Batteries And Fuel Cells.ppt

Batteries and Fuel Cells

Introduction • A battery is a device that converts the chemical energy contained in its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. • In the case of a rechargeable system, the battery is recharged by a reversal of the process. This type of reaction involves the transfer of electrons from one material to another through an electric circuit. • In a nonelectrochemical redox reaction, such as rusting or burning, the transfer of electrons occurs directly and only heat is involved. • As the battery electrochemically converts chemical energy into electric energy, it is not subject, as are combustion or heat engines, to the limitations of the Carnot cycle dictated by the second law of thermodynamics. • Batteries, therefore, are capable of having higher energy conversion efficiencies.

• While the term ‘‘battery’’ is often used, the basic electrochemical unit being referred to is the ‘‘cell.’’ • A battery consists of one or more of these cells, connected in series or parallel, or both, depending on the desired output voltage and capacity. The cell consists of three major components: • 1. The anode or negative electrode—the reducing or fuel electrode— which gives up electrons to the external circuit and is oxidized during the electrochemical reaction. • 2. The cathode or positive electrode—the oxidizing electrode—which accepts electrons from the external circuit and is reduced during the electrochemical reaction. • The electrolyte—the ionic conductor—which provides the medium for transfer of charge, as ions, inside the cell between the anode and cathode. The electrolyte is typically a liquid, such as water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity. Some batteries use solid electrolytes, which are ionic conductors at the operating temperature of the cell.

Cell vs. Battery: • A cell is the basic electrochemical unit providing a source of electrical energy by direct conversion of chemical energy. • The cell consists of an assembly of electrodes, separators, electrolyte, container and terminals.

• A battery consists of one or more electrochemical cells, electrically connected in an appropriate series / parallel arrangement to provide the required operating voltage and current levels, including, if any, monitors, controls and other ancillary components (e.g. fuses, diodes), case, terminals and markings. • (Although much less popular, in some publications, the term ‘‘battery’’ is considered to contain two or more cells.)

CLASSIFICATION OF BATTERIES • Electrochemical cells and batteries are identified as primary (nonrechargeable) or secondary (rechargeable), depending on their capability of being electrically recharged. Primary Cells or Batteries • These batteries are not capable of being easily or effectively recharged electrically and, hence, are discharged once and discarded. Many primary cells in which the electrolyte is contained by an absorbent or separator material (there is no free or liquid electrolyte) are termed ‘‘dry cells.’’ Ex: Zinc-carbon cells, Alkaline battery, Lithium battery, Mercury battery, Silver oxide battery Secondary or Rechargeable Cells or Batteries • These batteries can be recharged electrically, after discharge, to their original condition by passing current through them in the opposite direction to that of the discharge current. • They are storage devices for electric energy and are known also as ‘‘storage batteries’’ or ‘‘accumulators.’’ Ex: Lead acid battery, Nickel-cadmium battery, Lithium ion battery, Nickel metal hydride battery

The applications of secondary batteries • The secondary battery is used as an energy-storage device, generally being electrically connected to and charged by a prime energy source and delivering its energy to the load on demand. Examples are automotive and aircraft systems, emergency no-fail and standby (UPS) power sources, hybrid electric vehicles and stationary energy storage (SES) systems for electric utility load leveling. • The secondary battery is used or discharged essentially as a primary battery, but recharged after use rather than being discarded. Secondary batteries are used in this manner as, for example, in portable consumer electronics, power tools, electric vehicles, etc., for cost savings (as they can be recharged rather than replaced), and in applications requiring power drains beyond the capability of primary batteries.

Primary Battery

• In the primary batteries, the reaction occurs only once and after use over a period of time battery becomes dead and cannot be reused again. The most familiar example of this type is the dry cell which is used commonly in our transistors and clocks. • The cell consists of a zinc container that acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon. The space between the electrodes is filled by a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2). • The electrode reactions as follows : Anode: Zn(s) ⎯→ Zn2+ + 2e– Cathode: MnO2+ NH4++ e–⎯→ MnO(OH) + NH3 • In the reaction at cathode, manganese is reduced from the + 4 oxidation state to the +3 state. Ammonia produced in the reaction forms a complex with Zn2+ to give [Zn(NH3)4]2+. The cell has a potential of nearly 1.5 V.

• Mercury cell, suitable for low current devices like hearing aids, watches, etc. consists of zinc – mercury amalgam as anode and a paste of HgO and carbon as the cathode. • The electrolyte is a paste of KOH and ZnO. The electrode reactions for the cell are given below:

Anode: Zn(Hg) + 2OH– ⎯→ ZnO(s) + H2O + 2e– Cathode: HgO + H2O + 2e– ⎯→ Hg(l ) + 2OH– The overall reaction is represented by Zn(Hg) + HgO(s) ⎯→ ZnO(s) + Hg(l ) The cell potential is approximately 1.35 V and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time.

Secondary Batteries

• The most important secondary cell is the lead storage battery (Fig. commonly used in automobiles and invertors. • It consists of a lead anode and a grid of lead packed with lead dioxide (PbO2 ) as cathode. A 38% solution of sulphuric acid is used as an electrolyte. The cell reactions when the battery is in use are given below: Anode: Pb(s) + SO42–(aq) → PbSO4(s) + 2e– Cathode: PbO2(s) + SO42–(aq) + 4H+(aq) + 2e– → PbSO4 (s) + 2H2O (l ) i.e., overall cell reaction consisting of cathode and anode reactions is: Pb(s)+PbO2(s)+2H2SO4(aq)→ 2PbSO4(s) + 2H2O(l) On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode is converted into Pb and PbO2, respectively.

Redox Batteries The term ‘‘redox’’ is obtained from a contraction of the words ‘‘reduction’’ and ‘‘oxidation.’’ Although reduction and oxidation occur in all battery systems, the term ‘‘redox battery’’ is used for those electrochemical systems where the oxidation and reduction involves only ionic species in solution and the reactions take place on inert electrodes. This means that the active materials must be mostly stored externally from the cells of the battery. Although redox systems are capable of long life, their energy density is low because of the limited solubility of the active materials typically involved.

Vanadium Redox Battery or VRB as it is known, has any significant development continuing as of 2001.

The electrolytes in the positive and negative electrode compartments of Vanadium Redox Batteries are different valence states of vanadium sulfate. Both solutions are 2 M in concentration and contain sulfuric acid as a supporting electrolyte. The electrode reactions occur in solution, with the reaction: at the negative electrode in discharge being:

Both reactions are reversible on the carbon felt electrodes that are used. An ion-selective membrane is used to separate the electrolytes in the positive and negative compartments of the cells. Cross-mixing of the reactants would result in a permanent loss in energy storage capacity for the system because of the resulting dilution of the active materials. Migration of other ions (mainly H) to maintain electroneutrality, however, must be permitted. Thus, ion-selective membranes are required.

LITHIUM-ION BATTERIES Lithium-ion (Li-ion) batteries are comprised of cells that employ lithium intercalation compounds as the positive and negative materials. As a battery is cycled, lithium ions (Li) exchange between the positive and negative electrodes. They are also referred to as rocking chair batteries as the lithium ions ‘‘rock’’ back and forth between the positive and negative electrodes as the cell is charged and discharged. The positive electrode material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material with a tunneled structure, such as lithium manganese oxide (LiMn2O4), on a current collector of aluminum foil. The negative electrode material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge/ discharge process, lithium ions are inserted or extracted from interstitial space between atomic layers within the active materials.

Schematic of the electrochemical process in a Li-ion cell.

In this scheme, LiMO2 represents the metal oxide positive material, such as LiCoO2, and C the carbonaceous negative material, such as graphite. The reverse happens on discharge. As metallic lithium is not present in the cell, Liion batteries are chemically less reactive, safer, and offer longer cycle life than possible with rechargeable lithium batteries that employ lithium metal as the negative electrode material. The charge/discharge process in a Li-ion cell is further illustrated graphically in Fig. In the figure, the layered active materials are shown on metallic current collectors.

Electrode and cell reactions in a Li-ion cell.

Advantages and Disadvantages of Li-ion Batteries

Fuel Cells • Fuel cells, like batteries, are electrochemical galvanic cells that convert chemical energy directly into electrical energy and are not subject to the Carnot cycle limitations of heat engines.

• Fuel cells are similar to batteries except that the active materials are not an integral part of the device (as in a battery), but are fed into the fuel cell from an external source when power is desired. •

• The fuel cell differs from a battery in that it has the capability of producing electrical energy as long as the active materials are fed to the electrodes. • The battery will cease to produce electrical energy when the limiting reactant stored within the cell is consumed. • The electrode materials of the fuel cell are inert and have catalytic properties which enhance the electroreduction or electrooxidation of the reactants (the active materials).

• The anode active materials used in fuel cells are generally gaseous or liquid (compared with the metal anodes generally used in most batteries) and are fed into the anode side of the fuel cell. • Oxygen or air is the predominant oxidant and is fed into the cathode side of the fuel cell. Fuel cell technology can be classified into two categories

• Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly in the fuel cell • Indirect systems in which the fuel, such as natural gas or other fossil fuel, is first converted by reforming to a hydrogen-rich gas which is then fed into the fuel cell

• One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water. The cell was used for providing electrical power in the Apollo space programme. • The water vapours produced during the reaction were condensed and added to the drinking water supply for the astronauts. • In the cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution. • Catalysts like finely divided platinum or palladium metal are incorporated into the electrodes for increasing the rate of electrode reactions.

The electrode reactions are given below: Anode: 2H2 (g) + 4OH–(aq) ⎯→ 4H2O(l) + 4e– Cathode: O2(g) + 2H2O(l ) + 4e–⎯→ 4OH–(aq) Overall reaction being: 2H2(g) + O2(g) ⎯→ 2 H2O(l )

• The cell runs continuously as long as the reactants are supplied. • Fuel cells produce electricity with an efficiency of about 70 % compared to thermal plants whose efficiency is about 40%. • There has been tremendous progress in the development of new electrode materials, better catalysts and electrolytes for increasing the efficiency of fuel cells. • These have been used in automobiles on an experimental basis. • Fuel cells are pollution free and in view of their future importance, a variety of fuel cells have been fabricated and tried.

Alkaline Fuel Cells Alkaline fuel cells differ from other types of fuel cells in the chemical reaction and the operating temperature. Hydrogen and oxygen are reactants and Potassium hydroxide and electrolyte The chemical reaction that occurs at the anode is:

−

2H2 + 4OH

−

4H2O + 4e

The reaction at the cathode occurs when the electrons pass around an external circuit and react to form hydroxide ions, OH−, as shown: O2 + 4e− + 2H2O

4OH−

Alkaline fuel cells have some major advantages over other types of fuel cells. The rest is that the activation overvoltage at the cathode is usually less than with an acid electrolyte fuel cell. The second advantage is that the electrodes do not have to be made of precious metals.

Molten Carbonate Fuel Cell

The defining characteristic of a molten carbonate fuel cell (MCFC) is the material used for the electrolyte. The material is a molten mixture of alkali metal carbonates. The electrolyte is usually a binary mixture of lithium and potassium, or lithium and sodium carbonates which is held in a ceramic matrix of LiAlO2. A highly conductive molten salt is formed by the carbonates at very high temperatures.

Direct Methanol Fuel Cells The use of pure hydrogen in fuel cells is not the only way to convert hydrogen into useful electric energy. A variety of reactions can produce hydrogen indirectly, thus enabling the classic hydrogen fuel cell chemical reaction to take place. The fuel is a mixture of water and of methanol; it reacts directly at the anode according to:

CH3OH + H20

6H+ + 6e− +CO2

As mentioned above the boiling point of methanol at atmospheric pressure is 65oC, thus the cells requires an operating temperature around 70oC (to avoid a too high vapor pressure). The reaction mechanism is much more complex with the appearance of species adsorbed as well as with HCOH and HCOOH. If one considers this reaction on a Pt/Ru catalyst, it can be represented with the following stages.

The compounds PtCOH and PtCO are poisons for Platinum, and after research it was found that the addition of Ruthenium makes it possible to cure the Pt, and prevent poisoning.

The above reactions give the production of the hydrogen, which can in turn be used by the cathode. The cathode undergoes the typical fuel cell reaction with hydrogen combining with oxygen. The total DMFC equation, representing only the initial and final products for both the cathode and anode is as follows:

Difference between a fuel cell and a battery Batteries • A battery stores the chemical reactants, usually metal compounds like lithium, zinc or manganese. • Batteries will produce electricity as long as reactive materials are present. • Once used up, you must recharge or throw away the battery.

Fuel Cell • A fuel cell creates electricity through reactants (hydrogen and oxygen) stored externally. • A fuel cell will produce electricity as long as it has a fuel supply. • In short, a fuel cell vehicle is refueled instead of recharged.