Phosphoric acid fuel cell cathode catalyst layer

Yang SC (2000) Modeling and simulation of steady-state polarization and impedance response of phosphoric acid fuel-cell cathodes with catalyst-layer microstructure consideration. J Electrochem Soc 147 71-77... 

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

Electrochemical corrosion of carbon supports was widely studied in the context of phosphoric acid fuel cell development (Antonucci et al. 1988 Kinoshita 1988), but recently also the low-temperature fuel cell community paid more attention to this process (Kangasniemi et al. 2004 Roen et al. 2004). Carbon corrosion in fuel cell cathodes in the form of surface oxidation leads to functionalization of the carbon surface (e.g., quinones, lactones, carboxylic acids, etc.), with a concomitant change in the surface properties, which clearly results in changes of the hydrophobicity of the catalyst layer. Additionally, and even more severe, total oxidation of the carbon with the overall reaction... 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

Kim, J.H., Kim, H.J., Lim, T.H., Lee, H.I. (2007) Dependence of the performance of a high-temperature polymer electrolyte fuel cell on phosphoric acid-doped polyhenzimi-dazole ionomer content in cathode catalyst layer. Journal of Power Sources, 170, 275-280. 

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