Oxygen-permeable membrane-covered electrode

An alternative approach places the Pt electrodes directly on a flexible polymer carrier [80]. Eh Lilly developed a three-electrode transducing system based on a polyimide carrier with electroadsorbed enzyme and a highly oxygen-permeable membrane covering the sensor. Such a system was tested in vivo and published results seem encouraging although the company cancelled the project. 

In 1954 Leland Clark demonstrated that a platinum cathode would measure the oxygen concentration of blood when it and a reference electrode were covered by an oxygen permeable membrane. Later in that same year Stow and Severinghaus showed that carbon dioxide could be estimated in blood with a glass electrode fitted with a gas permeable membrane. In the seventies the Huchs demonstrated that mechanical adaptations of these devices could be utilized to provide transcutaneous (non-invasive) measurement of arterial blood gas concentration if the skin area surrounding the sensor was heated to 44 - 45°C. 

Oxygen-sensitive membrane electrodes are commercially available. The electrode in such a system is covered with an oxygen-permeable plastic membrane, thus protecting it from impurities. The current is proportional to the activity of dissolved molecular oxygen, and at low concentration, to the amount of DO. Before the measurement of DO in the sample, calibrate the electrodes using standards of known DO concentrations (determined from iodometric titrations). 

While most gas sensors rely on potentiometric detection, the important oxygen probe is based on amperometric measurements. In particular, membrane-covered oxygen probes based on the design of Clark et al. (105) have found acceptance for many applications. The sensor is based on a pair of electrodes immersed in an electrolyte solution and separated from the test solution by a gas-permeable hydrophobic membrane (Fig. 6.22). The membrane is usually... 

Specially designed electrodes in which the electrode tip is covered in a gel containing the appropriate oxidase are commercially available. A semi permeable membrane retaining the gel, and permitting oxygen diffusion, completes the assembly. 

In addition to the iodometric method more modern physico-chemical methods are now available, which are also used for the construction of automatic oxygen analysers. These are potentiometric methods with a membrane electrode (probe), based on the measurement of the current generated during the cathode depolarization (hydrogen electrode) with dissolved oxygen. A suitable electrode pair, for example, Ag/Pt covered with a gas permeable membrane, is used [14]. 

There are also nongalvanic oxygen sensors that are based on the same principles. This type of oxygen sensor typically consists of an inert metallic cathode covered with a gas-permeable membrane and a silver anode. These sensors are usually connected in series with a 1.5 V battery and an external potential of 0.8 V is applied between the electrodes to initiate the amperometric measurement of oxygen. One such sensor employs a gold cathode, a silver anode, and a potassium chloride gel as the electrolyte. 

Some gases may be selectively detected they need to be either electroactive (oxygen, bromine) or acido-basic (carbon dioxide, sulfhydric acid, ammonia) and diffuse through an hydrophobic membrane (teflon, polyethylene). Thus such GSE are generally built with a metal or a pH electrode covered with a gas permeable membrane[17,18,29,30]. 

Cobalt-based macrocyclic complexes are some of the catalysts that exhibit significant catalytic activity against DO. Vitamin B12 is a cobalt-based complex and inherits excellent electrocatalytic properties. It can be easily screen printed to modify the electrode providing DO sensors better than the Clark-type electrode. The latter should be covered with a permeable membrane and must be properly maintained. Modifying the electrode with vitamin B12 creates a membrane-free DO sensor. Tedious maintenance is not required with this kind of sensor. Moreover, poisoning of the membrane is not an issue anymore. The electrochemical behavior of the vitamin Bi2-modified electrode involved in the reduction of oxygen can be explained by the following equations ... 

The so-called oxygen electrode is based on a different principle an electrolysis cell is used, and the current measured at the inert cathode is, under standardized conditions, a function of the oxygen concentration (activity). Selectivity is enhanced by covering the cathode with a membrane permeable to molecules only. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

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