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Title: Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Abstract

The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. In this work, nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C–H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H 2. Support for the theoretically predicted mechanism is providedmore » by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.« less

Authors:
 [1];  [2];  [2];  [2];  [3]
  1. Univ. of Illinois, Urbana-Champaign, IL (United States). Dept. of Chemistry; Humboldt Univ. of Berlin (Germany). Inst. of Chemistry
  2. Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology
  3. Univ. of Illinois, Urbana-Champaign, IL (United States). Dept. of Chemistry
Publication Date:
Research Org.:
Harvard Univ., Cambridge, MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); Alexander von Humboldt Foundation; National Science Foundation (NSF)
OSTI Identifier:
1235503
Alternate Identifier(s):
OSTI ID: 1469107
Grant/Contract Number:  
SC0009758; CHE-1305124
Resource Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 113; Journal Issue: 3; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; electrocatalysis; metalloporphyrin; proton transfer; dearomatization

Citation Formats

Solis, Brian H., Maher, Andrew G., Dogutan, Dilek K., Nocera, Daniel G., and Hammes-Schiffer, Sharon. Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism. United States: N. p., 2015. Web. doi:10.1073/pnas.1521834112.
Solis, Brian H., Maher, Andrew G., Dogutan, Dilek K., Nocera, Daniel G., & Hammes-Schiffer, Sharon. Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism. United States. doi:10.1073/pnas.1521834112.
Solis, Brian H., Maher, Andrew G., Dogutan, Dilek K., Nocera, Daniel G., and Hammes-Schiffer, Sharon. Thu . "Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism". United States. doi:10.1073/pnas.1521834112.
@article{osti_1235503,
title = {Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism},
author = {Solis, Brian H. and Maher, Andrew G. and Dogutan, Dilek K. and Nocera, Daniel G. and Hammes-Schiffer, Sharon},
abstractNote = {The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. In this work, nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C–H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H2. Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.},
doi = {10.1073/pnas.1521834112},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 3,
volume = 113,
place = {United States},
year = {2015},
month = {12}
}

Journal Article:
Free Publicly Available Full Text
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DOI: 10.1073/pnas.1521834112

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