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Direct-ethanol fuel cells or DEFCs are a category of fuel cell in which ethanol is fed directly into the cell. They have been used as a model to investigate a range of fuel cell concepts including the use of PEM. [1]
DEFC uses Ethanol in the fuel cell instead of the more toxic methanol. Ethanol is an attractive alternative to methanol because it comes with a supply chain that's already in place. Ethanol also remains the easier fuel to work with for widespread use by consumers.
Ethanol is a hydrogen-rich liquid and it has a high specific energy (8.0 kWh/kg) compared to methanol (6.1 kWh/kg). Ethanol can be obtained in great quantity from biomass through a fermentation process from renewable resources like from sugar cane, wheat, corn, or even straw. Bio-generated ethanol (or bio-ethanol) is thus attractive since growing crops for biofuels absorbs much of the carbon dioxide emitted into the atmosphere from fuel used to produce the biofuels, and from burning the biofuels themselves. This is in sharp contrast to the use of fossil fuels. The use of ethanol would also overcome both the storage and infrastructure challenge of hydrogen for fuel cell applications. In a fuel cell, the oxidation of any fuel requires the use of a catalyst in order to achieve the current densities required for commercially viable fuel cells, and platinum-based catalysts are some of the most efficient materials for the oxidation of small organic molecules.
The DEFC, similar to the DMFC, relies upon the oxidation of ethanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Protons (H+) are transported across the proton exchange membrane to the cathode where they react with oxygen to produce water. Electrons are transported through an external circuit from anode to cathode, providing power to connected devices.
The half-reactions are:
| Equation | |
|---|---|
| Anode | oxidation |
| Cathode | reduction |
| Overall reaction | redox reaction |
Platinum-based catalysts are expensive, so practical exploitation of ethanol as fuel for a PEM fuel cell requires a new catalyst. New nanostructured electrocatalysts (HYPERMEC by ACTA SpA for example) have been developed, which are based on non-noble metals, preferentially mixtures of Fe, Co, Ni at the anode, and Ni, Fe or Co alone at the cathode. With ethanol, power densities as high as 140 mW/cm2 at 0.5 V have been obtained at 25 °C with self-breathing cells containing commercial anion exchange membranes. [2] This catalyst does not contain any precious metals. In practice tiny metal particles are fixed onto a substrate in such a way that they produce a very active catalyst.
A polymer acts as electrolyte. The charge is carried by the hydrogen ion (proton). The liquid ethanol (C2H5OH) is oxidized at the anode in the presence of water, generating CO2, hydrogen ions and electrons. Hydrogen ions travel through the electrolyte. They react at the cathode with oxygen from the air and the electrons from the external circuit forming water.
Bio-Ethanol based fuel cells may improve the well-to-wheel balance of this biofuel because of the increased conversion rate of the fuel cell compared to the internal combustion engine. But real world figures may be only achieved in some years since the development of direct methanol and ethanol fuel cells is lagging behind hydrogen powered fuel cells. [3]
On 13 May 2007 a team from the University of Applied Sciences in Offenburg presented the world's first vehicle powered by a DEFC at Shell's Eco-marathon in France. The car "Schluckspecht" completed a successful test drive on Nogaro Circuit, powered by a DEFC stack giving an output voltage of 20 to 45 V (depending on load). [4]
Various prototypes of Direct Ethanol Fuel Cell Stack mobile phone chargers have been built [5] featuring voltages from 2V to 7V and powers from 800 mW to 2W [6] were built and tested.
An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic cells or galvanic cells and those that generate chemical reactions, via electrolysis for example, are called electrolytic cells. A common example of a galvanic cell is a standard 1.5 volt cell meant for consumer use. A battery consists of one or more cells, connected in parallel, series or series-and-parallel pattern.
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from metals and their ions or oxides that are commonly already present in the battery, except in flow batteries. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean either "breakdown of electricity" or "breakdown via electricity".
Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.
Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells in which methanol is used as the fuel. Their main advantage is the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions.
A solid oxide fuel cell is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton-exchange membrane fuel cell or of a proton-exchange membrane electrolyser: separation of reactants and transport of protons while blocking a direct electronic pathway through the membrane.
Electrolysis of water is the process of using electricity to decompose water into oxygen and hydrogen gas. Hydrogen gas released in this way can be used as hydrogen fuel, or remixed with the oxygen to create oxyhydrogen gas, which is used in welding and other applications.
Formic acid fuel cells are a subcategory of proton-exchange membrane fuel cells where the fuel, formic acid, is not reformed, but fed directly to the fuel cell. Their applications include small, portable electronics such as phones and laptop computers as well as larger fixed power applications and vehicles.
A protonic ceramic fuel cell or PCFC is a fuel cell based around a ceramic, solid, electrolyte material as proton conductor from anode to cathode. Currently available PCFC have high enough proton conductivity at elevated temperatures.
A microbial fuel cell (MFC) is a bio-electrochemical system that drives an electric current by using bacteria and a high-energy oxidant such as O2, mimicking bacterial interactions found in nature. MFCs can be grouped into two general categories: mediated and unmediated. The first MFCs, demonstrated in the early 20th century, used a mediator: a chemical that transfers electrons from the bacteria in the cell to the anode. Unmediated MFCs emerged in the 1970s; in this type of MFC the bacteria typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode. In the 21st century MFCs have started to find commercial use in wastewater treatment.
Hydrogen technologies are technologies that relate to the production and use of hydrogen. Hydrogen technologies are applicable for many uses.
Lanthanum strontium cobalt ferrite (LSCF), also called lanthanum strontium cobaltite ferrite is a specific ceramic oxide derived from lanthanum cobaltite of the ferrite group. It is a phase containing lanthanum(III) oxide, strontium oxide, cobalt oxide and iron oxide with the formula La
xSr
1-xCo
yFe
1-yO
3, where 0.1≤x≤0.4 and 0.2≤y≤0.8.
A Direct Carbon Fuel Cell (DCFC) is a fuel cell that uses a carbon rich material as a fuel such as bio-mass or coal. The cell produces energy by combining carbon and oxygen, which releases carbon dioxide as a by-product. It is also called coal fuel cells (CFCs), carbon-air fuel cells (CAFCs), direct carbon/coal fuel cells (DCFCs), and DC-SOFC.
An enzymatic biofuel cell is a specific type of fuel cell that uses enzymes as a catalyst to oxidize its fuel, rather than precious metals. Enzymatic biofuel cells, while currently confined to research facilities, are widely prized for the promise they hold in terms of their relatively inexpensive components and fuels, as well as a potential power source for bionic implants.
A membrane electrode assembly (MEA) is an assembled stack of proton-exchange membranes (PEM) or alkali anion exchange membrane (AAEM), catalyst and flat plate electrode used in fuel cells and electrolyzers.
The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.
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.
An alkaline anion exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion exchange membrane to separate the anode and cathode compartments.
Polymer electrolyte membrane(PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the issues of partial load, low current density, and low pressure operation currently plaguing the alkaline electrolyzer. It involves a proton-exchange membrane.
