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Metal-organic frameworks (MOFs), featuring porous crystalline structures with coordinated metal nodes and organic linkers, have recently found increasing interest in diverse applications. By virtue of their versatile and highly tunable compositions and structures, constructing hollow architectures will further endow MOFs with enhanced properties and designability, exceeding the molecular scale. MOFs could be considered as promising building units to fabricate complex hollow nanocomposites with faster mass transport, multiple active components, more exposed active sites, and better compatibility than bulk MOFs. To construct a promising blueprint for hollow pristine MOFs, this review provides a comprehensive overview for structural design strategies and applications of hollow pristine MOFs. We will highlight the merits, challenges and future potential by structuring and applying MOFs in sensing, separation, storage, catalysis, environmental remediation, photochemical and electrochemical energy conversion. This review might pave a new avenue for future development of novel pristine hollow MOFs.

Transition metal nitrides (TMNs) are predicted to be highly promising electrocatalysts for the nitrogen reduction reaction (NRR) by theoretical calculations. TMNs follow a special Mars-van Krevelen (MvK) mechanism during the NRR, involving surface lattice nitrogen exchange and regeneration. It is hence essential to identify the source of nitrogen, especially taking the stability of lattice nitrogen into consideration. Herein, we investigated the deactivation process of a benchmark CrN nanoparticle catalyst (CrN NPs) with a phase-pure cubic rocksalt (RS) structure for the electrochemical NRR. The current work identifies two possible deactivation pathways for CrN: (i) potential-induced structural collapse of the catalyst including both lattice N leaching and metal dissolution and (ii) ammonia poisoning due to the accumulation and strong chemical bonding of the produced ammonia on the CrN surface. This work provides new perspectives for understanding the structural evolution of nitrogen-containing electrocatalysts for NRR research.

[EN] Supercapacitors (SCs), showing excellent power density, long service life, and high reversibility, have received great attention because of the increasing demand for energy storage devices. To further improve their performance, it is essential to develop advanced electrode materials. One group of materials, porous crystalline solids referred to as metal-organic frameworks (MOFs), have proved to be excellent templates for synthesizing functional materials to be employed in the preparation of electrodes for SCs. In comparison to monometallic MOFs, bimetallic MOFs and their derivatives offer a number of advantages, including tunable electrochemical activity, high charge capacity, and improved electrical conductivity. This review focuses on the use of MOF-derived bimetallic materials in SCs, the origin of the improved performance, and the latest developments in the field. Furthermore, the challenges and perspectives in this research area are discussed., This work was supported by Tarbiat Modares University and College of Engineering, Peking University. Financial support by the Spanish Ministry of Science and Innovation (Severo Ochoa and RTI2018-98237-CO2-1) and Generalitat Valenciana (Prometeo 2017-083) is gratefully acknowledged.

Solar-thermal energy storage using latent heat of phase change materials (PCMs) offers renewable penetration in wide range of smart applications. The limiting solar energy harvesting efficiency of existing photo-thermal dopant materials and their negative impact on thermal storage capacity have remained fundamental impediment to further advancement. Herein, we explore a phosphorene based bandgap engineered broadband photonic nanoheater for efficient thermal charging of solid-solid PCMs. In our devised composite system, we benefitted from the synergistic effect of highly efficient photonic energy harvesting characteristic of exfoliated phosphorene nanoflakes (PNF) and latent heat storage capability as well as encapsulating feature of PCM matrix. The solar-thermal energy storage efficiency of our developed materials exceeds 95 % even at lower phosphorene doping level (1 wt. %) and under full solar spectrum with improved latent heat storage capacity (150 J g−1). The achieved efficiency is highest among all photo-thermal storage materials and attributed to the intense and broadband solar absorbance of PNF featured by wide thickness distribution. Further, coating of developed solar heat storage material on fabric substrate has shown promising results toward real world applications.

Porous carbon materials doped with atomically dispersed metal sites (ADMSs) are promising electrocatalysts for oxygen reduction reaction (ORR) electrocatalysis. In this work, we fabricated hierarchical porous nitrogen-doped carbon nanofibers with atomically dispersed Fe-N4 sites by carbonization of electrospinning iron-based metal-organic frameworks (MOFs)/polyacrylonitrile nanofibers for ORR electrocatalysis. Remarkably, the resultant carbon nanofibers with atomically dispersed Fe-N4 sites exhibit extraordinary electrochemical performance with an onset potential of 0.994 V and a halfwave potential of 0.876 V in alkaline electrolyte, comparable to the benchmark commercial Pt/C catalyst. The high catalytic performance is originated from the unique hierarchically porous 1D carbon structure and abundant highly active atomically dispersed Fe-N4 sites.

Antiperovskite Co3 InC0.7 N0.3 nanomaterials with highly enhanced oxygen reduction reaction (ORR) performance were prepared by tuning nitrogen contents through a metal-organic framework (MOF)-derived strategy. The nanomaterial surpasses all reported noble-metal-free antiperovskites and even most perovskites in terms of onset potential (0.957 V at J=0.1 mA cm-2 ) and half-wave potential (0.854 V). The OER and zinc-air battery performance demonstrate its multifunctional oxygen catalytic activities. DFT calculation was performed and for the first time, a 4 e- dissociative ORR pathway on (200) facets of antiperovskite was revealed. Free energy studies showed that nitrogen substitution could strengthen the OH desorption as well as hydrogenation that accounts for the enhanced ORR performance. This work expands the scope for material design via tailoring the nitrogen contents for optimal reaction free energy and hence performance of the antiperovskite system.

Metal–organic frameworks (MOFs) have emerged as desirable cross-functional platforms for electrochemical and photochemical energy conversion and storage (ECS) systems owing to their highly ordered ...

Polyurethane (PU) based phase change materials (PCMs) undergo the solid–solid phase transition and offer state-of-the-art thermal energy storage (TES). Nevertheless, the exploration of these PCMs in real-life applicable smart devices is generally hindered by the technical bottleneck of structural rigidity, low thermal storage capacity and lack of functionalities. Herein, for the first time, we systematically tuned the thermal storage capacity and flexibility of PU-based PCMs (PU-PCMs) by controlling the molecular weight of polyethylene glycol and figured out the optimum selection through the tradeoff between flexibility and thermal storage capacity. We further incorporated functionalized carbon nanotubes (CNTs) in flexible PU-PCMs to improve the thermal conductivity and solar-driven thermal conversion and storage capability. The results demonstrate that the thermal conductivity of the PU–CNT composite is improved by 2.3 times with only 5% weight content of CNTs, and the solar-thermal energy storage efficiency reaches 85.89% at 125 mW cm−2 of irradiation power. Benefitting from the unique mechanical and thermal properties, we further explored our developed composite PCM for solar-driven thermal management of human body parts.

The challenges of energy depletion and environmental pollution raise the demands for the development of new energy technologies, such as metal-air batteries (MABs). The performance of a metal-air battery heavily relies on two fundamental electrocatalytic reactions, that is, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which take place at the air cathode during battery discharging and charging, respectively. Metal-organic frameworks (MOFs) and their derivatives stand out as promising candidate catalysts for the once sluggish redox reactions at the cathodes of MABs, due to their unique merits of crystalline porous structures and tunable chemistry. These distinct characteristics of MOFs offer an effective way to introduce bifunctionality (catalytic activity for ORR and OER in the case of MABs) through the design and synthesis of MOF-based catalysts with large surface area, well-developed porous networks, smooth electron and mass transfer pathways and active sites with different catalytic preferences. Herein, this review summarizes recent advances in the design and synthesis of MOF-based catalysts for a range of MABs, including Li-, Zn-, Al-, Fe- and Na-air batteries. By the demonstration of representative examples, this review also discusses the underlying mechanisms for the origin of bifunctional performance and enhanced catalytic activity with MOF-based catalysts. Future challenges and prospects for both MOFs and MABs are also proposed at the end of this review.

Transition-metal-based phosphides (TMPs) have been considered as attractive electrocatalysts for water splitting due to their earth-abundance and remarkable catalytic activity. As a representative type of precursors, metal-organic frameworks (MOFs) provide ideal plateaus for the design of nanostructured TMPs. In this work, the hierarchically structured iron phosphide nanobundles (FeP-500) were fabricated by one-step phosphorization of an iron-based MOF (MET(Fe)) precursor. The derived FeP-500 nanobundles were constructed by quasi-paralleled one-dimensional nanorods with uneven surface, which provided channels for electrolyte penetration, mass transport, and effective exposure of active sites during the water-splitting process. With the addition of conductive Super P, the obtained FeP-500-S exhibited a good electrocatalytic performance towards the hydrogen evolution reaction in alkaline electrolyte (1 mol L-1 KOH). Furthermore, to investigate the influence of secondary metal doping, a series of isoreticular MOF precursors and bimetallic TMPs were fabricated. The results indicated that the catalytic performance is structure dominated.

The widespread utilization of phase change materials (PCMs) in thermal energy storage technologies is often limited by the shape instability, rigidity, low conductivity and lack of multi-driven capabilities. Therefore, the functionalization of PCMs in order to overcome the aforementioned issues has remained an elusive goal. Herein, we infiltrate a polyethylene glycol (PEG) based solid-solid PCM into the pores of carbon nanotube sponge (CNTS) to fabricate a dual form-stable, flexible and highly conductive phase change composites (PCCs). The developed PCCs undergo nanopore-confined solid-solid phase transition triggered by a low voltage or sunlight with high electro/photo to thermal energy storage efficiency (>94%). The reported energy conversion efficiencies for both electro and photo to thermal energy storage is highest among all functionalized PCCs and attributed to the excellent energy conversion/transfer performance of aligned carbon nanotubes (CNTs) network in the composite structure. In addition to extra shape stability, the solid-solid PCC present high axial thermal conductivity (2.4 W m−1k−1) as well as high-energy storage density 132 J g−1 which is close to solid-liquid PCCs. Therefore, this study provides routes towards the development of real-life applicable PCCs for thermal energy applications.

The development of highly exposed active sites and highly active inexpensive electrocatalysts is important and challengeable for electrocatalytic hydrogen evolution reaction (HER). Herein, we report a bimetallic metal-organic frameworks based strategy to fabricate Ru-based electrocatalysts with high exposure of the Ru active sites (Ru-HPC, ruthenium-decorated hierarchically porous carbon) for high efficient hydrogen evolution. Remarkably, Ru-HPC achieves a current density of 25 mA cmgeo−2 at an overpotential of 22.7 mV, and shows an ultrahigh turnover frequency of 1.79 H2 s−1 at 25 mV, which is almost twice higher than that of commercial Pt/C. The electrochemical HER performance of Ru-HPC surpasses most of the electrocatalysts reported so far in alkaline solutions. Besides, a two-electrode water splitting device is constructed with Ru-HPC and porous RuO2 (obtained by oxidizing Ru-HPC) as electrode materials, which achieves a current density of 10 mA cm−2 at only 1.53 V. This work provides a facile strategy to fabricate high-performance metal-carbon hybrid electrocatalysts with abundant exposed active sites from bimetallic MOFs, which can hopefully be applied to many other metals-carbon hybrids for various electrochemical applications in the foreseeable future.

Fe-based catalysts have been discovered as the best elementary metal-based heterogeneous catalysts for the ammonia synthesis in industrial application during the last century. Herein, a novel and scalable strategy is developed to prepare the K-promoted Fe/C catalyst with extremely high Fe loading (> 50 wt.%) through pyrolysis of the Fe-based metal-organic framework (MOF) xerogel. The obtained K-Fe/C catalysts exhibited superior activity and stability towards ammonia synthesis. The weight-specific reaction rate of Fe/C with K2O as promoter can achieve 12.4 mmol·g−1·h−1 at 350 °C and 30.4 mmol·g−1·h−1 at 400 °C, approximately four and two times higher than that of the commercial fused-iron catalyst (3.4 mmol·g−1·h−1 at 350 °C and 16.7 mmol·g−1·h−1 at 400 °C) under the same condition, respectively. The excellent performance of K-Fe/C can be ascribed to the inherited structure derived from the metal-organic frame precursors and the promotion of potassium, which can modify the binding energy of reactant molecules on the Fe surface, transfer electrons to iron for effective activation of nitrogen, prevent agglomeration of Fe nanoparticle (NPs) and restrain side reaction of carbon matrix to methane.

Due to the severe environmental issues, many advanced technologies, typically fuel cells and metal-air batteries have aroused widespread concerns and been intensively studied in recent years. However, oxygen redox reactions including oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) as the core reactions suffer from sluggish kinetics of the multiple electron transfer process. Currently, Pt, RuO2, and IrO2 are considered to be the benchmark catalysts for ORR and OER, but their high price, scarcity and instability hinder them from large-scale application. To overcome these limits, exploring alternative electrocatalysts with low cost, high activity, long-term stability, and earth-abundance is of extreme urgency. Metal-organic frameworks (MOFs) are a family of inorganic-organic hybrid materials with high surface areas and tunable structures, making them proper as catalyst candidates. Herein, the recent progress of MOFs and MOF-derived materials for ORR and OER is systematically reviewed, and the relationship between compositions and electrochemical performance is discussed. It is expected that this review can be helpful for the future development of related MOF-based materials with excellent electrochemical performance.

Transition metal and heteroatom doping porous carbon exhibits a promising perspective as non-noble metal electrocatalyst. In this work, we develop a novel and scalable strategy to synthesize hierarchically porous nitrogen-doped carbon nanofibers embedded with abundant Fe3C nanoparticles from continuous electrospun of iron-based metal organic frameworks (MOFs) and polyacrylonitrile nanofibrous precursors. The as-prepared Fe3C/NCNFs (NCNFs for nitrogen-doped carbon nanofibers) exhibit high graphitization degree, high specific surface area, hierarchically meso-microporous structures and well-dispersion of carbon-shielded ultrafine Fe3C nanoparticles. The Fe3C/NCNFs exhibit superior electrocatalytic performance towards oxygen reduction reaction with an onset potential of 1.012 V and a half-wave potential of 0.873 V in alkaline media, exceeding the benchmark commercial Pt/C catalyst by ca. 13 and 46 mV, respectively. Moreover, the onset and half-wave potentials of Fe3C/NCNF are 0.832 and 0.664 V in acid media, respectively, which are also comparable to commercial Pt/C catalyst. The dramatically improved electrocatalytic performance is attributed to the continuous one-dimensional (1D) structure of Fe3C/NCNFs enhancing electro-conductivity compared with simple MOF-derived carbon. Our study demonstrates the Fe3C/NCNF nanofibers a promising candidate for efficient, robust electrocatalyst for economic platinum-free fuel cells.

Nitrogen is a fundamental constituent for all living creatures on the Earth and modern industrial society. The current nitrogen industry is largely powered by fossil fuels with huge energy consumption and carbon dioxide emission, and nitrogen pollution in surface water bodies induced by the indiscriminate discharge of industrial and domestic wastewater has become a worldwide environmental concern. Electrochemical techniques for nitrogen fixation and transformation under mild conditions are promising approaches to meet the challenge of efficiently managing and balancing the nitrogen cycle, where the rational design of advanced electrocatalysts from both structural and compositional aspects down to the nanoscale plays the most essential role. Herein, important nitrogen species including dinitrogen (N2), ammonia (NH3) and hydrazine (N2H4), their transformation processes between each other including the nitrogen reduction reaction (NRR), ammonia oxidation reaction (AOR) and hydrazine oxidation reaction (HzOR), and research progress on the development of related electrocatalysts are systematically summarized, aiming at establishing a general picture of the whole nitrogen cycle instead of a certain single reaction. Strategies combining theoretical computations and experimental optimizations are proposed to improve the catalytic performance including activity, efficiency, selectivity and stability, finally contributing to a self-sufficient and carbon-free "green" nitrogen economy.

Nano-confinement of metal-based nanostructures in one-dimensional carbon nanotubes (M@CNTs) is an interesting and effective way to achieve new functional materials with unique physical and chemical properties for various energy applications with enhanced performance. In this unique structure, the inner cavity of CNTs can act as a nanoreactor with a diameter of a few nanometers and length in microns. The nano-confinement of metals into CNTs not only improves the local environment of CNTs but also prevents metal leaching and aggregation. This critical review summarizes recent advances in developing advanced nano-confinement methods for different functional nanoparticles, such as metals, metal oxides, metal sulfides, metal phosphides, metal carbides, etc. Particular attention is paid to their catalytic activities and stabilities, strategies for the enhancement of storage capacities of Zn–air batteries, Li–O2 batteries and lithium ion batteries. The demonstrated examples provide an understanding of M@CNTs for use as electrocatalysts and electrodes for batteries. Finally, challenges and future prospects of M@CNTs are highlighted for electrochemical energy conversion and storage devices.

Large carbon networks featuring hierarchical pores and atomically dispersed metal sites (ADMSs) are ideal materials for energy storage and conversion due to the spatially continuous conductive networks and highly active ADMSs. However, it is a challenge to synthesize such ADMS-decorated carbon networks. Here, an innovative fusion-foaming methodology is presented in which energetic metal-organic framework (EMOF) nanoparticles are puffed up to submillimeter-scaled ADMS-decorated carbon networks via a one-step pyrolysis. Their extraordinary catalytic performance towards oxygen reduction reaction verifies the practicability of this synthetic approach. Moreover, this approach can be readily applicable to a wide range of unexplored EMOFs, expanding scopes for future materials design.

Phase change materials (PCMs) provide a state-of-the-art thermal energy storage capability and offer enormous potential for solar energy storage systems. However, the widespread adaptation of PCMs in advanced energy systems is often limited by low energy harvesting efficiency and poor shape stability. Thus, developing shape-stable PCMs for high-efficiency solar-thermal energy storage has remained an impediment to further advancement. Herein, we devised novel shape-stable composite PCMs based on monodispersed CuS disk-like nanoparticles and solid-solid PCM polyurethane (PU). In our devised composite system, the incorporated CuS nanoparticles act as a photonic nanoheater and the PU matrix acts as the heat reservoir which can store thermal energy via the latent heat while the phase transition occurs. The fabricated CuS@PU composite with 4 wt % doping of CuS nanodisks exhibits a phase change enthalpy of around 120 J/g, which is only 14% lower than that of the neat PU PCM. Owing to the solid-state phase transition of the PU PCM, only 0.6% of energy storage loss occurred over 100 repeated heating and cooling cycles. Besides, the solar-thermal energy storage efficiency of the CuS@PU composite exceeds 92% at 1 sun illumination under the full solar spectrum. Based on these outstanding thermophysical properties such as excellent shape stability, thermal stability, and thermal reliability, the developed CuS@PU composite PCMs are imperative candidates for real-world applications.

Summary Metal-organic frameworks (MOFs) and their derivatives are two developing families of functional materials for energy storage and conversion. Their high porosity, versatile functionalities, diverse structures, and controllable chemical compositions offer immense possibilities in the search for adequate electrode materials for rechargeable batteries. Despite these advantageous features, MOFs and their derivatives as electrode materials face various challenging issues, which impede their practical applications. From this perspective, we present both the opportunities and challenges that MOFs/MOF composites and MOF-derived materials bring to rechargeable batteries, including lithium-ion batteries, lithium-sulfur batteries, lithium-oxygen batteries, and sodium-ion batteries. By discussing the development of MOFs/MOF composites and MOF-derived materials in each battery system, some design principles that dominate the specific electrochemical behaviors are outlined, with the key requirements that a practical electrode should fulfill. At the end, a basic guidance and future directions for further development are provided.

Oxygen reduction and evolution reactions as two important electrochemical energy conversion processes in metal-air battery devices have aroused widespread concern. However, synthesis of low-cost non-noble metal-based bifunctional high-performance electrocatalysts is still a great challenge. In this work, we report on the design and synthesis of a novel Co-B/N codoped carbon with core-shell-structured nanoparticles aligned on graphene nanosheets (denoted as CoTIB-C/G) derived from cobalt tetrakis(1-imidazolyl)borate (CoTIB) and graphene oxide hybrid template. Compared with pristine CoTIB-derived bulk structure (CoTIB-C), CoTIB-C/G particles with an average size of 25 nm are uniformly dispersed on highly conductive graphene sheets in the hybrid material, thus dramatically increasing the utilization efficiency and activity of the active components upon oxygen reduction and evolution. After all, because of the "barrier effect" of graphene sheets toward CoTIB-C/G and the synergistic effect between Co nanoparticles and carbon shells linked to the graphene sheets, as well as heteroatoms' doping effect, the as-obtained bifunctional electrocatalyst exhibits remarkable oxygen reduction and evolution reaction activities in alkaline media, indicating its feasibility and potential in practical applications.

Boron and nitrogen co-doped graphene (BCN) nanotubes have tremendous properties for energy storage devices. Herein, we first report a BCN nanotubes architecture entangled on a three dimensional (3D) melamine foam derived carbon skeleton with high surface area, hierarchical porosity and heteroatoms (B, C, N) extant. Having such efficacious properties, 3D-BCN-950 (calcinated under 950 °C) exhibited excellent capacitance of 344 F g−1 at a current density of 1 A g−1. Furthermore, 3D-BCN-950 nanotubes are utilized as electrodes in a symmetric and negative electrode in asymmetric hybrid supercapacitors. The symmetric supercapacitor presented a high energy density of 19.8 W h kg−1 and elevated power density of 5074 W kg−1, the asymmetric supercapacitor also demonstrated a high energy density of 72 W h kg−1 and power density of 22 732 W kg−1. These results indicate the as-synthesized heteroatoms doped graphene nanotubes architecture could be a potential negative electrode materials for the fabrication of future high energy density hybrid supercapacitors.

A high-performance “green” carbon-based supercapacitor electrode material is synthesized from selected algae microspheres, which are grown under controlled cultivation conditions. The best-performing sample possesses a high specific surface area of 1337.9 m2 g−1 with a hierarchically porous structure and naturally intrinsic nitrogen dopant. This leads to an excellent specific capacitance of 353 F g−1 at 1 A g−1 and 92% capacitance retention even after 10 000 charge–discharge cycles at 20 A g−1, which makes it superior to many recently reported biomass- and synthetically derived carbon electrode materials. It is found that residual nitrogen and metal contents in algae-derived carbon are highly influenced by biomass components, such as proteins. The content of these components can be controlled by adjusting the concentrations of nutrients in cultivation media. However, nitrogen in algae proteins is not pyrolytically stable, and studies indicate that excessive residual metal content plays the role of “dead mass” and results in a less-developed porous structure. Therefore, this suggests that both biomass selection and cultivation should aim for proteins with a more stable nitrogen content and minimized content of electrochemically inactive metals such as Mg and Ca. Hence, this study does not just demonstrate a green candidate electrode material for high-performance supercapacitors, but also provides an innovative selection and cultivation strategy to improve the capacitive performance of biomass-derived carbon.

Hierarchically porous electrodes made of electrochemically active materials and conductive additives may display synergistic effects originating from the interactions between the constituent phases, and this approach has been adopted for optimizing the performances of many electrode materials. Here we report our findings in design, fabrication, and characterization of a hierarchically porous hybrid electrode composed of α-NiS nanorods decorated on reduced graphene oxide (rGO) (denoted as R-NiS/rGO), derived from water-refluxed metal–organic frameworks/rGO (Ni-MOF-74/rGO) templates. Microanalyses reveal that the as-synthesized α-NiS nanorods have abundant (101) and (110) surfaces on the edges, which exhibit a strong affinity for OH− in KOH electrolyte, as confirmed by density functional theory-based calculations. The results suggest that the MOF-derived α-NiS nanorods with highly exposed active surfaces are favorable for fast redox reactions in a basic electrolyte. Besides, the presence of rGO in the hybrid electrode greatly enhances the electronic conductivity, providing efficient current collection for fast energy storage. Indeed, when tested in a supercapacitor with a three-electrode configuration in 2 M KOH electrolyte, the R-NiS/rGO hybrid electrode exhibits a capacity of 744 C g−1 at 1 A g−1 and 600 C g−1 at 50 A g−1, indicating remarkable rate performance, while maintaining more than 89% of the initial capacity after 20 000 cycles. Moreover, when coupled with a nitrogen-doped graphene aerogel (C/NG-A) negative electrode, the hybrid supercapacitor (R-NiS/rGO/electrolyte/C/NG-A) achieved an ultra-high energy density of 93 W h kg−1 at a power density of 962 W kg−1, while still retaining an energy density of 54 W h kg−1 at an elevated working power of 46 034 W kg−1.

Phase change materials (PCMs) have been extensively characterized as constant temperature latent heat thermal energy storage (TES) materials. Nevertheless, the widespread utilization of PCMs is limited due to the flow of liquid PCMs during melting, phase separation, supercooling and low heat transfer rate. In order to overcome these inherent problems and to improve thermo-physical properties, the confinement of PCMs at the nanoscale has been identified as a versatile strategy, which ensures the encapsulation of PCMs in much smaller nano-containers. Such strategies including core–shell, longitudinal, interfacial and porous confinement have been widely presented in recent years to efficiently encapsulate PCMs in nanospaces and are presenting attractive ways to enhance thermal performance. This review summarizes the recent advancement and critical issues of nanoconfinement technologies of PCMs from the point of view of material design. In addition, the potential applications of nanoconfined PCMs in diverse fields, including energy conversion and storage, thermal rectification and temperature controlled drug delivery systems, are presented in detail. Finally, the major drawbacks associated with nanoconfined PCMs and their prospective solutions are also provided.

Electrochemical energy conversion via electrocatalysis offers promising solutions to tackle problems of global energy crisis and environmental issues. The rational design of efficient electrocatalysts requires an in-depth understanding of the underlying mechanism and the structure-activity relationship of key electrocatalytic processes such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). To gain a comprehensive understanding of these electrochemical systems, in situ characterization techniques capable of determining catalyst states as well as important reaction intermediates under operando conditions are necessary. Vibrational spectroscopic methods, including Raman and FTIR-based techniques, are powerful tools to monitor surface transformation, active sites, and interfacial intermediates under reaction conditions. Here, recent developments of in situ/operando vibrational spectroscopic techniques in the probing of various electrochemical reactions and processes are described and summarized. The outlook for future research directions in this field is also discussed. The aim is to attract increasing attention of the wider community to in situ/operando characterization techniques and promote the fundamental understanding of reaction mechanisms and structure-activity relationships in electrocatalytic energy conversion systems.

A novel Fe3O4 nanoparticles/nitrogen-doped graphene aerogel (denoted as Fe3O4@NGA) hybrid material with high porosity was prepared by using iron-based metal-organic frameworks (MOFs), [Fe3O(H2N-BDC)3] (H2N-BDC=2-aminoterephtalic acid), denoted as MIL-88B-NH2, as templates and nitrogen-doped graphene aerogel (NGA) as the substrate. The obtained Fe3O4 nanoparticles demonstrate a rich edge area with abundant exposed active sites and defects, indicating great potential for oxygen adsorption and activation. When used as an electrocatalyst in alkaline solution, the Pt-free Fe3O4@NGA exhibited excellent oxygen reduction reaction (ORR) performance and dramatically enhanced durability and tolerance towards methanol compared to commercial Pt/C catalyst, revealing its great viability and potential in practical application.

Cobalt oxides, typically Co3O4, have received considerable attention due to their high theoretical capacity as anode materials for Li-ion batteries. However, their poor electron conductivity and large volume change upon the insertion/removal of Li+ ions limit their practical application. Carbon coating is widely used to improve the electrochemical performance of materials and release the strain during the lithiation/delithiation processes, in which the thickness of the coating carbon shell has a vital role in determining the performance of the material. In this study, Co3O4 nanoparticles coated with a thin carbon shell are obtained from the metal–organic framework (MOF) precursor Co-MOF-74 via a sequential two-step carbonization process, where carbon oxides, e.g., CO2, are used as the oxidation atmosphere in the second step. The carbon content and shell thickness are controlled by changing the calcination time. The electrode containing a certain carbon content (3.17 wt%) exhibits a capacity of 1137 mA h g−1 after 100 cycles tested at 100 mA g−1 between 0.005 and 3.0 V. This enhanced electrochemical performance is attributed to the well-dispersed nanosized Co3O4 particles and thin carbon shell coating on the electrode surface, which shorten the Li+ ion diffusion length and enhance the electron conductivity of the hybrid.

The influence of pore texture and nitrogen species of the carbon support for the Fischer–Tropsch synthesis was investigated using well-defined catalysts derived from metal–organic frameworks (MOFs). Two typical MOFs were employed in the carbonization process to prepare the target catalysts, i.e. nitrogen-rich ZIF-67 and nitrogen-free Co-MOF-74. The Co-MOF-74-derived nanocomposite (Co@C) showed a carbon monoxide (CO) conversion of 30%, whereas the ZIF-67-derived nanocomposite (Co@NC) exhibited a CO conversion of 10%. The nitrogen-free Co@C composite showed 65% selectivity for long-chain hydrocarbons (C5+) and 10% selectivity for short-chain hydrocarbons (C2–C4) after 100 h on stream; on the other hand, the Co@NC composite showed 31% selectivity for C5+ products and 37% selectivity for short-chain hydrocarbons (C2–C4) after 100 h on stream. The excellent CO conversion was attributed to the large pore size of the carbon support, which facilitates the diffusion of the hydrocarbons. The high C2–C4 selectivity originates from the influence of nitrogen species in the carbon support. This study is expected to open a new avenue for the design of new catalysts for the Fischer–Tropsch synthesis with high activity and superior selectivity via choosing suitable MOFs precursors.

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

The greater availability of sodium with large storage capacity motivates the fabrication of advanced electrodes materials for sodium ion batteries (SIBs) to promote in potential market of smart appliances. Multiple metals atomic dispersion into carbon framework can enhance the adsorption property of sodium for high performance SIBs. In this work, rationalized atomic/clusters of Fe/Se/Al are dispersed on N-doped amorphous carbon fibers through electrospinning and direct selenization (DS) processes. The developed product of DS-Fe/Se/Al@NC-650 nanofibers are employed as anode materials of SIBs, exhibitinga high specific capacity of 432 mAh g−1 at 100 mA g−1 a desirable cycling stability with a capacity retention of 99 % after 700 cycles. Such an excellent storage capacity initiates through synergistic effects of the uniformly distributed Fe/Se/Al and the interconnected nitrogen-doped carbon network. For the apprehension of capacity and stability of DS-Fe/Se/Al@NC-650 nanofibers for SIBs, the theoretical calculations were carried out and provide strong evidences which proves the electron/ion transport in experimental results.

The search for highly efficient earth-abundant carbon nanomaterials with Pt-like electrocatalytic activity for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is still a great challenge. Herein, we present a new catalyst-free synthetic strategy of self-squeezing and rolling of B, N co-doped graphene nanosheets to nanotubes with tunable dimensions and atomic bonds as metal-free electrocatalysts for enhancing the ORR and HER using (polyethylene glycol (PEG)) as the directing agent. We found that the PEG with a higher molecular weight favors the formation of B, N co-doped graphene nanosheets with a high concentration of B–N bonds in a carbon framework whereas the one with a lower molecular weight leads to B, N co-doped graphene nanotubes (BCN nanotubes) with segregated B–C and N–C bonds. The as-prepared graphene nanostructures show interesting atomic bonds and dimension-dependent electrocatalytic activity towards the ORR and HER with BCN nanotubes being the best. The BCN nanotubes show Pt-like ORR activity and much better ORR stability than commercial Pt/C catalysts. They also exhibit excellent HER activity with a very low overpotential and a small Tafel slope of 92 mV dec−1. The present work highlights the importance of tuning atomic bonds and dimensions of carbon nanomaterials for achieving highly efficient electrocatalysts for the ORR and HER.

Constructed with both capacitive and battery-like electrodes, hybrid supercapacitor devices have been adopted as promising energy storage devices for their favorable power and energy densities. Design and fabrication of capacitive and battery-like electrode materials endowed with high specific capacitances/capacities, high rate performance and desirable durability are crucial to improve the overall energy storage performances of hybrid supercapacitors. As emerging porous crystalline materials, metal-organic frameworks with favorable porous properties, tunable chemical compositions and adjustable structures/morphologies can lead to desirable energy storage performances of hybrid supercapacitors. In this review, started from the classification of supercapacitors and electrode materials, the recent advances of pristine metal-organic frameworks, metal-organic framework composites and metal-organic frameworks derived materials applied for hybrid supercapacitor were elaborated. Furthermore, based on previous contributions, challenges and perspectives of metal-organic framework-based materials for hybrid supercapacitor application were summarized.

Transition-metal phosphides have flourished as promising candidates for oxygen evolution reaction (OER) electrocatalysts. Herein, it is demonstrated that the electrocatalytic OER performance of CoP can be greatly improved by constructing a hybrid CoP/TiOx heterostructure. The CoP/TiOx heterostructure is fabricated using metal-organic framework nanocrystals as templates, which leads to unique hollow structures and uniformly distributed CoP nanoparticles on TiOx . The strong interactions between CoP and TiOx in the CoP/TiOx heterostructure and the conductive nature of TiOx with Ti3+ sites endow the CoP-TiOx hybrid material with high OER activity comparable to the state-of-the-art IrO2 or RuO2 OER electrocatalysts. In combination with theoretical calculations, this work reveals that the formation of CoP/TiOx heterostructure can generate a pathway for facile electron transport and optimize the water adsorption energy, thus promoting the OER electrocatalysis.

Three Co-based isostructural MOF-74-III materials with expanded pores are synthesized, with varied extent of fused benzene rings onto sidechain of same-length ligands to finely tune the pore sizes to 2.6, 2.4, and 2.2 nm. Gas sorption results for these highly mesoporous materials show that alternately arranged fused benzene rings on one side of the ligand could serve as extra anchoring sites for CO2 molecules with π-π interactions, conspicuously enhancing CO2 uptake and CO2 /CH4 and CO2 /N2 selectivity; while more steric hindrance effect towards open CoII sites were imposed by ligands flanked with fused benzene rings on both sides, compromising such extra-sites enhancement. In the catalytic conversion of CO2 with propylene oxide to form propylene carbonate, the as-synthesized MOF-74-III(Co) with desired properties of highly exposed and accessible open CoII centers, large mesopore apertures and multi-interactive sites, demonstrated higher catalytic activity compared with other two MOFs, with benzene rings fused to ligands hampering the functionality of CoII centers as Lewis acid sites. Our results highlight the viability of finely tuning the expanded pores of MOF-74 isostructure and the effect of fused benzene rings as functional groups onto selective CO2 capture and conversion.

Metal sites play an essential role in both electrocatalytic and photocatalytic energy conversion. The highly ordered arrangements of the organic linkers and metal nodes as well as the well-defined pore structures of metal-organic frameworks (MOFs) make them ideal substrates to support atomically dispersed metal sites (ADMSs) located in their metal nodes, linkers, and pores. Porous carbon materials doped with ADMSs can be derived from these ADMS-incorporating MOF precursors through controlled treatments. These ADMSs incorporated in pristine MOFs and MOF-derived carbon materials possess unique advantages over molecular or bulk metal-based catalysts and bridge the gap between homogeneous and heterogeneous catalysts for energy-conversion applications. This Review presents recent progress in the design and incorporation of ADMSs in MOFs and MOF-derived materials for energy-conversion applications.

Yolk-shell nanostructures have received great attention for boosting the performance of lithium-ion batteries because of their obvious advantages in solving the problems associated with large volume change, low conductivity, and short diffusion path for Li+ ion transport. A universal strategy for making hollow transition metal oxide (TMO) nanoparticles (NPs) encapsulated into B, N co-doped graphitic nanotubes (TMO@BNG (TMO = CoO, Ni2 O3 , Mn3 O4 ) through combining pyrolysis with an oxidation method is reported herein. The as-made TMO@BNG exhibits the TMO-dependent lithium-ion storage ability, in which CoO@BNG nanotubes exhibit highest lithium-ion storage capacity of 1554 mA h g-1 at the current density of 96 mA g-1 , good rate ability (410 mA h g-1 at 1.75 A g-1 ), and high stability (almost 96% storage capacity retention after 480 cycles). The present work highlights the importance of introducing hollow TMO NPs with thin wall into BNG with large surface area for boosting LIBs in the terms of storage capacity, rate capability, and cycling stability.

Metal-organic frameworks (MOFs), a new class of crystalline porous organic-inorganic hybrid materials, have recently attracted increasing interest in the field of energy storage and conversion. Herein, recent progress of MOFs and MOF composites for energy storage and conversion applications, including photochemical and electrochemical fuel production (hydrogen production and CO2 reduction), water oxidation, supercapacitors, and Li-based batteries (Li-ion, Li-S, and Li-O2 batteries), is summarized. Typical development strategies (e.g., incorporation of active components, design of smart morphologies, and judicious selection of organic linkers and metal nodes) of MOFs and MOF composites for particular energy storage and conversion applications are highlighted. A broad overview of recent progress is provided, which will hopefully promote the future development of MOFs and MOF composites for advanced energy storage and conversion applications.

Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM = 2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon–graphene composite electrode (d...

To cater for the demands of electrochemical energy storage system, the development of cost effective, durable and highly efficient electrode materials is desired. Here, a novel electrode material based on redox active β-Co(OH)2 and B, N co-doped graphene nanohybrid is presented for electrochemical supercapacitor by employing a facile metal-organic frameworks (MOFs) route through pyrolysis and hydrothermal treatment. The Co(OH)2 could be firmly stabilized by dual protection of N-doped carbon polyhedron (CP) and B/N co-doped graphene (BCN) nanosheets. Interestingly, the porous carbon and BCN nanosheets greatly improve the charge storage, wettability, and redox activity of electrodes. Thus the hybrid delivers specific capacitance of 1263 F g−1 at a current density of 1A g−1 with 90% capacitance retention over 5000 cycles. Furthermore, the new aqueous asymmetric supercapacitor (ASC) was also designed by using Co(OH)2@CP@BCN nanohybrid and BCN nanosheets as positive and negative electrodes respectively, which leads to high energy density of 20.25 Whkg−1. This device also exhibits excellent rate capability with energy density of 15.55 Whkg−1 at power density of 9331 Wkg−1 coupled long termed stability up to 6000 cycles.

Materials with atomically dispersed metal sites (ADMSs) have been recently intensively studied as electrocatalysts for various energy conversion reactions. The well-defined and tunable compositions and structures of the ADMSs offer great opportunities for rational design and fabrication of ADMS-based electrocatalysts with unique electrochemical properties superior to conventional electrocatalysts. Recent progress in this field has demonstrated the potentials and superiority of the practical use of ADMS-based electrocatalysts for energy conversion. However, this field is still in its infancy with a large room for improvement. This review provides general strategies to improve the electrocatalytic performances of ADMS-based electrocatalysts such as precisely controlling the compositions and structures of ADMSs and finely tuning the interactions between ADMSs and supports. An overview of recent developments of ADMS-based electrocatalysts for specific energy conversion reactions including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO 2 RR), and nitrogen reduction reaction (NRR) is presented, while the main challenges and future directions in this field are also discussed.

Rational design of metal compounds in terms of the structure/morphology and chemical composition is essential to achieve desirable electrochemical performances for fast energy storage because of the synergistic effect between different elements and the structure effect. Here, an approach is presented to facilely fabricate mixed‐metal compounds including hydroxides, phosphides, sulfides, oxides, and selenides with well‐defined hollow nanocage structure using metal–organic framework nanocrystals as sacrificial precursors. Among the as‐synthesized samples, the porous nanocage structure, synergistic effect of mixed metals, and unique phosphide composition endow nickel cobalt bimetallic phosphide (NiCo‐P) nanocages with outstanding performance as a battery‐type Faradaic electrode material for fast energy storage, with ultrahigh specific capacity of 894 C g−1 at 1 A g−1 and excellent rate capability, surpassing most of the reported metal compounds. Control experiments and theoretical calculations based on density functional theory reveal that the synergistic effect between Ni and Co in NiCo‐P can greatly increase the OH− adsorption energy, while the hollow porous structure facilitates the fast mass/electron transport. The presented work not only provides a promising electrode material for fast energy storage, but also opens a new route toward structural and compositional design of electrode materials for energy storage and conversion.

The large-scale application of sodium/potassium-ion batteries is severely limited by the low and slow charge storage dynamics of electrode materials. The crystalline carbons exhibit poor insertion capability of large Na+ /K+ ions, which limits the storage capability of Na/K batteries. Herein, porous S and N co-doped thin carbon (S/N@C) with shell-like (shell size ≈20-30 nm, shell wall ≈8-10 nm) morphology for enhanced Na+ /K+ storage is presented. Thanks to the hollow structure and thin shell-wall, S/N@C exhibits an excellent Na+ /K+ storage capability with fast mass transport at higher current densities, leading to limited compromise over charge storage at high charge/discharge rates. The S/N@C delivers a high reversible capacity of 448 mAh g-1 for Na battery, at the current density of 100 mA g-1 and maintains a discharge capacity up to 337 mAh g-1 at 1000 mA g-1 . Owing to shortened diffusion pathways, S/N@C delivers an unprecedented discharge capacity of 204 and 169 mAh g-1 at extremely high current densities of 16 000 and 32 000 mA g-1 , respectively, with excellent reversible capacity for 4500 cycles. Moreover, S/N@C exhibits high K+ storage capability (320 mAh g-1 at current density of 50 mA g-1 ) and excellent cyclic life.

Construction of multifunctional highly active earth-abundant electrocatalysts on a large scale is a great challenge due to poor control over nanostructural features and limited active sites. Here, a simple methodology to tailor metal-organic frameworks (MOFs) to extract highly active multifunctional electrocatalysts on a large scale for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution reaction (HER) is presented. The N, S codoped Fe2 N decorated highly porous and defect-rich carbon nanosheets are grown using MOF xerogels, melamine, and polyvinylpyrollidone. The resulting catalyst exhibits excellent activity for ORR with an onset (0.92 V) and half-wave (0.81 V) potential similar to state-of-the-art Pt/C catalysts. The catalyst also shows outstanding OER and HER activities with a small overpotential of 360 mV in 1 m KOH and -123 mV in 0.5 m H2 SO4 at a current density of 10 mA cm-2 , respectively. Excellent catalytic properties are further supported by theoretical calculations where relevant models are built and various possible activation sites are identified by first-principles calculations. The results suggest that the carbon atoms adjacent to heteroatoms as well as Fe2 -N sites present the active sites for improved catalytic response, which is in agreement with the experimental results.

Currently, metal-organic frameworks (MOFs) are intensively studied as active materials for electrochemical energy storage applications due to their tunable structure and exceptional porosities. Among them, water stable pillared MOFs with dual ligands have been reported to exhibit high supercapacitor (SC) performance. Herein, the "One-for-All" strategy is applied to synthesize both positive and negative electrodes of a hybrid SC (HSC) from a single pillared MOF. Specifically, Ni-DMOF-TM ([Ni(TMBDC)(DABCO)0.5 ], TMBDC: 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid, DABCO: 1,4-diazabicyclo[2.2.2]-octane) nanorods are directly grown on carbon fiber paper (CFP) (denoted as CFP@TM-nanorods) with the help of triethylamine and function as the positive electrode of HSC under alkaline electrolyte. Meanwhile, calcinated N-doped hierarchical porous carbon nanorods (CFP@TM-NPCs) are produced and utilized as the negative counter-electrode from a one-step heat treatment of CFP@TM-nanorods. After assembling these two electrodes together to make a hybrid device, the TM-nanorods//TM-NPCs exhibit a wide voltage window of 1.5 V with a high sloping discharge plateau between 1-1.2 V, indicating its great potential for practical applications. This as-described "One-for-All" strategy is widely applicable and highly reproducible in producing MOF-based electrode materials for HSC applications, which shortens the gap between experimental synthesis and practical application of MOFs in fast energy storage.

Replacing the rare and precious platinum (Pt) electrocatalysts with earth-abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high-performance sustainable energy devices. However, the challenging issues associated with non-Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth-abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth-abundant materials for boosting ORR.