Metal-Free Motifs for Solar Fuel Applications

Ilić, Stefan; Zoric, Marija R.; Kadel, Usha Pandey; Huang, Yunjing; Glusac, Ksenija D.

doi:10.1146/annurev-physchem-052516-050924

Cited by 15 publications

(25 citation statements)

References 145 publications

(225 reference statements)

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“…This paradigm was developed for the interpretation of photoelectrochemical water splitting with transition‐metal oxides ,. In view of large exciton dissociation energies,,, significant polaron stabilization energies, and comparatively low charge carrier mobility in organic polymers, it appears questionable whether this paradigm is also applicable for triazine‐based covalent organic frameworks or heptazine‐based graphitic carbon nitrides. We have sketched in this article an alternative paradigm which emerged from the exploration of the specific photochemistry of aromatic heterocycles in hydrogen‐bonded complexes with water molecules using first‐principles electronic‐structure computational methods.…”

Section: Discussionmentioning

confidence: 99%

“…A nanodisperse noble-metal catalyst (usually Pt) is required for H 2 evolution with all N-heterocyclic polymeric materials. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] It is generally assumed that the noble-metal nanoparticles act as electron acceptors and catalyze thereby the neutralization of protons by photogenerated electrons. In recent work by Lotsch and co-workers it was shown that long-lived reduced species ("trapped electrons") could be accumulated under irradiation of cyanamide-functionalized carbon nitride in the presence of a sacrificial electron donor, but in the absence of nano-Pt.…”

Section: Photodissociation Of Heterocyclic Radicalsmentioning

confidence: 99%

“…[66][67][68] In the fused heterocycle heptazine, on the other hand, ring-puckering in excited states is suppressed and the excited states of heptazine (as well as the excited states of the heptazine-based polymer melon) have long lifetimes with high fluorescence quantum yields. [24][25][26][27][28][29][30] This implies that photohydration of heptazine can only occur as a follow-up reaction of photoinduced homolytic water splitting, that is, by the reaction of the photogenerated hydroxyl radical with the heptazinyl radical via the intermediate structures shown in Figure 7c. The spontaneous hydrolysis of heptazine (in the presence of light) is thus a witness of heptazine's ability to split water with visible light.…”

Section: Reactivity Of Oh Radicals With the Reduced Chromophoresmentioning

confidence: 99%

“…Melon and related materials are currently extensively explored as photocatalysts for water splitting as well as for the oxidation of organic pollutants. [24][25][26][27][28][29][30] The heptazine molecule itself has been synthesized only relatively recently and its structure confirmed by X-ray diffraction. [35] The monomeric heptazine molecule reacts readily with water in the presence of light (see Section 5) and is therefore not routinely available.…”

Section: Introductionmentioning

confidence: 99%

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Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

2018

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The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. For decades, transition metal oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen‐evolution photocatalysts. The mechanism of hydrogen evolution with these materials is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this Minireview an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N‐heterocycles in aqueous environments. Using ab initio electronic‐structure calculations, it is shown that a water molecule which is hydrogen‐bonded to an N‐heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by the discussion of excited‐state reaction paths and their energy profiles for hydrogen‐bonded complexes of pyridine, triazine and heptazine with a water molecule. It is shown that the excited‐state hydrogen‐transfer and hydrogen‐detachment reactions are essentially barrierless, in sharp contrast to water oxidation in the electronic ground state, where high barriers prevail. We also discuss in some detail the products of possible reactions of the highly reactive photogenerated hydroxyl radicals with the chromophores. We hypothesize that the challenge of efficient solar hydrogen generation with carbon nitride materials is less the decomposition of water as such, but rather the controlled recombination of the photogenerated radicals to the closed‐shell products H2 and H2O2.

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Section: Discussionmentioning

confidence: 99%

Section: Photodissociation Of Heterocyclic Radicalsmentioning

confidence: 99%

Section: Reactivity Of Oh Radicals With the Reduced Chromophoresmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

2018

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“…Our group is interested in metal‐free motifs capable of catalyzing electrochemical oxidation and reduction reactions . In our studies of the oxygen evolution reaction (OER), catalytic behavior was observed in an electrode‐assisted mechanism, where selected electrodes, such as glassy carbon and platinum, exhibited improved oxygen evolution performance in the presence of iminium or oxonium cations .…”

Section: Introductionmentioning

confidence: 99%

Conformational analysis of diols: Role of the linker on the relative orientation of hydroxyl groups

Zoric

Singh

Zeller

et al. 2019

J of Physical Organic Chem

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Conformational flexibility of three covalently linked alcohol dimers, composed of xanthenol units and diphenyl ether (DPE(OH)2), 9,9‐dimethylxanthene (XAN(OH)2), or biphenyl (BP(OH)2) linkers was studied. The relative orientation of the two alcohol units (In‐In, In‐Out, and Out‐Out conformers) in the model compounds was investigated using NMR spectroscopy, X‐ray crystallography, and density functional theory (DFT) calculations. Diols containing rigid linkers, such as XAN(OH)2 and BP(OH)2, favor the formation of an In‐In conformer in solution, and the interconversion between different conformers was slow on the NMR timescale. Interestingly, XAN(OH)2 adopted an In‐Out conformation in the crystalline solid state, possibly due to additional intermolecular interactions. More flexible DPE(OH)2 was found to freely interconvert on the NMR timescale. This study provides important structural information that can be utilized in catalysis, metal‐ligation, and other applications.

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Novel insight from in‐/ex‐situ investigation toward intercalated water in high‐performance oxygen evolution electrocatalysts and their mechanisms

Wang,

Lin,

Wang

et al. 2023

J Chinese Chemical Soc

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Hydrogen is a green energy source with zero carbon emissions and renewable properties. Green hydrogen, produced via water electrolysis, can efficiently harness excess renewable energy during peak periods, making it a key player in grid stabilization through processes like power‐to‐gas. Consequently, there is a pressing need to develop catalysts with high activity, stability, and cost‐effectiveness in the energy sector. The development of electrochemical (EC) water splitting, a promising path to meet alternative energy demands, however, is hindered by two main challenges that persist in water splitting: the anodic oxygen evolution reaction (OER) catalysts still need lower overpotential, and they must have enough stability with high catalytic activity. Both factors determine water electrolysis reactions' overall energy consumption and commercialization potential. This article introduces several significant parameters for assessing catalyst performance; three primary OER mechanisms are also briefly reviewed. Thereby, numerous electrocatalysts for OER are categorized by their composition and morphology. Furthermore, we generalize a common phenomenon that occurs at the surface of catalysts during the OER process. It is deduced that the surface transformation from as‐prepared to an activated state, that is, the surface‐environment change of the active site due to redox‐induced dissolution and re‐deposition, plays a critical role in OER. On the other hand, generating a layered structure leads to the accommodation of intercalated water molecules, which may enhance the activity and robustness via hydrogen bonds and dominate the absorption energy of oxygen species on the metal site. Lastly, we enumerate several in‐/ex‐situ methodologies that can discriminate the real active sites of the bulk, near‐surface region, interface, and intermediates adsorbed on the electrocatalyst surface. Further investigation is needed to unveil the interaction between active sites and embedded water molecules in EC catalytic processes. This paper provides a novel perspective of the intercalated water for future development of OER electrocatalysts, simultaneously considering performance and stability.

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Metal-Free Motifs for Solar Fuel Applications

Cited by 15 publications

References 145 publications

Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

Conformational analysis of diols: Role of the linker on the relative orientation of hydroxyl groups

Novel insight from in‐/ex‐situ investigation toward intercalated water in high‐performance oxygen evolution electrocatalysts and their mechanisms

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