Hydrogen is the lightest and simplest known chemical element constituting about 75% of the universe's mass. It is represented by the symbol H.

At normal temperature and pressure conditions, hydrogen gas exists as a combination of two atoms held by a chemical bond, with the molecular formula H2. It is colorless, odorless, and highly flammable gas. The electrically neutral atom of H contains a single positively-charged proton and a single negatively-charged electron.


H2 can be used as a fuel because it releases substantial energy in the form of heat when burned. It can also be used to produce electricity through an electrochemical reaction. In both processes, the only chemical by-product is water. H2 has the highest energy content of any common fuel per unit mass: 1kg of H2 when burned releases about the same amount of energy as a gallon of gasoline (which weights almost 3 kg). However, H2 gas is very diffused and therefore has low energy content by volume.

For example, even when compressed to the high pressure of 10,000 psi, it takes more than 6 gallons of H2 to match the energy containing in 1 gallon of gasoline.


The term "hydrogen generator" usually refers to the systems that either produce H2 or are powered by H2. Note that practically pure H2 is not found on Earth: it occurs primarily in combination with oxygen in water and in some chemical compounds such as methane. Industrially H2 is produced mainly from natural gas by a process called steam methane reforming. In small-scale applications, hydrogen can be made by passing an electric current through water. This process is called electrolysis. H2 can also be made by a variation on conventional electrolysis called steam electrolysis that uses heat, instead of electricity, to provide some of the energy needed to split water. The gaseous mixture generated via electrolysis contains molecules of H2 and O2 in a 2:1 ratio. Such mixture is often casually referred to as HHO, although its technical term is oxyhydrogen. HHO can be used for torches and for welding. When ignited, HHO converts to water and releases heat energy.

Since H2 is not found on Earth by itself and it takes certain energy to produce it from other substances, it is referred to as an energy carrier rather then an primary energy source. Note that because H2 is odorless and colorless and poses an explosion hazard even in low concentrations, a practical hydrogen generator needs special sensors (such as color-changing sensors) to quickly detect even low levels of leaking H2.


A fuel cell in general is an electrochemical device that produces electrical energy by combining a fuel, usually H2, with oxygen (O2). It normally contains two electrodes on either side of an electrolyte. There are various kinds of fuel cells classified primarily by the kind of electrolyte used. Hydrogen-fueled cars usually use so called Polymer Electrolyte Membrane (PEM) cells. Unlike a conventional battery, a fuel cell is not an energy source. It does not store much energy and does not require recharging: it converts chemical energy into electricity as long as H2 and O2 are supplied. H2 fuel is supplied to the anode (negative terminal) and oxygen is supplied to the cathode (positive terminal). Anode and cathode act as catalysts. The molecule of H2 is split at the anode into an electron and a proton. The electrolyte membrane allows only the positive ions to pass through it to the cathode but does not let electrons to pass through. This creates voltage between anode and cathode. Typical open circuit voltage of a single cell is 1-1.2 VDC, but this voltage is unregulated. If an external circuit is connected to the cell's electrodes, electric current can flow through it thus delivering power to the load. The protons that passed through the electrolyte to the cathode, bond with O2 and the electrons arriving via an external circuit to form water as a byproduct. This water then flows out of the cell through an exhaust and can be reused in its cooling system. Technically speaking, during the fuel cell chemical reaction two hydrogen molecules combine with one oxygen molecule and produce two water molecules, heat, and electrical energy.

Mulitiple cells can be stack in series for higher voltages or paralleled for higher currents. A fuel cell generator is efficient, simple, has virtually no emissions, and runs silently. It is capable of producing power anywhere from 1 watt to 10 megawatt and can be used in virtually any application that requires power: in cell phones, PCs, vehicles, as well as in electric grid. Unlike electric generators, fuel cells converts the chemical energy of a fuel directly to electricity without converting it first into mechanical energy and without combustion. Their main disadvantage is higher cost relative to other forms of power generation (currently it's about $45/kW). If the industry manage to reduce their cost, fuel cells could become a dominant type of pollution-free energy converters.

Additional information and references:
Introduction to fuel cell technology: how it works, efficiency, PEM cells.
Hydrogen and fuel cell basics for non-technical audiences: production, storage, safety, research.


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