Fuel Cells (Glossary)

Similar as in batteries and cells, chemically collected energy stored in fuel cells is converted into electricity directly, i.e. by-passing the additional way such as a heat cycle. Unlike as in the case of the first-mentioned energy stores, however, the standard fuel cell components (e.g. electrodes, electrolyte) ideally are not consumed, and if necessary, the proper energy carrier can be re-used as self-consuming "fuel". Hence we can talk here about batteries rechargeable any number of times.

The chemical reaction used in fuel cells is the purest form of hydrogen combustion in oxygen. The water electrolysis reaction is reversed here. The combustion product from the reaction, also called "cold combustion", is pure water, or optionally water vapour:

Formula: Fuel Cells

The basic working principle behind a battery (Why a battery supplies electric current?), i.e. the spatially split flow of reaction components and the equalising of charges with electronic and ionic conductors, applies to fuel cells, as well.

Theory and practise

In theory, the parameters that fuel cells can reach (e.g. energy density, efficiency coefficient) are very advantageous. In addition to that, the residual product of hydrogen combustion – pure water – is completely unharmful for environment. In practise, however, storing and transporting as well as handling, especially with aggressive hydrogen, cause substantial problems that can only be solved in a very costly manner. To avoid such problems, the principle of a fuel cell has been considerably modified: air can be used instead of oxygen, and, e.g. methanol, methane or natural gas can be used instead of hydrogen; however the price for that is a lower efficiency coefficient or the much more complicated cell construction and, thus higher susceptibility to consumption. For this reason, although the first experimental fuel cells were built as early as in the half of the 19th century and proved their efficiency in many fields of application, e.g. in astronautics, their economic applications have been usually very rare so far – despite regular and flashy press declarations promising something quite in contrary.

Types of fuel cells

The fuel cells utilised in research projects, tests and practical applications can be classified according to various criteria. According to their operating temperature, one can distinguish between low-temperature fuel cells (up to approx. 100ºC), medium-temperature cells (approx. 100 to approx. 600ºC) and high-temperature cells (approx. 600 to 1,000ºC). To quote just three of out many types of applications: the PEMFC type (Proton Exchange Membrane Fuel Cell) fuel cells used in the Gemini’s space mission, where hydrogen is incinerated at a room temperature, or DMFC (Direct Methanol Fuel Cell) cells used in low temperatures where methanol is used as fuel, and, finally SOFC (Solid Oxide Fuel Cell) high-temperature cells for stationary use only, where natural gas and other hydrogens in form of gas can be the gaseous fuel.

© Marc Stenzel

	See also:
		Glossary overview Batteries - Accumulators - Cells Battery-Packs

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