Completing the Loop Fuel Cells

The high-energy chemicals such as H2 that form in the reactions considered in the preceding reaction, can be recombined in fuel cells to extract the stored chemical energy as electricity. A fuel cell is an electrochemical device that converts the chemical energy in a fuel (such as hydrogen) and an oxidant (oxygen, pure or in air) directly to electricity, water, and heat. Fuel cells are classified according to the electrolyte that they use (Table 6). For automobile applications, the polymer-electrolyte-membrane (PEM) type of fuel cell is the leading candidate for developing zero-emission vehicles. Other types of fuel cells (e.g., solid oxide fuel cells or SOFCs)

Renewable Energy and the Hydrogen Economy 15 Table 6. Types of fuel cells.a

Type Electrolyte

Operating temperature, °C

Polymer-electrolyte membrane (PEM) Sulfuric acid impregnated in membrane 60-80

Alkaline KOH 70-120

Phosphoric acid Phosphoric acid 160-200

Molten carbonate Lithium/potassium carbonate 650

Solid oxide Yttria-stabilized zirconia 1,000

aAdapted from Ref. 39.

have been considered for stationary power needs. Figure 7 contains the schematic diagram of a PEM fuel cell.39

The major virtue of a fuel cell, other than its clean emissions, is its high electrical conversion efficiency. This is not Carnot-limited (unlike in heat engines) and for an ideal hydrogen-oxygen fuel cells, can approach an impressive 83%.40 In practical devices, up to 60% of the energy content in H2 can be converted to electricity, the remainder being dissipated as heat. For comparison, practical internal combustion engines using H2 fuel achieve efficiencies of only 45%.40

The principle of fuel cells has been known since 1838 thanks to William Grove. However, widespread deployment did not begin till the 1960s and 70s when fuel cells were used in space and for military (e.g., submarine) applications. Nowadays, fuel cells are being considered for low-polluting co-generation of heat and power in buildings and for transportation applications.

As with the technologies considered earlier, the main deterrent is cost. Today's fuel cell demonstration cars and buses are custom-made prototypes that cost about $ 1 million apiece.41 Economies of scale in mass manufacture would bring this cost to a more reasonable $6,000-10,000 range. This translates to about $125 per kilowatt of engine power, which is about four times as high as the $30 per kilowatt cost of a comparable gasoline-powered internal combustion engine.41 A major cost component in the PEM fuel cell is the noble metal (usually Pt) electrocatalyst. Efforts are underway in many laboratories to find less expensive substitutes (see for example, Refs. 42-44).

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000-40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be overcome.

High temperature membranes, that can operate at temperatures above 100 °C, are desirable to promote heat rejection, speed up electrode reaction rates, and to improve tolerance to impurities. This is an active area of materials research. Unfortunately, space constraints preclude a detailed description of fuel cell technologies and the underlying issues. Instead, the reader is referred to excellent reviews and books that exist on this topic.45-47

Fuel Cell Submarines
Fig. 7. Schematic diagram of a PEM fuel cell. Reproduced from Ref. 39. Copyright (2004), by permission of The Electrochemical Society.

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