Electrocatalytic property of carbon nanostructures and its effects on counter electrode for Dye Sensitized Solar Cells

Abstract

Due to the mass consumption and exhaustion of unsustainable energy, such as oil, coal and natural gas, human beings are paying much attention to sustainable energy, such as wind, hydropower and solar energy. As far as solar energy is concerned, silicon based solar cells dominate up to 80% of the photovoltaic market. But dye-sensitized solar cells (DSCs) have emerged as a viable alternative to solid-state silicon solar cell because of its low-cost, non-vacuum and high energy conversion efficiency. Commonly, fluorine doped tin oxide (FTO) coated glass is used as a substrate and Platinum (Pt) is used as a counter electrode (CE) catalyst for tri-iodide reduction in standard DSCs. Despite the good catalytic activity of Pt-CE in DSCs, the high cost, poor stability in corrosive electrolytes, and high processing temperatures necessitate the development of alternative CE materials. FTO glass substrates have several limitations, such as high cost, high sheet resistance and brittleness, which make them nonviable for flexible large scale DSCs. To develop large-scale applications and wider applications, abundant-materials is preferred. In the recent past, various carbon materials have been described for stable CE and conducting catalyst. To elucidate its advantages

electron transfer kinetics and, device properties were analysed systematically. Here, we used several carbon materials, such as sub-micron size graphite (CG), Micron size graphite (AG20), carbon-nanofiber (CNF), and nano-carbon for effective tri-iodide reduction in DSCs. In addition, CNF and nanocarbon were studied as a low-temperature flexible counter electrode catalyst for DSCs with the stable quasi-solid state electrolyte.In chapter 1 and 2, theoretical background of alternative energy source, different varieties of photovoltaic’s, working principles of various solar cells, device fabrication, characterization, and materials used are reviewed. Then, objective of the research was also explained and analyzed with the other reported works. Catalytic mechanism in DSCs has been mainly focused and their respective mechanism was discussed.In chapter 3, submicron-size colloidal graphite (CG) was tested as a conducting electrode to replace transparent conducting oxide (TCO) electrodes and as a catalytic material to replace Pt for tri-iodide (I3–) reduction in DSCs. CG paste was used to make a film via the doctor-blade process. The 9 micron thick CG film showed a comparable resistivity with the widely used fluorine-doped tin oxide TCO (8-15W.□–1). The catalytic activity of this graphite film was measured and compared with the corresponding properties of Pt. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies clearly showed a decrease in the charge transfer resistance with the increase in the thickness of the graphite layer from 3 to 9 microns. Under 1 sun illumination (100 mW cm–2, AM 1.5), DSCs with submicron-size graphite as a catalyst on fluorine-doped tin oxide TCO showed an energy conversion efficiency greater than 6.0%, comparable to the conversion efficiency of Pt. DSCs with a graphite counter electrode (CE) on TCO-free bare glass showed an energy conversion efficiency greater than 5.0%, which demonstrated that the graphite layer could be used both as a conducting layer and as a catalytic layer.In chapter 4, carbon-nanofibers (CNFs) with antler and herringbone structures are studied as an I3– reduction electrocatalyst in combination with the liquid electrolyte or an alternative stable quasi-solid state electrolyte. The catalytic properties of the counter electrode (CE) were characterized by CV and EIS. The doctor bladed low temperature CNFs-CE has faster I3– reduction rate and promising charge transfer resistance (Rct) of ~29.8 W with Pt (~6.5 W) due to the nanofiber stacking morphology. Herringbone and antler structures with opened graphitic layers lead to defect rich edge planes and larger diameter of CNFs facilitate the electron transfer kinetics. The cells with CNF counter electrodes are showing promising energy conversion efficiency greater than 7.0 % for the glass based devices and 5.0 % for the flexible cells filled with the quasi-solid state electrolyte, which is similar to Pt performance. Application of CNFs-CE for flexible and quasi-solid state electrolyte increases the possibility of roll to roll process, low cost and stable DSCs.In chapter 5, Dye-sensitized solar cells (DSCs) were fabricated with carbon counter electrode deposited at low temperature. Carbon powders with an average particle size of 30-50nm were ultrasonically dispersed in ethanol and the solution was used for spray coating onto FTO glass substrate and also onto transparent conducting oxide coated plastic substrate at 100C. Thickness of the carbon electrode was controlled by varying the spray deposition time from 10 s to 420 s. Catalytic property of carbon material was measured by CV and EIS. The Rct of carbon electrode deposited for 420 s was found out to be ~67.5  in iodide/tri-iodide redox electrolyte which was little higher than Pt electrode (~6.5). Current-voltage characteristic of the DSCs was measured using a solar simulator under one sun illumination (100 mW.cm–2, AM 1.5). DSCs with spray coated carbon counter electrode on glass and plastic showed energy conversion efficiency of more than 6.0%, which is comparable with the Pt counter electrode. This optimized condition was also successfully applied with the quasi-solid state electrolyte, which also illustrates promising performance. In chapter 6, three different carbon nanomaterials were compared and scrutinized for nobel metal free counter electrodes for DSCs. Three carbons are 0-D nano carbon, 1-D carbon nanofiber (CNF), and 2-D graphite. All the carbon counter electrode catalytic properties were measured from electrochemical impedance spectroscopy, cyclic voltammetry, conductivity, defective sites and scanning electron microscopy. Graphite showed very high conductivity and moderate catalytic activity which solves the TCO and Pt free counter electrode issues. CNF electrode showed amazing catalytic activity and promising conductivity due to its 1-D structure, defect rich edge planes and dense film with high surface area. The 1-D structure will be more preferable for catalytic material in DSCs

it has high defect rich edge planes. Since with the 1-D structure electron transport will be much faster and electron trapping will not be a problem.