SCIENCE
Polymer Conducts Protons At Room Temperature Blend of polyvinyl alcohol and phosphoric acid invented by Signal Research Center scientists is being tested in hydrogen-sensor device Scientists at Allied-Signal's Signal Research Center in Des Plaines, 111., have invented a polymer that con ducts protons at room temperature. The new polymer—a blend of poly vinyl alcohol and phosphoric acid— is already being tested as a hydro gen sensor. The developers—phys icist Anthony J. Polak, research chemical e n g i n e e r Sande PettyWeeks, and polymer chemist Allyson J. Beuhler—say that the discov ery could lead to similar materials suitable for use in batteries, fuel cells, and electrochromic display devices. Solid ionic conductors are a fer tile field for research (C&EN, May 20, page 42). Much of the work deals with crystalline inorganic solids. Sta
bilized zirconias, for example, are used as oxygen sensors in combus tion control systems in autos. How ever, most of these materials are efficient ion conductors only at tem peratures of 200 to 500 °C or even higher. And for some uses, such as solid-state batteries, the rigidity of the inorganic materials also puts them at a disadvantage. Because of dimensional changes in the elec trodes during charging and dis charging, it is difficult, if not im possible, to maintain intimate con tact between the electrodes and the solid electrolyte. Consequently, scientists also have been looking for polymers and polymer-salt complexes that will act as ionic conductors. Used as solid electrolytes, the flexible polymers can deform to maintain the elec trode-electrolyte interface, even as the electrodes grow or shrink. Also, the polymers can be cast in thin films. That minimizes the size as well as the electrical resistance of the electrolyte. Polak notes that a number of ionconducting polymers have been
ι
Petty-Weeks (left) and Polak test conducting polymer they invented 28
November 25. 1985 C&EN
identified. Examples include com plexes of alkali metal salts dissolved in polar polymers such as p o l y e t h ylene oxide) or polyether-substituted polyphosphazenes. Again, however, these materials become usefully con ductive only at elevated tempera tures, on the order of 70 to 220 °C. The Allied-Signal team has been working to develop solid proton conductors. Early efforts involved blending various polymers with compounds like dodecamolybdophosphoric acid and uranyl o-phosphate. These efforts weren't success ful. Either the products required a source of water vapor to remain con ductive, or conductivities were too low. Success finally came in the form of polyvinyl alcohol (PVA) and plain old phosphoric acid. In a simple procedure, acid is added to PVA dissolved in deionized water. Then the mixture is evaporated to leave a thin transparent film of the poly mer complex. According to tests, the proton conductivity of films made in this manner ranges from 10~3 to 10~6 (ohm-cm) -1 , depending on the acid-PVA ratio. No external source of water is needed. The polymer's conductivity isn't so good as that of the best solid inorganic ion conductors, Polak notes. However—and it's a very im portant however—the H3PO4-PVA complex is conductive at room tem perature. In fact, it's conductive from —40 to 40 °C. That operating range makes the material attractive for many applications. For example, Polak says, the abil ity to measure hydrogen concentra tions quickly and accurately is im portant to many research studies and to many chemical processes, includ ing oil refining, semiconductor man ufacture, and electroplating. To test the membrane's utility as
Proton-conducting membrane forms basis of hydrogen sensor Voltmeter
Recomb hydrogen
Proton- · · conducting * membrane
Proton "si
Electron flow
Ι
\· ·
···
H2 molecule
Platinum coating Hydrogen molecules from the side of higher concentration try to cross the membrane. But they strike its platinum coating and break into their constitu ent protons and electrons. The protons pass through the membrane. The elec trons flow along a wire to the other side. There, protons and electrons recombine to form molecular hydrogen. Hydrogen concentration can be calcu lated by measuring voltage
a hydrogen sensor, Polak and asso ciates sputtered both sides of the film with platinum to form elec trodes. The device was exposed on one side to a reference stream of pure hydrogen and on the other side to a hydrogen-containing mix ture to be analyzed. The electrodes were connected to a voltmeter. The electrochemical concentration cell performed as hoped. Protons migrated from the pure hydrogen side across the membrane to the region of lower partial hydrogen pressure on the other side. Elec trons, unable to pass the membrane, traveled around the circuit and through the voltmeter to the other side, there to recombine with the protons. The resulting voltage was measured, and the hydrogen con centration in the analyte mixture was calculated from the voltage. The system has many desirable
properties for a hydrogen sensor, Polak says. The response is specific and response time is only about six seconds. Measured voltages deviate less than 1% from theoretical val ues. Response is linear from the parts-per-million range to superatmospheric pressures. The system re mained stable over a period of about 100 days of continuous testing. In a "poisoning study," hydrogen sul fide, hydrogen chloride, butane, and carbon dioxide had no measurable effects. The system isn't perfect, however. Carbon monoxide at concentrations greater than 0.1% causes interfer ence by competing with hydrogen for surface sites on the platinum electrode. If the analyte gas stream contains oxygen, a hydrogen-oxygen fuel cell is formed, and that also interferes. And the membrane, in its original form, is soluble in water. So the work goes on. "Sande al ready has been able to form a proton-conducting complex polymer that can be more conductive than the original one," Polak says, add ing that the new material is also water-insoluble, and sturdier to boot. Other efforts aim at increasing the useful operating temperature of the polymers. The team also has devised another membrane-based hydrogen sensing system in which a layer of palladium hydride serves as a refer ence electrode. That eliminates the need for a reference stream of hy drogen gas. In addition, studies are under way to learn more about the basic mechanisms involved in pro ton conductivity in the membranes. Aside from the hydrogen-sensing applications, Allied-Signal is closemouthed about its plans for the new materials. However, the company does mention one obvious poten tial use: electrochromic display de vices. The membranes would medi ate electrochemical reactions that cause materials to change color. Un like liquid crystal displays, which go blank when the power is off, displays based on the chemically altered materials would retain their images, even in the absence of an applied voltage. But the images could be erased by passing current through in the other direction, thus reversing the chemical reaction. Ward Worthy, Chicago
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November 25, 1985 C&EN
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