Pore and Heteroatom Engineered Carbon Foams for Supercapacitors

Carbonaceous materials are attractive supercapacitor electrode materials due to their high electronic conductivity, large specific surface area, and low cost. Here, a unique hierarchical porous N,O,S‐enriched carbon foam (KNOSC) with high level of structural complexity for supercapacitors is reported. It is fabricated via a combination of a soft‐template method, freeze‐drying, and chemical etching. The carbon foam is a macroporous structure containing a network of mesoporous channels filled with micropores. It has an extremely large specific surface area of 2685 m² g^?1. The pore engineered carbon structure is also uniformly doped with N, O, and S. The KNOSC electrode achieves an outstanding capacitance of 402.5 F g^?1 at 1 A g^?1 and superior rate capability of 308.5 F g^?1 at 100 A g^?1. The KNOSC exhibits a Bode frequency at the phase angle of ?45° of 18.5 Hz, which corresponds to a time constant of 0.054 s only. A symmetric supercapacitor device using KNOSC as electrodes can be charged/discharged within 1.52 s to deliver a specific energy density of 15.2 W h kg^?1 at a power density of 36 kW kg^?1. These results suggest that the pore and heteroatom engineered structures are promising electrode materials for ultrafast charging.     

Tailoring Electrode/Electrolyte Interfacial Properties in Flexible Supercapacitors by Applying Pressure

Electrode/electrolyte interfacial properties of flexible supercapacitors assembled with nanostructured activated carbon fabric (ACF) electrodes can be tailored by applying a pressure and tuning electrolyte ion size relative to electrode pore size. Experimental results reveal that increasing pressure between the supercapacitor electrodes can significantly improve capacitive performance. The ratio of solvated ion size in the electrolyte to the pore size on the electrodes determines the minimum pressure necessary to achieve an optimum performance. For a specific electrode material, this minimum pressure for optimum performance is primarily governed by the size of the larger solvated ions (either the anions or cations), and is lower (～689 KPa) when the ratio of the solvated ion size to the pore size is higher than 0.6, and is higher (at least 1379 KPa) when the ratio is lower than 0.6. An analytical model capable of predicting the experimental performance data has been developed. These results together provide a fundamental understanding of pressure dependence of electrode/electrolyte interfacial properties and pave the way for practical applications of flexible supercapacitors.     

Interconnected Frameworks with a Sandwiched Porous Carbon Layer/Graphene Hybrids for Supercapacitors with High Gravimetric and Volumetric Performances

A facile approach to synthesize porous disordered carbon layers as energy storage units coating on graphene sheets to form interconnected frameworks by one‐step pyrolysis of the mixture of graphene oxide/polyaniline and KOH is presented. As effective energy storage units, these porous carbon layers play an important role in enhancing the electrochemical performances. The obtained porous carbon material exhibits a high specific surface area (2927 m² g^?1), hierarchical interconnected pores, moderate pore volume (1.78 cm³ g^?1), short ion diffusion paths, and a high nitrogen level (6 at%). It displays both unparalleled gravimetric (481 F g^?1) and outstanding volumetric capacitance (212 F cm^?3) in an aqueous electrolyte. More importantly, the assembled symmetrical supercapacitor delivers not only high gravimetric (25.7 Wh kg^?1 based on total mass of electroactive materials) but also high volumetric energy densities (11.3 Wh L^?1) in an aqueous electrolyte. Furthermore, the assembled asymmetric supercapacitor yields a maximum energy density up to 88 Wh kg^?1, which is, to the best of our knowledge, the highest value so far reported for carbon//MnO₂ asymmetric supercapacitors in aqueous electrolytes. Therefore, this novel carbon material holds great promise for potential applications in energy‐related technological fields.     

Toward the Experimental Understanding of the Energy Storage Mechanism and Ion Dynamics in Ionic Liquid Based Supercapacitors

A series of salt‐templated carbons with gradually changed pore structure and their corresponding nitrogen‐doped analogues are synthesized and applied as model systems to thoroughly study the ion migration dynamics and energy storage mechanism in hierarchical pore structures with different surface functionalization in electric double‐layer capacitors with a model ionic liquid electrolyte (1‐ethyl‐3‐methylimidazolium tetrafluoroborate). Ion conformation and phase variation during the charging/discharging process and their contribution to the energy storage mechanism are investigated. A significant contribution of structural changes in the bulk of the ionic liquid electrolyte strengthening charge storage in the electric double‐layer beyond the usual expectations is uncovered. Furthermore, a quantitative model of the structure–dynamics relationship is proposed, in which the optimal ratio of mesopores to micropores is determined to be 3:1 in pore volume. Below this ratio, the ion dynamics can be promoted by increasing mesopore content and/or doping with nitrogen, while those parameters show only minor influence when the ratio is surpassing 3:1. Nitrogen doping in this system improves the rate capability (due to the enhanced ion transport dynamics) rather than the amount of energy stored.     

Ordered Hierarchical Mesoporous/Microporous Carbon Derived from Mesoporous Titanium‐Carbide/Carbon Composites and its Electrochemical Performance in Supercapacitor

Novel ordered hierarchical mesoporous/microporous carbon (OHMMC) derived from mesoporous titanium‐carbide/carbon composites was prepared for the first time by synthesizing ordered mesoporous nanocrystalline titanium‐carbide/carbon composites, followed by chlorination of titanium carbides. The mesostructure and microstructure can be conveniently tuned by controlling the TiC contents of mesoporous TiC/C composite precursor, and chlorination temperature. By optimal condition, the OHMMC has a high surface area (1917 m²g^?1), large pore volumes (1.24 cm³g^?1), narrow mesopore‐size distributions (centered at about 3 nm), and micropore size of 0.69 and 1.25 nm, and shows a great potential as electrode for supercapacitor applications: it exhibits a high capacitance of 146 Fg^?1 in noaqueous electrolyte and excellent rate capability. The ordered mesoporous channel pores are favorable for retention and immersion of the electrolyte, providing a more favorable path for electrolyte penetration and transportation to achieve promising rate capability performance. Meanwhile, the micropores drilled on the mesopore‐walls can increase the specific surface area to provide more sites for charge storage.     

Combining Nature‐Inspired,Graphene‐Wrapped Flexible Electrodes with Nanocomposite Polymer Electrolyte for Asymmetric Capacitive Energy Storage

Two kinds of free‐standing electrodes, reduced graphene oxide (rGO)‐wrapped Fe‐doped MnO₂ composite (G‐MFO) and rGO‐wrapped hierarchical porous carbon microspheres composite (G‐HPC) are fabricated using a frozen lake‐inspired, bubble‐assistance method. This configuration fully enables utilization of the synergistic effects from both components, endowing the materials to be excellent electrodes for flexible and lightweight electrochemical capacitors. Moreover, a nonaqueous HPC‐doped gel polymer electrolyte (GPE‐HPC) is employed to broad voltage window and improve heat resistance. A fabricated asymmetric supercapacitor based on G‐MFO cathode and G‐HPC anode with GPE‐HPC electrolyte achieves superior flexibility and reliability, enhanced energy/power density, and outstanding cycling stability. The ability to power light‐emitting diodes also indicates the feasibility for practical use. Therefore, it is believed that this novel design may hold great promise for future flexible electronic devices.     

Recent Developments on and Prospects for Electrode Materials with Hierarchical Structures for Lithium‐Ion Batteries

Since their successful commercialization in 1990s, lithium‐ion batteries (LIBs) have been widely applied in portable digital products. The energy density and power density of LIBs are inadequate, however, to satisfy the continuous growth in demand. Considering the cost distribution in battery system, it is essential to explore cathode/anode materials with excellent rate capability and long cycle life. Nanometer‐sized electrode materials could quickly take up and store numerous Li⁺ ions, afforded by short diffusion channels and large surface area. Unfortunately, low thermodynamic stability of nanoparticles results in electrochemical agglomeration and raises the risk of side reactions on electrolyte. Thus, micro/nano and hetero/hierarchical structures, characterized by ordered assembly of different sizes, phases, and/or pores, have been developed, which enable us to effectively improve the utilization, reaction kinetics, and structural stability of electrode materials. This review summarizes the recent efforts on electrode materials with hierarchical structures, and discusses the effects of hierarchical structures on electrochemical performance in detail. Multidimensional self‐assembled structures can achieve integration of the advantages of materials with different sizes. Core/yolk–shell structures provide synergistic effects between the shell and the core/yolk. Porous structures with macro‐, meso‐, and micropores can accommodate volume expansion and facilitate electrolyte infiltration.     

Alveoli‐Inspired Facile Transport Structure of N‐Doped Porous Carbon for Electrochemical Energy Applications

Heteroatom‐doped porous carbon materials have attracted much attention because of their extensive application in energy conversion and storage devices. Because the performance of fuel cells and the rate capability of supercapacitors depend significantly on multiple factors, such as electrical conductivity and transport rate of ions and reactants, designing these carbon‐based materials to optimize performance factors is vital. In order to address these issues, alveoli that possess a hollow cavity where oxygen exchange can occur are synthesized, inspired by N‐doped carbon materials with a high surface area and low transport resistance. By incorporating a dopamine coating on zeolitic imidazolate framework (ZIF), pore size is modified and electrical conducting pathways are constructed, resulting in changes to the reaction kinetics. These highly interconnected electron connection channels and proper pore sizes facilitate the diffusion of reactants and the conduction of electrons, leading to high activity of the oxygen reduction reaction (ORR), which is comparable to Pt, and high rate performance in supercapacitors.     

Design Principles for Optimum Performance of Porous Carbons in Lithium–Sulfur Batteries

A series of experiments is presented that establishes for the first time the role of some of the key design parameters of porous carbons including surface area, pore volume, and pore size on battery performance. A series of hierarchical porous carbons is used as a model system with an open, 3D, interconnected porous framework and highly controlled porosity. Specifically, carbons with surface areas ranging from ≈500–2800 m² g^?1, pore volume from ≈0.6–5 cm³ g^?1, and pore size from micropores (≈1 nm) to large mesopores (≈30 nm) are synthesized and tested. At high sulfur loadings (≈80 wt% S), pore volume is more important than surface area with respect to sulfur utilization. Mesopore size, in the range tested, does not affect the sulfur utilization. No relationship between porosity and long‐term cycle life is observed. All systems fail after 200–300 cycles, which is likely due to the consumption of the LiNO₃ additive over cycling. Moreover, cryo‐scanning transmission electron microscopy imaging of these carbon–sulfur composites combined with X‐ray diffraction (XRD) provides further insights into the effect of initial sulfur distribution on sulfur utilization while also revealing the inadequacy of the indirect characterization techniques alone in reliably predicting distribution of sulfur within porous carbon matrices.     

Strain‐Based In Situ Study of Anion and Cation Insertion into Porous Carbon Electrodes with Different Pore Sizes

The expansion of porous carbon electrodes in a room temperature ionic liquid (RTIL) is studied using in situ atomic force microscopy (AFM). The effect of carbon surface area and pore size/pore size distribution on the observed strain profile and ion kinetics is examined. Additionally, the influence of the potential scan rate on the strain response is investigated. By analyzing the strain data at various potential scan rates, information on ion kinetics in the different carbon materials is obtained. Molecular dynamics (MD) simulations are performed to compare with and provide molecular insights into the experimental results; this is the first MD work investigating the pressure exerted on porous electrodes under applied potential in a RTIL electrolyte. Using MD, the pressure exerted on the pore wall is calculated as a function of potential/charge for both a micropore (1.2 nm) and a mesopore (7.0 nm). The shape of the calculated pressure profile matches closely with the strain profiles observed experimentally.     

High Performance,Flexible, Solid‐State Supercapacitors Based on a Renewable and Biodegradable Mesoporous Cellulose Membrane

A flexible, transparent, and renewable mesoporous cellulose membrane (mCel‐membrane) featuring uniform mesopores of ≈24.7 nm and high porosity of 71.78% is prepared via a facile and scalable solution‐phase inversion process. KOH‐saturated mCel‐membrane as a polymer electrolyte demonstrates a high electrolyte retention of 451.2 wt%, a high ionic conductivity of 0.325 S cm^?1, and excellent mechanical flexibility and robustness. A solid‐state electric double layer capacitor (EDLC) using activated carbon as electrodes, the KOH‐saturated mCel‐membrane as a polymer electrolyte exhibits a high capacitance of 110 F g^?1 at 1.0 A g^?1, and long cycling life of 10 000 cycles with 84.7% capacitance retention. Moreover, a highly integrated planar‐type micro‐supercapacitor (MSC) can be facilely fabricated by directly depositing the electrode materials on the mCel‐membrane‐based polymer electrolyte without using complicated devices. The resulting MSC exhibits a high areal capacitance of 153.34 mF cm^?2 and volumetric capacitance of 191.66 F cm^?3 at 10 mV s^?1, representing one of the highest values among all carbon‐based MSC devices. These findings suggest that the developed renewable, flexible, mesoporous cellulose membrane holds great promise in the practical applications of flexible, solid‐state, portable energy storage devices that are not limited to supercapacitors.     

Fabrication of Ordered Macro‐Microporous Single‐Crystalline MOF and Its Derivative Carbon Material for Supercapacitor

Because of their good performance in diffusion‐limited processes, ordered macro‐microporous single‐crystalline metal‐organic frameworks (MOFs) have potential for use in various fields. However, there are still very few reports of the synthesis of such MOFs. A general synthesis methodology for ordered macro‐microporous single‐crystalline MOFs is highly desired. Here, a novel strategy is reported for synthesizing single‐crystalline ordered macro‐microporous MOFs by monodentate‐ligand‐induced in situ crystallization within a 3D ordered hard template in a double‐solvent system. A space‐confined growth model is proposed to clarify the shaping effect of the template; the role of the monodentate ligand is also analyzed. Moreover, a carbon material derived from the macro‐microporous MOF inherits the ordered interconnected macroporous structure. The improved diffusion and lower resistance, as well as the structural robustness, endow the derivative carbon material with superior rate performance and excellent cycling stability when prepared as electrodes for a supercapacitor. It is anticipated that the method will provide new paths to the synthesis of such macro‐microporous materials for applications in energy‐related fields and beyond.     

Quantifying the Role of Nanotubes in Nano:Nano Composite Supercapacitor Electrodes

Many promising supercapacitor electrode materials have high resistivity and require conductive additives to function effectively. However, the detailed role of the additive is not understood. Here, this question is resolved by applying a quantitative model for resistance‐limited supercapacitor electrodes to Co(OH)₂‐nanosheet/carbon nanotube composites. Without nanotubes, theory predicts and experiments show that while the low‐rate capacitance increases linearly with electrode thickness, the high rate capacitance decreases with thickness due to slow charging. Experiments supported by theory show that nanotube addition has two effects. First, the nanotube network effectively distributes charge, increasing the intrinsic electrode performance to the limit associated with its accessible surface area. Second, at high‐rate, the increased electrode conductivity shifts the rate‐limiting resistance from electrode to electrolyte, thus removing the thickness‐dependent capacitance falloff. Furthermore, the analysis quantifies the out‐of‐plane conductivity of the nanotube network, identifies the cross‐over from resistance‐limited to diffusion‐limited behavior, and allows full electrode modeling, facilitating rational design.     

Tomographic Reconstruction and Analysis of a Silver CO2 Reduction Cathode

Tomographic reconstruction has been well established as a valuable tool in the analysis of polymer electrolyte fuel cell (PEMFC) electrodes. While forays have been made into applying it to polymer electrolyte water electrolyzer (PEMWE) electrodes, CO₂ electrolyzer electrodes are still new ground. Here a tomographic analysis of an electrochemical CO₂ reduction gas diffusion electrode by means of focused ion beam scanning electron microscope tomography is presented. The reconstruction shows a porosity of 68%. While most of the porosity is on the nanoscale, a broad tail of micropores is observed in the distribution. The spatial distribution of the pores is nonuniform. The large pores are concentrated in the center of the layer in the through‐plane direction. From the reconstruction, an effective diffusivity factor of 0.5 for the catalyst layer is calculated. The Knudsen number of 0.19 obtained from the later shows that the diffusion is mostly in the bulk regime. Flooding of the catalyst layer is likely to decrease the effective diffusivity factor substantially.     

3D Holey‐Graphene Architecture Expedites Ion Transport Kinetics to Push the OER Performance

The kinetics process of heterogeneous catalysis involves several steps including adsorption, diffusion, and surface chemical reactions. Current studies generally aim at increasing active site amount and improving intrinsic activity. However, the ion diffusion kinetics at the electrode/electrolyte interface as a bottleneck has been rarely directly addressed. Here, a 3D holey‐graphene framework is demonstrated as a catalyst‐loading platform, with nanoscale holes that can be elaborately tuned via facile aqueous‐phase chemical etching. This enables the ions to be efficiently transported to deeply buried active sites to mitigate their insufficient supply. With systematical electrochemical investigations tuned by varied pore structures, a series of models from a simplified equivalent circuit to complicate realistic one are proposed to figure out the modulation rules of weakened electrochemical diffusion domination and identify the ion transport resistance as well. Moreover, given the inevitable negative effect on the conductivity of graphene skeleton by introducing nanoscale holes, the balance between the outside ion transport and the inside charge transport of electrode is highlighted. Such a protocol represents a synergistic modulation of catalytic performance from both the supply side (reactive ion transport) and the consuming side (active site), and provides striking information for the precise design of catalyst electrodes toward further pushing the oxygen evolution reaction performance limit.     

Passive and facilitated transport in nuclear pore complexes is largely uncoupled

Nuclear pore complexes provide the sole gateway for the exchange of material between nucleus and cytoplasm of interphase eukaryotic cells. They support two modes of transport: passive diffusion of ions, metabolites, and intermediate-sized macromolecules and facilitated, receptor-mediated translocation of proteins, RNA, and ribonucleoprotein complexes. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the nuclear pore complex. To test this hypothesis, we studied the mutual effects between transporting molecules utilizing either the same or different modes of translocation. We find that the two modes of transport do not interfere with each other, but molecules utilizing a particular mode of transport do hinder motion of others utilizing the same pathway. We therefore conclude that the two modes of transport are largely segregated.     

Single‐Particle Performances and Properties of LiFePO4 Nanocrystals for Li‐Ion Batteries

It has been recently reported that the solution diffusion, efficiency porosity, and electrode thickness can dominate the high rate performance in the 3D‐printed and traditional LiMn_0.21Fe_0.79PO₄ electrodes for Li‐ions batteries. Here, the intrinsic properties and performances of the single‐particle (SP) of LiFePO₄ are investigated by developing the SP electrode and creating the SP‐model, which will share deep insight on how to further improve the performance of the electrode and related materials. The SP electrode is generated by fully scattering and distributing LiFePO₄ nanoparticles to contact with the conductive network of carbon nanotube or conductive carbon to demonstrate the sharpest cyclic voltammetry peak and related SP‐model is developed, by which it is found that the interfacial rate constant in aqueous electrolyte is one order of magnitude higher, accounting for the excellent rate performance in aqueous electrolyte for LiFePO₄. For the first time it has been proposed that the insight of pre‐exponential factor of interface kinetic Arrhenius equation is related to desolvation/solvation process. Thus, this much higher interfacial rate constant in aqueous electrolyte shall be attributed to the much larger pre‐exponential factor of interface kinetic Arrhenius equation, because the desolvation process is much easier for Li‐ions jumping from aqueous electrolyte to the Janus solid–liquid interface of LiFePO₄.     

Solid‐State Lithium/Selenium–Sulfur Chemistry Enabled via a Robust Solid‐Electrolyte Interphase

Lithium/selenium‐sulfur batteries have recently received considerable attention due to their relatively high specific capacities and high electronic conductivity. Different from the traditional encapsulation strategy for suppressing the shuttle effect, an alternative approach to directly bypass polysulfide/polyselenide formation via rational solid‐electrolyte interphase (SEI) design is demonstrated. It is found that the robust SEI layer that in situ forms during charge/discharge via interplay between rational cathode design and optimal electrolytes could enable solid‐state (de)lithiation chemistry for selenium‐sulfur cathodes. Hence, Se‐doped S_22.2Se/Ketjenblack cathodes can attain a high reversible capacity with minimal shuttle effects during long‐term and high rate cycling. Moreover, the underlying solid‐state (de)lithiation mechanism, as evidenced by in situ ⁷Li NMR and in operando synchrotron X‐ray probes, further extends the optimal sulfur confinement pore size to large mesopores and even macropores that have been long considered as inferior sulfur or selenium host materials, which play a crucial role in developing high volumetric energy density batteries. It is expected that the findings in this study will ignite more efforts to tailor the compositional/structure characteristics of the SEI layers and the related ionic transport across the interface by electrode structure, electrolyte solvent, and electrolyte additive screening.     

Reactive conducting polymers as actuating sensors and tactile muscles

Films of conducting polymers when used as electrodes in an electrolytic solution oxidize and reduce under the flow of anodic and cathodic currents, respectively. The electrochemical reactions induce conformational movements of the chains, generation or destruction of free volume and interchange of ions and solvent with the electrolyte giving a gel that reacts, swells or shrinks. Electric pulses acting on reactive gels constituted by polymers, solvent and ions are the closest artificial materials to those that constitute actuating biological organs. The electrochemical reaction under the flow of a constant current promotes a progressive change of color, volume, porosity, stored charge and storage or release of ions. The reaction is kinetically controlled by the conformational movements or by the diffusion of counterions through the gel; it works under electrochemical equilibrium and defines, at any intermediate oxidation state, equilibrium potentials. Any variable (mechanical, chemical, optical, magnetic, etc) acting on the equilibrium will induce a change in the working potential of any device, driven by a constant current, based on this reaction; actuating-sensing devices based on the electrochemical properties are expected. Artificial muscles able to sense pushed weights, electrolyte concentration or ambient temperature during actuation are described. The activation energy of the reaction includes structural information and allows the obtention of the conformational energy, the heart of both actuating and sensing properties.     

Thick Binder‐Free Electrodes for Li–Ion Battery Fabricated Using Templating Approach and Spark Plasma Sintering Reveals High Areal Capacity

The templating approach is a powerful method for preparing porous electrodes with interconnected well‐controlled pore sizes and morphologies. The optimization of the pore architecture design facilitates electrolyte penetration and provides a rapid diffusion path for lithium ions, which becomes even more crucial for thick porous electrodes. Here, NaCl microsize particles are used as a templating agent for the fabrication of 1 mm thick porous LiFePO₄ and Li₄Ti₅O₁₂ composite electrodes using spark plasma sintering technique. These sintered binder‐free electrodes are self‐supported and present a large porosity (40%) with relatively uniform pores. The electrochemical performances of half and full batteries reveal a remarkable specific areal capacity (20 mA h cm^?2), which is 4 times higher than those of 100 µm thick electrodes present in conventional tape‐casted Li–ion batteries (5 mA h cm^?2). The 3D morphological study is carried out using full field transmission X‐ray microscopy in microcomputed tomography mode to obtain tortuosity values and pore size distributions leading to a strong correlation with their electrochemical properties. These results also demonstrate that the coupling between the salt templating method and the spark plasma sintering technique turns out to be a promising way to fabricate thick electrodes with high energy density.