Outline and Objectives of This Chapter

This chapter will cover major topics of CL research, focusing on (i) electrocatalysis of the ORR, (ii) porous electrode theory, (iii) structure and properties of nanoporous composite media, and (iv) modern aspects in understanding CL operation. Porous electrode theory is a classical subject of applied electrochemistry. It is central to all electrochemical energy conversion and storage technologies, including batteries, fuel cell, supercapacitors, electrolyzers, and photoelectrochemi-cal cells, to name a few examples. Discussions will be on generic concepts of porous electrodes and their percolation properties, hierarchical porous structure and flow phenomena, and rationalization of their impact on reaction penetration depth and effectiveness factor. 

In the section Structure Eormation in Catalyst Layers and Effective Properties aspects related to the self-organization phenomena in CL inks will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Thereafter, catalyst layer performance models that involve parameters related to structure, processes, and operating conditions will be presented. 

In this chapter, connections will be established between electrocatalytic surface phenomena and porous media concepts. The underlying logics appear simple, at least at first sight. Externally provided thermodynamic conditions, operating parameters, and transport processes in porous composite electrodes determine spatial distributions of reaction conditions in the medium, specifically, reactant and potential distributions. Local reaction conditions in turn determine the rates of surface processes at the catalyst. This results in an effective reactant conversion rate of the catalytic medium for a given electrode potential. 

From the insights presented, conclusions will be drawn about conceivable improvements of catalyst effectiveness, voltage efficiency, power density, water handling capabilities, and stability through optimized operating conditions and advanced stractural design. 

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