A Broad Spectrum of Fuel Cell Concepts
By Donald Georgi
The image of the Power Sources Conference in June of 2002, having support from the Army, opens the door to fuel cell topics which relate to military handheld devices. In such devices, the Direct Methanol Fuel Cell (DMFC) provided the greatest number of presentations. This popularity may first be due to the elimination of small or nonexistent reformer hardware which could compromise size or reliability. Next, the fuel source, methanol, is easy to obtain and safely transported. While the poisonous aspects of methanol are present, the quantities and concentrations in portable fuel cells may not be life threatening. DMFCs may be the first fuel cells to achieve operational status both in the military and commercial environments. Although simple in configuration, DMFCs are still in the research and development stage. The devices function in laboratory environments but have not yet demonstrated the long-term reliability in the field which will be necessary to compete with or complement Lithium-ion batteries.
A presentation by MTI Microfuel Cells identified the ballpark for device performance to provide one Watt and 30 Wh in a 30 cc package. Such a small device uses an air breathing cathode, which although uncomplicated, may get in the way of Army requirements to operate in wet locations, or even be blocked off in an enclosure. The MTI devices use a polymer electrolyte (NafionR). With fuel conversion efficiency of 85%, the overall efficiency can exceed 30%. A complete power system is envisioned to include a DC-DC Voltage converter and a peak power source such as a supercapacitor. A 2 Watt prototype has been operated for 1000 hours and has been demonstrated to power a cell phone. Projected commercialization is in 2004.
Motorola is designing a 1 Watt DMFC using Co-fired Ceramic Technology to provide a microfluidic control system which will mix fuel cell exhaust water with pure methanol to reduce the concentration to less than 1 molar (M) to the anode thus minimizing crossover. This system is complicated in operation, but with microminiaturization reliability already proven successful in electronic integrated circuits, such complexity may not impact processor reliability.
More Energy Ltd. (a subsidiary of MEDIS TECHNOLOGIES Ltd.) has demonstrated a power density of 50 mW in a handheld device which uses a proprietary liquid alkaline electrolyte. (See BD # 66, pp. 1-4.) This material allows the departure from platinum group catalysts, the use of organic alcohols and negates methanol crossover. The company considers itself in the Ďprovingí mode for the rest of this year and then will begin to exploit production in 2003-2004. With projected costs to manufacture of $2.50-$3.50 per Watt, one might suspect that the under $10 retail device may be a possibility.
Stepping up to DMFCs with 15 Watt capabilities, Lynntech Inc. has constructed a prototype by using a common metal grid as the core of an anode and the cathode of the adjacent cell, thereby creating a monopolar construction. This arrangement allows for reduced flow resistance for both air and methanol at a 4 M concentration. Taking only 1/15th of the output, the piezoelectric pump and fan provide for high overall efficiency.
Dr. Karl Kordesch at the Technical University of Graz proposes combining a circulating acidic electrolyte with hydrophilic flow-through anodes for methanol cleaning and a polymer electrode membrane. The performance is targeted to provide between 20 and 150 Watts with the benefit being increased performance, efficiency and cost savings.
As acronyms continue to proliferate, QinetiQ presented an intermediate temperature proton conducting fuel cell (ITPCFC) which employs a ceramic electrolyte similar to the solid oxide fuel cell, but at a much reduced operating temperature. Since performance is targeted for methane and methanol, it falls into the category of DMFCs. Testing of materials with good protonic conductivity at 5000 C and good reformate tolerance is needed. Current results are limited by low power densities, but feasibility is promising.
There actually is life beyond research in DMFCs. The University of Illinois is investigating small formic acid micro fuel cells. These cells run successfully with formic acid concentrations between 5 and 20 M with little crossover or degradation in performance. Formic acid has a lower power density than methanol , but it provides higher peak power and easier water management with little crossover. Formic acid fuel cells also produce higher open circuit potentials and current densities.
A development in air breathing Proton Exchange membrane (PEM) fuel cells constructed as a 2-dimensional fuel cell (2DFC) was reported by QinetiQ. The construction is as a cylinder so that hydrogen can be ported through the core and air circulated around the outside. The tubular membrane electrode assembly (MEA) can be a composite of materials and metals. A PEM layer is coated on both sides with a thin coat of platinum. Power densities are in excess of 120 W/kg and manufacturing costs are projected to be low.
When PEM fuel cells are operating on pure hydrogen, there is a high possibility that significant amounts of carbon monoxide could be present which would poison the catalystís surface diminishing the fuel cellís performance. Work at the University of Iowa has focused on including micrometer diameter magnetite iron oxide particles on the cathode and anode. With this addition, cells functioning with hydrogen having over 100 ppm of CO performed as well as cells fueled with pure hydrogen. It is believed that the magnetic fields facilitate electron transfer increasing the mechanism of carbon monoxide oxidation. Comparative test data was presented so that these results can be considered science rather than just market hype.
When constructing a high power multi-cell stack, the use of bipolar plates (cathode on one side of a metal plate, anode on the other) improves packaging density. Composites used have high volume, leaving the pure metals to be the conductor of choice. These metals suffer from formation of passivating oxide layers which continually reduce the performance of the stack. INEOS Chlor Ltdís. presentation focused on a coating to apply to the metal surface prior to anode and cathode placement. Results show that fuel cells constructed with the coating showed no reduction in power output in operation over times in excess of 10,000 hours.
Motorola is investigating the feasibility of building a 20 Watt fuel cell powered by methanol, using an on-board steam reformer. Using monolithic ceramic lamination structures, the hydrogen is extracted from methanol at temperatures between 180-2300 C. While the ceramic fabrication technology is well established, significant challenges stand in the way of acceptable performance of such a fuel cell.
Moving away from classical hydrogen powered fuel cells, the Lawrence Livermore National Laboratory presented the possibility of operating with carbon as a fuel. In a molten carbonate system, particulate carbon contacts an anode made of sparsely wetted finely-divided carbons in molten carbonate which melts at 4880 C. The separator is a wetted porous ceramic material and the cathode is a lithiated nickel oxide. Air and carbon dioxide react on the cathode forming CO32- which migrates across the separator to combine with carbon at the anode to form carbon dioxide. Theoretical efficiencies approach 100%, and specific energies in the region of 1000-4000 Wh/kg are possible when operating at power densities of 100 mW/cm2.
If one charts the progress of earlier technologies such as solid state electronics, the initial products were discrete elements consisting of single transistors. Then as the technology evolved, higher densities of integrated circuits allowed greater processing power at lower costs. A group at the University of Illinois is taking fuel cells to that next level of higher density with a microfluidic cell which eliminates the polymer membrane. A planar construction of electrodes on an elastomeric polymer is separated by a channel which directs the flow of oxygen in water past the cathode and the fuel in water past the anode. Using laminar flow principles, the streams do not mix in the reaction area, eliminating the need for a separator. While still in the research mode, energy densities of 0.22 mW/cm2 have been observed.
While PEM fuel cells have made dramatic improvements in PEM materials and reduction in catalyst loading from 10 mg Pt/cm2 to less than 0.35 mg Pt/cm2, better understanding of the thermal characteristics are needed to further improve designs. Auburn University has developed a heat transfer model with a corresponding test system which evaluates warm-up, cool down and pulse operation of a 13 MEA stack. Model coefficients can be quantitatively obtained from the test data. .
Semi Fuel Cells
Semi fuel cells are defined as having a magnesium anode, a cathode which reduces hydrogen peroxide and a seawater electrolyte. This configuration, investigated by the Naval Undersea Warfare Center, has a limitation in the power density because of slow mass transport at the cathode. To improve performance, a new cathode with electrostatically flocked carbon microfibers was developed. It significantly improves the cathode rate capability. Future improvements are seen at this time.
Another semi fuel cell combines aluminum with hydrogen peroxide to form aluminum hydroxide. It finds application in underwater applications requiring high power and energy density. One power source, developed by Fuel Cell Technologies Ltd., is targeted to provide 80 Kwh at 300 W in a deep-sea underwater vehicle for 11 days of uninterrupted full-power operation. The targeted performance is three times that of Lithium-ion which has a theoretical energy density of 663 Wh/kg with a cell Voltage of 4.2 Volts. By contrast, the Aluminum-hydrogen peroxide system has a theoretical energy density of 3,418 Wh/kg at 3.3 Volts. Potassium hydroxide as a catholyte and anolyte is circulated through the cell with a 50% hydrogen peroxide solution which supplies oxygen to the cathode. Another application utilizes the Aluminum-oxygen energy system to power a dry suit diverís heating system for six hours. In a package 12 inches in diameter by 24 inches long, the unit allows for rapid recharge in 10 minutes. Commercialization is planned for 2003.
Taking advantage of the available resources when hydrogen is needed in marine locations, the Pennsylvania State University is experimenting with lithium metal as a reactant combined with water to produce hydrogen. A comparison of densities shows that compressed hydrogen at 350 bar contain 23.2 g of hydrogen/liter while 92% Lithium contains 78.4 g of hydrogen/liter. Because of the highly reactive nature of lithium with water, a substantial amount of heat and pressure must be controlled. Using 92% (by mole) of lithium, a reaction with water produces LiH, which when saturated, frees the hydrogen leaving lithium oxide. Early experimentation found that the reaction has two distinct periods. In the first period high heat is produced at ambient pressure. This period has little hydrogen release. In the second period, the system reacts with the oxygen producing high pressure hydrogen gas at a reduced rate of heat generation. The subsequent test reactor consists of a pressure vessel holding 6 kg of lithium alloy for the reaction and a cooling jacket while water is being taken in and hydrogen gas is removed. During operation the reaction temperature was maintained between 1000-15000 K. Actual hydrogen generated correlated well with calculated values.
Another University of Illinois contribution was in the realm of microreactors which convert ammonia to hydrogen. Tiny aluminum posts, 300 microns square and 3 mm high, are anodized with ruthenium in a field 9.5 mm square. The 250 posts of one microreactor convert 15 standard ccs of ammonia to 5.4 Watts of hydrogen with 61.6 % efficiency at 650 0C. This work only addresses the catalytic separation of hydrogen from ammonia and does not yet address the balance of plant required for a successful separator.
Carbon nanotubes for use in hydrogen storage and as electrodes for alkaline fuel cells are being investigated at The Indian Institute of Technology. Using the carbon with high porosity, conductivity and hydrophobicity may be a way to eliminate noble metal catalysts. A low cost porous ceramic support material is first coated with conducting carbon and then electroplated with nickel. Carbon nanotubes are grown over the nickel coating. To date, electrode resistance, hydrogen dissociation Voltage ( 987 mV), current density (1818 A/m2) and chemical stability have been measured. These results suggest promising options for fuel cell electrodes.
In another presentation by the Indian Institute of Technology, carbon microbeads are being produced from cashew nut shell oil. The source material is readily available. Carbon beads and carbon spheres find increasing application as electrodes for Lithium-ion batteries and fuel cells. This initial work focused on the production of microbeads and their measurement of sheet resistance with temperature. Interestingly, the carbon film shows semiconducting properties.
Next month, BD will launch into Power Sources polymer subjects and later conclude with molten salts and COTS batteries. Never heard of COTS ? Stay tuned