Young Hye Lee

Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf

A stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem a CH3NH3PbI3 perovskite single junction solar cell with a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm(2) at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performance to best known cobalt phosphate (Co-Pi) (5.0 mA/cm(2) at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but with significantly better stability. A tandem artificial-leaf-type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3% (wired), 3.0% (wireless) under simulated 1 sun illumination. Hence, our device based on a D4 tandem photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.

Hetero-type dual photoanodes for unbiased solar water splitting with extended light harvesting

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO4 and α-Fe2O3 as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm−2 at 1.23 VRHE under 1 sun irradiation. A tandem cell composed with the dual photoanodes–silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forward en route to the goal of >10% efficiency required for practical solar hydrogen production.

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]

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.

Isopropylation of naphthalene by isopropanol over conventional and Zn- and Fe-modified USY zeolites

Catalytic performances of USY, MOR, and BEA zeolites were compared for the isopropylation of naphthalene by isopropyl alcohol in a high-pressure, fixed-bed reactor. The USY catalyst showed a high conversion of 86% and good stability but a low 2,6-/2,7-DIPN shape selectivity ratio of 0.94. In contrast, over the MOR catalyst, 2,6-DIPN was selectively synthesized with a high 2,6-/2,7-DIPN ratio of 1.75, but low naphthalene conversions and fast deactivation of the catalyst were observed. The USY catalyst was modified by Zn and Fe using the wet impregnation method to enhance the selectivity for 2,6-DIPN. The highest conversion (~95%) and selectivity for 2,6-DIPN (~20%) were achieved with 4% Zn/USY catalyst. It appeared that small metal oxide islands formed in the USY pores to decrease the effective pore size and thus render it mildly shape-selective. Zn loading also decreased the number of strong acid sites responsible for coke formation and increased the number of weak acid sites. The high conversion and stability of Zn-modified catalysts were ascribed to the presence of a suitable admixture of weak and strong acid sites with less coke deposition. The Fe-modified USY catalysts were less effective because the modification increased the number of the strong acid sites.

MOR/SBA‐15 Composite Catalysts with Interconnected Meso/Micropores for Improved Activity and Stability in Isopropylation of Naphthalene

The isopropylation of naphthalene with isopropyl alcohol was studied over composites of MOR/SBA‐15 in a high‐pressure, fixed‐bed reactor. The MOR catalyst showed a high 2,6‐/2,7‐diisopropyl naphthalene (DIPN) ratio of 1.75, but a low naphthalene conversion (54 %) and fast deactivation of the catalyst. The composites of MOR/SBA‐15 were prepared by a hydrothermal recrystallization process to obtain hierarchical micro/mesopores. During the process, MOR recrystallized in the mesopores of SBA‐15, the structure of which was stabilized by carbon coating formed on the pore walls as a template. The best prepared MOR/SBA‐15 catalyst achieved a high conversion of 85 %, high stability, and less coking, while maintaining the high 2,6‐/2,7‐DIPN ratio (≈1.8). The modified micro/mesopore structure allowed facile diffusion of bulky molecules to and from active catalytic sites located in the small MOR pores as a result of the connection between the two types of pores in the MOR/SBA‐15 composite.

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.

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.