Hierarchical model agglomerate

The assumption of constant Ch+ fails in water-filled agglomerates, considered in the section Hierarchical Model of CCL Operation in Chapter 4 and in water-filled... 

As for the first assumption, the electrolyte phase must be treated as a mixed phase. It consists of a thin-film structure of ionomer at the surface of Pt/C agglomerates and of water in ionomer-free intra-agglomerate pores. The proton density is highest at the ionomer film (pH 1 or smaller), and it is much smaller in water-filled pores (pH > 3). However, the proton density distribution is not incorporated in the statistical utilization Tstat, but in an agglomerate effectiveness factor, defined in the section Hierarchical Model of CCL Operation. ... 

As concerns the second assumption, direct access to the gas pore network is not a stringent requirement for keeping the Pt surface active. The condensed phases of water and ionomer possess finite oxygen permeabilities, which will render oxygen concentrations finite at electrolyte-covered Pt particles. Again, this is an issue of nonuniform reaction rate distributions that will be dealt with in the section Hierarchical Model of CCL Operation. In that section, a two-scale performance model will be presented, which couples transport and distribution of protons and oxygen at the agglomerate level and at the macroscopic scale. 

In the section Hierarchical Model of CCL Operation, a hierarchical modeling framework will be presented it couples multiple effects of proton and oxygen transport at the mesoscopic scale of agglomerates and at the macroscopic scale of the layer. In that general case, the CL effectiveness factor can be defined by... 

In the hierarchical model, macroscale transport processes will be coupled to transport and reaction at the mesoscale, as illustrated in Figure 4.1 an explicit treatment of agglomerate effects is needed to properly assess the effectiveness factor of Pt utilization, which transpires as the key parameter in the structural optimization of CCLs. 

Whether one is considering an agglomerate of aggregated zeoHte crystalUtes, or a pellet, a hierarchical model [89, 90] allows one to distinguish the different transport and/or rate processes which operate at different length scales. 

For the purpose of physical modeling of CL operation, it is expedient to treat the layer as a two-scale system as illustrated in Figure 4.1. Referring to the hierarchical scheme in Figure 1.16, the emphasis will be on the two rightmost scales. At the mesoscopic scale, operation is determined by the properties of agglomerates composed... 

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