Page 149 - Introduction to Transfer Phenomena in PEM Fuel Cells

138 Introduction to Transfer Phenomena in PEM Fuel Cells
4.4.3. The effect of temperature on the performance of the
PEMFC
The temperature is of paramount importance, directly or indirectly
influencing the performance of a PEMFC. Indeed, variations in temperature
affect phenomena related to the reaction kinetics, the transport of water, the
humidity, the membrane conductivity, the tolerance of the catalyst and the
dissipation of heat. A good review of the role of temperature in PEMFCs
was presented by Zhang et al. [ZHA 06] also aimed at the development of
high temperature PEMFCs. Song et al. [SON 07] experimentally studied the
performance of the Nafion membrane [TAN 97], which constitutes a
PEMFC. The authors concluded that battery performance increases with
increasing temperature from ambient to 80°C. They also found that at low
current density (< 0.4 A/cm²), an increase in temperature decreases the
performance of the fuel cells; for high current densities (> 0.4 A/cm²), an
increase in temperature implies an increase in performance. In their
experiments, they found that the best performance was at around 80°C, with
an absolute pressure of 3 bar and a relative humidity of 100%.

4.5. Heat sources in the PEMFC

Wang [WAN 04] mentioned in his review of PEMFC modeling that, in
most modeling studies, the heat released due to entropy change has always
been neglected. There is a good deal of fuel cell modeling going from simple
empirical models to complex CFD models. The existence of journals in the
literature such as [BLY 05, SIE 08, WEB 04a] clearly indicates the extent of
ongoing research around the world on the subject of PEMFCs. In work by
Hashmi [HAS 10], it is well explained that, in the literature, there is no
consensus on the magnitude and location of the reversible heat production at
each electrode. Several modeling studies (non-isothermal) of fuel cells
[BAC 08, NGU 04, SAD 09] have been based on Lampinen results
[LAM 93] for the entropy change if all the reversible heat is produced at the
cathode.

Wöhr et al. [WÖH 98] used different values for entropy change at the
anode and at the cathode, except that the change in total entropy (for the
whole reaction) did not correspond to the change in entropy of the two half
reactions.