Air electrode

In the experimental systems studied the iron electrode has been of the siatered type and the oxygen —air electrodes have been of the bifunctional type. 

In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at potentials where carbon corrosion occurs. However, in alkaline electrolytes carbon is sufficiently electrocatalytically active for oxygen evolution to occur simultaneously with carbon corrosion at potentials corresponding to charge conditions for a bifunctional air electrode in metal/air batteries. In this situation, oxygen evolution is the dominant anodic reaction, thus complicating the measurement of carbon corrosion. Ross and co-workers [30] developed experimental techniques to overcome this difficulty. Their results with acetylene black in 30 wt% KOH showed that substantial amounts of CO in addition to C02 (carbonate species) and 02, are... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

Figure 5. Polarization curves for bifunctional air electrode in 1.5Ah Zn/air cell with 12 KOH at 27 °C. From Ross 1351.

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

Thus, using PANI-type catalysis gives a possibility to exclude basically the typical side reaction of oxygen evolution during charging of battery, which usually conducts to destruction of catalytic active Air electrode. 

The discharge curve for Air electrode, which was measured using additional third Ag AgCl reference electrode, is ideally horizontal since it is determined by the oxygen reduction potential in according medium... 

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). 

It is found that some types of active carbons possess enough catalytic activity to be used as catalysts in air electrodes operating at low c.d. 

Catalysts from active carbon additionally activated with cobalt- or iron- phthalocyanines are also studied [7], The results show that at current densities up to 50 mA/cm2, the polarization of the air electrodes with catalyst from active carbon promoted with FePc is lower than that of the electrode with catalyst from active carbon promoted with CoPc. At higher current density the polarization of the electrode with catalyst from active carbon promoted with CoPc is lower, which is probably connected to the lower transport hindrances, due to the more favorable structure of this catalyst. 

Several types of experimental magnesium-air cells were tested. These cells varied in their size (the working area of the air electrodes used) [10]. The current-voltage curves of an experimental Mg-air cell with two air electrodes (Sair = 80 cm2) with pyrolyzed CoTMPP catalyst and sandwich-type Mg anode (MA8M06) operating in NaCl-electrolytes with different concentrations are presented by Figure 2. 

The testing of the different magnesium anodes is performed in magnesium-air cells with two air electrodes with pyrolyzed CoTMPP catalyst (S = 65 cm2). One-sheet Mg anodes were used in 4N NaCl media. 

Mechanically rechargeable magnesium-air cells were designed and investigated. The most important feature of these cells is that the cell-case with the air electrodes can be used many times. 

Two types of mechanically rechargeable Mg-air cells for operation at high nominal currents are designed. The air electrodes in these cells are with pyrolyied CoTMPP catalyst. Mg anodes from the alloy MA8M06 were used in this design. 

Even more powerful Mg-air cell was constructed with total working area of the air electrodes S = 660 cm2. Current loads up to 100 A could be reached with this cell at a voltage higher than 1 V. 

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. 

Iliev I. Air electrodes for primary metal-air batteries, 160th Meeting of the Electrochemical Society, Denver, Colorado, Oct. 1981, Extended Abstracts, p. 268-269. 

Iliev I., Air electrodes for aluminium- air batteries, Bull.Soc.Chim.Beograd, 1983 47 (Supplement) S317-S338. 

The hydrophobic gas layer of the air electrode [4] possesses high porosity (ca. 0,9 cm2/g), such that an effective oxygen supply through this layer is obtained. From the experimental porogrames measured by both mercury and 7 N KOH-porometiy the contact angle 0en of the hydrophobic material with water electrolytes is obtained (0eff =116° 118°). Because of... 

The catalytic layer of the air electrode is made from a mixture of the same hydrophobic material and porous catalyst [2]. It comprises hydrophobic zones through which the oxygen is transported in gas phase and zones containing catalyst where the electrochemical reduction of oxygen is taking place. It must be noted that the overall structure of the electrode is reproducible when various kinds of carbon-based catalysts are used. 

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. 

Figure 4. Tafel portions of the polarization curves of air electrodes with carbon-based catalysts.

Nevertheless, the comparison (Figure 5) of the polarization curves of the same air electrodes show that the polarization at the high current density range of the most active electrode is much higher that that of the electrodes... 

Figure 5. Polarization curves of air electrodes with catalysts from active carbon and with active carbon promoted with 5% and 30% of silver.

Figure 6. Polarization curve of an air electrode operating with air and with pure oxygen.

The value of AE is obtained by the extraction of the potential of the two curves at one and the same current density, so it is free from the 1R drop between the air electrode and the reference electrode. 

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