Membraneless Fuel Cells convert stored chemical energy into electrical energy without the use of a conducting membrane as with other types of fuel cells. 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. 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 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. The efficiency of these cells is generally much higher than modern electricity producing sources. For example, a fossil fuel power plant system can achieve a 40% electrical conversion efficiency while an outdated nuclear power plant is slightly lower at 32%.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 cycle{{Citation needed|reason=again, the numbers seem way off. The best achieved efficiency for the initial cycle is about 30%. The capture of residual thermal energy is at best 30% to date, which comes to an overall efficiency of 51% at best |date=June 2022}}. Fuel cell 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.
Overview
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 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 proton exchange membrane(PEM) (also known as Polymer Electrolyte Membrane) and a ceramic or solid oxide electrolyte (often used in Solid oxide fuel cells). Although hydrogen and oxygen are very common reactants, a plethora of other reactants exist and have been proven effective. Hydrogen for fuel cells can be produced in many ways. The most common method in the United States (95% of production) is via Gas reforming, 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, methanol, ethanol, and methane can all be used in the reforming process. Electrolysis 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. However, since these methods of hydrogen production are often energy and space intensive, it is often more convenient to use the chemicals directly in the fuel cell. Direct methanol fuel cells (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 H2-O2 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.
Membraneless fuel cells and operating principles 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 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.
Laminar flow LFFC's overcome the problem of unwanted crossover through the manipulation of the Reynolds number, 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 fuel efficiency resulting in higher power output.
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