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WIREs Comput Mol Sci
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The electronic structure underlying electrocatalysis of two‐dimensional materials

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Abstract The understanding and development of advanced electrochemical catalysts have attracted intensive studies and achieved tremendous progress in the past decades. Two‐dimensional (2D) materials, such as doped graphene and atomically‐thin transition metal compounds, have shown great promise as electrocatalysts for various renewable energy conversion and storage reactions. Their further developments require improved understanding of the catalytic mechanisms at atomic level. Currently, most of the understandings are based on the formation free energies of the intermediates, which are determined by their binding strengths with the catalyst, usually calculated from density functional theory (DFT). These energies/binding strengths have been used as descriptor to describe the activity of many catalysts. However, it remains less explored why different catalysts have different binding strengths and what are the underlying factors controlling them, requiring studies going beyond atomic level to electronic level. This review aims to provide such links, focusing on 2D electrocatalysts for hydrogen evolution reaction (HER), oxygen reduction reaction/oxygen evolution reaction (ORR/OER), CO2 reduction reaction (CO2R) and nitrogen reduction reaction (NRR). We also discuss some of the significant issues that need to be addressed in DFT calculations, including the effects of varying charge and fixed potential of the catalyst, the passivation of active sites, and the solvation effects. This article is categorized under: Structure and Mechanism > Reaction Mechanisms and Catalysis Structure and Mechanism > Computational Materials Science
(a) Linear relation between adsorption energies of various species on metal and sulfur sites of various transition metal (Mo, W, Nb, Ta) sulfide edges and the d‐band center of the transition metal site at zero coverage, reproduced with permission from Reference (copyright 2014 American Chemical Society); (b) Illustration of energy of lowest unoccupied states and its linear relation with H adsorption energies on basal planes of transition‐metal dichalcogenides, reprinted by permission from Macmillan Publishers Ltd: Nature Energy (Reference ), copyright 2017; (c) Example of p band peak and its linear relation with H adsorption energy on doped graphene, reprinted by permission from Macmillan Publishers Ltd: Nature Energy (Reference ), copyright 2016; (d) Relation between the proposed (see text for details) and adsorption energy of H on single metal‐(N)‐graphene structures, reprinted by permission from Macmillan Publishers Ltd: Nature Catalysis (Reference ), copyright 2018
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(a) Schematic of molecular N2 binding to transition metal and nonmetal B sites through d‐orbital and sp3 orbital interaction. (Reproduced from Reference , copyright 2018 American chemistry society) (b) The top and side view of spin density distribution of FeN3 without and with N2 adsorption. (Reproduced from Reference , copyright 2016 American chemistry society) the atomic orbital distribution of BC3 for N2 and the corresponding the lowest unoccupied molecular orbital and the highest occupied molecular orbital distribution of undoped and B‐doped graphene. (Reproduced from Reference , copyright 2018 Elsevier Inc.)
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(a) Minimum ORR/OER overpotential versus the descriptor Φ for X‐doped graphene. (b) Measured limiting current density from the LSV curves, normalized by Pt/C electrode current density at 0.5 under the same conditions in the same experiment, and the predictions. (c) Adsorption free energy of OH versus the descriptor φ for all single TM atoms supported on graphene. (d) Theoretical and corresponding experimental onset potentials for ORR versus the descriptor φ. (a) and (b), reproduced with permission from Reference copyright 2010 John Wiley & Sons, Inc. (c) and (d), reproduced with permission from Reference copyright 2018 Springer Nature: Nature Catal
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(a) The linear relationship between ΔGOH* and Ediff data were collected for the most active site of various doped graphene models. (b) Scheme of orbital hybridization of valence band from active sites and adsorbates bonding orbital. EF refers to highest valence orbital energy of the entire graphene cluster. (c) Relative pz occupancy versus ΔGOH on the various active sites of N‐, B‐, and co‐doped graphene. (d) Contour plot showing the dependence of pz occupancy of the active sites of N‐doped and B2N‐cluster codoped graphene on ΔGOH and ORR activity (negative of overpotential). (a) and (b) reproduced with permission from Reference . Copyright 2014 American Chemical Society, (c) and (d) reproduced with permission from Reference , copyright 2018 John Wiley & Sons, Inc
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Illustration of different doping schemes for CO2R. The dopants can be nonmetal element such as N (a–c), metals (f–i), or their complex (d–e). C atoms are shown in gray, N in blue, metal in green or orange/red. Note that in (g–h), two metal atoms are supported on graphene quadrovacancy and two adjacent graphene single vacancies. (a) and (b) reproduced with permission from References (copyright 2015 American Chemical Society), (c) reproduced with permission from Reference (copyright 2017 American Chemical Society), (d–g) reproduced with permission from Reference (copyright 2018 Macmillan Publishers Ltd: Nature Catalysis), (h‐j) reproduced with permission from Reference (copyright 2015 American Chemical Society), respectively
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Selectivity of CO2R against HER catalyzed by transition metal ions immobilized on graphene: (a) Most of the embedded single metal are CO2R selective, (b) Illustration of a decoupled elementary proton−electron transfer mechanism for CO formation, (c) Experimental observation of the CO selectivity dependence on pH at −0.9 V versus NHE, (d) rate‐limiting potential difference UL(CO2) − UL(H2) of N‐doped graphene and Ni‐N4‐C motif, (e) CO selectivity comparison between different ions that immobilized on graphene, (f) CO temperature programing desorption (TPD) spectra of different transition metal ions immobilized on N‐doped graphene. (a) Reproduced with permission from Reference (copyright 2016 Royal Chemical Society), (b) and (c) reproduced with permission from Reference (copyright 2018 American Chemical Society), (d) reproduced with permission from Reference (copyright 2017 American Chemical Society), (e) and (f) reproduced with permission from Reference (copyright 2018 John Wiley & Sons, Inc.)
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