Min Wook Chung

B, N- and P, N-doped graphene as highly active catalysts for oxygen reduction reactions in acidic media

Graphene has been highlighted recently as a promising material for energy conversion due to its unique properties deriving from a two-dimensional layered structure of sp2-hybridized carbon. Herein, N-doped graphene (NGr) is developed for its application in oxygen reduction reactions (ORRs) in acidic media, and additional doping of B or P into the NGr is attempted to enhance the ORR performance. The NGr exhibits an onset potential of 0.84 V and a mass activity of 0.45 mA mg−1 at 0.75 V. However, the B, N- (BNGr) and P, N-doped graphene (PNGr) show onset potentials of 0.86 and 0.87 V, and mass activities of 0.53 and 0.80 mA mg−1, respectively, which are correspondingly 1.2 and 1.8 times higher than those of the NGr. Moreover, an additional doping of B or P effectively reduces the production of H2O2 in the ORRs, and shows much higher stability than that of Pt/C in acidic media. It is proposed that the improvement in the ORR activity results from the enhanced asymmetry of the spin density or electron transfer on the basal plane of the graphene, and the decrease in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphene through additional doping of B or P.

Long-Range Electron Transfer over Graphene-Based Catalyst for High-Performing Oxygen Reduction Reactions: Importance of Size, N-doping, and Metallic Impurities

N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior stability.

Rational Design of a Hierarchical Tin Dendrite Electrode for Efficient Electrochemical Reduction of CO2

Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2 . However, successful CO2 reduction still suffers from the lack of affordable catalyst design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx /Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2 (.-) intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi-branched conifer-like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 μmol h(-1)  cm(-2) ) at -1.36 VRHE without any considerable catalytic degradation over 18 h of operation.

Doping of chalcogens (sulfur and/or selenium) in nitrogen-doped graphene–CNT self-assembly for enhanced oxygen reduction activity in acid media

N-doped carbon has been recognized as a promising electro-catalytic material for oxygen reduction reactions (ORRs). Herein, as a promoter of ORRs in acid media, sulfur and/or selenium atoms are additionally decorated onto the N-doped graphene–CNT self-assembly (NGCA) by heat-treatment with diphenyldisulfide and/or diphenyldiselenide. It is demonstrated that S and Se are successfully doped in the carbon lattice with dominant phases of –C–S–C– and –C–Se–C–, respectively. In the ORRs, the prepared materials exhibit similar onset potentials at ∼0.85 V (vs. RHE) regardless of chalcogenation. However, the additional doping of S and/or Se in the NGCA increases the current from ORRs in acid media. Specifically, additional Se-doping demonstrates significantly improved ORR activity with a high methanol tolerance and long-term stability in acid media compared to Pt/C. It is suggested that the high ORR activity of the carbon materials is related to the asymmetric spin and charge densities of the carbon atoms, which are enhanced by the additional doping of S and/or Se.

Enhanced electrochemical oxygen reduction reaction by restacking of N-doped single graphene layers

Graphene, which is a two-dimensional layer structure of sp2-hybridized carbon, has been highlighted recently as a promising material for energy conversion. Herein, graphene-derived catalysts are developed for application in oxygen reduction reactions (ORRs) in acidic media via heat-treatment with dicyandiamide and a small amount (<1 wt.%) of transition metal. In ORRs, bare graphene exhibits 0.58 V (vs. RHE) of the onset potential; however, it increases to ∼0.9 V through modification steps and records a mass activity of 1.28 mA mg−1 at 0.75 V. Through the correlation curve between the ORR activities and the number of restacked graphene layers, it is proposed that the stacking of a few layers is desirable in the ORRs rather than a single layer catalyst. The graphene-derived catalysts exhibit graphite properties of facile electron transfer as the restacking of graphene layers increases, without degradation of the pyridinic-N on the graphene edge.

Diamond@carbon-onion hybrid nanostructure as a highly promising electrocatalyst for the oxygen reduction reaction

The modification of nanodiamond derived carbon nano-onions by the formation of additional edge- and defect-sites through the rupturing of surface graphene layers is investigated for its application towards the oxygen reduction reaction (ORR) in acidic media. The catalyst, which had a high degree of edge- and defect-sites on its surface, demonstrated a remarkably enhanced ORR performance compared to that of an edge- and defect-sites poor catalyst, where the onset potential increased from 0.53 to 0.91 V, with a mass activity of 2.70 mA mg−1 (at 0.8 V). According to our electrochemical impedance spectroscopy study, the enhancement in the catalytic performance between the two catalysts could have originated from the charge transfer resistance. Moreover, an accelerated degradation test revealed the outstanding stability of the edge- and defect-rich catalyst, compared to that of Pt/C performed in harsh conditions, which could have originated from the diamond core. The selection of carbon material with adequate modifications to enhance the catalytic activity and stability towards the ORR drafted a scheme for potential catalysts.

Dimensional tailoring of nitrogen-doped graphene for high performance supercapacitors

Due to its unique properties, graphene has been regarded as a promising electrode material in various fields of energy conversion and storage devices. In supercapacitors, however, the graphene electrodes show unexpectedly poor energy densities due to low transferability of charge carriers in the randomly overlaid graphene electrodes. For efficient charge transfers, construction of three-dimensional graphene structures has been generally considered. In this study, contrary to previous strategies, the graphene structures are sequentially tailored from two-dimensional sheets to one-dimensional ribbons and zero-dimensional dots, and then their capacitive behaviors are investigated in a symmetric unit cell. Dimensionality of the graphene determines the local pore structure and morphology of the fabricated graphene electrodes. Hence, it strongly affects the transfer rate of charge carriers and capacitive performance. One-dimensional ribbons, which have a high length-to-width ratio and a consequent net-like porous structure in the fabricated electrode, demonstrate an efficient charge transferability with 378 F g−1 specific capacitance at 1 A g−1 current density in 6 M KOH electrolyte. Additionally, a durability study coupled with X-ray photoelectron spectroscopy (XPS) reveals that performance degradation of the graphene-based electrodes mainly results from surface oxidation which inhibits facile electron transfers.