Mg-air batteries, with their intrinsic advantages such as high theoretical volumetric energy density, low cost, and environmental friendliness, have attracted tremendous attention for electrical energy storage systems. However, they are still in an early stage of development and suffer from large voltage polarization and poor cycling performance. At present, Mg-air batteries with high rechargeability remain difficult to achieve, mainly because the discharge products [Mg(OH)₂, MgO and MgO₂] are thermodynamically and kinetically difficult to decompose at moderate voltage ranges. Therefore, it is crucial to optimize the reaction paths and kinetics from the electrodes to the batteries via the combination of materials design and first-principles calculations. In this review, remarkable progress is highlighted regarding the currently used materials for Mg-air batteries, including anodes, electrolytes, and cathodes. In addition, the corresponding reaction mechanisms are comprehensively surveyed. Finally, future perspectives for rechargeable Mg-air batteries with decreased voltage polarization and improved cycling performance are also described for further practical applications.

Ambient heat, slightly above or at room temperature, is a ubiquitous and inexhaustible energy source that has typically been ignored due to difficulties in its utilization. Recent evidence suggests that a class of azobenzene (Azo) photoswitches featuring a reversible photoinduced crystal-to-liquid transition could co-harvest photon energy and ambient heat. Thus, a new horizon has been opened for recovering and regenerating low-grade ambient heat. Here, a series of unilateral para-functionalized photoinduced liquefiable Azo derivatives is presented that can co-harvest and convert photon energy and ambient heat into chemical bond energy and latent heat in molecules and eventually release them in the form of high-temperature utilizable heat. A straightforward crystalline-to-liquid phase transition achieved with ultraviolet light irradiation (365 nm) is enabled by appending a halogen/alkoxy group on the para-position of the Azo photoswitches, and the release of thermal energy is triggered by short-wavelength visible-light irradiation (420 nm). The phase transition properties of the trans- and cis-isomers and the energy density, storage lifetime and heat release performance of the cis-liquid are investigated with differential scanning calorimetry, ultraviolet-visible absorption spectroscopy, and an infrared (IR) thermal camera. The experimental results indicate a high energy density of 335 J/g, a long lifetime of 5 d and a heat release of up to 6.3 °C due to the coupled photochemical-thermophysical mechanism. This work presents a new model for utilizing renewable energy, i.e., the photoinduced conversion of ambient thermal energy.

Transition metal molybdates have been studied as anode materials for high-performance lithium-ion batteries, owing to their high theoretical capacity and low cost, as well as the multivalent states of molybdenum. However, their electrochemical performance is hindered by poor conductivity and large volume changes during charge and discharge. Here, we report lithium molybdate (Li₂MoO₄) composited with carbon nanofibers (Li₂MoO₄@CNF) as an anode material for lithium-ion batteries. Li₂MoO₄ shows a shot-rod nanoparticle morphology that is tightly wound in the fibrous CNF. Compared with bare Li₂MoO₄, the Li₂MoO₄@CNF composite demonstrates superior high specific capacity and cycling stability, which are attributed to the reversible Li-ion intercalation in the Li_xMo_yO_z amorphous phase during charge and discharge. The capacity of the Li₂MoO₄@CNF anode material can reach 830 mAh g^-1 in the second cycle and 760 mAh g^-1 after 100 cycles at a charge/discharge current density of 100 mA g^-1, which is much better than the bare Li₂MoO₄. This work provides a simple method to prepare a high-capacity and stable lithium molybdate anode material for lithium-ion batteries.

In recent years, energy storage and conversion have become key areas of research to address social and environmental issues, as well as practical applications, such as increasing the storage capacity of portable electronic storage devices. However, current commercial lithium-ion batteries suffer from low specific energy and high cost and toxicity. Conversion-type cathode materials are promising candidates for next-generation Li metal and Li-ion batteries (LIBs). Metal fluoride materials have shown tremendous chemical tailorability and exhibit excellent energy density in LIBs. Batteries based on such electrodes can compete with other envisaged alternatives, such as Li-air and Li-S systems. However, conversion reactions are typically multiphase redox reactions with mass transport phenomena and nucleation and growth processes of new phases along with interfacial reactions. Therefore, these reactions involve nonequilibrium reaction pathways and significant overpotentials during the charge-discharge process. In this review, we summarize the key challenges facing metal fluoride cathode materials and general strategies to overcome them in cells. Different synthesis methods of metal fluorides are also presented and discussed in the context of their application as cathode materials in Li and LIBs. Finally, the current challenges and future opportunities of metal fluorides as electrode materials are emphasized. With continuous rapid improvements in the electrochemical performance of metal fluorides, it is believed that these materials will be used extensively for energy storage in Li batteries in the future.

The oxygen evolution reaction (OER) is of fundamental importance as a half reaction and rate-controlling step that plays a predominant function in improving the energy storage and conversion efficiency during the electrochemical water-splitting process. In this review, after briefly introducing the fundamental mechanism of the OER, we systematically summarize the recent research progress for nonprecious-metal-based OER electrocatalysts of representative first-row transition metal (Fe, Co and Ni)-based composite materials. We analyze the effects of the physicochemical properties, including morphologies, structures and compositions, on the integrated performance of these OER electrocatalysts, with the aim of determining the structure-function correlation of the electrocatalysts in the electrochemical reaction process. Furthermore, the prospective development directions of OER electrocatalysts are also illustrated and emphasized. Finally, this mini-review highlights how systematic introductions will accelerate the exploitation of high-efficiency OER electrocatalysts.

Hydrogen peroxide (H₂O₂) has been widely used in environmental cleaning, hospital disinfecting and chemical engineering. Compared to the traditional anthraquinone oxidation method, the electrocatalytic two-electron oxygen reduction reaction (2e^-ORR) to produce H₂O₂ has become a promising alternative due to its green, safety and reliability. However, its industrial application is still limited by the slow reaction kinetics and low selectivity due to the competitive reaction of the 4e^-ORR to H₂O. Herein, we prepare a novel photoresponsive metal-free electrocatalyst based on oxidized g-C₃N₄/carbon nanotubes (OCN/CNTs) and introduce an external light field to realize the high-performance electrocatalytic 2e^-ORR to produce H₂O₂. Impressively, the OCN/CNT electrocatalyst exhibits an outstanding H₂O₂ productivity of 30.7 mmol/g_cat/h with a high faradaic H₂O₂ efficiency of 95%. The oxygen-containing groups of the OCN/CNTs promote the adsorption of oxygen intermediates and the photo-coupled electrocatalysis simultaneously improves the electron transport efficiency and enhances the electrocatalytic selectivity.

Commercial carbonate electrolytes with poor oxidation stability and high flammability limit the operating voltage of Li-ion batteries (LIBs) to ~4.3 V. As one of the most promising candidates for electrolyte solvents, sulfolane (SL) has received significant interest because of its wide electrochemical window, low flammability and high dielectric permittivity. Unfortunately, SL-based electrolytes with normal concentrations cannot achieve highly reversible Li⁺ intercalation/deintercalation in graphite anodes due to an ineffective solid electrolyte interface, thus undermining their potential application in LIBs. Here, a low-concentration SL-based electrolyte (LSLE) is developed for high-voltage graphite||LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) full cells. A highly reversible graphite anode can be achieved through the preferential decomposition of the dual-salt LiDFOB-LiBF₄ in the LSLE. The addition of fluorobenzene further restrains the decomposition of SL, endowing uniform, robust and inorganic-rich interphases on the electrode surfaces. As a result, the LSLE with improved thermal stability can support the MCMB||NCM811 full cells at 4.4 V, evidenced by an excellent cycling performance with capacity retentions of 83% after 500 cycles at 25 ℃ and 82% after 400 cycles at 60 ℃. We believe that the design of this fluorobenzene-containing LSLE offers an effective routine for next-generation low-cost and safe electrolytes for high-voltage LIBs.

Semiconductor membrane fuel cells are a new promising R&D for solid oxide fuel cells and proton ceramic fuel cells. There is a challenge of the electronic short circuit issue by using semiconductor to replace conventional electrolyte membrane. In this work, type II band alignment of the semiconductor heterostructure based on Mg-doped ZnO and ZnO can, on one hand, block electrons passing through the junction, and on the other hand, trigger the ionic properties of membrane to boost the fuel cell performance. The Mg doping into ZnO creates more oxygen vacancies at the surface of ZnO, leading to enhanced ionic transport, and meaningful fuel cell performance of 673 mW/cm²; while the Mg-doped ZnO/ZnO heterostructure fuel cell has delivered 997 mW/cm² and OCV 1.04 V at 520 ^oC. It is worth highlighting that the constructed heterostructure interface, especially the band bending and constituted build-in electric field, plays a pivotal role in enhancing the ionic transport and suppressing the electron passing through the internal device. First principal calculations using density functional theory confirmed the doping of Mg and the formation of heterostructure with ZnO to help for enhancing charge carriers and separations. This work suggests that the constructed type II band alignment or the semiconductor heterostructure is useful for developing advanced fuel cells.