Oxygen reduction reaction on active sites of heteroatom-doped graphene (original) (raw)
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Journal of Catalysis, 2014
Electrocatalysts are essential to two key electrochemical reactions, oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in renewable energy conversion and storage technologies such as regenerative fuel cells and rechargeable metal-air batteries. Here, we explored N-doped graphene as costeffective electrocatalysts for these key reactions by employing density functional theory (DFT). The results show that the substitution of carbon at graphene edge by nitrogen results in the best performance in terms of overpotentials. For armchair nanoribbons, the lowest OER and ORR overpotentials were estimated to be 0.405 V and 0.445 V, respectively, which are comparable to those for Pt-containing catalysts. OER and ORR with the minimum overpotentials can occur near the edge on the same structure but different sites. These calculations suggest that engineering the edge structures of the graphene can increase the efficiency of the N-doped graphene as efficient OER/ORR electrocatalysts for metal-air batteries, water splitting, and regenerative fuel cells.
Oxygen reduction reaction mechanism on nitrogen-doped graphene: A density functional theory study
Journal of Catalysis, 2011
Nitrogen-doped graphene (N-graphene) was reported to exhibit a good activity experimentally as an electrocatalyst of oxygen reduction reaction (ORR) on the cathode of fuel cells under the condition of electropotential of $0.04 V (vs. NHE) and pH of 14. This material is promising to replace or partially replace the conventionally used Pt. In order to understand the experimental results, ORR catalyzed by N-graphene is studied using density functional theory (DFT) calculations under experimental conditions taking the solvent, surface adsorbates, and coverages into consideration. Two mechanisms, i.e., dissociative and associative mechanisms, over different N-doping configurations are investigated. The results show that N-graphene surface is covered by O with 1/6 monolayer, which is used for reactions in this work. The transition state of each elementary step was identified using four different approaches, which give rise to a similar chemistry. A full energy profile including all the reaction barriers shows that the associative mechanism is more energetically favored than the dissociative one and the removal of O species from the surface is the rate-determining step.
A DFT Study of the Oxygen Reduction Reaction Mechanism on Be Doped Graphene
2021
Graphene despite its high surface area has very limited activity towards the oxygen reduction reaction (ORR), demonstrating selectivity towards the unfavorable two-electron mechanism. We have employed the spin polarized density functional theory (DFT) method to investigate the effect of the heteroatom p-type beryllium (Be) dopant on the oxygen reduction reaction activity of graphene. The preferred doping sites, active sites and reaction mechanism available on the doped graphene surfaces were investigated with increasing Be concentrations of 0.03 ML, 0.06 ML and 0.09 ML. Our results reveal that oxygen does not bind strongly to bare graphene, and Be at the lattice sites provides site for the oxygen adsorption and ORR. Oxygen binds dissociatively on the doped surfaces preferentially in the order 0.06 ML > 0.09 ML > 0.03 ML. From this studies introduction of Be impurities in a single honeycomb ring of graphene has significant impact on the binding and adsorbate-adsorbent interacti...
Nitrogen-Doped Graphene Oxide Electrocatalysts for the Oxygen Reduction Reaction
ACS Applied Nano Materials
Platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) often exhibit a complex functionalized graphitic structure. Because of this complex structure, limited understanding exists about the design factors for the synthesis of high-performing materials. Graphene, a two-dimensional hexagonal structure of carbon, is amenable to structural and functional group modifications, making it an ideal analogue to study crucial properties of more complex graphitic materials utilized as electrocatalysts. In this paper, we report the synthesis of active nitrogen-doped graphene oxide catalysts for the ORR in which their activity and four-electron selectivity are enhanced using simple solvent and electrochemical treatments. The solvents, chosen based on Hansen's solubility parameters, drive a substantial change in the morphology of the functionalized graphene materials by (i) forming microporous holes in the graphitic sheets that lead to edge defects and (ii) inducing 3D structure in the graphitic sheets that promotes ORR. Additionally, the cycling of these catalysts has highlighted the multiplicity of the active sites, with different durability, leading to a highly selective catalyst over time, with a minimal loss in performance. High ORR activity was demonstrated in an alkaline electrolyte with an onset potential of ∼1.1 V and half-wave potential of 0.84 V vs RHE. Furthermore, long-term stability potential cycling showed minimal loss in half-wave potential (<3%) in both N 2-and O 2-saturated solutions with improved selectivity toward the four-electron reduction after 10000 cycles. The results described in this work provide additional understanding about graphitic electrocatalysts in alkaline media that may be utilized to further enhance the performance of PGM-free ORR electrocatalysts.
Science Advances, 2016
Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical to renewable energy conversion and storage technologies. Heteroatom-doped carbon nanomaterials have been reported to be efficient metal-free electrocatalysts for ORR in fuel cells for energy conversion, as well as ORR and OER in metal-air batteries for energy storage. We reported that metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) doped with nitrogen exhibited superb bifunctional electrocatalytic activities for both ORR and OER, with an excellent stability in alkaline electrolytes (for example, KOH). For the first time, it was experimentally demonstrated that the electrondonating quaternary N sites were responsible for ORR, whereas the electron-withdrawing pyridinic N moieties in N-GRW served as active sites for OER. The unique 3D nanoarchitecture provided a high density of the ORR and OER active sites and facilitated the electrolyte and electron transports. As a result, the as-prepared N-GRW holds great potential as a low-cost, highly efficient air cathode in rechargeable metal-air batteries. Rechargeable zinc-air batteries with the N-GRW air electrode in a two-electrode configuration exhibited an open-circuit voltage of 1.46 V, a specific capacity of 873 mAh g −1 , and a peak power density of 65 mW cm −2 , which could be continuously charged and discharged with an excellent cycling stability. Our work should open up new avenues for the development of various carbon-based metal-free bifunctional electrocatalysts of practical significance.
Computational and Theoretical Chemistry, 2020
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Eco-Friendly Nitrogen-Doped Graphene Preparation and Design for the Oxygen Reduction Reaction
Molecules, 2021
Four N-doped graphene materials with a nitrogen content ranging from 8.34 to 13.1 wt.% are prepared by the ball milling method. This method represents an eco-friendly mechanochemical process that can be easily adapted for industrial-scale productivity and allows both the exfoliation of graphite and the synthesis of large quantities of functionalized graphene. These materials are characterized by transmission and scanning electron microscopy, thermogravimetry measurements, X-ray powder diffraction, X-ray photoelectron and Raman spectroscopy, and then, are tested towards the oxygen reduction reaction by cyclic voltammetry and rotating disk electrode methods. Their responses towards ORR are analysed in correlation with their properties and use for the best ORR catalyst identification. However, even though the mechanochemical procedure and the characterization techniques are clean and green methods (i.e., water is the only solvent used for these syntheses and investigations), they are t...
Quantum chemistry of the oxygen reduction reaction (ORR) on Fe-G iron doped graphene for fuel cells
International Journal of Hydrogen Energy, 2019
In this work, we used a dissociative mechanism to perform calculations based on Density Functional Theory, DFT, and electronic structure in order to study the oxygen reduction reaction on graphene doped with iron. The model takes into account some of the operating conditions of a proton exchange membrane fuel cell such as the equilibrium of hydrogen oxidation reaction, electrode potential of 1.23 V and 0 V, solvation effects, and corrections to the energy at the zero point (ZPE) and the entropic one. However, in this approach, we neglect the effects on the free energy of the interaction of the adsorbed species with the electric field due to the double electrochemical layer, the pH of the acid medium, and the oxygen coverage. The free energy diagrams for different intermediate steps of the oxygen reduction reaction, ORR, the oxygen adsorption energy on sites close to those occupied by the Fe-atoms, as well as the activity calculations indicated that Fe-Graphene (Fe-G) system, may possess catalytic properties close to either Pt, as catalyst, as its alloys since they can favor the ORR in a fuel cell proton exchange membrane. The results have been compared with other theoretical studies which use graphene as a central element as will be established in the present manuscript.
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Metal-free carbon electrodes with well-defined composition and smooth topography were prepared via sputter deposition followed by thermal treatment with inert and reactive gases. XPS and Raman spectroscopies show that three carbons of similar N/C content that differ in Nsite composition were thus prepared: an electrode consisting of almost exclusively graphitic-N (NG), an electrode with predominantly pyridinic-N (NP) and one with ca. 1:1 NG:NP composition. These materials were used as model systems to investigate activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity towards 4e-reduction of O2 is strongly influenced by the NG/NP site composition, with the material possessing nearly uniform NG/NP composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters were carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NGdoping or NP-doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed NG/NP sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.