88
Ind. Eng. Chem. Res. 1997, 36, 88-91
Development of a Novel Manganese Oxide-Clay Humidity Sensor Kazuhide Miyazaki,* Masaharu Hieda, and Takafumi Kato Department of Chemical Engineering, Faculty of Engineering, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-80, Japan
By mixing certain clay mineral samples into finely divided EMD (electrolytic manganese dioxide) and by using the mixtures as the solid electrolyte of a galvanic potential type humidity sensor, which consisted of a Pt/EMD/Cu or Al system, it was found that the clay minerals exhibited a new function to markedly improve the performance of the EMD sensing element, which otherwise responded only to high-humidity regions. The response time profiles of the galvanic potentials produced by the clay-admixed sensor elements were different from the EMD’s. It was observed that different effects took place depending on the nature of the additives, and kaolinite and muscovite enhanced the humidity-sensing characteristics of EMD, showing good linearity and fast response. A mechanism for the enhancement of humidity-sensing characteristics was proposed, taking into account the role of interlayer water molecules present in the EMD-additive system. Introduction
Experimental Section
humidity sensor (Miyazaki et al., 1994). As the additives to the EMD, each of the following materials was examined: kaolinite, bentonite, activated clay, zeolite, mica, and silica sand. Here, reagent-grade materials available were used, except for mica. As for the mica sample, a muscovite species called the “white grade” was available industrially in a powdery form. This sample had been evaluated in a previous study (Miyazaki and Hieda, 1996) and was employed for comparison. Silica sand was used as a reference material in relating the humidity adsorption in the experimental runs. The mixing ratio of each additive to the EMD matrix was 15 wt %, since this quantity was found to be optimal, in our preliminary experimentation (Miyazaki and Hieda, 1996), in order to minimize the dilution of the bulk of EMD. The above additives were, in turn, mixed thoroughly with EMD material in an agate mortar and wetted with deionized water. A quantity of the resulting paste was mounted (approximately 0.3 mm in thickness) onto a mullite tube (1.0 mm in diameter and 0.3 mm in thickness). Two electrodes of different metal filaments (0.25 mm in diameter) were embedded (3 mm apart) in the mounted mixture paste to ensure an electrical contact was made and then dried at room temperature. As in previous studies (Miyazaki et al., 1994; Miyazaki and Xu, 1995; Miyazaki and Hieda, 1996), the galvanic cell system thus formed is written as M1/MnOx/M2, where M1 and M2 act as the cathode and the anode, respectively, and MnOx acts as the solid electrolyte. The sensor potentials were measured by means of an ADVANTEST-TR2114 digital voltmeter (internal impedance > 109 Ω), and the impedance of the sensor element containing EMD was on the order of 104 Ω (Miyazaki et al., 1994), which is well within the meaningful range of measuring impedance in this case. The humiditycontrolling and electrical measurement systems used were similar to the ones described in our previous studies (Xu and Miyazaki, 1992; Miyazaki et al., 1994; Miyazaki and Xu, 1995; Miyazaki and Hieda, 1996).
International Common EMD Sample No. 14 (I.C. No. 14 EMD) was used as the main component of the
Results and Discussion
* Author to whom correspondence should be addressed. Telephone: +81-92-871-6631 ext. 6422. Fax: +81-92-8656031.
Figure 1 shows typical potential response curves for the mixed system I.C. No. 14 EMD plus 15% kaolinite (b), in comparison with the initial, single system of I.C.
Electrolytic manganese dioxide (EMD) has been mainly used as a dry battery cathode, and it is well-known that the structural water of γ-MnO2 (the MnO2 species of EMD) will play a vital role in the MnO2 battery performance (Miyazaki, 1975). Also, EMD is a typical example of multifunctional materials, and one of our recent research interests is to add a new function to this material, other than as a dry battery cathode (Miyazaki, 1993). With this background, how EMD and its derivative MnOx (manganese oxides) would respond electrochemically to H2O in the ambient atmosphere is of great interest and importance from the viewpoint of electric batteries as well as gas sensors. In our previous studies, resistance-type (Xu and Miyazaki, 1992) and potential-type (Miyazaki et al., 1994) humidity sensors were developed utilizing EMD derivatives as a sensing medium. As for the latter type, the galvanic cell system M1/MnOx/M2 was employed as the sensing element, where M1/M2 were two dissimilar metal electrodes and MnOx functioned as the solid electrolyte which was actually a mixture of MnO2 and the lower oxide(s) derived thereform (Miyazaki et al., 1994). The addition of the lower manganese oxide(s) in a controlled quantity proved very effective in improving the humidity-sensing performance of EMD, which otherwise responded only to such a high humidity region as above 60-80% RH (relative humidity) (Miyazaki and Xu, 1995). Recently, the search for effective additives extended into the field of mica, in spite of it being a typical electric insulator. Very interestingly, certain types of mica powders developed a new function to improve the humidity-sensing characteristics of EMD (Miyazaki and Hieda, 1996). The present study is exploring a certain type of clay as a brethren of mica. Hopefully, a similar principle will apply in establishing a novel system of M1/MnOx/M2 type humidity sensor.
S0888-5885(96)00239-4 CCC: $14.00
© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 89
Figure 1. Typical potential-time response curves for the mixed system of EMD plus 15% kaolinite (b), in comparison with the initial, single system of EMD (a).
Figure 3. Typical effects of additives, the same as in Figure 2, with use of more active Pt/Al electrode pairs.
Figure 2. Typical effects of a series of clay samples, together with silica sand as a standard, as additives to a MnO2 humidity sensor on producing galvanic potentials of Pt/Cu electrodes.
Figure 4. Typical potential-time profiles developed by EMD plus 15% kaolinite (a), EMD plus 15% zeolite (b), EMD plus 15% silica sand (c), and single EMD (d) systems with Pt/Al electrodes in response to 90% RH ambient.
No. 14 EMD (a), upon turning on and off the 90% RH ambient atmosphere. Two features are observed: first, the Pt/Cu potential produced with the kaolinite-mixed EMD electrolyte was higher (ca. 520 mV) than the one with the EMD single system (ca. 450 mV), and, second, the moistening response of the mixed system was quicker than that of the single system (ca. 5 min vs ca. 10 min) while the desiccating response was just reversed (ca. 10 min vs 2 min). It should be pointed out that the mixed system did not react quickly to changes in air while desiccating. Presumably, this is because a considerable portion of H2O molecules, adsorbed under the humid condition, were trapped between the lamellar structures of kaolinite present, so that the release of the H2O molecules was hindered by the narrow paths to any extent after the ambient atmosphere had been switched to dry air. Typical effects of a series of additives are shown in Figure 2, on facilitating the generation of the Pt/Cu galvanic potentials, with EMD as the solid electrolyte. By using such additives as kaolinite, muscovite, activated clay, bentonite, and zeolite, it is seen that the potentials generally increased with increasing humidity. In the cases of zeolite and quartz sand, no appreciable potentials were developed in the low humidity regions of 20-60% RH, and these materials were even worse than in the case of no additives to the EMD matrix. If so, would zeolite and quartz sand act as if they were just diluents of the EMD matrix? This question is answered in Figure 3, where the results with Pt/Al electrodes instead of Pt/Cu are shown. It is seen from Figure 3 that zeolite definitely played a favorable role in generating the galvanic potentials from the lowhumidity region such as 20% RH on working together
with the more active electrode pair Pt/Al. This behavior was quite different from that of quartz sand, as an additive, since quartz sand remained inert in this case, too. It is to be noted that the Pt/MnOx/Al sensor systems with various additives produced high galvanic potentials reaching 1200-1300 mV at 90% RH, although a beautiful linear relationship was difficult to obtain between the relative humidity and electric potentials. In order to examine the differences in behavior of kaolinite and zeolite in more detail, Figure 4 compares typical examples of moistening and desiccating response profiles developed by EMD plus 15% kaolinite and by EMD plus 15% zeolite sensors, with reference to the blank tests with 15% silica sand addition and EMD alone, in the case of the Pt/Al couple being exposed to 90% RH. We can see the following features from Figure 4: (1) Since aluminum is less noble than copper, the potentials generated with Pt/Al were generally higher than those with the preceding Pt/Cu pair. (2) In the moistening response, the zeolite-mixed sensor required about 6 min to reach the attainable potential value, whereas the kaolinite-mixed sensor required only 3 min. (3) In the desiccating cycle, the zeolite-mixed sensor was again slower than the kaolinite-mixed sensor. The former showed an asymptotic approach, after an inactive period of 3-4 min, to return to the zero value. The latter, on the other hand, after a similar inactive period of 3-4 min, followed by a rather sharp potential drop to the zero value. (4) Silica sand behaved just as a diluent of EMD, rather than as an effective additive. From item (1), observed with the Pt/Al couple, as well as from the foregoing results with the Pt/Cu couple, it can be deduced that the potentials generated by the experimental sensor elements were without a doubt
90
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
Figure 5. Schematic of H2O adsorption onto the MnO2 surface.
galvanic. As for items (2) and (3), a steric hindrance to H2O molecules will likely occur within the zeolite intercrystalline pore structure, which is an open space called a supercagesthree-dimensional networks of nanopores (Armor, 1995), much smaller in size than the micropores present in kaolinite. The mobility of H2O molecules can be considerably limited due to the zeolite cavity during the moistening and desiccating processes. Silica sand of item (4) proved that the smooth surface of this material was apparently inert to such processes, except for interparticle voids present among the particles. The following half-cell reactions will contribute to the generation of the potentials by the Pt/MnOx/Cu and Pt/ MnOx/Al galvanic cell sensors.
Cu ) Cu2+ + 2e-
(1)
Al ) Al3+ + 3e-
(1′)
O2(MnO2) f 1/2O2 + 2H+ + 2e- ) H2O
(2)
The role of Pt electrode here will be a source or sink of electrons, permitting electron transfer without itself entering into the reactions; i.e., Pt is a “redox” or “inert electrode” (Brett and Brett, 1993). The standard electrode potentials for eqs 1, 1′, and 2 are 0.34, -1.66, and 1.23 V, respectively. Accordingly, the Pt/MnOx/Cu galvanic cell will produce a potential of approximately 0.9 V, and the Pt/MnOx/Al, approximately 2.9 V, in the standard state with plenty of aqueous solution. However, the corresponding voltages obtained with the experimental galvanic cell-type humidity sensors were 0.55 and 1.25 V at most. Due to unavoidable microscopic inhomogeneity of the electrode surface, corrosive dissolution and/or passivation thereof, the side reactions, although in a minute amount in any case, including hydrogen and oxygen evolution will bring about the mixed potential in a given electrochemical cell (Berndt, 1993; Tuck, 1991). It may be worthy to note in this connection that the open circuit voltage of an Al/air battery is between 1.6 and 1.8 V (Tuck, 1991), values which agree well with the potential values observed in the Pt/EMD/Al sensor system. In addition to this, the diminished concentration of protons under the lowhumidity conditions in the ambient atmosphere would contribute to further lower the galvanic potential of the sensor. Figure 5 is a schematic representation of how H2O molecules are adsorbed onto the surface of EMD particles (Nitta et al., 1987). The manganese dioxide particles will have an immediate outer layer of exposed OH- groups and protons originating from the dissociation of chemisorbed H2O molecules. Next to this array, the H2O molecules will be physisorbed, layer by layer from the ambient atmosphere. These chemical and physical adsorptions of water will contribute to the
Figure 6. Proposed model for a potential-type humidity sensor, consisting of a MnO2-lamellar aluminosilicate mixture element with dissimilar metal electrodes M1 and M2.
electric conductivity of the humidity sensor element, giving rise to the galvanic potentials of the metal pairs. This effect may have been enhanced by the presence of interlayer water molecules of clay minerals. Figure 6 is a proposed model that illustrates how the M1/MnOx/M2 humidity sensor works with the aid of clay mineral additives. The interlayer water molecules are seen to bridge some of the EMD particles of the sensor element. Besides, the capillary condensation of water can also be taken into account; that is, the clay-admixed EMD system is more sensitive to the ambient humidity than the single EMD system because the cavity of lamellar structure of clay minerals is on the order of several angstroms (Klein and Hurlbut, 1993), while the average diameter of micropores of EMD is around 20 Å (Kozawa, 1974). The condensation of water molecules from the ambient air into the smaller-sized capillaries will be earlier and smoother than into the larger-sized capillaries, as is dictated by the Kelvin equation for capillary condensation. Although the mechanism suggested for interlayer water molecules is a working hypothesis, we would like to introduce another experimental fact which is consistent with our understanding; that is, a muscovite sample, which had been heat-treated at around 900 °C to release the interlayer water molecules, did not retain the enhancement effect as a sensor additive anymore (Miyazaki and Hieda, 1996). It can be said that the presence of interlayer molecules is essential. The mechanism whereby the interlayer molecules enhance the effect is not fully understood at the present stage of investigation, and further study is required to test this suggestion. Conclusions A novel characteristic of clay minerals has been explored as a component of the potential-type humidity sensor, M1/MnOx/M2 galvanic cell system, where MnOx is a EMD-clay mineral mixture, M1 is Pt, and M2 is Cu or Al. Clay mineral samples included kaolinite, bentonite, activated clay, zeolite, and muscovite. Silica sand was used as a reference material. Of these, kaolinite was found to be a good additive to EMD, which otherwise responded to the high-humidity region such as above 60-80% RH. The EMD-kaolinite mixture responded in a wider humidity region of 20-90% RH. The zeolite-mixed system, on the other hand, was rather slow in moistening and desiccating actions. The galvanic potential values obtained by the Pt/Cu and Pt/ Al electrochemical cells in the presence of MnO2 were
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 91
evaluated in terms of standard electrode potentials. For the first time an investigation on interlayer molecules involved in clay minerals was conducted in relation to humidity-sensing characteristics of EMD. We are continuing our research effort to reveal in detail the new aspects of interlayer water molecules, aiming at a higher-performing humidity sensor. Acknowledgment The experiment was partially carried out by Mr. H. Takakubo and Mr. H. Tazawa. The muscovite powder sample was kindly supplied by Okabe Mica Co., Ltd. (Japan). Abbreviations EMD ) electrolytic manganese dioxide EMF ) electromotive force RH ) relative humidity
Literature Cited Armor, J. N. Molecular Sieves for Air Separation. In Materials ChemistrysAn Emerging Discipline; Interrante, L. V., Caspar, L. A., Ellis, A. B., Eds.; American Chemical Society: Washington, DC, 1995. Berndt, D. Maintenance-Free BatteriessLead-Acid, Nickel/Cadmium, Nickel/HydridesA Handbook of Battery Technology; Research Studied Press: Taunton, U.K., 1993. Brett, C. M. A.; Brett, A. M. O. ElectrochemistrysPrinciples, Method, and Applications; Oxford University Press: New York, 1993. Klein, C.; Hurlbut, C. K., Jr. Manual of Mineralogy (after James D. Dana), 21st ed.; John Wiley & Sons: New York, 1993.
Kozawa, A. Electrochemistry of Manganese Dioxide and Production and Properties of Electrolytic Manganese Dioxide (EMD). In Batteries. Manganese Dioxide; Kordesch, K. V., Ed.; Marcel Dekker: New York, 1974; Vol. 1. Miyazaki, K. Release Pattern of Combined Water of MnO2 under a Vacuo-Heated Condition. Manganese Dioxide Symposium, Cleveland, OH, Oct 1975; Paper 6. Miyazaki, K. R&D Relating to Enterprise Activities in the Inorganic Material Industry in Japan. Fukuoka Univ. Rev. Technol. Sci. 1993, 50, 277. Miyazaki, K.; Xu, C.-N. Novel Effects of Mn2O3 Addition on a New M1/MnOx/M2 Humidity Sensing System. The 2nd East Asia Conference on Chemical Sensors, Xi’an, China, Oct 1995; Paper H-06. Miyazaki, K.; Hieda, M. A New Function of Mica Powders as a Humidity Sensor Element. Fukuoka Univ. Rev. Technol. Sci. 1996, 56, 177 (in Japanese). Miyazaki, K.; Xu, C.-N.; Hieda, M. A New Potential-Type Humidity Sensor Using EMD-Based Manganese Oxides as a Solid Electrolyte. J. Electrochem. Soc. 1994, 141, L35. Nitta, M.; Takeda, Y.; Haradome, M. Gas Sensors and Their Applications; Power-Sha: Tokyo, 1987 (in Japanese). Tuck, C. D. S. Aluminum-Air Batteries. In Modern Battery Technology; Tuck, C. D. S., Ed.; Ellis Horwood: New York, 1991. Xu, C.-N.; Miyazaki, K. Humidity Sensing Characteristics of the EMD-Derivatives. J. Electrochem. Soc. 1992, 139, L111.
Received for review April 26, 1996 Revised manuscript received October 1, 1996 Accepted October 12, 1996X IE960239H
X Abstract published in Advance ACS Abstracts, December 1, 1996.
