Electrochemical cell

An electrochemical cell is a device that facilitates the movement of charge through chemical reactions.

These cells are essential for modern life because they manage the relationship between chemical energy and electrical energy. There are two fundamental types of cells based on how they handle this energy. A galvanic cell, also known as a voltaic cell, generates electrical energy from spontaneous redox reactions. In contrast, an electrolytic cell uses external electrical energy to induce non-spontaneous chemical reactions. When multiple electrochemical cells are connected in series or parallel, they form a battery.

To understand the mechanism, one must examine the two half-cells that make up a full cell. Each half-cell contains an electrode and an electrolyte, which is a substance like a water-based solution or molten salt that conducts current. The process relies on redox reactions, which are combinations of oxidation and reduction. In the oxidation half-cell, species lose electrons at the anode, which is the negative electrode in a galvanic cell. In the reduction half-cell, species gain electrons at the cathode, which is the positive electrode.

As electrons travel through an external wire from the anode to the cathode, they create an electric current.

Maintaining electrical neutrality is critical for a steady flow of electricity. To prevent charge from building up, electrochemical cells use a salt bridge or a porous membrane. A salt bridge, such as filter paper soaked in potassium nitrate (KNO3) or sodium chloride (NaCl), connects the two electrolytes. This allows ions to move between the half-cells to balance the charge without letting the solutions mix. If the solutions mixed directly, unwanted side reactions might occur. Other methods for separation include using porous pots or gelled solutions to keep the components distinct while allowing ionic contact.

History and discovery have shaped our understanding of these systems. The galvanic cell is named after Luigi Galvani, though Alessandro Volta is also a key figure in this field. The potential of a cell, or its voltage, can be predicted by looking at electrode potentials. These are measured relative to a standard hydrogen electrode (SHE), which is assigned a value of 0 volts. The total cell potential is the difference between the voltages of the two half-cells.

While cell potentials can range from roughly 0 to 6 volts, water-based electrolytes are usually limited to less than 2.5 volts. This limit exists because powerful chemicals can react too strongly with the water itself. However, lithium cells can reach 3 volts by using different solvents.

We can categorize batteries into two main groups: primary and secondary cells. A primary cell consists of single-use galvanic cells that rely on irreversible chemical reactions. Once the chemicals are used up, the battery stops producing electricity and cannot be recharged. These are often used for small household items like flashlights or portable radios.

On the other hand, secondary cells use reversible reactions. A lead-acid battery used in automobiles is a common secondary cell. It functions as a galvanic cell when discharging power and as an electrolytic cell when the car's alternator recharges it.

Electrolytic cells serve different, specific purposes, such as electrolysis. The term "lysis" comes from the Greek word for "loosing" or "setting free." This process is used to decompose stable chemical compounds. For example, electrolysis can decompose water into hydrogen and oxygen, or bauxite into aluminum. It is also used in electroplating to coat objects with metals like copper, silver, nickel, or chromium.

In an electrolytic cell, an external voltage is applied to the electrodes to force the reaction to occur. The ions in the electrolyte are attracted to the electrode with the opposite potential, where charge-transfer reactions take place.

A unique category is the fuel cell, which differs from a standard battery because it requires a continuous supply of fuel.

A fuel cell reacts hydrogen fuel with an oxidizing agent, like oxygen from the air, to convert chemical energy into electricity. At the anode, a catalyst causes the fuel to undergo oxidation, creating protons and electrons. The electrons flow through an external circuit to produce direct current, while the protons move through the electrolyte to the cathode. At the cathode, the protons, electrons, and oxygen react to form water. Because individual fuel cells produce small potentials of about 0.7 volts, they are often "stacked" in series to meet power needs. This technology is used for everything from backup power in buildings to fueling buses, boats, and submarines.