Catalyst layer operation composition effects

How are we going to disentangle this mess The strategy of the modeling approaches reviewed in this contribution is to start from appropriate structural elements, identify relevant processes, and develop model descriptions that capture major aspects of catalyst layer operation. In the first instance, this program requires theoretical tools to relate structure and composition to relevant mass transport coefficients and effective reactivities. The theory of random... 

Optimum thickness. At fixed composition a phase diagram of the catalyst layer can be generated, which establishes a relation between the optimum thickness interval of the catalyst layer and the target current density jo (or jo interval) of fuel cell operation. The optimum compromise between kinetic losses and mass transport losses is realized in the intermediate regime. The existence of an upper limit on the thickness beyond which the performance would start to deteriorate is due to the concerted impact of oxygen and proton transport limitations, whereas considered separately each of the effects would only serve to define a minimal thickness. 

High catalyst activity and utilization of sputtered thin films was demonstrated in operating fuel cells. Optimal sputter-deposition conditions for platinum-ruthenium alloys have been determined. The effect of composition on the performance of Pt-Ru films was studied, and optimal composition has been determined. Novel methods of enhancing surface area and improving porosity have been identified. Co-sputtered ruthenium oxide has been demonstrated not to have any significant beneficial effect on the activity of the catalyst layers. While cost presents a major obstacle to commercialization of DMFCs for mobile applications, this project demonstrates novel means to reduce the catalyst costs in DFMC fuel cells. Efficiency enhancements that are also necessary for DMFCs to be viable will be addressed... 

Due to the thickness range, L 10 — 20pm, the operation of conventional catalyst layers depends decisively on the availability of sufficient gas porosity for the transport of reactants. The pertinent theory of gas diffusion electrodes dates back to the 1950s with major contributions by A.N. Frumkin and O.S. Ksenzhek [79-81]. Specifically, catalyst utilization and specific effective surface area in composite electrodes were always the focus of attention (see, e.g. [82,83] and the articles quoted therein). 

On the same topic of DMFC performance with supported vs. unsupported catalysts Smotkin and co-workers concluded that at 363 Kthe supported PtRu (1 1) catalyst with a toad of 0.46 mg cm performed as welt as an unsupported PtRu (1 1) with over four times higher load, i.e., 2 mg cm [266]. It is likely that these differences between various studies are related not only to the intrinsic activity of the respective anode catalys layers but also to the manufacturing procedures such as catalyst layer preparation and application techniques, MEA hot pressing conditions (temperature, pressure and time), presence or absence of other binders (such as PTFE) and fuel cell compression. All these MEA manufacturing variables can affect, in a poorly understood manner at present, the structure, morphology and composition of the catalyst layer in the operating fuel cell. Therefore, in fuel cell experiments it is difficult to isolate the truly physico-chemical effect of the support on the catalytic activity. 

The performance of a HT-PEMFC depends mainly on the amount of phosphoric acid in the polymer membrane and in the porous catalyst layer as well as on the temperature. Furthermore, it is well known that phosphoric acid dehydrates at low water vapour partial pressure and rehydrates with increasing partial pressure, effects which can be observed in HT-PEMFCs under operating conditions [1, 2]. The composition change of phosphoric acid results in a variation of the ionic conductivity as well as of the viscosity. 

The application of permeable composite monolith membranes for the FT synthesis has been tested [122]. An overview of concepts associated with this reactor type has been presented (Figure 12.25) [123]. Novel uses of this concept have been advanced, and some experimental results have demonstrated the ability to operate at high CO conversion with metal FT catalysts by removal of the water produced during the synthesis [ 124] and the encapsulation of an FT catalyst by a zeolite membrane layer to effect upgrading reactions in the FT reactor [125]. The potential of this technique merits further studies to evaluate the ability to scale to a commercial level. 

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