In another aspect, the metal used in combination with either the first and/or the second metal will have at least one of the following properties: no phase change occurs during the redox reaction with the other metal, no side reactions or decomposition reactions occur during the redox reaction with the other metal, the metal is compatible with an acidic supporting electrolyte, and the ΔE will be at least 0.5 V. In some aspects, the metal used with either the first and/or the second metal will have two or more of these properties. In some aspects, the metal used with either the first and/or the second metal will have three or more of these properties. In some aspects, the metal used with either the first and/or the second metal will have all four of these properties. For example, the first metal is selected (e.g., titanium), and the second metal is selected such that it exhibits one or more of the above listed properties. In another non-limiting example, the second metal is selected (e.g., cerium), and the first metal is selected such that it exhibits one or more of the above listed properties.

In some aspects, the first electrolyte and the second electrolyte have at least one ion in common. In some aspects, the first electrolyte and the second electrolyte have at least one anion in common. In some aspects, the first electrolyte and/or the second electrolyte is an acid. Acid selection is done as described elsewhere herein, but in all cases the acid must be electrochemically stable. As used herein, “electrochemically stables” means that a 1 M solution of the acid does not undergo a decomposition reaction at a voltage below 1.5V relative to a standard hydrogen electrode. In some aspects, the acid is stable at a voltage below 2.0V. For example, as illustrated in FIG. 2A, hydrochloric acid exhibits a decomposition peak at about 1.5V in the cyclic voltammogram.