Catalyst layer engineering

Advances in Anode Catalyst Layer Engineering Example Analysis... 

The electrode performance in any electrochemical system depends on the complex interaction between intrinsic kinetics and various transport processes involving reactants, products, and the electrolyte. In the particular case of direct fuel cells, the catalyst support (when employed), the hydrophobic-hydrophilic properties of the diffusion substrate, die ionomer load in the catalyst layer, and the electrode design, including the current collector, all have a great impact on die power output. The goal in the present section is to give an overview of the experimental advances in the area of catalyst layer engineering and anode structures. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

The necessity of having a post catalyst layer which can eliminate slipping ammonia (in addition, since CO and HC also must be eliminated, current catalytic SCR-urea systems applied to diesel engine emissions are composed typically of five catalytic layers, making the size of the catalytic converter quite large and therefore applicable essentially only to heavy-duty trucks and buses). 

Rajalakshmi, N., and Dhathathreyan, K. S. Catalyst layer in PEMFC electrodes— Fabrication, characterization and analysis. Chemical Engineering Journal 2007 129 31 0. 

Pt is, however, an expensive and limited resource. For a 60 kW fuel cell vehicle, the cost of Pt would be over 2,400 at current cost and loading of Pt. Even worse, replacing combustion engines in all existing vehicles by fuel cell drive systems at no penalty in power would exceed the known reserves of Pt. Catalyst layer design, therefore, strives to reduce the Pt loading markedly at no penalty in the fuel cell voltage. 

Figure 1.6. PEM fuel cell catalyst layer structure [13]. (Reproduced from Journal of Power Sources, 102, Costamagna P, Srinivasan S, Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000 Part II. Engineering, technology development and application aspects, 253-69 2001, with permission from Elsevier.)...

Once one chooses to create a catalyst layer with finite thickness, however, one must then consider the transport of reactants through the thickness of that layer. At the most fundamental level, an examination of the two defining reactions of a fuel cell reveals the engineering challenge that must be confronted in designing a catalyst layer. These reactions are, of course... 

Approaches for mass production of catalyst layers, i.e. reducing the coating cost are rarely reported. A cheap possibility has been demonstrated by O CormeU et al. (2011) by screen printing in the application of micro-engineered fuel reformers for hydrogen production. Such a coating is shown in Fig. 2. 

Current produced by the cell increases with the active surface of catalyst. To increase this surface a huge number of tiny catalyst particles are mixed with ionic and electron conductors in the catalyst layers. A void pore in the close vicinity of the catalyst particle is needed to provide fast delivery of feed molecules. Over the past decade, an enormous work has been done to engineer stable and efficient catalyst layers. However, even in the best layers a large amount of precious particles are disconnected from the electroljde or located far from the void pore and are, therefore, dead for the reaction. 

Dr. Qianpu Wang received his Diploma in Metallurgy Engineering from the Central South University, China in 1986 and his Ph.D. in Multiphase Flow from the Norwegian University of Science and Technology, Norway in 2001. He then joined the National Research Council of Canada Institute for Fuel Cell Innovation (NRC-IFCI) as a post-doctoral fellow until 2004. Thereafter he has been a research officer at NRC-IFCI. His research interests include fundamental understanding of mass transport limitations, catalyst utilization in catalyst layers, and fuel cell/stack modeling and optimization. 

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