Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups
- 25 February 2021
- journal article
- research article
- Published by Wiley in ChemSusChem
- Vol. 14 (8), 1886-1892
- https://doi.org/10.1002/cssc.202002756
Abstract
In the search for replacement of the platinum‐based catalysts for fuel cells, MN4 molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum‐group metal‐free (PGM‐free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e− to 4e− pathway in the pH range of 3.5‐6. This phenomenon was shown by density functional theory (DFT) calculations to be related to the protonation of the nitrogen atoms and carboxylic acid groups on the corroles indicated by the pKa values of the protonation sites in the vicinity of the ORR active sites. Control of the electron‐withdrawing nature of these groups characterized by the pKa values could switch the ORR from the H+ to e− rate‐determining step mechanisms and from 2e− to 4e− ORR pathways and also controlled the durability of the corrole catalysts. The results suggest that protonation of the nitrogen atoms plays a vital role in both the ORR activity and durability for these materials and that pKa of the N atoms at the active sites can be used as a descriptor for the design of high‐performance, durable PGM‐free catalysts.Keywords
Funding Information
- Ministry of Energy, Israel
This publication has 41 references indexed in Scilit:
- Effect of pH on the Activity of Platinum Group Metal-Free Catalysts in Oxygen Reduction ReactionACS Catalysis, 2018
- Active sites and factors influencing them for efficient oxygen reduction reaction in metal-N coordinated pyrolyzed and non-pyrolyzed catalysts: a reviewJournal of Materials Chemistry A, 2017
- Flexible and Lightweight Fuel Cell with High Specific Power DensityACS Nano, 2017
- A Model for the pH-Dependent Selectivity of the Oxygen Reduction Reaction Electrocatalyzed by N-Doped Graphitic CarbonJournal of the American Chemical Society, 2016
- Earth‐Abundant Nanomaterials for Oxygen ReductionAngewandte Chemie, 2015
- Platinfreie Nanomaterialien für die SauerstoffreduktionAngewandte Chemie, 2015
- Recent advancements in Pt and Pt-free catalysts for oxygen reduction reactionChemical Society Reviews, 2015
- Mechanistic and kinetic implications on the ORR on a Au(100) electrode: pH, temperature and H–D kinetic isotope effectsPhysical Chemistry Chemical Physics, 2014
- A thermally self-sustained micro solid-oxide fuel-cell stack with high power densityNature, 2005
- Fuel cells for portable applicationsJournal of Power Sources, 2002