Performance cathode

The cathode reaction efficiency depends on a variety of factors, including the concentration and species of fhe oxidant (electron acceptor), catalyst performance, electrode structure and operational conditions. 

Generally, the reaction rate (r) increases linearly with the oxidant concentration [O] according to the relationship r = fc[0] , where k is the rate constant and a is the stoichiometric coefficients of the oxidant. 

The operating temperature can also affect the kinetics of oxidant reduction, mass and proton transfer, thus determining the cathode performance. Liu 

To account for the observed temperature dependence, it is necessary to assume that the cathode performance of full cells is much better than measured on separate cathodes on thicker electrolytes, prepared by a very similar procedure. The main point is that the activation energy of the cathode reaction must be 

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V (2 ), Cr ( ), Zr (1 ), or Ta (1 ). The role of these promoters in the air cathode is unclear, and some have suggested that the active catalysts are alloys of the Ft with the transition metal (1,4) which form during heat-treatment of the oxide impregnated precursor. In the first section of this paper, we review the work from the Lawrence Berkeley Laboratory on the study of the mechanism of promotion of air cathode performance by these transition metal additives. 

Eickes C, Piela P, Davey J, Zelenay P. 2006. Recoverable cathode performance loss in direct methanol fuel cells. J Electrochem Soc 153 A171-A178. 

The suitability of lanthanum nickelate as an SOFC cathode has been examined by Virkar s group [138], They showed that LN performed poorly as a single-phase cathode in an anode-supported YSZ cell. However, with an SDC/LN composite interlayer the performance of the LN cathode increased substantially and the maximum power density of the cell with a YSZ thin electrolyte (-8 pm) was -2.2 Wear2 at 800°C, considerably higher than 0.3 to 0.4 Wcm-2 of similar cells with only LN or SDC interlayer. The results are significant as it shows that the composite MIEC cathodes perform much better than single-phase MIEC in the case of LN despite its mixed ionic and electronic conductivity. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

A similar trend was found for LSC-SSC composite cathodes, with a sintering temperature of 1000°C recommended by the authors, and a temperature of 1200°C observed to cause excessive sintering and lower cathode performance [81]. 

J. Lee, Y. Park, S.K. Lee, E.J. Cho, D.Y. Kim, H.Y. Chu, H. Lee, L.M. Do, and T. Zyung, Tris-(8-hydroxyquinoline)aluminum-based organic light-emitting devices with Al/CaF2 cathode performance enhancement and interface electronic structures, Appl. Phys. Lett., 80 3123-3125 (2002). 

Fig. 6.4 Scheme illustrating how the chemical interactions result in cathode performance issues. 

A number of technical and cost issues facing polymer electrolyte fuel cells at the present stage of development have been recognized by managers and researchers (6, 27, 28, 29). These issues concern the cell membrane, cathode performance, and cell heating limits. 

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. ... 

Oxidant The oxidant composition and utilization are parameters that affect the cathode performance, as evident in Figure 2-3. Air, which contains -21% Oi, is the oxidant of choice for PAFCs. The use of air with -21% Oi instead of pure Oi results in a decrease in the current density of about a factor of three at constant electrode potential. The polarization at the cathode increases with an increase in Oi utilization. Experimental measurements (38) of the change in overpotential (Aric) at a PTFE-bonded porous electrode in 100% H3PO4 (191°C, atmospheric pressure) as a function of O2 utilization is plotted in Figure 5-4 in accordance with Equation (5-7) ... 

Typical cathode performance curves obtained at 650°C with an oxidant composition (12.6% 02/18.4% C02/69% N2) that is anticipated for use in MCFCs, and a common baseline composition (33% 02/67% CO2) are presented in Figure 6-3 (20,49). The baseline composition contains O2 and CO2 in the stoichiometric ratio that is needed in the electrochemical reaction at the cathode (Equation (6-2)). With this gas composition, little or no diffusion limitations occur in the cathode because the reactants are provided primarily by bulk flow. The other gas composition, which contains a substantial fraction of N2, yields a cathode performance that is limited by gas phase diffusion from dilution by an inert gas. 

Oxidant The electrochemical reaction at the cathode involves the consumption of two moles CO2 per mole O2 (see Equation (6-2)), and this ratio provides the optimum cathode performance. The influence of the [C02]/[02] ratio on cathode performance is illustrated in Figure 6-7 (46). As this ratio decreases, the cathode performance decreases, and a limiting current is discernible. In the limit where no CO2 is present in the oxidant feed, the equilibrium involving the dissociation of carbonate ions becomes important. 

Figure 6-7 Effect of CO2/O2 Ratio on Cathode Performance in an MCFC, Oxygen Pressure is 0.15 atm (20, Figure 5-10, Pgs. 5-20)...

For DMFC systems, Pt cathodes are also used as the catalyst of choice however, given Pt s ability to reduce oxygen and oxidize methanol, this lack of selectivity makes them sensitive to methanol crossover from anode to cathode via the membrane. This methanol crossover can have a depolarizing effect on cathode performance, reducing overall cathode activity. To combat this, an extensive effort has been made to identify and develop selective oxygen/reduction catalysts unaffected by MeOH crossover. 

Song, Y, Wei, Y, Xu, H., Williams, M., Liu, Y., Bonville, L. J., Kunz, H. R., and Fenton, J. M. Improvement in high-temperature proton exchange membrane fuel cells cathode performance with ammonium carbonate. Journal of Power Sources 2005 141 250-257. 

This review focuses on the factors governing SOFC cathode performance—advances we have made over... 

One of the most heavily studied factors thought to influence cathode performance has been the issue of reactivity between the electrode material and the electrolyte (usually YSZ) to form insulating secondary phases. This subject is sufficiently broad and complex to warrant its own review, and readers having a detailed interest in this topic are encouraged to read previous literature reviews in papers by Kawada and Mitterdorfer. Our main focus here is on how these secondary phases (or other impurities) appear to retard the reaction, particularly electrochemical kinetic processes occurring at the interface. 

Exactly how these secondary phases influence cathode performance remains somewhat circumstantial. Labrincha et al. studied the electrical properties of LZ over a range of temperature and Pq and found it to have low conductivity under typical SOFC cathode operating conditions Q cm at... 

Water Balance, Maximum Air Feed Rate and Implications for Cathode Performance... 

From the foregoing discussion, it is clear that, in a DMFC, the air cathode has to be operated under rather challenging conditions, that is, with a low air feed rate at nearly full water saturation. This type of operating conditions can easUy lead to cathode flooding and thus poor and unstable air cathode performance. To secure better air cathode performance, we have made great efforts to improve the ell cathode structure and cathode flow field design to facilitate uniform air distribution and easy water removal. The performance of our 30-cell DMFC stacks operated with dry air feed at low stoichiometry is reported in the following section. 

Modifications of the chemical nature of the catalyst under cathodic load are also possible. Sulphides can be reductively dissolved with liberation of H2S [139]. Oxides can be progressively reduced with loss of the specific activity [140]. In the latter case, an additive can be used to diminish the rate of reduction. Intermetallic compounds or alloys may exhibit preferential dissolution of one of the components during cathodic performances in concentrated alkali [141],... 

High operating temperatures are needed to achieve sufficient electrolyte conductivity. Most MCFC stacks operate at 650°C, as a compromise between high performance and stack life, because, above 650°C there are increased electrolyte losses due to evaporation and increased material corrosion. The voltage of MCFCs varies with the composition of the reactant gases. Increasing the reactant gas utilisation generally decreases cell performance. A compromise leads to typical utilisations of 75 to 85% of the fuel. The electrochemical reaction at the cathode involves the consumption of two moles C02 per mole 02, and this ratio provides the optimum cathode performance (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

The beneficial effects on the cathode performance by alloying and on producing an ordered alloy structure are shown in Fig 7. 

Electrocatalytic investigations (185) on the preparation, properties, and long-term cathode performance of spongy Raney Ni type materials show that secondary structure (fine pores) and tertiary structure (coarser pores and cracks) depend on the chosen preparation procedure, and these factors determine the effective catalytic activity for the HER in a material way. Long-term performance is remarkably improved by controlled leaching of the Raney Ni alloy and oxidative aging (181,182,184) of the developed porous Raney Ni matrix. 

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