Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn–MnO2 batteries. Herein, direct evidence was provided of the importance of Mn2+ in this type of battery by using a bespoke cell. Without pre‐addition of Mn2+, the cell exhibited an abnormal discharge–charge profile, meaning it functioned as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO2. Additionally, a dynamic pH variation was observed during the discharge–charge process, with a precipitation of Zn4(OH)6(SO4)⋅5H2O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 was first converted into MnOOH, which reverted to MnO2 through disproportionation, resulting in the dissolution of Mn2+. The charging process occurred by the electrodeposition of MnO2, thus improving the reversibility through the availability of Mn2+ ions in the solution.
Strain sensors for smart wearable textiles have recently attracted great attention due to their potential in the healthcare applications, specifically, functions to track heartbeat, pulse signals, and movements of limbs and joints. Traditional methods typically require complicated procedures including dip-coating in different solutions to prepare sensing fibers before weaving into fabrics. In this study, we used an ultraviolet picosecond laser to directly induce graphene on polyimide (PI) fabric to produce a strain sensor. The process is mask-free, easy-to-fabricate, and the graphene tracks are well-adhered to the substrate. High-quality 3D-porous graphene was produced directly using appropriate laser parameters with a sheet resistance as low as 20 Ω/sq. The graphene strain sensors showed high sensitivity within a small strain range (strain below 4%) (GFmax = 27), good linearity, a low threshold value (strain = 0.08%), and high stability (4% resistance loss after 1000 cycles). Furthermore, reliable signals gathered from various human motions demonstrated the potential for health-care monitoring.
Conventional electrode preparation techniques of supercapacitors such as tape casting or vacuum filtration often lead to the restacking or agglomeration of two‐dimensional (2D) materials. As a result, tortuous paths are created for the electrolyte ions and their adsorption onto the surfaces of the active materials can be prevented. Consequently, maintaining high rate performance while increasing the thickness of electrodes has been a challenge. Herein, a facile freeze‐assisted tape‐casting (FaTC) method is reported for the scalable fabrication of flexible MXene (Ti3C2Tx) supercapacitor electrode films of up to 700 μm thickness, exhibiting homogeneous ice‐template microstructure composed of vertically aligned MXene walls within lamellar pores. The efficient ion transport created by the internal morphology allows for fast electrochemical charge–discharge cycles and near thickness‐independent performance at up to 3000 mV s−1 for films of up to 300 μm in thickness. By increasing the scan rate from 20 to 10,000 mV s−1, Ti3C2Tx films of 150 μm in thickness sustain 50% of its specific capacitance (222.9 F g−1). When the film thickness is doubled to 300 μm, its capacitance is still retained by 60 % (at 213.3 F g−1) when the scan rate is increased from 20 to 3000 mV s−1, with a capacitance retention above 97.7% for over 14,000 cycles at 10 A g−1. They also showed a remarkably high gravimetric and areal power density of 150 kW kg−1 at 1000 A g−1 and 667 mW cm−2 at 4444 mA cm−2, respectively. FaTC has the potential to provide industry with a viable way to fabricate electrodes formed from 2D materials on a large scale, while providing promising performance for use in a wide range of applications, such as flexible electronics and wearable energy storage devices.
Modulation of the grain boundary properties in thermoelectric materials that have thermally activated electrical conductivity is crucial in order to achieve high performance at low temperatures. In this work, we show directly that the modulation of the potential barrier at the grain boundaries in perovskite SrTiO 3 changes the low-temperature dependency of the bulk material’s electrical conductivity. By sintering samples in a reducing environment of increasing strength, we produced La 0.08 Sr 0.9 TiO 3 (LSTO) ceramics that gradually change their electrical conductivity behavior from thermally activated to single-crystal-like, with only minor variations in the Seebeck coefficient. Imaging of the surface potential by Kelvin probe force microscopy found lower potential barriers at the grain boundaries in the LSTO samples that had been processed in the more reducing environments. A theoretical model using the band offset at the grain boundary to represent the potential barrier agreed well with the measured grain boundary potential dependency of conductivity. The present work showed an order of magnitude enhancement in electrical conductivity (from 85 to 1287 S cm –1 ) and power factor (from 143 to 1745 μW m –1 K –2 ) at 330 K by this modulation of charge transport at grain boundaries. This significant reduction in the impact of grain boundaries on charge transport in SrTiO 3 provides an opportunity to achieve the ultimate “phonon glass electron crystal” by appropriate experimental design and processing.
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