Two tributaries of the electrical double layer

This special issue of Journal of Physics: Condensed Matter focuses on physical aspects of capacitive energy storage and harvesting devices based on electrical double layers. An electrical double layer (EDL) is an interfacial structure in which charges are separated between two neighboring phases or layers. Originally introduced by Helmholtz in 1850, the concept of EDLs has found numerous applications in various areas. Perhaps two of the fastest growing areas are the capacitive energy storage and energy harvesting. This special issue covers a wide range of topics in these areas, ranging from solving fundamental problems to proposing specific working devices. Electrical double layer capacitors (i.e., capacitive energy storage systems based on EDLs) were first developed in the labs of General Electric (late 1950s) and Standard Oil of Ohio (1960s). However unusual it may sound, they were abandoned as unusable, but luckily reborn by the NEC Corporation who marketed them as ‘supercapacitors’ in 1978. Supercapacitors consist of porous electrodes immersed in an electrolyte medium and store energy in the EDLs at the electrode/electrolyte interfaces [1, 2]. As a result of this simple setup that typically involves no chemical reaction, supercapacitors are distinguished by high power densities and unusual cyclability (reaching more than 1000 000 cycles), but have only moderate energy densities. Largely for the latter reason, supercapacitors did not play an important role in the mainstream energy storage applications historically. However, recent breakthroughs [3, 4] in developing novel electrode materials and room-temperature ionic liquids (ILs) have given a new life to supercapacitors, which now complement and in some cases even compete with the conventional energy storage devices such as batteries. Indeed, the market of then-abandoned supercapacitors was worth about $1.8 billion in 2014 and was estimated to show a year-on-year growth rate of about 9.2% according to some sources [5]. Works on supercapacitors appear now in a wide spectrum of journals covering chemical, material, energy, nanotechnology and applied physics sciences (for the latest review see e.g. [6]). In this special issue we attempted to collect papers focusing specifically on the physical aspects of the supercapacitor research, in all their complexity and depth, highlighting new challenges and yet unresolved problems. A distinct focus is however on electrolytes and electrolyte-electrode interactions. Why is this important? The energy stored in a supercapacitor is roughly CV2/2, where C is capacitance and V the applied voltage. Now, by increasing the voltage from 1 V (electrochemical window of a typical aqueous electrolyte) to 3 V or more (room-temperature ionic liquids), we increase the stored energy almost by an order of magnitude! On the other hand, it has recently become clear that adding an aqueous or organic component to room-temperature ILs reduces their viscosity and can dramatically accelerate charging [7]. However, this comes at a cost of the decreased stored energy because solvent can narrow the electrochemical window of an electrolyte. A sound understanding of the fundamental mechanisms of electrode-electrolyte interactions is thus needed to enable a significant leap in the performance of supercapacitors. This is where the physics can help improve supercapacitors as is the goal of many works including this special issue. A brief read-map is provided below to navigate the readers through this special issue. We start from the works by Docampo-Álvarez et al [8] and Vatamanu et al [9] who approach the discussed problems by using atomistic molecular dynamics simulations. Docampo-Álvarez et al deals specifically with the behaviour of water-IL mixtures in charged nanopores, while Vatamanu et al focus on the effects due to polar organic solvents (viz. acetonitrile). Both studies conclude that the presence of solvent can have a profound and sometimes unexpected effect on charging. For instance, confinement increases the amount of water in the positively charged electrode, but a decrease is observed on neutral and negatively charge surfaces [8]. Such effects must be accounted for when designing supercapacitors and choosing an optimal voltage regime. The experimental study of Ohkubo et al [10] deals with the hydration structures of calcium ions in non-polarized nanopores. They find that calcium ion-assisted adsorption of water falls off significantly when the pores become narrow (0.63 nm). This finding therefore S Kondrat et al