High-performance Energy Storage Research Articles

Synthesis and characterization of coprecipitated layered NCM oxide as cathode for sodium-ion batteries: Kinetics and hydrodynamics studies

Interest in the development of high-performance sodium-ion batteries (SIBs) as a sustainable alternative to lithium-ion batteries (LIBs) for emerging energy storage systems has motivated to delve into a novel approach for the synthesis of layered transition metal oxide (LTMO) type cathode for SIBs batteries. In this study, we introduce novel chemical synthesis method that diverges from traditional methodologies reported till date. Rather than relying on sulfur, nitrate, or acetate metal precursor salts, we employ metal carbonate salt coupled with oxalic acid as a chelating agent, and sodium carbonate as a precipitating agent. This novel chemistry enhances the microstructure of the cathode material inherited from its precursor exhibiting spherical morphology, influence on the electrochemical properties, bestowing the material with high specific capacity, extended cycle life, and an elevated charge–discharge rate. The primary focus of this work centers on a comprehensive investigation of the hydrodynamic and kinetics aspects of the co-precipitation reaction, guided by the principles of chemical engineering. The effects of key parameters such as impeller speed, impeller type, and scale-up effects on the co-precipitation process have been systematically explored. This research introduces insights into optimizing reaction conditions such as pH, reaction temperature, metal precursor salt ratio, precipitating agent, through rigorous experimentation and characterization reports. The notable effect of precipitating agent on morphology has been demonstrated by electron microscopy images. The kinetics study shows the activation energy of 8.9 kJ mol−1 depicts the reaction is controlled by mass transfer resistance and to overcome this effect, impeller type and speed plays significant role and experimentation results shows the axial mixing pattern created by pitched blade impeller is effective to improvise electrochemical properties. The synthesized cathode material exhibits remarkable electrochemical properties, including tap density of approximately 1.3 g cc−1 with reversible capacity of 215 mAh g−1, along with excellent cyclic stability (90 %) over 60 cycles. Furthermore, it shows the high energy density up to 1300 Wh/L and a rapid charge–discharge rate. These findings contribute to the advancement of SIBs technology and offer promising prospects for future researchers to efficient development of high-performance energy storage solutions.

NiFe2O4@NiCo2O4 hollow algae-like microspheres enabled by Mott-Schottky for electrochemical energy storage

Designing hybrid structure of transition metal oxides (TMOs) with controlled morphology can adjust the electronic structure and create more kinetics reactions. Herein, a new kind of NiFe2O4@NiCo2O4 nanocomposite was synthesized with one pot hydrothermal route. The unique hollow algae-like microspheres with plenty nanowires and the synergistic effect factors promote high electroactive sites and generate a built-in electric field. The constituted Mott-Schottky heterostructure creates strong interfacial interaction, paves the way for the electrolyte ions diffusion and results high charges transfer abilities by reducing the energy barriers in the heterointerfaces. As cathode material, the NiFe2O4@NiCo2O4 presents satisfactory specific capacity of 996C·g−1 at 0.5 A·g−1 and maintains ultrahigh capacity retention of 81 % at 30 A·g−1 with marvellous stability of only 6 % loss during 20,000 cycles. Interestingly, asymmetric hybrid supercapacitor device based NiFe2O4@NiCo2O4//active carbon (AC) provides tremendous energy density of 117.6 Wh·kg−1 at 438 W·kg−1 power density. What’s more, all-solid-state device offers ultra-low self-discharge process of 9.5 % through 24 h. Therefore, the combination of NiFe2O4@NiCo2O4 to form Mott-Schottky heterojunction was a promising strategy to develop electrode material with high energy storage performance.

Heterostructures Assembled from Bi2O2CO3 and MXene for Boosted Potassium-Ion Storage by Arousing the Built-in Electric Field.

Bismuth-based materials have been recognized as the appealing anodes for potassium-ion batteries (PIBs) due to their high theoretical capacity. However, the kinetics sluggishness and capacity decline induced by the structure distortion predominately retard their further development. Here, a heterostructure of polyaniline intercalated Bi2O2CO3/MXene (BOC-PA/MXene) hybrids is reported via simple self-assembly strategy. The ingenious design of heterointerface-rich architecture motivates significantly the interior self-built-in electric field (IEF) and high-density electron flow, thus accelerating the charge transfer and boosting ion diffusion. As a result, the hybrids realize a high reversible specific capacity, satisfying rate capability as well as long-term cycling stability. The in/ex situ characterizations further elucidate the stepwise intercalation-conversion-alloying reaction mechanism of BOC-PA/MXene. More encouragingly, the full cell investigation further highlights its competitive merits for practical application in further PIBs. The present work not only opensthewayto thedesignof other electrodes with an appropriate working mechanism but also offers inspiration for built-in electric-field engineering toward high-performance energy storage devices.

Boosting the Energy Density of Bowl-Like MnO2@Carbon Through Lithium-Intercalation in a High-Voltage Asymmetric Supercapacitor with "Water-In-Salt" Electrolyte.

Highly concentrated "'water-in-salt"' (WIS) electrolytes are promising for high-performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO2 in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO2 nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO2/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO2/MBC and activated carbon (AC) and using 21m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2Whkg-1 at 700W kg-1, and still retains 24.8Whkg-1 when the power density is increased to 28kWkg-1. The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO2 + xLi+ + xe-↔LixMnO2 takes place during charging and discharging. Therefore, the asymmetric MnO2/MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium-ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.

Rational Design of a Graphene Oxide–Coated Separator for Thermally and Mechanically Stable Li Metal Anode

Lithium-ion (Li-ion) batteries are widely used in high-performance energy storage applications because of their high energy density. However, safety concerns related to thermal runaway remain a significant challenge for Li-ion batteries. Electrolyte leakage and dendrite formation can trigger thermal runaway, and these factors typically damage traditional polyethylene (PE) separators. Consequently, these separators struggle under extreme conditions and fail to control dendrite growth. In this study, we proposed a solution by coating PE separators with graphene oxide (GO) layers. GO, as a ceramic material, provides superior thermal and mechanical stability compared with polymers. Moreover, GO-coated PE separators (GO-S) do not compromise the advantages of PE separators and effectively manage dendrite growth. In this study, the as-prepared GO-S exhibits excellent electrochemical properties in terms of high ionic conductivity, suppression of Li dendrite growth during charge/discharge process, and long-term cyclability for 7,000 h (3,500 cycles) as well as high thermal stability even after heat treatment of 100°C. Thus, we expect that this research can highlight the potential application of functionalized GO sheets in addressing the thermal and mechanical limitations of polymer-based separators, thereby enhancing the safety and reliability of Li-ion batteries.

Resolving the tradeoff between energy storage capacity and charge transfer kinetics of sulfur-doped carbon anodes for potassium ion batteries by pre-oxidation-anchored sulfurization

Sulfur-heteroatom doping is an effective approach to improve the capacity of carbon anodes for potassium-ion batteries due to the reversible K+-sulfur reaction. However, the random sulfur doping induces unresolved tradeoff between capacity and kinetics, which results from the fundamental difficulty in controllable sulfur incorporation using traditional approaches where breaking robust carbon-containing bonds is necessary. Herein, a pre-oxidation-anchored sulfurization approach is proposed to realize site-controlled and content-adjustable sulfur doping, demonstrating the optimum sulfur content of 9.8 at.% in sulfur-doped hollow carbon sphere (S-HCS). Intentional pre-oxidation introduces easy-to-break C-O/C-O-C bond-included oxygen-containing functional groups serving as anchoring sites for substitution with sulfur atoms. Consequently, S-HCS-9.8 % achieves superior performance for K+ storage (406.1 mA h g–1 at 0.1 A g–1, 112.6 mA h g–1 at 5 A g–1 and 170 mA h g–1 after 2,000 cycles at 2 A g–1). The enhanced electrochemical performances are attributed to the largest interlayer spacing and highest disorder degree resulting from the thiophene-sulfur-dominated configuration at high sulfur content, which realizes high structure reversibility and stable solid-electrolyte-interface layer. This work provides new insights and guiding principles for the development of heteroatom-doped carbon anodes for next-generation high-performance energy storage applications.

Innovative design of silicon-core mesoporous carbon composite for high performance anode material in advanced lithium-ion batteries

Silicon is widely recognized as an exceptionally auspicious anode material for lithium−ion batteries due to its remarkable specific capacity, suitable operational potential, and ecologically innocuous characteristics. This study presents a novel silicon@void@FC (fibrous carbon) composite anode material for lithium-ion batteries. Utilizing a one-step synthesis approach, the resulting hollow core–shell structure, combined with a mesoporous carbon shell, demonstrates exceptional electrochemical performance. The Si@Void@FC electrode exhibits remarkable cycle stability and rate performance, maintaining structural integrity over 500 cycles without observable degradation. The outer carbon shell acts as a protective layer and accommodates silicon volume expansion during cycling, preventing excessive contact with the electrolyte. The rich mesoporous structure facilitates efficient lithium-ion diffusion, enhancing lithiation and delithiation processes. The simplified one-step synthesis method further streamlines the preparation process, eliminating the need for complete silica template formation. This innovative Si@Void@FC composite material holds promise for advancing lithium-ion battery technology, addressing challenges associated with silicon-based anode materials, and offering insights into the design and synthesis of high-performance energy storage materials.

Achieving High Energy Storage Performance under a Low Electric Field in KNbO3-Doped BNT-Based Ceramics.

Ceramic capacitors have great potential for application in power systems due to their fantastic energy storage performance (ESP) and wide operating temperature range. In this study, the (1 - x)Bi0.5Na0.47Li0.03Sn0.01Ti0.99O3-xKNbO3 (BNLST-xKN) energy storage ceramics were synthesized through the solid-phase reaction method. The addition of KN disrupts the long-range ferroelectric order of the BNLST ceramic, inducing the emergence of polar nanoregions (PNRs), which enhances the ESP of the ceramics. The BNLST-0.2KN ceramic demonstrates a high recovered energy density (Wrec ∼ 3.66 J/cm3) and efficiency (η ∼ 85.8%) under a low electric field of 210 kV/cm. Meantime, it exhibits a large current density (CD ∼ 831.74 A/cm2), high power density (PD ∼ 78.86 MW/cm3), and fast discharge rate (t0.9 ∼ 0.1 μs), along with good temperature stability and excellent fatigue stability. These properties make the BNLST-0.2KN ceramic a promising candidate for energy storage applications in low electric fields.

Ant-nest-inspired porous structure for MXene composites with high-performance energy-storage and actuating multifunctions

Ant-nest-inspired porous structure for MXene composites with high-performance energy-storage and actuating multifunctions

Achieving High Energy Storage Performance in Lead-Free Relaxor Ferroelectric Ceramics of (1-x)Bi0.88Nd0.10Sm0.02FeO3-xBaTiO3

BiFeO3-based lead-free ferroelectrics have attracted great interest in energy storage applications due to their high spontaneous polarisation strength, however, the high residual polarisation strength (Pr) has become a serious obstacle to their practical application. In this work, (1-x)Bi0.88Nd0.10Sm0.02FeO3-xBaTiO3+0.1 wt% MnO2 ceramics were designed by ion doping and solid solution modification strategies, where Sm3+ and Nd3+ promote ionic disorder and reduce leakage current, and the introduction of nano-BaTiO3 induces the crushing of long-range ferroelectric structure into polar nano-regions with enhanced relaxation. As a result, the ceramic is able to maintain a high maximum polarisation intensity while possessing a low residual polarisation.The recoverable energy density (Wrec) of the 0.50BSNF-0.50BT ceramic is as high as 3.61 J/cm3 with an efficiency of 70.11%. In addition, the 0.50BSNF-0.50BT has good thermal stability (Wrec variation <7%) at 25-120°C. In the charge/discharge test, the Wdis was 2.58 J/cm3 and the discharge time was 273 ns. These results indicate that the 0.50BSNF-0.50BT ceramic is an ideal candidate as a material for energy storage applications.

Radially oriented Ni3(HITP)2 microspheres as high-performance anode materials for Li-ion capacitors with exceptional energy density and cycling stability

Developing of high-energy, high-power-density energy storage devices is challenging. Despite being promising electrode materials for these devices, metal–organic frameworks (MOFs) have poor electrical conductivity and weak ligand–metal coordination. Here, the highly conductive MOF, Ni3(HITP)2, which comprises radially oriented microspheres, is adopted as the anode for Li-ion capacitors (LICs). The half-cell demonstrates high reversible capacity (834 mAh g−1) at 50 mA g−1 with minimal capacity reduction, attributable to the participation of C and N functional groups of the ligand and Ni2+ in the redox reactions during charge/discharge. Especially, density functional theory calculations show that, apart from the inner rings, participation of the outer rings of Ni3(HITP)2 in the redox reactions is responsible for the high Li + storage capacity. An LIC with a Ni3(HITP)2 anode and commercial activated-carbon cathode demonstrates high specific energy (120.7 Wh kg−1) at 89.2 W kg−1, nearly twice as high as that of graphite-based LICs, and maintains specific energy of 25.8 Wh kg−1 even at high specific power (7.16 kW kg−1), over the range 1–4.4 V, along with high cycling stability (76% capacity retention over 10000 cycles). The proposed radially oriented Ni3(HITP)2 microspheres have potential for application as the anode in high-performance energy storage devices.

Boosting the pseudocapacitance of BiFeO3 by Zn doping for flexible battery-supercapacitor hybrid devices

Perovskite BiFeO3 (BFO) as a type of battery-type electrode materials, usually suffers from poor electrolyte ion transport in the BFO crystal. We propose a novel strategy of Zn doping to improve the ion transport and boost the faradaic pseudocapacitance of BFO for flexible battery-supercapacitor hybrid (BSH) devices. The Zn-doped BFO (Z-BFO-1) with the low Zn doping content, can maintain the crystal structure of BFO and the uniform distributions of elements. The oxygen vacancy content can be increased from BFO to Z-BFO-1, reflecting the influence of the Zn doping on the crystal structure of BFO. The Z-BFO-1 electrode with the stable crystal structure in the charge/discharge process, can provide the highest specific capacitance (223 F g−1 at 0.2 A/g) and the largest Li+ diffusion coefficient (2.386 × 10−11 cm2 s−1) among these BFO-based electrodes with different Zn doping contents. The first-principles calculations reveal that the Zn doping in BFO can significantly enhance the built-in electric fields and greatly lower the Li+ migration energy barriers (from 2.08 eV to 1.43 eV). This flexible MnO2//Z-BFO-1 BSH device with the high energy density (37.5 Wh kg−1 at 0.23 kW kg−1), opens up a new avenue for developing high-performance electrochemical energy storage devices.

Superior energy-storage density and ultrahigh efficiency in KNN-based ferroelectric ceramics via high-entropy design

The rapidly advancing energy storage performance of dielectric ceramics capacitors have garnered significant interest for applications in fast charge/discharge and high-power electronic techniques. Simultaneously improving the recoverable energy storage density Wrec and efficiency η becomes more prominent at the present time for their practical applications. Herein, a high-entropy concept is implemented on the (K0.5Na0.5)NbO3 (KNN)-based ferroelectric ceramics to design the high-performance dielectric capacitors. First, the strong lattice distortion can absorb some electric energy during the electrical loading process and result in the delayed polarization saturation. Additionally, the large composition fluctuations induce the weak correlation between polar nanoregions and enhance the η. Finally, the high-entropy design and viscous polymer processing method reduce the grain size and improve the Eb. In consequence, excellent Wrec of 11.14 J/cm3 with high η of 87.1% are achieved under an electric field of 750 kV/cm in the high-entropy component. These results demonstrate that the high-entropy concept is a potential avenue to design the KNN-based high-performance dielectric energy storage capacitors.

Conformal Engineering of Both Electrodes Toward High-Performance Flexible Quasi-Solid-State Zn-Ion Micro-Supercapacitors.

The severe Zn-dendrite growth and insufficient carbon-based cathode performance are two critical issues that hinder the practical applications of flexible Zn-ion micro-ssupercapacitors (FZCs). Herein, a self-adaptive electrode design concept of the synchronous improvement on both the cathode and anode is proposed to enhance the overall performance of FZCs. Polypyrrole doped with anti-expansion graphene oxide and acrylamide (PPy/GO-AM) on the cathode side can exhibit remarkable electrochemical performance, including decent capacitance and cycling stability, as well as exceptional mechanical properties. Meanwhile, a robust protective polymeric layer containing reduced graphene oxide and polyacrylamide is self-assembled onto the Zn surface (rGO/PAM@Zn) at the anode side, by which the "tip effect" of Zn small protuberance can be effectively alleviated, the Zn-ion distribution homogenized, and dendrite growth restricted. Benefiting from these advantages, the FZCs deliver an excellent specific capacitance of 125mFcm-2 (125Fcm-3) at 1mAcm-2, along with a maximum energy density of 44.4µWhcm-2, and outstanding long-term durability with 90.3% capacitance remained after 5000 cycles. This conformal electrode design strategy is believed to enlighten the practical design of high-performance in-plane flexible Zn-based electrochemical energy storage devices (EESDs) by simultaneously tackling the challenges faced by Zn anodes and capacitance-type cathodes.

Ultrahigh energy storage performance in AN-based superparaelectric ceramics

Ceramics capacitors, especially featuring antiferroelectric (AFE) structure, are widely used in pulsed power electronic systems due to distinctive high-power density and external field stability. Lead-free AFE material AgNbO3 has seized substantial research attention owing to its unique temperature driven multi-level phase transitions, and many works have greatly improved the energy storage properties by engineering these temperature-dependent phase boundaries. In this work, an ever-unadopted strategy was proposed to modulate the relaxor structure in AgNbO3 by designing room-temperature (RT) superparaelectric phase via Bi incorporation into AN lattice matrix. Structural analysis shows M phase (space group Pbcm) evolves into O phase (space group Cmcm) with 12 mol.% Bi-substitution, breaking the long-range AFE order into polar nanoregions. Hence, an ultra-high recoverable energy density (7.6 J/cm3) and a high efficiency (79 %) are simultaneously achieved in the Ag0.64Bi0.12NbO3 ceramics under 52.2 kV/mm. Moreover, the excellent energy storage properties are accompanied with good temperature and frequency stability, with the variation of Wrec less than ± 15% (over 25–120 °C) and ± 10% (over 1–200 Hz) under 25 kV/mm, respectively. The novel approach reported herein provides a new guidance for designing AgNbO3-based and other AFE materials for high-performance energy storage applications.

Low-crystallinity conductive multivalence iron sulfide-embedded S-doped anode and high-surface area O-doped cathode of 3D porous N-rich graphitic carbon frameworks for high-performance sodium-ion hybrid energy storages

Sodium-ion hybrid energy storages (SIHESs) are promising electrochemical energy storages for many applications, but their low energy and power densities are yet to be overcome. Herein, we report a strategy to realize ultrahigh-energy density and fast-rechargeable SIHESs. Ultrafine iron sulfide-embedded S-doped carbon/graphene (FS/C/G) anode materials are synthesized from iron-based metal-organic framework (MOF)/graphene oxide heterostructures via graphitic carbon formation and sulfidation. Operando and ex-situ analyses reveal that cycled iron sulfides are rescaled into low-crystallinity conductive fragments with Fe vacancies and multivalence Fe2+/Fe3+ states. Size reduction to fragments inside a 3D porous S-doped N-rich graphitic carbon framework induces high-capacity/high-rate FS/C/G performance. Moreover, 3D porous O-doped carbon cathode materials are synthesized from zeolitic imidazolate frameworks (ZIFs) via pyrolysis-assisted micropore and KOH-assisted mesopore formations. This ZIF-derived porous carbon (ZDPC) has a ∼20-fold higher surface area (3972 m2/g) than conventional ZDCs, O-induced micropores/N-rich sites for high capacity, heteroatom-induced ion-accessible defects/mesopores, and N-rich conductive graphitic carbon networks. Additionally, FS/C/G//ZDPC SHHES benefits from diffusion-controlled and capacitive reactions, as demonstrated by its hitherto highest energy density of 247 Wh kg-1 outperforming state-of-the-art SIHESs, fast-rechargeable power density (up to 34,748 W kg-1) exceeding battery-type reactions by more than 100 folds, and cycle stability with ∼100 % Coulombic efficiency over 5000 charge-discharge cycles.

Lithium-Ion Batteries under the X-ray Lens: Resolving Challenges and Propelling Advancements

The quest for high-performance lithium-ion batteries (LIBs) is at the forefront of energy storage research, necessitating a profound understanding of intricate processes like phase transformations and thermal runaway events. This review paper explores the pivotal role of X-ray spectroscopies in unraveling the mysteries embedded within LIBs, focusing on the utilization of advanced techniques for comprehensive insights. This explores recent advancements in in situ characterization tools, prominently featuring X-ray diffraction (XRD), X-ray tomography (XRT), and transmission X-ray microscopy (TXM). Each technique contributes to a comprehensive understanding of structure, morphology, chemistry, and kinetics in LIBs, offering a selective analysis that optimizes battery electrodes and enhances overall performance. The investigation commences by highlighting the indispensability of tracking phase transformations. Existing challenges in traditional methods, like X-ray absorption spectroscopy (XAS), become evident when faced with nanoscale inhomogeneities during the delithiation process. Recognizing this limitation, the review emphasizes the significance of advanced techniques featuring nanoscale resolution. These tools offer unprecedented insights into material structures and surface chemistry during LIB operation, empowering researchers to address the challenges posed by thermal runaway. Such insights prove critical in unraveling interfacial transport mechanisms and phase transformations, providing a roadmap for the development of safe and high-performance energy storage systems. The integration of X-ray spectroscopies not only enhances our understanding of fundamental processes within LIBs but also propels the development of safer, more efficient, and reliable energy storage solutions. In spite of those benefits, X-ray spectroscopies have some limitations in regard to studying LIBs, as referred to in this review.

Synergistic effects of CoS-decorated Ni-MOF with hierarchical structures for high-performance supercapacitor electrode materials

To further improve the electrochemical performance of supercapacitor, the construction of composite based electrode material was proved to be an effective strategy. Herein, CoS-decorated Ni-MOF composites with hierarchical structures and obvious synergistic effects were synthesized by one-step hydrothermal and subsequent electrodeposition route. Benefitting from the increased surface roughness with more exposed active sites, the hierarchical structure with high space utilization rate for rapid ion transport as well as the improved conductivity and entire robustness, the optimal composites exhibited an excellent specific capacitance (2167.5 F g−1 at 1 A g−1), outstanding rate performance (up to 70.1% even at 5 A g−1) and the good cycle stability (80.12% capacity retention after 3500 cycles). The corresponding assembled asymmetric supercapacitors could reach an energy density 92.18 Wh kg−1 when the power density of was 1626.71 W kg−1, which overwhelmed most other reported works. What is more, the capacity retention could maintain 73.34% even after 7000 cycles, indicating the excellent cycle stability. Overall, to our knowledge, this is the first report on a synergistic effect of a combination of Ni-MOF and CoS. The constructed composites exhibited great potential as promising candidates for high-performance energy storage devices.

Simultaneous impregnation and microencapsulation of CaCl2 using silica gel and methyl cellulose for thermal energy storage applications

Thermal energy storage utilizing the adsorption of moisture from air is a promising energy storage technology due to its high energy density and minimum heat losses. Salt hydrates and salt hydrate composites, such as calcium chloride (CaCl2) and CaCl2-based composites, have shown favourable energy storage properties in this area of research. However, these materials have shown issues with stability due to swelling and deliquescence. In this work, CaCl2 was stabilized using three methods: impregnation into silica gel, encapsulation in methyl cellulose, and both impregnation and encapsulation stabilization techniques used simultaneously. Therefore, three CaCl2-based composites were synthesized. For the first composite, silica gel was impregnated with CaCl2. For the second composite, CaCl2 was encapsulated by methyl cellulose. For the third composite, silica gel was impregnated with CaCl2 and the CaCl2 was encapsulated with methyl cellulose. These samples were structurally characterized using scanning electron microscopy as well as Brunauer-Emmett-Teller (BET) to determine surface area, pore size distribution and nitrogen adsorption isotherms at 77 K. Water vapour adsorption isotherms were also determined at 25 °C for different relative humidities by dynamic vapor sorption (DVS). Similarly, LiCl-based composites were also synthesized and examined in this work, but issues of deliquescence, swelling, and agglomeration made the materials impractical to work with. To determine the prepared materials’ thermal energy storage performance, 2–6 g of each sample was tested in a lab-scale apparatus. This process uses the exothermic adsorption of moisture from ambient air in an open thermal energy storage system. The CaCl2 impregnated silica gel that was encapsulated in methyl cellulose showed reasonably high stability and energy storage performance after 3 hydration and dehydration cycles with minimum agglomeration. An energy storage density of 241 kWh/m3 (0.87 GJ/m3) and specific energy of 630 Wh/kg (2268 kJ/kg) was achieved with this material for 90% inlet relative humidity after a regeneration at 90 °C.

Confined grotthuss proton-conduction along polyoxometalate chains inside carbon nanotubes for high-rate charge storage

Redox-active materials with excellent rate capability and cycling performance are in highly demand for electrochemical energy storage and conversion applications. Here, we unveil the confined proton-conduction behavior of one-dimensional polyoxometalate chains inside single-walled carbon nanotubes (SWNTs). The polyoxometalate molecules including phosphomolybdic acid {HPMo} and phosphotungstic acid {HPW} are encapsulated within SWNTs via host–guest recognition, driven by the electron transfer from nanotubes to polyoxometalates. Impressively, the redox hybrids of polyoxometalate@SWNTs deliver abnormal rate performance in acidic solution with capacity retentions over 73 % at high sweep rate of 1000 mV s−1, relative to the capacity at 10 mV s−1. This value is comparable to that of the pure SWNTs, typically known as double-layer capacitive materials. Electrochemical analyses reveal the Grotthuss proton-conduction mechanism of {HPMo} in SWNTs with a low activation energy of 0.11 eV. Therefore, the {HPMo}@SWNT hybrid demonstrates quasi diffusion-free characteristic with dominant capacitive contribution over 60 % at different sweep rates. Molecular dynamic simulations and density functional theory calculations evidence the strong burst-like transport of protons in the {HPMo}@SWNT hybrid. This process is dynamically favored in the continuous and long-range hydrogen-bond network along polyoxometalate chains with bound/free water molecules inside SWNTs. The superiority of nanoconfined Grotthuss mechanism may engender an exciting avenue for developing high-performance energy storage materials.