Batteries

Katy Swancutt

ENGR 302I

December 2007

What is a battery?

• It is something that converts chemical energy into electrical energy.

• They produce current via redox reactions:– Red=reduction (gaining electrons)– Ox=oxidation (losing electrons)

• Batteries are essentially Voltaic Cells connected in series.

What is a voltaic cell?Major components:• Metals ZINC

COPPER

What is a voltaic cell?Major components:• Metals• Electrolytes

ZINC

COPPER

CuSO4(aq)

ZnSO4(aq)

What is a voltaic cell?Major components:• Metals• Electrolytes• “Salt Bridge”• Wire

ZINC

COPPER

CuSO4(aq)

1.0 Molar

ZnSO4(aq

1.0 Molar

WIRE

AGAR

Battery lifetime depends on the mass of metal in the electrodes.

What is a voltaic cell?

ZINC

COPPER

CuSO4(aq)

ZnSO4(aq)

WIRE

AGAR

ANODE

CATHODE

How does it make energy?

• The solid zinc anode oxidizes and becomes Zn2+

• Electrons travel through the wire toward the cathode

• The aqueous Cu2+ is reduced and adds to the solid copper cathode

How much energy can it make?

• You can predict the voltage (direct current) between any two materials based on reduction potentials.

• A reduction potential, Eocell, is the

difference between the standard reduction potential of the cathode (Eo

red(cathode)) and the anode (Eo

red(anode)).

Eocell= Eo

red(cathode)-Eored(anode)

How much energy can it make?

Step 1: Write the reaction. Zn(s)+Cu2+

(aq)→Zn2+(aq)+Cu(s)

Step 2: Look up values.

Eored(cathode)= 0.337V

Eored(anode)= -0.763V

Eocell= 0.337V-(-0.763V)

Eocell= 1.10V

Actual voltage=1.044V

1.044 Volts

How can you get more energy?

1. Connect several cells in series (1.044V+1.044V+1.044V…)

2. Work outside of standard conditions (concentration, temp, etc.)

3. Use different compounds for the electrodes.

The greatest voltages are from a combination of the compound with the most negative potential (anode) and the most positive (cathode). These happen to be Li and F, respectively.

Electrode CombinationsBelow: Magnesium anode and Zinc cathode

Expected Vocell=1.607V

Observed Vocell=0.63V

Above: Zinc anode and IronIII cathode

Expected Vocell=1.534V

Observed Vocell=0.70V

Electrode Combinations

Above: IronIII anode and Copper cathode

Expected Vocell=0.434V

Observed Vocell=0.432V

Below: Magnesium anode and Copper cathode

Expected Vocell=2.707V

Observed Vocell=1.923V

Electrode Combinations

Above: Magnesium anode and IronIII cathode

Expected Vocell=3.141V

Observed Vocell=1.373V

Environmental Impacts

• Some electrodes are made of hazardous materials– Cadmium in Ni-Cd batteries– Lead in lead-acid batteries (like a car battery)

– Mercury-electrode batteries (banned in 1996)

– Arsenic and Antimony in lead-acid batteries

Environmental Impacts

• When batteries are not recycled, they ultimately end up contaminating soil, air, water, and finally end up in the food chain.

• Using secondary (rechargeable) batteries helps to prevent pollution.

THANK YOU!