Ionic liquid based EDLCs: influence of carbon porosity on electrochemical performance

Electrochemical double layer capacitors (EDLCs) are a category of supercapacitors; devices that store charge at the interface between electrodes and an electrolyte. Currently available commercial devices have a limited operating potential that restricts their energy and power densities. Ionic liquids (ILs) are a promising alternative electrolyte as they generally exhibit greater electrochemical stabilities and lower volatility. This work investigates the electrochemical performance of EDLCs using ILs that combine the bis(trifluoromethanesulfonyl)imide anion with sulfonium and ammonium based cations. Different activated carbon materials were employed to also investigate the influence of varying pore size on electrochemical performance. Electrochemical impedance spectroscopy (EIS) and constant current cycling at different rates were used to assess resistance and specific capacitance. In general, greater specific capacitances and lower resistances were found with the sulfonium based ILs studied, and this was attributed to their smaller cation volume. Comparing electrochemical stabilities indicated that significantly higher operating potentials are possible with the ammonium based ILs. The marginally smaller sulfonium cation performed better with the carbon exhibiting the largest pore width, whereas peak performance of the larger sulfonium cation was associated with a narrower pore size. Considerable differences between the performance of the ammonium based ILs were observed and attributed to differences not only in cation size but also due to the inclusion of a methoxyethyl group. The improved performance of the ether bond containing IL was ascribed to electron donation from the oxygen atom influencing the charge density of the cation and facilitating cation–cation interactions

Energy applications of ionic liquids

Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy related applications. Cation–anion combinations that exhibit low volatility coupled with high electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermoelectrochemical cells. In the field of water splitting to produce hydrogen they have been used to synthesize some of the best performing water oxidation catalysts and some members of the protic ionic liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of fuel cells operating in the optimum temperature region above 100 °C. Beyond electrochemical applications, the low vapour pressure of these liquids, along with their ability to offer tuneable functionality, also makes them ideal as CO2 absorbents for post-combustion CO2 capture. Similarly, the tuneable phase properties of the many members of this large family of salts are also allowing the creation of phase-change thermal energy storage materials having melting points tuned to the application. This perspective article provides an overview of these developing energy related applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities

Electrowetting of Ionic Liquid on Graphite: Probing via in Situ Electrochemical X-ray Photoelectron Spectroscopy

Thin films of ionic liquid 1-ethyl-3-methylimidazolium bis(fluoromethylsulfonyl)imide ([EMIm][FSI]) vapor-deposited on highly oriented pyrographite (HOPG) were studied by X-ray photoelectron spectroscopy and atomic force microscopy. The results revealed a reversible morphological transition from a “drop-on-layer” structure to a “flat-layer” structure at positive, and not at negative, polarization. The effect is rationalized in terms of electric-field-induced reduction of the liquid–solid transition temperature in the ionic liquid film, when its thickness is comparable to the charge screening length. The observed bias asymmetry of [EMIm][FSI] electrowetting on HOPG is tentatively explained by the bilayer structure at the interface driven by the affinity of the imidazolium ring to the HOPG surface

Robust NiCo2O4/Superactivated Carbon Aqueous Supercapacitor with High Power Density and Stable Cyclability

Herein, we investigate the performance of an aqueous asymmetric supercapacitor (AAS) assembled by using novel nanostructured NiCo2O4 as the positive electrode and a polymer‐derived superactivated carbon (SAC) as the negative electrode. The combination of both the nanostructured NiCo2O4 and the carbon with hierarchical porosity and ultrahigh specific surface area (above 3000 m2 g−1) led to excellent rate performances and long stability of the system. The optimization of the AAS device is further achieved through the variation of mass ratio between positive and negative electrodes. The optimized AAS full cell exhibits reversibility within the 0.0–1.5 V operative voltage region, delivering a specific cell capacity of 24.6 mAh g−1 at a current density of 1 A g−1. This results in a remarkable energy density of 13 Wh kg−1 at a power density of 26.2 kW kg−1 and an excellent cycling durability above 87 % of the initial capacity after 10,000 charge–discharge cycles.Spanish Ministry of Economy and Competitiveness. Grant Numbers: MINECO/FEDER, MAT2015-64617-C2-2-R Basque Government. Grant Number: CICe Elkartek 201

Relation between texture and high-rate capacitance of oppositely charged microporous carbons from biomass waste in acetonitrile-based supercapacitors

A biomass-derived activated carbon with a systematic control over the pore size distribution is used to decode the effect of pore size distribution on charge dynamics in organic acetonitrile-based supercapacitors. Distinct trends in the high-current capacitance of the positive and negative electrodes are revealed by isolating the ion-specific accessible pore width and specific surface area from the total values calculated on the basis of low-temperature nitrogen adsorption/desorption isotherms. A size match between ions and pores for each separate electrode is established to maximize gravimetric capacitance under high current load. Most importantly, the high-current gravimetric capacitance demonstrates the existence of an optimum micropore width depending on polarization as well as no need for wide micropores or mesopores for ensuring rapid capacitive response.The work was supported by the Basque Government under the Etortek Energigune'12 (Project Reference IE12-335) and Elkartek 2015 (Project CICE2015, Reference KK-2015/0000100) Program. The donation of olive mill waste from Agropecuaria Ecológica Sierra de Alcaraz (Albacete, Spain) is gratefully acknowledged. The authors also thank Dr. I~naki Orue (University of The Basque Country) for help in measuring the electrical conductivity of carbon electrodesPeer Reviewe

Increasing the efficiency and cycle life of Na-O2 batteries based on graphene cathodes with heteroatom doping

To overcome the challenges of Na-O2 batteries with respect to efficiency, capacity, and cycle life as well as to develop cheap, metal-free, and abundant electrocatalysts, we explored boron and nitrogen-functionalized graphene aerogels prepared by the hydrothermal self-assembly of graphene oxide with subsequent thermal reduction. The results showed an improve of both the cycling overpotential and the coulombic efficiency for the functionalized graphene aerogels. However, the nitrogen-containing cathode presented a shortened cycle life and decreased charging stability. The postmortem analysis of the full discharge, and the full discharge and charge cathodes demonstrated that nitrogen functionalization triggered the formation of solid parasitic products that passivate the cathode surface, thus resulting in a poorer electrochemical performance. By contrast, functionalization with boron-containing groups demonstrated to be a more promising strategy due to minimized parasitic products, leading to lower oxygen reduction and evolution overpotentials with a concomitantly enhanced cell efficiency vs. the undoped cathodes. This resulted in a cycle life of 70 cycles at a relatively high current density of 0.1 mA cm−2 with a capacity cut-off of 0.5 mAh cm−2. Our study underscores that functionalization with heteroatoms simultaneously alters multiple characteristics of graphene-based materials, including their chemistry, texture and morphology, which in turn presents a critical impact on the electrochemical response of the resultant Na-O2 cells.This work was funded by the European Union (Graphene Flagship-Core 3, Grant number 881603) and the Basque Country Government (CIC energiGUNE’20 of the ELKARTEK program, N° Exp. KK-2020/0078). J.I.P. and S.V.-R. acknowledge funding by the Spanish Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (MCIN/AEI/ 10.13039/501100011033) as well as the European Regional Development Fund (ERDF, A way of making Europe) through grant PID2021-125246OB-I00, and by Plan de Ciencia, Tecnología e Innovación (PCTI) 2018-2022 del Principado de Asturias and the ERDF through grant IDI/2021/000037. N. Ortiz-Vitoriano thanks Ramon y Cajal grant (RYC-2020-030104-I) funded by MCIN/AEI/10.13039/501100011033 and by FSE invest in your future.Peer reviewe