Electrospun fibers as Catalyst

Sputtering in progress. [Some rights reserved by Engenharia de Superficies, licensed under CC BY 2.0]

High surface area of electrospun nanofibers makes it highly attractive for use in applications where chemical reactions are involved. Unlike nanoparticles, a mesh of fibers is self-supporting and its high porosity enables numerous exposed reaction sites. There are also no issues with agglomeration and the stable structure ensures predictable and consistent reaction rate.

There are a few ways which electrospun fibers may be used to form a catalytic material. First, the electrospun fiber itself may be made of catalytic substance. Inorganic nanofibers are routinely fabricated by sintering of electrospun precursors. Next, electrospun fibers may be loaded with catalytic nanoparticles or substances. In this case, the electrospun fibers act as a carrier. Lastly, the electrospun fibers may function as a template in which catalytic substances are coated over it.

There are numerous catalysts that are made of inorganic material. The ease of using electrospinning to produce inorganic fibers has made it possible to produce catalytic membranes. Platinum is a well known catalyst and electrospinning has been used to fabricate pure Pt wires after sintering its precursor in fibrous form [Shui 2010]. Huang et al (2014) used electrospinning to fabricate La_0.75Sr_0.25MnO₃ nanofibers and tested for oxidation of CO and CH₄. Their study showed that the catalytic activity of the inorganic nanofibers for CO oxidation and CH₄ combustion was better than its nanoparticles form. This has been attributed to the high surface area and porosity of the membrane which offers more reactive surface.

Electrospun membrane may also be used as a carrier for catalytic nanoparticles. Soukup et al (2014) used wet impregnation technique to load palladium and platinum nanoparticles onto poly(2,6-dimethyl-1,4-phenylene) oxide electrospun membranes. While an increased loading of palladium nanoparticles into the nanofibers resulted in an increase in catalytic activity, the catalytic activity of platinum nanoparticles loaded nanofibers were unaffected by the its loading. Larger platinum nanoparticle size was found to increase catalytic activity but the size-catalysis correlation was not found in palladium nanoparticles. To create a palladium (Pd) nanoparticles coated nanofibers, Wang et al (2019) first electrospun a mixture of polyethyleneimine/polycaprolactone (PEI/PCL). The addition of PEI resulted in the formation of pits on the surface of the fiber and helped to bind the hydrophilic Pd nanoparticles on its surface by dipping the nanofibers in a solution containing Pd nanoparticles. Compared with Pd nanoparticles, the Pd nanoparticles coated nanofibers showed better catalytic performance in the reduction of 4-nitrophenol (4-NP) with the presence of NaBH₄. It only took 100s to completely reduce 4-NP to 4-AP by PEI/PCL@PdNPs nanofibers but it took 10 minutes for Pd nanoparticles. The superior catalytic performance has been attributed to the better dispersion of nanoparticles on the nanofibers. The PEI/PCL@PdNPs nanofibers were able to maintain 95% of its catalytic efficiency for 4-NP after 8 cycles of repeated catalytic reduction using fresh solutions. Hong et al (2008) used an in-situ metallization technique to bind platinum onto the surface of electrospun polystyrene (PS) fibers. The method involves sulfonation of PS fibers followed by reaction with Pt salt solution. The Pt ions that were bonded on the PS fibers surface was metallized by reduction in NaBH₄ solution. Using a similar concept, Guo et al (2017) electrospun a composite fiber comprising of polyvinyl alcohol (PVA), poly(acrylic acid) (PAA) and Fe₃O₄ nanoparticles followed by reduction of HAuCl₄ to Au nanoparticles on the surface of the composite nanofibers. The Au nanoparticles (AuNPs) were evenly distributed on the surface of the composite fibers with high catalytic reduction activity on p-nitrophenol and 2-nitroaniline solutions. The PVA/PAA/Fe₃O₄/AuNPs nanocomposites also showed excellent stability and regeneration capability toward reduction of p-nitrophenol.

Apart from organic nanofibers carrier, inorganic nanofibers may also be used as carrier for catalytic nanoparticles. Im et al (2008) prepared carbon nanofibers containing vanadium by electrospinning a solution mixture of polyacrylonitrile and vanadium pentoxide. Heat treatment was carried out to reduce the composite to carbon/vanadium nanofiber. Potassium hydroxide was used to activate the carbon nanofibers by generating ultra-micropores on the nanofibers. Comparing the hydrogen storage capacity of activated carbon nanofibers with and without vanadium, the former was able to give a higher hydrogen storage of 2.41 wt% compared to pure carbon nanofibers of 1.78 wt%. Similar concept has been used to prepare other inorganic nanofiber carrier containing catalytic nanoparticles such as silver/silica nanofibers [Kang et al 2010], palladium/silica nanofibers [Wen et al 2015] and Pt/TiO₂ [Formo et al 2008].

Inorganic nanofibers with catalytic function has also been constructed for use in quasi-solid-state zinc-air batteries (ZABs). Pan et al (2019) fabricated CuCo₂O₄nanoparticles@N-carbon nanofiber (CuCo₂O₄NPs@N-CNFs) film by electrospinning of their precursors blend followed by carbonization/oxidation processes. The resultant CuCo₂O₄NPS@N-CNFs undergo a room-temperature in situ sulfurization by immersing into 2.0 m Na₂S solution for a couple of hours. The .CuCo₂S₄ NSs@N-CNFs) films showed remarkable bifunctional catalytic performance (Ej= 10 (OER) - E1/2 (ORR) = 0.751 V) with excellent mechanical flexibility.

Electrospun fibers may also be used as a sacrificial template for forming catalytic nanotubes. Pantojas et al (2008) coated electrospun polyethylene oxide fibers with palladium using sputtering. A sputtering duration of more than 250s is required to form complete tubes after sintering. However, the thickness of the wall section of the cylinder is not uniform due to line-of-sight deposition. Other methods such as electrodeless deposition may also be used to coat nanofibers [Ochanda and Jones Jr. 2005]. Chen et al (2017) tested the catalytic efficiency of electrospun solid, hollow and fiber in tube antimony-doped tin oxide nanofibers (ATONF) with electrodeposited Pt nanoparticles. Solid, hollow and fiber in tube electrospun ATONFs were formed by controlling the heating rate during calcination of its precursors. Following electrodeposition of Pt nanoparticles, it was found that for hollow tube ATONFs, the Pt nanoparticles were located in the hollow channels of the nanofibers instead of on the exposed surface. While this phenomena require further investigation, it may be due to lower nucleation energy on the concave inner wall of the tube. Catalytic activity on oxygen reduction reaction (ORR) was the highest on the Pt nanoparticles coated hollow ATONFs compared to Pt nanoparticles coated solid ATONFs and conventional antimony-doped tin oxide powders. Higher ORR in hollow ATONF versus solid ATONF despite having the same Pt loading and nanoparticles size may be attributed to enhanced probability of O₂ collision onto the catalytic Pt surface within the confines of the nanotube.