This study reports a novel hydrogel synthesized using only water and the inorganic salts of FeCl3.6H2O and (NH4)6Mo7O24.4H2O, which offers a stable host for various ions (including Li+, Na+, Mg2+, Zn2+, Mn2+, or Ca2+), affording high ionic conductivity. More interestingly, the redox pair Fe2+/Fe3+ of the gel renders considerable pseudo-capacitance, delivering a high volumetric energy density (4.8 mWh cm−3, based on the one-piece half-cell) and cycling stability. This simple one-piece approach is convenient and effective—by pairing the mineral gel-based half-cell with another matching electrode, a novel type of charge storage device is formed, with the gel serving as one electroactive material, the electrolyte, and the membrane separator. Furthermore, the mineral hydrogel reported here is of low cytotoxicity, self-bondable and healable, and highly resistant against swelling and disintegrating, with no collapse or volume expansion observed even after being soaked in water for 60 days. To our knowledge, this is the first time that mineral hydrogels have been synthesized from all-inorganic agents in a fully biocompatible setting, which also sheds light on the myth-ridden topic of pre-cell evolution in the prebiotic age.
