Anthony J. R. Rennie

Energy storage in electrochemical capacitors: designing functional materials to improve performance

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomass-derived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

Ether-Bond-Containing Ionic Liquids as Supercapacitor Electrolytes

Electrochemical capacitors (ECs) are electrical energy storage devices that have the potential to be very useful in a wide range of applications, especially where there is a large disparity between peak and average power demands. The use of ionic liquids (ILs) as electrolytes in ECs can increase the energy density of devices; however, the viscosity and conductivity of ILs adversely influence the power density of the device. We present experimental results where several ILs containing different cations have been employed as the electrolyte in cells containing mesoporous carbon electrodes. Specifically, the behavior of ILs containing an ether bond in an alkyl side chain are compared with those of a similar structure and size but containing purely alkyl side chains. Using electrochemical impedance spectroscopy and constant current cycling, we show that the presence of the ether bond can dramatically increase the specific capacitance and reduce device resistance. These results have the important implication that such ILs can be used to tailor the physical properties and electrochemical performance of IL-based electrolytes.

Influence of Particle Size Distribution on the Performance of Ionic Liquid-based Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs) employing ionic liquid electrolytes are the subject of much research as they promise increased operating potentials, and hence energy densities, when compared with currently available devices. Herein we report on the influence of the particle size distribution of activated carbon material on the performance of ionic liquid based EDLCs. Mesoporous activated carbon was ball-milled for increasing durations and the resultant powders characterized physically (using laser diffraction, nitrogen sorption and SEM) and investigated electrochemically in the form of composite EDLC electrodes. A bi-modal particle size distribution was found for all materials demonstrating an increasing fraction of smaller particles with increased milling duration. In general, cell capacitance decreased with increased milling duration over a wide range of rates using CV and galvanostatic cycling. Reduced coulombic efficiency is observed at low rates (<25 mVs−1) and the efficiency decreases as the volume fraction of the smaller particles increases. Efficiency loss was attributed to side reactions, particularly electrolyte decomposition, arising from interactions with the smaller particles. The effect of reduced efficiency is confirmed by cycling for over 15,000 cycles, which has the important implication that diminished performance and reduced cycle life is caused by the presence of submicron-sized particles.

Improved Performance of Ionic Liquid Supercapacitors by using Tetracyanoborate Anions

Abstract Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN)4] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9 mS cm−1). Herein, we report the use of ILs containing the [B(CN)4] anion for such an application. They presented a high maximum operating voltage of 3.7 V, and two‐electrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20 F g−1 @ 15 A g−1). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.

A review of magnesiothermic reduction of silica to porous silicon for lithium-ion battery applications and beyond

Increasing demands for portable power applications are pushing conventional battery chemistries to their theoretical limits. Silicon has potential as an anode material to increase lithium-ion cell capacity. The associated volume change during lithiation/delithiation leads to a decline in capacity during cycling and low lithium diffusion rates within silicon limit high rate performance. Porous silicon can potentially address the poor cyclability and rate capabilities simultaneously by minimising stresses and providing smaller silicon substructures for lithium diffusion. Template assisted synthesis and magnesiothermic reduction of silica to silicon offers a facile and scalable route for the production of porous silicon structures even when using a non-porous feedstock. This review collates the available literature concerning the effects of reaction conditions through the reduction reaction. We highlight that it is important to report in detail all reaction conditions and complete characterisation of both the reactant and the product. The battery performance of these porous silicon structures is discussed and future research directions are identified. These outcomes will enable the identification of a clear design pathway for the bespoke production of porous silicon.

Biotemplating: A Sustainable Synthetic Methodology for Na-ion Battery Materials

Dextran biotemplating is a novel, sustainable and reduced-temperature synthetic approach that allows a high level of control over the size and shape of particles formed. This article discusses the application of this technique to the synthesis of an important candidate sodium-ion positive electrode material, Na2/3Ni1/3Mn2/3O2 (or ‘NNM’), with a high theoretical specific capacity (173 mA h g−1). While a solid state reference sample prepared at 850 °C exhibited a specific capacity of ∼80 mA h g−1 after 10 cycles, samples made via our dextran biotemplating route with final calcination at 550 °C for 12 h showed a large and significant improvement at 103.1 mA h g−1, under the same operating conditions.

Chapter 3 – CO2-Derived Fuels for Energy Storage

Abstract This chapter examines the manner in which CO2-derived fuels, synthesised using renewable energy, can help with decarbonisation of several sectors. Three areas are discussed: the electrical generation system, transport fuels and energy for heating. In addition, the advantages and disadvantages of such CO2-derived fuels are considered with respect to other energy storage technologies. An introduction to the variations in daily demand on an electrical grid is illustrated by considering a single day experienced by the Spanish grid and the concept of a “generation mix” is presented. Synthetic fuels for transportation are explored alongside alternatives to the internal combustion engine, such as fuel cells and batteries. Finally, the area of heat provision, where CO2-derived fuels can be expected to play a vital role is introduced.