The sluggish kinetics of the oxygen evolution reaction (OER) at the anode severely limit hydrogen production at the cathode in water splitting systems. Although electrocatalytic systems based on ...cheap and earth‐abundant copper catalysts have shown promise for water oxidation under basic conditions, only very few examples with high overpotential can be operated under acidic or neutral conditions, even though hydrogen evolution in the latter case is much easier. This work presents an efficient and robust Cu‐based molecular catalyst, which self‐assembles as a periodic film from its precursors under aqueous conditions on the surface of a glassy carbon electrode. This film catalyzes the OER under neutral conditions with impressively low overpotential. In controlled potential electrolysis, a stable catalytic current of 1.0 mA cm−2 can be achieved at only 2.0 V (vs. RHE) and no significant decrease in the catalytic current is observed even after prolonged bulk electrolysis. The catalyst displays first‐order kinetics and a single site mechanism for water oxidation with a TOF (kcat) of 0.6 s−1. DFT calculations on of the periodic Cu(TCA)2 (HTCA=1‐mesityl‐1H‐1,2,3‐triazole‐4‐carboxylic acid) film reveal that TCA defects within the film create CuI active sites that provide a low overpotential route for OER, which involves CuI, CuII−OH, CuIII=O and CuII−OOH intermediates and is enabled at a potential of 1.54 V (vs. RHE), requiring an overpotential of 0.31 V. This corresponds well with an overpotential of approximately 0.29 V obtained experimentally for the grown catalytic film after 100 CV cycles at pH 6. However, to reach a higher current density of 1 mA cm−2, an overpotential of 0.72 V is required.
Copper can do it: A self‐assembled complex of CuII efficiently catalyzes water oxidation under neutral conditions with a very low overpotential and excellent stability in prolonged electrolysis. Based on DFT calculations, structural defects in the periodic catalyst film could explain the experimentally observed low overpotential of the deposited molecular film in the oxygen evolution reaction.
The computational hydrogen evolution activity of Pt(111) remains controversial due to apparent discrepancies with experiments concerning rate-determining activation free energies and equilibrium ...hydrogen coverages. A fundamental source of error may lie within the static representations of the metal–water interface commonly employed in density functional theory (DFT)-based kinetic models neglecting important entropic effects on reaction dynamics. In this work, we present a dynamic reassessment of the Volmer–Tafel hydrogen evolution pathway on Pt(111) through DFT-based constrained molecular dynamics simulations and thermodynamic integration. Hydrogen coverage effects are gauged at two distinct surface saturations, while the critical potential dependence and constant potential conditions are accounted for using a capacitive model of the electrified interface. The uncertainty in the highly nontrivial treatment of the electrode potential is carefully examined, and we provide a quantitative estimation of the error associated with dynamically simulated electrochemical barriers. The dynamic description of the electrochemical interface promotes a substantial decrease of the Tafel free energy barrier as the coverage is increased to a full monolayer. This follows from a decreased entropic barrier due to suppressed adlayer dynamics compared to the unsaturated surface, a detail easily missed by static calculations predicting notably higher barriers at the same coverage. Due to observed endergonic adsorption of active hydrogen intermediates, the Tafel step remains rate-determining irrespective of the coverage as illustrated by composed Volmer–Tafel free energy landscapes. Importantly, our explicitly dynamic approach avoids the ambiguous choice of frozen solvent configuration, decreasing the reliance on error cancellation and paving the way for less biased electrochemical simulations.
Diazo compounds are commonly employed as carbene precursors in carbene transfer reactions during a variety of functionalization procedures. Release of N2 gas from diazo compounds may lead to carbene ...formation, and the ease of this process is highly dependent on the characteristics of the substituents located in the vicinity of the diazo moiety. A quantum mechanical density functional theory assisted by machine learning was used to investigate the relationship between the chemical features of diazo compounds and the activation energy required for N2 elimination. Our results suggest that diazo molecules, possessing a higher positive partial charge on the carbene carbon and more negative charge on the terminal nitrogen, encounter a lower energy barrier. A more positive C charge decreases the π-donor ability of the carbene lone pair to the π* orbital of N2, while the more negative N charge is a result of a weak interaction between N2 lone pair and vacant p orbital of the carbene. The findings of this study can pave the way for molecular engineering for the purpose of carbene generation, which serves as a crucial intermediate for many chemical transformations in synthetic chemistry.
Although nitrogen-doped nanocarbon systems have recently received intense attention, the mechanism for the observed highly efficient oxygen reduction is still under debate. To address this issue, we ...investigated the adsorption and dissociation of an oxygen molecule on three pristine or nitrogen-doped nanocarbon systems: graphene, single-walled and double-walled carbon nanotubes using density functional theory calculations. The adsorption and dissociation energies were determined for both pristine and N-doped single-walled carbon nanotubes of different diameters with graphitic-like N substitutions in order to see the effect of diameter on oxygen dissociation. It was found that the energy barrier for oxygen dissociation, chemisorption energy and reaction energy are a function of carbon nanotube diameter, but independent of the number of walls. We also investigated the energy barrier of oxygen dissociation on single-walled carbon nanotubes with different types of nitrogen doping (i.e.pyridinic and graphitic). It was observed that higher nitrogen concentrations greatly reduce the energy barrier for graphitic nitrogen. Our results contribute towards a better understanding of the reaction mechanism for nitrogen-doped carbon nanomaterials involving oxygen molecule dissociation in the first step.
The dissolution of NaCl has been systematically investigated by employing
ab initio
molecular dynamics (AIMD) on different NaCl nanocrystals as well as on a surface system immersed in water. We ...discovered a complex dissolution process simultaneously involving multiple ions initiated at the corner sites of the crystal. Our simulations indicated a difference in the dissolution rates of sodium and chlorine. While sodiums readily became partially solvated, chlorines more frequently transitioned into the fully solvated state leading to an overall greater dissolution rate for Cl. We determined that this difference arises due to faster water mediated elongations of individual ionic bonds to Na, but a significantly slower process for the last bond in comparison to Cl. In an attempt to investigate this phenomenon further, we performed metadynamics based free energy simulations on a surface slab presenting corner sites similar to those in cubic crystals, aiming to extract the dissolution free energy profile of corner ions. In qualitative agreement with the nanocrystal simulations, this revealed a shallower first free energy minimum for Na, but no statistically significant difference in the corresponding barriers and inconclusive results for the latter stage. Finally, simulations of smaller NaCl crystals illustrated how dissolution proceeds beyond the point of crystal lattice collapse, highlighting the strength of solvated ion interactions.
NaCl nanocrystal dissolution was investigated in atomistic detail revealing a difference in the solvation of two different ionic species.
The oxygen evolution reaction (OER) is a critical reaction in electrochemical water splitting and rechargeable metal-air batteries to generate and store clean energy. Therefore, the development of ...efficient and low cost electrocatalysts for the OER with high activity and stability is of great technological and scientific interest. We demonstrate here for the first time that maghemite ( gamma -Fe sub(2)O sub(3)) nanoparticles decorated on carbon nanotubes (CNTs) function as low cost, highly active and durable OER electrocatalysts. The material generates a current density of 10 mA cm super(-2) at overpotentials of 0.38 and 0.34 V in 0.1 and 1 M NaOH, respectively. These values are comparable to those of the best OER electrocatalysts reported so far. Moreover, gamma -Fe sub(2)O sub(3)/CNTs show a stable performance at a potential of similar to 1.64 V vs.RHE during 25 h stability tests. The gamma -Fe sub(2)O sub(3) nanoparticles are formed from carbon encapsulated iron nanoparticles (CEINs) during the first OER measurements of the CEIN/CNT electrode. The CEIN/CNT material itself is synthesized by a fast and low cost floating catalyst chemical vapor deposition method in a one-step synthesis with a similar growth process to that of CNTs.
Oxygen reduction catalyzed by cofacial metalloporphyrins at the 1,2-dichlorobenzene–water interface was studied with two lipophilic electron donors of similar driving force, 1,1′-dimethylferrocene ...(DMFc) and tetrathiafulvalene (TTF). The reaction produces mainly water and some hydrogen peroxide, but the mediator has a significant effect on the selectivity, as DMFc and the porphyrins themselves catalyze the decomposition and the further reduction of hydrogen peroxide. Density functional theory calculations indicate that the biscobaltporphyrin, 4,5-bis5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)-9,9-dimethylxanthene, Co2(DPX), actually catalyzes oxygen reduction to hydrogen peroxide when oxygen is bound on the “exo” side (“dock-on”) of the catalyst, while four-electron reduction takes place with oxygen bound on the “endo” side (“dock-in”) of the molecule. These results can be explained by a “dock-on/dock-in” mechanism. The next step for improving bioinspired oxygen reduction catalysts would be blocking the “dock-on” path to achieve selective four-electron reduction of molecular oxygen.
In this study, we investigate the structure of the Pt(111)-water interface in an alkaline environment with large OH coverages of 1/3, 2/3 and 1 monolayer using a large well-equilibrated system. We ...observe that the OH coverage influences both the orientational distribution of the water molecules and their density, with more structure associated with higher coverage. At the same time, there is evidence of a highly dynamic hydrogen bond network on the lower coverage systems with substantial exchange of water between the surface and the solvent. In addition to OH and H
2
O species, which are preferentially located at the top sites, the 1/3 and 2/3 monolayer surfaces also contain O atoms, which are relatively stable and prefer the hollow sites. In contrast, the 1 monolayer surface shows none of these dynamics, and is unlikely to be active. The dynamic coexistence of O, OH and H
2
O on Pt(111) electrodes in alkaline conditions necessitates the investigation of several possible reaction paths for processess like ORR and water splitting. Finally, the exchange processes observed between the solvent and the interface underscore the need to explicitly include liquid water in simulations of systems similar to Pt(111).
OH and H
2
O species prefer the top sites of the 1/3 and 2/3 OH monolayer Pt(111) surface while O atoms prefer the hollow sites. The surfaces possess a dynamic hydrogen bond network with water exchange between the surface and the solvent.
In this study, we investigate the structure of the Pt(111)–water interface in an alkaline environment with large OH coverages of 1/3, 2/3 and 1 monolayer using a large well-equilibrated system. We ...observe that the OH coverage influences both the orientational distribution of the water molecules and their density, with more structure associated with higher coverage. At the same time, there is evidence of a highly dynamic hydrogen bond network on the lower coverage systems with substantial exchange of water between the surface and the solvent. In addition to OH and H 2 O species, which are preferentially located at the top sites, the 1/3 and 2/3 monolayer surfaces also contain O atoms, which are relatively stable and prefer the hollow sites. In contrast, the 1 monolayer surface shows none of these dynamics, and is unlikely to be active. The dynamic coexistence of O, OH and H 2 O on Pt(111) electrodes in alkaline conditions necessitates the investigation of several possible reaction paths for processess like ORR and water splitting. Finally, the exchange processes observed between the solvent and the interface underscore the need to explicitly include liquid water in simulations of systems similar to Pt(111).
Atomic layer deposition (ALD) is a coating technology used to produce highly uniform thin films. Aluminiumoxide, Al
2
O
3
, is mainly deposited using trimethylaluminium (TMA) and water as precursors ...and is the most studied ALD-process to date. However, only few theoretical studies have been reported in the literature. The surface reaction mechanisms and energetics previously reported focus on a gibbsite-like surface model but a more realistic description of the surface can be achieved when the hydroxylation of the surface is taken into account using dissociatively adsorbed water molecules. The adsorbed water changes the structure of the surface and reaction energetics change considerably when compared to previously studied surface model. Here we have studied the TMA-H
2
O process using density functional theory on a hydroxylated alumina surface and reproduced the previous results for comparison. Mechanisms and energetics during both the TMA and the subsequent water pulse are presented. TMA is found to adsorb exothermically onto the surface. The reaction barriers for the ligand-exchange reactions between the TMA and the surface hydroxyl groups were found to be much lower compared to previously presented results. TMA dissociation on the surface is predicted to saturate at monomethylaluminium. Barriers for proton diffusion between surface sites are observed to be low. TMA adsorption was also found to be cooperative with the formation of methyl bridges between the adsorbants. The water pulse was studied using single water molecules reacting with the DMA and MMA surface species. Barriers for these reactions were found to reasonable in the process conditions. However, stabilizing interactions amongst water molecules were found to lower the reaction barriers and the dynamical nature of water is predicted to be of importance. It is expected that these calculations can only set an upper limit for the barriers during the water pulse.
A comprehensive density functional study on the reaction mechanisms during the atomic layer deposition of alumina
via
trimethylaluminium-waterprocess.
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