Hunmin Park

Highly active and stable hydrogen evolution electrocatalysts based on molybdenum compounds on carbon nanotube-graphene hybrid support.

Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.

A highly efficient transition metal nitride-based electrocatalyst for oxygen reduction reaction: TiN on a CNT–graphene hybrid support

Transition metal nitrides of group 4–6 (Mo2N, W2N, NbN, Ta3N5, and TiN) were synthesized by the urea-glass route and screened for oxygen reduction reaction (ORR) electrodes in PEMFCs. In terms of electrochemical stability and activity, TiN was selected as the most promising candidate as a catalyst for ORR. To further enhance the activity for ORR, TiN was modified with nanostructured carbon supports including CNTs, graphene (GR), and CNT–GR hybrid. The obtained nanocarbon-supported TiN catalysts exhibited small particle sizes of TiN (<7 nm) and a good TiN–support interaction with reduced aggregation and no free-standing TiN particles away from the supports compared to bare TiN. In particular, TiN supported on the CNT–GR hybrid (TiN/CNT–GR) showed greatly enhanced ORR activity than bare TiN and other supported TiN catalysts. It exhibited a high onset potential (0.83 V) and the highest current density among the reported nitride-based electrocatalysts. The enhancement was ascribed to a synergistic effect between TiN nanoparticles (NPs) and CNT–GR hybird support, roles of which were to provide active sites for ORR and a facile electron pathway to NPs, respectively. Besides, TiN/CNT–GR exhibited large mesopores that could allow easy access of the electrolyte due to the formation of a 3-D CNT–GR structure assembled between 2-D graphene and 1-D CNTs. Further, it showed an excellent methanol tolerance compared to the commercial Pt/C catalyst. Thus, our TiN/CNT–GR could be a promising ORR electrocatalyst for PEMFCs and DMFCs.

Simple Synthesis of Nanocrystalline Tin Sulfide/N-Doped Reduced Graphene Oxide Composites as Lithium Ion Battery Anodes

Composites of nitrogen-doped reduced graphene oxide (NRGO) and nanocrystalline tin sulfides were synthesized, and their performance as lithium ion battery anodes was evaluated. Following the first cycle the composite consisted of Li2S/LixSn/NRGO. The conductive NRGO cushions the stress associated with the expansion of lithiation of Sn, and the noncycling Li2S increases the residual Coulombic capacity of the cycled anode because (a) Sn domains in the composite formed of unsupported SnS2 expand only by 63% while those in the composite formed of unsupported SnS expand by 91% and (b) Li percolates rapidly at the boundary between the Li2S and LixSn nanodomains. The best cycling SnS2/NRGO-derived composite retained a specific capacity of 562 mAh g-1 at the 200th cycle at 0.2 A g-1 rate.

Catalytic CO2 hydrogenation to formic acid over carbon nanotube-graphene supported PdNi alloy catalysts

Pure formic acid was successfully produced via CO2 hydrogenation for the first time over a heterogeneous catalyst of PdNi alloy on a carbon nanotube-graphene (CNT-GR) support in water as an eco-friendly solvent without a base additive. The highest formic acid yield obtained was 1.92 mmol with a turnover number of 6.4 and a turnover frequency of 1.2 × 10−4 s−1 under mild reaction conditions of 40 °C and 50 bar. Alloying Pd with Ni brought a significant enhancement in catalytic activity compared to the monometallic Pd catalyst. In addition, the CNT-GR composite as a catalytic support improved the dispersion of Pd–Ni alloy particles, which exhibited good stability under the reaction conditions.

Phase transition-induced band edge engineering of BiVO4 to split pure water under visible light

Significance Hydrogen has been recognized as one of the most promising energy carriers for the future, because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO4 by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of monoclinic BiVO4, which induced partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4. Through phase transition-induced band edge engineering by dual doping with In and Mo, a new greenish BiVO4 (Bi1-XInXV1-XMoXO4) is developed that has a larger band gap energy than the usual yellow scheelite monoclinic BiVO4 as well as a higher (more negative) conduction band than H+/H2 potential [0 VRHE (reversible hydrogen electrode) at pH 7]. Hence, it can extract H2 from pure water by visible light-driven overall water splitting without using any sacrificial reagents. The density functional theory calculation indicates that In3+/Mo6+ dual doping triggers partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening.

Facile Synthesis of Ge/N-Doped Carbon Spheres with Varying Nitrogen Content for Lithium Ion Battery Anodes

The simple fabrication of composites of germanium nanoparticles dispersed on nitrogen-doped carbon nanospheres (Ge/NC) of varying nitrogen content and their performance in lithium ion battery anodes are reported. A heavily nitrogen-doped carbon gel was formed by condensing m-phenylenediamine with formaldehyde (PF-gel); a less heavily N-doped gel was formed by condensing resorcinol and m-phenylenediamine with formaldehyde (RPF-gel); and an undoped gel was formed by condensing resorcinol with formaldehyde (RF-gel). Pyrolises of the gels with GeCl4 at 750 °C produced nanocrystalline Ge composites with 7.5 atom % N-doped carbon, termed Ge/NC (PF), with 3.9% N-doped carbon, termed Ge/NC (RPF) and undoped carbon, termed Ge/C (RF). The heavily N-doped Ge/NC (PF) anode retained a reversible capacity of 684 mAhg-1 at a specific current of 0.2 Ag-1 after 200 cycles, versus 337 mAhg-1 retained by anode made with Ge/NC (RPF) and 278 mAhg-1 retained by anode made with undoped Ge/C (RF). At a specific current 2.0 Ag-1, the capacity of the Ge/NC (PF) anode was 472 mAhg-1, versus the 210 mAhg-1 of the Ge/NC (RPF) anode and 83 mAhg-1 of the Ge/C (RF) anode. The enhanced performance of the Ge/NC (PF) anode is attributed to the better electrical conductivity of Ge/NC (PF) and to the higher density of Li+ binding defects in its N-doped carbon.

Selective Formation of Hägg Iron Carbide with g‐C3N4 as a Sacrificial Support for Highly Active Fischer–Tropsch Synthesis

The Fischer–Tropsch synthesis (FTS) is a feasible pathway to chemicals and fuels from underutilized resources like coal, natural gas, biomass, and shale gas, instead of the current petroleum‐based production. Owing to its high activity and low price, the iron‐based catalysts are widely used in FTS, yet catalysts with higher activity and better selectivity should be developed for widespread applications. Herein, we report a unique strategy to synthesize an efficient iron catalyst for FTS by applying a graphitic carbon nitride (g‐C3N4) as a sacrificial support. The iron catalyst on g‐C3N4 is effectively reduced to a state that is rapidly and selectively converted to highly crystalline and pure Hägg carbide (χ‐Fe5C2) phase during the FTS reaction. The obtained catalyst exhibits outstanding CO conversion, and high selectivity for C5+ products, outperforming most of the recently reported carbon‐based iron catalysts.

Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane

In the dry reforming of methane with CO2, coke formation in the catalyst during the reaction is the most serious problem. As a coke resistant reforming catalyst, Ni/Al2O3 nanosheets were synthesized by a solvothermal method. The synthesized nanosheet catalysts demonstrated highly stable methane conversion, although the amount of deposited carbon was similar to that in Ni/Al2O3 with a random morphology that deactivated rapidly. The critical effect of nanosheet morphology has been demonstrated on the coke tolerance of the nickel-based dry reforming catalyst.

A highly active and stable palladium catalyst on a g-C3N4 support for direct formic acid synthesis under neutral conditions

Graphitic carbon nitride (g-C3N4) is applied as a support of the Pd catalyst for direct HCOOH synthesis by CO2 hydrogenation under neutral conditions. The high CO2 affinity of g-C3N4 is responsible for the enhanced catalytic activity and stability relative to the inert support such as a carbon nanotube.

Highly Active and Coke-Tolerant Hierarchical Mordenite Catalysts Synthesized by Recrystallization for the Isopropylation of Naphthalene

In the isopropylation of naphthalene for the production of 2,6‐naphthalenedicarboxylate, the monomer of polyethylene naphthalate plastic, a shape‐selective mordenite (MOR) zeolite catalyst provides the best selectivity for the desired 2,6‐diisopropylnaphthalene. However, the small pore size of the zeolite limits the naphthalene conversion and lowers the stability because of pore‐mouth blocking by coke. We discovered that these problems could be mitigated by synthesizing a micro‐meso hierarchical pore structure in the MOR zeolite by the recrystallization of MOR with cetyltrimethylammonium bromide as a mesopore‐forming surfactant. The recrystallized catalysts allow the facile diffusion of bulky molecules through connected meso‐ and micropores to and from active catalytic sites located in the small MOR pores. Relative to microporous MOR, the hierarchical MOR catalyst demonstrated a greatly enhanced activity, stability, and coke tolerance, and the intrinsic high shape selectivity of MOR for 2,6‐diisopropylnaphthalene was maintained. Mild desilication enlarged the pore volume and formed additional acid sites to increase the activity further.