Hongju Wang

A green and scalable route to yield porous carbon sheets from biomass for supercapacitors with high capacity

A green and scalable route is developed to prepare hierarchical porous carbon sheets (HPCS) from biomass (cornstalk) directly under an air atmosphere instead of inert gas protection. In addition, the method adopts non-toxic NaCl and KCl mixed salt as reaction media to regulate the activation process, which avoids the corrosive and poisonous KOH and ZnCl2 reagents. Meanwhile, the porous structures of HPCS can be well-manipulated by the salt templates and the etching effects of the oxygen atom as well as the high energy molten salt media. The as-obtained HPCS consist of an ultra-thin sheet structure (∼4.6 nm in thickness) and abundant hierarchical pores, which facilitates electrolyte diffusion and ion transfer. In addition, the high specific surface area of 1588 m2 g−1 provides sufficient active sites and the rich O doping introduces pseudocapacitance. Therefore, the as-prepared HPCS exhibit a high specific capacitance of 407 F g−1 at 1 A g−1 in a three-electrode system. Moreover, the assembled symmetric supercapacitor in a two-electrode system also exhibits an outstanding specific capacitance of 413 F g−1 at 0.5 A g−1, an excellent rate capacity and a long-term cycling stability with 92.6% of initial capacitance retention after 20 000 cycles at 5 A g−1, which are comparable to those of the porous carbons activated with traditional methods. It is expected that this green and facile technique could serve as an effective and scalable method to prepare optimized HPCS with tunable porous structures from biomass for electrochemical energy storage applications.

Plasmon resonance energy transfer and hot electron injection induced high photocurrent density in liquid junction Ag@Ag2S sensitized solar cells.

An in situ technique was developed to deposit Ag@Ag2S core-shell quantum dots on a SnO2 mesoporous film for solar energy conversion. When adopted as a photoanode, an impressive high photocurrent density of ∼25.6 mA cm-2 was demonstrated in a cell configuration using polysulfide S2-/Sn2- as an electrolyte and Cu2S/brass as a counter electrode, which leads to a power conversion efficiency of ∼0.784% for this environmentally benign device. Optical measurements showed that Ag nanoparticles could be employed as plasmon resonance centers to enhance the harvesting efficiency of incident light at the visible and near-infrared range. Moreover, photoluminescence spectra demonstrated fast charge transfer at Ag@Ag2S/SnO2 interfaces, which facilitates direct hot electron injection from sensitizers to the SnO2 matrix and finally gives rise to the high photocurrent density.

Tunable synthesis of single-crystalline-like TiO2 mesocrystals and their application as effective scattering layer in dye-sensitized solar cells

Single-crystalline-like TiO2 mesocrystals (TMCs) with spherical and spindle-like shapes were selectively prepared in acetic acid system using benzoic acid as structural directing regent. It was found that the intermediate butyl-benzoic acid could interval the oriented assembly of the primary nanoparticles as porogen and shape regulator, which results in spherical TMCs with greater pore size distribution compared with the spindle-like TMCs. When used as scattering layer in dye sensitized solar cells (DSSCs), the spherical TMCs with long-range ordered stacking pattern results in characteristic photonic reflection, which enhances the scattering effect of the photoanode and leads to a high short circuit current density of 16.6 mA cm(-2). Therefore, Cell-spherical TMCs demonstrated a high power conversion efficiencies of 8.10%, indicating substantial improvements compared with Cell-spindle-like TMCs (7.58%) and Cell-nanoparticle (6.59%).

In-situ synthesis of molybdenum sulfide/reduced graphene oxide porous film as robust counter electrode for dye-sensitized solar cells

Molybdenum sulfide/reduced graphene oxide (MoS2/RGO) porous film was in-situ deposited on fluorine-doped tin oxide (FTO) substrates via a one-pot hydrothermal method. Due to the oxygen-containing groups distributing on graphene oxide (GO) surface, the MoS2 sheets could nucleate and grow taking GO as substrates and the MoS2/RGO film can be strongly linked to the FTO. Based on the electrochemical investigations, the enhanced cell performance could be ascribed to the improved electrical conductivity, catalytic active sites and electrolyte diffusion rate, which finally contribute to the high catalytic performance on the reduction of I-/I3- couples in the electrolyte. Therefore, the cell adopting as-prepared MoS2/RGO as counter electrode demonstrated high power conversion efficiencies (PCE) of 7.63%, which indicates ∼14% enhancement compared with the MoS2-based (6.68%) device.

Biomass derived nitrogen-doped hierarchical porous carbon sheets for supercapacitors with high performance

A facile potassium chloride salt-locking technique combined with hydrothermal treatment on precursors was explored to prepare nitrogen-doped hierarchical porous carbon sheets in air from biomass. Benefiting from the effective synthesis strategy, the as-obtained carbon possesses a unique nitrogen-doped thin carbon sheet structure with abundant hierarchical pores and large specific surface areas of 1459 m2 g-1. The doped nitrogen in carbon framework has a positive effect on the electrochemical properties of the electrode material, the thin carbon sheet structure benefits for fast ion transfer, the abundant meso-pores provide convenient channels for rapid charge transportation, large specific surface area and lots of micro-pores guarantee sufficient ion-storage sites. Therefore, applied for supercapacitors, the carbon electrode material exhibits an outstanding specific capacitance of 451 F g-1 at 0.5 A g-1 in a three-electrode system. Moreover, the assembled symmetric supercapacitor based on two identical carbon electrodes also displays high specific capacitance of 309 F g-1 at 0.5 A g-1, excellent rate capacity and remarkable cycling stability with 99.3% of the initial capacitance retention after 10,000 cycles at 5 A-1. The synthesis strategy avoids expensive inert gas protection and the use of corrosive KOH and toxic ZnCl2 activated reagents, representing a promising green route to design advanced carbon electrode materials from biomass for high-capacity supercapacitors.

Ultrasonic-assisted synthesis of two dimensional BiOCl/MoS2 with tunable band gap and fast charge separation for enhanced photocatalytic performance under visible light

Janus shaped BiOCl/MoS2 composites with two dimensional configuration are successfully prepared via a facile pulse ultrasonic assisted method, which spontaneously introduces oxygen vacancies on the BiOCl surface and builds well-defined heterojuction at the BiOCl/MoS2 interfaces. The as-prepared BiOCl/MoS2 composites possess reduced band gap and defect energy levels due to the incorporation of MoS2 and the oxygen vacancies, which permits the enhanced light harvesting efficiency in the visible range. In addition, because of the formed BiS bonds at the BiOCl/MoS2 interface, the composites demonstrate improved charge separation of the photo-generated carriers. Therefore, when used as photocatalyst for Rhodamine B photodegradation, the optimized composite demonstrates a degradation rate of 0.078 min-1, which is much enhanced compared with that of pure BiOCl (0.052 min-1). Mechanism investigation indicates the degradation is a hole mediated process. In addition, the composite shows good stability and outstanding organic carbon removal efficiency, which could serve as a promising photocatalyst for water remediation under visible light.