Advanced Redox Electrochemical Capacitor Diode (CAPode) based on Parkerite (Ni3Bi2S2) with High Rectification Ratio for Iontronic Applications

Recently, CAPodes were introduced as a new type of capacitive diode analogues. This device realizes unidirectional charging of ultracapacitors based on ion sieving mechanisms. Here, we present a new redox CAPode system in which hydroxide ions react with nickel bismuth sulfide on nickel foam as a battery-like electrode. This redox reaction leads to a high capacitance in a specific polarization window. However, for reversed bias this Faradaic electrode becomes inactive as it does not react with potassium cations, resulting efficient blocking. By adapting counter electrode materials, mass loading, utilized substrates, and electrolyte concentrations a very high rectification ratio (RI = 37 and RII = 0.96 at 10 mV s-1) was achieved, significantly surpassing the performance of other reported redox-type CAPodes. Moreover, this system shows an outstanding long-term stability of a rectification retention of 90% for RII over 5000 repolarization cycles. Additionally, operando electrochemical measurements provide deep insights into the charge/discharge mechanism leading to a rationalization of the proposed concept. The integrated device, tested in logic gate circuits such as AND and OR gates, shows prospective results in ionologic applications.