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Carbon electrodes are very useful in some electrochemical reactions because of several favorable properties. Electrochemical reactions involve the transport of electrons from one place to another and, if configured appropriately, yield useful electrical currents as a result. Electricity may be stored, produced, or consumed in the reactions, which are used in batteries, the production of industrial metals and chemicals, capacitors, and fuel cells.
Electrodes are the solid surface at which fluid electrochemical reactions occur. The anode has a positive charge and attracts electrons, while the cathode has a negative charge and attracts the positive ions. By the exchange of electrons from the cathode to the anode, electrical current is established. Carbon can serve both an anode or cathode; however, in both applications, carbon is usually combined with other elements to increase conductivity.
In organic chemistry, carbon is often thought of in its hydrocarbon molecules, the key compounds of living and previously living matter. Electrochemists think of carbon in its solid states of graphite and similar, almost pure carbon forms. Carbon bonded almost exclusively to other carbon atoms achieves a high degree of delocalized electrons that make it a good conductor. A carbon anode is a favored choice in electrochemistry for other reasons including non-toxicity, low cost, and flexibility.
The first use of carbon as a cathodic material was demonstrated in 1792 when graphite successfully replaced metals in some experimental early batteries. In a battery, energy is stored in the electrochemical potential of the reactants and released as necessary.
Electronic capacitors made of carbon store electrical charge between double carbon electrodes of immense surface area. One side of the double layer acts as a carbon cathode and the other as a carbon anode. The positively charged ions cling to the cathodic side, and negatively charges cling to the anodic side. When discharged, electrons are released to the circuit.
A molded carbon anode made from artificial graphite is used in the large-scale production of chlorine, aluminum, and silicon. Production of calcium carbide, yellow phosphorus, and ferroalloys uses carbon anodes. These processes require energy. The carbon anode is gradually consumed in the process, losing carbon as carbon dioxide. The anodes lose efficiency as they degrade, prompting the use of metal oxide anodes in modern chlorine manufacturing plants.
In fuel cells, electricity is drawn directly from the anode of the electrochemical reaction — a very efficient conversion when compared to energy generated indirectly through combustion of fuels to drive mechanical equipment. The fuel is usually hydrogen gas, and the oxidant is oxygen from the air. The anode-electrolyte mix-cathode cell is very thin and is packaged in blocks of more than 400 cells arranged in series. The carbon anode performs an electrochemical function, but also serves as a way to disperse expensive metallic catalysts.
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