Electrocatalysis of ORR

With the four-electron ORR process in the fuel cell cathode well recognized as the principal challenge at both the theoretical and experiniental/technical levels, it is interesting to examine the effects of the most recent theoretical developments on the fundamental understanding of ORR electrocatalysis. Such examination would naturally focus on the nature and quahty of the links with experimental work, as reflected by ... 

Need for Higher Surface Areas in the Context of ORR Electrocatalysis... 

For electrocatalysis of an ORR, two of the most important criteria are catalyst s activity and stability. The activity measures how fast the electrochemical reaction can be speeded up by the catalyst. The faster the catalyzed reaction, the higher the activity of that catalyst would be. For example, a high active ORR... 

Extensive research related to electrocatalysis of oxygen reduction reactions (ORRs) and hydrogen or methanol or ethanol oxidation reactions (HORs, MORs, or EORs) in alkaline media have been conducted worldwide in the last five decades. Spendelow and Wieckowski have provided an in-depth review of electrocatalysis for O2 reduction on Pt-group and Ag metal and alloys, for methanol oxidation on... 

Chapters 7-12 focus on the electrocatalysis of carbon-based non-precious metal catalysts. The unique properties and fuel cell applications of various carbon based catalysts are intensively discussed in these chapters. Chapter 7 summarizes the fundamental studies on the electrocatalytic properties of metallomacrocyclic and other non-macrocyclic complexes. Chapter 8 and 9 review the progress made in the past 5 years of pyrolyzed carbon-supported nitrogen-coordinated transition metal complexes. Chapter 10 gives a comprehensive discussion on the role of transitional metals in the ORR electrocatalysts in acidic medium. Chapter 11 introduces modeling tools such as density functional theory (DPT) and ah initio molecular dynamics (AIMD) simulation for chemical reaction studies. It also presents a theoretical point of view of the ORR mechanisms on Pt-based catalysts, non-Pt metal catalysts, and non-precious metal catalysts. Chapter 12 presents an overview on recent progresses in the development of carbon-based metal-free ORR electrocatalysts, as well as the correlation between catalyst structure and their activities. 

Chemical reactions are temperature sensitive, and indeed, chemical rate constants and reactions mechanism are expected to vary considerably with temperature. Most investigations on the electrocatalysis of the ORR are usually performed at ambient conditions, which do not necessarily represent the behavior of the materials and the reaction at the conditions of practical interest. For example, in proton exchange membrane fuel cells, the temperature of operation is between 80 and 100 °C. Significant discrepancy in behavior may arise if reactions and materials are tested at ambient conditions and their behavior at high temperatures is merely deduced firom extrapolation. Schafer et al. introduced variable temperature SECM, with an operational range of 0-100 °C, by integrating a temperature control unit (Peltier element) into an SECM setup, as shown in the schematic of Fig. 23 [66]. At the heart of the temperature control unit is the Peltier element, which is housed in a stainless steel block. 

Based on the extensive research performed over the past 20 years, of which only a small fraction has been presented here, there is no doubt that real advances have been made in understanding the concept of electron reservoirs created in the chevrel-type cluster units for electrocatalysis of reactions like the ORR. The investigated catalytic materials are indeed novel, and seem to have the ability to provide coordination (through their catalytic centers) for oxygen reactivity and selectivity. They have certainly thrown light on the nature of the complicated ORR, and have provided researchers with a eontrolled system for the systematic investigation of this best known cathodic reaction. 

Specific processes involved in the ORR will be dealt with in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum in Chapter 3. Here, the focus is on basic phenomenological concepts that are required to incorporate electro-catalytic reactions in device-level modeling of PEFCs. For a more detailed treatment of fundamental concepts in electrocatalysis, see the recent edition of the textbook by Schmickler and Santos (2010), as well as the classical textbook by Bard and Faulkner (2000). 

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. 

Pathways, mechanisms, and corresponding kinetic parameters of the ORR have been discussed in the section Electrocatalysis of the Oxygen Reduction Reaction at Platinum. In a highly simplified picture, derived originally on the basis of a series of experimental studies by Damjanovic and coworkers (Damjanovic, 1992 Gatrell and MacDougall, 2003 Sepa et al., 1981,1987), it was proposed that the rate-determining reaction step is the initial adsorption. 

In conclusion, the main challenge in the design of nanoporous materials for UTCLs is to fine-tune the proton concentration in order to optimize the interplay of ORR and Pt dissolution. This fundamental principle has not been widely realized, as most efforts in fuel cell electrocatalysis compare candidate catalyst materials at identical and normally high proton concentration. Proton concentration is usually not considered a parameter to tinker with although it is the key card in the game. 

Electrocatalysis of the ORR of various Chevrel type chalcogenide compounds was first studied by Alonso-Vante and Tributsch [19] in an acid medium and an enhancement in the ORR activity was observed when Mo was partially substituted by Ru. The compounds containing non-substituted octahedral Mo show metallic behavior while Ru substituted Mo octahedral (Moe-rM Xg, where X = S, Se, Te) systems show semiconducting behavior. The compound Mo4.2Rui,8Se8 displayed catalytic behavior comparable to that of Pt. The ORR overpotentials on catalysts of the series MogXg followed the trend Te[Pg.468]

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