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MICRO FUEL CELLS
PRINCIPLES AND APPLICATIONS
Academic Press
Copyright © 2009 Elsevier Inc.
All right reserved.
ISBN: 978-0-08-087887-4
Chapter One
Electrolytes for Long-Life, Ultra Low-Power Direct Methanol Fuel Cells Shruti Prakash, William E. Mustain, and Paul A. Kohl
1.1 Introduction 2 1.1.1 Potential Applications for Micro Fuel Cells 3 1.1.2 Direct Methanol Fuel Cells 4 1.1.3 Energy Efficiency and Device Life 6 1.2 Perfluorinated Polymer Proton Exchange Membranes 9 1.2.1 Nafion 11 1.2.2 Nafion on Composite Membranes for Direct Methanol Fuel Cells 12 1.3 Non-Nafion Polymer Proton Exchange Membranes 24 1.3.1 Polyvinyl Alcohol Blends 25 1.3.2 Sulfonated Poly (Ether Ketone)s 27 1.3.3 Sulfonated Poly (Phenylene Oxide) 29 1.3.4 Polybenzimidazoles 30 1.3.5 Polyimides 33 1.3.6 Non-Nafio Polymer Organic/Inorganic Composites 34 1.4 Inorganic Membranes 37 1.4.1 Silicate Glasses 37 1.4.2 Phospho-Silicate Glasses 39 1.4.3 P2O5-ZrO2-SiO2 and P2O5-TiO2-SiO2 Glasses 41 1.4.4 Inorganic/Organic Nano Composite Membranes 41 1.5 Conclusions 43
Portable direct methanol fuel cells are potentially excellent power sources for small electronic devices because of the high energy density of pure methanol. For example, wireless sensors are valuable in monitoring and control situations. The ability of wireless sensors to form self-assembled networks may provide rapid growth for the technology. In each of the small electronic applications, the cost, lifetime, size, and weight of the power source is a critical part of the value of the overall system. These devices may require tens of milliwatts for milliseconds to acquire or transmit data, and tens of microwatts for long periods in sleep mode. This style of operation (low intermittent power over a long time period) is far different from the power sources for transportation, high-power electronic devices, or electric power. The fuel cells must have very low energy losses, including low methanol permeability, and must allow the use of highly concentrated fuels. In this chapter, the existing fuel cell technologies are examined in light of these new requirements.
1.1 INTRODUCTION
The fuel cell market can be divided into different segments of our energy infrastructure based on power level and end use. These areas (with example power levels) are (i) stationary plug power (hundreds of megawatts), (ii) back-up power (tens to hundreds of kilowatts), (iii) traction power (portable supplies at 10 to 100kW), (iv) small portable power (1 to 100W) and (v) mini or micro power (10W to 1W). Fuel cells have the potential to provide clean, efficient, sustainable power in all market segments. However, in each market segment, there are multiple energy conversion devices available. Heat engines (Rankine & Brayton cycles of stationary power and Otto & Diesel cycles for traction) are highly competitive in high power-density applications where device size and fuel infrastructure are not primary concerns. However, at smaller sizes, electrochemical devices become more competitive because they are simpler than heat engines, requiring no moving parts, while providing higher energy densities since they scale as a function of their surface area, not volume.
1.1.1 Potential Applications for Micro Fuel Cells
Energy sources for portable devices, such as electronic devices, are under continual pressure to decrease both consumed power and weight. The scaling of electronic devices has produced products with higher functionality (requiring more power) and smaller form-factors. The increase in popularity of portable electronics has continued to put pressure on increasing the energy density and lowering the cost of portable power sources. The average power can range from microwatts to watts, depending on function and duty cycle.
One growing market segment is the use of electronic sensors. Some devices communicate wirelessly and are deployed in locations where plug-in power is difficult or impossible to implement and a portable power source is essential to its implementation. Among these, industrial sensors are the largest market segment. Sensors such as temperature, pressure, gas composition, smoke, motion, humidity, and light are needed in mining, construction, utilities, manufacturing, transportation, and warehouse locations. A second area where wireless sensors will have an impact is in commercial buildings. The sensing and control of heating, ventilation, and air-conditioning systems will improve the energy efficiency and make buildings more environmentally benign. Also, wireless security systems can be easily added to commercial and residential properties.
A third technological area where small wireless sensors are valuable is in the implementation of automated meter readers, such as water and gas meters. The ability of wireless sensors to form self-assembled networks allows utilities to accurately monitor and bill communities at low cost. A fourth area of interest is in home automation. Wireless sensors can be used to monitor remote door opening as well as smoke, gas, flame, and water situations. Again, energy efficiency can be improved by monitoring and selectively controlling heating, air conditioning, and lighting.
Finally, wireless sensors, especially those forming networks, are valuable in environmental and homeland security situations. Simple sensors can be implemented for the detection of biological, chemical, and radiation hazards. In addition, environmental sensors can be used for weather forecasting, agricultural monitoring (fertilizer and water monitoring), and motion tracking at borders and other secure locations.
In each of these applications, the cost, lifetime, and size and weight of the power source are a critical part of the value of the overall system. Many of the monitor-style sensors, as described above, have low duty cycles (need to acquire data only occasionally) and require low power to operate due to their simple function. Devices may require tens of milliwatts for milliseconds to acquire or transmit data, and tens of microwatts for long periods in sleep mode. This style of operation—low, intermittent power over a long time period—is far different from the power sources for transportation, high-power electronic devices, or electric power. The general approach for low-power applications is for the fuel cell to provide the constant power for the sleep mode (e.g., tens of microwatts), and a secondary storage device, which is maintained by the fuel cell, provides the burst-power for events including sensing and transmission. Thus, the design parameters for the fuel cell shift from the traditional high-power mode, to low-power and high energy efficiency. The parameters which affect energy efficiency will be explained in the following section, and the ability of existing PEM systems to provide high efficiency during ultra low-power operation will be the subject of the remaining sections of this chapter.
1.1.2 Direct Methanol Fuel Cells
In the direct methanol fuel cell (DMFC), liquid methanol is fed directly to the anode compartment of the PEM fuel cell. This provides several advantages over its hydrogen counterpart, namely that 1) liquid methanol has a higher volumetric energy density than hydrogen and 2) the generation, storage, transportation of methanol is facile. However, unlike the hydrogen oxidation, the direct oxidation of methanol requires a considerable amount of water in the anolyte since the water is oxidized along with methanol to form carbon dioxide (Eq. 1.1).
CH3OH + H2 [right arrow] CO2 + 6H+ + 6e- (1.1)
Equivalent to the PEM fuel cell, the proton in Eq. 1.1 is transported through the electrolyte membrane as a hydrated ion and the electrons travel through an external circuit (the device). The protons then meet molecular oxygen at the cathode, forming water via the acidic oxygen reduction reaction (Eq. 1.2).
O2 + 4H+ + 4e- [right arrow] 2H2O (1.2)
This process is illustrated in Figure 1.1.
Theoretically, a 17M methanol anode feed is possible. However, in conventional systems, the highest currents and powers are achieved with dilute methanol solutions in the 0.5M to 2M methanol range. The concentration optimization is a balance of three effects. First, if the methanol concentration is high, permeation of fuel through the electrolyte is prohibitive. Second, if the methanol concentration is too low, the reaction kinetics reduces and mass transport of the methanol reactant to the anode limits the current density. Finally, up to 15 water molecules can be transported by electro-osmotic drag from the anode to the cathode for each methanol molecule oxidized. This dilution of the fuel is highly undesirable because it decreases the energy density of the cell and can cause water management problems.
1.1.3 Energy Efficiency and Device Life
Fuel cells have very high theoretical energy densities because concentrated liquid fuels with high equivalence (e.g., six electrons from methanol) can be used, and the oxidizing agent (species to be reduced at the cathode) does not have to be carried within the cell if oxygen from air is reduced. The theoretical energy density of pure methanol is 6100Whr/kg whereas lead-acid and nickel-cadmium batteries offer 30–85Whr/kg, and lithium ion generally offers between 110–160Whr/kg. However, 6100Whr/kg is not achievable because one cannot discharge a methanol fuel cell at the theoretical thermodynamic voltage and pure methanol is not acceptable in the PEM fuel cell since water must also be provided as a reactant at the anode, thus diluting the fuel. A more realistic goal may be a discharge voltage of 0.5V and use of 12M methanol. Under these conditions, one would have an energy density of over 1200Whr/ kg. Figure 1.2 shows the lifetime of such a cell as a function of duty cycle and average power level. One milliliter of fuel theoretically could last 10 years at an average power of 100µW and 10% duty cycle.
However, there are two primary energy loss mechanisms which must be considered and mitigated in order to achieve long operating life. The first is the permeation of fuel, methanol in the direct methanol fuel cell, through the electrolyte. In this case, the mass transport of methanol from the anode to cathode results in not only a simple fuel loss, but the electrochemical performance of the cathode is decreased as well. Second, the ionic conductivity of the electrolyte is important as ohmic-type heating losses must be minimized.
An expression for the energy conversion efficiency of a fuel cell can be derived by considering the energy available relative to the energy delivered. The useful energy delivered from a fuel cell, EU, is given in Eq. 1.3.
Eu = iVop (1.3)
where i is the fuel cell current and Vop is the operating voltage. Resistive loss caused by ionic transport through the proton exchange membrane, ER], is expressed by Eq. 1.4:
ER] = (i2 ρδ 1/A1) (1.4)
where ρ is the ionic resistivity of the electrolyte, δ1 is ionic path length, A1 is the electrochemically active area.
Fuel can be lost by permeation through the electrolyte, often referred to as methanol crossover. This loss, Ex, is given by Eq. 1.5.
Ex =(P1Δp A2/δ2) nFVocv (1.5)
where P1 is the permeation coefficient of the membrane, Δp is the pressure drop across the membrane, A2 is the exposed membrane area available for fuel transport through the membrane, δ2 is the electrolyte thickness, n is the number of electrons transferred in the reaction, and F is Faraday's constant. It should be noted that generally δ1 = δ2; however, A1 and A2 need not be the same. Appropriately engineering the electrode structure may block the membrane from crossover loss while maintaining a large membrane area for low resistive losses (i.e., A2 < A1).
Combining Eqs. 1.3–1.5, the energy efficiency, ε is given by Eq. 1.6.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.6)
Fuel loss from permeation through the PEM membrane is especially important in ultra low-power fuel cells compared to intermediate and high-power cells because 1) the rate of fuel consumption through electrochemical oxidation is orders of magnitude lower; 2) the ohmic loss is significantly reduced since it is a function of the square of the operating current; and 3) the electrolyte aspect ratio (α = A/δ) is high. That is, the current in the numerator of Eq. 1.6 is smaller (can be much smaller) than in high-power systems, making it more important to have tight control of losses (terms in the denominator of Eq. 1.6), especially the permeation losses.
It should be noted here that the device life is a significant function of the chemical stability of the components used. Though the chemical stability of the electrode-electrolyte interface will not be discussed in detail in this review, a few examples are listed below. The most common limitation observed in the DMFC is the chemical stability of the solid electrolyte membrane. In many cases, the polymer backbone is degraded by either heat treatment or contact with peroxide radicals generated at the cathode. Second, it has been found that the PtRu anode catalyst degrades during DMFC operation. Evidence suggests that the ruthenium is electrochemically oxidized, transported in its solvated ionic form through the electrolyte and electrodeposits on the cathode. This leads to a loss of activity for both the anode and cathode. DMFC lifetime can also be affected by the corrosion of the carbon catalyst support at each of the electrodes. When the carbon support is oxidized, especially at the positive electrode, the platinum or other alloy particles peel away from the support and electrical contact is lost, resulting in an increase in electrode resistance. This process is exacerbated at high currents, high temperature, and low relative humidity conditions.
1.2 PERFLUORINATED POLYMER PROTON EXCHANGE MEMBRANES
Several fluorinated membranes have been developed for use as proton exchange membranes in both the PEM and DMFC systems. The fluorinated polymers are typically synthesized by copolymerization of tetrafluoroethylene and a perfluorinated vinyl ether with sulfonyl acid fluoride. The sulfonyl fluoride groups, -SO2F, are converted to sulfonic acid (-SO3-H+) by consecutive soaking in hot sodium hydroxide (to yield (-SO3-Na+), hydrogen peroxide, and sulfuric acid. The resulting polymer contains a fluorocarbon backbone, which is hydrophobic, and perfluoroether side chains containing hydrophilic sulfonic acid ionic groups. The sulfonic acid groups form internal clusters so that small channels (ca. 4nm) form. When hydrated, these channels provide ionic pathways for protons. The general structure for the fluorinated membranes is presented as Figure 1.3.
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