Helmer Fjellvåg

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2024). Unraveling the (De)sodiation Mechanisms of BiFeO<inf>3</inf> at a High Rate with Operando XRD. ACS Applied Materials & Interfaces. ISSN 1944-8244. 16(10), s. 12428–12436. doi: 10.1021/acsami.3c17296. Fulltekst i vitenarkiv Vis sammendrag

  Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials’ characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO3 in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO3 is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g–1, when cycled at 0.1 A g–1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g–1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na–Fe–O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na3Bi reaction is less damaging to the cycling stability of the material.

• Thøgersen, Rasmus Vester; Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein Slagtern & Fjellvåg, Helmer (2023). Structural and magnetic properties of antifluorite-type Li5+xFe1−xCoxO4. Zeitschrift für Anorganische und Allgemeines Chemie. ISSN 0044-2313. 649(22). doi: 10.1002/zaac.202300146. Fulltekst i vitenarkiv
• Hempel, Frida Sveen; Martineau-Corcos, Charlotte; Bianchini, Federico; Fjellvåg, Helmer & Arstad, Bjørnar (2023). Dynamics of Interlayer Na-Ions in Ga-Substituted Na2Zn2TeO6 (NZTO) Studied by Variable-Temperature Solid-State 23Na NMR Spectroscopy and DFT Modeling. ACS Physical Chemistry Au. 3(4), s. 320–405. doi: 10.1021/acsphyschemau.3c00012. Fulltekst i vitenarkiv Vis sammendrag

  Local Na-coordination and dynamics of Na2–xZn2–xGaxTeO6; x = 0.00 (NZTO), 0.05, 0.10, 0.15, 0.20, were studied by variable-temperature, 23Na NMR methods and DFT AIMD simulations. Structure and dynamics were probed by NMR in the temperature ranges of 100–293 K in a magnetic field of 18.8 T and from 293 up to 500 K in a magnetic field of 11.7 T. Line shapes and T1 relaxation constants were analyzed. At 100 K, the otherwise dynamic Na-ions are frozen out on the NMR time scale, and a local structure characterization was performed for Na-ions at three interlayer sites. On increasing the temperature, complex peak shape coalescences occurred, and at 293 K, the Na NMR spectra showed some averaging due to Na-ion dynamics. A further increase to 500 K did not reveal any new peak shape variations until the highest temperatures, where an apparent peak splitting was observed, similar to what was observed in the 18.8 T experiments at lower temperatures. A three-site exchange model coupled with reduced quadrupolar couplings due to dynamics appear to explain these peak shape observations. The Ga substitution increases the Na-jumping rate, as proved by relaxation measurements and by a decrease in temperature for peak coalescence. The estimated activation energy for Na dynamics in the NZTO sample, from relaxation measurements, corresponds well to results from DFT AIMD simulations. Upon Ga substitution, measured activation energies are reduced, which is supported, in part, by DFT calculations. Addressing the correlated motion of Na-ions appears important for solid-state ion conductors since benefits can be gained from the decrease in activation energy upon Ga substitution, for example.

• Banu S, Lathifa; Veerapandy, Vasu; Fjellvåg, Helmer & Ponniah, Vajeeston (2023). First-Principles Insights into the Relative Stability and Physical and Chemical Properties of MoSe2. ACS Omega. ISSN 2470-1343. 8(15), s. 13799–13812. doi: 10.1021/acsomega.2c08217. Fulltekst i vitenarkiv
• Choudhary, Mukesh K.; Fjellvåg, Helmer & Ponniah, Ravindran (2023). First principle studies on electronic and thermoelectric properties of Fe2TiSn based multinary Heusler alloys. Computational Materials Science. ISSN 0927-0256. 216. doi: 10.1016/j.commatsci.2022.111856. Fulltekst i vitenarkiv
• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein; Vaughan, Gavin B. M.; Di Michel, Marco & Fjellvåg, Helmer [Vis alle 9 forfattere av denne artikkelen] (2022). 5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 24(44), s. 27075–27085. doi: 10.1039/d2cp03892g. Fulltekst i vitenarkiv
• Hempel, Frida Sveen; Bianchini, Federico; Arstad, Bjørnar & Fjellvåg, Helmer (2022). Effects of Ga Substitution on the Local Structure of Na2Zn2TeO6. Inorganic Chemistry. ISSN 0020-1669. 61(33), s. 13067–13076. doi: 10.1021/acs.inorgchem.2c01431. Fulltekst i vitenarkiv Vis sammendrag

  In the work presented here, we prepared Ga-substituted NZTO (Na2–xZn2–xGaxTeO6, x = 0.00, 0.05, 0.10, 0.15, 0.20) layered materials with a soft chemical, citric acid-based synthesis method and characterized these by means of X-ray diffraction (XRD), 23Na and 125Te NMR, and by density functional theory (DFT) modeling. The influence of randomly distributed Ga cations on the 125Te NMR spectra confirms the successful synthesis. With DFT-based linear response computations, we show that the local distribution of Na ions in the two neighboring interlayers influences the 125Te chemical shift, consistent with observations. DFT modeling suggests that some of the Na sites are rarely occupied in pure NZTO but become favorable upon Ga substitution. There are clear indications that Ga substitution gives an uneven distribution of Na ions in neighboring interlayers and that the Na structure in one layer affects the adjacent layers.

• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, J. & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2022). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Materials Today Communications. ISSN 2352-4928. 33. doi: 10.1016/j.mtcomm.2022.104084.
• Premkumar, T.; Vidya, R. & Fjellvåg, Helmer (2022). Stabilizing p-type conductivity in CuYO2 by co-doping: A first-principles study. Materials Chemistry and Physics. ISSN 0254-0584. 285. doi: 10.1016/j.matchemphys.2022.126101.
• Thøgersen, Rasmus Vester; Bianchini, Federico; Fjellvåg, Helmer & Ponniah, Vajeeston (2022). Antifluorite-type Na<inf>5</inf>FeO<inf>4</inf> as a low-cost, environment-friendly cathode with combined cationic/anionic redox activity for sodium ion batteries: a first-principles investigation. RSC Advances. ISSN 2046-2069. 12(27), s. 17410–17421. doi: 10.1039/d2ra01834a. Fulltekst i vitenarkiv
• Eggert, Bruno G. F.; Delczeg-Czirjak, Erna K.; Maccari, Fernando; Kumar, Susmit; Gutfleisch, Oliver & Fjellvåg, Helmer [Vis alle 8 forfattere av denne artikkelen] (2022). Exploring V-Fe-Co-Ni-Al and V-Fe-Co-Ni-Cu high entropy alloys for magnetocaloric applications. Journal of Alloys and Compounds. ISSN 0925-8388. 921. doi: 10.1016/j.jallcom.2022.166040. Fulltekst i vitenarkiv
• Balakrishnan, Kanimozhi; Veerapandy, Vasu; Fjellvåg, Helmer & Ponniah, Vajeeston (2022). First-Principles Exploration into the Physical and Chemical Properties of Certain Newly Identified SnO<inf>2</inf>Polymorphs. ACS Omega. ISSN 2470-1343. 7(12), s. 10382–10393. doi: 10.1021/acsomega.1c07063.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Nanotechnology. ISSN 0957-4484. 33(18). doi: 10.1088/1361-6528/ac4eb5. Fulltekst i vitenarkiv Vis sammendrag

  Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi2MoO6) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report an operando x-ray diffraction (XRD) investigation of Bi2MoO6-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi2MoO6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na3Bi phase. Preliminary electrochemical evaluation in half-cells versus Na metal demonstrated specific capacities for Bi2MoO6 to be close to 300 mAh g−1 during the initial 10 cycles, followed by a rapid capacity decay. Operando XRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na3Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na–Mo–O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Saha, Dipankar; Kumar, Susmit; Gonano, Bruno Raymond Gino; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Total Scattering Study of Local Structural Changes in MnAs1-xPx Influenced by Magnetic Interactions. Chemistry of Materials. ISSN 0897-4756. 33, s. 2576–2584. doi: 10.1021/acs.chemmater.1c00128. Fulltekst i vitenarkiv
• Bernal, Fabian Leonardo Martinez; Lundvall, Fredrik; Kumar, Susmit; Hansen, Per-Anders S.; Wragg, David S. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2021). Jahn-Teller active fluoroperovskites ACrF3 (A=Na+, K+): Magnetic and thermo-optical properties. PHYSICAL REVIEW MATERIALS. ISSN 2475-9953. 5(6). doi: 10.1103/PhysRevMaterials.5.064420. Fulltekst i vitenarkiv Vis sammendrag

  Chromium (II) fluoroperovskites A CrF 3 ( A = Na + , K + ) are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the A -site ion plays in this family of fluoroperovskites using both experimental methods (x-ray diffraction, optical absorption spectroscopy, and magnetic fields) and density functional theory simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions E 2 and E 3 only merge completely for A = Na at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in NaCrF 3 increases with increasing applied field. Direct magneto-optical correlations suppress the spin-flip transitions for KCrF 3 below its Néel temperature. In NaCrF 3 the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations possibly linked to crystal structure changes. This suggests that as the long range ordering is reduced, local Jahn-Teller effects in the individual CrF 4 − 6 octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic A x MnF 3 + x system. The size of the A -site cation is shown to be central in dictating the physical properties and phase transitions in A CrF 3 , opening up the possibility of varying the composition to create novel states of matter with tunable properties.

• Periyasamy, Manimuthu; Patra, Lokanath; Fjellvåg, Øystein; Ravindran, Ponniah; Sørby, Magnus Helgerud & Kumar, Susmit [Vis alle 8 forfattere av denne artikkelen] (2021). Effect of electron doping on the crystal structure and physical properties of an n = 3 Ruddlesden-Popper compound La4Ni3O10. ACS Applied Electronic Materials. ISSN 2637-6113. 3(6), s. 2671–2684. doi: 10.1021/acsaelm.1c00270. Fulltekst i vitenarkiv
• Gonano, Bruno Raymond Gino; Fjellvåg, Øystein; Steciuk, Gwladys; Guillou, Francois; Saha, Dipankar & Pelloquin, Denis [Vis alle 7 forfattere av denne artikkelen] (2021). Tuning the Magnetically Segregated Nanolayering in Mn–Ni–As Intermetallics. Chemistry of Materials. ISSN 0897-4756. 33(8), s. 3002–3010. doi: 10.1021/acs.chemmater.1c00760. Fulltekst i vitenarkiv
• Schuler, Raphael; Bianchini, Federico; Norby, Truls Eivind & Fjellvåg, Helmer (2021). Near-Broken-Gap Alignment between FeWO4 and Fe2WO6 for Ohmic Direct p–n Junction Thermoelectrics. ACS Applied Materials & Interfaces. ISSN 1944-8244. 13(6), s. 7416–7422. doi: 10.1021/acsami.0c19341.
• Sønsteby, Henrik Hovde; Skaar, Erik; Fjellvåg, Helmer & Nilsen, Ola (2021). Effect of Subcycle Arrangement on Direct Epitaxy in ALD of LaNiO3. ACS Applied Electronic Materials. ISSN 2637-6113. 3(1), s. 292–298. doi: 10.1021/acsaelm.0c00855. Fulltekst i vitenarkiv
• Karthikeyan, Jayakumar; Fjellvåg, Helmer; Bundli, Silje & Sjåstad, Anja Olafsen (2021). Efficient Exfoliation of Layered Double Hydroxides; Effect of Cationic Ratio, Hydration State, Anions and Their Orientations. Materials. ISSN 1996-1944. 14(2). doi: 10.3390/ma14020346.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Sottmann, Jonas; Koposov, Alexey & Fjellvåg, Helmer (2021). Understanding the (De)Sodiation Mechanisms in Na‐Based Batteries through Operando X‐Ray Methods. Batteries & Supercaps. ISSN 2566-6223. 4(7), s. 1039–1063. doi: 10.1002/batt.202000294. Fulltekst i vitenarkiv Vis sammendrag

  Progress in the field of Na‐based batteries strongly relies on the development of new advanced materials. However, one of the main challenges of implementing new electrode materials is the understanding of their mechanisms (sodiation/desodiation) during electrochemical cycling. Operando studies provide extremely valuable insights into structural and chemical changes within different battery components during battery operation. The present review offers a critical summary of the operando X‐ray based characterization techniques used to examine the structural and chemical transformations of the active materials in Na‐ion, Na‐air and Na‐sulfur batteries during (de)sodiation. These methods provide structural and electronic information through diffraction, scattering, absorption and imaging or through a combination of these X‐ray‐based techniques. Challenges associated with cell design and data processing are also addressed herein. In addition, the present review provides a perspective on the future opportunities for these powerful techniques.

• Hu, Yang; Miikkulainen, Ville; Mizohata, Kenichiro; Norby, Truls Eivind; Nilsen, Ola & Fjellvåg, Helmer (2020). Ionic conductivity in LixTaOy thin films grown by Atomic Layer Deposition (ALD). Electrochimica Acta. ISSN 0013-4686. 361. doi: 10.1016/j.electacta.2020.137019. Fulltekst i vitenarkiv
• Bratvold, Jon E.; Fjellvåg, Helmer & Nilsen, Ola (2020). Phase and Orientation Control of NiTiO3 Thin Films. Materials. ISSN 1996-1944. 13(1), s. 1–19. doi: 10.3390/ma13010112. Fulltekst i vitenarkiv
• Sønsteby, Henrik Hovde; Bratvold, Jon Einar; Killi, Veronica Anne-Line Kathrine; Choudhury, Devika; Elam, Jeffrey W. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2020). tert-butoxides as precursors for atomic layer deposition of alkali metal containing thin films. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 38, s. 1–15. doi: 10.1116/6.0000589. Fulltekst i vitenarkiv
• Dietzel, Pascal D.C.; Blom, Richard & Fjellvåg, Helmer (2020). Variability in the formation and framework polymorphism of metal-organic frameworks based on yttrium(III) and the bifunctional organic linker 2,5-dihydroxyterephthalic acid. Zeitschrift für Anorganische und Allgemeines Chemie. ISSN 0044-2313. 647(1), s. 15–25. doi: 10.1002/zaac.202000276. Fulltekst i vitenarkiv Vis sammendrag

  Depending on the solvothermal reaction conditions, we obtained three different metal‐organic frameworks with yttrium(III) as metal component and 2,5‐dihdyroxyterepthalic acid (H4dhtp) as bifunctional organic linker: Y2(H2dhtp)3(dmf)4·(dmf)2 (CPO‐29) contains dinuclear, paddle‐wheel like inorganic secondary building units (SBUs) connected by the organic linker to a network with α‐Po topology, while Y2(H2dhtp)(dhtp)(dmf)2 (CPO‐30) and Y2(H2dhtp)(dhtp)(dmf)2(H2O)2·(H2O)4 (CPO‐31) contain one‐dimensional inorganic SBUs that differ in how the half‐ and fully deprotonated ligands are connected to and arranged around them. Only the carboxylic acid groups of the organic linker are deprotonated in CPO‐29, while CPO‐30 and CPO‐31 contain both 2,5‐dihydroxyterephthalate (H2dhtp2–) linkers and fully deprotonated 2,5‐dioxidoterephthalate (dhtp4–) linkers. All three compounds contain large volumes filled with solvent, but we were able to demonstrate permanence of porosity only for CPO‐30. Variable temperature powder X‐ray diffraction reveals that CPO‐29 and CPO‐31 undergo discontinuous phase transitions upon heating, and the flexibility of the framework structure indicated by these might be the reason for the inability to access the pore volume. Desolvated CPO‐30 and CPO‐31 are polymorphs, whose network structures differ in whether the H2dhtp2– and dhtp4– linkers are located in cis or trans arrangement around the inorganic SBU.

• Bernal, Fabian Leonardo Martinez; Gonano, Bruno Raymond Gino; Lundvall, Fredrik; Wragg, David; Fjellvåg, Helmer & Veillon, Fabien [Vis alle 8 forfattere av denne artikkelen] (2020). Canted antiferromagnetism in high-purity NaFeF3 prepared by a novel wet-chemical synthesis method. PHYSICAL REVIEW MATERIALS. ISSN 2475-9953. 4(11). doi: 10.1103/PhysRevMaterials.4.114412. Fulltekst i vitenarkiv
• Arstad, Bjørnar; Blom, Richard; Håkonsen, Silje Fosse; Pierchala, Joanna; Cobden, Paul & Lundvall, Fredrik [Vis alle 10 forfattere av denne artikkelen] (2020). Synthesis and Evaluation of K-Promoted Co3-xMgxAl-Oxides as Solid CO2 Sorbents in the Sorption-Enhanced Water−Gas Shift (SEWGS) Reaction. Industrial & Engineering Chemistry Research. ISSN 0888-5885. 59(40), s. 17837–17844. doi: 10.1021/acs.iecr.0c02322. Fulltekst i vitenarkiv Vis sammendrag

  Hydrogen is essential in a variety of large-scale chemical processes. As a carbon-free energy carrier, hydrogen has a potential for wide use within power production and transportation. However, most of the recent production methods involve the release of CO2 as a by-product. Hence, decarbonization of hydrogen production is one step to reduce CO2 emission into the Earth’s atmosphere. Several process schemes have been suggested for low-carbon emission production of hydrogen. In this work, we show how to improve solid sorbents for the sorption-enhanced water–gas shift (SEWGS) process, which is a process that exploits a solid sorbent in the water–gas shift reactor to capture CO2 in situ and drive the process toward an improved hydrogen yield. We report herein a series of CoxMg3-xAl materials based on hydrotalcites, promoted with various loadings of K. The materials have been characterized by BET, XRD, and NMR and tested for their CO2 adsorption performance in three adsorption–desorption cycles in a lab-scale fixed-bed reactor (20–22 bar, CO2 + steam as reactant gas, and isothermal conditions at 375 and 400 °C). The most promising material was subjected to a long-term test (120 adsorption–desorption cycles at similar conditions). This test indicates that a K-promoted Co1.5Mg1.5Al (22 wt % of added K2CO2 to the oxide) material has a higher cyclic capacity for CO2 than standard reference cases. We have estimated that the volumetric capacity (in mol/L unit) of this sorbent will be 23–26% higher than a standard reference material at 400 °C and 30–39% higher at 375 °C. This would, in fixed-bed columns, lead to significant reduction in the needed column volumes in the final process and reduce costs.

• Li, Xinyu; Bianchini, Federico; Wind, Julia; Pettersen, Christine; Vajeeston, Ponniah & Wragg, David [Vis alle 7 forfattere av denne artikkelen] (2020). Insights into Crystal Structure and Diffusion of Biphasic Na2Zn2TeO6. ACS Applied Materials & Interfaces. ISSN 1944-8244. 12(25), s. 28188–28198. doi: 10.1021/acsami.0c05863. Fulltekst i vitenarkiv
• Gonano, Bruno Raymond Gino; Fjellvåg, Øystein; Steciuk, Gwladys; Saha, Dipankar; Pelloquin, Denis & Fjellvåg, Helmer (2020). Exotic Compositional Ordering in Manganese–Nickel–Arsenic (Mn-Ni-As) Intermetallics. Angewandte Chemie International Edition. ISSN 1433-7851. 132(50), s. 22568–22573. doi: 10.1002/anie.202006135. Fulltekst i vitenarkiv
• Dietzel, Pascal D.C.; Georgiev, Peter A.; Frøseth, Morten; Johnsen, Rune E.; Fjellvåg, Helmer & Blom, Richard (2020). Effect of larger pore size on the sorption properties of isoreticular metal–organic frameworks with high number of open metal sites. Chemistry - A European Journal. ISSN 0947-6539. 26, s. 13523–13531. doi: 10.1002/chem.202001825. Fulltekst i vitenarkiv Vis sammendrag

  Four isostructural CPO‐54‐M metal‐organic frameworks based on the larger organic linker 1,5‐dihydroxynaphthalene‐2,6‐dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO‐27 (MOF‐74) materials. Desolvated CPO‐54‐Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2, H2, CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO‐27. The plateau of high heat of adsorption decreases earlier in CPO‐54‐Mn as a function of loading per mole than in CPO‐27‐Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

• Sønsteby, Henrik Hovde; Skaar, Erik; Bratvold, Jon Einar; Freeland, John W.; Yanguas-Gil, Angel & Elam, Jeffrey W. [Vis alle 8 forfattere av denne artikkelen] (2020). Tuning electronic properties in LaNiO3 thin films by B-site Cu-substitution. Journal of Materials Chemistry C. ISSN 2050-7526. 8(36), s. 12662–12668. doi: 10.1039/D0TC03406A.
• Sønsteby, Henrik Hovde; Killi, Veronica Anne-Line Kathrine; Storaas, Thomas Aarflot; Choudhuy, Devika; Elam, Jeffrey W. & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2020). Understanding KOtBu in atomic layer deposition–in situ mechanistic studies of the KNbO3 growth process. Dalton Transactions. ISSN 1477-9226. 49(38), s. 13233–13242. doi: 10.1039/D0DT02324H.
• Zacharaki, Eirini; Bremmer, Marien G.; Vajeeston, Ponniah; Kalyva, Maria Evangelou; Fjellvåg, Helmer & Kooyman, Patricia J. [Vis alle 7 forfattere av denne artikkelen] (2020). One-pot synthesis of cobalt-rhenium nanoparticles taking the unusual beta-Mn type structure. Nanoscale Advances. 2, s. 1850–1853. doi: 10.1039/d0na00097c. Fulltekst i vitenarkiv
• Pal, Santosh Kumar; Frommen, Christoph; Kumar, Susmit; Hauback, Bjørn; Fjellvåg, Helmer & Helgesen, Geir (2020). Enhancing giant magnetocaloric effect near room temperature by inducing magnetostructural coupling in Cu-doped MnCoGe. Materials & design. ISSN 0264-1275. 195, s. 1–11. doi: 10.1016/j.matdes.2020.109036. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Ruud, Amund; Sønsteby, Henrik Hovde; Nilsen, Ola & Fjellvåg, Helmer (2020). Crystallization, Phase Stability, and Electrochemical Performance of β-MoO3 Thin Films. Crystal Growth & Design. ISSN 1528-7483. 20(6), s. 3861–3866. doi: 10.1021/acs.cgd.0c00156. Fulltekst i vitenarkiv Vis sammendrag

  Insight into the crystallization process of functional materials deposited as thin films in an amorphous state is essential when phase pure samples of specific polymorphs are sought for utilization of performance aspects or fundamental studies. In this work, we report on controlled crystallization of α- and β-MoO3 polymorphs from amorphous MoO3 thin films produced by atomic layer deposition (ALD). The MoO3 films are amorphous as deposited. Our experiments show that β-MoO3 starts to crystallize from amorphous MoO3 at a surprisingly low temperature of 185 °C, only 20 °C above the adopted ALD deposition temperature. Thin films of the β-MoO3 polymorph start to slowly convert into α-MoO3 at temperatures above 300 °C; however, long holding times are required to obtain phase pure products. High-quality thin film samples (electrodes) were characterized by electrochemical cycling toward lithium, demonstrating that α- and β-MoO3 transfers into the same chemical state during cycling and that amorphous MoO3 undergoes less change upon cycling.

• Schuler, Raphael; Norby, Truls Eivind & Fjellvåg, Helmer (2020). Defects and polaronic electron transport in Fe2WO6. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 22(27), s. 15541–15548. doi: 10.1039/d0cp01588a. Fulltekst i vitenarkiv
• Sønsteby, Henrik Hovde; Skaar, Erik; Fjellvåg, Øystein; Bratvold, Jon E.; Fjellvåg, Helmer & Nilsen, Ola (2020). A foundation for complex oxide electronics -low temperature perovskite epitaxy. Nature Communications. ISSN 2041-1723. 11(1). doi: 10.1038/s41467-020-16654-2. Fulltekst i vitenarkiv
• El-Kharbachi, Abdelouahab; Wind, Julia; Ruud, Amund; Høgset, Astrid Bygdås; Nygård, Magnus Moe & Zhang, Junxian [Vis alle 14 forfattere av denne artikkelen] (2020). Pseudo-ternary LiBH4-LiCl-P2S5 system as structurally disordered bulk electrolyte for all-solid-state lithium batteries. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 22(25), s. 13872–13879. doi: 10.1039/d0cp01334j. Fulltekst i vitenarkiv
• Periyasamy, Manimuthu; Fjellvåg, Øystein; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2020). Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 511, s. 1–8. doi: 10.1016/j.jmmm.2020.166949. Fulltekst i vitenarkiv
• Bernal, Fabian Leonardo Martinez; Sottmann, Jonas; Wragg, David; Fjellvåg, Helmer; Fjellvåg, Øystein & Drathen, Christina [Vis alle 8 forfattere av denne artikkelen] (2020). Structural and magnetic characterization of the elusive Jahn-Teller active NaCrF3. PHYSICAL REVIEW MATERIALS. ISSN 2475-9953. 4(5). doi: 10.1103/PhysRevMaterials.4.054412. Fulltekst i vitenarkiv
• Duparc, Marion Jeanne Louise; Sønsteby, Henrik Hovde; Nilsen, Ola; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2020). Atomic Layer Deposition of GdCoO3 and Gd0.9Ca0.1CoO3. Materials. ISSN 1996-1944. 13(1), s. 1–10. doi: 10.3390/ma13010024. Fulltekst i vitenarkiv
• Kalantzopoulos, Georgios N.; Lundvall, Fredrik; Thorshaug, Knut; Lind, Anna Maria; Vajeeston, Ponniah & Dovgaliuk, Iurii [Vis alle 9 forfattere av denne artikkelen] (2020). Factors Determining Microporous Material Stability in Water: The Curious Case of SAPO-37. Chemistry of Materials. ISSN 0897-4756. 32(4), s. 1495–1505. doi: 10.1021/acs.chemmater.9b04510. Fulltekst i vitenarkiv
• Sønsteby, Henrik Hovde; Wind, Julia; Jensen, Martin; Storaas, Thomas Aarflot; Chernyshov, Dmitry & Fjellvåg, Helmer (2019). In situ synchrotron X-ray diffraction of thin films under perturbation by an electric field. Ferroelectrics (Print). ISSN 0015-0193. 537, s. 20–26. doi: 10.1080/00150193.2018.1528953.
• Li, Xinyu; Bianchini, Federico; Wind, Julia; Vajeeston, Ponniah; Wragg, David & Fjellvåg, Helmer (2019). P2 Type Layered Solid-State Electrolyte Na2Zn2TeO6: Crystal Structure and Stacking Faults . Journal of the Electrochemical Society. ISSN 0013-4651. 166(15), s. A3830–A3837. doi: 10.1149/2.1231915jes.
• Sønsteby, Henrik Hovde; Nilsen, Ola & Fjellvåg, Helmer (2019). Chemical Uniformity in Ferroelectric KxNa1−xNbO3 Thin Films. Global Challenges. ISSN 2056-6646. 3(10). doi: 10.1002/gch2.201800114. Fulltekst i vitenarkiv
• Ruud, Amund; Sottmann, Jonas; Vajeeston, Ponniah & Fjellvåg, Helmer (2019). Direct observation of reversible conversion and alloying reactions in a Bi2(MoO4)3-based lithium-ion battery anode. Journal of Materials Chemistry A. ISSN 2050-7488. 7(30), s. 17906–17913. doi: 10.1039/c9ta03459e.
• Patra, Lokanath; Vidya, Ravindran; Fjellvåg, Helmer & Ravindran, Ponniah (2019). Giant Magnetoelectric Coupling in Multiferroic PbTi1–xVxO3 from Density Functional Calculations. ACS Omega. ISSN 2470-1343. doi: 10.1021/acsomega.9b01176.
• Kumar, Susmit; Fjellvåg, Øystein; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2019). Physical properties of Ruddlesden-Popper (n = 3) nickelate: La4Ni3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 496. doi: 10.1016/j.jmmm.2019.165915. Fulltekst i vitenarkiv
• Varunaa, R; Fjellvåg, Helmer & Ponniah, Ravindran (2019). Zinc substituted MgH2 - a potential material for hydrogen storage applications. International Journal of Hydrogen Energy. ISSN 0360-3199. 44(26), s. 13632–13646. doi: 10.1016/j.ijhydene.2019.04.016.
• Kiruthika, S.; Fjellvåg, Helmer & Ravindran, P. (2019). Amphoteric behavior of hydrogen (H+1 and H-1) in complex hydrides from van der Waals interaction-including ab initio calculations. Journal of Materials Chemistry A. ISSN 2050-7488. 7(11), s. 6228–6240. doi: 10.1039/c8ta11685g.
• Vajeeston, Ponniah; Bianchini, Federico & Fjellvåg, Helmer (2019). First-Principles Study of the Structural Stability and Dynamic Properties of Li2MSiO4 (M = Mn, Co, Ni) Polymorphs . Energies. ISSN 1996-1073. 12(224), s. 1–24. doi: 10.3390/en12020224. Fulltekst i vitenarkiv
• Bratvold, Jon E.; Sønsteby, Henrik Hovde; Nilsen, Ola & Fjellvåg, Helmer (2019). Control of growth orientation in as-deposited epitaxial iron-rich nickel ferrite spinel. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 37(2), s. 1–7. doi: 10.1116/1.5082012. Fulltekst i vitenarkiv
• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2019). Nonhexagonal Na Sublattice Reconstruction in the Super-Ionic Conductor Na2Zn2TeO6: Insights from Ab Initio Molecular Dynamics . Journal of Physical Chemistry C. ISSN 1932-7447. 123(8), s. 4654–4663. doi: 10.1021/acs.jpcc.8b10362.
• Kumar, Susmit; Waller, David; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2019). Development of custom made bimetallic alloy model systems based on platinum-rhodium for heterogeneous catalysis. Journal of Alloys and Compounds. ISSN 0925-8388. 786, s. 1021–1029. doi: 10.1016/j.jallcom.2019.02.026. Fulltekst i vitenarkiv Vis sammendrag

  The current study extends the insight into solid-state diffusion in alloys of the Platinum Group Metals (PGMs). Thereby, we pave the way for tailor making surface compositions of binary platinum (Pt) – rhodium (Rh) model catalysts, with deposited rhodium layers on the surface of Pt single crystals (SCs) as the starting point. Platinum single crystals with (100), (110) and (111) orientations were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). In addition, X-ray Photoelectron spectroscopy (XPS) characterization was carried out on Pt(110). The crystallographic orientation has a prominent effect on diffusion characteristics. The slowest moving Kirkendall – plane was observed for Pt(110) at xK = 250 nm from reference point x0, while, the fastest for Pt(100) was at xK = 450 nm, indicating that the inter-diffusion rate with respect to orientation increases in the sequence Pt(110) < Pt(111) < Pt(100). The diffusion coefficient of Rh into Pt is of the order ~ 1017 – 1018 m2/s, whereas it was slightly less than ~ 1019 m2/s for Pt into Rh; albeit comparable to the vacancy diffusion rate from Pt into Rh. Our experimental findings show good agreement with calculated inter-diffusion coefficients using Darken analysis. A theoretical direction is proposed for improved fitting of data.

• Bundli, Silje; Dhak, Prasanta; Jensen, Martin; Gunnæs, Anette Eleonora; Nguyen, Phuong Dan & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2019). Controlled alloying of Pt-Rh nanoparticles by the polyol approach. Journal of Alloys and Compounds. ISSN 0925-8388. 779, s. 879–885. doi: 10.1016/j.jallcom.2018.11.301.
• Lundvall, Fredrik; Kalantzopoulos, Georgios N.; Wragg, David; Arstad, Bjørnar; Blom, Richard & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2019). Characterization and evaluation of synthetic Dawsonites as CO2 sorbents. Fuel. ISSN 0016-2361. 236, s. 747–754. doi: 10.1016/j.fuel.2018.09.057. Fulltekst i vitenarkiv
• Pal, Santosh Kumar; Frommen, Christoph; Kumar, Susmit; Hauback, Bjørn; Fjellvåg, Helmer & Woodcock, T. G. [Vis alle 8 forfattere av denne artikkelen] (2019). Comparative phase transformation and magnetocaloric effect study of Co and Mn substitution by Cu in MnCoGe compounds. Journal of Alloys and Compounds. ISSN 0925-8388. 775, s. 22–29. doi: 10.1016/j.jallcom.2018.10.040. Fulltekst i vitenarkiv
• Bianchini, Federico; Fjellvåg, Helmer & Vajeeston, Ponniah (2018). A first-principles investigation of the Li difusion mechanism in the super-ionic conductor lithium orthothioborate Li3BS3 structure. Materials Letters. ISSN 0167-577X. 219, s. 186–189. doi: 10.1016/j.matlet.2018.02.083. Fulltekst i vitenarkiv
• Sønsteby, Henrik Hovde; Bratvold, Jon E.; Weibye, Kristian; Fjellvåg, Helmer & Nilsen, Ola (2018). Phase Control in Thin Films of Layered Cuprates. Chemistry of Materials. ISSN 0897-4756. 30(3), s. 1095–1101. doi: 10.1021/acs.chemmater.7b05005.

Se alle arbeider i Cristin

• Brennhagen, Anders; Skurtveit, Amalie; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2024). Revealing the (de)sodiation mechanisms of Bi-metallates through operando X-ray characterisation. Vis sammendrag

  Na-ion batteries is entering the battery market as an alternative to Li-ion batteries. To improve their overall performance it is crucial to develop new types of anode materials with high capacity and long cycle life. Materials combing conversion and alloying mechanisms (CAMs) are promising anodes with their high capacity, but obtaining good cycling stability is still challenging. A comprehensive understanding of the cycling and degradation mechanism of these materials is crucial to improve their performance. Bi-metallates, with a general formula of Bi−TM−O (TM = transition metal) is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction forming nanoparticles of Bi-metal embedded in a Na-TM-O matrix during the first sodiation, followed by a reversible two-step alloying reaction forming Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction (XRD), pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS) to investigate the desodiation mechanisms of Bi2MoO6 and BiFeO3. Through this work, we discovered that Bi2MoO6 forms the cubic version of Na3Bi (c-Na3Bi) while BiFeO3 forms hexagonal Na3Bi (h-Na3Bi) in addition to c-Na3Bi during the first sodiation. In the desodiated state, the Bi-particles are partially oxidised, while still maintaining the Bi-metal structure, indicating that it is only the Bi atoms at the interface between the Bi nanoparticles and the Na−TM−O matrix that is oxidised. During cycling the NaxBi particles grow larger thus increasing the distance between them and increasing the impedance in the material. This is considered to be the main driver for the capacity degradation that was observed during the first 20 cycles. The operando XAS data also revealed that Mo6+ in Bi2MoO6 does not change oxidation state during cycling, but changes coordination between tetrahedral and distorted octahedral coordination during cycling. The cycling and degradation mechanisms of Bi2MoO6 is summarised in Figure 1.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Skurtveit, Amalie; Wragg, David Stephen & Koposov, Alexey [Vis alle 9 forfattere av denne artikkelen] (2023). Revealing the Cycling and Degradation Mechanism of Bi-Metallates as Anode Materials fro Na-ion Batteries. Vis sammendrag

  Na-ion batteries (NIBs) represent an emerging alternative to modern Li-ion batteries. To improve the overall performance of NIBs it is crucial to develop new types of anode materials with high capacity and long cycle life. Among the anode candidates, materials operating under combined conversion and alloying mechanisms (CAMs) are considered as promising. They are capable of delivering high capacity, however, despite theoretical predictions the cycling stability is still below expectations. Therefore, a comprehensive understanding of the cycling and degradation mechanisms of CAMs is crucial to improve their performance. However, these materials undergo multiple chemical transformations, which often involves formation of amorphous and nanocrystalline phases during cycling. Such chemical complexity substantially impedes their full characterization and understanding. Bismuth metallates, with a general formula of Bi-TM-O (TM = transition metal), is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction, which leads to formation of Bi nanoparticles embedded in a Na-TM-O matrix during the first sodiation. This is followed by a reversible two-step alloying reaction leading to formation of Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction over the course of ∼30 cycles to study the cycling and degradation mechanisms of two CAMs - Bi2MoO6 and BiFeO3. With the help of ex situ X-ray absorption spectroscopy and pair distribution function (PDF) analysis, we have revealed new insights into these materials in addition to confirming the general cycling mechanism. Both materials form the cubic phase of Na3Bi (c-Na3Bi) in the sodiated state with some noticeable differences. While, the hexagonal Na3Bi (h-Na3Bi) phase starts to form after 10 cycles in Bi2MoO6, a combination of c-Na3Bi and h-Na3Bi can be observed after the first cycle in BiFeO3. The formation of c-Na3Bi is kinetically favorable in these systems, while h-Na3Bi is the more thermodynamically stable and, therefore, forms later in the cycling process. Both materials shows three distinct regions of capacity degradation, where the first is initiated by a growth in the crystallite sizes of Bi-particles. This is followed by a disappearance of the Bi-phase as the second step. The last degradation phase is related to the disappearance of the NaBi-phase and locking the system in the sodiated state. The present talk will discuss the mechanisms of these degradation steps in view of the future development of CAMs in greater details.

• Sottmann, Jonas; Ruud, Amund; Fjellvåg, Øystein Slagtern; Vaughan, Gavin B. M.; Di Michel, Marco & Fjellvåg, Helmer [Vis alle 9 forfattere av denne artikkelen] (2023). Erratum: 5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode (Phys. Chem. Chem. Phys. (2022) 24 (27075-27085) DOI: 10.1039/D2CP03892G). Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. doi: 10.1039/d3cp90076b.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2023). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Vis sammendrag

  Na-ion batteries (NIBs) generally have slightly lower energy densities than Li-ion batteries (LIBs), but the abundancy of Na could make NIBs cheaper and more sustainable. The search for new anode materials is essential for further development of NIBs. Anode materials combining conversion and alloying mechanisms (CAMs) are promising because of their high capacity and high rate capabilities, but their complex cycling mechanisms involving amorphous phases are challenging to study [1]. Bi2MoO6 is an example of a CAM that has high initial capacity, but struggles with cycling stability and undergoes major changes during cycling. Detailed understanding of its cycling and degradation mechanism will hopefully enable us to improve the cycling stability of Bi2MoO6 and CAMs in general. Therefore, we have studied Bi2MoO6 using a combination of operando X-ray diffraction (Fig. 1), pair distribution function, X-ray absorption spectroscopy and transmission electron microscopy [3]. This revealed that during the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na, forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. During these cycles, we observe a growth in the crystallite size of the alloying particles, which is probably the leading cause of the rapid capacity decay after cycle 10 where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Corrigendum: Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries (Nanotechnology (2022)33 (185402) DOI: 10.1088/1361-6528/ac4eb5). Nanotechnology. ISSN 0957-4484. 33(35). doi: 10.1088/1361-6528/ac72b2.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (de)sodiation of Bi2MoO6 through good and bad times with operando XRD. Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study1. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs2. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism (Figure 1)3. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process.
• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2021). Operando XRD study on Bi2MoO6 as anode material for Na-ion batteries. Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Sjåstad, Anja Olafsen; Ponniah, Vajeeston & Fjellvåg, Helmer (2021). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Bi4V2O11/RGO composite as conversion and alloying anode for Na-ion batteries.
• Brennhagen, Anders; Wragg, David; Fjellvåg, Helmer; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Cavallo, Carmen (2021). Beyond Insertion: Conversion alloying Bi/RGO-based materials as novel anodes for Na-ion batteries.
• Hempel, Frida Sveen; Arstad, Bjørnar & Fjellvåg, Helmer (2020). Layered hexagonal materials - Differences in ionic conductivity for solid-state electrolyte .
• Hval, Halvor Høen; Tripathi, Alok Mani; Forseth, Sissel & Fjellvåg, Helmer (2020). Understanding the details of crystal structure and electronic states in LNMO.
• Wragg, David; Fjellvåg, Øystein; Fjellvåg, Helmer; Sottmann, Jonas; Vaughan, Gavin B M & Di Michel, Marco [Vis alle 7 forfattere av denne artikkelen] (2020). Operando Total Scattering Computed Tomography: Making the Invisible Visible .
• Cavallo, Carmen; Brennhagen, Anders; Wragg, David & Fjellvåg, Helmer (2020). Beyond Insertion: Conversion alloying Bi/RGO-based materials as anode for Na-ion batteries .
• Fjellvåg, Øystein; Sønsteby, Henrik Hovde; Nilsen, Ola; Wolff, Max; Kapaklis, Vassilios & Fjellvåg, Helmer [Vis alle 8 forfattere av denne artikkelen] (2020). Towards Multiferroic Thin Film Composites by Atomic Layer Deposition (ALD) .
• Wragg, David; Fjellvåg, Øystein; Ruud, Amund; Fjellvåg, Helmer; Vajeeston, Ponniah & Vaughan, Gavin B M [Vis alle 9 forfattere av denne artikkelen] (2019). XRD/PDF computed tomography and how to get the best possible operando data from ion batteries.
• Wragg, David; Ruud, Amund; Fjellvåg, Øystein; Fjellvåg, Helmer; lebedev, Oleg & Di Michel, Marco [Vis alle 9 forfattere av denne artikkelen] (2019). PDF Analysis of Batteries Using Diffraction Computed Tomography.
• Wragg, David; Ruud, Amund; Fjellvåg, Helmer; Fjellvåg, Øystein; Vajeeston, Ponniah & Sottmann, Jonas [Vis alle 9 forfattere av denne artikkelen] (2019). PDF Analysis of Batteries Using Diffraction Computed Tomography.
• Hempel, Frida Sveen; Arstad, Bjørnar; Fjellvåg, Helmer & Li, Xinyu (2019). Na-dynamics in Na2Zn2TeO6, a possible solid-state electrolyte.
• Hval, Halvor Høen; Tripathi, Alok Mani; Ruud, Amund; Emerich, Hermann; Forseth, Sissel & Nilsen, Ola (2019). Make Batteries Safe Again.
• Hval, Halvor Høen; Tripathi, Alok Mani & Fjellvåg, Helmer (2019). High Energy Density Electrodes for Lithium-ion Batteries and Operando Studies.
• Hval, Halvor Høen; Ruud, Amund; Forseth, Sissel; Emerich, Hermann; Nilsen, Ola & Fjellvåg, Helmer (2019). Operando investigation of batteries.
• Hval, Halvor Høen; Tripathi, Alok Mani; Ruud, Amund; Emerich, Hermann; Forseth, Sissel & Nilsen, Ola (2019). Make Batteries Safe Again.
• Hval, Halvor Høen; Ruud, Amund; Forseth, Sissel; Emerich, Hermann; Nilsen, Ola & Fjellvåg, Helmer (2018). Operando investigation of batteries.
• Hval, Halvor Høen; Ruud, Amund; Forseth, Sissel; Emerich, Hermann; Nilsen, Ola & Fjellvåg, Helmer (2018). Operando investigation of batteries.
• Wragg, David; Sottmann, Jonas; Ruud, Amund; Fjellvåg, Helmer; Fjellvåg, Øystein & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2018). Operando X-ray Diffraction Studies of Battery Materials.
• Wragg, David; Ruud, Amund; Sottmann, Jonas; Vajeeston, Ponniah; Fjellvåg, Helmer & Fjellvåg, Øystein [Vis alle 8 forfattere av denne artikkelen] (2018). New Anodes for Sodium and Lithium Ion Batteries.
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2024). Bi metallates as conversion-alloying anodes for Na-ion batteries. Universitetet i Oslo.
• Hempel, Frida Sveen & Fjellvåg, Helmer (2023). Substituted Na-based Layered Tellurates: Synthesis, Local Structure and Na-Dynamics. University of Oslo, Faculty of Mathematics and Natural Sciences, Department of Chemistry. ISSN 1501-7710. 2023(2688). Vis sammendrag

  Solid-state batteries are currently limited by low ionic conductivity compared to the liquid analogue, and improving ionic conductivity is a major concern for commercialization. This thesis focuses on the local structure and ionic conductivity mechanism of a solid-state electrolyte candidate (the layered oxide Na2Zn2TeO6). The Na content of layered materials is well known to influence conductivity and phase stability, and in this work this is changed by substituting the framework cations Zn and Te with Ga and Sb. The first work of this thesis establishes a reliable synthesis route for the substituted materials, ensuring and confirming homogeneous distribution of the substituent by nuclear magnetic resonance spectroscopy (NMR). The decreased Na content is found to increase local motion of Na, but this is not translated to long range motion of ionic conductivity. Decreasing the Na-content is therefore not enough to improve ionic conductivity. This work finally made an in-depth study of complex ordering schemes that may occur in multicomponent layered oxides. The real situation can indeed be very complex, compared to the rather simple average picture description.

• Amedi, Salah Bra & Fjellvåg, Helmer (2023). Syntese av lavvalent kompleksfluorider. Kjemisk Institutt, Universitetet i Oslo.
• Skautvedt, Casper; Fjellvåg, Helmer; Koposov, Alexey; Brennhagen, Anders; Cavallo, Carmen & Sjåstad, Anja Olafsen (2022). Syntese og karakterisering av BiFeO3 som anodemateriale i Na-ionebatterier. Universitetet i Oslo. Vis sammendrag

  Na-ionebatterier (NIB-er) er foreløpig en umoden teknologi, men har de siste åra blitt forska på i økende grad. Et stort forskningsfelt innafor NIB-er er på anoden, da det ikke finnes noen som fungerer optimalt. I dette arbeidet har vi sett på BiFeO3 som anodemateriale i NIB-er. Bi-baserte anoder har tidligere blitt undersøkt i forskningsgruppa og de har vist lovende resultater. Valget av BiFeO3 var basert på den interessante mekanismen som skjer under opp- og utladning når den brukes som anode. Mekanismen er en omsettingslegeringsreaksjon og kombinerer omsettings- og legeringsreaksjoner når BiFeO3 natrieres. Vi har undersøkt denne mekanismen i detalj med kombinert røntgendiffraksjon (XRD) og galvanostatisk sykling (GS) med operando-XRD på RECX-labben ved Kjemisk institutt. Som en del av arbeidet blei BiFeO3 syntetisert ved keramisk faststoffsyntese og våtkjemisk sol-gelsyntese. Det har tidligere blitt framstilt med disse syntesene, men har vist seg å være vanskelig å få fasereint. Produkta av syntesene har blitt karakterisert med XRD og forfina med Rietveldmetoden. Den keramiske faststoffsyntesen ga lite utbytte av BiFeO3 og flere ureinheter, men sol-gelsyntesen ga derimot et høyt utbytte av BiFeO3 og få ureinheter. Det blei undersøkt ulike behandlinger av BiFeO3, blant annet kulemølling, syrevasking og doping, for å gi økt ytelse som batterimateriale. Disse testene viste at kulemølling av materialet ga best resultater, det var likevel ikke mulig å finne en optimalisering som unngikk et stort fall i kapasiteten de første 30 syklusene. For å karakterisere BiFeO3 som anodemateriale testa vi det i halvseller mot Na-metall. Disse halvsellene blei elektrokjemisk analysert med GS og syklisk voltammetri (SV). Vi testa også ulike blandinger av elektrodesammensetning og elektrolytter for å optimalisere batteriegenskapene. Disse viste at NaPF6 var det beste saltet og propylenkarbonat (PC) + 5 vt% fluoretylenkarbonat (FEC) ga de mest stabile batterisellene. Dataene fra den elektrokjemiske karakteriseringa og operando-XRD bruktes sammen for å tolke reaksjonsmekanismene til BiFeO3. Vi så ut fra disse at det var likheter i legeringsreaksjonen mellom Na og Bi med andre Bi-baserte anoder, med NaBi og Na3Bi som ulike ledd i reaksjonen. Det var dog ingen indikasjoner på fasen/-e til Fe etter omsettingsreaksjonen.

• Raniseth, Amund Midtgard; Hval, Halvor Høen; Fjellvåg, Helmer & Samarasingha, Pushpaka (2022). Synthesis, structural characterization, electrochemical properties, and thermal stability of Al-substituted LiMn1.5Ni0.5O4. Universitetet i Oslo. Vis sammendrag

  One of the interesting lithium-ion battery cathode materials which have been developed over the last 20 years is the lithium manganese nickel oxide (LMNO). However, problems with lifetime remain. Increasing the lifetime of LMNO is crucial to its commercial adoption, and one of the promising techniques to do this is substituting different elements of the structure. In this thesis, the structure, electrochemistry, and thermal stability of LMNO have been investigated with different amounts of aluminum substitution attempted to replace either manganese, nickel, or both elements evenly. The findings suggest that substituting manganese is not easily done, as impurities form. However, by replacing mainly nickel, aluminum is successfully incorporated into the structure at a stoichiometric amount of 0.2. This type of substitution was also found to change the electrochemical voltage plateau of the material, in addition to prolonging its cycle life. The aluminum substituted material also exhibited better thermal stability above 100℃.

• Schuler, Raphael; Fjellvåg, Helmer & Norby, Truls Eivind (2021). Oxide thermoelectrics - materials, junctions and modules. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. ISSN 1501-7710. 2021(2364). Vis sammendrag

  Thermoelectric materials are able to directly convert heat into electricity, which makes them predestined candidates for the recuperation of waste heat and contributing to a more efficient energy economy. They hold enormous potential for energy generation, since all thermodynamic processes lose a major portion of their energy in the form of waste heat. However, conventional thermoelectric materials are often toxic, scarce and/or expensive, and show limited high temperature stability, which currently limits their relevance for large-scale applications. Recently, oxide thermoelectric materials, composed of abundant elements have attracted increased interest, as they offer a cheaper, environmental-friendly, and high-temperature stable alternative. They are usually discovered and researched individually, and only later combined to a p-n pair in a module, which often leads to complications, like thermal mismatch or reactions. In this work the approach was taken to design a compatible pair of new thermoelectric oxides that preclude complications in advance. This novel approach is demonstrated on a model pair of iron tungstates, which show promising thermoelectric properties and an exceptional band alignment, which could prove to be key properties for future thermoelectrics.

• Nygård, Magnus Moe; Hauback, Bjørn Christian; Sørby, Magnus Helgerud & Fjellvåg, Helmer (2020). Local and average structure in high-entropy alloys and associated metal deuterides. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. ISSN 1501-7710. 2020( 2305).
• Perizat, Berdiyeva; Hauback, Bjørn Christian; Deledda, Stefano & Fjellvåg, Helmer (2020). Characterization of Heat Storage Materials with Neutron Scattering and Imaging Techniques. Matematisk Naturvitenskapelig fakultet, Universitetet i Oslo. ISSN 1501-7710. 2020(2304). Vis sammendrag

  Roughly, three-quarters of the energy produced in the world ends up as waste heat. However, the waste heat can be harvested, stored and reused using specially built heat storage reactors for high- or low-temperature applications. This thesis attempts to provide new insight into metal halide ammines as a potential heat storage system with a high energy efficiency. These materials undergo desorption and absorption of NH3-gas in a thermochemical reaction, which results in an uptake or release of heat, respectively. This work shows the study of the thermochemical heat storage reactor prototype with the metal halide ammine salt Sr(NH3)xCl2 for low-temperature applications below 100 °C. The changes in kinetics and chemical- and physical properties during NH3 cycling were characterized by neutron imaging and scattering techniques. A honeycomb-shaped heat exchanger within the reactor was optimized based on the results from the neutron imaging investigations. The mechanical stresses and heat transfer within the heat exchanger were studied in detail. Overall, this project demonstrates for the first time the possibility of applying neutron imaging and scattering techniques for analysis of thermochemical heat storage materials and systems.

• Li, Xinyu; Fjellvåg, Helmer & Nilsen, Ola (2020). Na-Based Layered Tellurate Na2M2TeO6 (M = Zn, Mg): Synthesis, Crystal Structure and Na+ Ion Diffusion . Universitetet i Oslo.
• Skaar, Erik; Fjellvåg, Helmer & Sønsteby, Henrik Hovde (2019). Structure-Property Relations in Thin Films of LaNiO3 and LaNi1-xCuxO3. Universitetet i Oslo.
• Bratvold, Jon E.; Nilsen, Ola & Fjellvåg, Helmer (2019). Towards a Model System for Lamellar Magnetism by Atomic Layer Deposition. University of Oslo, Faculty of Mathematics and Natural Sciences, Department of Chemistry. ISSN 1501-7710. 2019(2093).

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