In the Laboratory
An Undergraduate Laboratory Experiment Using a Simple Photoassisted Fuel Cell To Remediate Simulated Wastewater
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Faiza Touati and Kevin G. McGuigan Department of Physiology and Medical Physics, Royal College of Surgeons in Ireland, Dublin 2, Ireland John Cassidy* School of Chemical and Pharmaceutical Sciences and FOCAS, Dublin Institute of Technology, Dublin 8, Ireland;
[email protected] Titanium dioxide has been used as a suspension for water remediation (1) or has been immobilized on a support to allow its easy recovery to mineralize organic compounds in aqueous solutions (2). Work has been carried out with nanocrystalline TiO2 coatings for formic acid degradation with UV light (3). Such coatings have also been used for the degradation of humic acid (4), phenol (5), and in a clever cell that allows the oxidation of organics at the photoanode along with the reduction of metals, specifically copper(II), at the cathode (6). This latter system allowed the reduction of chemical oxygen demand, COD, in aqueous solutions along with the removal of toxic metals. Grätzel cells have used light-sensitized TiO2 layers on indium tin oxide (ITO) electrodes (7). Substrates for oxidation have included p-nitrophenol (8) and I− (7) or Br− (9). In the present experiment organic compounds are degraded at a photoanode while oxygen is reduced at a cathode of a fuel cell. An extension of the work is to immobilize TiO2, as has been done previously (10). The use of simple composites with polymers has been carried out with zeolites (11) and mediating cobalt complexes (12). In this experiment, an electrode coated with TiO2 is used to oxidize organic compounds, along with an air electrode where oxygen reduction occurs. In addition, a tungsten light source is used that is safe and cheap in comparison with UV light sources. There are a number of commercial fuel-cell kits that are based on a hydrogen–oxygen system (13). In addition, methanol–air fuel cells have been demonstrated with precious-metal electrodes (14), in which case a coupling of the degradation of organic compounds with a fuel-cell configuration provides an attractive environmentally friendly system. The advantages of this system are that a safe light source is used, there is no requirement for ITO electrodes, and the TiO2 layer can be readily cast. Students will be typically thirdor fourth-year undergraduate students from environmental science or chemistry. Supporting lecture material on the operation of photovoltaic devices would be required (7). Learning outcomes from this experiment include: • Introduction to the electrochemistry of fuel cells; a broader discussion of the uses and limitations of fuel cells would be possible • Introduction to the interaction between light and semiconductors; a broader discussion of the band-gap model could be undertaken • Introduction to environmental remediation and chemical analysis using HPLC
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Chemicals, Equipment, and Method Composite layers were prepared by using a suspension of TiO2 (Degussa P25, 2.00 g L᎑1) in tetrahydrofuran (THF) along with polyvinylchloride (PVC; 0.7 g L᎑1) . This suspension was sonicated for 10 min and typically 0.162 mL of this suspension was measured onto a glassy carbon electrode, (3.8 cm2) and dried at room temperature. The light source was a standard 60 W tungsten spot light, (a lamp used for recessed ceiling fittings with an inbuilt mirror that focuses the radiation, Tesco, Dublin). Glassy carbon sheets (Tokai) and platinum foil (Goodfellow) were silver epoxied to shielded copper wire and then encapsulated with Araldite adhesive (Radionics, Dublin). Carbon disks (3mm diameter, Metrohm), a saturated calomel electrode (SCE), and a platinum wire formed the three-electrode onecompartment cell for voltammetry. The potentiostat was a CHI model 602 linked to a PC. Current from the fuel cell was passed through a resistance (typically 1 kΩ) and the voltage fed directly to a Recorderlab XYT chart recorder. The air electrode was taken from a mini fuel cell (Electrochem-technic, Oxford) consisting of a porous platinum electrode of geometric area equal to 9.6 cm2. Analysis of formic acid was carried out by HPLC as detailed in the Supplemental Material.W Hazards Small quantities (10 mL) of the suspension in THF are prepared, which minimizes the risk since THF is flammable. The potentiostat should be operated under supervision. The acid, H2SO4, is corrosive and appropriate precautions should be taken. Results and Discussion In this experiment a fuel cell remediates wastewaters by oxidizing the organics and lowering the COD, while a corresponding level of reduction happens at the other electrode where the substrate is molecular oxygen. A solution of formic acid was used to simulate wastewater. Formic acid has a rapid and simple electrode reaction under the conditions used in the fuel cell. Such simple organic acids are products of the decomposition of organic compounds in landfills and are frequently found in leachates. Figure 1 shows the cyclic voltammogram of a carbon disk modified with the PVC兾TiO2 composite in a three-electrode system, in ambi-
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In the Laboratory
Figure 1. Cyclic voltammogram for a carbon-disk electrode (area = 0.071 cm2 ) in a solution of 6 mM HCOOH in 0.1 M KCl. A three-electrode one-compartment cell was used and potentials are quoted with respect to SCE: (A) in ambient laboratory light and (B) exposed to 60 W tungsten lamp, scan rate = 100 mV s᎑1.
ent laboratory light (A) and when exposed to light from the 60 W tungsten lamp (B). It can be clearly seen that there is an enhancement of the current over a wide range of potentials associated with the enhanced oxidation of formic acid in solution. There is quite a similarity between the emission of a tungsten light source and the spectral profile of sunlight (15). The cyclic voltammetry experiment is performed to characterize the photoactivity of the electrode. The mechanism of this enhancement is well-documented using UV light (8) and it is replicated in Figure 1 using a simple composite TiO2兾PVC layer along with an easily accessible light source. Figure 2 shows the photoactive behavior of a fuel cell consisting of a TiO2兾PVC composite on a carbon plate (3.8 cm2) linked to a commercial air electrode as described in the experimental section. On exposure to light there is an enhancement of the current. The current increases from 20 µA to 115 µA on exposure to the light, which is not as energetic as UV light typically used in photoelectrochemistry (3, 8, 10). This enhancement is consistent with the cyclic voltammetry results. On exposure to light the current is constant indicating a kinetically controlled system that is reversible. Once the cell is running, it is possible to examine the effect of the position of the light source with respect to the electrodes along with the relative position of the electrodes. Figure 3 shows the decrease in the concentration of formic acid monitored chromatographically as a function of time for a fuel cell with a carbon electrode (3.8 cm2) coated with the TiO2兾PVC composite as an anode and a Pt electrode (4 cm2) as cathode. Typically the current level changed from 0.2 µA to 0.9 µA over a period of 4 hours. The concentration decreases linearly indicating a zeroth-order reaction as has been found previously for HCOOH (3). When a similar HCOOH solution was not exposed to light, there was no decrease in formic acid concentration over the same period. Conclusion
Figure 2. A current transient at a fuel cell consisting of a glassy carbon electrode coated with the composite acting as an anode (area = 3.8 cm2 ) along with a porous air electrode as cathode in a one-compartment cell with a solution of 10 mM HCOOH and 0.1 M KCl on exposure to light.
This work describes the behavior of coated electrodes using a simple method of casting with a TiO2兾PVC composite layer. A fuel cell with this novel electrode along with an air electrode can be operated in solutions with a readily available light source. This system demonstrated that even with low energy light sources, an enhancement of the oxidation efficiency is possible. This would encourage the development of systems in climates where there is continuous sunshine; obviously not Ireland! Much work has to be done for the development of optimized cell configurations that would be compatible with large water masses. Most simply the device would float on the surface of the water to employ the sun’s radiation and work is ongoing in the area. Acknowledgment JC acknowledges a Team Research Scheme (TERS, 2004) grant from DIT. FT acknowledges funding from the Government of Libya.
Figure 3. Decrease in HCOOH concentration (initial concentration = 0.1 mM) in a fuel cell consisting of a TiO2/PVC anode (3.8 cm2), and a cathode that is a Pt sheet (4 cm2) in 15 mL of solution, exposed continuously to 60 W tungsten lamp.
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Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
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In the Laboratory
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8. Herrera-Melian, J. A.; Dona-Rodriguez, J. M.; Tello Rendon, E.; Soler Vila, A.; Brunet Quetglas, M.; Alvera Azcarate, A.; Pascual Pariente, L. J. Chem. Educ. 2001, 78, 775–777. 9. Bohrman-Linde, C.; Tausch, M. W. J. Chem. Educ. 2003, 80, 1471–1473. 10. Li, J.; Peter, L. M.; Potter, R. J. Applied Electrochem 1983, 14, 493–504. 11. Cassidy, J.; O’Donoghue, E.; Breen, W. Analyst 1989, 114, 1509–1510. 12. De Smet, S.; Cassidy, J.; McCormac, T.; Maes, N. Electroanalysis 1995, 7, 782–784. 13. Dipl. Ing. Udo Michelfelder, Steigersbrünnle 4, 74632 Neuenstein, Germany. http://www.udomi.de/home/home-e.html (accessed Oct 2006). 14. Zerbinati, O. J. Chem. Educ. 2002, 79, 829–831. 15. Goode, S. R.; Metz, L. A. J. Chem. Educ. 2003, 80, 1455– 1459.
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