The PEMFC uses a solid polymer membrane as an electrolyte. The main components of this fuel cell are an electron-conducting anode consist­ing of a porous gas diffusion layer as an electrode and an anodic cata­lyst layer; a proton-conducting electrolyte, a hydrated solid membrane; an electron-conducting cathode consisting of a cathodic catalyst layer and a porous gas diffusion layer as an electrode; and current collectors with the reactant gas flow fields (see Fig. 9.3).

In the PEMFC, platinum or platinum alloys in nanometer-size par­ticles are used as the electrocatalysts with NafionTM (a DuPont trade­mark) membranes [3, 10—12]. The polymer electrolyte membranes have some unusual properties: In a hydrated membrane, the negative ions are rigidly held within its structure and are not allowed to pass through.

Electric current

,-WV

Only the positive ions contained within the membrane are mobile and free to carry a positive charge through the membrane. The proton exchange membrane (PEM) is a good conductor of hydrogen ions (pro­tons), but it does not allow the flow of electrons through the electrolyte membrane. As the electrons cannot pass through the membrane, elec­trons produced at the anode side of the cell must travel through an external wire to the cathode side of the cell to complete the electrical circuit in the cell.

In the PEMFC, the positive ions moving through the electrolyte are hydrogen ions, or protons. Therefore, the PEMFC is also called a proton exchange membrane fuel cell. The polymer electrolyte membrane is also an effective gas separator; it keeps the hydrogen fuel separated from the oxidant air. This feature is essential for the efficient operation of a fuel cell.

The heart of a PEMFC is the membrane electrode assembly (MEA), consisting of the anode-electrolyte-cathode assembly that is only a few hundred microns thick [11].

Electrochemistry of PEM fuel cells. All electrochemical reactions consist of two separate reactions: an oxidation half reaction occurring at the anode and a reduction half reaction occurring at the cathode.

Oxidation half reaction: 2H2 ^ 4H+ + 4e~

Reduction half reaction: O2 + 4H+ + 4e~ ^ 2H2O

Overall cell reaction: 2H2 + O2 ^ 2H2O

The H2 half reaction. At the anode, hydrogen (H2) gas molecules diffuse through the porous electrode until they encounter a platinum (Pt) par­ticle. Pt catalyzes the dissociation of the H2 molecule into two hydrogen atoms (H) bonded to two neighboring Pt atoms; here each H atom releases an electron to form a hydrogen ion (H+). These H+ ions move through the hydrated membrane to the cathode while the electrons pass from the anode through the external circuit to the cathode, resulting in a flow of current in the circuit.

The O2 half reaction. The reaction of one oxygen (O2) molecule at the cathode is a four-electron reduction process that occurs in a multistep sequence. The catalysts capable of generating high rates of O2 reduction at relatively low temperatures (~80°C) appear to be the Pt-based expen­sive catalysts. The performance of the PEMFCs is limited primarily by the slow rate of the O2 reduction half reaction, which is many times slower than the H2 oxidation half reaction.

Electrolyte. The polymer electrolyte membrane is a solid organic poly­mer, usually poly-[perfluorosulfonic] acid. Atypical membrane material used in the PEMFC is Nation [11, 12]. It consists of three regions:

1. The teflon-like fluorocarbon backbone, hundreds of repeating —CF2—CF—CF2— units in length

2. The side chains, — O-CF2-CF-O-CF2-CF2-, which connect the molec­ular backbone to the third region

3. The ion clusters consisting of sulfonic acid ions, SO3~ H+

The negative ion SO3~ is permanently attached to the side chain and cannot move. However, when the membrane becomes hydrated by absorbing water, the hydrogen ion becomes mobile. Ion movement occurs by protons (H ) bonded to water molecules, hopping from one SO3 site to another within the membrane. Because of this mechanism, the solid hydrated electrolyte is an excellent conductor of hydrogen ions.

Electrodes. The anode and the cathode are separated from each other by the electrolyte, the PEM. Each electrode consists of porous carbon to which very small Pt particles are bonded. The porous electrodes allow the reactant gases to diffuse through each electrode to reach the catalyst. Both platinum and carbon are good conductors, so electrons are able to move freely through the electrode [13, 14].

Catalyst. The two half reactions occur very slowly under normal con­ditions at the low operating temperature (~80°C) of the PEMFC. Therefore, catalysts are needed on both the anode and cathode to increase the rates of each half reaction. Although platinum is a very expensive metal, it is the best material for a catalyst on each electrode.

The half reactions occurring at each electrode can occur only at a high rate at the surface of the Pt catalyst. A unique feature of Pt is that it is sufficiently reactive in bonding H and O intermediates, as required to facilitate the electrode processes, and is also capable of effectively releas­ing the intermediate to form the final product. The anode process requires Pt sites to bond H atoms when the H2 molecule reacts; next, these Pt sites release the H atoms, as follows:

2H+ + 2e~ ^ H2
H2 + 2Pt ^ 2(Pt-H)

2(Pt-H) ^ 2Pt + 2H+ + 2e~

This optimized bonding to H atoms (neither very weak nor very strong) is a unique property of the Pt catalyst. To increase the reaction rate, the catalyst layer is constructed with the highest possible surface area. This is achieved by using very small Pt particles, about 2 nm in diameter, resulting in an enormously large total surface area of Pt that is accessible to gas molecules. The original MEAs for the Gemini space program used 4 mg of platinum per square centimeter of membrane area (4 mg/cm2). Although the technology varies with the manufacturer, the total platinum loading has decreased from the original 4 mg/cm2 to about 0.5 mg/cm2. Laboratory research now uses platinum loadings of 0.15 mg/cm2. For catalyst layers containing Pt of about 0.15 mg/cm2, the thickness of the catalyst layer is ~10 ^m; the MEA with a total thickness ~200 ^m can generate more than half an ampere of current for every square centimeter of the MEA at a voltage of 0.7 V between the cath­ode and the anode [2, 3, 10-12]. Recently, scientists at Los Alamos National Laboratory, USA have developed a new class of hydrogen fuel cell catalysts that exhibit promising activity and stability. The cata­lysts, cobalt-polypyrrole-carbon (Co-PPY-XC72) composite, are made of low-cost metals entrapped in a heteroatomic-polymer structure.

The cell hardware. The hardware of the fuel cell consists of backing layers, flow fields, and current collectors. These are designed to maxi­mize the current that can be obtained from an MEA. The backing layers placed next to the electrodes are made of a porous carbon paper or carbon cloth, typically 100-300 ^m thick. The porous nature of the back­ing material ensures effective diffusion of the reactant gases to the cat­alyst. The backing layers also assist in water management during the operation of the fuel cell; too little or too much water can halt the cell operation. The correct backing material allows the right amount of water vapor to reach the MEA and keep the membrane humidified.

Carbon is used for backing layers because it can conduct the electrons leaving the anode and entering the cathode. Apiece of hardware, called a plate, is pressed against the outer surface of each backing layer. The plate serves the dual role of a flow field and current collector. The side of the plate next to the backing layer contains channels machined into the plate. The plates are made of a lightweight, strong, gas-impermeable, electron-conducting material; graphite or metals are commonly used, although composite material plates are now being developed. Electrons produced by the oxidation of hydrogen move through the anode, through the backing layer, and through the plate before they can exit the cell, travel through an external circuit, and reenter the cell at the cathode plate. In a single fuel cell, these two plates are the last of the compo­nents making up the cell.

In a fuel cell stack, current collectors are the bipolar plates; they make up over 90% of the volume and 80% of the mass of a fuel cell stack [11, 15, 16].

Water and air management. Although water is a product of the fuel cell reaction and is carried out of the cell during its operation, it is nec­essary that both the fuel and air entering the fuel cell be humidified. This additional water keeps the polymer electrolyte membrane hydrated. The humidity of the gases has to be carefully controlled, as too little water dries up the membrane and prevents it from conducting the H+ ions and the cell current drops. If the air flow past the cathode is too slow, the air cannot carry all the water produced at the cathode out of the fuel cell, and the cathode “floods.” Cell performance deteri­orates because not enough oxygen is able to penetrate the excess liquid water to reach the cathode catalyst sites. Cooling is required to maintain the temperature of a fuel cell stack at about 80°C, and the product water produced at the cathode at this temperature is both liquid and vapor.

Performance of the PEM fuel cell [3,11,16]. Energy conversion in a fuel cell is given by the relation:

Chemical energy of the fuel = electric energy + heat energy

Power is the rate at which energy (E) is made available (P = dE/dt, or AE = PAt). The power delivered by a cell is the product of the current (I) drawn and the terminal voltage (V) at that current (P = IV watts). In order to compute power delivered by a fuel cell, we have to know the cell voltage and load current. The ideal (maximum) cell voltage (E) for the hydrogen/air fuel cell reaction (H2 + 1/2O2 ^ H2O) at a specific tem­perature and pressure is calculated from the maximum electrical energy

AG

Wel = — AG = nFE or E = —

TP

where AG is the change in Gibbs free energy for the reaction, n is the number of moles of electrons involved in the reaction per mole of H2, and F (Faraday’s constant) = 96,487 C (coulombs = joules/volt). At a constant pressure of 1 atm, the change in Gibbs free energy in the fuel cell process (per mole of H2) is calculated from the reaction temperature (T) and from changes in the reaction enthalpy (H) and entropy (S).

AG = AH — T AS

= -285,800 J — (298 K)(-163.2 J/K) = -237,200 J

For the hydrogen-air fuel cell at 1 atm pressure and 25°C (298 K), the cell voltage is

AG

nF

As temperature rises from room temperature to the PEM fuel cell operating temperature (80°C or 353 K), the change in values of H and S is very small, but T changes by 55°C. Thus the absolute value of AG decreases. Assuming negligible change in the values of H and S,

AG = -285,800 J/mol — (353 K)(163.2 J/mol • K)

= -228,200 J/mol

Therefore,

Thus, for standard pressure of 1 atm, the maximum cell voltage decreases from 1.23 V at 25°C to 1.18 V at 80°C. An additional correc­tion is needed for using air instead of pure oxygen, and also for using humidified air and hydrogen instead of dry gases. This further reduces the maximum voltage from the hydrogen-air fuel cell to 1.16 V at 80°C and 1 atm pressure. With an increase in load current, the actual cell potential is decreased from its no-load potential because of irreversible losses, which are often called polarization or overvoltage (h). These origi­nate primarily from three sources:

■ Activation polarization (hact)

■ Ohmic polarization (hohm)

■ Concentration polarization (hconc)

The polarization losses result in a further decrease in actual cell volt­age (V) from its ideal potential E (V = E — potential drop due to losses). The activation polarization loss is dominant at low current density. This is because electronic barriers have to be overcome prior to current and ion flows. Activation polarization is present when the rate of an electro­chemical reaction at an electrode surface is controlled by sluggish elec­trode kinetics. Therefore, activation polarization is directly related to the rates of electrochemical reactions. In an electrochemical reaction with hact > 50 — 100 mV, activation polarization is described by a semi­empirical equation known as the Tafel equation:

ha“ = ІЗ) ln (t)

where a is the electron transfer coefficient of the reaction at the elec­trode (anode or cathode), and i0 is the exchange current density. The Tafel slope for the PEMFC electrochemical reaction is about 100 mV/decade at room temperature. Thus there is an incentive to develop electrocat­alysts that yield a lower Tafel slope [2, 3, 11, 12].

Ohmic losses occur because of the resistance to the flow of ions in the electrolyte and resistance to the flow of electrons through the electrode materials. Decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte can reduce the ohmic losses. Both the electrolyte and fuel cell electrodes obey Ohm’s law; the ohmic losses can be expressed by the equation: hohm = iR, where i is the current flowing through the cell and R is the total cell resistance, which includes ionic, electronic, and contact resistance. See Figure 9.4.

Due to the consumption of reactants at the electrode by an electro­chemical reaction, the surrounding material is unable to maintain the

Figure 9.4 Activation losses in a PEM fuel cell [1].

initial concentration of the bulk fluid and a concentration gradient is formed, resulting in a loss of electrode potential. Although several processes contribute to concentration polarization, at practical current densities, slow transport of reactants and products to and from the elec­trochemical reaction site is a major contributor to concentration polar­ization. The effect of polarization is to shift the potential of the electrode: For the anode,

and for the cathode,

kathode Ecathode I hcathode I

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential. This reduces the cell voltage. The cell voltage includes the contribution of the anode and cathode potentials and ohmic polarization. See Figure 9.5.

Fcell kathode Fanode iR; or

Fcell Ecathode ^cathode 1 (Eanode ^ |^anode 1) iR; or

Fcell Ecell |Ecathode 1 |^anode 1

where Ecell Ecathode Eanode

The goal of fuel cell developers is to minimize the polarization losses so that the Fcell approaches the Ecell by modifications to the fuel cell

Figure 9.5 PEM fuel cell voltage versus current density curve [3].

design by improvement in the electrode structures, better electrocata­lysts, more conductive electrolytes, thinner cell components, and so forth. It is possible to improve the cell performance by modifying the operat­ing conditions such as higher gas pressure, higher temperature, and a change in gas composition to lower the gas impurity concentration [3].