**1. Introduction**

Most of the power generators use conventional fuel and they obey the same operation principle that is transforming chemical energy to mechanical energy [1]. The heat engine (which is a very common conventional generator) burns or oxidizes the fuel in the air [2]. The chemical energy of fossil fuel is converted into thermal energy and this is the first step of its operation. The rising temperature and pressure lead to expanding engine gases. These excited gases rotate a crankshaft (conversion energy from thermal to mechanical energy step). Finally, the electricity is generated by the mechanical work done by the crankshaft [2]. Now it becomes clear that the production of electricity by heat engine passes through three stages. The overall conversion efficiency declines as the number of stages increases and this is one of the main disadvantages of heat engines [3].

Thermal engines have not been at the top of the list of energy sources despite their ability to provide high energy. This is due to several reasons, including that thermal engines cause increased global warming and environmental pollution by emitting carbon dioxide. Thermal engines often use fossil fuels which are originally threatened with depletion. Also, the sustainability of damaged mechanical parts adds an economic cost to the manufacture and operation of conventional engines. All these reasons prompted the search for new and sustainable energy sources that can compensate for the deficiencies in traditional sources of energy generation and storage. The solution embodies the use of fuel cells, which are not only a source of green energy but also fill the lack of advantages of both the thermal engines and rechargeable batteries. Their simple construction and non-use of fossil fuels gave them the ability to work for a long time without the need to replace their damaged parts. Fuel cells have unlimited operating time, higher power density and therefore greater storage capacity to outperform the advantages of a simple battery and rechargeable battery [4].

Fuel cells are zero pollution' power generators. They work as an electrochemical converter to chemically produce electrical energy from gas fuels. The conversion occurs directly by the chemical reactions and without burning of the fuel. Even better, they are not following the thermodynamic laws that limit most of the power plants. This chapter presents the operation principle of fuel cells, their main components, and finally the newer trends in developing these cells for commercial uses.
