Energy Storage Data Reporting in Perspective—Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems

Due to the tremendous importance of electrochemical energy storage, numerous new materials and electrode architectures for batteries and supercapacitors have emerged in recent years. Correctly characterizing these systems requires considerable time, effort, and experience to ensure proper metrics are reported. Many new nanomaterials show electrochemical behavior somewhere in between conventional double‐layer capacitor and battery electrode materials, making their characterization a non‐straightforward task. It is understandable that some researchers may be misinformed about how to rigorously characterize their materials and devices, which can result in inflation of their reported data. This is not uncommon considering the current state of the field nearly requires record breaking performance for publication in high‐impact journals. Incorrect characterization and data reporting misleads both the materials and device development communities, and it is the shared responsibility of the community to follow rigorous reporting methodologies to ensure published results are reliable to ensure constructive progress. This tutorial aims to clarify the main causes of inaccurate data reporting and to give examples of how researchers should proceed. The best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed

Tunable atomic force microscopy bias lithography on electron beam induced carbonaceous platforms

Tunable local electrochemical and physical modifications on the carbonaceous platforms are achieved using Atomic force microscope (AFM) bias lithography. These carbonaceous platforms are produced on Si substrate by the technique called electron beam induced carbonaceous deposition (EBICD). EBICD is composed of functionalized carbon species, confirmed through X-ray photoelectron spectroscopy (XPS) analysis. AFM bias lithography in tapping mode with a positive tip bias resulted in the nucleation of attoliter water on the EBICD surface under moderate humidity conditions (45%). While the lithography in the contact mode with a negative tip bias caused the electrochemical modifications such as anodic oxidation and etching of the EBICD under moderate (45%) and higher (60%) humidity conditions respectively. Finally, reversible charge patterns are created on these EBICD surfaces under low (30%) humidity conditions and investigated by means of electrostatic force microscopy (EFM)

Supercapacitors

Efficient clean energy storage is developing at a fast pace in the 21st century, aimed at building a fossil fuel-free society. Due to high theoretical efficiency in converting chemical to electrical energy, promise of electrochemical energy storage technologies has triggered numerous efforts in improving energy and power performance metrics. Electrochemical capacitors, also known as supercapacitors, keep evolving into new domains of research and development with focus on fast charging, high energy density, and long-lasting energy storage technologies. Specifically, supercapacitors can be a better choice over batteries in high power density applications with typical charging time scales of a few seconds for low energy density applications (5-10Wh/kg). In this chapter, we dwell upon fundamental aspects and historical developments of electrochemical capacitors. It covers fundamental understanding of charge storage processes, general properties, and transition from electrical double-layer capacitors to high rate-high energy pseudocapacitive energy storage. This chapter also provides progress on state-of-the-art capacitive energy storage devices including hybrid capacitors and on-chip microsupercapacitors. At the end, perspectives on new materials development and new mechanisms for improving energy storage technologies will be discussed. © 2022 Elsevier Inc. All rights reserved

Micro-electrochemical capacitors: Progress and future status

Self-powered energy autonomy drives the sustainable operation of miniaturized electronics and wireless sensor networks in the current era of emerging internet of things (IoTs). Development and integration of on-chip energy storage with the harvesting modules enables autonomous functioning of microsensors for health tracking and environmental monitoring among many other micro-world requirements. By the virtue of high-power density, ultrahigh rate capabilities and longevity, microsupercapacitors (MSCs) turn out to be the maintenance-free micro-power sources. In this review, we discuss major breakthroughs in the field of MSCs over the past decade in terms of fabrication techniques, processing of electrode materials towards achieving optimal electrochemical performance metrics. The essence of moving from two-dimensional (2D) to three-dimensional (3D) electrode designs, symmetric to asymmetric devices and hybrid metal-ion capacitors is emphasized. Energy harvesting by solar, vibrational, and wireless charging show promise in developing self-powered MSCs in compatible manner. The design of MSCs for alternating current line-filtering applications, which potentially replace bulky low energy density electrolytic capacitors is highlighted. Scalable manufacturing of MSCs, ease of integration and packaging open the avenues for the maintenance-free operation of remote sensors, biomedical implantable chips, and wearable electronic gadgets in a self-sufficient manner. © 2022 Elsevier Lt

Ambipolar Electrochemistry of Pre-Intercalated Ti3C2Tx MXene in Ionic Liquid Electrolyte

Cation intercalation with or without redox remains the dominant charge storage mechanism for two-dimensional (2D) Ti3C2Tx MXene. Anion-based charge storage remains unexplored due to intrinsic negative surface charge of MXenes preventing spontaneous intercalation of anions and irreversible oxidation of Ti at anodic potentials in aqueous electrolytes. In this work, we report on the ambipolar electrochemical behavior of the Ti3C2Tx in ionic liquid electrolyte over a 2.5 V electrochemically stable window. The experiments are conducted on a thin Ti3C2Tx film current collector coated with an electroactive layer of small flakes (∼150 nm) of Ti3C2Tx pre-intercalated with 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide (EMIM-TFSI) ionic liquid. Couples of redox peaks with a very small potential separation during the voltage sweep are observed at high negative (−0.75 V vs. Ag wire) and high positive (+0.75 V vs. Ag wire) potentials. Our experimental electrochemical data combined with density functional theory (DFT) calculations suggest feasibility of pseudo-intercalation of TFSI anions between Ti3C2Tx flakes. This study provides a pathway for elucidating anion intercalation for different MXene chemistries in solvent-free electrolytes, which can lead to development of MXene based energy storage devices with improved performance

Pencil-on-paper: electronic devices

Paper based electronics have been rapidly growing in recent years. Drawing with a pencil on paper is perhaps the simplest and easiest way of establishing graphitic circuitry in a solvent-free manner, which in the post-graphene years, has attracted an unusual interest. Here in this focus article, we highlight the recent efforts in the literature employing pencil drawings in various ways including sensors, microfluidics, energy storage and microanalytical devices. Even active devices such as piezo and chemiresistive devices as well as field effect transistors have been realised by utilizing pencil-traces. Pencil-on-paper may offer a viable route for developing lab-on-paper applications through suitable integration of the passive and active roles of the pencil-trace

Doped micro-silicon and vanadium carbide MXene composite as anode for high stability and high capacity Li-ion batteries

The demand for high-energy lithium-ion batteries (LIBs) has been rising exponentially. Silicon (Si) is gaining increased attention and popularity as an anode material due to its high theoretical capacity (4200 mAhg−1, Li4.4Si) and ample abundance, but the huge volume expansion of Si restricts its use in practical applications. Herein, we propose a composite consisting of nitrogen (N) and phosphorus (P) doped micron Si/graphite with vanadium carbide (V2C) MXene, which effectively helps to buffer the mechanical stresses initiated by the volume expansion of Si. The lithium storage specific capacity of the composite is 2003 mAhg−1 (based on the weight of Si) after a long-term cycling of 500 cycles (1C rate) along with a good high rate performance. The improved performance of the composite electrode can be attributed to V2C as well as N/P doping, which significantly enhance the electron/ion conduction pathways. Also, low-cost micron Si can provide high tap density in practical applications where volumetric performance is desired. Thus, this work provides an approach to develop high-performance micron Si-based materials for LIBs

Field effect transistors and RC filters from pencil-trace on paper

We report the fabrication of Resistor-Capacitor (RC) filters and field effect transistors (FETs) based on pencil drawings on paper, which contain turbostratic graphite crystallites as evidenced from Raman analysis. Pencil drawings have been employed as resistor and an ion gel, 1-butyl-3-methylimidazolium octyl sulfate mixed with polydimethylsiloxane (PDMS) as dielectric, for the fabrication of RC filters with a cut-off frequency of 9 kHz. With ion gel as gate dielectric, an ambipolar electric field effect has been obtained from the pencil-trace at low operating voltages. The carrier mobilities were found to be ∼106 and 59 cm2 V⁻¹ s⁻¹ for holes and electrons, respectively. The mobility value showed only 15% variation among the devices tested, truly remarkable given the simplicity of the fabrication process

Solution processed sun baked electrode material for flexible supercapacitors

We report a new strategy for making electrode materials for supercapacitors based on a nanocrystalline Pd/carbon (nc-Pd/C) composite, obtained by thermolyzing a thin film of Pd hexadecylthiolate under sunlight. Thus obtained nc-Pd/C composite is porous (BET specific surface area, 67 m2 g-1), hydrophilic (contact angle, 49°) and of good electrical conductivity (resistivity ∼2 μΩ m), all these properties being highly suitable for supercapacitor electrodes. The autocatalytic nature of the nc-Pd/C composite was exploited for electrolessly depositing MnO2 bearing a nanowall morphology to serve as a pseudocapacitive material. Using MnO2/nc-Pd/C electrodes in 1 M Na2SO4, a specific capacitance of ∼450 F g-1 was obtained at 10 mV s-1. An asymmetric supercapacitor was fabricated by employing MnO2/nc-Pd/C as a positive electrode and nc-Pd/C as a negative electrode, where the potential window could be enhanced to 1.8 V with an energy density of 86 W h kg-1. The electrode precursor being a direct write lithography resist allowed fabrication of a planar micro-supercapacitor with an ionic liquid as electrolyte, exhibiting a cell capacitance of 8 mF cm-2. As our recipe does not make use of an additional charge collecting layer and binder, nor does it use any external energy during fabrication, the only cost consideration is related to Pd; however, given the extremely small amount of Pd consumed per device (∼0.18 mg), it is highly cost effective