A fuel cell is an electrochemical energy conversion device that takes stored chemical energy and converts it into electrical energy directly.
In the presence of an electrolyte (which also acts as the reactant separator) and electrodes (which act as the catalyst), a fuel cell produces electricity from fuels such as hydrogen and oxygen. The current is collected externally at the bipolar plates.
A battery has all its chemicals stored inside it, and it too converts those chemicals into electricity but unlike a fuel cell, batteries eventually "go dead" through depletion and have to be either discarded or recharged.
With a fuel cell however, chemicals constantly flow into the cell so it never goes dead. A continuous replenishment of fuel into the cell, provides near continuous electrical production. Most fuel cells in use today use hydrogen and oxygen as the chemicals and as such, the conversion of fuel into electricity via the electrochemical process is considered clean, quiet and highly efficient typically two to three times more efficient than combustion.
In addition to low, zero or near zero emissions, other benefits include high efficiency and reliability, multi-fuel capability, scalability and ease of maintenance. Fuel cells operate quietly, so they reduce noise pollution as well as air pollution and the waste heat from a fuel cell can be used to provide hot water or space heating for a home or office.
Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide etc.