NOVELTY - An electrode, comprises a regenerative catalytic material.    USE - In electrolytic device, fuel cell, and regenerative fuel cell (claimed).    ADVANTAGE - The electrode catalytically produces oxygen gas from water optionally at about neutral pH, with an overpotential of less than 0.4 (preferably less than 0.35, especially less than 0.325) volts at an electrode current density of at least 1 mA/cm2. The electrode catalytically produces oxygen gas from water with a Faradaic efficiency of about 100 (preferably at least about 99, especially at least about 95, particularly at least about 90)%. The electrode is also catalytically forms water from oxygen gas. The electrode is also catalytically produces water from oxygen gas; oxygen from gaseous water; and oxygen from liquid water.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a system (S1) for electrolysis of water;    (2) a system (S2) for catalytically producing oxygen gas from water;    (3) a composition for an electrode comprising: cobalt ions; and anionic species comprising phosphorus, where the ratio of cobalt ions to anionic species comprising phosphorus is 10:1-1:10, where the composition is capable of catalytically forming oxygen gas from water;    (4) a composition able to catalyze the formation of oxygen gas from water, obtainable by a process, involving: exposing at least one surface of a current collector to a source of cobalt ions and anionic species comprising phosphorus; and applying a voltage to the current collector for a period of time to accumulate, proximate the surface of the current collector, where the composition comprises at least a portion of the cobalt ions and anionic species comprising phosphorus;    (5) method (M1) involving either: producing oxygen gas from water at a water pH of 3-11, at an overpotential of less than 0.4 volts at an electrode current density of at least 1 mA/cm2, where the water contains NaCl, the water is obtained from an impure water source, and is not purified in a manner that changes its resistivity by a factor of more than 25% after being drawn from the source prior to use in the electrolysis, the water comprises at least one impurity that is substantially non-participative in the catalytic reaction, present in an amount of at least 1 part per million in the water; or producing oxygen gas from water at an overpotential of less than 0.4 volts at an electrode current density of at least 1 mA/cm2, using water from a water source having a resistivity of less than 16 mega Omega .cm that is not purified in a manner that changes its resistivity by a factor of more than 25% after being drawn from the source prior to use in the electrolysis;    (6) method (M2) of catalytically producing oxygen gas from water, involving: providing an electrochemical system comprising an electrolyte, a first electrode comprising a current collector, metal ionic species, and anionic species, where the current collector does not contain platinum and the current collector has a surface area of greater than about 0.01 m2/g, and a second electrode biased negatively with respect to the first electrode; and causing the electrochemical system to catalyze the production of oxygen gas from water, where the metal ionic species and the anionic species participate in a catalytic reaction involving a dynamic equilibrium in which at least a portion of the metal ionic species are cyclically oxidized and reduced, thus associating and disassociating, respectively, from the current collector, and the system is capable of catalyzing the producing oxygen gas from water with an overpotential of less than about 0.4 volts at an electrode current density of at least 1 mA/cm2;    (7) method (M3) for making an electrode, involving: providing a solution comprising metal ionic species and anionic species; providing a current collector, where the current collector does not contain platinum and the current collector has a surface area of greater than about 0.01 m2/g; and causing the metal ionic species and the anionic species to form a composition associated with the current collector by application of a voltage to the current collector, where the metal ionic species and anionic species are able to catalytically producing oxygen gas from water with an overpotential of less than 0.4 volts at an electrode current density of at least 1 mA/cm2, the metal ionic species and anionic species are able to catalyze water electrolysis at a pH of 5.5-9.5, the composition does not contain metal oxide or metal hydroxide, and the electrode can catalytically produce oxygen gas from water.