In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Direct Methanol Fuel Cell submitted by:
Orfeo Zerbinati Dipartimento di Scienze e Tecnologie Avanzate, Università del Piemonte Orientale, Corso T. Borsalino, 54, I-15100 Alessandria, Italy;
[email protected] checked by:
Ali Mardan Applied Chemistry Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan
checked by:
Mark M. Richter Department of Chemistry, Southwest Missouri State University, Springfield, MO 65804
More than 160 years after the first demonstration of fuel cells, performed in 1839 by William R. Grove, these devices are rapidly coming of age for the production of electricity from fuels, including non-petroleum-derived ones. Among the most developed types of fuel cells are alkaline (AFC), phosphoric acid (PAFC), molten carbonate (MCFC), solid oxide (SOFC), and proton exchange membrane (PEM) cells (also referred to as solid polymer electrolyte or SPE cells). Hydrogen is actually the most common fuel, because of its relevant electrochemical reactivity and energy density, although its storage and safe handling are troublesome. Alcohols, particularly methanol, have been envisaged as convenient fuels, and in recent years the electrical characteristics of direct methanol fuel cells (DMFCs) have been greatly improved. There are many sources of additional information at various levels of complexity both in the literature (1–6 ) and on the Web (7, 8). Several electrochemical demonstrations have been proposed to illustrate the conversion of chemical into electrical energy for the hydrogen/oxygen system (9–13). This paper will deal with a simple apparatus for demonstrating DMFC operation. The direct electrochemical oxidation of methanol in acidic solution takes place at the anode according to the following equation: CH3OH + H2O → CO2 + 6H+ + 6e᎑; E ° = 0.016 V/SHE Protons migrate to the cathode, where oxygen is reduced according to the following reaction: O2 + 4H+ + 4e᎑ → 2H2O; E ° = 1.229 V/SHE The overall reaction is CH3OH + 3Ú2O2 → CO2 + 2H2O ∆G ° = ᎑702 kJ/mol and ∆H ° = ᎑726 kJ/mol On a theoretical basis (∆G ° = ᎑nFE °), the equilibrium standard electromotive force (emf ) of an ideal DMFC is thus 1.21 V, but the emf of practical devices is much lower than the ideal value because of incomplete oxidation of methanol, which can stop at the formaldehyde or formic acid stage. In fact, even at very low current densities, the emf drops to less than 0.8 V.
Materials and Methods A demonstration DMFC for didactic purposes was built and tested. The housing of the DMFC described in this work was constructed very simply by connecting the luer tips of two 10-mL plastic syringes (without pistons) by means of 5cm segments of polypropylene or silicone tubing, 2 mm i.d. Each syringe was filled with 6 mL of 1 M sulfuric acid electrolyte, taking care to avoid the formation of bubbles inside the tube. Thus the ion conductive membrane usually present in DMFCs was replaced by this plastic tubing filled with sulfuric acid, which acted as a salt bridge. Several suitable catalysts have been developed for use with the DMFC. Platinum–ruthenium alloys perform better than any currently known material. Pure platinum can also work effectively, although it is more sensitive than platinum–ruthenium to poisoning by the products of incomplete methanol oxidation (14 ). For simplicity, the platinum catalyst was used for both the anode and cathode, and it was deposited onto 0.3-mmdiameter 80:20 nickel–chromium wires commonly used for fabricating electric resistors or thermocouples. These electrodes had 2-cm2 active areas, obtained by winding 21.2 cm of wire onto a 4-mm-diameter core, leaving a 6-cm lead for attaching electrodes. Before platinizing, these 2-cm2 spirals were electrocleaned at 0.1 A/cm2 current density for 10 s in 1 M sulfuric acid, using a graphite rod cathode, and then washed with distilled water. A 9-V dry battery can provide the 200-mA current necessary for 2-cm2 electrodes, with the addition of a 27-Ω–2-W resistor in series. Subsequently, platinum catalyst was electrodeposited on the Ni–Cr spirals from a solution of 10 g/ L of H2PtCl6⭈6H2O (3.75 g/L Pt) in 1 M HCl, with a graphite rod as anode. Fifty milliliters of this solution, corresponding to 0.5 g of chloroplatinic acid, was enough to platinize several dozen electrodes; therefore the expense due to platinum consumption was limited. Two methods with different degrees of complexity were devised for platinizing electrodes. The simpler one required only a 9-V dry battery and a 470-Ω resistor, which connected the negative pole of the battery to the nickel–chromium wire; the positive pole was connected to a graphite electrode. Electrolysis of this platinizing solution for 30 min at a current of approximately 15 mA with the 2-cm2 electrodes resulted in
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
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In the Classroom
2
Reference
Working
-5V
-5V UA741
4
V+ + 3 7
2
R1
− V−
6
10k
R1 100k
POT
U1
+5V +5V
R3 10k
Q2
0.0
BC107
4
− V−
3 +
V+ 7
U2
Counter 6
Q1
-5V +5V
BC177 +5V
Potential Drop / V
-5V LF351N
-0.2
-0.4
-0.6
Figure 1. Schematic of the potentiostat used for platinizing electrodes. Two 4.5-V dry batteries can be used as power supply. U1 is an FET operational amplifier with high input impedance, such as LF351N, 13741 or equivalent; Q1 and Q2 can be any complementary pair of low-power, bipolar transistors (e.g., BC107/ BC177); the R3 potentiometer regulates cell voltage. The internal reference of a combined glass electrode can be conveniently exploited by connecting the shield of its cable to the “Reference” input.
the deposition of a suitable amount of platinum catalyst over the metal electrode. The second method uses a three-electrode potentiostat capable of providing at least 20 mA at 1.3 V, in order to operate under controlled conditions, as reported in the literature (9, 13). Laboratories that are not equipped with such an electrochemical instrument but can access electronic facilities could assemble the inexpensive circuit of Figure 1, which can also be used for general electrochemistry experiments. By this method, platinum catalyst was electrodeposited on the Ni–Cr spirals by immersing them in the platinizing solution together with a graphite rod as anode and a saturated Ag/AgCl reference electrode. The Ni–Cr electrode was held at ᎑250 mV vs Ag/AgCl (᎑53 mV vs RHE) for 30 min by means of the potentiostat. After the potential was applied, the current gradually reached 15 mA and about 2–5 mg of Pt was deposited on the Ni–Cr wire. Platinized electrodes were washed with 1 M sulfuric acid and stored in distilled water, taking care to avoid damage to the delicate platinum layer. If they were stored in 1 M sulfuric acid, the Ni–Cr alloy slowly dissolved, probably owing to the combined effects of the platinum catalyst and dissolved oxygen. When platinized electrodes were positioned in the fuel cell compartments in the absence of methanol, a few-millivolt potential difference was measured by a digital multimeter, but this potential rose to 0.7 V as soon as 0.2 mL of methanol was added to the anode, which assumed negative electric polarity. Methanol can be introduced by means of 1-mL syringes with 18-gauge needles, since the exact quantity is not critical. After the addition of methanol the cell soon ceased generating electricity because the oxygen initially dissolved in the electrolyte was consumed, but electricity was produced again if air was bubbled into the cathodic compartment by means of small plastic tubing connected to a cylinder or another source of compressed air. When the same experiment was carried out with nonplatinized nickel–chromium electrodes, no significant potential was obtained in either the presence or absence of methanol, demonstrating that the cell could produce electricity only if methanol, atmospheric oxygen, and catalyst were all present. The short-circuit current of the cell was about 600 µA, and its voltage to current curve was estimated with the aid of 830
0
100
200
300
400
500
600
Current / µA Figure 2. Voltage drop vs current for one DMFC element. Each voltage drop is referred to the potential measured immediately before the insertion of each resistor load.
resistance loads ranging from 220 Ω to 2.2 MΩ by measuring the potential difference across the cell and calculating current according to the Ohm’s law. An example of such a curve is shown in Figure 2. It can be seen that generated potentials decrease when current increases and the gradient is slightly smaller in the central portion of the curve, as is observed in devices developed for power production (14). Since this cell’s potential showed some instability, together with memory effects that required some time for recovery after current was drawn, it is preferable to plot current against the voltage drop occurring immediately after connecting the load. Measurements under load were restricted to the shortest time possible, especially at the largest current values. No carbon dioxide evolution at anodes was observed. However, the amounts of fuel consumed and consequently the amounts of CO2 produced by this device are very low, as simple calculations based on electrochemical equivalents can demonstrate. Consequently, the CO2 produced can easily remain in solution considering its water solubility. To supply an electric clock, which required a 200-µA average current at 1.5 V concentrated into 40-ms, 5-mA pulses, five fuel cell elements were placed in series and a 470-µF–25 V electrolytic capacitor was placed in parallel with the terminals of the clock, to level its current absorption from the electric generator. A photograph of this device is shown in Figure 3. Air coming from a cylinder was bubbled into the cathode chambers by means of plastic tubing. This generator was capable of powering a clock for several hours, until voltage degradation occurred owing to electrode poisoning and methanol migration into the cathodic compartment. Initial conditions could be restored by washing the electrodes with distilled water and renewing the electrolyte. Hazards Safety concerns during the demonstration involve the manipulation of methanol and 1 M sulfuric acid, for which the wearing of safety goggles, latex gloves and a lab coat is recommended. After use, the acidic electrolyte with low methanol content (3%) can be disposed of as is usual for diluted acidic solutions. A possible disposal procedure could
Journal of Chemical Education • Vol. 79 No. 7 July 2002 • JChemEd.chem.wisc.edu
In the Classroom
voltage-to-current curve is easily determined by those who wish to explore the topic of electrical energy more deeply. For didactic purposes, the effects of electrolyte and methanol concentration or the type of fuel (ethanol in place of methanol) on DMFC performance can be investigated. Literature Cited
Figure 3. Photograph of the didactic fuel cell assembly, showing the 470 µF–25 V electrolytic capacitor used for leveling the current absorption of the clock, the plastic tubing for bubbling air at the cathodes, the pseudo salt bridge plastic tubing, and the platinized Ni–Cr electrodes.
consist of neutralizing the acidic electrolyte under a fume hood by careful addition of a stoichiometric amount of sodium hydrogen carbonate, which simultaneously neutralizes acid and strips out the volatile organic component. It must be stressed that methanol is toxic if ingested; however, according to European legislation, it may be tolerated in wine up to a concentration of 0.3% (vs ethanol). Discussion In conclusion, this DMFC is very simple to prepare, and several cells in series can power an electric clock, making an attractive demonstration of an alternative power source. The
1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14.
Sammells, A. F. J. Chem. Educ. 1983, 60, 320. Appleby, A. J. Sci. Am. 1999, 281 (1), 74–79. Lloyd, A. C. Sci. Am. 1999, 281 (1), 80. Dyer, C. K. Sci. Am. 1999, 281 (1), 88–93. Kordesch, K., Simader, G. Fuel Cells and Their Applications; VCH: Weinheim, 1996. Weissbart, J. J. Chem. Educ. 1961, 38, 267. Beyond the Internal Combustion Engine; The Methanol Institute: Washington, DC; http://www.methanol.org/fuelcell/special/ promise.html (accessed Apr 2002). Direct Methanol Fuel Cell; Giner Electrochemical Systems; http://www.ginerinc.com/direct_methanol.htm (accessed Apr 2002). Revised Nuffield Chemistry Teachers’ Guide II; Dawson, B. E., Ed.; Longman: London, 1978; p 315. Feinstein, H. I.; Gale, V. J. Chem. Educ. 1977, 54, 432. Skinner, J. F. J. Chem. Educ. 1977, 54, 619. Gilbert, G. L. J. Chem. Educ. 1980, 57, 216. Roffia, S.; Concialini, V.; Paradisi, C. J. Chem. Educ. 1988, 65, 272. Lamy, C.; Léger, J.-M. In Proceedings of New Materials for Fuel Cell Modern Battery Systems II; Savadogo, O., Ed. Montreal, Canada, 1997; 477.
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