Catalyst layer operation nanoparticles

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

The modeling of structure and operation of CLs is a multiscale problem. The challenges for the theory and modeling of catalyst layer operation are, however, markedly reduced if we realize that the main structural effects occur at well-separated scales, viz. at catalyst nanoparticles (a few nm), at agglomerates of carbon/Pt ( 100 nm), and at the macroscopic device level. 

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 ... 

This chapter gives an overview of the state of affairs in physical theory and molecular modeling of materials for PEECs. The scope encompasses systems suitable for operation at T < 100°C that contain aqueous-based, proton-conducting polymer membranes and catalyst layers based on nanoparticles of Pt. 

One critical issue facing the commercialization of low-temperature fuel cells is the gradual decline in performance during operation, mainly caused by the loss of the electrochemical surface area (EGA) of carbon-supported platinum nanoparticles at the cathode. The major reasons for the degradation of the cathodic catalyst layer are the dissolution of platinum and the corrosion of carbon under certain operating conditions, especially those of potential cycling. Cycling places various loads on... 

Table 11.2 and assume A=100, which is rather conservative value, to compute J via Eq. (11.32) and O via Eq. (11.22). The results show t p 0.91 which implies that the O2 backspillover mechanism is fully operative under oxidation reaction conditions on nanoparticle metal crystallites supported on ionic or mixed ionic-electronic supports, such as YSZ, Ti02 and Ce02. This is quite reasonable in view of the fact that, as already mentioned an adsorbed O atom can migrate 1 pm per s on Pt at 400°C. So unless the oxidation reaction turnover frequency is higher than 103 s 1, which is practically never the case, the O8 backspillover double layer is present on the supported nanocrystalline catalyst particles. 

Time courses of rate of hydrogen generated from decalin with carbon-supported platinum catalyst at various feed rates in bench-scale continuous operation. Catalyst platinum nanoparticles supported on ACC (5 wt-metal%), 0.29 g (one layer, ), 0.58 g (two layers, A), and 0.87 g (three layers, O). Feed rate of decalin 1.5, 2.0, 2.5, 3.0, and 5.0 mL/min. Reaction conditions boiling and refluxing by heating at 280°C and cooling at 25°C. 

Figure 8.1 summarizes the operation principle and the main mechanisms occurring at multiple scales in a DAFC alcohol and water transport in the anode (air and water in the cathode) at the macroscale (within the distributor), mesoscale (within the secondary pores formed by C in the electrodes) and microscale (within the primary pores of the C), nanoscale electrochemical double layer formation around the catalyst nanoparticles, alcohol electrochemical oxidation in the anode and ORR in the cathode. 

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