View source for Direct methanol fuel cell

{{Short description|Type of fuel cell}}
[[Image:Fuel cell NASA p48600ac.jpg|thumb|250px|right|Direct methanol fuel cell]]
'''Direct methanol fuel cells''' or '''DMFCs''' are a subcategory of [[proton-exchange membrane fuel cell]]s in which [[methanol]] is used as the fuel and a special proton-conducting polymer as the [[proton-exchange membrane|membrane]] (PEM).
Their main advantage is low temperature operation and the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions.

Whilst the thermodynamic theoretical [[energy conversion efficiency]] of a DMFC is 97%;<ref name="thermodynamics">{{cite journal|author=Umit B. Demirci|doi=10.1016/j.jpowsour.2007.03.050|title=Review: Direct liquid-feed fuel cells: Thermodynamic and environmental concerns|year=2007|journal=Journal of Power Sources|volume=169}}</ref> as of 2014 the achievable energy conversion efficiency for operational cells attains 30%<ref>{{cite book|doi=10.1016/B978-0-12-383860-5.00004-3|chapter=4.4.7 Direct Methanol Fuel Cells|author=Ibrahim Dincer, Calin Zamfirescu|date=2014|title=Advanced Power Generation Systems}}</ref> – 40%.<ref>{{cite book|author=Keith Scott, Lei Xing|doi=10.1016/B978-0-12-386874-9.00005-1|title=Fuel Cell Engineering|chapter=3.1 Introduction|page=147|year=2012}}</ref> There is intensive research on promising approaches to increase the operational efficiency.<ref name="effincrease">{{cite journal|title=Determination of the efficiency of methanol oxidation in a direct methanol fuel cell|author=Pasha Majidi |display-authors=etal |journal=Electrochimica Acta|volume=199|date=1 May 2016}}</ref>

A more efficient version of a direct fuel cell would play a key role in the theoretical use of methanol as a general energy transport medium, in the hypothesized [[methanol economy]].

== The cell ==
In contrast to [[indirect methanol fuel cell]]s, where methanol is reacted to [[hydrogen]] by [[steam reforming]], DMFCs use a methanol solution (usually around 1[[molarity|M]], i.e. about 3% in mass) to carry the reactant into the cell; common operating temperatures are in the range {{cvt|50 to 120|°C}}, where high temperatures are usually pressurized.
DMFCs themselves are more efficient at high temperatures and pressures, but these conditions end up causing so many losses in the complete system that the advantage is lost;<ref>Dohle, H.; Mergel, J. & Stolten, D.: Heat and power management of a direct-methanol-fuel-cell (DMFC) system, Journal of Power Sources, 2002, 111, 268-282.</ref> therefore, atmospheric-pressure configurations are currently preferred.

Because of the methanol cross-over, a phenomenon by which methanol diffuses through the membrane without reacting, methanol is fed as a weak solution: this decreases efficiency significantly, since crossed-over methanol, after reaching the air side (the cathode), immediately reacts with air; though the exact kinetics are debated, the result is a reduction of the cell voltage. Cross-over remains a major factor in inefficiencies, and often half of the methanol is lost to cross-over. Methanol cross-over and/or its effects can be alleviated by (a) developing alternative membranes (e.g.<ref>{{cite journal|last=Wei|first=Yongsheng|title=A novel membrane for DMFC – Na2Ti3O7 Nanotubes/Nafion composite membrane: Performances studies|journal=International Journal of Hydrogen Energy|year=2012|volume=37|issue=2|pages=1857–1864|doi=10.1016/j.ijhydene.2011.08.107|display-authors=etal}}</ref><ref>{{cite web |title=Safe space: improving the "clean" methanol fuel cells using a protective carbon shell |url=https://bioengineer.org/safe-space-improving-the-clean-methanol-fuel-cells-using-a-protective-carbon-shell/ |website=Bioengineer.org |date=4 December 2020 |access-date=30 December 2020}}</ref>), (b) improving the electro-oxidation process in the catalyst layer and improving the structure of the catalyst and gas diffusion layers (e.g.<ref>{{cite journal|last=Matar|first=Saif|author2=Hongtan Liu|title=Effect of cathode catalyst layer thickness on methanol cross-over in a DMFC|journal=Electrochimica Acta|year=2010|volume=56|issue=1|pages=600–606|doi=10.1016/j.electacta.2010.09.001}}</ref> ), and (c) optimizing the design of the flow field and the membrane electrode assembly (MEA) which can be achieved by studying the current density distributions (e.g.<ref>{{cite journal|last=Almheiri|first=Saif|author2=Hongtan Liu|title=Separate measurement of current density under land and channel in Direct Methanol Fuel Cells|journal=Journal of Power Sources|year=2014|volume=246|pages=899–905|doi=10.1016/j.jpowsour.2013.08.029|bibcode=2014JPS...246..899A}}</ref> ).

Other issues include the management of [[carbon dioxide]] created at the [[anode]], the sluggish dynamic behavior, and the ability to maintain the solution water.

The only waste products with these types of fuel cells are [[carbon dioxide]] and water.

== Application ==
Current DMFCs are limited in the power they can produce, but can still store a high energy content in a small space.  This means they can produce a small amount of power over a long period of time. This makes them ill-suited for powering large vehicles (at least directly), but ideal for smaller vehicles such as forklifts and tuggers<ref>[https://abcnews.go.com/Business/wireStory?id=8409053 Tenn. Nissan Plant to Use Methanol to Cut Costs ] by ABC News.</ref> and consumer goods such as [[mobile phone]]s, [[digital camera]]s or [[laptop]]s. Military applications of DMFCs are an emerging application since they have low noise and thermal signatures and no toxic effluent. These applications include power for man-portable tactical equipment, battery chargers, and autonomous power for test and training instrumentation. Units are available with power outputs between 25 watts and 5 kilowatts with durations up to 100 hours between refuelings. Especially for power output up to 0.3&nbsp;kW the DMFC is suitable. For a power output of more than 0.3&nbsp;kW the [[Reformed methanol fuel cell|indirect methanol fuel cell]] presents a higher efficiency and is more cost-efficient.<ref>{{Cite journal|last1=Simon Araya|first1=Samuel|last2=Liso|first2=Vincenzo|last3=Cui|first3=Xiaoti|last4=Li|first4=Na|last5=Zhu|first5=Jimin|last6=Sahlin|first6=Simon Lennart|last7=Jensen|first7=Søren Højgaard|last8=Nielsen|first8=Mads Pagh|last9=Kær|first9=Søren Knudsen|date=2020|title=A Review of The Methanol Economy: The Fuel Cell Route|journal=Energies|language=en|volume=13|issue=3|pages=596|doi=10.3390/en13030596|doi-access=free}}</ref> Freezing of the liquid methanol-water mixture in the stack at low ambient temperature can be problematic for the membrane of DMFC (in contrast to indirect methanol fuel cell).

== Methanol ==
Methanol is a liquid from {{cvt|−97.6 to 64.7|°C}} at atmospheric pressure.
The volumetric [[energy density]] of methanol is an order of magnitude greater than even highly [[compressed hydrogen]], about two times greater than liquid hydrogen and 2.6 times higher than [[Lithium-ion battery|lithium-ion batteries]].{{when|date=January 2022}} The energy density per mass is a tenth of that of hydrogen, but 10 times higher than that of lithium-ion batteries.<ref>{{cite journal|doi=10.1016/j.enpol.2008.09.036|title=Hydrogen and fuel cells: Towards a sustainable energy future |journal=Energy Policy |volume=36 |issue=12 |date=December 2008 |pages=4356–4362|last1=Edwards |first1=P.P. |last2=Kuznetsov |first2=V.L. |last3=David |first3=W.I.F. |last4=Brandon |first4=N.P. |bibcode=2008EnPol..36.4356E }}</ref>

Methanol is slightly [[toxicity|toxic]] and highly [[Flammability|flammable]]. However, the International Civil Aviation Organization's (ICAO) Dangerous Goods Panel (DGP) voted in November 2005 to allow passengers to carry and use micro fuel cells and methanol fuel cartridges when aboard airplanes to power [[laptop computer]]s and other consumer electronic devices.
On September 24, 2007, the [[US Department of Transportation]] issued a proposal to allow airline passengers to carry fuel cell cartridges on board.<ref>[http://www.fuelcelltoday.com/online/news/articles/2007-09/US-Department-of-Transportation-moves-to-approve-fuel-cells-for-aircraft-use ''US Department of Transportation moves to approve fuel cells for aircraft use''] {{webarchive|url=https://web.archive.org/web/20090211170242/http://www.fuelcelltoday.com/online/news/articles/2007-09/US-Department-of-Transportation-moves-to-approve-fuel-cells-for-aircraft-use |date=2009-02-11 }}, by FuelCellToday.</ref>
The Department of Transportation issued a final ruling on April 30, 2008, permitting passengers and crew to carry an approved fuel cell with an installed methanol cartridge and up to two additional spare cartridges.<ref>[http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Federal%20Register/Hazmat/HM-215J%20Final%20Rule%2012-30-08.pdf ''Hazardous Materials: Revision to Requirements for the Transportation of Batteries and Battery-Powered Devices; and Harmonization with the United Nations Recommendations, International Maritime Dangerous Goods Code, and International Civil Aviation Organization's Technical Instructions''] {{Webarchive|url=https://web.archive.org/web/20110725072442/http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Federal%20Register/Hazmat/HM-215J%20Final%20Rule%2012-30-08.pdf |date=2011-07-25 }}, by the US department of transportation.</ref>
It is worth noting that 200 ml maximum methanol cartridge volume allowed in the final ruling is double the 100 ml limit on liquids allowed by the Transportation Security Administration in carry-on bags.<ref>[http://www.tsa.gov/press/happenings/311_intl_acceptance.shtm ''3-1-1 Gains International Acceptance''] {{webarchive|url=https://web.archive.org/web/20080509155910/http://www.tsa.gov/press/happenings/311_intl_acceptance.shtm |date=2008-05-09 }}, by the US transport security administration.</ref>

== Reaction ==
The DMFC relies upon the [[redox|oxidation]] of [[methanol]] on a [[catalyst]] layer to form [[carbon dioxide]]. Water is consumed at the [[anode]] and produced at the [[cathode]]. [[Proton]]s (H<sup>+</sup>) are transported across the proton exchange membrane - often made from [[Nafion]] - to the cathode where they react with [[oxygen]] to produce water. [[Electron]]s are transported through an external circuit from anode to cathode, providing power to connected devices.

The [[half-reaction]]s are:

{| border="1" cellspacing="0" cellpadding="10" style="border-collapse:collapse;"
 !
 !Equation
 |-
 !Anode
 |<math>\mathrm{CH_3OH + H_2O \to 6\ H^+  + 6\ e^- + CO_2}</math><br /><small>oxidation</small>
 |-
 !Cathode
 |<math>\mathrm{\frac{3}{2} O_2 + 6\ H^+  + 6\ e^- \to 3\ H_2O}</math><br /><small>reduction</small>
 |-
 !Overall reaction
 |<math>\mathrm{CH_3OH + \frac{3}{2} O_2 \to 2\ H_2O + CO_2}</math><br /><small>redox reaction</small>
 |-
 |}

Methanol and water are adsorbed on a catalyst usually made of [[platinum]] and [[ruthenium]] particles, and lose protons until carbon dioxide is formed.  As water is consumed at the [[anode]] in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back [[diffusion]] ([[osmosis]]), or [[active transport]] such as pumping. The need for water limits the energy density of the fuel.

Platinum is used as a catalyst for both half-reactions. This contributes to the loss of cell voltage potential, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of these cells.

During the methanol oxidation reaction [[carbon monoxide]] (CO) is formed, which strongly adsorbs onto the platinum catalyst, reducing the number of available reaction sites and thus the performance of the cell. The addition of other metals, such as [[ruthenium]] or [[gold]], to the platinum catalyst tends to ameliorate this problem. In the case of platinum-ruthenium catalysts, the oxophilic nature of ruthenium is believed to promote the formation of [[hydroxyl radicals]] on its surface, which can then react with carbon monoxide adsorbed on the platinum atoms. The water in the fuel cell is oxidized to a hydroxy radical via the following reaction: H<sub>2</sub>O → OH• + H<sup>+</sup> + e<sup>−</sup>. The hydroxy radical then oxidizes [[carbon monoxide]] to produce [[carbon dioxide]], which is released from the surface as a gas: CO + OH• → CO<sub>2</sub> + H<sup>+</sup> + e<sup>−</sup>.<ref>{{cite journal|last=Motoo|first=S.|author2=Watanabe, M.|title=Electrolysis by Ad-Atoms Part II. Enhancement of the Oxidation of Methanol on Platinum by Ruthenium Ad-Atoms|journal=Electrochemistry and Interfacial Electrochemistry|year=1975|volume=60|pages=267–273}}</ref>

Using these OH groups in the half reactions, they are also expressed as:

{| border="1" cellspacing="0" cellpadding="10" style="border-collapse:collapse;"
 !
 !Equation
 |-
 !Anode
 |<math>\mathrm{CH_3OH + 6\ OH^- \to 5\ H_2O  + 6\ e^- + CO_2}</math><br /><small>oxidation</small>
 |-
 !Cathode
 |<math>\mathrm{\frac{3}{2} O_2 + 3\ H_2O  + 6\ e^- \to 6\ OH^-}</math><br /><small>reduction</small>
 |-
 !Overall reaction
 |<math>\mathrm{CH_3OH + \frac{3}{2} O_2 \to 2\ H_2O + CO_2}</math><br /><small>redox reaction</small>
 |-
 |}

=== Cross-over current ===
Methanol on the anodic side is usually in a weak solution (from 1M to 3M), because methanol in high concentrations has the tendency to diffuse through the membrane to the cathode, where its concentration is about zero because it is rapidly consumed by oxygen. Low concentrations help in reducing the cross-over, but also limit the maximum attainable current.

The practical realization is usually that a solution loop enters the anode, exits, is refilled with methanol, and returns to the anode again. Alternatively, fuel cells with optimized structures can be directly fed with high concentration methanol solutions or even pure methanol.<ref>{{cite journal|last=Li|first=Xianglin|author2=Faghri|title=Amir|journal=Journal of Power Sources|volume=226|pages=223–240|doi=10.1016/j.jpowsour.2012.10.061}}</ref>

=== Water drag ===
The water in the anodic loop is lost because of the anodic reaction, but mostly because of the associated water drag: every proton formed at the anode drags a number of water molecules to the cathode. Depending on temperature and membrane type, this number can be between 2 and 6.

== Ancillary units ==
A direct methanol fuel cell is usually part of a larger system including all the ancillary units that permit its operation. Compared to most other types of fuel cells, the ancillary system of DMFCs is relatively complex. The main reasons for its complexity are:

* providing water along with methanol would make the fuel supply more cumbersome, so water has to be recycled in a loop;
* CO<sub>2</sub> has to be removed from the solution flow exiting the fuel cell;
* water in the anodic loop is slowly consumed by reaction and drag; it is necessary to recover water from the cathodic side to maintain steady operation.

== See also ==
{{cmn|
* [[Alkali anion-exchange membrane]]
* [[Dynamic hydrogen electrode]]
* [[Fuel cell]]
* [[Glossary of fuel cell terms]]
* [[Liquid fuels]]
* [[Methanol (data page)]]
* [[Methanol economy]]
* [[Portable fuel cell applications]]
* [[Rudolf Schulten]]
* [[SymPowerco]]
}}

== References ==
{{reflist}}

== Further reading ==
* Merhoff, Henry and Helbig, Peter. Development and Fielding of a Direct Methanol Fuel Cell; ''ITEA Journal'', March 2010

== External links ==
*[https://fuelcellnewstoday.com Fuel Cell News Today. An internet portal of news and articles of fuel cell developments]
*[https://www.knowledgefoundation.com/viewevents.php?event_id=209&act=evt 12th Small Fuel Cells. Annual conference on portable fuel cell technology developments] {{Webarchive|url=https://web.archive.org/web/20120303214859/http://www.knowledgefoundation.com/viewevents.php?event_id=209&act=evt |date=2012-03-03 }}

{{Fuel cells}}

[[Category:Fuel cells]]
[[Category:Methanol]]