High-temperature electrolysis
1110875
220308255
2008-06-19T06:19:25Z
74.68.135.3
[[Image:High-temperature electrolysis.png|right|350px|thumb|High-temperature electrolysis schema.]]
'''High-temperature electrolysis''' (also '''HTE''' or '''steam electrolysis''') is a method currently being investigated for the production of [[hydrogen]] from water with [[oxygen]] as a by-product.
==Efficiency==
High temperature electrolysis is more efficient economically than traditional room-temperature [[electrolysis of water|electrolysis]] because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures. In fact, at 2500°C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through [[thermolysis]]. Such temperatures are impractical; proposed HTE systems operate between 100°C and 850°C.
The efficiency improvement of high-temperature electrolysis is best appreciated by assuming the electricity used comes from a [[heat engine]], and then considering the amount of heat energy necessary to produce one kg hydrogen (141.86 megajoules), both in the HTE process itself and also in producing the electricity used. At 100°C, 350 megajoules of thermal energy are required (41% efficient). At 850°C, 225 megajoules are required (64% efficient).
A brief explanation on calculating the efficiency of an HTE electrolytic cell is available on the website of Solid Cell Inc at: http://www.solidcell.net/electrolyzer
==Materials==
The selection of the materials for the electrodes and electrolyte is essential. One currently investigated option for the process<ref>Kazuya Yamada, Shinichi Makino, Kiyoshi Ono, Kentaro Matsunaga, Masato Yoshino, Takashi Ogawa, Shigeo Kasai, Seiji Fujiwara, and Hiroyuki Yamauchi "High Temperature Electrolysis for Hydrogen Production Using Solid Oxide Electrolyte Tubular Cells Assembly Unit", presented at AICHE Annual Meeting, San Francisco, California, November 2006.
[http://aiche.confex.com/aiche/2006/techprogram/S3253.HTM abstract]</ref> used [[yttria-stabilized zirconia]] (YSZ) electrolytes, nickel-[[cermet]] steam/hydrogen electrodes, and mixed oxide of lanthanum, strontium and cobalt oxygen electrodes.
==Economic potential==
HTE does not provide a means to bypass the inherent inefficiency of a heat engine, by producing hydrogen which is then converted back to electricity in a fuel cell. (Any such efficiency improvement would allow the theoretical construction of a [[perpetual motion]] machine, which would violate the [[Second Law of Thermodynamics]], which is impossible.) Thus any economic advantage to be gained from using HTE must come from supplying chemical processes which use hydrogen as a feedstock and not as a power source (such as the petrochemical or fertilizer industries), or motive processes for which hydrogen is a better energy carrier than electricity (rockets are an example, cars are not yet an example).
High-temperature electrolysis cannot compete with the chemical conversion of hydrocarbon or coal energy into hydrogen, as none of those conversions are limited by heat engine efficiency. Thus the possible supplies of cheap high-temperature heat for HTE are all nonchemical, including [[nuclear reactor]]s, [[solar power|concentrating solar thermal collectors]], and [[Geothermal power|geothermal]] sources.
===The market for hydrogen production===
Given a cheap, high-temperature heat source, other hydrogen production methods are possible. In particular, see the thermochemical [[sulfur-iodine cycle]]. Thermochemical production might reach higher efficiencies than HTE because no heat engine is required. However, large-scale thermochemical production will require significant advances in materials that can withstand high-temperature, high-pressure, highly-corrosive environments.
The market for hydrogen is large (50 million metric tons/year in 2004, worth about $135 billion/year) and growing at about 10% per year (see [[hydrogen economy]]). The two major consumers are currently oil refineries and fertilizer plants (each consume about half of all production). Should hydrogen-powered cars become widespread, their consumption would greatly increase the demand for hydrogen.
==Electrolysis and thermodynamics==
During electrolysis, the amount of electrical energy that must be added equals the change in [[Gibbs free energy]] of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum [[thermodynamics|thermodynamic]] efficiency of any electrochemical process equals 100%. In practice, the efficiency is given by electrical work achieved divided by the Gibbs free energy change of the reaction.
In most cases, such as room temperature water electrolysis, the electric input is larger than the enthalpy change of the reaction, so some energy is released as waste heat. In some other cases however, for instance in the electrolysis of [[steam]] into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the [[heating value]] of the produced hydrogen is higher than the electric input. In this case the efficiency relative to electric energy input can be said to be greater than 100%. The maximum theoretical efficiency of a [[fuel cell]] is the inverse of that of electrolysis. It is thus impossible to create a [[perpetual motion]] machine by combining the two processes.
==Mars ISRU==
High temperature electrolysis has also been proposed to produce oxygen on [[Mars]] from atmospheric carbon dioxide, using zirconia electrolysis devices.
(See [[In-Situ_Resource_Utilization |In-situ Resource Utilization]].)
==References==
*[http://www1.eere.energy.gov/solar/pdfs/doctor.pdf U.S. DOE high-temperature electrolysis]
==Footnotes==
{{reflist}}
{{electrolysis}}
[[Category:Electrolysis]]
[[Category:Hydrogen production]]
[[fr:Électrolyse à haute température]]
[[it:Elettrolisi ad alta temperatura]]
[[nl:Hoge-temperatuurelektrolyse]]