1. Effect of PTFE on Lower Platinum Loading Electrospun Electrospray Electrodes John Derner, Maximilian Doan, Ian Echols, Zach Ellis CHEN 313 Group 05

2. Background: Electrospinning In 1952, Vonnegut and Neubauer used variable high voltage in a glass capillary tube to produce streams of highly electrified uniform droplets. Drozin (1955) observed aerosols that resulted from the dispersed droplets. Simons (1966) created an apparatus that produced non-woven, ultra thin & light fabrics via electrical spinning. Baumgarten (1971) built on this apparatus to create electrospun acrylic fibers. The term “electrospinning” was coined in 1994 and has since gained interest due to developments in nanotechnology.

3. Electrospinning of Polymer Nanofibers The three basic components needed are a high voltage supplier, a capillary tube with a pipette, and a metal collecting screen. A high voltage is implemented to cause an electrically charged jet of polymer solution. The jet solidifies before hitting the metal collector, forming a web of small fibers. One electrode is placed into the solution and the other on the grounded collector.

4. Past Results from Electrospinning Prior work with Electrospinning has shown results with about a 10% improvement in the performance of the fuel cell from control fuel cells. The results suggest that Nafion nanofibers increased the TPB and reduced the amount of Pt required. Single Nafion nanofibers exhibit high proton conductivity. The improvement on conductivity is on an order of magnitude higher than the proton conductivity of bulk Nafion. The proton conductivity of Nafion nanofibers was found to increase with decreasing Nafion nanofiber diameter size

5. Comparing the E/E Technique Electrospraying or Electrospinning alone: Nafion and Pt/C mixture are expelled from same needle E/E Technique: Nafion polymer solution is expelled from Electrospinning needle Pt/C catalyst ink is expelled from Electrospraying needle Allows for higher level of control over fiber size and Pt loading

6. Factors Affecting E/E Fiber Size Amount of catalyst loading Solution composition and concentration Applied Voltage Typically higher voltage results in lower nanofiber diameter size Slight changes in environmental conditions (temperature, relative humidity, etc.) Tends to affect electrospraying more than electrospinning

7. Factors Affecting E/E Fiber Size Cont. Electrospraying flow rate Typically set higher in the E/E set-up Allows for greater catalyst:Nafion ratio Electrospinning flow rate Duration of E/E process Difference in Electrospraying and Electrospinning flow rates Larger differences may result in higher particle agglomerate sizes

8. Non-woven Membrane to Single Nanofiber The electric field is subjected to the end of the capillary tube containing solution, causing a charge on the surface of the liquid. Charge repulsion causes a force opposite to surface tension. As the intensity of the electric field increases, the fluid elongates to form a Taylor cone. The electric field eventually reaches a critical value, surpassing surface tension, and the jet is ejected from the Taylor cone. The jet undergoes an elongation and instability process and the solvent evaporates, leaving a single nanofiber.

9. Scanning Electron Microscopy (SEM) SEM utilizes a focused electron beam over a surface to generate an image. The electron beam will interact with the surface of the sample to produce signals that can be used to obtain information about surface topography and composition.

10. Scanning Electron Microscopy (SEM) Cont. SEMs do not rely on wavelength to generate images. Because wavelength can be a limiting factor in generating images in optical microscopy, SEMs operate at a higher magnification than optical microscopes. This means that SEMs are better used for analyzing samples that require heavy magnification. The E/E results are observed via SEM.

11. Nanofiber Diameter When the jet is traveling to the metal collector, it may split into multiple jets, causing varying fiber diameters. If there is no splitting, fiber diameter is increased when solution viscosity is higher. When using a solid polymer dissolved in a solvent, viscosity is proportional to polymer concentration. As such, diameter increases with polymer concentration.

12. Bead Defects Beads cause the fiber to be rough and prevent uniformity. Bead formation is affected by polymer concentration. Higher polymer concentrations have fewer beads, but the beads have larger diameters. Reducing surface tension or adding filler material into the polymer solution are options to obtain fibers with no beads. Increased electrical potential causes rougher fibers.

13. Fiber Alignment Via Cylinder Collector Fiber bundles have a wider array of uses than nonwoven nanofibers. Cylinder collectors are spun at high speeds to obtain aligned fibers. When linear speed of the rotating cylinder surface matches the evaporated jet depositions, fibers attach to the surface in a circumferential manner. This occurs at alignment speed. Below this speed, fibers will be randomly deposited. Above it, the fiber jet will break.

14. Fiber Alignment Via Electrical Field A collection mandrel is placed asymmetrically between two charged plates. The auxiliary electrical field improves fiber alignment. Electrospun fibers with large diameter can be oriented circumferentially, while fibers with small diameters are still randomly oriented. The difference caused by the electrical field is shown in the bottom picture.

15. Frame Collector A frame collector can be used to obtain a single nanofiber. A rectangular frame is placed under a spinning jet and collects electrospun fibers. The material of the frame influences the quality of fiber alignments. For example, aluminum provides better results than wood. The angle of the frame, the distance between the frame rods, and the size and shape of the rods also affects the fiber alignments.

16. Multiple Field Technique Using multiple fields, the polymer jet can be straightened somewhat. This allows for a more controlled deposition of the electrospun fibers. Fiber yarns are collected by using this technique.

17. Background: Fuel Cell Fuel cells provide exceptionally clean, steady, and reliable power generation However, fuel cells are more expensive than other sources of energy and are much more complex ●Key research interests include making fuel cells less expensive

18. Background: Fuel Cell Investment Iceland opened the world’s first hydrogen refueling station in 2003 and introduced hydrogen fed, fuel cell powered buses. They then set their sights on expanding to cars and boats. Also in 2003, the U.S. launched a $1.2 Billion Hydrogen Fuel Initiative to develop cars powered by fuel cells. After electricity generation, transportation is the largest producer of CO ₂. Fuel cells have zero CO ₂ emission and, if massively incorporated, would significantly reduce global greenhouse gas emissions.

19. Background: Platinum Requirement for Fuel Cells In a fuel cell, a fuel such as hydrogen gas (H ₂ ) is oxidized at the anode [left side]. Oxygen gas (O ₂ ), typically from air in the atmosphere, is reduced at the cathode [right side]. The overall redox reaction generates an overall voltage. A catalyst, typically platinum (Pt), is used to speed up the both the reduction and oxidation reactions.

20. Hydrogen peroxide (H ₂ O ₂ ) is an intermediate formed during the reduction of O ₂ at the cathode. Without a catalyst, the decomposition of this intermediate is relatively slow. H ₂ O ₂ corrodes carbonaceous electrodes and lowers the fuel cell’s overall voltage, decreasing performance and the sustainability of the cell. Because Pt increases the reduction reaction rate, the decomposition rate of the H ₂ O ₂ also increases and there is less buildup that could harm the electrodes or impact the voltage. Background: Platinum Requirement for Fuel Cells Cont.

21. Platinum Disadvantages Platinum, although very effective, is quite an expensive catalyst. Pt catalysts are also sensitive to H ₂ S and CO impurities, which must be removed for long term operation There is a finite amount of Pt in the world. It is estimated that for 500 million fuel cell vehicles, the total amount of Pt resources known would only last for 15 years.

22. Platinum Disadvantages Cont. There are several other markets and applications for Pt including: ●Stationary power fuel cells ●Industrial catalysts ●Catalytic converters in automobiles ●Jewelry Research is being done to find suitable replacements for Pt, while still competing with the efficacy that Pt provides. In the meantime, Pt is being incorporated into the electrode material in creative ways in an effort to minimize the utilization of Pt while still providing adequate catalysis.

23. What Makes Platinum a Good Fuel Cell Catalyst? Generally, a good metal catalyst will follow these three trends: 1.Has a low affinity to catalytic poisons (substrates that damage the catalyst) 2.Can be efficiently recycled in a process 3.Have a relatively high melting point In addition to the three trends above, Pt has an ideal affinity for Hydrogen adsorption. Hydrogen atoms do not bond too strongly with Pt. The H-Pt bond is strong enough to occur on a frequent basis for catalysis, but weak enough to allow the generation of H+ as a product. These characteristics make Pt an ideal candidate for a fuel cell catalyst.

24. Platinum Utilization To save on catalyst costs, Pt is often distributed on the porous surface area of carbon electrodes. Much research is being done to reduce the Pt loading and thus develop more cost effective fuel cells. This involves utilizing composites of Pt and nanomaterials or polymer membranes to increase current density. This is achieved by using composites that improve mass transfer across the membranes.

25. Past Attempts to Reduce Platinum Loadings Changing the Metal-Catalyst Morphology: 1.Dispersing Pt nanoparticles on high surface area Carbon supports 2.Using a non-noble metal catalyst (Pt is considered a noble metal, along with Pd and Rh) 3.Using Pt alloys 4.Using Pt core-shell structures 5.Using Pt-supported thin films Pt/C Catalyst Pt Core-Shell Morphology

26. Past Attempts to Reduce Platinum Loading Cont. Introducing a Nafion ionomer increases the triple phase boundary (TPB) TPB- junction point where catalytic and electron conduction sites, reactant gases, and proton conducting Nafion ionomers meet Attempts to expose more Pt surface area for O 2 reduction and increase TPB have been explored in alternative electrode designs. This includes electrospraying and electrospinning (E/E) catalyst/ionomer mixtures over an electrode.

27. Ideas to Replace Platinum Because the two main problems with Pt are ●Pt is a finite, precious metal ●Pt is widely used One replacement for platinum in fuel cells could be another precious metal such as palladium (Pd) or ruthenium (Ru). ●These materials don’t have the same catalytic activity as platinum ●Palladium and ruthenium are also precious, finite materials and will eventually run out just like platinum

28. Ideas to Replace Platinum Cont. The more attractive alternative to Pt is a metal that is both abundant and non- precious.Research must be done to find a metal of this criteria with sufficient catalytic activity Some candidates that have attracted interest in the scientific community include: ●Pyrolized metal porphyrins, where porphyrin is heated with Co or Fe ●Heterocyclic polymer polypyrrole with entrapped metal ions, a stable material that generates active sites for O ₂ reduction ●Polyanaline polymerized on carbon black nanoparticles, treated with Co or Fe species. This candidate seems to have the most promising activity as a cheap, abundant alternative. Platinum remains the most active catalyst, but hopefully developments will continue in its replacement ?

29. Background: PTFE use in fuel cells The experiment uses a polymer electrolyte membrane fuel cell (PEMFC). Poly(tetrafluoroethylene) (PTFE) forms the backbone of the proton conducting polymer membrane for a PEMFC. The polymer membrane acts as electrolyte for the fuel cell. The membrane allows protons from dissociated acids to pass through and generate current but has low permeability to H ₂ and O ₂, allowing high coulombic efficiency.

30. Why Poly(tetrafluoroethylene)? Poly(tetrafluoroethylene), or PTFE, was chosen to as a polymer to reduce the Pt loading in fuel cells because of its hydrophobicity. The high electronegativity of the Fluorine atoms results in enhanced London Dispersion Forces, secondary attractive forces. Thus, the Carbon- Fluorine bond is very strong and unreactive. Hydrophobic materials assist in water removal from the Gas Diffusion Layer (GDL), and aids in the gas transport through the fuel cell.

31. Results From Experiment MEA TypeMax. Power Density (H 2 /O 2 ) Max. Power Density (H 2 /air) Platinum (Pt) Loading Reduction in Pt Loading Reduction in Max. Power Density Control1.420 W/cm 2.839 W/cm 2.570 mg Pt /cm 2 N/A E/E w/o PTFE 1.090 W/cm 2.647 W/cm 2.112 mg Pt /cm 2 20% of control77% of control E/E w/ PTFE 1.240 W/cm 2.725 W/cm 2.094 mg Pt /cm 2 16% of control86-87% of control Control: A hand-painted electrode without Polytetrafluoroethylene (PTFE) E/E: Electrosprayed/Electrospun PTFE: poly(tetrafluoroethylene)

32. Explaining the Experimental Results The improved results of the E/E MEA with PTFE is related to the hydrophobicity of the nanofibers. The hypothesis was that more hydrophobic nanofibers will enhance the mass transfer and therefore the fuel cell performance. The image below shows the hydrophobicity of different electrode assemblies. An increased angle of contact denotes a higher hydrophobicity. Hand-painted electrode w/o PTFE E/E electrode w/o PTFE E/E electrode w/ PTFE

33. Fuel Cell Performance of Different Electrodes Fuel Cell Performance of: E/E #1: no PTFE E/E #3: no PTFE E/E #4: 1% wt PTFE Conditions: a)H2O2 172 kPa back pressure b)H2/O2 ambient pressure c)H2/air 172 kPa back pressure d)H2/air ambient pressure

34. Effect of Hydrophobicity on Fuel Cell Performance Small amounts of PTFE can affect the hydrophobicity of E/E electrodes. PTFE typically increases the hydrophobicity of E/E electrodes Increased hydrophobicity in the pores of the catalyst layer assists in the water removal and gas transport. Improved mass transfer is shown in the polarization curve. E/E #4, the electrode with PTFE, demonstrates a higher current density and power density than the other E/E without PTFE.

35. Fuel Cell Tests and Cyclic Voltammetry Membrane Electrode Assemblies (MEAs) were placed between two serpentine flow field graphite plates. Fuel cell performance of each MEA was characterized via polarization curves (voltage vs. current density) with a Fuel Cell Test Station. These tests were carried out at ambient pressure and 172 kPa. The tests were also carried out with H 2 gas and air. Voltage scans were performed on the test cell until steady- state was achieved. Serpentine flow field graphite plates Fuel Cell Test Station

36. Future Uses of Electrospun Polymer Nanofibers Polymer nanofibers have a wide array of uses. One of the biggest prospective fields for possible applications is in biomedical sciences. The various applications for nanofibers in this field include medical prostheses, tissue template, wound dressing, drug delivery, and cosmetics. Spraying the polymer provides a thin, protective coating, making it ideal for covering prostheses and wounds.

37. Filtration with Polymer Nanofibers Filtration is a growing market that is important in numerous engineering fields. A need to remove particles in the submicron range had made electrospinning a solution for filtration. The high surface area to volume ratio of the nanofibers provides surface cohesion that can trap small particles. Filter efficiency is inversely proportional to fiber diameter. Nanofibers also allow for lower air resistance.

38. How Research Could be Improved Test the effect of ●Electrospraying only without electrospinning ●Electrospinning only without electrospraying The primary research combined the two techniques, but it would be beneficial to find how much each method contributes Replicate the experiment using solutions with polymer backbones other than PTFE ?

39. Future Research Suggestions Explore alternatives to platinum catalyst ●Test alternatives in conjunction with electrospinning/electrospraying ○Test electrospinning and electrospraying both independently and together Explore possibility of electrospinning/electrospraying materials that incorporate Pt on the nano-level Combination Experiment E/E Method Pt Replacement

40. References Hou, Xianghui. "Hydrophobicity Study of Polytetrafluoroethylene Nanocomposite Films." Hydrophobicity Study of Polytetrafluoroethylene Nanocomposite Films. Thin Solid Films, n.d. Web. 13 Apr. 2016. Huang, Zheng-Ming, Y.-Z. Zhang, M. Kotaki, and S. Ramakrishna. "A Review on Polymer Nanofibers by Electrospinning and Their Applications in Nanocomposites." Composites Science and Technology 63.15 (2003): 2223-253. Web. Sealy, C. (2008) ‘The problem with platinum’, Materials Today, 11(12), pp. 65–68. doi: 10.1016/S1369-7021(08)70254-2. Wang, Xuhai, et al. "Effect of Polytetrafluoroethylene on Ultra-Low Platinum Loaded Electrospun/Electrosprayed Electrodes in Proton Exchange Membrane Fuel Cells." Effect of Polytetrafluoroethylene on Ultra-Low Platinum Loaded Electrospun/Electrosprayed Electrodes in Proton Exchange Membrane Fuel Cells. Electrochimica Acta, n.d. Web. 13 Apr. 2016. Wang, Xuhai, et al. "Ultra-low Platinum Loadings in Polymer Electrolyte Membrane Fuel Cell Electrodes Fabricated via Simultaneous Electrospinning/electrospraying Method." Ultra-low Platinum Loadings in Polymer Electrolyte Membrane Fuel Cell Electrodes Fabricated via Simultaneous Electrospinning/electrospraying Method. Journal of Power Sciences, n.d. Web. 13 Apr. 2016.