Although fuel cells have been around for more than a century (William Grove in 1839 first discovered the principle of the fuel cell), it was not until the National Aeoronautics and Space Administration (NASA) demonstrated its potential applications in providing power during space flights in the 1960s that fuel cells became widely known and the indus­try began to recognize the commercial potential of fuel cells. Initially, fuel cells were not economically competitive with existing energy tech­nologies; but with advancements in fuel cell technology, it is now becom­ing competitive for some niche applications [6].

The main components of a fuel cell are anode, anodic catalyst layer, electrolyte, cathodic catalyst layer, and cathode, as shown in Fig. 9.1. The anode and cathode consist of porous gas diffusion layers, usually made of high-electron-conductivity materials such as thin layers of porous graphite. The most common catalyst is platinum for low — temperature fuel cells. Nickel is preferred for high-temperature fuel
cells. Some other materials (Pt-Pt/Ru, Perovskites, etc.) are also used, depending on the fuel cell type [3].

The electrolyte is made up of materials that provide high proton con­ductivity and zero or very low electron conductivity. The charge carriers (from the anode to the cathode or vice versa) are different, depending on the type of fuel cell. A fuel cell stack is obtained by connecting such fuel cells in series/parallel to yield the desired voltage and current out­puts (see Fig. 9.2). The bipolar plates (or interconnects) collect the elec­trical current and also distribute and separate reactive gases in the fuel cell stack. Sometimes, gaskets for sealing/preventing leakage of gases between anode and cathode are also used.

The anode reaction in hydrogen fuel cells is direct oxidation of hydro­gen. For fuel cells using hydrocarbon fuels, the anodic half reaction con­sists of indirect oxidation through a reforming step.

In most fuel cells, the cathode reaction is oxygen (air) reduction. The overall reaction for hydrogen fuel cells is

H2 + 1 O2 s H2O with AG = —237.2 kJ/mol

where, AG is the change in Gibbs free energy of formation. The reaction product is water released at the cathode or anode, depending on the type of fuel cell.

For an ideal fuel cell, the theoretical voltage E0 under standard con­ditions of 25°C and 1 atm pressure is 1.23 V, whereas typical operating voltage for high-performance fuel cells is ~0.7 V. Stack voltage depends on the number of cells in a series in a stack. Cell current depends on the cross-sectional area (the size) of a cell.

Fuel cell systems are not limited by Carnot cycle efficiency. Therefore, a fuel cell system with a combined cycle and/or cogeneration has very high efficiency (55-85%) as compared to the efficiency of about 30-40% of cur­rent power generation systems. In a distributed generation system, fuel cells can reduce costly transmission line installation and transmission losses. There are no moving parts in a fuel cell and very few moving parts (compressors, fans, etc.) in a fuel cell system. Therefore, it has higher reli­ability compared to an internal combustion or gas turbine power plant.

Fuel cell-based power plants have no emissions when pure hydrogen and oxygen are used as fuel. However, if fossil fuels are used for gener­ating hydrogen, fuel cell power plants produce CO2 emissions. Compared to a steam power plant, a fuel cell plant has very low water usage; water/steam is a reaction product in a fuel cell. This clean water/steam does not require any pretreatment and can be used for reactant humid­ification and cogeneration. Another advantage of the fuel cell power plant is that it does not produce any solid waste and its operation is very silent as compared to a steam/gas turbine power plant. The noise gen­erated in a fuel cell power plant is only from the fan/compressor used for pumping/pressurizing the fuel and the air supply to the cathode.

A fuel cell power plant has good load-following capability (it can quickly increase or decrease its output in response to load changes). The modular construction of fuel cell plants provides good planning flexibility (new units can be added to meet the growth in electric demand when needed), and its performance is independent of the power plant size (efficiency does not vary with variation in size from W to MW size).

The major technical challenges in fuel cell commercialization at pres­ent are (1) high cost, (2) durability, and (3) hydrogen availability and infrastructure. For fuel cells to compete with contemporary power gen­eration technology, they have to become competitive in terms of the cost per kilowatt required to purchase and install a power system. A fuel cell system needs to cost ~$30/kW to be competitive for transportation appli­cations and for stationary systems; the acceptable price range is $400-$750/kW for widespread commercial application [9]. Fuel cell tech­nology needs a few breakthroughs in development to become competi­tive with other advanced power generation technologies.