Carbon-based air electrode

Lai-jcSr MnOs). Unlike its effect in fuel cells, platinum metal is not a very effective catalyst for metal-air batteries, especially for charging. Instead, silver has demonstrated high reduction efficiency and good stability. The most inexpensive catalyst is activated carbon, which has a very high surface area. For oxygen reduction in carbon-based air electrodes, manganese dioxide is the catalyst most extensively used. 

Both nonaqueous and aqueous electrolyte-based Hthium-air batteries have similar theoretical specific energies ( 11000 Whkg based on Hthium alone), as shown in Table 22.1. Zheng et al. [39] simulated both aqueous and nonaqueous electrolyte-based Hthium-air batteries where the total weight of the Hthium, the carbon-based air electrode, and the electrolyte were considered. Their analysis showed that the maximum theoretical specific capacities of the cells are 435 and 940 mAh for lithium-air batteries with aqueous- and nonaqueous-based electrolytes, respectively. The main difference between these two kinds of Hthium-air batteries originates from the fact that the solvent is consumed in aqueous electrolyte-based lithium-air batteries, but it is not consumed in the nonaqueous electrolyte-based Hthium-air batteries. 

The electrolyte used by the fuel cell is a solid gas—impermeable zirconia known as zirconium oxide (ZrOj). This ZrOj is doped with calcium oxide (CaO) to supply enough oxide ions to carry the cell current. The oxidant air or oxygen is bubbled through the molten silver cathode, which is held inside the zirconia cup. At the fuel electrode or the carbon-based anode electrode, the oxide ions are combined with carbon monoxide (CO) and give up their electrons to an external circuit. The cell by-products CO and hydrogen, which are formed in the initial fuel decomposition, are burned outside the cell to keep the fuel cell at operating temperature. The hydrogen is not involved in the electrochemical cell reaction. 

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... 

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. 

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). 

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. 

Gas-diffusion electrode metal-air cells gas-transport in porous media carbon-based catalysts. 

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. 

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],... 

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

The investigations of various types of carbon-based catalysts allow suitable air electrodes to be developed for use in the large variety of metalair cells and batteries designed in this laboratory. 

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. 

Lu, L., et al., Highly stable air working bimorph actuator based on a graphene nanosheet/carbon nanotube hybrid electrode. Advanced Materials, 2012. 24(31) p. 4317-4321. 

The improvement in air performance of catalyzed carbon based (0.5 mg Pt/cm ) porous cathodes with cell temperature is illustrated in Figure 4-4 (17). As expected, the electrode potential at a given current density decreases at lower temperatures, and the decrease is more significant at higher current densities. In the temperature range of 60 to 90°C, the cathode performance increases by about 0.5 mV/°C at 50 to 150 mA/cm. ... 

An ambitious review of carbon applications in chemical power sources (see also Section 5.3.5) was offered by Fialkov [94], It is disconcerting, however, that the author discusses the influence exerted by... surface properties without citing even one of the well-known—or well-cited or more recent—studies on carbon surface chemistry. And yet, in conjunction with the use of carbon in air (oxygen) electrodes, he speculates that the oxygen electroreduction kinetics depend on... the degree to which side faces of carbon crystallites are developed because base groups are formed there and presumably interact (e.g., with HjOj) in the following manner ... 

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