The direct methanol fuel cell or DMFC is emerging as a future winner in many of the applications that fuel cells can satisfy. DMFC is an important subset of the proton exchange membrane (PEM) fuel cell technology.
Direct Methanol Fuel Cells
Anna Welch Crull | STIX
|The direct methanol fuel cell or DMFC is emerging as a future winner in many of the applications that fuel cells can satisfy. DMFC is an important subset of the proton exchange membrane (PEM) fuel cell technology.|
|Anna Welch Crull, STIX|
Fuel cells are electrochemical devices capable of converting the energy of a chemical reaction into electrical energy without combustion and with virtually no pollution. Fuel cells are unlike batteries because fuel cells use chemical energy to directly create electrical energy as chemical reactants are continuously delivered to the fuel cell. Batteries store energy. Primary batteries such as alkaline cells are disposable. Secondary batteries such as lead-acid batteries are rechargeable. Still, the distinction is that a battery stores energy. Fuel cells are delivered fuel that is used to generate energy. When the fuel cell is not fueled, it has zero electrical potential. Fuel cells are typically used to produce a continuous source of electrical energy that competes with other forms of continuous electrical energy production, depending on the scale, such as the internal combustion engine, nuclear power, and coal-fired power stations as well as competing with primary and secondary batteries.
A reaction that produces electrons is an oxidation, and a reaction that consumes electrons is a reduction. These types of reactions must happen together, because electrons do not generally exist in the environment by themselves. An anode is the electrode where oxidation happens, and a cathode is the electrode where reduction happens, so electrons leave the anode and arrive at the cathode.
Different types of fuel cells include phosphoric acid, alkaline, molten carbonate, solid oxide, metal-air, and the proton exchange membrane and its important subset the direct methanol fuel cell (DMFC). The direct methanol fed fuel cell is a subset of the PEM fuel cell but is becoming a separate universe of its own. The most promising type of fuel cell in most situations is the proton exchange membrane (PEM) fuel cell assembled into a membrane electrode assembly (MEA). The market environment for PEM fuel cells addresses not only needs satisfied by hydrocarbon fuels, but also satisfied by stored energy sources such as batteries. There are unsatisfied and unrealized needs and desires in all the energy consuming and energy generating sectors. To satisfy these needs and desires, the economics and availability and distribution of energy, stored or generated, are now making an impact on the emerging market environment for the PEM fuel cells and the DMFC is starting to find its niche.
The innovation driver for direct methanol fuel cells' (DMFC) development is the need, want, and desire for sustainable energy resources. Every developed country in the world wants a cheap and stable supply of energy for an ever-increasing number of energy-consuming portable devices. How to have DMFCs meet this need at an affordable cost is problematic. Increasing power requirements and desires drive the DMFC as a way to meet the increasing demands. A new reality is emerging as the DMFC is found to be complementary to secondary batteries when used as a trickle charge, full charger, or in some other hybrid fuel cell combination.
As DMFCs are scaled down to efficiently and reliably perform, the technology takes a big step forward. Solving the issues of fuel cells for transportation will have to come some time later. The storage or generation of hydrogen is often considered an obstacle to the widespread commercial use of fuel cells. Problems remain, including the lower electrochemical activity of methanol solutions as compared to pure hydrogen this gives rise to lower cell voltages and efficiencies.
Primary components of a fuel cell are an ion conducting electrolyte, a cathode, and an anode. Taken together these thee components are often referred to as the membrane electrode assembly or MEA. This is a simple single cell fuel cell. Several simple single cell combined make up a more useful MEA. In the simplest example, a fuel is brought into the anode compartment and an oxidant, typically oxygen from the air, enters the cathode compartment. Direct chemical combustion is prevented by the electrolyte that separates the fuel from the oxidant. The electrolyte serves as a barrier to gas diffusion, but lets ion migrate across it.
The direct methanol fuel cell falls is an important subset of the proton exchange membrane (PEM) type fuel cell. Most commonly, platinum nanoparticles on carbon are utilized for both the anode and cathode. In direct methanol fuel cells a platinum-ruthenium (PtRu) alloy in a 50:50 molar ration is used at the anode. Ruthenium has the ability to electro-oxidize carbon monoxide that is adsorbed onto the platinum.
In DMFCs methanol as the fuel supplied to the anode. The electrochemical reactions are essentially as follows: first, a methanol molecule's carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule's oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air (supplied to the cathode) is simultaneously reduced at the cathode. The ions (protons) formed at the anode migrate through the interposing electrolyte and combine with the oxygen at the cathode to form water.
The following equations sum up the net reaction for a hydrogen fed fuel cell and for a methanol fed fuel cell.
CH3OH + H2O =>CO2 + 6H+ + 6e-
2H2=> 4H+ + 4e- (anode side)
1.5O2 + 6H+ + 6e- =>3H2O
O2 + 4H+ + 4e- => 2H2O (cathode side)
CH3OH + 1.5 O2 =>CO2 + 2H2O
2H2 + O2 => 2H2O
As noted above, the generalized formula for the steps in methanol oxidation is that methanol plus water gives carbon dioxide six protons and six electrons. This takes place in several stages. First there is the dehydrogenation of methanol to carbon monoxide. Water is dehydrogenated to oxygen. This is followed by an oxidative recombination forming the carbon-oxygen bond and a desorption of the products.
With pure platinum, carbon monoxide is the thermodynamic sink and will poison the surface if not removed. Research has shown that the bifunctional mechanism of a platinum-ruthenium catalyst is best because methanol dehydrogenates best of platinum and water dehydrogenation is best facilitated on ruthenium. Osmium may combine both capabilities of the bifunctional reaction.
Another alternative is the Millennium Cell concept of using fuel cartridges containing sodium borohydride. The U.S. subcommittee on transport of dangerous goods has regulations in the works that would allow the commercial transport of this cartridge.
In general, the catalysts for DMFC are platinum-ruthenium alloys. A major change has been initiated by encouraging and implementing the use of nanotechnology. Technion (Israel Institute of Technology, Haifa, Israel) has been known for its multidisciplinary nanoscience and nanotechnology programs.
It is the anode catalyst that provides the foundation for converting the chemical energy of the fuel into electrical energy. Platinum (Pt) is the best anode for hydrogen oxidation, but in the presence of methanol, CO, formed as a reaction intermediate, irreversibly absorbs to the Pt surface, rapidly lowering its activity. Pt/Ru bifunctional catalysts are presently the most active for methanol oxidation, with ruthenium (Ru) believed to serve the role of removing the absorbed carbon monoxide as carbon dioxide gas. This is shown in the equation below.
Ru-OH + Pt-CO ® Ru + Pt + CO2 + H+ + e-
The majority of scientific literature indicates that metallic ruthenium is preferred, while some groups have proposed that ruthenium oxide is the active species in this reaction. More research has been conducted on dispersing the catalyst on carbon supports to increase the efficiency of use and lower the catalyst cost, as well as on optimizing the ratio of Pt to Ru. Research has focused primarily on binary platinum (Pt) and ruthenium (Ru) catalysts, with little attention on ternary or quaternary catalysts. Some promising results have been reported for Pt-Ru-A systems (where A= a molybdenum oxide such as MoOx, a tungsten oxide such as WOx, a vanadium oxide such as VOx or an iridium oxide such as IrOx) as the next evolutionary step for fuel cell catalyst development. Ternary catalysts for direct methanol oxidation are a focus of some research efforts.
DMFCs do not have some of the fuel storage problems typical of other fuel cell types. Methanol has a higher energy volume density than hydrogen, but a lower energy volume density than gasoline or diesel fuel. The efficiency of the fuel in a fuel cell may be greater than in combustion types devices. Methanol is easier to transport and supply to the public using our current infrastructure because it is a liquid. Direct methanol fuel cell technology is relatively new compared to that of fuel cells powered by pure hydrogen, and research and development are roughly two to three years behind that of other fuel cell types. DMFCs appear to be the most promising battery replacement for portable applications and small-scale devices.
Direct methanol fuel cells (DMFCs) must compete or complement advanced battery technologies such as the lithium-ion (Li-ion) and its flexible subset the lithium-polymer. Also entering as a contender is the zinc-air fuel cell that is often used like a primary battery. Another use in the mix is the use of DMFCs as chargers for secondary batteries. DMFCs are expected to provide twice the amount of power delivered currently by batteries. The 7%/year improvement in efficiency of new lithium-based batteries is insufficient to close the gap on the increasing energy demand of portable electronic devices alone according to PolyFuel CEO Jim Balcom.
Crude oil supplies are imperceptibly drying up. With good chemistry and catalysts, natural gas or methane, can be converted into a variety of products, including gasoline for mid- to large-scale applications. This costs money. Batteries for premium portable supplies also cost money. There are environmental issues with fluorocarbon fuel cell membranes and there are environmental issues with lithium-ion/polymer issues and their manufacture. There are issues with wind farms for stationary power generation. It does not take a great stretch of the imagination to envision that there will be some issues raised over DMFCs.
An unresolved problem is the environmental impact of disposing of the fluorinated polymers used in most fuel cells. Preferred disposal options for fluoropolymer membranes are recycling and landfill. Incineration is possible only if incinerator is capable of scrubbing out hydrogen fluoride and other acidic combustion products. The composite structure and individual layers of these membranes can pick up a strong static electricity charge. Unless this charge is dissipated as it forms by using ionizing radiation devices or special conducting metal tinsel, it can build to thousands of volts and discharge to people or metal equipment. Caution is needed to prevent static accumulation when using flammable solvents while coating membrane surfaces.
Advanced lithium-ion battery technology, wind turbines, and photovoltaic technology all have environmental issues. Some of these are real and involve the manufacturing process. Other concerns with these technologies are only esthetically perceived and may not be actual environmental action items.
This niche market and technology report projects that the value of DMFCs in the U.S. will grow from the 2006 value of $14 million to $63 million by 2011. This is an average annual growth rate of 35%/year. Such a sizeable increase is significant but it is growing from a relatively small base number. This $63 million is the value of the unit itself and excludes the fuel container, the methanol delivery or reforming system as well as the device that the fuel cell might be included in and any balance of plant options for water control. The earliest adapter and the first big winner in DMFCs will be laptop computers. Military applications, enhanced cell phones and other hand held devices would follow adoption of DMFCs into the consumer acceptance.
Small portable devices are well suited, in terms of storage, safety, and energy density, to use of methanol as a fuel for fuel cells. Direct hydrogen feed for fuel cells requires complicated storage and would take much more space in small portable devices. There is also the safety issue of compressed hydrogen being allowed on airplanes. Cartridges of methanol can fit into existing retail channels or be available from OEMs. Methanol cartridges could be available through any number of delivery channels and accepted without difficulty into the consumer market. DMFCs are adapted to a hybrid system of portable devices such as trickle charging or recharging or the backup of secondary batteries inside a portable device. Recharging is a hybrid transitional approach or even a long-term approach to future commercialization of DMFCs.
Asia, principally Japan, is emerging as a big player in fuel cell development and especially in DMFC development and commercialization. Their predominance in the electronics and related manufacturing sectors has led these companies toward a vertical integration. Both in the U.S. and Japan there has been seen a need for the development of reliable and supplemental power sources for ever-improving and multi-faceted electronics and big companies prefer for this development to be under their own control and not dependent on an outside supply train at the early stages of development. These large corporations either acquire licensed technology or developing their own.
One of the first hurdles for fuel delivery is that gaseous diffusion layers were originally designed to handle, as the name suggests, gases. In DMFCs, the feed is a liquid and presents a new set of parameters. If methanol is to be converted to pure hydrogen, that presents another set of problems in handling, storing or delivering the gaseous reforrmate fuel.
The direct methanol fuel cell or DMFC is emerging as a future winner in many of the applications that fuel cells can satisfy. DMFC is an important subset of the proton exchange membrane (PEM) fuel cell technology. DMFC emergence is especially viable in the portable device sector. Commercialization is driven by consumer demands and desires for a superior power source that can operate alone or as a supplement or synergist with existing advanced battery technologies.
STIX [www.stix-market-research.com] is a source for scientific, technical and information analysis bringing together experts with experience in advanced technology. The factual evaluations are predominantly with advanced technology niche markets that are emerging as shaping business opportunities.
ANNA WELCH CRULL
Anna Crull is a business analyst with industrial and private sector experience in advanced technology consulting and analysis. She graduated from the School of Engineering, University of Mississippi and has a MS in chemistry from the University of Missouri. Her early career was at Redstone Arsenal, AL for the U.S. Army researching rocket/missile systems, solid fuels and explosives, and later water systems for Gemini spacecraft in Houston, TX. This experience led to a continuing interest in fuel cells, membranes and advanced materials technology. She has been employed by the U.S. government and private industry as an analyst. Over the past 30 years she has authored over 85 technical /marketing multi-client studies published by Business Communications Co., Inc. and conducted hundreds of special projects for individual clients. Most recent studies have been in fuel cells and membranes science.
The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag
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