lectrochemistry: Primary and Secondary Batteries

Goal: to understand the construction and operation of several common batteries

Working Definitions:

A primary battery is a portable voltaic cell that is not rechargeable.

A secondary battery is a portable voltaic cell that is rechargeable.

Introduction:

Voltaic cells constructed from metal strips and beakers, though relatively easy to build, are not convenient sources of electricity. Batteries, constructed from one or more voltaic cells, are usually compact and sturdy devices that are easily stored and utilized. With one or two minor variations, primary and secondary batteries operate exactly as do simpler voltaic cells!

A Primary Battery: The Leclanche Dry Cell

	Move mouse over cell to see more detail.

A flashlight battery, or dry cell, is constructed with a zinc shell that serves as the anode; a graphite rod which serves as the cathode; and a moist mixture of ammonium chloride {NH₄Cl}, zinc chloride {ZnCl₂}, and manganese dioxide {MnO₂}. A schematic representation of a dry cell is shown on the right. The half-reaction that occurs on the anode when the battery delivers current is the oxidation of zinc atoms:

Zn(s, shell) Zn²⁺(aq) + 2e^–

The half-reaction that occurs simultaneously on the cathode is the reduction of ammonium ions:

2 e^– + 2 NH₄⁺(aq, moist paste) 2 NH₃(g) + H₂(g)

A porous graphite electrode is embedded in the moist paste and readily conducts electrons from the external circuit to the aqueous ammonium ions. Take another look at the products of the reduction that occurs at the graphitic cathode. Two gases are being produced in a sealed container! Not to fear, our battery will not explode as additional reactions essentially fix the two gases:

2 NH₃(g) + Zn⁺² (aq) [Zn(NH₃)₂]²⁺(aq)

In this reaction, zinc ion, primarily from ZnCl₂, is acting as a Lewis acid; the complex formed solubilizes the gas.

Note: This reaction serves another purpose; if ammonia molecules build up around the cathode, they can have an insulating effect, decreasing the amount of current that can be delivered to the reactants. This process is known as polarization, and can occur when the dry cell is under high current loads. Disconnecting a "tired" battery from its load and allowing it to "rest" will allow time for the zinc ions to react with the ammonia molecules and can restore the battery to a more active state.

The reaction that fixes the hydrogen gas involves the reduction of manganese dioxide. Where does the hydrogen end up? As part of a water molecule!

H₂(g) + 2 MnO₂(s) Mn₂O₃(s) + H₂O(l)

Primary and Secondary Batteries