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  • 1.
    Fan, Junpeng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Panecatl-Bernal, Yesmin
    Universidad Interserrana Del Estado de Puebla-Ahuacatlán San Andrés Tlayehualancingo, Puebla, Mexico.
    Ashok, Anumol
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
    Koroidov, Sergey
    Department of Physics, Stockholm University, Stockholm, Sweden.
    β-Mo2C Nanoparticles Produced by Carburization of Molybdenum Oxides with Carbon Black under Microwave Irradiation for Electrocatalytic Hydrogen Evolution Reaction2021In: ACS Applied Nano Materials, E-ISSN 2574-0970, Vol. 4, no 11, p. 12270-12277Article in journal (Refereed)
    Abstract [en]

    The synthesis of electrochemically active β-Mo2C nanoparticles for hydrogen production was achieved by a fast and energy-efficient microwave-assisted carburization process from molybdenum oxides and carbon black. With the use of microwave-based production methods, we aim to reduce the long-time high-temperature treatments and the use of hazardous gases often seen in traditional molybdenum carbide synthesis processes. In our process, carbon black not only serves as a carbon source but also as a susceptor (microwave absorber) and conductive substrate. The irradiation power, reaction time, and Mo:C ratio were optimized to achieve the highest electrocatalytic performance toward hydrogen production in an acidic electrolyte. A complete transformation of MoO3 to β-Mo2C nanoparticles and an additional graphitization of the carbon black matrix were achieved at 1000 W, 600 s, and Mo:C ratio above 1:7.5. Under these conditions, the optimized composite exhibited an excellent HER performance (η10 = 156 mV, Tafel slope of 53 mV·dec-1) and large turnover frequency per active site (3.09 H2·s-1 at an overpotential of 200 mV), making it among the most efficient non-noble-metal catalysts. The excellent activity was achieved thanks to the abundance of β-Mo2C nanoparticles, the intimate nanoparticle-substrate interface, and enhanced electron transport toward the carbon black matrix. We also investigated the flexibility of the synthesis method by adding additional Fe or V as secondary transition metals, as well as the effect of the substrate.

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  • 2.
    Liu, Yong-feng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. College of Physical Science and Technology, Yangzhou University, Yangzhou, China.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Umeå, Sweden.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Umeå, Sweden.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Carbon nanodots: a metal-free, easy-to-synthesize, and benign emitter for light-emitting electrochemical cells2022In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000, Vol. 15, no 6, p. 5610-5618Article in journal (Refereed)
    Abstract [en]

    Light-emitting electrochemical cells (LECs) can be fabricated with cost-efficient printing and coating methods, but a current drawback is that the LEC emitter is commonly either a rare-metal complex or an expensive-to-synthesize conjugated polymer. Here, we address this issue through the pioneering employment of metal-free and facile-to-synthesize carbon nanodots (CNDs) as the emitter in functional LEC devices. Circular-shaped (average diameter = 4.4 nm) and hydrophilic CNDs, which exhibit narrow cyan photoluminescence (peak = 485 nm, full width at half maximum = 30 nm) with a high quantum yield of 77% in dilute ethanol solution, were synthesized with a catalyst-free, one-step solvothermal process using low-cost and benign phloroglucinol as the sole starting material. The propensity of the planar CNDs to form emission-quenching aggregates in the solid state was inhibited by the inclusion of a compatible 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene host compound, and we demonstrate that such pristine host-guest CND-LECs turn on to a peak luminance of 118 cd·m−2 within 5 s during constant current-density driving at 77 mA·cm−2.

  • 3.
    Miranda la Hera, Vladimir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mena, Josué
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ashok, Anumol
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
    Koroidov, Sergey
    Department of Physics, Stockholm University, Stockholm, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Controlled synthesis of tellurium nanowires by physical vapor deposition2022In: Nanomaterials, E-ISSN 2079-4991, Vol. 12, no 23, article id 4137Article in journal (Refereed)
    Abstract [en]

    One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour–solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 μm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 μm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.

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  • 4.
    Piñeiro-García, Alexis
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Benchmarking molybdenum-based materials as cathode electrocatalysts for proton exchange membrane water electrolysis: can these compete with Pt?2023In: ACS Sustainable Chemistry and Engineering, E-ISSN 2168-0485, Vol. 11, no 20, p. 7641-7654Article in journal (Refereed)
    Abstract [en]

    Proton exchange membrane water electrolysis (PEMWE) is a promising technology to produce high-purity renewable hydrogen gas. However, its operation efficiency is highly dependent on the usage of expensive noble metals as electrocatalysts. Replacing, decreasing, or simply extending the operational lifetime of these precious metals have a positive impact on the hydrogen economy. Mo-based electrocatalysts are often praised as potential materials to replace the Pt used at the cathode to catalyse the hydrogen evolution reaction (HER). Most electrocatalytic studies are performed in traditional three-electrode cells with different operational conditions than those seen in PEM systems, making it difficult to predict the expected material’s performance under industrially relevant conditions. Therefore, we investigated the viability of using three selected Mo-based nanomaterials (1T′-MoS2, Co-MoS2, and β-Mo2C) as HER electrocatalysts in PEMWE systems. We investigated the effects of replacing Pt on the catalyst loading, charge transfer resistance, kinetics, operational stability, and hydrogen production efficiency during the PEMWE operation. In addition, we developed a methodology to identify the individual contribution of the anode and cathode kinetics in a PEMWE system, allowing to detect the cause behind the performance drop when using Mo-based electrocatalysts. Our results indicate that the electrochemical performance in three-electrode cells might not strictly predict the performance that could be achieved in PEMWE cells due to differences in interfaces and porosity of the macroscopic catalyst layers. Among the catalysts studied, 1T′-MoS2 is truly an excellent candidate to replace Pt as an HER electrocatalyst due to its low overpotential, low charge transfer resistance, and excellent durability, reaching a high efficiency of ∼75% at 1 A cm-2 and 1.94 V. Our study highlights the importance of a continuous development of efficient noble-metal free HER electrocatalysts suitable for PEMWE systems.

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  • 5.
    Piñeiro-García, Alexis
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rafei, Mouna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mörk, Paul Jonathan
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of physics and astronomy, Julius-Maximilians-Universität Würzburg, Würzburg, Germany.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A Quaternary mixed oxide protective scaffold for ruthenium during oxygen evolution reaction in acidic media2023In: Communications Engineering, E-ISSN 2731-3395, Vol. 2, no 1, article id 28Article in journal (Refereed)
    Abstract [en]

    Proton exchange membrane water electrolysis is widely used in hydrogen production, but its application is limited by significant electrocatalyst dissolution at the anode during the oxygen evolution reaction (OER). The best performing electrocatalysts to date are based on ruthenium and iridium oxides, but these experience degradation even at moderate cell potentials. Here we investigate a quaternary Sn-Sb-Mo-W mixed oxide as a protective scaffold for ruthenium oxide. The acid-stable mixed oxide consists of an interconnected network of nanostructured oxides capable of stabilizing ruthenium into the matrix (Ru-MO). In combination with titanium fibre felt, we observed a lower degradation in the oxygen evolution reaction activity compared to unprotected ruthenium oxide after the electrochemical stress test. The superior stability of Ru-MO@Ti is attributed to the presence of MO which hinders the formation of reactive higher valence ruthenium (Ru+8). Our work demonstrates the potential of multi-metal oxides to extend the lifetime of the OER active metal and the titanium support.

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  • 6.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydrogen evolution mediated by sulfur vacancies and substitutional Mn in few-layered molybdenum disulfide2024In: Materials Today Energy, ISSN 2468-6069, Vol. 41, article id 101524Article in journal (Refereed)
    Abstract [en]

    MoS2 is widely praised as a promising replacement for Pt as an electrocatalyst for the hydrogen evolution reaction (HER), but even today, it still suffers from low performance. This issue is tackled by using Mn3+ as a surface modifier to trigger sulfur vacancy formation and enhance electron transport in few-layered 2H MoS2. Only 10% of Mn is sufficient to transform the semiconductive MoS2 into an active HER electrocatalyst. The insertion of Mn reduces both HER onset potential and Tafel slope which allows reaching 100 mA/cm2 at an overpotential of 206 mV, ten times larger of what undoped MoS2 can achieve. The enhanced activity arises because Mn3+ introduces electronic states near the conduction band, promotes sulfur vacancies, and increases the hydrogen adsorption. In addition to its facile production and extended shelf-life, Mn–MoS2 exhibits an efficiency of 73% at 800 mA/cm2 and 2.0 V when used in proton exchange membrane water electrolyzers.

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  • 7.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Miranda la Hera, Vladimir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Non-stoichiometric NiFeMo solid solutions; tuning the hydrogen adsorption energy via molybdenum incorporation2022In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 9, no 34, article id 2201214Article in journal (Refereed)
    Abstract [en]

    Solution precursor plasma spraying is used to produce catalytic trimetallic coatings containing Ni, Fe and Mo directly onto stainless-steel mesh, Ni foam and carbon paper. The resulting material is mostly comprised of face centered cubic FeNi3 alloy forming a highly porous coating with nanostructured features. The addition of Mo (up to ≈14 at%) generates no new crystal phases but only an increase in the lattice parameter, indicating the formation of FeNi3Mox solid solutions. The FeNi3Mox solid solutions are used as electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media. The addition of Mo increases the HER activity significantly reaching an optimum performance at ≈9 at% Mo (FeNi3Mo0.40) with an overpotential at −10 mA cm−2 of 112 mV and a Tafel slope of 109 mV dec−1. The enhanced HER activity is attributed to the formation of a FeNi3Mox solid solution with an increased work function that is correlated to smaller hydrogen adsorption energies. Theoretical activity maps reveal that sites near superficial Mo atoms forms catalytic hot spots and are responsible for the observed activity.

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  • 8.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Plasma spray coatings as catalysts for water splitting: exploring novel materials and strategies2024Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Today, fossil fuels still play a dominant role in the global energy systems. However, they are depleting quickly and the combustion of them causes many environmental concerns, including global warming, air pollution, ozone layer depletion, and acid rain. In response to these environmental challenges, a transition from fossil fuel energy sources towards sustainable alternatives is urgent and necessary. Unlike traditional fossil fuels, hydrogen serves as an environmentally friendly fuel with exceptional energy density, and its combustion generates no greenhouse gases. Moreover, hydrogen holds the versatility to be produced, stored, and utilized by various sectors, including transportation, industry, and electricity generation. Electrolyzer technology offers a sustainable pathway for clean hydrogen production when using electricity generated from renewable sources such as solar and wind power. The integration of hydrogen into energy systems holds significant potential for a decarbonized and sustainable future.

    In this thesis, we focused on creating affordable coatings using earth-abundant transition metals and explored their application as electrocatalysts for hydrogen and oxygen production in alkaline and acidic environments. We developed novel synthetic routes and new materials, we studied their intricate structure and composition, and we were able to fine-tune their catalytic activity and durability. Our findings demonstrated that plasma spray technology offers a scalable approach for producing highly active catalysts, while also developing coatings that can tolerate acidic environments and extend the lifetime of the state-of-the-art oxygen evolution catalysts. Furthermore, we tested and discussed alternative materials aiming to offer cost-effective substitutes for expensive Pt-based electrocatalysts for hydrogen production.

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  • 9.
    Wu, Xiuyu
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rafei, Mouna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Canto-Aguilar, Esdras Josué
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Scalable production of foam-like nickel-molybdenum coatings via plasma spraying as bifunctional electrocatalysts for water splitting2023In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, no 31, p. 20794-20807Article in journal (Refereed)
    Abstract [en]

    Foam-like NiMo coatings were produced from an inexpensive mixture of Ni, Al, and Mo powders via atmospheric plasma spraying. The coatings were deposited onto stainless-steel meshes forming a highly porous network mainly composed of nanostructured Ni and highly active Ni4Mo. High material loading (200 mg cm−2) with large surface area (1769 cm2 per cm2) was achieved without compromising the foam-like characteristics. The coatings exhibited excellent activity towards both hydrogen evolution (HER) and oxygen evolution (OER) reactions in alkaline media. The HER active coating required an overpotential of 42 mV to reach a current density of −50 mA cm−2 with minimum degradation after a 24 h chronoamperometry test at −10 mA cm−2. Theoretical simulations showed that several crystal surfaces of Ni4Mo exhibit near optimum hydrogen adsorption energies and improved water dissociation that benefit the HER activity. The OER active coating also consisting of nanostructured Ni and Ni4Mo required only 310 mV to achieve a current density of 50 mA cm−2. The OER activity was maintained even after 48 h of continuous operation. We envisage that the development of scalable production techniques for Ni4Mo alloys will greatly benefit its usage in commercial alkaline water electrolysers.

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  • 10.
    Wu, Xiuyu
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rafei, Mouna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kuzhikandathil Mohamed, Alice
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Highly active nickel-molybdenum coating as hydrogen electrocatalysts via solution precursor plasma sprayingManuscript (preprint) (Other academic)
1 - 10 of 10
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