Anode extended reaction zone

A. Bauer, E. L. Gyenge, and C. W. Oloman. Direct methanol fuel cell with extended reaction zone anode PtRu and PtRuMo supported on graphite felt. Journal of Power Sources 167 (2007) 281-287. 

The mass specific activities of the catalysts for ethanol electrooxidation are unfortunately quite low, between about 5 mW mg at 323 K and 9-11.5 mW mg at 353 K based on peak power outputs [183-184, 186]. Zhou et al. reported somewhat higher mass specific activities, reaching 23 mW mg at 333 K [188b]. Utilizing extended reaction zone anodes could also improve the mass specific activity (see also Section 4.3) [190]. 

Figure 4.62. Examples of three-dimensional supports for extended reaction zone anodes in direct liquid fuel cells, (a) unpressed graphite felt UGF, (h) pressed graphite felt GF, (c) reticulated vitreous earhon RVC [250, 305]. (Reprinted from Electrochimica Acta, 51(25), Bauer A, Gyenge EL, Oloman CW, Eleetrodeposition of Pt-Ru nanoparticles on fibrous carbon substrates in the presence of nonionie surfactant Apphcation for methanol oxidation, 5356-64, 2006, with permission from Elsevier, and reproduced by permission of ECS— The Electrochemical Society, Gyenge EL, Oloman CW. The surfactant-promoted electroreduction of oxygen to hydrogen peroxide.)...

Figure 4.65. Performance comparison of DMFC equipped with extended reaction zone anode composed of pressed graphite felt with PtRu and PtRuMo, obtained by electrodeposition from a colloidal solution [86, 250], a) 333 K, b) 343, and 353 K. Anode catalyst characteristics are given in Table 4.4. Anolyte 1 M CH3OH - 0.5 M H2SO4, 5 mL min, ambient pressure. Cathode 4 mg cm Pt black, O2 flow rate 500 nil min at 2 atm (abs). [86]. (Reprinted from Journal of Power Sources, 167(2), Bauer A, Gyenge EL, Oloman CW, Direct methanol fuel cell with extended reaction zone anode PtRu and PtRuMo supported on graphite felt, 281-7, 2007, with permission from Elsevier.)...

Lycke DR, Gyenge EL. Electrochemically assisted organosol method for Pt-Sn nanopartiele synthesis and in situ deposition on graphite felt support extended reaction zone anodes for direct ethanol fuel cells. Electrochim Acta 2007 52 4287-98. 

Bauer A, Gyenge EL, Oloman CW. Direct methanol fuel eeU with extended reaction zone anode. ECS Trans 2006 3 1271-7. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

A macrohomogeneous electrode can be established in different dimensional structures and the resulting models, which can present analytical or numerical solutions, could relate the global performance of the cathodic or anodic layer to unmeasurable local distributions of reactants, electrode potential, and reaction rates. These unmeasurable local distributions define a penetration depth of the active zone and suggest an optimum range of current density and electroactive layer thickness with minimal performance losses and highest electroactive effectiveness. In addition, the macrohomogeneous theory can be extended to include concepts of percolation theory. 

The most common anode material is nickel-yttria stabilized zirconia (Ni-YSZ) Cermet or a mixture of nickel and YSZ. While nickel serves for the required catalytic activity and electron conductivity, YSZ lowers the effective coefficient of expansion to march with adjacent YSZ electrolyte. However, use of YSZ extends the active zone for the anode reaction. Depending on the design, Ni to YSZ volume ratio varies in the range from 30% to 50% in the composite mixture. Electronic conductivity varies in the range of 0.1-1000 S cm"i depending on the porosity and composition of Ni-YSZ composition. 

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