140:, which describes the behavior of a fluid. In general, at low Reynolds numbers, flow is laminar whereas turbulence occurs at a higher Reynolds number. In laminar flow, two fluids will interact primarily through diffusion which means mixing is limited. By choosing the correct fuel and oxidizing agents in LFFC's, protons can be allowed to diffuse from the anode to the cathode across the interface of the two streams. The LFFC's are not limited to a liquid feed and in certain cases, depending on the geometry and reactants, gases can also be advantageous. Current designs inject the fuel and oxidizing agent into two separate streams which flow side by side. The interface between the fluids acts as the electrolytic membrane across which protons diffuse. Membraneless fuel cells offer a cost advantage due to the lack of the electrolytic membrane. Further, a decrease in crossover also increases
518:
micro fuel cells, these pumps and separators need to be miniaturized and packaged into a small volume (under 1 cm). Associated with this process is a so-called "packaging penalty" which results in higher costs. Further, pumping power drastically increases with decreasing size (see
Scaling Laws) which is disadvantageous. Efficient packaging methods and/or self-pumping cells (see Research and Development) need to be developed to make this technology viable. Also, while using high concentrations of specific fuels, such as methanol, crossover still occurs. This problem can be partially solved by using a nanoporous separator, lowering fuel concentration or choosing reactants which have a lower tendency towards crossover.
582:
automobile power applications, macro fuel cells can be used because space is not necessarily the limiting constraint. However, for portable devices such as cell phones and laptops, macro fuel cells are often inefficient due to their space requirements lower run times. LFFCs however, are perfectly suited for these types of applications. The lack of a physical electrolytic membrane and energy dense fuels that can be used means that LFFC's can be produced at lower costs and smaller sizes. In most portable applications, energy density is more important than efficiency due to the low power requirements.
552:
pumping. This variation causes fluctuations at the reactant interfaces which can disrupt laminar flow and affect diffusion and crossover. However, self-pumping mechanisms can be difficult and expensive to produce on the macro-scale. In order to take advantage of hydrophobic effects, the surfaces need to be smooth to control the contact angle of water. To produce these surfaces on a large scale, the cost will significantly increase due to the close tolerances which are needed. Also, it is not evident whether using a carbon-dioxide based pumping system on the large scale is viable.
535:
vented, fresh fuel is also drawn in at the same through the check valve and the cycle begins again. Thus, the fuel cell pumping is regulated by the reaction rate. This type of cell is not a two stream laminar flow fuel cell. Since the formation of bubbles can disrupt two separate laminar flows, a combined stream of fuel and oxidant was used. In laminar conditions, mixing will still not occur. It was found that using selective catalysts (i.e. Not platinum) or extremely low flow rates can prevent crossover.
23:. In Laminar Flow Fuel Cells (LFFC) this is achieved by exploiting the phenomenon of non-mixing laminar flows where the interface between the two flows works as a proton/ion conductor. The interface allows for high diffusivity and eliminates the need for costly membranes. The operating principles of these cells mean that they can only be built to millimeter-scale sizes. The lack of a membrane means they are cheaper but the size limits their use to portable applications which require small amounts of power.
125:
564:), self pumping can be difficult. Thus, external pumps are required. However, for a rectangular channel, the pressure required increases proportional to the L, where L is a length unit of the cell. Thus, by decreasing the size of a cell from 10 cm to 1 cm, the required pressure will increase by 1000. For micro fuel cells, this pumping requirement requires high voltages. Although in some cases,
48:
116:
shortcomings of proton exchange membranes. For example, fuel crossover means that low concentrations need to be used which limits the available power of the cell. In solid oxide fuel cells, high temperatures are needed which require energy and can also lead to quicker degradation of materials. Membraneless fuel cells offer a solution to these problems.
98:(DMFC's), for example, use methanol as the reactant instead of first using reformation to produce hydrogen. Although DMFC's are not very efficient (~25%), they are energy dense which means that they are quite suitable for portable power applications. Another advantage over gaseous fuels, as in the H
55:
A fuel cell consists of an electrolyte which is placed in between two electrodes – the cathode and the anode. In the simplest case, hydrogen gas passes over the cathode, where it is decomposed into hydrogen protons and electrons. The protons pass through the electrolyte (often NAFION – manufactured
551:
For example, laminar flow is a necessary condition for these cells. Without laminar flow, crossover would occur and a physical electrolytic membrane would be needed. Maintaining laminar flow is achievable on the macro scale but maintaining a steady
Reynolds number is difficult due to variations in
508:
In most fuel cell configurations with liquid feeds, the fuel and oxidizing solutions almost always contain water which acts as a diffusion medium. In many hydrogen-oxygen fuel cells, the diffusion of oxygen at the cathode is rate limiting since the diffusivity of oxygen in water is much lower than
152:
Diffusion across the interface is extremely important and can severely affect fuel cell performance. The protons need to be able to diffuse across both the fuel and the oxidizing agent. The diffusion coefficient, a term which describes the ease of diffusion of an element in another medium, can be
517:
The promise of membraneless fuel cells has been offset by several problems inherent to their designs. Ancillary structures are one of the largest obstacles. For example, pumps are required to maintain laminar flow while gas separators can be needed to supply the correct fuels into the cells. For
534:
is formed while fuel is consumed. The bubble begins to propagate towards the outlet of the cell. However, before the outlet, a hydrophobic vent allows the carbon dioxide to escape while simultaneously ensuring other byproducts (such as water) do not clog the vent. As the carbon dioxide is being
106:
cells, is that liquids are much easier to handle, transport, pump and often have higher specific energies allowing for greater power extraction. Generally gases need to be stored in high pressure containers or cryogenic liquid containers which is a significant disadvantage to liquid transport.
26:
Another type of membraneless fuel cell is a Mixed
Reactant Fuel Cell (MRFC). Unlike LFFCs, MRFCs use a mixed fuel and electrolyte, and are thus not subject to the same limitations. Without a membrane, MRFCs depend on the characteristics of the electrodes to separate the oxidation and reduction
115:
The majority of fuel cell technologies currently employed are either PEM or SOFC cells. However, the electrolyte is often costly and not always completely effective. Although hydrogen technology has significantly evolved, other fossil fuel based cells (such as DMFC's) are still plagued by the
34:
system can achieve a 40% electrical conversion efficiency while an outdated nuclear power plant is slightly lower at 32%. GenIII and GenIV Nuclear
Fission plants can get up to 90% efficient if using direct conversion or up to 65% efficient if using a magnetohydrodynamic generator as a topping
504:
In order to increase the diffusion flux, the diffusivity and/or concentration need to be increased while the length needs to be decreased. In DMFC's for example, the thickness of the membrane determines the diffusion length while the concentration is often limited due to crossover. Thus, the
581:
The thermodynamic potential of a fuel cell limits the amount of power that an individual cell can deliver. Therefore, in order to obtain more power, fuel cells must be connected in series or parallel (depending on whether greater current or voltage is desired). For large scale building and
71:, specifically using methane, which produces hydrogen from fossil fuels by running them through a high temperature steam process. Since fossil fuels are primarily composed of carbon and hydrogen molecules of various sizes, various fossil fuels can be utilized. For example,
668:
Verhallen, P., L. Oomen, A. Elsen, and A. Kruger. "The
Diffusion Coefficients of Helium, Hydrogen, Oxygen and Nitrogen in Water Determined from the Permeability of a Stagnant Liquid Layer in the Quasi-steady State." Chemical Engineering Science 39.11 (1984): 1535–541.
678:
Hollinger, Adam S., R. J. Maloney, L. J. Markoski, P. J. Kenis, R. S. Jayashree, and D. Natarajan. "Nanoporous
Separator and Low Fuel Concentration to Minimize Crossover in Direct Methanol Laminar Flow Fuel Cells." Journal of Power Sources 195.11 (2010): 3523–528.
547:
area. These cell sizes are suited for the small scale due to the limit of their operating principles. The scale-up of these cells to the 2–10 Watt range has proven difficult since, at large scales, the cells cannot maintain the correct operating conditions.
35:
cycle{{Citation needed|reason=again, the numbers seem way off. The best achieved efficiency for initial cycle is about 30%. The capture of residual thermal energy is at best 30% to date, which comes to overall efficiency of 51% at best |date=June 2022}}.
505:
diffusion flux is limited. A membraneless fuel cell is theoretically the better option since the diffusion interface across both fluids is extremely thin and using higher concentrations does not result in a drastic effect on crossover.
56:
by DuPont) across to the anode to the oxygen. Meanwhile, the free electrons travel around the cell to power a given load and then combine with the oxygen and hydrogen at the anode to form water. Two common types of electrolytes are a
525:
is produced in the reaction in the form of bubbles. The bubbles nucleate and coalesce on the anode. A check valve at the supply end prevents any fuel entering while the bubbles are growing. The check valve is not mechanical but
697:
Meng, D. D., J. Hur, and C. Kim. "MEMBRANELSS MICRO FUEL CELL CHIP ENABLED BY SELF-PUMPING OF FUEL-OXIDANT MIXTURE." Proc. of 2010 IEEE 23rd
International Conference on Micro Electro Mechanical Systems, Wanchai, Hong Kong.
555:
Membraneless fuel cells can utilize self-pumping mechanisms but requires the use of fuel which release GHG's (greenhouse gases) and other unwanted products. To use an environmentally friendly fuel configuration (such as
219:
572:
effects also become significantly more important. For the fuel cell configuration with a carbon dioxide generating mechanism, the surface tension effects could also increase the pumping requirements drastically.
305:
476:
641:
Kin, T., W. Shieh, C. Yang, and G. Yu. "Estimating the
Methanol Crossover Rate of PEM and the Efficiency of DMFC via a Current Transient Analysis." Journal of Power Sources 161.2 (2006): 1183–186. Print.
39:
systems are capable of reaching efficiencies in the range of 55%–70%. However, as with any process, fuel cells also experience inherent losses due to their design and manufacturing processes.
707:
Abruna, H., and A. Stroock. "Transport
Phenomena and Interfacial Kinetics in Planar Microfluidic Membraneless Fuel Cells." Hydrogen Program. U.S. Department of Energy. Web. 25 Nov. 2010. <
402:
87:
and high temperature combination cycles are also used to provide hydrogen from water whereby the heat and electricity provide sufficient energy to disassociate the hydrogen and oxygen atoms.
521:
Date: January 2010: Researchers developed a novel method of inducing self-pumping in a membraneless fuel cell. Using formic acid as a fuel and sulfuric acid as an oxidant, CO
132:
around a cylinder. At the beginning of the vortex, both fluids are separate. This indicates laminar flow with minimal mixing. Picture courtesy, Cesareo de La Rosa
Siqueira.
27:
reactions. By eliminating the membrane and delivering the reactants as a mixture, MRFCs can potentially be simpler and less costly than conventional fuel cell systems.
625:
424:
498:
348:
325:
245:
530:
in nature. By creating micro structures which form specific contact angles with water, fuel cannot be drawn backwards. As the reaction continues, more CO
659:
Fukada, Satoshi. "Analysis of Oxygen Reduction Rate in a Proton Exchange Membrane Fuel Cell." Energy Conversion and Management 42.9 (2000): 1121. Print.
688:
D. D. Meng and C.-J. Kim, “Micropumping of liquid by directional growth and selective venting of gas Bubbles”, Lab on a Chip, 8 (2008), pp. 958- 968.
163:
650:
1. E.R. Choban, L.J. Markoski, A. Wieckowski, P.J.A. Kenis, Micro-Fluidic Fuel Cell Based on Laminar Flow. J. Power Sources, 2004,128, 54–60.
738:
250:
776:
429:
544:
919:
621:
Ragheb, Magdi. "Steam Reforming." Lecture. Energy Storage Systems. University of Illinois, 3 Oct. 2010. Web. 12 Oct. 2010. <
64:). Although hydrogen and oxygen are very common reactants, a plethora of other reactants exist and have been proven effective.
67:
Hydrogen for fuel cells can be produced in many ways. The most common method in the United States (95% of production) is via
874:
622:
914:
797:
967:
884:
827:
363:
19:
convert stored chemical energy into electrical energy without the use of a conducting membrane as with other types of
543:
Membraneless fuel cells are currently being manufactured on the micro scale using fabrication processes found in the
154:
904:
766:
94:
are often energy and space intensive, it is often more convenient to use the chemicals directly in the fuel cell.
30:
The efficiency of these cells is generally much higher than modern electricity producing sources. For example, a
894:
812:
771:
95:
31:
899:
807:
731:
57:
568:
can be induced. However, for liquid mediums, high voltages are also required. Further, with decreasing size,
822:
802:
708:
509:
that of hydrogen. As a result, LFFC performance can also be improved by not using aqueous oxygen carriers.
909:
848:
817:
781:
68:
61:
832:
60:(also known as Polymer Electrolyte Membrane) and a ceramic or solid oxide electrolyte (often used in
598:
988:
853:
724:
565:
91:
327:
measures the amount of substance that will flow through a small area during a small time interval.
157:
which addresses the effects of a concentration gradient and distance over which diffusion occurs:
761:
207:
169:
408:
954:
949:
944:
939:
356:
629:
569:
527:
141:
137:
623:
https://netfiles.uiuc.edu/mragheb/www/NPRE%20498ES%20Energy%20Storage%20Systems/index.htm
482:
332:
124:
310:
230:
982:
869:
129:
879:
84:
136:
LFFC's overcome the problem of unwanted crossover through the manipulation of the
20:
747:
36:
47:
214:{\displaystyle {\bigg .}J=-D{\frac {\partial \phi }{\partial x}}{\bigg .}}
931:
709:
http://www.hydrogen.energy.gov/pdfs/review10/bes017_abruna_2010_o_web.pdf
72:
80:
76:
500:
is the diffusion length i.e. the distance over which diffusion occurs
426:(for ideal mixtures) is the concentration in dimensions of , example
300:{\displaystyle \left({\tfrac {\mathrm {mol} }{m^{2}\cdot s}}\right)}
51:
Fuel Cell Diagram. Note: Electrolyte can be a polymer or solid oxide
123:
46:
720:
716:
471:{\displaystyle \left({\tfrac {\mathrm {mol} }{m^{3}}}\right)}
438:
372:
259:
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411:
366:
335:
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166:
930:
862:
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418:
396:
342:
319:
299:
239:
213:
247:is the diffusion flux in dimensions of , example
397:{\displaystyle \left({\tfrac {m^{2}}{s}}\right)}
111:Membraneless Fuel Cells and Operating Principles
599:"MRFC Technology - Mantra Energy Alternatives"
732:
8:
83:can all be used in the reforming process.
739:
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185:
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590:
7:
777:Proton-exchange membrane fuel cell
447:
444:
441:
268:
265:
262:
196:
188:
144:resulting in higher power output.
14:
90:However, since these methods of
920:Unitized regenerative fuel cell
577:Potential Applications of LFFCs
1:
915:Solid oxide electrolyzer cell
58:proton exchange membrane(PEM)
798:Direct borohydride fuel cell
885:Membrane electrode assembly
828:Reformed methanol fuel cell
360:in dimensions of , example
1005:
905:Protonic ceramic fuel cell
875:Electro-galvanic fuel cell
767:Molten carbonate fuel cell
603:Mantra Energy Alternatives
96:Direct Methanol Fuel Cells
963:
895:Photoelectrochemical cell
813:Direct methanol fuel cell
772:Phosphoric acid fuel cell
900:Proton-exchange membrane
808:Direct-ethanol fuel cell
513:Research and development
155:Fick's laws of diffusion
890:Membraneless Fuel Cells
823:Metal hydride fuel cell
803:Direct carbon fuel cell
419:{\displaystyle \,\phi }
32:fossil fuel power plant
17:Membraneless Fuel Cells
910:Regenerative fuel cell
849:Enzymatic biofuel cell
494:
472:
420:
398:
344:
321:
301:
241:
215:
133:
62:Solid oxide fuel cells
52:
818:Formic acid fuel cell
782:Solid oxide fuel cell
495:
473:
421:
399:
352:diffusion coefficient
345:
322:
302:
242:
216:
127:
50:
483:
430:
409:
364:
333:
311:
251:
231:
164:
854:Microbial fuel cell
566:Electroosmotic flow
493:{\displaystyle \,x}
343:{\displaystyle \,D}
92:hydrogen production
762:Alkaline fuel cell
628:2012-12-18 at the
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211:
134:
53:
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320:{\displaystyle J}
290:
240:{\displaystyle J}
203:
996:
833:Zinc–air battery
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630:Wayback Machine
620:
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579:
570:surface tension
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541:
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273:
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162:
161:
150:
142:fuel efficiency
138:Reynolds number
122:
113:
105:
101:
45:
12:
11:
5:
1002:
1000:
992:
991:
981:
980:
974:
973:
971:
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964:
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769:
764:
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755:By electrolyte
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671:
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539:Scaling Issues
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153:combined with
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99:
44:
41:
13:
10:
9:
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4:
3:
2:
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842:Biofuel cells
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130:vortex street
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69:Gas reforming
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24:
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606:. Retrieved
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120:Laminar Flow
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89:
85:Electrolysis
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870:Blue energy
528:hydrophobic
357:diffusivity
989:Fuel cells
748:Fuel cells
608:2015-10-27
586:References
21:Fuel Cells
545:MEMS/NEMS
414:ϕ
284:⋅
197:∂
192:ϕ
189:∂
180:−
148:Diffusion
37:Fuel cell
983:Category
968:Glossary
932:Hydrogen
626:Archived
73:methanol
43:Overview
955:Vehicle
950:Storage
945:Station
940:Economy
791:By fuel
350:is the
81:methane
77:ethanol
863:Others
698:Print.
679:Print.
669:Print.
224:where
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