Yunzhen Chang

High performance of Pt-free dye-sensitized solar cells based on two-step electropolymerized polyaniline counter electrodes

A two-step cyclic voltammetry (CV) approach is employed in a quick and controllable electropolymerization of polyaniline (PANI) nanofibers with a short-branched structure onto fluorinated tin oxide (FTO) glass substrates as counter electrodes (CEs) for Pt-free dye-sensitized solar cells (DSSCs). In the two-step CV method, a small quantity of PANI as a function of the crystal nucleus in the crystal growth, is pre-electropolymerized under a suitably high potential for one cycle at the first-step, then subjected to the second-step for PANI electropolymerization at a low potential for a small number of scans. The well-controlled PANI nanofibers with high performance can be electropolymerized quickly using the two-step mode. The extensive CVs demonstrate the two-step PANI CE has superior electrocatalytic activity for the I3− reduction. Moreover, electrochemical impedance spectroscopy shows that the two-step PANI CE has a lower series resistance and charge-transfer resistance than the PANI CE prepared by conventional one-step CV electropolymerization. Therefore, the DSSC assembled with the two-step PANI CE exhibits an enhanced photovoltaic conversion efficiency of 6.21% (compared to 5.01% for the DSSC with the one-step PANI CE), up to ∼97% of the level of the DSSC using Pt CE. As the result, the two-step CV electropolymerized PANI CE can be considered as a promising alternative CE for Pt-free DSSCs.

Preparation of high performance perovskite-sensitized nanoporous titanium dioxide photoanodes by in situ method for use in perovskite solar cells

Perovskite-sensitized nanoporous TiO2 photoanode of several micrometers thickness was prepared for the first time by an in situ technique. The excellent contact between the TiO2 and perovskite could be beneficial to the separation and transmission of the electron and hole while the sufficient filling and abundant CH3NH3PbI3 could enhance the absorption and utilization of sunlight to finally obtain an efficient perovskite solar cell. Note that 10.03% of cell efficiency was achieved without adding hole transporting material into the device.

A simple route to prepare a Cu2O–CuO–GN nanohybrid for high-performance electrode materials

Cu2O–CuO–GN nanohybrid is prepared by a simple electrochemical method by applying a positive and negative pulse electric signal on a dispersion of a mixture of GO and Cu(NO3)2. During the process, reduction of graphene oxide and deposition of Cu2O–CuO on GN occur simultaneously. The nanohybrid is systematically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cu2O–CuO nanoparticles with diameters around 100 nm are uniformly distributed on the GN. In addition, the electrochemical properties of the Cu2O–CuO–GN nanohybrid are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (DC) and electrochemical impedance spectrometry measurements (EIS). The Cu2O–CuO–GN nanohybrid shows a higher specific capacitance (222 F g−1 at 1 A g−1) than that of pure GN (143 F g−1 at 1 A g−1) prepared under the same conditions. This approach opens up the possibility for fabrication of high-performance GN-based electrode materials.

A comparative study of graphene oxide reduction in vapor and liquid phases

Abstract Graphene oxide (GO) was reduced by formaldehyde or formic acid in vapor or liquid phases below 200 °C. The influence of the concentrations of reducing agents, reaction temperature and time on the electrical conductivity of the reduced graphene oxide (rGO) was investigated. The rGOs were characterized by XRD, XPS and Raman spectroscopy. Results show that the optimum reaction temperatures are 150 and 175 °C in the vapor phase and the liquid phase, respectively, based on the electrical conductivities of the rGOs. The ratio of the areas of the C1s peaks related to the C-C and C-O (Rcc/co) from XPS decreases with reaction time from 9 to 24 h in the vapor phase, and increases from 2 to 24 h in the liquid phase, which are in agreement with the electrical conductivities and Raman results. Gasification of carbon atoms in GO sheets may be responsible for the decrease of Rcc/co with prolonged reaction time in the vapor phase.

Stabilities of flexible electrochemical capacitors based on polypyrrole/carbon fibers in different gel electrolytes

Flexible electrochemical capacitors (ECs) are vitally important as the emerging flexible electronics. We have prepared polypyrrole doped with counter ion of Cl− on carbon fibers (PPy-Cl/CFs) via the simple electrochemical deposition method. The flexible ECs based on PPy-Cl/CFs electrodes have been assembled by using H3PO4/poly(vinyl alcohol) (PVA), H2SO4/PVA, LiClO4/PVA, KCl/PVA and LiCl/PVA as gel electrolytes. The capacitive properties of ECs have been systematically evaluated with electrochemical methods. The results show that the cells with H3PO4/PVA electrolyte exhibit good cyclic stability (91.5% retention after 15000 cycles) although PPy-Cl/CFs has moderate specific capacitance in H3PO4/PVA (41.6 mF·cm−1 or 52.0 F·cm−3). However, the ECs in other electrolytes have poor cyclic stability. Further electrochemical analysis reveals that the doping/dedoping processes of PPy-Cl/CFs are different in the five electrolytes, and X-ray photoelectron spectra demonstrate that the ratios of counter anions in PPy’s oxidized states to those in reduced states are obviously different when PPy-Cl/CFs are kept in reduced or oxidized states in the five electrolytes. The results also illustrate why the capacitors using H3PO4/PVA as electrolyte exhibit good cyclic stability. Furthermore, a light emitting diode (LED) with a threshold voltage of 2.5 V can be lighted by three ECs connected in series.