Perspective—A Guideline for Reporting Performance Metrics with Electrochemical Capacitors: From Electrode Materials to Full Devices

Abstract

Over the past decade, interest in electrochemical capacitors as an energy-storage technology has increased enormously, spurring the development and evaluation of a large number of new materials and device configurations. This perspective article aims to propose guidelines by which new materials and devices should be evaluated, and how resulting data should be reported with respect to critical metrics such as capacitance, energy and power. In recent years the number of publications dedicated to electrochemical capacitors (ECs) has increased enormously, while drawing from a diverse community of researchers in the fields of electrochemistry, material science, and engineering.1,2 Contributions from such different perspectives have been invaluable to the advancement of EC science and technology, but has also resulted in some confusion in the corresponding scientific literature because of the wide range of materials (active and inactive), device configurations, and electrochemical testing protocols that are used. Current StatusUnfortunately, a comparison of the electrochemical properties of materials for EC is often challenging because key experimental parameters and procedures are not fully described in literature reports, and in some cases data from single electrode experiments are improperly extrapolated to projected device performance. Inconsistency in reporting key performance metrics hinders the comparison of data from different laboratories on otherwise related materials and devices. 1,2The present perspective article identifies some useful guidelines for correctly evaluating the electrochemical performance of EC materials and devices and reporting on the resulting findings. Future Needs and ProspectsMaterials for ECs (electrode and electrolyte).-When a new (active) material is proposed, its synthesis procedure should be described in detail sufficient to allow for replication in another laboratory; basic material properties should be also reported (particle/crystallite/film morphology, crystal structure, etc). Any electrode preparation using the resulting materials must be clearly described, including information on the respective ratios (or weight %) between active material, conductive agent (if present) and binder(s) used in electrode fabrication. Furthermore, one should report the active material mass loading per electrode area, for example in units of mg cm −2 . This information is essential for readers to clearly evaluate the electrochemical performance of both the active material and full device, particularly when comparing against results from other laboratories or from commercial products. Authors should consider that the mass loading of commercialized activated carbon electrodes is ∼10 mg cm −2 . * Electrochemical Society Member. z E-mail: andrea.balducci@uni-jena.de When a new electrolyte component (solvent or salt) is proposed, the composition of the investigated electrolyte should be clearly described, for example with concentration of the salts noted in units of mol L −1 . The viscosity and ionic c...

Ni(OH)2 and NiO Based Composites: Battery Type Electrode Materials for Hybrid Supercapacitor Devices

“…In this configuration, the ability to store energy is characterized by its capacitance and is expressed in Farads, i.e., C·V −1 (Equation (1)), or F. It must be pointed out that the value of the capacitance must remain constant all over the given potential window for ideal capacitive storage. The capacitance unit can be used to easily compare various capacitors, but such value always needs to be accompanied with the corresponding cell voltage in which the capacitor can be safely operated [5]. The same goes for a single electrode: the capacitance value measured in a three-electrode cell needs to be quoted with the potential window in which the electrode can be safely used to store charge.…”

“…However, even at such fast cycling rates a faradaic behavior of the electrode is observed, and it is virtually impossible to calculate a constant value that can be assimilated to a capacitance for such electrode. Unfortunately, there are a large number of reports in the literature for which such a calculation is still performed and a capacitance (in F) is given instead of the capacity (in mAh) of the electrode which is the correct parameter to be used [5,10]. …”

Grafting of Quinones on Carbons as Active Electrode Materials in Electrochemical Capacitors

“…Indeed, in the case of the functionalized carbon electrode it is no longer possible to determine a capacitance since such a value will no longer be a constant all over the potential window as it was the case for the pristine carbon electrode. 20 …”

“…20 Typically, the stability profile consists of the evolution of the specific charge with the number of cycles. In acidic electrolyte, only a 17% loss of the Faradaic charge of the quinone molecules has been observed following 10000 charge/discharge cycles.…”

Value Quantification of Electrochemical Capacitor Active Material

“…A guideline for reporting electrochemical capacitor performance metrics has recently been published. 3 Although all such measurements provide important information, they do not express the dynamic performance of an active material, which is especially important when tasked with creating a minimum-size storage system for a specific power profile and thus quantify the storage system value. Other reported information often includes Ragone plots, which have only minor engineering importance-these plots are better used to compare different energy storage technologies than to design a storage system.…”