# How are battery reactions chosen?

Why are batteries not made of oxidants/reductants at the extreme ends of the reduction potential table?

It's possible, in principle, to use any spontaneous redox reaction to make a battery. Couple a pair of reactions with very high and very low standard reduction potentials together, and you obtain a cell with a very high emf at standard state, since the difference

cell = E°cathode - E°anode

will be very large.

But if you want to use such a reaction to produce a commercial battery, there are many other factors to consider. Here are just a few of them:

1. The cell won't be at standard state in real life. Making a battery with pH 0, all electrolyte activities at 1 M, and all partial pressures at 1 atm isn't always convenient or practical. You're looking for a cell that delivers a high emf under actual working conditions- not at standard state:

Ecell = E°cathode - E°anode + correction for nonstandard conditions

You can calculate the correction for nonstandard conditions from the concentrations of reactants and products using the Nernst equation.
2. The cell's internal resistance will lower the output voltage. A battery produces an electric current. The cell has some resistance to current flow. Some voltage will be necessary to overcome the cell's innate resistance if a current is to flow. If the current is symbolized by I and the resistance by R, the cell voltage will be decreased by IR:

Ecell = E°cathode - E°anode + correction for nonstandard conditions - IR

Different cells have different resistances. The internal resistance is affected by the materials (and the reaction) chosen for the cell.
3. The accessibility of the electrodes limits the cell's output voltage. The amount of current the cell will deliver is limited by how fast reactants can arrive at electrode surfaces (and how fast products leave). If reactant arrival and product departure at the electrodes is slow, you'll get a cell potential that's less than what you'd expect, even considering the IR drop. The decrease in cell voltage from this effect is called the concentration overvoltage:

Ecell = E°cathode - E°anode + correction for nonstandard conditions - IR - concentration overvoltage

4. The rate at which electrons are transferred at the electrode surfaces limits the cell's output voltage. The standard reduction potentials say nothing about how fast the electrons are transferred. Half reactions that involve gases have very slow electron transfer rates. The decrease in cell voltage from this effect is called the kinetic overvoltage:

Ecell = E°cathode - E°anode + correction for nonstandard conditions - IR - concentration overvoltage - kinetic overvoltage

The overvoltage for reactions that form hydrogen gas and oxygen gas can be on the order of 1 V, so this is not a small correction.
5. Some cells don't produce a steady voltage. Does the voltage fall rapidly as the battery discharges? The concentrations of the electrolytes and the condition of the electrode surfaces will change as the battery is used, changing the cell emf. Try running a clock using a cell with a voltage that drops off as it discharges.
6. How bulky is a cell based on the reaction? The reaction that works so well in a car battery wouldn't work as well in a heart pacemaker or a spacecraft.
7. How durable is the cell? Are the electrodes fragile? Can the battery be dropped? What's going to happen if someone breaks the casing on the battery?
8. Are there side reactions? Electrochemical cells usually involve many reactions. Some side reactions can foul the electrodes or otherwise lower the efficiency of the battery as it ages. For example, a fuel cell design for converting methanol to electricity being investigated at Lawrence Berkeley National Laboratory produces electricity for an instant, before being shut down by a side product of the reaction that coats the electrode. The problem has been studied for a decade without solution).
9. Is the reaction reversible? If it is, the battery can be recharged.
10. Does the cell have a reasonable shelf life? If the electrodes react with the electrolyte solution even when the cell isn't connected, the battery won't last very long.
11. Some cell reactions are too expensive. There are several cell reactions that work great- but if there are cheaper alternatives that perform just as well, no one will use them.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How are battery reactions chosen?