Catalyst layer nanoparticles

The catalyst inks were prepared by dispersing the catalyst nanoparticles into an appropriate amoimt of Millipore water and 5wt% Nafion solution. Then, both the anode and cathode catalyst inks were directly painted using a direct painting technique onto either side of a Nafion 117 membrane. A carbon cloth diffusion layer was placed on to top of both the anode and cathode catalyst layers [3-5]. The active cell area was 2.25cm. ... 

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 ... 

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. 

A series of experimental results on the ratio of heat recuperation is shown in Table 13.4. Here temperatures of the catalyst layer, composed of a sheet of ACC dispersed with platinum nanoparticles (5 wt-metal%, 3.0 g), were varied at the constant feed ratio (3.5 mmol/min) of liquid methylcyclohexane (MCH) [39]. At the catalyst-layer temperature of 285°C, the heat recuperation ratio of 63.3% was attained, being much larger than the magnitudes of 15.6 and 47.5% for the catalyst-layer temperatures of 210°C and 245°C, respectively. 

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. 

The generation of electric current in modern catalyst layers proceeds at nanoparticles of Pt that are randomly dispersed on a porous carbon substrate with pores of nanoscopic dimension (1-10 nm). A certain fraction of larger pores (10-100 nm) is needed for the supply of gaseous reactants and... 

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. 

Figure 1. From the macroscale to the nanoscale a membrane electrode assembly has a polymer electrolyte membrane sandwiched between two catalyst layers and gas diffusion layers. The catalyst layer is composed of carbon particles impregnated with catalyst nanoparticles. Effective utilization of the catalyst particles depends on their local environment.

An archetypal MEA consists of an electrolyte membrane sandwiched between two catalyst layers and two gas diffusion layers (GDLs) as shown in Fig. 1. The fuel and oxidant gases diffuse through the GDL to react in the catalyst layer between the electrode and electrolyte. The catalyst, typically Pt or Pt based alloy, are nanoparticles residing on carbon particles. In addition to its primary purpose as the center of reactivity, the catalyst must participate in the effective adsorption of the reactants, conduction of the electrons to/from the electrode and diffusion of protons to/from... 

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

Figure 2.1. Structure and composition of catalyst layers at three different scales At the nanoparticle level, anode and cathode processes are depicted, including possible anode poisoning by CO. At the agglomerate level, ionomer functions as binder and proton-conducting medium are indicated, and points with distinct electrochemical environments are shown (double- and triple-phase boundary). At the macroscopic scale, the interpenetrating percolating phases of ionomer, gas pores, and solid Pt/Carbon are shown, and the bimodal porous structure is indicated.

As discussed before, factors at the nanoparticle level in Eq. (2.21) further reduce catalyst utilization, viz. the atomistic surface-to-volume ratio of catalyst atoms, and the fraction of active sites. Besides, catalyst particle sizes and surface structure affect the specific activity, y of the catalyst layer, as corroborated in Section 2.4. 

The electrolyte membrane is in contact with catalyst layers consisting of platinum nanoparticles typically supported on a porous carbon black. The thickness of the catalyst layer is in the range from 5 to 20 pm which is contacted by a gas diffusion layer (GDL) of thickness 100-250 pm. The functional layers electrolyte membrane, catalytic layer and gas diffusion layer are making up the active part of the MEA. A detailed discussion of electro catalysts and catalyst layers is given in [9]. 

Electrons are transferred through the catalyst support to the catalyst nanoparticle while protons are transferred from the catalyst surface to the electrolyte phase. It is obvious that the ion conducting phase needs to be extended into the volume of the catalyst layer in order to minimize noble metal particles which are either not contacted or buried too deep in the electrolyte. It has to be noted that reactant presence at the reaction site can also be achieved by reactant diffusion from the gas phase through a thin electrolyte film covering the platinum particle. Reactant transport properties therefore have a significant influence on the design and... 

Figure 1.8 shows a scheme of the microstructxu e of the catalyst layer, where the presence of TPRs allows the simultaneous flows of and electrons toward the PEM and GDL, respectively. Note that the thickness of the proton conducting membrane covering the carbon-supported catalyst nanoparticles is of the order of nanometers, depending of the concentration of proton conducting binder (usually Nafion) in the ink used to prepare the MEA. 

As it was discussed above, the carbon with small mesopores used as support produce high dispersion of the PtRu nanoparticles. However, in the fuel cell test they can show a poorer performance than the catalysts supported on Vulcan. Catalyst nanoparticles deposited in a tight pore might be tmconnected to the perfluorosulfonate ionomer and inaccessible to the methanol. However, other factors affect the triple phase boundary and in consequence the performance of the cell using small mesopores carbon supports. For example, the method of catalyst layer formation, including ink formation and dispersion of the catalyst... 

In the case of gas-solid and liquid-solid heterogeneous reactions, catalyst nanoparticles themselves or a catalyst supported on nanoparticles can be suspended in a liquid phase and passed through the reactor. Doing so, however, may lead to clogging of microchannels. A solution to the latter problem is to coat the catalyst layer onto the microfluidic walls. The formation of a catalyst layer on a reactor wall can be achieved using... 

PEM fuel cells require catalysts for both cathode as well as anode reactions. Platinum nanoparticles are the conventionally used catalysts. Conventionally, Pt-nanoparticles are dispersed on a support material such as carbon black. The support material should have properties such as high-electronic conductivity, surface area and corrosion resistance. CNTs, owing to their superior properties, can provide superior performance as catalyst support. Superior cell performance and lifetime have been reported for Pt-nanoparticles-coated CNTs catalyst layers [160-162]. In addition, the presence of CNTs in the catalyst layer provides superior connectivity and easy charge transport. The requirement of high cost catalysts (platinum and its alloys) for both cathodic and anodic reactions makes them unsuitable for commercial applications. Development of efficient catalysts with reduced cost has drawn considerable scientific elforts. Recently, defective... 

The support needs to meet certain criteria. It should be a good electron conductor to allow electrons to move within the catalyst layer. It would be ideal if the support possesses both good electron and proton conductivities, because such a support can enhance catalyst utilization. It should be stable in the fuel cell environment otherwise, the Pt nanoparticles can detach from the support and potentially become useless. For example, the fuel cell... 

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