Photo-Assisted Rechargeable Metal Batteries: Principles, Progress, and Perspectives

The utilization of diverse energy storage devices is imperative in the contemporary society. Taking advantage of solar power, a significant environmentally friendly and sustainable energy resource, holds great appeal for future storage of energy because it can solve the dilemma of fossil energy depletion and the resulting environmental problems once and for all. Recently, photo-assisted energy storage devices, especially photo-assisted rechargeable metal batteries, are rapidly developed owing to the ability to efficiently convert and store solar energy and the simple configuration, as well as the fact that conventional Li/Zn-ion batteries are widely commercialized. Considering many puzzles arising from the rapid development of photo-assisted rechargeable metal batteries, this review commences by introducing the fundamental concepts of batteries and photo-electrochemistry, followed by an exploration of the current advancements in photo-assisted rechargeable metal batteries. Specifically, it delves into the elucidation of device components, operating principles, types, and practical applications. Furthermore, this paper categorizes, specifies, and summarizes several detailed examples of photo-assisted energy storage devices. Lastly, it addresses the challenges and bottlenecks faced by these energy storage systems while providing future perspectives to facilitate their transition from laboratory research to industrial implementation.

Synergistic Coupling of Charge Extraction and Sinking in Cu5FeS4/Ni3S2@NF for Photoassisted Electrocatalytic Oxygen Evolution

Exploring low-cost and high-performance oxygen evolution reaction (OER) catalysts has attracted great attention due to their crucial role in water splitting. Here, a bifunctional Cu5FeS4/Ni3S2@NF catalyst was in situ formed on a nickel (Ni) foam toward efficient photoassisted electrocatalytic (P-EC) OER, which displays an ultralow overpotential of 260 mV at 30 mA cm-2 in alkaline solution, outperforming most previously reported Ni-based catalysts. It also shows great potential in degradation of antibiotics as an alternative anode reaction to OER owing to the prompt transfer of photogenerated holes. The photocurrent test and transient photovoltage spectroscopy indicate that the synergistic coupling of charge extraction and sinking effects in Cu5FeS4 and Ni3S2 is critical for boosting the OER activity via photoassistance. Electrochemical active surface area and electrochemical impedance spectroscopy tests further prove that the photogenerated electromotive force can effectively compensate the overpotential of OER. This work not only provides a good guidance for integrating photocatalysis and electrocatalysis, but also indicates the key role of synergistic extraction and utilization of photogenerated charge carriers in P-EC.

Light-Assisted Metal-Air Batteries: Progress, Challenges, and Perspectives

Metal-air batteries are considered one of the most promising next-generation energy storage devices owing to their ultrahigh theoretical specific energy. However, sluggish cathode kinetics (O₂ and CO₂ reduction/evolution) result in large overpotentials and low round-trip efficiencies which seriously hinder their practical applications. Utilizing light to drive slow cathode processes has increasingly becoming a promising solution to this issue. Considering the rapid development and emerging issues of this field, this Review summarizes the current understanding of light-assisted metal-air batteries in terms of configurations and mechanisms, provides general design strategies and specific examples of photocathodes, systematically discusses the influence of light on batteries, and finally identifies existing gaps and future priorities for the development of practical light-assisted metal-air batteries.

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

Zinc-Air Battery-Based Self-Powered Sensor with High Output Power for Ultrasensitive MicroRNA let-7a Detection in Cancer Cells

Self-powered sensors do not require a power supply and are easy to miniaturize, which have potential for constructing wearable, portable, and real-time detection devices. However, it is challenging for the detection of low abundant targets due to the low output power density of fuel cells and much interference of complex biological environment. Herein, a new kind of photocatalytic zinc-air battery-based self-powered electrochemical sensor (ZAB-SPES) was constructed for the detection of microRNA let-7a (miRNA let-7a) by combining magnetic nanobeads (MBs) with a metal-organic framework loaded with glucose oxidase (MOFs@GOX). Poly(1,4-di(2-thienyl))benzene (PDTB) was used as the photocathode material, and the proposed ZAB-SPES had a high power density of 22.8 μW/cm², which was 2-3-fold of commonly used photofuel cells. MBs can capture and separate miRNA from complex samples quickly with a high separation efficiency of 99% within 60 s. The competitive reaction of oxygen reduction reaction between PDTB and MOFs@GOX would change the output power density of the ZAB-SPES. Based on the relationship between output power density and target concentration, the ZAB-SPES realized ultrasensitive detection of miRNA let-7a with a detection limit down to 1.38 fM. Furthermore, the successful detection of miRNA let-7a in A549 cancer cells indicated the great prospects of ZAB-SPES in clinical analysis and early diagnosis of cancers.

Light-Assisted Li-O2 Batteries with Lowered Bias Voltages by Redox Mediators

The enormous overpotential caused by sluggish kinetics of the oxygen reduction reaction and the oxygen evolution reaction prevents the practical application of Li-O₂ batteries. The recently proposed light-assisted strategy is an effective way to improve round-trip efficiency; however, the high-potential photogenerated holes during the charge would degrade the electrolyte with side reactions and poor cycling performance. Herein, a synergistic interaction between a polyterthiophene photocatalyst and a redox mediator is employed in Li-O₂ batteries. During the discharge, the voltage can be compensated by the photovoltage generated on the photoelectrode. Upon the charge with illumination, the photogenerated holes can be consumed by the oxidization of iodide ions, and thus the external circuit voltage is compensated by photogenerated electrons. Accordingly, a smaller bias voltage is needed for the semiconductor to decompose Li₂ O₂ , and the potential of photogenerated holes decreases. Finally, the round-trip efficiency of the battery reaches 97% with a discharge voltage of 3.10 V and a charge voltage of 3.19 V. The batteries show stable operation up to 150 cycles without increased polarization. This work provides new routes for light-assisted Li-O₂ batteries with reduced overpotential and boosted efficiency.

Photoelectrochemistry of oxygen in rechargeable Li-O2 batteries

Rechargeable lithium-oxygen (Li-O₂) batteries are promising energy storage devices due to their high theoretical energy density. However, the sluggish kinetics of the oxygen reduction and evolution reactions (ORR/OER) at the cathodes results in large polarization and low energy efficiency. Although advances have been achieved in electrode material designs and battery configurations, large discharge/charge voltage gaps remain. The introduction of light into Li-O₂ batteries has been demonstrated to boost the reaction kinetics of the ORR/OER, leading to enhanced electrochemical performances, but the understanding of the photoelectrochemical process at oxygen cathodes is limited. This tutorial review focuses on the recent findings regarding photoinvolved oxygen cathodes, battery configurations, and the stability of Li-O₂ batteries, aiming to provide a fundamental understanding of photoinvolved Li-O₂ batteries. The challenges and perspectives are discussed in light of the interdisciplinary nature of photochemistry, materials chemistry, electrochemistry, computation, spectroscopy, and surface science.

Photoelectrochemical energy storage materials: design principles and functional devices towards direct solar to electrochemical energy storage

Advanced solar energy utilization technologies have been booming for carbon-neutral and renewable society development. Photovoltaic cells now hold the highest potential for widespread sustainable electricity production and photo(electro)catalytic cells could supply various chemicals. However, both of them require the connection of energy storage devices or matter to compensate for intermittent sunlight, suffering from complicated structures and external energy loss. Newly developed photoelectrochemical energy storage (PES) devices can effectively convert and store solar energy in one two-electrode battery, simplifying the configuration and decreasing the external energy loss. Based on PES materials, the PES devices could realize direct solar-to-electrochemical energy storage, which is fundamentally different from photo(electro)catalytic cells (solar-to-chemical energy conversion) and photovoltaic cells (solar-to-electricity energy conversion). This review summarizes a critically selected overview of advanced PES materials, the key to direct solar to electrochemical energy storage technology, with the focus on the research progress in PES processes and design principles. Based on the specific discussions of the performance metrics, the bottlenecks of PES devices, including low efficiency and deteriorative stability, are also discussed. Finally, several perspectives of potential strategies to overcome the bottlenecks and realize practical photoelectrochemical energy storage devices are presented.

A Dual Photoelectrode Photoassisted Fe-Air Battery: The Photo-Electrocatalysis Mechanism Accounting for the Improved Oxygen Evolution Reaction and Oxygen Reduction Reaction of Air Electrodes

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-TiO₂ heterostructure and polyterthiophene (pTTh)-coated CuO (pTTh-CuO) grown on fluorine-doped 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 output of the PFAB can reach 34.28 mA cm^-2 , 1.15 V, and 5.69 mW cm^-2 upon illumination, respectively. The photoassisted Fe-air battery exhibits a low charge voltage of 0.64 V for ZnO-TiO₂ as photoelectrode and a discharge voltage of 1.38 V for pTTh-CuO as a photoelectrode at 0.1 mA cm^-2 .

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.

Semiconductor Electrochemistry for Clean Energy Conversion and Storage

Semiconductors and the associated methodologies applied to electrochemistry have recently grown as an emerging field in energy materials and technologies. For example, semiconductor membranes and heterostructure fuel cells are new technological trend, which differ from the traditional fuel cell electrochemistry principle employing three basic functional components: anode, electrolyte, and cathode. The electrolyte is key to the device performance by providing an ionic charge flow pathway between the anode and cathode while preventing electron passage. In contrast, semiconductors and derived heterostructures with electron (hole) conducting materials have demonstrated to be much better ionic conductors than the conventional ionic electrolytes. The energy band structure and alignment, band bending and built-in electric field are all important elements in this context to realize the necessary fuel cell functionalities. This review further extends to semiconductor-based electrochemical energy conversion and storage, describing their fundamentals and working principles, with the intention of advancing the understanding of the roles of semiconductors and energy bands in electrochemical devices for energy conversion and storage, as well as applications to meet emerging demands widely involved in energy applications, such as photocatalysis/water splitting devices, batteries and solar cells. This review provides new ideas and new solutions to problems beyond the conventional electrochemistry and presents new interdisciplinary approaches to develop clean energy conversion and storage technologies.

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Capturing Visible Light in Low-Band-Gap C4 N-Derived Responsive Bifunctional Air Electrodes for Solar Energy Conversion and Storage

We report facile synthesis of low-band-gap mesoporous C₄ N particles and their use as responsive bifunctional oxygen catalysts for visible-light-sensitive (VLS) rechargeable Zn-air battery (RZAB) and polymer-air battery (RPAB). Compared to widely studied g-C₃ N₄ , C₄ N shows a smaller band gap of 1.99 eV, with a larger photocurrent response, and it can function as visible-light-harvesting antenna and bifunctional oxygen reduction/evolution (ORR/OER) catalysts, enabling effective photocoupling to tune oxygen catalysis. The C₄ N-enabled VLS-RZAB displays a low charge voltage of 1.35 V under visible light, which is below the theoretical RZAB voltage of 1.65 V, corresponding to a high energy efficiency of 97.78 %. Pairing a C₄ N cathode with a polymer anode also endows an VLS-RPAB with light-boosted charge performance. It is revealed that the ORR and OER active sites in C₄ N are separate carbon sites near pyrazine-nitrogen atoms and photogenerated energetic holes can activate OER for improved reaction kinetics.

Solar-enhanced hybrid lithium-oxygen batteries with a low voltage and superior long-life stability

Lithium-oxygen batteries (LOBs) can suffer from large charge overpotentials during the sluggish oxygen evolution reaction (OER). Here, a strategy used in the photo-electrochemical (PEC) water oxidation area is integrated into hybrid LOBs. Coating α-Fe₂O₃ with NiOOH results in enhanced electrochemical properties, and the as-assembled hybrid LOBs deliver a low charge voltage of 3.03 V, high energy efficiency of 88%, and long-term stability for over 350 hours.

Recent advances in electrochemical 2e oxygen reduction reaction for on-site hydrogen peroxide production and beyond

The electroproduction of H₂O₂ through 2e oxygen reduction reaction (ORR) as an alternative strategy for the conventional anthraquinone process is highly energy-efficient and environment-friendly. Different kinds of electrocatalysts with high selectivity, activity, and stability have been recently reported, and are an essential part of the whole electroproduction process of H₂O₂. In this review, we expound the ORR mechanism and introduce some methods to screen out potential electrocatalysts through theoretical calculations and experimental verifications. In addition, recent advances in reactor design for large-scale on-site production of H₂O₂ and integrated systems for electricity-H₂O₂ co-generation are mentioned. With ideal electrocatalysts and rational reactor design, different concentrations of H₂O₂ can be obtained depending on the practical applications. Utilizing the solar or chemical energy, it can promote energy efficiency and sustainability of the process. Finally, we make a brief conclusion about recent developments in electrocatalysts, device design, as well as integrated systems, and give an outlook for future research challenges, which are meaningful for advancing the electrochemical on-site production of H₂O₂via 2e ORR to the marketplace.

A competitive self-powered sensing platform based on a visible light assisted zinc-air battery system

We developed a novel competitive self-powered sensing platform based on the discharge process of a visible light assisted zinc-air battery system for the detection of targets with a high open circuit potential signal (E^ocpt = 1.5 V), which is a splendid signal among state-of-the-art self-powered sensing platforms.

Integrated Photo-Responsive Batteries for Solar Energy Harnessing: Recent Advances, Challenges, and Opportunities

Photo-responsive batteries that enable the effective combination of solar harvesting and energy conversion/storage functionalities render a potential solution to achieve the large-scale utilization of unlimited and cost-effective solar energy and alleviate the limits of conventional energy storage devices. The internal integration of photo-responsive electrodes into rechargeable batteries with the simplest two-electrode configuration is regarded as a reliable and appealing strategy for highly-efficient and low-cost utilization of solar energy by simplifying the device architecture and improving the energy efficiency. This progress report provides a brief review on photo-responsive batteries with integrated two-electrode configuration that can achieve solar energy conversion/storage in one single device. The basic device architecture, operating principles and practical performance of various photo-responsive systems based on solar energy harvesting in various batteries including Li ion batteries, Li-S batteries, Li-I batteries, dual-liquid redox batteries, Li-O₂ batteries, non-Li anode-O₂ /air batteries are summarized and discussed. Finally, the future opportunities and challenges regarding the two-electrode photo-responsive batteries are proposed.

Simultaneous Photoelectrocatalytic Water Oxidation and Oxygen Reduction for Solar Electricity Production in Alkaline Solution

The photoelectrochemical (PEC) method offers an alternative approach to photovoltaic devices for solar electricity generation. The water oxidation reaction (WOR) on the anode and oxygen reduction reaction (ORR) on the cathode is an ideal design for energy transfer owing to their superiority in terms of cleanliness, eco-friendliness, and natural abundance. However, solar electricity production based on O₂ circulation by a fuel-free PEC cell is very challenging because it is extremely hard to extract electrons from water molecules owing to the uphill and sluggish WOR together with enormous overpotential for the cathodic ORR. Herein, a PEC cell based on the OH^- /O₂ redox pair is reported for efficient and sustainable solar electricity production by using two photoelectrodes of TiO₂ and polyterthiophene in alkaline electrolyte. This fuel-free PEC cell delivers an open-circuit voltage up to 0.90 V and a maximum power density of 222 μW cm^-2 with O₂ -saturated NaOH electrolyte under AM 1.5 G solar irradiation. A record solar-to-electricity conversion efficiency of 0.22 % is achieved in the case of tandem illumination of the two photoelectrodes. In addition, the dual photoelectrode remains robust in accelerated and day-night cycling operation under natural atmosphere for more than a week. This PEC cell is free of fuel, separating membranes, and cocatalyst, which may guide future designs for clean and simple devices for solar energy conversion.

Origin and Perspectives of the Photochemical Activity of Nanoporous Carbons

Even though, owing to the complexity of nanoporous carbons' structure and chemistry, the origin of their photoactivity is not yet fully understood, the recent works addressed here clearly show the ability of these materials to absorb light and convert the photogenerated charge carriers into chemical reactions. In many aspects, nanoporous carbons are similar to graphene; their pores are built of distorted graphene layers and defects that arise from their amorphicity and reactivity. As in graphene, the photoactivity of nanoporous carbons is linked to their semiconducting, optical, and electronic properties, defined by the composition and structural defects in the distorted graphene layers that facilitate the exciton splitting and charge separation, minimizing surface recombination. The tight confinement in the nanopores is critical to avoid surface charge recombination and to obtain high photochemical quantum yields. The results obtained so far, although the field is still in its infancy, leave no doubts on the possibilities of applying photochemistry in the confined space of carbon pores in various strategic disciplines such as degradation of pollutants, solar water splitting, or CO2 mitigation. Perhaps the future of photovoltaics and smart-self-cleaning or photocorrosion coatings is in exploring the use of nanoporous carbons.

Photoelectrochemical Response of WO3/Nanoporous Carbon Anodes for Photocatalytic Water Oxidation

This work demonstrates the ability of nanoporous carbons to boost the photoelectrochemical activity of hexagonal and monoclinic WO3 towards water oxidation under irradiation. The impact of the carbonaceous phase was strongly dependent on the crystalline structure and morphology of the semiconductor, substantially increasing the activity of WO3 rods with hexagonal phase. The incorporation of increasing amounts of a nanoporous carbon of low functionalization to the WO3 electrodes improved the quantum yield of the reaction and also affected the dynamics of the charge transport, creating a percolation path for the majority carriers. The nanoporous carbon promotes the delocalization of the charge carriers through the graphitic layers. We discuss the incorporation of nanoporous carbons as an interesting strategy for improving the photoelectrochemical performance of nanostructured semiconductor photoelectrodes featuring hindered carrier transport.

Optically Matched Semiconductor Quantum Dots Improve Photophosphorylation Performed by Chloroplasts

A natural-artificial hybrid system was constructed to enhance photophosphorylation. The system comprises chloroplasts modified with optically matched quantum dots (chloroplast-QD) with a large Stokes shift. The QDs possess a unique optical property and transform ultraviolet light into available and highly effective red light for use by chloroplasts. This favorable feature enables photosystem II contained within the hybrid system to split more water and produce more protons than chloroplasts would otherwise do on their own. Consequently, a larger proton gradient is generated and photophosphorylation is improved. At optimal efficiency activity increased by up to 2.3 times compared to pristine chloroplasts. Importantly, the degree of overlap between emission of the QDs and absorption of chloroplasts exerts a strong influence on the photophosphorylation efficiency. The chloroplast-QD hybrid presents an efficient solar energy conversion route, which involves a rational combination of a natural system and an artificial light-harvesting nanomaterial.

Atomic-Level Co3O4 Layer Stabilized by Metallic Cobalt Nanoparticles: A Highly Active and Stable Electrocatalyst for Oxygen Reduction

Developing atomic-level transition oxides may be one of the most promising ways for providing ultrahigh electrocatalytic performance for oxygen reduction reaction (ORR), compared with their bulk counterparts. In this article, we developed a set of atomically thick Co₃O₄ layers covered on Co nanoparticles through partial reduction of Co₃O₄ nanoparticles using melamine as a reductive additive at an elevated temperature. Compared with the original Co₃O₄ nanoparticles, the synthesized Co₃O₄ with a thickness of 1.1 nm exhibits remarkably enhanced ORR activity and durability, which are even higher than those obtained by a commercial Pt/C in an alkaline environment. The superior activity can be attributed to the unique physical and chemical structures of the atomic-level oxide featuring the narrowed band gap and decreased work function, caused by the escaped lattice oxygen and the enriched coordination-unsaturated Co²⁺ in this atomic layer. Besides, the outstanding durability of the catalyst can result from the chemically epitaxial deposition of the Co₃O₄ on the cobalt surface. Therefore, the proposed synthetic strategy may offer a smart way to develop other atomic-level transition metals with high electrocatalytic activity and stability for energy conversion and storage devices.

Heterojunctions of silver–iron oxide on graphene for laser-coupled oxygen reduction reactions

We report a two-step hybridization of N-doped graphene and Ag-decorated Fe₂O₃ hematite to realize a balanced oxygen adsorption/desorption equilibrium and a laser-coupled ORR (LORR).

In situ hybridization of CoOX nanoparticles on N-doped graphene through one step mineralization of co-responsive hydrogels

Involving GO into Co-PT hydrogels can enhance the strength of hydrogels. The electrocatalytic properties of CoO_X/N-rGO700 aerogels are improved by annealing.