Air gas-diffusion electrode

With these solid-oxide electrolytes, designed to operate in relatively 02-rich feed (e.g. air), gas-diffusion electrodes with their enhanced contact area, are not necessary, and electrode materials can be applied directly onto the electrolyte surfaces in thin films. 

The mechanisms and reasons of catalytic activity of polyaniline (PANI)-type conducting polymers toward oxygen reduction in acidic and saline solutions are investigated by electrochemical and quantum-chemical methods. The PANI/thermally expanded graphite compositions were developed for realization of fully functional air gas-diffusion electrodes. Principally new concept for creation of rechargeable metal-air batteries with such type of catalysts is proposed. The mockups of primary and rechargeable metal-air batteries with new type of polymer composite catalysts were developed and tested. 

Metal-air cells are developed with air gas-diffusion cathodes and Mg-anodes. Non-aggressive NaCl-solution is used as electrolyte. Carbon based catalysts for the oxygen reduction are selected and tested in the air gas-diffusion electrodes. Various Mg-alloys are tested as anodes. The V-A, power and discharge characteristics of the Mg-air cells are investigated. 

Electrochemical energy sources Magnesium anode Air gas-diffusion electrode Neutral electrolyte. 

The magnesium-air cells under investigation are electrochemical cells containing two air gas-diffusion electrodes (cathodes) and a magnesium anode. NaCl-solution is used as an electrolyte. 

The air gas-diffusion electrode developed in this laboratory [5] is a double-layer tablet (thickness ca.1.5 mm), which separates the electrolyte in the cell from the surrounding air. The electrode comprises two layers a porous, from highly hydrophobic, electrically conductive gas layer (from the side of the air) and a catalytic layer (from the side of the electrolyte). The gas layer consists of a carbon-based hydrophobic material produced from acetylene black and PTFE by a special technology [6], The high porosity of the gas layer ensures effective oxygen supply into the reaction zone of the electrode simultaneously the leakage of the electrolyte through the electrode... 

Various carbon-based catalysts for the electrochemical oxygen reduction have been tested in the air gas-diffusion electrodes [7]. The polarization curves of the air electrodes were measured when operating against an inert electrode in 2 N NaCl-solution. The potential of the air electrodes was measured versus saturated calomel electrode (SCE). 

Figure 1. Polarisation curves of air gas-diffusion electrodes with various catalysts.

Pyrolyzed catalysts obtained by heat treatment in Argon of active carbon impregnated with solution of the compound Co-tetramethoxyphenylporphirine (CoTMPP) are studied [9], Air gas-diffusion electrodes with this catalyst show low polarisation in a wide interval of current densities (up to 100 mA/cm2) and stable long-term performance. These catalysts are more suitable for use in magnesium-air cells operating at high current drains, but unfortunately their price is comparatively high. 

In Figure 1 we compare the polarization characteristics of the air gas-diffusion electrodes with the described catalysts. 

Magnesium-air air cells with NaCl-electrolyte were developed and investigated. The current-voltage and the discharge characteristics of the cells with were studied. Air gas-diffusion electrodes suitable for operation in NaCl-electrolytes were designed. Various carbon-based catalysts for the electrochemical reduction were tested in these air electrodes. Magnesium alloys suitable for use as anodes in Mg-air cells were found. 

Kaisheva A., Gamburzev S., Iliev I. Carbon air gas-diffusion electrodes for operation in neutral electrolytes containing mixed metal oxides, Elektrokhimija 1981 17 1362-66... 

Metal-air batteries combine a metal anode (similar to that used in the conventional primary batteries) and an air gas-diffusion electrode (cathode) similar to that used in the fuel cells. During operation the metal anode is electrochemically oxidized for the expense of the oxygen from the air, which is reduced on the air gas-diffusion electrode. 

The air gas-diffusion electrode possesses two advantages over the metal-oxide cathode in the conventional primary batteries infinite charge... 

The mechanism of gas transport through the porous hydrophobic layer of the developed air gas-diffusion electrodes is theoretically and experimentally investigated [5],... 

It must be noted that the effective diffusion coefficient (Di)eff is obtained by electrochemical measurements of air gas-diffusion electrodes with sufficiently thick gas layer so that the limiting process is the gas... 

One of the main problems in the development of air gas-diffusion electrodes for metal-air cells is to find active and stable catalysts for the electrochemical reduction of oxygen. Carbon-based catalysts are mostly used, because of their highly developed surface area and capability for adsorption of 02, suitable morphology, chemical stability, good electric conductivity and comparatively low price. 

Various carbon-based catalysts were tested in the investigated air gas-diffusion electrodes pure active carbon [6], active carbon promoted with silver [7] or with both silver and nickel. Catalysts prepared by pyrolysis of active carbon impregnated with a solution of the compound Co-tetramethoxyphenylporphyrine (CoTMPP) are also studied [8],... 

It is well known that the performance of the air gas-diffusion electrode is influenced not only by the activity of the catalyst, but also by all transport processes taking place in its porous structure. In addition, the transport hindrances in the electrode are function not only of its overall structure, but also of the porous structure and the surface properties of the catalyst. Methods for diagnostic of the activity and the transport properties of air gas-diffusion electrodes were proposed [9]. 

It was experimentally found that the polarization curves of the investigated air gas-diffusion electrodes in a semi-logarithmic scale at low current densities (below 10 mA/cm2) are straight lines, which can be treated as Tafel plots. At these low current densities the transport hindrances in the air electrode are negligible so that activation hindrances only are available. 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

Generally, the hindrances in the transport processes, which take place in the porous structure of the air gas-diffusion electrodes, influence strongly the electrode performance and are of prime importance for their practical application. 

An experimental method is proposed for estimation of the transport hindrances in air gas-diffusion electrodes. As a measure of the transport hindrances in the air gas-diffusion electrodes is introduced the difference AE between the potentials of one and the same electrode when operating with air and with pure oxygen at one and the same current density. The difference AE can be theoretically described by the equation ... 

The described method for the diagnostic of the activity and the transport hindrances in air gas-diffusion electrodes is very useful in the research of porous catalysts for air electrodes. The comparison of the activity and the transport hindrances of air electrodes with catalysts from various types of active carbon allow a proper selection to be accomplished. 

Various types of metal-air cells are developed using the described air gas-diffusion electrodes. In the most cases zinc is used as an anode [13]. 

Air gas-diffusion electrodes with similar overall structure suitable for operation in neutral electrolytes are developed. On the basis of these air electrodes metal-air cells are developed using aluminum and magnesium as an anode. 

Air gas-diffusion electrodes were developed, suitable for use in metal-air cell with alkaline or with saline electrolytes. A variety of carbon-based catalysts are used in these air electrodes. Methods for the estimation of the activity and the transport hindrances are proposed and used successfully for the optimization of the carbon-based catalysts. 

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