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Gracia-Espino, EduardoORCID iD iconorcid.org/0000-0001-9239-0541
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Publications (10 of 38) Show all publications
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
Sharifi, T., Gracia-Espino, E., Chen, A., Hu, G. & Wågberg, T. (2019). Oxygen Reduction Reactions on Single- or Few-Atom Discrete Active Sites for Heterogeneous Catalysis. Advanced Energy Materials, Article ID 1902084.
Open this publication in new window or tab >>Oxygen Reduction Reactions on Single- or Few-Atom Discrete Active Sites for Heterogeneous Catalysis
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2019 (English)In: Advanced Energy Materials, ISSN 1614-6832, article id 1902084Article in journal (Refereed) Epub ahead of print
Abstract [en]

The oxygen reduction reaction (ORR) is of great importance in energy-converting processes such as fuel cells and in metal-air batteries and is vital to facilitate the transition toward a nonfossil dependent society. The ORR has been associated with expensive noble metal catalysts that facilitate the O-2 adsorption, dissociation, and subsequent electron transfer. Single- or few-atom motifs based on earth-abundant transition metals, such as Fe, Co, and Mo, combined with nonmetallic elements, such as P, S, and N, embedded in a carbon-based matrix represent one of the most promising alternatives. Often these are referred to as single atom catalysts; however, the coordination number of the metal atom as well as the type and nearest neighbor configuration has a strong influence on the function of the active sites, and a more adequate term to describe them is metal-coordinated motifs. Despite intense research, their function and catalytic mechanism still puzzle researchers. They are not molecular systems with discrete energy states; neither can they fully be described by theories that are adapted for heterogeneous bulk catalysts. Here, recent results on single- and few-atom electrocatalyst motifs are reviewed with an emphasis on reports discussing the function and the mechanism of the active sites.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH, 2019
Keywords
active sites, catalyst motifs, electrocatalysis, oxygen reduction reaction, single active atom catalysts, transition metals, X-ray adsorption spectroscopy
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:umu:diva-164137 (URN)10.1002/aenm.201902084 (DOI)000486795700001 ()
Available from: 2019-10-17 Created: 2019-10-17 Last updated: 2019-10-17
Han, X.-B., Tang, X.-Y., Lin, Y., Gracia-Espino, E., Liu, S.-G., Liang, H.-W., . . . Zheng, L.-S. (2019). Ultrasmall Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts. Journal of the American Chemical Society, 141(1), 232-239
Open this publication in new window or tab >>Ultrasmall Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts
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2019 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 141, no 1, p. 232-239Article in journal (Refereed) Published
Abstract [en]

The oxygen evolution reaction is a crucial step in water electrolysis to develop clean and renewable energy. Although noble metal-based catalysts have demonstrated high activity for the oxygen evolution reaction, their application is limited by their high cost and low availability. Here we report the use of a molecule-to-cluster strategy for preparing ultrasmall trimetallic clusters by using the polyoxometalate molecule as a precursor. Ultrafine (0.8 nm) transition-metal clusters with controllable chemical composition are obtained. The transition-metal clusters enable highly efficient oxygen evolution through water electrolysis in alkaline media, manifested by an overpotential of 192 mV at 10 mA cm–2, a low Tafel slope of 36 mV dec–1, and long-term stability for 30 h of electrolysis. We note, however, that besides the excellent performance as an oxygen evolution catalyst, our molecule-to-cluster strategy provides a means to achieve well-defined transition-metal clusters in the subnanometer regime, which potentially can have an impact on several other applications.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2019
National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-155964 (URN)10.1021/jacs.8b09076 (DOI)000455561800040 ()30540181 (PubMedID)
Funder
Carl Tryggers foundation , CTS-16-161
Available from: 2019-02-07 Created: 2019-02-07 Last updated: 2019-02-07Bibliographically approved
Gracia-Espino, E., Barzegar, H. R. & Zettl, A. (2018). Coronene-based graphene nanoribbons insulated by boron nitride nanotubes: electronic properties of the hybrid structure. ACS Omega, 3(10), 12930-12935
Open this publication in new window or tab >>Coronene-based graphene nanoribbons insulated by boron nitride nanotubes: electronic properties of the hybrid structure
2018 (English)In: ACS Omega, ISSN 2470-1343, Vol. 3, no 10, p. 12930-12935Article in journal (Refereed) Published
Abstract [en]

We present a theoretical study on the formation of graphene nanoribbons-via polymerization of coronene molecules-inside the inner cavity of boron nitride nanotubes. We examine the electronic property of the hybrid system, and we show that the boron nitride nanotube does not significantly alter the electronic properties of the encapsulated graphene nanoribbon. Motivated by previous experimental works, we examine graphene nanoribbons with two different widths and investigate probable scenarios for defect formation and/or twisting of the resulting graphene nanoribbons and their effect on the electronic properties of the hybrid system.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-153661 (URN)10.1021/acsomega.8b01617 (DOI)000449026500069 ()
Funder
Carl Tryggers foundation , CTS-16-161Swedish Research Council, 2015-00520
Available from: 2018-11-26 Created: 2018-11-26 Last updated: 2018-11-26Bibliographically approved
diva2:1261501
Open this publication in new window or tab >>Double donor Sb5+doped hematite (Fe3+) photoanodes for surface-enhanced PEC water splitting
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2018 (English)In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 256Article in journal, Meeting abstract (Other academic) Published
Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
National Category
Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-153144 (URN)000447600002312 ()
Conference
256th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nanoscience, Nanotechnology and Beyond, AUG 19-23, 2018, Boston, MA
Available from: 2018-11-07 Created: 2018-11-07 Last updated: 2018-11-07Bibliographically approved
Subramanian, A., Gracia-Espino, E., Annamalai, A., Lee, H. H., Lee, S. Y., Choi, S. H. & Jang, J. S. (2018). Effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting. Applied Surface Science, 427, 1203-1212
Open this publication in new window or tab >>Effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting
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2018 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 427, p. 1203-1212Article in journal (Refereed) Published
Abstract [en]

In this paper, the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on the hematite (alpha-Fe2O3) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (beta-FeOOH) nanorods on FTO (fluorine-doped tin-oxide) substrates via a simple dipping method for which the respective metal-precursor solution was used, followed by a high-temperature (800 degrees C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is similar to 0.81 mA/cm(2) at 1.23 V-RHE, with an onset potential of 0.72 V-RHE; however, the tetravalent dopants on the hematite nanostructures alter the properties of the pristine photoanode. The Si4+-doped hematite photoanode showed a slight photocurrent increment without a changing of the onset potential of the pristine photoanode. The Sn4+- and Ti4+-doped hematite photoanodes, however, showed an anodic shift of the onset potential with the photocurrent increment at a higher applied potential. Interestingly, the Zr4+-doped hematite photoanode exhibited an onset potential that is similar to those of the pristine and Si4+-doped hematite, but a larger photocurrent density that is similar to those of the Sn4+- and Ti4+-doped photoanodes was recorded. The photoactivity of the doped photoanodes at 1.23 V-RHE follows the order Zr > Sn > Ti > Si. The onset-potential shifts of the doped photoanodes were investigated using the Ab initio calculations that are well correlated with the experimental data. X-ray diffraction (XRD) and scanning-electron microscopy (FESEM) revealed that both the crystalline phase of the hematite and the nanorod morphology were preserved after the doping procedure. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the tetravalent dopants on the hematite nanostructure. The charge-transfer resistance at the various interfaces of the doped photoanodes was studied using impedance spectroscopy. The doping on the hematite photoanodes was confirmed using the Mott-Schottky (MS) analysis. 

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Hematite, Akaganeite, Sintering, Nanorods, Onset potential
National Category
Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-152142 (URN)10.1016/j.apsusc.2017.09.042 (DOI)000415219100149 ()
Available from: 2018-10-01 Created: 2018-10-01 Last updated: 2018-10-01Bibliographically approved
Annamalai, A., Sandström, R., Gracia-Espino, E., Boulanger, N., Boily, J.-F., Mühlbacher, I., . . . Wågberg, T. (2018). Influence of Sb5+ as a Double Donor on Hematite (Fe3+) Photoanodes for Surface-Enhanced Photoelectrochemical Water Oxidation. ACS Applied Materials and Interfaces, 10(19), 16467-16473
Open this publication in new window or tab >>Influence of Sb5+ as a Double Donor on Hematite (Fe3+) Photoanodes for Surface-Enhanced Photoelectrochemical Water Oxidation
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2018 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 19, p. 16467-16473Article in journal (Refereed) Published
Abstract [en]

To exploit the full potential of hematite (α-Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface need to be studied in greater detail. Ex situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as the ex situ dopant because it is an effective electron donor and reduces recombination effects and concurrently utilize the possibility to tuning the surface charge and wettability. In the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a 10-fold increase in carrier concentration (1.1 × 1020 vs 1.3 × 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (∼990 vs ∼3700 Ω). Furthermore, a broad range of surface characterization techniques such as Fourier-transform infrared spectroscopy, ζ-potential, and contact angle measurements reveal that changes in the surface hydroxyl groups following the ex situ doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increasing the surface charge and enhancing the electron/hole transport properties. To a greater extent, the Sb5+- surface-doped determines the interfacial properties of electrochemical charge transfer, leading to an efficient water oxidation mechanism.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
hematite, ex situ doping, Fe2O3-Sb, water splitting, Sb5+, Fe3+, surface charge, double donors
National Category
Materials Chemistry
Identifiers
urn:nbn:se:umu:diva-148990 (URN)10.1021/acsami.8b02147 (DOI)000432753800027 ()29663796 (PubMedID)2-s2.0-85046257587 (Scopus ID)
Funder
Swedish Research Council, 2017-04862Carl Tryggers foundation , CTS-16-161Swedish Energy Agency, 45419-1
Available from: 2018-06-14 Created: 2018-06-14 Last updated: 2018-06-19Bibliographically 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
Sandström, R., Gracia-Espino, E., Hu, G., Shchukarev, A., Ma, J. & Wågberg, T. (2018). Yttria stabilized and surface activated platinum (PtxYOy) nanoparticles through rapid microwave assisted synthesis for oxygen reduction reaction. Nano Energy, 46, 141-149
Open this publication in new window or tab >>Yttria stabilized and surface activated platinum (PtxYOy) nanoparticles through rapid microwave assisted synthesis for oxygen reduction reaction
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2018 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 46, p. 141-149Article in journal (Refereed) Published
Abstract [en]

The enhancement of platinum (Pt) based catalysts for the oxygen reduction reaction (ORR) by addition of rare earth metals represents a promising strategy to achieve high activity yet low content of the precious metal and concurrently addresses stability issues experienced by traditional late transition metal doping. Improvement in Pt utilization is essential for vehicular applications where material cost and abundancy is a great concern. Here we report a fast and efficient production route of yttria-stabilized platinum nanoparticles (PtxYOy) using a conventional household microwave oven. ORR performance showed a significant improvement and an optimum activity at a 3:1 Pt:Y ratio outperforming that of commercial Pt-Vulcan with a doubled specific activity. Incorporation of Y is evidenced by extended X-ray absorption fine structure and energy dispersive X-ray analysis, while significant amounts of integrated Y2O3 species are detected by X-ray photoelectron spectroscopy. Density functional theory calculations suggest surface migration and oxidation of Y, forming stable superficial yttrium oxide species with low negative enthalpies of formation. The robustness of PtxYOy is shown experimentally and through theoretical arguments demonstrating that surface yttria acts as a stabilizing agent and promoter of highly active ORR sites on the remaining Pt surface, surpassing even the Pt3Y alloy configuration.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Yttrium, Platinum, Nanoparticles, Catalysis, Hydrogen fuel cells, Oxygen reduction
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-144503 (URN)10.1016/j.nanoen.2018.01.038 (DOI)000427924000017 ()
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2019-04-29Bibliographically approved
Projects
Disorder and Stoichiometry Imbalance as Tools to Improve Catalytic Activity in Multimetallic Chalcogenides [2018-03937_VR]; Umeå University
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9239-0541

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