Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox‐active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need‐tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide–based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

Consecutive layers of Ni(OH)₂ and Co(OH)₂ were electrodeposited on stainless steel current collectors for preparing charge storage electrodes of high specific capacity with potential application in hybrid supercapacitors. Different electrodes were prepared consisting on films of Ni(OH)₂, Co(OH)₂, Ni_1/2Co_1/2(OH)₂ and layered films of Ni(OH)₂ on Co(OH)₂ and Co(OH)₂ on Ni(OH)₂ to highlight the advantages of the new architecture. The microscopy studies revealed the formation of nanosheets in the Co(OH)₂ films and of particles agglomerates in the Ni(OH)₂ films. Important morphological changes were observed in the double hydroxides films and layered films. Film growth by electrodeposition was governed by instantaneous nucleation mechanism. The new architecture composed of Ni(OH)₂ on Co(OH)₂ displayed a redox response characterized by the presence of two peaks in the cyclic voltammograms, arising from redox reactions of the metallic species present in the layered film. These electrodes revealed a specific capacity of 762 C g⁻¹ at the specific current of 1 A g⁻¹. The hybrid cell using Ni(OH)₂ on Co(OH)₂ as positive electrode and carbon nanofoam paper as negative electrode display specific energies of 101.3 W h g⁻¹ and 37.8 W h g⁻¹ at specific powers of 0.2 W g⁻¹ and 2.45 W g⁻¹, respectively.

This work reports the preparation of γ-FeOOH and amorphous Ni–Mn hydroxide on carbon nanofoam paper (CNFP) by galvanostatic electrodeposition for application in hybrid FeOOH-CNFP∥Ni–Mn hydroxide-CNFP supercapacitors working in alkaline electrolytes. The γ-FeOOH on CNFP displayed a porous morphology composed of fluffy nanoflakes. It showed an enhanced redox response in the negative working potential range of −1.5–0 V vs. SCE with a storage capacity of 3.48 C cm⁻² at 10 mA cm⁻² and a response rate of 71% when the current densities increased from 10 mA cm⁻² to 50 mA cm⁻². The amorphous Ni–Mn hydroxide on CNFP displayed a porous morphology composed of fine nanoflakes. It showed an enhanced redox response in the positive working potential range of 0–0.5 V vs. SCE with a storage capacity of 1.54 C cm⁻² at 10 mA cm⁻² and a response rate of 73% when the current densities increased from 10 mA cm⁻² to 50 mA cm⁻². The hybrid FeOOH-CNFP∥Ni–Mn hydroxide-CNFP supercapacitor delivered a high energy density of 1515 mW h cm⁻², at a power density of 9 mW cm⁻² at a working voltage of 1.8 V and a long cycle life with a capacity retention of 95% after 10 000 charge–discharge cycles. Self-discharge studies revealed the main contribution of the FeOOH-CNFP electrode to the voltage drop of the supercapacitor cell. Activation-controlled processes governed the self-discharge of the FeOOH-CNFP electrode, while mixed activation-controlled and diffusion-controlled processes governed the self-discharge of the Ni–Mn hydroxide-CNFP electrode.

Structural evolution of electrodeposited Mn–Co hydroxide into spinel MnCo₂O₄nanosheets by thermal annealing.

Nickel manganese (Ni–Mn) oxide films were prepared by potentiostatic electrodeposition over stainless steel and post-thermal annealing at 250 °C. Morphological and structural changes were observed depending on the Ni to Mn ratio in the electrolyte. Single-phase Ni_1−xMn_xO oxide was formed at low Mn content, whereas two phases composed of Ni_1−xMn_xO and Ni_xMn_3−xO₄ were formed at high Mn content. Electrochemical studies revealed an increase of the specific capacitance of the mixed Ni–Mn oxide electrodes compared to the single metal oxides, thus pin-pointing a synergistic effect. The Ni–Mn oxide film displayed a specific capacitance of about 300 F g⁻¹ in a potential window of 0.5 V at an applied current density of 1 A g⁻¹. The rate capability of the oxides was 58% when the applied current was increased from 1 A g⁻¹ to 10 A g⁻¹. The Ni–Mn oxide film exhibits excellent cycling stability with 100% capacitance retention after 1500 cycles.

This work reports the fabrication, by electrodeposition and post-thermal annealing, of hybrid electrodes for high rate electrochemical energy storage composed of nickel manganese oxide (Ni_0.86Mn_0.14O) nanosheets over 3D open porous dendritic NiCu foams. The hybrid electrodes are made of two different percolation networks of nanosheets and dendrites, and exhibit a specific capacitance value of 848 F g⁻¹ at 1 A g⁻¹. The electrochemical tests revealed that the electrodes display an excellent rate capability, characterized by capacitance retention of approximately 83% when the applied current density increases from 1 A g⁻¹ to 20 A g⁻¹. The electrodes also evidenced high charge–discharge cycling stability, which attained 103% after 1000 cycles.