Solid-state electrochemical oxygen sensor with reduced combustible gases cross sensitivity

Abstract

A solid state electrochemical sensor to measure oxygen partial pressure, comprises; at least one solid state electrolyte 801 sandwiched between first electrode 804 for exposure to sample gas 802 and s

Description

FIELD OF THE INVENTION

[0001] This patent specification describes methods and apparatus using solid electrolytes in an electrochemical cell to measure oxygen in the gas phase, in a background which may also contain combustible gases. The described methods and apparatus enable measurement of oxygen in a gas mixture with reduced cross sensitivity to combustible gases, which would normally consume some or all of the oxygen present though burning and hence lead to an inaccurate, lower oxygen reading. The gas mixture may be produced by a natural or artificial process or a combination of these. The gas mixture may be from an industrial and / or medical process and the measurement of oxygen may be utilised for monitoring health, quality control, pollution management or process optimisation, thereby reducing the carbon footprint of industrial processes. BACKGROUND OF THE INVENTION

[0002] Early investigations and developments into using zirconia (zirconium oxide) at high temperature for industrial products were performed in the late 1800s and beginning of the 1900s by Walter Nernst and documented in his papers and patents for using zirconia as a light source. In 1937 Baur and Preis (Uber Brennstoff-Ketten mit Festleitern, Z. Elektrochem., 1937) showed yttria stabilised zirconia (YSZ) to be an ionic transfer material at elevated temperatures. Weissbart and Ruka in 1961 described (Rev. Sci. Instrum. 32, 593-595 (1961)) utilising a high temperature galvanic cell to measure oxygen partial pressure. This is now the fundamental building block for many commercial oxygen sensors such as the "lambda" cell used in the analysis of exhaust gases from internal combustion engines in the automotive industry. The oxygen sensor will typically comprise a YSZ layer, sandwiched between two porous, catalytic electrodes, with one side exposed to a known reference oxygen composition and the other with the sample to be measured. The voltage difference (EMFsampie) generated between the two sides will normally be approximated by the Nernst equation: EMFsamvte = —ln p°2ref                                (1) sample zF [po2sampleJ V 1 where R is the molar gas constant, T is the temperature (K), z is the number of charge units transferred (4 for oxygen molecules), F is the Faraday constant and pO2ref is the oxygen partial pressure of the reference side and pO2sampie is the oxygen partial pressure of the sample side. Although zirconia cells for measuring percentage oxygen have been around for many years, their usage in trace oxygen measurements (approximately, but not limited to, 1 part per billion (ppb) to 1,000 parts per million (ppm) oxygen) has been limited by the influence of background combustible gases, which can consume oxygen before it is detected by the sensor, leading to significant measurement error. Trace levels of oxygen measurements are impractical using standard paramagnetic sensors, and infrared laser-based measurements would require long absorption pathlengths due to the weak absorption lines with associated increased complexity and cost. Existing alternative technologies using liquid electrolytes have limitations on operating ambient temperature, may have consumable constituent parts, are affected by acidic background gases such as carbon dioxide and require electrolyte maintenance. Hence there remains a need for an oxygen sensor which is low maintenance, long-lasting and suitable for measuring oxygen at trace levels even with the presence of background combustible gases. SUMMARY

[003] Described herein are solid-state electrochemical sensors for measuring oxygen partial pressure in a gas sample. An example sensor comprises a solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas and a second electrode for exposure to a reference gas, wherein the first and second electrodes and the solid-state electrolyte are each porous to oxygen ions, and the first and second electrodes each comprise at least one active catalytic component for reversible ionisation of oxygen molecules to oxygen ions. The electrodes and sandwiched electrolyte enable transport of oxygen ions. The first and second electrodes each have at least one added electronically conductive component for the conduction of electrons between the electrodes and electronic circuitry of the sensor, and the active catalytic component and the electronically conductive component are each provided as particles interspersed with each other within at least the first electrode. In an embodiment, the active catalytic component and the electronically conductive component are each provided as particles interspersed with each other in a three-dimensional electrode structure, within each of the first and second electrodes. The electrode structure, formed with the active catalytic component and the electronically conductive component each provided as particles interspersed with each other in at least the first electrode, enables the use of materials for the active catalytic component which are less catalytically active for ionisation of oxygen molecules than has previously been considered necessary in solid-state electrochemical oxygen sensors, which enables selection of a catalytic component that is less catalytically active for burning combustible gases such as hydrocarbons. The invention enables a reduction in cross-sensitivity to combustible gases, which would otherwise lead to an inaccurate, lower oxygen reading. In various example sensors according to the invention, the catalytic component and the electronically conductive component are selected to be catalytically inactive for burning combustible gases, relative to their catalytic effect for reversible oxidation of oxygen molecules to oxygen ions. In particular, the catalytic and electronically conductive components are selected to be catalytically inactive for burning combustible gases relative to platinum and platinum alloys. The inventors have determined that electrodes that omit platinum and comprise bismuth or a bismuth compound or bismuth alloy as their active catalytic component achieve a better combination of catalytic effects within a relatively wide temperature range than electrodes containing platinum. In a first example sensor, the at least one active catalytic component of at least the first electrode comprises bismuth or a bismuth compound or bismuth alloy. The inventors have determined that these materials have a desired catalytic effect for ionisation of oxygen molecules across a relatively wide range of operating temperatures, but with a relatively low catalytic effect for the burning of combustible gases. The three-dimensional electrode structure with interspersed particulate components allows use of bismuth as a catalyst, despite bismuth sometimes being described as a suitable inhibitor for use in combination with more active catalysts. The first and second electrodes preferably have an equal composition, but this is not essential if it is known that negligible combustible gases are present in the reference gas relative to the oxygen partial pressure. Examples are given below where the at least one active catalytic component is selected to have reduced catalytic activity for the burning of combustible gases. In particular, an active catalytic component is selected to have reduced catalytic activity for the burning of combustible gases within a selected operating temperature range, relative to its catalytic activity for ionisation of oxygen, compared to a platinum-based electrode or other catalysts known for ionisation of oxygen at similar temperatures. The effect is that, when the sensor is used, there is incomplete combustion of combustible gases that are present, and therefore a reduction of oxygen measurement errors resulting from combustion. Bismuth, bismuth alloy or a bismuth compound are example materials for use as the active catalytic component that has reduced catalytic activity for burning of combustible gases. In some of the described examples, the at least one electrically conductive component of the first and second electrodes also has reduced catalytic activity for the burning of combustible gases, compared to a platinum-based electrode. Stabilised zirconia may be used as the electrolyte. In some examples, the reference-side electrode is not required to have reduced catalytic activity for burning of combustibles (e.g. if the reference gas does not contain combustible gases and / or if there is a high oxygen partial pressure relative to the combustible gases). The reference gas may have a known oxygen partial pressure, e.g. in a sensor based on a galvanic oxygen concentration cell. In an example potentiometric (voltage measuring) oxygen sensor such as a galvanic oxygen sensor, the reference gas has a known oxygen partial pressure and a measured voltage between the first and second electrodes is used to determine the oxygen partial pressure in the sample gas. Alternatively, in an ion pump sensor, a voltage between the first and second electrodes pumps oxygen molecules across the electrolyte via the reversible ionisation of oxygen molecules and transport of oxygen ions, and a measured electrical current is used to determine the oxygen partial pressure in the sample gas. Oxygen sensors that measure current flow are known as amperometric oxygen sensors. An example solid-state electrochemical sensor to measure oxygen partial pressure comprises: at least one solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas and a second electrode for exposure to a reference gas; wherein the first and second electrodes and the at least one solid-state electrolyte are each porous to oxygen ions, and the electrodes each comprise at least one active catalytic component for the reversible ionisation of oxygen molecules, wherein the at least one catalytic component of at least the first electrode comprises at least one of bismuth, a bismuth compound or a bismuth alloy, and the solid-state electrolyte enables transport of oxygen ions. In another example, there is provided a solid-state electrochemical sensor to measure oxygen partial pressure, comprising at least one solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas and a second electrode for exposure to a known oxygen partial pressure gas mixture, wherein the first and second electrodes are each porous to oxygen ions (and may also be porous to oxygen molecules as explained below); and the electrodes each comprise at least one active catalytic component for the reversible ionisation of oxygen molecules and transport of oxygen ions, wherein the at least one active catalytic component of at least the first electrode is selected to have reduced catalytic activity for the burning of combustible gases in the presence of oxygen within a selected operating temperature range, relative to its catalytic activity for ionisation of oxygen, as compared to platinum-based electrodes. The electrolyte is preferably maintained at an operating temperature in the temperature range between 300 and 1200 degrees centigrade, and preferably between 500 and 800 degrees centigrade for stabilised zirconia which exhibits Nernstian behaviour in the temperature range 500-800 degrees centrigrade. The electrodes may comprise a three-dimensional particulate structure that is porous to oxygen molecules, to provide a high surface area to enhance ionisation by the active catalytic component. In the case of low catalytic ionisation, a high surface-area three-dimensional electrode structure has the advantage that it provides an adequate number of ionisation sites for sufficient ionisation to occur. The solid-state electrolyte is porous to oxygen ions but not to oxygen molecules. The first and second electrodes preferably have an equivalent porous electrode composition and thickness. In example sensors according to the invention, the catalytic component comprises an alloy or compound of a post-transition group metal which is selected to have a high enough melting point for use at high temperatures (e.g. above 500 degrees centigrade), such as a bismuth oxide. However, other compounds and / or alloys of bismuth and / or other post-transition metals including compounds and / or alloys of aluminium and / or gallium and / or indium and / or tin can also be used in this invention. Although some lead and / or thallium and / or polonium compounds and / or alloys also have suitable catalytic properties, their use in commercial products is not preferred due to their known toxicity and environmental issues. One effect of use of the invention is that, when the inventive sensor is used, there is incomplete combustion of any combustible gases that are present, and therefore a reduction of oxygen measurement errors resulting from combustion. In an example, the sensor further comprises electronic circuitry for measuring a voltage between the first and second electrodes, and the electrodes also each comprise at least one electronically conductive component for the conduction of electrons between the electrodes and electronic circuitry of the sensor. The at least one electronically conductive component also preferably has reduced catalytic activity for the burning of combustible gases within the selected operating temperature range in the presence of oxygen. Such a sensor has reduced catalytic activity for the burning of combustible gases compared to a sensor working under similar ambient conditions that instead uses platinum as both the catalyst and electronically conductive components within the electrodes. In an example sensor according to the invention, incomplete combustion of combustible gases occurs even in the presence of excess oxygen, compared to stochiometric complete burn requirements, such as illustrated by exemplary equations 4 to 6. In an example sensor, the solid electrolyte comprises stabilised zirconia. In an example sensor, the stabilising component of the stabilised zirconia comprises at least one of yttria, hafnia, calcia, magnesia, ceria, scandia or alumina. In an example sensor, the stabilised zirconia comprises approximately 8mol% yttria stabilised zirconia. In an example sensor, the electrolyte comprises gadolinia and / or yttria doped ceria, gadolinia and / or yttria and / or erbia doped bismia or yttrium doped barium zirconate. In an example sensor, the active catalytic component contains powdered bismuth (III) oxide Bi20s and / or stabilised bismuth (III) oxide. In an example sensor, the at least one electronically conductive component of the first and second electrodes is sufficiently electronically conductive that the impedance of the sensor is dominated by the impedance of the solid electrolyte and / or electrode interface with the solidstate electrolyte. In an example sensor, the electronically conductive component comprises a metal or metal alloy wire and at least one of a powdered or deposited metal or metal alloy. In an example, metal and / or metal alloy wire that would be exposed to the sample gas is partially or fully covered by a low catalytic activity sheath and / or cover. In an example, the sheath and / or cover comprises at least one of a ceramic, glass or metal oxide. In an example, the electronically conductive component comprises at least one of gold, silver or gold or silver alloy. This avoids the problems associated with platinum group metals and their alloys, which are highly catalytic for the burning of combustible gases in the presence of oxygen. In an example sensor, the electrode additionally comprises an electrode stabilisation and / or ionic oxygen conduction component. This electrode stabilisation and / or ionic conduction component may comprise at least one of a ceramic, glass, metal oxide or solid-state electrolyte. In one example sensor, the stabilisation and / or ionic conduction component consists of at least one of bismuth (III) oxide, stabilised bismuth (III) oxide or stabilised zirconia. Note that bismuth (III) oxide and / or stabilised bismuth (III) oxide can be used as the catalyst and / or for stabilisation and / or ion conduction. In one example, stabilised zirconia is used as the ionic conduction component and the stabilised zirconia is approximately 8mol% yttria stabilised zirconia. An ionic conduction component comprising stabilised zirconia has a higher melting temperature than gold, its particles act as spacers between the interspersed particles of the catalytic component and the electronically conductive components, and its particles are porous to oxygen ions. Therefore, particles of stabilized zirconia (e.g. yttria-stabilized zirconia) help to maintain both the structural integrity and the ionic conduction of the first and second electrodes. In an example sensor, at least one electrode component comprises a powder or porous deposited structure. In an example, any powdered components are provided as powders with particle sizes in the range from lnm to 10mm. In an example sensor, the electrode components are mixed into a paste or ink and deposited onto the electrolyte surface. The paste or ink may comprise at least one of the electrode powdered components and a solvent or solvent mixture. The paste or ink may comprise at least one binder and / or flow modifying agent to enhance stabilisation and / or modify the flow properties of the paste or ink. An example sensor comprises a temperature monitor and heater for maintaining a working temperature to approximately conform with Nernstian behaviour. In an example sensor, the electrolyte comprises stabilised zirconia and is maintained at a controlled working temperature within the range 500 to 800 °C. In an example sensor, the solid electrolyte comprises yttria stabilised zirconia, the active catalytic electrode component comprises unstabilised and / or stabilised bismuth (III) oxide, and the working temperature is controlled to be between 500 and 800°C. In an example sensor, the electronically conductive component of the electrodes comprises sintered particulate gold and / or gold wire. In an example sensor, the catalytic component loading level is between 0.1 and 99.9% by mass of the electrode composition. In an example sensor comprising an electronically conductive component for the conduction of electrons, the electronically conductive component is between 0.1 and 99.9 % by mass of the electrode composition. In an example sensor, the first and second electrodes further comprise at least one spacer and / or adhesive component and / or ionic conductive component. In an example sensor, the thickness of the solid-state electrolyte sandwiched between the first and second electrodes is between 1 micron and 100mm. In an example sensor, the surface of the solid-state electrolyte which is in contact with the electrode is treated to enhance adhesion of the electrode and / or increase surface area of the interface contact between with the solid-state electrolyte and electrode. In an example, at least one keying-in feature is implemented to avoid delamination of the electrode at the electrode / solid-state electrolyte interface. In an example, surface roughening is implemented to avoid delamination of the electrode and / or enhance surface area at the electrode / solid-state electrolyte interface. In an example, the surface of the solid-state electrolyte is treated using at least one of bead-blasting, laser ablation, water-jet cutting, chemical etching or other suitable method. In an example, the at least one surface feature's dimensions after treatment are between lnm and 100mm. In an example sensor, the thickness of each of the first and second electrodes is between lnm and 100mm. In an example sensor, at least one electronically conductive component and a catalytically active component and an ionically-conductive spacer is pre-assembled and / or pre-formed as a porous structure and then either positioned and bonded onto the electrolyte interface to form an electrode / solid electrolytic interface or subsequently loaded with at least one additional component and then positioned and bonded onto the solid electrolyte interface to form an electrode / solid electrolytic interface. In an example, the at the least one additional component is at least one of the electronically conductive component and / or catalytically active component and / or spacer and / or adhesive component and / or ionically conductive component. The at least one additional component may be added from a solid and / or fluid phase, or the at least one additional component may be loaded via an intermediary compound or state and reduced thermally and / or chemically to the desired physical state and chemical composition. BRIEF DESCRIPTION OF DRAWINGS

[004] Apparatus and methods according to the invention are described below, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a Flow-Through Tube Sensor, Figure 2 shows a Test-Tube Sensor, Figure 3 shows a Disc Sensor, Figure 4 shows a Twin-Chamber Sensor, Figure 5 shows a Sealed-Reference Sensor, Figure 6 shows the Limiting Current Sensor, Figure 7A and 7B show the Fixed Volume Sensor (or Pump-Gauge Sensor), Figure 8 shows a schematic of a Twin Chamber Sensor design with more detail at electrode / electrolyte interface, Figure 9 shows a detailed implementation of the twin chamber cell used for the new electrode measurements, Figure 10 illustrates the results for four sensors made using a new electrode formulation containing Bi20s as the active catalytic component, Figure 11 illustrates the same results for the four sensors with a linear y axis of the measured mV signal and a log scale x axis of the oxygen concentration, Figure 12 illustrates the same results for the four sensors with a log scale y axis of the fitted Nernst equation derived oxygen concentrations and a log scale x axis of the applied oxygen concentrations, Figure 13 illustrates, for unit 1 from figures 10-12, the applicability of the Nernst equation linearisation even for low ppm concentrations, Figure 14 illustrates the performance of a sensor with a new electrode formulation in the extreme condition where 50ppm hydrogen is added to a background gas containing lOppm oxygen. Figure 15 illustrates the reaction of approximately 1 ppm of methane with lOppm oxygen, Figure 16 shows the reduction in oxygen signal for a high fixed hydrogen background for relative catalyst loading levels up to lOx the original loading level, Figure 17 illustrates the mV output for relative catalyst loading levels up to lOx by weight for a 3,000ppm oxygen in nitrogen sample mix and Figure 18 illustrates the results for an in-house time response test for relative catalyst loading levels up to lOx by weight. DETAILED DESCRIPTION

[005] In this patent specification, the following are described: • a device for measuring levels of oxygen, which is suitable for use where combustible gases may also be present, especially for low oxygen concentrations where trace combustibles are present; • components and materials for such a device; • a method of manufacturing components for such a device; and • methods of use of such a device. Solid-state electrochemical oxygen sensors make use of reversible catalytic ionisation of oxygen molecules and their diffusion through a solid electrolyte. Such sensors typically run at high temperatures (approximately, but not limited to greater than 500°C), although the optimum working temperature range will be dependent on the electrolyte and catalyst used, and different temperatures may be chosen for different sensor implementations (e.g. a high-surface area three-dimensional catalytic electrode structure at a given temperature may achieve ionisation as successfully as an electrode with a surface layer of catalyst at a higher temperature). The measurement is normally performed either in-situ, where it is either in or close-coupled to the process stream, or extractively, where a sample is drawn from a process and presented to the sensor outside the process stream - e.g. at a remote location. Close monitoring and / or control of the operating temperature is maintained to achieve accurate measurements, and this is normally achieved through at least one accurate temperature measurement and electrical heating means with feed-back control. The two main ways of driving the electrochemical cell are the galvanic concentration cell and ion pump cell. Various implementations of the measurement principle may be used, such as a flow through tube sensor, test tube sensor, disc sensor, twin chamber design and sealed reference design and ion pump cell, and implementations may include limiting current and fixed volume configurations. Example structural implementations are described below and are also shown and described in the International Electrotechnical Commission standards document IEC 61207-2. Exemplary implementations will first be used to help illustrate the Galvanic cell measurement principle, which is the most popular commercial implementation. Although there is a plethora of potential implementations for electrochemical sensors, most commercial sensors have two gas volumes, one of which contains a sample (or measure) gas and the other a reference gas containing oxygen, and these volumes are separated from each other by a solid electrolyte which is capable of allowing the diffusion of oxygen ions through its structure, whilst remaining non-porous to other gases. The solid electrolyte is sandwiched on each side by porous electrodes which contain a catalyst capable of reversibly creating oxygen ions. The reference gas contains a known partial pressure of oxygen and this may be air or other arbitrary oxygen partial pressure. A closely monitored and / or controlled high temperature (typically >500°C) is maintained so that the solid electrolyte and electrodes function with Nernstian-type behaviour. A voltage (EMF) will develop between the electrodes which will be related to the ratio of the oxygen partial pressures on each side of the solid electrolyte interface (equation 1) whereby the net passage of charge through the solid electrolyte is the diffusion of negatively charged oxygen ions due to the oxygen partial pressure difference either side of the boundary. Since the working temperature and reference oxygen partial pressure are known, the sample gas oxygen partial pressure may be calculated. This will be explained further in later pages of this Detailed Description. Note that it is the ratio of the partial pressures on each side of the solid electrolyte boundary which is important and not the relative mole fraction, thus if the sample gas contains half the mole fraction compared to the reference, but at twice the absolute pressure, this would give OmV across the solid electrolyte. Some sensor formats will now be illustrated: Figure 1 shows a Flow-Through Tube Sensor design. A hollow cylinder (tube) of solid electrolyte (101) allows the passage of sample gas (102) through the central region (103), whilst the gas surrounding the tube (104) contains a reference gas (such as air). The measured voltage (105) generated between the porous inner (106) and outer (107) electrodes is related to the ratio of the sample gas and reference gas oxygen partial pressures. Note that the sample gas and reference gas may be inter-changed, if desired, leading to an inversion of the voltage measured. A temperature sensor (108) is used to monitor the temperature and a heater (109) is used to reach the working temperature. Figure 2 shows a Test-Tube Sensor, where a solid electrolyte tube (201) is sealed at one end (202) and has a reference gas in the hollow inner section (203) and the sample gas (204) on the outside of the tube. Porous electrodes are present on the inner (205) and outer (206) surfaces and a voltage (207) is measured between the electrodes related to the ratio of the oxygen partial pressure of the sample and reference gases. This is a convenient format for a probe-type measurement, where the oxygen sensor may be placed in-situ in a sample flue or stream to measure the oxygen partial pressure. A temperature sensor (208) is used to monitor the temperature and a heater (209) is used to reach the working temperature. Figure 3 shows a Disc Sensor. A tube (301), which has a temperature coefficient of expansion that is approximately matched with the chosen solid electrolyte, is sealed at one end by a disc of solid electrolyte (302). The reference gas (303) is fed to the inside of the tube, whilst the sample gas (304) is present on the outside of the disc and tube. The voltage (305) measured between the porous electrodes on the inside (306) and outside (307) surfaces of the disc is related to the ratio of the partial pressures of the sample and reference gases. This is also suitable for an in-situ probe type measurement. A temperature sensor (308) is used to monitor the temperature and a heater (309) is used to reach the working temperature. Figure 4 shows a Twin-Chamber Sensor. The sample (401) and reference (402) gases are presented to either side of a solid electrolyte boundary (403). The sample (404) and reference (405) chambers may be flow and / or diffusionaIly fed by the respective gases. A voltage (406) is measured across the porous sample (407) and reference (408) electrodes. The heating is provided by a band heater (409) or equivalent, which provides an isothermal region around the interface and a temperature sensor (410) provides an accurate temperature reading. The voltage is related to the ratio of the partial pressures of the sample and reference gases. This is best suited for an extractive or close coupled design. Figure 5 shows a Sealed-Reference Sensor. In this case, the reference gas (501) is present in a sealed chamber (502) and the reference gas partial pressure is maintained by an equilibrium reaction between the oxygen gas in the sealed chamber and metal oxide powders (503). The chamber is sealed by a disc of solid electrolyte (505) separating the sample (504) and reference gas (501). The voltage (506) measured between the sample electrode (507) and reference electrode (508) is related to the ratio of the partial pressures of the measure and reference gases. A temperature sensor (509) is used to monitor the temperature and a heater (510) is used to reach the working temperature. The above examples of implementations of the galvanic oxygen concentration cell (gauge cell) use the voltage generated by an oxygen partial pressure difference either side of the solid electrolyte boundary. However, if direct current is made to flow (or pumped) through the solid electrolyte between reference and sample sides, the current passed through the solid electrolyte will be directly proportional to the number of mols 1 of oxygen ions passing through the solid electrolyte barrier. This behaviour may also be used to measure oxygen partial pressures and two exemplary implementations will now be illustrated for the ion pump cell. Figure 6 shows the Limiting Current Sensor design. A diffusion restriction (601) (such as a pinhole) limits the diffusion rate of sample oxygen molecules (602) to the measurement electrode (603) side of the solid electrolyte barrier, where a constant voltage (604) held between the measurement electrode and reference electrode (605) pumps all of the oxygen molecules arriving to the other side of the electrolyte. This leads to a locally low partial pressure of oxygen at the measurement electrode and hence the current flowing will be determined by rate of oxygen diffusion through the restriction, which, for a fixed restriction, will be directly proportional to the oxygen partial pressure in the sample gas. The direct current (606) measured will be directly proportional to the number of oxygen molecules transferred. A temperature sensor (607) is used to monitor the temperature and an electrical heater (608) is used to maintain the sensor at the desired working temperature. Figure 7A and 7B show the Fixed Volume Sensor (or Pump-Gauge Sensor) design. This configuration is more complex, with two sets of electrode pairs (701 and 702) arranged across a solid electrolyte barrier (703) sealing in a small, fixed volume (704). The first set (701) comprises an oxygen partial pressure measurement cell (gauge cell), which can measure the ratio of oxygen partial pressures inside and outside the fixed volume, whilst the second pair of electrodes (702) comprise an oxygen pump. The oxygen ion pump initially pumps out the oxygen within the fixed volume to a pre-determined low level (Figure 7A) and then the pump action is reversed and oxygen ions pumped into the fixed volume (Figure 7B) until the gauge cell detects that equal oxygen partial pressure has been achieved between the inside volume and the outside sample gas (705). The integrated value of the current (706) and time will be directly proportional to number of oxygen molecules transferred and this in turn will be related to the oxygen partial pressure in the sample gas. A temperature sensor for monitoring the temperature (707) and heater (708) for maintaining the sensor at working temperature are also present. This patent specification includes results which have been obtained using a twin chamber design configuration (Figure 4), but the described methods, systems and novel electrode formulations are equally applicable for many other suitable configurations and no limitations in implementation of the principles of the described methods and systems are implied by these illustrations. The solid electrolyte may typically be zirconia containing a tri-valent dopant, such as yttria, but other suitable systems and dopant materials may be used, such as gadolinia and / or yttria doped ceria, gadolinia and / or yttria doped bismia or yttrium doped barium zirconate. Figure 8 shows a schematic of a Twin Chamber Sensor design with more detail at electrode / electrolyte interface. The solid electrolyte (801) creates the sample gas (802) and reference gas (803) chambers as well as forming the wall between the sample and reference sides. The electrodes (804) on each side of the solid electrolyte wall are shown in more detail in the expanded image in the top righthand corner. The electrode typically comprises a catalytic component (805), an electronically conductive component (806) and potentially at least one additional component whose function may be to act as an oxygen ion conductor and / or spacer element and / or adhesive component (807) and which may be comprised of more than one material. These components are present as particles or powders interspersed with each other within the electrode structure and may have a distribution of shapes and sizes. Ideally, the electrode structure retains sufficient porosity to allow relatively unimpeded diffusion of oxygen throughout the electrode. The electrode catalytic activity, which will be influenced by surface area, shape, porosity and working temperature of the catalytic component will ideally be sufficient to give a high reaction rate for the reversible ionisation of the oxygen molecules. The electrode electronic conductivity component should also ideally be sufficiently conductive such that the impedance of the circuit is dominated by that of the solid electrolyte and / or the solid electrolyte / electrode interfaces. Sufficient ionic conductivity function may take place via the catalytic component alone, or via a combination of the catalytic and electronic conductive components, but an additional ionic conductive component may be beneficial to allow relatively unimpeded passage of oxygen ions to and from the solid electrolyte / electrode interface. An additional spacer component may be present to prevent excessive sintering and / or maintain electrode porosity. An additional adhesive component may be present to enhance structural stability within the body of the electrode and / or prevent de-lamination of the electrode layer from the solid electrolyte / electrode interface. The electronically conductive electrode component may additionally comprise at least one metal or metal alloy wire (808) embedded or adhered to the electrode to conduct electrons to and from the peripheral circuitry as well as the powdered metal and / or metal alloy within the electrode matrix itself. As described earlier, an output voltage (809) will be generated by the passage of oxygen ions through the solid electrolyte barrier between the sample and reference side electrodes, which will be related to the ratio of the partial pressures of oxygen in the sample and references gases. The sensor reaches stable working temperature via an electrical heater (810) and feedback control via monitoring with a temperature sensor (811). Typically, an identical porous electrode composition is present on both sides of the zirconia interface (boundary). An identical electrode composition for both sides of the electrolyte boundary has advantages, since symmetry for any diffusion limiting stages and catalytic activation energies will mean that when the measure and reference sides have identical partial pressures of oxygen, the sensor will be in equilibrium (EMFSampie=0V from equation 1) and where the oxygen partial pressure differs, the voltage output will be given by the Nernst equation. In practice, the electrode compositions and topology may only be approximately identical and, for example, the electrodes may experience only approximately the same ambient temperature and / or a different temperature at the interface than the set-point temperature and so modifications may be applied to the idealised equation 1. One potential solution is to apply a corrective offset voltage and / or offset temperature setting and this is illustrated in equation 2: Z7 A# 17 _ Z7 I RlT+Tcorr] i P°2ref , ~» EMFSample Ecorr + / 11 (2) where ECOrr is the asymmetry / offset voltage and TCOrr is the asymmetry / offset temperature correction. In another example, different electrode compositions could be used on different sides of the electrolyte boundary, for example using a different catalyst (e.g. a known platinumbased catalyst) on the reference side of the boundary in a sensor in which combustible gases are not present on the reference side and / or if there is a high oxygen partial pressure relative to the combustible gases on the reference side of the boundary. It is possible to compensate for electrode compositions that are significantly different on each side of the boundary. If composition, topology, working temperature and / or other factors create differences on each side of the boundary, and equations 1 or 2 do not adequately describe the device behaviour, then theoretically and / or empirically derived output models and / or algorithms can be employed, such as a polynomial instead of or in conjunction with equation 1 or 2. The porous electrode must contain electrically conductive materials to conduct the flow of charge (electrons) to and from the external associated electronic circuitry as well as a catalyst to reversibly convert the oxygen molecule / oxygen ion, as illustrated in equation 3: 02 + 4e“ o 2O2~                                      (3) where e- represents an electron. The 4 electrons exchanged in this reversible reaction is why the value of z is 4 in equations 1 and 2. When the gaseous mixture contains gases capable of being oxidised (combusted or burnt), then the oxygen content measured will be lower than if the gaseous mixture did not contain combustible gases. This is due to the combustible gases consuming oxygen as they burn. Some examples are given below. The hydrogen combustion reaction is given by equation 4: 1 H2+-02^H20                               (4) where each hydrogen molecule burns half an oxygen molecule. Another example is shown in equation 5 for the presence of carbon monoxide: 1 CO + - O2 -> CO2 (5) where each molecule of CO consumes half a molecule of oxygen. The case of the presence in hydrocarbons is illustrated in equation 6: CxHy + (x + ^)O2 -» xC02 + h2o (6) where CxHy is a hydrocarbon molecule containing x atoms of carbon and y atoms of hydrogen. Note that each molecule of hydrocarbon will consume (x+y / 4) molecules of oxygen. The actual measured reduction in the oxygen partial pressure will be affected by amongst other factors the catalyst composition and topology, ambient temperature and diffusional rates of the gases involved and may not exactly match the stochiometric ratio of these equations and these are therefore indicative of the effects expected. If oxygen is being measured in the percentage range (10,000s of ppm) and the combustibles are present at a few 10s or low 100s of ppm (parts per million), which will typically be the case in an efficient combustion process, the oxygen measurement inaccuracy may be acceptable. Similarly, if the reference gas has very low combustibles with high oxygen concentration, this will have negligible impact on the measurement accuracy. However, for trace oxygen measurements in the ppm or lower range, even ppm levels of background combustibles will have a severe impact on the measurement accuracy. This could be an important measurement in the production and / or distribution of high purity gases where oxygen would be a contaminant. For example, high purity nitrogen and argon production may be used in semiconductor device fabrication and may be produced by industrial cryogenic air separation units (ASUs). The presence of oxygen in these high purity gases could affect the yield and / or functioning of the manufactured semiconductor devices. Since oxygen is the second largest constituent of air after nitrogen, trace contamination from the air separation process or from air leaks in pipework or containers may lead to trace contamination of the high purity gases with oxygen. An example measurement device comprises a solid-state electrochemical sensor to measure oxygen partial pressure in a gas sample. The device is maintained at a known and / or controlled temperature when in use. The device comprises a solid-state electrolyte sandwiched between a first (measure side) electrode exposed to a sample gas and a second (reference side) electrode exposed to a known oxygen partial pressure gas mixture. The first and second (measure side and reference side) electrodes are porous to oxygen molecules and / or oxygen ions. The electrodes comprise at least one active catalytic component for the reversible ionisation of oxygen molecules and transport of oxygen ions, and the at least one active catalytic component has reduced catalytic activity for the burning of combustible gases relative to its catalytic activity for ionisation of oxygen, compared to using standard platinum electrodes. Reduced catalytic activity towards the burning of combustible gases would be indicated by incomplete combustion of the combustible gases present, even when excess oxygen required for combustion is present. This can be illustrated by the measurement of identical partial pressure oxygen test samples with and without combustible gases present and comparing the reduction in measured oxygen partial pressure with what would be expected for complete combustion with regard to the type and partial pressure of combustible gases present (with reference to equations 4-6, for example). Additionally and / or alternatively, the benefits of the invention can be illustrated by a comparison with a sensor using platinum electrodes, such as typically employed in zirconia-based oxygen sensors. Platinum electrodes are highly efficient at burning combustible gases under normal working conditions and so the oxygen partial pressure measured with the present-invention electrodes could be compared with that measured with the platinum electrode device when combustible gases were present. A higher oxygen measurement using the electrodes of the present invention compared with the platinum-based sensor illustrates the fact that the catalytic activity for the burning of combustible gases is reduced by the electrodes of the invention. The sample gas and reference gas side electrodes each have at least one electronically conductive component for the conduction of electrons between the device and any associated external electronic circuitry. The at least one electronically conductive component also has reduced catalytic activity for the burning of combustible gases which may be present. In one example device, at least one active catalytic component has intrinsic reduced catalytic activity for the burning of combustible gases. As described earlier, reduced catalytic activity is where the combustible gas burn is incomplete and / or reduced from that expected from the combination of available oxygen and combustible gases at high temperature in the presence of a catalyst such as illustrated by example equations 4, 5, and 6. For example, platinum electrodes under normal high temperature working conditions would approximately burn as much of the combustible gases present as the available oxygen would support - this could mean, in some instances, that approximately all of the oxygen would be consumed, if insufficient oxygen were present to sustain full combustion of the combustible gases present. An example method for determining the reduction of catalytic activity for the burning of combustible gases may be assessed by performing otherwise identical measurements with the same level of oxygen with and without the presence of combustible gas and comparing the signals for standard platinum and present-invention electrode formulations. The reduction in catalytic activity would be evidenced by the electrodes with the present-invention electrode formulation having a measurably higher oxygen measurement with the presence of combustible gases that a platinum electrode formulation. Where the oxygen partial pressure exceeds that required for complete combustion of any combustible gases present, the relative reduction in catalytic activity of the new electrode formulation may be quantified, for example, by the ratio of the reduction in oxygen level with the new present-invention electrode formulation divided by the reduction in oxygen level with a standard platinum formulation. For example, if oxygen is present in the sample gas and methane is added to the mixture and the signal falls by 5ppm with a platinum electrode formulation, but only falls by lppm for the new present-invention electrode formulation, then the relative ratio of catalytic activity would be 0.2 (=lppm / 5ppm), i.e. an 80% reduction in catalytic activity. A ratio value of less than 1 would signify a reduced catalytic burn activity and the closer the ratio is to zero, the more invariant the oxygen measurement would be to the presence of combustible gases. This is illustrated in equation 7: where Rca is the reduced catalytic activity ratio, Bnf is the consumed oxygen in the burn with the new present-invention formulation and Bpf is the oxygen consumed in the burn with a platinum electrode formulation. Bnf may be expressed as: ^NF ~ [^2NoC“ 02c]nF (8) where O2Noc is the measured oxygen signal with no combustibles present and O2C is the measured oxygen signal with the same level of oxygen, but with combustible gases present. The square brackets with lower case NF refers to the fact that this is for the new electrode formulation. Bpf may be expressed as: BpF — [^NoC- 02c]pF (9) where O2Noc is the measured oxygen signal with no combustibles present and O2C is the measured oxygen signal with the same level of oxygen, but with combustible gases present. The square brackets with lower case PF refers to the fact that this is for the platinum electrode formulation. In one example device, at least one active catalytic component contains at least one compound of bismuth. In one example device, the solid electrolyte comprises partially or fully stabilised zirconia. In one example device, the stabilising component of the zirconia solid electrolyte comprises at least one of yttria, hafnia, calcia, magnesia, ceria, scandia or alumina. In one example device, the stabilised zirconia electrolyte comprises approximately 8mol% yttria stabilised zirconia (YSZ). YSZ has high oxygen ion conductivity and is stable at high temperatures. In one example device, the electrolyte is gadolinia and / or yttria doped ceria, gadolinia and / or yttria and / or erbia doped bismia, or yttrium doped barium zirconate. In one example device, the active catalytic component contains powdered bismuth (III) oxide (Bi20s) and / or stabilised bismuth (III) oxide. In one example device, the electronically conductive component comprises a metal or metal alloy wire and at least one of a powdered or deposited metal or metal alloy. In one example device, the metal and / or metal alloy wire exposed to the sample gas is partially or fully covered by a low catalytic activity sheath and / or cover. In one example device, the sheath and / or cover comprises at least one of a ceramic, glass or metal oxide. In one example device, the electronically conductive component comprises at least one of gold, silver or gold alloy. In one example device, the electrode additionally comprises an electrode stabilisation and / or ionic oxygen conduction component. In one example device, the electrode stabilisation and / or ionic conduction component consists of at least one of a ceramic, glass, metal oxide or solid-state electrolyte. In one example device, the stabilisation and / or ionic conduction component consists of at least one of bismuth (III) oxide, stabilised bismuth (III) oxide or stabilised zirconia. In one example device, the stabilisation and / or ionic conduction component consists of fully stabilised cubic zirconia and where the stabilised zirconia component is approximately 8 mol% yttria stabilised zirconia. Zirconia is also available with other doping levels, but an approximate 8 mol% doping with yttria provides effective stabilisation. In one example device, at least one electrode component comprises a powder or porous deposited structure. In one example device, the powdered components are powders with particle sizes in the range from lnm to 10mm. A device where the electrode components are mixed into a paste or ink and deposited onto the electrolyte surface. A device where the paste or ink consists at least of the electrode powdered components and a solvent or solvent mixture. A device where binder and / or flow modifying agents are added to paste or ink to enhance stabilisation and / or modify the flow properties of the paste or ink. Binders enhance the ability of particulates to remain integrated within the paste or ink, without depositing out of the matrix and flow modifiers may be used to adjust the viscosity of the paste or ink to reduce or increase viscosity and / or induce thixotropic qualities, which may enhance the ease of application and / or yield of the electrode formulation by hand, thick film printing or suitable automated or semiautomated method. When manufacturing an example sensor device as described herein, a multi-stage thermal firing cycle is used to establish and / or stabilise and / or adhere the electrode structure, through evaporation and / or chemical breakdown and / or sintering and / or melting. The maximum temperature of the thermal cycle is kept below the melting temperature of the catalytic and conductive electrode components that are required for operation of the sensor. Thermal firing cycles may be used, typically in conjunction with pastes or ink, to eliminate superfluous materials such as solvents, binders and flow modifiers through thermal break down and / or evaporation and may condition components and / or break down precursor chemicals, where appropriate, to leave only the required constituents of the electrode. The thermal cycles may additionally through localised sintering and / or annealing create long-term physical and chemical stabilisation of a porous, catalytically active structure. An example method of manufacture including a multistage firing cycle is described below. For an example sensor as described, at least one of the electronically conductive component and / or catalytically active component and / or spacer and / or adhesive component and / or ionically conductive component can be pre-assembled and / or pre-formed as a porous structure and then either positioned and bonded onto the electrolyte interface to form an electrode / solid electrolytic interface and / or subsequently loaded with at least one additional component and then positioned and bonded onto the solid electrolyte interface to form an electrode / solid electrolytic interface. The additional component may be an electronically conductive component and / or catalytically active component and / or spacer and / or adhesive component and / or ionically conductive component. For the configuration and use of a sensor as described herein, the working temperature is chosen to approximately conform with Nernstian behaviour, with a logarithmic response as per equations 1 and 2. This will require maintenance of the operating temperature to fall within an agreed range. For example, a minimum lower temperature (solid electrolyte composition dependent) may be 500°C, with temperatures above 500°C typically being suitable for the ionisation to occur in the presence of the catalytic component. The upper temperature limit will be limited by material composition of the solid electrolyte (such as ionic behaviour and crystalline stabilisation) and catalyst and the properties of the surrounding embodiment to the solid-state electrolyte, such as the integrity of glass-to-metal / ceramic seals or thermal sheathing. For a device as described herein, the Nernst equation (equation 1) or a modified Nernst equation (equation 2) can be used to calibrate and / or calculate the oxygen partial pressure on the sample side. Equation 1 is the special case of equation 2, where the values of ECOrr and TCOrr are zero. The values of ECOrr and TCOrr may be determined experimentally by sequentially applying two known oxygen partial pressure sample gases and using simultaneous equations in equation 2 to determine their values. This is shown in equations 10 and 11 below, where, for convenience, the natural logarithm ratio has been expanded: EMFcau — Ecorr + [lnpO2ref lnpO2caZl] (10) and 5 EMFcai2 — Ecorr H — [lnp02ref — lnp@2cal2] (H) where EMFcaii and EMFcai2 are the measured output voltages corresponding to sample calibration gas partial pressures pO2caii and pO2Cai2 respectively. Equation 11 minus equation 10 gives: EMFcal2 - EMFcall = [in po2call _ ln pO2cal2\ (12) Zr 10 which by re-arrangement enables the calculation of TCOrr: i _ ZF\EMF Cai2 EMFCgii] corr R[\npO2Cali~ lnP^2cal2] (13) Once Tcorr is known, Ecorr may be calculated by re-arrangement of equation 10 or 11 to give: ECOrr ~ EMFcall zF [^nP^2ref ^nP^2call] (14) Alternatively, in a simpler arrangement, the ECOrr factor may be determined from equation 2 by 15 applying an identical reference oxygen partial pressure to the sample side and measuring the voltage output signal. This is shown in equation 15 below: EMF f=E +MT+Tcorr] po2ref CMrref n^rr zp tn po2rpf corr corr (15) where EMFref represents the voltage when the reference gas is applied to the sample side. If a second, known oxygen concentration is then applied to the sample side, the TCOrr term may then be determined by re-arrangement of equation 2: corr zF [EMFcai E corrl -T (16) P°2cal. where EMFcai and pO2Cai are the output voltage and oxygen partial pressure respectively when a known calibration oxygen partial pressure is applied to the sample side electrode. Once Ecorr and TCOrr are determined then any sample oxygen partial pressure may be calculated from the voltage signal. This may be illustrated by a re-arrangement of equation 2: ( zF [EMFsampie — Psample ~ P^2ref^Py [T+T ] j (17) Thus, the invention provides a method of measuring oxygen partial pressure using a sensor as described within this patent specification, wherein the Nernst equation or a modified Nernst equation is used to calibrate and / or calculate the oxygen partial pressure on the sample side. The invention provides a method of measuring oxygen partial pressure using a sensor according to any one of the claims of this patent specification, wherein a theoretical and / or empirical algorithm is used to calculate the oxygen partial pressure on the sample side. This may be used in the case where equations (10) to (17) are insufficient to accurately mathematically describe the system voltage output and may involve a polynomial equation either alone or in conjunction with the Nernst or modified Nernst equation. An example of such as equation is shown in equation 18 below: ( zp P^2sample — P^2ref^-^P ' — „rT,T fallEMFcal Ecorr] + KL2 "*■ 1corrl k a2[EMFcat - Ecorr\ + FLn[EMFcal - ECOrr]nH              (18) where ai, 32 and an are variables for the best fit solution for the power series in the inner brackets. In an example device, the working temperature is chosen to have required power usage, measurement sensitivity, material structural stability and reduced sensitivity to background combustible gases. In some devices the available power supply may be limited and / or for ecological or other reasons the power supply may be desired to be below a certain threshold. Since the device requires a high working temperature, radiation, natural and forced convection losses and thermal conduction losses will be significant and these are all dependent on the working temperature. The thermal losses may be reduced by design considerations, such as using materials with low thermal conductivities and thermal insulation, however, a lower set-point temperature will also reduce thermal losses. The advantage of maintaining a lower working temperature (although still sufficient to allow ionic conductivity through the solid electrolyte) can be weighed against the disadvantage of the reduction in the generated measured voltage (see equations 1 and 2) and hence the decreased signal-to-noise ratio. In some cases, raising the working temperature too high may result in a permanent or reversible crystalline phase change in the solid electrolyte which may be undesirable due to changing electrolytic properties and / or any accompanying physical dimensional changes may damage the device structure, for example, potentially leading to gas leaks or circuit rupture. The efficacy in the suppression or reduction of catalytic activity of the novel electrode material towards combustible gases may be decreased by increasing working temperature since the gas molecules will have more energy available for a combustion reaction. A more detailed description will now be made of the electrode and electrolyte structure and principles behind the presented invention. Pure zirconia is present in three different crystalline forms: monoclinic, tetragonal and cubic. Pure zirconia only has limited oxygen ion conductivity properties and crystalline structural instability may cause mechanical stress within the structure. Hence some form of crystalline stabilisation and increased ionic conductivity is normally required. Zirconia may be stabilised in various crystal forms by adding a dopant, which substitutes for some of the zirconium ions throughout the lattice. The type of dopant and loading level needed for partial or full stabilisation in a particular crystal format will be application dependent, and a dopant may not be required for some electrolyte materials. For example, zirconia may be partially stabilised by adding yttria as a dopant and is fully stabilised in the cubic format across a wide range of ambient conditions and temperatures when 8 mol% yttria is added as a dopant and this is the format used where experimental results are presented within this patent specification unless otherwise stated. This doping by yttria also results in a large increase in the ionic conductivity of the zirconia. However, the principles of this invention are not limited to this composition and configuration of solid electrolyte; and partially stabilised zirconia and / or alternative electrolytes may also be used, in appropriate sensor implementations. The yttrium ions within the zirconia lattice have a slightly higher ionic radius than zirconium and also have a valency of 3+ instead of 4+. Other stabilisation systems of zirconia may be achieved, such as by the addition of at least one of yttria, hafnia, calcia, magnesia, ceria, scandia or alumina. The addition of yttria (or other suitable dopant(s)) to pure zirconia generates oxygen vacancies within the lattice, due to the lower valency value of the yttrium ion relative to zirconium ion. These vacancies greatly increase the ionic conductivity properties of doped zirconia compared to pure zirconia and enable the propagation of oxygen ions through the structure and make the material suitable for a solidstate electrolytic oxygen sensor. In addition to doping, zirconia's use as a solid electrolytic material for oxygen sensing requires a controlled high temperature to be maintained for efficient operation, preferably under approximately Nernstian behaviour conditions (equations 1 and 2) and a catalyst to reversibly generate the oxygen ions which then penetrate into the electrolytic structure. The solid electrolyte may typically be zirconia containing a tri-valent dopant, such as yttria, but other suitable systems and dopant materials may be used, such as gadolinia and / or yttria doped ceria, gadolinia and / or yttria doped bismia or yttrium doped barium zirconate and this invention is not limited to a zirconia electrolyte. Ideally, the electrode on either side of the solid electrolytic barrier should be approximately identical (composition, size and structure) and held at the same temperature and should perform several functions simultaneously: • Have a porous structure to allow the gas to diffuse to / from the interface. • Have a stable structure over time and over gas sample / temperature changes. • React catalytically to reversibly convert the oxygen molecules to oxygen ions. • Be able to provide electronic conductivity and connect to the associated external electronic circuitry. • Be able to transport and inject / receive oxygen ions to / from the interface. • Be able to mechanically inter-connect within its own structure and also with the interface. The electrodes may be composed of several different materials. A suitable electrode composition for systems and methods according to the present invention contains the following: • A catalytic component: to reversibly convert oxygen molecules into ions - this may typically be at least one of bismuth metal, bismuth oxide, bismuth compound or a mixture of these, and may be present as a raw powder, pre-loaded onto a ceramic carrier or a deposited layer. The reversible generation of oxygen ions and molecules is crucial to the functioning of the oxygen sensor. The bivalent negatively charged oxygen ions travelling across the electrolytic barrier, in conjunction with the externally connected electronic electrodes, complete the electrical circuit and generate the measured output voltage. • An ionic conductive / diffusional component: to allow oxygen molecules and / or ions to move to / from the interface with the solid electrolyte - this functionality may be provided by the catalytic component and / or by another component, including a powdered solid-state electrolyte. The oxygen ions need to be able to reach the electrolytic interface in order to penetrate into the electrolyte and hence diffuse through the barrier and any reduction or inability for this to occur could have a negative impact on the measured signal. Equally, the oxygen ions migrating out through the electrolyte must be able to recombine into oxygen molecules and diffuse away, such that a dynamic equilibrium is maintained under steady state conditions on both sides of the electrolytic barrier. • An electronically conductive / diffusional component: which may be composed of at least one metal, metal compound or alloy. The interface between the electrolyte and the external circuitry is completed by electronic electrodes, since the oxygen ions are only conducting charge across the electrolytic barrier, whilst the charge carriers to complete the rest of the circuit are electrons and need a medium to travel through. The electrode may also potentially contain one or both of the following components: • Adhesive component(s): to adhere within the electrode itself to enhance internal structural strength within the depth of the electrode and at the interface with the solid electrolyte to avoid de-lamination of the electrode structure from the electrolytic interface. This component might be at least one of a glass frit, metal, metal oxide or ceramic component. • A stabilising component: this may be required to maintain a stable electrode structure over time, such as a ceramic spacer to inhibit ongoing sintering between electrode particles. Ongoing sintering within the electrode structure may decrease the exposed catalytic surface area and / or the porosity of the electrode, both of which may have a negative impact on the device performance, since it may impede the reversible production of oxygen ions and / or their diffusion to / from the electrolytic interface. At least one of these components may perform more than one function. In practice, the electrode material mix may be placed in a suitable vehicle to convert it into an ink or paste to enable its application onto the solid-state electrolyte. This could allow it to be printed as a thick film ink, applied by hand under a microscope or deposited by an automated or semi-automated process. In systems and methods described herein, the electrode material mix preferably includes: • active components, as listed above; and • a carrier component, typically an organic solvent or solvent mixture, potentially with a binder component, to maintain the paste / ink consistency and inhibit the solids separating out from the liquid phase. Additionally, flow modifying agents may be used to make the paste / ink more or less viscous and / or to introduce thixotropic qualities to the flow. After application, it is advantageous for the paste / ink to go through a multi-stage firing cycle including: • Burn out volatile / organic components stage: The initial drying and / or firing stages will evaporate and / or break down any solvent(s), binder(s) and flow modifying agents. • Sintering stage: This is to enable sintering (localised melting and adhesion between particles) to occur, creating a porous two or three-dimensional electrode matrix. • Annealing / stabilisation stage: This is to stabilise the electrode structure so that it will be able to cope with long-term, high-temperature operation with relatively low changes to its physical and chemical structure and properties. It is important that the carrier phase is fully burnt out as this may cause contamination of the catalyst matrix. The sintering stage should be at high enough temperature and duration to allow sufficient adhesion to occur within the particles and this could include sintering between catalyst particles and / or ionic conductor particles and / or electronically conductive particles and / or glass frit and / or ceramic spacer particles. However, the sintering stage should avoid generalised melting and / or excessive loss of catalytic surface area and porosity. The annealing / stabilisation stage, which may be integrated within the sintering stage, is to stabilise the structure for the desired application, but still to enable sufficient porosity, catalytic area and catalytic activity to occur. Sufficient porosity, catalytic area and catalytic activity means that these factors will not create a significant restriction in the diffusional rate and generational rate of oxygen ions and molecules within the electrode and hence the dynamic equilibrium at the electrolytic interface will ideally be dominated by the oxygen partial pressure gradient across the electrolytic boundary and not overly influenced or dominated by these secondary factors. Oxygen ion mobility may take place across the surface and / or within an electrode component, such as the catalyst and / or powdered electrolyte and / or electronic conductive component. Amongst other factors, the catalytic activity for any individual catalyst will be affected by the loading level, surface area, particle size, shape and any interaction with the carrier particles and / or interfaces with the surrounding material / particles. For example, finer particles will have higher surface area for the same mass loading than coarser particles and sharp features will have higher surface energy than blunt features. Any carrier and / or interface surface may put lattice strain and / or affect the surface energy of the deposited catalyst. Typical catalysts, such as precious metals and transition metals and their compounds may be used in a powdered form directly or may be deposited on a suitable carrier such as a ceramic like alumina or stabilised zirconia. Platinum group metals could also be used as the catalyst for the electrode that is to be exposed to the reference gas, if that is known to include negligible combustible gases. Depositing a catalyst on a ceramic carrier (typically at a few % by weight loading) may reduce production cost, whilst simultaneously maintaining a high surface area of exposed catalyst. A ceramic carrier for the catalyst may also reduce the likelihood of excessive sintering, due to the high melting temperature of ceramics, but will also decrease electronic conductivity, if the catalyst is being used additionally as an electronic conductor. A higher surface energy catalyst may be able to initiate catalytic reactions at a lower temperature and / or at faster rate than the same catalyst with a lower surface energy. Before a chemical reaction may occur, there is normally an energy barrier to overcome called the activation energy. If the average energy of reactant molecules is insufficient to overcome this activation energy barrier then only a very slow reaction rate will occur. A catalyst may offer an alternative reactive route from reactants to products with a lower activation energy which results in an increased reaction rate. The reaction rate k(t) is described by the Arrhenius equation: k(t) = Aexp(-^ (19) where A is a reaction constant, Ea is the activation energy, R is the gas constant and T is the ambient temperature in Kelvin. It can be seen from equation (19) that if the activation energy is reduced, the reaction rate will be increased at a given temperature up to the point where the reaction becomes diffusion limited. The extent of the reduction in activation energy will depend amongst other factors on the chemical reaction, the catalyst used, the catalytic surface energy and the catalytic surface area and accessibility. The catalytic surface energy will be influenced by the size and shape of the catalyst particles and small sharp particles will have higher surface energy per unit area compared to a blunt bulk catalyst. The surface energy and / or surface area of the catalyst may be increased by using a finely divided catalyst (fine powder or even nanoparticles) or a catalyst which has been deposited and / or distributed on the surface of a carrier ceramic, such as used in catalytic car exhaust systems. An important property of a catalyst is that it remains unchanged at the end of the reaction and is therefore able to initiate many consecutive and ongoing catalytic reactions. Under ideal conditions, the choice of catalyst, surface area, surface energy and porosity at the given working temperature will be such that the desired reaction(s) will occur at very high efficiency and therefore the reaction rate is only limited by the free diffusion rates of the reactants to and products from the catalytic surface. Within a gas mixture, there will be a distribution of molecular energies whether they are free or adsorbed / absorbed in / on the electrode. Dependent on the ambient temperature, this may mean that even in cases where the average energy is significantly below the activation energy, the reaction may still take place, but at a very slow rate. The use of a catalyst may mean that the average energy of the gas molecules is sufficient to allow the reaction to occur extremely efficiently and the reaction rate then becomes limited by the diffusion rate of the gas to / from the catalytic surface (constant A in equation 19). In the case of using a solid-state electrolyte to measure oxygen, a catalyst and a high operating temperature is required for a reasonable signal to be acquired. Some materials, especially metals, have been recognised to be effective to ionise the oxygen molecules and there are commercially available sensors using a platinum catalyst-based electrode in conjunction with a stabilised zirconia solid-state electrolyte. Since it is a metal, platinum can advantageously fulfil the dual roles of catalyst and electronic conductor simultaneously. However, such catalysts are also very effective at oxidising combustible gases such as hydrogen, carbon monoxide and hydrocarbons in the presence of oxygen at high temperature. For example, platinum group metal catalysts may be used to measure combustible gases in pellister-type applications and for oxidising combustible gases in catalytic automotive exhaust systems. As previously described, this would cause significant measurement errors when making trace oxygen measurements in the ppm or lower range, even if the combustible gases were themselves only present in trace amounts. The inventors of the present invention have determined that it is possible to develop a solid-state electrochemical oxygen sensor with electrodes comprising a catalyst that will be sufficiently active to reversibly ionise the oxygen molecules, whilst having a reduced catalytic effect with regard to the combustible gases. The formation of oxygen ions on the surface of the catalyst may of itself be a facilitation of a combustible reaction, so that the adsorption and interaction of the corresponding combustible molecules on the catalyst should ideally either be so weak that the combustible reaction is unlikely to occur or so strong that the molecule is tightly bound to the surface and also unlikely to react with the oxygen molecules / ions. Some metals, either alone or in compounds, have the ability to inter-change between oxidation states by donating and receiving electrons and this can make them effective catalysts. Traditional "good" catalysts, such as various transition or platinum group metals or metallic compounds are used in many catalysed industrial processes, but they can also be very effective at promoting undesired side reactions, such as previously described for platinum above, and this approach is therefore unlikely to achieve the desired outcome. The inventors have identified potential options for selective de-activation of specific catalytic pathways for particular catalysts, for example through focussed inhibition or poisoning. This can mean, for example, that at least one metal and / or metal alloy and / or metal compound from at least one transition metal could be used with the appropriate catalytic de-activation in place. However, although this approach might be able achieve some inhibition of catalytic pathways, it has extra complexity and limited applicability compared to the use of a catalyst with low intrinsic catalytic activity for burning of combustible gases. In addition, some traditional de-activation chemicals may contain at least one of a heavy metal and / or heavy metal compound and / or other compound, where toxicity and / or industrial restrictions may have to taken into account such as REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) and RoHS (Restriction of Hazardous Substances) register of restricted materials. As an alternative to selective de-activation of traditional over-active catalysts, the inventors have identified and selected a suitable catalytic element and compounds of which have intrinsically low-activity for combustion and therefore are able to ionise the oxygen molecules with relatively little cross-reactivity to combustible gases. The identified and preferentially selected catalysts include bismuth, atomic number 83, and compounds of bismuth. Unlike metallic platinum, metallic bismuth has a relatively low melting point of 271°C, which makes it unsuitable for use as the catalyst and / or electronic conductive material in the electrode for typical working temperatures above 500°C, although it may be suitable with an appropriate electrolyte whose functional temperature range begins below the bismuth melting temperature. Many compounds of bismuth have elevated melting points compared to the bismuth metal and also exhibit sufficient catalytic properties for reversible oxygen ion conversion, but bismuth (III) oxide has been selected as a preferred catalyst because of its exceptionally high relative melting temperature, chemical stability and commercial availability. The inventors also determined that alternative appropriate bismuth compounds to the oxide may also be selected - after taking into consideration, amongst other factors, the application and theoretical and / or empirical data for their catalytic activity and this patent specification is not limited to the bismuth (III) oxide. Bismuth (III) oxide (Bi20s) has a melting point of 824°C, and the working range of typical solidstate electrolytic oxygen sensors includes temperatures significantly less than this melting point temperature. Bismuth (III) oxide has 5 crystalline phases a (monoclinic), P (tetragonal), 6 (face-centred cubic), y (body-centred cubic) and s (mixed phases), whose relative ratios and stability is affected by the production processes and ambient conditions, especially pressure and temperature. P, 6 and y crystalline forms have much higher ionic conductive properties than a and s, with oxygen ions being the main charge carriers. The 6 crystalline form has an exceptionally high oxygen ionic conductivity, much higher than that for stabilised zirconia, but its use as the electrolyte (instead of or in addition to its use as the catalyst) would require a higher operating temperature than stabilised zirconia due to thermally-induced crystalline phase changes and therefore potentially a higher combustibles cross-sensitivity and greater heater power usage. Due to the different crystalline structures and their thermal instability, utilisation of bismuth (III) oxide as the solidstate electrolyte could be improved by using a stabilised doped format, such as with yttria and / or erbia doped stabilisation or co-doped dysprosia and zirconia stabilisation. The use of bismuth (III) oxide as the catalyst for reversible oxygen ion conversion gives more flexibility as to the crystalline format used and temperature range than its use as an electrolyte. A stabilised format for the bismuth (III) oxide catalyst is preferable in some implementations, dependent on, amongst other factors, the relative catalyst loading level within the electrode mix, working temperature range and thermal cycling. Stabilisation of the bismuth (III) oxide catalyst may be achieved by doping such as with yttria and / or erbia doped stabilisation or co-doped dysprosia and zirconia stabilisation. For the experimental results illustrated in this patent specification, pure (unstabilised) bismuth (III) oxide was used and this was found to be adequately catalytic and have low cross-sensitivity to combustible gases and suitable for long-term operation. Although bismuth is a heavy metal, it is unusual in that the metal and its compounds have low toxicity and are not on the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) or RoHS (Restriction of Hazardous Substances) register of restricted materials. Bismuth (III) oxide has been shown to have sufficient catalysis to allow the reversible ionisation of oxygen molecules, whilst maintaining a reduced cross sensitivity to burning of background combustible gases. The preferred working temperature chosen for the solid-state electrolytic sensor is where approximately Nernstian behaviour (given by equations 1 and 2) occurs. Although equations 1 and 2 indicate that a higher working temperature will give a larger signal, an optimal working temperature range can be determined by a compromise between the power usage, the material properties of the device (such as physical and chemical properties and melting temperatures), the required sensitivity of the device and also the required lower cross sensitivity to combustible gases. Even for a lower catalytic activity electrode, increased ambient temperature may result in combustible gases burning at an increased rate. Unlike the case for a platinum electrode, since Bi2Os is an electrical insulator, the electrode requires a separate electronic conductor as part of its constituent structure. The choice of electronic conductor requires careful consideration, since it will be held at high temperature and possibly exposed to corrosive gases, as well as combustible gases. As has been noted earlier, many metals and metallic alloys have catalytic properties and could negate the advantages of reduced catalytic activity of the active catalytic component. The inventors have determined that a suitable electronic conductor component for some preferred embodiments is gold, although other potential candidates, such as silver or alloys of gold can be used in particular applications. Gold may be used in this invention, since it is has very high electrical conductivity, high resistance to corrosion, a high melting point (l,064°C) relative to Bi20s and low catalytic activity for combustible gases. The experimental results illustrated in this patent specification were taken using gold as the electronic conductor in the electrode (as particles and wires to the external circuitry). Alternatively, at least some of the metal and / or metal alloy wire exposed to the sample gas may be partially or fully covered by a low catalytic activity sheath and / or cover, where the sheath and / or cover may comprise at least one of a ceramic, glass or metal oxide. As mentioned above, the sensor may also comprise one or more structural stabilisation spacer components, to prevent excessive sintering and / or maintain porosity and / or provide structural stability, and / or one or more ionic conduction components. This component or components may be a metal oxide, glass or ceramic and may be the same ionic conductor as the electrolyte or catalyst, such that in some preferred embodiments this is unstabilised or stabilised zirconia and / or Bi20s. The experimental results in this patent specification have used 8mole% yttria fully stabilised zirconia as this component. Results will now be presented for some preferred electrode embodiments containing Bi20s as catalyst for different electrode compositions and with exposure to various sample gases. As a comparison, some examples of the response from a commercially available platinum-based electrode under standard working conditions will also be shown alongside preferred embodiment results. This comparison illustrates that a standard platinum electrode zirconia sensor would not be suitable for trace oxygen measurements when combustible gases are present. Figure 9 shows a detailed implementation of the twin chamber cell used for these measurements. Although this gauge cell type design was used, it will be appreciated by someone skilled in the art that the electrode structure described within this patent specification would be equally applicable to other gauge type and / or oxygen ion pump implementations, such as described earlier in this specification, or other suitable solid-state electrolyte-based oxygen gas sensors. The sample gas enters the sample chamber via the sample gas inlet (901) and exits the chamber via the sample gas outlet (902). The sample inlet and sample outlet are held and sealed in position via the sample gas coupling (903). The reference gas enters the reference chamber via the reference gas inlet (904) and exits the chamber via the reference gas outlet (905). The reference gas inlet and reference gas outlet are held and sealed in position via the reference gas coupling (906). Due to the design of the implemented flow regime and flow rate used, the oxygen molecules reach the electrolyte interface (907) primarily via diffusion and therefore the signal is approximately independent of flow rate. These couplings are designed not to induce significant back pressure effects, which would be flow dependent, since this is a partial pressure device. In some implementations, for convenience, the reference gas (air) may be left to diffuse into and out of the reference chamber at atmospheric pressure, rather than be flowed through and so the reference side could merely have an open cap with a mesh or coarse filter to protect from dust ingress. If calibrated in this format, accurate oxygen measurements may still be maintained, and this is the format used to obtain the experimental results illustrated in this patent specification. If the sample gas side is not at approximately atmospheric pressure, then pressure compensation may have to be applied using at least one pressure sensor to measure the sample pressure relative to atmospheric pressure, since this is an oxygen partial pressure sensor. The solid electrolyte barrier (907) has sample and reference electrodes attached on either side (908). A high temperature heater (909) is wound around the barrel of the solid electrolyte and the temperature is measured using a thermocouple (910). In order to be able to measure the voltage signal (911) between the electrodes, wires from the electrodes are fed through holes in the electrolyte's (916) tubular wall into electrically insulating alumina tubes (912) and supported and sealed in place using a glass to ceramic seal (913). The cell is sealed at each side from the outside world by welded joins (914) onto an outer steel body and a stainless-steel diaphragm (915) which is glass to ceramic sealed onto the barrel of the solid electrolyte. The structure of the solid electrolyte (916) (a hollow cylinder with a mid-section wall) provides the sample and reference chamber cavities as well as the solid electrolyte boundary between the sample and reference electrodes. The tubular format also approximates an iso-thermal zone in the mid-section of the solid electrolyte (907). The stainless-steel diaphragm also acts as a flexure, reducing the stresses on the sealed joints due to thermal expansion coefficient mismatches between the different materials. Thermal isolation, which reduces heating power requirements and may also affect which materials may be used in the design, is enhanced by low thermal conductivity materials such as fully stabilised zirconia for the solid electrolyte (916), thin stainless-steel diaphragms (915) and also there is an enclosure air gap around the outside of the solid electrolyte (916). Although this is an oxygen partial pressure measurement, since both the sample and reference chambers were at atmospheric pressure at all times, the sample oxygen "partial pressure" will instead be listed as parts-per-million concentration (ppm). This is because by Dalton's Law the oxygen partial pressure will be directly proportional to the concentration (i.e. mole fraction) of the gas present for a constant total pressure. Figure 10 illustrates the results for four sensors made using a new electrode formulation containing Bi20s as the active catalytic component with a linear y axis of the measured mV signal and a linear x axis with the oxygen concentration provided by different gas bottle mixtures with a nitrogen background. This clearly shows the ability of the cells to respond to different applied sample oxygen concentrations (i.e. partial pressures) over a very wide range and that the response is non-linear with respect to the oxygen concentration. Figure 11 illustrates the same results for the four sensors with a linear y axis of the measured mV signal and a log scale x axis of the oxygen concentration. This shows the logarithmic dependence as is predicted by the Nernst equation (equation (1)). Figure 12 illustrates the same results for the four sensors with a log scale y axis of the fitted Nernst equation derived oxygen concentrations and a log scale x axis of the applied oxygen partial pressure. This graph again shows the very wide range of applicability of the Nernst-type behaviour to obtain a useful oxygen measurement. Figure 13 illustrates, for unit 1 from figures 10-12, the applicability of the Nernst equation linearisation even for low ppm concentrations. These results were obtained by blending an oxygen in nitrogen mixture with nitrogen after calibrating using air and 10.7ppm oxygen in nitrogen mixture and then using the modified Nernst equation (equation 2). Hydrogen is a good indicator of catalytic activity, since it may be easily combusted in the presence of oxygen and a catalyst, such as a typical platinum electrode used in a standard solid electrolyte oxygen sensor. Initiation of hydrogen burn is typically easier to achieve than, for example, with hydrocarbons and the hydrogen molecule's low mass and small size, means that it can diffuse quickly and deeply into the porous electrode structure and so the hydrogen reaction may be chosen as a means to assess and / or quality check reduced catalytic activity for the new electrode formulation. Figure 14 illustrates the performance (estimated experimental uncertainty of + / -0.3ppm) of a sensor with a new electrode formulation in the extreme condition where 50ppm hydrogen is added to a background gas containing lOppm oxygen. Since the hydrogen is in excess to what is needed to consume all of the oxygen present (compare with equation 4), the oxygen molecules would normally be expected to be completely eliminated. This can be seen to be the case where a platinum-based electrode consumes all of the oxygen present with an average reduction of lOppm (an average reading of Oppm (1401)), but the new electrode formulation only reduces by an average of 1.53 ppm (an average reading of 8.47 ppm (1402)), thus showing the substantially reduced catalytic activity towards combustion of hydrogen. Figure 15 illustrates the reaction of approximately 1.00 + / - O.OSppm of methane with lOppm oxygen. Methane is of interest since it is frequently formed and / or present from natural and / or artificial processes and more difficult to chemically trap or filter out compared to non-methyl hydrocarbons. The platinum-based electrode consumes an average of 1.88 ppm of oxygen (8.82ppm average (1501)), but the new electrode only consumes an average of 0.04 ppm oxygen (9.96ppm average (1502)), thus showing the substantially reduced catalytic activity towards combustion of methane for the new formulation electrode. From equation 7, this gives a reduced catalytic burn ratio of 0.02. Figure 16 shows the reduction in oxygen signal for a high fixed hydrogen background for relative catalyst loading levels up to lOx the original loading level. It can be seen that the catalytic activity is approximately constant over a wide range of relative loading levels by weight. Figure 17 illustrates the mV output for relative catalyst loading levels up to lOx by weight for a 5 3,000ppm oxygen in nitrogen sample mix. It can be seen that the mV voltage output is also constant over a wide range of relative loading levels by weight. Figure 18 illustrates the results for an in-house time response test for relative catalyst loading levels up to lOx by weight. The time response will be implementation dependent, however, it can be seen that the time response is also approximately constant over a wide range of relative 10 loading levels by weight.

Claims

1. A solid-state electrochemical sensor to measure oxygen partial pressure, comprising at least one solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas and a second electrode for exposure to a reference gas, wherein:the first and second electrodes and the at least one solid-state electrolyte are each porous to oxygen ions;the first and second electrodes each comprise at least one active catalytic component for the reversible ionisation of oxygen molecules to oxygen ions, and the active catalytic component of at least the first electrode comprises at least one of bismuth, a bismuth compound or a bismuth alloy;the first and second electrodes each comprise at least one electronically conductive component; andthe active catalytic component and the electronically conductive component are each provided as particles interspersed with each other in at least the first electrode.

2. A sensor according to claim 1, wherein the active catalytic component and the electronically conductive component are each provided as particles interspersed with each other within a three-dimensional electrode structure, in each of the first and second electrodes.

3. A sensor according to claim 1, wherein the active catalytic component of at least the first electrode comprises at least one of bismuth, a bismuth compound or a bismuth alloy and does not include platinum.

4. A sensor according to claim 1, wherein the active catalytic component and the electronically conductive component of at least the first electrode are each selected to be catalytically inactive for burning combustible gases, relative to their catalytic effect for reversible oxidation of oxygen molecules to oxygen ions; and wherein the active catalyticcomponent of at least the first electrode and the electronically conductive component are catalytically inactive for burning combustible gases relative to platinum and platinum alloys.

5. A sensor according to claim 1, wherein the reference gas has a known oxygen partial pressure and a measured voltage between the first and second electrodes is used to determine the oxygen partial pressure in the sample gas.

6. A sensor according to claim 1, wherein a voltage between the first and second electrodes pumps oxygen molecules across the electrolyte via the reversible ionisation of oxygen molecules and transport of oxygen ions, and a measured electrical current is used to determine the oxygen partial pressure in the sample gas.

7. A sensor according to anyone one of claims 1 to 6, wherein each of the first and second electrodes comprise at least one active catalytic component comprising at least one of bismuth, a bismuth compound or a bismuth alloy, for the reversible ionisation of oxygen molecules and transport of oxygen ions.

8. A sensor according to claim 7, wherein the first and second electrodes have an equivalent porous electrode composition and thickness.

9. A sensor according to any preceding claim, wherein the first and second electrodes are porous to both oxygen molecules and oxygen ions.

10. A sensor according to any one of claims 1 to 9, wherein the solid-state electrolyte is stabilised zirconia.

11. A sensor according to claim 10, wherein the stabilising component is at least one of yttria, hafnia, calcia, magnesia, ceria, scandia or alumina.

12. A sensor according to claim 11, wherein the stabilised zirconia is approximately 8mol% yttria stabilised zirconia.

13. A sensor according to any one of claims 1 to 9, wherein the electrolyte is gadolinia and / or yttria doped ceria, gadolinia and / or yttria and / or erbia doped bismia or yttrium doped barium zirconate.

14. A sensor according to any one of the preceding claims, wherein the active catalytic component contains powdered bismuth (III) oxide, Bi20s, and / or stabilised bismuth (III) oxide.

15. A sensor according to any one of claims 1 to 14, wherein the sensor further comprises electronic circuitry for measuring a voltage between the first and second electrodes, and wherein the first and second electrodes each further comprise at least one electronically conductive component for the conduction of electrons between the electrodes and the electronic circuitry of the sensor.

16. A sensor according to claim 15, wherein the at least one electronically conductive component of the first and second electrodes is sufficiently electronically conductive that the impedance of the sensor is dominated by the impedance of the solid electrolyte and / or electrode interface with the solid-state electrolyte.

17. A sensor according to claim 15 or claim 16, wherein the electronically conductive component comprises a metal or metal alloy wire and at least one of a powdered or deposited metal or metal alloy.

18. A sensor according to claim 17, wherein metal and / or metal alloy wire exposed to the sample gas is partially or fully covered by a low catalytic activity sheath and / or cover.

19. A sensor according to claim 18, wherein the sheath and / or cover comprises at least one of a ceramic, glass or metal oxide.

20. A sensor according to any one of claims 17 to 19, wherein the electronically conductive component comprises at least one of gold, silver or gold or silver alloy.

21. A sensor according to any one of claims 1 to 20, wherein the electrode additionally comprises an electrode stabilisation and / or ionic oxygen conduction component.

22. A sensor according to claim 21, wherein the electrode stabilisation and / or ionic conduction component consists of at least one of a ceramic, glass, metal oxide or solid-state electrolyte.

23. A sensor according to claim 21 or claim 22, where the stabilisation and / or ionic conduction component consists of at least one of bismuth (III) oxide, stabilised bismuth (III) oxide or stabilised zirconia.

24. A sensor according to claim 23, wherein the stabilised zirconia is approximately 8mol% yttria stabilised zirconia.

25. A sensor according to any preceding claim, wherein at least one electrode component comprises a powder or porous deposited structure.

26. A sensor according to claim 25, wherein any powdered components are powders with particle sizes in the range from lnm to 10mm.

27. A sensor according to any preceding claim, where the electrode components are mixed into a paste or ink and deposited onto the electrolyte surface.

28. A sensor according to claim 27, wherein the paste or ink comprises at least one of the electrode powdered components and a solvent or solvent mixture.

29. A sensor according to claim 27 or claim 28, wherein the paste or ink comprises at least one binder and / or flow modifying agent to enhance stabilisation and / or modify the flow properties of the paste or ink.

30. A sensor according to any preceding claim, further comprising a temperature monitor and heater for maintaining a working temperature to approximately conform with Nernstian behaviour.

31. A sensor according to claim 30, wherein the electrolyte comprises stabilised zirconia and is maintained at a controlled working temperature within the range 500 to 800 °C.

32. A sensor according to any one of claims 1 to 31, wherein the solid electrolyte comprises yttria stabilised zirconia, the active catalytic electrode component comprises unstabilised and / or stabilised bismuth (III) oxide, and the working temperature is controlled to be between 500 and 800°C.

33. A sensor according to claim 32, where the electronically conductive component of the electrodes comprises sintered particulate gold and / or gold wire.

34. A sensor according to any one of the preceding claims, wherein the electrodes comprise an electronically conductive component for the conduction of electrons, the electronically conductive component having sufficient electronic conductivity that the impedance of the sensor is dominated by the impedance of the solid electrolyte and / or electrode interface with the solidstate electrolyte.

35. A sensor according to any one of the preceding claims, wherein the catalytic component loading level is between 0.1 and 99.9% by mass of the electrode composition.

36. A sensor according to any one of the preceding claims, comprising an electronically conductive component for the conduction of electrons, wherein the electronically conductive component is between 0.1 and 99.9 % by mass of the electrode composition.

37. A sensor according to any one of the preceding claims, wherein the first and second electrodes further comprise at least one spacer and / or adhesive component and / or ionic conductive component.

38. A sensor according to any one of the preceding claims, wherein the thickness of the solidstate electrolyte sandwiched between the first and second electrodes is between 1 micron and 100mm.

39. A sensor according to any one of the preceding claims, wherein the surface of the solid-state electrolyte which is in contact with the electrode is treated to enhance adhesion of the electrode and / or increase surface area of the interface contact between with the solid-state electrolyte and electrode.

40. A sensor according to claim 39, wherein at least one keying-in feature is implemented to avoid delamination of the electrode at the electrode / solid-state electrolyte interface.

41. A sensor according to any one of claims 39 and 40, wherein surface roughening is implemented to avoid delamination of the electrode and / or enhance surface area at the electrode / solid-state electrolyte interface.

42. A sensor according to any one of claims 39 to 41, wherein the surface of the solid-state electrolyte is treated using at least one of bead-blasting, laser ablation, water-jet cutting, chemical etching or other suitable method.

43. A sensor according to any one of claims 39 to 42, wherein the at least one surface feature's dimensions after treatment are between lnm and 100mm.

44. A sensor according to any one of the preceding claims, wherein the thickness of each of the first and second electrodes is between lnm and 100mm.

45. A sensor according to any one of the preceding claims, wherein at least one electronically conductive component and / or catalytically active component and / or spacer and / or adhesive component and / or ionically conductive component is pre-assembled and / or pre-formed as a porous structure and then either positioned and bonded onto the electrolyte interface to form an electrode / solid electrolytic interface or subsequently loaded with at least one additional component and then positioned and bonded onto the solid electrolyte interface to form an electrode / solid electrolytic interface.

46. A sensor according to claim 45, wherein the at the least one additional component is at least one of the electronically conductive component and / or catalytically active component and / or spacer and / or adhesive component and / or ionically conductive component.

47. A sensor according to claim 46, wherein the at least one additional component is added from a solid and / or fluid phase.

48. A sensor according to claim 47, wherein the at least one additional component is loaded via an intermediary compound or state and reduced thermally and / or chemically to the desired physical state and chemical composition.02 10 2549. A solid-state electrochemical sensor to measure oxygen partial pressure, comprising at least one solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas and a second electrode for exposure to a known oxygen partial pressure gas mixture, wherein:the first and second electrodes and the solid-state electrolyte are each porous to oxygen 5 ions;the first and second electrodes each comprise at least one active catalytic component for the reversible ionisation of oxygen molecules to oxygen ions, wherein the active catalytic component of at least the first electrode comprises a component selected to have reduced catalytic activity for the burning of combustible gases in the presence of oxygen within a selected 10 operating temperature range, relative to its catalytic activity for ionisation of oxygen;the first and second electrodes each comprise at least one electronically conductive component; andthe active catalytic component and the electronically conductive component are each provided as particles interspersed with each other in at least the first electrode and wherein the 15 active catalytic component of at least the first electrode comprises at least one of bismuth, a bismuth compound or a bismuth alloy, without platinum.

50. A solid-state electrochemical sensor to measure oxygen partial pressure, comprising at least one solid-state electrolyte sandwiched between a first electrode for exposure to a sample gas 20 and a second electrode for exposure to a reference gas, wherein:the first and second electrodes and the solid-state electrolyte are each porous to oxygen ions;the first and second electrodes each comprise at least one active catalytic component for the reversible ionisation of oxygen molecules to oxygen ions, and wherein the active catalytic 25 component of at least the first electrode comprises at least one of a post-transition metal, posttransition metal compound or post-transition metal alloy, excluding lead, thallium and polonium, wherein said at least one active catalytic component is selected to have reduced catalytic activity for the burning of combustible gases in the presence of oxygen compared with platinum;the first and second electrodes each comprise at least one electronically conductive component; andthe active catalytic component and the electronically conductive component are each provided as particles interspersed with each other in at least the first electrode.

551. A sensor according to claim 50, wherein the post-transition metal, post-transition metal compound or alloy comprises at least one of bismuth and / or aluminium and / or gallium and / or indium and / or tin.10 52. A solid-state electrochemical sensor to measure oxygen partial pressure, according to anypreceding claim, wherein the sensor functions as a galvanic concentration cell and / or ion pump cell.02 10 2515

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