Total Capacitance Research Articles

Studies of structural, dielectric and impedance spectroscopy of Bi4Ti2.8Sn0.2O12

The polycrystalline sample Bi4Ti2.8Sn0.2O12 was synthesized using the solid-state reaction method at 850 °C. The sample was formed in a single-phase orthorhombic structure and having space group B2ab, according to the XRD study at room temperature. The dielectric behaviors of the sample have been studied for the frequency range 1 kHz-1 MHz and a wide range of temperature 25-500 °C, which provide some important properties of the synthesized sample. The Nyquist plots provide the effect of grains and grain boundaries to the total resistance and capacitance of the synthesized compound. From ac-conductivity studies, various kinds of conduction mechanisms have been examined. The J–E characteristics support the Ohmic character of the prepared sample. The ferroelectric property at room temperature at frequency 50 Hz has been studied from polarization study.

Manipulation of electrochemical properties of MXene electrodes for supercapacitor applications by chemical and magnetic disorder.

The potential of two-dimensional MXenes as electrodes in supercapacitor applications has been studied extensively. However, the role of chemical and magnetic disorder in their electrochemical parameters, e.g., capacitance, has not been explored yet. In this work, we have systematically addressed this for V2-xMnxCO2 MXene solid solutions with an analysis based upon the results from first-principles electronic structure calculations. We find that the variations in the total capacitance over a voltage window depend on the degree of chemical and magnetic disorder. In the course of our investigation, it was also found that the magnetic structure on the surface can substantially influence the redox charge transfer, an as yet unexplored phenomenon. A significantly large charge transfer and thus a large capacitance can be obtained by manipulating the chemical composition and the magnetic order of the surfaces. These findings can be useful in designing operational supercapacitor electrodes with magnetic constituents.

Negative capacitance based on isomeric polythiophene in action.

In this study, we demonstrated that arranging positive and negative capacitance materials in series, based on isomeric polythiophene, led to a notable increase in total capacitance. Furthermore, the application of negative capacitance (NC) technology in supercapacitor (SC) construction offers the potential for energy storage capabilities surpassing fundamental limits. Our findings suggest the feasibility of developing all-organic based energy storage devices with high capacitance.

Parasitic Capacitance Model for Stacked Gate-All-Around Nanosheet FETs

Gate-all-around (GAA) nanosheet field-effect transistors (NSFETs) are hailed as the most promising architecture for the incessant scaling of MOSFETs to the sub-5-nm technology node and beyond. Although the GAA structure and the ultra-thin channel lead to a significantly improved static performance owing to the enhanced electrostatic integrity and higher immunity against the short-channel effects, the vertical stacking and the inherent 3-D-geometry result in a large parasitic capacitance and may degrade the dynamic performance. Therefore, it becomes imperative to analyze the parasitic capacitance components in GAA NSFETs and provide design guidelines for optimizing the dynamic performance. To this end, in this work, we have formulated an analytical model for the total gate parasitic capacitance in GAA NSFETs, considering contributions from different parasitic capacitance components. Furthermore, we have also investigated the variation in parasitic capacitance components with the structural parameters for design optimization of GAA NSFETs from a dynamic performance perspective. The result of the developed model is in good agreement with TCAD simulation result indicating the efficacy of the analytical model.

Supercapacitance Performance of Mesoporous Mono- and Bi-Metallic Cobalt and Nickel Hydroxides Nanoflakes Chemically Deposited from Foam-Surfactant Liquid Crystal Dual Templates

Mesoporous 2D nanoflakes architectures of mono- and bi-metallic cobalt and nickel hydroxide (meso-Co-Ni(OH)2) hybrids were synthesized using novel chemical precipitation of foam-surfactant liquid crystal dual template (FSDT). The dissolved cobalt and nickel ions within the aqueous region of the Brij®78 surfactant template was reduced by the excess of sodium borohydride reducing agent that simultaneously generates excessive hydrogen foam as a double template. The physicochemical characterizations of the mono- and bi-metallic nanostructured Co-Ni(OH)2 hybrids performed using X-ray diffraction, surface area analyzer, scanning and transmission electrons microscopic techniques showed the formation of highly mesoporous hydroxides 2D nanoflakes architectures having an amorphous structure and surface area up to 253.50 m2/g. The electrochemical and supercapacitance evaluation of the mono-, and bi-metallic mesoporous hydroxides showed excellent Faradaic supercapacitance performance within a wider potential window and a higher charge and discharge rate. In particular the mesoporous monometallic of Co(OH)2, Ni(OH)2, bimetallic Co-Ni(OH)2 with Co/Ni ratios of (1:1), and (1:3), hydroxides showed total capacitances of 204, 869, 943, and 1384, F/g at charging current density of 5.0 A g−1 respectively and capacity retention rate of approximately 99.4% after 300 cycles. This suggests that these materials hold promise as electrode materials for energy storage applications.

Metal-free platforms for molecular thin films as high-performance supercapacitors.

Controlling chemical functionalization and achieving stable electrode-molecule interfaces for high-performance electrochemical energy storage applications remain challenging tasks. Herein, we present a simple, controllable, scalable, and versatile electrochemical modification approach of graphite rods (GRs) extracted from low-cost Eveready cells that were covalently modified with anthracene oligomers. The anthracene oligomers with a total layer thickness of ∼24 nm on the GR electrode yield a remarkable specific capacitance of ∼670 F g-1 with good galvanostatic charge-discharge cycling stability (10 000) recorded in 1 M H2SO4 electrolyte. Such a boost in capacitance is attributed mainly to two contributions: (i) an electrical double-layer at the anthracene oligomer/GR/electrolyte interfaces, and (ii) the proton-coupled electron transfer (PCET) reaction, which ensures a substantial faradaic contribution to the total capacitance. Due to the higher conductivity of the anthracene films, it possesses more azo groups (-N[double bond, length as m-dash]N-) during the electrochemical growth of the oligomer films compared to pyrene and naphthalene oligomers, which is key to PCET reactions. AC-based electrical studies unravel the in-depth charge interfacial electrical behavior of anthracene-grafted electrodes. Asymmetrical solid-state supercapacitor devices were made using anthracene-modified biomass-derived porous carbon, which showed improved performance with a specific capacitance of ∼155 F g-1 at 2 A g-1 with an energy density of 5.8 W h kg-1 at a high-power density of 2010 W kg-1 and powered LED lighting for a longer period. The present work provides a promising metal-free approach in developing organic thin-film hybrid capacitors.

Relation between Double Layer Structure, Capacitance, and Surface Tension in Electrowetting of Graphene and Aqueous Electrolytes.

Deciphering the mechanisms of charge storage on carbon-based materials is pivotal for the development of next-generation electrochemical energy storage systems. Graphene, the building block of graphitic electrodes, is an ideal model for probing such processes on a fundamental level. Herein, we investigate the thermodynamics of the graphene/aqueous electrolyte interface by utilizing a multiscale quantum mechanics-classical molecular dynamics (QM/MD) approach to provide insights into the effect of alkali metal ion (Li+) concentration on the interfacial tension (γSL) of the charged graphene/electrolyte interface. We demonstrate that the dependence of γSL on the applied surface charge exhibits an asymmetric behavior relative to the neutral surface. At the positively charged graphene sheet, the electrowetting response is amplified by electrolyte concentration, resulting in a strongly hydrophilic surface. On the contrary, at negative potential bias, γSL shows a weaker response to the charging of the electrode. Changes in γSL greatly affect the total areal capacitance predicted by the Young-Lippmann equation but have a negligible impact on the simulated total areal capacitance, indicating that the EDL structure is not directly correlated with the wettability of the surface and different interfacial mechanisms drive the two phenomena. The proposed model is validated experimentally by studying the electrowetting response of highly oriented pyrolytic graphite over a wide range of electrolyte concentrations. Our work presents the first combined theoretical and experimental study on electrowetting using carbon surfaces, introducing new conceptual routes for the investigation of wetting phenomena under potential bias.

A 10-bit 33.3-kS/s 3.2-fJ/conversion-step single-ended counter-type SAR ADC with dual 5-bit CDAC arrays and counters in 65-nm CMOS

In this paper, an area-and-energy-efficient 10-bit single-ended (SE) counter-type (CT) analog-to-digital converter (ADC) with dual 5-bit capacitive digital-to-analog converter (DAC) arrays and dual 5-bit counters is presented. The 10-bit capacitive-DAC (CDAC) array and counter of the conventional counter-type structure are divided into two 5-bit CDAC arrays and counters, respectively. Significant improvements are achieved in comparison to the conventional counter-type ADC as follows: the capacitance of the most-significant-bit (MSB) capacitor is reduced by 96.88 % from 512C to 16C; the total capacitors are reduced by 93.75 % from 1024C to 64C; the total clock cycles required in the conversion phase are reduced by 93.94 % from 1023 to 62; the average switching energy is reduced by 98.55 % from 2390CV2 to 35CV2. The proposed ADC occupies a smaller area, performs much faster, and its power consumption is greatly reduced. This structure has a simple design with small sampling capacitor (32C) and does not require a rail-to-rail comparator compared to SAR ADCs. It is suitable for biomedical and multi-channel applications. The proposed ADC is post-layout simulated in 65-nm CMOS technology which occupies 0.016 mm2 area. This ADC has 85.43 nW power consumption, 9.64 ENOB on 33.33 kS/s sampling rate, and 3.23 fJ/conversion-step FOM.

Prominent Structural Dependence of Quantum Capacitance Unraveled by Nitrogen-Doped Graphene Mesosponge.

Porous carbons are important electrode materials for supercapacitors. One of the challenges associated with supercapacitors is improving their energy density without relying on pseudocapacitance, which is based on fast redox reactions that often shorten device lifetimes. A possible solution involves achieving high total capacitance (Ctot), which comprises Helmholtz capacitance (CH) and possibly quantum capacitance (CQ), in high-surface carbon materials comprising minimally stacked graphene walls. In this work, a templating method is used to synthesize 3D mesoporous graphenes with largely identical pore structures (≈2100 m2 g-1 with an average pore size of ≈7nm) but different concentrations of oxygen-containing functional groups (0.3-6.7 wt.%) and nitrogen dopants (0.1-4.5 wt.%). Thus, the impact of the heteroatom functionalities on Ctot is systematically investigated in an organic electrolyte excluding the effect of pore structures. It is found that heteroatom functionalities determine Ctot, resulting in the cyclic voltammetry curves being rectangular or butterfly-shaped. The nitrogen functionalities are found to significantly enhance Ctot owing to increased CQ.

A 22.3-Bit Third-Order Delta-Sigma Modulator for EEG Signal Acquisition Systems

This paper presents a high resolution delta-sigma modulator for continuous acquisition of electroencephalography (EEG) signals. The third-order single-loop architecture with a 1-bit quantizer is adopted to achieve 22.3-bit resolution. The effects of thermal noise on the performance of the delta-sigma modulator are analyzed to reasonably allocate the switched-capacitor sizes for optimal signal to noise ratio (SNR) and minimum chip area. The coefficients in feedback path and input path are optimized to avoid the signal distortion under the full-scale input voltage range with almost no increase in total capacitance sizes. Fabricated in 0.5 µm CMOS technology and powered by a 5 V voltage supply, the proposed delta-sigma modulator can achieve 136 dB peak SNR with 16 Hz input and 137 dB dynamic range in 100 Hz signal bandwidth with an oversampling ratio of 512. The modulator dissipates 700 µA. The core chip area is 1.96 mm2. The modulator occupies 1.41 mm2 and the decimator occupies 0.55 mm2.

Analysis on the impact of interface Trap distributions on SOI DMG FinFETs: Overlap/underlap configurations

This paper investigates the significance of interface trap charges (ITCs) distribution by introducing localized charges (donor/acceptor) at the interface of semiconductor/insulator on dual-material gate (DMG) FinFETs in gate Overlap and Underlap configurations in terms of DC and analog/RF parameters. Different charge density profiles like Uniform and Gaussian distribution of traps are considered at different energy levels for both donor and acceptor ITCs to evaluate the devices reliability. It is observed that the donor traps mainly affects the OFF-state current (IOFF) by insignificantly, while, acceptor traps mainly impacts ON state current significantly. The impact of Acceptor ITCs on various parameters such as transfer characteristics, electron density, conduction band energy, transconductance (gm), total gate capacitance (Cgg), device efficiency (gm/ID), and cut-off frequency (fT), for DMG FinFETs have been comprehensively analysed at trap concentration N0=1×1013 and compared with no traps among the three devices. An acceptor trap charges with Gaussian distribution has larger impact on devices performance. It degrades the ON state-current by 36.09 %, 25.72 %, and 41.9 % for conventional, Overlap, and Underlap configurations, respectively. It is evident from the results DMG underlap configuration is more vulnerable to ITCs, attributed to its larger interface charge region due to the increase in effective channel length while, the overlap configuration is more immune towards ITCs as, lesser electrons get trapped. Thus, the overlap configuration is the most reliable among underlap and conventional configurations.

Facile synthesis of Mn3O4–ZnO composite for photocatalytic dye removal and capacitive applications

Researchers are focusing on the designing a material's chemistry with novel composition to enhance the efficiency of wastewater treatment and energy storage applications with sufficient stability. Current manuscript presents a detailed comparative study of Mn3O4–ZnO composite for photocatalytic and capacitive applications. The composite samples were prepared through chemical precipitation route to understand the effect of concentration of each component (Mn3O4 and ZnO) on structural and optical characteristics. X-ray diffraction (XRD) and Fourier Transformed Infra-Red spectroscopy (FTIR) confirmed the successful development of both phases with structural modifications in composite sample. Optical absorbance spectroscopy supported the structural amendments with variation in optical band gap (2.71–2.92 eV). Further, the equal concentration (1:1) of Mn3O4 and ZnO exhibited ∼98 % removal of methylene blue dye under UV–visible lamp (Hg lamp) with highest rate of removal (0.037 min−1) among all the prepared samples. Here, the degradation efficiency has been reduced significantly (to 30 %) in alkaline dye solution at pH = 11. Further, the capacitive measurements revealed the highest total capacitance (0.97 F/g) among all the mixed compositions.

Improved frequency performance in AlGaN/GaN HEMTs on Si using hydrogen silsesquioxane-assisted gate

AlGaN/GaN HEMTs have been regarded as the most promising option for achieving high-frequency power amplifier in various applications, including wireless communication, aerospace, and radar systems. To enable their use at even higher frequency, we conducted a study to enhance the frequency characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) on a silicon substrate. By employing a hydrogen silsesquioxane (HSQ)-assisted gate, AlGaN/GaN HEMT with an LG of 0.17 μm and LSD of 5 μm exhibited a VTH of 4.5 V, ID.max of 885 mA/mm, and gm.max of 253 mS/mm. The planar-gate without HSQ shows VTH of 4.4 V, ID.max of 876 mA/mm, and gm.max of 258 mS/mm. The DC characteristics are comparable to those observed in planar-gate HEMT with a conventional gate structure. Though the DC characteristics of both devices are similar, the HSQ-assisted gate HEMT exhibits a higher fT/fmax of 55/89 GHz, whereas the planar-gate HEMT yields an fT/fmax of 38/64 GHz. The 32% reduction in the total extrinsic gate capacitance (Cgs + Cgd) is the major contributor to the improved frequency performance of the HSQ-assisted gate AlGaN/GaN HEMT. The adoption of HSQ-assisted gate AlGaN/GaN HEMTs presents itself as a highly advantageous choice for achieving higher-frequency operations. © 2023 Elsevier Inc. All rights reserved.

Pore engineering in robust carbon nanofibers for highly efficient capacitive deionization

Capacitive deionization (CDI) is considered as a revolutionary desalination method to alleviate the global water crisis. However, the current carbon-based CDI cells are constrained by the inferior desalination efficiency, resulting from the undesirable pore structure and sluggish electron/ion transport. Herein, a new porogen of silicalite-1 is proposed for the tailorable fabrication of flexible porous carbon nanofibers (PCNFs) by electrospinning, carbonization and alkali etching, where the surface area, total pore volume, mechanical robustness, specific capacitance, and electrosorption performance of the developed PCNFs are associated with the silicalite-1 dosage. The symmetric CDI unit assembled by the freestanding PCNF membrane electrodes shows a state-of-the-art desalination capacity (38.64 mg g−1), a remarkable salt removal rate (12.1 mg g−1 min−1), a desirable charge efficiency (82.36 %), an acceptable energy consumption (0.668 Wh g−1), and a satisfactory cyclic durability, and such excellent deionization performance can be explained by that the abundant hierarchical pores, the large accessible surface area, and the interconnected conductive carbon network effectively expedite the electron/ion transport. Additionally, the home-made CDI device has a favorable desalination capability toward the high-ion concentration seawater from the Yellow Sea (Xiaomai Island). This work not only validates the feasibility of silicalite-1 as the pore-forming agent for the targeted preparation of flexible PCNFs, but also it highlights the potential of pore engineering in freestanding PCNFs for advanced CDI.

Effects of the ground-electrode temperature on electrical and optical characteristics of a coaxial dielectric barrier discharge in atmospheric pressure air

The applied power and electron energy are believed to play important roles in plasma applications, which can be significantly altered by controlling the ground-electrode temperature in a coaxial dielectric barrier discharge (DBD). The effects of the ground-electrode temperature on the electrical and optical characteristics of the DBD were investigated in atmospheric pressure air. The results show that the breakdown voltage decreases, and the number of discharge pulses, the transferred charges, and the applied power increase with the electrode temperature, while the peak current tends to decrease when the electrode temperature is below −35 °C and above 25 °C, respectively. When the electrode temperature remains constant, the breakdown voltage increases and the applied power decreases with the gap distance. The applied power increases with the applied voltage. The dielectric capacitance, the total capacitance, and the gap capacitance at the electrode temperature below −35 °C are generally higher than those at the electrode temperature above 25 °C. Moreover, they change in the same trend when the electrode temperature and the applied voltage increase. The dielectric and total capacitances increase while the gap capacitance decreases. The relative intensities of the spectral lines increase with the electrode temperature. The electron, vibrational, and rotational temperatures increase with the electrode temperature above 25 °C. However, the electron and vibrational temperatures increase and decrease with the electrode temperature below −35 °C. The secondary emission coefficients and electron energy distributions with the electrode temperature were discussed.

An integrated thermal and RF energy harvesting system with rectifying combination and storage controller for IoT devices

This work presents an integrated thermal/RF energy harvester. The harvesting system can combine energy from two sources simultaneously (one DC and one AC) and deliver a regulated combined voltage of 1.75 V to a load and a storage controller. The output power of the harvester is in the range of hundreds of μWatts which is suitable for low-power wireless sensor networks. Maximum power point tracking (MPPT) is achieved by reconfiguring the number of stages of the DC path, the frequency driving the charge pump, and the capacitors of the matching network in the RF-path. The storage controller charges a supercapacitor to 3.5 V to power up the system during power shortage periods. The system is implemented in a 180 nm CMOS technology and utilizes a total on-chip capacitance of 2.4 nF. The peak end-to-end efficiency of the system reaches 60.3 % at output load power of 1.275 mW.

Insights into the impact of random dopant fluctuation on ferroelectric germanium source vertical TFET

This paper investigates the effect of random dopant fluctuation (RDF) on the performance of ferroelectric Germanium source vertical tunnel field effect transistor (FE GeS-vTFET) using 3-D device simulation. The variation in the on-state current (σION), off-state leakage current (σIOFF), current ratio σ(ION/IOFF), subthreshold swing (σSS), threshold voltage (σVTH), total gate capacitance (σCgg), transconductance (σgm) and cut-off frequency (σfT) for different source doping concentration and source thickness due to the effect of RDF are evaluated. The findings indicate an increasing tendency in RDF with higher doping concentration in the source region. The variation in the on-current is from 0.2173 mA to 0.2337 mA (∼1.075 times higher), and σIOFF varies from 4.939 fA to 16.545 fA (∼3.349 times higher) from 1 × 1020 /cm3 to 3 × 1020 /cm3 doping concentration, respectively for Germanium source thickness of 12 nm. RDF within the source region mainly contributes to the threshold voltage variation of about 92 % in all regions. The randomized dopant-induced σVTH for the FE GeS-vTFET compared with other field effect transistors of the same technology node showed a substantial reduction in σVTH, signifying that the device is more immune to the effects of RDF. Furthermore, the effect of temperature on the switching properties of the FE GeS-vTFET has been studied.

Nano-tablets and rods-shaped Fe and Ni oxide/hydroxide encapsulated graphene for asymmetric supercapacitor and OER applications

Ternary hybrid electrodes have been spotlighted in the energy storage and conversion domain due to their superior physicochemical and electrochemical attributes. Yet, elevating their electrochemical properties remains a challenge. Herein, gamma‑iron oxide nano-tablets and nickel hydroxide nano-rods encapsulated graphene sheets (γ-Fe2O3@Ni(OH)2@G/NF) nanocomposite (NC) electrodes are synthesized, using PVP as a surfactant by a simple, cost-effective and low-temperature synthesis procedure for supercapacitor (SC) and OER applications. These interconnected graphene sheets with γ-Fe2O3 and Ni(OH)2 are seen to enhance the electrical conductivity of NCs, leading to outstanding supercapacitive performances. The developed NC electrode displays the highest specific capacitance (Csp) of ∼937.5 F g−1 in the alkaline electrolyte. Moreover, Dunn's investigations, revealing the surface capacitive controlled contribution (77 %@10 mV s−1), predominates in the total capacitance, thus affirming the capacitor behavior of the electrodes. Using the NC electrode, an asymmetric supercapacitor (ASC) is constructed, and exhibits a maximum Csp of ∼120.3 F g−1 with an outstanding stability performance of 87.3 % capacitance retention. Moreover, the ASC displays an energy density of 22 Wh kg−1 under a power density of 1100 W kg−1. Overall, the results demonstrate that the developed ternary-hybrid composite shows promise as a worthy electrode for future energy demands.

Flexible, Thermally Stable, and Ultralightweight Polyimide-CNT Aerogel Composite Films for Energy Storage Applications.

Polyimide (PI) aerogels are promising in various fields of application, ranging from thermal insulators to aerospace. However, they are typically in the form of a bulk monolith, which suffers from a lack of conformability and drapability. Moreover, their electrical conductivity is limited, and they mainly display an insulative behavior. These shortcomings can limit the applications of PI aerogels in energy storage systems, which require ultralightweight flexible conductive films, which at the same time offer high thermal stability, ultralow density, and high surface area. To overcome these obstacles, the present study reports the fabrication of PI-carbon nanotube (PI-CNT) aerogel composite films with varying CNT content prepared through a sol-gel preparation method, followed by a supercritical drying procedure. Compared to pristine PI aerogels, which displayed a large shrinkage and density of 18.3% and 0.12 g cm-3, respectively, the incorporation of only 5 wt % CNTs resulted in a significant reduction of both shrinkage and density to only 11.5% and 0.10 g cm-3, respectively. This suggests the importance of CNTs in improving the dimensional stability of aerogels and creating a robust network. Further characterizations showed that incorporation of 5 wt % CNTs also resulted in the highest pore volume (1.25 cm3 g-1), highest surface area (324 m2 g-1), highest real permittivity (80), highest electrical conductivity (3 × 10-1 S m-1), and ultrahigh service temperature (575 °C). It was also shown that the aerogel films can withstand a large degree of bending, can be twisted, and can be fully rolled with no obvious cracks propagated in the structure. The combined outstanding properties of the developed aerogel composite films make them promising potential candidates for supercapacitor electrodes. Therefore, the electrochemical performance of the devices based on aerogel electrodes was further studied. The device demonstrated a high energy density of 2.6 Wh kg-1 at a power density of 303.8 W kg-1. The total capacitance after 5000 cycles was 91.8% of the initial capacitance, which indicated excellent stability and durability of the device. Overall, this work provides a facile yet effective methodology for the development of high-performance aerogel materials for energy storage applications.

In-Depth Investigation of Manganese Dioxide as Pseudocapacitive Electrode in Lithium- and Sodium-Doped Ionic Liquids

We investigate the contribution of pseudocapacitance to the overall capacitance of MnO2 electrodes in pure and alkaline-doped ionic liquids via two spectroscopic methods: step potential electrochemical spectroscopy (SPECS) and in situ Raman spectroscopy. For both characterization methods, thin-film electrodes of birnessite-like amorphous MnO2 were cycled in Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, pure or doped with lithium or sodium. SPECS allows determination of the influence of the electrolyte composition on the electrochemical behavior of the MnO2 electrodes. Pseudocapacitive charge storage can account for over half of the total capacitance with alkaline-doped ionic liquids. In situ Raman spectroscopy provided insight into the reversible ion intercalation in the MnO2 structure, which appears to be controlled by EMIm+ cations. These findings are supported by density functional theory (DFT) calculations, which further help unveil the charge storage mechanism in birnessite-like amorphous MnO2 thin films operated in pure and alkaline-doped ionic liquids.