Hsisheng Teng

Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics

Abstract Oxygen treatment at 250°C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon–oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H 2 SO 4 , using potential sweep cyclic voltammetry and constant current charge–discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g −1 at a current density of 0.5 mA cm −2 ). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition.

Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination

Nitrogen-doped graphene oxide quantum dots exhibit both p- and n-type conductivities and catalyze overall water-splitting under visible-light irradiation. The quantum dots contain p-n type photochemical diodes, in which the carbon sp(2) clusters serve as the interfacial junction. The active sites for H2 and O2 evolution are the p- and n-domains, respectively, and the reaction mimics biological photosynthesis.

Influence of mesopore volume and adsorbate size on adsorption capacities of activated carbons in aqueous solutions

Liquid-phase adsorption of phenol, iodine and tannic acid onto commercial grade granular activated carbons and activated carbon fabric was conducted to explore the influence of the mesopore volume and the size of adsorbates on the adsorption capacity. These carbons have different mesopore volumes, while possessing similar surface areas and micropore volumes, which are believed to determine the adsorption capacity in the liquid phase. The adsorption isotherms reflect that the adsorption capacity is an increasing function of the mesopore volume. Further analysis using the Langmuir and the Dubinin-Radushkevich equations shows increases of both the equilibrium constant and the adsorption energy with the mesopore volume, indicating that mesopores facilitate the adsorption of the adsorbates in the inner and narrow micropores, which contain adsorptive sites with higher adsorption energies. It was also found that the influence of mesopore volume on the capacity was enhanced by the increase in the adsorbate size. The results of the present study have demonstrated the importance of mesopores in enhancing the adsorption capacity of microporous carbons in aqueous solutions, especially for the adsorption of large adsorbates.

High-performance quantum dot-sensitized solar cells based on sensitization with CuInS2 quantum dots/CdS heterostructure

A high-performance quantum dot-sensitized solar cell (QDSSC) is reported, which consists of a TiO2/CuInS2-QDs/CdS/ZnS photoanode, a polysulfide electrolyte, and a CuS counter electrode. The sensitization process involves attaching presynthesized CuInS2 QDs (3.5 nm) to a TiO2 substrate with a bifunctional linker, followed by coating CdS with successive ionic layer adsorption and reaction (SILAR) and ZnS as the last SILAR layer for passivation. This process constructs a sensitizing layer that comprises CdS nanocrystals, closely packed around the earlier-linked CuInS2 QDs, which serve as the pillars of the layer. The CuS counter electrode, prepared via successive ionic solution coating and reaction, has a small charge transfer resistance in the polysulfide electrolyte. The QDSSC exhibits a short-circuit photocurrent (Jsc) of 16.9 mA cm−2, an open-circuit photovoltage (Voc) of 0.56 V, a fill factor of 0.45, and a conversion efficiency of 4.2% under one-sun illumination. The heterojunction between the CuInS2 QDs and CdS extends both the optical absorption and incident photon conversion efficiency (IPCE) spectra of the cell to a longer wavelength of approximately 800 nm, and provides an IPCE of nearly 80% at 510 nm. The high TiO2 surface coverage of the sensitizers suppresses recombination of the photogenerated electrons. This results in a longer lifetime for the electrons, and therefore, the high Voc value. The notably high Jsc and Voc values demonstrate that this sensitization strategy, which exploits the quantum confinement reduction and other synergistic effects of the CuInS2-QDs/CdS/ZnS heterostructure, can potentially outperform those of other QDSSCs.

Zinc-doping in TiO2 films to enhance electron transport in dye-sensitized solar cells under low-intensity illumination

A nanocrystalline TiO(2) film with highly dispersed Zn-doping shows its capability for efficient electron transport in dye-sensitized solar cells (DSSCs). The Zn-doping is conducted via Zn(2+) introduction into a layered titanate followed by hydrothermal treatment and calcination. The Zn-doped films exhibit an elevated electron Fermi level, which may enhance band bending to lower the density of empty trap states. Because of this Zn-doping, the consequent DSSCs can alleviate the decay of light-to-electric energy conversion efficiency due to light intensity reduction. Intensity-modulated spectroscopic analysis reveals that enhanced transport of photogenerated electrons as a result of the trap density minimization is responsible for the high cell performance under low-intensity illumination. A Zn-doping content of ca. 0.4 at% Zn/Ti can enhance the light conversion efficiency by 23% at a solar light intensity as low as 11 mW cm(-2). This technique can significantly extend the indoor application of DSSCs.

Preparation of porous carbons from phenol–formaldehyde resins with chemical and physical activation

Porous carbons with high porosities were prepared from phenol–formaldehyde resins using chemical and physical activation methods. The resin precursor employed was synthesized with an initial formaldehyde-to-phenol ratio of 1.33 by mole. The chemical activation process consisted of KOH impregnation followed by carbonization in nitrogen at 500–900°C for 0–3 h, whereas the physical activation was performed by carbonizing the resins at 900°C followed by gasifying the char in CO2 to different degrees of burn-off. Both activation methods can produce carbons with surface areas and pore volumes greater than 2000 m2/g and 1.0 cm3/g, respectively. The influence of different parameters during chemical activation, such as carbonization temperature and time, KOH/resin ratio and heating rate, on the carbon yield and the surface characteristics was explored, and the optimum preparation conditions were determined. With physical activation the resulting carbons are mainly microporous and their porosity is an increasing function of the degree of burn-off. At similar porosity levels the carbon yield during physical activation was found to be lower than that with chemical activation. An SEM study showed that carbons produced from CO2 activation have a more compact surface than those produced from KOH activation.

Solution synthesis of high-quality CuInS2 quantum dots as sensitizers for TiO2 photoelectrodes

This study reports the solvothermal synthesis of colloidal CuInS2 quantum dots (QDs) for use as sensitizers for photoelectrochemical cells. The synthesis is conducted in an autoclave containing CuCl, InCl3, and S at a Cu/In/S ratio of 1/1/100. This high sulfur-excess environment leads to burst nucleation of CuInS2 at relatively low temperatures. For synthesis conducted at 110–150 °C for 1 h, the atomic ratio of the CuInS2 products is Cu : In : S = 1.1 : 1.0 : 2.1 and the particle size increases with the temperature from 3.5 to 4.3 nm, with a narrow size distribution within 7–11%. The as-prepared colloidal CuInS2 exhibits the quantum confinement effect in the optical absorption spectra. The photoluminescence emission of the resulting CuInS2 QDs has high energy, which may result from excited electrons falling from quantized levels to the ground states. Under illumination of simulated AM 1.5 G at one sun intensity, the CuInS2-sensitized TiO2 electrodes in aqueous sulfide/sulfite electrolyte show light-to-chemical energy conversion efficiencies of 1.9% at a +0.23 V bias and 1.2% at short-circuit. These encouraging conversion efficiencies are attributed to the high energy state of the photoexcited electrons in the CuInS2 QDs.

Performance of electric double-layer capacitors using carbons prepared from phenol–formaldehyde resins by KOH etching

Phenol–formaldehyde resins were used to produce carbons with different porosities by KOH etching. Through the introduction of a binder, carbon-binder composites were prepared to serve as electrodes in electric double-layer capacitors. Nitrogen adsorption was used to characterize the porous structure of the carbons and electrodes. The performance of the capacitors in 1 M H2SO4 was investigated with charge–discharge cycling experiments. The faradaic leakage current and the specific capacitance of the electrodes were found to increase with the specific surface area, while the resistance determined from the IR drop showed an opposite trend. The increased electrode resistance, which would probably result from the increased diffusion path in the pores upon etching, has caused a decrease in the capacitance per unit carbon area. A specific capacitance larger than 100 F/g was achieved with an electrode consisting of 80% carbon particles with a specific surface area of 1900 m2/g and 20% polyvinylidenefluoride as the binder.

Photoactive p-type PbS as a counter electrode for quantum dot-sensitized solar cells

A photoactive PbS film synthesized by successive cycles of coating with ionic solutions and reaction can function as a performance-promoting counter electrode for quantum dot-sensitized solar cells (QDSSCs). The PbS film has a wide absorption spectrum that extends to the near infra-red region, making it capable of absorbing the long-wavelength light that penetrates the photoanode of a QDSSC. Under simulated one-sun illumination, this PbS film exhibits a p-type photovoltaic response in a polysulfide electrolyte, showing a quasi-Fermi level shift of +0.25 V. For QDSSCs consisting of a TiO2/CuInS2/CdS/ZnS photoanode and a polysulfide electrolyte, the PbS film outperforms Pt and CuS films as a counter electrode even though CuS has a much higher electrocatalytic activity in the polysulfide electrolyte than PbS. The photoactive characteristics of the PbS electrode increase the photocurrent of the resulting QDSSC. The p-type conductivity of the PbS forms a partial tandem junction between the PbS and the anode, increasing the photovoltage and the fill factor. Under one-sun illumination, a QDSSC assembled with the photoactive p-type PbS counter electrode achieves a maximum power conversion efficiency of 4.7%, which is more than 15% greater than that of a cell assembled with the highly electrocatalytically active CuS.

Nitrogen-containing carbons from phenol-formaldehyde resins and their catalytic activity in NO reduction with NH3

Abstract Porous carbons with controlled nitrogen contents were prepared from phenol–formaldehyde resins impregnated with different amounts of m-phenylenediamine. The chemical compositions of the resin precursors and the resulting carbons were characterized. Comparison of results from X-ray photoelectron spectroscopic and elemental analysis showed that the nitrogen functional groups in the carbons are more numerous in the internal part and are mainly of the pyridine type. The catalytic activity of the carbons in NO reduction with NH3 increased upon nitrogen impregnation. The activity showed a clear correlation with the nitrogen content obtained using X-ray photoelectron spectroscopy, indicating that the reaction mainly occurred at the external part of the carbon particles. Under a low temperature regime (