THE PERFORMANCE OF PLANAR SOLID OXIDE FUEL CELLS USING
A thesis presented to
the faculty of
the Russ College of Engineering and Technology of Ohio University
in partial fulfillment
of the requirements for the degree
Master of Science
David D. Burnette
November 2007 2 This thesis titled
THE PERFORMANCE OF PLANAR SOLID OXIDE FUEL CELLS USING
HYDROGEN-DEPLETED SYNGAS
by
DAVID D. BURNETTE
has been approved for
the Department of Mechanical Engineering
and the Russ College of Engineering and Technology
______
Gregory G. Kremer
Associate Professor of Mechanical Engineering
______
Dennis Irwin
Dean, Russ College of Engineering and Technology 3
Abstract
BURNETTE, DAVID D., M.S., November 2007, Mechanical Engineering
THE PERFORMANCE OF PLANAR SOLID OXIDE FUEL CELLS USING
HYDROGEN-DEPLETED SYNGAS (139 pp)
Director of Thesis: Gregory G. Kremer
Since solid oxide fuel cells can operate on fuel containing both hydrogen and
carbon monoxide, it may prove possible to remove hydrogen from syngas streams for other purposes and allow the fuel cell to operate with higher carbon monoxide levels. In
this study, electrolyte-supported solid oxide fuel cells were tested using hydrogen, syngas,
and hydrogen-depleted syngas (HDS) as fuel sources. It was found that reducing the
hydrogen flow rate by 50% while maintaining an equivalent fuel utilization rate increases
the polarization of the electrode by less than 5%. Carbon deposition was avoided when
the water content of the fuel reflected that of actual syngas. The drop in the ideal voltage
plus the increase in the resistance of the cell equated to a measured loss in power density
of 7.8%.
Approved: ______
Gregory G. Kremer
Associate Professor of Mechanical Engineering 4
Acknowledgments
I would like to convey my sincerest thanks to Dr. Greg Kremer for his constant support, patience, and wisdom over the past two years. I am very grateful for the time and work provided by all who have supported me in this undertaking, especially Dr. David
Bayless and the faculty and staff of the Ohio Coal Research Center for providing me with this opportunity and aiding me at every turn. I would like to thank my other thesis committee members, Dr. Gerardine Botte and Dr. Hugh Richardson, for their time and commitment. I also want to thank Dr. Ben Stuart, Mr. Shyler Switzer, and Mr. Micah
McCreery, all of whom were vital in making this work possible. Table of Contents
Abstract...... 3 Acknowledgments ...... 4 List of Tables ...... 7 List of Figures...... 8 Chapter 1 ...... 11 1.1 ENERGY DEMAND AND SOURCES...... 11 1.2 ENVIRONMENTAL CONCERNS ...... 15 1.3 SOFC OPERATION ...... 22 1.4 SIGNIFICANCE OF RESEARCH ...... 33 1.5 RESEARCH OBJECTIVES ...... 34 Chapter 2 ...... 35 2.1 INTRODUCTION ...... 35 2.2 LITERATURE REVIEW...... 35 Chapter 3 ...... 52 3.1 EXPERIMENT APPARATUS ...... 52 Chapter 4 ...... 66 4.1 CELL PREPARATION ...... 66 4.2 GAS COMPOSITIONS...... 71 4.3 ELECTROCHEMICAL MEASUREMENTS...... 75 4.4 MATERIALS ANALYSIS...... 75 Chapter 5 ...... 76 5.1 BCS1 RESULTS...... 76 5.1.1 Trial 1 Results...... 76 5.1.2 Trial 2 Results...... 80 5.1.3 Trial 3 Results...... 88 5.1.4 Trial 4 Results...... 89 5.1.5 Trial 5 Results...... 98 5.2 MATERIALS ANALYSIS...... 104 5.2.1 SEM / EDX...... 104 5.3 DISCUSSION ...... 111 5.3.1 Open Circuit Voltage ...... 111 5.3.2 Polarizations...... 114 5.3.3 Carbon Deposition...... 121 5.3.4 SOFC Systems...... 124 Chapter 6 ...... 127 6.1 CONCLUSIONS...... 127 6
6.2 RECOMMENDATIONS...... 129 6.2.1 Testing Methods...... 129 6.2.2 Future Work...... 130 References...... 133 Appendix A: OCV Calculations Using EES...... 137 Appendix B: Example Uncertainty Caclulation ...... 139
7
List of Tables
Table 4.1 Composition of Oxygen-Blown Pittsburgh No. 8 Syngas...... 72 Table 4.2 Experimental Flow Rates...... 74 Table 5.1 Calculated OCV for Various Fuels...... 112 Table 5.2 Average Measured OCV for Trials 4 and 5...... 113 Table 5.3 Calculated OCV for Trial 5 ...... 114 Table 5.4 Average ASR for all Trials...... 116 Table 5.5 Trial 4 Quick Change Uncertainy Calculations...... 118 8
List of Figures
Figure 1.1 U.S. Electricity Consumption Growth ...... 11 Figure 1.2 Electricity Generation Sources in the U.S...... 12 Figure 1.3 Average Consumer Price of Natural Gas ...... 13 Figure 1.4 IGCC Plant Diagram ...... 15 Figure 1.5 Gavin Power Plant Overview...... 18 Figure 1.6 IR-SOFC-GT System ...... 21 Figure 1.7 Diagram of Planar SOFC ...... 23 Figure 1.8 Equivalent Circuit Models...... 26 Figure 1.9 Siemens-Westinghouse Tubular SOFC...... 30 Figure 2.1 NETL Dusty Gas Model Results for Syngas...... 37 Figure 2.2 Typical Results for Fuel Dilution...... 38 Figure 2.3 Performance Losses with Increasing CO Content...... 40 Figure 2.4 SOFC Performance with High CO Conent ...... 41 Figure 2.5 Equivilant Circuit Model for EIS ...... 45 Figure 2.6 Impdence Spectroscopy Results by Jiang ...... 46 Figure 2.7 Siemens-Westinghouse SOFC System Design ...... 49 Figure 2.8 Biomass SOFC System ...... 50 Figure 2.9 SOFC-GT and SOFC-ST Designs...... 51 Figure 3.1 Diagram of SOFC Testing Facility ...... 53 Figure 3.2 Test System Flowchart ...... 54 Figure 3.3 Solartron Cart ...... 55 Figure 3.4 Button Cell Test Stand #1 ...... 56 Figure 3.5 MFCs in Gas Box...... 57 Figure 3.6 BCS1 Gas Box Setup ...... 59 Figure 3.7 Bubbler ...... 60 Figure 3.8 Vaisala HMT337 in Gas Box ...... 62 Figure 3.9 Temperature and Humidty Probe Placement...... 63 Figure 3.10 BCS1 Furnace Cross-section...... 64 Figure 3.11 SOFC Testing Facility Photo ...... 65 9
Figure 4.1 SOFC Photos ...... 66 Figure 4.2 Current Collector Diagram...... 68 Figure 4.3 Solartron Lead Connections ...... 69 Figure 4.4 Cell Assmbly Diagram ...... 69 Figure 4.5 Cell Assembly Loading...... 70 Figure 5.1 VI Scans for Humidified Hydrogen During Trial 1 ...... 77 Figure 5.2 Expected Nextcell Performance ...... 79 Figure 5.3 Change in ASR during Trial 1...... 80 Figure 5.4 VI Scans during Phase 1 of Trial 2...... 82 Figure 5.5 ASR Trend for Phase 1 of Trial 2...... 83 Figure 5.6 VI Scans for Syngas during Trial 2...... 84 Figure 5.7 ASR Trend for Syngas during Trial 2 ...... 85 Figure 5.8 VI Scans for HDS during Trial 2...... 86 Figure 5.9 ASR Trend for HDS during Trial 2...... 87 Figure 5.10 ASR Trend for Hydrogen during Trial 3...... 88 Figure 5.11 ASR Trend for Syngas during Trial 3 ...... 89 Figure 5.12 VI Scans for Hydrogen during Trial 4...... 91 Figure 5.13 Trial 4 Perfomance Comparison for Syngas and Hydrogen...... 92 Figure 5.14 Trial 4 Performance Comparison for HDS and Hydrogen...... 93 Figure 5.15 Trial 4 VI Scans over Time using Hydrogen ...... 94 Figure 5.16 Trial 4 Fast Performance Comparison for Syngas and Hydrogen...... 95 Figure 5.17 Trial 4 Potential Difference Between Hydrogen and Syngas...... 96 Figure 5.18 Impedance Spectra for Trial 4 Fuels ...... 97 Figure 5.19 Trial 5 Fast Performance Comparison for HDS and Hydrogen ...... 99 Figure 5.20 Trial 5 Impedance Specta for Hydrogen at Different Potentials...... 100 Figure 5.21 Impedance Spectra for Trial 5 Fuels ...... 101 Figure 5.22 Trial 5 Change in Impedance over Time...... 102 Figure 5.23 Trial 5 VI Scans over Time using Syngas...... 103 Figure 5.24 Trial 5 ASR ...... 104 Figure 5.25 Cell1 SEM Image ...... 105 Figure 5.26 Cell 1 Nickel Paint Composition...... 106 10
Figure 5.27 Cell 1 Composition Maps...... 107 Figure 5.28 EDXS Results for Platinum Wire...... 108 Figure 5.29 Cell 3 Composition Maps...... 109 Figure 5.30 Cell 5 Composition Maps...... 110 Figure 5.31 Cell 5 Composition Maps of Carbon Concentration...... 111 Figure 5.32 Electrode Polarization for Trials 4 and 5...... 120 Figure 5.33 C-H-O Ternary Diagram ...... 122
11
Chapter 1 Introduction
1.1 Energy Demand and Sources
In the United States, the demand for electricity increases every year. In 2005, the demand increased to 4,055 billion kWh, and has continued to rise ever since, as shown in
Figure 1.1 [1]. With a growing population, changing weather conditions, and a more energy-dependent culture, the U. S. will continue to experience this trend in the future.
As the demand for energy increases, scientists will be under greater pressure to find energy solutions that can support those requirements.
Figure 1.1: U.S. Electricity Consumption Growth [1]
12
As can be seen in Figure 1.2, the combustion of coal accounts for approximately
half of the electricity produced in the United States [1]. Other sources include nuclear
fission, natural gas, and renewable resources such as hydroelectric, wind, and solar power.
From this data it can be concluded that improving the way in which we utilize coal would
have the greatest overall impact on meeting energy demands.
Figure 1.2: Electricity Generation Sources in the U.S. [1]
Coal is very abundant in North America and thus is much more economical than other sources. In 2005, the cost of coal averaged $1.54/MBtu while natural gas was
$8.21/MBtu [2]. Furthermore, the price of coal has remained fairly stable compared to 13
other sources. One reason for this is that coal mines are located all throughout the country
such that the supply is not affected by natural disasters to the degree of other sources like
natural gas, which increased in price by 52% in winter 2005 because of damage done to
refineries by Hurricane Katrina [2]. The price of natural gas continues to be volatile as
shown in Figure 1.3.
Figure 1.3: Average Consumer Price of Natural Gas [2]
Many factors affect what fuel types are used to generate electricity in the United
States. Nuclear power faces much opposition and a new atomic reactor has not been constructed since the early 1970s. Renewable sources, on the other hand, appear very favorable but are limited by location and sometimes cost. Both of these issues are less of 14 a concern with coal and natural gas, which can be shipped and pumped respectively.
Additionally, both natural gas and gasified coal have been used for power generation with alternative technologies, such as high-temperature fuel cells, which could prove necessary as the demand for energy outpaces the construction of transmission lines.
Since coal is the most abundant and economical source of electricity, it is likely that it will remain as the primary means of power generation in the United States for some time. Because of this, finding better ways of utilizing coal and also allowing for distributed generation will have long-term advantages.
Integrated gasification combined cycle (IGCC) power plants are a way in which to use coal to produce electricity in a more efficient and environmentally friendly manner.
IGCC plants are expected to be the dominant utility plant in the future. They operate by producing a gaseous fuel from oxygen and solid fuel that is later combusted to operate a gas turbine and steam turbine. Furthermore, as concerns over greenhouse gases increase the construction of IGCC plants, which allow for easier sequestration of greenhouse gases, is much more likely.
15
Figure 1.4: IGCC Plant Diagram [3]
Figure 1.4 is a diagram of a theoretical IGCC plant. Here, pulverized coal is converted to a synthetic gas (syngas) in the gasifier. Pollutants are reduced through processes during gasification and also downstream prior to entering the gas turbine.
Waste heat is recovered and used to create steam, which provides more power via a steam turbine.
1.2 Environmental Concerns
There has been significant work done in the last few decades on producing a more environmentally friendly means of generating electricity. The price and availability of coal has made it the primary source of electricity in the U.S., but the negative environmental impacts have long been cause for concern. The combustion of coal results in the formation of sulfur dioxides (SOX), nitrous oxides (NOX), and particulate matter, 16
all of which have harmful effects on humans and their surroundings when released into the air [4-6].
Sulfur dioxide, after combining with water in the atmosphere, becomes acid rain, which causes deforestation and is harmful to marine life. Sulfur dioxide is also harmful to humans if exposed, causing minor irritations, respiratory problems, and possibly other, more serious conditions such as pulmonary diseases. People with asthma can have severe reactions at even low concentrations [4].
Nitrous oxide, which becomes nitrogen dioxide, is also harmful to vegetation and humans. NO2 appears as the yellowish-brown color seen as smog. Prolonged exposure to
nitrogen dioxide can cause chronic-lung disease. An increased rate of hospitalization and
respiratory disease is common in children living in areas with a high concentration of
nitrogen dioxide. Nitrous oxides also cause the formation of ozone (O3) at ground level,
which can affect the cardiac and respiratory systems. Increased hospital admissions and
mortality rates have been linked to high ozone levels, especially in the elderly and those with pre-existing respiratory problems [5].
Particulate matter may pose the biggest risk to humans. Problems similar to
sulfur dioxide and nitrogen oxide are also associated with the particulate matter released
from power plants. Emphysema and other severe illnesses have been linked to exposure of fine particulate matter, and recent research has linked PM with premature death, indicating children, the elderly, and those with existing heart conditions are the most at
risk. Repeated exposure increases the chance of chronic respiratory disease and
cardiorespiratory mortality [6]. 17
Another pollutant causing concern today is mercury. Mercury is very toxic to humans, causing neurological problems to those exposed. Coal contains a small amount of mercury naturally, which is released into the atmosphere from power plants and eventually finds its way into lakes and rivers where it forms methylmercury and enters the food chain. Consuming fish tainted with mercury can cause serious problems to humans, especially to pregnant women. In addition to delayed neurotoxicity and other problems, mercury also has a severe impact on the reproduction system because developing nervous systems are more susceptible than that of adults [7].
Because of these problems, there has been much work on removing these pollutants from coal plant emissions. Electrostatic precipitators, scrubbers, SCRs, controlled combustion, and other techniques have been very successful in lowering harmful emissions. Still, approximately 11.5 million tons of SO2 and 5.6 million tons of
NO2 were emitted in 2000 [8]. 18
Figure 1.5: Gavin Power Plant Flowchart [9]
Figure 1.5, an illustration of the power generation cycle at Gavin Power Plant, shows the complexity of conventional pollution control approaches. In this scenario, controls for oxides of sulfur and nitrogen as well as particulates occur after the combustion of the pulverized coal. As would be expected, the design and operation of one control device is not independent of the others. Technologies that increase the overall efficiency of the system would reduce the cost associated with these devices. As shown in Figure 1.4, these types of controls are also needed, to a certain extent, in gasification plants.
Most of the improvements in emission control technology have been the result of federal legislation. The EPA was created in 1969 to address such issues. A year later the 19
Clean Air Act was passed, which greatly expanded existing legislation [10]. There have been many updates to the Clean Air Act over the years and tougher emission regulations have been a historical trend since its enactment. It is because of this pressure on the power industry by the government that there is currently a vast amount of research being conducted into cleaner energy sources.
Since coal is both the chief provider of electric power and the primary contributor of many types of air pollution, it is clear that there is a dire need for improving the way in which we get power from coal. Coal gasification offers a way to improve the efficiency of electric power plants. Used conventionally, coal gasification plants are predicted to have an efficiency of up to 60%, which is twice that of some existing power plants [11].
One very promising approach is using gasified coal as fuel in planar solid oxide fuel cells.
Gasification of coal is a thermal process in which the coal is converted into a gas made up of potential fuels. Two of the products of gasification are hydrogen and carbon monoxide, both of which can be used as fuels in a solid oxide fuel cell and make up the bulk of coal syngas [11].
Additionally, heat produced by the fuel cell can be used elsewhere for power generation. Heat being released from the fuel cell can be recovered from the exiting gas and used to create steam for power generation. Very efficient systems have been developed which combine solid oxide fuel cells with secondary systems. The SOFC may be combined with Stirling engine technology, which creates an effective hybrid system.
Models have predicted an increase in efficiency when SOFCs are combined with Stirling engines [12-13]. One analysis showed that operating the fuel cell at lower electrical 20
efficiencies was favorable due to a greater amount of heat produced for use by the engine
[13].
Solid oxide fuel cells operated at high pressures in conjunction with gas turbines
result in a very efficient power generating system. Fuel cells operate at a higher
efficiency at elevated pressures and models have predicted extremely efficient systems
for this configuration. Chan's model predicted efficiencies above 80% [14]. This system can be found in Figure 1.6. Internal reforming (IR) refers to the reformation of methane within the fuel cell stack. Coal syngas could also be used in a similar fashion. The extremely high efficiency of this system equates to a large reduction in pollutants released into the atmosphere. The presence of multiple recuperators here emphasizes the large amount of high quality waste heat produced in the system, which isn’t the case with other fuel cell types. 21
Figure 1.6: Schematic IR-SOFC-GT System [14]
Another benefit of fuel cell technology is in the distribution of electricity. SOFC
stacks can be operated at any size without sacrificing performance. In fact, most fuel cell
experimental results are reported as energy per unit of area. This is in contrast to turbines
found in power plants that lose efficiency at smaller scales, which indicates that Stirling
engines could be better suited for coupling with fuel cells in some situations [15]. This would allow smaller facilities, such as businesses and apartment complexes, to generate their own power, saving losses in electrical efficiency due to transmission over power
lines. 22
The most notable aspect of the solid oxide fuel cell over competing technologies is its ideal fit into the current infrastructure. Unlike other types of fuel cells, SOFCs are more tolerant to sulfur and can run on a variety of fuels. The SOFC’s ability to operate at high efficiencies with existing fuels such as natural gas, propane, diesel fuel and gasified coal sets it apart from other types of fuel cells. Introducing the SOFC into coal-burning power plants to be operated alongside more conventional methods is a logical first step toward converting to cleaner power sources.
The solid oxide fuel cell has a very high electrical efficiency, typically operating above 50% at reasonable current densities, and can operate using the most abundant fossil fuel in the country. The waste heat it generates can be used to produce even more electricity, increasing the plant's performance. The end result would be a very efficient method of generating electricity with an immense reduction in harmful emissions [16].
1.3 SOFC Operation
A fuel cell is a device in which two or more chemicals are combined to produce electricity. In a solid oxide fuel cell powered by syngas, hydrogen, carbon monoxide, and methane can all be used as fuel. In a planar solid oxide fuel cell, the fuel is sent to the anode, which is normally made up of Ni/ZrO2 or another cermet. The other chemical, oxygen, is sent to the cathode, typically in the form of air. The cathode is made from doped lanthanum manganate. Figure 1.7 depicts a solid oxide fuel cell using hydrogen as fuel. 23
Figure 1.7: Diagram of Planar SOFC [17]
Between the anode and cathode is the electrolyte. Made from yttrium-stabilized zirconia or another ionic conductor, the electrolyte's role is to conduct oxygen ions from the cathode to the anode. The reaction of hydrogen and oxygen, shown in Equation 1, is used to produce electrical energy [18]. SOFCs using syngas also oxidize carbon monoxide as shown in Equation 2. Notice that the oxidation of hydrogen produces water and the oxidation of carbon monoxide produces carbon dioxide (CO2) [18].
H + 1 O → H O (1) 2 2 2 2
CO + 1 O → CO (2) 2 2 2 24
As oxygen enters the cathode, it undergoes the reaction in Equation 3. The two electrons present in the reaction enter the cathode from the anode after passing through an external load. After the reaction, the negative ions are then conducted across the electrolyte [18].
1 O + 2e − → O 2− (3) 2 2
When the oxygen ions reach the anode, they meet up with the fuel, which is oxidized. The oxidation of hydrogen and carbon monoxide, which are the products of coal gasification, are shown in Equation 4 and Equation 5, respectively. The two
electrons produced leave the anode through the external load before entering the cathode,
thus supplying the electrical load [18].
2− − H 2 + O → H 2O + 2e (4)
2− − CO + O → CO2 + 2e (5)
In fuels containing both carbon monoxide and hydrogen, another reaction will occur, shown here in Equation 6. Although the oxidation of both hydrogen and carbon monoxide is possible, this water-gas shift reaction usually occurs prior to CO oxidation because of the faster rate of reaction. This occurs even in dry fuels, since water is formed in the anode. The hydrogen formed in this reaction is then oxidized in the fuel cell. The shift reaction also occurs for hydrocarbons, which are broken down into hydrogen and carbon monoxide in the SOFC and can be oxidized [18]. Methane is also present in coal gas, but to a much lesser extent. Equation 7 shows the reforming of methane that is common at high temperatures.
CO + H 2O → CO2 + H 2 (6) 25
CH 4 + H 2O → CO + 3H 2 (7)
The reaction in Equation 6 is likely at high temperatures. A solid oxide fuel cell is
considered a high temperature fuel cell, typically operating between 800°C and 1000°C.
Lowering the temperature of the fuel cell causes the ions to move slower across the
electrolyte. Since the SOFC does not require rare catalysts, it is cheaper and more
efficient than other fuel cell types.
Equation 8 is commonly referred to as the Nearnst Equation and is used to predict the open circuit voltage of the fuel cell. Equation 8 describes the voltage produced in a fuel cell during the oxidation of hydrogen. It can be seen that the potential of the cell is a function of the Gibbs free energy at standard pressure. The second half of the equation adjusts the value for non-standard pressure, in which "a" represents the activity of the
named species. Equation 9 shows Nernst Equation for pure carbon monoxide oxidation.
ΔG is the change in Gibbs energy, n is the number of electrons per mole, R is the
universal gas constant, T is the temperature in Kelvin units, and F is Faraday's constant
[18].
1/ 2 ΔG RT ⎡aH 2 (aO2 ) ⎤ V = + ln⎢ ⎥ (8) nF nF ⎣ aH 2O ⎦
ΔG RT ⎡a (a )1/ 2 ⎤ V = + ln⎢ CO O2 ⎥ (9) nF nF a ⎣⎢ CO2 ⎦⎥
When using multiple fuels, the potential of the cell can be modeled by assuming
the two voltages exist in a parallel circuit [19]. However, much of the carbon monoxide is
converted to hydrogen during the shift reaction so many models predict the operating 26 voltage based on the oxidation of hydrogen alone. Figure 1.8 shows an equivalent circuit model for both approaches. The resistances given here are calculated from the equilibrium of the reactions above, previously measured data, and modeling.
Figure 1.8: (a) Equivalent circuit for multiple reactions and (b) single reaction [19]
Another method of predicting the OCV of a fuel cell is by using the Wagner equation, shown here in Equation 9 [20]. With this equation it is clear to see why SOFCs are sometimes called “oxygen concentration” cells, since the ideal voltage is determined by the amount of oxygen present on each side of the conductor. In this equation, the oxygen partial pressure on the cathode side is found in the numerator of the natural log function and the anode side oxygen partial pressure is found in the denominator.
RT ⎡ p'' ⎤ V = ln⎢ O2 ⎥ (10) 4F p' ⎣⎢ O22 ⎦⎥ 27
The result of the above equations is called the ideal or reversible voltage. This
potential is not the actual cell voltage. There are several sources of voltage loss, which
are called polarizations. The most dominant of these in a solid oxide fuel cell is the ohmic
polarization. The ohmic loss is a drop from the ideal voltage caused by resistances to
current flow within the fuel cell. The greatest source of ohmic loss comes from the
electrolyte, especially in electrolyte-supported cells. This loss does decrease with
increasing temperature, but construction materials for fuel cells limit the operating
temperature. The Ohmic loss is given in Equation 11. Here, "r" is the area specific
resistance and "i" is the current density [18].
ΔVohmic = i × r (11)
Another loss is called the activation polarization. The Tafel equation, provided here as Equation 12, is used to model this. The exchange current density, io, is unique to the type of fuel cell being considered. The activation loss is a function of the current density, this value, and the constant A, which has been found experimentally. This polarization is the result of slow reactions on the electrodes, such that a portion of the voltage produced is used to force the reactions that push the electrons [18]. The Tafel
equation is only valid at current densities greater than the exchange current density.
Typically, activation losses are very small for solid oxide fuel cells.
⎛ i ⎞ ⎜ ⎟ ΔVact = Aln⎜ ⎟ (12) ⎝ io ⎠
The Tafel equation is an empirical equation. The value of A can be expressed by
Equation 13, in which α is called the charge transfer coefficient. Equation 11 shows that 28
as temperature increases, the value of A increases. However, the exchange current
density also increases with temperature, so the drop in voltage is actually reduced at
higher temperatures. It's important to note that the reactions taking place on the
electrolyte are assumed to be equilibrium [18].
RT A = (13) 2αF
A more correct form of Equation 12 is the Butler-Volmer Equation, which relates
the activation polarization to the current density and is shown here in Equation 14. It
must be noted that there are activation losses on the anode and cathode side, both of
which have different current exchange densities. However, these are typically lumped
together in one equation.
⎡ ⎛αne FΔVact ⎞ ⎛ (1−α)ne FΔVact ⎞⎤ i = io ⎢exp⎜ ⎟ − exp⎜ ⎟⎥ (14) ⎣ ⎝ RT ⎠ ⎝ RT ⎠⎦
The concentration polarization, sometimes called mass transport loss, occurs due
to the change in concentration of the reactant at the electrode. In the case of SOFCs, this
occurs from insufficient oxygen at the cathode or fuel at the anode. Equation 15 shows
how the drop in voltage can be modeled using an exponential curve fit. Here, m and n
are constants. The concentration losses are much more noticeable at higher current
densities [18].
ΔVconc = mexp(n ⋅i) (15)
Obviously, fuel utilization plays a role in mass transport losses. Operating at high fuel utilizations increases the likelihood that there will be insufficient fuel at the triple
phase boundary needed for the reaction. For this reason, higher current densities will 29 result in greater concentration losses, as would be expected in Equation 15. However, mass transport losses are also related to how quickly the molecules of fuel or oxidant diffuse through their respective electrodes. Using a fuel with a low diffusivity will cause greater concentration losses than when using a fuel with a high diffusivity.
Another loss in performance in fuel cells is due to fuel crossover at the electrolyte or internal currents, both of which are referred to as leakage. Typically, the leakage current is combined into the Tafel equation. However, this loss is very small for high temperature fuel cells. The loss due to leakage can be ignored since the thick electrolyte reduces the leakage to less than 1% [13] on SOFCs and even less on electrolyte- supported cells.
It is important to note that the planar shape is not the only type of SOFC. Tubular fuel cells, as Figure 1.9 depicts, have shown huge potential for power generation. Unlike planar cells, the tubular design can be easily pressurized, thereby increasing performance
[18]. Planar cells rely more heavily on seals, which are less reliable at high temperatures and pressures.
The tubular design is not without drawbacks though. Although the basic theory is the same for tubular cells as for planar cells, the interconnection of tubular cells increases the resistance and lowers the efficiency. While planar cells do have interconnects with considerable resistance, they are typically much lower than the tubular shape. This is due to the longer electron path shown here in Figure 1.9. 30
Figure 1.9: Tubular SOFC by Siemens-Westinghouse [21]
The efficiency of a fuel cell is expressed in many ways. The maximum efficiency possible is shown in Equation 16, and is sometimes called the thermodynamic or Gibbs efficiency. However, another term must be applied to account for the polarizations.
Therefore, this value is multiplied by the ratio of the observed voltage to the ideal voltage.
Another factor is fuel utilization. For the most part fuel utilization is not considered as part of the efficiency calculation, mostly because fuel utilization is typically about 50% in fuel cells. Incorporating this value into the total efficiency calculation will result in
Equation 17, where ηcurrent is the current efficiency defined as the current divided by the total amount of electrons flowing through the cell. Therefore, if fuel utilization was 100% 31
this value would be 1. However, since concentration losses are greater at higher fuel
utilizations this will rarely be the case. It is worth noting that the theoretical efficiency for
fuel cells is dependent on the phase of the product and efficiency claims should be
examined closely to see what is actually being reported [18]. Since solid oxide fuel cells
are purposely operated at lower (~50%) fuel utilization under the assumption that there is
downstream combustion and power generation, the overall system efficiency is a much
better description of the technology.
ΔG f ηGibbs = (16) Δh f
ηtotal = ηGibbs ⋅ηvoltage ⋅ηcurrent (17)
Solid oxide fuel cells may be run on gasified coal. The composition of this gas varies depending on the method of gasification, but carbon monoxide and hydrogen are
always components [18]. Relative to carbon monoxide, which is harmful to humans,
hydrogen has many more uses. Hydrogen is a source of clean combustion and is the only
fuel for many other types of fuel cells. Because of this, the idea of using a portion of the
hydrogen from coal gas for other purposes is very attractive. Hydrogen could be used in
the automotive industry to power cars through both combustion and electrochemical
reactions in other fuel cells.
The one drawback to removing hydrogen from the fuel stream is that the
performance of SOFCs decreases as the ratio of carbon monoxide to hydrogen increases
[22]. The oxidation of pure carbon monoxide is certainly possible, but a steady operating
condition has yet to be achieved. This is because of additional losses associated with CO 32
oxidation, the changing reaction rates involved, and because of carbon deposition on the anode that decreases the number of oxidation sites [22-24]. When hydrogen is present,
the water produced can initiate the shift reaction with the CO, creating hydrogen that
oxidizes at a stable rate. As the amount of hydrogen in the fuel decreases, the shift
reaction does as well. For this reason, fuels with higher concentrations of carbon
monoxide could benefit from adding steam into the anode.
Steam addition cannot prevent all carbon deposition though. SOFCs have been
shown to "coke up" over time at many levels of CO concentration. This is because the
carbon present in the fuel cell is likely to form elemental carbon at the temperatures
typical for solid oxide fuel cell use [22]. Equation 18 shows the typical mechanism for
carbon deposition. From this it can be concluded that carbon deposition will likely occur
on some level because even if the shift reaction occurs, carbon dioxide and carbon
monoxide still exist in larger quantities. However, the extent may be minimal.
CO2 + C ↔ 2CO (18)
Another source of carbon deposition comes from the use of methane. Equation 19
shows what is referred to as methane decomposition. This reaction is very probable at
high temperatures. For syngas compositions with little or no methane, this is a smaller
concern compared to using natural gas as a fuel. Avoiding methane decomposition is an
argument in support of using an external reformer. This way, solid carbon formed will
not poison the fuel cell anode.
CH 4 ↔ C + 2H 2 (19)
33
1.4 Significance of Research
Planar solid oxide fuel cells (PSOFC) are an efficient means of generating
electricity without the harmful pollutants emitted in traditional power plants. Before
PSOFCs can be used for distributed power, a reliable fuel source must be tested. Gasified coal provides the necessary reactants needed for operation of the PSOFC. However, the higher concentration of carbon monoxide found in coal gas causes higher losses and may lead to coking, which results in an even greater loss in performance and cell life. The fuel cell may also become less stable since the hydrogen present does not create sufficient water in the anode to react with the CO via the shift reaction.
Since there are numerous other uses for pure hydrogen, including use in other types of fuel cells, removing as much of it as possible from the gas stream would be beneficial. However, this would have to be weighed against the loss of performance in the PSOFC operating on higher CO concentrations.
Determining the effects of changes in fuel quality is crucial to establishing coal- powered PSOFCs as a viable source of distributed power. There has been some testing of planar solid oxide fuel cells with various fuel qualities, but additional testing with high
CO concentrations is greatly needed in order to examine the effects on performance and cell life. Another important aspect of this work was to identify and separate ohmic losses from electrode polarization losses.
Electrolyte-supported cells were used during testing. In these cells, the electrolyte is, by far, the thickest portion of the cell. For this reason, it was expected that the ohmic polarization would be far higher than other contributors. This is usually the case with 34 solid oxide fuel cells but even more apparent with electrolyte-supported cells. For this reason it was believed that similar performance might be attained for coal syngas as seen with pure hydrogen. Proving this and determining the effects of reducing the hydrogen content in the syngas will help determine the appropriateness of using coal syngas and hydrogen-depleted syngas in solid oxide fuel cells.
1.5 Research Objectives
The goal of this research was to examine the change in the performance of a planar solid oxide fuel cell when switching from syngas to hydrogen-depleted syngas.
Voltage measurements were taken as a function of current. Impedance spectroscopy was used to isolate the effects on the electrodes. The percent change in the measured voltage at different fuel qualities was compared across trials and used to determine the effects of the fuel change.
Material analysis techniques were used to determine if any negative changes occurred during operation that might result in the degradation or destruction of the cell over time. This will determine if any carbon deposition occurred and to what extent it may have occurred.
The principal objective of these experiments was to determine the relationship between the measured voltage of the fuel cell and the type of fuel gas being used. Since the effects of other impurities, such as hydrogen sulfide, were neglected, these findings were used to conclude what the change in voltage was as a result of higher CO concentrations using hydrogen-depleted coal gas in planar solid oxide fuel cells. 35
Chapter 2 Literature Review
2.1 Introduction
Solid oxide fuel cell research has become incredibly popular in recent years
because of its suitability for use in distributed electricity generation and the existing
energy infrastructure. Much of the current work focuses on finding materials better suited
for use in SOFCs, such as thin films with lower ohmic losses that allow operation at
lower temperatures, and also better ways of testing them. An important issue facing those
working in the SOFC field is making the cells more resistant to contaminants such as
sulfur and mercury, which are found in coal syngas. In addition to this, researchers are
also working on materials and methods that reduce the losses in the SOFC. This includes
lowering the electrode polarizations and area-specific resistance.
2.2 Literature Review
The ability to oxidize carbon monoxide makes solid oxide fuel cells advantageous
in power generation. The ability to use carbon monoxide as a fuel is not found in low
temperature fuel cells and is beneficial since fuels derived from hydrocarbons and coal
are rich in CO. In fact, the ideal voltage created during the oxidation of CO is greater
than that of hydrogen [18]. For example, methane can be reformed internally, externally,
or by some combination of the two depending on the cooling requirements of the system. 36
There are problems associated with using carbon monoxide as a fuel. First of all, the concentration polarization is increased. Research has shown that the concentration polarization is higher because of the adsorption of carbon monoxide into the anode [22-
24]. Jiang and Virkar also showed that the adsorption of the gas into the anode increased the time it took for carbon monoxide to reach oxidation sites and for carbon dioxide to return to the surface of the anode [25]. This causes a slower overall reaction time.
Mass transport losses vary with many factors. The porosity, tortuosity, and temperature as well as the fuel used are all factors. Multiple gases diffuse through the anode or cathode at different rates as a result of different mechanisms and overall diffusivity. Researchers at the National Energy Technology Laboratory (NETL) have calculated the equilibrium compositions of coal syngas in the anode from the cell surface to the electrolyte border [26]. This method can be used to predict concentration losses during operation. Applying this data to the Wagner equation, which was shown above and will be discussed later, shows how a small concentration polarization can exist even at open circuit conditions. 37
Figure 2.1: Dusty Gas Model Results for Coal Syngas by NETL [26]
Figure 2.1 shows the results for the NETL’s dusty gas model (DGM). Here, it is clearly seen that different gas species migrate through the anode at different rates.
Therefore, it can be concluded that mixed gas fuels having components with lower diffusivities will have higher concentration losses.
Mass transport losses are not only the result of diffusion through an electrode.
Excessive dilution of the desired gas increases this effect. Figure 2.2 shows how increasing the amount of dilution gas decreases the operating voltage of the cell for cells with thick anodes. 38
Figure 2.2: Typical Results for Fuel Dilution [25]
Although activation polarization is typically low in solid oxide fuel cells, the slow
reaction time of carbon monoxide oxidation equates to a higher activation polarization
compared to using pure hydrogen. Holfappels showed that the multiple and periodically
changing reaction rates for the oxidation of carbon monoxide on Ni-YSZ electrodes are a
function the “passivation and reactivation of oxidation sites” on the electrode [22].
Matasuzaki and Yasuda found that the rate of oxidation of hydrogen was much better
than that of carbon monoxide [24], and Jiang and Virkar documented the activation polarization was much higher when using carbon monoxide as a fuel than when using hydrogen [25]. Matasuzaki and Yasuda concluded that the lower potential caused by the 39
high activation made the use of pure carbon monoxide as fuel undesirable. However, the
same conclusions were not stated for all fuels containing CO [24].
Fuels containing both carbon monoxide and hydrogen do not result in the same performance losses as pure carbon monoxide. This is because the oxidation of hydrogen
results in water, which can react with the CO in the water-gas shift reaction described
earlier. Yongtaek modeled the kinetics of the reaction at different temperatures to show
CO conversion for use of the hydrogen in fuel cells [27]. Because of this reaction, Jiang and Virkar concluded that results typical of hydrogen could be achieved in a mixed fuel if the H2/CO ratio was greater than 1:1 [25]. This way, each mole of H2O formed from the
oxidation of hydrogen would react with carbon monoxide to form a new mole of H2 and no CO would actually be oxidized. This is beneficial for fuels containing water vapor, such as gasified coal. The performance of anode supported SOFCs with different hydrogen to carbon monoxide ratios is shown in Figure 2.3. 40
Figure 2.3: Performance Losses with Increasing CO Content [25]
There is much interest in using reformed hydrocarbons in place of pure hydrogen as fuel in SOFCs because of their availability. Reformed methane and coal gas both contain high levels of CO and H2. Scientists have tested gases with different H2/CO ratios
in order to determine the change in performance of the fuel cell. Sasaki et al. tested
different H2/CO ratios in a solid oxide fuel cell and concluded that higher water vapor
concentrations in the fuel led to higher H2/CO ratios, which increased cell performance,
because of the shift reaction [28]. Weber et al. tested a solid oxide fuel cell using various
ratios and determined that there was no noticeable difference in the performance of pure 41
H2 and 25% CO and 75% H2. They found that there was no change in current density
when the CO content was as high as 85% [29]. This is reproduced in Figure 2.4.
Figure 2.4: SOFC Performance with High CO Content [29]
However, the results of this study showed severe degradation at very high concentrations of CO, which occurs because of carbon deposition, and anode polarization losses increased. The authors also injected pure methane into the cell and determined that
stable operation was possible with an appropriate steam to carbon ratio [29]. Trembly
tested a PSOFC with simulated coal gas and showed that increasing the concentration of
water in the coal gas can successfully prevent coking [30]. 42
Other researchers have also tested pure hydrocarbons with PSOFCs. Although oxidation of methane is possible, hydrocarbons are typically reformed at the anode into
CO and H2 mixtures. Researchers at Northwestern University tested PSOFCs with pure
methane and found that stable operation without coking occurred at lower temperatures,
but high current densities or steam would be required to reduce carbon deposition at high
temperatures [31]. Reformed methane has also been tested on Ni-YSZ electrodes at
Kyoto University, where researchers concluded that developing an anode catalyst that is
not susceptible to carbon deposition would increase efficiency because the level of water
vapor required to remove carbon from the cell would be lower [32].
Researchers at Ohio University have successfully tested PSOFCs using a
simulated coal syngas containing hydrogen sulfide, determining that ASR degradation
can be severe with higher concentrations [30]. Later work demonstrated that trace
elements in the syngas have a smaller effect on cell performance [33].
Some materials prevent carbon deposition. A CaO catalyst was found to prevent carbon deposition and encourage steam reforming of the methane [31]. Murray and
Barnett were able to operate a PSOFC on dry methane without carbon deposition when using a nickel and ceria oxide anode [34], which has since become a popular material in
commercial fuel cells. Propane has also been tested in SOFCs with Ni-YSZ anodes and
no carbon deposition was observed [35]. Typically, a higher current density prevents this.
Much of the research in the fuel cell industry has involved developing new anode
materials that are cheaper, more conductive, catalyze the oxidation reactions better, and
tolerate higher levels of contaminants. However, the majority of anodes being sold 43
commercially are made up of Ni-YSZ, Ni-GDC, or some combination of the two. Doping
zirconia with other elements is common but YSZ and GDC anodes are the most common.
Ceria has been proven to be a suitable anode material because it is a mixed conductor, meaning it conducts ions and also electrons at high temperatures. Having a
mixed conductor as the anode increases performance by expanding the triple phase
boundary (TPB) where the reaction takes place. Also, carbon deposition tends to occur
less often on ceria anodes. However, reduced ceria is an ineffective anode for
hydrocarbons and the expansion caused by this can cause the anode to peel away from the electrolyte [36]. Sometimes a second layer, usually YSZ, will cover a GDC anode in order to prevent reduction. Ceria anodes have also been used with copper in place of nickel since nickel tends to catalyze carbon deposition. These anodes are considered to have the best potential for the direct oxidation of hydrocarbons [37].
Scandia-stabilized zirconia (ScSZ) is another fuel cell material that has been tested for use with hydrocarbon fuels. Research has shown that Ni-ScSZ anodes can be operated at low steam to carbon ratios with little carbon deposited. Deposition can be avoided completely at high temperatures and current density with these cells [38]. Also, the addition of GDC increases the performance of such cells by expanding the TPB [39].
Because there is so much potential in using fossil fuels with PSOFCs, the development of new materials and operating states that can accommodate them is important. Since the losses in the cell decrease with temperature, developing temperature- resistant materials is very attractive. Reducing these losses at lower temperatures is more favorable though. There are new, less proven, electrolyte materials that also have 44 improved ionic conductivity. However, Leng tested a planar solid oxide fuel cell with a thin film YSZ electrolyte which allowed the cell to be operated at a lower temperature because of the decreased resistance [40]. Thin YSZ electrolytes have also been successfully tested without deposition at Northwestern University at temperatures less than 700°C. The cells performed well at low temperatures but required higher current densities to prevent coking at high temperatures [35].
The role of the electrolyte is to conduct oxygen ions from the cathode to the anode, and sometimes to give structural support. Other materials besides YSZ have been used for this. GDC has a very high ionic conductivity but being a mixed conductor causes leakage currents that lower the cell voltage. Leakage currents also occur when using a material called LSGM, a very good ionic conductor, as the electrolyte. During cell preparation, nickel from the anode interacts with the electrolyte, crosses to the cathode, and creates leakage currents via small short circuits in the cell. The nickel and LSGM can also react to produce a non-conducting phase [41]. ScSZ electrolytes do not have these drawbacks, show favorable ionic conductivity, and have good mechanical strength.
In order to optimize the electrolyte performance, scientists have created electrolytes consisting of multiple layers. The resistance of ScSZ electrolytes can be lowered by the addition of a ceria interlayer [42]. This method takes advantage of the good conductivity of the ceria interlayer while allowing the insulative layers to avoid its drawbacks. ScSZ electrolytes are the basis for an existing 1kW SOFC system [43].
Fuel cell performance is often evaluated using impedance spectroscopy.
MacDonald described the process, which involves using a small amplitude alternating 45 current and measuring the total impedance in the fuel cell. This procedure is repeated for a spectrum of frequencies and the results are fitted to an equivalent circuit model that is used to identify values such as conductivities or capacitances at the interfaces [44].
Mogensen studied the kinetics of SOFC anodes using impedance spectroscopy and found three separate processes contributing to the polarizations, which are seen as arcs in the impedance plot. He stated that the conductive and capacitive parts of the low frequency arcs decreased when water was added to the gas [45]. Wagner also tested SOFCs, finding only two impedance arcs, and described the effects of water on cell performance. He used an equivalent circuit in which the electrolyte was modeled with a resistor, the cathode was modeled with a resistor and constant phase element (CPE) in parallel, and the anode processes were modeled with two R/CPE processes [46]. The anode, electrolyte, and cathode processes were arranged in series, as shown in Figure 2.5.
Figure 2.5: Equivalent Circuit Model for EIS [44]
46
Jiang's impedance study also found two processes, shown in Figure 2.6, at the anode of the SOFC and he noted that the two of the three arcs claimed by Mogensen behaved in the same fashion and were essentially the same. Like Mogensen, Jiang found that the conductivity of the anode increased when water was added to dry hydrogen although the open circuit voltage was lower. Both impedance arcs were reduced in size, although the higher arc, which was also more temperature dependent, was reduced to a greater extent. This was possibly due to the dissociation of water aiding in hydrogen diffusion or bridging the interface. From his research, Jiang concluded that the low frequency arc was the result of hydrogen dissociative adsorption/diffusion on the surface of Ni particles and that the high frequency arc was due to hydrogen transfer from the electrode surface to the electrolyte surface, followed by a charge transfer process at the electrolyte surface [44].
Figure 2.6: Impedance Spectroscopy Results by Jiang [44] 47
Experimental methods have varied little when testing solid oxide fuel cells.
Impedance spectrometry is commonly used to measure SOFC performance [44-46].
Several types of mircrostructural analyses, such as SEM [24,30], can be used to
determine the extent of carbon deposition and nickel agglomeration caused by the fuel’s
ability to break the nickel bonds of the anode. Due to the space requirements, size, and the effects of cell interconnects in stacks, much of the fuel cell research has been
conducted using single cells. For mixed fuel testing, simulated gases have been used in
place of actual gasified coal or reformed methane [30,33]. This enables the researcher to
verify the effect of individual components of gas and to also more accurately know the
components of the fuel. Simulated gases usually consist of components of interest and
inert gases. However, existing components in supplied fuel can affect the performance of
the cell even if it does not play a role in the electrochemical oxidation. Jiang and Virkar
showed that because of porosity, the anodic concentration polarization is lower when using lighter inert gas [25].
It is clear from previous research that increasing the carbon monoxide concentration in a mixed fuel results in greater polarizations. However, impedance spectroscopy has shown that the presence of water vapor at the Ni-YSZ anode reduces these polarizations by converting more carbon monoxide into hydrogen. This is extremely important for coal gas, which contains a large amount of carbon monoxide and water vapor. If SOFCs can operate effectively using hydrogen-depleted coal gas then the excess hydrogen could be used elsewhere, possibly in mobile sources. 48
Because of the nature of gas transport and the very valuable (high temperature) waste heat produced in solid oxide fuel cells, the net efficiency of any SOFC system is heavily reliant on the mechanical design. Many designs have been presented in literature.
Some of these, including various flow designs and peripherals, are tailored for use with specific fuels or for a specific setting.
Although the Chan model [14] showed SOFC-GT systems to have a possible efficiency of over 80%, existing systems are not this efficient. Siemens-Westinghouse currently manufactures a 250kW tubular SOFC-GT system that operates at 55%, which is still better than existing power plants. Larger power plant versions are expected to be around 70% efficient. The Siemens-Westinghouse design is shown in Figure 2.7.
49
a)
b)
Figure 2.7: The Siemens-Westinghouse SOFC-GT Design [21]
50
As shown above, the Siemens-Westinghouse design was built to operate on
natural gas, but similar systems can be constructed for use with syngas, which is usually
delivered to the system at a higher temperature. Figure 2.8 is a diagram of a fuel cell
power system operated on syngas derived from biomass. In this scenario, extra effort
must be taken to cool the system because of the high temperature of the gasifier and lack
of reforming reaction with methane. Otherwise, the cell would be damaged.
Figure 2.8: A Biomass SOFC System [47]
Another study of a SOFC-GT system resulted in an efficiency of 70% which the authors believed could be used for automotive purposes. The addition of a Stirling cycle, they claimed, would raise the system efficiency to 80% [48]. Figure 2.9 shows two 51
configurations that were presented. In this study, it was argued that Stirling engines, external combustion engines that run on any heat source, surpass microturbines in efficiency and would therefore be a better match for high temperature fuel cell systems
[48]. This would be especially true at very small scales such as in mobile applications, reinforcing the fuel cell’s advantages in a distributed generation setting.
Figure 2.9: Proposed SOFC-GT and SOFC-ST Designs [48] 52
Chapter 3 Experimental Apparatus
3.1 Experiment Apparatus
The experiments proposed in this work were conducted in the SOFC Testing
Facility located within the Ohio Coal Research Center. This facility was designed to test high temperature fuel cells using coal syngas. Figure 3.1 shows the placement of the various components of the safety system, which detects and monitors all test parameters and gas levels within the cabinets, canopy, and room. In the event that conditions in the lab violate the established operating limits, the lab can be sealed and ventilated and the experiment can be terminated. Details on the laboratory and established safety limits can be found in the facility’s documentation [49].
53
SOFC Canopy SOFC Canopy Solartron Cart (mobile) Exterior Door Gas Delivery Cabinets S olar Gas Storage Cabinets tron Safety System Interface Inj1/Air SCBA Masks R.O. Storage (above) Gas Storage CO2 O Hydrogen Sulfide Scrubber Cabinets 2 N2 Hydrogen Sulfide Sensors N 2 Magnetically Locked Doors
O2 Safety System Warning Light CO2 O2 CO CO H2 Gas Delivery CO Cabinets H2
H2 RO H2
H2S/CO Inj 2
OCRC Entrance
Figure 3.1: Diagram of SOFC Testing Facility within the OCRC [49]
As shown below in Figure 3.2, gas stored in the delivery cabinets is fed into the fuel cell test stand via mass flow controllers. Cathode gases, air or pure oxygen, are delivered directly to the stand while the anode gases are first plumbed through a bubbler that creates the desired humidity level. The cell voltage is measured by the Solartron Cart and stored electronically. 54
N2 air MFCs SOFC Solartron Gas Storage O2
CO H2 N2
Pitt. No.8 MFCs Bubbler
dry coal gas
PC measured voltage
Figure 3.2: Test Setup
The Solartron Cart is a mobile station consisting of two potentiostats, a frequency analyzer, and a PC with display. All VI scans, impedance sweeps, and applied loads are created with this device. Although this cart is located within the SOFC Testing Facility,
either in or outside of the canopy, remote connection is possible from outside of the
laboratory. The Solartron can also perform impedance sweeps, applying small AC signals
between given frequencies and recording the results. Data was analyzed using CorrWare
software.
55
Frequency analyzer
Potentiostat 1
Potentiostat 2
Figure 3.3: Solartron Cart
The fuel cell tests were performed on Button Cell Test Stand #1 (BCS1). BCS1 is an advanced test stand design capable of testing fuel cells under numerous conditions.
These include temperature, flow rates, humidity, cell size, and current density. A picture of BCS1 is shown in Figure 3.4. 56
Figure 3.4: Button Cell Test Stand #1 in SOFC Canopy 57
As mentioned above, the desired fuel flow rates are delivered by mass flow controllers located on the test stand. The stand houses eight mass flow controllers, each plumbed to a specific gas cabinet. Gases include hydrogen, carbon monoxide, carbon dioxide, nitrogen, and air. There are also two controllers designated for injection gases.
MFCs
Furnace Furnace limit
Bubbler Anode Other Inlet
Figure 3.5: MFCs in Gas Box 58
The MFCs are located within the gas box, which also houses solenoid valves to shut off flow in the event that the furnace loses power. Anode gases are combined into
the “anode mix” just before the solenoid valve. The “cathode mix” also runs through a
solenoid valve. A third gas path and valve exists for blends containing hydrogen sulfide, which is intentionally injected further downstream after humidification. 59
RH Sensor
MFCs
Anode
Cathode
Inj Anode
Figure 3.6: BCS1 Gas Box Setup
60
Coal syngas contains a significant amount of water and pure hydrogen is often
humidified as well. Therefore, a water bubbler is installed onto BCS1 to produce the
specified water content. The bubbler, shown in Figure 3.7, is a stainless steel pressure vessel precision machined for use at high pressures. Gases enter the top of the bubbler and are piped to the bottom and through an aerator. Gas bubbles travel up through the bubbler and are forced out of the exit tube which travels out of the bottom of the bubbler cylinder. This avoids the need for excess insulation on gas tubing. TC
From MFCs
float
Refill sensor
frit
To furnace
Figure 3.7: Bubbler 61
The correct partial pressure of the fuel gas exiting the bubbler was achieved by
controlling the temperature of the water within the bubbler. The thermocouple within the
bubbler feeds a voltage signal to a heat tape controller. The desired bubbler temperature
is set on the front panel of the stand, shown in Figure 3.5
Since a small temperature profile could exist in the bubbler, the humidity
delivered to the fuel could vary depending on the height of the water within the cylinder.
Because of this and the desire to automate the testing process, an interference sensor
triggers the automatic refilling of the bubbler. The water used to refill the bubbler is
plumbed into the canopy from a reverse osmosis system.
The delivered humidity is measured with a Vaisala HMT337 humidity sensor
which is designed for high humidity use. The bubbler temperature can be offset from the
theoretical value in order to achieve the correct performance. Since the sensor cannot be
exposed to syngas, data for equivalent flow rates of nitrogen and hydrogen can be used to
predict the performance for syngas. The sensor is shown in Figure 3.8. The sensor is
inserted into the flow path after the bubbler. During syngas testing, it is bypassed.
After the fuel gas is humidified a drop in temperature could cause water to condense out of the stream. To prevent this, gas lines are heat traced and insulated
between the bubbler and furnace. A heat tape is used and the temperature is maintained
with a Watlow controller and thermocouple. The gas lines are typically kept several
degrees above the bubbler temperature.
62
Probe leads
T, RH outputs
Figure 3.8: Vaisala HMT337
Figure 3.9 shows the setup of the humidity sensor probes. Since the gas may be near saturation, the humidity probe is heated and prevents condensation from forming and offsetting the measurement. In order to calculate an accurate humidity, the temperature is measured as well. The temperature and humidity probes must be in close proximity for an accurate calculation. Inserting the humidity sensor vertically also prevents condensation within the sensor.
The sensor is programmed to transmit signals corresponding to both temperature and the partial pressure of water. These signals are relayed through the data acquisition 63 system that records all data from the stand onto the PC. A pressure transducer and the partial pressure reading are used to calculate the water content of the gas.
Humidity
Temperature
Figure 3.9: Temperature and Humidity Probes
64
Solid oxide fuel cells operate at very high temperatures and therefore need a heat source before the reaction can take place. A cross-section of the furnace of BCS1 is shown in Figure 3.10. The furnace temperature is set using the thermocouple inside the furnace yet outside of the cell flanges. The temperatures within the gas streams on the anode and cathode sides are both recorded as well. air
cell
syngas
Figure 3.10: BCS1 Furnace Cross-section 65
Due to the long testing times in fuel cell research and the use of low fuel
utilization rates for button cells, a large amount of fuel gas is needed. To ensure a
continuous supply of fuel, ventilated gas cabinets, shown in Figure 3.11, contain redundant gas canisters connected to a switchover valve. When the pressure in any single bottle drops below a specified value, the valve automatically switches to a fresh one. An alert is sent to the operator electronically when bottles need replaced.
Canopy
Solartron Cart
Gas Delivery Cabinets
Figure 3.11: SOFC Testing Facility Photo
66
Chapter 4 Experimental Methods
4.1 Cell Preparation
The cells selected for this testing were electrolyte-supported fuel cells. This means that the electrolyte provides most of the structural support for the cell. While the thicker electrolyte means a greater ohmic resistance, mass transport losses and leakage losses are lower than anode-supported cells. Figure 4.1 shows the anode of the cell.
a) b)
Figure 4.1: SOFC with platinum leads on a) cathode and b) anode sides
These button cells, purchased from Nextech Materials, were made up of a tri-layer sandia-stabilized zirconia electrolyte to provide excellent ionic conductivity. The tri-layer, scandia arrangement reduces leakage currents and has exceptional mechanical strength. A proprietary diffusion barrier exists on both the anode and cathode side. 67
The electrodes are very different in design. The anode is made up of Ni-GDC
with a layer of protective Ni-YSZ, mirroring an LSM-based cathode. A diagram of the
cell layers and testing arrangement is below. Note that the diameter of the electrodes measures 1.3 centimeters while the electrolyte is 2.8 centimeters. This allows for easier handling.
After the cells were constructed, current collection meshes were added. A platinum mesh was used to provide good conductivity and a lack in reactivity without encouraging carbon deposition. The mesh was attached to the anode with nickel paint and to the cathode with LSM. A close look at Figure 4.2 will reveal the mesh and conductive wire. 68
Electrolyte
Anode Cathode
NiPt paint paint LSM paint
Pt mesh & wire
Figure 4.2: Current Collector Diagram
The platinum wires were welded to silver wires which were run out of the bottom of the furnace. Here, they are connected to the Solartron for simple VI scans
69
Ref 1
Cathode CE (+)
Ref 2 Anode
WE (-)
Figure 4.3: Solartron Connections
The leads of the SOFC must not touch the surface of the cell or there will be a risk of fracture. To avoid this and prevent fuel leakage, mica seals were used as shown in the
Figure 4.4. These are compression seals and are tightened when the cell is put into the furnace. When conducting impedance sweeps, an alumina gasket can be used to keep the leads separated.
Alumina gasket
Pt leads SOFC
mica seals
Figure 4.4: Cell Assembly Diagram
70
The cell and seals were sandwiched between alumina flanges to complete the assembly. The completed assembly and flanges are shown below. Notice how the excess electrolyte is sandwiched between the flanges while the electrodes are exposed to the gas flow. The assembly is loaded into the furnace and kept under compression during testing.
a) b)
c) d)
Figure 4.5: Cell Assembly Loading 71
4.2 Gas Compositions
Coal syngas forms as a result of the incomplete combustion of solid coal. The components of the gas are a product of the elements present in the coal, as well as the temperature and pressure of the gasifier. The ideal coal syngas would be made up of high quantities of hydrogen and carbon monoxide. However, other factors such as the rate of diffusion of oxygen within the coal and the physical design of the system also have a large impact on the gasifier’s results.
Typically, coal syngas contains significant amounts of carbon dioxide and steam.
In the case of SOFCs, the precedence of steam is actually favorable to increase the hydrogen content via the shift reaction while the carbon dioxide has no benefit and actually dilutes the fuel. Water can also be added to the process if needed.
Since the gasifier bed is a reducing environment there are additional volatiles formed. These are included in Table 3.1, which shows syngas results for a Pittsburgh No.
8 coal chosen by the OCRC for these tests. Although these make up a small fraction of the syngas they can have a large effect on the performance of some SOFC materials.
These have been studied elsewhere and are not within the scope of this work [26, 27].
Therefore, nitrogen will be substituted in their place.
72
Table 4.1: Composition of Oxygen Blown Pittsburgh No. 8 [26]
Species Mole Fraction (%)
CO 37.73
CO2 15.38
H2 24.9
H2O 16.21
CH4 4.53
H2S .95
COS .02
NH3 .27
HCN .02
The experiment consisted of six trials. First, the cell was heated to temperature at a rate of 1.7 °C per minute under air and nitrogen flows. After the desired temperature of
800°C was reached, humidified hydrogen was slowly introduced over the course of one
hour. Open circuit data was then collected for several hours before any load was applied. 73
Any change between gas compositions was performed over the course of one hour.
After testing syngas, the hydrogen concentration was reduced by half, which also
increases the concentration of the other components. The mass flow controllers were
adjusted to accommodate the new fuel composition and to maintain the needed fuel to be
oxidized at the test current at the stated fuel utilization. The bubbler temperature was
increased such that the new water vapor content was reached. After the system reached
the desired output, electrochemical measurements were conducted as in the other trials.
The flow rates for all trials are given in Table 3.2. Since data from the preceding
test influenced the planning of the following test, a further explanation of these values will be discussed in the following chapter.
74
Table 4.2: Experiment Flow Rates
Fuel Component Flow Rate Percentage (mL/min)
351.09 83.8% Humidified H2 Hydrogen 67.87 16.2% H2 O
139.65 24.9% Syngas H2
90.86 16.2% H2 O
CO 211.44 37.7%
86.37 15.4% CO2
32.53 5.8% N2
87.16 14.22% HDS H2
113.41 18.50% H2 O
CO 263.93 43.06%
107.81 17.59% CO2
40.60 6.63% N2
75
4.3 Electrochemical Measurements
The voltage was recorded via the data acquisition device described in the previous
chapter. The data for each trial was compared in order to determine initial voltage
changes after a change in fuel quality and also to determine the rate of change in voltage
or cell resistance over time.
A VI scan was conducted every hour. The current was increased at a rate of 1mA
per second while the corresponding voltage was measured several times per second. The
slope of this line was considered the area-specific resistance (ASR) while the y-intercept at zero current density was the open circuit voltage. Impedance spectra were taken across the entire cell at an operating potential of .5V.
4.4 Materials Analysis
Several methods were used to study the fuel cells in question. A scanning electron
microscope (SEM) was used to examine cells before and after testing to look for any
changes in the surface or cross section. Since SEM images are easier to obtain on
conductive materials, detached anodes were used rather than entire cells. Electron
Dispersion X-ray Spectroscopy (EDXS) was used in an attempt to look for the existence of carbon. Spectra were compared for unused cells, cells tested with hydrogen, and cells tested with syngas.
76
Chapter 5 Results & Discussion
5.1 BCS1 Results
5.1.1 Trial 1 Results
The cell used in Trial 1 was prepared and loaded into BCS1 using the method
described in the preceding chapter. The nitrogen flow rate of 350 mL/min was incrementally replaced by an equivalent flow of hydrogen. The bubbler remained at room temperature for the duration of this trial.
Once the transition from nitrogen to hydrogen was complete, a current of .995 amps, corresponding to .75 amps per square centimeter, was applied to the cell for one
hour. Immediately following this, a galvanodynamic sweep was performed from the initial value of .995 amps to the maximum value of 1.99 amps before sweeping back to open circuit conditions and returning to the initial load of .995 amps. From here, the cell was operated galvanostatically for another hour. This cycle was repeated constantly
during all of Trial 1. VI sweeps were performed at a rate of 1 milliamp per second with
data points being recorded every second.
It was noted during this test that the furnace temperature as measured close to the
anode was 820°C during the entire testing cycle. This was because the furnace temperature in BCS1 is controlled with the thermocouple located at the wall of the furnace, as shown in Figure 3.10. Since temperature close to the cell was constantly monitored and consistent, this value was used for theoretical calculations. 77
The open circuit voltage was measured before each measurement described above.
This value was consistently found to be 1.0653 V. This corresponds quite well with the
theoretical value of 1.069 V. Calculations for ideal voltages can be found in Appendix 1.
For Trial 1, the bubbler was operated at room temperature. However, since this value was not controlled and was not continually recorded it is only assumed to be constant.
As expected, increasing the current during the VI sweeps did lower the voltage and resulted in a fairly linear relationship. Non-linear aspects are the result of activation and concentration losses, dynamic effects during scanning, or failure to reach a stable operating point for the cell. Figure 5.1 shows the results of VI sweeps near the beginning and end of Trial 1 using humidified hydrogen.
Figure 5.1: VI Scans for Humidified Hydrogen during Trial 1 78
Despite having a stable open circuit voltage, the performance of this cell did
change over time. The term area-specific resistance (ASR) is sometimes used to define
the slope of this line over a given range such as the most linear portion of the curve.
However, in this work ASR will indicate the average slope of the linear curve fit over the
entire range of data. As seen in Figure 5.2, the ASR increased over time from .577 Ω-cm2 to .608 Ω-cm2. This increase is typical of SOFCs that have yet to reach a stable operating
point. Figure 5.3 shows a slight increase in the ASR of these cells as reported by the
manufacturer for approximately the same time period. Although the change in the ASR
during Trial 1 was only around 5%, this was higher than expected based on the data in
Figure 5.3, which indicated that the change in the ASR value varies from cell to cell or
that there was a problem during the testing which caused a greater drop in performance.
79
Figure 5.2: Change in Nextcell Performance over Time [46]
The cell failed approximately 24 hours into testing and corresponded to an attempt to increase the fuel utilization from 2% to 20%. The method of failure cannot be discussed here due to a nondisclosure agreement with the manufacturer. However, this failure did impact the way in which further tests were carried out as the cause was not known at the time.
When looking at the ASR data for Trial 1 in Figure 5.2, it can be seen that the increased value was a linear trend and likely not the result of a change in fuel utilization.
The blue line provided in this figure is the slope of a linear line fit to the data within the given voltage range. The lower overall value is a result of the removal of the non-linear 80 portion of Figure 5.2 which typically corresponds to the activation polarization. However, unlike the ASR value taken for the complete data set, the second resistance value appeared more volatile. This indicated that the shape of the trend itself was changing and the conclusion was made that the cell had not reached a stable operating point.
Figure 5.3: Change in ASR during Trial 1
5.1.2 Trial 2 Results
The cell used in Trial 2 was assembled in the same fashion as the cell in Trial one.
Instead of a switchover between nitrogen and hydrogen when the furnace reached 800°C, the use of syngas was attempted and gas compositions were changed over time in three 81
phases. However, since Phase 1 was the first syngas trial in the SOFC Testing Facility the
carbon monoxide shutoff valves had yet to be open. Therefore, only diluted hydrogen was supplied to the cell at first. Hydrogen was fed to the cell at a rate of 139.65 mL/min while carbon dioxide flowed at 86.37 mL/min and nitrogen entered at 32.53 mL/min. The bubbler temperature was maintained at 55.7°C, which corresponds to a theoretical water vapor pressure of 16.2 kPa or 16.2% assuming an atmospheric pressure of 100 kPa.
The open circuit voltage for this gas mixture was .944 volts, slightly higher than the ideal potential of .94 volts. While this difference could be due in part to experimental error, it could also indicate lower water content than anticipated during this phase. This will be discussed in the next section.
The resistance of this cell was slightly higher than that in Trial 1. The fact that the fuel is diluted means that higher concentration losses are expected, but a direct comparison cannot be made since the variation from cell to cell has not been established.
However, the change in the ASR for Trial 2 was comparable to that of Trial 1 and the
drop is clear in Figure 5.4
Figure 5.5 shows the ASR values for the entire 35 hours of this phase of the test.
Unlike Trial 1, the cell used in this trial does seem to reach a stable operating point. The
figure shows how the value converges but unlike Trial 1 the value calculated for the
limited range also remains stable. This means that even though the curve is not completely linear it is still consistent.
82
Figure 5.4: VI Scans from Trial 2 Phase 1
83
Figure 5.5: ASR Trend for Phase 1 of Trial 2
Once carbon monoxide was allowed to enter the cell (at 211.44 mL/min) and establish the desired fuel mixture, the open circuit potential climbed to .97 volts, which matched the theoretical value. The area-specific resistance of the cell was mostly constant over time, typically around .66 ohm-cm^2 with little variation during this ~24 hour phase of the test.
Figure 5.6 shows the characteristic curve of the cell at two points in time during the test. Part of the second data set has been removed so that the first data set can be seen.
In Figure 5.7, the measured ASRs for different ranges are given. While not identical, both 84 lines appear stable, showing little change after the first data point, which was likely taken before the new fuel mixture had reached the cell.
Figure 5.6: VI Scans for Syngas during Trial 2
85
Figure 5.7: ASR Trend for Syngas during Trial 2
The next phase of Trial 2 called for the reduction of hydrogen in the fuel stream.
The hydrogen flow rate was reduced by 50%. No method of hydrogen reduction from a
syngas stream is being presented here so it is assumed that there would be no effect to the
other components of the gas. This causes an increase in the mole percentage of the other
components, including water since the high temperatures found in gasification would
prevent condensation of the water during the removal of hydrogen. This means that the
bubbler temperature on BCS1 needed to be increased in order to accommodate the new theoretical mixture. This was not done at first but was corrected during this phase of the
test. 86
The measured OCV fell to .94 volts after switching to this hydrogen-depleted syngas (HDS). During this ~24 hour period of testing, the ASR remained constant as it did during the syngas phase. Figure 5.8 shows the characteristic curve at two points in time. It is important to note that the curves are nearly identical, with the later curve having only a small offset in voltage. This is due to the increase in the bubbler temperature that took place during the test.
Figure 5.8: VI Scans for Trial 2 Using HDS
87
The measured ASR, taken over the course of this phase, is shown in Figure 5.9.
There is little change in the total ASR. There was an increase beginning at VI sweep
number 5, which corresponds to the increase in the bubbler temperature. This appears to
have stabilized by the end of the test. Regardless, the average of the ASR values was
calculated to within 2 mV of the beginning and end values.
After this phase of testing, the intent was to operate the cell on humidified hydrogen. However, the cell failed during the time between gas changes. The failure
mode was identical to the previous trial and cannot be discussed here.
Figure 5.9: ASR Trend for Trial 2 using HDS 88
5.1.3 Trial 3 Results
The cell preparation and startup procedure for Trial 3 was identical to Trial 1. The
first phase of the experiment entailed the use of humidified hydrogen. This was to be followed by testing with syngas and also HDS. There was no variation in cell temperature or flow rates.
The open circuit voltage when using humidified hydrogen was .99 volts, slightly lower than the ideal value because of leakages. As can be seen in Figure 5.10, the ASR of the cell remained nearly constant after the fourth VI sweep. So despite having a lower than expected OCV, the stabilization time for this cell was very short.
Figure 5.10: ASR Trend over Time for Trial 3 using Hydrogen
89
There was no large change in the ASR when the cell was exposed to syngas.
Figure 5.11 shows that there was some change in the ASR over time but no clear
conclusions can be drawn from this since the cell failed after the fifth VI sweep. Post-test
observations of the cell assembly revealed that the sealing of the assembly was quite poor.
This was corrected in later trials.
Figure 5.11: ASR Trend over Time for Trial 3 using Syngas
5.1.4 Trial 4 Results
Following Trial 3, operation within the SOFC Testing Facility was halted for
laboratory upgrades. When Trial 4 began, the temperature in the room was significantly 90
higher than in previous tests. As a consequence, the bubbler temperature was also higher
which led to a lower OCV when using pure hydrogen. When the bubbler temperature was
maintained the measured value of 1.01 volts fit closely with the theoretical value.
Due to cell failures in previous trials, extra care was taken to avoid undue stress
on the electrodes. Previously, three silver wires were used to deliver current to the potentiostat. These were replaced by single wires on each electrode in order to reduce
stress on the cell. Also, gas changes were conducted only during open circuit conditions and electrochemical measurements were taken in a fashion to avoid sudden shock.
As expected, the ASR of the cell increased initially (Figure 5.12). In order to account for any changes in resistance over time, gas changes were performed quickly and instantaneous measurements were taken. After this, the fuel gas was replaced with hydrogen again. Since the bubbler temperature does not reach the desired value instantaneously, it had to be kept at a single temperature during these tests. Therefore, the hydrogen was humidified at higher temperatures (55.7°C) than in previous trials, which corresponds to the temperature used to simulate syngas.
91
Figure 5.12: VI Scans for Trial 4 using Hydrogen
When exposed to syngas, the OCV dropped to .98 volts. As can be seen in Figure
5.13, the cell performance was fairly consistent when using hydrogen or syngas with the exception of the theoretical voltage difference. There is a slight difference in the slope of the characteristic curve depending on the direction of the VI sweep, likely due to cell kinetics and bottlenecking of ions in the electrolyte. This occurs when using faster sweep rates. Better results are given when decreasing the current.
92
Figure 5.13: Trial 4 Performance Comparison for Syngas and Hydrogen
After data was taken during the syngas phase, the cell was operated on hydrogen.
A complete recovery in the voltage was observed. After this, an HDS mixture was sent to the cell anode. Since the HDS mixture involved increasing the bubbler temperature several degrees, there was a short delay for this measurement. As was the case for syngas, the cell resistance remained almost constant after switching from hydrogen to the new fuel. Aside from the theoretical difference in voltage, cell performance was basically the same.
93
Figure 5.14: Trial 4 Performance Comparison for HDS and Hydrogen
The cell was then allowed to operate using humidified hydrogen for seven days.
Aside from an approximately 20mV increase in the OCV after the third day, performance remained consistent. The only difference in the characteristic curves, taken hourly over the course of an entire week, is small potential offsets. It is possible that the increase in
OCV is due to further reduction of the anode but this is more likely due to a variation in the humidity level of the fuel gas. As mentioned in Chapter 3, it is possible that a temperature profile exists in the bubbler, meaning the humidity delivered to the anode gases is dependent on how full the bubbler is. The sudden change seen after the third day 94
of testing was thought to be due to the automatic refilling of the bubbler. Figure 5.15
shows the characteristic curve for the cell taken over a period of several days.
Figure 5.15: Trial 4 VI Scans over Time using Hydrogen
On the last day of testing, different fuel gas mixtures were tested again in the same fashion as before. As shown in Figure 5.16, the characteristic curves are nearly identical if the offset is neglected. However, the two lines do not appear perfectly parallel, with the offset increasing slightly at high current densities.
Figure 5.17 shows the difference between the measured potentials when using hydrogen and syngas. There is clearly a linear increase, indicating a slight difference in
ASR. Because of this, impedance spectroscopy was used to examine the cells further. 95
Figure 5.16: Trial 4 Fast Performance Comparison for Syngas and Hydrogen 96
Figure 5.17: Trial 4 Potential Difference between Hydrogen and Syngas
Impedance measurements were taken of the cell while operating on each fuel. A
0.5 VDC potential was applied with a 10mV AC signal sweeping between 0.1 Hz and
100 kHz. Figure 5.18 displays the response for hydrogen, syngas, and HDS. Here, the x- axis indicates the real impedance while the y-axis is the negative imaginary impedance. 97
Figure 5.18: Impedance Spectra for Trial 4 Fuels
The impedance data clearly shows that there is an increase in real impedance when using syngas or HDS. However, the increase was small enough to go unnoticed when only looking at the characteristic curves and uncertainty of the ASR. In this case, the x-intercept corresponds to the pure resistance, or ionic resistance, while the two semi- circles correspond to the activation and concentration loses found in the cell, which are typically the non-linear portions of the characteristic curve.
As noted in Chapter 2, Jiang concluded that the high frequency arc was related to a charge transfer process while the low frequency arc was a result of adsorption and diffusion through the electrodes. Since a reference electrode was not used, the exact 98 values for the anode polarizations cannot be determined. However, both semi-circles did increase, specifically the high frequency arc.
Following the impedance measurements, Trial 4 was concluded by reducing the furnace temperature over the same time interval as the initial heat up. Nitrogen and air flows continued until the cell was removed for study.
5.1.5 Trial 5 Results
After the standard startup procedure, the cell in Trial 5 was exposed to humidified hydrogen. The bubbler temperature was maintained at 55.7°C as in the previous trial. A series of gas changes took place during which VI scans and impedance sweeps were recorded, yielding results similar to Trial 4. Figure 5.19 shows VI scans when using humidified hydrogen and HDS. As in Trial 4, the curves are almost identical without the potential offset. 99
Figure 5.19: Trial 5 Fast Performance Comparison for HDS and Hydrogen
Since the fuel cells used in this study have shown both a significant part to part
variation and also a change in ASR over time, impedance spectroscopy was used
extensively in Trial 5. Impedance sweeps were carried out at 0.5 V and 0.75 V for
different fuels. Figure 5.20 shows the difference in the impedance spectra at the different
potentials. The value of the total impedance on the x-axis corresponds to the ASR at the specified point. Since the characteristic curve is not linear, the value at 0.75 V is larger
than that at 0.5 V. However, the leftmost x-intercept point, which can be considered the
ohmic resistance, remains constant at different potentials. The increase in impedance at
the higher potentials is the result of activation losses that dominate the overall resistance 100
at those points. This corresponds with Jiang’s findings, which state that the high frequency arc is a product of a charge transfer process [44].
Figure 5.20: Trial 5 Impedance Spectra at Different Potentials
Impedance sweeps were carried out for each fuel source as done in Trial 4. Once
again, there was an increase in the impedance when switching from hydrogen to syngas.
This value also increased when reducing the hydrogen content by half. However, this difference was much smaller. Figure 5.21 shows impedance spectra for all gasses. 101
Figure 5.21: Impedance Spectra for Trial 5 Fuels
The cell was then allowed to operate using humidified hydrogen for an extended
period of time. As in Trial 4, there was a change in the ASR over time. Impedance
sweeps were conducted periodically during this phase of the testing. Figure 5.22 shows
that while the overall ASR did increase over time, the change was only due to the ohmic resistance. The two semi-circles corresponding to non-linear polarizations did not change
in magnitude. 102
Figure 5.22: Trial 5 Change in Impedance over Time
The next step of the experiment was to let the cell operate using syngas for an extended period of time. During this phase of the test, the ASR remained stable. The characteristic curve for syngas is given for three different days in Figure 5.23. Following the first day, thre was no change in this curve.
103
Figure 5.23: Trial 5 VI Scans using Syngas
Following the syngas phase of the test, the hydrogen flow rate was reduced by half. As in Trial 4, the cell was allowed to operate for several hours before the bubbler temperature was increased. Figure 5.24 shows the ASR over the course of Trial 5. During the HDS phase of the test, the ASR increases rapidly. After further measurements, the test was shut down and the cell was studied for damage. This will be discussed in more detail in the materials analysis section. 104
Figure 5.24: Trial 5 ASR Over Time
5.2 Materials Analysis
5.2.1 SEM / EDX
SEM and EDX were used to study the spent cells in this research. Figure 5.25 is an SEM image of the cell used in Trial 1. The platinum mesh is clearly visible although partially covered with nickel paint, as described in Chapter 4. Since the nickel paint has a different porosity than the anode and the amount to which this covers the anode varies from cell to cell, it is difficult to take into account these layers in modeling. Also, the platinum wires cover a large portion of the anode themselves. This would be less of an 105
issue on larger cells using the same current collector mesh. Therefore, gas transport issues are better addressed on industrial-sized fuel cells.
Figure 5.25: Cell 1 SEM Image
EDX was used to determine the composition of the exposed surfaces. As expected, the NiO layer (paint and surface layer of the anode) was confirmed. Since the goal of the
EDX measurements was to look for deposited carbon, it was possible to only concentrate
on the nickel paint layer. They concentration of nickel in this area and lack of available
oxygen make it the most likely place for deposition to occur.
Since this cell had only been exposed to humidified hydrogen, carbon was not expected to be found. However, small amounts of carbon were detected in this region, as shown in Figure 5.26. The very small concentrations in this area fall below what can be 106 reliably deemed carbon. Other possibilities were that the carbon detected was present on the platinum just under the NiO layer or that it was the result of a carbon-containing adhesive used to attach the removed anode to the holder.
Element Wt.
%
C 3.74
O 16.51
Ni 79.50
Pt .25
Figure 5.26: Cell 1 Nickel Paint Composition
Figure 5.27 shows the location of different elements on the sample. Notice that nickel and oxygen, pictures c and d, respectively, are more uniformly distributed over the surface than carbon, which is shown in picture b. An important observation made was that the carbon seems almost nonexistent in regions completely covered by nickel paint and appears to match the platinum mesh in other areas. This implies that the carbon present is likely within the platinum wire. 107
a) b)
c) d)
Figure 5.27: Cell images for Trial 1 including SEM image (a), carbon content map (b), nickel content map (c), and oxygen content map (d)
To confirm this, a portion of the platinum wire was measured using EDXS. The platinum wire did show much a higher carbon composition than the overall cell (~8%).
Given this and the fact that the carbon distribution follows the platinum mesh well, it can be easily said that the majority of carbon found on this sample was a result of the mesh.
The small amounts of carbon on other parts of the cell was more difficult to account for since it could have been present in the nickel paint, adhesive, or just a product of the platinum wire underneath the NiO layer. 108
Figure 5.28: EDXS Results for Platinum Wire
SEM images and composition maps were also taken for the cell used in Trial 3 and are shown in Figure 5.29. In Trial 3 syngas was used so carbon deposition was possible. In this case, however, the nickel paint layer on the surface of the platinum mesh sufficiently covered the mesh such that less carbon was visible on the surface than for the previous cell. There appeared to be minor scatterings (~4%) of carbon as found in the previous cell but were ignored. Instead, the large concentrations were focused on. For this cell, every possible carbon deposit studied was also accompanied by higher concentrations of contaminant materials such as sulfur and silicon. Because of this, these areas, despite containing high concentrations of carbon, were deemed pre-existing contamination rather than carbon deposits.
109
a) b)
c) d)
Figure 5.29: Cell images for Trial 3 including an SEM image (a), carbon content map (b), nickel content map (c), and oxygen content map (d)
Next, the same tests were conducted on the cell used in Trial 5. Since the water content in this trial was in question and the resistance climbed so high during the end of the testing, carbon was expected to be found. In Figure 5.30a, the carbon is distributed more uniformly than in Trial 1. Since the carbon is found at higher levels even between the platinum mesh it was concluded that this was the result of carbon deposition rather than having existed prior to the test.
110
a) b)
c) d)
Figure 5.30: Cell images for Trial 5 including an SEM image (a), a carbon content map with bare platinum wire highlighted (b), a nickel content map (c), and an oxygen content map (d)
Despite being heavily distributed, the carbon did appear to have higher concentrations in several areas. Upon further inspection, these areas were revealed to be
exposed portions of the platinum mesh. Figure 5.31 shows the same distributions for this
area. Since areas of exposed platinum appear to have higher concentrations of carbon
than the NiO layers, this indicates that the surface layers must have a lower carbon
content than 8%, which was the value seen for the bare wire. 111
a) b)
c) d)
Figure 5.31: Cell images for Trial 5 showing higher carbon content in platinum wire. SEM image (a), carbon map with platinum highlighted (b), nickel map (c), and oxygen map (d)
5.3 Discussion
5.3.1 Open Circuit Voltage
Although it has been proposed that solid oxide fuel cells could maintain
approximately the same performance using HDS as when using hydrogen, this is in regards to cell resistance only. Familiarization with gas kinetics and Wagner equation leads one to the conclusion that using HDS in fuel cells results in a lower OCV than that 112
seen for hydrogen or even syngas. Table 5.1 shows the calculated OCV values assuming
the cell is operated in the presence of air.
Table 5.1: Calculated OCV for Various Fuels
Reversible Cell Voltage at 820°C
Hydrogen (16.2% water) 1.01 V
Syngas (Pittsburgh No. 8) .97 V
Hydrogen-depleted Syngas (HDS) .95 V
Because of malfunctions in the data acquisition software and the failure to control
the bubbler temperature while testing with hydrogen, accurate OCV comparisons can
only be made for the last two trials. In fact, after reviewing the logged data it was noted
that the backpressure in the system averaged around .76 psi. While this has little effect on the theoretical value of the OCV once the gases are delivered to the cell, it can have a significant change on the water content of the gas. Although the OCV values for Trial 4,
given in Table 5.2, appear to match closely the theoretical values, correction for the
system pressure indicates that the water partial pressure delivered to the gas from the
bubbler was slightly higher than anticipated.
Table 5.2: Average Measured OCV for Trials 4 and 5 113
Average Measured OCV Values
Fuel Source Trial 4 Trial 5
Hydrogen (humidified) 1.011 V 1.058 V
Syngas (Pittsburgh No. 8) .978 V .989 V
Hydrogen-depleted Syngas (HDS) .955 V .977 V
Near the end of Trial 4, the OCV increased while using hydrogen to about 1.058 volts. This occurred with no other changes to the settings on BCS1. Upon starting Trial 5, this same OCV was recorded. The same method of calculating the OCV (see Appendix 2) was used to determine the water content in the hydrogen. It was found that the value of
1.058 volts equated to a water content of 6.7%, much lower than the expected value.
Assuming the same bubbler performance for the other fuel types, the same calculations were made, both yielding very close results. These results are shown in Table 5.3. Note that in this table the term HDS is used to refer to the period during which the hydrogen content was reduced in the syngas mixture but the bubbler temperature had not been increased. Based on these calculations, it was determined that the desired gas compositions were not reached. In fact, later examination of the bubbler revealed that it had not in fact been refilling. However, it is still apparent that operation with HDS results in a lower OCV that can be predicted with theoretical calculations.
Table 5.3: Calculated Cell Voltage for Trial 5 114
Calculated Reversible Cell Voltage for Trial 5
Hydrogen (6.7% water) 1.058 V
Syngas .984 V
Hydrogen-depleted Syngas (HDS) .976 V
This assessment cannot be made for the first three trials for two reasons. First, the bubbler temperature was not controlled during all hydrogen use. Second, data for those trials was taken such that any changes in the performance of the bubbler, and therefore the water content of the gas, could not be accounted for. However, OCV data for the last two trials was consistent enough to conclude that the fuel cells will operate with an OCV very close to theoretical predictions.
5.3.2 Polarizations
In order to make conclusions regarding the cell resistance, it is necessary to take into account the way in which the VI scans were conducted. This type of experimentation was a large focus during the first three trials. Typically, a scan rate of under 3mA/s resulted in approximately the same performance as the .1667 mV/s standard potentiodynamic sweep. Also, the direction of the scan did have a small affect as the
ASR calculated for forward sweeps was typically about 2% higher than reverse sweeps.
Although the reverse sweeps resulted in the most linear response, the forward sweeps corresponded well with the manufacturer’s data and are the source of all stated ASR 115
values in this document, mostly so that data from the early trials can be compared with
that of the last two.
Much of the data collected in this work is impossible to compare at face value.
The button cells used in this study showed enormous part-to-part variation. While some
of this difference is associated with the expected variation in the manufacturing of such
small cells, much of this is a product of a specific problem that was identified by the fuel
cell supplier. This not only led to premature failure of the first three trials, but was also the reason for the large difference in ASR between the first three and last two trials.
Although this predicament had no impact on the relative values of the data, it did lead to a large difference in the nominal values. Therefore, a direct comparison cannot be made.
The average ASR values for each trial are collected in Table 5.4.
116
Table 5.4: Average ASR for all Trials
Average Area-Specific Resistances for all Trials
Trial Fuel ASR (Ω cm2) Uncertainty for 95% confidence
1 Hydrogen .577-.608a N/A
2 Syngas .6592 .0099
2 HDS .6860 .0229b
3 Hydrogen .6434 .0049
3 Syngas .6304 .0228c
4 Hydrogen 1.2511 .0122
4 Syngas 1.266 d N/A
4 HDS 1.289 d N/A
5 Hydrogen 1.14-1.18 a N/A
5 Syngas 1.25-1.31 a N/A
5 HDS 1.30-1.50 a N/A a – no stable operation, range given b – temperature setting error, fewer data points available c – anode failure, fewer data points available d – continuous data not taken
117
As can be seen in Table 5.4, there were many hurdles during testing that prevented extensive data collection. First, since the ASR changed so significantly over time, data was only assessed after this value stabilized within an uncertainty of 5%. This did not occur in the first and last trials. Also, the data in Trial 4 was collected such that the variation in the ASR values for syngas and HDS cannot be calculated to within any acceptable standard. However, all of the data collected in this work appeared to be within expectations so legitimate claims can be drawn from that. Specifically, using diluted fuels at low fuel utilizations does result in a higher ASR, but only such that it requires extensive data collection to determine the extent of this.
As seen in Figure 5.22, the ohmic resistance can change over time but the polarizations due to the fuel type, activation and concentration losses stay approximately
the same. Because of this, differences in the data (for different gases) taken during testing phases that were considered to be in a period of stable operation, as described above, were considered to be a result of these mechanisms. However, since the total ASR did change over time, gas changes and measurements had to be conducted quickly, which reduced the amount of data available and therefore widened the confidence interval.
Since the cell used in Trial 4 was the only cell to be tested with all three fuel types and since the OCV values matched the theoretical value so well, this trial gives the most reliable comparison of the different fuel types at a fixed point in time.
A series of three VI scans were conducted for each fuel gas. In this case, ASR changes over time can be neglected because of the speed of the procedure and the fact that the ASR value for this cell returned to within 1% of the original value when the cell 118
was returned to hydrogen operation. Although fewer data points equate to a wider
confidence interval, deviations in the measured data were small enough so that important
conclusions can be made. By subtracting the confidence intervals for different fuels, a
worst case scenario can be stated for the different fuel types. Analyzing the data in Table
5.5 leads to the conclusion that switching from humidified hydrogen to syngas would result in a maximum ASR increase of 5.5% while a switch to HDS would cause a maximum difference of 6.8%.
Table 5.5: Trial 4 Quick Change Uncertainty
Worst Case Scenario for ASR Data – Trial 4 Quick Change Data
Fuel Mean (Ω cm2) Max (Ω cm2) Min (Ω cm2)
Hydrogen 1.363 1.387 1.338
Syngas 1.391 1.412 1.369
HDS 1.405 1.429 1.381
Because the overall cell resistance changed from cell to cell, comparisons
between fuels could only be made within the same trial. Reliable data for HDS was only
obtained in the second, fourth, and fifth trials, yet in all cases, as described above,
characteristic curves appeared virtually the same. Since this was observed in the second
trial, impedance spectroscopy was used in the fourth, and more so in the fifth for further
explanation. As shown in Figure 5.22, the change in the cell resistance over time seemed 119
almost completely due to changes in the ohmic resistance and therefore not a result of
changing fuels. Furthermore, there was a small increase in resistance when switching
from hydrogen to syngas and from syngas to HDS. Figure 5.18 shows that there is an
increase in the size of the impedance semi-circles that correspond to activation and
concentration losses. This total increase, however, when switching from hydrogen to
HDS was between 6% and 9%, depending on the overall cell performance. This
corresponds quite well with the study of the ASR values. It is important to note that there
was a significant resistance contributed from the leads during testing, as high as 4% in
some trials. Since the data herein is only compared within single trials, these values were
not removed from the reported numbers.
Using impedance spectroscopy, the ohmic resistance can be subtracted out of the
total resistance, leaving only the polarization of the electrodes. This value is provided in
Figure 5.31 for each fuel type. In Trial 4, the polarization of the electrodes increased
by .023 Ω (.03 Ωcm2) when switching from syngas to HDS but this data was insufficient
to make confident claims. In Trial 5, the mean difference was .02 Ω (.027 Ωcm2), which equates to 5%. This difference can be confidently (95%) stated to be within 7.4%.
Trial 2 was the only other test in which data was taken for both syngas and HDS.
If the change in the ASR when using the different fuels is assumed to be only a result of the electrode polarization, then the impedance data agrees very well since the difference in the mean value of the ASR was .027 Ωcm2 or 4.1%. 120
Figure 5.31: Electrode Polarization for Trials 4 and 5
Without the use of reference electrodes, it is impossible to completely distinguish losses between the two electrodes. In all cases, however, the increase in impedance was mostly seen as an increase in the size of the high frequency arc associated with activation losses. This was expected since prior modeling showed that concentration losses had little effect at normal operating conditions and especially since such high fuel utilizations were used in this study. Therefore, it can be concluded that based on this work and prior research, the increase in the polarization of the cell is likely due to increased carbon monoxide oxidation rather than hydrogen oxidation. This is an important point because this means the method of removing hydrogen from syngas is very crucial to cell 121
performance. In this study, it was assumed that no other changes were made. However, in practice the removal of hydrogen or other contaminants at lower temperatures would reduce the water content, decrease the shift reaction, and lead to higher activation losses,
as well as increase the likelihood of carbon deposition.
5.3.3 Carbon Deposition
A drawback to using syngas and hydrocarbon fuels is the formation of carbon
deposits on the surface of the anode under certain conditions. Since this would eventually
destroy the cell, SOFC systems must operate under conditions that do not favor carbon
deposition. The C-H-O ternary diagram in Figure 5.32 depicts the conditions under which
deposition is thermodynamically favored. 122 Methane Syngas
HDS (18.5% water)
HDS (16.2% water)
Syngas (.067% water)
HDS (.067% water)
900°C 820°C 700°C
Figure 5.32: C-H-O Ternary Diagram [27]
Examination of Figure 5.32 shows that increasing the mole fraction of carbon (left slope) will increase the likelihood of deposition while increasing the mole fraction of oxygen (right slope) pushes the operating point below the deposition area. The lines splitting the diagram correspond to temperature. When switching from syngas to HDS, there is little change in the location of the operating point on the diagram. Previous literature relates the likelihood of coking (carbon deposition) to the steam to carbon ratio 123
[22-25]. Reducing the hydrogen flow has a small impact on this value, hence the diagram
agrees with previous findings.
Although all of the operating conditions fall below the coking region of the
diagram, those with lower water content reside closer to the boundary. Since coking is believed to have occurred during Trial 5 and not others, the water content in that trial must have been lower, hence the reason those assumed values were used in this diagram.
Despite having lower water content then other phases of testing, those trials would still fall below the deposition region of the diagram. This can best be explained by the uncertainty of the testing conditions. Whether this was related to the fuel composition
or changes in the system pressure cannot be determined without further testing.
Even though carbon deposition likely occurred during testing, it is believed that
this can be limited or prevented with further testing. By improving testing methods the
deposition region can be avoided entirely. Also, methane, which has a much higher tendency for carbon deposition than syngas, has been tested successfully with no coking by maintaining a minimum current density. This method should be applicable for syngas as well. Since fuel cell testing involves maintaining the cell with little or no current for long periods of time, particularly at low fuel utilizations, carbon deposition is more likely to occur than under normal operating conditions, such as in a fuel cell power generation system, which would most likely operate at a much higher fuel utilization and current density.
Since carbon deposition is believed to have been avoided during the trials with higher humidity levels in the fuels, deposition in a large commercial system is unlikely 124
with these fuels. However, since fuel exists so close the deposition point and any
deposition would occur slowly, testing for extended periods of time would be needed to prove this.
5.3.4 SOFC Systems
The combined losses of a lower ideal potential and increased cell resistance are
seen when comparing the maximum power density of the cells. Figure 5.33 shows the
power density resulting from Trial 2 for syngas and HDS. The accompanying dashed
lines represent 95% confidence intervals. The mean peak power densities, found by
equating the derivatives of the power functions to zero, are located at different current
densities but are both very close to the operating point chosen for impedance
spectroscopy. When switching from syngas to HDS, the mean peak power density
decreased by 7.8%. However, given the low sample sizes the maximum difference,
obtained by subtracting the lower bound of the HDS phase from the upper bound of the
syngas phase, was 13.8%
Although an actual SOFC system could see larger concentration losses, the
electrolyte-supported cells used in this study at a low fuel utilization rate are assumed to
have a similar performance to those using a high utilization rate. This is because
concentration losses near the peak power density are small even on anode-supported cells
[22]. Since the anodes of these cells are typically 100 times thicker than those used in this
work, transport losses are assumed negligible in normal operating ranges and fuel
utilization rates. Aside from the interconnect resistances, the three losses mentioned in 125
this document are the only important inefficiencies in an electrolyte-supported fuel cell
stack.
Power Density Comparison 0.4
0.35
0.3
2) m0.25 /c W ( ty si 0.2 en Syngas D r e HDS w o 0.15 P
0.1
0.05 13.8% difference from Syngas upper bound and HDS lower bound
0 0 0.2 0.4 0.6 0.8 1 1.2 Current Density (A/cm2)
Figure 5.33: Trial 2 Power Density Comparison
In addition to cell performance, switching fuels can have a negative impact on the efficiency of a fuel cell power generator in other ways. The high temperature waste heat produced by a solid oxide fuel cell, especially using preheated fuels such as syngas, is a valuable source of energy. The fact that fuel cells must operate using excess fuel (in order to prevent large concentration losses) means that a secondary system or bottoming cycle 126 is essential. By removing hydrogen from the syngas stream or even by diluting the fuel with steam to prevent coking, the heating value of the remaining fuel is decreased.
Additionally, a fuel cell system must be self-sustaining, such that the reactions at the cell must provide enough energy to sustain the desired operating temperature. In order to operate different fuels at the same temperature, the fuel utilization and oxidant flow rates must be changed. Therefore, this type of direct comparison in single cell testing is not a completely even portrayal of the expected system performance.
Analysis and experimentation of many fuel cell system arrangements under realistic stack conditions using these fuels has yet to be conducted. This, along with projections for the economic value of the system and removed hydrogen, are needed before it can be determined if hydrogen could be removed from syngas for other purposes. 127
Chapter 6 Conclusions & Recommendations
6.1 Conclusions
The proposed testing methods proved effective and the SOFC Testing Facility within
the Ohio Coal Research Center was demonstrated capable of testing solid oxide fuel cells
under simulated coal syngas conditions. The average open circuit voltages, with the exception of Trial 5, all fell within 10mV of the expected value. The assumption that removing hydrogen from a syngas stream will decrease the operating voltage and therefore the possible power density was confirmed and found to be consistent with theoretical calculations. This means that even with no losses in the system there is an expected tradeoff of performance in exchange for the benefits of hydrogen removal.
The ASR values reported in this study are largely irrelevant for several reasons.
First, there was significant variation in the performance of different cells. Also, this value was not always stable over time. Most importantly, however, is that this test as well as the manufacturer’s reported data for these cells shows that the smaller, “button cells” used in this study typically give much higher ASR values. However, the polarization of the electrodes alone was found to be much more consistent and representative of the changes resulting from removing hydrogen from the syngas.
The increase in the polarization after switching from syngas to HDS was under 5% in all trials when operating near peak power density. This value is a result of an increase in activation losses as well as an increase in concentration losses from fuel dilution and changes in the average gas diffusion rates of the fuel. However, operating at a higher 128
current density also causes the cell to auto-dilute, meaning the product gases dilute the
incoming fuel. Since transport losses for syngas are believed to be extremely small at
normal operating conditions [22], the same can likely be said when using HDS. More testing is needed, but it is believed that the results presented from this research will be
consistent with electrolyte-supported cells used at higher fuel utilizations and at or below
the peak power density. Transport losses would likely prove more substantial for anode-
supported fuel sources and polarization from fuel dilution could be magnified.
The combined losses when switching from syngas to HDS while maintaining the
same fuel utilization equated to a 7.8% loss in peak power density. However, because of
the limited amount of data taken during the tests, this value can only be confidently
(95%) stated to be less than 13.8%.
Carbon deposition could not be confirmed on any cells except that used in Trial 5.
The high open circuit voltage, dramatic increase in the ASR, and carbon distribution on
the EDX map prove that carbon deposition likely occurred as a result of having lower
than expected water content. When the fuels were properly delivered carbon did not
deposit to any measureable extent.
Analysis of the data for all trials shows that losses in cell performance are within
expectations. However, since no system level performance or economic models are
currently available for HDS, it cannot be concluded whether or not hydrogen can be
removed from syngas in a commercial SOFC system.
129
6.2 Recommendations
6.2.1 Testing Methods
There are several improvements regarding the testing methodology and apparatus
that can improve future results. First, the temperature and pressure conditions within
BCS1, although fairly constant, did not match the desired set points. The furnace temperature is controlled by a separate thermocouple located much further from the cell.
Rather than using the cell thermocouple to control the furnace temperature, conducting
short tests to determine a relationship between the two measurements would allow the
operator to achieve the desired temperature easily.
The elevated system pressure, on the other hand, cannot be avoided. This pressure
has little impact on the reactions in the cell. However, it does affect the water delivered to the syngas mixture. By measuring the delivered partial pressure and establishing it as a function of flow rate, bubbler temperature, and possibly other factors such as the bubbler fill level, the operator can achieve an accurate and repeatable humidity level.
Finally, a greater effort should be made to fabricate fuel cells using the OCRC facilities. While there are benefits to testing professionally fabricated cells, proprietary agreements restrict full explanations of test results and limit the types of data presented.
Also, when detailed descriptions of the various cell layers is not provided interpretation of impedance spectroscopy and material analysis data can be difficult or impossible.
Agreements to overcome these issues would be beneficial but testing the repeatability of cells made within the OCRC would be a logical future step. 130
6.2.2 Future Work
As mentioned above, the nominal values obtained with button cells can be very different than larger, industrial sized cells. One reason for this is that the thickness of
larger fuel cell anodes does not change greatly whereas the shape of button cell anodes is
more convex, which is not accounted for in most models. Future work should include fuel
quality testing using larger fuel cells. This should be done in a stack arrangement since
the interconnect design does influence fuel delivery.
Realistic fuel utilization rates should be used. This is important because, aside
from establishing the influence of concentration losses, the fuel utilization rate has a large
impact on carbon deposition. Note that the fuel utilization rate is a relationship between
the theoretical amount of fuel needed for the operating current and the actual amount
needed and does not have anything to do with the size of the cell. Therefore, studies on
fuel delivery and anode microstructure may lead to methods of decreasing the size of
cells needed.
If coal syngas is to be used as a source of hydrogen with the remaining fuel
powering SOFCs, a complete study of the thermodynamics of different system designs,
including any necessary cleanup or water addition systems, is needed to determine the
complete impact of using HDS on a fuel cell power generation system. This information
should be used to create an economic model that can be used to determine the suitability
of different fuels in a fuel cell system. In order to do this, however, a complete cell-level
model must be created to predict cell performance as well as operating temperature. Since 131
fuel cell systems must be self sustaining, heat transfer across the cell and within the system cannot be avoided.
Further contaminant studies should be conducted. Reducing the hydrogen content in the fuel gas increases the partial pressure of contaminants by default. The effects of these higher levels should be considered in future contaminant studies. If it is deemed that coal syngas contaminants are unsuitable for fuel cell anodes and a cleanup system is required, then the product gases of cleanup systems should be used for fuel cell tests. The use of actual syngas directly from a coal gasifier has yet to be reported in academic literature.
In this study, methane was not used in the syngas mixture and was substituted with nitrogen. This dilution of the fuel effects the performance and, to a greater extent, the tendency for coking of the cell. Since methane makes up a substantial (~4.5%) part of the syngas chosen it should be included in future studies. However, at the operating temperatures used, this methane would reform into hydrogen and carbon monoxide.
Equilibrium calculations should be performed such that adjustments to the MFC flow rates and bubbler can be made to simulate the reformed conditions.
Since coking is a potentially damning issue for fuel cells, the lack of carbon deposition in these tests does not mean that future work is unwarranted. Since these fuel mixtures operate so close to the deposition region on the ternary diagram any deposition could occur slowly enough to not be seen within these testing times. Therefore, endurance testing is warranted even without contaminants. If deposition occurs, 132 researching materials that do not promote carbon deposition may be the only option aside from steam injection.
Finally, syngas may be a possible source of pure hydrogen but a reliable method of extracting the hydrogen must be identified. This method must be compatible with contaminants existing in syngas or in the product gas of cleanup procedure. Once a method is chosen, its effect on the syngas must be determined. It is unlikely that hydrogen can be removed from syngas with no other changes to the gas composition so future work should account for this. 133
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Appendix A: OCV Calculations using EES
{a_H2 + b_ H2O + c_CO + d_CO2 + e_CH4 + f_N2 + Z_O2 -> g_H2 + h_ H2O + i_CO + j_CO2 + k_CH4 + l_N2 + m_C + n_O2}
{Carbon Balance} c+d+e=i+j+k
{Oxygen Balance} b+c+2*d + 2*Z = h+i+2*j+2*n
{Hydrogen Balance} 2*a+2*b+4*e=2*g+2*h+4*k
{Nitrogen Balance} 2*f=2*l
{Feed Conditions} {a=.249} {b=.162} {c=.377} {d=.154} {e=.0453} {f= .0125} Z=0
{New Mole Fractions} tot = g + h + i + j + k + l + n Yg = g/tot Yh = h/tot Yi = i/tot Yj = j/tot Yk = k/tot Yl = l/tot
Yn = n/tot
{Reforming Reaction - Chan} Kr = (Yg^3*Yi)/(Yk*Yh) Ar = -2.63121e-11 Br = 1.24065e-7 Cr = -2.25232e-4 Dr = 1.95028e-1 Er = -6.61395e1 Ref = Ar*T^4 + Br*T^3 + Cr*T^2 + Dr*T + Er Kr = 10^Ref
{Shift Reaction - Chan} Ks = (Yg*Yj)/(Yi*Yh) As = 5.47301e-12 Bs = -2.57479e-8 Cs = 4.63742e-5 138
Ds = -3.91500e-2 Es = 1.32097e1 Sft = As*T^4 + Bs*T^3 + Cs*T^2 + Ds*T + Es Ks = 10^Sft
{Hydrogen Oxidation} Kho=exp(f1/(T*R)) f1=116.571095*T-501729.51179 Kho=(Yg^2*Yn/(Yh^2))
{Carbon Dioxide Oxidation} {Kco=exp(f2/(T*R))} {f2=164.245099*T-552388.252927} {Kco=(Yi^2*Yn/(Yj^2))}
R=8.314 T=820+273
EMF=R*T/(4*96485)*ln((Y_o2/Yn))
Example Solutions
H2 H2O CO CO2 CH4 N2 EMF 0 0.5 0.49 0 0 0.01 0.4934 0.163 0.067 0.493 0.201 0 0.065 0.9761 0.144 0.172 0.437 0.179 0 0.067 0.9576 0.838 0.162 0 0 0 0 1.011 0.249 0.162 0.377 0.154 0.0453 0.0125 0.9849 0.2708 0.3051 0.2206 0.1901 0.0083 0.0203 0.9371 0.838 0.162 0 0 0 0 1.011 0.249 0.162 0.377 0.154 0 0.0578 0.9659 0.1422 0.1851 0.4307 0.176 0 0.06604 0.9556
139
Appendix B: Example Uncertainty Calculation
Sample # ASR 1 0.65459 2 0.65219 3 0.65803 4 0.66031 5 0.66125 6 0.66694 7 0.65979 8 0.65542 9 0.65943 10 0.6639 Avg. 0.659185 Std. Dev 0.004395 t multiple 2.2622 Uncertainty 0.009943
df 0.1 0.05 0.025 0.01 2 2.92 4.3027 6.2054 9.925 3 2.3534 3.1824 4.1765 5.8408 4 2.1318 2.7765 3.4954 4.6041 5 2.015 2.5706 3.1634 4.0321 6 1.9432 2.4469 2.9687 3.7074 7 1.8946 2.3646 2.8412 3.4995 8 1.8595 2.306 2.7515 3.3554 9 1.8331 2.2622 2.685 3.2498 10 1.8125 2.2281 2.6338 3.1693 11 1.7959 2.201 2.5931 3.1058 12 1.7823 2.1788 2.56 3.0545 13 1.7709 2.1604 2.5326 3.0123 14 1.7613 2.1448 2.5096 2.9768 15 1.7531 2.1315 2.4899 2.9467 16 1.7459 2.1199 2.4729 2.9208 17 1.7396 2.1098 2.4581 2.8982 18 1.7341 2.1009 2.445 2.8784