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Title: Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts

Abstract

The design of electrocatalysts for the oxidation and production of H2 is important for the development of alternative energy sources. This paper focuses on the electrocatalysts, where denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R' covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to – e– → for H2 oxidation and the reverse reaction for H2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT–ET and ET–PT, are investigated for the following (R,R’) substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pKas, and PT pathways, and Marcus theory is used to describe the electrochemical electron transfer, including the effects of solute and solvent reorganization energies. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT–ET mechanism would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable or thermoneutral ET. The sequential ET–PT mechanism for these systems would requiremore » a relatively large initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET–PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research. This material is based upon work supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under FWP56073.« less

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1034571
Report Number(s):
PNNL-SA-83518
Journal ID: ISSN 1932-7447; KC0307010; TRN: US1200843
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Journal of Physical Chemistry C
Additional Journal Information:
Journal Volume: 116; Journal Issue: 4; Journal ID: ISSN 1932-7447
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ATOMS; DESIGN; ELECTROCATALYSTS; ELECTRODES; ELECTRON TRANSFER; ELECTRONS; ENERGY SOURCES; FREE ENERGY; FUNCTIONALS; NICKEL; NITROGEN; OXIDATION; PHOSPHORUS; PRODUCTION; PROTONS; SOLUTES; SOLVENTS

Citation Formats

Fernandez, Laura, Horvath, Samantha, and Hammes-Schiffer, Sharon. Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts. United States: N. p., 2012. Web. doi:10.1021/jp210690q.
Fernandez, Laura, Horvath, Samantha, & Hammes-Schiffer, Sharon. Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts. United States. doi:10.1021/jp210690q.
Fernandez, Laura, Horvath, Samantha, and Hammes-Schiffer, Sharon. Thu . "Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts". United States. doi:10.1021/jp210690q.
@article{osti_1034571,
title = {Theoretical analysis of the sequential proton-coupled electron transfer mechanisms for H2 oxidation and production pathways catalyzed by nickel molecular electrocatalysts},
author = {Fernandez, Laura and Horvath, Samantha and Hammes-Schiffer, Sharon},
abstractNote = {The design of electrocatalysts for the oxidation and production of H2 is important for the development of alternative energy sources. This paper focuses on the electrocatalysts, where denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R' covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to – e– → for H2 oxidation and the reverse reaction for H2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT–ET and ET–PT, are investigated for the following (R,R’) substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pKas, and PT pathways, and Marcus theory is used to describe the electrochemical electron transfer, including the effects of solute and solvent reorganization energies. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT–ET mechanism would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable or thermoneutral ET. The sequential ET–PT mechanism for these systems would require a relatively large initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET–PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research. This material is based upon work supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under FWP56073.},
doi = {10.1021/jp210690q},
journal = {Journal of Physical Chemistry C},
issn = {1932-7447},
number = 4,
volume = 116,
place = {United States},
year = {2012},
month = {2}
}