Electrochemical Cells: Automobile Batteries and Flashlight Cells

One way to understand electrochemical cells is to consider them in two commonly used items: automobile batteries and flashlights.

The automobile battery, or lead storage battery, consists of six electrochemical cells connected in series. The anode of each cell is lead, while the cathode is lead dioxide.

Lead dioxide is represented as:

The electrodes are immersed in a sulfuric acid solution. Sulfuric acid is represented as:

When you start your car, the following cell reactions take place:

When this reaction takes place, both electrodes become coated with solid lead (II) sulfate, and the sulfuric acid is used up.

After the automobile has been started, the alternator or generator takes over the job of producing electricity (for spark plugs, lights, and so on) and also recharges the battery. The alternator reverses both the flow of electrons into the battery and the original redox reactions, and regenerates the lead and lead dioxide:

The lead storage battery can be discharged and charged many times. But the shock of running over bumps in the road or into the curb flakes off a little of the lead (II) sulfate and eventually causes the battery to fail.

During charging, the automobile battery acts like a second type of electrochemical cell, an electrolytic cell, which uses electricity to produce a desired redox reaction.

Flashlight cells

The common flashlight cell, a dry electrochemical cell, is contained in a zinc housing that acts as the anode (the electrode at which oxidation takes place). The other electrode, the cathode (where reduction takes place), is a graphite rod in the middle of the cell.

A layer of manganese oxide and carbon black (one of the many forms of carbon) surrounds the graphite rod, and a thick paste of ammonium chloride and zinc chloride serves as the electrolyte. The diagram shows the structure of a dry cell. The dry cell reactions are shown in the following.

Anode reaction/oxidation:

Cathode reaction/reduction:

Note that the case of the dry cell is actually one of the electrodes; it’s being used up in the reaction. If there’s a thin spot in the case, a hole could form, and the cell could leak the corrosive contents. In addition, the ammonium chloride tends to corrode the metal case, again allowing for the possibility of leakage.

In the alkaline dry cell (alkaline battery), the acidic ammonium chloride of the regular dry cell is replaced by basic (alkaline) potassium hydroxide. With this chemical, corrosion of the zinc case is greatly reduced.

A battery is composed of two or more cells connected together. You put a battery in your car, but you put a cell into your flashlight.

Another cell with the same basic construction is the small mercury battery commonly used in watches, pacemakers, and so on. With this battery, the anode is zinc, as in the regular dry cell, but the cathode is steel. Mercury(II) oxide (HgO) and some alkaline paste form the electrolyte.

You should dispose of small mercury batteries (such as those used in watches) carefully, to keep the mercury from being released into the environment.

All these galvanic cells produce electricity until they run out of a reactant. Then they must be discarded. However, there are cells that can be recharged, as the redox reaction can be reversed to regenerate the original reactants. Nickel-cadmium (Ni-Cad) and lithium batteries fall into this category. The most familiar type of rechargeable battery is probably the automobile battery.