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| [[Image:Fc diagram pem.gif|thumb|Diagram of a PEM fuel cell]]
| | Friends contact her Claude Gulledge. To perform handball is the factor she loves most of all. Delaware is our beginning place. Bookkeeping is what I do for a living.<br><br>my webpage; [http://imagine-studiofx.biz/COM-3020/index.php?qa=4671&qa_1=auto-repair-secrets-you-should-know car warranty] |
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| '''Proton exchange membrane fuel cells''', also known as [[polymer electrolyte membrane]] (PEM) fuel cells ('''PEMFC'''), are a type of [[fuel cell]] being developed for transport applications as well as for [[stationary fuel cell applications]] and [[portable fuel cell applications]]. Their distinguishing features include lower temperature/pressure ranges (50 to 100 °C) and a special polymer electrolyte membrane. PEMFCs operate on a similar principle to their younger sister technology [[PEM electrolysis]]. They are a leading candidate to replace the aging alkaline fuel cell technology, which was used in the [[Space Shuttle]].<ref name=NTRS>{{cite web|last=Loyselle, Patricia; Prokopius, Kevin|title=Teledyne Energy Systems, Inc., Proton Exchange Member (PEM) Fuel Cell Engineering Model Powerplant. Test Report: Initial Benchmark Tests in the Original Orientation|url=http://hdl.handle.net/2060/20110014968|work=NASA|publisher=Glenn Research Center|accessdate=15 September 2011}}</ref>
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| ==Science==
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| ===Reactions===
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| {{Details|Fuel cell}}
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| A proton exchange membrane fuel cell transforms the [[chemical energy]] liberated during the [[electrochemical]] reaction of hydrogen and oxygen to [[electrical energy]], as opposed to the direct [[combustion]] of hydrogen and oxygen gases to produce [[thermal energy]].
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| A stream of hydrogen is delivered to the [[anode]] side of the [[membrane electrode assembly]] (MEA). At the anode side it is [[catalysis|catalytically]] split into [[proton]]s and [[electron]]s. This [[Redox|oxidation half-cell reaction]] or Hydrogen Oxidation Reaction (HOR) is represented by:<br>
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| '''At the Anode:'''
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| {| style="border:1px solid #ccc; width:50%;"
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| || <math>\mathrm{H}_2 \rightarrow \mathrm{2H}^+ + \mathrm{2e}^-</math>
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| | style="text-align: right;" | <math>E^o = 0\,V</math><sub>[[Standard hydrogen electrode|SHE]]</sub>
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| | style="text-align:right; vertical-align:top;"| <math>\left ( 1 \right )</math>
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| |}
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| The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external [[electrical network|load circuit]] to the [[cathode]] side of the MEA, thus creating the [[Current (electricity)|current]] output of the fuel cell.
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| Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This [[Redox|reduction]] half-cell reaction or [[oxygen reduction reaction]] (ORR) is represented by:<br>
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| '''At the cathode:'''
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| {| style="border:1px solid #ccc; width:50%;"
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| || <math>\mathrm1/2{O}_2 + {2H}^+ + {2e}^- \rightarrow \mathrm{H}_2\mathrm{O}</math>
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| | style="text-align: right;" | <math>E^o =1.229\,V</math><sub>[[Standard hydrogen electrode|SHE]]</sub>
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| | style="text-align:right; vertical-align:top;"| <math>\left ( 2 \right )</math>
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| |}<br>
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| '''Overall reaction:'''
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| {| style="border:1px solid #ccc; width:50%;"
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| || <math>\mathrm{H}_2 + \mathrm 1/2{O}_2 \rightarrow \mathrm{H}_2\mathrm{O}</math>
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| | style="text-align: right;" | <math>E^o = 1.229\,V</math><sub>[[Standard hydrogen electrode|SHE]]</sub>
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| | style="text-align:right; vertical-align:top;"| <math>\left ( 3 \right )</math>
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| |}
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| The reversible reaction is expressed in the equation and shows the reincorporation of the hydrogen protons and electrons together with the oxygen molecule and the formation of one water molecule.
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| ===Polymer electrolyte membrane===
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| {{Main|Polymer electrolyte membrane}}
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| [[Image:Pem.fuelcell2.gif|thumb|300px]]
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| To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "[[short circuit]]" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as '''gas crossover'''. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode. | |
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| Splitting of the hydrogen [[molecule]] is relatively easy by using a [[platinum]] catalyst. Unfortunately however, splitting the oxygen molecule is more difficult, and this causes significant electric losses. An appropriate catalyst material for this process has not been discovered, and platinum is the best option. One promising catalyst that uses far less expensive materials—iron, nitrogen, and carbon—has long been known to promote the necessary reactions, but at rates that are far too slow to be practical.<ref>Bullis, Kevin. (2009-04-02) [http://www.technologyreview.com/energy/22386/ A Catalyst for Cheaper Fuel Cells]. Technologyreview.com. Retrieved on 2013-04-19.</ref> Recently, a Canadian research institute has dramatically increased the performance of this type of iron-based catalyst. Their material produces 99 A/cm<sup>3</sup> at 0.8 volts, a key measurement of catalytic activity {{Citation needed|date=August 2009}}. That is 35 times better than the best nonprecious metal catalyst so far, and close to the Department of Energy's goal for fuel-cell catalysts: 130 A/cm<sup>3</sup>.<ref>[http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/fuel_cells.pdf U.S. Department of Energy, Technical Plan: Fuel Cells, 2007]. eere.energy.gov</ref> It also matches the performance of typical platinum catalysts. The only problem at the moment is its durability because after only 100 hours of testing the reaction rate dropped to half. Another significant source of losses is the resistance of the membrane to proton flow, which is minimized by making it as thin as possible, on the order of 50 [[micrometer (unit)|µm]].
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| The PEMFC is a prime candidate for vehicle and other mobile applications of all sizes down to [[mobile phones]], because of its compactness. However, the water management is crucial to performance: too much water will flood the membrane, too little will dry it; in both cases, power output will drop. Water management is a very difficult subject in PEM systems, primarily because water in the membrane is attracted toward the cathode of the cell through polarization. A wide variety of solutions for managing the water exist including integration of [[electroosmotic pump]]s. Furthermore, the platinum catalyst on the membrane is easily [[catalyst poisoning|poisoned]] by [[carbon monoxide]] (no more than one [[part per million]] is usually acceptable) and the membrane is sensitive to things like metal ions, which can be introduced by corrosion of metallic [[bipolar plate]]s, metallic components in the fuel cell system or from contaminants in the fuel/oxidant.
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| PEM systems that use reformed [[methanol]] were proposed, as in Daimler Chrysler Necar 5; reforming methanol, i.e. making it react to obtain hydrogen, is however a very complicated process, that requires also purification from the carbon monoxide the reaction produces. A platinum-[[ruthenium]] catalyst is necessary as some carbon monoxide will unavoidably reach the membrane. The level should not exceed 10 [[parts per million]]. Furthermore, the start-up times of such a reformer reactor are of about half an hour. Alternatively, methanol, and some other [[biofuels]] can be fed to a PEM fuel cell directly without being reformed, thus making a direct methanol fuel cell ([[DMFC]]). These devices operate with limited success.
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| The most commonly used membrane is [[Nafion]] by [[DuPont]], which relies on liquid water humidification of the membrane to transport protons. This implies that it is not feasible to use temperatures above 80 to 90 °C, since the membrane would dry. Other, more recent membrane types, based on [[Polybenzimidazole fiber|polybenzimidazole (PBI)]] or [[phosphoric-acid fuel cell|phosphoric acid]], can reach up to 220 °C without using any water management: higher temperature allow for better efficiencies, power densities, ease of cooling (because of larger allowable temperature differences), reduced sensitivity to carbon monoxide poisoning and better controllability (because of absence of water management issues in the membrane); however, these recent types are not as common.<ref>{{cite journal|author=Lee, J. S. ''et al.'' |url=http://www.cheric.org/PDF/JIEC/IE12/IE12-2-0175.pdf |title=Polymer electrolyte membranes for fuel cells |year=2006 |pages=175–183 |journal=Journal of Industrial and Engineering Chemistry |volume=12}}</ref>
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| ====Efficiency====
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| The maximal theoretical efficiency applying the [[Gibbs free energy]] equation ΔG=-237.13 kJ/mol and using the [[Lower Heating Value]] (LHV) of Hydrogen ΔH=-285.84 kJ/mol) is 83% at 298 K.
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| <math>\eta=\frac{\Delta G}{ \Delta H}=1-\frac{T\Delta S}{\Delta H}</math>
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| The practical efficiency of a PEM's is in the range of 40–60% using the [[Higher Heating Value]] of hydrogen (HHV).
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| Main factors that create losses are:
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| * Activation losses
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| * Ohmic losses
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| * Mass transport losses
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| ===Catalyst research===
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| Much of the current research on catalysts for PEM fuel cells can be classified as having one of two main objectives:
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| #to obtain higher catalytic activity than the standard carbon-supported platinum particle catalysts used in current PEM fuel cells
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| #to reduce the poisoning of PEM fuel cell catalysts by impurity gases
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| Examples of these two approaches are given in the following sections.
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| ====Increasing catalytic activity====
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| As mentioned above, platinum is by far the most effective element used for PEM fuel cell catalysts, and nearly all current PEM fuel cells use platinum particles on porous carbon supports to catalyze both hydrogen oxidation and oxygen reduction. However, due to their high cost, current Pt/C catalysts are not feasible for commercialization. The [[U.S. Department of Energy]] estimates that platinum-based catalysts will need to use roughly four times less platinum than is used in current PEM fuel cell designs in order to represent a realistic alternative to [[internal combustion engines]].<ref>[http://www.hydrogen.energy.gov/program_plans.html Hydrogen, Fuel Cells & Infrastructure Technologies Program Multi-Year Research, Development and Demonstration Plan], U.S. Department of Energy, October 2007.</ref> Consequently, one main goal of catalyst design for PEM fuel cells is to increase the catalytic activity of platinum by a factor of four so that only one-fourth as much of the precious metal is necessary to achieve similar performance.
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| One method of increasing the performance of platinum catalysts is to optimize the size and shape of the platinum particles. Decreasing the particles’ size alone increases the total surface area of catalyst available to participate in reactions per volume of platinum used, but recent studies have demonstrated additional ways to make further improvements to catalytic performance. For example, one study reports that high-index facets of platinum [[nanoparticles]] (that is [[Miller index]]es with large integers, such as Pt (730)) provide a greater density of reactive sites for oxygen reduction than typical platinum nanoparticles.<ref>{{cite journal|author=N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, Z. L. Wang|title=Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity|journal=Science|volume=316|year=2007|pages=732–735|doi=10.1126/science.1140484|pmid=17478717|issue=5825}}</ref>
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| A second method of increasing the catalytic activity of platinum is to [[alloy]] it with other metals. For example, it was recently shown that the Pt<sub>3</sub>Ni(111) surface has a higher oxygen reduction activity than pure Pt(111) by a factor of ten.<ref>{{cite journal|author=V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas, N. M. Marković. Activity on Pt<sub>3</sub>Ni(111) via increased surface site availability|journal=Science|volume=315|year=2007|pages=493–497|doi=10.1126/science.1135941|title=Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability|pmid=17218494|issue=5811}}</ref> The authors attribute this dramatic performance increase to modifications to the [[electronic structure]] of the surface, reducing its tendency to bond to oxygen-containing ionic species present in PEM fuel cells and hence increasing the number of available sites for oxygen [[adsorption]] and reduction.
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| ====Reducing poisoning====
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| The other popular approach to improving catalyst performance is to reduce its sensitivity to impurities in the fuel source, especially carbon monoxide (CO). Presently, pure hydrogen gas is not economical to mass-produce by [[electrolysis]] or any other means. Instead, hydrogen gas is produced by [[steam reforming]] light [[hydrocarbons]], a process which produces a mixture of gases that also contains CO (1–3%), CO<sub>2</sub> (19–25%), and N<sub>2</sub> (25%).<ref>{{cite book|author=G. Hoogers|title=Fuel Cell Technology Handbook|publisher=Boca Raton, FL: CRC Press|year=2003|pages=6–3|isbn=0-8493-0877-1}}</ref> Even tens of parts per million of CO can poison a pure platinum catalyst, so increasing platinum’s resistance to CO is an active area of research.
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| For example, one study reported that cube-shaped platinum nanoparticles with [[Miller index|(100) faces]] displayed a fourfold increase in oxygen reduction activity compared to randomly faceted platinum nanoparticles of similar size.<ref>{{cite journal|author=C. Wang, H. Daimon, T. Onodera, T. Koda, S. Sun. A general approach to the size- and shape-controlled synthesis of platinum nanoparticles and their catalytic reduction of oxygen|journal=Angewandte Chemie International Edition|volume=47|year=2008|pages=3588–3591|doi=10.1002/anie.200800073|title=A General Approach to the Size- and Shape-Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen|pmid=18399516|issue=19}}</ref> The authors concluded that the (111) facets of the randomly shaped nanoparticles bonded more strongly to [[sulfate]] ions than the (100) facets, reducing the number of catalytic sites open to oxygen molecules. The nanocubes they synthesized, in contrast, had almost exclusively (100) facets, which are known to interact with sulfate more weakly. As a result, a greater fraction of the surface area of those particles was available for the reduction of oxygen, boosting the catalyst’s oxygen reduction activity.
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| In addition, researchers have been investigating ways of reducing the CO content of hydrogen fuel before it enters a fuel cell as a possible way to avoid poisoning the catalysts. One recent study revealed that ruthenium-platinum core–shell nanoparticles are particularly effective at oxidizing CO to form CO<sub>2</sub>, a much less harmful fuel contaminant.<ref>{{cite journal|author=S. Alayoglu, A. U. Nilekar, M. Mavrikakis, B. Eichhorn. Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen|journal=Nature Materials|volume=7|year=2008|pages=333–338|doi=10.1038/nmat2156|title=Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen|pmid=18345004|issue=4}}</ref> The mechanism that produces this effect is conceptually similar to that described for Pt<sub>3</sub>Ni above: the ruthenium core of the particle alters the electronic structure of the platinum surface, rendering it better able to catalyze the oxidation of CO.
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| ==History==
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| Before the invention of PEM fuel cells, existing fuel cell types such as [[solid-oxide fuel cells]] were only applied in extreme conditions. Such fuel cells also required very expensive materials and could only be used for stationary applications due to their size. These issues were addressed by the PEM fuel cell. The PEM fuel cell was invented in the early 1960s by Willard Thomas Grubb and Leonard Niedrach of [[General Electric]].<ref>[http://americanhistory.si.edu/fuelcells/pem/pemmain.htm PEM Fuel Cells]. Americanhistory.si.edu. Retrieved on 2013-04-19.</ref> Initially, sulfonated polystyrene membranes were used for electrolytes, but they were replaced in 1966 by [[Nafion]] [[ionomer]], which proved to be superior in performance and durability to sulfonated polystyrene.
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| PEM fuel cells were used in the [[NASA]] [[Project Gemini|Gemini]] series of spacecraft, but they were replaced by [[Alkaline fuel cell]]s in the [[Project Apollo|Apollo]] program and in the [[Space shuttle]].
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| Parallel with [[Pratt and Whitney]] Aircraft, General Electric developed the first proton exchange membrane fuel cells (PEMFCs) for the [[Gemini program|Gemini space missions]] in the early 1960s. The first mission to use PEMFCs was [[Gemini 5|Gemini V]]. However, the [[Apollo program|Apollo space missions]] and subsequent [[Apollo-Soyuz Test Project|Apollo-Soyuz]], [[Skylab]] and Space Shuttle missions used fuel cells based on Bacon's design, developed by Pratt and Whitney Aircraft.
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| Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications. However, fuel cells were seen to be desirable due to the large amounts of fuel available (hydrogen and oxygen).{{Citation needed|date=September 2009}}
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| Despite their success in space programs, fuel cell systems were limited to space missions and other special applications, where high cost could be tolerated. It was not until the late 1980s and early 1990s that fuel cells became a real option for wider application base. Several pivotal innovations, such as low platinum [[catalyst]] loading and thin film electrodes, drove the cost of fuel cells down, making development of PEMFC systems more realistic.<ref name=RSC2>{{cite web|title=Fuel cell electric vehicles and hydrogen infrastructure: status 2012|url=http://www.researchgate.net/publication/233987484_Fuel_cell_electric_vehicles_and_hydrogen_infrastructure_status_2012?ev=prf_pub|last1=Eberle|first1=Ulrich|first2=Bernd|last2=Mueller|first3=Rittmar|last3=von Helmolt|publisher=[[Royal Society of Chemistry]] |date=2012-07-15| accessdate=2013-01-08}}</ref> However, there is significant debate as to whether hydrogen fuel cells will be a realistic technology for use in [[automobile]]s or other [[vehicle]]s. (See [[hydrogen economy]].)
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| ==See also==
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| {{Portal|Sustainable development}}
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| *[[Power-to-weight ratio]]
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| *[[Dynamic hydrogen electrode]]
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| *[[Gas diffusion electrode]]
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| *[[Hydrogen sulfide sensor]]
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| *[[Reversible hydrogen electrode]]
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| *[[Glossary of fuel cell terms]]
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| *[[Timeline of hydrogen technologies]]
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| ==References==
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| {{Reflist|35em}}
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| {{Fuel cells}}
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| [[Category:Fuel cells]]
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| [[ko:연료전지#고분자전해질 연료전지 (Polymer Electrolyte Membrane Fuel Cell, PEMFC)]]
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