Analysis of the Influence of Temperature and Gas Humidity on the Performance Stability of Polymer Electrolyte Membrane Fuel Cells

Abstract

Adequate water management represents one of the main challenges in the design and operation of polymer electrolyte membrane fuel cells. In this work, the influence of inlet gas humidification on cell performance is investigated by in-situ current density measurements obtained using the segmented cell approach. Particular attention is paid to the combined effect of cell temperature and relative humidity of the anode and cathode feed streams. When operated at 80 • C and low humidity conditions, the cell is seen to undergo a severe voltage decline that is not observed at 60 • C. The analysis shows that the variation with temperature of the water uptake rate of the gaseous streams plays a key role in determining the observed differences in performance stability. In the case of 60 • C operation, the water uptake rate of the cathode stream at 50% inlet relative humidity is roughly 30% of its value at 80 • C at the same humidification level, resulting in a significantly lower drying capacity. A simple balance of water model, able to explain the observed cell behavior, is finally presented and discussed. Energy demand has become one of the most serious concerns of modern society due to the problems related with greenhouse gas emissions and the depletion of fossil fuels. In this context, hydrogen is expected to play an important role as future energy vector, with polymer electrolyte membrane fuel cells (PEMFCs) being the leading candidates to provide efficient and clean electric energy conversion during the XXI century. Recently, significant progress has been made toward meeting the challenging cost and performance targets required for the widespread use of PEMFCs, specifically in the automotive industry. 1The state-of-the-art of polymer electrolyte membrane fuel cell technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature between 60• C and 80 • C. 2 Since the ionic conductivity of PFSA membranes depends on the water content of the membrane, 3,4 water management is one of the most important issues for successful operation, high performance, and good durability of PEMFCs. Excess inlet gas humidification as well as condensation processes within the cell are likely to produce the accumulation of liquid water in the porous electrodes and gas diffusion media (effect known as flooding), thereby decreasing cell performance. On the other hand, an insufficient level of gas humidification lowers the ionic conductivity of the membrane and also results in a performance reduction.Numerous studies have investigated the operation of PEMFC under dry conditions in order to simplify operation. 5,6 Early work to demonstrate stable performance for PEMFC using dry or slightly humidified gases has been reported by Büchi et al. 5 Strategies for operating polymer electrolyte fuel cells include also the reduction of humidification of both reactant gases 7-10 or the dry operation of the cathode 11,12 or anode 13 sides. In the last decade, a wide variety of diagnostic and visualization tools have b...

The effects of cathode flow channel size and operating conditions on PEM fuel performance: A CFD modelling study and experimental demonstration

“…Therefore, keeping the temperature at the inlet constant, the mixture composition at the anode/cathode inlet is varied in order to have a set of relative humidity (ie, 35%, 55%, and 75%) through changing the temperatures of the bubble humidifier; the large is the temperature of the humidifier, and the larger is the relative humidity. We have limited the relative humidity investigation to the cathode side as it, ie, the cathode relative humidity, has a substantially greater effect on the performance of the fuel cell than that of the anode side . It can be observed from Figure A that at high and medium voltages, there is virtually no difference between the results.…”

“…The parameters are normally optimized to achieve the best possible fuel cell performance. For example, the relatively high and low humidity of the inlet gases could cause water flooding and membrane dehydration, respectively; therefore, the humidity of the inlet gases must be optimized to prevent the occurrence of the above 2 detrimental phenomena . However, the optimization of the parameters influencing the fuel cell performance is not normally straightforward because of the interactions existing between them.…”

“…However, the optimization of the parameters influencing the fuel cell performance is not normally straightforward because of the interactions existing between them. For example, to achieve the optimum level of humidity, some other parameters, eg, temperature and flow rate, should be controlled; therefore, such interactions must be taken into account in the optimization procedures …”

“…For example, the relatively high and low humidity of the inlet gases could cause water flooding and membrane dehydration, respectively; therefore, the humidity of the inlet gases must be optimized to prevent the occurrence of the above 2 detrimental phenomena. [13][14][15][16] However, the optimization of the parameters influencing the fuel cell performance is not normally straightforward because of the interactions existing between them. For example, to achieve the optimum level of humidity, some other parameters, eg, temperature and flow rate, should be controlled; therefore, such interactions must be taken into account in the optimization procedures.…”

“…For example, to achieve the optimum level of humidity, some other parameters, eg, temperature and flow rate, should be controlled; therefore, such interactions must be taken into account in the optimization procedures. 13 Design wise, there have been many numerical and experimental studies to investigate the effects of the flow channel geometry on the PEM fuel cell performance; see, for example, other studies. [16][17][18][19][20] The effect of channel depth on the current distribution within the fuel cell was investigated numerically in Inoue et al 20 Three channel depths were analysed, namely, 0.5, 1.0, and 1.5 mm.…”