Chemistry Everyday for Everyone
Noble Metal–Membrane Composites for Electrochemical Applications Pierre Millet Laboratoire des Composés Non Stoechiométriques, Université de Paris Sud, Bâtiment 415, 91405 Orsay Cedex, France
Solid Polymer Electrolyte Concept and Related Applications An electrochemical cell can be used in two different ways: first, to make chemical transformations (an electrolyzer in which electric power is supplied to the cell); and second, to generate electric power (a generator supplying power to the user). Generally, the electrochemical cell consists of two metallic electrodes immersed in an electrolytic solution (1). The electrodes are disposed in parallel in the solution, at a typical distance of a few millimeters. In the case of a gasevolving reaction (e.g., water electrolysis), a separation diaphragm is added between the two electrodes to avoid the recombination of the electrolysis products. The ohmic resistance of the interpolar gap (i.e., the sum of the electrolytic solution and diaphragm resistances) can be significant and decreases the overall cell efficiency. Higher efficiencies therefore require a thinner cell. In the early 1950s, at the beginning of the U.S. space program, General Electric Company (GEC) first suggested the use of ion-exchange membranes as solid electrolytes in acidic fuel cells. At that time, experimentation related to the development of acidic fuel cells was limited to bench-top scale because of severe corrosion problems encountered in the management of acid electrolytes. The main advantage foreseen by GEC in using a solid electrolyte was as a solution to the problems of electrolyte leakage and corrosion. The fuel being gaseous hydrogen and oxygen, a significant improvement was expected in power-to-mass ratio and cell design. This advantage was of particular importance for space applications. Furthermore, pure water resulting from the electrochemical reaction could be used as drinking water by the space crew. Metal–membrane composite cells (usually referred to as solid polymer electrolyte or SPE) are pictured in Figure 1 for two classical applications: water electrolysis and H 2–O2 fuel cells. In both cases, the membrane plays the role of a solid electrolyte and acts as a separator to the reaction fuels or products. The electrodes are porous to allow fluid transport.
Fuel Cell Configuration In the H2–O 2 fuel cell (Fig. 1a), gaseous hydrogen and oxygen are supplied to the electrodes (2). Molecular hydrogen is oxidized at the negative electrode according to H 2 → 2 H+ + 2 e᎑ Electrons are collected by the external electric circuit, and protons migrate to the cathode through the membrane. Molecular oxygen is reduced at the positive electrode according to
Figure 1. Schematic diagram of (a) a hydrogen/oxygen fuel cell and (b) a water electrolysis cell.
Thus, the overall reaction is H 2 + 1/2O2 → H2O + electric power
Water Electrolysis Configuration In the case of water electrolysis (Fig. 1b), a water feed is broken down at the anode according to (3) H2O(liquid or vapor) → 1/2O2(gas) + 2H+ + 2e᎑ Molecular oxygen is extracted from the anodic compartment while electrons are collected by the external electric circuit. Protons migrate to the cathode through the membrane. On the cathodic side, they are reduced according to 2H+ + 2e ᎑ → H2(gas) The overall reaction can be written as electric power + H2O → H2 + 1/2O2 Each proton carries several (3 to 4) molecules of water. This flux is called the electroosmotic flux and is collected on the cathodic side. During electrolysis, an excess of water is supplied to the anodic and cathodic compartments to extract the gas products as well as the excess heat produced by the Joule effect. The SPE concept reached technological maturity in the early 1970s with the appearance of the Nafion membranes (Nafion is a registered trademark of E. I. du Pont de Nemours for its perfluorosulfonic membrane materials). Nafion is a polymeric material with fixed anionic sites allowing cationic diffusion. With typical cell thicknesses of ca. 200 µm, typical current densities of 1 A cm᎑2 can be afforded without significant ohmic losses. For comparison, alkaline cells are generally operated at 0.5 A cm᎑2.
/2O2 + 2 H+ + 2e᎑ → H 2O
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Chemistry Everyday for Everyone
Other Electrochemical Applications The SPE concept can be used in widespread electrochemical applications: for example, for brine electrolysis, chlorine recovery from diluted hydrochloric acid solutions, ozone generation, and organic electrosynthesis. A review of such applications can be found in ref 4. Plating Procedure The work performed at GEC on SPE technology provides a large body of information about plating procedures (5–7). Typically, the composites are prepared by coating, under heat and/or pressure, a mixture of a catalyst powder and a binder on each side of the membrane. A simpler way for preparing the composites is to use a “wet” procedure (8). The two-step wet procedure consists of (i) ion-exchanging the Nafion membrane initially in the H+ form with platinum tetramine ([Pt(NH 3) 4] 2+]), and (ii) reducing in situ the platinum salt to the metallic state. The reduction process is achieved by immersing the platinum tetramine–exchanged membrane in a reducing solution of sodium borohydride. Under specific experimental conditions (9), the precipitation of metallic platinum occurs preferentially near the membrane surfaces, thus creating two thin platinum electrodes. This procedure is used in a fourth-year Material Science course at the University of Orsay. A dry Nafion 117 membrane in H+ form (1 cm2) is first hydrated by immersion for one hour in boiling water. The membrane is then platinum-ionexchanged by immersion for 15 min in a well-stirred 10 ᎑2 M aqueous solution of platinum tetramine [Pt(NH3)4Cl2.H2O]. Then, the platinum-exchanged membrane is thoroughly rinsed with water and immersed for one hour in a 3 g L᎑1 reducing solution of sodium borohydride. A black precipitate of metallic platinum develops on each side of the membrane. Finally, the composite is immersed for a few minutes in a 1 M HNO3 solution to return to the H+ form. The loading–precipitation sequence can be repeated two times to increase the amount of platinum.
Figure 2. Electron microprobe concentration profiles across the membrane thickness (e) in arbitrary scale. (a) Nafion–platinum exchanged sample; (—) [Pt(NH3)4]2+ concentration profile. (b) Nafion– platinum exchanged sample treated for 2 h with a 15 g L ᎑1 NaBH4 solution; (- - -) Pt 0 and (—) Na+ concentration profiles.
Figure 3. Transmission electron micrograph of the cross-section of a Pt/Nafion/Pt composite showing the interpenetration of the electrode and the membrane.
Characterization of the Composites Figure 2 shows the concentration profiles obtained by electron microprobe analysis before and after completion of the precipitation. Data are plotted in arbitrary scale across the membrane thickness (e). Once the ion-exchange process is achieved, [Pt(NH 3)4]2+ is homogeneously distributed across the membrane thickness (Fig. 2a). After completion of the precipitation, metallic platinum is located mainly near the membrane surfaces, while the sodium ions are homogeneously distributed across the membrane thickness (Fig. 2b). Details of the microstructure of the precipitate are given in Figure 3. Inside the membrane, tiny particles with an average diameter of ca. 100 Å can be seen. At the membrane surface, a more continuous platinum deposit is observed (Fig. 4). Cracks delimit the gas-evolving regions during electrolysis. The requirements for an ideal electrode structure for electrolysis applications are as follows. The metallic and polymeric phases must strongly interpenetrate each other to improve the electrode adherence. The electrode has two different roles to play. The first is that of current collector and involves the external zone. The structure must be sufficiently porous to 48
Figure 4. Scanning electron micrograph of the surface of a platinum deposit after 2,500 h of continuous electrolysis at 1 A cm᎑2. View of the anodic side.
allow fluid transport (reactants and products), but mechanical stability and electronic conductivity must be preserved. The second is that of electrocatalyst and is realized by the internal zone. This part must have a high specific area in order to decrease the local current densities. This is realized by the dispersion of tiny metallic particles into the membrane matrix over a depth of several micrometers. Deeper under the membrane surface, isolated catalyst particles, which are of no use for the reaction, can be observed.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
The performances of the SPE cell can be further improved by using a more appropriate anodic catalyst (8). For example, results obtained with metallic iridium on the anodic side are plotted in Figure 6. A comparison with Figure 5 shows that for a given operating current density, the required cell voltage is significantly lower with iridium than with platinum. Summary
Figure 5. Cell voltage–current density relation with temperature (atmospheric pressure); (–––) platinum as anode and cathode; (- - -) alkaline electrolysis at 70 °C using Ni electrodes in a KOH solution.
Precipitation of metallic platinum at the surface of polymeric membranes allows the preparation of thin electrochemical cells. These cells can be used in widespread electrochemical applications, for example, water electrolysis and hydrogen–oxygen fuel cells. In spite of their cost due to the use of noble metals, they remain attractive because of significant gain in electrochemical performances. With typical noble-metal loadings of 0.2 mg cm᎑2, current densities of 1 A cm᎑2 are afforded. SPE cells are an interesting alternative to more classical cells. Literature Cited
Figure 6. Cell voltage–current density relation with temperature (atmospheric pressure). Iridium as anode and platinum as cathode.
1. Millet, P. J. Chem. Educ. 1996, 73, 956. 2. Hirano, S.; Kim, J.; Srinivasan, S. Electrochim. Acta 1997, 42, 1587. 3. Millet, P.; Andolfatto, F.; Durand, R. Int. J. Hydrogen Energy 1996, 21, 87. 4. Millet, P. A Review of Proton-Exchange-Membrane Based Applications; in Current Topics in Electrochemistry; Current Trends: Poojapura, India, in press. 5. Titterington W. A. Presented at ECS Fall Meeting, New York, 1974; extended abstract; p 576. 6. Lu P. W. T.; Srinivasan, S. J. Appl. Electrochem. 1979, 9, 269. 7. General Electric Co. Solid Polymer Electrolyte Water Electrolysis Technological Development for Large Scale Hydrogen Production; DOE Rep. No. DOE/ET/26-202-1; U.S. Department of Energy: Washington, DC, 1981. 8. Millet, P.; Durand, R.; Pinéri, M. Int. J. Hydrogen Energy 1990, 15, 245. 9. Millet, P.; Andolfatto, F.; Durand, R. J. Appl. Electrochem. 1995, 25, 233.
Electrochemical Performances in Water Electrolysis For electrochemical measurements, the composite is mounted in a Kell-F frame and immersed in deionized (18 Mohm cm at 20 °C) water. Two platinum wires are used as current collectors. Typical U(i) curves of Figure 5 are obtained at different operating temperatures. For comparison, the performances obtained with a more conventional alkaline cell are shown in dashed lines. The increased overall efficiency of the SPE cell can be attributed to (i) the better electrocatalytic activity of platinum over nickel, (ii) the protonic conductivity of the membrane, which is higher than KOH, and (iii) the thinner interpolar gap in the SPE configuration.
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