REACTION ENGINEERING The fuel cell is connected in series to an external load resistance R L . The voltage and current are measured across R L . The fuel cell was constructed such that the residence times of the reactant gases (Volume/Flow rate) are larger than the characteristic diffusion time (Volume 2/3 /D). Flow rates of the reactant gases (1-10 mL/min) were controlled with mass flow controllers. As the reaction progresses, the amount of water in the membrane changes and this alters the membrane resistance, R M (a dryer membrane indicates a higher value of R M ). We study the fuel cell behavior under the influence of four operating parameters: temperature (T), external load resistance (R L ), flow rates of reactants into the anode and cathode, and relative humidity of the inlet reactant streams. PERSPECTIVE OF PEM FUEL CELLS: THE STIRRED TANK REACTOR AS A BUILDING BLOCK Ee-Sunn J. Chia, Jay B. Benziger, and Ioannis G. Kevrekidis Department of Chemical Engineering Princeton University Princeton, NJ 08544 Introduction There has been widespread research on polymer electrolyte membrane (PEM) fuel cells since they have been touted as the next alternative power source. The PEM fuel cell consists of two reactors separated by an ion-conducting barrier (membrane). Protons produced at the anode are transported across the membrane to the cathode where water is produced. The two sequential chemical reactions in a PEM fuel cell are thus coupled to the transport of the intermediate products between the reactors. Before PEM fuel cells become commercially viable for mobile applications, predictive models of fuel cell performance that correctly incorporate the transient interplay of reaction and transport processes are critical. Figure 2. Stirred tank reactors in series approximate an integral reactor. The reactors are parallel electrically. PEM fuel cells are often constructed and modeled as two- and three- dimensional integral reactors. 1-4 These integral reactors may be modeled as differential (stirred tank) reactors in series. A differential fuel cell reactor has uniform compositions at the anode and cathode; the only spatial gradients are transverse through the membrane. This circumvents the complexities of the existing integral reactors since there are no spatial variations involved in the stirred tank reactor. From a differential PEM fuel cell, we are able to extract valuable information pertaining to the kinetics and the convective transport. We have shown that a stirred tank PEM fuel cell reactor exhibits ignition/extinction phenomena and have reproducibly demonstrated steady state multiplicity. 5-7 Integral Reactor. The other extreme of a well-mixed reactor is the plug flow (integral) reactor where the reactants flow downstream without dispersion. The stirred tank PEM fuel cell can be thought of as a differential element in an integral reactor. Therefore, we may approximate the standard serpentine flow PEM fuel cell integral reactor by placing many stirred tank PEM fuel cells in series as shown in Figure 2. The membrane hydration in each tank will vary downstream. Thus each tank is associated with a different R M . Although several reactors are placed in series, they are electrically in parallel as depicted in Figure 2. We present here a summary of the stirred tank PEM fuel cell reactor findings, the remarkable analogy to the autocatalycity in an exothermic stirred tank reactor, and an extension of the stirred tank PEM fuel cell to approximate the conventional integral reactor. Results and Discussion Multiple steady states. Water plays an important role in the fuel cell because it eases proton transport from the anode to the cathode. In fact, Yang et al. has shown that the membrane conductivity is exponentially dependent on the water activity in the membrane but only weakly dependent on temperature. 8 It is the water balance in the fuel cell that leads to the multiple steady states shown in Figure 3. The steady state current achieved in a stirred tank reactor fuel cell depends on the initial water content in the membrane. 5 Moreover, the water balance in the stirred tank reactor fuel cell is analogous to the energy balance in the classical chemical engineering exothermic stirred tank reactor. PEM Fuel Cell Reactor I Nafion Membrane Hydrogen in Relative Humidity Differential Reactor. The differential PEM fuel cell shown in Figure 1 consists of two stirred tank reactors separated by the membrane electrode assembly (MEA) with a membrane area of 1 cm 2 . The current collecting graphite plates contained gas plenums with volumes of 0.2 cm 3 . Pillars were machined onto the graphite plates to ensure a uniform sealing pressure on the MEA. The fuel cell was designed to ensure uniform gas compositions above the membrane. Relief RL Electrode assembly V Oxygen In Thermocouple Figure 1. Schematic depiction of the stirred tank PEM fuel cell reactor and its equivalent circuit. The membrane electrode assembly separates the anode and cathode chambers. Figure 3. Start-up of the PEM fuel cell (T=50 O C) with different initial membrane water contents (λ = H 2 O/SO 3 ) and dry feeds. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2004, 49(2), 794