Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential

Abstract

The selective four electron, four proton, electrochemical reduction of O2 to H2O in the presence of a strong acid (TFA) is catalyzed at a dicobalt center. The faradaic efficiency of the oxygen reduction reaction (ORR) is furnished from a systematic electrochemical study by using rotating ring disk electrode (RRDE) methods over a wide potential range. We derive a thermodynamic cycle that gives access to the standard potential of O2 reduction to H2O in organic solvents, taking into account the presence of an exogenous proton donor. The difference in ORR selectivity for H2O vs H2O2 depends on the thermodynamic standard potential as dictated by the pKa of the proton donor. The model is general and rationalizes the faradaic efficiencies reported for many ORR catalytic systems.

Mechanisms of Two‐Electron versus Four‐Electron Reduction of Dioxygen Catalyzed by Earth‐Abundant Metal Complexes

“…Binuclear cobalt‐1,10‐phenanthroline (phen) and cobalt‐2,2′‐bipyridine (bpy) complexes bridged by 1,8‐bis(2,2′:6′,2′′‐terpyrid‐4′‐yl)anthracene (btpyan) were reported to catalyze the electrochemical four‐electron/four‐proton reduction of O 2 to H 2 O without the generation of H 2 O 2 as a two‐electron reduction product through the binuclear cobalt‐ μ ‐1,2‐peroxo complex [Co( μ ‐O 2 )(L) 2 (btpyan)] 4+ (L=phen and bpy) . Dicobalt complexes with a diamond Co III 2 (OH) 2 stabilized by the six coordinate ligand, dipyridylethane naphthyridine (DPEN), also act as electrocatalysts for the four‐electron/four‐proton reduction of O 2 with CF 3 COOH in MeCN . In the case of a binuclear cobalt‐ μ ‐1,2‐peroxo complex with a decadentate dinucleating ligand containing a pyridazine bridging group and pyridylic arms, however, the peroxo complex catalyzed only two‐electron/two‐proton reduction of O 2 by Me 8 Fc in the presence of CF 3 COOH to produce H 2 O 2 .…”

Efficient Solar‐Assisted O2 Reduction Using a Cofacial Iron Porphyrin Dimer Catalyst Integrated into a p‐CuBi2O4 Photocathode

“…Indeed catalyzed ORR is achieved with high efficiency and selectivity both in nature by cytochrome c oxidases and related oxidases and in many fuel cells by precious Pt‐based catalysts . Recently, substantial progress has been achieved in the electrochemical ORR with various non‐precious Fe, Co, Mn, Cu, and Ni catalysts. …”

“…The results indicate that more than 3.7 electrons were consumed and <15 % H 2 O 2 were produced per O 2 reduction, indicating the high selectivity of the Fe 2 TPFPP‐TMP for ORR in non‐aqueous medium. A recently reported binuclear Co complex with dipyridylethane naphthyridine ligand showed in non‐aqueous CH 3 CN solution containing proton donors such as phenol a remarkable ORR activity with >40 % H 2 O 2 production …”

Sonochemical Synthesis of Two Dimensional C3N4 Nanosheets Supported Palladium Composites and Their Electrocatalytic Activity for Oxygen Reduction and Methanol Oxidation Reaction

“…Generally, the products of ORR are typically H 2 O and H 2 O 2 with the latter being the undesirable product owing to its lower cell voltage. [36] The 4e  process is considered to be more efficient than the 2e  process that produces H 2 O 2 . The H 2 O 2 production and the electron transfer number n were determined by using:…”