Faradaic Redox Reactions Research Articles

A Covalently Integrated Reduced Graphene Oxide–Ion‐Exchange Resin Electrode for Efficient Capacitive Deionization

AbstractCapacitive deionization (CDI) is an emerging, cost‐effective alternative for energy‐efficient desalination technology. Efficient electrode materials based on individual reduced graphene oxide (rGO) nanosheets are produced by functionalizing them with polystyrene (rGO–PS) through an in situ polymerization process involving rGO, styrene monomer, and divinylbenzene. The rGO–PS‐integrated composite nanostructures are subsequently functionalized with sulfonate and quaternary amine functionalities to achieve positively and negatively charged electro‐adsorbent ion‐exchange resins (EAIERs), respectively. These EAIERs ‘molecular constructs’ are used to fabricate CDI electrodes, and deionization is performed to remove various ions. These molecular constructs promote faster charge transfer at the electrode–electrolyte interface and maintained the electrical conductivity of the active rGO. This leads to a high electroadsorption capacity of 15.93 mg g−1 of Cl− using NaCl solution with a conductivity of 802 µS in laboratory batch experiments, which is approximately five times higher than the adsorption capacity of rGO electrodes reported earlier (≈2–3 mg g−1) in comparable experimental conditions. No significant Faradaic redox reactions or chemical changes are observed on the electrode surface, which make these electrodes exhibit excellent electrochemical stability even after multiple adsorption/desorption cycles.

Effect of ionic conductivity in polymer-gel electrolytes containing iodine-based redox mediators for efficient, flexible energy storage systems

Tailoring redox-mediators (RMs) and developing systematic fabrication methods for favorable electrochemical kinetics are essential to improve the energy storage performance of fiber-based supercapacitors. The effective use of RMs can provide a unique energy storage mechanism; additional Faradaic redox reactions and optimized ion diffusion between the electrodes and electrolyte can be achieved. Here, we successfully optimized the electrochemical performance of fiber-based supercapacitors using the iodine-based redox mediator (I-RM) potassium iodide (KI). The fiber-based symmetrically yarned supercapacitor cells (f-SYCs), incorporating the KI mediator at a concentration of 7.5mM, exhibit a high specific capacitance of 13.9mF at a current density of 10μA, which directly depicts its superior electrochemical performance compared to that of the previously reported fiber-based supercapacitors. Owing to the limited moisture content present in the polymer-gel electrolyte, the improved electrochemical performance of the f-SYCs containing I-RMs is attributed to the optimized ionic conductivity and diffusion kinetics, as a result of the well-engineered KI electrolyte properties. Synergistically, the results indicate that controlling the amount of RMs in the polymer-gel electrolyte is crucial to achieve excellent overall electrochemical properties in next-generation fiber-based supercapacitors.

Three-dimensional hierarchical core-shell CuCo2O4@Co(OH)2 nanoflakes as high-performance electrode materials for flexible supercapacitors

Rational design of composite electrode materials with novel nanostructures plays an important role in improving both high energy density and structure stability of flexible and wearable supercapacitors. Herein, numerous peculiar three-dimensional hierarchical core-shell CuCo2O4@Co(OH)2 nanoflakes directly grown on Ni foam are synthesized via a facile hydrothermal method and subsequent electrodeposition technique. Ultrathin Co(OH)2 nanosheets arrays vertically anchored on CuCo2O4 nanoflakes can not only improve the electrical conductivity, but also provide interconnected channels for ion diffusion and enrich electrochemical active sites to boost faradaic redox reaction, leading to the enhanced electrochemical behavior. Excellent electrochemical performance of CuCo2O4@Co(OH)2 electrode can be reflected on a higher specific capacitance of 1558 F/g and lower resistance compared with that of the pristine CuCo2O4 electrode. The asymmetric flexible supercapacitor assembled by the optimized CuCo2O4@Co(OH)2 electrode and activated carbon exhibits high energy density of 62.5 Wh/kg at 893 W/kg, outstanding cycle stability of 88.6% capacitance retention after 10,000 cycles and remarkable mechanical flexibility, performing the best electrochemical behavior among various metal oxides based asymmetric supercapacitors. All above results indicate that the resulted hierarchical core-shell CuCo2O4@Co(OH)2 electrode can be a promising candidate for flexible energy storage devices.

Benzoyl hydrazine-anchored graphene oxide as supercapacitor electrodes

In this study, benzoyl-hydrazine anchored graphene oxide (BHGO) is synthesised using graphene oxide (GO) and benzoyl hydrazine (BH) via a simple, cost effective ultrasonic assisted chemical route. BH acted as a nitrogen source, reducing agent, and morphology modifier resulting in good electrochemical performance of BHGO. The supercapacitor behaviour of BHGO is investigated in different aqueous electrolytes and it exhibits a specific capacitance of 170 F g−1 at a current density of 1 A g−1 in 1 M H2SO4 and capacitive retention of 85% over 5000 cycles at 5 A g−1. This high performance is attributed to the enrichment of electroactive sites of GO through nitrogen moieties enhancing faradaic redox reactions and thereby the polarization at the electrode surface.

High Magnetic Field-Engineered Bunched Zn-Co-S Yolk-Shell Balls Intercalated within S, N Codoped CNT/Graphene Films for Free-Standing Supercapacitors.

The abundant mass and charge transfer involved in Faradaic redox reactions are largely determined by microstructures including the surface area and porosity, elemental composition and electrical conductivity of bimetallic sulfides. Here, a high magnetic field (HMF) was introduced to tune these intrinsic characters for superior supercapacitor electrodes. We developed a novel HMF-controlled anion-exchange methodology to prepare the one-dimensional (1D) bunched Zn-Co-S yolk-shell balls (ZCS6T BYSBs). The HMF-induced directional growth and alignment of Zn0.76Co0.24S drive the directional 1D assembly. The as-obtained ZCS6T BYSBs possess larger surface area/pore volume, higher crystallinity and electrical conductivity, richer electroactive elements, and favorable axial electron and ion transport because of HMF-enhanced favorable ion diffusion and exchange kinetics. Flexible S, N codoped carbon nanotubes/graphene films embedded with the ZCS6T BYSBs (CZS6T/CNTs/SNGS) were fabricated by vacuum filtration and one-step S, N codoping and reduction of graphene oxides to improve structural stability and charge transport. The CZS6T/CNTs/SNGS electrode displayed impressive enhanced specific capacitance and rate capability with 78.7% capacitance retention at 30 A g-1. Furthermore, the CZS6T/CNTs/SNGS//CNTs/SNGS asymmetric supercapacitor delivered remarkable cycling stability with a high energy density of 41.1 W h kg-1 at a large power density of 9022 W kg-1.

MnOx nanosheets anchored on a bio-derived porous carbon framework for high-performance asymmetric supercapacitors

MnO2 is regarded as one of the most promising electrode materials for supercapacitors due to its high theoretical capacitance and low cost; however, it suffers from poor electronic and ionic conductivity. Herein, nanostructured manganese oxides (MnOx) are anchored on a lotus seedpod-derived carbon framework (LSCF) in situ using a one-step hydrothermal-based redox synthesis strategy. The as-prepared MnOx@LSCF hybrid features a 3D conductive network and ultrathin MnOx nanosheets with oxygen vacancies, which can enhance both electrons and ions transfer intrinsically and facilitate sufficient surface Faradaic redox reactions. Benefitting from the synergistic effect of the nanostructured MnOx and the conductive LSCF, the optimized MnOx@LSCF hybrid exhibits a high specific capacitance (406 F g−1 at 0.5 A g−1) and a remarkable discharging rate capability (270 F g−1 at 10 A g−1). Moreover, an asymmetric supercapacitor fabricated by using MnOx@LSCF as the positive electrode and LSCF as the negative electrode achieves a high energy density of 48.1 Wh kg−1 at 500 W kg−1 (which still retains 22.2 Wh kg−1 at 10 kW kg−1), and a considerable cyclic stability of 90.1% after 5000 cycles. These properties demonstrate the MnOx@LSCF hybrid would have a valuable commercial application prospects in high-performance supercapacitors.

Incorporation of electroactive NiCo2S4 and Fe2O3 into graphene aerogel for high-energy asymmetric supercapacitor

Incorporation of electroactive Fe2O3 and NiCo2S4 (NCS) into graphene aeorgel (GA) is presented in this work. The efficient adsorption of precursor ions on the graphene oxides sheets and subsequent hydrothermal gelation yield the GA-Fe2O3 and GA-NCS composites that can be used as negative and positive electrode, respectively, for constructing a high-energy asymmetric supercapacitor (ASC). Influences of mass contents of Fe2O3 and NCS on capacitive performances of GA-Fe2O3 and GA-NCS electrodes are systematically investigated in KOH aqueous electrolyte. The uniform distribution of Fe2O3 on the graphene sheets (GS) and the NCS nanoparticles wrapped by GS provide sufficient electroactive sites for Faradaic redox reactions, while the GA interconnected with macropores offers favorable channels for electrolyte transport. Thereby, the specific capacitance of 200 F g−1 (178 C g−1) and 386 F g−1 (170 C g−1) are achieved for GA-Fe2O3 (20 wt% Fe2O3) and GA-NCS (77 wt% NCS), respectively. A coin-type capacitor built with GA-Fe2O3 as negative electrode and GA-NCS as positive electrode delivers a specific capacitance of 93 F g−1 (128 C g−1) at 0.1 A g−1, corresponding to a maximum energy density of 25 Wh kg−1 at a power density of 54 W kg−1. Meanwhile, the ASC also exhibits a good rate capability and a satisfying cycling stability with 72.3 % capacitance retention after cycling at 2 A g−1 for 5000 cycles. These results show great potentials of GA-Fe2O3 and GA-NCS electrodes in electrochemical energy storage.

Efficient coupling of semiconductors into metallic MnO2@CoMn2O4 heterostructured electrode with boosted charge transfer for high-performance supercapacitors

Semiconductor heterostructures have emerged as highly promising candidates for energy storage and conversion owing to their tunable electronic and structural properties by rational design and controllable synthesis at a molecular level. The overall electrochemical performance of the semiconductor heterostructures often depends on the carrier mobility from the semiconductor interfaces and faradaic redox reactions from their active sites. Herein, we report a facile, low-cost, two-step method to controllably grow MnO2 nanorods directly on CoMn2O4 nanosheets (i.e., MnO2@CoMn2O4) with robust adhesion. An induced electric field resulted from the interface effect of heterostructures tailors the kinetic performance of electrons and ions during the charge-discharge process, enhancing the electron mobility and reducing diffusion barrier for charge carriers (OH−) ions migration. As a result, the rational design and controllable heterostructures exhibit significantly improved electrochemical capacitive performance, including remarkable specific capacitance, excellent rate capability, and cycling stability. Furthermore, exploring MnO2@CoMn2O4 as positive electrode and N-doping 3D reduction of graphene oxide (N-3DrGO) as negative electrode yields an asymmetric supercapacitor with high energy density (230.57 mWh cm−2), remarkable power density (3.91 mW cm−2 at 149.99 mWh cm−2), and excellent cycling stability (81.3% capacitance retention after 5000 cycles at 3 mA cm−2). This work offers new opportunities to explore high-performance electrode materials by providing methods to control the interface in nano-heterostructures.

Novel Ag configurations decorated CuO hybrid electrode for high-performance asymmetric supercapacitor

In situ integration of highly conductive metals into transitional metal oxide electrodes with enabling fast electron transfer kinetics is a plausible technique for improving electrochemical performances of electrode materials. Herein, for the first time, we demonstrate a rational configuration of conductive Ag flakes (FAg) that directly grown on CuO nanorods by in situ chemical approach. The obtained FAg–CuO hybrid electrode demonstrates a superior specific capacitance of 812 F/g at a current density of 2 A/g which is retained of about 110.37% after 5000 cycles. The solid-state asymmetric supercapacitor (ASC) based on FAg–CuO and active carbon is further fabricated. The ASC device similarly demonstrates remarkable performances with a high energy density of 40.02 Wh/kg at a power density of 1095.8 W/kg and also excellent cycling ability with stable capacitances output beyond 5000 cycles. The obtained superior performances are attributed to the rational Ag configurations which could effectively reduce the interfacial barriers and meanwhile enhance the electron transport and ion diffusion, with achieving the high-efficiency utilization for the active materials during Faradaic redox reactions.

Incorporating Fe into Bismuthic Anode Systems: A Smart "Merits Combination/Complementation" Route to Build Better Ni-Bi Batteries.

The advancement of Ni-Bi batteries has turned sluggish because of impenetrable barriers related to physicochemical instability of bismuthic species under thermal conditions. This directly makes Bi-based anodes impossible to hybridize with graphitic carbons for a longer-term cyclic lifespan. To break this constraint, we herein propose an effective strategy by incorporating Fe into bismuthic systems to form multielement anodes. The smart Bi/Fe merits combination/complementation can drastically promote the tolerable temperature of Bi-containing nanomaterials over 500 °C, enabling carbon encapsulation without altering their geometric properties and in the meantime endowing the anodes with inherited electrochemical superiorities. The as-built BiFeO3@carbon anodes exhibit prominent electrode performances with excellent electrochemical activity (both Bi- and Fe-based components act as faradaic redox reaction sites), excellent rate capabilities, and impressive capacity retention (∼83.4% after 2000 cycles). We further unveil the anodic phase conversions of "BiFeO3 → Bi2O3/Fe2O3" (via the transition state of Bi2O3(222)) based on the real-time characterizations/post-analysis at distinct cyclic stages. The packed full cells exhibit max. energy/power densities of ∼90.72 W h kg-1/∼1.3 kW kg-1. Our study may offer a promising engineering route to promote the development of safe and applicable Ni-Bi batteries in near-future applications.

Ultra-long cycle life and binder-free manganese-cobalt oxide supercapacitor electrodes through photonic nanostructuring.

We report a novel photonic processing technique as a next-generation cost-effective technique to instantaneously synthesize nanostructured manganese-cobalt mixed oxide reduced graphitic oxide (Mn-Co-rGO) for supercapacitor electrodes in energy storage applications. The active material was prepared directly on highly conductive Pt–Si substrate, eliminating the need for a binder. Surface morphological analysis showed that the as-prepared electrodes have a highly porous and resilient nanostructure that facilitates the ion/electron movement during faradaic redox reactions and buffers the volume changes during charge–discharge processes, leading to the improved structural integrity of the electrode. The presence of distinct redox peaks, due to faradaic redox reactions, at all scan rates in the cyclic voltammetry (CV) curves and non-linear nature of the charge–discharge curves suggest the pseudocapacitive charge storage mechanism of the electrode. The electrochemical stability and the life cycle were examined by carrying out galvanostatic charge–discharge (GCD) measurements at 0.40 mA cm−2 constant areal current density for 80 000 cycles, and the electrode showed 95% specific capacitance retention, exhibiting excellent electrochemical stability and an ultra-long cycle life. Such remarkable electrochemical performance could be attributed to the enhanced conductivity of the electrode, the synergistic effect of metal ions with rGO, and the highly porous morphology, which provides large specific surface area for electrode/electrolyte interaction and facilitates the ion transfer.

Green synthesis of CoWO4 powders using agar-agar from red seaweed (Rhodophyta): Structure, magnetic properties and battery-like behavior

This work reports a proteic sol-gel green method that uses agar-agar from red seaweed (Rhodophyta) for synthesizing cobalt tungstate (CoWO4) powders for battery-like electrodes. For comparison, CoWO4 is also prepared by a similar proteic chemical route that uses flavorless gelatin. The as-prepared powders were calcined at 800 °C for 2 h and further characterized by XRD (with Rietveld refinement), SEM/EDS, FTIR/Raman/UV–Vis–NIR spectroscopies, VSM, and various electrochemical techniques for assessing the battery-like behavior in alkaline media (3 M KOH). The results indicate that samples produced by gelatin show average particle size of 150 nm and crystallite size of 68 nm, against 284 nm (particle) and 84 nm (crystallite) for agar-agar processed materials. Magnetic assessment at 300 K showed a paramagnetic behavior for both samples. The analysis of these curves showed an effective magnetic moment per cobalt atom of 4.908 μB and 4.926 μB for samples prepared using gelatin and agar-agar, respectively. The performance of CoWO4 as battery-like electrodes is ascribed due to a surface faradaic redox reaction mechanism related to reversible valence state between Co2+ and Co3+. Discharging curves indicate no substantial difference of electrochemical performance, with maximum specific capacity of 77 C g−1 at a specific current of 1 A g−1. The long-term stability of electrodes is confirmed by a capacity retention of around 98% over 1000 charge-discharge cycles at 1 A g−1.

Design of reduced graphene oxide supported NiMoS4 to enhance energy capacity of hybrid supercapacitors

The energy storage material NiMoS4 with Faradaic redox reactions was grown on the reduced graphene oxide (RGO) lamella by a facile hydrothermal method, considering that RGO with the high surface area and conductivity can provide some channels for charge/ion transfer and active sites for redox reactions. The optimal NiMoS4/RGO composite can exhibit the high specific capacity of 124 mA h/g at 1 A/g, and be selected as positive electrode materials of hybrid supercapacitors. The assembled NiMoS4/RGO//activated carbon hybrid device possesses a high current density of 63 Wh/kg at a power density of 1125 W/kg and good cyclic stability with the capacity retention of 81 % after 5000 cycles. The prepared NiMoS4/RGO composite is a promising electrode material for supercapacitors.

Reagents assisted ZnCo2O4 nanomaterial for supercapacitor application

ZnCo2O4 (ZC-UAH) nanomaterials fabricated using an essential-route hydrothermal technique followed by a post-annealing procedure and used as the materials for high performance supercapacitors (SCs) by varying three reagents: urea (UA), ammonium fluoride (AF), and hexamethylenetetramine (HT). A reagent composite (ZC-UAH) served as a large surface area, high electrical conductivity, growth modifier during fabrication, and provide active abundant active sites for Faradaic redox reactions. The resulting products of the crystalline structural, composition, morphological, surface analysis behaviors were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and Brunauer-Emmett-Teller analysis. The electrochemical properties of the as-prepared materials were tested using a three-electrode system. As a result, the reagent-assisted composite of the ZC-UAH electrode material showed a higher specific capacity of 462.5C g−1 at a current density of 1 A g−1 within the potential range of 0.0–0.4 V in the 2 M KOH electrolyte solution. In addition, the ZC-UAH composite delivered excellent cycling stability, retaining 97.4% of its initial capacitance after 5000 cycles. These outcomes show that the reagent-assisted composite of the ZC-UAH electrode material has remarkable application potential and can achieve better characteristics for high-performance supercapacitors in energy storage applications.

Redox Tuning in Crystalline and Electronic Structure of Bimetal-Organic Frameworks Derived Cobalt/Nickel Boride/Sulfide for Boosted Faradaic Capacitance.

The development of efficient electrode materials is a cutting-edge approach for high-performance energy storage devices. Herein, an effective chemical redox approach is reported for tuning the crystalline and electronic structures of bimetallic cobalt/nickel-organic frameworks (Co-Ni MOFs) to boost faradaic redox reaction for high energy density. The as-obtained cobalt/nickel boride/sulfide exhibits a high specific capacitance (1281 F g-1 at 1 A g-1 ), remarkable rate performance (802.9 F g-1 at 20 A g-1 ), and outstanding cycling stability (92.1% retention after 10 000 cycles). An energy storage device fabricated with a cobalt/nickel boride/sulfide electrode exhibits a high energy density of 50.0 Wh kg-1 at a power density of 857.7 W kg-1 , and capacity retention of 87.7% (up to 5000 cycles at 12 A g-1 ). Such an effective redox approach realizes the systematic electronic tuning that activates the fast faradaic reactions of the metal species in cobalt/nickel boride/sulfide which may shed substantial light on inspiring MOFs and their derivatives for energy storage devices.

Influence of carboxymethyl cellulose content on structures and electrochemical behaviors of reduced graphene oxide films

Film-like charge-storage devices have attracted considerable attention owing to the emergence of flexible, bendable and wearable devices. Reduced graphene oxide (rGO), which is similar to graphene, is a promising deformable electrode for charge storage. However, film-like rGO has a densely stacked structure and poor wettability to the electrolyte, which are challenging issues. In this study, carboxymethyl cellulose/graphene oxide (CG) films with different amounts of carboxymethyl cellulose (CMC) were prepared. CMC, which is a derivative of cellulose and a water-soluble polymer, functioned as an activation agent. The CG films were converted into reduced CG (rCG), which had enhanced electrochemical properties because of the increase in the accessible areas to the electrolyte ions and the pseudocapacitance due to the thermal reduction process. The CG films possessed bending property and were retained after reduction. However, the bending property decreased after reduction when the CMC was added in excess. The thickness of the rCG films increased as vacancies were formed between stacked rGO layers, and basic oxygen functional groups capable of electrochemically performing a Faradaic redox reaction were created by CMC activation. The areal specific capacitance of the rCG films increased by a factor of 30 compared with the rGO film before cycling. With increasing charging/discharging cycles, the specific capacitance of the rCG films drastically increased owing to synergistic effects of electro-activation and pseudocapacitance.

Morphology-dependent binder-free CuNiO2electrode material with excellent electrochemical performances for supercapacitors

Abstract Rational design for structure and morphology of multi-component metal oxides is an efficient and promising way to enhance energy storage performance of electrode materials. In this present work, nanosheet-like CuNiO2 heterostructures are fabricated using facile one-step hydrothermal route by introducing various amounts of ammonium fluoride (NH4F) as structure-directing agent. The NH4F assisted synthesis of CuNiO2 materials on Ni foam current collector can be effectively utilized as binder-free battery-type electrode materials for supercapacitors. With an assistance of NH4F, the structural, morphological and composition evolutions of CuNiO2 electrodes are discussed effectively using X-ray diffraction, scanning electron microscopy and transmission electron microscopy and X-ray photoelectron spectroscopy characterizations. The CuNiO2 electrode material prepared with 0.4 M NH4F provides large number of active sites, superior conductivity and rapid charge transfer, which are promote fast Faradaic redox reactions. As a battery-type material, the optimized 0.4-CuNiO2 electrode material (NH4F is 0.4 M) exhibits a high specific capacity (~153.02 mA h g−1 at 2 A g−1), excellent rate capability (~87.4% retains even at 10 A g−1), and outstanding cycling stability (~94.14% at 6 A g−1 over 3000 cycles), respectively. Thereby, this study paves the path into rational design for structure and morphology of multi-component metal oxides for improving energy storage performance.

Facile synthesis of NiCo2O4 quantum dots for asymmetric supercapacitor

Binary metal oxides have great potential as electrode materials for supercapacitors. Three-dimensional flower-like nickel-cobalt oxide (NiCo2O4) nanostructures have been obtained by a simple and cost-effective hydrothermal synthesis followed by annealing at distinct temperatures. The effect of calcination temperature, on the structure and the electrochemical behaviour of the nanostructures, has also been investigated. Highly porous, hollow and continuous structures of NiCo2O4 quantum dots (NCO-QDs), formed at the calcination temperature of 300 °C, delivered a specific capacitance of 362 F g−1 at a current density of 0.5 A g−1. The good electrochemical behaviour of these nanostructures may be related to their unique, highly porous, integrated nanostructures that provide a good electroactive surface for faradaic redox reactions to occur. Also, an asymmetric-supercapacitor with NCO-QDs as positive electrode and reduced graphene oxide (rGO) as negative electrode has been fabricated which exhibits a good charge storage capacity (81 F g−1 at the scan rate of 5 mV s−1) along with very good cycling stability (86% capacitance retention after 1000 cycles). The energy density and power density of the supercapacitor have been checked at a potential window of 2.5 V after 1000 charge-discharge cycles. The device has shown a very good energy density of 69.5 W h kg−1 along with high power density of 2.22 kW kg−1 at a current density of 1.5 A g −1. So, this study suggests that the prepared NCO-QDs are prospective candidates for fabricating supercapacitor electrodes with good electrochemical performance in an extended potential window.

Pseudocapacitive Storage in Nanolayered Ti2NTxmxene Using Mg-Ion Electrolyte

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges viapseudocapacitive mechanisms. Nitride MXenes such as Ti2NTxare predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTxnitride MXene. Successful formation of nanolayered Ti2NTxMXene is characterized by XRD, SEM, and N2physisorption analyses. The identity of the surface terminating groups Txare assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTxMXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 uF cm–2 in 1 M MgSO4aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 uF cm–2) indicating that charge is stored in the material viaa pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg2+> Al3+> H+> Li+> Na+> K+, that matches theoretical predictions. Remarkably, nanolayered Ti2NTxMXene exhibits >200 F g–1 capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles owing to the two-electron reaction and the unique multilayer adsorption behavior of the Mg2+cation on the Ti2NTxMXene. These findings identify Ti2NTxMXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.

A self-supported electrode as a high-performance binder- and carbon-free cathode for rechargeable hybrid zinc batteries

Developing highly efficient and flexible air-breathing electrodes is of significant importance for various energy conversion and storage technologies. In this work, we report the synthesis of the self-supported sandwich-structured electrode with Co3-xNixO4/Co3O4 nanowire arrays (NS@Co3-xNixO4/Co3O4). This novel nanostructure demonstrates very high reversible Faradaic redox reaction of CoNi−O ↔ CoNi−O−OH and excellent bifunctional activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, the air cathode, prepared by pressing the stainless steel (SS)@Co3O4 sandwiched between two Ni foam (NF)@Co3-xNixO4, possesses a dense and interconnected structure with a high loading of active catalyst. The Ni foam in this work not only acts as a substrate, but also serves as a nickel precursor for the formation of the Co3-xNixO4 catalyst. Meanwhile, combining the 3D porous structure of NF and intrinsic high OER activity of SS, the assembled porous air electrode can take full advantage of the sandwich structure to promote the electrocatalytic reaction kinetics of NS@Co3xNixO4/Co3O4. Rechargeable battery system assembled from this NS@Co3-xNixO4/Co3O4 electrode based on both Zn−Co3-xNixO4 and Zn−air electrochemical reactions exhibits much higher energy efficiency and durability than those of commercial Pt/C and RuO2 electrocatalysts. This protocol opens a new avenue for the rational design of highly efficient and stable self-supported air electrode for metal-air batteries.