Electrolysis is the process for breaking water into its constituent elements of hydrogen (H 2 ) and
oxygen (O 2 ) gas by supplying electrical energy. The advantage of the process is that it supplies a
very clean hydrogen fuel that is free from carbon and sulfur impurities. The disadvantage is that
the process is expensive relative to steam-reforming of natural gas, because of the cost of the
electrical energy required to drive the process.
Several reasons argue in favor of hydrogen as a chemical fuel and electrolysis as a means of
making this hydrogen fuel (NRC and NAE 2004). Water, and the hydrogen that it contains, is
more abundant than hydrocarbons and is available wherever there is practicable human
habitation. Electrolysis also represents a means to convert fuels such as coal into a higher-value
fuel for transport. In addition, the electrolysis technology can be scaled over a broad range of
distributed applications, from existing fueling stations for automobiles, to high-volume,
centralized hydroelectric or nuclear power plants. Electrolysis also represents a way to transform
the electrical power generated from renewable sources such as hydro, wind, or solar into a fuel.
Electrolysis technologies have been in use for decades to meet industrial chemical needs. They
have also played a critical role in life support in space and submarine applications over the past
decades. Presently, however, commercial electrolyzers for production of hydrogen are
commercially viable only in niche industrial applications. They are sometimes employed at
isolated locations where there is no available natural gas for production of fertilizers. There are
also small distributed market demands for limited amounts of hydrogen at truckload quantities,
where the customers absorb the high $12/kg cost of electrolytically produced material. In
contrast, the U.S. Department of Energy (DOE) estimates that in order to compete for
transportation fuels, cost of hydrogen must be pushed below $3.00/kg, where a kg of hydrogen is
seen to power an automobile for 60 miles of travel.
Electrolyzers for production of hydrogen fall into two basic categories: solid polymer and
alkaline liquid electrolyte. The difference in technologies also defines a difference in scale of
application: the alkaline electrolysis units can be scaled up to high-volume production, whereas
the solid polymer technology is best suited to small-volume production.
Solid polymer electrolyzers are also referred to as proton exchange membrane (PEM)
electrolyzers. General Electric and other companies developed this technology in the 1950s and
1960s to support the U.S. space program. When water is introduced into the PEM electrolyzer
cell, hydrogen ions are drawn into and through the membrane, where they recombine at the
cathode with electrons to form hydrogen atoms. Oxygen is produced at the anode on the other
side of the membrane from the hydrogen and is removed as the water is recirculated.
Alkaline industrial electrolysis units are produced in two forms today, in a unipolar or bipolar
form, which describe a parallel or series connections of the electrodes used in the electrolysis.
Both, however, utilize aqueous potassium hydroxide solutions. An ion exchange membrane is
placed between the cathode and anode, separating the hydrogen and oxygen as they are
produced, but allowing the passage of ions between the two sides.
Both technologies are governed by the same electrolysis reaction:
H 2 O → ½ O 2 + H 2 (1)