Iron molybdates show many interesting properties due to their special structures and flexible chemical valences. Moreover, amorphous iron molybdates (Fe(OH)(MoO₄)•H₂O) can be facilely and quickly prepared by mixing two salt solutions containing Fe³⁺ and MoO4^2- respectively: Fe³⁺ and MoO4^2- are bridged by hydroxyl; the abundant oxygen atoms of the metal-oxygen cluster (MoO₄^2-) can easily hydrogen-bonded with the hydrogen atom of water, thus partially disrupting the hydrogen-bonding network of water. Fe(OH)(MoO₄)•H₂O can further crystalize into different phases of Fe₂(MoO₄)₃ (e.g., orthorhombic, monoclinic, or triclinic), where MoO₆ octahedra and FeO₄ tetrahedra are both included, and the arrangement of these polyhedra greatly determines the properties of the host phase. For example, the corner-shared polyhedral network of the orthorhombic phase generally has a smaller packing density. On the other hand, the iron can be Fe²⁺ or Fe³⁺, or both, and the different ratios of Fe²⁺ to Fe³⁺ can generate different phases. These properties warrant iron molybdates many stunning performances in energy storage, anti-freezing, and phase transformation.

In this thesis, various iron molybdates (FeMo₂O_x•nH₂O) with different morphologies (nanowires or nanoparticles) and states (gel, paste or precipitate) were prepared by a biocompatible sol-gel method: just mixing the solutions of two common inorganic salts FeCl₃•6H₂O and (NH₄)₆Mo₇O₂₄•4H₂O. We observed many interesting phenomena: a mineral hydrogel with a redox center could be an all-in-one charge storage device; mineral gels directly turned into residual stress-strengthened monolithic ceramics by dehydration; MoO₄^2- can also work as a sacrificial anion in alkaline to self-reconstruct unary and binary Fe-based electrocatalyst for OER with high performance. This thesis explores all these impressing findings and investigates the corresponding mechanisms.

Chapter I introduces the definition, classification, crystalline structures, intrinsic properties, and applications of iron molybdates.

Chapter II presents chemicals, synthetic methodology, characterization, and analysis methods involved in this thesis.

Chapter III reports a novel mineral hydrogel (FeMo₂O_x(OH)_y•nH₂O) synthesized from all-inorganic agents in a fully biocompatible setting, 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 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.

In Chapter IV, we report that ceramic objects can be directly produced and strengthened by drying purely inorganic hydrogels (PIGs): we imitated the stunning biological tactic to fabricate continuous monoliths from amorphous precursors. The whole process is easy and biocompatible: two common salt solutions are mixed to generate a PIG, which, upon drying under mild temperature, turns into a strengthened ceramic block of high mechanical performance. Analogous to the famous Prince Rupert’s tear reinforced by the thermal residual stress upon quenching, the uneven volume shrinkage from outside inward during the dehydration builds up residual stress that in turn enables mineral fusion and strengthening, and dramatic bandgap reduction due to the local structural changes of the Fe atoms. This PIG-dehydration approach holds promise for green ceramic manufacturing.

In Chapter V, unary Fe- and binary FeNi- based catalysts, FeOOH and FeNiO_x(OH)_y, were successfully prepared by self-reconstruction of Fe₂(MoO₄)₃ and FeNiMo_2.4O_x. The highest OER performance of evo-FeOOH among all the unary iron oxides- and hydroxides- based powder catalysts reported to date support Fe can be catalytically active for OER. Besides, Fe was found to be oxidized to 3.5+ during the *OOH process in the binary FeNiO_x(OH)_y, thus, Fe is identified to be the active site in this new layered double hydroxide (LDH) structure with Fe:Ni=1:1. Furthermore, FeNiO_x(OH)_y@NF (nickel foam) serves as low-cost bifunctional electrodes for overall water-splitting, delivering excellent performance comparable to commercial electrodes based on precious metals, which overcomes a major obstacle to the commercialization of bifunctional electrodes: prohibitive cost.

Chapter VI gives a conclusion and outlook for this thesis.