As another example, FIG. 4 provides a schematic overview of a solid-state energy storage device 400 making use of hydrogen ions (protons, H⁺) as the working ion. The configuration depicted in FIG. 4 is not to scale and is provided as an example to facilitate description of the general structure processes taking place. A first electrode 405 is in contact with a solid electrolyte 410, which is in contact with a second electrode 415. Second electrode 415 is exemplified as including a metal hydride (MH₂) and solid electrolyte 410 is exemplified as a proton conducting ceramic. First electrode 405 is exemplified as a composite structure including a water accommodating transition metal oxide active material and a gas storage structure, such as a gas-absorbing solid-state material. Optionally, the gas storage structure may flank and/or be in gaseous communication with the solid electrolyte 410 for providing gas to interstices within a crystalline structure of the solid electrolyte 410. For example, the gas storage structure may be or comprise an oxygen absorbing solid-state material, such as an O₂ sponge material. For example, the water accommodating transition metal oxide active material may be in gaseous communication or contact with the O₂ sponge material. It will be appreciated that the O₂ sponge material may take up oxygen gas generated during charging of the energy storage device 400 and may provide oxygen gas as needed during discharging of the energy storage device 400. Example O₂ sponge materials include, but are not limited to, cobalt-, cobalt salt-, cerium-, and ceria-based oxygen absorbing materials, such as strontium cobaltite, and [{(bpbp)Co₂^II(NO₃)}₂(NH₂bdc)](NO₃)₂·H₂O, where “bpbp” is 2,6-bis(N,N-bis(2-pyridylmethyl)-aminomethyl)-4-tert-butylphenolato, and “NH₂bdc” is 2-amino-1,4-benzenedicarboxylato. See, for example, Chem. Sci., 2014, 5, 4017-4025 (DOI:10.1039/C4SC01636J), hereby incorporated by reference, which describes crystalline salts of a series of cationic multimetallic cobalt complexes that reversibly, selectively and stoichiometrically chemisorb dioxygen in a process involving the two electron oxidation of dimetallic sites with concurrent reduction of two equivalents of sorbed O₂ to form μ-η¹,η²-peroxide ligands. The coordinating ability of counteranions, ClO₄^?, PF₆^?, BF₄^?, CF₃SO₃^? and NO₃^? determine the O₂ affinity of the deoxygenated forms. These crystals undergo reversible single-crystal-to-single-crystal transformations on the stoichiometric uptake of O₂. During this process O₂ replaces the two counterion ligands. Thus the Co ions are six coordinated in both the oxy and deoxy states. Rapid hydration/dehydration processes involving several molecules of co-crystallized water per unit cell accompany the reaction. The following cationic oxy complexes can be synthesized by mixing the dinucleating pro-ligand, bpbpH, the appropriate carboxylic acid and base in stoichiometric amounts in mixtures of methanol, acetone and H₂O under aerobic conditions: [(bpbp)Co₂(O₂)(O₂CR)](A)₂ where R is methyl, phenyl, CH₂Cl, CHCl₂, CCl₃, or CH₂CH₂S^? and [{(bpbp)Co₂(O₂)}₂(bdcR₄)](A)₄ where R is