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The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium-air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium-based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion-exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs.

The nickel content of the layered lithium transition metal oxide cathodes is proportional to the discharge capacity of energy storage systems. Ni2+ ion, the oxidation state of Ni, moves to the Li sites when Li+ ion is deintercalated during discharge process due to the similar size of ion (Ni2+: 0.069 nm, Li+: 0.076 nm). The spatial transition of Ni2+ ion causes gradual cation mixing and structure degradation of cathode, resulting in the failure of the battery. Also, thick cathode-electrolyte interphase (CEI) is formed on the cathode surface because of the transition metal dissolution, the degradation of electrolyte, and the residual gas such as H2O and CO2. Coating technology is one of the strategies for the physical protection of the cathode surface and the control of CEI layer thickness. Polyaniline(PANi) is a conductive polymer with stable oxidation state. PANi can enhance the electrochemical property and structural stability as a protective layer. In this study, we synthesized PANi material by the oxidative polymereization and coated NCM with the synthesized PANi by sonication method. The PANi-coated NCM(PANi@NCM) has uniform coating layer with 5 nm thickness. PANi@NCM exhibits an initial discharge capacity of 208 mAhg-1 and a capacity retention of 81% after 50 cycles at 45 oC in Figure 1(a) and (b). After the cycling test, pristine and coated cathodes were disassembled for the comparison of the microstructure. Pristine NCM only shows the structural transition from layered structure to structure in the XRD pattern of Figure 1(c). Cross-sectional SEM image of the pristine NCM after cycling test shows micro and macro cracks are distributed from the bulk to the surface of the cathode as shown in Figure 1(d). On the other side, cracks are barely observed on the PANi@NCM particles in Figure 1(e). The layered structure of PANi@NCM is maintained even after cycling test, which are confirmed by TEM analysis with FFT patterns and HAADF images. Figure (f) and (g) show the FFT pattern of the two cathode surfaces after cycling test. Mixed phase is observed on the pristine cathode surface, while the layered structure is maintained on the PANi@NCM surface. Also, the PANi@NCM maintains a thinner passivation layer compared with that of the pristine NCM (6 nm vs 35 nm). In conclusion, the PANi coating layer prevents the degradation of structure and the formation of thick CEI layer. The structural stability enhances the electrochemical performance. Figure 1

Predicting and preventing disasters in difficult-to-access environments, such as oceans, requires self-powered monitoring devices. Since the need to periodically charge and replace batteries is an economic and environmental concern, energy harvesting from external stimuli to supply electricity to batteries is increasingly being considered. Especially, in aqueous environments including electrolytes, coiled carbon nanotube (CNT) yarn harvesters have been reported as an emerging approach for converting mechanical energy into electrical energy driven by large and reversible capacitance changes under stretching and releasing. To realize enhanced harvesting performance, experimental and computational approaches to optimize structural homogeneity and electrochemical accessible area in CNT yarns to maximize intrinsic electrochemical capacitance (IEC) and stretch-induced changes are presented here. Enhanced IEC further enables to decrease matching impedance for more energy efficient circuits with harvesters. In an ocean-like environment with a frequency from 0.1 to 1 Hz, the proposed harvester demonstrates the highest volumetric power (1.6-10.45 mW cm

A novel 3D interconnected web-like carbon material with high electrocatalytic activities toward oxygen reduction reaction (ORR) has been developed for the first time via direct carbonization of a composite (Zn-MOF-74@CNFs) comprising Zn-MOF-74s grown on carbon nanofibers (CNFs) web. The hexagonal pillar shaped Zn-MOF-74s with a diameter which ranges from 300 to 600 nm grow along the CNFs web by solvothermal method. After carbonization of Zn-MOF-74@CNFs, effective interconnections promoting electron transfer are successfully formed between carbonized Zn-MOF-74 (C-Zn-MOF-74) and on CNFs as well as C-Zn-MOF-74 themselves. The extraordinary 3D structure thus fabricated significantly improves the electrocatalytic activity toward ORR. The calculated electron transfers number (n) values for carbonized Zn-MOF-74@CNFs (C-Zn-MOF-74@CNFs) are nearly 4 at potentials ranging from 0.4 to 0.6 V (vs. reversible hydrogen electrode), demonstrating that the ORR process occurs dominantly through a direct four-electron pathway. Tafel slope of C-Zn-MOF-74@CNFs at low over-potential are lower than those from C-Zn-MOF-74 and even commercial Pt/C. Durability is also found to exceed that of commercial Pt/C. This study provides a novel 3D interconnected carbon material as a non-metal ORR electrocatalyst and design strategy for a large-area, self-standing and binder-free carbon-based electrochemical electrode.

Many efforts are continuously devoted to developing high-efficiency, low-cost, and highly scalable oxygen reduction reaction (ORR) electrocatalysts to replace precious metal catalysts. Herein, we successfully synthesize Co- and defect-rich carbon nanofibers (CNFs) using an efficient heat treatment approach involving the pyrolysis of electrospun fibers at 370 °C under air. The heat treatment process produces Co-decorated CNFs with a high Co mass ratio, enriched pyridinic N, Co-pyridinic Nx clusters, and defect-rich carbon structures. The synergistic effects from composition and structural changes in the designed material increase the number of catalytically active sites for the ORR in an alkaline solution. The prepared Co- and defect-rich CNFs exhibit excellent ORR activities with a high ORR onset potential (0.954 V vs. RHE), a large reduction current density (4.426 mA cm−2 at 0.40 V), and a nearly four-electron pathway. The catalyst also exhibits a better long-term durability than commercial Pt/C catalysts. This study provides a novel hybrid material as an efficient ORR catalyst and important insight into the design strategy for CNF-based hybrid materials as electrochemical electrodes.

Herein, we describe a microwave-assisted hydrothermal process to synthesize α-Fe2O3 nanotubes/SnO2 nanorods/reduced graphene oxide (FNT/S/RGO) for application as a high-performance anode in lithium-ion batteries (LIBs). The composite products exhibit anisotropic growth because of heteronucleation and the preferred orientation of SnO2. SnO2 nanorods on the FNT surfaces are converted into Sn metal during the alloying/dealloying reaction, which offers improved electrical conductivity. The FNT/S/RGO show substantially enhanced electrochemical properties because of the reduced volume expansion effect, which improves the electrical and Li-ion conductivity and provides a large surface area. As a consequence, the FNT/S/RGO anode delivers a high reversible capacity of 883 mA h g−1 even at a current density of 200 mA g−1, with a capacity retention of 90% between the 1st and 220th cycles, excellent high-rate capacity (382 mA h g−1 at 4320 mA g−1), and long-term cycle durability (maintaining 629 mA h g−1 at 1000 mA g−1 for 1000 cycles). The presented FNT/S/RGO electrodes are the most efficient SnO2- and Fe2O3-based anode electrodes reported thus far for LIBs. The origin of the synergistic effect and the reaction mechanism of the FNT/S/RGO was thoroughly investigated using various in situ transmission electron microscopy, electrochemical impedance spectroscopy, and X-ray diffraction analysis methods.

Compared to conventional electrode, a self-standing structure electrode is an effective way to achieve high performance of supercapacitor by maximizing the use of active material. We present a new combination of nickel oxide–carbon composites fabricated by directly carbonizing a nickel metal–organic framework (Ni-MOF)@carbon nanofiber (CNF) for a self-standing electrode of the supercapacitor application. The new scheme utilizes the CNF film as a substrate with high electron transferring capability and as a backbone of a self-standing electrode as well. It is observed that the MOF-derived NiOs with a diameter of ~ 8 nm are uniformly distributed in the carbon matrix and result in the improvement of the electrical conductivity. The self-standing electrode, NiO/C@CNF composite, provides a high rate capability with high specific capacitances of 742.2 and 671.1F g−1 (at 1 and 10 A g−1), respectively. An asymmetric supercapacitor (ASC) constructed from the NiO/C@CNF and the activated carbon exhibits an excellent specific energy density of 58.43 Wh kg−1 at a power density of 1,947 W kg−1. It is also confirmed that the ASC shows a good cycle stability from the long-term cycling test. It is demonstrated that the proposed nickel oxide–carbon composite has a potential as promising self-standing electrode materials for supercapacitors.

Metal-organic framework(MOF)-derived carbon material is a promising material for lithium-air battery cathode, but its pores are small (