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Ekspong, Joakim
Publications (9 of 9) Show all publications
Sandström, R., Annamalai, A., Boulanger, N., Ekspong, J., Talyzin, A., Mühlbacher, I. & Wågberg, T. (2019). Evaluation of Fluorine and Sulfonic Acid Co-functionalized Graphene Oxide Membranes in Hydrogen Proton Exchange Membrane Fuel Cell Conditions. Sustainable Energy & Fuels, 3(7), 1790-1798
Open this publication in new window or tab >>Evaluation of Fluorine and Sulfonic Acid Co-functionalized Graphene Oxide Membranes in Hydrogen Proton Exchange Membrane Fuel Cell Conditions
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2019 (English)In: Sustainable Energy & Fuels, ISSN 2398-4902, Vol. 3, no 7, p. 1790-1798Article in journal (Refereed) Published
Abstract [en]

The use of graphene oxide (GO) based membranes consisting of self-assembled flakes with a lamellar structure represents an intriguing strategy to spatially separate reactants while facilitating proton transport in proton exchange membranes (PEM). Here we chemically modify GO to evaluate the role of fluorine and sulfonic acid groups on the performance of H2/O2 based PEM fuel cells. Mild fluorination is achieved by the presence of hydrogen fluoride during oxidation and subsequent sulfonation resulted in fluorine and SO3- co-functionalized GO. Membrane electrode assembly performance in low temperature and moderate humidity conditions suggested that both functional groups contribute to reduced H2 crossover compared to appropriate reference membranes. Moreover, fluorine groups promoted an enhanced hydrolytic stability while contributing to prevent structural degradation after constant potential experiments whereas sulfonic acid demonstrated a stabilizing effect by preserving proton conductivity.

Place, publisher, year, edition, pages
Royal Society of Medicine Press, 2019
Keywords
Proton exchange membrane, Fuel Cell, Graphene oxide, Hydrogen, Fluorine, Sulfonic acid
National Category
Nano Technology Other Chemical Engineering Other Materials Engineering Energy Systems
Research subject
nanomaterials
Identifiers
urn:nbn:se:umu:diva-158496 (URN)10.1039/C9SE00126C (DOI)000472980200014 ()
Funder
Swedish Research Council, 2017-04862Swedish Energy Agency, 45419-1ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 15-483Interreg Nord
Note

Originally included in thesis in manuscript form

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-08-05Bibliographically approved
Sandström, R., Ekspong, J., Gracia-Espino, E. & Wågberg, T. (2019). Oxidatively Induced Exposure of Active Surface Area during Microwave Assisted Formation of Pt3Co Nanoparticles for Oxygen Reduction Reaction. RSC Advances, 9(31), 17979-17987
Open this publication in new window or tab >>Oxidatively Induced Exposure of Active Surface Area during Microwave Assisted Formation of Pt3Co Nanoparticles for Oxygen Reduction Reaction
2019 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 9, no 31, p. 17979-17987Article in journal (Refereed) Published
Abstract [en]

The oxygen reduction reaction (ORR), the rate-limiting reaction in proton exchange membrane fuel cells, can efficiently be facilitated by properly manufactured platinum catalysts alloyed with late 3d transition metals. Herein we synthesize a platinum:cobalt nanoparticulate catalyst with a 3:1 atomic ratio by reduction of a dry organometallic precursor blend within a commercial household microwave oven. The formed nanoparticles are simultaneously anchored to a carbon black support that enables large Pt surface area. Two separate microwave treatment steps were employed, where step one constitutes a fast oxidative treatment for revealing active surface area while a reductive secondary annealing treatment promotes a Pt rich surface. The resulting Pt3Co/C catalyst (~3.4 nm) demonstrate an enhanced ORR activity directly attributed to incorporated Co with a specific and mass activity of 704 μA cm-2Pt and 352 A g-1Pt corresponding to an increase by 279 % and 66 % respectively compared to a commercial Pt/C (~1.8 nm) catalyst measured under identical conditions. The method´s simplicity, scalability and novelty is expected to further assist in Pt-Co development and bring the catalyst one step closer toward commercialization and utility in fuel cells.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2019
Keywords
Proton exchange membrane fuel cell, platinum cobalt, Oxygen reduction reaction, Microwave synthesis
National Category
Nano Technology Other Materials Engineering
Research subject
nanomaterials; nanoparticles; Materials Science
Identifiers
urn:nbn:se:umu:diva-158492 (URN)10.1039/c9ra02095k (DOI)000471914300054 ()
Funder
Swedish Research Council, 2017-04862ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 15-483Swedish Energy Agency, 45419-1Swedish Research Council, 2018-03937Stiftelsen Olle Engkvist Byggmästare, 186-0637
Note

Originally included in thesis in manuscript form 

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-07-11Bibliographically approved
Ekspong, J. & Wågberg, T. (2019). Stainless Steel as A Bi-Functional Electrocatalyst – A Top-Down Approach. Materials, 12(13), Article ID 2128.
Open this publication in new window or tab >>Stainless Steel as A Bi-Functional Electrocatalyst – A Top-Down Approach
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 13, article id 2128Article in journal (Refereed) Published
Abstract [en]

For a hydrogen economy to be viable, clean and economical hydrogen production methods are vital. Electrolysis of water is a promising hydrogen production technique with zero emissions, but suffer from relatively high production costs. In order to make electrolysis of water sustainable, abundant, and efficient materials has to replace expensive and scarce noble metals as electrocatalysts in the reaction cells. Herein, we study activated stainless steel as a bi-functional electrocatalyst for the full water splitting reaction by taking advantage of nickel and iron suppressed within the bulk. The final electrocatalyst consists of a stainless steel mesh with a modified surface of layered NiFe nanosheets. By using a top down approach, the nanosheets stay well anchored to the surface and maintain an excellent electrical connection to the bulk structure. At ambient temperature, the activated stainless steel electrodes produce 10 mA/cm(2) at a cell voltage of 1.78 V and display an onset for water splitting at 1.68 V in 1M KOH, which is close to benchmarking nanosized catalysts. Furthermore, we use a scalable activation method using no externally added electrocatalyst, which could be a practical and cheap alternative to traditionally catalyst-coated electrodes.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
water splitting, electrolysis, bifunctional, electrocatalysts, hydrogen evolution reaction, oxygen olution reaction, sustainable, stainless steel, nano
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-162337 (URN)10.3390/ma12132128 (DOI)000477043900092 ()31269744 (PubMedID)2-s2.0-85068826298 (Scopus ID)
Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-08-22Bibliographically approved
Ekeroth, S., Münger, E. P., Boyd, R., Ekspong, J., Wågberg, T., Edman, L., . . . Helmersson, U. (2018). Catalytic nanotruss structures realized by magnetic self-assembly in pulsed plasma. Nano letters (Print), 18(5), 3132-3137
Open this publication in new window or tab >>Catalytic nanotruss structures realized by magnetic self-assembly in pulsed plasma
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2018 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 18, no 5, p. 3132-3137Article in journal (Refereed) Published
Abstract [en]

Tunable nanostructures that feature a high surface area are firmly attached to a conducting substrate and can be fabricated efficiently over significant areas, which are of interest for a wide variety of applications in, for instance, energy storage and catalysis. We present a novel approach to fabricate Fe nanoparticles using a pulsed-plasma process and their subsequent guidance and self-organization into well-defined nanostructures on a substrate of choice by the use of an external magnetic field. A systematic analysis and study of the growth procedure demonstrate that nondesired nanoparticle agglomeration in the plasma phase is hindered by electrostatic repulsion, that a polydisperse nanoparticle distribution is a consequence of the magnetic collection, and that the formation of highly networked nanotruss structures is a direct result of the polydisperse nanoparticle distribution. The nanoparticles in the nanotruss are strongly connected, and their outer surfaces are covered with a 2 nm layer of iron oxide. A 10 μm thick nanotruss structure was grown on a lightweight, flexible and conducting carbon-paper substrate, which enabled the efficient production of H2 gas from water splitting at a low overpotential of 210 mV and at a current density of 10 mA/cm2.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
nanotrusses, nanowires, nanoparticles, iron, electrocatalysis, pulsed sputtering
National Category
Nano Technology
Identifiers
urn:nbn:se:umu:diva-148834 (URN)10.1021/acs.nanolett.8b00718 (DOI)000432093200055 ()29624405 (PubMedID)
Available from: 2018-06-12 Created: 2018-06-12 Last updated: 2018-06-12Bibliographically approved
Sandström, R., Ekspong, J., Annamalai, A., Sharifi, T., Klechikov, A. & Wågberg, T. (2018). Fabrication of microporous layer - free hierarchical gas diffusion electrode as a low Pt-loading PEMFC cathode by direct growth of helical carbon nanofibers. RSC Advances, 8(72), 41566-41574
Open this publication in new window or tab >>Fabrication of microporous layer - free hierarchical gas diffusion electrode as a low Pt-loading PEMFC cathode by direct growth of helical carbon nanofibers
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2018 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 8, no 72, p. 41566-41574Article in journal (Refereed) Published
Abstract [en]

Improving interfacial contact between each component in the proton exchange membrane fuel cell (PEMFC) can lead to a significant increase in power density and Pt utilization. In this work, the junction between the catalyst layer and gas diffusion layer (GDL) is greatly enhanced through direct attachment of helical carbon nanofibers, giving rise to a hierarchical structure within the electrical interconnections. The alternative novel GDL is produced by spraying a thin layer of Pd2C60 precursor on commercial carbon paper, followed by chemical vapor deposition growth resulting in a surface morphology of well-attached nanofibers surrounding the microfibers present in the commercial carbon paper. Subsequent solvothermal deposition of platinum nanoparticles allowed evaluation of its suitability as gas diffusion electrode in cathodic H-2/O-2 PEMFC environment. A combination of lowered charge transfer resistance and enhanced Pt-utilization is attributed to its unique wire-like appearance and its robust properties. The fabricated microporous layer - free GDL is suitable for relatively aggressive membrane electrode assembly fabrication procedures and is produced by industrially favorable techniques, rendering it capable of efficiently supporting small amounts of precious metal catalyst nanoparticles in various PEM applications.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2018
National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-155124 (URN)10.1039/c8ra07569g (DOI)000453914300053 ()
Funder
The Kempe FoundationsSwedish Energy AgencySwedish Research Council
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-04-29Bibliographically approved
Ekspong, J., Sandström, R., Rajukumar, L. P., Terrones, M., Wågberg, T. & Gracia-Espino, E. (2018). Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst. Advanced Functional Materials, 28(46), Article ID 1802744.
Open this publication in new window or tab >>Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst
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2018 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 46, article id 1802744Article in journal (Refereed) Published
Abstract [en]

The metastable 1T′ polymorph of molybdenum disulfide (MoS2) has shown excellent catalytic activity toward the hydrogen evolution reaction (HER) in water‐splitting applications. Its basal plane exhibits high catalytic activity comparable to the edges in 2H MoS2 and noble metal platinum. However, the production and application of this polymorph are limited by its lower energetic stability compared to the semiconducting 2H MoS2 phase. Here, the production of stable intercalated 1T′ MoS2 nanosheets attached on graphitic nanoribbons is reported. The intercalated 1T′ MoS2 exhibits a stoichiometric S:Mo ratio of 2.3 (±0.1):1 with an expanded interlayer distance of 10 Å caused by a sulfur‐rich intercalation agent and is stable at room temperature for several months even after drying. The composition, structure, and catalytic activity toward HER are investigated both experimentally and theoretically. It is concluded that the 1T′ MoS2 phase is stabilized by the intercalated agents, which further improves the basal planes′ catalytic activity toward HER.

Place, publisher, year, edition, pages
WILEY-VCH VERLAG GMBH, 2018
Keywords
DFT calculations, hydrogen evolution reaction, intercalation, MoS2, transition metal chalcogenides
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-154948 (URN)10.1002/adfm.201802744 (DOI)000449887300019 ()
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-01-07Bibliographically approved
Ekspong, J., Boulanger, N. & Gracia-Espino, E. (2018). Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study. Carbon, 137, 349-357
Open this publication in new window or tab >>Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study
2018 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 137, p. 349-357Article in journal (Refereed) Published
Abstract [en]

Introducing heteroatoms and creating structural defects on graphene is a common and rather successful strategy to transform its inert basal plane into an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR). However, the intricate atomic configuration of defective graphenes difficult their optimization as ORR electrocatalysts, where not only a large density of active sites is desirable, but also excellent electrical conductivity is required. Therefore, we used density functional theory to investigate the current-voltage characteristics and the catalytic active sites towards ORR of nitrogen-doped and defective graphene by using 8 zig-zag graphene nanoribbons as model systems. Detailed ORR catalytic activity maps are created for ten different systems showing the distribution of catalytic hot spots generated by each defect. Subsequently, the use of both current-voltage characteristics and catalytic activity maps allow to exclude inefficient systems that exhibit either low electrical conductivity or have adsorption energies far from optimal. Our study highlights the importance of considering not only the interaction energy of reaction intermediates to design electrocatalysts, but also the electrical conductivity of such configurations. We believe that this work is important for future experimental studies by providing insights on the use of graphene as a catalyst towards the ORR reaction. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2018
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:umu:diva-151037 (URN)10.1016/j.carbon.2018.05.050 (DOI)000440661700035 ()2-s2.0-85047971117 (Scopus ID)
Available from: 2018-09-05 Created: 2018-09-05 Last updated: 2018-09-05Bibliographically approved
Ekspong, J., Sharifi, T., Shchukarev, A., Klechikov, A., Wågberg, T. & Gracia-Espino, E. (2016). Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction. Advanced Functional Materials, 26(37), 6766-6776
Open this publication in new window or tab >>Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction
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2016 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 37, p. 6766-6776Article in journal (Refereed) Published
Abstract [en]

Finding an abundant and cost-effective electrocatalyst for the hydrogen evolu-tion reaction (HER) is crucial for a global production of hydrogen from water electrolysis. This work reports an exceptionally large surface area hybrid catalyst electrode comprising semicrystalline molybdenum sulfi de (MoS 2+ x) catalystattached on a substrate based on nitrogen-doped carbon nanotubes (N-CNTs), which are directly grown on carbon fiber paper (CP). It is shown here that nitrogen-doping of the carbon nanotubes improves the anchoring of MoS 2+ xcatalyst compared to undoped carbon nanotubes and concurrently stabilizes a semicrystalline structure of MoS 2+ x with a high exposure of active sites for HER. The well-connected constituents of the hybrid catalyst are shown to facilitate electron transport and as a result of the good attributes, the MoS 2+ x/N-CNT/CPelectrode exhibits an onset potential of −135 mV for HER in 0.5 M H2SO4, a Tafel slope of 36 mV dec −1, and high stability at a current density of −10 mA cm −2.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2016
Keywords
carbon paper, hydrogen evolution reaction, molybdenum disulfide—MoS2, nitrogen doped carbon nanotubes, water splitting catalysts
National Category
Inorganic Chemistry Other Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-128753 (URN)10.1002/adfm.201601994 (DOI)000384810300006 ()
Available from: 2016-12-14 Created: 2016-12-14 Last updated: 2018-06-09Bibliographically approved
Sandström, R., Gracia-Espino, E., Annamalai, A., Persson, P., Persson, I., Ekspong, J., . . . Wågberg, T.Microwave-Induced Structural Ordering of Resilient Nanostructured L10-FePt Catalysts for Oxygen Reduction Reaction.
Open this publication in new window or tab >>Microwave-Induced Structural Ordering of Resilient Nanostructured L10-FePt Catalysts for Oxygen Reduction Reaction
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We show how structurally ordered L10 face-centered tetragonal (fct) FePt nanoparticles are produced by a solid-state microwave-assisted synthesis method. The structural phase as well as the incorporated Fe into the nanoparticles is confirmed by X-ray diffraction and high resolution high-angle annular dark field scanning transmission electron microscopy experiments. The prepared particles exhibit a remarkable resilience toward crystallite growth at high temperatures. Directly correlated to the L10 phase, the best oxygen reduction reaction (ORR) characteristics are achieved for particles with a 1:1 Fe:Pt atomic ratio and an average size of ~2.9 nm where Pt-specific evaluation provided a high mass and specific activity of ~570 A/gPt and ~600 μA/cm2Pt respectively. Our results demonstrate that well-structured catalysts possessing activities vastly exceeding Pt/C (~210 A/gPt & ~250 μA/cm2Pt), can be synthesized through a fast and highly eco-friendly method. We note that the achieved mass activity represent a significant leap toward the theoretical maximum for fully ordered FePt nanoparticles.

Keywords
Proton exchange membrane fuel cell, platinum iron, Oxygen reduction reaction, microwave synthesis
National Category
Nano Technology Other Materials Engineering
Research subject
Materials Science; nanomaterials; nanoparticles; Solid State Physics
Identifiers
urn:nbn:se:umu:diva-158495 (URN)
Funder
Swedish Research Council, 2017-04862Swedish Energy Agency, 45419-1Interreg NordÅForsk (Ångpanneföreningen's Foundation for Research and Development), 15-483Swedish Research Council, 2016‐04412Swedish Foundation for Strategic Research , RIF 14‐0074Swedish Research Council, 2018-03937Stiftelsen Olle Engkvist Byggmästare, 186-0637
Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-04-29
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