Pulsed Potential Amperometric Electrochemical Gas Sensors
Authors: Scheffler Towner 
Published: February 2020

Pulsed electrochemical techniques are well known and are well reviewed. The various discussions and derivations of the theory of pulsed voltammetry or polarography extant make clear the motivations behind the development of these techniques: increasing the analytical sensitivity of these methods by separating, in time, the charging current and the Faradaic current. There are at least three critical criteria for the success of pulsed voltammetric methods:

The potential pulse should be small: > 0.059/n V @ 25C.

The time between pulses should be long: for electrode area of 1 cm2, several seconds.

The electrode area should be minimized: < 1 cm2, down to 1 µm2.

All of these efforts had one goal in mind; minimizing the charging current with respect to the Faradaic current. Pulsed voltammetric techniques lowered the typical useful concentration range of electroanalytical methods from parts-per-thousand (10-3) to parts per million (ppm, 10-6) and lower.

The above discussion applies to classic solution-oriented electroanalytical techniques. Amperometric electrochemical gas sensors differ in several important ways which should limit or eliminate the usefulness of pulse techniques in this application.

Amperometric electrochemical gas sensors are ubiquitous in the modern industrial workplace, commonly deployed for worker protection against toxic and noxious gases. The heart of any good electrochemical gas sensor is the gas diffusion electrode, which is typically the working electrode in sensors. The gas diffusion electrode allows the easy confluence of three things, the electrocatalyst of which the electrode is comprised, usually a metal, the internal electrolyte of the sensor, usually a liquid solution, and the gas of interest. This confluence is often referred to as the “triple point.” To maximize the sensitivity of a sensor, maximize the number of triple points. This is commonly accomplished by using finely divided, high surface area catalytic metals, such as platinum (Pt) or iridium (Ir) “blacks.” As gas diffusion electrodes commonly use such high surface area metal particles, their actual, usable electrochemically accessible surface area, and hence, electrical capacitance often far exceed their geometric area, and vary greatly from sensor (electrode) type to sensor type, and may approach 0.5 m2.

We have recently applied pulsed potential voltammetric techniques to the high surface area electrodes commonly employed in electrochemical gas sensors with great utility. The accompanying graph depicts the results of a typical experiment. This sensor utilized a rough gold working electrode with an electrochemically accessible surface area of approximately 100 cm2. The potential was switched between -150 and +250 mV (vs an internal Pt|air pseudo-reference electrode) every 0.5 sec. The traces in the figure are offset for clarity.

The upper trace in the figure is the raw response recorded with a Bio-Logic VMP3 potentiostat. The data was collected every 0.1 sec. Ten (10) ppm nitrogen dioxide (NO2), in air, was applied to the sensor at a flow rate of 0.25 l/min, beginning at the 5 minute time and switched back to clean air at the same flow rate at the 10 minute mark. The bottom traces depict the results of current sampling the sensor response just before the potential of the working electrode was switched to either -150 mV or 250 mV (a 400 mV pulse). The pulse rate was 0.5 seconds; therefore, the current was sampled approximately 0.49 sec after the application of the potential pulse.

As can be seen in the lower traces, very serviceable sensor responses were obtained in this manner for the application of 10 ppm nitrogen dioxide (NO2) in air (open circles, reduction) or 10 ppm nitric oxide (NO) in air (open diamonds, oxidation). The response time, noise, other figures of merit are very similar to data obtained from this same sensor operated at a single potential of -150 mV (for the reduction of nitrogen dioxide) or at +250 mV (for the oxidation of nitric oxide).

Using this technique, we have produced a single sensor that allows the simultaneous detection of both nitric oxide and nitrogen dioxide at concentrations important for worker protection. We have extended this technique to the detection of other gases, including oxygen at and near ambient conditions (20.8 vol-% at sea level and 25 C).

Figure 1