(254ax) Electrochemical Behavior of Ni2bpy(TPTZ)2 Complex on Carbon Paste Electrode Modified with Nanocrystalline ZnO Powder for Usability in Dye-Sensitized Solar Cells

Recently, nanocrystalline ZnO powder has been used as promising alternative photoanode material in dye-sensitized solar cells DSSCs due to suitable band gap energy, electron affinity, high electron mobility and stability against photocorrosion^1,2. The most efficient DSSCs demonstrated to date have all been based on Ru(II) bipyridyl (Bpy) complex dyes as sensitizers in which metal-ligand charge transfer takes place at a much faster rate than the back recombination reaction^3,4. Alternative cost-effective transition metal bipyridyl complexes have been developed as sensitizer for photoanodes as well as redox mediator instead of almost exclusively employed iodide/ triiodide redox couple⁵.

In this study, chemical structure of Ni(II) complex with the mixed N-donor ligands 2,2â??-bipyridyl-1,3,5-tripyridyl-striazine (TPTZ) and Bpy was electrochemically determined using different metal/ligand ratios. The square-wave voltammetry (SWV) results on glassy carbon electrode (GCE) confirmed that Ni₂Bpy(TPTZ)₂complex was stable at pH 7.8 in phosphate buffer. The electrochemical behavior of the complex was investigated using a conventional three-electrode system, consisting of carbon paste (CPE) and ZnO modified carbon paste (ZnO/CPE) working electrodes to understand its possible use as a redox mediator and/or sensitizer in DSSCs.

The mechanisms of the electrode processes were elucidated by using SWV and cyclic voltammetry (CV) techniques. The number of electrons transferred in the electrode processes (n) and transfer coefficients (α) was determined from the effect of frequency on the SW voltammograms. Two reduction peaks were observed on the SWVs at both electrode surfaces whereas a quasi-reversible redox couple appeared corresponding to anodic oxidation of central Ni(II) ion on ZnO powders. The scan rate dependence of the peak currents and potentials showed that electrode process was adsorption controlled. The rate constant for oxidation process was found to be much faster than the reduction one suggesting that electron injection from the excited complex to the ZnO nanoparticles may also be greater than the recombination with the oxidized complex in DSSCs.

It can be concluded that the complex can be employed as a photosensitizer for ZnO powder photoanode as well as the redox mediator in DSSCs used CPE cathode.

REFERENCES

1. Seema Rani, Poonam Suri, P.K. Shishodia, R.M. Mehra, Synthesis of nanocrystalline ZnO powder via solâ??gel route for dye-sensitized solar cells. Solar Energy Materials & Solar Cells 92 (2008) 1639â??1645.
2. Lanlan Lu, Renjie Li, Ke Fan, Tianyou Peng. Effects of annealing conditions on the photoelectrochemical properties of dye-sensitized solar cells made with ZnO nanoparticles. Solar Energy 84 (2010) 844â??853.
3. Seigo Ito (2011). Investigation of Dyes for Dye-Sensitized Solar Cells: Ruthenium-Complex Dyes, Metal-Free Dyes, Metal-Complex Porphyrin Dyes and Natural Dyes, Solar Cells - Dye-Sensitized Devices, Prof. Leonid A. Kosyachenko (Ed.), ISBN: 978-953-307-735-2, InTech, Available from: http://www.intechopen.com/books/solar-cells-dye-sensitized-devices/investigation-of-dyes-for-dye-sensitizedsolar-cells-ruthenium-complex-dyes-metal-free-dyes-metal-co.
4. Jooyoung Chung, June young Lee, Sangwoo Lim. Annealing effects of ZnO nanorods on dye-sensitized solar cell efficiency, Physica B 405 (2010) 2593â??2598.
5. Joel T. Kirner, C. Michael Elliott Are High-Potential Cobalt Tris(bipyridyl) Complexes Sufficiently Stable to Be Efficient Mediators in Dye-Sensitized Solar Cells? Synthesis, Characterization, and Stability Tests, J. Phys. Chem. C 2015, 119, 17502â??17514.