Layer-by-layer polyelectrolyte assembly for the protection of GaP surfaces from photocorrosion. - In: ACS applied nano materials, ISSN 2574-0970, Bd. 4 (2021), 1, S. 425-431
Polyelectrolyte layer-by-layer assemblies are known as protective coatings for corrosion inhibition. Here, we demonstrate that polyelectrolyte multilayers of poly(ethyleneimine) (PEI) and poly(styrene sulfonate) (PSS)-(PEI/PSS)x-adsorbed at the GaP(100) photocathode surface remarkably mitigate the photocorrosion of GaP without decreasing its photoconversion efficiency. The activity of the polybase-polyacid complex is based on buffering pH changes at the solid-liquid interface. We carried out ab initio molecular dynamics-based simulations of the GaP(100) surface in contact with liquid water and demonstrated that an increase in the proton concentration enhances GaP dissolution. We used the scanning vibrating electrode technique (SVET) to characterize the distribution of photocorrosion activity areas over bare and polyelectrolyte-coated GaP surfaces and we showed that a polyelectrolyte coating impedes the dissolution kinetics. Data obtained using the SVET were compared to photoetched pores on the semiconductor surface. Voltammetric and chronoamperometric measurements were also performed to evaluate photoconversion efficiencies before and after the application of the protective coatings.
https://doi.org/10.1021/acsanm.0c02768
Proximity measures based on KKT points for constrained multi-objective optimization. - In: Journal of global optimization, ISSN 1573-2916, Bd. 80 (2021), 1, S. 63-86
An important aspect of optimization algorithms, for instance evolutionary algorithms, are termination criteria that measure the proximity of the found solution to the optimal solution set. A frequently used approach is the numerical verification of necessary optimality conditions such as the Karush-Kuhn-Tucker (KKT) conditions. In this paper, we present a proximity measure which characterizes the violation of the KKT conditions. It can be computed easily and is continuous in every efficient solution. Hence, it can be used as an indicator for the proximity of a certain point to the set of efficient (Edgeworth-Pareto-minimal) solutions and is well suited for algorithmic use due to its continuity properties. This is especially useful within evolutionary algorithms for candidate selection and termination, which we also illustrate numerically for some test problems.
https://doi.org/10.1007/s10898-020-00971-3
Finding any given 2-factor in sparse pseudorandom graphs efficiently. - In: Journal of graph theory, ISSN 1097-0118, Bd. 96 (2021), 1, S. 87-108
https://doi.org/10.1002/jgt.22576
Nonconvex constrained optimization by a filtering branch and bound. - In: Journal of global optimization, ISSN 1573-2916, Bd. 80 (2021), 1, S. 31-61
A major difficulty in optimization with nonconvex constraints is to find feasible solutions. As simple examples show, the [alpha]BB-algorithm for single-objective optimization may fail to compute feasible solutions even though this algorithm is a popular method in global optimization. In this work, we introduce a filtering approach motivated by a multiobjective reformulation of the constrained optimization problem. Moreover, the multiobjective reformulation enables to identify the trade-off between constraint satisfaction and objective value which is also reflected in the quality guarantee. Numerical tests validate that we indeed can find feasible and often optimal solutions where the classical single-objective [alpha]BB method fails, i.e., it terminates without ever finding a feasible solution.
https://doi.org/10.1007/s10898-020-00956-2
Expensive multi-objective optimization of electromagnetic mixing in a liquid metal. - In: Optimization and engineering, ISSN 1573-2924, Bd. 22 (2021), 2, S. 1065-1089
This paper presents a novel trust-region method for the optimization of multiple expensive functions. We apply this method to a biobjective optimization problem in fluid mechanics, the optimal mixing of particles in a flow in a closed container. The three-dimensional time-dependent flows are driven by Lorentz forces that are generated by an oscillating permanent magnet located underneath the rectangular vessel. The rectangular magnet provides a spatially non-uniform magnetic field that is known analytically. The magnet oscillation creates a steady mean flow (steady streaming) similar to those observed from oscillating rigid bodies. In the optimization problem, randomly distributed mass-less particles are advected by the flow to achieve a homogeneous distribution (objective function 1) while keeping the work done to move the permanent magnet minimal (objective function 2). A single evaluation of these two objective functions may take more than two hours. For that reason, to save computational time, the proposed method uses interpolation models on trust-regions for finding descent directions. We show that, even for our significantly simplified model problem, the mixing patterns vary significantly with the control parameters, which justifies the use of improved optimization techniques and their further development.
https://doi.org/10.1007/s11081-020-09561-4
Functional models for entire symmetric operators in rigged de Branges Pontryagin spaces. - In: Journal of functional analysis, ISSN 1096-0783, Bd. 280 (2021), 2, 108776
The theory of operator extensions in rigged Pontryagin spaces is used to develop two functional models for closed symmetric entire operators S with finite deficiency indices (p,p) acting in a separable Pontryagin space K. In the first functional model it is shown that every such operator S is unitarily equivalent to the multiplication operator in a de Branges-Pontryagin space B(E) of p×1 vector valued entire functions. The second functional model is used to parametrize a class of compressed resolvents of extensions ÜÞS of S in terms of the range of a linear fractional transformation that is associated with the model. This approach is independent of the methods used by Krein and Langer to parameterize a related class of extensions.
https://doi.org/10.1016/j.jfa.2020.108776
Port-Hamiltonian formulation of nonlinear electrical circuits. - In: Journal of geometry and physics, Bd. 159 (2021), 103959, insges. 15 S.
We consider nonlinear electrical circuits for which we derive a port-Hamiltonian formulation. After recalling a framework for nonlinear port-Hamiltonian systems, we model each circuit component as an individual port-Hamiltonian system. The overall circuit model is then derived by considering a port-Hamiltonian interconnection of the components. We further compare this modeling approach with standard formulations of nonlinear electrical circuits.
https://doi.org/10.1016/j.geomphys.2020.103959
Coordinate transformation and its uncertainty under consideration of a non-orthogonal coordinate base. - In: Measurement science and technology, ISSN 1361-6501, Bd. 32 (2021), 4, 045001, insges. 6 S.
Nanopositioning and nanomeasuring machines are 3D coordinate measuring systems with nanometer precision at measurement volumes in the cubic centimeter range. The coordinate base is formed by an interferometer system with a common mirror corner. The orthogonality deviations of the mirror corner require a coordinate transformation of the measuring axes. The uncertainty of the coordinate transformation must be taken into account in the overall measurement uncertainty budget. Starting from a complete transformation model, the result of model simplications on the transformation behaviour is analysed and discussed.
https://doi.org/10.1088/1361-6501/aba3f5
Hollow submicrospheres of trimetallic selenides for high-capacity lithium and sodium ion batteries. - In: The chemical engineering journal, ISSN 1873-3212, Bd. 405 (2021), 126724
Highly conductive metal selenides have drawn increasing attention in the field of energy storage. Unfortunately, their application is severely limited by the inferior capacity contribution as well as unsatisfactory cycling stability. Here, we propose a simple and practical way to prepare hollow nickel-cobalt-manganese selenides (NCMSe) submicrospheres. The NCMSe submicrospheres exhibit rich redox reactions during the reaction process into which much more alkali metal ions can be inserted, leading to high reversible capacity and their hollow structure facilitates the contact between the active material and electrolyte to accelerate the redox kinetics. Benefiting from these features, the hollow NCMSe submicrospheres show superior Li-storage capacity (1600 mAh g^-1 after 1000 cycles at 2 A g^-1) and Na-storage capacity (695 mAh g^-1 after 200 cycles at 0.1 A g^-1). This work offers a novel insight to the remarkable electrochemical performance anode materials for both lithium and sodium ion batteries.
https://doi.org/10.1016/j.cej.2020.126724
Well-defined nanostructures for electrochemical energy conversion and storage. - In: Advanced energy materials, ISSN 1614-6840, Bd. 11 (2021), 15, 2001537, insges. 53 S.
Electrochemical energy conversion and storage play crucial roles in meeting the increasing demand for renewable, portable, and affordable power supplies for society. The rapid development of nanostructured materials provides an alternative route by virtue of their unique and promising effects emerging at nanoscale. In addition to finding advanced materials, structure design and engineering of electrodes improves the electrochemical performance and the resultant commercial competitivity. Regarding the structural engineering, controlling the geometrical parameters (i.e., size, shape, hetero-architecture, and spatial arrangement) of nanostructures and thus forming well-defined nanostructure (WDN) electrodes have been the central aspects of investigations and practical applications. This review discusses the fundamental aspects and concept of WDNs for energy conversion and storage, with a strong emphasis on illuminating the relationship between the structural characteristics and the resultant electrochemical superiorities. Key strategies for actualizing well-defined features in nanostructures are summarized. Electrocatalysis and photoelectrocatalysis (for energy conversion) as well as metal-ion batteries and supercapacitors (for energy storage) are selected to illustrate the superiorities of WDNs in electrochemical reactions and charge carrier transportation. Finally, conclusions and perspectives regarding future research, development, and applications of WDNs are discussed.
https://doi.org/10.1002/aenm.202001537