Chang Hyuck Choi

Binary and Ternary Doping of Nitrogen, Boron, and Phosphorus into Carbon for Enhancing Electrochemical Oxygen Reduction Activity

N-doped carbon, a promising alternative to Pt catalyst for oxygen reduction reactions (ORRs) in acidic media, is modified in order to increase its catalytic activity through the additional doping of B and P at the carbon growth step. This additional doping alters the electrical, physical, and morphological properties of the carbon. The B-doping reinforces the sp(2)-structure of graphite and increases the portion of pyridinic-N sites in the carbon lattice, whereas P-doping enhances the charge delocalization of the carbon atoms and produces carbon structures with many edge sites. These electrical and physical alternations of the N-doped carbon are more favorable for the reduction of the oxygen on the carbon surface. Compared with N-doped carbon, B,N-doped or P,N-doped carbon shows 1.2 or 2.1 times higher ORR activity at 0.6 V (vs RHE) in acidic media. The most active catalyst in the reaction is the ternary-doped carbon (B,P,N-doped carbon), which records -6.0 mA/mg of mass activity at 0.6 V (vs RHE), and it is 2.3 times higher than that of the N-doped carbon. These results imply that the binary or ternary doping of B and P with N into carbon induces remarkable performance enhancements, and the charge delocalization of the carbon atoms or number of edge sites of the carbon is a significant factor in deciding the oxygen reduction activity in carbon-based catalysts.

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.

Heteroatom doped carbons prepared by the pyrolysis of bio-derived amino acids as highly active catalysts for oxygen electro-reduction reactions

Heteroatom (nitrogen and sulfur)-doped carbons were synthesized via the pyrolysis of composites composed of iron chloride, cobalt chloride and five different amino acids (alanine, cysteine, glycine, niacine and valine), and their electrocatalytic activity towards oxygen reduction reactions (ORR) compared with each other for fuel cell applications. In all of the prepared catalysts, carbon was doped by nitrogen, and, in particular, a catalyst synthesized from cysteine was dual-doped with nitrogen and sulfur. Among all the catalysts, the dual-doped carbon showed the highest onset potential (0.55 V, vs. Ag/AgCl) and electrochemical activity in acidic media, − 0.2 mA (at 0.2 V, vs. Ag/AgCl), which is about 43% of that of commercial Pt/C (40 wt%). XPS revealed that sulfur was doped in the carbon as sulfate or sulfonate, and it is surmised that not only nitrogen doping but also sulfur doping of carbon plays a key role in improving its electrocatalytic activity towards ORR.

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.

Designed Synthesis of Well-Defined Pd@Pt Core-Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

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.

Oxygen reduction activity of Pd–Mn3O4 nanoparticles and performance enhancement by voltammetrically accelerated degradation

Electrochemical properties of Pd-Mn3O4 nanoparticles toward oxygen reduction reaction (ORR) in acidic media were investigated. The catalysts were prepared by polyol reduction of Pd(acac)2 and thermal decomposition of Mn2(CO)10. Surface composition and structure of the particles vary depending on the injection temperature of Mn2(CO)10 and are closely related to the electrochemical properties. The presence of Mn3O4 promotes the performance towards ORR by facilitating oxygen transfer to the active sites of Pd. Through an accelerated degradation test (ADT), nanoparticles with a Pd-rich shell are obtained by dissolution of surface exposed Mn3O4 and at 0.57 V (vs. Ag/AgCl) this catalyst shows the highest activity towards ORR, 149% in mass activity and 142% in specific activity compared to that of Pd/C.

Synergism between CdTe semiconductor and pyridine - photoenhanced electrocatalysis for CO2 reduction to formic acid

The cooperative effect of pyridine catalysis with photoenhanced electrochemical reduction by CdTe/FTO of CO2 is explored. The enhancement of overall formic acid production and high faradaic efficiency is due to the increased photocurrent of CdTe/FTO upon irradiation and the decrease in activation barrier due to the formation of a pyridine–CO2 intermediate.

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.