Page 117 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Electrochemistry 195

(1.3 vs 2.0 V), (3) a constant cell voltage during discharge, The alkaline cell has lower internal resistance, longer shelf
and (4) almost unlimited life. In contrast to the sulfuric- life, and a larger open-circuit voltage (about 1.9 V) than
acid electrolyte of the lead-acid battery, the Edison battery the dry cell.
uses concentrated potassium hydroxide (KOH) as the elec-
IV
troyte. The cathode is nickel dioxide [Ni O 2 (s)] and the
D. High-Performance Batteries
anode is metallic iron. As the cell reaction confirms, the
(Nickel/Cadmium, Silver/Zinc, Lithium)
KOH concentration remains constant during discharge,
which accounts for the constant cell voltage during dis- 1. Nickel/Cadmium
charge (only water is consumed during discharge and re-
This is the most common of the rechargable batteries and
stored during recharge).
hasextensiveusein“cordless”powertools,electricrazors,
IV
Cathode (+ terminal): Ni O 2 (s) + 2H 2 O + 2e − and electronics. It has similarities to the Edison cell, but
better performance in its areas of application.
II
Ni (OH) 2 (s) + 2HO − (200)
IV
Cathode: Ni O 2 (s) + 2H 2 O + 2e −
Anode (−terminal): Fe (s) + 2HO −
II
Ni (OH) 2 (s) + 2HO − (209)
II
Fe (OH) 2 (s) + 2e − (201)
II
Anode: Cd (s) + 2HO − Cd (OH) 2 (s) + 2e − (210)
IV
Cell: Ni O 2 (s) + Fe (s) + 2H 2 O
IV
Cell: Ni O 2 (s) + Cd (s) + 2H 2 O
II
II
Ni (OH) 2 (s) + Fe (OH) 2 (s) (202)
II
II
Ni (OH) 2 (s) + Cd (OH) 2 (s) (211)
C. “Dry-Cell” Batteries The main problem with Ni/Cd batteries is their reduced
charge capacity if they are not fully discharged before
1. LeClanch´e Cell
recharging. Because of the toxicity of cadmium, responsi-
This traditional dry cell consists of a carbon-rod cathode ble disposal of worn-out batteries is important (but often
IV
(positive terminal) immersed in a moist paste of Mn O 2 , ignored).
II
Zn Cl 2 ,NH 4 Cl, and powdered carbon, which is contained
in a metallic zinc-can anode (negative terminal). The volt- 2. Silver/Zinc
age (without load) of these cells is about 1.6 V, which have
limited shelf life because of corrosion of the zinc can and Forms of this battery type range from those used in hear-
increased internal resistance. ing aids, watches, and cameras to those used as portable
power sources in space vehicles. The cathode/anode and
IV
Cathode: 2 Mn O 2 (s) + H 2 O+2e −
electrolyte system is similar to that for the alkaline cell,
III
IV
Mn O 3 (s) + 2HO − (203) except the Mn O 2 cathode material is replaced with much
2
I
more expensive silver oxide (Ag O). However, it has high
2
Anode: Zn (s) + 2NH 4 Cl + 2HO −
stability, a long shelf life, and a large charge density with
II
Zn (NH 3 ) 2 Cl 2 + 2H 2 O+2e − (204) a stable voltage (about 1.8 V).
I
IV
Cell: 2 Mn O 2 (s) + Zn (s) + 2NH 4 Cl Cathode: Ag O (s) + H 2 O + 2e − 2Ag(s) + 2HO −
2
III
II
Mn O 3 (s) + Zn (NH 3 ) 2 Cl 2 + H 2 O (205) (212)
2
II
Anode: Zn (s) + 2HO − Zn (OH) 2 (s) + 2e − (213)
2. Alkaline Cell Cell: Ag O (s) + Zn (s) + H 2 O
I
2
II
This modern replacement for the dry cell uses sodium 2Ag(s) + Zn (OH) 2 (s) (214)
or potassium hydroxide as the electrolyte (in place of the
II
acidic Zn Cl 2 /NH 4 Cl electrolyte of the dry cell). Its cath-
ode and anode are essentially the same as the dry cell. 3. Lithium
IV
Cathode: 2 Mn O 2 (s) + H 2 O + 2e − The development of the lithium/managanese dioxide bat-
tery has revolutioned the performance of modern cameras
III
Mn O 3 (s) + 2HO − (206)
2 and electronic devices. Because of the use of a dry sol-
II
Anode: Zn (s) + 2HO − Zn (OH) 2 (s) + 2e − (207) vent/electrolyte system (e.g., acetronitrile/LiX), these bat-
teries have operating lifetimes of at least five years, high
IV
Cell: 2 Mn O 2 (s) + Zn (s) + H 2 O
current capacity, and stable voltages (about 2.5 V) under
III
II
◦
Mn O 3 (s) + Zn (OH) 2 (s) (208) load and can be used at temperatures as low as −40 C.
2