Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials

Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer

Redox reactions of aqueous colloidal TiO₂ 4 nm nanoparticles (NPs) have been examined, including both citrate-capped and uncapped NPs (c-TiO₂ and uc-TiO₂). Photoreduction gave stable blue colloidal c-TiO₂^R NPs with 10-60 electrons per particle. Equilibration of these reduced NPs with soluble redox reagents such as methylviologen (MV²⁺) provided measurements of the colloid reduction potential as a function of pH. The potentials of c-TiO₂ from pH 2-9 varied linearly with pH, with a slope of -60 ± 5 mV/pH. Estimates of the potential at pH 12 were consistent with extrapolating that line to high pH. The reduction potentials did not correlate with the zeta potentials (ζ) or the surface charge of the NPs across this pH range. Similar reduction potentials were observed for c- and uc-TiO₂ at low pH even though they have quite different ζ potentials. These results show that the common surface-charging explanation of the pH dependence is not tenable in these systems. Oxidation of reduced c-TiO₂^R with the electron-transfer oxidant potassium triiodide (KI₃) occurred with a significant drop in pH, showing that protons were released when the electrons were removed from the NPs. Smaller pH drops were observed for the proton-coupled electron transfer (PCET) reagents O₂ (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethylpiperine-1-oxy radical). The difference in the number of protons released with KI₃ vs O₂ and 4-MeO-TEMPO was roughly one proton per electron removed. Thus, the thermodynamically preferred reactivity of these colloidal TiO₂ NPs is PCET over the pH 2-13 range studied. The measured redox potentials refer to the chemical process TiO₂ + H⁺ + e^- → TiO₂·e^-,H⁺; and therefore they do not correspond with an electronic energy such as a conduction band edge or flat band potential. The 1e^-/1H⁺ stoichiometry means that the TiO₂ reduction potentials correspond to a TiO₂-H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol^-1. The PCET description is consistent with the pH dependence of E(TiO₂/TiO₂·e^-,H⁺), the release of protons upon oxidation, the lack of correlation with ζ potentials, the similarity of capped and uncapped NPs, and the small change in the potential and BDFE from the first to the last electron/proton pair (H atom) removed. This behavior is suggested to be the norm for redox-active oxide/water interfaces.

Interface-Driven Pseudocapacitance Endowing Sandwiched CoSe2/N-Doped Carbon/TiO2 Microcubes with Ultra-Stable Sodium Storage and Long-Term Cycling Stability

Cobalt diselenide (CoSe₂) has drawn great concern as an anode material for sodium-ion batteries due to its considerable theoretical capacity. Nevertheless, the poor cycling stability and rate performance still impede its practical implantation. Here, CoSe₂/nitrogen-doped carbon-skeleton hybrid microcubes with a TiO₂ layer (denoted as TNC-CoSe₂) are favorably prepared via a facile template-engaged strategy, in which a TiO₂-coated Prussian blue analogue of Co₃[Co(CN)₆]₂ is used as a new precursor accompanied with a selenization procedure. Such structures can concurrently boost ion and electron diffusion kinetics and inhibit the structural degradation during cycling through the close contact between the TiO₂ layer and NC-CoSe₂. Besides, this hybrid structure promotes the superior Na-ion intercalation pseudocapacitance due to the well-designed interfaces. The as-prepared TNC-CoSe₂ microcubes exhibit a superior cycling capability (511 mA h g^-1 at 0.2 A g^-1 after 200 cycles) and long cycling life (456 mA h g^-1 at 6.4 A g^-1 for 6000 cycles with a retention of 92.7%). Coupled with a sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃)@C cathode, this assembled full cell displays a specific capacity of 281 mA h g^-1 at 0.2 A g^-1 for 100 cycles. This work can be potentially used to improve other metal selenide-based anodes for rechargeable batteries.

Dual-Ion Intercalation and High Volumetric Capacitance in a Two-Dimensional Non-Porous Coordination Polymer

Intercalation is a promising ion-sorption mechanism for enhancing the energy density of electrochemical capacitors (ECs) because it offers enhanced access to the electrochemical surface area. It requires a rapid transport of ions in and out of a host material, and it must occur without phase transformations. Materials that fulfil these requirements are rare; those that do intercalate almost exclusively cations. Herein, we show that Ni₃ (benzenehexathiol) (Ni₃ BHT), a non-porous two-dimensional (2D) layered coordination polymer (CP), intercalates both cations and anions with a variety of charges. Whereas cation intercalation is pseudocapacitive, anions intercalate in a purely capacitive fashion. The excellent EC performance of Ni₃ BHT provides a general basis for investigating similar dual-ion intercalation mechanisms in the large family of non-porous 2D CPs.

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Stabilization of Polyoxometalate Charge Carriers via Redox-Driven Nanoconfinement in Single-walled Carbon Nanotubes

We describe the preparation of hybrid redox materials based on polyoxomolybdates encapsulated within single-walled carbon nanotubes (SWNTs). Polyoxomolybdates readily oxidize SWNTs under ambient conditions in solution, and here we study their charge-transfer interactions with SWNTs to provide a detailed mechanistic insight into the redox-driven encapsulation of these - and similar - nanoclusters. We are able to correlate the relative redox potentials of the encapsulated clusters with the level of SWNT oxidation in the resultant hybrid materials and use this to show that precise redox tuning is a necessary requirement for successful encapsulation. The host-guest redox materials described here exhibit exceptional electrochemical stability, retaining up to 86% of their charge capacity over 1000 oxidation/reduction cycles despite the typical lability and solution-phase electrochemical instability of the polyoxomolybdates we have explored. Our findings illustrate the broad applicability of the redox-driven encapsulation approach to the design and fabrication of tunable, highly conductive, ultra-stable nanoconfined energy materials.

Catalytic oxidation and deionization of nitrite and nitrate ions using mesoporous carbon-supported nano-flaky cobalt and nickel oxyhydroxides

The composite electrode of NiCo oxide supported by porous carbon was synthesized for nitrite oxidation and nitrate electro-sorption. The crystal structure and chemical state of the Co and Ni oxyhydroxides which were precipitated on loofah-derived activated carbon (AC) using hypochlorite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The voltammetry showed that the redox couple of Co(II)/Co(III) and Ni(II)/Ni(III) as the mediator catalytically transferred the electrons of NO₂^-/NO₃^-; the Ni site had a relatively high transfer coefficient and diffusive current, while the Co site was better in the capacitive removal of the nitrite and nitrate compounds. A batch electrolysis of nitrite ions was operated under constant anodic potential mode (0 to + 1.5 V vs. Ag/AgCl) to assess the performance of the composite electrodes. The adsorption capacity of NiCo/AC (Ni = 5% and Co = 5% on AC by weight) was 23.5 mg-N g^-1, which was twice that of AC substrate (7.5 mg-N g^-1), based on a multilayer adsorption model. The steady-state kinetics of the consecutive reaction were derived to determine the rate steps of the electrochemical oxidation of NO₂^- and adsorption of NO₃^-.

Boosting the lithium storage performance by synergistically coupling ultrafine heazlewoodite nanoparticle with N, S co-doped carbon

Transition metal sulfides, as an important class of inorganics, have been shown to be potential high-performance electrode candidates for lithium-ion batteries (LIBs) in account of their high activity towards lithium storage, rich types and diverse structures. Despite these advantages, structure degradation related with volume variations upon electrochemical cycling restricts their further development. In this present study, a unique hybrid structure with ultrafine heazlewoodite nanoparticles (less than 10 nm) in-situ confined in nitrogen and sulfur dual-doped carbon (Ni₃S₂@NSC) was constructed though a facile pyrolysis process, using a novel Ni-based metal chelates as the precursor. Specifically, enhanced structure stability, shortened Li⁺ migration distance and improved reaction dynamics can be obtained simultaneously in the designed structure, thereby allowing to realize high lithium storage performance. Consequently, a remarkable reversible capacity of 955.9 mAh g^-1 (0.1 A g^-1 after 100 cycles) and a superior long-term cycling stability up to 1200 cycles (863.7 mAh g^-1 at 1.0 A g^-1) are obtained. Importantly, the fundamental understanding on the improved lithium storage of Ni₃S₂@NSC based on the synergistic coupling reveals that the combination between Ni₃S₂ and NSC at the hetero-interface through the doped sulfur atoms contributes to the integrity of electrode and improved kinetics.

Nano-dendrite structured cobalt phosphide based hybrid supercapacitor with high energy storage and cycling stability

The supercapacitors possessing high energy storage and long serving period have strategic significance to solve the energy crisis issues. Herein, fluffy nano-dendrite structured cobalt phosphide (CoP) is grown on carbon cloth through simple hydrothermal and electrodeposition treatments (CoP/C-HE). Benefit from its excellent electrical conductivity and special structure, CoP/C-HE manifests a high specific capacity of 461.4 C g^-1at 1 A g^-1. Meanwhile, the capacity retention remains 92.8% over 10 000 cycles at 5 A g^-1, proving the superior cycling stability. The phase conversion of Co₂P during the activation process also contributes to the improved performance. The assembled two-electrode asymmetric supercapacitor demonstrates excellent performance in terms of energy density (42.4 W h kg^-1at a power density of 800.0 W kg^-1) and cycling stability (86.3% retention over 5000 cycles at 5 A g^-1), which is superior to many reported cobalt-based supercapacitors. Our work promotes the potential of transition metal phosphides for the applications in supercapacitors.

Effects of interlayer confinement and hydration on capacitive charge storage in birnessite

Nanostructured birnessite exhibits high specific capacitance and nearly ideal capacitive behaviour in aqueous electrolytes, rendering it an important electrode material for low-cost, high-power energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both Faradaic (involving redox) and non-Faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite's response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy and operando atomic force microscope dilatometry to provide a holistic understanding of its structural, gravimetric and mechanical responses. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite, ReaxFF reactive force field-based molecular dynamics and ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite-water-electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.

Embellishing hierarchical 3D core-shell nanosheet arrays of ZnFe2O4@NiMoO4 onto rGO-Ni foam as a binder-free electrode for asymmetric supercapacitors with excellent electrochemical performance

Tailoring hierarchical hybrid core-shell electrodes with impartial microstructural features and excellent electroactive constituents is crucial for the design of high-performance supercapacitors (SCs). Herein, for the first time, we fabricate uniformly aligned porous ZnFe₂O₄ (ZFO) nanosheet arrays onto reduced graphene oxide-garnished conductive Ni foam (rGO-NF) substrates and subsequently embellish the first layer of ZFO nanosheets with morphology-controlled secondary NiMoO₄ nanosheets to achieve a hierarchical 3D core-shell structure of ZnFe₂O₄@NiMoO₄ nanosheet arrays (NSAs) onto rGO-NF for SC applications. Improving the synergistic effect of the core-shell nanoarchitecture with a conductive rGO-NF substrate, the hierarchical 3D ZFO@NMO NSAs tend to have superb electronic conductivity, tailoribility, effective nanoporous channels, and appropriate roadways for rapid ion/electron transfer, which are required for rapid reversible redox reactions, thus reflecting the excellent electrochemical features, including the excellent specific capacitance, good rate performance, and prolonged cyclic performance of the three electrode assemblies for SCs. An asymmetric supercapacitor (ASC) device composed of ZFO@NMO NSAs@rGO-NF as the cathode and MOF-derived hollow porous carbon (MDHPC) as the anode exhibits a high energy density of 58.6 Wh kg^-1 at a power density of 799 W kg^-1 with prolonged cyclic durability (89.6 % after 7000 cycles), thus indicating its potential applicability towards advanced hybrid SCs.

Defect-rich CeO2 in a hollow carbon matrix engineered from a microporous organic platform: a hydroxide-assisted high performance pseudocapacitive material

A microporous organic polymer (MOP) was utilized for the engineering of nanoparticulate CeO₂ in a hollow carbon matrix (H-C/CeO₂). After CeO₂ nanoparticles were incorporated into a hollow MOP platform (H-MOP) through the decomposition of cerium acetate, successive carbonization produced H-C/CeO₂. The redox feature of defective CeO₂ in a conductive carbon matrix induced promising pseudocapacitive behavior. In particular, the H-C/CeO₂ showed excellent electrochemical performance in an alkaline electrolyte (KOH), due to the hydroxide ion-assisted redox behavior of defective CeO₂. H-C/CeO₂-3 with an optimized amount of CeO₂ showed specific capacitances of up to 527 (@0.5 A g^-1) and 493 F g^-1 (@1 A g^-1). Even at high current densities of 10 and 20 A g^-1, the H-C/CeO₂-3 maintained high capacitances of 458 and 440 F g^-1, respectively. After 10 000 cycling tests, the H-C/CeO₂-3 retained the 94-95% capacitance of the first cycle.

Solvent Co-Intercalation-Induced Activation and Capacity Fade Mechanism of Few-/Multi-Layered MXenes in Lithium Ion Batteries

MXenes attract tremendous research efforts since their discovery in 2011 due to their unique physical and chemical properties, allowing for application in various fields. One of them is electrochemical energy storage due to their pseudocapacitive (=redox) behavior, high electronic conductivity, and charge storage versatility regarding the cationic species (e.g., Li⁺ ). MXenes typically display stable charge/discharge cycling behavior over hundreds of cycles in numerous electrolytes, however, a drastic loss of reversible capacity is detectable during the initial cycles. Furthermore, an electrochemical "activation" is also reported in the literature, especially for free-standing electrodes. Here, these electrochemical phenomena are investigated by electrochemical and analytical means to decipher the responsible mechanism by comparing few-layered and multi-layered Ti₃ C₂ T_x . A change in the pseudocapacitive behavior of MXenes during cycling can be explained by in situ X-ray diffraction studies, revealing solvent co-intercalation in the first cycle for the morphologically different MXenes. This co-intercalation is responsible for the capacity decay detected in the first cycles and is also responsible for the ongoing "activation" occurring in later cycles.

3D Cross-linked Ti3C2Tx-Ca-SA films with expanded Ti3C2Tx interlayer spacing as freestanding electrode for all-solid-state flexible pseudocapacitor

Ti₃C₂T_x, a member of the MXene, has attracted extensive interest because of its high conductivity, unique two-dimensional (2D) structure and intrinsic pseudocapacitance for supercapacitors. Flexible and freestanding Ti₃C₂T_x films are promising electrodes for functioned supercapacitors used in wearable and portable electronic devices. However, the severe self-restacking of 2D Ti₃C₂T_x nanosheets restraints their practical application. Herein, freestanding and flexible three-dimensional (3D) cross-linked Ti₃C₂T_x-Ca-SA (sodium alginate) films with expanded Ti₃C₂T_x interlayer spacing are reported. The expanded interlayer spacing allows more electrolyte ions to quickly intercalate providing more intercalation pseudocapacitance, while the 3D cross-linked microstructure ensures a continuous conductive network facilitating charges transport. Attributing to the unique structure, the Ti₃C₂T_x-Ca-SA film delivers an outstanding areal capacitance (633 mF cm^-2 at 5 mV s^-1). Meanwhile, the assembled all-solid-state pseudocapacitor shows good flexibility and capacity stability under various bending conditions. The device exhibits a high energy density up to 12.6 µWh cm^-2 at the power density of 375 µW cm^-2 and excellent cycling stability, which are much better than prior reported state-of-the-art supercapacitors. This research exploits a simple method to optimize the structure of MXene as state-of-the-art electrodes for high-performance flexible energy-storage devices.

Host-Guest Intercalation Chemistry in MXenes and Its Implications for Practical Applications

The ever-increasing demand on developing layered materials for practical applications, such as electrochemical energy storage, responsive materials, nanofluidics, and environmental remediation, requires the profound understanding and artful exploitation of interlayer engineering or intercalation chemistry. The past decade has witnessed the massive exploration of a recently discovered 2D material-transition metal carbides, carbonitrides, and nitrides (referred to as MXenes), which began to take hold of a myriad of applications owing to the abundant possibilities on their compositions and intercalation states. However, application-targeted manipulation of the material performance of MXenes is constrained by the dearth of deep comprehension on fundamental intercalation chemistry/physics. To this end, the aim of this review is to provide a holistic discussion on the intercalation chemistry in MXenes and the physical properties of MXene intercalation compounds. On the basis of this, potential solutions for the challenges confronted in the synthesis, tuning of material properties, and practical applications are proposed, which are also expected to reinvigorate the exploration of layered materials that are similar to MXenes.

Enhancing capacity and transport kinetics of C@TiO2core-shell composite anode by phase interface engineering

In nanocomposite electrodes, besides the synergistic effect that takes advantage of the merits of each component, phase interfaces between the components would contribute significantly to the overall electrochemical properties. However, the knowledge of such effects is far from being well developed up to now. The present work aims at a mechanistic understanding of the phase interface effect in C@TiO₂core-shell nanocomposite anode which is both scientifically and industrially important. Firstly, amorphous C, anatase TiO₂and C@anatse-TiO₂electrodes are compared. The C@anatase-TiO₂shows an obvious higher specific capacity (316.5 mAh g^-1at a current density of 37 mA g^-1after 100 cycles) and Li-ion diffusion coefficient (4.0 × 10^-14cm²s^-1) than the amorphous C (178 mAh g^-1and 2.9 × 10^-15cm²s^-1) and anatase TiO₂(120 mAh g^-1and 1.6 × 10^-15cm²s^-1) owing to the C/TiO₂phase interface effect. Then, C@anatase/rutile-TiO₂is obtained by a heat treatment of the C@anatase-TiO₂. Due to an anatase-to-rutile phase transformation and diffusion of C along the anatase/rutile phase interface, additional abundant C/TiO₂phase interfaces are created. This endows the C@anatase/rutile-TiO₂with further boosted specific capacity (409.4 mAh g^-1at 37 mA g^-1after 100 cycles) and Li-ion diffusion coefficient (3.2 × 10^-13cm²s^-1), and excellent rate capability (368.6 mAh g^-1at 444 mA g^-1). These greatly enhanced electrochemical properties explicitly reveal phase interface engineering as a feasible way to boost the electrochemical performance of nanocomposite anodes for Li-ion batteries.

Hybrid Li-Ion Capacitor Operated within an All-Climate Temperature Range from -60 to +55 °C

Lithium-ion capacitors (LICs) have been considered as an advanced energy storage system owing to their high energy and power densities. However, their application in a wide temperature range is still a great challenge due to the reduced ionic conductivity of the electrolyte and the poor electric conductivity of the battery-type transition metal oxide electrodes. Herein, an all-climate LIC is well-fabricated with TiNb₂O₇@expanded graphite as the anode and activated carbon as the cathode in an optimized electrolyte, which can be operated within a wide temperature range from -60 to +55 °C. Benefitting from the synergetic effect of the improved electrode and electrolyte, the LIC exhibits an outstanding energy density of 119 W h kg^-1 and a power density of 5110 W kg^-1 based on the total mass of both negative and positive electrodes. Moreover, it can deliver a capacity retention of as high as 42% at -60 °C and function at a superior rate capability at a high temperature of +55 °C, which exhibits an all-climate feature and the potential for wide applications under some extreme conditions.

Titanium Carbide MXene Shows an Electrochemical Anomaly in Water-in-Salt Electrolytes

Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti₃C₂T_x MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

Grotthuss Proton-Conductive Covalent Organic Frameworks for Efficient Proton Pseudocapacitors

Herein, we describe the synthesis of two highly crystalline, robust, hydrophilic covalent organic frameworks (COFs) that display intrinsic proton conduction by the Grotthuss mechanism. The enriched redox-active azo groups in the COFs can undergo a proton-coupled electron transfer reaction for energy storage, making the COFs ideal candidates for pseudocapacitance electrode materials. After in situ hybridization with carbon nanotubes, the composite exhibited a high three-electrode specific capacitance of 440 F g^-1 at the current density of 0.5 A g^-1 , among the highest for COF-based supercapacitors, and can retain 90 % capacitance even after 10 000 charge-discharge cycles. This is the first example using Grotthuss proton-conductive organic materials to create pseudocapacitors that exhibited both high power density and energy density. The assembled asymmetric two-electrode supercapacitor showed a maximum energy density of 71 Wh kg^-1 with a maximum power density of 42 kW kg^-1 , surpassing that of all reported COF-based systems.

Probing the In Situ Pseudocapacitive Charge Storage in Ti3C2 MXene Thin Films with X-ray Reflectivity

MXenes are a large family of two-dimensional materials that are attractive for energy storage due to their high-rate charging capabilities as well as for electrochemical actuators, water purification, and many other technologies. Ion intercalation during electrochemically driven charge and discharge processes is the fundamental process associated with MXene functionality, which we have characterized using in situ and operando X-ray reflectivity (XRR). Experiments performed at the Advanced Photon Source at Argonne National Laboratory monitored the changes in the structure of a Ti₃C₂ MXene film on a platinum current collector as a function of static applied potential between 0.3 and -0.7 V vs Ag/AgCl in an aqueous 0.1 M Li₂SO₄ electrolyte. Negative potential sweeps lead to a contraction of 1.2 Å in the interlayer spacing and a loss of electron density between the layers, likely due to Li⁺ ion insertion and water removal. The change in lattice spacing includes a continuous variation vs potential as well as an additional discrete contraction that occurs near -0.35 V that has the characteristics of a first-order transition. The continuous change in the MXene interlayer spacing is associated with the capacitive charge, while the discrete change in structure correlated to the weak feature in the cyclic voltammogram at -0.35 V can be interpreted as either a pseudocapacitive charging process or a potential-dependent change in capacity.

Ultra-dispersed nickel-cobalt sulfides on reduced graphene oxide with improved power and cycling performances for nickel-zinc batteries

Rechargeable alkaline nickel-zinc (Ni-Zn) batteries are attracting increased attention owing to their exceptional inherent safety and high specific capacity. Unfortunately, the limited power and cycling performances of these Ni-Zn batteries are still challenging. Herein, bimetal nickel-cobalt sulfide/ reduced graphene oxide (NiCo-S/RGO) composites with tunable compositions are synthesized by rational designing precursor and subsequent sulfidation treatment. NiCo-S is evenly anchored on RGO surface, resulting in increased number of electrochemical active sites, accelerated electrolyte ion diffusion, and enhanced electrical conductivity. Particularly, by tuning the Ni and Co composition ratios in NiCo-S, NiCo-S/RGO with a Ni to Co ratio of 2:1 (NiCo-S-2/RGO) shows a specific capacity of 145.7 mA h g^-1 at 1 A g^-1 and long-life cycling retention of 84.7% after 1000 cycles, and the above performances are superior than the controlled samples with other Ni to Co ratios. Furthermore, the as-assembled alkaline zinc batteries of NiCo-S-2/RGO//Zn deliver an impressive specific energy of 333.2 W h kg^-1, showing great potential in practical applications. This experiment hopefully provides new idea for construction of high-performance electrodes of aqueous rechargeable batteries.

Facile Synthesis of Graphene with Fast Ion/Electron Channels for High-Performance Symmetric Lithium-Ion Capacitors

With the battery-type anode and capacitor-type cathode, lithium-ion capacitors (LICs) are expected to exhibit both high energy and high power density but suffer from the mismatch of the electrode reaction kinetics and capacity. Herein, to alleviate the mismatch between the two electrodes and synergistically enhance the energy/power density, we design a method of microwave irradiation reduction to prepare graphene-based electrode material (MRPG/CNT) with fast ion/electron pathway. The three-dimensional structure of CNT intercalation to graphene inhibits the restacking of graphene sheets and improves the conductivity of the electrode material, resulting a rapid ion and electron diffusion channel. Due to its specific properties, MRPG/CNT materials can be used as both anode and cathode electrodes of LICs at the same time. As anode, MRPG/CNT shows a high capacity of 1200 mAh g^-1 as well as high rate performance. As cathode, MRPG/CNT displays a high capacity of 108 mAh g^-1 and the capacity retention of 100% after 8000 cycles. Coupling the prelithiated MRPG/CNT anode with MRPG/CNT cathode gives a full-graphene-based symmetric LIC, which achieves a high energy density of 232.6 Wh kg^-1 at 226.0 W kg^-1, 111.2 Wh kg^-1 at the ultrahigh power density of 45.2 kW kg^-1, and superior capacity retention of 86% after 5000 cycles. The structure design of this electrode provides a new strategy for alleviating the mismatch of LIC electrodes and constructing high-performance symmetrical LICs.

α-TiPO4 as a Negative Electrode Material for Lithium-Ion Batteries

To realize high-power performance, lithium-ion batteries require stable, environmentally benign, and economically viable noncarbonaceous anode materials capable of operating at high rates with low strain during charge-discharge. In this paper, we report the synthesis, crystal structure, and electrochemical properties of a new titanium-based member of the MPO₄ phosphate series adopting the α-CrPO₄ structure type. α-TiPO₄ has been obtained by thermal decomposition of a novel hydrothermally prepared fluoride phosphate, NH₄TiPO₄F, at 600 °C under a hydrogen atmosphere. The crystal structure of α-TiPO₄ is refined from powder X-ray diffraction data using a Rietveld method and verified by electron diffraction and high-resolution scanning transmission electron microscopy, whereas the chemical composition is confirmed by IR, energy-dispersive X-ray, electron paramagnetic resonance, and electron energy loss spectroscopies. Carbon-coated α-TiPO₄/C demonstrates reversible electrochemical activity ascribed to the Ti³⁺/Ti²⁺ redox transition delivering 125 mAh g^-1 specific capacity at C/10 in the 1.0-3.1 V versus Li⁺/Li potential range with an average potential of ∼1.5 V, exhibiting good rate capability and stable cycling with volume variation not exceeding 0.5%. Below 0.8 V, the material undergoes a conversion reaction, further revealing capacitive reversible electrochemical behavior with an average specific capacity of 270 mAh g^-1 at 1C in the 0.7-2.9 V versus Li⁺/Li potential range. This work suggests a new synthesis route to metastable titanium-containing phosphates holding prospective to be used as negative electrode materials for metal-ion batteries.

Recent Advances in Graphene and Conductive Polymer Composites for Supercapacitor Electrodes: A Review

Supercapacitors (SCs) have generated a great deal of interest regarding their prospects for application in energy storage due to their advantages such as long life cycles and high-power density. Graphene is an excellent electrode material for SCs due to its high electric conductivity and highly specific surface area. Conductive polymers (CPs) could potentially become the next-generation SC electrodes because of their low cost, facile synthesis methods, and high pseudocapacitance. Graphene/CP composites show conspicuous electrochemical performance when used as electrode materials for SCs. In this article, we present and summarize the synthesis and electrochemical performance of graphene/CP composites for SCs. Additionally, the method for synthesizing electrode materials for better electrochemical performance is discussed.

Hierarchical porous carbon materials synthesized from the castor oil/MgO solids for high-performance supercapacitors

A kind of biomass-based hierarchical porous carbons (HPCs) is easily synthesized by the direct pyrolysis of castor oil/MgO solids, in which the solids can be prepared by mixing appropriate amounts of MgO into castor oil. The morphology, microstructure, phase structure, textural property, surface element composition and thermal stability properties are studied for the achieved HPCs. It is demonstrated that the HPCs belong to a type of high-graphitization, graphene-like and foamed carbon materials with high specific surface area and wide pore size distribution. The HPC obtained at 900 °C (HPC-900) displays the highest specific surface area of 1013.17 m²g and more reasonable pore size distribution. Without the need of conductive agents, the HPC-900 exhibits a maximum capacitance (340 F g^-1at 0.5 A g^-1), excellent rate performance (70.1% of capacitance retention at high current density of 10 A g^-1) and a remarkable long-term cycling stability (about 99% capacitance retention after 9000 cycles) in the aqueous electrolyte of 6 M KOH. Meanwhile, assembled as-prepared sample into symmetrical supercapacitor, the HPC-900 provides a high energy and power density, with 8.6 Wh kg^-1and 426.7 W kg^-1in 1 M Na₂SO₄, respectively. The HPCs prepared based on castor oil show high potential for the electrode materials of supercapacitors.

Modeling and Experimental Study of the Electron Transfer Kinetics for Non-ideal Electrodes Using Variable-Frequency Square Wave Voltammetry

The knowledge of nonequilibrium electron transfer rates is paramount for the design of modern hybrid electrocatalysts. Herein, we propose a general simulation-based approach to interpret variable-frequency square wave voltammetry (VF-SWV) for heterogeneous materials featuring reversible redox behavior. The resistive and capacitive corrections, inclusion of the frequency domain, and statistical treatment of the surface redox kinetics are used to account for the non-ideal nature of electrodes. This approach has been validated in our study of Co^II/Co^I redox transformation for Co tetraphenylporphyrin (CoTPP) immobilized on carbon cloth and multiwalled carbon nanotubes (CNTs) - one of the most active heterogeneous molecular catalysts in carbon dioxide (CO₂) electroreduction. It is demonstrated that the modeling of experimental data furnishes the capacitance of the surface double layer C, uncompensated resistance R_u, symmetry coefficients α, kinetic constants k₀, and equilibrium redox potentials E⁰ in one experiment. Moreover, the proposed method yields a stochastic map of the redox kinetics rather than a single value, thus exposing the inhomogeneous nature of the electrochemically active layer. The computed parameters are in excellent agreement with the results of the classic methods such as cyclic voltammetry and fall in line with the reported CoTPP catalytic activity. Thus, VF-SWV is suitable for the study of high-level composites such as covalent organic frameworks and organometallic-CNT mixtures. The resulting insights into the electron transfer mechanisms are especially useful for the rational development of the catalyst-support interfaces and immobilization methods.

Interlayer Modification of Pseudocapacitive Vanadium Oxide and Zn(H2 O)n 2+ Migration Regulation for Ultrahigh Rate and Durable Aqueous Zinc-Ion Batteries

The interlayer modification and the intercalation pseudocapacitance have been combined in vanadium oxide electrode for aqueous zinc-ion batteries. Intercalation pseudocapacitive hydrated vanadium oxide Mn1.4 V10 O24 ·12H2 O with defective crystal structure, interlayer water, and large interlayer distance has been prepared by a spontaneous chemical synthesis method. The inserted Mn2+ forms coordination bonds with the oxygen of the host material and strengthens the interaction between the layers, preventing damage to the structure. Combined with the experimental data and DFT calculation, it is found that Mn2+ refines the structure stability, adjusts the electronic structure, and improves the conductivity of hydrated vanadium oxide. Also, Mn2+ changes the migration path of Zn2+ , reduces the migration barrier, and improves the rate performance. Therefore, Mn2+ -inserted hydrated vanadium oxide electrode delivers a high specific capacity of 456 mAh g-1 at 0.2 A g-1 , 173 mAh g-1 at 40 A g-1 , and a capacity retention of 80% over 5000 cycles at 10 A g-1 . Furthermore, based on the calculated zinc ion mobility coefficient and Zn(H2 O)n 2+ diffusion energy barrier, the possible migration behavior of Zn(H2 O)n 2+ in vanadium oxide electrode has also been speculated, which will provide a new reference for understanding the migration behavior of hydrated zinc-ion.

Engineering triangular bimetallic metal-organic-frameworks derived hierarchical zinc-nickel-cobalt oxide nanosheet arrays@reduced graphene oxide-Ni foam as a binder-free electrode for ultra-high rate performance supercapacitors and methanol electro-oxidation

The rigorous fabrication of electrode materials using upper-ranked porous precursor especially metal organic frameworks (MOFs) are challenging but appealing task to procure electrochemical energy storage and conversion system with altitudinous performance. Herein, we replenish the rational construction of atypical electrode of hollow Zn-Ni-Co-oxide (ZNCO) nanosheet arrays onto rGO garnished Ni foam (rGO/NF) via two step solution based method. Firstly, 2D Zn-Co-MOFs derived nanoleave arrays are prepared by co-precipitation method. Next, hollow and porous ZNCO nanostructure from 2D solid nanoleave arrays are achieved by ion-exchange and etching process conjoined with post annealing treatment. The as-fabricated hierarchical ZNCO nanosheet arrays offer large numbers of electroactive sites with short ion-diffusion pathways, reflecting the outstanding electrochemical performance in-terms of excellent specific capacity (267 mAh g^-1) ultra-high rate capability (83.82% at 50 A/g) and long-term cycling life (~90.16%) in three electrode configuration for supercapacitor (SCs). Moreover, the hollow and porous ZNCO nanostructure responds as immensely active and substantial electrocatalyst for methanol oxidation with lowest onset potential of 0.27 V. To demonstrate the practicability, hybrid supercapacitor (HSC) device is constructed using ZNCO@rGO-NF nanostructure as positive and rGO decorated MOF derived porous carbon (rGO-MDPC) as negative electrode. The as-assembled ZNCO//rGO-MDPC ASC device delivers higher energy density of 61.25 Wh kg^-1 at the power density of 750 W kg^-1 with long-term cyclic stability (<6% to its initial specific capacity value) after 6000 cycles.

A vacancy-rich perovskite fluoride K0.79Ni0.25Co0.36Mn0.39F2.83@rGO anode for advanced Na-based dual-ion batteries

A novel concept of Na-based dual-ion batteries (Na-DIBs) has been designed via a perovskite K0.79Ni0.25Co0.36Mn0.39F2.83@reduced graphene oxide (KNCMF@rGO) hetero-nanocrystal anode, showing surface conversion and insertion hybrid mechanisms. The KNCMF@rGO//graphite (KS6) DIBs deliver superior energy/power densities and cycling stability and have a significant impact on developing energy storage devices.

Scalable Synthesis of MAX Phase Precursors toward Titanium-Based MXenes for Lithium-Ion Batteries

MXenes have emerged as one of the most interesting material classes, owing to their outstanding physical and chemical properties enabling the application in vastly different fields such as electrochemical energy storage (EES). MXenes are commonly synthesized by the use of their parent phase, i.e., MAX phases, where "M" corresponds to a transition metal, "A" to a group IV element, and "X" to carbon and/or nitrogen. As MXenes display characteristic pseudocapacitive behaviors in EES technologies, their use as a high-power material can be useful for many battery-like applications. Here, a comprehensive study on the synthesis and characterization of morphologically different titanium-based MXenes, i.e., Ti₃C₂ and Ti₂C, and their use for lithium-ion batteries is presented. First, the successful synthesis of large batches (≈1 kg) of the MAX phases Ti₃AlC₂ and Ti₂AlC is shown, and the underlying materials are characterized mainly by focusing on their structural properties and phase purity. Second, multi- and few-layered MXenes are successfully synthesized and characterized, especially toward their ever-present surface groups, influencing the electrochemical behavior to a large extent. Especially multi- and few-layered Ti₃C₂ are achieved, exhibiting almost no oxidation and similar content of surface groups. These attributes enable the precise comparison of the electrochemical behavior between morphologically different MXenes. Since the preparation method for few-layered MXenes is adapted to process both active materials in a "classical" electrode paste processing method, a better comparison between both materials is possible by avoiding macroscopic differences. Therefore, in a final step, the aforementioned electrochemical performance is evaluated to decipher the impact of the morphology difference of the titanium-based MXenes. Most importantly, the delamination leads to an increased non-diffusion-limited contribution to the overall pseudocapacity by enhancing the electrolyte access to the redox-active sites.

High-power-energy proton supercapacitor based on interface-adapted durable polyaniline and hexagonal tungsten oxide

Supercapacitors are high power energy storage devices, however, their application are remain limited by the low energy density. Developing high capacity electrode materials and constructing devices with high operating voltage are effective ways to solve this problem. Herein, performance of polyaniline (PANI) electrode materials is dramatically enhanced by engineering robust PANI/carbon interfaces, through assembling PANI nanorod array on rose petals derived carbon network (RPDCN). The structure of the PANI is optimized by adjusting the concentration of the aniline precursor. The unique structure enables the prepared PANI/RPDCN composite show a high capacitance of 636 F g^-1 at 0.5 A g^-1, based on the total weight of PANI and RPDCN substrate. The robust interface effectively prolonged the composite electrode stably cycled for over 2000 cycles at 2 A g^-1 with a capacity retention of 89%. When coupled with a hexagonal tungsten oxide (h-WO₃) anode, a high-power asymmetric proton supercapacitor with high energy densities (29.0 Wh kg^-1/0.61 kW kg^-1 and 21.4 Wh kg^-1/19.51 kW kg^-1) was assembled. This work provides an effective and eco-friendly route toward superior PANI electrodes and proposes a promising high-power energy storage system using proton as working ion.

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Nanocrystalline anatase TiO₂ is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li insertion and electron accumulation, respectively, and these optical signals are used to deconvolute bulk Li insertion from other electrochemical responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO₂ systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concentration, and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO₂ nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO₂ particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochemistry.

Application of 2D Materials to Potassium-Ion Hybrid Capacitors

Metal-ion hybrid supercapacitors (MICs) are a new type of electrochemical energy storage (EES) device, consisting of a battery-type electrode and a supercapacitor (SC)-type electrode. Exhibiting the advantages of both batteries and SCs (e. g., good energy density, excellent power density and long cycle life), these advanced energy storage devices have considerable commercial application prospects. Among MICs, potassium-ion hybrid supercapacitors (PICs) have several further advantages, including abundancy of resources, low standard electrode potential, and low cost. PICs are regarded as potential substitutes for lithium- or sodium-ion hybrid supercapacitors. However, the practical applications of PICs remain limited, owing to the imbalance of kinetics and capacity between the electrodes, the slow ion/electron diffusion rate, and the poor electrode structural stability. Recently, 2D materials with distinct structures and fascinating features have elicited widespread attention for application in PICs, thus achieving significant enhancements, ranging from charge storage capacity to reaction kinetics. This Review discusses research progress in 2D materials for PICs. Firstly, the energy storage principle and development requirements of MICs are introduced. The pivotal advantages and significant roles of 2D materials in the fabrication of PICs are then discussed in detail. Lastly, the challenges and prospects of the application of 2D materials to high-performance PICs are presented.

Hybrid carbon spherogels: carbon encapsulation of nano-titania

Extraordinarily homogeneous, freestanding titania-loaded carbon spherogels can be obtained using Ti(acac)₂(OiPr)₂ in the polystyrene sphere templated resorcinol-formaldehyde gelation. Thereby, a distinct, crystalline titania layer is achieved inside every hollow sphere building unit. These hybrid carbon spherogels allow capitalizing on carbon's electrical conductivity and the lithium-ion intercalation capacity of titania.

Gelation of organic liquid electrolyte to achieve superior sodium-ion full-cells

The irreversible consumption of active sodium in sodium-ion full-cells (SIFCs) becomes particularly serious due to the existence of unavoidable interface or side reaction, which has become the key to restrict the development of high-performance sodium-ion batteries (SIBs). Interface design and electrolyte optimization have been proved to be effective strategies to improve or solve this problem. In this work, on the basis of traditional organic liquid electrolytes, a novel gel polymer electrolyte with high ionic conductivity (1.13 × 10^-3 S cm^-1) and wide electrochemical stability window (~4.7 V) was designed and synthesized using bacterial cellulose film as precursor. Compared with the liquid electrolyte, the obtained electrolyte can endow better sodium storage performance in both half- and full-cells. When coupled with sodium hexacyanoferrate cathode and hard carbon anode, a capacity of 94.2 mA h g^-1 can be obtained with a capacity retention of 75% after 100 cycles at a current density of 100 mA g^-1, while those of with conventional liquid electrolyte can deliver a capacity of 99.0 mA h g^-1 but only accompany 58% capacity retention under the same conditions. Significantly, when the current density increases to 800 mA g^-1, their capacity difference reaches 23.4 mA h g^-1.

Self-Supported Sheets-on-Wire CuO@Ni(OH)2/Zn(OH)2 Nanoarrays for High-Performance Flexible Quasi-Solid-State Supercapacitor

Transition metal hydroxides have attracted a lot of attention as the electrode materials for supercapacitors owing to their relatively high theoretical capacity, low cost, and facile preparation methods. However, their low intrinsic conductivity deteriorates their high-rate performance and cycling stability. Here, self-supported sheets-on-wire CuO@Ni(OH)2/Zn(OH)2 (CuO@NiZn) composite nanowire arrays were successfully grown on copper foam. The CuO nanowire backbone provided enhanced structural stability and a highly efficient electron-conducting pathway from the active hydroxide nanosheets to the current collector. The resulting CuO@NiZn as the battery-type electrode for supercapacitor application delivered a high capacity of 306.2 mAh g−1 at a current density of 0.8 A g−1 and a very stable capacity of 195.1 mAh g−1 at 4 A g−1 for 10,000 charge–discharge cycles. Furthermore, a quasi-solid-state hybrid supercapacitor (qss HSC) was assembled with active carbon, exhibiting 125.3 mAh g−1 at 0.8 A g−1 and a capacity of 41.6 mAh g−1 at 4 A g−1 for 5000 charge–discharge cycles. Furthermore, the qss HSC was able to deliver a high energy density of about 116.0 Wh kg−1. Even at the highest power density of 7.8 kW kg−1, an energy density of 20.5 Wh kg−1 could still be obtained. Finally, 14 red light-emitting diodes were lit up by a single qss HSC at different bending states, showing good potential for flexible energy storage applications.