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Funding Information: This work was supported by Business Finland through project Molecular Modelling in Industrial Research and Development (MM-IRD). The authors wish to acknowledge CSC – IT Center for Science, Finland, for computational resources. Publisher Copyright: © 2023 The Author(s) Oxygen and water are the most reactive gases of the ambient air. The adsorption of both molecules on transition metal surfaces have been studied extensively, but mostly separately. However, water and oxygen usually co-exist, and therefore realistic systems need to take into consideration both simultaneously. As these adsorption reactions are so common, state-of-the-art results are beneficial as they capture large trends as accurately as possible. A comprehensive study of oxygen and water co-adsorption and dissociation on Ag(111)-, Au(111)-, Pd(111)-, Pt(111)-, Rh(111)- and Ni(111)-surfaces have been performed using density functional theory. We present a very strong general trend, where dissociated oxygen systematically lowers the activation energy of water dissociation on transition metal surfaces. This makes the oxygen dissociation the rate-determining step of the water dissociation reaction. The effect is caused by the additional pathway that the dissociated oxygen enables for the dissociation of water molecule.

The uploaded data contains (i) "01_Data"optimized molecular structure in XYZ format and theprimary attributes and SMILES, (ii)"02_Datasets" datasets used in the publication, and (iv) "03_pynb_script" a Jupyter-Notebook. The01_Data directory contains more than 8000 subdirectories. Each is for a molecule that undergoes a two-proton two-electron transfer reaction. In each subdirectory, one finds the following files: (1) directories named corresponding to the ones in Figure 1 of the paper. Inside each, there are geometries and properties in XYZ and CSV format, respectively. (2) "freeEnergy.dat"contains the free energy of different states. (3) "schemesquare.dat" hasthe parameters of the electrochemical scheme of square representation. ├── A │├── info.csv │└── pos.xyz ├── A1- │├── info.csv │└── pos.xyz ├── A2- │├── info.csv │└── pos.xyz ├── AH │├── info.csv │└── pos.xyz ├── AH1+ │├── info.csv │└── pos.xyz ├── AH1- │├── info.csv │└── pos.xyz ├── AH2 │├── info.csv │└── pos.xyz ├── AH21+ │├── info.csv │└── pos.xyz ├── AH22+ │├── info.csv │└── pos.xyz ├── freeEnergy.dat └── schemesquare.dat &nbsp

Funding Information: Calculations were performed at the Finnish IT centre for science (CSC). All authors acknowledge financial support from the Jane and Aatos Erkko Foundation through the “Renewable energy storage to high value chemicals” project. M. B. is additionally grateful for the support through the Dr. Barbara Mez‐Stark foundation. Open Access funding enabled and organized by Projekt DEAL. Publisher Copyright: © 2023 The Authors. ChemCatChem published by Wiley-VCH GmbH. CO2 reduction is typically performed at neutral pH. Under these conditions CO2 is in equilibrium with H2CO3, HCO3− and CO32−. However, despite their presence so far most studies solely focus on the contribution of CO2 while carbonate species as alternative reactants are generally neglected. Using density functional theory (DFT) modelling we explore the possible contribution of these carbonate species to the overall CO2 reduction activity for a Fe porphyrin model catalyst. Considering only reaction Gibbs free energies, we find the reduction of carbonic acid (H2CO3), bicarbonate (HCO3−) and CO2 to be equally likely. However, owing to a very high activation barrier for the initial adsorption of CO2 onto the catalyst, bicarbonate and carbonic acid reduction are found to be several orders of magnitude faster. These data are used to model the pH dependence of the reaction rates of the different reactants. These results confirm that carbonic acid and bicarbonate are the most likely reactants independent of the pH and reactor setup.

openaire: EC/H2020/722614/EU//ELCOREL We are thankful for the financial support from European Union under the project ELCOREL ITN Horizon 2020 (721624-ELCOREL), Academy of Finland (Profi-5), and Jane and Aatos Erkko Foundation (USVA), Finland. We also acknowledge the generous computer resources provided by the Finnish National Supercomputer Centre CSC and Aalto University Raw Materials Infrastructure (RAMI) and OtaNano Nanomicroscopy Center. The mixture of CO and H2, known as syngas, is a building block for many substantial chemicals and fuels. Electrochemical reduction of CO2 and H2O to syngas would be a promising alternative approach for its synthesis due to negative carbon emission footprint when using renewable energy to power the reaction. Herein, we present temperature-controlled syngas production by electrochemical CO2 and H2O reduction on a cobalt tetraphenylporphyrin/multiwalled carbon nanotube (CoTPP/MWCNT) composite in a flow cell in the temperature range of 20–50 °C. The experimental results show that for all the applied potentials the ratio of H2/CO increases with increasing temperature. Interestingly, at −0.6 VRHE and 40 °C, the H2/CO ratio reaches a value of 1.2 which is essential for the synthesis of oxo-alcohols. In addition, at −1.0 VRHE and 20 °C, the composite shows very high selectivity toward CO formation, reaching a Faradaic efficiency of ca. 98%. This high selectivity of CO formation is investigated by density functional theory modeling which underlines that the potential-induced oxidation states of the CoTPP catalyst play a vital role in the high selectivity of CO production. Furthermore, the stability of the formed intermediate species is evaluated in terms of the pKa value for further reactions. These experimental and theoretical findings would provide an alternative way for syngas production and help us to understand the mechanism of molecular catalysts in dynamic conditions.

R.K. is supported by the School of Chemical Engineering of Aalto University through a doctoral scholarship. The authors thank Heikki Lappalainen for assistance. Computational resources were provided by CSC – IT Center for Science, Finland. Facile solvent reorganization promoting ion transfer across the solid−liquid interface is considered a prerequisite for efficient electrocatalysis. We provide first-principles insight into this notion by examining water self-ion dynamics at a highly rigid NaCl(100)−water interface. Through extensive density functional theory molecular dynamics simulations, we demonstrate for both acidic and alkaline solutions that Grotthuss dynamics is not impeded by a rigid water structure. Conversely, decreased proton transfer barriers and a striking propensity of H₃O⁺ and OH⁻ for stationary interfacial water are found. Differences in the ideal hydration structure of the ions, however, distinguish their behavior at the water contact layer. While hydronium can maintain its optimal solvation, the preferentially hypercoordinated hydroxide is repelled from the immediate vicinity of the surface due to interfacial coordination reduction. This has implications for alkaline hydrogen electrosorption in which the formation of undercoordinated OH⁻ at the surface is proposed to contribute to the observed sluggish kinetics.

R.K. acknowledges the School of Chemical Engineering of Aalto University for funding in the form of a doctoral scholarship. The authors wish to thank CSC – IT Center for Science, Finland, for the generous computational resources provided through the Mahti supercomputer pilot project. 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.

Icosahedral Au clusters with three and four shells of atoms are found to deviate significantly from the commonly assumed Mackay structures. By introducing additional atoms in the surface shell and creating a vacancy in the center of the cluster, the calculated energy per atom can be lowered significantly, according to several different descriptions of the interatomic interaction. Analogous icosahedral structures with five and six shells of atoms are generated using the same structural motifs and are similarly found to be more stable than Mackay icosahedra. The lowest energy per atom obtained here is for clusters containing 310, 564, 928 and 1426 atoms, as compared with the commonly assumed magic numbers of 309, 561, 923 and 1415. Some of the vertices in the optimized clusters have a hexagonal ring of atoms, rather than a pentagon, with the vertex atom missing. An inner shell atom in some cases moves outwards by more than an Ångström into the surface shell at such a vertex site. This feature, as well as the wide distribution of nearest-neighbor distances in the surface layer, can strongly influence the properties of icosahedral clusters, for example catalytic activity. The structural optimization is initially carried out using the GOUST method with atomic forces estimated with the EMT empirical potential function, but the atomic coordinates are then refined by minimization using electron density functional theory (DFT) or Gaussian approximation potential (GAP). A single energy barrier is found to separate the Mackay icosahedron from a lower energy structure where a string of atoms moves outwards in a concerted manner from the center so as to create a central vacancy while placing an additional atom in the surface shell.

We are grateful for the generous computing resources from CSC-IT Center for Scientific Computing and Mikko Hakala for useful scripts and tips throughout the project. In this study, we looked at the hydrogen evolution reaction on the doubly doped Ni3P2 terminated Ni2P surface. Two Ni atoms in the first three layers of the Ni2P surface model were exchanged with two transition metal atoms. We limited our investigation to combinations of Al, Co, and Fe based on their individual effectiveness as Ni2P dopants in our previous computational studies. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that the combination of Co and Fe dopants most improves the catalytic activity of the surface through the creation of multiple novel and active catalytic sites.

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.

pKa values in non-aqueous solvents are of critical importance in many areas of chemistry. Our knowledge is, despite their relevance, still limited to the most fundamental properties and few pKa values in the most common solvents. Taking advantage of a recently introduced computationally efficient procedure we computed the pKa values of 182 compounds in 21 solvents. This data set is used to establish for the first time universal trends between all solvents. Our computations indicate, that the total charge of the molecule and the charge of the acidic group combined with the Kamlet-Taft solvatochromic parameters are sufficient to predict pKa values with at least semi- quantitative accuracy. We find, that neutral acids such as alcohols are strongly affected by the solvent properties. This is contrasted by cationic acids like ammonium ions whose pKa is often almost completely independent from the choice of solvent., publishedVersion

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the p

Publisher Copyright: © 2022 The Authors. Published by American Chemical Society. Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pKa of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pKa's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pKa of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pKa of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pKa's computed using an implicit solvation model on the other hand differ significantly from the experimental data. Thesedifferences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

Density functional theory (DFT) based computational electrochemistry has the potential to serve as a tool with predictive power in the rational development and screening of electrocatalysts for renewable energy technologies. It is, however, of paramount importance that simulations are conducted rigorously at a level of theory that is sufficiently accurate in order to obtain physicochemically sensible results. Herein, we present a comparative study of the performance of the static climbing image nudged elastic band method (CI-NEB) vs. DFT based constrained molecular dynamics simulations with thermodynamic integration in estimating activation and reaction (free) energies of the Volmer–Heyrovský mechanism on a nitrogen doped carbon nanotube. Due to cancellation of errors within the CI-NEB calculations, static and dynamic activation barriers are observed to be surprisingly similar, while a substantial decrease in reaction energies is seen upon incorporation of solvent dynamics. This finding is attributed to two competing effects; (1) solvent reorganization that stabilizes the transition and, in particular, the product states with respect to the reactant state and (2) destabilizing entropic contributions due to solvent fluctuations. Our results highlight the importance of explicitly sampling the interfacial solvent dynamics when studying hydrogen evolution at solid–liquid interfaces.

Platinum (Pt)-free catalysts for the hydrogen evolution reaction (HER) is currently a blooming research topic in view of the high cost and scarcity of Pt. Experiments on single-shell carbon-encapsu...

Complex machine learning (ML) models applied within computational chemistry and materials science tend to be seen as black boxes, yielding property predictions given some input features. While the purpose of ML methods is often to circumvent computationally expensive first-principles calculations, the fact that the inner workings of the models are not understood conceals chemical insight and knowledge regarding the underlying data and physical correlations within it. Knowing what a model is learning from the data and how outputs are formed is also useful in facilitating the justification and wider adoption of ML solutions. Here, we present an important contribution in this direction by exploring and explaining the hydrogen adsorption properties of defective nitrogen-doped carbon nanotubes (NCNTs) through density functional theory simulations and machine learning-based data analysis. As the main highlight, we demonstrate the application of a recent game-theoretic approach to deconvolute and interrogate the trained ML models, revealing how various structural, chemical, and electronic features contribute toward the hydrogen affinities of roughly 6500 different NCNT adsorption sites. The employed method of Shapley additive explanations (SHAP) attributes locally accurate importances to the investigated features, unraveling high spin polarization, narrow highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap, small dopant–adsorption site separation, and diverse angle and coordination effects as particularly impactful for increasing hydrogen adsorption strengths. The SHAP method is shown capable of promoting a deep understanding of complex feature–activity relationships, facilitating research efforts such as rational catalyst design for energy conversion applications.

Calculations were performed at the Chalmers Centre for Computational Science and Engineering (C3SE) and the Finnish IT centre for science (CSC). M. B. and K. L. acknowledge financial support from the Jane and Aatos Erkko Foundation through the “Renewable energy storage to high value chemicals” project. We describe a simple and efficient procedure to compute a conversion factor for the absolute potential of the standard hydrogen electrode in water to any other solvent. In contrast to earlier methods our procedure only requires the pK(a) of an arbitrary acid in water and few simple quantum chemical calculations as input. Thus, it is not affected adversely by experimental shortcomings related to measurements in non-aqueous solvents. By combining this conversion factor with the absolute potential in water, the absolute potential in the solvent of interest is obtained. Based on this procedure a new generalized computational standard hydrogen electrode for the computation of electron transfer and proton-coupled electron transfer potentials in non-aqueous solvents and ionic liquids is developed. This enables for the first time the reliable prediction of redox potentials in any solvent. The method is tested through calculation of absolute potentials in 36 solvents. Using the Kamlet-Taft linear solvation energy model we find that the relative absolute potentials consistently increase with decreasing polarisability and decreasing hydrogen bonding ability. For protic solvents good agreement with literature is observed while significant deviations are found for aprotic solvents. The obtained conversion factors are independent of the quantum chemical method, while minor differences are observed between solvation models. This does, however, not affect the global trends.

openaire: EC/H2020/721065/EU//CREATE Nanoparticles comprising of transition metals encapsulated in an ultrathin graphene layer (NiFe@UTG) are utilized to anchor very low amount of finely dispersed pseudo-atomic Pt to function as a durable and active electrocatalyst (Pt/NiFe@UTG) for the hydrogen evolution reaction (HER) in acidic media. Our experiments show the vital role of the carbon shell thickness for efficient utilization of Pt. Furthermore, density functional theory calculations suggest that the metal-core has a crucial role in achieving promising electrocatalytic properties. The thin carbon shell allows the desired access of Pt atoms to the vicinity of the NiFe core while protecting the metallic core from oxidation in the harsh acidic media. In acidic media, the performance of this Pt/NiFe@UTG catalyst with 0.02 at% Pt is the same as that of commercial Pt/C (10 and 200 mV overpotential to reach 10 and 200 mA cm−2, respectively) with promising durability (5000 HER cycles). Our electrochemical characterization (cyclic voltammetry) shows no Pt specific peaks, indicating the existence of a very low Pt loading on the surface of the catalyst. Hence, this conductive core-shell catalyst support enables efficient utilization of Pt for electrocatalysis.

A protocol for the accurate computation of electron transfer (ET) potentials from ab initio and density functional theory (DFT) calculations is described. The method relies on experimental pKa values, which can be measured accurately, to compute a computational setup dependent effective absolute potential. The effective absolute potentials calculated using this protocol display strong variations between the different computational setups and deviate in several cases significantly from the "generally accepted" value of 4.28 V. The most accurate estimate, obtained from CCSD(T)/aug-ccpvqz, indicates an absolute potential of 4.14 V for the normal hydrogen electrode (nhe) in water. Using the effective absolute potential in combination with CCSD(T) and a moderately sized basis, we are able to predict ET potentials accurately for a test set of small organic molecules (σ = 0.13 V). Similarly we find the effective absolute potential method to perform equally good or better for all considered DFT functionals compared to using one of the literature values for the absolute potential. For, M06-2X, which comprises the most accurate DFT method, standard deviation of 0.18 V is obtained. This improved performance is a result of using the most appropriate effective absolute potential for a given method.

Atomically flat, single-crystal solid-liquid interfaces attract considerable interest through their electrochemical relevance and well-defined structure facilitating controlled atomistic characterization. Yet, crucial details especially regarding the nanoscale adlayer-water dynamics remain uncertain. Here, the influence of adsorbate coverage on the interfacial structure and solvent relaxation on hydrogenated Pt(111) is examined by extensive density functional molecular dynamics simulations. Pronounced water dynamics is observed with increasing hydrogen coverage, for which an interpretation based on displacement of specifically co-adsorbed water and strong screening of the electrostatic interaction across the interface is proposed. However, the magnitude of the solvent fluctuations is argued to be partly overestimated by the employed RPBE-D3 exchange-correlation functional, which impedes water chemisorption and charge transfer to sparsely hydrogenated platinum. This manifests as overestimated equilibrium electrode potentials compared to experimental adsorption isotherms, which are conversely well reproduced by static calculations invoking the computational hydrogen electrode formalism. By coupling the interfacial structure with electrostatic properties, our work underscores the profound importance of functional choice as well as the persisting value and comparable precision of carefully employed static approximations in electrochemical simulations.

openaire: EC/H2020/686053/EU//CritCat In this study, we looked at the hydrogen evolution reaction on Mg-, Mo-, Fe-, Co-, V-, and Cu-doped Ni3P2 and Ni3P2 + P terminated Ni2P surfaces. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that doping can substantially improve the catalytic activity of the Ni3P2 terminated surface. In contrast, the Ni3P2 + P terminated one seems to be catalytically active irrespective of the type of doping, including in the absence of doping. Based on our doping energy and adsorption free energy calculations, the most promising dopants are iron and cobalt, whereas copper is less likely to function well as a doping element.

Atomic layer deposition (ALD) has emerged as an important technique for thin-film deposition in the last two decades. Zinc oxide thin films, usually grown via diethylzinc (DEZ) and water process, have seen much interest both in application and in theoretical research. The surface processes related to the growth of the thin film are not entirely understood, and the conceptual picture of the ALD process has been contradicted by recent experiments where ligands from the zinc pulse persist on the surface even after extended water pulse exposures. In this work, we investigate the overall growth of the zinc oxide thin films grown via DEZ/H2O process by modeling the surface chemistry using first-principles kinetic Monte Carlo for the first time. The kinetic Monte Carlo allows us to implement density functional theory calculations conducted on the zinc oxide (100) surface into a kinetic model and extract data directly comparable to experimental measurements. The temperature-dependent growth profile obtained from our model is in good qualitative agreement with the experimental data. The onset of thin-film growth is offset from the experimental data because of the underestimation of the reaction barriers within density functional theory. The growth per cycle of the deposited film is overestimated by 18% in the kinetic model. Mass gain during an ALD cycle is in qualitative agreement with the experimental quartz-crystal microbalance data. The main mass gain within an ALD cycle is obtained during the DEZ pulse and mass change during the water pulse is negligible. The cause of low film growth at low temperatures is due to the high reaction barriers for ethyl-elimination during the water pulse. This kinetic barrier results in low film growth as no new DEZ can adsorb to the ethyl-saturated surface. At elevated temperatures, ethyl-elimination becomes accessible, resulting in the ideal layer-by-layer growth of the film. However, a large fraction of ethyl-ligands persist on the surface after each ALD cycle even at high temperatures. This results in ethyl-ligands being encapsulated into the film lattice. This is likely due to an incomplete set of reaction pathways, and it is likely that some yet unidentified process is responsible for the elimination of the ethyl-ligands from the surface as the deposition process progresses.

Designing earth-abundant element based efficient and durable electrocatalysts for hydrogen evolution reaction (HER) is attracting growing attention as the renewable electricity supply sector urgently needs sustainable methods for storing energy. Nitrogen functionalized carbon nanomaterials are an interesting electrocatalysts option because of their attractive electrical properties, excellent chemical stability and catalytic activity. Hence, this study reports the HER mechanism on nitrogen functionalized few-walled carbon nanotubes (N-FWCNT). With this earth-abundant element based catalyst 250mV overpotential is required to reach 10mAcm-2 current density and so its HER activity is comparable to other non-noble metal catalysts, and clearly among the highest previously reported for N-FWCNTs. To gain fundament insight on their functioning, computational analysis has been carried out to verify the effect of nitrogen and to analyze the reaction mechanism. The reaction mechanism has also been analyzed experimentallywith a pH series, and both the methods suggest that the HER proceeds via the Volmer-Heyrovský mechanism. Overall hydrogen surface coverage on N-FWCNT is also suggested to affect the HER rate. Interestingly, in the studied structure, carbons in vicinity of nitrogen atoms, but not directly bound to nitrogen, appear to promote the HER most actively. Furthermore, durability of N-FWCNTs has been demonstrated by operating a full electrolyzer cell for five weeks.

Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production. Chemical modification by doping can be used to fine-tune the electrocatalytic activity, but this path requires theoretical, atomic-level support which has not been widely available for Ni-P. We present a density functional theory analysis of Al-doped Ni2P surfaces to identify structural motifs that could contribute to the improved behavior of the catalyst. Based on the formation energies of substitutionally Al-doped Ni sublattices, we find doping to take place preferably at the topmost layers. The Ni-Ni bridge and the P-top sites are the optimal ones in terms of hydrogen bonding energies. The Ni-Ni bridge site is not present on pristine surfaces but is a consequence of Al doping and provides a candidate to explain the experimentally observed high activities in doped Ni-P nanoparticles. Similar structural motifs can be recommended to be engineered for other Ni-P structures for improved electrocatalytic activity.

Among current technologies for hydrogen production as an environmentally friendly fuel, water splitting has attracted increasing attention. However, the efficiency of water electrolysis is severely limited by the large anodic overpotential and sluggish reaction rate of the oxygen evolution reaction (OER). To overcome this issue, the development of efficient electrocatalyst materials for the OER has drawn much attention. Here, we show that organometallic Ni(II) complexes immobilized on the sidewalls of multiwalled carbon nanotubes (MWNTs) serve as highly active and stable OER electrocatalysts. This class of electrocatalyst materials is synthesized by covalent functionalization of the MWNTs with organometallic Ni bipyridine (bipy) complexes. The Ni-bipy-MWNT catalyst generates a current density of 10 mA cm–2 at overpotentials of 310 and 290 mV in 0.1 and 1 M NaOH, respectively, with a low Tafel slope of ∼35 mV dec–1, placing the material among the most active OER electrocatalysts reported so far. Different ...

The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years, molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2-Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2-Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, which suggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant-vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2-Co than for MoS2-Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour.

peer-reviewed The full text of this article will not be available in ULIR until the 29/07/2021 The oxygen evolution reaction (OER) is the limiting factor in an electrolyzer and the oxygen reduction reaction (ORR) the limiting factor in a fuel cell. In OER, water is converted to O2 and H+/e- pairs, while in ORR the reverse process happens to form water. Both reactions and their efficiency are important enablers of a hydrogen economy where hydrogen will act as a fuel or energy storage medium. OER and ORR can both be described assuming a 4-step electrochemical mechanism with coupled H+/e- transfers between 4 intermediates (M-*, M-OH, M=O, M-OOH, M = active site). Previously, it was shown that an unstable M-OOH species can equilibrate to an MOO species and a hydrogenated acceptor site (M-OOH/eq), enabling a bifunctional mechanism. Within OER, the presence of Fe within an NiOOH acceptor site was found to be beneficial to lower the required overpotential (Vandichel et al. Chemcatchem, 2020, 12 (5), 1436-1442). In this work, we present the first proof-of-concept study of various possible mechanisms (standard and bifunctional ones) for OER and ORR, i.e. we include now the active edge sites and hydrogen acceptor sites in the same model system. Furthermore, we consider water as solvent to describe the equilibration of the M-OOH species to M-OOH/eq, a crucial step that enables a bifunctional route to be operative. Additionally, different single Fe-dopant positions in an exfoliated NiOOH model are considered and four different reaction schemes are studied for OER and the reverse ORR process. The results are relevant in alkaline conditions, where the studied model systems are stable. Certain Fe-dopant positions result in active Ni-edge sites with very low overpotentials provided water is present within the model system.

openaire: EC/H2020/686053/EU//CritCat The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years,molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2–Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2–Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, whichsuggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant–vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2–Co than for MoS2–Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour.

The Cluster Expansion formalism based on Density Functional Theory (DFT) simulation data has been applied for Al Mg alloys with high accuracy ( ∼ 1 meV/atom). The atomistic simulations are used to model the Al Mg phase diagram, phase boundaries and the initial solute clustering at different compositions and temperatures. The obtained free energies of formation for the FCC, HCP and γ-phase are in accordance with the experimental phase diagram. The calculations demonstrate the formation of Guinier-Preston (GP) zones of Al 3 Mg (L 1 2 phase) within the Al matrix under varying conditions. The computed transition temperatures where the ordered structures dissolve are approximately 50 K higher than experimental data. The free energy barriers associated with the formation of GP-zones increase as the solute (Mg) concentrations are reduced and the temperature is increased.

We report a comprehensive computational study of the intricate structure–property relationships governing the hydrogen adsorption trends on MoS2 edges with varying S- and H-coverages, as well as provide insights into the role of individual adsorption sites. Additionally, the effect of single- and dual S-vacancies in the basal plane on the adsorption energetics is assessed, likewise with an emphasis on the H-coverage dependency. The employed edge/site-selective approach reveals significant variations in the adsorption free energies, ranging between ∼±1.0 eV for the different edges-types and S-saturations, including differences of even as much as ∼1.2 eV between sites on the same edge. The incrementally increasing hydrogen coverage is seen to mainly weaken the adsorption, but intriguingly for certain configurations a stabilizing effect is also observed. The strengthened binding is seen to be coupled with significant surface restructuring, most notably the splitting of terminal S2-dimers. Our work links the energetics of hydrogen adsorption on 2H-MoS2 to both static and dynamic geometrical features and quantifies the observed trends as a function of H-coverage, thus illustrating the complex structure/activity relationships of the MoS2 catalyst. The results of this systematical study aims to serve as guidance for experimentalists by suggesting feasible edge/S-coverage combinations, the synthesis of which would potentially yield the most optimally performing HER-catalysts.

Nanoparticles of iron and nickel are promising candidates as nanosized soft magnetic materials and as catalysts for carbon nanotube synthesis and CO methanation, among others. To understand geometry- and size-dependent properties of these nanoparticles, phase diagram of Fe/Ni alloy nanoparticles was calculated by density functional theory and cluster expansion method. Ground state convex is presented for face-centered cubic (FCC), body-centered cubic (BCC), and icosahedral (ICO) particles. Previous experimental observations were explained by using multiscale model for particles with realistic size (diameter ≥2 nm). At size 1.5 nm, geometry changes from BCC at low X(Ni) to icosahedral at high X(Ni). FCC is stabilized over icosahedral geometry by increasing number of atoms from 561 to 923. In large FCC particles, there is enrichment of Fe atoms from core to shell beneath surface, while surface and core are enriched by Ni atoms. Catalytic enhancement effect in CO methanation was found to be due to Ni incorpo...

openaire: EC/FP7/610446/EU//PAMS Hydration structures at crystal surfaces play important roles in crystal growth or dissolution processes in liquid environments. Recently developed two-dimensional (2D) and three-dimensional (3D) force mapping techniques using frequency-modulation atomic force microscopy (FM-AFM) allow us to visualize the hydration structures at the solid-liquid interfaces at angstrom-scale resolution in real space. Up to now, the experimental and theoretical studies on local hydration structures have mainly focused on those on the terrace, but little work has looked at step edges, usually the key areas in dissolution and growth. In this study, we measured local hydration structures on water-soluble alkali halide crystal surfaces by 2D force mapping FM-AFM. The atomic-scale hydration structures observed on the terraces agree well with molecular-dynamics (MD) simulations. We also measured the hydration structures at the step edge of the NaCl(001) surface, which was constantly dissolving and growing, leading to the clear observation of atomic fluctuations. We found, with the support of MD simulations, that the hydration structures measured by FM-AFM at a time scale of a minute can be interpreted as the time-average of the hydration structures on the upper terrace and those on the lower terrace.

The structures and energetics of CuNiPd ternary alloys were studied by the ab initio-based cluster expansion method and compared to the results of experimental studies. It is demonstrated that the environments of Ni and Pd remains unchanged when going from dilute Cu1−xatomx to Cu1−xNi0.5xPd0.5x, with Ni forming pure Ni phase and Pd being coordinated by 12 Cu atoms. In Cu57Ni1Pd42, a phase transition β ( ordered ) → γ ( disordered ) occurs at approximately 865 K. There are no ordered ternary compounds in the fcc or bcc lattice at 0 K.

peer-reviewed The full text of this article will not be available in ULIR until the embargo expires on the 30/11/2020 Oxygen evolution (OER) via mixed metal oxy hydroxides [M(O)(OH)] may take place on a large variety of possible active sites on the actual catalyst. A single site computational description assumes a 4-step electrochemical mechanism with coupled H+/e- transfers between 4 intermediates (M-*, M-OH, M=O, M-OOH). We also consider bifunctional routes, in which an unstable M-OOH species converts via a proton shuttling pathway to a thermodynamically more favourable bare M-* site, O2 and a hydrogenated acceptor site; the acceptor site takes up the proton forming a hydrogenated acceptor site after recombination with an electron from the catalyst material. Here, we combine pure metal γM(O)(OH) edge sites (M = Fe, Co, Ni) with as proton-acceptor sites different threefold coordinated oxygens on β-(M,M’)(O)(OH) terraces (M,M’ = Fe, Co, Ni). The acceptor sites on these terraces have of a M’2MO motif. Our combinatorial study results in a ranking of their bifunctional OER activity on a 3D-volcano plot. Via various bi- and tri-metallic oxy hydroxide combinations, we show that their excellent experimental OER activity results from bifunctionality and provide a roadmap to construct innovative low overpotential OER catalysts

We have studied the mechanism of the initial stages of nitrogen-doped single-walled carbon nanotube growth illustrated for the case of a floating catalyst chemical vapor deposition system, which uses carbon monoxide (CO) and ammonia (NH(3)) as precursors and iron as a catalyst. We performed first-principles electronic-structure calculations, fully incorporating the effects of spin polarization and magnetic moments, to investigate the bonding and chemistry of CO, NH(3), and their fragments on a model Fe(55) icosahedral cluster. A possible dissociation path for NH(3) to atomic nitrogen and hydrogen was identified, with a reaction barrier consistent with an experimentally determined value we measured by tandem infrared and mass spectrometry. Both C-C and C-N bond formation reactions were found to be barrierless and exothermic, while a parasitic reaction of HCN formation had a barrier of over 1 eV.

openaire: EC/H2020/686053/EU//CritCat Platinum is the most active anode and cathode catalyst in next-generation fuel cells using methanol as liquid source of hydrogen. Its catalytic activity can be significantly improved by alloying with 3d metals, although a precise tuning of its surface architecture is still required. Herein, we report the design of a highly active low-temperature (below 0 °C) methanol dehydrogenation anode catalyst with reduced CO poisoning based on ultralow amount of precisely defined PtxNi1-x (x = 0 to 1) bimetallic clusters (BCs) deposited on inert flat oxides by cluster beam deposition. These BCs feature clear composition-dependent atomic arrangements and electronic structures stemming from their nucleation mechanism, which are responsible for a volcano-type activity trend peaking at the Pt0.7Ni0.3 composition. Our calculations reveal that at this composition, a cluster skin of Pt atoms with d-band centers downshifted by subsurface Ni atoms weakens the CO interaction that in turn triggers a significant increase in the methanol dehydrogenation activity.

In this study, we looked at the hydrogen evolution reaction on Mg-, Mo-, Fe-, Co-, V-, and Cu-doped Ni3P2 and Ni3P2 + P terminated Ni2P surfaces. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that doping can substantially improve the catalytic activity of the Ni3P2 terminated surface. In contrast, the Ni3P2 + P terminated one seems to be catalytically active irrespective of the type of doping, including in the absence of doping. Based on our doping energy and adsorption free energy calculations, the most promising dopants are iron and cobalt, whereas copper is less likely to function well as a doping element.

The accuracy of density functional theory (DFT) based kinetic models for electrocatalysis is diminished by spurious electron delocalization effects, which manifest as uncertainties in the predicted values of reaction and activation energies. In this work, we present a constrained DFT (CDFT) approach to alleviate overdelocalization effects in the Volmer-Heyrovsky mechanism of the hydrogen evolution reaction (HER). This method is applied a posteriori to configurations sampled along a reaction path to correct their relative stabilities. Concretely, the first step of this approach involves describing the reaction in terms of a set of diabatic states that are constructed by imposing suitable density constraints on the system. Refined reaction energy profiles are then recovered by performing a configuration interaction (CDFT-CI) calculation within the basis spanned by the diabatic states. After a careful validation of the proposed method, we examined HER catalysis on open-ended carbon nanotubes and discovered that CDFT-CI increased activation energies and decreased reaction energies relative to DFT predictions. We believe that a similar approach could also be adopted to treat overdelocalization effects in other electrocatalytic proton-coupled electron transfer reactions, e.g., in the oxygen reduction reaction.

The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3-1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone-Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone-Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties. © 2018 American Chemical Society.

Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production. Chemical modification by doping can be used to fine-tune the electrocatalytic activity, but this path requires theoretical, atomic-level support which has not been widely available for Ni-P. We present a density functional theory analysis of Al-doped Ni2P surfaces to identify structural motifs that could contribute to the improved behavior of the catalyst. Based on the formation energies of substitutionally Al-doped Ni sublattices, we find doping to take place preferably at the topmost layers. The Ni-Ni bridge and the P-top sites are the optimal ones in terms of hydrogen bonding energies. The Ni-Ni bridge site is not present on pristine surfaces but is a consequence of Al doping and provides a candidate to explain the experimentally observed high activities in doped Ni-P nanoparticles. Similar structural motifs can be recommended to be engineered for other Ni-P structures for improved electrocatalytic activity.