| 1. |
- Langhammer, David Michael, 1991-
(författare)
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Capturing Air Pollutants : Photochemical Adsorption and Degradation of SO2, NO2 and CO2 on Titanium Dioxide
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Titanium dioxide (TiO2) is a material with many useful properties. It is used most widely as a pigment in white paint, although in technological research it is better known for its ability to catalyze chemical reactions during light absorption. This process is referred to as photocatalysis, where the energy of the light is used to power the chemical reactions. This has enabled several interesting applications of TiO2, where it can for instance be applied to windows or façade walls to make their surfaces self-cleaning. Another implementation that has received much attention lately is artificial photosynthesis, where the light energy is used to transform CO2 and H2O into synthetic fuels. This thesis work contributes to the development of both these applications, although the main ambition is to show how three of the most common ambient air pollutant molecules, SO2, NO2 and CO2, can be captured at the surface of TiO2 by means of photocatalysis. Specifically, infrared (IR) spectroscopy and density functional theory (DFT) has been used as complementary tools of analysis to study the photocatalytic reactions that enable transformation of SO2, NO2 and CO2 into strongly bound sulfates, nitrates and carbonates, respectively. This combined experimental and theoretical approach has enabled a detailed description of how these reactions proceed and has further shown how the fundamental reactivity of the TiO2 surface changes upon light absorption.The work presented herein contributes to an increased understanding of photocatalysis and shows, quite generally, how molecules can be effectively captured at the surface of metal oxides by forming surface-integrated ionic adsorbates.
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| 2. |
- Ahmed, Taha, 1984-
(författare)
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Nanostructured ZnO and metal chalcogenide films for solar photocatalysis
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increasing demand for clean energy and safe water resources has driven the development of efficient and sustainable technologies. Among these technologies, photocatalysis using semiconducting materials has emerged as a promising solution for both solar hydrogen generation and water purification. Low-dimensional ZnO, including nanorods, nanoparticles, and quantum confined particles (so called quantum dots), has demonstrated excellent photocatalytic properties due to their large surface area, high electron mobility, and tunable band gap.The work in this thesis aims to investigate the potential of low-dimensional ZnO alone and in combination with CdS and Fe2O3 for solar hydrogen generation and photocatalytic water purification. The thesis includes a comprehensive analysis of the synthesis, characterization, and optimization of low-dimensional ZnO-based photocatalyst systems for solar hydrogen generation and photocatalytic water purification. Additionally, the thesis will evaluate the performance of the ZnO-based photocatalysts under different experimental conditions, either as photoelectrodes or as distributed particle systems for water purification. The work includes detailed size control of ZnO by itself in dimensions below 10 nm using a hydrothermal method, to provide an increased total surface area and introduce quantum confinement effects that increase the band gap to enable degradation of chemical bonds in a model pollutant in a distributed system for water purification. The work also includes a relatively detailed study of the phonon–phonon and electron–phonon coupling as a function of dimension from 10 nm to 150 nm for ZnO using non-resonant and resonant Raman spectroscopy. Ultimately, the thesis aims to provide insight into the potential of low-dimensional ZnO alone and in combination with other inorganic materials for solar hydrogen generation and photocatalytic water purification and pave the way for the development of efficient and sustainable technologies for clean energy and safe water resources.
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| 3. |
- Ahmed, Taha, et al.
(författare)
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Phonon–phonon and electron–phonon coupling in nano-dimensional ZnO
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermal losses through vibrational coupling are critical bottlenecks limiting several materials classes from reaching their full potential. Altering the phonon–phonon and electron–phonon coupling by controlled suppression of vibrational degrees of freedom through low-dimensionality are promising but still largely unexplored approaches. Here we report a detailed study of the first- and second-order Raman processes as a function of size for low-dimensional ZnO. Wurtzite ZnO nanoparticles were synthesised into 3D frameworks of ZnO crystallites, with tailored crystallite diameters from 10 nm to 150 nm and characterised by electron microscopy, X-ray diffraction and non-resonant and resonant Raman spectroscopy.We present a short derivation of how resonance Raman and the relation between the longitudinal optical (LO) phonons can be utilised to quantify the electron–phonon coupling, its merits, and limitations. Theoretical Raman response using density functional theory is corroborating the experimental data in assigning first- and second-order Raman modes. The Lyddane-Sachs-Teller equation was applied to the measured LO–TO split and revealed no change in the ratio between the static and high-frequency dielectric constant with changing ZnO dimension from 10 nm to 150 nm. The second-order Raman revealed a phonon–phonon coupling that generally increased with particle size and markedly so for differential modes. Resonance Raman showed the fundamental LO mode and the 2nd, 3rd, and 4th overtones. The intensity relation between the fundamental LO mode and its overtones enabled the extraction of the change in electron–phonon coupling via the Huang-Rhys parameter as a function of particle size, which showed an increase with particle size.
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| 4. |
- Grånäs, Oscar, 1979-, et al.
(författare)
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Femtosecond bond breaking and charge dynamics in ultracharged amino acids
- 2019
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 151:14
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Tidskriftsartikel (refereegranskat)abstract
- Historically, structure determination of nanocrystals, proteins, and macromolecules required the growth of high-quality crystals sufficiently large to diffract X-rays efficiently while withstanding radiation damage. The development of the X-ray free-electron laser has opened the path toward high resolution single particle imaging, and the extreme intensity of the X-rays ensures that enough diffraction statistics are collected before the sample is destroyed by radiation damage. Still, recovery of the structure is a challenge, in part due to the partial fragmentation of the sample during the diffraction event. In this study, we use first-principles based methods to study the impact of radiation induced ionization of six amino acids on the reconstruction process. In particular, we study the fragmentation and charge rearrangement to elucidate the time scales involved and the characteristic fragments occurring.
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| 5. |
- Johansson, Fredrik
(författare)
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Core-hole Clock Spectroscopy Using Hard X-rays : Exciting States in Condensed Matter
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is about how electrons move from one place to another, that is charge transfer dynamics. Charge transfer dynamics is an important property governing chemical and physical changes that form the base for many applications such as electronics, optoelectronics and catalysis. The fundamental aspect is how charge transfer manifests in the constituent materials and their interfaces building up these devices. The basic method used is synchrotron radiation based electron spectroscopies.Using core-hole clock spectroscopy it is possible to study dynamic processes in the femtosecond and attosecond regimes - here we study the if the core-excited electron decays back into the core hole (local decays), or if the core excited electron have been tunneled away from the atomic site before the core-hole decays. Spectroscopically we can discern the two situations since one of the processes is photon energy dependent and one is not. Knowledge of the life-time of the core hole, and measuring the probability of the core-excited system decaying one way or the other makes it possible to calculate a charge transfer time. Using hard X-rays to create excited state with deep core-holes allow us to study high kinetic energy Auger electrons, also deep core-holes tend to be short lived, which gives access to short time-scales.Bulk crystals of 2D materials have been used as model systems here owing to their well-known properties. Using those it has been demonstrated that the regime of observable times using the mentioned method can be extended with an order of magnitude compared to previous studies. Our results present themselves on time-scales on par with the atomic unit of time. The highly selective nature of resonant X-ray excitations allows the anisotropic unoccupied electronic structure of bulk 2D crystals to be mapped out, here the example of SnS2 is presented. This shows that this is a direct probe of the unoccupied band structure.With core-hole clock spectroscopy the charge transfer time dependence on relative concentrations of blends between the low band-gap polymer PCPDTBT, with PCBM (functionalized fullerenes). This is a common prototypical system for organic photovoltaics. The charge transfer time decreases with increasing intermixing, up to a point where is starts getting slower, the same trend as the efficiency of solar cell devices made with the same mixing. The method employed here is chemically specific and probes the local surrounding energy landscape at the site of excitation – this is different from other techniques that utilize optical excitations which are non-local in character.The synthetization of bulk heterostructures and thin films, and the disentanglement of core-ionized states are also investigated using spectroscopic and scattering techniques.
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| 6. |
- Qiu, Zhen
(författare)
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Transition Metal-Based Electrocatalysts for Alkaline Water Splitting and CO2 Reduction
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With excessive usage of fossil fuels and ever-increasing environmental issues, numerous efforts have been devoted to the development of renewable energies for the replacement of traditional fossil fuels to reduce greenhouse gas emission and realize the rapidly growing demand for global energy. Renewable energies, however, often show diurnal and seasonal variations in power output, forming a need for energy storage to meet people’s continuous energy supply. One approach is to use electrolysis and produce a fuel that can be used on demand at a later stage. A full realization of effective electricity-to-fuel conversion, however, is still limited by the large overpotential requirements as well as concerns with the usage of scarce platinum group elements. This thesis presents studies on transition metal-based electrocatalysts for alkaline water splitting and CO2 reduction, which are two technologies to produce a chemical fuel from renewable electricity. Our aim is to develop efficient, inexpensive, and robust electrocatalysts based on earth-abundant elements with high energy conversion efficiencies.In the first part, we develop and investigate three different electrocatalysts intended for high-performance electrocatalysis of water; NiO nanoflakes (NFs) with tuneable surface morphologies, Fe doped NiO nanosheets (NSs), and self-optimized NiFe layered double hydroxide (LDH) NSs. The self-assembled NiO NFs show drastically different performance for the oxygen evolution reaction (OER). Besides the morphology effect on the catalytic property, the presence of Fe is also functional to improve the catalytic activity for both OER and hydrogen evolution reaction (HER). The NiFe LDH NSs form the most effective system for the overall catalytic performance and is dramatically improved via a dynamic self-optimization, especially for HER, where the overpotential decreases from 206 mV to 59 mV at 10 mA cm-2. In order to get insight into the interfacial reaction processes, a variety of techniques were performed to explore the underlying reasons for the catalytic improvement. Ex-situ X-ray photoelectron spectroscopy, transmission electron microscope and in-situ Raman spectroscopy were utilized to characterize and understand the oxidations states, the crystallinity and the active phases. Electrochemical impedance spectroscopy was applied to investigate the dominating reaction mechanisms during high-performance and stable electrocatalysis.In the second part, dynamically formed CuInO2 nanoparticles were demonstrated to be high-performance electrocatalysts for CO2 reduction. In-situ Raman spectroscopy was utilized to reveal and understand the formation of CuInO2 nanoparticles based on the Cu2O pre-catalyst onto an interlayer of indium tin oxide under the electrochemical reaction. Density function theory calculation and ex-situ X-ray diffraction further prove the formation of CuInO2 nanoparticles during vigorous catalysis. The findings give important clues on how Cu-based electrocatalysts can be formed into more active materials and can provide inspiration for other Cu-based intermetallic oxides for high-efficiency CO2 reduction.
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| 7. |
- Thyr, Jakob, 1979-
(författare)
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Low dimensional Zinc- and Copper Oxides and their Electronic, Vibrational and Photocatalytic Properties
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pollution of water resources is a growing problem in the world and this has drawn the attention to photocatalysis, which is an emerging technology for water purification. In this thesis, low dimensional zinc oxide and copper oxides, which are promising photocatalytic materials, have been studied. In the initial work, an approach for determining the crystal orientation in ZnO nanomaterials was developed based on polarized Raman spectroscopy. The approach was extended to non-polarized Raman spectroscopy for convenient crystal orientation determination. The results were corroborated by density functional theory (DFT) calculations providing a full vibrational mode analysis of ZnO, including higher-order Raman scattering. Photocatalyst materials based on both ZnO and copper oxides were synthesized, starting with visible light absorbing Cu2O prepared by low temperature thermal oxidation of flat and 3D structured Cu-foils. Defect induced Raman scattering revealed Raman activity in modes that are only IR active or optically silent in pristine Cu2O, with mode assignments supported by DFT calculations. Experiment with solar light illuminated Cu2O showed efficient degradation of organic water-soluble molecules and degradation rates could be further increased by 3D structuring into nanopillars. With the aim of creating a combined photocatalyst that use favourable properties from several materials, nanoparticles of ZnO were synthesized and deposited onto Cu2O, Cu4O3 and CuO. ZnO of sufficiently small size exhibit quantum confinement, which allowed for tuning of the electronic and optical properties of ZnO and this was utilized for energy level alignment in heterojunctions with copper oxides. The heterojunctions were shown to facilitate charge transfer which improved the photocatalytic properties of the dual catalysts compared to the single components. The quantum confinement effects in ZnO nanoparticles were further investigated by more detailed electrochemical measurements. The main finding was that quantum confinement results in a large decrease in the available electronic density of states which has clear implications on the capacitance and photon absorption in the material. Raman spectroscopy has been a central tool in all work, and the thesis ends with a study that goes through and explain spurious Raman signals. The contribution shows how to identify and avoid spectral artefacts and other light generating processes that compete with the Raman signal and guide the acquisition of good quality spectra.
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| 8. |
- Ahmed, Taha, et al.
(författare)
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Optical Quantum Confinement in Ultrasmall ZnO and the Effect of Size on Their Photocatalytic Activity
- 2020
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 124:11, s. 6395-6404
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Tidskriftsartikel (refereegranskat)abstract
- Zinc oxide is a well-known metal oxide semiconductor with a wide direct band gap that offers a promising alternative to titanium oxide in photocatalytic applications. ZnO is studied here as quantum dots (QDs) in colloidal suspensions, where ultrasmall nanoparticles of ZnO show optical quantum confinement with a band gap opening for particles below 9 nm in diameter from the shift of the band edge energies. The optical properties of growing ZnO QDs are determined with Tauc analysis, and a system of QDs for the treatment and degradation of distributed threats is analyzed using an organic probe molecule, methylene blue, whose UV/vis spectrum is analyzed in some detail. The effect of optical properties of the QDs and the kinetics of dye degradation are quantified for low-dimensional ZnO materials in the range of 3-8 nm and show a substantial increase in photocatalytic activity compared to larger ZnO particles. This is attributed to a combined effect from the increased surface area as well as a quantum confinement effect that goes beyond the increased surface area. The results show a significantly higher photocatalytic activity for the QDs between 3 and 6 nm with a complete decolorization of the organic probe molecule, while QDs from 6 nm and upward in diameter show signs of competing reduction reactions. Our study shows that ultrasmall ZnO particles have a reactivity beyond that which is expected because of their increased surface area and also demonstrates size-dependent reaction pathways, which introduces the possibility for size-selective catalysis.
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| 9. |
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| 10. |
- Araujo, Rafael B., et al.
(författare)
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High-entropy alloy catalysts : Fundamental aspects, promises towards electrochemical NH3 production, and lessons to learn from deep neural networks
- 2023
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Ingår i: Nano Energy. - : Elsevier. - 2211-2855 .- 2211-3282. ; 105
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Tidskriftsartikel (refereegranskat)abstract
- A computational approach to judiciously predict high-entropy alloys (HEAs) as an efficient and sustainable material class for the electrochemical reduction of nitrogen is here presented. The approach employs density functional theory (DFT), adsorption energies of N atoms and N2 molecules as descriptors of the catalytic activity and deep neural networks. A probabilistic approach to quantifying the activity of HEA catalysts for nitrogen reduction reaction (NRR) is described, where catalyst elements and concentration are optimized to increase the probability of specific atomic arrangements on the surfaces. The approach provides key features for the effective filtering of HEA candidates without the need for time-consuming calculations. The relationships between activity and selectivity, which correlate with the averaged valence electron concentration and averaged electronegativity of the reference HEA catalyst, are analyzed in terms of sufficient interaction for sustained reactions and, at the same time, for the release of the active site. As a result, a complete list of 3000 HEAs consisting of quinary components of the elements Mo, Cr, Mn, Fe, Co, Ni, Cu, and Zn are reported together with their metrics to rank them from the most likely to the least likely active catalysts for NRR in gas diffusion electrodes, or for the case where non-aqueous electrolytes are utilized to suppress the competing hydrogen evolution reaction. Moreover, the energetic landscape of the electrochemical NRR transformations are computed and compared to the case of Fe. The study also analyses and discusses how the results would translate to liquid-solid reactions in aqueous electrochemical cells, further affected by changes in properties upon hydroxylation, oxygen, hydrogen, and water coverages.
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| 11. |
- Araujo, Rafael, et al.
(författare)
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N-2 adsorption on high-entropy alloy surfaces : unveiling the role of local environments
- 2023
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 11:24, s. 12973-12983
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Tidskriftsartikel (refereegranskat)abstract
- Developing highly active catalysts to electrochemically reduce N-2 to NH3 under ambient conditions is challenging but bears the promise of using ammonia as a potential energy vector in sustainable energy technology. One of the scientific challenges concerns the inertness of N-2 emanating from the highly stable triple bonds and the lack of dipole moments, making N-2 fixation on catalytic surfaces difficult. Another critical challenge is that electrons are more prone to reduce hydrogen than N-2 at the surface, forming a scaling relationship where the reduction ability of the catalyst most often benefits hydrogen reduction instead of nitrogen reduction. Here we show that high-entropy alloys (HEA) - a new class of catalysts with vast compositional and structural possibilities, can enhance N-2 fixation. More specifically, we investigate the role of the local environment in the first and second solvation shell of the adsorbing elements in the bond strength between the dinitrogen molecules and the HEA surfaces. Density functional theory using a Bayesian error estimation functional and vdW interactions is employed to clarify the properties dictating the local bonding. The results show that although the main property calibrating the N-2 bond strength is the d-band centers of the adsorbing elements, the value of the d-band centers of the adsorbing elements is further regulated by their local environment, mainly from the elements in the first solvation shell due to electron donor-acceptor interactions. Therefore, there exists a first solvation shell effect of the adsorbing elements on the bond strength between N-2 molecules and the surface of HEAs. The results show that apart from the direct active site, the indirect relation adds further modulation abilities where the local interactions with a breath of metallic elements could be used in HEAs to engineer specific surface environments. This is utilized here to form a strategy for delivering higher bond strength with the N-2 molecules, mitigating the fixation issue. The analysis is corroborated by correlation analysis of the properties affecting the interaction, thus forming a solid framework of the model, easily extendable to other chemical reactions and surface interaction problems.
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| 12. |
- Araujo, Rafael, et al.
(författare)
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Supervised AI and Deep Neural Networks to Evaluate High-Entropy Alloys as Reduction Catalysts in Aqueous Environments
- 2024
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 14:6, s. 3742-3755
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Tidskriftsartikel (refereegranskat)abstract
- Competitive surface adsorption energies on catalytic surfaces constitute a fundamental aspect of modeling electrochemical reactions in aqueous environments. The conventional approach to this task relies on applying density functional theory, albeit with computationally intensive demands, particularly when dealing with intricate surfaces. In this study, we present a methodological exposition of quantifying competitive relationships within complex systems. Our methodology leverages quantum-mechanical-guided deep neural networks, deployed in the investigation of quinary high-entropy alloys composed of Mo-Cr-Mn-Fe-Co-Ni-Cu-Zn. These alloys are under examination as prospective electrocatalysts, facilitating the electrochemical synthesis of ammonia in aqueous media. Even in the most favorable scenario for nitrogen fixation identified in this study, at the transition from O and OH coverage to surface hydrogenation, the probability of N2 coverage remains low. This underscores the fact that catalyst optimization alone is insufficient for achieving efficient nitrogen reduction. In particular, these insights illuminate that system consideration with oxygen- and hydrogen-repelling approaches or high-pressure solutions would be necessary for improved nitrogen reduction within an aqueous environment.
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| 13. |
- Ardo, Shane, et al.
(författare)
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Pathways to electrochemical solar-hydrogen technologies
- 2018
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 11:10, s. 2768-2783
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Forskningsöversikt (refereegranskat)abstract
- Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/ or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.
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| 14. |
- Atak, Gamze, et al.
(författare)
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Nitrogen doped W oxide films for electrochromic applications
- 2019
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Ingår i: EMRS Spring Meeting 2019.
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Konferensbidrag (refereegranskat)abstract
- Electrochromic (EC) materials are able to change their optical properties such as transmission, absorption and reflection reversibly by application of an external voltage. EC metal oxides are divided into two groups: cathodic (coloring under ion insertion) and anodic (coloring under ion extraction). W oxide is a well-known cathodic EC material and its color changes from transparent to dark blue when ions are inserted.A desirable electrochromic material must have and maintain a high optical modulation, high coloration efficiency, fast coloration/bleaching switching kinetics and a stable charge/ discharge reversibility. In this study, W oxide films with different nitrogen levels were deposited by using reactive DC sputtering onto glass and ITO coated glass in Ar+O2+N2 atmosphere. For all films, the total gas pressure was set to 4.0 Pa, the Ar flow rate was kept at 50 ml/min, and the O2+N2 flow rate was kept at 7.5 ml/min. The optical, structural and electrochromic properties of undoped and N-doped W oxide films were investigated. The optical studies revealed that the average optical transmittance and band gap decreased (from 3.43 to 3.08 eV) due to N doping. It is shown that a small amount of nitrogen has promising effects on the EC performance (i.e. charge/discharge reversibility, optical modulation, coloration efficiency) of the WO3 films. It is observed that CE values increased by increasing N2 flow rate and its maximum value was 33.8 cm2/C. The maximum ΔT at 537 nm was 73.6% for an optimized N doped W oxide film.
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| 15. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Bifunctional solar electrocatalytic water splitting using CIGS solar modules and WO3-based electrolyzers
- 2019
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Ingår i: EMRS Spring Meeting 2019.
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Konferensbidrag (refereegranskat)abstract
- Using energy from the sun to produce a fuel and finally obtaining only water as an exhaust is a promising future technology for renewable energy and environmental sustainability. Solar driven water splitting is a method to produce hydrogen from solar energy. Coupling a solar cell with an electrolyzer is the approach with highest technological readiness. CuInxGa1-xSe2 (CIGS) is here a promising solar cell material for water splitting because it is possible to tune the band gap between 1.0 and 1.7 eV by changing the ratio between Ga and In, thus enabling maximum power point matching with an electrolyzer. Tungsten oxide is known as a photocatalytic material and mainly used for the oxygen evolution reaction in a water splitting process. However, WO3 films also show electrochromic activity together with hydrogen evolution. This result is interesting because it shows that WO3 films can be used as bifunctional materials for both hydrogen and oxygen evolution in water splitting, and provide additional functionalities to the system. In this study, WO3 films coated at different sputtering conditions on Ni foam and indium tin oxide substrates were investigated in the potential range of the hydrogen evolution reaction. The best overpotential of 164 mV vs. RHE at 10 mA/cm2 was obtained for WO3 films on Ni foam in 0.5 M H2SO4. The lowest potential needed for 10 mA/cm2 was measured 1.768 V for the electrolyzers consisting WO3 films on Ni foam as the cathode and non-coated Ni foam as the anode. Optimum solar-to-hydrogen (STH) efficiency of the CIGS solar cell modules and the electrolyzers was examined for different band gaps of the CIGS modules and sputtering conditions of WO3 films. Operation points of the combined system were calculated from the intersection of the voltage-current density curves for the CIGS modules and the electrolyzers. The results showed that the detailed sputtering conditions were not very critical to obtain high STH efficiency, indicating that the system could be robust and easily manufactured. The best-matched band gap of the CIGS was 1.19 eV and the highest STH efficiency of the CIGS driven WO3-based electrolyzers was 12.98 %.
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| 16. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Electrochromic solar water splitting using a cathodic WO3 electrocatalyst
- 2021
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Ingår i: Nano Energy. - : Elsevier. - 2211-2855 .- 2211-3282. ; 81
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Tidskriftsartikel (refereegranskat)abstract
- Solar-driven water splitting is an emerging technology with high potential to generate fuel cleanly and sustainably. In this work, we show that WO3 can be used as a cathodic electrocatalyst in combination with (Ag,Cu) InGaSe2 solar cell modules to produce hydrogen and provide electrochromic functionality to water splitting devices. This electrochromic effect can be used to monitor the charge state or performance of the catalyst for process control or for controlling the temperature and absorbed heat due to tunable optical modulation of the electrocatalyst. WO3 films coated on Ni foam, using a wide range of different sputtering conditions, were investigated as cathodic electrocatalysts for the water splitting reaction. The solar-to-hydrogen (STH) efficiency of solar-driven water electrolysis was extracted using (Ag,Cu)InGaSe2 solar cell modules with a cell band gap varied in between 1.15 and 1.25 eV with WO3 on Ni foam-based electrolyzers and yielded up to 13% STH efficiency. Electrochromic properties during water electrolysis were characterized for the WO3 films on transparent substrate (indium tin oxide). Transmittance varied between 10% and 78% and the coloration efficiency at a wavelength of 528 nm and the overpotential of 400 mV was 40 cm(2) C-1. Hydrogen ion consumption in ion intercalation for electrochromic and hydrogen gas production for water electrolysis processes was discussed.
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| 17. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Impedance Spectroscopy Modeling of Nickel–Molybdenum Alloys on Porous and Flat Substrates for Applications in Water Splitting
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:39, s. 23890-23897
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen production by splitting water using electrocatalysts powered by renewable energy from solar or wind plants is one promising alternative to produce a carbon-free and sustainable fuel. Earth-abundant and nonprecious metals are, here, of interest as a replacement for scarce and expensive platinum group catalysts. Ni–Mo is a promising alternative to Pt, but the type of the substrate could ultimately affect both the initial growth conditions and the final charge transfer in the system as a whole with resistive junctions formed in the heterojunction interface. In this study, we investigated the effect of different substrates on the hydrogen evolution reaction (HER) of Ni–Mo electrocatalysts. Ni–Mo catalysts (30 atom % Ni, 70 atom % Mo) were sputtered on various substrates with different porosities and conductivities. There was no apparent morphological difference at the surface of the catalytic films sputtered on the different substrates, and the substrates were classified from microporous to flat. The electrochemical characterization was carried out with linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in the frequency range 0.7 Hz–100 kHz. LSV measurements were carried out at direct current (DC) potentials between 200 and −400 mV vs the reversible hydrogen electrode (RHE) in 1 M NaOH encompassing the HER. The lowest overpotentials for HER were obtained for films on the nickel foam at all current densities (−157 mV vs RHE @ 10 mA cm–2), and the overpotentials increased in the order of nickel foil, carbon cloth, fluorine-doped tin oxide, and indium tin oxide glass. EIS data were fitted with two equivalent circuit models and compared for different DC potentials and different substrate morphologies and conductivities. By critical evaluation of the data from the models, the influence of the substrates on the reaction kinetics was analyzed in the high- and low-frequency regions. In the high-frequency region, a strong substrate dependence was seen and interpreted with a Schottky-type barrier, which can be rationalized as being due to a potential barrier in the material heterojunctions or a resistive substrate–film oxide/hydroxide. The results highlight the importance of substrates, the total charge transfer properties in electrocatalysis, and the relevance of different circuit components in EIS and underpin the necessity to incorporate high-conductivity, chemically inert, and work-function-matched substrate–catalysts in the catalyst system.
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| 18. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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NiMoV and NiO-based catalysts for efficient solar-driven water splitting using thermally integrated photovoltaics in a scalable approach
- 2021
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Ingår i: iScience. - : Cell Press. - 2589-0042. ; 24:1
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Tidskriftsartikel (refereegranskat)abstract
- In this work, a trimetallic NiMoV catalyst is developed for the hydrogen evolution reaction and characterized with respect to structure, valence, and elemental distribution. The overpotential to drive a 10 mA cm−2 current density is lowered from 94 to 78 mV versus reversible hydrogen electrode by introducing V into NiMo. A scalable stand-alone system for solar-driven water splitting was examined for a laboratory-scale device with 1.6 cm2 photovoltaic (PV) module area to an up-scaled device with 100 cm2 area. The NiMoV cathodic catalyst is combined with a NiO anode in alkaline electrolyzer unit thermally connected to synthesized (Ag,Cu) (In,Ga)Se2 ((A)CIGS) PV modules. Performance of 3- and 4-cell interconnected PV modules, electrolyzer, and hydrogen production of the PV electrolyzer are examined between 25°C and 50°C. The PV-electrolysis device having a 4-cell (A)CIGS under 100 mW cm−2 illumination and NiMoV-NiO electrolyzer shows 9.1% maximum and 8.5% averaged efficiency for 100 h operation.
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| 19. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Optimum Band Gap Energy of ((Ag),Cu)(InGa)Se2 Materials for Combination with NiMo–NiO Catalysts for Thermally Integrated Solar-Driven Water Splitting Applications
- 2019
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Solar-driven water splitting is considered one of the promising future routes to generate fuel in a sustainable way. A carbon-free solar fuel, molecular hydrogen, can here be produced along two different but intimately related routes, photoelectrochemical (PEC) water splitting or photovoltaic electrolysis (PV-electrolysis), where the latter builds on well-established solar cell and electrolysis materials with high efficiency. The PV-electrolysis approach is also possible to construct from an integrated PEC/PV-system avoiding dc-dc converters and enabling heat exchange between the PV and electrolyzer part, to a conventionally wired PV-electrolysis system. In either case, the operating voltage at a certain current needs to be matched with the catalyst system in the electrolysis part. Here, we investigate ((Ag),Cu)(In,Ga)Se-2 ((A)CIGS)-materials with varying Ga-content modules for combination with NiMo-NiO catalysts in alkaline water splitting. The use of (A)CIGS is attractive because of the low cost-to-performance ratio and the possibility to optimize the performance of the system by tuning the band gap of (A)CIGS in contrast to Si technology. The band gap tuning is possible by changing the Ga/(Ga + In) ratio. Optoelectronic properties of the (A)CIGS materials with Ga/(Ga + In) ratios between 0.23 and 0.47 and the voltage and power output from the resulting water splitting modules are reported. Electrolysis is quantified at temperatures between 25 and 60 degrees C, an interval obtainable by varying the thermal heat exchange form a 1-sun illuminated PV module and an electrolyte system. The band gaps of the (A)CIGS thin films were between 1.08 to 1.25 eV and the three-cell module power conversion efficiencies (PCE) ranged from 16.44% with 1.08 eV band gap and 19.04% with 1.17 eV band gap. The highest solar-to-hydrogen (STH) efficiency was 13.33% for the (A)CIGS-NiMo-NiO system with 17.97% module efficiency and electrolysis at 60 degrees C compared to a STH efficiency of 12.98% at 25 degrees C. The increase in STH efficiency with increasing temperature was more notable for lower band gaps as these are closer to the overpotential threshold for performing efficient solar-driven catalysis, while only a modest improvement can be obtained by utilizing thermal exchange for a band gap matched PV-catalysts system. The results show that usage of cost-effective and stable thin film PV materials and earth abundant catalysts can provide STH efficiencies beyond 13% even with PV modules with modest efficiency.
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| 20. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Scalable and thermally-integrated solar water-splitting modules using Ag-doped Cu(In,Ga)Se2 and NiFe layered double hydroxide nanocatalysts
- 2022
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 10:22, s. 12079-12091
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Tidskriftsartikel (refereegranskat)abstract
- Photovoltaic (PV) electrolysis is an important and powerful technology for environmentally-friendly fuel production based on solar energy. By directly coupling solar cell materials to electrochemical systems to perform water electrolysis, solar energy can be converted into hydrogen fuel utilizing locally-generated heat and avoid losses from DC-DC convertors and power grid transmission. Although there have been significant contributions to the photoelectrochemical and PV-electrolysis field using isolated laboratory cells, the capacity to upscale and retain high levels of efficiency in larger modules remains a critical issue for widespread use and application. In this study, we develop thermally-integrated, solar-driven water-splitting device modules using AgCu(In,Ga)Se2 (ACIGS) and an alkaline electrolyzer system with NiFe-layered double hydroxide (LDH) nanocatalysts with devices of 82-100 cm2 area. The Ga-content in the ACIGS solar cells is tuned to achieve an optimal voltage for the catalyst system, and the average efficiencies and durability of the PV-electrolyzer were tested in up to seven-day indoor and 21 day outdoor operations. We achieved a solar-to-hydrogen (STH) module efficiency of 13.4% from gas volume measurements for the system with a six-cell CIGS-electrolyzer module with an active area of 82.3 cm2 and a 17.27% PV module efficiency under 100 mW cm−2 illumination, and thus 77% electricity-to-hydrogen efficiency at one full sun. Outdoor tests under mid-Europeen winter conditions exhibited an STH efficiency between 10 and 11% after the initial activation at the installation site in Jülich, Germany, in December 2020, despite challenging outdoor-test weather conditions, including sub-zero temperatures.
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| 21. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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The climatic response of thermally integrated photovoltaic-electrolysis water splitting using Si and CIGS combined with acidic and alkaline electrolysis
- 2020
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Ingår i: Sustainable Energy & Fuels. - : ROYAL SOC CHEMISTRY. - 2398-4902. ; 4:12, s. 6011-6022
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Tidskriftsartikel (refereegranskat)abstract
- The Horizon 2020 project PECSYS aims to build a large area demonstrator for hydrogen production from solar energy via integrated photovoltaic (PV) and electrolysis systems of different types. In this study, Si- and CIGS-based photovoltaics are developed together with three different electrolyzer systems for use in the corresponding integrated devices. The systems are experimentally evaluated and a general model is developed to investigate the hydrogen yield under real climatic conditions for various thin film and silicon PV technologies and electrolyser combinations. PV characteristics using a Si heterojunction (SHJ), thin film CuInxGa1-xSe2, crystalline Si with passivated emitter rear totally diffused and thin film Si are used together with temperature dependent catalyst load curves from both acidic and alkaline approaches. Electrolysis data were collected from (i) a Pt-IrO2-based acidic electrolysis system, and (ii) NiMoW-NiO-based and (iii) Pt-Ni foam-based alkaline electrolysis systems. The calculations were performed for mid-European climate data from Julich, Germany, which will be the installation site. The best systems show an electricity-to-hydrogen conversion efficiency of 74% and over 12% solar-to-hydrogen (STH) efficiencies using both acidic and alkaline approaches and are validated with a smaller lab scale prototype. The results show that the lower power delivered by all the PV technologies under low irradiation is balanced by the lower demand for overpotentials for all the electrolysis approaches at these currents, with more or less retained STH efficiency over the full year if the catalyst area is the same as the PV area for the alkaline approach. The total yield of hydrogen, however, follows the irradiance, where a yearly hydrogen production of over 35 kg can be achieved for a 10 m(2) integrated PV-electrolysis system for several of the PV and electrolyser combinations that also allow a significant (100-fold) reduction in necessary electrolyser area for the acidic approach. Measuring the catalyst systems under intermittent and ramping conditions with different temperatures, a 5% lowering of the yearly hydrogen yield is extracted for some of the catalyst systems while the Pt-Ni foam-based alkaline system showed unaffected or even slightly increased yearly yield under the same conditions.
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| 22. |
- Benesperi, Iacopo, et al.
(författare)
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Dynamic dimer copper coordination redox shuttles
- 2022
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Ingår i: Chem. - : Elsevier. - 2451-9308 .- 2451-9294. ; 8:2, s. 439-449
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Tidskriftsartikel (refereegranskat)abstract
- Summary Conventional redox mediators based on metal coordination complexes undergo electron transfer through the change in oxidation state of the metal center. However, electron transfer kinetics are offset toward preferred oxidation states when preorganized ligands constrain the reorganization of the coordination sphere. In contrast, we report here on dimeric copper(II/I) redox couples, wherein the extent of oxidation/reduction of two metal centers dictates the dynamic formation of dimer and monomer complexes: the dimeric (Cu(I))2 transitions to monomers of Cu(II). The bis(thiazole/pyrrole)-bipyridine tetradentate ligands stabilize both oxidation states of the unique redox systems. The dynamic dimer redox mediators offer a viable two-electron redox mechanism to develop efficient hybrid solar cells through inhibited recombination and rapid charge transport. Density functional theory calculations reveal inner reorganization energies for single-electron transfer as low as 0.27 eV, marking the dimeric complexes superior redox systems over single complexes as liquid and potentially solid-state electrolytes.
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| 23. |
- Calnan, Sonya, et al.
(författare)
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Development of Various Photovoltaic‐Driven Water Electrolysis Technologies for Green Solar Hydrogen Generation
- 2021
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Ingår i: Solar RRL. - : John Wiley & Sons. - 2367-198X. ; 6:5
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Tidskriftsartikel (refereegranskat)abstract
- Direct solar hydrogen generation via a combination of photovoltaics (PV) and water electrolysis can potentially ensure a sustainable energy supply while minimizing greenhouse emissions. The PECSYS project aims at demonstrating asolar-driven electrochemical hydrogen generation system with an area >10 m2 with high efficiency and at reasonable cost. Thermally integrated PV electrolyzers(ECs) using thin-film silicon, undoped, and silver-doped Cu(In,Ga)Se2 and silicon heterojunction PV combined with alkaline electrolysis to form one unit are developed on a prototype level with solar collection areas in the range from 64 to2600 cm2 with the solar-to-hydrogen (StH) efficiency ranging from 4 to 13%. Electrical direct coupling of PV modules to a proton exchange membrane EC test the effects of bifacially (730 cm2 solar collection area) and to study the long-term operation under outdoor conditions (10 m2 collection area) is also investigated. In both cases, StH efficiencies exceeding 10% can be maintained over the test periods used. All the StH efficiencies reported are based on measured gas outflow using mass flow meters.
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| 24. |
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| 25. |
- D'Amario, Luca, et al.
(författare)
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Towards time resolved characterization of electrochemical reactions : electrochemically-induced Raman spectroscopy
- 2022
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Ingår i: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 13:36, s. 10734-10742
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Tidskriftsartikel (refereegranskat)abstract
- Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.
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