ABSTRACT
The oxygen reduction reaction (ORR) represents one of the most important electrochemical reactions, which is crucial to a variety of electrochemical energy storage and conversion technologies, particularly low-temperature polymer electrolyte membrane fuel cells (PEMFCs). The ORR in acidic and alkaline media can be correspondingly described by the following electrochemical reactions and standard reaction potentials (E°):
O2 + 4H+ + 4e− → 2H2O E° = 1.229 V (acidic media) (7.1)
O2 + 2H2O + 4e− → 4OH− E° = 0.401 V (alkaline media) (7.2)
CONTENTS
7.1 Introduction ................................................................................................ 195 7.2 Roles of Transition Metals ........................................................................ 198 7.3 Roles of Nitrogen ....................................................................................... 201 7.4 Roles of Supporting Templates ................................................................ 204
7.4.1 Carbon Black Nanoparticles ......................................................... 204 7.4.2 Carbon Nanotubes ......................................................................... 205 7.4.3 Graphene and Graphene Oxide ................................................... 207 7.4.4 Noncarbon Supports ..................................................................... 207
7.5 In Situ Formed Graphitized Carbon Nanostructures ........................... 209 7.5.1 Nitrogen Precursors ...................................................................... 210 7.5.2 Transition Metals ........................................................................... 214 7.5.3 Heating Temperature .................................................................... 215
7.6 Conclusion .................................................................................................. 216 References ............................................................................................................. 217
Unfortunately, the ORR electrode kinetics is quite sluggish with a high over-potential, which has been the main obstacle to its effective usage in PEMFCs. In order to speed up the ORR kinetics, electrocatalysts are necessary. In practice, platinum (Pt)-based catalysts are the state of the art in terms of activity and durability for the ORR. However, prohibitive cost and scarcity have limited widespread implementation of Pt metal-based catalysts [1]. To fully realize these technologies, such as PEMFCs, highly active, durable, and inexpensive catalysts based solely on earth-abundant elements are desperately needed for the ORR. In the most recent decade, some promising nonprecious metal catalysts (NPMCs) for ORR have been studied intensively, including organometallic components, nonprecious metal chalcogenides, and nitrogen-doped carbon nanotubes (CNTs) and graphene catalysts [2-4]. Although some of the best performance NPMCs have exhibited ORR catalytic activity close or even superior to Pt-based catalysts in alkaline electrolyte [5-7], the grand challenge of developing an NPMC for Naon membrane and ionomer-based acidic PEMFCs has remained for decades due to both insufcient activity and durability. Recent progresses in the development of high-performance NPMCs for PEMFCs have suggested that a type of catalyst synthesized from Fe, Co, N, and C (abbreviated as M-N-C with M=Fe or Co) has the potential to efciently catalyze ORR, showing approaching activity and durability to Pt catalysts [8,9].
