Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study
Here we used density functional theory to investigate the current-voltage characteristics and the distribution of catalytic active sites towards oxygen reduction of nitrogen-doped and defective graphene. The study highlights the importance of considering not only the interaction energy of reaction intermediates, but also the electrical conductivity of such configurations. The results were published in the journal of Carbon.
Joakim Ekspong, Nicolas Boulanger, Eduardo Gracia-Espino
Carbon (2018). DOI: 10.1016/j.carbon.2018.05.050
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.
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.
Atomically FeN2 Moieties Dispersed on Mesoporous Carbon: A New Atomic Catalyst for Efficient Oxygen Reduction Catalysis
We now report a template casting strategy to easily introduce atomically dispersed FeN2 moieties onto the surface of N-doped ordered mesoporous carbon with extraordinary catalytic activity towards ORR. This work was carried out in collaboration with Prof. Guo (College of Engineering, Peking University) and Prof. Hu (Xinjiang Technical Institute of Physics and Chemistry). The results are published in the journal of Nano Energy.
H. Shena, E. Gracia-Espino, J. Mac, H. Tang, X. Mamat, T. Wagberg, G. Hua, S. Guoe. Nano Energy, (2017)
Earth-abundant materials with Fe-N-C centers have been identified as promising catalysts for oxygen reduction reaction (ORR), but these alternatives for Pt catalysts are usually the porphyrin-like FeN4 configuration. The density functional theory (DFT) calculations reveal that FeN2 outperforms FeN4 due to its lower interaction with ⁎O2 and ⁎OH intermediates and enhanced electron transport, however, achieving an optimum design of these earth-abundant materials with the enriched FeN2 catalytic centers is still a great challenge. Here, we report an intriguing template casting strategy to introduce a mass of atomically dispersed FeN2 moieties onto the surface of N-doped ordered mesoporous carbon for boosting ORR electrocatalysis. One of unique parts herein is to pre anchor Fe precursor on the surface of template (SBA-15) during catalyst synthesis, preventing Fe from penetrating into the carbon skeleton and facilitating the removal of excessive Fe-based particles during silica elimination by HF etching, resulting in a desirable model structure comprising only highly active atomically dispersed FeN2 sites, as confirmed by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine structure (EXAFS) and Mößbauer spectroscopy analysis. The well-defined structure prompts us to understand the nature of the catalytic active sites, and to demonstrate that the catalyst activity is linearly proportional to the concentration of FeN2 sites. The obtained atomic electrocatalyst exhibits superior electrocatalytic performance for ORR with a more positive half-wave potential than that of Pt/C catalyst. We further establish a kinetic model to predict the ORR activity of these single-atom dispersed catalysts. The present work elaborates on a profound understanding for designing low-cost, highly efficient FeN2-based electrocatalyst for boosting ORR.
Behind the Synergistic Effect Observed on Phosphorus-Nitrogen co-Doped Graphene during the Oxygen Reduction Reaction
In our most recent publication we performed ab initio calculations to construct ORR overpotential maps and describe the availability and spatial distribution of catalytic active sites on phosphorus-nitrogen co-doped graphene. The results are published in the Journal of Physical Chemistry C.
Eduardo Gracia-Espino. J. Phys. Chem. C, 120, 27849–27857 (2016)
Ab initio calculations are performed to investigate how the simultaneous introduction of phosphorus and nitrogen into graphene modifies the availability and spatial distribution of catalytic active sites for oxygen reduction reaction (ORR). A phosphoryl group (R3-P=O) is selected as a representative for the phosphorus doping, and the ORR is studied under alkaline conditions where a 4e- mechanism is used to determine the limiting step and overpotential (ηORR) along the entire graphene surface. A scanning procedure is used to construct ηORR maps for pristine-, N-, P-, and diverse PN co-doped graphenes. The results indicate that a single N (P) atom activates up to 17 (3) C atoms, while the simultaneous introduction of P and N activates up to 55 C atoms equivalent to 57% of the surface. Additionally, PN co-doped graphenes reveals that the relative location of both dopants has significant effects on the ORR performance, where a P-N separation distance of at least 4 Å minimize the localization of electronic states on the neighboring C atoms and improves the quantity and distribution of active sites. The results shows the importance of designing synthesis procedures to control the dopant concentration and spatial distribution to maximize the number of active sites. Furthermore, the ηORR maps reveal features that could be obtained by scanning tunneling microscopy allowing to experimentally identify and possibly quantify the catalytic active sites on carbon-based materials.
Nano for Energy group
Comprehensive Study of an Earth-Abundant Bifunctional 3D Electrode for Efficient Water Electrolysis in Alkaline Medium.
ACS Appl. Mater. Interfaces, 2015, 7, 28148
C60/Collapsed Carbon Nanotube Hybrids - A Variant of Peapods.
Nano Lett., 2015, 15 (2), pp 829–834
Fabrication of One-Dimensional Zigzag [6,6]-Phenyl-C61-Butyric Acid Methyl Ester Nanoribbons from Two-Dimensional Nanosheets.
ACS Nano, 2015, 9, 10516
Hierarchical self-assembled structures based on nitrogen-doped carbon nanotubes as advanced negative electrodes for Li-ion batteries and 3D microbatteries.
J. P. Sources, 2015, 279, 581
.Self-Assembly Synthesis of Decorated Nitrogen-Doped Carbon Nanotubes with ZnO Nanoparticles: Anchoring Mechanism and the Effects of Sulfur.
J. Phys. Chem. C, 120, 27849 (2016)
Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be2+ as co-dopant.
Sci Rep. 2016; 6: 23183.
Atomistic understanding of the origin of high oxygen reduction electrocatalytic activity of cuboctahedral Pt3Co–Pt core–shell nanoparticles.
Catal. Sci. Technol., 2016, 6, 1393-1401
Photocatalytic reduction of CO2 with H2O over modified TiO2 nanofibers: Understanding the reduction pathway.
Nano Res. (2016) 9: 1956.