The fuel cell is a so-called electrochemical cell that generates electricity through chemical reactions between a fuel source and an oxidant. The fuel source is generally hydrogen and the oxidant oxygen, but versions that work on hydrocarbons and alcohols or chlorine and chlorine dioxides also exist. These substances are simply inserted into the fuel cell and are there consumed, being converted into water or carbon dioxide and electricity. Although often compared to batteries, fuel cells differ from these in how they rely on external fuel and oxidant supplies; which essentially means that a fuel cell can operate indefinitely as long as the flow of the fuel and the oxidant is maintained.
Granted, the long-term efficiency of the cell depends in part on the amount of power that is drawn from it. More power effectively translates to a greater electrical current, which increases the energy loss of the process and results in lower efficiency in terms of lower cell voltage. Thus fuel cells could technically be worn down over time. This threatens to compromise the amount of power that may be generated from a cell. However this problem is worked around by the fact that fuel cells are often installed in series or parallel circuits, the former permitting higher voltage and the latter a stronger electrical current. Larger cells with greater surface areas also produce this effect. Taking this into account the fuel cell is not only a powerful energy source, but its lack of major moving parts also means that it requires minimal maintenance and minimal downtime in comparison to alternative power generators.
The fuel cell's comparable simplicity moreover makes it conveniently compact and lightweight. Consequentially fuel cells are ideal as portable power sources or as stationary ones in remote or inaccessible locations. Spacecrafts, weather stations, rural locations and military submarines are a few examples of where fuel cells are currently employed for electricity generation. It is less ideal for vehicles in how it does not store energy, for which sizeable storage systems are needed to maintain the fuel supply. This is one of the foremost reasons for why fuel cell technology has not yet been extensively implemented in vehicle propulsion. Suffice it to say that while a space craft or submarine can carry the extra weight, it is not as easy to add it to a normal-sized automobile. Yet solutions are underway in this department as well, with several fuel cell-driven vehicles currently existing in either limited editions or as demonstration models of future methods of propulsion. They still suffer from the fuel cell's incompatibility with cold weather, but this too is being addressed by more recent models thereof by a proton exchange membrane that prevents the fuel cell's liquid stores from freezing.
This potential to supplement or even supplant gasoline has ensured that hydrogen power cell technology has been subject to considerable investments worldwide as a preventative measure against greenhouse gas omissions and global warming. It was particularly favoured as an alternative by the Bush administration of the US, but it has since been de-prioritized as other alternatives are thought likely to become fully developed - and thus more likely to make an impact - much sooner than hydrogen fuel cell cars. This might have set back the arrival of hydrogen-run vehicles by years or even decades, but it has far from stopped it dead in its tracks. In the meantime fuel cells will continue to serve as primary or emergency power generators for hospitals, universities and office buildings and etcetera, as well as potential power sources for portable electric devices that are unlikely to see many opportunities for recharging.