. Oosthuizen et al. [24]

numerically tested the effect of channel-to-channel gas crossover on the pressure and temperature distribution. The results revealed that flow crossover is only significant when the porosity of the GDL exceeds approximately 0.65. And flow crossover tends to decrease the pressure drop across the flow channel.

Also, the dominant factor in determining the temperature is the thermal conductivity of the flow plate material instead of the crossover. Shimpalee et al. [25] investigated various channel path lengths to estimate the impact of flow path length on temperature distributions, current density distributions and the performance. According to their results, local temperature, water content and current density distributions become more uniform under serpentine flow-field designs with shorter path lengths or greater number of channels. Karvonen et al. [26] proposed a parallel flow pattern that had uniform flow distribution by both numerical and experimental analyses. The inlet distributor of flow pattern is narrowed that leads to a better equality of flow velocities in different channels.

The difference between the largest and the smallest velocities is decreased from 16% to 8%. Sun et al. [27] presented a model considering a serpentine flow channel with trapezoidal cross-section shape. The obtained results indicated

that an increase in the trapezoidal cross-section shape ratio R is associated with an increase in the flow-cross through GDL. And R has a significant effect on the pressure variation in the flow field. As R value increases, the pressure drop increases slightly for the cross-over case. Also, an increase in Re is associated with a slight increase in the flow cross-over. Park and Li [28] numerically and experimentally studied the characteristics and effect of cross flow through the porous electrode structure between two adjacent flow channels. The results indicated that the thickness and permeability of the GDL are the two most important parameters influencing the cross flow and the resultant pressure drop.

Another numerical investigation on the influence of flow pattern geometry was carried out by Jeon et al. [29] for four 10 cm² serpentine flow-fields with single channel, double channel, cyclic-single channel and symmetric-single channel patterns at 100% and 64% inlet HR. According to their results, the double channel flow-field was found to have the highest performance at 100% inlet HR and to have most uniform current density distribution. However, for 64% inlet HR, there were little difference on performance and current density uniformity among the four serpentine flow-field patterns.