J. Phys. E: Sci. Instrum. 20 (1987) 1156 - 1168. Printed in the UK
Inorganic solid state chemically sensitive devices : electrochemical oxygen gas sensors
W C Maskell Energy Technology Centre, Middlesex Polytechnic, Bounds Green Road, London N 11 2NQ. UK
Abstract. The principles of operation of potentiometric, amperometric, coulometric and impedance-based oxygen gas sensors are reviewed. Factors influencing the speed of response are considered in detail and related to the information obtainable from impedance measurements. The response of a sensor to gaseous mixtures not in equilibrium is discussed, particularly in relation to measurements in gases generated by the combustion of fossil fuels.
1. Introduction Several reviews of solid state gas sensors have been published during the last decade (Dietz et a1 1977. Fouletier 1982/3, Williams and McGeehin 1984, Maskell and Steele 1986) and the reader is referred to these for background information including practical details and design of devices. The purpose of the present review is to concentrate on the scientific principles underlying the operation of the major device types rather than to describe the multitude of configurations available for technological applications. Where possible the discussion is generalised so as not to be specific to the sensing of a particular gas. whereas the examples refer to the sensing of oxygen. The attention devoted to the various device types reflects the relative volume of published literature relating to each. Many of the principles discussed for potentiometric sensors are applicable to devices operating in the alternative modes.
applications. On the other hand, elevated temperature of operation can be a disadvantage so that, for example, an oxygen sensor operating at 700 "C may require the addition of a flame trap to prevent the ignition of potentially explosive mixtures of gases.
Oxygen sensors are widely used at the present time for controlling the air -fuel (A/F) ratio in combustion processes. Examples include detecting the stoichiometric point for the operation of a three-way catalyst in internal combustion engines and controlling fossil-fuelled boilers for maximisation of thermal efficiency while minimising the generation of carbon monoxide.
In the near future oxygen sensors are likely to be used to control the air-gas ratio in fully premixed condensing domestic boilers; if excess air levels are reduced the dew point of water vapour in the flue is raised so that the recovery of latent heat by condensation is increased.
Following Fouletier (1982/3) the potentiometric solid electrolyte gas sensor can be depicted as follows:
X 2 ( ~ 2 ) :M ~ I S E I MX2, ( p l )
MI and M2 are electronic conductors and act as cell electrodes to which electrical connections may be made; the solid electrolyte (SE) should be physically impervious to the gas X2 which is at partial pressures pi and p 2 : the electrolyte usually has a mobile ion which is a charged species of the gaseous atom, e.g. 02/02 but this is not always the case. The electrode
reaction (for the case involving an anion) is
Solid state devices operating on electrochemical principles have gained popularity in some applications compared. for example. with aqueous electrochemical sensors for a number of reasons: the electrolyte in the former devices is non-volatile enabling operation in environments above ambient without solvent loss, so that operating life is not terminated prematurely because of this; elevated temperature of operation can facilitate the catalysis of reactive gases. so that equilibrium gas concentrations may be measured where required: response time
to a change in gas concentration zirconia oxygen sensor in more
can be rapid (e.g.
50 ms for a
0:at 700 "C);
the sensor can be highly selective showing little interference from the presence of other gases; the potentiometric devices have a logarithmic response and can measure gas concentrations over an extremely wide range (106-10-25 Pa of 0 2 with a zirconia- based electrolyte under appropriate conditions); alternatively the amperometric devices (based upon current measurement) deliver a linear response, which is preferable in some applications. and thus solid state devices are versatile and can be tailored to the
X, + ne +2X"'
The EMF of cell (1) assuming reversibility is given by
E=(RT/nF) ui': tiond Inp.
t,"" is the transport number of the mobile ion while R . T and F have their usual significance. The electrolytes generally chosen have a very low electronic transport number under the conditions of operation. so that tic" is close to unity. Equation (3) then becomes
E=(RT/nF) h ( p l/ p 2 ) .
Hence if p 2 is known (reference gas) then p , may be determined from a measurement of E (at a given T ) .The reference gas may originate from a gaseous source, e.g. ambient air or a gas cylinder, or may be generated from a solid as follows:
0022-37?5/87/101l56 + 13 S02.50 3 1987 IOP Publishing Ltd