T-00139: Carbon Nanofibers as a Low Cost Alternative to Existing Platinum Counter Electrodes

Summary of Technology

The following is an analysis of the technology described in the invention disclosure submitted to the Technology Transfer Office on October 4, 2010. The invention disclosure describes a process invented by Qiquan Qiao, Hao Fong, and Lifeng Zhang that uses carbon nanofibers as an alternative to expensive platinum counter electrode in dye sensitized solar cells (DSSCs).  Dye sensitized solar cells (DSSCs) have been studied as an alternative to conventional solar cells. DSSCs’ simple fabrication process, high energy conversion efficiency, and potentially low cost motivate companies to utilize DSSCs in photovoltaic applications. Current dye sensitized solar cells often contain an expensive platinum counter electrode. As a solution, SDSU researchers have developed a technology that uses electrospun carbon nanofibers as an electrocatalyst and low-cost alternative to platinum for triiodide reduction in DSSCs.

Inventor: Qiquan Qiao

Description of Technology

Photovoltaic devices are based on the concept of charge separation at an interface of two materials of different phase for electron and hole transport. The solar cell market has been dominated by silicon solar cells. However, Gratzel’s work on DSSCs offers a different option in the field of photovoltaics (Gratzel).

In DSSCs, platinum (Pt) is generally used as a counter electrode. However, Pt is a rare metal and expensive. Alternative methods of providing an electrocatalyst and low-cost alternative to platinum for triiodide reduction in DSSCs are being researched. In general, both a titanium dioxide (TiO2) electrode and a counter electrode are formed with an electrolyte filled between the electrodes. A porous network of nanosize TiO2 particles serve as a charge-transport medium on which a monolayer of dye molecules is chemically absorbed. When placed under illumination, electrons are absorbed by the photoexcited dye into the conduction band of the TiO2 and then move through the electrolyte until the electrons reach the counter electrode. The counter electrode is generally made by depositing a thin layer of a platinum catalyst on a fluorine-doped tin oxide conducting glass followed by heat treatment (Lee).

This invention uses electrospun carbon nanofibers (ECN) as a low cost alternative to platinum. Measurements have shown that ECN used as counter electrodes exhibit lower charge transfer resistance, larger capacitance, and faster reaction rates for triiodide reduction over existing platinum electrodes. Further, short circuit current density and open circuit voltage measurements were comparable. One drawback to the invention was a slight loss of energy conversion efficiency. The primary reason for this lower performance was attributed to a series resistance three times larger than that of the Pt based devices. Most importantly, ECN are significantly less expensive than platinum. As a result, this technology could have application where lower production costs are needed.

Market

Nanofibers have a large surface-to-volume ratio which improves performance for many applications. With collaboration with electrospinning technology, nanofibers can be produced with unique physical properties and relatively high production rates. A variety of nanofibers can be produced with varying applications. An article in Materials Today written by Seeram Ramakrishna describes energy storage, healthcare, biotechnology, environmental engineering, and defense and security as areas of science potentially creating a market for nanofiber-based technologies. “Startups such as eSpin Technologies, NanoTechnics, and Kato Tech are just some of the companies seeking to reap the unique advantages offered by electrospinning, while companies such as Donaldson Company and Freudenberg have been using electrospun fibers in their air filtration products for the last two decades” (Ramakrishna). Although these companies are not directly in the business of photovoltaics, they might have interest in an additional technology that uses electrospun nanofibers.

Carbon nanotubes are also being researched for applicability in dye sensitized solar cells. Carbon nanotubes are unique nanoscale objects with the combined advantages of large surface area, high electrical conductivity, and chemical stability. Carbon nanotubes used as an alternative to platinum electrodes are by researchers from the Energy Conversion Devices Research Center at the Korea Electrotechnology Research Institute. An article in ACS Applied Materials & Interfaces written by Won Jae Lee reports successful application of multiwalled carbon nanotubes as counter electrodes in DSSCs. Results show energy conversion efficiency comparable to that of conventional DSSCs using platinum counter electrodes. Although similar in scope, the SDSU invention differs in that carbon nanofibers are used instead of carbon nanotubes.

Sources

• Gratzel, M. (2003). Dye-Sensitized solar cells. Journal of Photochemistry and Photobiology, 4 , 145-153.
• Ramakrishna, S., Fujihara, K., Teo, W., Yong, T., Ma, Z., Ramaseshan, R. (2006). Electrospun nanofibers: solving global issues. Materials Today, 9 (3), 40-50.
• Lee, W.J., Ramasamy, E., Lee, D.Y., Song, J.S. (2009). Efficient dye-sensitized solar cells with catalytic multiwall carbon nanotube counter electrodes. ACS Applied Materials & Interfaces, 1 (6), 1145-1149.