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Seeking advanced energy conversion devices, which are sustainable as well as environment friendly, is extremely challenging in our modern society. [3,4] In this regard, the metal-air battery is among the alternative costeffective battery technologies with high energy density, environmentally friendly, and which is composed of an air electrode and a metal electrode, generating electricity through the redox reaction. [5] Among the various metal-air batteries, Li-air and Zn-air batteries have received the most extensive attention. [6] Although Li-air battery exhibits extremely high theoretical energy density (3500 Wh kg −1 ), Li is so reactive that the Li-air battery inevitably faces security risks. [7,8] Iron (Fe) is the fourth most abundant element in the world. In addition, Fe is also a cheap, safe, and environmentally friendly anode material for metal-air batteries. [9] Therefore, compared with the Zn anode, Fe as the anode can reduce battery cost for its abundant reserves. [10] The Fe-air battery has been considered as a low-cost, environmentally benign electrochemical energy-storage system. [11][12][13] Fe-air battery can sustain more charge-discharge cycles than Zn-air battery due to its lower susceptibility to dendrite formation. [8] Intrinsically, the Fe-air battery can store/ release energy repeatedly as needed via oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) but this process is always plagued by slow kinetics of OER/ORR. [14] Therefore, the study on improving reaction kinetics or lowering overpotential of OER/ORR has garnered considerable research interest in recent years. [5,15] Yu et al. used IrO 2 /Ti as the electrode of Fe-air battery to reduce the overpotential of OER and it can achieve a charge voltage of 1.65 V at 1 mA cm −2 . [12] To further reduce the OER/ORR overpotential of metal-air battery, the combination of solar energy and battery is a new research field with fascinating prospects. [16] Solar energy has been utilized in various electrochemical devices for improved reaction kinetics and lowered overpotential, such as hydrogen evolution reaction, water spilling, and pollutant degradation. [17][18][19][20][21] For the aqueous metal-air battery, it usually involves a semiconductor that, upon illumination, could generate Effective utilization of solar energy in battery systems is a promising solution to achieve sustainable and green development. In this work, a photoassisted Fe-air battery (PFAB) with two photoelectrodes of ZnO-TiO 2 heterostructure and polyterthiophene (pTTh)-coated CuO (pTTh-CuO) grown on fluorinedoped tin oxide (FTO) is proposed. The band structure of semiconductors and the charge-transfer mechanism of heterostructure are studied. The electrochemical results show that the photogenerated electrons and holes play key roles in reducing the oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) overpotential in the discharging and charging processes, respectively. The short-circuit current density, the open-circuit voltage, and the maximum power outpu...

Solar energy generation and storage are two distinct processes and integrating them in a single device is of great challenge. Herein, BiOCl hydrogel film electrode featuring excellent photocorrosion and regeneration properties acts as the anode to construct a novel type of smart Solar-Metal-Air Batteries (SMABs), which combine the characteristics of solar cell (direct photovoltaic conversion) and metal-air battery (electric energy storage and release interacting with atmosphere). The cyclic photocorrosion processes between BiOCl (Bi 3+ ) and Bi can simply be achieved by solar light illuminating and standing in dark, corresponding to the charging and discharging processes of the battery, respectively. Upon illumination, the device takes open-circuit configuration to charge itself from the sunlight. Photogenerated electrons in the conduction band of BiOCl reduce Bi 3+ to Bi 0 following the photocorrosion process of BiOCl; and in the meantime, photogenerated positive charges (holes) initiate the oxygen evolution reaction to produce O2. Notably, in this system, the converted solar energy can be stored in the SMABs without the need of external batteries to store the electricity like those for the traditional solar cells. In the discharging process in the dark, Bi 0 spontaneously turns back to Bi 3+ producing electrons to induce oxygen reduction reaction occurring at the counter electrode (Pt/C) like metal-air battery. With an illumination of 15 min, the battery with an electrode area of 1 cm 2 can be continuously discharged for approximately 3,000 s, demonstrating a theoretical capacity of 384.75 mAh•g -1 , which is higher than the theoretical capacity of lithium-ion batteries (LiCoO2, 274 mAh•g -1 ). This novel type of SMABs is developed for the first time based on the unique photocorrosive and self-oxidation reaction of BiOCl to achieve photochemical energy generation and storage. The revealed fundamental mechanism and proposed device design create new solutions to the renewable energy harvesting and storage field. This class of solar light direct-charging battery is an effective step to fulfill the need for green and sustainable energy developments and exhibits great promise for the commercial market.

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