Introduction: Over the past few decades, there has been increasing interest in the design and construction of energy conversion and storage systems that can simultaneously convert and store various forms of energies from nature. A large number of energy conversion and storage systems have been developed with different energy conversion technologies such as solar cells and energy storage devices such as rechargeable batteries and supercapacitors. This work reports the characterization and photoelectrochemical studies on two different photocapacitive devices fabricated with Activated Carbon (AC)/Silver-Metal Oxide/SnS heterostructured photoanodes (AC/Ag₂MoO₄/SnS and AC/Ag₂CrO₄/SnS) which allow direct solar energy harvesting and storage. In these devices, Silver-Metal Oxide provides the photoactive core of the device, while SnO_x nanoparticles (formed by the controlled oxidation of SnS nanoparticles) deliver a capacitive platform by redox reaction. The synergistic coupling of these two systems leads to high capacitance under illumination, which could be utilized subsequently.

Methods and Results: Initially, Ag₂MoO₄ and SnS were synthesized by a simple hydrothermal method whereas Ag₂CrO₄ nanostructures were synthesized using ultrasonication technique. The synthesized nanostuctures were used to develop the AC/Ag₂MoO₄/SnS and AC/Ag₂CrO₄/SnS photoanodes by doctor blade coating method. Then, the developed photoanodes were characterized by SEM, and XRD, and UV-Visible spectroscopies. The SEM images of Ag₂MoO₄ and Ag₂CrO₄ films on AC revealed peeled potato-like and rod-like morphologies respectively while the corresponding photoanodes showed flake-like structures. The XRD pattern of bare Ag₂MoO₄ indicated cubic spinel structure whereas those of bare Ag₂CrO₄ and bare SnS revealed orthorhombic structure. From the UV-Visible spectroscopy, optical band gaps of Ag₂MoO₄, Ag₂CrO₄ and SnS were calculated as 3.32, 1.94 and 1.74 eV respectively using direct absorption edge in the Kubelka-Munk plots. Subsequently, Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were carried out with a three electrode system consisting of the developed photoanode as working electrode, Pt as counter electrode, and Ag/AgCl as reference electrode with 0.1 M sodium phosphate buffer solution as the electrolyte. The heterostructured AC/Ag₂MoO₄/SnS and AC/Ag₂CrO₄/SnS photoanodes exhibited specific capacitances of 20 and 54 mF/g respectively at the scan rate of 50 mVs^-1 when the devices were subjected to visible light illumination.

Conclusions: The high visible light harvesting behaviour demonstrated by the novel photocapacitors fabricated with the above heterostructured photoanodes may be attributed to the large specific area and high conductivity of the Silver-Metal Oxide/SnS films employed in this study and will lead to promising research avenues in the development of novel solar energy harvesting and storage strategies.

Keywords: Solar energy harvest and storage, Silver-Metal Oxide, Specific capacitance

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