Origin Of Oxygen Reduction Reaction Activity Research Articles

Defect-Engineered Mesoporous Undoped Carbon Nanoribbons for Benchmark Oxygen Reduction Reaction.

For large-scale fuel cell applications, it is significant to replace expensive Pt-based oxygenreductionreaction (ORR) electrocatalysts with nonprecious metal- or metal-free carbon-based catalysts with high activity. However, it is still challenging to deeply understand the role of intrinsic defects and the origin of ORR activity in pure nanocarbon. Therefore, a novel self-assembly and a pyrolysis strategy to fabricate defect-rich mesoporous carbon nanoribbons are presented. Due to the effective regulation of nanoarchitecture, a vast number of defective catalytic sites (edge defects and holes) are exposed, which thereby enhances the electron transfer kinetics and catalytic activity. Such undoped nanoribbons display a large half-wave potential of 0.837V, excellent long-term stability, and exceptional methanol tolerance, surpassing the most undoped ORR catalysts and the commercial Pt/C (20wt.%) catalyst. Structural characterizations and density functional theory (DFT) calculations confirm that the zigzag edge defects and the armchair pentagon at the hole defect are responsible for outstanding ORR performance.

Pulsed electrocatalysis enables the stabilization and activation of carbon-based catalysts towards H2O2 production

Hydrogen peroxide (H2O2) electrosynthesis through oxygen reduction reaction (ORR) provides an environmentally-friendly alternative to the traditional anthraquinone process. While most studies focus on the original construction of active sites in electrocatalysts, it is also necessary to optimize the microenvironment in the dynamic catalytic process. Here, we proposed a pulse-induced strategy to achieve the in-situ regulation of active sites and interface microenvironment in ORR process, enabling a 210 % leap in H2O2 yield and a 74 % increase in Faraday efficiency. A series of operando measurements revealed the stabilization effect on the catalyst morphology and oxygen-containing functional groups distribution, and the activation effect on the basal defect sites to strengthen the interaction with *O2 and *OOH. Density functional theory calculations were further employed to reveal a unique ORR reaction pathway which decoupled the proton transfer and electron transfer process in pulsed electrocatalysis, providing new insights into the origin of ORR activity and selectivity.

Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction

Single-atom Fe-N-C catalysts has attracted widespread attentions in the oxygen reduction reaction (ORR). However, the origin of ORR activity on Fe-N-C catalysts is still unclear, which hinder the further improvement of Fe-N-C catalysts. Herein, we provide a model to understand the ORR activity of Fe-N4 site from the spatial structure and energy level of the frontier orbitals by density functional theory calculations. Taking the regulation of divacancy defects on Fe-N4 site ORR activity as examples, we demonstrate that the hybridization between Fe 3dz2, 3dyz (3dxz) and O2 π* orbitals is the origin of Fe-N4 ORR activity. We found that the Fe–O bond length, the d-band center gap of spin states, the magnetic moment of Fe site and *O2 as descriptors can accurately predict the ORR activity of Fe-N4 site. Furthermore, these descriptors and ORR activity of Fe-N4 site are mainly distributed in two regions with obvious difference, which greatly relate to the height of Fe 3d projected orbital in the Z direction. This work provides a new insight into the ORR activity of single-atom M-N-C catalysts.

Facile Synthesis of Mayenite Electride Nanoparticles Encapsulated in Graphitic Shells Like Carbon Nano Onions: Non-noble-metal Electrocatalysts for Oxygen Reduction Reaction (ORR).

This manuscript presented a large scale synthesis of Graphitic Shells like carbon nano onions (GS-CNOs) by direct solution method using mayenite electride as a catalyst for synthesis of CNOs. Thermal characterization, microstructural analysis, and high resolution electron microscopy have confirmed the graphitization and revealed the resulting GS-CNOs with particle size about 15 nm, maximum BET surface area of 214 m2.g−1, and moderate conductivity of 250 S.cm−1, thus providing a new approach to synthesize GS-CNOs. The reported GS-CNOs, which acts as more active but less expensive electrocatalysts with onset potential of 1.03 V, half wave potential of 0.88 V vs. the reversible hydrogen electrode (RHE), and limited current density of 5.9 mA.cm−2, higher than that of benchmark 20% Pt/C (1.02 eV, 0.82 V, 5.2 mA.cm−2). The synthesized nano-powder acts as an origin of ORR activity via a four electron (4e−) pathway, along with significantly enhanced stability, in alkaline media. The high ORR activity is ascribed to GS-CNOs embedded sufficient metallic C12A7:e− particles, which favor faster electron movement and better adsorption of oxygen molecules on catalyst surface. Hence, we explored first time large scale synthesis of GS-CNOs with gram level and provide efficient approach to prepare novel, lowest cost, potential non-noble metals catalyst for fuel cells.

Exploring Active Sites in Nitrogen Doped Carbon Nanotube Based Electrocatalyst

Carbon nanotube based electrocatalysts have been expected to be the alternative catalysts for the Oxygen Reduction Reaction (ORR) due to their competitively high activity and low cost compared with the conventional Pt/C catalysts. Although pristine carbon nanotubes are known to exhibit poor ORR activity, doping heteroatoms, introducing transition metals and controlling carbon structure can be applied to enhance their ORR activity extensively.[1-4] Nevertheless, the origin of the ORR activity is still not well understood. The Metal-Nitrogen-Carbon complex (MNC) and Nitrogen containing carbon (CNx) related sites have been proposed to be the possible active sites in nitrogen doped carbon materials. In both MNC and CNx structures, it is widely accepted that pyridinic nitrogen plays an important role in the ORR. To further verify the ORR mechanism, in this study, we conduct durability tests to study the relationship between ORR activity and nitrogen structures. The LSV of HA-N900, HA-N900 after durability test (Dur-HA-N900) and re-nitrogen doping of Dur-HA-N900 (Re-N900) in 0.1M HClO4 were shown in Figure 1. As pyridinic nitrogen is always related to the high ORR activity, it is reasonable that along with the ORR activity decreases after the durability test, the pyridinc nitrogen concentration also decreases. Indeed, this phenomenon was confirmed by conducting XPS measurements for the samples. On the other hand, interestingly, we found that even though by introducing pyridinc nitrogen to the same level as that before the durability test through re-nitrogen doping, the ORR activity could not return to the initial state. The ORR activity, along with the role of different kind of nitrogen composition and the metal impurities’ effect will be discussed further on the conference through detailed characterizations with X-ray Photoelectron Spectroscopy (XPS), Temperature Programed Desorption (TPD) and Raman Spectroscopy. The MWCNTs used were provided by Showa Denko KK Japan (VGCF-X, diameter: 15nm, approximate length: 3μm; containing Fe impurities <1 wt%). As a precursor, functionalized and purified MWCNT was obtained by heat and acid treatment in the mixture of H2SO4/HNO3 (HA). [5] Then HAMWCNTs were nitrogen doped in 10% NH3/ Argon gas at 900℃(HA-N900). The durability test was conducted by cycling the catalysts under Ar gas bubbling in 0.1M HClO4 with a scan rate of 500mV/s, voltage window [1.0-1.5] V vs. RHE for 1600 cycles. Acknowledgements This work partially was supported by the Center for Low Carbon Society Strategy (LCS) and Shin-Etsu Chemical Co. Ltd. , Japan. Their contribution is greatly appreciated. Reference [1] Waki, Keiko. "Nitrogen Doped Defective Carbon Nanotube Electrocatalyst for Oxygen Reduction Reaction." PRiME 2016/230th ECS Meeting (October 2-7, 2016). Ecs, 2016. [2] Waki, Keiko, et al. "Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity." Energy & Environmental Science 7.6 (2014): 1950-1958. [3] Li, Yanguang, et al. "An oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes." Nature nanotechnology 7.6 (2012): 394-400. [4]Tao, Li, et al. "Edge-rich and dopant-free graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction." Chemical Communications 52.13 (2016): 2764-2767. [5] Oktaviano, Haryo S, et al. "Nano-drilled multiwalled carbon nanotubes: characterizations and application for LIB anode materials." Journal of Materials Chemistry 22.48 (2012): 25167-25173. Figure 1

The Effect of Oxidation Temperature for the Oxygen Reduction Reaction Activity of Defective Multi-Walled Carbon Nanotubes (MWCNT)

1. IntroductionRecently, carbon based catalysts have attracted much attention as an excellent cathode catalyst for proton exchange membrane fuel cell (PEMFC), because of its low cost and high performance. Multi walled carbon nanotubes (MWCNT) is a special kind of carbon material with higher durability and better cost-efficiency than noble metal catalyst. There are a lot of reports claim that nitrogen doping or adding metal impurity to form M-C-N moiety are important to obtain high activity.[1][2] However, there is another opinion which claimed the non-metal MWCNT itself could also show proper activity and work as the cathode catalyst in PEMFC.[3] Previously, we have reported that only by making defect structure on the surface of MWCNT with catalyst-assistant oxidation, the oxygen reduction reaction (ORR) property could greatly improve.[4] In this study, we are interested in the relationship between oxidation and the ORR activity of the defective carbon nanotube. The further investigation on the defect making indicates that during chemical drilling process, different temperature will lead to an obviously change of the ORR property. A high activity MWCNT with defective structures has been successfully made and its onset potential could reach to 0.76V vs RHE. 2. ExperimentalThe MWCNT in experiment is VGFX-XA (diameter: 15 nm, approximate length: 1 mm, provided by Showa Denko KK, Japan). Pretreatments including heat and acid treatment have been applied in order to remove the impurities. The defective MWCNT is prepared by chemical drilling process, depositing Cobalt on the surface of MWCNT and followed by oxidation. Here various conditions with different temperature have been applied which will lead to different defective structures. These differences cause essential influences on the ORR property of MWCNT. Cyclic voltammetry, temperature programmed desorption and BET analysis are conducted to investigate those MWCNT and study the ORR mechanism. 3. ResultThe onset potential of the defective MWCNT significantly increase with the increasing of the temperature during oxidation from the linear sweep voltammetry measurement as Fig. 1. The oxidation temperature of 600℃ could result in the highest onset potential up to 0.76V vs. RHE, without N doping or metal impurities. Temperature program desorption results also proved that different defect making methods would lead to different functional groups. The details of the defective structures will be furtherly studied by XPS, Raman and ICP analysis. Acknowledge:This work partially was supported by the Center of low carbon society strategy (LCS) and Shin-Etsu Chemical Co., Ltd. Japan. Their contribution is greatly appreciated. Reference:[1] Z.-Y. Yang, Y.-X. Zhang, L. Jing, Y.-F. Zhao, Y.-M. Yan, and K.-N. Sun, “Beanpod-shaped Fe–C–N composite as promising ORR catalyst for fuel cells operated in neutral media,” J. Mater. Chem. A, vol. 2, no. 8, pp. 2623–2627, 2014.[2] J. Shui, M. Wang, F. Du, and L. Dai, “N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells.,” Sci. Adv., vol. 1, no. 1, p. e1400129, 2015.[3] W. Wei et al., “Unusual high oxygen reduction performance in all-carbon electrocatalysts.,” Sci. Rep., vol. 4, p. 6289, 2014.[4] K. Waki et al., “Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity,” Energy Environ. Sci., vol. 7, no. 6, pp. 1950–1958, 2014. Figure 1

Non-Nitrogen Doped Defective Multi-Walled Carbon Nanotubes (MWCNT) As the Cathode Catalyst for PEMFC

Recently, non-noble metal catalysts has been intensively studied for proton exchange membrane fuel cell to achieve low cost and high performance. Among them, carbon nanotubes have attracted much attention because of their lower cost and recyclable property. It is suggested that defective structures on the carbon surface with nitrogen doping and small amount of transient metal to form Fe-C-N moiety are important to have high activity.[1][2] However, the role of carbon defects for the oxygen reduction reaction (ORR) is still not clarified. We have been interested in the ORR activity of the defective pure carbon nanotube and found that with defective structures even without nitrogen doping or annealing, the onset potential can also reach to 0.73V vs RHE. [3] So far high ORR activity always achieved by high temperature annealing process in Argon or ammonia atmosphere. In this study, aiming to further investigate the characteristics of the defective carbon nanotube, optimization of the chemical drilling processes conduct to control the defect structure on MWCNT. By this method high ORR activity MWCNT material could be obtained without annealing or heating treatment. The MWCNT were provided by Showa Denko KK Japan, acid and heat treatment has been done before experiment. The procedure of making defect on MWCNT is prepared by cobalt deposition and oxidation. Here various conditions have been applied which will lead to different defect structure. These differences cause essential influences on the ORR property of MWCNT. To evaluate the performance of defected MWCNT, both cyclic voltammetry and power generation of cathode catalyst in PEMFC have been measured. Linear sweep voltammetry test shows that high defect density and shallow defect structure could bring benefits to the ORR property, and 0.65V vs. RHE onset potential has already been achieved by pure MWCNTs in Fig 1, without N doping, metal impurities or annealing process. Temperature program desorption results also proved that different defect making methods would lead to different characteristics of functional group. The details of different defect structures and principles will be furtherly studied by XPS, Raman, ICP analysis and Power Generation. Acknowledge: This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co., Ltd. Japan. Their contribution is greatly appreciated. Reference: [1] Michel Lefèvre, Eric Proietti, Frédéric Jaouen, Jean-Pol Dodelet. "Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells" Science, 324, 71-74, 2009. [2] Kuanping Gong, Feng Du, Zhenhai Xia, Michael Durstock, Liming Dai. "Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction" Science, 323, 760-764, 2009. [3] K. Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada. "Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity" Energy Environ. Sci., 7, 1950-1958, 2014. Figure 1

Nitrogen-Doped Graphene with Pyridinic Dominance as a Highly Active and Stable Electrocatalyst for Oxygen Reduction.

The nitrogen-doped graphene (NG) with dominance of the pyridinic-N configuration is synthesized via a straightforward process including chemical vapor deposition (CVD) growth of graphene and postdoping with a solid nitrogen precursor of graphitic C3N4 at elevated temperature. The NG fabricated from CVD-grown graphene contains a high N content up to 6.5 at. % when postdoped at 800 °C but maintains high crystalline quality of graphene. The obtained NG exhibits high activity, long-standing stability, and outstanding crossover resistance for electrocatalysis of oxygen reduction reaction (ORR) in alkaline medium. The NG treated at 800 °C shows the best ORR performance. Further study of the dependence of ORR activity on different N functional groups in these metal-free NG electrodes provides deeper insights into the origin of ORR activity. Our results reveal that the pyridinic-N tends to be the most active N functional group to facilitate ORR at low overpotential via a four-electron pathway.