Stability In Aqueous Electrolyte Research Articles

Stable n-Type Perylene Derivative Ladder Polymer with Antiambipolarity for Electrically Reconfigurable Organic Logic Gates.

Organic electrochemical transistors (OECTs) are one of the promising building blocks to realize next-generation bioelectronics. To date, however, the performance and signal processing capabilities of these devices remain limited by their stability and speed. Herein, the authors demonstrate stable and fast n-type organic electrochemical transistors based on a side-chain-free ladder polymer, poly(benzimidazoanthradiisoquinolinedione). The device demonstrated fast normalized transient speed of 0.56 ± 0.17 msum-2 and excellent long-term stability in aqueous electrolytes, with no significant drop in its doping current after 50 000 successive doping/dedoping cycles and 2-month storage at ambient conditions. These unique characteristics make this polymer especially suitable for bioelectronics, such as being used as a pull-down channel in a complementary inverter for long-term stable detection of electrophysiological signals. Moreover, the developed device shows a reversible anti-ambipolar behavior, enabling reconfigurable electronics to be realized using a single material. These results go beyond the conventional OECT and demonstrate the potential of OECTs to exhibit dynamically configurable functionalities for next-generation reconfigurable electronics.

Ultrastable halide perovskite CsPbBr3 photoanodes achieved with electrocatalytic glassy-carbon and boron-doped diamond sheets

Halide perovskites exhibit exceptional optoelectronic properties for photoelectrochemical production of solar fuels and chemicals but their instability in aqueous electrolytes hampers their application. Here we present ultrastable perovskite CsPbBr3-based photoanodes achieved with both multifunctional glassy carbon and boron-doped diamond sheets coated with Ni nanopyramids and NiFeOOH. These perovskite photoanodes achieve record operational stability in aqueous electrolytes, preserving 95% of their initial photocurrent density for 168 h of continuous operation with the glassy carbon sheets and 97% for 210 h with the boron-doped diamond sheets, due to the excellent mechanical and chemical stability of glassy carbon, boron-doped diamond, and nickel metal. Moreover, these photoanodes reach a low water-oxidation onset potential close to +0.4 VRHE and photocurrent densities close to 8 mA cm−2 at 1.23 VRHE, owing to the high conductivity of glassy carbon and boron-doped diamond and the catalytic activity of NiFeOOH. The applied catalytic, protective sheets employ only earth-abundant elements and straightforward fabrication methods, engineering a solution for the success of halide perovskites in stable photoelectrochemical cells.

Interfacial Chemistry in Aqueous Lithium-Ion Batteries with Dilute Aqueous Electrolytes: A Case Study with V2O5

Aqueous lithium ion batteries (ALIBs) are promising candidates for large-scale energy storage systems, benefiting from the intrinsic safe and environmentally friendly properties of aqueous electrolytes. However, its practical application is impeded due to the narrow electrochemical stability windows (ESWs) of aqueous electrolytes. Despite that increasing electrolyte concentration can achieve a wide ESW with the protection of SEI layer, it brings numerous disadvantages induced by excessive salt as well. Therefore, understanding the SEI formation mechanism and further widening the ESWs of dilute aqueous electrolytes are critical for the breakthrough of next-generation ALIBs.Herein, the interphase chemistry of V2O5 electrode with dilute aqueous electrolyte was systematic investigated by conducting various Operando characterizations with LMO||V2O5 cell system. Dynamic SEI, which consists of Li2CO3, LiF and V2O4, was formed during charge and disappeared during the following discharge, which is strongly affected by electrolyte pH fluctuation. Moreover, its evolution process is reversible during cycling. There are several impacts affect the pH fluctuation during cycling. First, V2O5 dissolution, which turns electrolyte into acidic before cycling, and is aggravated by electrolyte concentration decrease. Second, proton/hydronium ion insertion, which takes place on electrode surface in dilute electrolyte, while it is suppressed by concentration increase due to reduced proton concentration. Third, the redox reaction of dissolved V2O5, which is controlled by both potential and electrolyte pH change according to the V-H2O poubaix diagram. Along with the anode potential decrease and the alkalinisation of electrolyte, V2O4 is formed on anode surface by the reduction of HV10O28 5- and V4O12 4- and this reduction process further produces hydroxide ion into electrolyte. Last, the carbon oxidation, which takes place on cathode side during charge and on anode side during the final discharge. Therefore, the acidification of electrolyte during cycling is detrimental to SEI stability in aqueous electrolytes. This systematic research work provides new perspectives to the understanding of the interphase chemistry evolution in aqueous Li-ion batteries using dilute electrolytes, which guides the building of effective SEI layer in aqueous Li-ion batteries and beyond. Figure 1

Redox-Active Polymers Designed for the Circular Economy of Energy Storage Devices

The decarbonization of the transport and energy sector will accelerate the demand for electrochemical energy storage devices. While past and current research efforts strongly focused on improving the performance of cells (energy density, power density, and stability), little attention has been paid to how these devices are recycled after their end of life, leaving the recycling of devices as an afterthought. Thus, to limit the accumulation of electric waste and the depletion of important raw materials for energy storage devices, there is an urgency to develop a circular economy for materials and integrate recyclability in device architectures. In my talk, I will present a new concept where we applied chemical design strategies to intentionally develop redox-active materials for a circular economy.[1] We developed solution-processible redox-active conjugated polymers that function as binder- and additive-free electrodes with high stability in aqueous electrolytes, achieving >98% retention of the capacity after 500 charging/discharging cycles. The tuning of the local environment of the polymer further enables fast charging of micron-thick single-phase electrodes with cell voltages > 1.2 V in aqueous electrolytes. Finally, we demonstrate the recyclability of the devices, achieving >85% capacity retention after recycling (76 % retention after recycling the device twice). Our work is a demonstration of how material chemistry enables the development of a circular economy for electrochemical energy storage devices.[1] S. T. M. Tan, T. J. Quill, M. Moser, G. LeCroy, X. Chen, Y. Wu, C. J. Takacs, A. Salleo, A. Giovannitti, ACS Energy Lett. 2021, 6, 3450.

Modified MnO2-Based Cathode for Zinc-Ion Batteries Using Facile Processing and Easily Available Materials

With the fast development of diverse electronics, the demand for safe energy storage systems with high energy density and high stability has increased rapidly. So far, lithium-ion batteries (LIBs) have dominated the market, from small smart electronics to electric vehicles. Nevertheless, LIBs have several limitations, such as high cost, limited raw material resources, high flammability, and harsh environmental impact. Therefore, novel rechargeable batteries which can mitigate these shortcomings must be explored. Aqueous zinc-ion batteries (ZIBs) have shown a promising future. Zinc with high theoretical volumetric capacity (5855 mAh cm-3) and high stability in aqueous electrolytes should enable high-performing, safe, and low-cost batteries. Additionally, the simple assembly of ZIBs in the ambient environment results in low capital costs. Despite these attractive merits, developing cathode materials with high capacity, rate performance, and stability in order to build commercial high-performing ZIBs, remains a great challenge.Cathode materials directly influence the electrochemical performance of ZIBs. Manganese oxide (MnO2) has been investigated as a promising cathode in ZIBs research due to its good specific capacity, low cost and high safety. However, the low ionic conductivity, slow diffusion kinetics, and low stability of MnO2 severely deteriorate the electrochemical performance and cycling stability of zinc-ion batteries. Recently, a series of studies have revealed the high effectiveness of coating on improving MnO2 cathode stability, including polymer coatings, carbon-based material coatings, artificial cathode-electrolyte interfaces, and metal oxide coatings.[1] Nevertheless, the improvement of other parameters, such as the rate performance, could be further improved for practical usage. In this research, we develop a novel modification method combining coating and doping strategies to induce MnO2 cathode with high capacity, high rate performance and low capacity decay after long-term operation. First, we utilize a facile one-step hydrothermal synthesis for Ag-doped α-MnO2. Different amounts of AgNO3 are added into reactors with KMnO4 and MnSO4 (n(Ag):n(Mn)=1:100-4:100) to investigate the influence of doping on capacity and rate performance. Subsequently, the doped tunnel-type MnO2 cathodes are mixed with precursor solutions of Al2O3 and then thoroughly stirred. Finally, an annealing process is conducted to achieve a nanoscale Al2O3 coating on the tunnel type MnO2. The coating configurations influence the electrochemical performance of cathodes;[2] thus, we test precursor solutions with different concentrations (1-3 wt% Al2O3) and annealing temperatures (500-700 °C) to investigate the influence of coating thickness and annealing treatment.With the assistance of scanning and transmission electron microscopes, the nanoscale Al2O3 layer coating on the doped active material is verified. The stable Al2O3 coating protects active materials from dissolution and structural collapse, improving cycling stability and resulting in high capacity retention after long-term operation. Moreover, a significant improvement in rate performance compared to unmodified MnO2 is verified through electrochemical tests. The X-ray photoelectron spectroscopy (XPS) and EPR spectra showed that the doped Ag+ ions lead to oxygen vacancies due to the formation of Ag-O-Mn.[3] The abundant vacancies act as active sites, significantly improving conductivity and ion insertion rates. The ex-situ X-ray diffractometer (XRD) and electrochemical tests are used to investigate the ion insertion mechanism during charging/discharging operations. The Ag-MnO2 @ Al2O3 cathode is synthesized through a facile methodology with environment-friendly materials. The modified cathode exhibits promising performance, especially regarding cycling stability and rate performance, which will be a crucial step for developing high-performing cathodes in aqueous zinc-ion batteries.Reference:[1] Shi, W., Lee, W. S. V., & Xue, J. (2021). Recent development of Mn‐based oxides as zinc‐ion battery cathode. ChemSusChem, 14(7), 1634-1658.[2] Zhou, A., Xu, J., Dai, X., Yang, B., Lu, Y., Wang, L., ... & Li, J. (2016). Improved high-voltage and high-temperature electrochemical performances of LiCoO2 cathode by electrode sputter-coating with Li3PO4. Journal of Power Sources, 322, 10-16.[3] Pu, X., Li, X., Wang, L., Maleki Kheimeh Sari, H., Li, J., Xi, Y., ... & Wu, Y. (2022). Enriching Oxygen Vacancy Defects via Ag–O–Mn Bonds for Enhanced Diffusion Kinetics of δ-MnO2 in Zinc-Ion Batteries. ACS Applied Materials & Interfaces, 14(18), 21159-21172. Figure 1

Bistate-type ion storage of azo polymer for aqueous zinc ion battery

Azo organic molecules are emerging as promising electrode candidates for energy storage systems due to their high theoretical specific capacity and excellent kinetic performance. In general perception, conventional azo organic small molecules used as cathode material in aqueous batteries remain a challenge because of the poor cycling stability in aqueous electrolytes. Herein, azo organic polymers (AOPs) were synthesized using a diazo-coupling reaction under mild conditions and developed as cathode materials for aqueous zinc-ion batteries. Owing to the long conjugate structure, the AOPs afford extra-long stability with outstanding cyclability of >1000 cycles at 2 A g−1, a comparable capacity (170 mAh g−1 at 0.5 A g−1), and excellent rate performance from 0.5 g−1 to 10 A g−1 in aqueous electrolytes. Significantly, the hydroxyl oxygen and azo functional groups on AOPs can spontaneously form metal heterocyclic complexes with zinc ions in 1 M Zinc trifluoromethanesulfonate (Zn(OTF)2) electrolyte, leading to a notable increase in voltage (by 0.2 V) and capacity (by 43 mAh g−1 at a current density of 2 A g−1) in aqueous zinc-ion battery. Additionally, mechanistic analysis reveals that the AOPs are an active material for bistate storage of zinc ions during the charging and discharging process. The AOPs not only store zinc ions in fully charged state by forming metal heterocyclic complexes but also store zinc ions by a redox reaction between azo functional groups and zinc ions at the discharge process. The results put forward a novel storage mechanism for zinc ions and will pave the way for AOPs’ development in aqueous batteries.

Highly redox-active oligomers between graphene sheets as ultrahigh capacitance/rate and stable electrodes for supercapacitors

Organic electrodes have emerged as promising candidates for supercapacitors due to their sustainability, structural diversity, and high electrochemical activity. However, the intrinsic poor electronic conductivity and solubility in electrolytes impede their further applications. Herein, we propose a facile strategy for the synthesis of pyrogallic (PG) oligomer confined in-between graphene layers (FRGO) as high-performance electrodes for supercapacitors. The PG oligomer exhibits both physiochemical properties of small molecules and organic polymers, such as fast reaction kinetics and structural stability in aqueous electrolyte. Conjugated sp2 carbon plane with abundant electroactive groups (-OH and -C=O) possess high electrochemical activity and construct fast electron transfer tunnels crossing the longitudinal graphene layers. Accordingly, FRGO delivers an excellent rate performance (835 F g−1 at 0.5 A g−1, 233 F g−1 at 200 A g−1) and ultra-long cycle stability. The symmetric supercapacitor exhibits an ultra-high energy density of 82.4 Wh kg−1 at 300 kW kg−1, dramatic power density of 90 kW kg−1 at 4 Wh kg−1, and remarkable stability of 90% retention after 20000 cycles. This work may open a new window for the rational design of organic oligomers in energy storage and conversion fields.

Binder-free fabricated transparent Nb4+ implanted Nb2O5-x ultra-thin negatrode with enhanced areal capacitance and exceptional cyclic stability in aqueous electrolyte

Direct coating and surface engineering of Nb2O5-based active storage materials are important techniques for improving their interfacial interactions with the current collector and electrolyte ions, which have been difficult to achieve in a facile and energy-efficient way. Herein, we report the binder-free coating of shear-structured Nb4+ implanted Nb2O5 (Nb2O5-x) sub-micron thin pseudocapacitive negatrode with wide negative operating voltage (−1.20 V vs Ag/AgCl), improved electronic and ionic conductivity, reduced charge transfer resistance and enhanced energy storage capacity. The Nb4+ implanted electrode exhibits 3.5 mF/cm2 c. a. At 5 mV/s and 100% capacity retention after 3000 constant charge-discharge cycles. The unique electrode was realized via mild de-oxidation of electrodeposited Nb2O5 in a vacuum-less, low-temperature surface engineering process. This simple strategy is suitable for improving current collector-Nb2O5-x-electrolyte interfacial interactions and for industrial-scale production of improved pseudocapacitive negatrodes.

Confined Ionic-Liquid-Mediated Cation Diffusion through Layered Membranes for High-Performance Osmotic Energy Conversion.

Ion-selective membranes act as the core components in osmotic energy harvesting, but remain with deficiencies such as low ion selectivity and a tendency to swell. 2D nanofluidic membranes as competitive candidates are still subjected to limited mass transport brought by insufficient wetting and poor stability in water. Here, an ionic-liquid-infused graphene oxide (GO@IL) membrane with ultrafast ion transport ability is reported, and how the confined ionic liquid mediates selective cation diffusion is revealed. The infusion of ionic liquids endows the 2D membrane with excellent mechanical strength, anti-swelling properties, and good stability in aqueous electrolytes. Importantly, immiscible ionic liquids also provide a medium, allowing partial dehydration for ultrafast ion transport. Through molecular dynamics simulation and finite element modeling, that GO nanosheets induce ionic liquids to rearrange, bringing in additional space charges, which can be coupled with GO synergistically, is proved. By mixing 0.5/0.01m NaCl solution, the power density can achieve a record value of ≈6.7W m-2 , outperforming state-of-art GO-based membranes. This work opens up a new route for boosting nanofluidic energy conversion because of the diversity of the ILs and 2D materials.

Electrochemical performance of aqueous electrolytes in porous carbon derived cassava peel electrode material-based for sustainable symmetrical supercapacitor

Electrolytes have been generally recognized as one of the most important components in enhancing the electrochemical performance of supercapacitors. On the other hand, aqueous electrolytes are considered prime candidates for the development of the next generation of symmetric supercapacitors due to their low-cost, environmentally friendly, high ionic conductivity, fine ionic size, and high capacitance. Herein, the symmetrical supercapacitor of the sustainable porous carbon-based electrode material was confirmed through various aqueous electrolytes consisting of neutral, basic, and acidic Na2SO4, KOH, and H2SO4. Activated carbon is obtained from high potential biomass sources of cassava peel waste. Activated carbon synthesis was performed with a comprehensive approach in order to obtain abundant pore structure, high porosity, and improved wettability through a combination of high-temperature chemical and physical activation. in addition, the electrode material is designed to resemble a solid disc without the addition of a synthetic binder. The evaluation of the disc dimensions showed high porosity in the obtained activated carbon. Furthermore, the symmetrical supercapacitor of the optimized electrode material exhibit excellent specific capacitances of 112, 150, and 183 F g-1 at 1 mV s-1 in the electrolytes Na2SO4, KOH, and H2SO4, respectively. In addition, the highest rate capability of 70% was confirmed in the H2SO4 acid electrolyte. Moreover, their coulombic efficiency can be maintained around 89% with low equivalent series resistance 0.21-0.42 ?. Therefore, the activated carbon-based supercapacitor symmetric cell device from cassava peel shows high performance for developing high-performance supercapacitor applications with guaranteed stability in aqueous electrolytes.

Improving the Selectivity of Nitrogen Reduction Reaction through the Mars-Van Krevelen Mechanism

The electrochemical nitrogen reduction reaction (NRR) process is an attractive alternative to minimizing the energy and greenhouse gas footprint from current ammonia (NH3) production processes. Most NRR catalysts operate through utilizing an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. In this presentation, we report on a new active catalyst for NRR that operates through the Mars-van Krevelen (MvK) mechanism to increase the selectivity of NRR towards NH3. This new catalyst, two-dimensional (2D) Ti2N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques, including Raman, X-ray diffraction, and UV-Vis. The Ti2N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH3 at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm2/hr in a 0.1 m hydrochloric acid (HCl) N2-saturated electrolyte. Electrocatalytic activity and selectivity obtained in an Ar-saturated electrolyte confirm that the new catalyst operates through an MvK mechanism. These results can be expanded to a broad class of systems enabling the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

Hierarchical nickel oxalate superstructure assembled from 1D nanorods for aqueous Nickel-Zinc battery

Hierarchical superstructures in nano/microsize can provide improved transport of ions, large surface area, and highly robust structure for electrochemical applications. Herein, a facile solution precipitation method is presented for synthesizing a hierarchical nickel oxalate (Ni-OA) superstructure composed of 1D nanorods under the control of mixed solvent and surfactant of sodium dodecyl sulfate (SDS). The growth process of the hierarchical Ni-OA superstructure was studied and indicated that the product had good stability in mixed solvent. Owing to smaller size, shorter pathway of ion diffusion, and abundant interfacial contact with electrolytes, hierarchical Ni-OA superstructure (Ni-OA-3) showed higher specific capacity than aggregated micro-cuboids (Ni-OA-1) and self-assembled micro/nanorods (Ni-OA-2). Moreover, the assembled Ni-OA-3//Zn battery showed good cyclic stability in aqueous electrolytes, and achieved a maximum energy density of 0.42 mWh cm−2 (138.75 Wh kg−1), and a peak power density of 5.36 mW cm−2 (1.79 kW kg−1). This work may provide a new idea for the investigation of hierarchical nickel oxalate-based materials for electrochemical energy storage.

(Digital Presentation) Achieving High Selectivity for the Nitrogen Reduction Reaction through the Mars-Van Krevelen Mechanism

Electrochemical nitrogen reduction reaction (NRR) technology is a viable alternative to reducing the energy and greenhouse gas footprint from the current ammonia (NH3) production technology. Most NRR catalysts suffer from low selectivity towards NH3 because they operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER). In this presentation, we report on a new catalyst and untapped mechanism for NRR to increase the selectivity towards NH3. This untapped Mars-van Krevelen (MvK) mechanism reduces the competition between NRR and HER by eliminating the sluggish hydrogenation reactions of the dissolved N2 molecule. The new catalyst, two-dimensional (2D) Ti2N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques, including x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and cyclic voltammetry (CV). Through an MvK mechanism, the Ti2N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH3 at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm2/hr in a 0.1M hydrochloric acid (HCl) N2-saturated electrolyte. These results constitute the foundation of NRR technology based on MXenes and can be expanded to a broad class of systems evoking the MvK mechanism.

Polyoxometalate/ZIF-67 composite with exposed active sites as aqueous supercapacitor electrode

Polyoxometalates (POMs) owing to their distinctive structural and highly stable redox properties are emerging as advanced electrode material for energy storage applications. The imminent concern with POMs is their long-term stability in aqueous electrolytes and conductivity which can be effectively tackled by structural engineering at molecular level. Here, we report smart assembly of polyoxometalates (POMs) and zeolitic imidazole framework (ZIF-67) as promising tool for structural modulation. The complementing dimensions of POM and ZIF-67 results in successful encapsulation of POM which not only enhances the active sites for charge storage, but also prevents the dissolution of POMs. Resulting nanocomposites were investigated in three different electrolytes where ammonium decavanadate intercalated nanocomposite (ZADV) is found to exhibit excellent electrochemical performance in 1 M Na2SO4 owing to its ordered structure with enhanced number of active sites. Further, to meet the demand of high energy density supercapacitor for modern applications, structure modulated POM (positive potential electrode) is integrated with a negative potential material “loofah sponge derived carbon (LSC)” that extends the potential from −1 to 0.6 V. The resulting integrated electrode ([email protected]) displays a maximum specific capacitance of 250.1F g−1 @ 0.8 A g−1 with good columbic efficiency. Interestingly, [email protected]||[email protected] works well upto 1.6 V, delivers energy density of 19.7 Wh kg−1 @ 586 W kg−1 with excellent cycling stability (89% after 5000 cycles @ 1.5 A g−1). This work is a keystone for the upcoming studies on enhancing the performance of polyoxometalate based electrodes with improved active sites and energy density.

Molecular design of an electropolymerized copolymer with carboxylic and sulfonic acid functionalities

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most researched conjugated polymer in the field of organic bioelectronics. The conjugated PEDOT backbone features good redox stability in aqueous electrolyte, and low oxidation potential. However, PEDOT:PSS has two major drawbacks. The PEDOT backbone lacks biofunctionality, limiting the fine tuning of its interface with the biological environment. The dopant PSS is insulating, resulting in a decrease in the capacitance of the polymer. Here, we describe the design of a random copolymer, P(EDOTCOOH-co-EDOTS), based on EDOT monomers functionalized with sulfonic and carboxylic acid groups. The copolymer was successfully synthesized by electropolymerization as confirmed by X-ray photoelectron spectroscopy. Contact angle measurements illustrated the high hydrophilicity of the P(EDOTCOOH-co-EDOTS) (28 ± 6º), attributed to the sulfonate group in the side chains. This in turn led to a higher water penetration into the copolymer film, enhancing significantly its volumetric capacitance (69 ± 4 F cm−3), and thereby, its performance when used as an active channel in an organic electrochemical transistor. Of note, we incorporated the sulfonate group in its sodium salt form retaining its highly ionized properties. This is the first instance of utilizing an uncapped, ionized sulfonate group covalently bound to the backbone of a polymer, where the resultant polymer is oxidized at very low potentials, as well as stable and electroactive in aqueous electrolytes. Furthermore, our molecular design to incorporate carboxylic acid groups paves the way for the development of conjugated polymers that can be tailored for bioelectronic applications.

Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Atomically Dispersed Manganese Lewis Acid Sites Catalyze Electrohydrogenation of Nitrogen to Ammonia

Stable α-MoO3 Electrode with a Widened Electrochemical Potential Window for Aqueous Electrochemical Capacitors

α-MoO3 is a promising electrode material for electrocapacitive energy storage. However, it has poor stability in aqueous electrolytes due to dissolution in water. Here, we demonstrate that an α-MoO3 electrode prepared using the electrochemical deposition method is stable over a wide potential window of 2.2 V in a neutral aqueous electrolyte. The improved stability of the α-MoO3 electrode in water is attributable to enhanced electron conductivity, which minimizes the polarization-induced structural collapse, and the dense microstructure of the α-MoO3 film electrode. The widened potential window is related to the electrocatalytic inactivity of the α-MoO3 toward water electrolysis and the high overpotentials of the neutral electrolyte toward H2 and O2 evolution reactions. A symmetric supercapacitor cell fabricated with the electrochemically deposited α-MoO3 electrode delivers a specific energy as high as ∼22 Wh/kg at a specific power of ∼301 W/kg. This work demonstrates that the electrochemical deposition method is a viable approach to fabricating α-MoO3 electrodes for aqueous electrochemical energy storage.

The effect of different surfactants as electrolyte additives on NaTi2(PO4)3/C anode

Aqueous rechargeable batteries have attracted attention because of their low cost and high security, but the electrode materials have poor cycling stability in aqueous electrolytes. In this paper, different surfactants (sodium dodecyl sulfate, SDS; sodium laurylsulfonate, SLS; carboxymethyl cellulose sodium, SCC; hexadecyl trimethyl ammonium bromide, CTAB; poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), F127) were used as additives to the electrolyte (0.5 M Na2SO4 aqueous solution). The effect of surfactant additives on the electrochemical parameters such as the conductivity, diffusion coefficient (DNa), transfer coefficient (α), and self-corrosion current (icorr) of NaTi2(PO4)3/C was characterized by cyclic voltammetry, galvanostatic charging/discharging, electrochemical impedance spectroscopy, and Tafel polarization tests. It is found that the addition of surfactant reduced the capacity decay of NaTi2(PO4)3/C anode and improved its capacity retention rate, especially SDS, which was 81.2% after 100 cycles. Possible reasons were analyzed as follows: (1) surfactants inhibited the decomposition of water in the electrolyte and (2) surfactants adsorbed on the surface of NaTi2(PO4)3/C reduced its contact with H2O molecules and relieved the dissolution of NaTi2(PO4)3/C.

Improvement in Charge Storage Capability By Synergetic Effect between RuO2 Nanosheets and MnO2 Particles

MnO2 shows fast redox behavior (Mn(Ⅲ)/Mn(Ⅳ)) and has a large overpotential for oxygen evolution reaction, hence has been used as an active material for aqueous electrochemical capacitors. MnO2 electrodes generally consists of MnO2 powder, conductive carbon and polymeric binders. However, conductivity additives and polymer binders are electrochemically non-active or poorly active for energy storage. The reduction in the amount of such additives and polymeric binder will lead to the improvement in the energy and power density. Conductive nanosheets such as MXene has been recently studied as an active binder, acting as a redox active conductive additive.1 Unfortunately, MXene has poor stability in aqueous electrolyte at high potentials due to irreversible oxidation and is not sufficient for use with MnO2 positive electrodes.2 In this work, RuO2 nanosheets with high electronic conductivity and specific capacitance is studied as an active and stable binder. 3 RuO2 nanosheets4 and MnO2 particles5 were synthesized according to literature. The MnO2-RuO2 nanosheet composite electrodes (MnO2(100-x)-RuO2(x); x=100, 80, 60, 50, 30, 20, 10, 0, x indicates the molar ratio of RuO2 ) were prepared by dropping dispersions of composite ink on a mirror-polished glassy carbon electrode. Electrochemical characterization was conducted by cyclic voltammetry using a three-electrode cell consisting of Pt mesh as a counter electrode and a Ag/AgCl (KCl sat.) reference electrode. 1.0 M Li2SO4 (25°C) was used as the electrolyte.The morphologies of MnO2(50)-RuO2(50) and MnO2(80)-RuO2(20) were analyzed by SEM-EDX. The SEM images show the presence both of particles and nanosheets. The EDX mappings show a homogeneous distribution of ruthenium atoms, and the distribution of manganese atoms agrees with the location of the particles. Figure 1 show the specific capacitance of MnO2(100-x)-RuO2(x) as a function of RuO2 nanosheet additive at scan rates of 2 and 50 mV s-1. The specific capacitance of the MnO2 particle and the RuO2 nanosheet at 2 mV s-1 were 144 and 233 F (g-material)-1, respectively. By adding 20 mol% RuO2 nanosheet to the MnO2 particle, the specific capacitance increased 188 F (g-material)-1, which was 1.3 times higher than pure MnO2 particle. While the specific capacitance of MnO2 was 23 F (g-material)-1 at 50 mV s-1, by adding 20 mol% RuO2 nanosheet, the specific capacitance increased to 155 F (g-material)-1. This is ~6.5 times higher than pure MnO2. It should be noted that the specific capacitance of MnO2 can easily be improved by adding only 20 mol% RuO2 nanosheets. To clarify the reason of the improved specific capacitance, the specific capacitances per mass of MnO2 or RuO2 were calculated. Based on the results, it is concluded that RuO2 nanosheet acts as a conductive additive for MnO2 while MnO2 acts as a spacer to suppress re-stacking of RuO2 nanosheets, thereby affording high utilization of both MnO2 and RuO2 nanosheets.This work was supported in part by a JST program on Open Innovation Platform with Enterprise Institute and Academia (OPERA JPMJOP 1843) and ISHIFUKU Metal Industry Co. Ltd. Yu, L. Hu, B. Anasori, Y. T. Liu, Q. Zhu, P. Zhang, Y. Gogotsi and B. Xu, ACS Energy Lett., 3, 1597 (2018).R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall’Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum and Y. Gogotsi, Science, 341, 1502 (2013). Saito, Y. Sato, D. Takimoto, S. Hedeshima and W. Sugimoto, Electrochem. In pressFukuda, T. Saida, J. Sato, M. Yonezawa, Y. Takasu and W. Sugimoto, Inorg. Chem., 49, 4391 (2010).Devaraj and N. Munichandraiah, J. Phys. Chem. C, 112, 4406 (2008). Figure 1

Mesoporous Fe3O4@C nanoarrays as high-performance anode for rechargeable Ni/Fe battery

Rechargeable aqueous batteries with high power density and energy density are highly desired for electrochemical energy storage. Despite the recent reports of various cathode materials with ultrahigh pseudocapacitance exceeding 3000 F g−1 (or 800 mA h g−1), the development of anode materials is relatively insufficient, which limits the whole performance of the devices far from practical applications. Herein, we report the preparation of mesoporous Fe3O4@C nanoarrays as high-performance anode for rechargeable Ni/Fe battery by a self-generated sacrificial template method. ZnO/Fe3O4 composite was first synthesized by a co-deposition process, and ZnO was subsequently removed by alkali etching to construct the mesoporous structure. A thin carbon film was introduced onto the surface of the electrode by the carbonization of glucose to increase the structural stability of the electrode. The unique mesoporous nanoarray architecture endows the electrode with larger specific surface area, faster charge/mass transport and higher utilization of Fe3O4, which shows an ultrahigh specific capacity (292.4 mA h g−1 at a current density of 5 mA cm−2) and superior stability in aqueous electrolyte (capacitance retention of 90.8% after 5000 cycles). After assembled with hierarchical mesoporous NiO nanoarray as a cathode, an optimized rechargeable Ni/Fe battery with double mesoporous nanoarray electrodes was fabricated, which provided high energy/power densities (213.3 W h kg−1 at 0.658 kW kg−1 and 20.7 kW kg−1 at 113.9 W h kg−1, based on the total mass of the active materials) in the potential window of 1.5 V with excellent cyclability (81.7% retention after 5000 charge/discharge cycles).