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An ionic redox transistor is an ionic device in which ion transport from source to drain is regulated by the electrochemical redox state of the membrane between source and drain. The electrical signal required to switch the device between reduced and oxidized state is applied directly at the gate port that is connected directly to the membrane. In our prior demonstration, we had demonstrated that polypyrrole doped with dodecylbenzenesulfonate [PPy(DBS)] formed over a pore demonstrated a maximum conductance of 30μS/cm and a current gain of 60X as the polymer switches between oxidized (Vm>‐200mV) and reduced state (Vm<‐600mV).
The PPy(DBS), PPy(TFSI) and PPy(PF6) membranes are formed on various porous substrates relevant for energy storage (CelgardTM, carbon paper and microporous carbon filter). These membranes are assembled into a custom-made Sawgelok cell with Lithium, lithiated graphite or potassium metal electrodes as required for the energy storage device. Transmembrane ion transport is characterized using HEKA ELP3 SF+SECM hardware and we demonstrate controlled ion transport across suspended PPy(DBS) for various thicknesses, cation concentrations, Vm and VAC. Thickness of PPy(DBS) formed over the porous substrate is varied by controlling areal charge density (AE = 0.05 ‐ 1.5 C/cm2) during potentiostatic electropolymerization. For thickness (AE > 0.15 C/cm2), it is observed from SEM images that PPy(DBS) spans the pores underneath and forms a physical barrier for ion transport across the porous substrate. The experimental setup to investigate ion transport across the membrane. The ionic circuit was set up using the membrane as septum and transmembrane currents were recorded for reduced and oxidized states of the PPy(DBS).
The cyclic voltammogram (CV) of PPy(DBS) (areal density of 0.3 C/cm2) in various concentrations of Li+ or K+ ions. The CV appears as expected, with the reduction potential shifting to the right and the peak reduction current increasing with concentration. A typical temporal response of transmembrane current (IAC [mA/cm2]) at different redox potentials (Vm = ‐0.4V to ‐0.9V) and periodic VAC (±100mV). In this figure, it is observed that transmembrane currents are negligibly small (IAC ≈ 0) till the onset of reduction (Vm > ‐0.5V) and the membrane is in the OFF state. As the applied membrane potential is decreased beyond the onset of reduction (Vm < ‐0.5V), the transmembrane current (IAC) begins to increase. As the membrane potential decreases beyond the reduction peak (Vm < ‐0.8V), the transmembrane current reaches a steady state, indicating that it is in the ON state.
We define a new metric, amplification factor (ß), to capture the increase in transmembrane current (IAC) as the membrane switches between fully ON and fully OFF states. The amplification factor as a function of Li+ concentration is calculated. At smaller concentrations, below 200mM, the amplification factor is relatively small, and it has a linear dependence between 200mM and 1000mM. At lower Li+ concentrations, smaller number of charge carriers in the electrolyte and comparable size of Li+ ion to porous pathways in PPy(DBS) results in a smaller amplification factor. In this article, we compare the functionality of an ionic redox transistor in organic electrolytes and its application as a smart membrane separator in various energy storage devices (Lithium ion batteries, metal-air battery and super capacitors). We show that the polarity of the device changes due to the electrolyte and hence affects the control strategy required for regulating power output in energy storage devices that uses organic electrolytes.
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