Gas-diffusion electrode

The solubility of CO increases under elevated pressure, leading to enhanced CO2 transport rate. Thus CO2 reduction will naturally proceed at higher current density. CO2 reduction with simple bulk metal electrodes under elevated pressure was discussed in the previous Sections from the viewpoint of the electrocatalytic aspects. The present Section mentions enhancement effect of the transport process. 

Ito and his coworkers applied elevated pressure up to 20 atm to CO2 reduction in aqueous and nonaqueous electrolyte solutions with Zn, In, Sn and Pb electrodes. 2 They showed enhanced current density of 30 mA cuT at -1.7 V vs. SCE under 20 atm. The faradaic efficiency of HCOOH formation increased with the increase of CO2 pressure. 

Sakata and his coworkers studied CO2 reduction under elevated pressure extensively. They could enhance the current density to 163 mA cm with a Cu electrode at -1.64 V vs. Ag/AgCl under 30 atm with major products of CH4, C2H4 and CO. The maximum partial current density of HCOOH formation amounted to 560 mA cm with an In electrode imder 60 atm.  

The solubility of CO2 is liigh in methanol. Fujishima and his coworkers employed CO2 methanol mixtures imder elevated pressure as the electrolyte solution. Tetraalkyl ammonium salts were used for the supporting electrolytes. They showed that CO2 reduction can proceed with a Cu electrode at 200 to 500 mA cm imder 40 to 60 atm. The major products were CO and methyl formate.  

Later Mazin et al., and Li and Prentice also published studies on CO2 reduction at Cu electrodes in methanol based electrolyte and ethanol based one under elevated pressure.  

Air electrode plays an important role in the operation of Li-air battery. A successful air cathode must satisfy many requirements such as good electrical conductivity, high surface area, fast oxygen diffusion, stable electrode integrity, and fast ionic conductivity. In nonaqueous Li-air batteries, the air cathode is a carbon-supported porous structure, which acts as a gas transport channel involving the formation and storage of discharge product. GDE helps to accomplish an air cathode to complete the cell such as that in PEM 

It is well known that a more important factor in achieving high performance is the ability to make effective use of pores, not the size or total volume of pores. Keeping this in mind, Prabaharan et al have synthesized a hierarchically porous carbonaceous 

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The macrokinetics of processes in gas-diffusion electrodes is analogous to that in liquid-phase electrodes. In calculations, one must take into account, however, that the electric current and the solute species will be carried only through that part of pore space which is electrolyte filled, whereas gas supply is accomplished primarily not by diffusion through the liquid but by flow in the gas channels. 

Gonzalez ER, TicianeUi EA. 2005. Studies of CO tolerance on modified gas diffusion electrodes containing ruthenium dispersed on carbon. J Electroanal Chem 575 53-60. 

Figure 11.14 E versus time curve recorded during the ORR on a FePc/C gas diffusion electrode j = —25 mAcm ). The FePc electrode was fed with ambient air without any convection (temperature 20 °C) [Baranton et al., 2005].

Willsau J, Heitbaum J. 1984. The influence of Pt-activation on the corrosion of carbon in gas diffusion electrodes A DEMS study. J Electroanal Chem 161 93-101. 

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

Significant advances have been made in this decade in electrochemical H2 separation, mostly through the use of solid polymer electrolytes. Since the overpotentials for H2 reduction and oxidation are extremely low at properly constructed gas diffusion electrodes, very high current densities are achievable at low total polarization. Sedlak [13] plated thin layer of Pt directly on Nafion proton conductors 0.1-0.2cm in thickness, and obtained nearly 1200 mA/cm2 at less than 0.3 V. The... 

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 mechanism is quite complex. In free electrolyte the reaction order in C02 is actually negative [30] the order in a functioning fuel cell, with gas-diffusion electrodes, rises to near zero [31]. This low order in C02 is essential in the efficient operation at very low C02 pressures as would be encountered in life-support. The MCFC has been... 

Fig. 30. Cathodic polarization of a nickel gas-diffusion electrode before and after sulfidation with H2S.

Interesting results have been obtained using metallophthalocyanines supported on porous carbon gas diffusion electrodes.132-136 In the case of CoPc and NiPc, CO is formed with a current efficiency of almost 100%.135 With Sn, Pb, and In phthalocyanines, mainly HC02H is formed, while Cu and Ti phthalocyanines promote the formation of CH4. The reason why some metal Pc complexes give CO or CH4, while others yield HC02H, has been interpreted in terms of the electron configuration in the metal.137 A rather different type of reaction is the very recent demonstration of the simultaneous reduction of C02 and N02 to give urea (NH2)2CO, which can be achieved with an efficiency up to 40% at similar gas-diffusion electrode devices with a NiPc supported catalyst.138... 

Electroreduction of N2 to NH3 has also been examined using gas-diffusion electrodes modified by 14 different metal phthalocyanines.320 It was found that the Sn-Pc complex is the best catalyst in terms of current efficiency and stability of the electrode for the electrochemical dinitrogen activation. 

CARBON MATERIALS FOR GAS DIFFUSION ELECTRODES, METAL AIR CELLS AND BATTERIES... 

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. 

The next important stage in the development of porous gas-diffusion electrode is an investigation of influence of thickness of PANI layer (or more easy controlled parameters like PANI mass or electrochemical capacity) on the local currents of O2 electroreduction (table 3). 

Figure 6. The galvanostatic discharge curves and self-charge curve for Zn-Air coin battery with PANI/TEG gas-diffusion electrode.

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. 

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