ELECTROCHEMICAL APPLICATIONS OF CARBON FOAM ELECTRODES

Transcription

1 ELECTROCHEMICAL APPLICATIONS OF CARBON FOAM ELECTRODES by Wen Nee Yeo A THESIS Submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN CHEMICAL ENGINEERING Michigan Technological University May, 2003 Copyright Wen Nee Yeo 2003

2 This thesis, Electrochemical Applications of Carbon Foam Electrodes, is hereby approved in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN CHEMICAL ENGINEERING. Department of Chemical Engineering Thesis Advisor Dr. Tony N. Rogers Department Chair Dr. Michael E. Mullins Date

3 ABSTRACT Graphitic carbon foams produced from Powellton coals, which combine good electrical conductivity with low weight and cost, have been investigated as electrochemistry electrodes. The foams have been heat-treated to 2200 O C with different foaming and calcining temperatures. Electrochemical performance and morphology of these carbon foam materials were examined to determine their potential in electrochemical applications. The electrochemical behavior of these foam electrodes was studied by cyclic voltammetry (CV) using the ferricyanide redox couple in 1M potassium chloride solution. Based on the diagnostics tests, the carbon foam materials satisfy the criteria for a quasi-reversible reaction and exhibit kinetic rates consistent with literature for glassy carbon. The peak current at a low voltage scan rate was compared for different foam materials to investigate mass transfer limitation. Following the CV tests, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to understand the morphology of carbon foam electrodes. The SEM images of the surface showed that all carbon foam materials have interconnected pores, except a sample cleaned by solvent extraction to remove impurities. EDS was conducted for the carbon foam materials before and after electrochemical service. In general, appreciable aluminum (Al), silicon (Si), sulfur (S), calcium (Ca), titanium (Ti), iron (Fe), copper (Cu), and zinc (Zn) were found in the materials prior to service. Finally, a phosphoric acid test fuel cell was fabricated from non-conductive Teflon to compare the performance of carbon foams to conventional working electrode materials in phosphoric acid fuel cell applications. Assembly and testing of the fuel cells are planned in follow-up research. Based on the analyses to date, the coal-based foams have been shown to perform well due to their high degree of graphitization, which corresponds to good electrical and strength properties. Their light weight and low cost make them attractive as fuel cell and battery electrodes.

4 ACKNOWLEDGMENTS I wish to thank Dr. Tony N. Rogers, the advisor, for the guidance and support he provided throughout the study. I also wish to thank Dr. Michael E. Mullins, Dr. Carl Nesbitt, and Dr. John A. Jaszczak for the instruction and service as the committee member. Thanks also extended to Mr. Owen P. Mills for his assistance in SEM and EDS, Dr. Daniel A. Crowl for his advice in safety issues that associate with experiments, Dr. Jason M. Keith for his input in my thesis writing, Mr. Alamjeet S. Bhatia and Mr. Bovornlak Oonkhanond for their help in the experiment and discussion in electrochemistry. Thanks also go to Touchstone Research Laboratory, Ltd. for funding this research under U.S Department of Energy SBIR Phase II Grant and the opportunity for me to work in the project. I also would like to thank to the colleagues in the Department of Chemical Engineering for their friendship and kind assistance. Finally, I would like to thank to my family for their mental support through out my study at Michigan Technological University.

5 Table of Contents Table of Contents...i List of Figures...iii List of Tables.vi Chapter 1. Introduction Carbon Foam Carbon Foam as an Electrode Current Collectors Prescreening and Screening Phases Phosphoric Acid Fuel Cells...5 Chapter 2. Experimental Cyclic Voltammetry Screening Tests Carbon Foam Morphology 8 Chapter 3. Results and Discussion Electrochemical Performance Cyclic Voltammetry Screening Tests Discharge Characteristics Stability of Electrodes versus Electrolyte and Service Conditions Carbon Foam Morphology Scanning Electron Microscopy (SEM) Energy Dispersive Spectroscopy (EDS) Chapter 4. Conclusions Chapter 5. Extension of this Work Reference..64 i

6 Appendix A. Cyclic Voltammograms for All Electrode Materials 68 A.1. Cyclic Voltammograms with Varying Scan Rates 68 A.2. Cyclic Voltammograms with Varying K 3 Fe(CN) 6 76 Appendix B. SEM Images of Carbon Foam Samples..84 Appendix C. EDS Spectrums of Carbon Foam Samples Appendix D. Job Safety Assessment (JSA) Forms...99 D.1. JSA of Cyclic Voltammetry Screening Tests D.2. JSA of Electrochemical Deposition of Platinum onto Carbon Foam D.3. JSA of Phosphoric Acid Sol-Gel..112 D.4. JSA of Operation of Fuel Cell..124 ii

7 List of Figures Figure 1. Phosphoric acid fuel cell operation..5 Figure 2. Experiment set-up for electrochemistry study.7 Figure 3. Photo of the JEOL JSM Figure 4. Desired sample attributes.9 Figure 5. A cyclic voltammentry potential waveform with switching potentials at 1.0 and 0.4 V versus silver-silver chloride electrode 11 Figure 6. Cyclic voltammogram obtained from the potential waveform in Figure 4 12 Figure 7. Working curve for the determination of rate constant for a quasi-reversible system 16 Figure 8. Voltammogram of sample PocoFoam TM with varying scan rate...23 Figure 9. Voltammogram of sample Mitsubishi with varying scan rate...24 Figure 10. Voltammogram of sample CO2 with varying scan rate Figure 11. Voltammogram of sample III-480 with varying scan rate...25 Figure 12. Voltammogram of sample IV-460 with varying scan rate...25 Figure 13. Voltammogram of sample PocoFoam TM with varying K 3 Fe(CN) 6 concentration 28 Figure 14. Voltammogram of sample Mitsubishi with varying K 3 Fe(CN) 6 concentration 28 Figure 15. Voltammogram of sample CO2 with varying K 3 Fe(CN) 6 concentration 29 Figure 16. Voltammogram of sample III-480 with varying K 3 Fe(CN) 6 concentration 29 iii

8 Figure 17. Voltammogram of sample IV-460 with varying K 3 Fe(CN) 6 concentration 30 Figure 18. Effect of concentration on sample PocoFoam TM peak current Figure 19. Effect of concentration on sample Mitsubishi peak current Figure 20. Effect of concentration on sample CO2 peak current..32 Figure 21. Effect of concentration on sample III-480 peak current Figure 22. Effect of concentration on sample IV-460 peak current.. 33 Figure 23. Voltammogram of sample PocoFoam TM at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s Figure 24. Voltammogram of sample Mitsubishi at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s..34 Figure 25. Voltammogram of sample CO2 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s..35 Figure 26. Voltammogram of sample III-480 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s..35 Figure 27. Voltammogram of sample IV-460 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s..36 Figure 28. SEM image of sample PocoFoam TM at 50x magnification..40 Figure 29. SEM image of sample Mitsubishi at 50x magnification..41 Figure 30. SEM image of sample E1 at 50x magnification...42 Figure 31. SEM image of sample CO2 at 50x magnification Figure 32. SEM image of sample C4 at 50x magnification...46 Figure 33. SEM image of sample III-480 at 50x magnification 47 iv

9 Figure 34. SEM image of sample IV-460 at 50x magnification...48 Figure 35. EDS spectrum of sample PocoFoam TM before and after service.50 Figure 36. EDS spectrum of sample Mitsubishi before and after service.51 Figure 37. EDS spectrum of sample CO2 before and after service...51 Figure 38. EDS spectrum of sample III-480 before and after service...52 Figure 39. EDS spectrum of sample IV-460 before and after service...52 Figure 40. EDS spectrum of sample E-1 before and after service.53 Figure 41. Preparation procedures of composites composed of H 3 PO 4 -doped silica gel and SEBS elastomer 61 Figure 42. Schematic of phosphoric acid test fuel cells using Cfoam electrodes Figure 43. Flow diagram of phosphoric acid test fuel cells...63 v

10 List of Tables Table 1. Diagnostic tests for cyclic voltammograms at 25 O C.. 14 Table 2. Summary of electrode performance in 5mM ferricyanide system for Phase I project..18 Table 3. Summary of electrode performance in 5mM ferricyanide system for Phase II project..21 Table 4. Diagnostic tests for PocoFoam TM, Poco HTC TM, Mitsubishi, and C Coal-based foam samples Table 5. Electron transfer rate constants for sample PocoFoam TM at varying scan rates..26 Table 6. Electron transfer rate constants for sample Mitsubishi at varying scan rates..27 Table 7. Electron transfer rate constants for sample CO2 at varying scan rates...27 Table 8. Electron transfer rate constants for sample III-480 at varying scan rates 27 Table 9. Electron transfer rate constants for sample IV-460 at varying scan rates...27 Table 10. SEM analysis of carbon foam materials 38 Table 11. Ultimate analysis of C coal Table 12. Conductivity of produced sol-gel..60 vi

11 Chapter 1. Introduction 1.1. Carbon Foam Carbon foam is an enabling technology for a host of next generation material systems and components. Carbon foam can be made from a variety of precursor materials. The precursor and the foaming methodology determine the properties of the foam. A porous carbon foam, CFOAM, developed by Touchstone Research Laboratory, Ltd. (Triadelphia, WV), has been studied in this research. CFOAM is produced from inexpensive bituminous coal powders by a controlled coking process. After the coal is foamed, its mechanical, thermal, and physical properties can be improved by further heat treatment. At this stage, a substantial quantity of organic matter, such as small aliphatic groups that would affect its electrochemical performance, still remains in the foam. Calcining at 1000 to 2000 O C under inert gas can removed these materials. The produced foam gives high electrical conductivity, compressive strength, impact resistance, and low thermal conductivity. [1] The produced foam is further heat-treated above 1700 O C (graphitization process) to increase graphitic ordering and thus increase electrical conductivity, thermal conductivity, and elastic modulus. [1] Consequently, the characteristics of foam can be designed under three criteria: precursor selection, foaming process conditions, and heat treatment conditions. [2] The foam can be graded according to density and pore structure throughout its thickness to provide localized stiffness and thermal expansion control while maintaining an overall weight-efficient structure. 1

12 1.2. Carbon Foam as an Electrode Current Collector The applications of carbon foam are varied due to its unique properties, such as low material cost and light weight. These applications include heat-shielding systems in aerospace, bone surgery materials, battery and fuel cell electrodes, etc. Among these applications, battery and fuel cell electrodes have been of significant interest to this research. The electrodes used in fuel cells usually have the characteristic of good gas diffusion. The morphology and composition of the electrode material, the mass transport and electrical resistance, and the distribution of catalyst and surface area are critical for fuel cell electrodes. [3, 4] Carbon foam materials offer great benefits for fuel cell applications due to their electrical conductivity, cell size, and connectivity. [1] In addition, the low material cost of carbon foam helps to reduce the cost concerns of fuel cells. Carbonaceous materials such as natural graphite, cokes, Meso Carbon Micro Bead (MCMB), and non-graphitizable carbon foams (e.g., glassy carbons) have been proposed as possible anode materials for rechargeable batteries, e.g., Lithium-ion batteries. [1] This is because these materials have reliability, are non toxic, and have a long cycle life. However, the critical properties of these materials (for instance, surface area and porosity) are difficult to predict and control. These properties are important for battery applications since the formation of decomposition products on the surface of the carbon may lead to a passivating layer (the reactions between the carbon with the electrolyte). [1] Carbon foams having different cell sizes, connectivities, densities, and surface 2

13 chemistries can be fabricated with only sight process modifications, offering a high potential for electrode properties design Prescreening and Screening Phases Touchstone Research Laboratory, Ltd. has collaborated with Michigan Technological University (MTU) to explore the potential of carbon foams as porous electrodes for fuel cells and electrochemistry applications. The objectives for this research are: (1) examine whether carbon foams support electrochemistry, (2) investigate the effect of precursor and processing conditions on performance as current collectors, and (3) obtain data to support commercialization of carbon foams by Touchstone Research Laboratory, Ltd. The project was divided into two phases. Under the prescreening phase, several foams made from different precursor coals and having very different cellular structures were produced by Touchstone Research Laboratory, Ltd for the scoping study. These coals are labeled as A, B, C, and D. The A, B, and C series coals contain high-volatile impurities, and the D series coal has less of these contaminants. These materials were heat-treated to certain temperatures (the temperatures are labeled as 1 to 6, with 1 to 3 designating above 1600 O C and 4 to 6 designating above 2200 O C). The effect of graphitic ordering and chemical composition on the electrode performance was done using cyclic voltammetry and a series of morphological and surface analytical techniques. The actual coal types and heat treatment conditions cannot be disclosed since these are considered proprietary information. Based on the experimental feedback, series B coal-based foams heat-treated 3

14 to temperature 4 and C coal-based foams heat-treated to temperature 4 offered the most promising electrochemical response. These were comparable in performance with the Mitsubishi sample, a synthetic foam that served as a benchmark. These precursors are less than 1/100 th the cost of Mitsubishi material. [1] These coal-based foams are therefore attractive to serve as electrodes if their performance is similar to synthetic resin-based foams. The information collected under the prescreening phase is used to guide the foam electrode design and the required processing conditions under the screening phase. Under the screening phase, modifications of C coal-based foams that were heat-treated to temperature 4 were investigated. These modifications included precursor cleaning through solvent extraction to remove aromatic sulfur, activating using CO 2, calcining at different temperatures to drive off aliphatic material from the carbon foam, and foaming to a range of temperatures to produce cellular materials with different densities. By analyzing these different samples using cyclic voltammetry and a series of morphological and surface analytical techniques, optimum processing conditions for carbon foam electrodes were determined. Conventional electrode materials, such as Mitsubishi, PocoFoam TM and Poco Higher Thermal Conductivity (Poco HTC TM ) were also analyzed under the same test conditions. These materials served as benchmarks to compare with those samples in the screening phase. In addition, a phosphoric acid test fuel cell was fabricated to further evaluate carbon foam in electrode applications. 4

15 1.4. Phosphoric Acid Fuel Cells Phosphoric acid fuel cells (PAFC) use phosphoric acid as the electrolyte and operate at temperatures around 150 to 200 O C. [5] PAFC consists of two electrodes sandwiched around the electrolyte. These electrodes are often carbon mixed with a catalyst such as platinum. The principle of operation of the cell is schematically represented in Figure 1. [5] Hydrogen ions migrate through the electrolyte from the anode to the cathode. Electrons generated at the anode travel through an external circuit and return to the cathode. From there, the electrons, hydrogen ions, and oxygen form water and are expelled from the cell. Figure 1. Phosphoric acid fuel cell operation. [5] A hydrocarbon fuel in an external reformer can be used to produce hydrogen that is consumed in the cell. If the hydrocarbon fuel is gasoline, sulfur must be removed to avoid damage to the electrode catalyst. 5

16 The conversion efficiencies of fuel-bound energy to electricity of PAFC are about 40 to 80 percent. One of the advantages of PAFC is that the cells can tolerate 1 to 2 percent carbon monoxide at the operating temperature of 200 O C. Another advantage is that concentrated phosphoric acid electrolyte can operate above the boiling point of water, a limitation on other acid electrolytes that require water for conductivity. The original idea of PAFC systems was for peak shaving or as spinning reserves in electric utilities. However, the system has a competitor, gas turbines, for this application. Another application of the PAFC system is for on-site integrated energy systems to provide electrical and heat energy for buildings. [5] The most recent application of PAFC is powered commercial vehicles because they could be refueled quickly and conveniently. 6

17 Chapter 2. Experimental 2.1. Cyclic Voltammetry Screening Tests The electrochemical properties of carbon foam materials have been studied with a computerized potentiostat PARSTAT 2263 (Princeton Applied Research). The carbon foam materials were hand cut into pieces 1 x 1 x 0.3 cm 3 at the same position relative to bulk for all samples to ensure identical segment in each sample. A platinum wire was then assembled to the carbon foam materials and served as working electrodes in the cells. For cyclic voltammetry tests, a nickel plate was used as the counter electrode; while a silver-silver chloride (Ag / AgCl) electrode was used as the reference electrode. A detailed operating procedure can be found in Appendix D.1. The experimental set-up is shown in Figure 2. Cell that contains electrolyte and electrodes PARSTAT 2263 Computer Figure 2. Experiment set-up for electrochemistry study. 7

18 2.2. Carbon Foam Morphology Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to study the morphology of carbon foam materials. The carbon foam materials were placed on an aluminum round cylindrical mount with diameter 12 mm and height 5 mm. These samples were then coated with a thin layer of carbon to enhance the quality of the image. [6] A JEOL JSM-6400 Scanning Electron Microscope equipped with X-ray detector was used to evaluate microstructure and chemical compositions in the materials. The magnification board range from 10x-300,000x of the JEOL JSM-6400 makes it ideal for study of morphology of the carbon foam materials at both low and high magnifications. The SEM images and EDS spectra can be collected by the software coupled with the instrument. Figure 3 shows a photo of the instrument. Figure 3. Photo of the JEOL JSM

19 Chapter 3. Results and Discussion The ideal foam material for electrode service would have the characteristics shown in Figure 4. The cyclic voltammetry (CV) and morphology measurements in this work are intended to find those samples with optimum characteristics, recognizing that there are trade-offs, e.g., a high porosity may promote ion transport while reducing the electrical conductivity through intact cell walls. Highest Electron Transfer Rate Highest Surface Area Cell Wall Integrity Largest Electron Transfer Rate Constant Small Pore Size High Porosity Ion Transport Scan Rate SEM Observations (Morphology) Figure 4. Desired sample attributes. CV Analysis 3.1. Electrochemical Performance Carbon materials are good conductors and are inert in most electrochemical systems. In addition, carbon materials are readily available and inexpensive compared with platinum. Both advantages explain the wide use of carbon materials in electrochemical systems, such as fuel cells. The most commonly used carbon electrodes are made from graphite or glassy carbon. Electrodes made from carbon foam materials have not been extensively studied. In order to gain a better understanding of the electrochemical performance of carbon foam materials, cyclic voltammetry screening tests, discharge characteristics, and 9

20 stability of electrodes versus electrolyte and service conditions of carbon foam materials are studied Cyclic Voltammetry Screening Tests The electrochemistry properties of carbon foam materials were examined using cyclic voltammetry (CV) cell and ferricyanide (Fe 2+ / Fe 3+ red-ox couple) testing in 1 M potassium chloride (KCl) solution. The CV is a widely used electrochemical method, and it is always the first test to be carried out in a new subject in electrochemical study. [7] In CV, the potential is linearly scanned from an initial value, E initial, to a second value and then back to E initial or some other final potential, E final. The potential excitation is a triangular waveform as demonstrated in Figure 5 for a potential from 1.0 to 0.4 V versus the reference electrode, silver-silver chloride electrode. [8] As shown in this figure, the scan starting at a potential of 1.0 V and reversed in direction at a potential of 0.4 V versus silver-silver chloride electrode, returning back to the original potential of 1.0 V. This displays the cathodic (reduction) and anodic (oxidation) waveforms. 10

21 Figure 5. A cyclic voltammentry potential waveform with switching potentials at 1.0 and 0.4 V versus silver-silver chloride electrode. [8] A typical cyclic voltammogram is shown in Figure 6. This is the voltammogram that illustrated the current response signal obtained when the potential excitation waveform in Figure 4 is applied to a platinum electrode immersed in an electroactive species, 2.0 mm potassium hexachloroiridate (IV) (K 2 IrCl 6 ), in a supporting electrolyte, aqueous 0.1 M potassium nitrate (KNO 3 ). [8] 11

22 Figure 6. Cyclic voltammogram obtained from the potential waveform in Figure 4. [8] As shown in Figure 6, the scan begins at point A with a negative direction. When the potential becomes sufficiently negative, hexachloroiridate (IV) is reduced to hexachloroiridate (III) and cathodic current is initiated at point B. [8] The cathodic current increases rapidly until the surface concentration of hexachloroiridate (IV) approaches zero. At this point (point C), the current reaches maximum, where the diffusion is limited. From there, the current decays as a function of t 1/2, according to the Cottrell equation, [9] as the solution next to the electrode surface is depleted of hexachloroiridate (IV) (which means it has been electrochemically reduced to hexachloroiridate (III)). [8] At point D, the scan is reversed to a positive direction. When the potential becomes sufficiently positive compared to the reference electrode at point E, the electrode oxidizes the reductant (hexachloroiridate (III)) that had been accumulated near the electrode surface back to hexachloroiridate (IV). [8] This generated anodic current and follow similar fashion as described for the cathodic current. Therefore, the anodic current reaches a maximum at point F and decays as the solution adjacent to the electrode is depleted of 12

23 hexachloroiridate (III). [8] The first cycle is completed when the potential returns to the initial potential. At this potential, hexachloroiridate (III) is still present at the surface of the electrode and oxidizes back to the initial form of the couple. [8] The reduction and oxidation process for hexachloroiridate (IV) is analog to the ferricyanide system (Fe 2+ / Fe 3+ redox couple) that was used in the study of electrochemistry properties of carbon foam materials. After completing a cyclic voltammetry test, four important parameters are obtained: 1. The anodic peak current (I pa ) 2. The anodic peak potential (E pa ) 3. The cathodic peak current (I pc ) 4. The cathodic peak potential (E pc ) The peak currents that are obtained from the voltammogram provide information about the analyte concentration and the stability of the electrogenerated species. The peak potentials that are obtained from the voltammogram provide information about the identity of the analyte and the kinetics of the oxidation-reduction process. [8] The voltammograms that are collected from an electrochemical study for the first time can be classified into three types according to the electron transfer at the electrode surface: reversible (Nernstian) system, totally irreversible system, and quasi-reversible system. In the case of a reversible system, the electron transfer rates at all potentials are significantly greater than the mass transport rates, and therefore Nernstian behavior is 13

24 always maintained at the electrode surface. In the case of a totally irreversible system, the electron transfer rates are not sufficient to maintain the surface equilibrium and cause the change in the shape of voltammograms. The most characteristic feature for a totally irreversible system is the absence of a reverse peak. However, the absence of a reverse peak could be due to a fast following chemical reaction. Therefore, other diagnostic tests for an irreversible system must be done. When a reversible system is observed at low scan rates and an irreversible system is observed at high scan rates, this system is called a quasi-reversible system. Differentiating the type of system is usually done by recording voltammograms over a wide range of scan rates at 25 O C and performing diagnostic tests at that temperature. Table 1 shown the diagnostic tests for cyclic voltammograms at 25 O C. [7, 9, 10, 11, 12] Table 1. Diagnostic tests for cyclic voltammograms at 25 O C Totally Irreversible Quasi-Revesible Test Reversible System System System E = E E 59/n mv (No reverse peak) > 59/n mv p pa pa pc pc I / I 1 (No reverse peak) Variable ** I pc vs. v 1/ 2 I v 1/ 2 I v Increases with E p vs. v Independent (No reverse peak) Increases E pc vs. v Independent Shifts by 30 /α C n * Decreases 1/ 2 v * α C is the transfer coefficient and n is the number of electrons transferred [9] ** I I 1 provided α C = α A = 0.5 (subscripts a and c indicate anodic and cathodic process pa / = pc respectively) [7] 14

25 The identification of the type of system leads to different governing equations which determine the peak current density at 25 O C or the rate constant. For example, the peak current density for a reversible system is represented by Randles-Sevcik equation. [7, 10] The value of the electron transfer rate constant, k o, is commonly determined using CV. For a quasi-reversible system, k o can be obtained from Ep values following the method of Nicholson and Shain, which makes use of a working curve that relate Ep to the variable ψ, which is defined as [13] 1/ 2 α / 2 Ψ = k RT Do π o nfdo v D Eqn. 1 R where ψ k o R T n F v = kinetic parameter = electron transfer rate constant, cm/s = gas constant, J/(Kmol) = temperature, K = number of electrons involved in overall electrode reaction = Faraday constant, C/mol = scan rate, V/s D o = diffusion coefficient for ferrocyanide (Fe(CN) 6 4- ), cm 2 /s D R = diffusion coefficient for ferricyanide (Fe(CN) 6 3- ), cm 2 /s α = transfer coefficient 15

26 The working curve for the ferricyanide system is shown in Figure 7. [13] By comparing experimental Ep values with the working curve for several scan rates, the k o values are determined from the rearrangement of Eqn.1. Plot of ψ vs. Ep ψ Ep (mv) Figure 7. Working curve for the determination of rate constant for a quasi-reversible system. [13 ] The analysis of voltammograms of carbon foam materials was used to check for the stability of the voltammogram shape (i.e. scan rate pattern). A summary table of the electrode performance in a 5 mm ferricyanide system for the prescreening phase samples is illustrated in Table 2. [1] The actual coal types and heat treatment conditions cannot be disclosed since it is considered proprietary information. Based on the comparison in terms of the peak current (electron transfer rate), samples A1, B4, C4, and D2 produced the highest peak currents in each series, respectively. Since sample D2 has the lowest peak current among the four different samples, it was not considered further in the study. Foam that was produced from the A series was found to contain a significant quantity of 16

27 iron (from EDS). [1] Hence, this series of foam materials was not included in further study due to its performance in the ferricyanide system being artificially enhanced. As a result, samples B4 and C4 offered the most promising electrochemical performance among the four different carbon foam materials. The performance of samples B4 and C4 were also comparable to the performance of Mitsubishi, a synthetic foam that served as a benchmark in the CV test. These materials are less than 1/100 th the cost of synthetic pitch, [1] making these materials more attractive if their performance is similar to the synthetic pitch. Based on the results in prescreening phase, Touchstone Research Laboratory, Ltd. decided only C coal-based foam materials heat-treated to temperature 4 were to be further investigated under screening phase. The reason for heat treatment to a moderate temperature is because the current reaches a peak for the temperature 4 graphitization sample (refer to Table 2). A lower graphitization temperature produces too little graphitic ordering. At higher temperatures, the graphitic ordering is disrupted by the off-gassing of the impurities from the structure. 17

28 of electrode performance in a 5mM ferricyanide system for prescreening phase [1] ion Rate (mv/sec.) 1M KCl w/ 5mM K 3 Fe(CN) le * Ip (A) Epa (mv) Epc (mv) Ip (A) Epa (mv) Epc (mv) Ip (A) Epa (mv) Epc (mv) Ip (A) Epa (mv) Epc (mv) Ip (A) Epa (mv) Epc (mv) Glassy C 1.872E E E E E A E E E E E A E E E E A E E E E A E E E E E A E E E E E A E E E E B E E E E E B E E E E B E E E E B E E E E B E E E E B E E E E E C E E E E E C E E E E E C E E E E E C E E E E E C E E E E E C E E E D E E E E D E E E E D E E E E D E E E E MB 5.778E E G E E E E G E E E Note: A = Foam produced from coal A (high-volatile materials) B = Foam produced from coal B (high-volatile materials) C = Foam produced from coal C (high-volatile materials) D = Foam produced from coal D (low-volatile materials) MB = "standard" foam produced from Mitsubishi ARA24 synthetic resin (mesophase pitch derived from napthalene) * Number associated with each sample represents the graphtization temperature, with "1" to "3" disignating above 1600 O C and "4" to "6" designating above 2200 O C 18

29 Touchstone Research Laboratory, Ltd. produced nine samples of C coal-based foam materials that were heat-treated to temperature 4 with different foaming and calcining temperatures, and this was tested during the screening phase. The actual calcining and foaming temperatures cannot be disclosed due to their nature as being proprietary information. Two other C coal-based foam samples produced by Touchstone Research Laboratory, Ltd., with the same process conditions as prescreening phase but modified through solvent extraction and thermal activation process using carbon dioxide (CO 2 ) were also included in the tests. Other foams that were prepared by different precursors, used as benchmarks, were also tested. These included Mitsubishi and two foams from Poco Graphite Inc., PocoFoam TM and Poco High Thermal Conductivity (Poco HTC TM ). Altogether, these 14 samples were first analyzed using CV in ferricyanide system with a voltage scan rate of mv/s to generate a closed-loop voltammogram. The ferricyanide system contained 1 M potassium chloride (KCl) and ferricyanide. Table 3 shows a summary of electrode performance in a 5 mm ferricyanide system. Based on the data obtained in Table 3, diagnostic tests were performed to classify the type of reaction system. Table 4 lists the diagnostic tests for the 14 samples. As demonstrated in the table, all the samples satisfy the tests required for quasi-reversible system. This is true for ferricyanide system according to the chemical literature. [14, 15] A passivating layer of Prussian blue (KFe II [Fe III (CN) 6 ]) may be deposited on the surface electrode during the cyclic voltammentry experiment to reduce the electron transfer rate to the extent that the electrochemical behavior of ferricyanide becomes quasi-reversible. [16] 19

30 As noticed from Table 3, all the C coal-based foam samples graphitized at temperature 4 produced a side reaction in the CV screening test. The peak current in Table 3 was compared among the C coal-based foam materials to examine the electron transfer rate. The comparison shows that sample CO2 has the highest peak current whereas sample E1 has the lowest peak current. Samples III-480 and IV-460 were next in performance after sample CO2. When the performance of samples CO2, III-480, and IV-460 were compared with the benchmarks, such as PocoFoam TM and Mitsubishi, these carbon foam materials were comparable to one another. When comparing the nine samples with different foaming and calcining temperatures, these samples produced similar peak current. Hence, different foaming and calcining temperatures do not influence the performance significantly. 20

31 Table 3. Summary of electrode performance in a 5 mm ferricyanide system for screening phase 21

32 Table 4. Diagnostic tests for PocoFoam TM, Poco HTC TM, Mitsubishi, and C coal-base foam samples Sample* I pa / I pc 1 ** vs v I pc. *** E p vs. v v E pc vs. Reaction System PocoFoam TM Yes Increases with v 1/2 Increases Decreases Quasi-reversible Poco HTC TM Yes Increases with v 1/2 Increases Decreases Quasi-reversible Mitsubishi Yes Increases with v 1/2 Increases Decreases Quasi-reversible E1 Yes Increases with v 1/2 Increases Decreases Quasi-reversible CO2 Yes Increases with v 1/2 Increases Decreases Quasi-reversible I-410 Yes Increases with v 1/2 Increases Decreases Quasi-reversible II-440 Yes Increases with v 1/2 Increases Decreases Quasi-reversible III-480 Yes Increases with v 1/2 Increases Decreases Quasi-reversible IV-460 Yes Increases with v 1/2 Increases Decreases Quasi-reversible IX-450 Yes Increases with v 1/2 Increases Decreases Quasi-reversible V-520 Yes Increases with v 1/2 Increases Decreases Quasi-reversible VI-540 Yes Increases with v 1/2 Increases Decreases Quasi-reversible VII-420 Yes Increases with v 1/2 Increases Decreases Quasi-reversible VIIA-500 Yes Increases with v 1/2 Increases Decreases Quasi-reversible * The diagnostic tests were performed for voltage scan rates between 15 mv/s and 100mV/s. ** Ratios between 0.95 and 1.05 satisfy the stated criterion. *** R is the criterion for determining whether Ip is proportional to v ½ (where R is the coefficient of determination). 22

33 Among all the samples tested under screening phase, PocoFoam TM, Mitsubishi, CO2, III- 480, and IV-460 offered the most promising electrochemical response. The voltammograms with varying scan rate of these samples are presented in the following figures. Cyclic Voltammogram of PocoFoam TM with Varying Scan Rate Current (A) Potential (V) PocoFoam-5-15 PocoFoam-5-25 PocoFoam-5-50 PocoFoam PocoFoam Figure 8. Voltammogram of sample PocoFoam TM with varying scan rate.(scan rates of 15, 25, 50, 100, and 500 mv/s, K 3 Fe(CN) 6 concentration of 5 mm.) 23

34 Cyclic Voltammogram of Sample NMB with Varying Scan Rate Current (A) Potential (V) NMB-5-15 NMB-5-25 NMB-5-50 NMB NMB Figure 9. Voltammogram of sample Mitsubishi with varying scan rate. Cyclic Voltammogram of Sample CO2 with Varying Scan Rate Current (A) Potential (V) CO CO CO CO CO Figure 10. Voltammogram of sample CO2 with varying scan rate. 24

35 Cyclic Voltammogram of Sample III-480 with Varying Scan Rate Current (A) Potential (V) III III III III III Figure 11. Voltammogram of sample III-480 with varying scan rate. Cyclic Voltammogram of Sample IV-460 with Varying Scan Rate Current (A) Potential (V) IV IV IV IV IV Figure 12. Voltammogram of sample IV-460 with varying scan rate. 25

36 From these figures, it can be seen that quasi-reversible waveforms are very well developed, especially for sample CO2. These materials also offer comparable peak current to Mitsubishi and PocoFoam TM materials, which served as benchmarks for the CV tests. A collection of voltammograms with varying scan rate for all samples is presented in Appendix A.1. The electron transfer rate constant at different scan rates can be determined from the CV by applying Eqn.1 and Figure 7. Tables 5 through 9 show the rate constant for samples PocoFoam TM, Mitsubishi, CO2, III-480, and IV-460 at varying scan rates. The electron transfer rate constants for these samples are comparable with those of glassy carbon (typically in the range of 0.01 to cm/s). [17] Thus, the electron transfer rate constants of these carbon foam materials are as good as glassy carbon. When the electron transfer rate constant is compared at the single voltage scan rate (i.e., 25 mv/s), the rate constant decreases in the order of samples Mitsubishi, PocoFoam TM, III-480, IV-460, and CO2. The decreasing in the rate constant is because the electron transfer rate of reaction was slowed and thus the equilibrium was not established rapidly in comparison to the voltage scan rate. This occurred because the current takes more time to respond to the applied voltage. This also justifies the reason why sample CO2 showed a very well quasireversible waveform. Table 5. Electron transfer rate constants for sample PocoFoam TM at varying scan rates v (mv/s) Ep(mV) Ψ ko (cm/s) E E E E-03 26

37 Table 6. Electron transfer rate constants for sample Mtisubishi at varying scan rates v (mv/s) Ep(mV) Ψ ko (cm/s) E E E-03 Table 7. Electron transfer rate constants for sample CO2 at varying scan rates v (mv/s) Ep(mV) Ψ ko (cm/s) E E E Table 8. Electron transfer rate constants for sample III-480 at varying scan rates v (mv/s) Ep(mV) Ψ ko (cm/s) E E E E-03 Table 9. Electron transfer rate constants for sample IV-460 at varying scan rates v (mv/s) Ep(mV) Ψ ko (cm/s) E E E The voltammograms with varying K 3 Fe(CN) 6 concentration for the carbon foam materials were also recorded. Figures 13 through 17 show the voltammograms for samples PocoFoam TM, Mitsubishi, CO2, III-480, and IV

38 Current (A) Cyclic Voltammogram of PocoFoam TM with Varying K 3 Fe(CN) 6 Concentration Potential (V) PocoFoam-1-15 PocoFoam-2-15 PocoFoam-5-15 PocoFoam PocoFoam Figure 13. Voltammogram of sample PocoFoam TM with varying K 3 Fe(CN) 6 concentration.(k 3 Fe(CN) 6 concenctrations of 1, 2, 5, 10, and 20 mm, scan rate of 15 mv/s.). Cyclic Voltammogram of Sample NMB with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) NMB-1-15 NMB-2-15 NMB-5-15 NMB NMB Figure 14. Voltammogram of sample Mitsubishi with varying K 3 Fe(CN) 6 concentration. 28

39 Cyclic Voltammogram of Sample CO2 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) CO CO CO CO CO Figure 15. Voltammogram of sample CO2 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample III-480 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) III III III III III Figure 16. Voltammogram of sample III-480 with varying K 3 Fe(CN) 6 concentration. 29

40 Cyclic Voltammogram of Sampl IV-460 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) IV IV IV IV IV Figure 17. Voltammogram of sample IV-460 with varying K 3 Fe(CN) 6 concentration. In all cases, the voltammograms are very stable for varying K 3 Fe(CN) 6 concentrations, with the separation of cathodic and anodic peaks with increasing concentration. These samples also show very similar peak currents with respect to one another. The effect of concentration can be illustrated by plotting either I pa or I pc versus concentration. For a single charge reaction, as the bulk concentration of K 3 Fe(CN) 6 increases, the peak current increases linearly. [18] Figures 18 through 22 show the i pc versus concentration for samples PocoFoam TM, Mitsubishi, CO2, III-480, and IV-460. As noticed from those figures, the relationship between the peak current and K 3 Fe(CN) 6 concentrations is linear. A collection of voltammograms with varying K 3 Fe(CN) 6 concentration for all samples is presented in Appendix A.2. 30

41 Effect of Concentration on Sample PocoFoam TM Peak Current I pc (A) y = x R 2 = Concentration (mm) Figure 18. Effect of concentration on sample PocoFoam TM peak current. Effect of Concentration on Sample Mitsubishi Peak Current Ipc(A) y = x R 2 = Concentration (mm) Figure 19. Effect of concentration on sample Mitsubishi peak current. 31

42 Effect of Concentration on Sample CO2 Peak Current Ipc (A) y = x R 2 = Concentration (mm) Figure 20. Effect of concentration on sample CO2 peak current. Effect of Concentration on Sample III-480 Peak Current Ipc (A) y = x R 2 = Concentration (mm) Figure 21. Effect of concentration on sample III-480 peak current. 32

43 Effect of Concentration on Sample IV-460 Peak Current I pc (A) y = x R 2 = Concentration (mm) Figure 22. Effect of concentration on sample IV-460 peak current Discharge Characteristics The voltammograms produced from CV are also used to check the discharge characteristics of carbon foam materials. The test performed was similar to the CV with varying scan rates, except that it was performed at a constant scan rate with seven iterations. In the experiment, the reference electrode was Ag /AgCl / NaCl electrode instead of Ag / AgCl / KCl electrode. The explanation for this difference is that the Ag/AgCl / KCl reference electrode used in the previous experiment was contaminated. Five samples were tested for their discharge characteristics. These are the best samples based on the CV analysis. Figure 23 through 27 show the voltammograms of samples PocoFoam TM, Mitsubishi, CO2, III-480, and IV-460 at a K 3 Fe(CN) 6 concentration of 5 mm and a voltage scan rate of 50 mv/s. 33

44 Cyclic Voltammogram of PocoFoam TM Current (A) Potential (V) Cycle #1 Cycle #2 Cycle #3 Cycle #4 Cycle #5 Cycle #6 Cycle #7 Figure 23. Voltammogram of sample PocoFoam TM at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s. Cyclic Voltammogram of Sample NMB Current (A) Potential (V) Cycle #1 Cycle #2 Cycle #3 Cycle #4 Cycle #5 Cycle #6 Cycle #7 Figure 24. Voltammogram of sample Mitsubishi at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s. 34

45 Cyclic Voltammogram of Sample CO Current (A) Potential (V) Cycle #1 Cycle #2 Cycle #3 Cycle #4 Cycle #5 Cycle #6 Cycle #7 Figure 25. Voltammogram of sample CO2 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s. Cyclic Voltammogram of Sample III Current (A) Potential (V) Cycle #1 Cycle #2 Cycle #3 Cycle #4 Cycle #5 Cycle #6 Cycle #7 Figure 26. Voltammogram of sample III-480 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s. 35

46 Cyclic Voltammogram of Sample IV Current (A) Potential (V) Cycle #1 Cycle #2 Cycle #3 Cycle #4 Cycle #5 Cycle #6 Cycle #7 Figure 27. Voltammogram of sample IV-460 at a K 3 Fe(CN) 6 concentration of 5mM and a voltage scan rate of 50 mv/s. All of the voltammograms demonstrated reproducible cathodic and anodic peak potentials in the multi-cycle tests. However, other mechanisms were observed in the voltammograms. These mechanisms may include development of a Cl - Stern or hydration layer near the carbon surface that reduces surface redox reactions, oxidation of nongraphitic carbon (at the 0.5V position), and oxygen evolution (at the 0.8V position). Further investigation is needed to understand the features observed in these voltammograms Stability of Electrodes versus Electrolyte and Service Conditions The carbon foam materials were immersed in different electrolytes at room temperature for 30 minutes to examine the chemical compatibility. These electrolytes included 85 % concentrated phosphoric acid, 1M lithium iodide, and potassium ferricyanide / potassium 36

47 chloride solutions used in the CV analysis. All of these electrolytes have shown no visible chemical interactions of oxidation to carbon foam materials. Hence, these carbon foam materials showed a good chemical compatibility to the electrolytes. The carbon foam materials also exposed to air. The exposure to air may cause the contamination of carbon foam. This can be validated by degradation performance was observed during retesting samples from prescreening phase Carbon Foam Morphology The CV results provide a good understanding of the electrode performance of carbon foam materials. However, it is not sufficient to draw a conclusion solely based on CV results. Hence, carbon foam morphology is studied to clarify the CV results. The surface morphology of carbon foams were studied through scanning electron microscopy (SEM) and the chemical compositions of carbon foams were studied through energy dispersive spectroscopy (EDS) Scanning Electron Microscopy SEM examines the structure by bombarding the specimen with a scanning electron beam and then collecting the electrons that the specimen generates. These are collected, amplified, and displayed on a cathode ray tube (CRT). The electron beam and the CRT scan synchronously so that an image of the surface of the specimen is formed. The surface morphology of carbon foam materials has been investigated with a scanning electron microscope. To enhance the quality of the image, a thin layer of carbon coating 37

48 was sputtered on the sample. All the images of carbon foam materials were taken at a magnification of 50x. SEM results are tabulated in Table 10. Table 10. SEM analysis of carbon foam materials Sample Precursor Graphitization Cell Pore Pore Temp. ( O C) Walls * Size ** Shape PocoFoam TM Mesophase pitch 2800 Intact Small Uniform Poco HTC TM Mesophase pitch 2800 Intact Small Uniform Mitsubishi Mitsubishi resin 2700 Intact Small Uniform E1 C coal 2200 Intact Medium Uniform CO2 C coal 2200 Disrupted Medium Variable I-410 C coal 2200 Disrupted Medium Uniform II-440 C coal 2200 Disrupted Large Uniform III-480 C coal 2200 Disrupted Medium Uniform IV-460 C coal 2200 Disrupted Large Variable IX-450 C coal 2200 Disrupted Medium Variable V-520 C coal 2200 Disrupted Large Uniform VI-540 C coal 2200 Disrupted Medium Variable VII-420 C coal 2200 Disrupted Large Uniform VIIA-500 C coal 2200 Disrupted Large Uniform * Cell wall disruption reflects reduced electrical conductivity. However, cell wall disruption may aid in ion transport because of interconnected pores. ** Pore size comparison is performed relative to PocoFoam TM pore size which is about 350 microns and considered to be small. As indicated in Table 10 and Figures 28 and 29, PocoFoam TM and Mitsubishi show cell wall integrity with small pore size and uniform pore shape, which is reflected in the high peak current of these carbon foam materials. Unlike PocoFoam TM, Poco HTC TM, and Mitsubishi, all the C coal-based foam samples show disruption of cell walls, except sample E1. The SEM image of sample E1 shows cell wall integrity. The explanation is that sample E1 was cleaned through solvent extraction to essentially removed the impurities in the coal. This also explains the well-developed CV shape of this sample 38

49 (refer to Figure A.1.4 in Appendix A.1). The reason that sample E1 gave the lowest peak current in CV tests may be its limited surface area for reaction (expected below 2 m 2 /g). Figure 30 presents the SEM image of sample E1. 39

50 Figure 28. SEM image of sample PocoFoam TM at 50x magnification. 40

51 Figure 29. SEM image of sample Mitsubishi at 50x magnification. 41

52 Figure 30. SEM image of sample E1 at 50x magnification. 42

53 According to the electrode performance, sample CO2 gave the highest peak current. This sample was thermally activated with CO 2 prior to graphitization. During activation, sintering increased cell wall integrity, and a micropore structure was created to provide a larger surface area, which is between 15 m 2 /g and 20 m 2 /g (information provided by Touchstone Research Laboratory, Ltd.), for the reaction to take place. Partial desruption of non-graphitic carbon regions at an elevated temperature may also have increased the graphitic ordering of the sample prior to the primary graphitization heat-treatment. The CO2 sample s surface area is considered large as compare to Mitsubishi, which has a surface area of 4 m 2 /g. [19] This also explains why sample CO2 has a small electron rate constant but high peak current because the surface area promotes the surface reaction and hence increases the overall rate of electron transfer. Figure 31 shows the SEM image of this sample. The SEM image of sample CO2 demonstrated the disruption of the cell walls. The disruption of the cell walls can help the migration of ions to / from the foam structure. The large surface area and cell walls disruption of sample CO2 explains why this sample offered the most promising electrochemical performance among the C coalbased foams. A comparison was also made between sample CO2 and sample C4 in the prescreening phase since both these samples were prepared under the same conditions except that sample CO2 was an activated carbon foam material. Figure 32 shows the SEM image of sample C4. As indicated in the figures, both samples show a good interconnecting between pores. However, sample CO2 has a good pore size distribution and smaller pore 43

54 size compared to sample C4. Figure 33 and 34 also show the SEM image of samples III- 480 and IV-460. A collection of SEM images for all samples is presented in Appendix B. 44

55 Figure 31. SEM image of sample CO2 at 50x magnification. 45

56 Figure 32. SEM image of sample C4 at 50x magnification. 46

57 Figure 33. SEM image of sample III-480 at 50x magnification. 47

58 Figure 34. SEM image of sample IV-460 at 50x magnification. 48

59 Energy Dispersive Spectroscopy EDS, using x-ray signals, was employed to analyze the chemical composition in a compound. First, electron hole pairs are created when the x-rays impinge on the intrinsic silicon crystal of the EDS detector. The hole pairs are attracted to a collector through a bias voltage across the crystal. Then, this comparatively weak signal is amplified, converted to a pulse and registered as a count. Finally, it is gathered with the other pulses into channels based on their respective energy. EDS was conducted for all the 14 samples before and after service in the electrolyte solution. In general, all the C coal samples showed appreciable aluminum (Al), silicon (Si), sulfur (S), calcium (Ca), titanium (Ti), iron (Fe), copper (Cu), and zinc (Zn) prior to service. The chlorine (Cl) and potassium (K) also showed in some of the C coal-based foam samples. All of these elements are likely obtained from soda ash and soil minerals present in the coal precursors based on the ultimate analysis performed by Touchstone Research Laboratory, Ltd. and Tradet Laboratories (Triadelphia, WV). [1] The ultimate analysis of C coal is presented in Table 11. [1] The presence of iron in C coals can artificially enhanced its performance for the ferricyanide CV tests. The PocoFoam TM, Poco HTC TM, and Mitsubishi samples have no metals in their EDS spectra, as these foams were prepared from very clean precursors (the presence of the Cu and Zn peaks are system peaks associated with the instrument). High intensities of Cl and K were found in the EDS spectra of post-cv samples. This is due to the remnants from CV service in the 1M KCl electrolyte solution. 49

60 Table 11. Ultimate analysis of C coal Element Percentage (%) Carbon Hydrogen 5.30 Nitrogen 2.44 Ash * 0.50 Sulfur 0.82 Oxygen 6.85 *Known to contain TiO 2. EDS spectra for samples PocoFoam TM, Mitsubishi, CO2, III-480 and IV-460 before and after service in the electrolyte solution are presented in Figures 35 through 39. The EDS spectra for all the samples are presented in Appendix C. Spectrum of Sample PocoFoam TM Intensity Cu 200 Zn Energy (kev) Before After Figure 35. EDS spectrum of sample PocoFoam TM before and after service. 50

61 Spectrum of Sample Mitsubishi Cl K 800 Intensity Cu Zn Energy (kev) Before After Figure 36. EDS spectrum of sample Mitsubishi before and after service. Spectrum of Sample CO Si 2000 Intensity Cl K Al 500 S Ca Ti Fe Cu Zn Energy (kev) Before After Figure 37. EDS spectrum of sample CO2 before and after service. 51

62 Spectrum of Sample III Si 2500 Intensity Al Cl K S Ca 500 Ti Fe Cu Energy (kev) Before After Figure 38. EDS spectrum of sample III-480 before and after service. Spectrum of Sample IV Si 2000 Intensity Al 800 Cl S K Fe 400 Ca Ti Cu Energy (kev) Before After Figure 39. EDS spectrum of sample IV-460 before and after service. 52

63 Spectrum of Sample E Cl K 3000 Intensity Si S 500 Al Fe Cu Energy (kev) Before After Figure 40. EDS spectrum of sample E1 before and after service. The EDS spectrum of sample E1 before and after service in the electrolyte solution is shown in Figure 40. The low intensities of Al, Si, and S in the spectrum are because the impurities in the coals were cleaned through solvent extraction. As a result, sample E1 shows increased cell wall integrity. This is supported by the SEM image in Figure

64 Chapter 4. Conclusions The electrochemical application of carbon foam materials produced from low-cost bituminous coals, which were produced by Touchstone Research Laboratory, Ltd., was investigated. From CV, carbon foam materials from the C coal precursor heat-treated to temperature 4 gave promising electrochemical response. C coal was chosen as the precursor because its performance is similar to synthetic pitch foams with a cost less than 1/100 th of the synthetic pitch. Touchstone Research Laboratory, Ltd. produced additional modifications to C coal-based foams heat-treated to temperature 4 in order to determine the optimum process for obtaining electrodes. These foam improvements included altering the foaming and calcining temperatures, cleaning through solvent extraction, and thermally activating the carbon foam (i.e., creating a micropore structure) by exposure to CO 2. During this research, the electrochemical performance and morphology of carbon foam materials were studied through CV, SEM, and EDS. This research has demonstrated: quasi-reversible behavior of the ferricyanide redox couple in potassium chloride solution, improved current density of coal-based carbon foam materials as compared to the expensive synthetic foam samples, electron transfer rate constant of carbon foam materials comparable to that of glassy carbon (literature), stability of the voltammogram shape at different voltage scan rates and K 3 Fe(CN) 6 concentrations, 54

65 foaming and calcining temperatures do not influence peak current significantly, and graphitization temperature (i.e., extent of graphitic ordering) and surface area are the main factors governing electrochemical performance of carbon foams. Among the C coal-based foam samples considered in this study, sample CO2 (thermally activated sample) produced the highest peak current. Samples labeled III-480 and IV-460 were next in performance. The performance of these samples was comparable to Mitsubishi and PocoFoam TM, which served as benchmarks for the CV tests. The voltammogram of sample CO2 also exhibits a classic quasi-reversible shape. An explanation for the high peak current of this sample is that it was thermally activated to create micropore structure to provide a larger surface area for the redox reaction to take place. According to the CV analysis, the solvent cleaned sample E1 gave the lowest peak current, which may be the result of its limited surface area for reaction combined with a lack of pore interconnectivity. Qualitatively, the characteristics that caused the CO2 sample to give the highest electron transfer rate are: highest peak current, disruption cell walls, and largest surface area. Drawbacks included a lower electron transfer constant and metallic impurities identified by EDS. SEM images for all the C coal-based foam samples showed disrupted cell walls, except for sample E1. Sample E1 was cleaned through solvent extraction to remove the impurities, and hence it has intact cell walls compared to the other carbon foam materials from the same precursor and degree of heat treatment. Due to its cell wall integrity, a 55

66 well-developed CV shape is observed in the voltammograms, although the peak current is reduced. EDS was conducted for all the samples before and service in the electrolyte solution. In general, the C coal-based carbon foam materials contained aluminum (Al), silicon (Si), sulfur (S), calcium (Ca), titanium (Ti), iron (Fe), copper (Cu), and zinc (Zn) prior to service. All of these elements are likely from soda ash and soil minerals present in the coal precursors. The presence of iron in C coal-based carbon foam materials can artificially enhanced its performance in the ferricyanide CV tests. The PocoFoam TM, Poco HTC TM, and Mitsubishi samples have no metals in their EDS spectra, as these foams were prepared from very clean precursors. High intensities of chlorine (Cl) and potassium (K) were found in the EDS spectra of post-cv samples. This is due to remnants from CV service in the 1M KCl electrolyte. A phosphoric acid test fuel cell was fabricated using non-conductive Teflon to compare the performance of carbon foam materials to conventional working electrodes in fuel cell applications. Development of this test fuel cell included three steps: deposit platinum onto potential carbon foam materials, produce a highly conductive phosphoric acid solgel, and operate the fuel cell. All of the design and safety issues associated with these steps have been addressed, and fuel cell operation will be conducted in the future in a follow-on research effort. 56

67 Based on the analysis obtained from the ferricyanide CV tests, SEM, and EDS, carbon foams are suitable electrodes, especially samples CO2, III-480, and IV-460 for the ferricyanide redox system. In this study, these samples offered comparable performance to the more expensive synthetic foam samples. 57

68 Chapter 5. Extension of this Work The primary extension of this research will be to use a test fuel cell to investigate promising carbon foams identified by the CV, SEM, and EDS measurements and observations. To begin this effort, a phosphoric acid test fuel cell was fabricated using non-conductive Teflon to compare the performance of CFOAM to conventional working electrode materials in phosphoric acid fuel cell applications. The electrodes in the test fuel cell contain a mixture of the platinum electrocatalyst supported on carbon foam materials. The porous structure of carbon foam materials allows rapid gas permeability and also provides storage for additional acid to replace the acid lost by evaporation during the operating life of the cell. In addition, it also serves as a structural support for the electrocatalyst layer and acts as a current collector. The platinum is deposited onto the carbon foam materials that will serve as electrodes in the test fuel cell. Deposition can be done chemically or electrochemically using hydrogen hexachloroplatinate (IV) hydrate solution. Chemical deposition can be done by immersing the carbon foam materials into hexachloroplatinate (IV) hydrate solution for few hours and then determine the weight of platinum on the carbon foam materials. These procedures are repeated until the weight of platinum on the carbon foam materials is approximately 0.5g. For electrochemical deposition, a detailed operating procedure and schematic of the experiment can be found in Appendix D.2. The electrolyte used in the test fuel cell is concentrated phosphoric acid. The advantage of using concentrated phosphoric acid (H 3 PO 4 ) electrolyte is that it can operate above the 58

69 boiling point of water, a limitation on other acid electrolytes that require water for conductivity. The phosphoric acid must be held in a matrix, whose function is to hold the acid by capillary action. [20] The requirements for the matrix containing electrolyte are [20] : 1. High capillary action to retain the acid 2. Acts as an electronic insulator 3. Prevents the crossover of reactant gases within the cell structure 4. High thermal conductivity 5. Chemical stability at high temperature working conditions (>200 O C) 6. Sufficient mechanical strength Several methods are found in the literature to produce H 3 PO 4 -doped gel electrolytes. These include synthesis using propylene carbonate in a network based on methyl methacrylate with H 3 PO 4, [21] dimethylformamide in a network based on glycidyl methacrylate with H 3 PO 4, [21] polymer gels based on polyacrylamide matrix with H 3 PO 4, [22] and phosphoric acid-doped silica gel and styrene-ethylene-butylene-styrene (SEBS) elastomer. [23] Among these methods, only the last method fulfills all the requirements for a suitable acid matrix. Therefore, this method was chosen to prepare the electrolyte gel. Figure 41 shows the preparation procedure of composites composed of H 3 PO 4 - doped silica gel and SEBS elastomer. [23] A detailed preparation procedure is presented in Appendix D.3. The preparation of sol-gel was completed and the conductivity is comparable to the reported value of 10-5 S cm -1 for the sol-gel. The measured conductivity is presented in Table

70 Table 12. Conductivity of produced sol-gel Run R (ohm) C (S cm -1 ) E E E-05 Average 1.048E-05 To operate the fuel cell to test various carbon foam electrode materials, the cell will be connected to a PARSTAT 2263 potentiostat to measure the generated current. The electrodes used in the fuel cell are both platinum impregnated foams. A rich hydrogen (H 2 ) gas mixture (80% H 2 and 20% N 2 ) is used as the reactant at the anode. Since this gas mixture above the Upper Flammability Limit (UFL) of H 2 of 74% (see the flammability diagram in Appendix D, Figure D.2), precautions have to be taken when operating the fuel cell. First, ensure there is no air inside the body of fuel cell by flushing nitrogen (N 2 ) into it before heating the cell. Another precaution is diluting the unreacted H 2 before releasing it out of the cell. This is done by mixing N 2 with excess H 2 in the outlet. A check valve is used to ensure the flow of N 2 in only one direction. A detailed operating procedure and flow diagram of the experiment is given in Appendix D.4. In addition, a schematic and a flow diagram of the phosphoric acid test fuel cell are shown in Figure 42 and Figure 43 respectively. According to the analysis of CV, SEM, and EDS, samples CO2, III-480, and IV-460 are potential carbon foam electrode materials. Therefore, these samples are to be further investigated using the phosphoric acid test fuel cell. In addition, samples PocoFoam TM 60

71 and Mitsubishi will be tested in the fuel cell to serve as benchmark electrodes. The generated current of these samples will be compared at various H 2 feed rates. Si(OEt) 4 + EtOH + H 2 O +HCl + [(C 2 H 5 ) 4 N]BF 4 Stirring at room temp. H 3 PO 4 Gelation at room temp. Drying in vacuum oven at 60 to 80 O C for 8 hrs H 3 PO 4 -doped SiO 2 gel Heat-treatment at 100 O C SEBS in toluene Composite composed of H 3 PO 4 -doped SiO 2 gel and SEBS Figure 41 Preparation procedures of composites composed of H 3 PO 4 - doped silica gel and SEBS elastomer. 61

72 Specifications: o PTFE Machined with dimensions 3 x 3 x 3 o Swagelok tubing and thermocouple fittings o Gas feeds: forming gas (H2 in N2), air o Operates between C o Top is flanged for SS bolts o Removable PTFE electrode holder o Machined 2mm CFOAM electrodes o Teflon Gasket o Phosphoric acid gel Connect to Cathode of PARSTAT 2263 Connect to Anode of PARSTAT 2263 Figure 42. Schematic of phosphoric acid test fuel cells using CFOAM electrodes (drawn by Janelle Meyer, M.S candidate, MTU. 2003). 62 Thermocouple Port

73 Figure 43. Flow diagram for phosphoric acid test fuel cell. 63

74 References 1. Darren K. Rogers, Tailorable, Inexpensive Carbon Foam Electrodes for High-Efficiency Fuel Cell and Electrochemical Applications, Touchstone Research Laboratory, Ltd., 2001, pp. 2-4, 10-1, 24, 26-7, 29, 41, E~ Rogers, D. K., Plucinski, J. W., Handley, R. A., Preparation and Graphitization of High Performance Carbon Foams from Coals, American Carbon Society Carbon 2001 Conference, July 14-19, 2001, Lexington, Kentucky, USA 3. T. Tang and K.-Y. Chan, Microfabricated gas-diffusion electrodes, J. Electroanal. Chem., 344, 1992, pp K.-Y. Chan, G. S. Efthymiou, and J. F. Cocchetto, Modelling gas-diffusion electrode performance with a wedge-meniscus model, Electrochimica Acta, 32, 1987, pp S. Srinivasan et al., Overview of Fuel Cell Technology, edited by Leo J.M.J. Blomen and Michael N. Mugerwa, Fuel Cell Systems, Plenum Press, New York, 1993, pp , Joseph I. Goldstein et al., Scanning Electron Microscopy and X-Ray Microanalysis, 2 nd ed., Plenum Press, New York, 1992, pp. 299, 349, 574, Southampton Electrochemistry Group, University of Southampton, Instrumental Methods in Electrochemistry, Ellis Horwood Limited, 1985, pp. 178, Bamforth, C. W., Cyclic Voltammetry of Hexachloroiridate (IV) CH 426 Laboratory Experiment, Chemical Educator, 5, 2000, 231-5, DOI /s a 64

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76 16. Steven Petrovic, Cyclic Voltammetry of Hexachloroiridate (IV): An Alternative to the Electrochemical Study of the Ferricyanide Ion, Chemical Educator, 5, 2000, S (00) , /s a 17. Bioanalytical Systems Inc., Instruction Manual for BAS Epsilon for Electrochemistry, version , accessed March 26, Dr. Huang Pty. Ltd., Electrochemistry Software Polargraph.com 5.1, accessed April 30, Klett J. W., T. D. Burchell, High Thermal Conductivity Mesophase Pitch-derived Carbon Foams, 43rd International SAMPE Symposium, May 31 - June 4, Aneheim, CA, 1998, accessed May 1, Rioji Anahara, Research, Development, and Demonstration of Phosphoric Acid Fuel Cell Systems, edited by Leo J.M.J. Blomen and Michael N. Mugerwa, Fuel Cell Systems, Plenum Press, New York, 1993, pp D. Raducha, W. Wieczorek, Z. Florjanczyk, and J. R. Stevens, Nonaqueous H 3 PO 4 -Doped Gel Electrolytes, J. Phys. Chem., 100, 1996, pp W. Wieczorek, Z. Florjanczyk, and J.R. Stevens, Proton Conducting Polymer Gels Based on a Polyacrylamide Matrix, Electromchimica Acta, 40, 13-14, 1995, pp

77 23. Kazuki Hirata et al., Preparation and Characterization of Highly Proton-Conductive Composites Composed of Phosphoric Acid-Doped Silica Gel and Styrene-Ethylene- Butylene-Styrene Elastomer, Journal of Sol-Gel Science and Technology, 17, 2000, pp

78 Appendix A. Cyclic Voltammograms for All Electrode Materials A.1. Cyclic Voltammograms with Varying Scan Rate Cyclic Voltammogram of PocoFoam TM with Varying Scan Rate Current (A) Potential (V) PocoFoam-5-15 PocoFoam-5-25 PocoFoam-5-50 PocoFoam PocoFoam Figure A.1.1. Voltammogram of sample PocoFoam TM with varying scan rate. 68

79 Cyclic Voltammogram of Poco HTC TM with Varying Scan Rate Current (A) Potential (V) PocoHTC-5-15 PocoHTC-5-25 PocoHTC-5-50 PocoHTC PocoHTC Figure A.12. Voltammogram of sample Poco HTC TM with varying scan rate. Cyclic Voltammogram of Sample NMB with Varying Scan Rate Current (A) Potential (V) NMB-5-15 NMB-5-25 NMB-5-50 NMB NMB Figure A.1.3. Voltammogram of sample Mitsubishi with varying scan rate. 69

80 Cyclic Voltammogram of Sample E1 with Varying Scan Rate Current (A) Potential (V) E E E E E A.1.4. Voltammogram of sample E1 with varying scan rate. Figure Cyclic Voltammogram of Sample CO2 with Varying Scan Rate Current (A) Potential (V) CO CO CO CO CO A.1.5. Voltammogram of sample CO2 with varying scan rate. Figure 70

81 Cyclic Voltammogram of Sample I-410 with Varying Scan Rate Current (A) Potential (V) I I I I I A.1.6. Voltammogram of sample I-410 with varying scan rate. Figure Cyclic Voltammogram of Sample II-440 with Varying Scan Rate Current (A) Potential (V) II II II II II A.1.7. Voltammogram of sample II-440 with varying scan rate. Figure 71

82 Cyclic Voltammogram of Sample III-480 with Varying Scan Rate Current (A) Potential (V) III III III III III A.1.8. Voltammogram of sample III-480 with varying scan rate. Figure Cyclic Voltammogram of Sample IV-460 with Varying Scan Rate Current (A) Potential (V) IV IV IV IV IV A.1.9. Voltammogram of sample IV-460 with varying scan rate. Figure 72

83 Cyclic Voltammogram of Sample IX-450 with Varying Scan Rate Current (A) Potential (V) IX IX IX IX IX A Voltammogram of sample IX-450 with varying scan rate. Figure Cyclic Voltammogram of Sample V-520 with Varying Scan Rate Current (A) Potential (V) V V V V V A Voltammogram of sample V-520 with varying scan rate. Figure 73

84 Cyclic Voltammogram of Sample VI-540 with Varying Scan Rate Current (A) Potential (V) VI VI VI VI VI Figure A Voltammogram of sample V1-540 with varying scan rate. Cyclic Voltammogram of Sample VII-420 with Varying Scan Rate Current (A) Potential (V) VII VII VII VII VII Figure A Voltammogram of sample VII-420 with varying scan rate. 74

85 Cyclic Voltammogram of Sample VIIA-500 with Varying Scan Rate Current (A) Potential (V) VIIA VIIA VIIA VIIA VIIA Figure A Voltammogram of sample VIIA-500 with varying scan rate. 75

86 A.2. Cyclic Voltammograms with Varying K 3 Fe(CN) 6 Concentration Figure A.2.1. Voltammogram of sample PocoFoam TM with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of PocoFoam TM with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) PocoFoam-1-15 PocoFoam-2-15 PocoFoam-5-15 PocoFoam PocoFoam

87 Cyclic Voltammogram of Poco HTC TM with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) PocoHTC-1-15 PocoHTC-2-15 PocoHTC-5-15 PocoHTC PocoHTC Figure A.2.2. Voltammogram of sample Poco HTC TM with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample NMB with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) NMB-1-15 NMB-2-15 NMB-5-15 NMB NMB Figure A.2.3. Voltammogram of sample Mitsubishi with varying K 3 Fe(CN) 6 concentration. 77

88 Cyclic Voltammogram of Sample E1 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) E E E E E Figure A.2.4. Voltammogram of sample E1 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample CO2 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) CO CO CO CO CO Figure A.2.5. Voltammogram of sample CO2 with varying K 3 Fe(CN) 6 concentration. 78

89 Cyclic Voltammogram of Sample I-410 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) I I I I I Figure A.2.6. Voltammogram of sample I-410 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample II-440 with Varying K 3Fe(CN) 6 Concentration Current (A) Potential (V) II II II II II Figure A.2.7. Voltammogram of sample II-440 with varying K 3 Fe(CN) 6 concentration. 79

90 Cyclic Voltammogram of Sample III-480 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) III III III III III Figure A.2.8. Voltammogram of sample III-480 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sampl IV-460 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) IV IV IV IV IV Figure A.2.9. Voltammogram of sample IV-460 with varying K 3 Fe(CN) 6 concentration. 80

91 Cyclic Voltammogram of Sample IX-450 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) IX IX IX IX IX Figure A Voltammogram of sample IX-450 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample V-520 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) V V V V V Figure A Voltammogram of sample V-520 with varying K 3 Fe(CN) 6 concentration. 81

92 Cyclic Voltammogram of Sample VI-540 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) VI VI VI VI VI Figure A Voltammogram of sample VI-540 with varying K 3 Fe(CN) 6 concentration. Cyclic Voltammogram of Sample VII-420 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) VII VII VII VII VII Figure A Voltammogram of sample VII-420 with varying K 3 Fe(CN) 6 concentration. 82

93 Cyclic Voltammogram of Sample VIIA-500 with Varying K 3 Fe(CN) 6 Concentration Current (A) Potential (V) VIIA VIIA VIIA VIIA VIIA Figure A Voltammogram of sample VII-500 with varying K 3 Fe(CN) 6 concentration. 83

94 Appendix B. SEM Images of Carbon Foam Samples Figure B.1. SEM image of Sample C4 at 50x magnification. Figure B.2. SEM image of sample PocoFoam TM at 50x magnification. 84

95 Figure B.3. SEM image of sample Poco HTC TM at 50x magnification. Figure B.4. SEM image of sample Mitsubishi at 50x magnification. 85

96 Figure B.5. SEM image of sample E1 at 50x magnification. Figure B.6. SEM image of sample CO2 at 50x magnification. 86

97 Figure B.7. SEM image of sample I-410 at 50x magnification. Figure B.8. SEM image of sample II-440 at 50x magnification. 87

98 Figure B.9. SEM image of sample III-480 at 50x magnification. Figure B.10. SEM image of sample IV-460 at 50x magnification. 88

99 Figure B.11. SEM image of sample IX-450 at 50x magnification. Figure B.12. SEM image of sample V-520 at 50x magnification. 89

100 Figure B.13. SEM image of sample VI-540 at 50x magnification. Figure B.14. SEM image of sample VII-420 at 50x magnification. 90

101 Figure B.15. SEM image of sample VIIA-500 at 50x magnification. 91