An electrochemical cell manages chemical and electrical energy, facilitating the conversion between the two forms in a controlled environment. This conversion is driven by chemical reactions involving the transfer of electrons, which can be harnessed to do work or forced to occur by an external power source. The cell acts as a controlled system where chemical reactions are physically separated, directing the movement of electrons through an external circuit. This capability makes electrochemical cells foundational technology for devices ranging from standard batteries to large-scale industrial processes like metal refining.
Essential Internal Components
An electrochemical cell requires several distinct physical components, beginning with the electrodes, the conductive surfaces where chemical reactions take place. These electrodes are named the anode and the cathode. The anode is the site where oxidation occurs (chemical species lose electrons), while the cathode is the site where reduction occurs (chemical species gain electrons).
Both electrodes are immersed in an electrolyte, a liquid or paste containing free-moving ions. The electrolyte completes the internal circuit by allowing a path for ionic movement between the two electrode areas. This ionic conductivity ensures that charge neutrality is maintained within the cell as electrons move through the external circuit. Without the electrolyte, the buildup of charge at the electrodes would quickly halt the entire reaction.
In many cell designs with two separate half-cells, a salt bridge or porous separator connects the two electrolyte solutions. This bridge prevents the bulk mixing of the solutions while permitting the migration of ions. The primary purpose of the salt bridge is to balance the accumulation of positive and negative charges that occur in the half-cells as the reactions proceed. By maintaining this charge balance, the salt bridge allows the continuous flow of electrons through the external circuit.
The Mechanism of Energy Conversion
The core of an electrochemical cell’s operation lies in the coordinated process of reduction-oxidation (redox) reactions. At the anode, the chemical species undergoes oxidation, releasing electrons into the electrode material. These electrons travel through the external wire to the cathode, creating the electrical current that can power an attached device.
Simultaneously, at the cathode, another chemical species undergoes reduction, accepting the electrons arriving from the external circuit. This transfer of electrons from the oxidized species at the anode to the reduced species at the cathode is the source of the electrical energy generated by the cell. The potential difference, measured in volts, is the driving force that pushes these electrons through the external circuit.
For the electrical current to remain steady, charge transfer must be balanced by internal ionic movement. As the anode releases positively charged ions into its solution and the cathode consumes positive ions from its solution, a charge imbalance begins to form. The electrolyte, or the salt bridge, neutralizes this imbalance by allowing its own ions to migrate—anions (negative ions) move toward the anode, and cations (positive ions) move toward the cathode. This internal ionic flow completes the circuit, ensuring that the half-cell solutions remain electrically neutral and allowing the electron transfer reactions to continue.
The Difference Between Galvanic and Electrolytic Cells
Electrochemical cells are categorized into two main types based on energy flow: galvanic (or voltaic) cells and electrolytic cells. Galvanic cells generate electrical energy from a spontaneous chemical reaction. In these systems, the chemical process naturally releases energy, which is then harnessed as electricity, making them the type used in standard batteries and fuel cells.
A key distinction in galvanic cells is the polarity: the anode is the negative electrode because it is the source of electrons flowing into the external circuit. Conversely, electrolytic cells require an external source of electrical energy to drive a non-spontaneous chemical reaction. They convert electrical energy into chemical energy, a process used in applications like electroplating or recharging a battery.
In an electrolytic cell, the polarity is reversed relative to a galvanic cell, with the anode being the positive electrode and the cathode being the negative electrode. The external power supply forces electrons to flow from the positive anode to the negative cathode, driving the non-spontaneous redox reaction. This difference highlights that electrochemical technology can either produce power from stored chemical energy or use power to force a chemical change.