Electrochemistry Dictionary - O

Ω: Symbol and abbreviation of ohm. (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)
OCP: Stands for open-circuit potential. See equilibrium potential.
OCV: Stands for open circuit voltage.
OER: Stands for oxygen evolution reaction.
Ohm: Measurement unit of the electrical resistance. Symbol: "Ω". (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)
Ohmic resistance (behavior): A system or system element is behaving "ohmically" if it follows Ohm's law. That is, the value of the resistance is independent of the current and the potential. Typically, metals and electrolyte solutions are "ohmic." Opposite: non-ohmic behavior.
Ohm's Law: The relation amongst the current flowing through a resistor and the potential difference between the two ends of the resistor. The potential difference is equal to the product of the current and the resistance (volt = ampere times ohm).
Ohmic drop: See ir drop.
Ohmic loss: The total ir drop in an electrochemical cell, including the ir drop in the solution between the electrodes and in any separator.
Ohmic overpotential (polarization): See solution ir drop and ohmic loss.
Ohmmeter: Instrument used for the measurement of electrical resistance.
OHP: Stands for "outer Helmholtz plane." See the Helmholtz model of the double layer.
One-shot battery: See reserve battery.
Open-circuit potential: See equilibrium potential. Abbreviated as "ocp."
Open circuit voltage (OCV): The voltage of a cell or battery under no-load condition, measured with a high impedance voltmeter or potentiometer. If the battery has recently been delivering or receiving current, then the voltage will probably be in the process of settling to its eventual equilibrium. This settling process can take hours for some batteries.
ORP electrode: Stands for oxidation/reduction potential electrode.
ORR: Stands for oxygen reduction reaction.
Outer Helmholtz plane: See the Helmholtz model of the double layer. Abbreviated as "OHP."
Outer-sphere charge-transfer reaction: A charge-transfer reaction with the reactants separated from each other by some solvent molecules due to the solvation of the reactants. Note that a "reactant" can also be an electrode. Contrast with inner-sphere charge-transfer reaction.
Overall reaction: See cell reaction.
Overcharge: The continued application of charging current to a cell or battery after it has reached its maximum state of charge.
Overcharging: During the charging of a rechargeable battery, eventually enough electrical charge is supplied to convert all the active material stored in the electrodes. If charging continues, the battery is said to be "overcharged." It very much depends on the battery system whether overcharging is detrimental to the battery or not.
Over discharge: Forced discharge of a cell or a battery past 100% of the available capacity. In the case of a multicell battery, the over discharge may cause cell reversal.
Overpotential: The difference in the electrode potential of an electrode between its equilibrium potential and its operating potential when a current is flowing. The overpotential represents the extra energy needed (an energy loss that appears as heat) to force the electrode reaction to proceed at a required rate (or its equivalent current density). Consequently, the operating potential of an anode is always more positive than its equilibrium potential, while the operating potential of a cathode is always more negative than its equilibrium potential. The overpotential increases with increasing current density, see Tafel equation. The value of the overpotential also depends on the "inherent speed" of the electrode reaction: a slow reaction (with small exchange current density) will require a larger overpotential for a given current density than a fast reaction (with large exchange current density). Also referred to as "polarization" of the electrode. See also overvoltage. An electrode reaction always occurs in more than one elementary step, and there is an overpotential associated with each step. Even for the simplest case, the overpotential is the sum of the concentration overpotential and the activation overpotential.
Overvoltage: The difference between the cell voltage (with a current flowing) and the open-circuit voltage (ocv). The overvoltage represents the extra energy needed (an energy loss that appears as heat) to force the cell reaction to proceed at a required rate. Consequently, the cell voltage of a galvanic cell (e.g., a rechargeable battery during discharging) is always less than its ocv, while the cell voltage of an electrolytic cell (e.g., a rechargeable battery during charging) is always more than its ocv. Occasionally also referred to as "polarization" of the cell. The overvoltage is the sum of the overpotentials of the two electrodes of the cell and the ohmic loss of the cell. Unfortunately, the terms "overvoltage" and "overpotential" are sometimes used interchangeably.
Oxidant: Alternative expression for oxidizing agent.
Oxidation: The loss of electrons by a chemical species.
Oxidation/reduction: In a narrow sense, oxidation means the reaction of a substance with oxygen. Hydrogen can react with oxygen to be oxidized to water. Hydrocarbon fuels (gasoline, natural gas, etc) can react with oxygen to be oxidized to carbon dioxide and water. Iron can react with oxygen to be oxidized to "rust." During oxidation, the oxygen itself is being reduced. Oxidation and reduction always occur simultaneously. During these reactions, electrons are transferred from the substance that is oxidized to the oxygen. In a wider sense, all electron-transfer reactions are considered oxidation/reduction. The substance gaining electrons ("oxidizing agent" or "oxidant") is oxidizing the substance that is losing electrons ("reducing agent" or "reductant"). In the process, the "oxidizing agent" is itself reduced by the "reducing agent." Consequently, the reduction process is sometimes called "electronation," and the oxidation process is called "de-electronation."
Oxidation/reduction potential: A measure of the oxidation/reduction capability of a solution. It is a redox potential measured with an inert electrode. An oxidizing solution (e.g., one saturated with oxygen) has a more positive potential than a reducing solution (e.g., one saturated with hydrogen).
Oxidation/reduction potential electrode: A measuring electrode used for the determination of the oxidation/reduction potential of a solution. Abbreviated as "ORP."
Oxidizing agent: A substance that is affecting oxidation by accepting electrons from another substance. See oxidation/reduction. Also called "oxidant."
Oxygen evolution reaction: An electrode reaction in which oxygen gas is produced at the anode of an electrolytic cell by the oxidation of hydroxyl (OH-) ions or the oxidation of the water molecules of an aqueous solution. Abbreviated as "oer." See also water electrolysis. It is the reverse reaction of oxygen reduction.
Oxygen production: See water electrolysis.
Oxygen reduction reaction: An electrode reaction in which oxygen gas is reduced at the cathode of an electrochemical cell. The product of the reduction can be hydroxyl (OH-) ions or water molecules (or occasionally hydrogen peroxide molecules). Abbreviated as "orr." It is the reverse reaction of oxygen evolution. It is a very important and much studied electrode reaction because it occurs at the cathode of practically all fuel cells and it occurs at the cathode of many (though not all) corrosion cells.