Huajun Tian

Enhanced photovoltaic performance of dye-sensitized solar cells using a highly crystallized mesoporous TiO2 electrode modified by boron doping

Highly crystallized boron-doped anatase TiO2 nanoparticles are prepared by a facile synthetic route and successfully used as the photoanode of dye-sensitized solar cells (DSCs). We have observed that the boron doping could improve the crystallinity of TiO2. Moreover, the highly crystallized anatase boron-doped TiO2 were analyzed by electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy, and the internal resistances of the boron-doped DSCs were studied by measuring the electrochemical impedance spectra (EIS). The improved photocurrent density of the boron-doped DSCs is due to a significant enhancement of IPCE in the range 370–650nm in comparison with that of the undoped DSC. Meanwhile, the energy-conversion efficiency of the cell based on the B-doped TiO2 electrode is enhanced significantly, by about 9%, compared to that of the undoped DSC. Overall, DSCs based on B-doped electrodes show good stability and remain over 95% of their initial efficiency under visible light soaking for more than 2400 h.

Characteristics of dye-sensitized solar cells based on the TiO2 nanotube/nanoparticle composite electrodes

The influence of TiO2 nanotubes on the charge collection efficiency and the dynamics of electron transport and recombination in dye-sensitized solar cells (DSCs) based on the TiO2 nanotube/nanoparticle composite films were investigated in this paper. Electrochemical impedance spectroscopy was employed to quantify the charge transfer resistance of DSCs. The different transport and recombination properties of DSCs were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. It was shown that the electrons had a longer lifetime in the nanotubes and retarded recombination more significantly than in the nanoparticles. But with increasing the amount of nanotube, the electron pathway was extended seriously in these network structures resulting in increased recombination chances. In addition, the 5 wt% nanotube DSC had the highest charge collection efficiency among all the DSCs, which yielded a high photovoltaic conversion efficiency of 9.79% under simulated AM 1.5 sunlight (100 mW cm−2).

High power rechargeable magnesium/iodine battery chemistry

Rechargeable magnesium batteries have attracted considerable attention because of their potential high energy density and low cost. However, their development has been severely hindered because of the lack of appropriate cathode materials. Here we report a rechargeable magnesium/iodine battery, in which the soluble iodine reacts with Mg2+ to form a soluble intermediate and then an insoluble final product magnesium iodide. The liquid–solid two-phase reaction pathway circumvents solid-state Mg2+ diffusion and ensures a large interfacial reaction area, leading to fast reaction kinetics and high reaction reversibility. As a result, the rechargeable magnesium/iodine battery shows a better rate capability (180 mAh g−1 at 0.5 C and 140 mAh g−1 at 1 C) and a higher energy density (∼400 Wh kg−1) than all other reported rechargeable magnesium batteries using intercalation cathodes. This study demonstrates that the liquid–solid two-phase reaction mechanism is promising in addressing the kinetic limitation of rechargeable magnesium batteries.

A facile synthesis of anatase N,B codoped TiO2 anodes for improved-performance dye-sensitized solar cells

A facile process to fabricate highly crystalline mesoporous N,B codoped TiO2 is introduced. The N,B codoped TiO2 nanomaterial, which has controlled crystallographic phase, size and unique nanoshape, is successfully applied to an enhanced device for dye-sensitized solar cells (DSCs) with an open-circuit photovoltage of 0.823 V and yielding a high overall energy conversion of 8.4%. A DSC based on the N,B codoped TiO2 electrode shows a competitive photovoltaic performance compared with the undoped DSC, which is attributed to the ideal combination of superior energy band structure and retarded electron recombination from the unique N,B codoped TiO2 particle structure.

Superior energy band structure and retarded charge recombination for Anatase N, B codoped nano-crystalline TiO2 anodes in dye-sensitized solar cells

This work reports the preparation of N, B codoped TiO2 (N, B–TiO2) electrodes in dye-sensitized solar cells (DSCs) by a facial modified sol–gel method. After the nitrogen and boron dopants incorporated into the TiO2 electrodes, the cubic-like TiO2 nanocrystallites with diameters of 22∼24 nm were obtained efficiently. The back electron transfer of the DSC based on the N, B–TiO2 electrode was studied by measuring the electrochemistry impedance spectra (EIS) and the EIS for the DSCs showed that the enhanced electron lifetime for the dye-sensitized B, N–TiO2 solar cells could be attributed to the formation of an O–Ti–B–N bond in the TiO2 photoelectrode, which retards electron recombination at the dyed N, B–TiO2 photoelectrode/electrolyte interface after N, B codoping as compared to the undoped DSC. We found that a high efficiency of 8.4% for the DSC (active area:4 cm2) based on the N, B–TiO2 anode under 0.2 sun illumination was received. In particular, the photovoltaic performance of the DSC under high temperature conditions (60 °C) and one-sun light soaking over a time of more than 1100 h showed that the DSC based on the N, B–TiO2 electrode exhibited a better stability compared to the undoped DSC. The excellent photoelectrochemical performance could be attributed to the ideal combination of retarded electron recombination and superior energy band structure from the unique N, B–TiO2 particle structure.

A lithiation/delithiation mechanism of monodispersed MSn5 (M = Fe, Co and FeCo) nanospheres

A designed Sn based alloy host as a higher capacity and longer cycle life next generation lithium-ion battery, consisting of monodisperse nanospheres of intermetallic MSn5 (M = Fe, Co and FeCo) phases was synthesized by a nanocrystal conversion chemistry method using preformed Sn nanospheres as templates. The same crystal structure, identical particle surface morphology and the similar particle size distribution (30-50 nm) of these intermetallic MSn5 (M = Fe, Co and FeCo) phases are ideal for comparison of the electrochemical performance, reaction mechanism, thermodynamics and kinetics during lithiation/delithiation. Importantly, MSn5 (M = Fe, Co and FeCo) phases with defect structures Fe0.74Sn5, Co0.83Sn5 and Fe0.35Co0.35Sn5, exhibit the highest theoretical capacity of >917 mA h g(-1) among the reported M-Sn (M is electro-chemically inactive) based intermetallic anodes. The ex situ XRD and XAFS illustrate the complete reversibility of MSn5 (M = Fe, Co and FeCo) phases during lithium insertion/extraction for the first cycle. The Fe0.35Co0.35Sn5 anode can take advantage of both high capacity of Fe0.74Sn5 and long cycle life of Co0.83Sn5, providing 736 mA h g(-1) and maintaining 92.7% of initial capacity after 100 cycles with an average capacity loss of only 0.07% per cycle. The excellent electrochemical performance of the Fe0.5Co0.5Sn5 system is attributed to higher reversibility, lower reaction resistance. This work provides a novel insight toward designing and exploring an optimal Sn based alloy anode for next generation Li-ion batteries.

Scalable fabrication of micro-sized bulk porous Si from Fe–Si alloy as a high performance anode for lithium-ion batteries

Silicon has been perceived as one of the most promising anodes in the next generation lithium-ion batteries (LIBs) due to its superior theoretical capacity. However, bulk silicon experiences an enormous volume expansion during the lithiation/delithiation process, resulting in rapid capacity fading. And, its high-cost and low coulombic efficiency also present significant challenges for applications. Here, we presented a facile and large-scale approach for preparing micro-sized porous silicon by acid etching the abundant and inexpensive metallurgical Fe–Si alloy as a high-performance anode in LIBs. Profiting from the unique micro-sized structure, it exhibited a fantastic first-cycle coulombic efficiency of 88.1% and an excellent reversible capacity of 1250 mA h g−1 at 500 mA g−1 after 100 cycles. Furthermore, the micro-sized porous silicon without carbon coating could deliver a reversible capacity of 558 mA h g−1 at a high current density of 5 A g−1 due to the unique porous structure. This work provides a promising route for a large-scale production of high-performance micro-sized Si as anode materials in LIBs.

Enhanced Electrochemical Performance of Fe0.74Sn5@Reduced Graphene Oxide Nanocomposite Anodes for Both Li-Ion and Na-Ion Batteries.

The recently found intermetallic FeSn5 phase with defect structure Fe0.74Sn5 has shown promise as a high capacity anode for lithium-ion batteries (LIBs). The theoretical capacity is as high as 929 mAh g(-1) thanks to the high Sn/Fe ratio. However, despite being an alloy, the cycle life remains a great challenge. Here, by combining Fe0.74Sn5 nanospheres with reduced graphene oxide (RGO) nanosheets, the Fe0.74Sn5@RGO nanocomposite can achieve capacity retention 3 times that of the nanospheres alone, after 100 charge/discharge cycles. Moreover, the nanocomposite also displays its versatility as a high-capacity anode in sodium-ion batteries (SIBs). The enhanced cell performance in both battery systems indicates that the Fe0.74Sn5@RGO nanocomposite can be a potential anode candidate for the application of Li-ion and Na-ion battery.

Multiple adsorption of tributyl phosphate molecule at the dyed-TiO2/electrolyte interface to suppress the charge recombination in dye-sensitized solar cell

Electron recombination and dye aggregation at the dyed-TiO2/electrolyte interface are still problems in dye-sensitized solar cell (DSC) research. In this paper, tributyl phosphate (TBpp) as a special additive to modify the dyed-TiO2/electrolyte interface was introduced to enhance the photovoltaic performance. The adsorption mode of TBpp and the interaction between cis-dithiocyanate-N,N′-bis-(4-carboxylate-4-tetrabutylammonium carboxylate-2,2′-bi-pyridine) ruthenium(II) (N719) and TBpp were investigated. It was found that one TBpp parent molecule split into several smaller fragments and formed four anchoring modes on the TiO2 surface. It was very interesting that the molecular cleavage of TBpp and adsorption of N719 assisted each other on the sensitized TiO2 surface. The fragments distributed around N719 result in steric hindrance, consequently hydrogen-bonding among N719 molecules was decreased. The unstable type N719 transformed into stable type N719 accompanied by molecular cleavage of TBpp and the N719 aggregation was reduced. Furthermore, these new fragments were multiply adsorbed on the non-sensitized TiO2 surface to form an insulating barrier layer. Therefore, the electron recombination at the dyed-TiO2/electrolyte interface was restrained. Besides the change of surface configuration, the TiO2 band edge negatively shifted and the rate of electron transport in the TiO2 films decreased with the addition of TBpp. As a result, an increase in the photoelectric conversion efficiency (η) was obtained of almost 40%.

High lithium electroactivity of boron-doped hierarchical rutile submicrosphere TiO2

We have reported a facile method to fabricate hierarchical boron-doped rutile submicrosphere TiO2 (SMT), whose primary particles are ∼20 nm in diameter. The as-synthesized boron-doped SMT shows excellent cycling performance and rate capability in comparison with undoped TiO2 as an anode material in Lithium-Ion Batteries (LIBs). It has a very stable capacity of ∼190 mA h g−1 for 500 cycles at 1C. In addition, the density functional theory (DFT) calculations are carried out to indicate that a low concentration (<1.0 at%) of boron doping could enhance the carrier mobility μ and electrical conductivity σ, and thus reveal the relationship between the electronic structure of boron-doped SMT and the performances of the boron-doped SMT anode in LIBs. Our results also clearly demonstrate the importance and advantage of the hierarchical submicrometer-sized spherical morphology of the TiO2 anode in LIBs.