Systems for electrochemical energy storage and conversion include batteries, fuel cells, and electrochemical capacitors. Although the energy storage and conversion mechanisms are different, there are a few similarities between these three systems. Common features include the energy-transfer processes taking place at the phase boundary of the electrode/electrolyte interface and electron and ion transport being separated. Batteries, fuel cells, and super-capacitors all consist of two electrodes in contact with an electrolyte solution. In batteries and fuel cells, electrical energy is generated by conversion of chemical energy via redox reactions at the anode and cathode. As reactions at the anode usually take place at lower electrode potentials than at the cathode, the terms negative and positive electrode (indicated as minus and plus poles) are used. The more negative electrode is designated the anode, whereas the cathode is the more positive one. The difference between batteries and fuel cells is related to the locations of energy storage and conversion. Batteries are closed systems, with the anode and cathode being the charge-transfer medium and taking an active role in the redox reaction as "active masses". In other words, energy storage and conversion occur in the same compartment. Fuel cells are open systems where the anode and cathode are just charge-transfer media and the active masses undergoing the redox reaction are delivered from outside the cell, either from the environment (e.g. oxygen from air), or from a tank (e.g. hydrogen and hydrocarbons).