Electric vehicles, powered by Li-ion batteries, are limited by short driving range, long recharge time and capacity fade to compete with fossil fuel-powered alternatives. Alternative to Li-ion batteries such as supercapacitors and redox flow batteries with comparably high specific power and rapid recharge/refill have poor energy density due to selfdischarge. In this article, we introduce an ionic redox transistor that regulates bidirectional ion transport across the membrane as a function of its redox state and applicable as a smart membrane separator in supercapacitors and redox flow batteries. The smart membrane separator would enable the design of a new category of rechargeable/refillable energy storage devices with high energy density and specific power and overcome contemporary limitations of electric vehicles.

Enzymes have evolved over hundreds of years through changes in ecosystems (climate, atmosphere, hydrology, etc). The evolutionary changes driven by the need to survive has led to enzymes with diverse functionality such as reduction of carbon dioxide and methane to other forms of carbon, fixation of nitrogen, and high temperature biochemical processes. While these enzymes have useful properties, engineering a scalable cell-free system with these enzymes will be useful for stable production of desired products without involving the vagaries of cellular metabolism. This article presents various approaches to incorporate ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in a conducting polymer (polypyrrole (PPy)) doped with a bulky anion (dodecylbenzenesulfonate (DBS)) in an effort to create functional devices for the conversion of carbon dioxide into precursors for high-value chemicals. We demonstrate that the tailored device creates an environment where the enzyme can retain its function while being protected from denaturing conditions. It is envisioned that the 3-PGA produced by RuBisCO will be converted into value-added products.

In this article, the saturation kinetics model that describes chronoamperometric response of PPy(DBS) in our recently published work is extended to study the effect of mass and charge density on the step response of PPy(DBS). The saturation kinetics model is based on a mechanistic approach for charge storage in conducting polymers and leads to the development of structure-dependent input-output relationships to develop a cation concentration sensor. In this article, we demonstrate the use of poles and residues in the saturation kinetics model to deconstruct the chronoamperometric and chronocoulometric response by seperating the contributions from double layer charge accumulation and faradaic ion transport. We show that: (i) the number of redox sites, and therefore the number of ingressing ions at saturation, is directly proportional to the mass of the conducting polymer, (ii) the accessibility of these redox sites associated with ion ingress is inversely proportional to the conducting polymer charge density, (iii) the rate of ion ingress is found to be inversely proportional to mass and charge density, due to the decrease in the driving force per unit redox site and redox site accessibility, respectively. For lower charge densities, the mass has a dominant effect on saturation and rate of ion ingress, with charge density effects becoming apparent as it increases. The saturation charges obtained are consistent with the peak charges during cyclic voltammetry, thus validating the mechanistic interpretations. The findings of this article highlight the trade-offs between charge storage and transport properties for conducting polymer devices.

This research introduces an integrated vibration energy harvester and electrochemical energy storage device that can effectively convert ambient vibrations directly into stored electrochemical energy. The electrochemical energy storage device is an electrical double layer capacitor (EDLC) with an ionic redox transistor as its membrane separator. This ‘smart’ membrane separator directly rectifies the electrical energy generated by the transduction from the nonlinear energy harvester, creating an ionic polarization across the membrane separator for storage. This electrochemical gradient can be subsequently used for powering sensor electronics as required in various applications, including structural condition monitoring. The alternating voltage developed by the energy harvester (+/−5V around 100 Hz) is connected to an aqueous supercapacitor fabricated from nanofibrous carbon paper electrodes and a polypyrrole-based (PPy(DBS)) smart membrane separator. A potential below −400mV from the energy harvester applied to the supercapacitor turns the smart membrane separator ‘ON’ and results in a unidirectional ionic current of Li+ ions. As the potential developed by the harvester cycles above ∼50 mV, the membrane separator switches ‘OFF’ and prevents the discharge of the rectified current. This leads to a continuous polarization of ions towards electrical fields relevant for powering electronics. This article is the first description and demonstration of an energy harvesting and storage system that can directly convert the electrical energy from a vibration energy harvester into electrochemical energy without the use of passive circuit components for power rectification.

In this article, it is proposed that a membrane with tunable ionic conductivity can be used as a separator between the electrodes of a supercapacitor to both allow normal charge/discharge operation and minimize self-discharge when not in use. It is shown that the redox active conducting polymer PPy(DBS), when polymerized on a porous substrate, will span across the pores of the membrane. PPy(DBS) is also shown to function as an ionic redox transistor, in which the transmembrane ionic conductivity of the polymer membrane is a function of its redox state. The PPy(DBS) ionic redox transistor is applied between the electrodes in a supercapacitor as a smart membrane separator. It is demonstrated that the maximum tunable ionic conductivity of the smart membrane separator is comparable in operation to an industry standard separator at maximum ionic conductivity, with a self-discharge leakage current of ∼0.12mA/cm2 at 1V. The minimum tunable ionic conductivity of the smart membrane separator is shown to decrease the supercapacitor self-discharge when not in use by a factor of 10, with a leakage current of 0.012mA/cm2 at 1V. This range of tunable ionic conductivity could lead to the emergence of redox transistor batteries with high energy density and low self-discharge for short and long-term storage applications.