How Fuel Cells Work

How Fuel Cells Work

In essence, a fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel though a circuit, hence converting them to electrical power.

In mobile applications, such as energy sources for vehicles and small appliances such as cell phones and laptop computers, fuel cells offer much the same advantages as batteries, namely, noiseless, non-polluting, and vibration-free operation. For this reason, they are natural candidates for use as battery replacements. However, fuel cells offer the additional advantages that they are much lighter in weight than batteries and are not energy storage devices that require recharging after use. Rather, fuel cells take advantage of the high energy density of fossil fuels compared with the conventional low energy density associated with batteries.

Fuel Cells are electrochemical devices that generate electricity by directly converting the chemical energy associated with the oxidation of a fuel into direct current (DC) electricity. The chemical processes by which the electric current is produced are electrochemical, rather than thermochemical, in nature. Since no combustion reactions are involved in these processes fuel cells do not produce any of the undesirable products normally associated with the oxidation of fossil fuels in conventional energy conversion systems. As a result, fuel cells do not emit CO2, SO2, oxides of nitrogen, or particulate matter. Thus, fuel cells are environmentally friendly.

The primary fuel used by fuel cells is hydrogen and the oxidizer is oxygen, either in its pure form or in its more readily available form mixed with nitrogen and other gases as air. In some fuel cell types, the hydrogen fuel is derived from a fossil fuel, such as natural gas or gasoline, and the oxidizer is supplied as air. In these cases, the fuel cell must be fitted with a device, known as a reformer, that extracts the hydrogen from the fossil fuel.

The hydrogen gas is continuously fed to the anode (negative electrode) compartment while the oxygen is continuously fed to the cathode (positive electrode) compartment. Inside the fuel cell, the anode and the cathode are separated from one another by an intervening medium known as the electrolyte that serves as an ion conductor. The anode and the cathode are also connected together electrically by means of an electric circuit external to the fuel cell that serves as an electron conductor. In a typical fuel cell design, at the anode, the hydrogen is broken down into two components: the hydrogen nucleus (also known as a proton) and an electron. The electron represents the electric current and is transferred from the anode to the cathode through the electric circuit. This electric contains the load such as an electric motor. The proton migrates from the anode to the cathode through the electrolyte. At the cathode, the proton meets up with the electron coming from the load and the oxygen. They all combine together to form water as the only chemical product of the fuel cell.

The electrochemical reactions that take place at the two electrodes (anode and cathode) need to be facilitated by another component of the fuel cell known as a catalyst, usually platinum. At the two electrodes, the electrochemical reactions occur on the catalyst surface where the electrolyte and fuel (or oxidizer) meet. This interface is often referred to as the triple interface, or three-phase interface, since in some fuel cell designs the fuel (or oxidizer) is a gas, the electrolyte is a liquid, and the electrode/catalyst surface is a solid. The state of this three-phase interface plays a critical role in determining the electrochemical performance of a fuel cell; hence, this triple interface has been the focus of much research in improving fuel cells. Figure 1.0 below illustrates the operation of a fuel cell.

Depending upon the manner in which the electrodes and the electrolyte are configured and the nature of the ionic species transported through the electrolyte, fuel cells can be grouped into six basic designs: (1) phosphoric acid fuel cells (PFAC) in which the electrolyte is phosphoric acid (H2PO4); (2) alkaline fuel cells (AFC) in which the electrolyte is a base such as potassium hydroxide (KOH); (3) proton exchange membrane fuel cells (PEMFC) in which a polymer membrane serves as the electrolyte; (4) molten carbonate fuel cells (MCFC) in which molten carbonate, primarily in the form of potassium carbonate (K2CO3), serves as the electrolyte transporting carbonate ions rather than protons; (5) solid oxide fuel cells (SOFC) in which the electrolyte is a solid such as yttria (Y2O3)-stabilized zirconia (ZrO2) that transports oxygen ions rather than protons; and (6) direct methanol fuel cells which are similar to PEMFC in that a polymer membrane serves as the electrolyte but the methanol/hydrogen reformer is built in to the anode.

Each of these fuel cell types has a particular set of characteristics with a set of applications for which it is appropriate. In general, the applications can be grouped into two broad categories: stationary applications and mobile applications.

In small scale stationary applications, such as distributed energy sources for residential application with power levels on the order of 10 kW or less, fuel cells, such as PEMFC, offer the advantages of noiseless, non-polluting, and vibration-free operation while at the same time eliminating the need for a loss making, wide area distribution network. In intermediate applications in the range of 10 kW to 250 kW, PAFC have found widespread use in facilities such as hospitals and telecommunications installations. For larger stationary applications at the level of 10 - 100 MW, MCFC and SOFC, offer the additional advantage over conventional power plants of co-generation of both electric power and low grade heating.