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[[File:Microbial electrolysis cell.png|thumb|right|200px|A microbial electrolysis cell]]
A '''microbial electrolysis cell''' ('''MEC''') is a technology related to [[Microbial fuel cell]]s (MFC). Whilst MFCs produce an [[electric current]] from the microbial decomposition of organic compounds, MECs partially reverse the process to generate [[hydrogen]] or [[methane]] from organic material by applying an electric current.<ref>{{cite journal|last1=Badwal|first1=SPS|title=Emerging electrochemical energy conversion and storage technologies|journal=Frontiers in Chemistry|volume=2|doi=10.3389/fchem.2014.00079|pmid=25309898|pmc=4174133|year=2014|page=79|bibcode=2014FrCh....2...79B|doi-access=free}}</ref> The electric current would ideally be produced by a renewable source of power. The hydrogen or methane produced can be used to produce electricity by means of an additional PEM fuel cell or internal combustion engine.

==Microbial electrolysis cells==
MEC systems are based on a number of components:

'''Microorganisms''' – are attached to the anode. The identity of the microorganisms determines the products and efficiency of the MEC.

'''Materials''' – The anode material in a MEC can be the same as an MFC, such as carbon cloth, carbon paper, graphite felt, graphite granules or graphite brushes. Platinum can be used as a catalyst to reduce the [[overpotential]] required for [[hydrogen production]]. The high cost of platinum is driving research into biocathodes as an alternative. Or as other alternative for catalyst, the stainless steel plates were used as cathode and anode materials.<ref>{{cite journal|title=Development of biohydrogen production by photobiological, fermentation and electrochemical processes: A review|first1=M. Y.|last1=Azwar|first2=M. A.|last2=Hussain|first3=A. K.|last3=Abdul-Wahab|date=1 March 2014|journal=Renewable and Sustainable Energy Reviews|volume=31|issue=Supplement C|pages=158–173|doi=10.1016/j.rser.2013.11.022}}</ref> Other materials include membranes (although some MECs are membraneless), and tubing and gas collection systems.<ref>{{cite web|url=http://www.greencarcongress.com/2008/11/study-concludes.html|title=Green Car Congress: Study Concludes That Microbial Electrolysis Cells Are a Promising Approach to Renewable and Sustainable Hydrogen Production|first=BioAge|last=Media|website=www.greencarcongress.com}}</ref>

==Generating hydrogen==
[[Exoelectrogen|Electrogenic microorganisms]] consuming an energy source (such as [[acetic acid]]) release electrons and protons, creating an [[voltage|electrical potential]] of up to 0.3 volts. In a conventional MFC, this voltage is used to generate electrical power. In a MEC, an additional voltage is supplied to the cell from an outside source. The combined voltage is sufficient to [[redox reaction|reduce]] protons, producing hydrogen gas. As part of the energy for this reduction is derived from bacterial activity, the total electrical energy that has to be supplied is less than for [[electrolysis of water]] in the absence of microbes. Hydrogen production has reached up to 3.12 m<sup>3</sup>H<sub>2</sub>/m<sup>3</sup>d with an input voltage of 0.8 volts. The efficiency of hydrogen production depends on which organic substances are used. Lactic and acetic acid achieve 82% efficiency, while the values for unpretreated cellulose or glucose are close to 63%.<br />
The efficiency of normal water electrolysis is 60 to 70 percent. As MEC's convert unusable biomass into usable hydrogen, they can produce 144% more usable energy than they consume as electrical energy.<br /> Depending on the organisms present at the cathode, MECs can also produce methane by a related mechanism.

'''Calculations'''<br />Overall hydrogen recovery was calculated as ''RH''<sub>2</sub> = ''C''<sub>E</sub>''R''<sub>Cat</sub>. The Coulombic efficiency is ''C''<sub>E</sub>=(''n''<sub>CE</sub>/''n''<sub>th</sub>), where ''n''<sub>th</sub> is the moles of hydrogen that could be theoretically produced and ''n''<sub>CE</sub> = ''C''<sub>P</sub>/(2''F'') is the moles of hydrogen that could be produced from the measured current, ''C''<sub>P</sub> is the total coulombs calculated by integrating the current over time, ''F'' is Faraday's constant, and 2 is the moles of electrons per mole of hydrogen. The cathodic hydrogen recovery was calculated as ''R''<sub>Cat</sub> = ''n''<sub>H2</sub>/''n''<sub>CE</sub>, where ''n''<sub>H2</sub> is the total moles of hydrogen produced. Hydrogen yield (''Y''<sub>H2</sub>) was calculated as ''Y''<sub>H2</sub> = ''n''<sub>H2</sub> /''n''<sub>s</sub>, where ''n''<sub>s</sub> is substrate removal calculated on the basis of chemical oxygen demand (22).<ref>{{cite journal|title=Sustainable and efficient biohydrogen production via electrohydrogenesis|date=20 November 2007|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=104|issue=47|pages=18871–18873|doi=10.1073/pnas.0706379104|pmid=18000052|pmc=2141869|author1=Shaoan Cheng|author2=Bruce E. Logan|bibcode=2007PNAS..10418871C|doi-access=free}}</ref>

==Uses==
Hydrogen and methane can both be used as alternatives to fossil fuels in [[internal combustion engines]] or for power generation. Like MFCs or [[bioethanol]] production plants, MECs have the potential to convert waste organic matter into a valuable energy source. Hydrogen can also be combined with the nitrogen in the air to produce ammonia, which can be used to make ammonium fertilizer. Ammonia has been proposed as a practical alternative to fossil fuel for internal combustion engines.<ref>{{cite web|url=http://live.psu.edu/story/27233|title=Penn State Live|access-date=2009-06-26|archive-url=https://web.archive.org/web/20090512141254/http://live.psu.edu/story/27233|archive-date=2009-05-12|url-status=dead}}</ref>

==See also==

*[[Hydrogen technologies]]
*[[Microbial electrosynthesis]]
*[[Microbial fuel cell]]s
*[[Microbial electrolysis carbon capture]]

==References==
{{Reflist}}{{Refbegin}}
* M.Y. Azwar, M.A. Hussain, A.K. Abdul-Wahab (2014). Development of biohydrogen production by photobiological, fermentation and electrochemical processes: A review. Renewable and Sustainable Energy Reviews.Volume 31, March 2014, Pages 158–173. Copyright  2017 Elsevier B.V. http://doi.org/10.1016/j.rser.2013.11.022
{{Refend}}

==External links==
* [https://www.nsf.gov/news/news_summ.jsp?cntn_id=110648 National Science Foundation]
* [http://www.awmc.uq.edu.au/index.html?page=84198 The University of Queensland]
* [http://www.scientificblogging.com/news_account/microbial_electrolysis_cell_turning_bacteria_into_hydogen_machines Scientific Blogging]
* [https://web.archive.org/web/20090512141254/http://live.psu.edu/story/27233]

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[[Category:Biotechnology]]
[[Category:Electric power]]
[[Category:Fuel cells]]
[[Category:Hydrogen production]]