Amperometric Fuel Cells

If the number and type of gaseous species to be detected can be narrowly defined and if they remain invariable, it is possible to use a brute-force approach and to construct a chemical sensor without detailed knowledge of the mechanism of its operation. In such a case, the output of the sensor - be it voltage, current, or any other physical 

An example is the sensor for methane (Stetter and Li, 2008), which also shows sensitivity to several other oxidizable species. The cell current is calibrated against the concentration of methane in air of 75% relative humidity. The key element in this sensor is the low volatility electrolyte, such as y-butyrolactone, propylene carbonate, 

The porous platinum/Teflon electrodes separate the electrolytic cell from the gaseous reference chamber on one side and the sample chamber on the working electrode side. The applied voltage is controlled by a potentiostat. The sample enters into the electrolytic cell through the porous electrodes, the pore size of which also needs to be closely controlled in order to prevent their flooding with the solvent. An example of an electrochemical reaction of interest is oxidation of methane under conditions of humid air. 

Using an 800 mV applied voltage, the response of the sensor has been evaluated in the range of 12-100% CH4 in air (Fig. 7.18). Major interferences have been caused by the presence of nitrous oxide, ethane, hydrogen, and carbon monoxide. Other sensors of this type have been described, but they differ only in the details of the design and not significantly in the concept. 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

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O2 Potentiometric, amperometric Amperometric fuel cell principle... 

Fig. 7.17 Fuel cell amperometric sensor (adapted from Stetter and Li, 2008)...

Figure 4 shows how the short circuit current depends on the concentration of H2 which is diluted with air or N2- It is noteworthy that for H2 in air the short circuit current is approximately in direct proportion to the H2 concentration. As mentioned before, this fact suggests that for practical purpose the amperometric sensor is more accurate than a potentiometric sensor. When H2 was diluted with N2, the sensor exhibited a very different behavior with far greater current values and a nonlinear dependence on H2 concentration. In this case, the cell is actually an H2 02 fuel cell which accounts for the greater current values. 

Different electrochemical sensors have been developed for cell concentration measurement. The most promising of these sensors are based on impedimetric measurements. A commercial version of a sensor that measures the frequency-dependent i)ermittivity is available from Aber Instruments Ltd [137-139]. Another type of electrochemical probe measures the potential changes in the cell suspension caused by the production of electroactive substances during cell growth [140-143]. To date, no on-line applications of these potentiometric sensors under real cultivation conditions have been reported. Other types of probes, such as amperometric and fuel-cell sensors, measure the current produced during the oxidation of certain compounds in the cell membrane. Mediators are often used to increase the sensitivity of the technique [143-145]. 

Electrochemical sensors include amperometric cells, especially galvanic fuel cells, and polarographic cells. Both types are available from several manufacturers world-wide in large numbers for clinical use. Higher stability with fewer technical problems makes the galvanic cell currently the most popular oxygen sensor in the operating room environment [12]. 

V and therefore the cell reaction occurs spontaneously. These cells are sometimes referred to as Mackereth sensors . Such galvanic sensors are also often termed fuel cell sensors and it is possible to measure either the resulting current or the cell voltage. In the former case, the term amperometric sensor in its widest definition is still correct, although the cell is fundamentally different from the usual Faradaic amperometric systems. Note that for the determination of atmospheric oxygen an alternative has become available in recent years in the form of optical sensors based on fluorescence. These sensors are very robust as they do not contain electrodes or a liquid phase and show very fast response times. 

Amperometric methods include all those in which current is the measured parameter. Arguably, the simplest of these methods uses the fuel cell in which no potential is applied current is generated in response to a thermodynamically favorable overall cell reaction that involves two electrodes in a solution although uncommon as an analytical method, the fuel cell approach... 

Lu X, Wu S, Wang L, Su Z (2005) Solid-state amperometric hydrogen sensor based on polymer electrolyte membrane fuel cell. Sens Actuators B 107 812-817... 

Dissolved hydrogen Amperometric probe Hydrogen/air fuel cell Mass spectrometry Silicon or Teflon membrane tubing to transfer dissolved hydrogen to gas phase Ktiroda et al. (1991) Pauss et al. (1990) Meyer and Heinzle (1998) Bjomsson et al. (2001)... 

Besides the broad applications of electrically contacted enzyme electrodes as amperometric biosensors, substantial recent research efforts are directed to the integration of these functional electrodes as biofuel cell devices. The biofuel cell consists of an electrically contacted enzyme electrode acting as anode, where the oxidation of the fuel occurs, and an electrically wired cathode, where the biocatalyzed reduction of the oxidizer proceeds (Fig. 12.4a). The biocatalytic transformations occurring at the anode and the cathode lead to the oxidation of the fuel substrate and the reduction of the oxidizer, with the concomitant generation of a current through the external circuit. Such biofuel cells can, in principle, transform chemical energy stored in biomass into electrical energy. Also, the use... 

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