Ionotronics for reverse actuation

In the midst of an ongoing energy crisis, the search for new methods of energy harvesting has never been more important. Here we explore, analyse and discuss principles of ionotronic reverse-actuator devices based on the effect of double-layer charging. The designs that we consider in this paper operate based on a common principle - using external mechanical work, which would otherwise be wasted, to produce changes in the contact area of electrode and electrolyte, translated into the time variation of the double-layer capacitance. Periodic variation of capacitance, when connected to a reference voltage source, produces alternating electric current through a load. This concept is not new and in some forms was realised in the early works of Boland, Krupenkin and several papers of our group. The goal of the present paper is to build a comprehensive analytical platform for a description of operation of such devices in terms of materials, generated power as a function of the frequency of variation of applied force, electrical load, and other factors; the understanding of which allows us to optimise these systems and navigate their construction. The first design, discussed in the paper, is based on flat electrodes. It is the simplest one and, as such, helps elucidate some key factors determining power generation. While being easy to realise experimentally, it generates relatively low power, even when optimised. The second design, based on microporous electrodes is more sophisticated and allows a much larger power harvest. The results are also compared to the recently proposed capacitive rotor device. The developed theory is set to capture the key factors that determine the functioning of the considered reverse actuators. The structures under study are matched to fit into the sole of a shoe and produce power from walking and running. However, they can also be scaled-up to larger operating systems and various external loads.