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Electrochemistry and X-ray absorption spectroscopy (XAS) of MnO2/reduced graphene oxide (RGO) were made to evaluate capacitive performance and electronic structure as a supercapacitor. MnO2 was deposited on the surface of RGO by a spontaneous redox reaction. At low current density (1 A g−1), the specific capacitance of MnO2/RGO (MRGO) greatly increased with increasing growth time from 281 to 462 F g−1. Electrochemical results show that the specific capacitance of RGO and MRGO arises from the combined contributions of electrochemical double-layer capacitance (EDLC) and pseudo-capacitance. To gain insights into the charge storage mechanism of RGO and MRGO, it was characterized using ex-situ soft XAS techniques at Mn L-edge and C, O K-edges in the charging/discharging process. The ex-situ soft XAS results provide evidence that the contribution of RGO to specific capacitance involves an oxygen-functionality-related contribution of pseudocapacitance and EDLC. In addition, the specific capacitance of MRGO reveals pseudocapacitive contributions from the redox processes that involve the Mn(III)/Mn(IV) redox reaction in MnO2. This work utilizes electrochemical and ex-situ XAS techniques to elucidate charge storage mechanism for supercapacitor applications.

Ni doped V2O5 nanorods were synthesized by simple sol–gel route. The effect of Ni doping has been studied through XRD, Micro-Raman, SEM and CV analysis. The XRD result shows that the synthesized samples were exhibits orthorhombic structured V2O5. SEM images illustrate the Ni particles are decorated on the V2O5 rod surfaces. From the Raman analysis, the prepared samples are observed in V-O vibrational modes without impurities. Both V2O5 and Ni:V2O5 electrodes are exhibits pseudocapacitance nature, which shows the superior specific capacitance values for 112 and 152 Fg−1 at 10 mVs−1, respectively.

Constructing high-performance electrodes with both wide potential window (e.g. ≥ 2 V in aqueous electrolyte) and excellent mechanical flexibility represents a great challenge for supercapacitors. Because of the outstanding conductivity and flexibility, carbon cloth (CC) has shown unlimited prospects for constructing flexible electrodes, but is rarely used directly as electrode material due to its electrochemical inertness and small specific surface area. To tackle these two critical limitations, we design a novel redox-etching strategy to synthesize CC-based electrode with 3D interconnecting pore structure. The sponge-like highly porous CC was further activated by strong oxidant to form abundant oxygenic groups, which occupy the interior and surface of current collector to render substantial pseudocapacitance. The as-synthesized CC electrode yielded an impressive capacitance of 4035 mF cm−2 at 3 mA cm−2 and satisfying cycling durability in a wide potential range of −1 – 1 V vs. SCE, which surpass the majority of reported CC-based electrodes. A symmetric supercapacitor with stable voltage of 2 V is assembled and delivers remarkable energy density of 6.57 mWh cm−3. Significantly, the device demonstrates an unparalleled flexibility with no capacitive decay after 100 bending cycles. This facile chemical etching and post-treatment processes are designed for large-scale manufacturing of the CC electrodes by providing high surface area and abundant electrochemically active sites, promising for industry application. The innovative synthetic strategy opens up new opportunities for high-performance flexible energy storage.

Metal antimony (Sb) is a promising anode material of potassium-ion batteries (PIBs) for its high theoretical capacity but limited by its inferior cycle stability due to the serious volume expansion during cycling. Herein, we design and construct a kind of low-crystalline Sb nanoparticles coated with amorphous Sb2O3 and dispersed into three-dimensional porous carbon via a strategy involving NaCl template-assisted in-situ pyrolysis and subsequent low-temperature heat-treated in air. Significantly, the crystallinity and ratio of Sb/Sb2O3 have been precisely tuned and controlled, and the optimized sample of HTSb@Sb2O3@C-4 displays a high reversible specific capacity of 543.9 mAh g−1 at 0.1 A g−1, superior rate capability and excellent cycle stability (~273 mAh g−1 at 2 A g−1 after 2000 cycles) as an anode of PIBs. The outstanding potassium-ion storage performance can be ascribed to the appropriate crystallinity and the multiple-buffer-matrix structure comprising an interconnected porous conductive carbon to relieve the volume changes and suppress the aggregation of Sb, a Sb nanoparticle core to shorten the ion transport pathways and decrease the mechanical stress, and a low-crystalline Sb2O3 as the shell to consolidate the interface between Sb and carbon as well as facilitate the rapid electron transport. The dynamic analysis shows that the composite is mainly controlled by pseudocapacitance mechanism. This work provides a novel thought to design high-performance composite electrode in energy storage devices.

It is challenging to create cation vacancies in electrode materials for enhancing the performance of rechargeable lithium ion batteries (LIBs). Herein, we utilized a strong alkaline etching method to successfully create Co vacancies at the interface of atomically thin Co3−xO4/graphene@CNT heterostructure for high-energy/power lithium storage. The creation of Co-vacancies in the sample was confirmed by high-resolution scanning transmission electron microscope (HRSTEM), X-ray photoelectron spectroscopy (XPS) and electron energy loss near-edge structures (ELNES). The obtained Co3−xO4/graphene@CNT delivers an ultra-high capacity of 1688.2 mAh g−1 at 0.2 C, excellent rate capability of 83.7% capacity retention at 1 C, and an ultralong life up to 1500 cycles with a reversible capacity of 1066.3 mAh g−1. Reaction kinetic study suggests a significant contribution from pseudocapacitive storage induced by the Co-vacancies at the Co3−xO4/graphene@CNT interface. Density functional theory confirms that the Co-vacancies could dramatically enhance the Li adsorption and provide an additional pathway with a lower energy barrier for Li diffusion, which results in an intercalation pseudocapacitive behavior and high-capacity/rate energy storage.

Zinc-based energy storage systems (zinc-air, zinc-nickel and zinc-ion batteries and zinc-ion hybrid supercapacitors (ZHSs) are considered as promising power sources for wide applications from personal electronic devices to electric vehicles. However, these systems, especially the Zn-based hybrid supercapacitors, display unsatisfying power density and energy density, which should be enhanced for their large-scale applications. In this work, aqueous alkaline zinc-carbon hybrid supercapacitors (A-ZCHS) were designed, consisting of B, N dual doped carbon cathode, Zn anode and KOH electrolyte. The B, N dual doped carbon was prepared via thermal treatment of metal-organic frameworks and boric acid, which exhibits abundant hierarchical pore structure (micropore, mesopore and macropore) and suitable defect construction, promoting ion diffusion/charge transfer and providing more rapid surface pseudocapacitance reaction. More obviously, when the optimized B, N dual doped carbon was used as cathode in A-ZCHS and ZHS, more capacitive charge storage and rapider electrochemical kinetics can be observed in A-ZCHS than in ZHS. Therefore, the optimized A-ZCHS displays a high energy density of 115.7 Wh kg−1 at the power density of 711.6 W kg−1 with excellent stability, which is much better than most of ZHSs reported previously. The A-ZCHS should be a promising candidate for energy storage applications.

Reinforcing electronic and ionic conductivity is very important to realize the high-rate Li2ZnTi3O8 (LZTO) anode for Lithium-ion batteries (LIBs). Here, we reported a synthetic strategy toward in situ growth of homodispersed Li2ZnTi3O8 on ultralight 3D carbon aerogels (CAs) via facile heat treatment and activation process taking LiOH as activating agent. Such an optimized composite obtains a specific surface area of 154.427 m2 g−1 and a high pore volume. The strong interaction between the LZTO and N-doped CAs, a highly electrical conductivity and multidimensional ion transport channels of the hybrid result in a high capacity, outstanding high-rate performance and long cycle life. As a result, the NPC-LZTO electrode (NPC-LZTO-B) delivers a commendable reversible capacity at high-rate (e.g. 162.4 mAh g−1 at 5 A g−1 (22 C) and 127.3 mAh g−1 at 10 A g−1) and durable long-term performance (capacity retention of 73% after 4500 cycles at 5 A g−1). Moreover, the kinetic analysis confirms that the feature of pseudocapacitance boosts lithium-ion storage performance at high rate. Porous carbon aerogels frame constructed LZTO nanocomposite electrodes provide a facile way to design high durable LIBs anode.

Aqueous zinc ion battery (AZIB) is a promising battery system developed in recent years, which has the advantages of safety, environmental protection and low price. However, it is still a puzzle to develop and improve cathode materials with satisfactory performance. In this paper, the chromium vanadate (CrVO3) electrode material was reported for the first time. The obtained CrVO3 have mesoporous structure (the mesopore sizes: 2–50 nm), excellent conductivity, high surface area (129.3 m2 g−1) and uniform thickness of 2 nm, which provides a short path for rapid transfer of zinc ions, a large surface area for high pseudocapacitance, and sufficient voids to mitigate volume expansion. Given these structural advantages, the CrVO3 cathode delivers high capacities of 188.8 and 112.8 mAh g−1 at 0.5 and 4 A g−1 and excellent long cycle stability, respectively. More importantly, the Zn//CrVO3 battery provided an energy density of 231.9 Wh kg−1 at a power density of 100.4 W kg−1. Meanwhile, insight into the formation mechanism and Zn2+ storage mechanism by ex situ methods. The results show that the porous CrVO3 is a promising cathode material for AZIBs, which provide a valuable idea for the design of porous vanadate with significantly enhanced performances in electrochemical energy storage.

Sluggish divalent charges (e.g., Zn2+) diffusion within MXene host electrodes due to the larger sizes and stronger coulomb interactions compared to those of dominant univalent charges is an essential issue in developing MXene-based microsupercapacitors (MSCs) of higher theoretical energy densities. Herein, MXene/bacterial cellulose fiber (BCF) hybrid films with effectively expanded interlayer spacing between re-stacked few-layered MXene sheets via homogeneous intercalation of BCF nanospacer were designed and prepared. The electrochemical testing and molecular dynamics simulation demonstrated the consequently widened ions-transport-channels between few-layered MXene sheets can effectively reduce the diffusion barrier of Zn2+ within the MXene/BCF host electrodes compared to pure MXene film electrodes without BCF nanospacer. Further in-situ Raman and ex-situ XPS spectroscopy characterizations suggest: the consequently accelerated Zn2+ diffusion involved adsorption/desorption featured charge storage, and followed electrochemical reaction of Zn2+ with oxygen terminal groups on the MXene flakes induced additional pseudocapacitance within the MXene/BCF host electrodes simultaneously contribute to the acquired superior areal capacitance of the fabricated symmetrical aqueous Zn-ions MSCs (ZMSCs) based on the MXene/BCF host electrodes and Zn(CF3SO3)2/polyacrylamide hydrogel electrolyte. Meanwhile, combined with doubled voltage window (1.2 V) benefiting from the cocurrently suppressed kinetics of hydrogen/oxygen evolution within the MXene/BCF host electrodes, a tremendously increased areal energy density (34.0 μWh cm−2) in comparison with that employing conventional H2SO4/polyacrylamide hydrogel electrolyte (0.6 V/8.6 μWh cm−2) is finally realized. The work demonstrates a simple and effective strategy to synchronously boost bivalent Zn2+ diffusion and depress hydrogen/oxygen evolution within MXene host electrodes toward symmetric ZMSCs of high areal energy density.

Enhancing pseudocapacitance of Ti3C2Tx MXene-based electrode materials induced by sufficient surface terminating groups (such as Ti–O bonding) becomes one promising way for fabricating high energy-density supercapacitors. However, most of terminating group activity cannot be excited well in the multilayer Ti3C2Tx due to the layer stacking feature, which limits its further performance improvement. In this paper, a CQDs@Ti3C2Tx hybrid material is synthesized by modifying few-layer Ti3C2Tx MXene matrix with highly dispersed carbon quantum dots (CQDs). In this unique hybrid model, the Ti3C2Tx MXene matrix with high conductivity and high specific surface area is just like the “paddy field”, which not only promotes the charge rapid transfer, but also provides the high double layer capacitance by ion adsorption. More importantly, the CQDs act as “rice seedlings” firmly anchored to the Ti3C2Tx matrix, and greatly stimulate the activity of the termination groups of Ti3C2Tx. As expected, this hybrid achieves a harmonious coexistence of high conductivity and high pseudocapacitance. As a result, the CQDs@Ti3C2Tx electrode delivers high reversible capacitances of 441.3 F g−1 at 1 A g−1 and 310.1 F g−1 at 20 A g−1, and a relatively stable cyclability with almost 100% capacitance retention after 10 000 cycles at 10 A g−1.

Graphite as a promising anode candidate of K-ion batteries (KIBs) has been increasingly studied currently, but corresponding rate performance and cycling stability are usually inferior to amorphous carbon materials. To protect the layer structure and further boost performance, tempura-like carbon/carbon nanocomposite of graphite@pitch-derived S-doped carbon (G@PSC) is designed and prepared by a facile and low-temperature modified molten salt method. This robust encapsulation structure makes their respective advantages complementary to each other, showing mutual promotion of electrochemical performances caused by synergy effect. As a result, the G@PSC electrode is applied in KIBs, delivering impressive rate capabilities (465, 408, 370, 332, 290, and 227 mA h g−1 at 0.05, 0.2, 0.5, 1, 2, and 5 A g−1) and ultralong cyclic stability (163 mA g−1 remaining even after 8000 cycles at 2 A g−1). On basis of ex-situ studies, the sectionalized K-storage mechanism with adsorption (pseudocapacitance caused by S doping)-intercalation (pitch-derived carbon and graphite) pattern is revealed. Moreover, the exact insights into remarkable rate performances are taken by electrochemical kinetics tests and density functional theory calculation. In a word, this study adopts a facile method to synthesize high-performance carbon/carbon nanocomposite and is of practical significance for development of carbonaceous anode in KIBs.

Electronic structure engineering on electrode materials could bring in a new mechanism to achieve high energy and high power densities in sodium ion batteries. Herein, we design and create Co vacancies at the interface of atomically thin CoSe 2 /graphene heterostructure and obtain Co 1-x Se 2 /graphene heterostructure electrode materials that facilitate significant Na + intercalation pseudocapacitance. Density functional theory (DFT) calculation suggests that the Na + adsorption energy is dramatically increased, and the Na + diffusion barrier is remarkably reduced due to the introduction of Co vacancy. The optimized electrode delivers a superior capacity of 673.6 mAh g -1 at 0.1 C, excellent rate capability of 576.5 mAh g -1 at 2.0 C and ultra-long life up to 2000 cycles. Kinetics analysis indicates that the enhanced Na + storage is mainly attributed to the intercalation pseudocapacitance induced by Co vacancies. This work suggests that the creation of cation vacancy could bestow heterostructured electrode materials with pseudocapacitive Na + intercalation for high-capacity and high-rate energy storage.

Synergistic effect improves the electrochemical properties of metal oxide anode materials for lithium ion-batteries. The influence of different ratios of nickel and manganese on the microscopic morphology and the contribution of pseudocapacitance were investigated. The ternary Prussian blue analog NixMny[Fe(CN)6]2(x + y = 3) manufactured by adjusting the ratio of Ni-Mn mixed salt was used to obtain the ternary transition metal oxide. When the ratio of Ni and Mn is 8:2 or 2:8, the synergistic effect of oxide anode materials between the ternary transition metals is strong. The corresponding materials have excellent cycle stability, higher reversible capacity, lower charge transfer resistance and higher proportion of pseudocapacitance contribution. The calcined product is NiFe2O4-FeMnO3, which has a core-shell structure when the ratio of Ni and Mn is 2:8, and the first cycle discharge capacity is 1697.6 mAh/g. It can provide a reversible capacity 536.1 mAh/g after 500 cycles at 500 mA/g. The analysis of electrode dynamics shows that the pseudocapacitance contribution can reach 46.8% at 0.1 mV/s.

To address the problem that capacitor electrode being low in capacitance and reliant on binder to retain shape, a binder-free, self-supporting supercapacitor electrode is made via pyrolysis of porous polyacrylonitrile (PAN) formed by phase separation, supported by hydrothermally treated nickel foam backbone. The role of hydrothermal reaction time and PAN solution concentration on electrochemical properties was thoroughly studied, and the results indicate a generally peaked capacitance at 4 h of hydrothermal reaction time, but after a certain threshold, a supposed micro-domain buffering mechanism would take over and enhance the pseudocapacitance from NiO to reach a specific capacitance as high as 152 F/g at a high active material mass load of 14.98 mg/cm2, providing the area specific capacitance up to 1.98 F/cm2. The use of temperature-controlled phase separation and incorporation of nickel oxide together guarantees the electrode sufficient ion and electron transportation pathway while improving its capacitance by redox electrochemical reactions.

In this study, the experiment was conducted assuming that the citrus fruits were contaminated with bacteria. Herein, orange peels (OP) and lemon peels (LP) can be used as a carbon source and have the advantage of using discarded materials and heteroatoms. Also, the nitrogen heteroatom is introduced by naturally doping the materials with bacteria (Escherichia Coli, E. coli). The as-prepared bacteria doped activated carbon showed an increase in nitrogen content and surface properties which led to an improvement in electrochemical properties. The specific capacitance of bacteria doped OP and LP was 92.4 and 139 F g−1 compared to the bare samples with a specific capacitance of 60.9 and 49.6 F g−1 at a current density of 0.2 A g−1 and capacity retention of 129% after 10,000 cycles for the bacteria-doped samples. This process which is simple, cheap, and environmentally friendly can be applied to discarded fruit peels for the fabrication of supercapacitor materials.

Electrochemical (EC) reduction of graphene oxide (GO) serves as a green chemistry pathway to produce reduced GO (rGO). However, the mechanistic understanding of this process is still limited due to the structural complexity of GO-based materials. In the present study, we employ an inductive approach that consists of investigating the structures afforded by alternating EC reduction and oxidation steps to elucidate the electrochemical reduction reactions in aqueous solution. The initial EC reduction of chemically derived GO mainly converts epoxides (C–O–C) to COH/CH pairs. Simultaneously, carbon-centered radicals (C•) are eliminated. Subsequent EC oxidation does not result in restoration of epoxides but reverse dehydrogenation and radical formation. The second EC reduction and oxidation cycle involves reversible C–H formation coupled with radical elimination, suggesting that C• serves as an active site for the hydrogenation. As a result of this study, COH–CH is revealed as a plausible characteristic structure in electrochemically derived rGO. The reversible radical hydrogenation reaction can offer an effective route to manipulate the chemical reactivities of GO-based materials. We demonstrate that the electrons transferred by EC reduction can delocalize in the rGO to some extent, activating the pseudocapacitance that probably originates from COH.

The electrode materials with high pseudocapacitance can enhance the rate capability and cycling stability of lithium-ion storage devices. Herein, we fabricated MoS2 nanoflowers with ultra-large interlayer spacing on N-doped hollow multi- nanochannel carbon nanofibers (F2-MoS2/NHMCFs) as freestanding binder-free anodes for lithium-ion batteries (LIBs). The ultra-large interlayer spacing (0.78∼1.11 nm) of MoS2 nanoflowers can not only reduce the internal resistance, but also increase accessible active surface area, which ensures the fast Li+ intercalation and deintercalation. The NHMCFs with hollow and multi-nanochannel structure can accommodate the large internal strain and volume change during lithiation/delithiation process, it is beneficial to improving the cycling stability of LIBs. Benefiting from the above combined structure merits, the F2-MoS2/NHMCFs electrodes deliver a high rate capability 832 mA h g−1 at 10 A g−1 and ultralong cycling stability with 99.29 and 91.60 % capacity retention at 10 A g−1 after 1000 and 2000 cycles, respectively. It is one of the largest capacities and best cycling stability at 10 A g−1 ever reported to date, indicating the freestanding F2-MoS2/NHMCFs electrodes have potential applications in high power density LIBs.

Highlights The iontronic pressure sensor achieved an ultrahigh sensitivity (Smin > 200 kPa−1, Smax > 45,000 kPa−1). The iontronic pressure sensor exhibited a broad sensing range of over 1.4 MPa. Pseudocapacitive iontronic pressure sensor using MXene was proposed. ABSTRACT Flexible pressure sensors are unprecedentedly studied on monitoring human physical activities and robotics. Simultaneously, improving the response sensitivity and sensing range of flexible pressure sensors is a great challenge, which hinders the devices’ practical application. Targeting this obstacle, we developed a Ti3C2Tx-derived iontronic pressure sensor (TIPS) by taking the advantages of the high intercalation pseudocapacitance under high pressure and rationally designed structural configuration. TIPS achieved an ultrahigh sensitivity (Smin > 200 kPa−1, Smax > 45,000 kPa−1) in a broad sensing range of over 1.4 MPa and low limit of detection of 20 Pa as well as stable long-term working durability for 10,000 cycles. The practical application of TIPS in physical activity monitoring and flexible robot manifested its versatile potential. This study provides a demonstration for exploring pseudocapacitive materials for building flexible iontronic sensors with ultrahigh sensitivity and sensing range to advance the development of high-performance wearable electronics.

Conversion/alloying type anodes have shown great promise for sodium-ion batteries (SIBs) because of their high theoretical capacity. However, the poor structural stability derived from the large volume expansion and short lifetime impedes their further practical applications. Herein, we report a novel anode with a pomegranate-like nanostructure of SnP₂O₇ particles homogeneously dispersed in the robust N-doped carbon matrix. For the first time, we make use of in situ self-nanocrystallization to generate ultrafine SnP₂O₇ particles with a short pathway of ions and electrons to promote the reaction kinetics. Ex situ transmission electron microscope (TEM) shows that the average particle size of SnP₂O₇ decreases from 66 to 20 nm successfully based on this unique nanoscale-engineering method. Therefore, the nanoparticles together with the N-doped carbon contribute a high pseudocapacitance contribution. Moreover, the N-doped carbon matrix forms strong interaction with the self-nanocrystallization ultrafine SnP₂O₇ particles, leading to a stable nanostructure without any particle aggregation under a long-cycle operation. Benefiting from these synergistic merits, the SnP₂O₇@C anode shows a high specific capacity of 403 mAh g–¹ at 200 mA g–¹ and excellent cycling stability (185 mAh g–¹ after 4000 cycles at 1000 mA g–¹). This work presents a new route for the effective fabrication of advanced conversion/alloying anodes materials for SIBs.

The conventional solid-state reaction suffers from low diffusivity, high energy consumption, and uncontrolled morphology. These limitations are competed by the presence of water in solution route reaction. Herein, based on concept of combining above methods, we report a facile solid-state reaction conducted in water vapor at low temperature along with calcium doping for modifying lithium vanadate as anode material for lithium-ion batteries. The optimized material, delivers a superior specific capacity of 543.1, 477.1, and 337.2 mAh g−1 after 200 and 1000 cycles at current densities of 100, 1000 and 4000 mA g−1, respectively, which is attributed to the contribution of pseudocapacitance. In this work, we also use experimental and theoretical calculation to demonstrate that the enhancement of doped lithium vanadate is attributed to particles confinement of droplets in water vapor along with the surface and structure variation of calcium doping effect. The conventional solid-state reaction suffers from the low diffusivity, high energy consumption, and uncontrolled morphology. Here, the authors report a low-temperature solid-state reaction under water vapour for synthesis of various electrodes for lithium ions batteries.

Cation vacancies can bring numerous surprising characters due to its multifarious electron and orbit distribution. In this work, δ-MnO2 with alkali-ion (K, Na, Li) associated manganese (Mn) vacancies is fabricated by a simple hydrothermal reaction, and the correlation between their electronic structure and pseudocapacitance are systematically investigated. FESEM/TEM images have shown that the morphology of MnO2 is obviously changed after the introducing of cation vacancies. The position of alkali-ion in MnO2 structure can be controlled by adjusting the ion concentration. XRD patterns and Raman spectra demonstrate that the alkali-ion is embedded in Mn vacancies at low concentration, while entered the interlayer of MnO2 at high concentration. The existence of Mn vacancies will resulting in the distortion of neighboring atoms, leading to the electronic delocalization, and thus enhancing the conductivity, pseudocapacitance and rate capability of MnO2. Accordingly, the specific capacitances of optimized 0.4KMO, 0.4NaMO and 0.4LiMO samples are enhanced about 1.9, 1.6 and 1.6 times compared to pure MnO2. Meanwhile, the rate performance has also been improved about 76%, 46% and 42%, respectively. Theoretical calculations further confirm that the Mn vacancies can generate additional occupancy states and cause an increase in carrier concentration, which will improve the conductivity and further boost the pseudocapacitance of MnO2. This result open up a promising approach to explore active and durable electrode materials.

This article reports the preparation of new ZnMn2O4/Mn2O3 composites by ultrasonication and microwave irradiation assisted hydrothermal method. The composite formation (ZnMn2O4/Mn2O3) is confirmed by powder X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. The four ‘petals’ flower and six ‘petals’ flower-shaped morphology of composite material are confirmed through scanning electron microscopic analysis. The synthesis method directly influences the morphology of the composites. Further, nanosized composite formation is confirmed through transmission electron microscopic analysis. The prepared composite materials' electrochemical properties are studied using a three-electrode system and it shows a pseudocapacitance nature. The high specific capacitance of 380 F/g @ 0.5 A/g is found for the composite material in galvanostatic charge-discharge studies. The prepared ZnMn2O4/Mn2O3 composite shows an excellent cycling stability of 92% of the initial specific capacitance at current density of 3 A/g after 2000 cycles. Electrochemical studies of the composite reveal that the ZnMn2O4/Mn2O3 composite can be used for supercapacitor applications.

Obtaining a comprehensive understanding of the energy storage mechanisms, interface compatibility, electrode-electrolyte coupling, and synergistic effects in carefully programmed nanoarchitectural electrodes and complicated electrolyte systems will provide a shortcut for designing better supercapacitors. Here, we report the intrinsic relationships between the electrochemical performances and microstructures or composition of complex nanoarchitectures and formulated electrolytes. We observed that isolated TiNb2O7 nanoparticles provided both a Faradaic intercalation contribution and a surface pseudocapacitance. The holey graphenes partitioned by nanoparticles not only fostered the fast transport of both electrons and ions but also provided additional electrical double-layer capacitance. The charge contributions from the diffusion-controlled intercalation process and capacitive behaviors, double-layer charging, and pseudocapacitance, were quantitatively distinguished in different electrolytes including a formulated ionic-liquid mixture, various nanocomposite ionogel electrolytes, and an organic LiPF6 electrolyte. A steered molecular dynamics simulation method was used to unveil the underlying principles governing the high-rate capability of holey nanoarchitectures. High energy density and high rate capability in solid-state supercapacitors were achieved using the Faradaic contributions from the lithium-ion insertion process and its surface charge-transfer process in combination with the non-Faradaic contribution from the double-layer effects. The work suggests that practical high-voltage supercapacitors with programmed performances and high safety can be realized via the efficient coupling between emerging nanoarchitectural electrodes and formulated high-voltage electrolytes.

Potassium-ion batteries (PIBs) are promising next-generation energy storage candidates due to abundant resources and low cost. Sb-based materials with high theoretical capacity (660 mAh·g−1) and low working potential are considered as promising anode for PIBs. The remaining challenge is poor stability and slow kinetics. In this work, FeSb@N-doped carbon quantum dots anchored in three-dimensional (3D) porous N-doped carbon (FeSb@C/N⊂3DC/N), a Sb-based material with a particular structure, is designed and constructed by a green salt-template method. As an anode for PIBs, it exhibits extraordinarily high-rate and long-cycle stability (a capacity of 245 mAh·g−1 at 3,080 mA·g−1 after 1,000 cycles). The pseudocapacitance contribution (83%) is demonstrated as the origin of high-rate performance of the FeSb@C/N⊂3DC/N electrode. Furthermore, the potassium storage mechanism in the electrode is systematically investigated through ex-situ characterization techniques including ex-situ transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Overall, this study could provide a useful guidance for future design of high-performance electrode materials for PIBs.

Increasing the performance of energy storage systems using different metal oxides and carbon nanomaterial as support scaffolds in electrode manufacture is of great importance. However, deposition of active material using binders or conductive agents results in reduced effective contact areas in the electrodes and between the electrolytes, lowering the energy storage capacity. In this work, a homogeneous and stable low current electrodeposition of binary metal oxides MnO₂/Co₃O₄ on superaligned electrospun carbon nanofibers (SA-ECNFs) greatly overcomes these shortcomings. The morphology tests revealed that the manganese oxide (MnO₂) and cobalt oxide (Co₃O₄) were uniformly wrapped around the carbon nanofibers to form a porous morphology, rendering high energy storage capacity from both the pseudocapacitance and electrochemical double layer (ECDL). Electrochemical tests indicated that the as-prepared MnO₂/Co₃O₄@SA-ECNFs electrode displays a specific capacitance of 728 F g–¹ in 6 M KOH electrolyte in CV vs 622 F g–¹ of MnO₂@SA-ECNFs electrode at 5 mV s–¹. The performance of galvanic charge–discharge (GCD) at 2 A g–¹ of the electrode demonstrated 64.5 Wh kg–¹ for energy density and 1276 W kg–¹ for power density and a capacity retention of 71.8% over 11 000 cycles.

Thanks to the combination of the advantages of batteries and supercapacitors, battery-supercapacitor hybrid devices (BSHD) with higher power densities than batteries and higher energy densities than supercapacitors have aroused extensive concerns. Through the reasonable design of the capacity and kinetic characteristics of the cathode and anode, BSHD can provide individual advantages such as excellent properties, low cost, and safety. Li-ion BSHD has been widely studied because of lithium’s low potential and small ionic radius. However, the anode with slow electrode process dynamics is a great challenge for developing high-performance Li-ion BSHD. Consequently, it is essential to develop an electrode material with better electrical conductivity and a faster electrochemical reaction process. Here, an inverse spinel oxide iron gallium oxide (FeGa₂O₄, FGO) is designed with Fe²⁺ occupying octahedral sites and Ga³⁺ occupying tetrahedral sites. FGO pure phase was successfully synthesized by controlling the calcination temperature and atmosphere, and the experimental results were verified by X-ray diffraction (XRD). Subsequently, the electrochemical test of the half-cell shows that the capacity is stable at 500 mAh g–¹ after 200 cycles at a current density of 0.1 A g–¹, showing that the electrode has extraordinary cycle stability. Meanwhile, the kinetics of the electrode process of the material was tested by pseudocapacitance calculations, electrochemical impedance spectra (EIS), and the galvanostatic intermittent titration technique (GITT). The results show that the material is suitable for Li-ion BSHD. The assembled Li-ion BSHD exhibits high energy density (up to 107.19 Wh kg–¹), high power density (up to 4126.01 W kg–¹), and long cycle life in a wide working voltage range (0.5–4.0 V). Therefore, this work has carried out a new research direction for Li-ion BSHD’s electrode materials with high performance.

Iron carbodiimide (FeNCN) belongs to a type of metal compounds with a more covalent bonding structure compared to common transition metal oxides. It could provide possibilities for various structural designs with improved charge-transfer kinetics in battery systems. Moreover, these possibilities are still highly expected for promoting enhancement in rate performance of sodium (Na)-ion battery. Herein, oriented FeNCN crystallites were grown on the carbon-based substrate with exposed {010} faces along the [001] direction (O-FeNCN/S). It provides a high Na-ion storage capacity with excellent rate capability (680 mAh g-1 at 0.2 A g-1 and 360 mAh g-1 at 20 A g-1), presenting rapid charge-transfer kinetics with high contribution of pseudocapacitance during a typical conversion reaction. This high rate performance is attributed to the oriented morphology of FeNCN crystallites. Its orientation along [001] maintains preferred Na-ion diffusion along the two directions in the entire morphology of O-FeNCN/S, supporting fast Na-ion storage kinetics during the charge/discharge process. This study could provide ideas toward the understanding of the rational structural design of metal carbodiimides for attaining high electrochemical performance in future.

The SnSe nanoparticles encapsulated in the carbon nanofibers (SnSe@C) with microrod morphology and core–shell structure are prepared by electrospinning and annealing process, and investigated as anode materials for sodium ion batteries. Benefiting from this unique structure, the SnSe@C can deliver a reversible capacity of 283.8 mAh g−1 after 500 cycles at a high current density of 1.0 A g−1. The sodium ion storage mechanisms of SnSe are further characterized by ex-situ X-ray diffraction, high-resolution transmission electron microscope and selected area electron diffraction measurements. Besides, the excellent electrochemical performance of the electrodes is investigated by pseudocapacitance and in situ electrochemical impedance spectroscopy measurements. This work may provide a new avenue for synthesis of metal selenides with core–shell structure and a good idea for studying the kinetics process.

Manganese phosphates have shown excellent performances and great potential in electrochemical energy storage, which are demonstrated by research works published in recent years. For manganese phosphates, the open-framework structures with large channels and cavities endow them with good ion conductivity and charge storage capacity. In this review, we present the recent progress on manganese phosphates, by focusing on their advantages/disadvantages and potential applications as a new class of electrode materials in supercapacitors. The structural characteristics, synthesis methods, and mineral sources to prepare these manganese phosphates are investigated, together with the modification, as they strongly affect the electrochemical energy storage performance. Attentions are paid to those hybrid-type materials, where strong synergistic effects exist. In the summary, interlayer engineering for the manganese phosphates and hybrid-types are proposed and discussed.

Heteroatom doped porous carbon materials have great application prospects in supercapacitors. In the present study, an approach of preparing N-doped porous carbon (NPC) was proposed from porous poly(resorcinol-formaldehyde-melamine) monoliths which were prepared by high internal phase emulsion (HIPE) template. Melamine was dissolved in the external phase and copolymerized, acting as the N source and porous structure regulator to provide micropore-dominant NPCs. The structure, morphology, specific surface area (SSA), and chemical composition of the samples were systematically studied. With melamine content increasing, N-doping content in NPC increased while the SSA of NPC increased at first and then decreased. When the content of N is 8.42 ​wt%, the obtained NPC showed the highest SSA of about 1670 ​m2 ​g−1. Furthermore, high N doping content could improve the electronic conductivity and provide additional pseudocapacitance of NPC. Under the combined influences of proper N content and high porosity, the prepared NPC electrodes revealed excellent specific capacitance (228.0 ​F ​g−1 at 1.0 ​A ​g−1), favorable circling stability, and prominent rate capability in a three-electrode system with 6 ​M KOH solution as the electrolyte.

Sodium-ion storage devices are highly desirable for large-scale energy storage applications owing to the wide availability of sodium resources and low cost. Transition metal nitrides (TMNs) are promising anode materials for sodium-ion storage, while their detailed reaction mechanism remains unexplored. Herein, we synthesize the mesoporous Mo3N2 nanowires (Meso-Mo3N2-NWs). The sodium-ion storage mechanism of Mo3N2 is systematically investigated through in-situ XRD, ex-situ experimental characterizations and detailed kinetics analysis. Briefly, the Mo3N2 undergoes a surface pseudocapacitive redox charge storage process. Benefiting from the rapid surface redox reaction, the Meso-Mo3N2-NWs anode delivers high specific capacity (282 mAh g−1 at 0.1 A g−1), excellent rate capability (87 mAh g−1 at 16 A g−1) and long cycling stability (a capacity retention of 78.6% after 800 cycles at 1 A g−1). The present work highlights that the surface pseudocapacitive sodium-ion storage mechanism enables to overcome the sluggish sodium-ion diffusion process, which opens a new direction to design and synthesize high-rate sodium-ion storage materials.

The demand for fast-charging of lithium-ion batteries (LIBs) in modern electric transportation and wearable electronics is rapidly growing. However, commercially available graphite anodes still suffer from slow kinetics of lithium-ion diffusion and severe safety concerns of lithium plating when achieving the fast-charging goal. Here, it is demonstrated that the Li-ion diffusion kinetics of orthorhombic Nb2O5 nanotubes (T-Nb2O5 NTs) is enhanced by atomically precise manufacturing of nanoarchitectures. The controlled fabrication of T-Nb2O5 NTs with wall thicknesses from 24 to 43 nm is realized via atomic layer deposition (ALD) using electrospun polyacrylonitrile nanofibers as a sacrificing template. The wall thickness of T-Nb2O5 NTs can be precisely tuned by adjusting the number of ALD cycles. The relationship between the wall thicknesses and electrochemical performances is investigated in detail. The electrochemical kinetic analysis suggests that the lithium storage in T-Nb2O5 NTs is dominated by surface and intercalation pseudocapacitance. The morphology of T-Nb2O5 crystallites is found to have significant effects on the Li-ion insertion/extraction kinetics and the performance of the electrodes in LIBs. The resulting T-Nb2O5 NTs exhibit fast charge-storage kinetics and enable highly reversible insertion/extraction of Li ions without a phase change. This work may open up a new avenue for further development of intercalation-pseudocapacitive nanostructured materials for high-rate and ultrastable energy-storage devices.

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g−1 at a rate of 0.5 A g−1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g−1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials. Pseudocapacitors exhibit charge-storage mechanisms leading to high-capacity and rapidly cycling devices. An organic system designed via molecular contortion is now shown to exhibit unprecedented electrochemical performance and stability.

The simple and low-cost supercapacitor preparation strategy has received great attention at present. In our work, the MnO2 nanosheets were equably grown on carbon foam by electrodeposition to synthesize electrode. The carbon foam (CF) is a three-dimensional (3D) conductive skeleton that guarantees the rapid transfer of ions and electrons. An appropriate amount of MnO2 nanosheets provide excellent pseudocapacitance properties. The composite material reaches the highest specific capacitance of 365 F g−1 at 1 A g−1 and the cycling test shows capacitance retention of 87.7% over 5000 cycles at a current density of 10 A g−1 in a three-electrode system. Meanwhile, the device revealed a maximum energy density of 22.2 Wh kg−1 at a power density of 1000 W kg−1. Two assembled supercapacitors were enough to light up a timer and the material has a certain practical value.

Although novel anode materials made of transition metal hydroxide can initially exhibit high capacity, their cycling performances would rapidly decline due to the poor structural stability and low electrical conductivity. Herein, we demonstrated a novel design strategy to prepare a Cu2+ and Ni3+ co-doped nickel-based layered double hydroxide (LDH) by a simple one-step co-precipitation method. The well-doped Cu2+ shortens the band gap and enhances the interlamellar spacing of the co-doped LDH, thus promoting the migration of electrons and lithium ions, and increasing the stability of layered structure. Moreover, in situ generated Ni3+ not only shorten the overall band gap but also improve the pseudocapacitance of the Ni-based LDH material. As a result, the co-doped nickel-based LDH exhibit an ultra-high capacity of 942.5 mAh g−1 after 1000 cycles under the rate of 2 A g−1, which behaves outstanding cycling performance in comparison with all the reported nickel-based hydroxide anode materials to our knowledge. In addition, the reversible discharge capacity at 1 A g−1 of the co-doped nickel-based layered double hydroxide anode at a low voltage window (0.01~1.5 V) is more than four times of that of the common Ni(OH)2 anode after 800 cycles.

Enhancing the pseudocapacitance of carbon electrodes by doping them with heteroatoms becomes one promising way for fabricating high energy density supercapacitors. Nevertheless, rich doping with he...

Metal organic framework (MOFs) of ternary metal precursors with different organic linkers are synthesized and fabricated on Ni-foam as the electrodes via hydrothermal reaction aiming to enhance the electrical conductivity and the specific capacitance, Cs. We report the preparation of ternary metal with different organic linkers named CoCuNi-bi using terephthalic acid (H2bdc), CoCuNi-tri using trimellitic acid and CoCuNi-tetra using pyromellitic acid with increasing of active sites on Ni-foam substrate respectively. The CoCuNi-tetra demonstrates the highest Cs of 740 F g-1 at 2 mV/s & 791 F g-1 (87.9 mAh g­­­-1) at 1 Ag-1 respectively followed by CoCuNi-tri (674 F g-1 at 1 A g-1; 74.9 mAh g-1) and CoCuNi-bi (591 F g-1 at 1 A g-1; 65.7 mAh g-1). CoCuNi-tetra shows the best electrochemical performance hence it could be the encouraging electrode for supercapacitor materials.

Capacitive deionization (CDI) is considered one of the most promising desalination technologies for obtaining fresh water from saline water. In this work, we synthesized a hollow core–shell Co-MOF@Fe/Co-LDH (Co-Fe-LDH) material by developing a strategy to simultaneously grow Co/Fe-LDH on the surface of a Co-MOF precursor in situ. Owing to the increase in the specific surface area of the hollow structure and the Faradaic process of a layered double hydroxide (LDH), the Co-Fe-LDH material exhibits high electrical double layer (EDL) capacitance and pseudocapacitance, which significantly improves the salt adsorption of the material during CDI (34.2 mg/g in a 600 mg/L NaCl solution at 1.2 V). The adsorption for NaCl in this work is approximately 2.5 times the maximum salt adsorption capacity (SAC) of LDH materials applied in nonmembrane CDI (NMCDI). This work may provide a promising model for the application of hollow LDH materials that exhibit pseudocapacitance in CDI.

Ni3[Fe(CN)6]2 nano-cubic precursors were prepared by chemical coprecipitation at room temperature with nickel acetate and potassium ferricyanide as raw materials. The corresponding NiFe2O4-NiO@C composites with excellent crystallization were prepared by two-stage oxidation at low temperature. The microstructure and electrochemical behavior of the materials showed that the Prussian blue analog was transformed into metal oxide while the carbon coating was maintained in the two-stage oxidation at low temperature. The existence of the carbon coating reduces the charge transfer impedance to 31.5 Ω. At the current density of 500 mA/g, the reversible capacity of 632.7 mAh/g is maintained after 500 cycles. At the same time, carbon cladding can also enhance the role of pseudocapacitance in the material. At the scanning rate of 0.1 mV/s, the pseudocapacitance account for 54.4% of the total discharge capacity, which is significantly higher than that of uncoated materials.

A layered triazinyl-COF(COF-TP) linked by − NH − bond was synthesized by condensation of 2,4,6 − trichloro − 1,3,5 − triazine and p − phenylenediamine. The BET specific surface area of COF-TP was 43.65 m2·g− 1 and the pore size was about 13.658 A. A N-doped porous carbon material(COF−TP − C) was prepared by heating the COF-TP at 700 °C under nitrogen protection. The BET specific surface area of COF−TP − C was 398.15 m2·g− 1 and the pore size was about 10 A. COF−TP exhibited a significant pseudocapacitance characteristic in 6 M K(OH)2 electrolyte with the highest specific capacitance of 283 F·g− 1 at 1 A·g− 1 current density in three electrode configuration. The specific capacitance retention was about 88 % after 20,000 cycles. COF−TP − C showed the maximum specific capacitance of 449.5 F·g− 1 at the current density 1 A·g− 1 and a good cyclic performance of 77 % after 3000 GCD cycles(93 % after 1000 cycles) at the current density of 5 A·g− 1, and retained 99.2 % after 5000 cycles at the current density of 1 A·g− 1. Both COF−TP and COF−TP − C showed good capacitance performance and potential application in supercapacitors.

Pseudocapacitances are being increasingly explored due to their potential applications to achieve higher capacitance and energy density than the traditional electric double-layer capacitances. It is one of the urgent challenges to realize the simple, fast, and good process compatibility manufacturing of pseudocapacitance for its wide industrial applications. A one-step laser-induced plasma-assisted ablation-based deposition (LIPAA-based deposition) method for fabrication of embedded planar pseudocapacitive supercapacitors on the ITO glass surface was proposed. The surface morphology and structural characteristics of the designed pseudocapacitance interdigital electrode surfaces were investigated using the XRD, SEM, and XPS. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) were used to analyze the electrochemical properties of the prepared glass-based planar pseudocapacitances, which achieved a potential range of − 0.5 to 0.4 V in the PVA/H2SO4 gel electrolyte due to the reaction induced by ions in the electrolyte intercalation/deintercalation process on the electrode surface. A maximum areal capacitance of 238.57 μF cm−2 was achieved by the manufactured capacitor and ~ 80.022% initial capacitance retention after 2000 cycles. The one-step preparation of planar supercapacitors with ITO conductive glass surfaces as electrodes is of great significance for the commercial development of pseudocapacitors in the field of electrochemical energy storage.

Lithium-ion battery (LIB) is the dominant energy storage device for the application on electric vehicles and portable electronic devices. Up to now, the enhancement of LIBs’ energy density mainly depends on the improvement of anode’s capacity. Due to the obstacle of large volume effect and low rate performance, the use of intrinsic high-capacity anodes, such as silicon, is still in the laboratory stage. Combining battery and capacitor to improve the electrochemical performance of the commercial carbon anode is an alternative and effective strategy in practical application. In this paper, a high nitrogen (N) content (8.2%) carbon is obtained by pyrolyzing the melamine formaldehyde (MF) foam resin at low temperature (600 °C). It delivers a stable specific capacity of around 600 mAh g−1 at 0.1C. No significant capacity degradation appears after 200 cycles, and the coulombic efficiency maintained over 99.5%. Even at 1.2 C, a specific capacity of 350 mAh g−1 is delivered and so a Li-ions diffusion coefficient of 1.8×10–13 cm2s-1. The reason of the superior electrochemical performance of the high N-doped carbon anode is the synergistic effect of battery and capacitor. The calculated pseudocapacitance proportion is from 32 to 61% at scan rate from 0.1 to 1mV s−1.

Sodium-ion batteries with abundant resources and low cost are considered as a potential candidate for energy application. However, the low rate performance and poor cyclability of cathodes always i...

Fe2O3 electrode materials exhibit excellent electrochemical performance in electrochemical energy storage system. However, its poor electrical conductivity limits its future practical application. The binder-free NiCo2O4@Fe2O3 composites was reasonably designed and fabricated on carbon fiber paper with NiCo2O4 nanowires as conductive scaffold in the present investigation. The three-dimensional nanostructure of the porous Fe2O3 nanorods coated the NiCo2O4 nanowire arrays showed the fascinating electrochemical performance, including high specific capacitance of 262 ​mF/cm2 at a current density of 1 ​mA/cm2, and remarkable cycle stability with ~74.2% capacitance retention after 4000 cycles. The excellent pseudocapacitance performance of NiCo2O4@Fe2O3 composite materials is due to synergistic effect between NiCo2O4 and Fe2O3. The results of the present work show that NiCo2O4@Fe2O3 core-shell composite electrode is expected to exhibit excellent performance in the field of supercapacitors.

MXenes are well known for their potential application in supercapacitors due to their high-rate intercalation pseudocapacitance and long cyclability. However, the reported low capacity of pristine MXenes hinders their practical application in lithium-ion batteries. In this work, a robust strategy is developed to control the functional groups of Nb2CTx MXene. The capacity of pristine Nb2CTx MXene can be significantly increased by Li+ intercalation and surface modification. The specific capacity of the treated Nb2CTx is up to 448 mAh g−1 at 0.05 A g−1, and at a large current density of 2 A g−1 remains a high reversible capacity retention rate of 75% after an ultra-long cycle of 2000 cycles. These values exceed most of the reported pristine MXenes (including the most studied Ti3C2Tx) and carbon-based materials. It demonstrates that this strategy has great help to improve the electrochemical performance of pristine MXene, and the results enhance the promise of MXenes in the application of lithium-ion batteries.

Here, we have fabricated the spinel binary-metal oxide (FeCo2O4) via a solvent-free and cost-effective approach. The nanocomposites of the as-fabricated binary-metal spinel oxide have been prepared with three different conductive-matrices, namely r-GO, CNTs, and PANI, via ultra-sonication approach. The spinel phase and surface functionalities of the fabricated FeCo2O4 sample have been confirmed via XRD and FT-IR analyses, respectively. The morphological-structure and elemental composition of the fabricated samples have been probed via FESEM and EDX results. The role of added conductive-matrices in the improvement of the electrical conductivities of the fabricated nanocomposites has been investigated via I–V experiments. The electrochemical experiments, conducted in half-cell configuration, showed that FeCo2O4/PANI nanocomposite exhibited the highest specific capacitance (658.9 Fg-1) than that of the remaining two nanocomposites. Furthermore, FeCo2O4/PANI nanocomposite exhibited excellent cyclic stability as it lost just 8.3% of its initial specific capacitance even after 3000 cyclic tests. The superior capacitive-activity of the FeCo2O4/PANI nanocomposite is accredited to its high conductivity, large surface area, and synergy effects between the pseudocapacitance derived from the PANI and FeCo2O4 nanostructure. The electrochemical and electrical measurements suggested that FeCo2O4/PANI nanostructure is an emerging contender for energy storage applications.

In this study, we have evaluated the effect of potassium hydroxide (KOH) on the energy storage performance of metal-free carbon-based materials prepared from molasses. Molasses are a renewable-resource biomass and economical by-product of sugar refinement, used here as a carbon precursor. Two co-doped carbon materials using molasses were synthesized via a time and cost-efficient microwave carbonization process, with ammonium polyphosphate as a phosphorus and nitrogen doping agent. The phosphorus and nitrogen co-doped carbon (PNDC) samples were prepared in the presence and absence of a chemical activating agent (KOH), to study the role of chemical activation on PNDCs. Physical characterizations were performed to gain insight into the composition, pore size and topographical data of each material. Electrochemical characterization via cyclic voltammetry in 1 M sulfuric acid (H2SO4) as well as in 6 M KOH as electrolytes, revealed high current density and specific capacitance for the chemically activated material (PNDC2) compared to one without chemical activation (PNDC1). The capacitance value of 244 F/g in KOH electrolyte was obtained with PNDC2. It is concluded that addition of KOH prior to carbonization increases the surface functionality, which significantly enhances the electrochemical properties of the PNDC material such as current density, stability, and specific capacitance.

We project banyan prop root biomass waste as an alternative zero-cost precursor for preparing activated carbon for fabricating electrodes for pseudocapacitors. Banyan prop root activated carbon (BPRAC) powder was obtained by activating the dried biomass powder in concentrated H2SO4 followed by thermal processing at 800 °C under N2 atmosphere for 1 h. Relevant physical characterization studies were done on BPRAC to assess its quality prior to examining the capacitive output in 1 M H2SO4 (acid) and 1 M Na2SO4 (neutral) electrolytes by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge cycling studies. The specific capacitance calculated by cyclic voltammetry in acid electrolyte is as high as 466 F/g at a scan rate of 10 mV/s. Significant electrochemical stability with 91% of the initial capacitance is retained at the end of the 500th cycle. Practical and acceptable electrochemical output (pseudocapacitance) that is tapped from BPRAC is presumed to be built upon certain favorable physical features like surface organic functional groups containing heteroatoms on the BPRAC. Thus, the ascendancy of waste biomass in energy storage devices provides a strategy for both recycling of wastes and eco-friendly capacitor electrodes.

Anode materials are critical for energy devices based on Li-ion batteries (LIBs). This work reports on a facile method to produce anodes based on carbon-coated Fe3O4 (CP-Fe3O4) that is encapsulated in reduced graphene oxide (rGO) layers forming a porous core-shell structure Fe3O4@carbon (rGO-CP-Fe3O4). First, Fe3O4 particles were coated with carbon by hydrothermal and carbothermal reduction methods leading to an intermediate product termed CP-Fe3O4. Next, CP-Fe3O4 was encapsulated by two-dimensional layered rGO to obtain CP-Fe3O4 composites with a three-dimensional structure. The Fe3O4 volume expansion during LIB cycling was inhibited by carbon and rGO and a three-dimensional electron transport network was generated by the introduction of rGO. The rGO-CP-Fe3O4 composite showed excellent electrochemical properties (839 mA h g-1 at 0.3 A g-1 after 200 cycles) and rate capacities (165 mA h g-1 at 6.0 A g-1). In addition, the rGO-CP-Fe3O4 pseudocapacitance was equal to 65% of the overall capacity at 5 mV s-1.

Li-ion batteries (LIBs) have become critical components in the manufacture of electric vehicles (EVs) as they offer the best all-round performance compared to competing battery chemistries. However, LIB performance at low temperature (LT) extremes of EV operation (typically −40 to 0 °C) suffers from a reduced output and diminished cycle life. LT cycling increases cell impedance, diminishing Li ion diffusion through the cell, exacerbating electrode polarisation, and hindering interfacial Li+ desolvation. Herein, we present a comprehensive review of (i) the factors that influence LT Li-ion performance, (ii) outline the shortcomings of the current state-of-the-art and (iii) discuss recent findings in the field, focusing on alternative anode materials with particular emphasis on high-capacity, fast charging alternatives to the archetypal carbon (graphite) anode. Different approaches to improve LT LIB performance are outlined in an in-depth analysis of recent improvements from the anode perspective. These include electrolyte-driven enhancements, the resurgence of Li metal batteries, the impact of conductive coatings, elemental doping and nanocomposite formation, substitution of intercalating anodes with high-capacity Li alloying and Li conversion materials, and fast redox pseudocapacitance.