Sustainable production of H2 through electrochemical water splitting is of great importance in the foreseeable future. Transition-metal metaphosphates (TMMPs) have a three-dimensional (3D) open-framework structure and a high content of P (which exists as PO3−), and therefore have been recognized as highly efficient catalysts for oxygen evolution reaction (OER) and the bottleneck of electrochemical water splitting. Furthermore, TMMPs can also contribute to hydrogen evolution reaction (HER) in alkaline and neutral media by facilitating water dissociation, and thus, overall water splitting can be achieved using this kind of material. In this timely review, we summarize the recent advances in the synthesis of TMMPs and their applications in OER and HER. We present a brief introduction of the structure and synthetic strategies of TMMPs in the first two parts. Then, we review the latest progress made in research on TMMPs as OER, HER, and overall water-splitting electrocatalysts. In this part, the intrinsic activity of TMMPs as well as the current strategy for improving the catalytic activity will be discussed systematically. Finally, we present the future opportunities and the remaining challenges for the application of TMMPs in the electrocatalysis field.

Small-molecule electrooxidation-boosted water electrolysis (WE) is an energy-saving method for hydrogen (H2) production. Herein, PdPt bimetallenes (PdPt BMLs) are obtained through the simple galvanic replacement reaction. PdPt BMLs reveal 2.93-fold enhancement in intrinsic electroactivity and 4.53-fold enhancement in mass electroactivity for the formate oxidation reaction (FOR) with respect to Pd metallenes (Pd MLs) at 0.50 V potential due to the synergistic effect. Meanwhile, the introduction of Pt atoms also considerably increases the electroactivity of PdPt BMLs for hydrogen evolution reaction (HER) with respect to Pd MLs in an alkaline medium, which even exceeds that with the use of commercial Pt nanocrystals. Inspired by the outstanding FOR and HER electroactivity of bifunctional PdPt BMLs, a two-electrode FOR-boosted WE system (FOR-WE) is constructed by using PdPt BMLs as the cathode and the anode. The FOR-WE system only requires an operational voltage of 0.31 V to achieve H2 production, which is 1.48 V lower than that (ca. 1.79 V) with the use of the traditional WE system.

Battery safety has attracted considerable attention worldwide due to the rapid development of wearable electronics and the steady increase in the production and use of electric vehicles. As battery failures are often associated with mechanical-thermal coupled behaviors, protective shielding materials with excellent mechanical robustness and flame-retardant properties are highly desired to mitigate thermal runaway. However, most of the thermal insulating materials are not strong enough to protect batteries from mechanical abuse, which is one of the most critical scenarios with catastrophic consequences. Here, inspired by wood, we have developed an effective approach to engineer a hierarchical nanocomposite via self-assembly of calcium silicate hydrate and polyvinyl alcohol polymer chains (referred as CSH wood). The versatile protective material CSH wood demonstrates an unprecedented combination of light weight (0.018 g cm−3), high stiffness (204 MPa in the axial direction), negative Poisson's ratio (−0.15), remarkable toughness (6.67 × 105 J m−3), superior thermal insulation (0.0204 W m−1 K−1 in the radial direction), and excellent fire retardancy (UL94-V0). When applied as a protective cover or a protective layer within battery packages, the tough CSH wood can resist high-impact load and block heat diffusion to block or delay the spread of fire, therefore significantly reducing the risk of property damage or bodily injuries caused by battery explosions. This work provides new pathways for fabricating advanced thermal insulating materials with large scalability and demonstrates great potential for the protection of electronic devices.

Colorless-to-black switching has attracted widespread attention for smart windows and multifunctional displays because they are more useful to control solar energy. However, it still remains a challenge owing to the tremendous difficulties in the design of completely reverse absorptions in transmissive and colored states. Herein, we report on an electrochemical device that can switch between colorless and black by using the electrochemical process of hybrid organic–inorganic perovskite MAPbBr3, which shows a high integrated contrast ratio of up to 73% from 400 to 800 nm. The perovskite solution can be used as the active layer to assemble the device, showing superior transmittance over the entire visible region in neutral states. By applying an appropriate voltage, the device undergoes reversible switching between colorless and black, which is attributed to the formation of lead and Br2 in the redox reaction induced by the electron transfer process in MAPbBr3. In addition, the contrast ratio can be modulated over the entire visible region by changing the concentration and the applied voltage. These results contribute toward gaining an insightful understanding of the electrochemical process of perovskites and greatly promoting the development of switchable devices.

With the increasing market demand for high-performance lithium-ion batteries with high-capacity electrode materials, reducing the irreversible capacity loss in the initial cycle and compensating for the active lithium loss during the cycling process are critical challenges. In recent years, various prelithiation strategies have been developed to overcome these issues. Since these approaches are carried out under a wide range of conditions, it is essential to evaluate their suitability for large-scale commercial applications. In this review, these strategies are categorized based on different battery assembling stages that they are implemented in, including active material synthesis, the slurry mixing process, electrode pretreatment, and battery fabrication. Furthermore, their advantages and disadvantages in commercial production are discussed from the perspective of thermodynamics and kinetics. This review aims to provide guidance for the future development of prelithiation strategies toward commercialization, which will potentially promote the practical application of next-generation high-energy-density lithium-ion batteries.

Herein, we report bifunctional molybdenum-doped nickel sulfide on nickel foam (Mo-NiSx/NF) for magnetic field-enhanced overall water splitting under alkaline conditions. Proper doping of Mo can lead to optimization of the electronic structure of NiSx, which accelerates the dissociation of H2O and the adsorption of OH− in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) processes, respectively. In addition, the magnetically active Mo-NiSx/NF can further enhance the HER and OER activity under an applied magnetic field due to the magnetoresistance effect and the ferromagnetic (FM) exchange-field penetration effect. As a result, Mo-NiSx/NF requires low overpotentials of 307 mV at 50 mA cm−2 (for OER) and 136 mV at 10 mA cm−2 (for HER) under a magnetic field of 10000 G. Furthermore, the electrolytic cell constructed by the bifunctional Mo-NiSx/NFs as both the cathode and the anode shows a low cell voltage of 1.594 V at 10 mA cm−2 with optimal stability over 60 h under the magnetic field. Simultaneous enhancement of the HER and OER processes by an external magnetic field through rational design of electrocatalysts might be promising for overall water splitting applications.

Carbon super-heterostructures with high nitrogen contents from the covalent hybrid precursors of covalent triazine frameworks (CTFs) and zeolitic imidazolic frameworks (ZIFs) are scarcely explored because of CTF's ordered structure and toxic superacid that dissolves or destabilizes the metal nodes. To solve this problem, herein, we report a straightforward two-step pathway for the covalent hybridization of disordered CTF (d–CTF)–ZIF composites via preincorporation of an imidazole (IM) linker into ordered CTFs, followed by the imidazole-site-specific covalent growth of ZIFs. Direct carbonization of these synthesized d–CTF−IM−ZIF hybrids results in unique hollow carbon super-heterostructures with ultrahigh nitrogen content (>18.6%), high specific surface area (1663 m2 g−1), and beneficial trace metal (Co/Zn NPs) contents for promoting the redox pseudocapacitance. As proof of concept, the obtained carbon super-heterostructure (Co–Zn–NCSNH–800) is used as a positive electrode in an asymmetric supercapacitor, demonstrating a remarkable energy density of 61 Wh kg−1 and extraordinary cyclic stability of 97% retention after 30,000 cycles at the cell level. Our presynthetic modifications of CTF and their covalent hybridization with ZIF crystals pave the way toward new design strategies for synthesizing functional porous carbon materials for promising energy applications.

Exploiting efficient urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) catalysts are significant for energy-saving H2 production through urea-assisted water electrolysis, but it is still challenging. Herein, carbon-encapsulated CoNi coupled with CoNiMoO (CoNi@CN-CoNiMoO) is prepared by solvothermal method and calcination to enhance the activity/stability of urea-assisted water electrolysis at large current density. It exhibits good activity for UOR (E10/1,000 = 1.29/1.40 V) and HER (E−10/−1000 = −45/−245 mV) in 1.0 M KOH + 0.5 M urea solution. For the UOR||HER system, CoNi@CN-CoNiMoO only needs 1.58 V at 500 mA cm−2 and shows good stability. Density functional theory calculation suggests that the strong electronic interaction at the interface between NiCo alloy and N-doping-carbon layers can optimize the adsorption/desorption energy of UOR/HER intermediates and accelerate the water dissociation, which can expedite urea decomposition and Volmer step, thus increasing the UOR and HER activity, respectively. This work provides a new solution to design UOR/HER catalysts for H2 production through urea-assisted water electrolysis.

Organometal halide perovskites are promising semiconducting materials for photodetectors because of their favorable optoelectrical properties. Although nanoscale perovskite materials such as quantum dots (QDs) show novel behavior, they have intrinsic stability issues. In this study, an effectively silane barrier-capped quantum dot (QD@APDEMS) is thinly applied onto a bulk perovskite photosensitive layer for use in photodetectors. QD@APDEMS is synthesized with a silane ligand with hydrophobic CH3-terminal groups, resulting in excellent dispersibility and durability to enable effective coating. The introduction of the QD@APDEMS layer results in the formation of a low-defect perovskite film with enlarged grains. This is attributed to the grain boundary interconnection effect via interaction between the functional groups of QD@APDEMS and uncoordinated Pb2+ in grain boundaries. By passivating the grain boundaries, where various trap sites are distributed, hole charge-carrier injection and shunt leakage can be suppressed. Also, from the energy point of view, the deep highest occupied molecular orbital (HOMO) level of QD@APDEMS can work as a hole charge injection barrier. Improved charge dynamics (generation, transfer, and recombination properties) and reduced trap density of QD@APDEMS are demonstrated. When this perovskite film is used in a photodetector, the device performance (especially the detectivity) stands out among existing perovskites evaluated for energy sensing device applications.

Metallic lithium (Li) is considered the “Holy Grail” anode material for the next-generation of Li batteries with high energy density owing to the extraordinary theoretical specific capacity and the lowest negative electrochemical potential. However, owing to inhomogeneous Li-ion flux, Li anodes undergo uncontrollable Li deposition, leading to limited power output and practical applications. Carbon materials and their composites with controllable structures and properties have received extensive attention to guide the homogeneous growth of Li to achieve high-performance Li anodes. In this review, the correlation between the behavior of Li anode and the properties of carbon materials is proposed. Subsequently, we review emerging strategies for rationally designing high-performance Li anodes with carbon materials, including interface engineering (stabilizing solid electrolyte interphase layer and other functionalized interfacial layer) and architecture design of host carbon (constructing three-dimension structure, preparing hollow structure, introducing lithiophilic sites, optimizing geometric effects, and compositing with Li). Based on the insights, some prospects on critical challenges and possible future research directions in this field are concluded. It is anticipated that further innovative works on the fundamental chemistry and theoretical research of Li anodes are needed.

Hydrogen peroxide (H2O2) production by the electrochemical 2-electron oxygen reduction reaction (2e− ORR) is a promising alternative to the energy-intensive anthraquinone process, and single-atom electrocatalysts show the unique capability of high selectivity toward 2e− ORR against the 4e− one. The extremely low surface density of the single-atom sites and the inflexibility in manipulating their geometric/electronic configurations, however, compromise the H2O2 yield and impede further performance enhancement. Herein, we construct a family of multiatom catalysts (MACs), on which two or three single atoms are closely coordinated to form high-density active sites that are versatile in their atomic configurations for optimal adsorption of essential *OOH species. Among them, the Cox–Ni MAC presents excellent electrocatalytic performance for 2e− ORR, in terms of its exceptionally high H2O2 yield in acidic electrolytes (28.96 mol L−1 gcat.−1 h−1) and high selectivity under acidic to neutral conditions in a wide potential region (>80%, 0–0.7 V). Operando X-ray absorption and density functional theory analyses jointly unveil its unique trimetallic Co2NiN8 configuration, which efficiently induces an appropriate Ni–d orbital filling and modulates the *OOH adsorption, together boosting the electrocatalytic 2e− ORR capability. This work thus provides a new MAC strategy for tuning the geometric/electronic structure of active sites for 2e− ORR and other potential electrochemical processes.

The rational synergy of chemical composition and spatial nanostructures of electrode materials play important roles in high-performance energy storage devices. Here, we designed pea-like MoS2@NiS1.03–carbon hollow nanofibers using a simple electrospinning and thermal treatment method. The hierarchical hollow nanofiber is composed of a nitrogen-doped carbon-coated NiS1.03 tube wall, in which pea-like uniformly discrete MoS2 nanoparticles are enclosed. As a sodium-ion battery electrode material, the MoS2@NiS1.03–carbon hollow nanofibers have abundant diphasic heterointerfaces, a conductive network, and appropriate volume variation-buffering spaces, which can facilitate ion diffusion kinetics, shorten the diffusion path of electrons/ion, and buffer volume expansion during Na+ insertion/extraction. It shows outstanding rate capacity and long-cycle performance in a sodium-ion battery. This heterogeneous hollow nanoarchitectures designing enlightens an efficacious strategy to boost the capacity and long-life stability of sodium storage performance of electrode materials.

The specific energy of Li metal batteries (LMBs) can be improved by using high-voltage cathode materials; however, achieving long-term stable cycling performance in the corresponding system is particularly challenging for the liquid electrolyte. Herein, a novel pseudo-oversaturated electrolyte (POSE) is prepared by introducing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) to adjust the coordination structure between diglyme (G2) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Surprisingly, although TTE shows little solubility to LiTFSI, the molar ratio between LiTFSI and G2 in the POSE can be increased to 1:1, which is much higher than that of the saturation state, 1:2.8. Simulation and experimental results prove that TTE promotes closer contact of the G2 molecular with Li+ in the POSE. Moreover, it also participates in the formation of electrolyte/electrode interphases. The electrolyte shows outstanding compatibility with both the Li metal anode and typical high-voltage cathodes. Li||Li symmetric cells show a long life of more than 2000 h at 1 mA cm−2, 1 mAh cm−2. In the meantime, Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) cell with the POSE shows a high reversible capacity of 134.8 mAh g−1 after 900 cycles at 4.5 V, 1 C rate. The concept of POSE can provide new insight into the Li+ solvation structure and in the design of advanced electrolytes for LMBs.

The performance sensitivity of the solid-state lithium cells to the synergistic interactions of the charge-transport and mechanical properties of the electrolyte is well acknowledged in the literature, but the quantitative insights therein are very limited. Here, the charge-transport and mechanical properties of a polymerized ionic-liquid-based solid electrolyte are reported. The transference number and diffusion coefficient of lithium in the concentrated solid electrolyte are measured as a function of concentration and stack pressure. The elastoplastic behavior of the electrolyte is quantified under compression, within a home-made setup, to substantiate the impact of stack pressure on the stability of the Li/electrolyte interface in the symmetric lithium cells. The results spotlight the interaction between the concentration and thickness of the solid electrolyte and the stack pressure in determining the polarization and stability of the solid-state lithium batteries during extended cycling.

Back cover image: It is challenging to recycle millions of tons of spent lithium-ion batteries efficiently and economically, because of the low valuation of commodity metals and materials, such as LiFePO4. In article number 10.1002/cey2.282, Li et al. demonstrate that the surplus energy of lithiated graphite obtained from spent LIBs can be applied to prepare high-value organolithium compounds (such as lithium ethoxide), thereby significantly improving the economic profitability of LIBs recycling.

Phase engineering is an efficient strategy for enhancing the kinetics of electrocatalytic reactions. Herein, phase engineering was employed to prepare high-performance phosphorous-doped biphase (1T/2H) MoS2 (P-BMS) nanoflakes for hydrogen evolution reaction (HER). The doping of MoS2 with P atoms modifies its electronic structure and optimizes its electrocatalytic reaction kinetics, which significantly enhances its electrical conductivity and structural stability, which are verified by various characterization tools, including X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, X-ray absorption near-edge spectroscopy, and extended X-ray absorption fine structure. Moreover, the hierarchically formed flakes of P-BMS provide numerous catalytic surface-active sites, which remarkably enhance its HER activity. The optimized P-BMS electrocatalysts exhibit low overpotentials (60 and 72 mV at 10 mA cm−2) in H2SO4 (0.5 M) and KOH (1.0 M), respectively. The mechanism of improving the HER activity of the material was systematically studied using density functional theory calculations and various electrochemical characterization techniques. This study has shown that phase engineering is a promising strategy for enhancing the H* adsorption of metal sulfides.

Silicon (Si) is widely used as a lithium-ion-battery anode owing to its high capacity and abundant crustal reserves. However, large volume change upon cycling and poor conductivity of Si cause rapid capacity decay and poor fast-charging capability limiting its commercial applications. Here, we propose a multilevel carbon architecture with vertical graphene sheets (VGSs) grown on surfaces of subnanoscopically and homogeneously dispersed Si–C composite nanospheres, which are subsequently embedded into a carbon matrix (C/VGSs@Si–C). Subnanoscopic C in the Si–C nanospheres, VGSs, and carbon matrix form a three-dimensional conductive and robust network, which significantly improves the conductivity and suppresses the volume expansion of Si, thereby boosting charge transport and improving electrode stability. The VGSs with vast exposed edges considerably increase the contact area with the carbon matrix and supply directional transport channels through the entire material, which boosts charge transport. The carbon matrix encapsulates VGSs@Si–C to decrease the specific surface area and increase tap density, thus yielding high first Coulombic efficiency and electrode compaction density. Consequently, C/VGSs@Si–C delivers excellent Li-ion storage performances under industrial electrode conditions. In particular, the full cells show high energy densities of 603.5 Wh kg−1 and 1685.5 Wh L−1 at 0.1 C and maintain 80.7% of the energy density at 3 C.

Silicon (Si) is a promising anode material for lithium-ion batteries (LIBs) owing to its tremendously high theoretical storage capacity (4200 mAh g−1), which has the potential to elevate the energy of LIBs. However, Si anodes exhibit severe volume change during lithiation/delithiation processes, resulting in anode pulverization and delamination with detrimental growth of solid electrolyte interface layers. As a result, the cycling stability of Si anodes is insufficient for commercialization in LIBs. Polymeric binders can play critical roles in Si anodes by affecting their cycling stability, although they occupy a small portion of the electrodes. This review introduces crucial factors influencing polymeric binders' properties and the electrochemical performance of Si anodes. In particular, we emphasize the structure–property relationships of binders in the context of molecular design strategy, functional groups, types of interactions, and functionalities of binders. Furthermore, binders with additional functionalities, such as electrical conductivity and self-healability, are extensively discussed, with an emphasis on the binder design principle.

Commercialization of Zn-metal anodes with low cost and high theoretical capacity is hindered by the poor reversibility caused by dendrites growth, side reactions, and the slow Zn2+-transport and reaction kinetics. Herein, a reversible heterogeneous electrode of Zn-nanocrystallites/polyvinyl-phosphonic acrylamide (Zn/PPAm) with fast electrochemical kinetics is designed for the first time: phosphonic acid groups with strong polarity and chelation effect ensure structural reversibility and stability of the three-dimensional Zn-storage-host PPAm network and the Zn/PPAm hybrid; hydrophobic carbon chains suppress side reactions such as hydrogen evolution and corrosion; weak electron-donating amide groups constitute Zn2+-transport channels and promote “desolvation” and “solvation” effects of Zn2+ by dragging the PPAm network on the Zn-metal surface to compress/stretch during Zn plating/stripping, respectively; and the heterostructure and Zn nanocrystallites suppress dendrite growth and enhance electrochemical reactivity, respectively. Thus, the Zn/PPAm electrode shows cycle reversibility of over 6000 h with a hysteresis voltage as low as 31 mV in symmetrical cells and excellent durability and flexibility in fiber-shaped batteries.

A fuel cell is an energy conversion device that can continuously input fuel and oxidant into the device through an electrochemical reaction to release electrical energy. Although noble metals show good activity in fuel cell-related electrochemical reactions, their ever-increasing price considerably hinders their industrial application. Improvement of atom utilization efficiency is considered one of the most effective strategies to improve the mass activity of catalysts, and this allows for the use of fewer catalysts, saving greatly on the cost. Thus, single-atom catalysts (SACs) with an atom utilization efficiency of 100% have been widely developed, which show remarkable performance in fuel cells. In this review, we will describe recent progress on the development of SACs for membrane electrode assembly of fuel cell applications. First, we will introduce several effective routes for the synthesis of SACs. The reaction mechanism of the involved reactions will also be introduced as it is highly determinant of the final activity. Then, we will systematically summarize the application of Pt group metal (PGM) and nonprecious group metal (non-PGM) catalysts in membrane electrode assembly of fuel cells. This review will offer numerous experiences for developing potential industrialized fuel cell catalysts in the future.