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17.5 Batteries and fuel cells  (Page 3/12)

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Each cell produces 2 V, so six cells are connected in series to produce a 12-V car battery. Lead acid batteries are heavy and contain a caustic liquid electrolyte, but are often still the battery of choice because of their high current density. Since these batteries contain a significant amount of lead, they must always be disposed of properly.

A diagram of a lead acid battery is shown. A black outer casing, which is labeled “Protective casing” is in the form of a rectangular prism. Grey cylindrical projections extend upward from the upper surface of the battery in the back left and back right corners. At the back right corner, the projection is labeled “Positive terminal.” At the back right corner, the projection is labeled “Negative terminal.” The bottom layer of the battery diagram is a dark green color, which is labeled “Dilute H subscript 2 S O subscript 4.” A blue outer covering extends upward from this region near the top of the battery. Inside, alternating grey and white vertical “sheets” are packed together in repeating units within the battery. The battery has the sides cut away to show three of these repeating units which are separated by black vertical dividers, which are labeled as “cell dividers.” The grey layers in the repeating units are labeled “Negative electrode (lead).” The white layers are labeled “Postive electrode (lead dioxide).”
The lead acid battery in your automobile consists of six cells connected in series to give 12 V. Their low cost and high current output makes these excellent candidates for providing power for automobile starter motors.

Visit this site for more information about lead acid batteries.

Fuel cells

A fuel cell    is a device that converts chemical energy into electrical energy. Fuel cells are similar to batteries but require a continuous source of fuel, often hydrogen. They will continue to produce electricity as long as fuel is available. Hydrogen fuel cells have been used to supply power for satellites, space capsules, automobiles, boats, and submarines ( [link] ).

A diagram is shown of a hydrogen fuel cell. At the center is a narrow vertical rectangle which is shaded tan and labeled “Electrolyte.” To the right is a slightly wider and shorter purple rectangle which is labeled “Cathode.” To the left is a rectangle of the same size which is labeled “Anode.” Grey rectangles that are slightly wider and longer are at the right and left sides, attached to the purple and blue rectangles. On the right side, a white region overlays the grey rectangle. This white region provides a pathway for O subscript 2 to enter at the upper left, move inward and along the interface with the purple region, and exit to the lower right. A similar pathway overlays the grey region on the left, allowing a pathway for the entry of H subscript 2 from the upper right along the interface with the blue rectangle, allowing for the exit of H subscript 2 O out to the lower left of the diagram. Black line segments extend upward from the blue and purple regions. These line segments are connected by a horizontal segment that has a yellow zig zag shape at the center. This shape is labeled “Electric power.” At the left of the diagram, in the upper left white region, 2 H subscript 2 is followed by an arrow that points right and down to H subscript 2. An arrow points right into the blue region to H subscript 2 O. A curved arrow point up to e superscript negative. Another e superscript negative is placed nearby and has an upward pointing arrow extending up to the left of the line segment extending from the purple region. A second arrow points upward along this segment with the label “e superscript negative” to its left. A curved arrow extends down and to the left from the H subscript 2 O into the white region. A second H subscript 2 O is shown below the first in the blue region repeating the arrow patterns established above. At the lower left, an arrow points left, to the exit of the white region. At the tip of this arrow is the label “2 H subscript 2 O.” In the central brown region, O superscript 2 negative is listed twice with arrows pointing left, to the H subscript 2 O formulas in the blue region. At the upper right, O subscript 2 is shown with an arrow pointing left and down to O subscript 2 in the white region. An arrow points left from this point into the purple region. From the tip of the arrow, two arrows point to the two O subscript 2 negative structures in the brown central region. An arrow, labeled “e superscript negative” points downward to the right of the line segment above the purple region. A second arrow extends down into the purple region, pointing to e superscript negative. Three additional e superscript negative symbols appear nearby. An arrow extends from them to the point where the arrows meet in the purple region.
In this hydrogen fuel-cell schematic, oxygen from the air reacts with hydrogen, producing water and electricity.

In a hydrogen fuel cell, the reactions are

anode: 2 H 2 + 2O 2− ⟶ 2 H 2 O + 4e − cathode: O 2 + 4e − ⟶ 2O 2− ¯ overall: 2 H 2 + O 2 ⟶ 2 H 2 O

The voltage is about 0.9 V. The efficiency of fuel cells is typically about 40% to 60%, which is higher than the typical internal combustion engine (25% to 35%) and, in the case of the hydrogen fuel cell, produces only water as exhaust. Currently, fuel cells are rather expensive and contain features that cause them to fail after a relatively short time.

Check out this link to learn more about fuel cells.

Key concepts and summary

Batteries are galvanic cells, or a series of cells, that produce an electric current. When cells are combined into batteries, the potential of the battery is an integer multiple of the potential of a single cell. There are two basic types of batteries: primary and secondary. Primary batteries are “single use” and cannot be recharged. Dry cells and (most) alkaline batteries are examples of primary batteries. The second type is rechargeable and is called a secondary battery. Examples of secondary batteries include nickel-cadmium (NiCd), lead acid, and lithium ion batteries. Fuel cells are similar to batteries in that they generate an electrical current, but require continuous addition of fuel and oxidizer. The hydrogen fuel cell uses hydrogen and oxygen from the air to produce water, and is generally more efficient than internal combustion engines.

Chemistry end of chapter exercises

What are the desirable qualities of an electric battery?

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List some things that are typically considered when selecting a battery for a new application.

Considerations include: cost of the materials used in the battery, toxicity of the various components (what constitutes proper disposal), should it be a primary or secondary battery, energy requirements (the “size” of the battery/how long should it last), will a particular battery leak when the new device is used according to directions, and its mass (the total mass of the new device).

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Consider a battery made from one half-cell that consists of a copper electrode in 1 M CuSO 4 solution and another half-cell that consists of a lead electrode in 1 M Pb(NO 3 ) 2 solution.

(a) What are the reactions at the anode, cathode, and the overall reaction?

(b) What is the standard cell potential for the battery?

(c) Most devices designed to use dry-cell batteries can operate between 1.0 and 1.5 V. Could this cell be used to make a battery that could replace a dry-cell battery? Why or why not.

(d) Suppose sulfuric acid is added to the half-cell with the lead electrode and some PbSO 4 ( s ) forms. Would the cell potential increase, decrease, or remain the same?

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Consider a battery with the overall reaction: Cu ( s ) + 2 Ag + ( a q ) ⟶ 2Ag ( s ) + Cu 2+ ( a q ) .

(a) What is the reaction at the anode and cathode?

(b) A battery is “dead” when it has no cell potential. What is the value of Q when this battery is dead?

(c) If a particular dead battery was found to have [Cu 2+ ] = 0.11 M , what was the concentration of silver ion?

(a) anode: Cu ( s ) ⟶ Cu 2+ ( a q ) + 2e − E anode ° = 0.34 V cathode: 2 × ( Ag + ( a q ) + e − ⟶ Ag ( s ) ) E cathode ° = 0.7996 V ; (b) 3.5 × 10 15 ; (c) 5.6 × 10 −9 M

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An inventor proposes using a SHE (standard hydrogen electrode) in a new battery for smartphones that also removes toxic carbon monoxide from the air:
Anode: CO ( g ) + H 2 O ( l ) ⟶ CO 2 ( g ) + 2H + ( a q ) + 2e − E anode ° = −0.53 V Cathode: 2 H + ( a q ) + 2e − ⟶ H 2 ( g ) E cathode ° = 0 V ¯ Overall: CO ( g ) + H 2 O ( l ) ⟶ CO 2 ( g ) + H 2 ( g ) E cell ° = +0.53 V

Would this make a good battery for smartphones? Why or why not?

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Why do batteries go dead, but fuel cells do not?

Batteries are self-contained and have a limited supply of reagents to expend before going dead. Alternatively, battery reaction byproducts accumulate and interfere with the reaction. Because a fuel cell is constantly resupplied with reactants and products are expelled, it can continue to function as long as reagents are supplied.

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Explain what happens to battery voltage as a battery is used, in terms of the Nernst equation.

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Using the information thus far in this chapter, explain why battery-powered electronics perform poorly in low temperatures.

E cell , as described in the Nernst equation, has a term that is directly proportional to temperature. At low temperatures, this term is decreased, resulting in a lower cell voltage provided by the battery to the device—the same effect as a battery running dead.

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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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