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Title: Surface Immobilization of Molecular Electrocatalysts for Energy Conversion

Abstract

Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical energy and chemical energy. Molecular catalysts offer precise control of their structure, and the ability to modify the substituents to understand structure-reactivity relationships that are more difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non-covalent interactions. Advantages of surface immobilization include the need for less catalyst, avoidance of bimolecular decomposition pathways, and easier determination of catalyst lifetime. Copper-catalyzed click reactions are often used to form covalent bonds to surfaces, and pi-pi stacking of pyrene substituents appended to the ligand of a molecular complex is a frequently used method to achieve non-covalent surface immobilization. This mini-review highlights surface confinement of molecular electrocatalysts for reduction of O2, oxidation of H2O, production of H2, and reduction of CO2.

Authors:
ORCiD logo [1];  [1]; ORCiD logo [1]
  1. Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, Richland WA 99352 USA
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1363987
Report Number(s):
PNNL-SA-121541
Journal ID: ISSN 0947-6539; KC0307010
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Chemistry - A European Journal; Journal Volume: 23; Journal Issue: 32
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Bullock, R. Morris, Das, Atanu K., and Appel, Aaron M. Surface Immobilization of Molecular Electrocatalysts for Energy Conversion. United States: N. p., 2017. Web. doi:10.1002/chem.201605066.
Bullock, R. Morris, Das, Atanu K., & Appel, Aaron M. Surface Immobilization of Molecular Electrocatalysts for Energy Conversion. United States. doi:10.1002/chem.201605066.
Bullock, R. Morris, Das, Atanu K., and Appel, Aaron M. Wed . "Surface Immobilization of Molecular Electrocatalysts for Energy Conversion". United States. doi:10.1002/chem.201605066.
@article{osti_1363987,
title = {Surface Immobilization of Molecular Electrocatalysts for Energy Conversion},
author = {Bullock, R. Morris and Das, Atanu K. and Appel, Aaron M.},
abstractNote = {Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical energy and chemical energy. Molecular catalysts offer precise control of their structure, and the ability to modify the substituents to understand structure-reactivity relationships that are more difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non-covalent interactions. Advantages of surface immobilization include the need for less catalyst, avoidance of bimolecular decomposition pathways, and easier determination of catalyst lifetime. Copper-catalyzed click reactions are often used to form covalent bonds to surfaces, and pi-pi stacking of pyrene substituents appended to the ligand of a molecular complex is a frequently used method to achieve non-covalent surface immobilization. This mini-review highlights surface confinement of molecular electrocatalysts for reduction of O2, oxidation of H2O, production of H2, and reduction of CO2.},
doi = {10.1002/chem.201605066},
journal = {Chemistry - A European Journal},
number = 32,
volume = 23,
place = {United States},
year = {Wed Mar 22 00:00:00 EDT 2017},
month = {Wed Mar 22 00:00:00 EDT 2017}
}
  • Molecular electrocatalysts can play an important role in energy storage and utilization reactions needed for intermittent renewable energy sources. This manuscript describes three general themes that our laboratories have found useful in the development of molecular electrocatalysts for reduction of CO 2 to CO and for H 2 oxidation and production. The first theme involves a conceptual partitioning of catalysts into first, second, and outer coordination spheres. This is illustrated with the design of electrocatalysts for CO 2 reduction to CO using first and second coordination spheres and for H 2 production catalysts using all three coordination spheres. The secondmore » theme focuses on the development of thermodynamic models that can be used to design catalysts to avoid high energy and low energy intermediates. In this research, new approaches to the measurement of thermodynamic hydride donor and acceptor abilities of transition metal complexes were developed. Combining this information with other thermodynamic information such as pKa values and redox potentials led to more complete thermodynamic descriptions of transition metal hydride, dihydride, and related species. Relationships extracted from this information were then used to develop models that are powerful tools for predicting and understanding the relative free energies of intermediates in catalytic reactions. The third theme is the control of proton movement during electrochemical fuel generation and utilization reactions. This research involves the incorporation of pendant amines in the second coordination sphere that can facilitate H-H bond heterolysis and heteroformation, intramolecular and intermolecular proton transfer steps, and the coupling of proton and electron transfer steps. Studies also indicate an important role for outer coordination sphere in the delivery of protons to the second coordination sphere. Understanding these proton transfer reactions and their associated energy barriers are key to the design of faster and more efficient molecular electrocatalysts for energy storage. Funding for the research described in this manuscript was provided as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science and through individual grants from the Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.« less
  • This review discusses the development of molecular electrocatalysts for H2 production and oxidation based on nickel. A modular approach is used in which the structure of the catalyst is divided into first second and outer coordination spheres. The first coordination sphere consists of the ligands bound directly to the metal center, and this coordination sphere can be used to control such factors as the presence or absence of vacant coordination sites, redox potentials, hydride acceptor abilities and other important thermodynamic parameters. The second coordination sphere is defined as functional groups such as pendant acids or bases that can interact withmore » bound substrates such as H2 molecules and hydride ligands, but that do not form strong bonds with the metal center. These functional groups can play diverse roles such as assisting the heterolytic cleavage of H2, controlling intra- and intermolecular proton transfer reactions, and provide a physical pathway for coupling proton and electron transfer reactions. By controlling both the hydride donor/acceptor ability of the catalysts using the first coordination sphere and the proton acceptor/donor abilities of the functional groups in the second coordination sphere, catalysts can be designed that are biased toward H2 production, H2 oxidation, or that are bidirectional (catalyzing both H2 oxidation and production). The outer coordination sphere is defined as that portion of the catalytic system that are not in the first and second coordination spheres. This coordination sphere can assist in the delivery of protons and electrons to and from the catalytically active site, thereby adding another important avenue for controlling catalytic activity. Many features of these simple catalytic systems are good models for enzymes and they provide the opportunity to probe certain aspects of catalysis that may be difficult in enzymes themselves, but that can provide insights into enzyme function and reactivity.« less