Jae Yul Kim

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.

Graphene–carbon nanotube composite as an effective conducting scaffold to enhance the photoelectrochemical water oxidation activity of a hematite film

The iron oxide photoanode was modified with a graphene–carbon nanotube (CNT) composite conducting scaffold for efficient charge transfer from Fe2O3 particles to transparent conducting oxide substrate in photoelectrochemical water splitting cells. The Fe2O3–composite photoanode showed a photocurrent increase of 530% compared with to the bare Fe2O3 photoanode at 1.23 V vs. RHE, while the increase was only 200 and 240% for Fe2O3–CNT and Fe2O3–graphene photoanodes, respectively. This remarkable performance enhancement by the composite scaffold was attributed to synergistic effects induced by the formation of a 3D-like architecture from 1D CNT and 2D graphene. They become a spacer for each other forming a more open and highly exposed structure, in which both 2D graphene and 1D CNT can exist in the forms with much less self-agglomeration, thus not only enlarging the contact area between the conducting scaffold and Fe2O3 particles but also recovering in part the intrinsic conducting ability of graphene and CNT.

Photoelectrochemical water splitting over ordered honeycomb hematite electrodes stabilized by alumina shielding

Highly ordered, honeycomb-like iron oxide (hematite) films were fabricated by double-step anodic oxidation of iron foil. The honeycomb structure obtained by double step anodization was found to be more effective in producing a large area film with homogeneous pore distribution compared to nanotubes fabricated by the conventional single-step anodic oxidation process. To prevent agglomeration of the hematite film during the annealing process, a thin alumina layer was deposited on the hematite film surface by atomic layer deposition. With this alumina shielding and subsequent removal by alkaline treatment, one-dimensional (1-D) hematite nanostructure was preserved perfectly after annealing at 550 °C. This highly ordered 1-D nanostructure film showed much enhanced photoelectrochemical cell performances relative to hematite films with low degrees of ordering.

A versatile photoanode-driven photoelectrochemical system for conversion of CO2 to fuels with high faradaic efficiencies at low bias potentials

A photoanode-driven photoelectrochemical system consisting of a WO3 photoanode under bias potential and Cu or Sn/SnOx as the cathode for the reduction of CO2 has been studied under visible light irradiation. The bias potentials typically required for the onset of oxygen evolution current at the photoanode were sufficient for the efficient reduction of CO2 at the metallic/composite counter electrodes. Using Cu as a cathode electrocatalyst, faradaic efficiencies of 67% for CH4 and 71.6% for all carbon-containing products were achieved. With Sn/SnOx, a combined faradaic efficiency (CO + HCOOH) of 44.3% was obtained at +0.8 V. The 2-electrode potential between the counter electrode and working electrode for the WO3 driven system was less than the lowest bias potential reported so far for conventional photocathode-driven systems. The results demonstrate for the first time that the intrinsically more stable photoanode-driven systems could accomplish the reduction of CO2 with higher efficiencies relative to the conventional photocathode-driven systems.

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.

Photocatalytic Synthesis of Pure and Water‐Dispersible Graphene Monosheets

Several research groups are actively investigating graphene, which consists of a one-atom thick planar sheet of sp bonded carbon, in attempts to understand its unusual characteristics, such as outstanding electronic properties, optical properties, thermal conductivity (5000 Wm 1 K ), high mechanical strength (200 times stronger than steel), and large surface area per unit mass (2,630 mg 1 calculated,) Novoselov et al. first obtained high quality monosheet graphene detached from natural graphite by the so-called Scotch tape method in 2004. Chemical vapor deposition (CVD) is frequently used to synthesize graphene in a practical scale, and it is suitable for producing a large area (on the inches scale) of high-quality graphene as good as that obtained from natural graphite. However, the CVD process requires high processing temperatures and etching processes, which remove Ni or Cu catalyst nanoparticle layers to yield mono layers or a few layers of graphene on the substrate. Thus, the process remains expensive. Its applications have focused on replacing indium tin oxide (ITO) or fluorinedoped tin oxide (FTO) in transparent conducting electrodes. If an efficient solution-based chemical reduction method were available, we could produce graphene by reducing graphene oxide (GO) on a large scale at low cost. Unfortunately, these types of graphene intrinsically contain many defects that cannot be removed by reduction or thermal treatment. Still, because reduced graphene oxide (RGO) can be obtained easily as powder or solution forms, its utility in, for example, photocatalysts, fuel cells, batteries, ultracapacitors, and hydrogen storage is highly valuable. And solution forms of RGO can be easily applied for inkjetprinting, spray or spin-coating on various substrates. In the solution phase synthesis of graphene, aggregation is a serious problem. Graphene layers tend to aggregate due to high van der Waals interactions. Li et al. solved this problem by reducing GO by hydrazine (RGOH2N NH2) in a basic solution, where RGOH2N NH2 remained dissolved by electrostatic repulsion between the negatively charged carboxylic groups on the graphene sheets. The advantage of this method is that water-dispersible graphene could be made without the use of surfactants or stabilizers. However, one of the problems is that hydrazine is harmful, explosive, and expensive. Furthermore, nitrogen impurities derived from hydrazine impose limits on the conductivity, and residual hydrazine in graphene solutions make its handling and further processing dangerous. Williams et al. introduced the use of ultraviolet (UV) photocatalysis to produce a composite of TiO2 and graphene. [16]

Isomorphic Ti substitution into SBA-15 without Ti loss and with lower TiO2 segregation.

A convenient method has been discovered to incorporate Ti atoms isomorphically into a SBA-15 lattice without Ti loss. By hydrolysis of a Ti precursor near neutral pH instead of conventional acidic conditions, Ti loss was almost eliminated and its segregation to form TiO2 particles was suppressed while the mesoporous structure remained intact.

Photocatalytic selective oxidation of the terminal methyl group of dodecane with molecular oxygen over atomically dispersed Ti in a mesoporous SiO2 matrix

Extraordinarily high selectivity (80–93%) for the oxyfunctionalization of the terminal methyl group was discovered in the photocatalytic selective oxidation of dodecane with molecular oxygen in a continuous flow system under mild gas phase reaction conditions over mesoporous TiO2–SiO2 mixed oxide photocatalysts. The oxygenated hydrocarbon products were mainly aldehydes with carboxylic acids and ketones as minor products. By dispersing most of the Ti atomically in a tetrahedral coordination in a SiO2 matrix, the oxygenated products were stabilized by diluting contiguous Ti sites present on the surface of TiO2 particles. The preferential oxidation of the terminal methyl group was ascribed to the extensive C–C bond breaking by photogenerated holes prior to oxyfunctionalization.