Electronic logic gates are the basic building blocks of every computing and micro controlling system. Logic gates are made of switches, such as diodes and transistors. Ion-selective, ionic switches may emulate electronic switches [1-8]. If we ever want to create artificial bio-chemical circuitry, then we need to move a step further towards ion-logic circuitry. Here we demonstrate ion XOR and OR gates with electrochemical cells, and specifically, with two wet-cell batteries. In parallel to vacuum tubes, the batteries were modified to include a third, permeable and conductive mid electrode (the gate), which was placed between the anode and cathode in order to affect the ion flow through it. The key is to control the cell output with a much smaller biasing power, as demonstrated here. A successful demonstration points to self-powered ion logic gates.
Introduction: Electrochemical reactions have been studied since the early eighteenth century [9][10]. Two half-cell reactions are considered. In one, oxidation of the anode takes place. Excess electrons then flow through an external load to the second half-cell, where reduction takes place at the cathode. The circuit is completed by ionic current in the electrolyte. The two-half cells are connected via a permeable membrane, which enables the passage of ions, yet, limits the flow of the bulk electrolyte molecules.
We wish to control the ion flow inside electrochemical cells, electrically. Control of a reaction near an electrode (working electrode) is routinely made with an auxiliary electrode and a saturated reference electrode using potentiostats or galvanostats. This approach may affect the surface potential of the working electrode and the control process could become nonlinear. Our approach is different: here, a third permeable electrode (the gate electrode) is placed between the anode and the cathode. Upon biasing of this mid-electrode we form an electrolyte barrier to the flowing ions.
Consequently, the external current and voltage of the cell are controlled [11][12][13].
Our approach is also different than what is accustomed to in the literature. The latter are typically conducted with functionalized membranes [14][15] for the purpose of ion separation. A bipolar Ion Transistor reported in Ref. 1 is a good examplethe design was based on ion-selective membranes, and hence was ion specific. In contrast, we aim at controlling both anions and cations by the same electrolyte barrier potential.
Simulations: Simulations employed a commercial tool, based on finite elements (COMSOL). We used a very simple Zn-Pt cell: a Zn electrode as the anode and a Pt electrode as the cathode. The model allowed us to deal with a single ion component (Zn 2+ ) and took into account the reactions at the anode (oxidation of Zn) and on the cathode (formation of hydrogen), yet assumed no reaction at the gate. The diffusion of ions in the cell has considered only excess Zn 2+ ions in the electrolyte. The local ion current density was assessed as the negative spatial derivative of the local electrolyte potential (which is proportional to the local electric field) multiplied by the electrolyte conductivity. The effective electrolyte-to-metallic volume ratio in the porous electrode was 1:1. Other simulation parameters were: electrical conductivity of Pt, Zn, porous electrode and Zn 2+ concentration in the electrolyte, respectively: 10 8 , 10 7 , 3x10 5 , 0.01 S/m. The upper tip of the Pt cathode was grounded and the upper tip of the Zn anode was kept at (-0.8) V, slightly lower than the standard potential of the Zn anode (E0 (Zn) =-0.82 V). This means that the cell’s voltage (between Pt cathode and the Zn anode) was +0.8 V. Results are shown in Figs. 1,2. In the absence of gate reaction, the gate voltage that stops the battery from functioning (the stopping potential) is 0 V when the gate is biased with respect to the grounded cathode. The stopping potential is -2 V when the gate is biased with respect to the grounded anode. The external electrical cell current is negative (meaning flowing towards the anode) and its slope is negative, similar to Fig. 1a.
We may conclude that: (1) changes in the electrolyte potential at the gate affect the external current. ( 2 The copper contact wire to the gate, which was hidden behind the plastic plate was visually inspected after the conclusions of the experiments: it was clean and did not corrode after 11 days. Previous experiments also indicated that the type of material used for electrical contact between the gate’s power supply and the gate electrode does not affect the stopping potential value provided that it is not exposed to the electrolyte [12].
Replacing the contact with a Co wire did not change the experimental results meaning that the contact material did not participate in any reaction at the gate. The gate electrode was biased with respect to the anode, which was also grounded. The gate membrane was tightly held between two plastic plates. A hole in each plate let the passage of ions. A copper wire, hidden behind the plates was providing a contact to the porous Au-Pd gate electrode (Fig. 1c). The copper wire remained clean and un-corroded The Ion Gate: The configuration for an ionic XOR gate is shown in Fig. 6a. The circuit was made of two anti-paralleled batteries, each incorporated with a gate electrode. If both ports, A and B were at ca 0 V, then their sum potential would be zero (see SI section).
If one of the ports, say A was biased with 1 V, while the other was biased with 0 V, then the circuit output would be the difference between open circuit voltage of the 1 V biased battery and the 0 V biased battery If both were biased with 1 V then the output of their sum potentials would be zero. The two 100 KOhms resistors were aimed at preventing current flowing directly from
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