Galvanic and Electrolytic Cells

Electrochemistry and electrochemical cells were introduced in our unit on diabetes monitoring. You can review that page here. Electrochemical cells are in a circuit that allows the transfer of electrons through it. The cell includes:
  • an anode, the electrode where oxidation takes place
  • a cathode, the electrode where reduction takes place
  • an electrolyte, to allow conduction of ions within the solution in each half cell
  • a salt bridge or semipermeable membrane, to allow conduction of ions between half cells
  • the external circuit that connects the two electrodes includes wires, a load, and meters

Galvanic Cells

An electrochemical cell that releases energy is called a galvanic cell. The electrochemical reaction has a negative value of the Gibbs free energy and a positive cell potential difference.

Below is a galvanic cell in which the reaction between A+ and B is exothermic, with a G of -10 kJ/mol under standard conditions and a value of E0 of 0.10 V.

Hydrogen Electrode and Half Reactions

We have tables that show reduction half reactions. How are these obtained?

An electrochemical reaction in an electrochemical cell requires both a reduction half reaction and an oxidation half reaction. A voltmeter in the circuit shows the total cell voltage (Etot = Ered + Eox). We can determine a half cell potentials by measuring the voltage of a cell that includes that half cell along with a standard half cell of known potential.

The accepted standard electrode is the hydrogen electrode. This consists of a half cell with an unreactive metal electrode, H2(g) at 1 atmosphere pressure, and a 1 M aqueous solution of acid.

In the reduction mode:
    2H+ + 2e- H2
    Ered = 0.00 V

In the oxidation mode:
    H2 2H+ + 2e-
    Ered = 0.00 V

You can see here that the potential of a half cell in which Cu+2 is reduced to Cu is 0.34 V.

reduction Cu+2 + 2e- Cu Ered = x
oxidation H2 2H+ + 2e- Eox = 0.00 V
net Cu+2 + H2 Cu + 2H+ Etot = 0.34 V

Electrolytic Cells

An electrolytic cell has an endothermic chemical reaction. The reaction is not spontaneous so a power source is required. Energy is stored in an electrochemical cell. That energy can be released when the cell is allowed to run in a galvanic mode.

Above right is the galvanic cell that we saw previously. The molecules A and B+ are produced and energy is released. The electrode on the left side of the cell is the cathode because A+ is reduced to A at that electrode.

Above left is the same cell in an electrolytic mode. An energy source causes the reaction to go in the direction opposite to the spontaneous direction. Now, A+ and B are produced. The electrode on the right is the cathode becuase B+ is reduced to B there.

Professor Patricia Shapley, University of Illinois, 2012