Showing papers on "Capacitance published in 2019"

3D Printing of Freestanding MXene Architectures for Current-Collector-Free Supercapacitors.

Abstract: Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1-3 nm) 2D Ti3 C2 Tx with large lateral size of about 8 µm possessing ideal viscoelastic properties are formulated for extrusion-based 3D printing of freestanding, high specific surface area architectures to determine the viability of manufacturing energy storage devices. The 3D-printed device achieves a high areal capacitance of 2.1 F cm-2 at 1.7 mA cm-2 and a gravimetric capacitance of 242.5 F g-1 at 0.2 A g-1 with a retention of above 90% capacitance for 10 000 cycles. It also exhibits a high energy density of 0.0244 mWh cm-2 and a power density of 0.64 mW cm-2 at 4.3 mA cm-2 . It is anticipated that the sustainable printing and design approach developed in this work can be applied to fabricate high-performance bespoke multiscale and multidimensional architectures of functional and structural materials for integrated devices in various applications.

Spatially resolved steady-state negative capacitance.

Abstract: Negative capacitance is a newly discovered state of ferroelectric materials that holds promise for electronics applications by exploiting a region of thermodynamic space that is normally not accessible1–14. Although existing reports of negative capacitance substantiate the importance of this phenomenon, they have focused on its macroscale manifestation. These manifestations demonstrate possible uses of steady-state negative capacitance—for example, enhancing the capacitance of a ferroelectric–dielectric heterostructure4,7,14 or improving the subthreshold swing of a transistor8–12. Yet they constitute only indirect measurements of the local state of negative capacitance in which the ferroelectric resides. Spatial mapping of this phenomenon would help its understanding at a microscopic scale and also help to achieve optimal design of devices with potential technological applications. Here we demonstrate a direct measurement of steady-state negative capacitance in a ferroelectric–dielectric heterostructure. We use electron microscopy complemented by phase-field and first-principles-based (second-principles) simulations in SrTiO3/PbTiO3 superlattices to directly determine, with atomic resolution, the local regions in the ferroelectric material where a state of negative capacitance is stabilized. Simultaneous vector mapping of atomic displacements (related to a complex pattern in the polarization field), in conjunction with reconstruction of the local electric field, identify the negative capacitance regions as those with higher energy density and larger polarizability: the domain walls where the polarization is suppressed. Imaging steady-state negative capacitance in SrTiO3/PbTiO3 superlattices with atomic resolution provides solid microscale support for this phenomenon.

High-Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bimetallic Transition Metal Phosphide Nanocrystals.

Abstract: Transition metal phosphides (TMPs) have recently emerged as an important type of electrode material for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic CoxNi1-xP ultrafine nanocrystals supported on carbon nanofibers (CoxNi1-xP/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of CoxNi1-xP/CNF, and CoxNi1-xP/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g-1/1405.6 C g-1 at a charge/discharge current density of 5 A g-1, which is the highest value for TMP-based electrode materials reported by far. Our density functional theory calculations demonstrate that the significant capacitance/capacity enhancement in CoxNi1-xP/CNF, compared to the monometallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using CoxNi1-xP/CNF as positive electrode material, activated carbon as negative electrode material, and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g-1/166.2 C g-1 at 20 mV s-1, delivers an energy density of 32.2 Wh kg-1 at 3.5 kW kg-1, and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

Ferroelectric negative capacitance

Abstract: The capacitor is a key element of electronic devices and is characterized by positive capacitance. However, a negative capacitance (NC) behaviour may occur in certain cases and implies a local voltage drop opposed to the overall applied bias. Therefore, a local NC response results in voltage enhancement across the rest of the circuit. Within a suitably designed heterostructure, ferroelectrics display such an NC effect, and various ferroelectric-based microelectronic and nanoelectronic devices have been developed, showing improved performance attributed to NC. However, the exact physical nature of the NC response and direct experimental evidence remain elusive or controversial thus far. In this Review, we discuss the physical mechanisms responsible for ferroelectric NC, tackling static and transient NC responses. We examine ferroelectric responses to voltage and charge, as well as ferroelectric switching, and discuss proof-of-concept experiments and possibilities for device implementation. Finally, we highlight different approaches for the optimization of the intrinsic NC response to maximize voltage amplification. Ferroelectrics-based materials can display a negative capacitance (NC) effect, providing an opportunity to implement NC in electronic circuits to improve their performance. In this Review, the authors discuss static and transient NC responses in ferroelectrics and highlight proof-of-concept experiments and possibilities for device implementation.

Origin of apparent light-enhanced and negative capacitance in perovskite solar cells

Abstract: So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current–voltage curve. The apparent negative capacitance remains elusive in the impedance analysis of metal halide perovskite solar cells. Here Ebadi et al. show that it can be attributed to slow transients in the injection current instead of classical capacitive effect, i.e. charge accumulation.

Electrochemiluminescence-Based Capacitance Microscopy for Label-Free Imaging of Antigens on the Cellular Plasma Membrane

Abstract: Electrochemiluminescence (ECL)-based capacitance microscopy using a square-wave voltage is established unprecedentedly to realize the label-free visualization of species on electrode surfaces and cellular plasma membranes. The drop in the local capacitance upon the binding of species to the surface or to a cellular membrane is derived to induce a relatively larger potential drop ( Vdl) across the double layer on the local electrode surface, which is utilized to prompt enhanced ECL at the binding position. The square-wave voltage with a frequency of as high as 1.5 kHz is proven to be favorable for the discrimination of the local ECL from the surrounding signal. Using this new detection principle and resultant capacitance microscopy, carcinoembryonic antigens (CEA) at amounts of as low as 1 pg can be visualized. Further application of this approach permits the direct imaging of CEA antigens on single MCF-7 cells through the capacitance change after the formation of the antigen-antibody complex. Successful visualization of the analyte without any ECL tag will allow not only special capacitance microscopy for label-free bioassays but also a novel ECL detection approach for the sensitive detection of biomolecules.

A New Free-Standing Aqueous Zinc-Ion Capacitor Based on MnO 2 –CNTs Cathode and MXene Anode

Abstract: Restricted by their energy storage mechanism, current energy storage devices have certain drawbacks, such as low power density for batteries and low energy density for supercapacitors. Fortunately, the nearest ion capacitors, such as lithium-ion and sodium-ion capacitors containing battery-type and capacitor-type electrodes, may allow achieving both high energy and power densities. For the inspiration, a new zinc-ion capacitor (ZIC) has been designed and realized by assembling the free-standing manganese dioxide–carbon nanotubes (MnO2–CNTs) battery-type cathode and MXene (Ti3C2Tx) capacitor-type anode in an aqueous electrolyte. The ZIC can avoid the insecurity issues that frequently occurred in lithium-ion and sodium-ion capacitors in organic electrolytes. As expected, the ZIC in an aqueous liquid electrolyte exhibits excellent electrochemical performance (based on the total weight of cathode and anode), such as a high specific capacitance of 115.1 F g−1 (1 mV s−1), high energy density of 98.6 Wh kg−1 (77.5 W kg−1), high power density of 2480.6 W kg−1 (29.7 Wh kg−1), and high capacitance retention of ~ 83.6% of its initial capacitance (15,000 cycles). Even in an aqueous gel electrolyte, the ZIC also exhibits excellent performance. This work provides an essential strategy for designing next-generation high-performance energy storage devices.

Boosting the supercapacitor performance of activated carbon by constructing overall conductive networks using graphene quantum dots

Abstract: Although high-surface-area activated carbons have been widely used for supercapacitors, they usually have limited capacitive and rate performances primarily because of the low conductivity and sluggish electrochemical kinetics caused by their amorphous microporous structure. Here, we report a simple strategy to boost the electrochemical performance of activated carbons by embedding highly crystallized graphene quantum dots. Benefiting from the formation of the overall conductive networks, the charge-transfer and ion migration kinetics of the activated carbon are significantly improved, facilitating electrolyte ion transport and storage in deep and branched micropores. As a result, the graphene quantum dot embedded activated carbon, possessing a microporous structure with a specific surface area of 2829 m2 g−1, achieves a remarkably high electric double-layer capacitance of 388 F g−1 at 1 A g−1 as well as excellent rate performance with 60% capacitance retention at 100 A g−1 in a two-electrode system. The capacitive and rate performances are much higher than not only those of the activated carbon without graphene quantum dots, but also those of most porous carbons reported in the literatures. This strategy provides a new route for designing advanced porous carbon materials for high performance energy storage.