Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

Abstract

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O. The rates and selectivities (for production of HO vs HO) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Free‐Standing 3D Electrodes for Electrochemical Detection of Hydrogen Peroxide

“…For further explorations, theoretical simulation about the interactions between the intermediates and the catalytic surface for thermodynamics and molecular dynamics could be an important approach for in‐depth understanding. Due to the intrinsic correlation between redox of H 2 O 2 and oxygen reduction reaction, the electrocatalyst with a low overpotential for ORR might be favorable for H 2 O 2 redox . It is worth noting that, most recent free‐standing 3D electrodes are tested in the typical three‐electrode system, real self‐standing devices integrating 3D electrodes with suitable reference electrode and counter electrode are in need for practical applications.…”

“…Due to the intrinsic correlation between redox of H 2 O 2 and oxygen reduction reaction, the electrocatalyst with a low overpotential for ORR might be favorable for H 2 O 2 redox. [175,176] It is worth noting that, most recent free- [170] Reprinted with permission from [170], copyright 2014, with permission from Elsevier. (c) Structure of Au-luminol based microelectrode for electrochemical imaging of intracellular H 2 O 2 ; (d) image of the microelectrode in the cell.…”

A Phthalocyanine‐Based Layered Two‐Dimensional Conjugated Metal–Organic Framework as a Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction

“…[15] In order to address these challenges,t he development of conductive two-dimensional (2D) conjugated MOFs,i .e., atomically ordered, planar metal-organic networks extending in two dimensions with fully in-plane p-delocalization and weak out-of-plane p-p stacking is of great significance in electrocatalysis.This is due to their unique 2D features as well as improved electron transfer capacity and the high utilization of exposed active sites,inaddition to the inherent advantages of traditional MOFs. [25][26][27] Recently,aNi 3 (hexaiminotriphenylene) 2 2D MOF with Ni-N 4 active sites was demonstrated as an ORR electrocatalyst with an onset potential of 0.82 Vv s. RHE in alkaline media. [25][26][27] Recently,aNi 3 (hexaiminotriphenylene) 2 2D MOF with Ni-N 4 active sites was demonstrated as an ORR electrocatalyst with an onset potential of 0.82 Vv s. RHE in alkaline media.…”

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

“…As asubclass of the broader coordination polymer family, MOFs have three important characteristics of crystallinity, porosity,a nd strong metal-ligand interactions,m aking them as ab ridge between homogeneous and heterogeneous catalysts. [24,25] More specifically,t he periodic structure of MOFs allows uniform dispersion of the active sites throughout the porous framework, which is awell-defined characteristic of homogeneous catalysts,while the solid nature in nano/ micro-scale endows MOFs as the promising heterogeneous catalysts. [16,[24][25][26][27] Therefore,the rational design of MOFs could meet the benefits of both heterogeneous and homogeneous catalysts in one system.…”