Chemistry
Fuel Cells
Fuel cells are devices that convert chemical energy directly into electrical energy through a chemical reaction. They typically consist of an anode, cathode, and electrolyte. Hydrogen fuel cells, for example, use hydrogen as the fuel and oxygen from the air as the oxidant, producing water and electricity as byproducts. This technology offers a promising alternative to traditional combustion-based power generation.
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12 Key excerpts on "Fuel Cells"
eBook - PDFCatalytic Oxidation: Principles And Applications - A Course Of The Netherlands Institute For Catalysis Research (Niok)
A Course of the Netherlands Institute for Catalysis Research (NIOK)
- Niok, R A Sheldon, Rutger A Van Santen(Authors)
- 1995(Publication Date)
- World Scientific(Publisher)
Fuel Cells J.A.R. VAN VEEN Shell Research B. V. (Koninkujke/Shell-Laboratorium, Amsterdam), P.O. Box 38000,1030BNAmsterdam, The Netherlands ABSTRACT The principles and present-day embodiments of Fuel Cells are discussed. Nearly all cells are hydrogen/oxygen ones, where the hydrogen fuel is usually obtained on-site from the reforming of methane or methanol. There exists a tension between the promise of high efficiency in the conversion of chemical into electrical energy and of very low emissions of noxious compounds, and the enormous difficulty of manufacturing the Fuel Cells cost-effectively. After three decennia of widespread effort to adapt the fuel cell to terrestrial applications, it is still too early to say whether their large-scale introduction will prove to be viable. 1. Introduction A fuel cell is an electrochemical device in which the chemical energy of the fuels is converted directly into electrical energy, i.e. without being first transformed into heat. A diagram of a fuel cell is shown in Fig. 1. At the fuel electrode, the anode, the fuel is oxidized. In principle, any fuel can be used, but of course a certain reactivity requirement has to be met. From this point of view, hydrogen is the best fuel, and indeed all practical cells to date are based on it. For stationary fuel-cell applications it is often envisaged to be produced through the steam reforming of methane or naphtha, eventually followed by shifting (CO being much less reactive than H 2 ). In the hydrogen case, then, the electrochemical oxidation can be simply written as, assuming an acidic electrolyte: H 2 -► 2H + + 2e (1) The electrons flow through the external circuit (where they can do their useful work), while protons sustain the current in solution. At the cathode electrons and protons combine again with the oxidizing agent.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Research World(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 3 Fuel Cell Demonstration model of a direct-methanol fuel cell. The actual fuel cell stack is the layered cube shape in the center of the image ________________________ WORLD TECHNOLOGIES ________________________ A fuel cell is an electrochemical cell that converts energy from a fuel into electrical energy. Electricity is generated from the reaction between a fuel supply and an oxidizing agent. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel Cells can operate continuously as long as the necessary reactant and oxidant flows are maintained. Fuel Cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished – a thermody-namically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system. Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide. Design Fuel Cells come in many varieties; however, they all work in the same general manner. They are made up of three segments which are sandwiched together: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load. At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot.
eBook - PDF- Nancy E. Carpenter(Author)
- 2014(Publication Date)
- Chapman and Hall/CRC(Publisher)
Managing these waste products is not a trivial matter, but treatment of this topic is beyond the scope of this text. But how do we get from fuel and oxidant to electric-ity? Since the generation of electrical energy by a fuel cell is based on electrochem-istry, a very brief review is in order. As noted above, some electron-rich compound Electrochemical cells Expendable Non-expendable Rechargeable Refuelable (fuel cell) FIGURE 6.1 The categories of electrochemical cells. Fuel (chemical energy) Combustion Conversion (heat engine) Generator Heat Mechanical energy Electrical energy FIGURE 6.2 The multiple conversions needed for conventional electricity generation. (Reprinted with permission from Li, X. 2006. Principles of Fuel Cells . New York: Taylor & Francis.) 141 Fuel Cells (the fuel) is oxidized at the anode while something with a high potential for reduction (the oxidant) is, in turn, reduced at the cathode. The pairing of these two half-cell reactions leads to the overall electrochemical equation. For the typical hydrogen fuel cell, the two half-cell reactions are H H aq of hydrogen at anode 2 2 2 ( ) ( ) ( ) g e oxidation → + + -(6.1) and 1 2 2 2 2 2 O H H O of oxygen at cathode + ( ) ( ) g e reduction + + → -(6.2) with the overall reaction being H O H O 2 2 1 2 2 ( ) ( ) g g + → (6.3) The cell potential ( E cell , also referred to as the electromotive force (EMF) or cell voltage ) is the force (in units of volts; 1 V = 1 joule/coulomb) that moves the electri-cal current through the circuit. The magnitude of E cell is a reflection of the difference in reduction potentials of the chemical reactants involved. A compound that is easily reduced has a high, positive reduction potential while a compound that is strongly prone to oxidation has a highly negative reduction potential.
eBook - PDFHydrogen and Fuel Cells
Emerging Technologies and Applications
- Bent Sorensen, Bent Sørensen(Authors)
- 2005(Publication Date)
- Academic Press(Publisher)
Fuel Cells Fuel Cells 3.1 Basic concepts 3.1.1 Electrochemistry and thermodynamics of Fuel Cells The electrochemical conversion of energy is a conversion of chemical energy to electrical energy or vice versa. An electrochemical cell is a device either converting chemical energy deposited within the device, or chemical energy supplied through channels from the outside to the cell, into electricity or it is operating in reverse mode, converting electricity to either stored chemical energy or an outward-going mass flow of chemical energy. At all stages, as-sociated heat may be generated or drawn from the surroundings. The gen-erallayout of such a device is shown in Fig. 3.1. According to thermodynamics (cf. textbooks, e.g., Callen, 1960), the maximum amount of chemical energy of the system that in a given situation can be converted into a high-quality energy form such as electricity is given by the free energy G, also called Gibbs energy, (3.1) where U is the internal energy of the system (such as chemical energy in the cases considered here), S is its entropy, and V is its volume, while (T ref , P rej ), the absolute temperature and pressure of the surroundings, defines what is meant by in a given situation. The classical law of energy conservation 13.1 BASIC CONCEPTS BENT S0RENSEN (which is also the first law of thermodynamics) states that the increase in in-ternal energy is given by the net energy added to the system from the out-side: AU = J dQ + J d W + J elM. (3.2) Here M is the net energy of material flowing into the device, W is the net amount of mechanical or electric work performed on the system by the sur-roundings, and Q is the net amount of heat received from the environment. ~ ~ '-----1------J m ~ JO, out t Reject heat reservOir at temperature 0ef Moss flow of specl flc energy win Power input p Heat source ot temperature T I I JQ in I ' , Energy converSion devIce, p t-----~ Power delivered Moss flow of specific energy Wout Figure 3.1.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
____________________ WORLD TECHNOLOGIES ____________________ Chapter- 1 Fuel Cell Demonstration model of a direct-methanol fuel cell. The actual fuel cell stack is the layered cube shape in the center of the image A fuel cell is an electrochemical cell that converts a source fuel into an electric current. It generates electricity inside a cell through reactions between a fuel and an oxidant, ____________________ WORLD TECHNOLOGIES ____________________ triggered in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel Cells can operate continuously as long as the necessary reactant and oxidant flows are maintained. Fuel Cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished – a thermod-ynamically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system. Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide. Design Fuel Cells come in many varieties; however, they all work in the same general manner. They are made up of three segments which are sandwiched together: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load. At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron.
eBook - PDF- Michael Frank Hordeski(Author)
- 2021(Publication Date)
- River Publishers(Publisher)
The chief characteristic of a fuel cell is its ability to convert chemical-energy directly into electrical en-ergy, giving high conversion efficiencies. Fuel Cells have much lower carbon dioxide emissions than fossil fuel based technologies for the same power output. They also produce negligible amounts of SO and NO , the main constituents of acid rain x x and photochemical smog. Several types of Fuel Cells are being developed around the world, the chief difference between each being the material used for the electro-lyte and the operating temperature. The types of Fuel Cells include solid oxide, molten carbonate, phosphoric acid, polymer, direct alcohol and alkaline (See Table 5-1) . All Fuel Cells use an electrolyte between the two electrodes. The different types of electrolytes have very different properties and the dif-ferent fuel cell types have been built around them and are mostly named after the electrolyte. When Fuel Cells transform the energy stored in a fuel into electricity and heat, the fuel is not burned in a flame but oxidized electrochemically. This means that Fuel Cells are not constrained by the law that governs heat engines, the Carnot limit, which specifies the maximum theoretical efficiency that a heat engine can reach. Their efficiency increases with a partial load. A fuel cell works similar to a battery. In a battery there are two electrodes which are separated by an electrolyte. At least one of the elec-trodes is generally made of a solid metal. This metal is converted to 1 8 4 New Technologies for Energy Efficiency Table 5-1.
eBook - PDF- Sunggyu Lee, James G. Speight, Sudarshan K. Loyalka, Sunggyu Lee, James G. Speight, Sudarshan K. Loyalka(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
A battery uses the chemical energy stored within the reactants inside the battery, whereas a fuel cell converts the chemical energy provided by an external fuel/oxidant mixture into electrical energy. Thus, batteries use chemical energy until the reactants are completely depleted, and at the end of their lifetime, they can be either recharged or just thrown away. Fuel 623 Fuel Cells cells, on the other hand, can provide electrical output as long as the supply of fuel and oxidant is maintained. Typically, hydrogen-rich fuels are used for operation, and the most common fuels include gases (i.e., hydrogen, natural gas, or ammonia), liquids (i.e., methanol, hydrocarbons, hydrazine), or coal. A preliminary conversion ( reforming ) process is required for all fuels, except for direct hydrogen. The oxidant used at the cathode is usually oxygen or air. 1,2 During operation, both hydrogen-rich fuel and oxygen/air are supplied to the elec-trodes. Hydrogen undergoes catalytic oxidation at one of the electrodes and splits into ions and electrons. Oxygen undergoes a reduction reaction at the other electrode. Both ions and electrons travel from one electrode to the other, using different path-ways. Ions travel through the electrolyte, and electrons are forced through a separate pathway via the current collectors to the other electrode, where they combine with oxygen to create water or other by-products, such as CO 2 . 19.2.3 T HERMAL E FFICIENCY Although the input (chemical energy, E ch ) and output (electrical energy, E e ) of the operation are the same for Fuel Cells and heat engines, the conversion process is Fuel in 2e Load H 2 De pleted fuel and pr oduct gases out Anode H 2 O H 2 O Positive ion or negative ion Oxidant in 1/2O 2 Depleted ox idant and product gases out Cathode Electrolyte (ion conductor) FIGURE 19.1 Design of a generic fuel cell.
eBook - PDF- Robert Schlögl(Author)
- 2012(Publication Date)
- De Gruyter(Publisher)
3.3.2 Components of a Fuel Cell Figure 3.3.3 schematically depicts the basic structure of an electrochemical fuel cell device. Generally, in electrochemical cells the overall chemical redox reaction proceeds via two coupled, yet spatially separated half-cell redox reactions at two separate electrodes. Fuel, hydrogen gas (red), comes in contact with a catalytically active electrode (the anode), on the surface of which the hydrogen molecule splits into protons and electrons in the hydrogen-oxygen reaction (HOR) according to H 2 → 2 H + + 2e − The protons travel across the ion-conducting (liquid) electrolyte to the opposite elec-trode (the cathode), where they recombine with the oxidant, here oxygen (blue), Fuel Cell Electricity H 2 O (l/g) Fuel (H 2 ) Oxygen H 2 + ½ O 2 → H 2 O Figure 3.3.2 Principle of a fuel cell as an electrochemical energy conversion device. Inside a fuel cell, fuel, e.g. hydrogen, and an oxidant, typically oxygen, combine electrochemically to form products, e.g. water, and electricity and some excess heat (not shown). Figure adapted from ref. [4] 3.3.2 Components of a Fuel Cell 165 and the electrons that traveled through the external circuit to water in the oxygen-reduction reaction (ORR) according to 1 2 O 2 + 2 H + + 2e − → H 2 O Table 3.3.1 shows an overview of the most common types of today ’ s Fuel Cells. Fuel Cells are typically categorized by the type of ion conductor or electrolyte employed. An important class of Fuel Cells is based on proton-conducting (acidic) electrolytes, either in the form of a solid membrane (PEMFCs) or a liquid acid, possibly absorbed inside a polymer matrix (phosphoric acid Fuel Cells).
- Nayan Kumar, Prabhansu Prabhansu(Authors)
- 2022(Publication Date)
- Wiley-Scrivener(Publisher)
Fuel Cell Utilization for Energy Storage 391 14.2 Fuel Cell Mechanism A fuel cell is a device that utilises electrochemical reactions to generate energy from chem- ical compounds. Here, we take the example of a hydrogen fuel cell (HFC) to explain the mechanism of a FC. The reactants involved in this FC are hydrogen as fuel and oxygen as oxidant. The FC is constructed of an anode, cathode and a thin electrolytic membrane. The mechanism of FC involves breaking down of hydrogen molecules into protons and electrons with the help of catalysts. This reduction process happens at anode (negative elec- trode) via catalysts. Once the breakdown of hydrogen molecules into atomic hydrogen (H) is done, the protons (H + ) flow through an electrolytic solution present in the membrane of the FC while the electrons (e – ) released will transfer to an external circuit which is used to generate electricity. The protons on the other hand reach to cathode (positive electrode), through membrane, where supply of oxygen takes place. These protons react with oxygen and produce heat and water. So, the final product of the reaction is water, heat and elec- tricity. Figure 14.1 depicts the fundamental principle of FC. The half reactions in the two electrode sides of a HFC are: • At anode: H 2 → 2H + + 2e – • At cathode: 2H + + ½ O 2 + 2e – → H 2 O 14.3 Efficiency of Fuel Cell The fundamental calculations of the efficiency of a FC are very crucial because of its poten- tiality to give extremely high efficiency. Let us mathematically find out the efficiency of Anode (–) Catalysts Cathode (+) O 2 (g) H 2 (g) Fuel (Input) H 2 O (I) (by-product) H 2 O (I) (by-product) Electricity (Output) H + O 2 Fuel Cell (Electrolyte) Membrane (Anode, +) Fuel Cell (Cathode, -) 4H + 2H 2 + 4e - 4H + 4e - 2H 2 O + + O 2 4e - Current Overall reaction 2H 2 +O 2 2H 2 O Figure 14.1 Basic principle of a fuel cell. 392 Renewable Energy for Sustainable Growth Assessment a HFC.
eBook - PDFElectrochemistry for Technologists
Electrical Engineering Division
- G. R. Palin, N. Hiller(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
104 FUEL AND OXIDANT 105 The fuel cell produces electricity from materials which are normally combusted to give power. It eliminates the middle steps in the series Chemical Energy -> Thermal Energy -> Mechanical Energy -> Electrical Energy There is no prospect of this occurring in large-scale power generation, with fuel cell systems replacing chemical or nuclear fuelled power stations. There are, however, prospects for the fuel cell in the field of smaller scale power generation. Apart from the increased simplicity, the increase in efficiency is considerable. The theoretical efficiency of a fuel cell may be as high as 100%, although the obtainable efficiency is nearer 50%. Even this compares very favourably with the efficiency of motor generators which is in the region of 20 %. Another possibility for the fuel cell is in the replacement of the sequence Chemical Energy -> Thermal Energy -» Mechanical Energy by Chemical Energy -* Electrical Energy -> Mechanical Energy Although far from realisation, the prospects for the fuel cell in traction and propulsion are enormous. A fuel cell consists basically of the following arrangement: FUEL / ELECTRODE / ELECTROLYTE / ELECTRODE / OXIDANT and these basic components will be discussed first. 4.2 Fuel and Oxidant A fuel cell causes the reactions making up a combustion process to occur in such a way that they produce potentials at electrodes. The simplest example of this is the reaction between oxygen and hydrogen. The combustion reaction is 2H 2 + 0 2 -> 2H 2 0 106 Fuel Cells 57-8 kcal are produced for every gram molecule of hydrogen which is combusted, and if the reaction occurs under adiabatic conditions, this energy appears as thermal energy of the steam.
eBook - ePub- Yves Brunet(Author)
- 2013(Publication Date)
- Wiley-ISTE(Publisher)
Chapter 6Fuel Cells: Principles and Function 1
6.1. What is a cell or battery?
A fuel cell is a system that produces electricity and heat using a chemical energy source: the fuel. Cells, batteries, and accumulators are all very precise electrochemical entities, but they are considered to be more or less equivalent or have vague boundaries. The cell is an electrochemical device invented by Alessandro Volta in 1800 to produce electricity using a stack of electrodes and compartments (cells formed by the electrodes), which contains chemical reagents and which has now become commonly available in cylindrical or disk form. These containers enclose an initially fixed quantity of chemical reagents and can, therefore, only produce a limited quantity of electricity, up to the point where the chemical energy is exhausted. Only if this process, known as discharge of the cell, is reversible, by recharging the cell (which is done by injecting electrical energy by connecting it to an electric power supply) can the system again have the same capacity as it had initially. An electrochemical cell that is rechargeable is known as an accumulator. The term battery, which is borrowed from artillery, is used for both rechargeable and non-rechargeable systems. Theoretically, it refers to a set of cells that have been joined together, but it is also used for a single cell, which is why the different terms can seem to mean the same thing. Electrochemists distinguish between the systems with the help of a notion of order (which is unfortunately of little clarification): a “primary battery” is not rechargeable, whereas a “secondary battery” is rechargeable (accumulator). Note that in French, the word pile is reserved for primary batteries, and the word batterie
- Lokesh Pandey(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
The condition of enhanced efficiency is driving the focus of research and development towards such systems where the fuel is provided directly to the anode chamber inside a fuel cell or where there is direct electrochemical oxidation of fuel. The reason behind this is that it permits the utmost conversion of chemical energy into electrical energy. Moreover, any thermal waste energy obtained during the operation can be used for either maintaining the operating temperature of the device or else it can be directly employed in the electrochemical or chemical reactions inside the fuel cell chamber. Additionally, there also exists an increased interest to lower the operating temperature of Fuel Cells in order to decrease the cost of overall system at the same time as increasing the life of the fuel cell. Table 1.1 provides a comparison between theoretical and actual electrical system efficiencies of various fuel cell systems that are operated on renewed hydrocarbon fuels (Giddey et al., 2012). All the energy from the fuel which is unable to convert into electrical power gets lost in the form of waste heat. Details regarding the method of calculation of total efficiency of a fuel cell system can be acquired from the following reference (Giddey et al., 2012). Such systems where the theoretical efficiency exceeds 100%, in that case, the fuel cell would need heat input for continuous operation.
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