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17.5 Batteries and fuel cells  (Page 2/12)

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anode: Zn ( s ) + 2 OH − ( a q ) ⟶ ZnO ( s ) + H 2 O ( l ) + 2e − E anode ° = −1.28 V cathode: 2 MnO 2 ( s ) + H 2 O ( l ) + 2e − ⟶ Mn 2 O 3 ( s ) + 2OH − ( a q ) E cathode ° = +0.15 V ¯ overall: Zn ( s ) + 2MnO 2 ( s ) ⟶ ZnO ( s ) + Mn 2 O 3 ( s ) E cell ° = +1.43 V

An alkaline battery can deliver about three to five times the energy of a zinc-carbon dry cell of similar size. Alkaline batteries are prone to leaking potassium hydroxide, so these should also be removed from devices for long-term storage. While some alkaline batteries are rechargeable, most are not. Attempts to recharge an alkaline battery that is not rechargeable often leads to rupture of the battery and leakage of the potassium hydroxide electrolyte.

A diagram of a cross section of an alkaline battery is shown. The overall shape of the cell is cylindrical. The lateral surface of the cylinder, indicated as a thin red line, is labeled “Outer casing.” Just beneath this is a thin, light grey surface that covers the lateral surface and top of the battery. Inside is a blue region with many evenly spaced small darker dots, labeled “M n O subscript 2 (cathode).” A thin dark grey layer is just inside, which is labeled “Ion conducting separator.” A purple region with many evenly spaced small darker dots fills the center of the battery and is labeled “ zinc (anode).” The very top of the battery has a thin grey curved surface over the central purple region. The curved surface above is labeled “Positive connection (plus).” At the base of the battery, an orange structure, labeled “Protective cap,” is located beneath the purple and blue central regions. This structure holds a grey structure that looks like a nail with its head at the bottom and pointed end extending upward into the center of the battery. This nail-like structure is labeled “Current pick up.” At the very bottom of the battery is a thin grey surface that is held by the protective cap. This surface is labeled “Negative terminal (negative).”
Alkaline batteries were designed as direct replacements for zinc-carbon (dry cell) batteries.

Visit this site to learn more about alkaline batteries.

Secondary batteries

Secondary batteries are rechargeable. These are the types of batteries found in devices such as smartphones, electronic tablets, and automobiles.

Nickel-cadmium , or NiCd, batteries ( [link] ) consist of a nickel-plated cathode, cadmium-plated anode, and a potassium hydroxide electrode. The positive and negative plates, which are prevented from shorting by the separator, are rolled together and put into the case. This is a “jelly-roll” design and allows the NiCd cell to deliver much more current than a similar-sized alkaline battery. The reactions are

anode: Cd ( s ) + 2OH − ( a q ) ⟶ Cd(OH) 2 ( s ) + 2e − cathode: NiO 2 ( s ) + 2H 2 O ( l ) + 2e − ⟶ Ni(OH) 2 ( s ) + 2OH − ( a q ) ¯ overall: Cd ( s ) + NiO 2 ( s ) + 2H 2 O ( l ) ⟶ Cd(OH) 2 ( s ) + Ni(OH) 2 ( s )

The voltage is about 1.2 V to 1.25 V as the battery discharges. When properly treated, a NiCd battery can be recharged about 1000 times. Cadmium is a toxic heavy metal so NiCd batteries should never be opened or put into the regular trash.

A diagram is shown of a cross section of a nickel cadmium battery. This battery is in a cylindrical shape. An outer red layer is labeled “case.” Just inside this layer is a thin, dark grey layer which is labeled at the bottom of the cylinder as “Negative electrode collector.” A silver rod extends upward through the center of the battery, which is surrounded by alternating layers, shown as vertical repeating bands, of yellow, purple, yellow, and blue. A slightly darker grey narrow band extends across the top of these alternating bands, which is labeled “Positive electrode collector.” A thin light grey band appears at the very bottom of the cylinder, which is labeled “Metal bottom cover (negative).” A small grey and white striped rectangular structure is present at the top of the central silver cylinder, which is labeled “Safety valve.” Above this is an orange layer that curves upward over the safety valve, which is labeled “Insulation ring.” Above this is a thin light grey layer that projects upward slightly at the center, which is labeled “Metal top cover (plus).” A light grey arrow points to a rectangle to the right that illustrates the layers at the center of the battery under magnification. From the central silver rod, the layers shown repeat the alternating pattern yellow, blue, yellow, and purple three times, with a final yellow layer covering the last purple layer. The outermost purple layer is labeled “Negative electrode.” The yellow layer beneath it is labeled “Separator.” The blue layer just inside is labeled “Positive electrode.”
NiCd batteries use a “jelly-roll” design that significantly increases the amount of current the battery can deliver as compared to a similar-sized alkaline battery.

Visit this site for more information about nickel cadmium rechargeable batteries.

Lithium ion batteries ( [link] ) are among the most popular rechargeable batteries and are used in many portable electronic devices. The reactions are

anode: LiCoO 2 ⇌ Li x − 1 CoO 2 + x Li + + x e − cathode: x Li + + x e − + x C 6 ⇌ x LiC 6 ¯ overall: LiCoO 2 + x C 6 ⇌ Li x − 1 CoO 2 + x LiC 6

With the coefficients representing moles, x is no more than about 0.5 moles. The battery voltage is about 3.7 V. Lithium batteries are popular because they can provide a large amount current, are lighter than comparable batteries of other types, produce a nearly constant voltage as they discharge, and only slowly lose their charge when stored.

This figure shows a model of the flow of charge in a lithium ion battery. At the left, an approximately cubic structure formed by alternating red, grey, and purple spheres is labeled below as “Positive electrode.” The purple spheres are identified by the label “lithium.” The grey spheres are identified by the label “Metal.” The red spheres are identified by the label “oxygen.” Above this structure is the label “Charge” followed by a right pointing green arrow. At the right is a figure with layers of black interconnected spheres with purple spheres located in gaps between the layers. The black layers are labeled “Graphite layers.” Below the purple and black structure is the label “Negative electrode.” Above is the label “Discharge,” which is preceded by a blue arrow which points left. At the center of the diagram between the two structures are six purple spheres which are each labeled with a plus symbol. Three curved green arrows extend from the red, purple, and grey structure to each of the three closest purple plus labeled spheres. Green curved arrows extend from the right side of the upper and lower of these three purple plus labeled spheres to the black and purple layered structure. Three blue arrows extend from the purple and black layered structure to the remaining three purple plus labeled spheres at the center of the diagram. The base of each arrow has a circle formed by a dashed curved line in the layered structure. The lowest of the three purple plus marked spheres reached by the blue arrows has a second blue arrow extending from its left side which points to a purple sphere in the purple, green, and grey structure.
In a lithium ion battery, charge flows between the electrodes as the lithium ions move between the anode and cathode.

Visit this site for more information about lithium ion batteries.

The lead acid battery    ( [link] ) is the type of secondary battery used in your automobile. It is inexpensive and capable of producing the high current required by automobile starter motors. The reactions for a lead acid battery are

anode: Pb ( s ) + HSO 4 − ( a q ) ⟶ PbSO 4 ( s ) + H + ( a q ) + 2e − cathode: PbO 2 ( s ) + HSO 4 − ( a q ) + 3H + ( a q ) + 2e − ⟶ PbSO 4 ( s ) + 2 H 2 O ( l ) ¯ overall: Pb ( s ) + PbO 2 ( s ) + 2 H 2 SO 4 ( a q ) ⟶ 2PbSO 4 ( s ) + 2 H 2 O ( l )

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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