Uk Sim

Hydrated Manganese(II) Phosphate (Mn3(PO4)2·3H2O) as a Water Oxidation Catalyst

The development of a water oxidation catalyst has been a demanding challenge in realizing water splitting systems. The asymmetric geometry and flexible ligation of the biological Mn4CaO5 cluster are important properties for the function of photosystem II, and these properties can be applied to the design of new inorganic water oxidation catalysts. We identified a new crystal structure, Mn3(PO4)2·3H2O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its high catalytic performance under neutral conditions. The bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn3(PO4)2·3H2O. Computational analysis indicated that the structural flexibility in Mn3(PO4)2·3H2O could stabilize the Jahn-Teller-distorted Mn(III) and thus facilitate Mn(II) oxidation. This study provides valuable insights into the interplay between atomic structure and catalytic activity.

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we suggest that monolayer graphene can be used as a catalyst for solar-driven hydrogen evolution reaction on Si-photocathodes, and its catalytic activity is boosted by plasma treatment in N2-ambient. The plasma treatment induces abundant defects and the incorporation of nitrogen atoms in the graphene structure, which can act as catalytic sites on graphene. The monolayer graphene containing nitrogen impurities exhibits a remarkable increase in the exchange current density and leads to a significant anodic shift of the onset of photocurrent from the Si-photocathode. Additionally, monolayer graphene shows the passivation effect that suppresses the surface oxidation of Si, thus enabling the operation of the Si-photocathode in neutral water. This study shows that graphene itself can be applied to a photoelectrochemical system as a catalyst with high activity and chemical stability.

Partially Oxidized Sub-10 nm MnO Nanocrystals with High Activity for Water Oxidation Catalysis

The oxygen evolution reaction (OER) is considered a major bottleneck in the overall water electrolysis process. In this work, highly active manganese oxide nano-catalysts were synthesized via hot injection. Facile surface treatment generated Mn(III) species on monodisperse 10 nm MnO nanocrystals (NCs). Size dependency of MnO NCs on OER activity was also investigated. Surprisingly, the partially oxidized MnO NCs only required 530 mV @ 5 mA cm−2 under near neutral conditions.

One‐Step Synthesis of N‐doped Graphene Quantum Sheets from Monolayer Graphene by Nitrogen Plasma

High-quality N-doped graphene quantum sheets are successfully fabricated from as-grown monolayer graphene on Cu using nitrogen plasma, which can be transferred as a film-like layer or easily dispersed in an organic solvent for further optoelectronic or photoelectrochemical applications.

N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production

Photoelectrochemical hydrogen production from solar energy has been attracting much attention in the field of renewable energy technology. The realization of cost-effective hydrogen production by water splitting requires electrolysis or photoelectrochemical cells decorated with highly efficient co-catalysts. A critical requirement for catalysts in photoelectrochemical cells is not only the ability to boost the kinetics of a chemical reaction but also to exhibit durability against electrochemical and photoinduced degradation. In the race to replace previous noble-metal catalysts, the design of carbon-based catalysts represents an important research direction in the search for non-precious, environmentally benign, and corrosion-resistant catalysts. Herein, we suggest graphene quantum sheets as a catalyst for the solar-driven hydrogen evolution reaction on Si nanowire photocathodes. The optimum nanostructures of the Si photocathodes exhibit an enhanced photocurrent and a lower overpotential compared to those of a planar Si surface. This significant enhancement demonstrates how graphene quantum sheet catalysts can be used to produce Si nanowire photocathodes as hydrogen evolution reaction catalysts with high activity.

An iron oxide photoanode with hierarchical nanostructure for efficient water oxidation

Hematite (α-Fe2O3) has been attracting attention for photoelectrochemical water oxidation due to its visible light photon absorption capacity and high chemical stability, but the short-diffusion length of holes and the large overpotential are still challenging to overcome. Here, in an effort to address these challenges, we develop a hierarchically nanostructured photoanode composed of iron-oxides; Ti-doped hematite nanorods are decorated with an undoped hematite underlayer and β-FeOOH nano-branches. The Ti-doped hematite nanorod array is prepared by hydrothermal synthesis, and this nanostructure offers enhanced separation of photogenerated charges. The underlayer not only increases the photocurrent density but also improves the onset potential. The photocurrent further increases by the epitaxially grown β-FeOOH nano-branches on the hematite, but the onset potential is positively shifted by the β-FeOOH due to increasing flat-band potential. The analyses of the photocurrent transients and electrochemical impedance spectra reveal that β-FeOOH improves the photocurrent by decreasing the resistance to charge transfer through the anode/electrolyte. This study demonstrates a new possibility for improving the efficiency of a hematite photoanode with the interface of other iron-oxides.

Nanostructural dependence of hydrogen production in silicon photocathodes

Hydrogen production from solar power energy is an important energy and environmental issue. Silicon (Si) has been widely studied as a photocathode for hydrogen production from water splitting. In this study, the electrochemical behavior of a Si photocathode for water splitting is highly dependent on its nanostructure. The optimum nanostructure of a Si photocathode exhibits an enhanced photocurrent and a lower overpotential compared to the planar bulk Si. The limiting current density of nanostructured Si is 1.58 times greater than that of the planar structure for p-type Si/aqueous electrolyte solution. Nanostructured Si without any catalyst notably produced a current density of −10.65 mA cm−2 under Air Mass 1.5 Global conditions with a light intensity of 100 mW cm−2 at the reversible potential vs. reversible hydrogen electrode, which is about 43 times higher than that of the untreated Si structure. The solar-to-hydrogen conversion efficiency of the optimized Si nanowire without depositing any catalyst has reached up to about 70% of the efficiency of planar Si decorated with Pt. This significant enhancement achieved in this study emphasizes the importance of a controlled nanostructure in the development of highly efficient photoelectrochemical devices for hydrogen production.

Enhanced performance of NaTaO3 using molecular co-catalyst [Mo3S4]4+ for water splitting into H2 and O2

Photocatalytic activity of NaTaO(3) was significantly improved by using a molecular co-catalyst [Mo(3)S(4)](4+). Its hydrogen production rate is 28 times higher than pure NaTaO(3). This study presents the potential of bioinspired molecular metal clusters as efficient co-catalysts.

Sulfur‐Modified Graphitic Carbon Nitride Nanostructures as an Efficient Electrocatalyst for Water Oxidation

There is an urgent need to develop metal-free, low cost, durable, and highly efficient catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using hydrothermal process. Sulfur-modified graphitic carbon nitride (S-modified g-CN x ) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. The sulfur modification in the g-CN x structure leads to excellent oxygen evolution reaction activity by lowering the overpotential. Compared with the previously reported nonmetallic systems and well-established metallic catalysts, the S-modified g-CN x nanostructures show superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm-2 and a Tafel slope of 120 mV dec-1 with long-term durability of 91.2% retention for 18 h. These inexpensive, environmentally friendly, and easy-to-synthesize catalysts with extraordinary performance will have a high impact in the field of oxygen evolution reaction electrocatalysis.

Correction: N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production

Correction for ‘N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production’ by Uk Sim et al., Energy Environ. Sci., 2015, DOI: 10.1039/c4ee03607g.