What is an Electrochemical Cell? - Structure & Uses

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  • 0:04 Electrochemical Cells
  • 0:29 Electrolytic vs Galvanic Cells
  • 3:05 Internal Resistance
  • 3:56 Connecting in Series &…
  • 5:27 Uses
  • 6:11 Lesson Summary
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Lesson Transcript
Instructor: Patricia Jankowski

Patricia has a BSChE. She's an experienced registered nurse who has worked in various acute care areas as well as in legal nurse consulting.

Electrochemical cells are devices with many useful applications that employ the principles of electrochemistry to generate an electric potential between electrodes. In this lesson, we'll learn how they're put together and how we use them.

Electrochemical Cells

Today we rely upon our laptops, cellphones, and hearing aids to make our lives easier and more enjoyable. But where does the energy for these things come from? Electrochemistry! Electrochemical cells use either chemical reactions to generate electricity, or, conversely, use electricity to energize chemical reactions. Let's go over the two main types of electrochemical cells: electrolytic and galvanic.

Electrolytic vs. Galvanic Cells

Electrolytic cells drive chemical reactions when electrical energy is applied to them. They consist of two electrodes that are immersed in a conducting liquid, usually an aqueous solution or a molten salt. An electrical supply is connected to the electrodes and provides the energy to drive a reaction in the solution.

This process is known as electrolysis. An example of electrolysis is the isolation of sodium (Na) and chlorine (Cl) from sodium chloride (NaCl). Two electrodes are immersed in molten NaCl, and chemical reactions occur at each electrode as electrons are pumped by the energy source from one electrode to the other.

The electrode at which electrons are lost is known as the anode, and the one at which electrons are gained is the cathode. Oxidation, or loss of electrons, is the process which occurs at the anode, and reduction, or gain of electrons, occurs at the cathode. The overall reduction-oxidation reaction, or redox reaction, for our electrolysis example is:

electrolytic cell reaction

Out of this process, elemental sodium and chlorine gas are obtained. Similarly, most of the metals that form positive ions can be obtained through electrolysis from a solution of their molten salts.

Galvanic cells (aka voltaic cells) use chemical reactions to generate electrical energy. In a galvanic cell, electrical energy is produced by a chemical redox reaction, instead of a chemical reaction being produced by electricity. The classic example of a redox reaction for a galvanic cell is the reaction between aqueous solutions of zinc (Zn) and copper (Cu):

galvanic cell equation

In this cell, the zinc is oxidized, and the copper is reduced. Initially, this produces a flow of electrons across a wire connected to the two separate electrode solutions, but as the zinc solution becomes positively charged from losing electrons and the copper solution becomes negatively charged from gaining them, that flow stops. No more negatively charged electrons want to flow toward the negatively charged copper solution!

To solve this problem, and to provide a continuous flow of electrons (which means a source of electricity), the electrode solutions must remain electrically neutral. This can be done with a salt bridge, a U-shaped tube filled with a concentrated salt solution. The solution in this tube provides a way for ions to travel between the two electrode solutions so that they can remain electrically neutral in charge. This enables the continuous flow of electrons.

Internal Resistance

Even in the world of electrochemistry, nothing good lasts forever.

Electrochemical cells are constructed of various materials, such as the wire, the solutions themselves, and the containers. All of these materials cause the cell to have a property called internal resistance. As the current flows in the cell over time, this resistance causes the cell to lose some of its potential, or voltage. What remains of the cell's voltage after subtracting the voltage lost due to internal resistance is called the terminal potential difference. This is expressed as:

Internal resistance equation

In this equation, Vt.p.d. is the terminal potential difference; emf is the electromotive force, or ideal amount of energy (in volts) provided by the cell before there is resistance; I is the current in amperes; and r is the internal resistance in ohms.

Connecting in Series & in Parallel

In the real world, energy for appliances, cellphones, and other useful technologies is not supplied by one simple electrochemical cell. Instead, the cells are cleverly arranged in various configurations to increase energy output (voltage) and current.

Placing voltaic cells in series means connecting the cells in a row, with the positive end of one terminal or electrode connected to the negative electrode of the next cell in the row. In this configuration, the voltage is always increased. So, for example, if a battery contains several cells in series, it can deliver the voltage of the sums of the voltages of all those cells. If more energy is desired from the battery, this is an advantage over containing only a single cell.

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