Author Archives: Andreas Röckert

Regulation of CO oxidation with Pd additives on Nb2CO2 MXene

Authors: Xilin Zhang, Chang Xu, Yiying Zhang, Cheng Cheng, Zongxian Yang, Kersti Hermansson,

Catalytic properties of the 2D material Nb2CO2 (MXene) with a surface oxygen vacancy and a supported single Pd atom in it (Pd/OV-Nb2CO2) is explored using DFT calculations. It is found that the single Pd atom can be stably anchored in the oxygen vacancy and out of three mechanisms for carbon monoxide oxidation were studied for our model catalyst and one of them shows promising features, namely CO oxidation via a tri-molecular Eley–Rideal (TER) mechanism which has a small activation barrier of 0.42 eV. This result may provide some guidance for the selection of anode materials with high CO-tolerance and high efficiency in removing CO from H2 for PEMFCs.

International Journal of Hydrogen Energy, Volume 46, Issue 12, 2021,

Multiparametric analysis by paper-assisted potentiometric sensors for diagnostic and monitoring of reinforced concrete structures

Authors: Noemi Colozza, Sara Tazzioli, Alessandro Sassolini, Lorenzo Agosta, Maria Giuseppina di Monted, Kersti Hermansson, Fabiana Arduini

Reinforced concrete has been employed worldwide as a leading building material for public and private structures as well as in modern sculptural art. Although the unrivalled mechanical strength and modelling versatility of this material, several interrelated processes are responsible for its progressive degradation (e.g., carbonation, penetration of aging-promoting agents), decreasing its long-last durability and representing a risk for the public security or the cultural heritage. With the aim to tackle this issue, the present work reports a novel configuration of a screen-printed sensor, obtained by the combination of flexible and robust polyester support and wax-printed filter paper device for the direct application on the concrete surface. Our sensor consists of a polyester-printed three-electrochemical cell that allows dual measurements on reinforced concrete, namely (i) the evaluation of corrosion probability of the metallic reinforcements (which outperforms the half-cell potential standard method) and (ii) the employment of a pH-sensitive iridium oxide film for the measurement of the pH of concrete. The paper was used as a porous material capable of ensuring the electrochemical connection between the Ag/AgCl printed electrode and the concrete solid matrix, acting also as a protective envelope for the electrode. After the laboratory tests, which revealed the noteworthy performances of the sensors in distinguishing among different levels of corrosion as well as measuring the pH of concrete, the developed sensor was applied for on-site measurement at the Giacomo Manzù Museum (Ardea, Italy), demonstrating its suitability for the real application to cultural heritage conservation. Overall, this easy-to-handle and non-invasive diagnostic device provides an innovative analytical approach for the on-site and prompt multiparametric monitoring of the physico-chemical phenomena that endanger the long-lasting preservation of reinforced concrete structures.

Sensors and Actuators B: Chemical
Volume 345, 15 October 2021, 130352

Space-Resolved OH Vibrational Spectra of the Hydration Shell around CO2

Authors: Pavlin D. Mitev, W. J. Briels, and Kersti Hermansson

The CO2 molecule is weakly bound in water. Here we analyze the influence of a dissolved CO2 molecule on the structure and OH vibrational spectra of the surrounding water. From the analysis of ab initio molecular dynamics simulations (BLYP-D3) we present static (structure, coordination, H-bonding, tetrahedrality) and dynamical (OH vibrational spectra) properties of the water molecules as a function of distance from the solute. We find a weakly oscillatory variation (“ABBA”) in the ‘solution minus bulk water’ spectrum. The origin of these features can largely be traced back to solvent–solute hard-core interactions which lead to variations in density and tetrahedrality when moving from the solute’s vicinity out to the bulk region. The high-frequency peak in the solute-affected spectra is specifically analyzed and found to originate from both water OH groups that fulfill the geometric H-bond criteria, and from those that do not (dangling ones). Effectively, neither is hydrogen-bonded.

J. Phys. Chem. B 2021, 125, 51, 13886–13895

Vertical-Flow Paper Sensor for On-Site and Prompt Evaluation of Chloride Contamination in Concrete Structures

Authors: Noemi Colozza, Sara TazzioliSara Tazzioli, Alessandro Sassolini, Lorenzo Agosta, Maria Giuseppina di Monte, Kersti Hermansson, and Fabiana Arduini

Corrosion occurring in reinforced concrete has turned into a primary concern of the current century, concrete being the most ubiquitous and predominant material used in the construction industry. Among the many interrelated processes that trigger corrosion of metallic reinforcements, the penetration of chloride ions into the concrete matrix is the most insidious threat. Herein, we developed the first electrochemical device entirely made of paper that allows for the direct, prompt, and noninvasive evaluation of free chloride ion contamination in concrete-based constructions. Our device is based on a three-layer wax-modified filter paper, consisting of two Ag/AgCl screen-printed electrodes that are interfaced by a junction pad in a sandwich-like configuration. Filter paper allows for generating a vertical-flow potentiometric device capable of measuring the electrochemical potential between two solutions containing different concentrations of chloride ions, which are separately drop-cast on the top and bottom layers. After demonstrating the analytical performance of the device, the same principle was applied to the evaluation of the chloride contents in different concrete samples, exploiting paper as a suitable interfacing material for potentiometric measurements on the cement solid surface. Laboratory-prepared concrete samples with known chloride contents were first assessed, and then, the paper-based vertical-flow device was applied to real concrete structures at the Giacomo Manzù Museum (Ardea, Italy) for the evaluation of chloride contamination caused by the proximity to the seaside. The capability of our device to provide timely warning of the risk conditions of concrete-based artifacts was demonstrated.

Anal. Chem. 2021, 93, 43, 14369–14374

Transition metal and alloy catalysts in the light of computational materials modelling

Author: Ageo Meier de Andrade

This thesis is built around two pillars. One is heterogeneous catalysis in the broader context of green chemistry. The focus here is on identifying catalytically active materials suitable for the valorization of renewable feedstocks. The second pillar deals with materials modelling itself, both its role to help identify the features responsible for certain desired material properties and the assessment of model quality and how to overcome challenges when modelling complex systems.

Density functional theory (DFT) has become a standard method in heterogeneous catalysis and materials science, as it generally combines good accuracy with an affordable computational cost. The choice of density functional (in relation to the system under study) strongly affect the accuracy of the DFT results. In this thesis, a subset of functionals have been tested and validated with respect to their ability to predict structural and energetic properties of single- and multi-component materials. It is shown that the inclusion of dispersion corrections by computing the nonlocal correlation self-consistently, as done in the vdW-DF-cx functional, increases the accuracy of computed results in relation to experimental data.

In the evaluation of the functionals, the computed properties were rationalized in terms of (i) the reduced density gradient distribution, unique for each material, and (ii) the exchange enhancement factor, unique for each density functional and dependent on the reduced density gradient distribution. Moreover, a tool is presented that can guide researchers towards the most appropriate density functional for the problem in question. This involves a protocol that brings DFT results into better agreement with experiment.

Heterogeneous catalysts are complex and catalyst research is often performed using model experiments and calculations. Here descriptors have important roles to play. Two descriptors for catalytic activity have been scrutinized in this thesis. The first is the work function of metal surfaces. Here, it is shown that the adsorption of selected ad-atoms on Ni surfaces provides a route to control the metal’s work function over a wide energy range. The second descriptor is the difference in stability between the enol and keto tautomers of a model lignin molecule on a model metal catalyst surface in the context of lignin depolymerization. The aim is to explore the reasons underlying the relative stabilities and to enhance the preference for enol in the keto-to-enol tautomerization. The modelling results show that a mixed PdPt alloy surface stabilizes the enol tautomer, suggesting that this could be an active catalyst for lignin depolymerization.

Doctorate thesis, Acta Universitatis Upsaliensis, 2021. , p. 74

Improving the transferability of density functional theory predictions through correlation analysis: Structural and energetic properties of NiX alloys (X=C, Si, Ge, and Sn)

Authors: Ageo Meier de Andrade, Jolla Kullgren, and Peter Broqvist

This work reports on the performance of density functional theory (DFT) for a series of single and binary systems, aiming for a quantitative description of NiX (X=C, Si, Ge, and Sn) alloys. Both semilocal GGA and a meta-GGA density functional, with and without dispersion corrections, are tested. We found in our study that no single functional simultaneously provides an accurate quantitative description of the investigated structural and energetic properties. However, the spread in computed DFT data could be rationalized in terms of the distribution of reduced density gradients and differences in the evolution of the exchange enhancement factors for different functionals. We demonstrate how to construct a regression model based on data from several density functionals that increases the predictivity of semilocal DFT. We foresee that the use of regression models (or extensions of it) can be valuable in the development of more accurate density functionals that in the future could provide a quantitative accuracy for complex multicomponent systems.

Phys. Rev. B 105, 085127 (2022)

Controlling the metal work function through atomic-scale surface engineering

Authors: Ageo Meierde Andrade, Jolla Kullgren, and Peter Broqvist

Adsorbate induced work function modification of Ni have been investigated by means of first-principles calculations. More specifically, the adsorption of Li, Na, Si, Zr, Pd, Pt, or Sn at various coverages on Ni low-index surface models have been considered. In the case of Sn, a more thorough investigation was performed comparing the adsorption as an overlayer structure with the case of surface alloy formation. Our calculations suggest that the most stable Sn@Ni configuration corresponds to a surface alloy, and here the Ni(100)c(2 × 2)-Sn, Ni(110)c(2 × 2)-Sn, and Ni(111)(sqrt(3) x sqrt(3))R30-Sn surface alloys were found to display similar stability. Concerning the induced work function change, a different behaviour as a function of coverage was observed depending on the nature of the Sn@Ni surface model. Both overlayer adsorption and surface alloying were found to induce a work function decrease already at relatively low coverages ( 0.05 atom Å −2), regardless of the underlying surface orientation. However, while the work function obtained for stable surface alloys was found to monotonously decrease as the coverage increases, the work function for the stable overlayer structures goes through a minimum. For all investigated surface modifications, the change in work function was found to be consistent with the orientation of the charge transfer at the adsorbate–surface interface. The computed data in this work may serve as handles for experimental endeavours aiming to optimize properties of active materials through atomic-scale surface engineering.

Applied Surface Science, Volume 589, (2022), 152932

Finite-field coupling via learning the charge response kernel

Authors: Yunqi Shao, Linnéa Andersson, Lisanne Knijff and Chao Zhang

Response of the electronic density at the electrode–electrolyte interface to the external field (potential) is fundamental in electrochemistry. In density-functional theory, this is captured by the so-called charge response kernel (CRK). Projecting the CRK to its atom-condensed form is an essential step for obtaining the response charge of atoms. In this work, the atom-condensed CRK is learnt from the molecular polarizability using machine learning (ML) models and subsequently used for the response-charge prediction under an external field (potential). As the machine-learnt CRK shows a physical scaling of polarizability over the molecular size and does not (necessarily) require the matrix-inversion operation in practice, this opens up a viable and efficient route for introducing finite-field coupling in the atomistic simulation of electrochemical systems powered by ML models.

Electronic Structure, 2022, 4, 1, 014012

Importance of the Ion-Pair Lifetime in Polymer Electrolytes

Authors: Harish Gudla, Yunqi Shao, Supho Phunnarungsi, Daniel Brandell, and Chao Zhang

imageIon pairing is commonly considered as a culprit for the reduced ionic conductivity in polymer electrolyte systems. However, this simple thermodynamic picture should not be taken literally, as ion pairing is a dynamical phenomenon. Here we construct model poly(ethylene oxide)–bis(trifluoromethane)sulfonimide lithium salt systems with different degrees of ion pairing by tuning the solvent polarity and examine the relation between the cation–anion distinct conductivity σ+–d and the lifetime of ion pairs τ+– using molecular dynamics simulations. It is found that there exist two distinct regimes where σ+–d scales with 1/τ+– and τ+–, respectively, and the latter is a signature of longer-lived ion pairs that contribute negatively to the total ionic conductivity. This suggests that ion pairs are kinetically different depending on the solvent polarity, which renders the ion-pair lifetime highly important when discussing its effect on ion transport in polymer electrolyte systems.

J. Phys. Chem. Lett. 2021, 12, 35, 8460–8464

Modelling Bulk Electrolytes and Electrolyte Interfaces with Atomistic Machine Learning

Authors: Yunqi Shao, Lisanne Knijff, Florian M. Dietrich, Kersti Hermansson,  Chao Zhang

imageBatteries and supercapacitors are electrochemical energy storage systems which involve multiple time-scales and length-scales. In terms of the electrolyte which serves as the ionic conductor, a molecular-level understanding of the corresponding transport phenomena, electrochemical (thermal) stability and interfacial properties is crucial for optimizing the device performance and achieving safety requirements. To this end, atomistic machine learning is a promising technology for bridging microscopic models and macroscopic phenomena. Here, we provide a timely snapshot of recent advances in this area. This includes technical considerations that are particularly relevant for modelling electrolytes as well as specific examples of both bulk electrolytes and associated interfaces. A perspective on methodological challenges and new applications is also discussed.

Batteries & Supercaps 2021, 4, 585.