Yttria stabilized and surface activated platinum (PtxYOy) nanoparticles through rapid microwave assisted synthesis for oxygen reduction reaction
We produced yttria-stabilized platinum nanoparticles (Pt3YOy) using a microwave assisted synthesis process for ORR. The robustness of PtxYOy is shown experimentally and through theoretical arguments demonstrating that surface yttria acts as an stabilizing agent and promoter of highly active ORR sites on the remaining Pt surface, surpassing even the Pt3Y alloy configuration.
Robin Sandström, Eduardo Gracia-Espino, Guangzhi Hu, Andrey Shchukarev, Jingyuan Ma, Thomas Wågberg. Nano Energy 46 (2018) 141-149
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 an stabilizing agent and promoter of highly active ORR sites on the remaining Pt surface, surpassing even the Pt3Y alloy configuration.
Cationic Vacancy Defects in Iron Phosphide: A Promising Route toward Efficient and Stable Hydrogen Evolution by Electrochemical Water Splitting
We created Fe vacancies as an approach to modulate the electronic structure and catalytic activity of iron phosphide (FeP). The Fe-vacancy-rich FeP nanoparticulate films showed excellent HER activity achieving a current density of 10 mA cm-2 at overpotentials of 108 mV in 1 M KOH, and 65 mV in 0.5 M H2SO4. This work was carried out in collaboration with Prof. Messinger (Uppsala University).
W. L. Kwong, E. Gracia-Espino, C. C. Lee, R. Sandström, T. Wågberg, and J. Messinger.
ChemSusChem (2017) DOI: 10.1002/cssc.201701565
Engineering the electronic properties of transition metal phosphides has shown great effectiveness in improving their intrinsic catalytic activity for the hydrogen evolution reaction (HER) in water splitting applications. Herein, we report for the first time, the creation of Fe vacancies as an approach to modulate the electronic structure of iron phosphide (FeP). The Fe vacancies were produced via chemical leaching of Mg that was introduced into FeP as 'sacrificial dopant'. The obtained Fe-vacancy-rich FeP nanoparticulate films, which were deposited on Ti foil, shows excellent HER activity as compared to pristine FeP and Mg-doped FeP, achieving a current density of 10 mA cm-2 at overpotentials of 108 mV in 1 M KOH and 65 mV in 0.5 M H2SO4, with a near-100% Faradaic efficiency. Our theoretical and experimental analyses reveal that the improved HER activity originates from the presence of Fe vacancies, which lead to a synergistic modulation of the structural and electronic properties that result in a near optimal hydrogen adsorption free energy and enhanced proton trapping. The success in catalytic improvement via the introduction of cationic vacancy defects has not only demonstrated the potential of Fe-vacancy-rich FeP as highly efficient, earth abundant HER catalyst, but also opened up an exciting pathway for activating other promising catalysts for electrochemical water splitting.
Effect of Tetravalent Dopants on Hematite Nanostructure for Enhanced Photoelectrochemical Water Splitting
This time the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on hematite nanostructure for enhanced photoelectrochemical water splitting is reported. The photoactivity of the doped photoanodes at 1.23 V RHE follows the order Zr > Sn > Ti > Si. The work was performed in collaboration with Prof. Jum Suk Jang (Chonbuk National University, Korea), and the results are published in the journal of Applied Surface Science.
A. Subramanian, E. Gracia-Espino, A. Annamalai, H. H. Lee, S Y. Lee, S. H. Choi, and J. S. Jang. Applied Surface Science (2017).
In this paper, the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on the hematite (α-Fe2O3) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (β-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° C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is ∼0.81 mA/cm2 at 1.23 VRHE, with an onset potential of 0.72 VRHE; 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 VRHE 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.
Synergistic Effect between the Atomically Dispersed Active Site of Fe-N-C and C-S-C for ORR in Acidic Medium
In this occasion, we investigated a sulfur-doped Fe-N-C (Fe/SNC) catalyst with a thiophene-like structure (C-S-C) that reduces the electron localization around the Fe center and improves the interaction with oxygenated species. The observed synergistic effect makes the Fe/SNC catalyst exhibits better ORR activity than sulfur free catalyst (Fe/NC) in 0.5 M H2SO4. The results were published in the journal of Angewandte Chemie International Edition.
Hangjia Shen, Eduardo Gracia- Espino, Jingyuan Ma, Ketao Zang, Jun Luo, Le Wang, Sanshuang Gao, Xamxikamar Mamat, Guangzhi Hu, Thomas Wagberg, and Shaojun Guo.
Angew. Chem. Int. Ed. (2017), DOI: 10.1002/anie.201706602
Various advanced catalysts of sulfur doped Fe-N-C materials have been recently designed for oxygen reduction reaction (ORR), however, the enhanced activity is still controversial and usually attributed to differences in surface area, improved conductivity, or to uncertain synergistic effects. Here, a sulfur-doped Fe-N-C catalyst (denoted as Fe/SNC) derived via a template sacrificing method is presented. The incorporated S gives a thiophene-like structure (C-S-C), reduces the electron localization around the Fe center, improves the interaction with oxygenated species, and therefore facilitates the complete 4e- ORR in acid solution. This synergistic effect makes the Fe/SNC catalyst exhibits much better ORR activity than sulfur free catalyst (Fe/NC) in 0.5 M H2SO4.
Nano for Energy group
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