A positive cathode is a type of cathode, a conductor where electrical current leaves an electric apparatus. It is commonly found in galvanic cells such as running batteries, where it supplies the electrons to power a circuit. In these electric cells, electrons move toward the positive cathode from the outside of the cell, while positive ions move toward the positive cathode from the inside of the cell. As a general rule, devices that supply electric power follow this pattern and contain positive cathodes.
Although the cathode is commonly known as the negative electrode, this is merely a common characteristic and not a true definition. This assumption is based on the motion of cations, or positively charged ions, toward the cathode, and anions, or negatively charged ions, toward the other electrode, which is called the anode. In a positive cathode, the motion of cations toward another source of positive charge may seem counter-intuitive. This motion, however, is driven by a chemical concentration gradient rather than an electric charge potential.
Positive cathodes are found frequently in galvanic cells, which are made up of two separate half-cells connected by a salt bridge or membrane. Each half-cell contains a metal in contact with an electrolyte, which can be either a material or a salt solution that contains ions of that particular metal. The movement of ions into the cathode and the progress of chemical reactions allow the galvanic cell to generate electric energy from chemical energy. The positive cathode will be the site of reduction when the cations in the solution become electrically neutral as they join the solid metal. This generates an outflow of electric current from the cathode, along with the influx of electrons needed to reduce the cations.
Batteries usually contain many of these galvanic cells to supply enough energy using the principle outlined above. Eventually, the cations will all move into the cathode, leaving no more ions to continue the reaction progress. At this point, the battery’s ability to generate energy will be used up.
Rechargeable batteries can run the current in the opposite direction to reset the electrochemical cells to their initial conditions. These batteries take advantage of the fact that many energy-creating chemical reactions are also reversible. In a recharging battery, the positive electrode switches from the cathode to the anode as the chemical reaction runs in reverse. The electrons flow away from the once-positive cathode, and the metal ions leave the cathode to rejoin the electrolyte or dissolve back into the salt solution.