Xinwei Sun

• Norby, Truls; Sun, Xinwei & Vøllestad, Einar (2023). A brick layer model for surface conduction in porous ceramics. Solid State Ionics. ISSN 0167-2738. 398. doi: 10.1016/j.ssi.2023.116269. Fulltekst i vitenarkiv
• Sun, Xinwei; Vøllestad, Einar; Rørvik, Per Martin; Prodinger, Sebastian; Kalantzopoulos, Georgios & Chatzitakis, Athanasios [Vis alle 7 forfattere av denne artikkelen] (2022). Surface protonic conductivity in chemisorbed water in porous nanoscopic CeO2. Applied Surface Science. ISSN 0169-4332. 611, s. 1–12. doi: 10.1016/j.apsusc.2022.155590. Fulltekst i vitenarkiv Vis sammendrag

  CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O^(1/2), pH2O^1, and pH2O^(3/2) dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

• Thøgersen, Annett; Sun, Xinwei; Jensen, Ingvild Julie Thue; Prytz, Øystein & Norby, Truls (2022). In-situ electron loss spectroscopy reveals surface dehydrogenation of hydrated ceria nanoparticles at elevated temperatures. Journal of Physics and Chemistry of Solids. ISSN 0022-3697. 170, s. 1–7. doi: 10.1016/j.jpcs.2022.110955. Fulltekst i vitenarkiv Vis sammendrag

  Ceria (CeO2) exhibits high reversible oxygen storage capacity at intermediate temperatures (500–800 °C) related to an extraordinary and not fully understood reduction of its surfaces. We have investigated pristine and alcohol-dispersed commercially available ceria nanoparticles by in-situ scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) to examine the dynamic changes during the initial redox reaction process of ceria nanoparticles in an ultra-high vacuum atmosphere using an in-situ heating holder. High spatially resolved EELS was used to estimate the amounts of Ce3+ and Ce4+ in the nanoparticles as a function of temperature, based on the white-line ratios M5/M4 of the EELS spectra. The results show a nm-range thick surface layer rich in Ce3+ on pristine particles prior to heating. During heating, this oxidises to Ce4+. Heating in high vacuum should normally not lead to oxidation, but the observed results can be understood if the surface layer has an oxyhydroxide composition such as CeOOH, which by heating in the vacuum dehydrogenates and hence oxidises to CeO2, a process that requires diffusion of hydrogen only. This process occurred for all samples, but was more pronounced for the particles that were previously dispersed in ethanol. Thermogravimetric analysis (TGA) by heating the pristine powder in dry atmosphere yielded a considerable weight loss confirming the content of hydroxide and probably water in and on the CeO2 particles. The results suggest that CeO2 surfaces are reduced to a layer of oxyhydroxide by hydrogen-containing molecules like water vapour or alcohols.

• Sun, Xinwei; Gu, Jie; Han, Donglin & Norby, Truls (2022). Quantifiable models for surface protonic conductivity in porous oxides - case of monoclinic ZrO2. Physical Chemistry, Chemical Physics - PCCP. ISSN 1463-9076. 24(19), s. 11856–11871. doi: 10.1039/d1cp05668a. Fulltekst i vitenarkiv
• Kang, Xiaolan; Chatzitakis, Athanasios; Aarholt, Thomas; Sun, Xinwei; Negri, Chiara & Norby, Truls (2022). Facet-engineered TiO<inf>2</inf>nanomaterials reveal the role of water-oxide interactions in surface protonic conduction. Journal of Materials Chemistry A. ISSN 2050-7488. 10(1), s. 218–227. doi: 10.1039/d1ta06075a. Fulltekst i vitenarkiv
• Sun, Xinwei; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2020). Photocatalytic generation of gas phase reactive oxygen species from adsorbed water: Remote action and electrochemical detection. Journal of Environmental Chemical Engineering. ISSN 2213-3437. doi: 10.1016/j.jece.2020.104809. Fulltekst i vitenarkiv Vis sammendrag

  The improvement of indoor environments is of great importance as it can significantly improve human health, comfort and productivity. Herein, different forms of TiO2 nanorods were used as the photocatalyst for generation of reactive oxygen species (ROS) in a gas phase photoreactor under controlled humidity. Several parameters were investigated by monitoring the remote decolourisation of Methylene Blue (MB) embedded in a Nafion film. A decolourisation of 26% under 80% relative humidity was observed when the MB film was 0.5 cm away from the photocatalyst. The length and ratio of light/dark intervals have major impacts on the efficiency of the gas phase photocatalytic process, which we link to the amount of water adsorbed on the photocatalyst, as the source for hydroxyl radicals. Furthermore, the photocatalytic production of ROS was quantified through a polyaniline electrochemical sensor and a rate of 1 · 10*12 of ROS molecules s−1 was estimated. This study contributes to the efficacy of the gas phase photocatalytic method in air decontamination, for the development of efficient air cleaning devices.

• Sun, Xinwei; Simonsen, Stian Christopher; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2019). Composite Membranes for High Temperature PEM Fuel Cells and Electrolysers: A Critical Review. Membranes. ISSN 2077-0375. 9(7). doi: 10.3390/membranes9070083.
• Sun, Xinwei; Xu, Kaiqi; Fleischer, Christian; Liu, Xin; Grandcolas, Mathieu & Strandbakke, Ragnar [Vis alle 9 forfattere av denne artikkelen] (2018). Earth-abundant electrocatalysts in proton exchange membrane electrolyzers. Catalysts. ISSN 2073-4344. 8:657(12), s. 1–41. doi: 10.3390/catal8120657. Fulltekst i vitenarkiv Vis sammendrag

  In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

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• Norby, Truls; Sun, Xinwei & Wang, Luyang (2024). Surface protonic conduction in porous oxides.
• Norby, Truls; Wu, Mengxin; Ewerhardt, Patrick & Sun, Xinwei (2024). Charge and Mass Transfer Polarisation of Electrodes for Proton Ceramic Electrochemical Cells .
• Wang, Luyang; Sun, Xinwei; Jiang, Bo & Norby, Truls (2024). Minority bulk and surface proton conduction in ceramic positrodes for proton ceramic electrochemical cells.
• Sun, Xinwei & Norby, Truls (2024). Charge and mass transfer polarisation of BGLC37 positrodes for proton ceramic electrochemical cells.
• Wang, Luyang; Sun, Xinwei & Norby, Truls (2023). Minority Bulk and Surface Proton Conduction in Positrodes for Proton Ceramic Electrochemical Cells. Vis sammendrag

  The positrode is a critical component for proton ceramic steam electrolyzers and fuel cells for hydrogen and ammonia. It constitutes a major contribution to the over-potentials and hence losses in the whole cell, resulting from its limited solubility and diffusivity of protons, which is challenging to characterise and improve. We are introducing a PhD project that aims to establish theoretical frameworks and experimental methodology for measuring protonic conductivities in the bulk and on surfaces of positrode materials. The approaches to reach the goal comprise the use of defect chemistry and transport theory for protons conduction, with the implementation of physiochemical experiments to distinguish signals of protons among other defects and charge carriers and paths on those predominantly electronic conductors. The results will be used as input to other project participants who perform computer simulations and, on that basis, seek strategies for optimization and effects on electrodes in scaled-up cells.

• Sun, Xinwei; Vøllestad, Einar & Norby, Truls (2022). Surface protonic conductivity in chemisorbed and physisorbed water layers in porous nanoscopic CeO2 .
• Sun, Xinwei; Vøllestad, Einar & Norby, Truls (2022). Surface protonics - a tutorial.
• Sun, Xinwei & Norby, Truls (2021). Surface protonic conductivity in chemisorbed and physisorbed water layers in porous nanoscopic CeO2.
• Norby, Truls; Kang, Xiaolan; Sun, Xinwei; Vøllestad, Einar; Gu, Jie & Han, Donglin (2021). Surface Protonics of Porous Oxide Ceramics.
• Norby, Truls; Sun, Xinwei; Kang, Xiaolan; Vøllestad, Einar; Gu, Jie & Han, Donglin (2021). Surface Protonics of Porous Oxide Ceramics .
• Norby, Truls Eivind & Sun, Xinwei (2020). Protonic conduction in water and hydrogen adsorbed in porous materials.
• Norby, Truls Eivind & Sun, Xinwei (2019). Hydrogen and clean air - Norway and China.
• Norby, Truls Eivind & Sun, Xinwei (2019). Novel UiO Technologies for Green Energy and Clean Air in China.
• Sun, Xinwei & Norby, Truls Eivind (2019). Hydrogen og hydrogenteknologi.
• Sun, Xinwei; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Composite membranes for high temperature PEM fuel cells and electrolysers. Vis sammendrag

  Polymer electrolyte membrane(PEM) fuel cells and electrolysers offer efficient use and production of H2 for emission-free transport and sustainable energy systems. In order to reduce the cost,complexity,flooding,catalyst poisoning and degradation,operation above 80°C and at lower relative humidities(RH) is preferable. Acid doped polybenzimidazole(PBI),sulfonated polyetheretherketone(SPEEK) and their composites are being investigated for high temperature and low RH operation with sufficient conductivities. We take a critical view and a surface protonic approach to gain fundamental insight.

• Sun, Xinwei; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Polymer-ceramic composite membranes for intermediate temperature PEM fuel cells.
• Sun, Xinwei; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2018). Can hydroxyl radicals travel far? Gas phase transport and detection after photocatalytic generation at TiO2 nanorods.
• Sun, Xinwei & Norby, Truls (2023). Quantification of surface protonic conduction in porous oxides. University of Oslo, Faculty of Mathematics and Natural Sciences, Department of Chemistry. ISSN 1501-7710. 2023(2604). Vis sammendrag

  The present work provides a fundamental and methodological framework for parameterisation of the thermodynamics and kinetics involved in surface protonic conduction of porous ionic oxides. A novel nomenclature for defect surface species is introduced. The surface protonic conductances have been estimated where various proton migration routes were considered. A brick layer model is developed that connects surface conductance with macroscopic sample conductivity via microstructural parameters. This allows quantitative discrimination between types of adsorption and surface transport in the different water layers on oxide surfaces, supported by water vapour partial pressure dependencies and enthalpies of adsorption and conductivity. The enthalpies for protonic surface mobility for oxides decrease systematically with lower temperatures (higher relative humidity) as the water layers grow in coverage and thickness and protons go from jumping between surface oxide ions to jumping between loosely bonded adsorbed hydroxide ions and water molecules. The findings and models developed in this work contribute to the understanding and control of surface protonics in porous oxides, which is believed to play an important role in electrochemical and photoelectrochemical cells, humidity sensors, and heterogeneous catalysis.

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