The Materials Chemistry of Fuel Cells: PEMS vs. SOFC

Introduction

Fuel cells are important part of our life since they are used to generate electric power, which is utilized in lighting bulbs, television, or any gadget with needs direct current power. Commercially used fuel cells can be categorized depending on the type of electrolyte used namely Solid Oxide Fuel Cell (SOFC) and Proton Exchange Membrane fuel cells (PEM).

Proton exchange membrane fuel cells “have as electrolyte a thin conductive plastic sheet such as perfluorosulphonic acid polymer, sandwiched between two permeable electrodes impregnate with an active catalysts“(Crawley 1). Since proton exchange fuel cell use membrane as an electrolyte, they operate at low temperature and therefore care should be taken not to expose them to temperatures above 80 degrees Celsius otherwise the membrane will be damaged rendering the cells inefficient or totally useless.

On the other hand, solid oxide fuel cells utilizes “a hard ceramic material as electrolyte, allowing operating temperatures up to 1000°C”( Crawley 1). Since the electrolyte is a hard solid substance, these kinds of fuel cells can operate on ethane, diesel, butane, methane and other fuels. They have wide application due to their robustness and being environmental friend. Sold oxide fuel cells have an efficiency of up to 60 percent and can withstand harsh environmental conditions. They are commonly used in large-scale power production. In this study, we will discuss how solid oxide fuel cells and proton exchange membranes polymers are synthesized, methods and features used to confirm them (characterization), and their properties.

Synthesis of Proton Exchange Membranes

These membranes are commonly known as Polymer Electrolyte Membrane Fuel Cell (PEMFC). These cells utilize a slim, porous polymeric film as the electrolyte with platinum electrode catalyst on both side of the membrane. These membranes have excellent proton conductivities, less porous towards reactant fuels and superior mechanical strength. Until now, costly and limited perfluorinated sulfonic acid polymers are still being used in commercial applications. We will focus on alternative polymer exchange membranes synthesized by modification of aromatic polymer “such as poly ether sulfones and poly ether ketones that have been chemically modified to contain sulfonic acid groups along the main chain” (Sankir et al 1).

Following research, it has emerged that managed sulfonation of the complete polymer chain by way of direct copolymerization produces high quality yield as compared to post-sulfonation by titration method(Sankir 1). Example of polymer exchange membrane include: Lignosulfonate membrane, polyamide composite membrane, poly (ethylene glycol) composite membrane and Sulfonated Poly (arylene ether benzonitrile) synthesized by “direct copolymerizing SDCDPS, 2, 6-dichlorobenzonitrile and 4, 4’-biphenol” (Sankir et al 2).

To produce 35mol percent of Sulfonated Poly (arylene ether benzonitrile) copolymer, 3.7654 g (0.0202 mol) 4, 4’-biphenol, 2.2609 g (0.0131 mol) 2, 6-dichlorobenzonitrile, 3.2139 g potassium carbonate and 3.4767 g (0.0071mol) SDCDPS was into 3-neck flask fitted a nitrogen gas inlet, Dean Stark entrap and motorized agitator. This was followed by adding 10ml of toluene and 20ml of dry NMP as polymerization solvent (Ehsani 17). To ensure that we have total dehydration of reactants, mixed reactants were heated at 150 ºC for 4 hours to ensure gradual removal of toluene solvent and then followed by bit by bit increase of temperature to 190 ºC and heating reactants for another 16 hours.

The product were then cooled and then mixed with NMP, followed by precipitation in ion free water. Deionized water was used to eliminate any external impurities. To remove all impurities, swollen fiber product was rinsed repeatedly with ion free water and again the impetuous copolymers heated for 4 hours in ion free water to get rid of any salts. Filter separated copolymers was put in the vacuum oven set at 120 ºC and left there for 24 hours.

To convert these membranes into potassium sulfonate variety, copolymer product was again dissolved in “copolymer in 5-10% (w/v) DMAc, filtered to afford, then cast onto clean glass substrates” (Sankir et al 2). The translucent mixture was cautiously desiccated using infrared heat at slowly rising temperatures up to 60 degrees Celsius under inert environment provided by constant flow of nitrogen. This final product can be transformed into acid-form by “boiling the cast membranes in 0.5 M sulfuric acid for 1.5 hours, followed by 1.5 hour extraction in boiling deionized water” (Sankir et al 2).

Initialization of is based on hydrophobic and hydrophilic principle where they interact and thereby start the reaction. This is then propagated by adding 20ml of NMP solvent and heated for 16 hours. To terminate the reaction, a proton is required, which replaces sodium and sodium oxide to form sulfic acid as a terminating chain. Therefore below, is the reaction mechanism through with the polymer is formed:

Synthesis of Proton Exchange Membranes

Characterization Proton Exchange Membrane

To confirm and characterize proton exchange membrane, Attenuated Total Reflectance Infrared Spectrometry comes in as powerful tool to study chemical structure these polymers. Various faces of the polymer are placed in contact with internal reflecting element (IRE) and any light that goes through is reflected at a short distance normal1µm into the polymer from the surface of the internal reflecting element. This is used to characterize the combined membrane since it is not possible to synthesis a uniform tablet with films or membranes and the amalgamated membranes or films are not adequately translucent to let light go through under the traditional infrared method.

Further characterization was done by use of Scanning Electron Microscopy and Energy Dispersive Spectrometry whereby the polymer membrane is cut into small pieces of approximately 1cm by 1cm and then put on top of carton tab. This is followed by dissolving the test sample into pure methanol to facilitate breaking of frozen membrane without altering the arrangement. Using an adhesive, it is mounted on a steel tab and then using electromagnetic lens, a beam of light is focused on the test sample to give information about fundamental composition of the polymer surface. From the results, we see that,” the membrane has two regions, called the top layer and the sublayer” (Sankir et al 7). This is a characteristic of a polymer.

Further analysis using Ubbelohde Viscometer at 25 ºC in solution of NMP and proton (1H) nuclear magnetic resonance utilizing Varian UNITY 400 Spectrometer analysis were done to determine inherent viscosities and functional group respectively. Finally most important test of determining polymer conductivity was performed on acidified films or membranes while immersed in ion free water “using a Hewlett Packard 4129A Impedance/Gain-Phase Analyzer recorded from 10 MHz to 10 Hz”(Sankir et al 3).

Results and Discussion

The reaction proceeds via “direct nucleophilic polycondensation in four hours dehydration step, achieved by toluene refluxing at 155 ºC and copolymerization step at 190 ºC for 16 h”(Sankir et al 3).

Synthesis of solid Oxide Fuel Cells

Practically, utilization of a slim covering deposition facilitates production fuel cell system in an incessant process. A systematic settlement beginning with the anode and ending cathode viaducts the complexities of linking electrolyte-covered anodes to complement cathodes (Ehsani 18). It should be note that “the ceramic material chosen for the electrode matrix is the same as that used for the vapor deposition of the electrolyte layer Synthesis of a dense, thin electrolyte layer is established by the rf-sputter deposition of a target” (Jankowski and Morse 2).

To produce a slim cover solid oxide fuel cell, a thin coating of silicon nitride was smeared to the substrate exterior by employing “low pressure chemical vapor deposition process” (Jankowski and Morse 2). This was followed by passing 33 cm3m-1 of ammonia gas at 33 pa and 112cm3m-1 of dichlorosilane into the solution resulting in formation of a 0.22µm thick coat. “An etch rate of 0.42 µmm-l is achieved during nitride removal using 500 W of power with a 2.7 Pa pressure at a 40 cm3m-1flow of CHF3 and a 80 cm3m-1 flow of CF4” (Jankowski and Morse 4).

Characterization of Solid Oxide Fuel Cell

To confirm the formation of solid oxide fuel cell electrolyte is done by analyzing a flawless electrolyte “electrolyte layer of cubic YSZ is confirmed through transmission electron microscopy (TEM) plan-view bright-field imaging and selected-area electron diffraction” (Jankowski and Morse 4). To further characterize solid oxide fuel cells, flow porometry technique is used whereby pores of the polymer synthesized is “pores of the sample are spontaneously filled with a wetting liquid and pressure of a non-reacting gas is increased on the sample to empty the pores and permit gas flow”(Jena and Gupta 2). Then the varying gas pressure required to displace wetting liquid in the polymer pore is calculated using the equation:

p = 4 γ cos θ / D

“In this equation p= differential gas pressure, γ = the surface tension of the wetting

Liquid, θ is the contact angle of the wetting liquid and D is the pore diameter” (Jena and Gupta 2). This measure and the procedure help determine accurately the rate of flow and polymer pore size over a wide range and thereby characterizing the polymer. In principle about characterize of solid oxide fuels, “Capillary Flow Polometry detects the presence of a pore when the wetting liquid is completely removed from the pore and gas starts flowing through the pore” ( Jena and Gupta 2). It is in this regard that whenever polymer pores turns out to be totally vacant is the equivalent pressure needed dislodge the wetting liquid and therefore this pressure is measured and computed to characterize the polymer.

All materials employed in solid oxide fuel cells must be chemically stable under intense reducing and oxidizing environment. Finally, at elevated temperatures, all materials used in these cells still posses thermo-mechanical properties (Singhal 1).

Results and Discussion

During the synthesis process, “a thin-film solid-oxide fuel cell (TFSOFC) consisting of a Ni-(YSZ) anode, a YSZ- electrolyte, and a Ag-(YSZ) cathode is formed through the continuous deposition process” (Jankowski and Morse 4). By using a Scanning Electron Microscopy, interrupted layers of both electrode and electrolyte coatings in the course of growth direction can be seen.

Multilayer settlement skill offers a way of manufacturing thin-coating solid oxide fuel cells capable of providing huge electric power. This method of manufacturing solid oxide fuel cells is novel in many ways since: ”

  1. the electrodes are co- sputter deposited thin films;
  2. a provision exists for the deposition of mixed conducting interfaces and
  3. the entire fuel cell is formed as a thin film through a continuous deposition process” (Jankowski and Morse 6).

Conclusion

Solid oxide fuel cells are made from a non-permeable tough pottery material as the electrolyte and therefore giving room for up to 1000°C operating temperature. In addition, solid oxide fuel cells are environmental friendly and long lasting. They are utilized in most in high-power functions for instance in high power electric production stations. On the other hand, Proton Exchange Membrane polymers are made from delicate membrane and therefore must be operated below 80 degrees Celsius. In terms of their applications, there are limited in use as opposed to solid oxide fuel cells, which withstands harsh conditions (Singhal 4).

In conclusion, both solid oxide fuel cells and proton exchange membrane fuels cells are important in our daily applications and care must be taken while handling them not to destroy the membrane and this is particularly for the later.

Works Cited

Crawley, Gemma. “Proton Exchange Membrane (PEM) Fuel Cells,” Fuel Today Journal, 6.1 (2006): 1-12.

Ehsani, Mehrdad. “Modern electric, hybrid electric, and fuel cell vehicles: fundamentals, theory, and design,” CRC Journal, 6. 1(2005):17-18.

Jankowski, Alan and Morse, Jeffrey. “Thin Film Synthesis of Novel Electrode Materials for Solid-Oxide Fuel Cells,” American Ceramic Society Journal, 87.4 (1997):1-7.

Jena, Akshaya and Gupta, Krishna. “Liquid Extrusion Techniques for Pore Structure Evaluation of Nonwovens,” International Nonwovens Journal, 12.2 (2003) :45-53.

Sankir, Mehmet et al. “Proton exchange membrane fuel cells: Synthesis and characterization of disulfonated poly (arylene ether benzonitrile) copolymers,” Journal of Chemical Society, 49.2 (2004):1-3.

Singhal, Subhash.” Solid Oxide Fuel Cells,” The Electrochemical Society Interface Journal, 45.1 (2007): 1-4.