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Why Do Some Electrochemical Gas Sensors Have Two Electrodes, and Some Have Three?
The short answer: It usually doesn't matter. In fact, you can operate almost any three-electrode sensor as a two-electrode sensor by shorting the Counter and Reference electrodes (CE and RE) together. You might lose a little response time, or lose linearity at the extreme upper end of the range, but that often doesn't matter.

The long answer requires a little preamble about electrochemical gas sensor operation. A typical amperometric gas sensor is shown schematically in Figure 1. Three electrodes are immersed in a liquid electrolyte.
Figure 1: Schematic of a three electrode sensor. It can be converted to a two electrode sensor by short-circuiting the reference and counter electrodes (RE and CE).
The key to the sensor's operation is the Working Electrode (WE), consisting of a catalytic material, usually platinum or gold, on a porous, hydrophobic polymer. The porous electrode allows the gas sample to reach the WE without allowing the liquid electrolyte to leak out. The catalytic reaction either produces or consumes electrons. When these flow in or out of the WE terminal of the sensor, the current is converted to your analytical signal.

You can't keep pumping electrons for long without building up a charge that will eventually stop the reaction. To keep the sensor electrically neutral, the excess or deficit of electrons has to be replaced with an equal and opposite current at another electrode. This electrode may be called the Counter Electrode (CE) or the Auxiliary Electrode.

The electrode reactions are controlled by the voltage between the metal of the electrode and the electrolyte solution. 'Way down at the atomic level, the voltage across the thin layer of molecules next to the electrode surface controls the rate of the reaction, and hence the amount of signal you measure. What is important, is that the reaction is controlled by the potential of the electrolyte, not one of the other electrodes.

Electrolyte potential is very difficult to measure accurately. Fortunately, for nearly all purposes, you can get a good-enough approximation of electrolyte potential by putting an electrode in the solution and measuring its potential without allowing a current to pass through it. When this electrode is included, it is called a Reference Electrode (RE).

The potential measured at the RE can be used to control the current driven through the CE. As long as that current is exactly the same as the catalytic, or Faradaic, current through the WE, the RE potential remains constant. Should the RE voltage drift up or down, the control circuitry will adjust the current through the CE until the RE's correct potential is restored.

Now, here is a surprise: the CE is intended to provide current, and the RE should not conduct any current. Yet, for nearly every application, the sensor will work just fine if the CE and RE are wired together! Even if the controlling circuit is unchanged.

Why is this? If you begin to pass a current through the RE, it no longer satisfies some of the assumptions you made when you designed it. Nevertheless, the signals from typical gas sensors are usually in the microamp range. These currents, it turns out, rarely perturb the chemistry of the RE, so it can remain a reliable indicator of electrolyte potential. If you lose at all, you lose in response time and linearity at high concentrations. Response time suffers because the small changes in electrode potential that do occur involve very large capacitive currents which must be satisfied as the signal increases. Linearity can be lost because the small change in the WE-RE potential difference might affect the catalytic activity at the WE.

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