Timo Grothe

Dye-Sensitized Solar Cells with Electrospun Nanofiber Mat-Based Counter Electrodes

Textile-based dye-sensitized solar cells (DSSCs) can be created by building the necessary layers on a textile fabric or around fibers which are afterwards used to prepare a textile layer, typically by weaving. Another approach is using electrospun nanofiber mats as one or more layers. In this work, electrospun polyacrylonitrile (PAN) nanofiber mats coated by a conductive polymer poly(3,4-ethylenedioxythiopene) polystyrene sulfonate (PEDOT:PSS) were used to produce the counter electrodes for half-textile DSSCs. The obtained efficiencies were comparable with the efficiencies of pure glass-based DSSCs and significantly higher than the efficiencies of DSSCs with cotton based counter electrodes. The efficiency could be further increased by increasing the number of PEDOT:PSS layers on the counter electrode. Additionally, the effect of the post treatment of the conductive layers by HCl, acetic acid, or dimethyl sulfoxide (DMSO) on the DSSC efficiencies was investigated. Only the treatment by HCl resulted in a slight improvement of the energy-conversion efficiency.

Electrospinning and stabilization of chitosan nanofiber mats

Chitosan is of special interest for biotechnological and medical applications due to its antibacterial, antifungal and other intrinsic physical and chemical properties. The biopolymer can, e.g., be used for biotechnological purposes, as a filter medium, in medical products, etc. In all these applications, the inner surface should be maximized to increase the contact area with the filtered medium etc. and thus the chitosans efficacy. Chitosan dissolves in acidic solutions, opposite to neutral water. Electrospinning is possible, e.g., by co-spinning with PEO (poly(ethylene oxide)). Tests with different chitosan:PEO ratios revealed that higher PEO fractions resulted in better spinnability and more regular fibre mats, but make stabilization of the fibre structure more challenging.

Functional Nanofiber Mats for Medical and Biotechnological Applications

Nanofiber mats from different polymers, possibly blended with other organic or inorganic components, can be created by the electrospinning technology. Such nanofiber mats possess large surface–volume ratios in comparison with other textile fabrics, such as common nonwovens. This property enables enhanced interactions with their environment, making them suitable for applications in wound dressing, drug delivery, biotechnological filter technology, etc., especially if prepared from biopolymers with intrinsically antimicrobial or other qualities. In a recent project, we investigate the possibilities to create such nanofiber mats from diverse (bio-) polymers by “green” electrospinning, i.e., electrospinning from aqueous solutions or other nontoxic solvents. The article gives an overview of the latest results from needleless electrospinning pure polymers and polymer blends as well as typical physical and chemical properties of the created nanofiber mats, especially for medical and biotechnological purposes, and shows diverse possibilities to crosslink water-soluble biopolymer nanofiber mats.

Electrospraying poloxamer/(bio-)polymer blends using a needleless electrospinning machine:

Electrospinning can be used to create nanofiber mats with high material purity and a large inner surface, applicable for medical or biotechnological filters, cell growth, and so on. Not each polymer which can be dissolved, however, can be spun in this way. Depending on the material, solution, and spinning parameters, especially on the molecular weight and the polymer concentration in the solution, either fibers or droplets can be formed. Both fibers and droplets are of technological interest for different applications. This article examines the possibilities of electrospinning or electrospraying poloxamer 188 (formerly known as Lutrol F 68, BASF, Germany) in combination with different biopolymers (dextran, gelatin, and agarose) as well as polyacrylonitrile (PAN) which belongs to the polymers spinnable from nontoxic solvents. Due to the similarity of the chemical structures of poloxamer and poly(ethylene glycol), a well-known spinning agent, and the relatively low molecular weight of poloxamer 188, it can be expected to work as a spraying agent. Our results show that electrospraying poloxamer/biopolymer blends is indeed possible, with the unexpected effect that for higher polymer concentrations, combining poloxamer 188 with dextran, fibers are formed additionally on parts of the substrate. Co-spinning poloxamer 188 with PAN, on the other hand, results in creation of a nanofiber mat whose morphology is mainly defined by the PAN content. The study shows that poloxamer can indeed be used as a spraying/spinning agent in electrospraying/electrospinning diverse biopolymers.

Stabilization of Electrospun PAN/Gelatin Nanofiber Mats for Carbonization

Due to their electrical and mechanical properties, carbon nanofibers are of large interest for diverse applications, from batteries to solar cells to filters. They can be produced by electrospinning polyacrylonitrile (PAN), stabilizing the gained nanofiber mats, and afterwards, carbonizing them in inert gas. The electrospun base material and the stabilization process are crucial for the results of the carbonization process, defining the whole fiber morphology. While blending PAN with gelatin to gain highly porous nanofibers has been reported a few times in the literature, no attempts have been made yet to stabilize and carbonize these fibers. This paper reports on the first tests of stabilizing PAN/gelatin nanofibers, depicting the impact of different stabilization temperatures and heating rates on the chemical properties as well as the morphologies of the resulting nanofiber mats. Similar to stabilization of pure PAN, a stabilization temperature of 280°C seems suitable, while the heating rate does not significantly influence the chemical properties. Compared to stabilization of pure PAN nanofiber mats, approximately doubled heating rates can be used for PAN/gelatin blends without creating undesired conglutinations, making this base material more suitable for industrial processes.

Influence of Salts on the Spinnability of Poly(Ethylene Glycol)

Electrospinning allows producing nanofiber mats from diverse polymers. In “green electrospinning”, aqueous and other non-hazardous solutions are used as spinning solutions, especially for biopolymers. Physical and chemical material properties of the solutions as well as the nanofiber mats can partly be tailored by co-spinning different materials. Especially for smart textile applications, conductive nanofiber mats are of high interest. However, electrospinning from highly conductive solutions is technically impossible. This article thus investigates the influence of different salts on the conductivity of poly(ethylene glycol) solutions and nanofiber mats and gives an estimate for the maximum possible conductivity of an aqueous polymer solution for electrospinning.