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Coupling Theory to Electrode Design for Electrocatalysis David P. Hickey and Shelley D. Minteer Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States
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Predicting the thermochemistry of interfacial proton-coupled electron-transfer reactions.
P
roton-coupled electron-transfer (PCET) reactions are ubiquitous in electrochemical energy conversion processes and underpin several biological pathways.1,2 While successful mechanistic investigations of PCET reactions only began relatively recently (seeing a major uptick starting around 2003), there have been widespread attempts to incorporate PCET schemes into newly developed electrocatalysts.3,4 The primary benefit to the concerted proton-/electron-transfer reactions is their ability to avoid high-energy intermediates that plague stepwise proton-/electron-transfer steps. A persistent question has been, given the dramatic expansion of methods to immobilize electrocatalysts at an electrode interface, what chemical knobs can be turned in tuning the energetics of new interfacial PCET electrocatalysts? The recent work by Jackson, Pegis, and Surendranath begins to provide an answer to this question.5 The thermodynamics of PCET can be described by the sum of free energies for its elementary component steps, proton transfer (PT) and electron transfer (ET), which are often visualized using a square reaction scheme, as shown in Figure 1. In this way it is easy to see that the PCET redox potential (EPCET) can be described as a function of both the ET and PT energies. For molecular PCET-dependent catalysts, where two of the three of these energies can be measured independently, catalytic activity can be predictably tuned by altering the energetics associated with either the ET step (in the form of redox potential, E0) or the PT step (in the form of pKa). Despite the popularity of heterogeneous electrocatalysts,6,7 a similar approach to catalyst tuning is not possible for interfacial PCET catalysts. This is because (1) the precise structure of the catalytic site is not known, and (2) it is often the case that only EPCET can be directly determined. Recent work by the Jackson et al. has aimed to provide such a platform for tuning interfacial © 2019 American Chemical Society
Figure 1. Square scheme illustrating the elementary steps of PCET.
PCET catalysts by addressing the first of these problems; they systematically modified an electrode surface to know the precise structure of the surface-bound PCET site. Over the last five years, the Surendranath group has developed a method for direct conjugation of electrocatalysts onto graphitic electrodes.8 In their most recent study, they utilized o-quinone functionalities present on heavily oxidized graphite electrodes to covalently attach a series of o-phenylenediamine derivatives, forming well-characterized, surface-conjugated PCET moieties.5 Thermochemistry of the resulting phenazinoid structures was compared to PT and ET energies of corresponding quinoxaline molecular analogues. Substantial differences were found between E0 of molecular analogues and EPCET of the graphite-conjugated species for all derivatives investigated; however, a striking correlation was observed between EPCET of the surface moieties at pH = 7 and the pKa of corresponding molecular analogues. Largely on the basis of this observation, Surendranath and co-workers were able to derive a mathematical formalism that describes EPCET as a function of the bulk pH, pKa of the molecular analogue, and the so-called potential of zero free charge (EPZFC). The EPZFC term correlates to the work function of the electrode and is meant to account for electrostatic driving forces caused by the buildup of charge at the electrode surface that can alter the protonation equilibrium during PCET. Taken together, their work suggests that the thermochemistry for interfacial PCET catalysts may be tuned by altering either the pKa or EPZFC Published: May 1, 2019 745
DOI: 10.1021/acscentsci.9b00387 ACS Cent. Sci. 2019, 5, 745−746
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Figure 2. Common varieties of interfacial electrocatalysts.
molecular descriptors to determine which structural dials can be turned to alter yield, rate, turnover frequency, and redox potential. Surendranath’s work correlating thermodynamics of surface-bound electrocatalysts to the chemical properties of corresponding molecular analogues potentially enables the use of relatively straightforward computational analysis to speed the development of new electrocatalysts.
terms. While a relatively limited number of structural derivatives were used to confirm the reported formalism, this work demonstrates a highly systematic platform to directly study the thermodynamics of interfacial PCET catalysis.
Taken together, their work suggests that the thermochemistry for interfacial PCET catalysts may be tuned by altering either the pKa or EPZFC terms.
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DavidP. P. Hickey and : 0000-0002-5297-6895 David Hickey: 0000-0002-5297-6895
The findings that, similar to molecular PCET catalysts, EPCET for surface-conjugated catalysts can be adjusted by changing either the proton binding affinity for the surface site or the work function of the electrode are not necessarily groundbreaking unto themselves. However, the mathematical formalism describing EPCET in terms of chemical descriptors for homogeneous molecular analogues enables the possibility to use this as a predictive tool for future catalyst design. The key to this work is the modular nature of graphite conjugation, which enables the systematic electrode modification necessary to draw correlations between small-molecule properties and corresponding heterogeneous analogues (Figure 2), which is critically important to the field of electrocatalysis from fuel cells to electrolyzers to electrofuels/electrosynthesis. Because chemical properties of molecular PCET derivatives are often readily computationally accessible, it is easy to imagine in silico screening being used to design novel interfacial PCET catalysts.
Shelley Minteer : 0000-0002-5788-2249 Shelley D. D. Minteer: 0000-0002-5788-2249
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the mathematical formalism describing EPCET in terms of chemical descriptors for homogeneous molecular analogues enables the possibility to use this as a predictive tool for future catalyst design The application of computational modeling and multivariate linear regression methods to small-molecular catalyst design has led to the discovery and foundational understanding of myriad catalytic motifs.9 These approaches require well-characterized and computationally accessible 746
DOI: 10.1021/acscentsci.9b00387 ACS Cent. Sci. 2019, 5, 745−746
