Study of Dehydrated Salts: Electrolyte for Intermediate Temperature Fuel Cell

Transcription

1 Portugaliae Electrochimica Acta 009, 7(), DI: /ea PUGALIAE ELECTRCHIMICA ACTA ISSN Study of Dehydrated Salts: Electrolyte for Intermediate Temerature Fuel Cell Kamlesh Pandey, 1,* Mrigank M. Dwivedi, 1 Mridula Triathi 1 National Centre of Exerimental Mineralogy and Petrology, University of Allahabad, Allahabad-11 00, India Deartment of Chemistry, C.M.P. Degree College, Allahabad, India Received 15 Setember 008; acceted 3 February 009 Abstract Fuel cells are receiving growing interest in recent years since they reresent one of the most romising energy source to reduce ollutant emission. We roose some new dehydrated salts as an electrolyte in the intermediate temerature fuel cell. The roton conduction in the dehydrated salts was established by the study of DTA/TGA, infrared sectroscoic study, transference number, bulk electrical conductivity measurement and emf study. The electrical conductivity of the dehydrated salts becomes ionic and increases times in the hydrogen ambient with resect to vacuum. Keywords: hydrogen-oxygen, fuel cell, roton conductor, dehydrated salt, electrochemical emf. Introduction In the resent scenario, because of the increasing energy demand and the growing concern about the limited fossil fuel, the scientific community has focused its attention towards the develoment of alternating ower generation technologies with the otential for low ollution. Usually the solar cells and the fuel cells are the main sources of the non conventional energy. These non conventional energy sources are advantageous because they are clean and economical. Recently, the roton conducting materials (as an electrolyte) have become the hot contenders for the electrochemical device alications like batteries, fuel cell and sensors [1-4]. Most of the good roton conductors are hydrates or hydrogen bonded materials which are unstable at high temeratures. For the fuel cell alications, generally 00 ºC or more is the referable * Corresonding author. address: k54831@gmail.com

2 K. Pandey et al. / Port. Electrochim. Acta 7 (009) temerature due to good thermodynamic efficiency. Many sintered oxides like SrCe 3, BaCe 3, SrZr 3 doed with Y 3, Yb 3, etc., which are -tye semiconductors, show roton conducting behavior at high temerature in H /humid ambient [5-7]. A series of materials KTa 3 doed with Ni, Fe or Li 5 Al 5 show rotonic behavior at intermediate temerature range ( ºC) [8-1]. In the resent communication, we reort the roton conducting behavior of some secific salt hydrates (i.e. Al (S 4 ) 3.16H, AlNH 4 (S 4 ).1H and FeS 4.7H ), which show a good conduction in the hydrated state as well as after the dehydration in the intermediate temerature range ( ºC). This unusual behavior of the dehydrated salts makes it a best candidate for electrochemical alication. The roton conduction in the dehydrated salts was established by the study of DTA/TGA, transference number, bulk electrical conductivity measurement and emf study. Exerimental techniques Dehydration of salt hydrates and ion transort study Salt hydrates (namely aluminum sulhate, aluminum ammonium sulhate and ferrous sulhate) are of analytical grade. These salts are dehydrated by heating the samles at 300 ºC, 0 ºC and 300 ºC, resectively, in an oven. The dehydration of the salts is confirmed by the TGA record as shown in Fig. 1. Figure 1. TGA sectra of three salts: Al (S 4 ) 3.16H, AlNH 4 (S 4 ).1H and FeS 4.7H. From the TGA, it is clear that all three salt hydrates are not comletely dehydrated in a single ste. For examle, in the first ste, aluminum sulhate losses two water molecules of crystallization u to 10 ºC; in the second ste (u to 145 ºC) next eight water molecules and finally the last six water molecules were removed from the lattice u to 300 ºC [13]. Dehydration of Al (S 4 ) 3.16H is given as 100

3 K. Pandey et al. / Port. Electrochim. Acta 7 (009) Al (S ).16H Al (S ).14H 10º C 145º C 4 3 H 4 3 8H Al (S ).6H Al (S ) 145º C 300º C 8H 4 3 6H 4 3 For aluminum ammonium sulhate, Duval (1959) [14], roosed a mechanism for weight loss by assigning AlNH 4 (S 4 ).1H the chemical formula NH 4 [Al(H ) (S 6 H 4 ) ]6H. n heating there is a loss of 6 molecules of water of crystallization u to 10 ºC, then a subsequent loss of 4 water molecules at ~140 ºC comes from grou (S 6 H 4 ), leaving behind the S 4 -- grou. Finally, the last two molecules of water attached to Al ion are removed at 0 ºC. Phase transition behaviour can be summarized as (1) AlNH (S ).1H AlNH (S ).6H 110º C 140º C 4 4 6H 4 4 4H AlNH (S ).H AlNH (S ) 140º C 0º C 4H 4 4 H 4 4 In ferrous sulhate, there is the loss of first three water molecules u to 66 ºC, next three are removed from the lattice u to 110 ºC, and final removable water of crystallization gets off at 87 ºC [15]. In ferrous sulhate, this can be summarized as () FeS.7H FeS.4H 66º C 110º C 4 3H 4 3H 110º C 87º C FeS 3 4.H FeS H 4 H If this owder is again ket in humid ambient, these water molecules may be reabsorbed. The dehydrated samles were ground and alletized in a evaccuable die (to avoid the rehydration roblem) in the form of a ellet (d = 6 mm and thickness mm). To determine the nature of mobile secies (i.e. transference number measurement), Wagner s method of olarization [16] has been used. In this, we record the current as a function of time on alication of a small dc otential on the samle. The initial current (i initial ) and the final current after the comlete olarization (i final ) were evaluated by the current - time lot. The ionic transference number (t ion ) is given by i initial i t final ion = i initial In the case of dehydrated salt, Wagner s method is unable to exactly redict whether the conductivity is due to electrons (σ e ) or holes (σ h ). The theory of nonionic contribution in the redominantly ionic conductor was roosed by Wagner and later used by Ilschner [17]. According to this theory, the current may be exressed as I e,h = I e + I h VF = 1 VF I A, + e e h σ e e σ h 1 LF where R is the gas constant, T is the absolute temerature, F is Faraday s constant, A and L are area and thickness of the ellet, resectively, V is the (3) (4) 101

4 K. Pandey et al. / Port. Electrochim. Acta 7 (009) alied voltage, I e,h is the total non ionic current (due to electrons and holes), i e and i h are the current due to the electrons and the holes, resectively. The variation of residual non ionic current (i.e. after full olarization) has been achieved as a function of alied otential. There are the following ossibilities: (i) If σ e >> σ h, then VF I = = A e e e, h Ie σ LF 1 For large value of V, the current saturates to I A e = [ σe] (5) LF (ii) If σ h >> σ e, then VF VF I = = A 1 e, h I h e LF σ h = A σ e LF h i.e., the current varies exonentially with V. When extraolated to V = 0, this gives the value of I h as I = A h σ (6) LF h The samle is olarized at some fixed voltage and the residual current was measured by Keithley SMU-36. The value of σ e and σ h could be calculated from the I-V characteristics using the above equations. The infrared sectra of dehydrated salts were recorded by Perkin-Elmer IR sectrohotometer. For the high temerature measurement, a temerature controlled evacuable cryostat with KBr window, suitable for the temerature range K (Secac, model 5757) was used. A samle (with KBr) allet was mounted inside the cell. This cell also has gas flow attachment. During the exeriment, hydrogen is flowing continuously in the cell. After the time of equilibration, the sectrum was recorded. The bulk electrical conductivity measurement was carried out by comlex imedance/admittance lot method with the hel of a Solartron 150 FRA couled with the 186 ECI. The high temerature electrical conductivity of dehydrated salts was measured with the hel of sring loaded samle holder laced in a closed furnace. Cell geometry used To establish a rotonic behavior at high temerature, the electrochemical emf measurement across the concentration cell is the best technique. Some imortant concentration cell geometries are (i) Cell-IA P H( ) Pt-electrode* Dehydrated salt Pt-electrode* P H( ) *To construct the orous Pt-electrode, it is necessary to fire the Pt-aste at 50 ºC. The otential difference develoed across it is given by Nernst s equation 10

5 K. Pandey et al. / Port. Electrochim. Acta 7 (009) E = t ln ' F '' where t is the transference number, and ' and " are the artial ressures of gases. (7) (ii) Cell-IB P H(I) P (I) Pt-electrode Dehydrated salt Pt-electrode P H(II) P (II) In the concentration cell having dry/ wet air combination, the emf of the cell is given by 1/ (8) E = ln F H H ( I ) ( II ) ( II ) ( I ) (iii) Cell-II Now, let us take another tye of concentration cell which uses H and in two comartments P H Pt-electrode Dehydrated salt Pt-electrode P P H(II)?? ) In this cell, the roton has tendency to migrate across the roton conductor towards the oxygen where it is discharged to form water vaor. Therefore, the emf is given by E = E 0 ln F H H ( I ) 1 where P H and P are the artial ressures of the hydrogen and oxygen, resectively. In this method, it is imortant to avoid the leakage between the anode and the cathode comartments. Two tyes of concentration cells were used for the resent study (9) Cell-1 Cell- Dry air // Dehydrated salt ellet // Wet air H (H) // Dehydrated salt ellet // air For the dry air we connect the assembly to P 5 (with R.H.< 5%) and for wet air it is connected to water bottle. Porous latinum aste was used as electrode on the both faces of the ellet. Results and discussion Transference number (t ion ) of dehydrated salts (i.e., Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4 ) was measured by Wagner s method of olarization at 50 ºC in vacuum and hydrogen ambient. In vacuum/low ressure air t ion of the dehydrated salts were found to be ~ zero. This is an indication of non ionic (or electronic) nature of mobile secies in vacuum. But in the hydrogen ambient, the nature of dehydrated salts was comletely changed and showed ionic behavior. The change 103

6 K. Pandey et al. / Port. Electrochim. Acta 7 (009) of non ionic to ionic nature in hydrogen ambient give the reliminary indication that dehydrated salts can sustain the rotonic conduction at elevated temeratures. The change in nature of dehydrated salts is exlained as follows: in the resence of hydrogen, it (rotons) is considered to be localized between the nearest neighboring oxide ions, forming a hydrogen bond with oxide in the lattice, roducing hydroxide grous and electrons as follows 1 H x * ' ( g) + o = Ho + e (10) * ne could believe that roton could migrate by hoing from the H o site to oxide ion site at a normal lattice site nearly causing the rotonic conductivity [18-1]. Calculated values of transference number in both atmosheres are given in Table 1. Table 1. Values of t ion in air and hydrogen for the dehydrated salts at T=50 ºC. S.No. Materials t ion (in air) t ion (in hydrogen) 1. Al (S 4 ) AlNH (S 4 ) FeS To determine the relative contribution of electrons and holes conduction searately, we use Ilschner s method. In this technique, the dehydrated salt ellet was sandwiched between ion blocking and a reversible electrode. A small dc voltage (always smaller than the dissociation otential of the material) was alied and the final residual current was measured. In I-V lot, the electronic art of the current tends to reach a saturation value (A/LF)σ e, while on the other hand the contribution of hole increases exonentially. The conductivities due to electrons and holes are given by and σ e = (LF/A) i e (11) σ h = (LF/A) i h (1) where i e is the measured saturation lateau current and i h is the value of current obtained by extraolating the exonentially increasing art at V = 0 (in I-V lot). The ellets of dehydrated salts with latinum electrodes were held in a sring loaded flat late cell assembly. The I-V characteristics of these salts (after allowing the sufficient time for olarization, if any) are given in Fig.. The variation of current with voltage in Al (S 4 ) 3 and FeS 4 shows only an exonential increase (giving the value of i h when extraolated to V = 0), but in AlNH 4 (S 4 ), a small lateau region (tyical of i e ) as well as an exonential increase of current, were observed. This infers that Al (S 4 ) 3 and FeS 4 both dehydrated salts are basically hole conductor in air/ vacuum (since the necessary lateau of i e is absent). The calculated values of σ e and σ h along with σ meas by comlex imedance method are listed in Table. 104

7 Current (Am) K. Pandey et al. / Port. Electrochim. Acta 7 (009) FeS i h 10-7 T=50 C In Air Al ( S 4 ) 3 i h 10-8 i e AlNH ( S 4 ) i h Alied Voltage (V) Figure. Variation of final saturation current with alied voltage in air for Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4. Table. Calculated values of electronic and hole conductivities in dehydrated salt electrolyte in air ambient at T=50 ºC. S.No. Electrolyte σ electronic (S/cm) σ hole (S/cm) Calculated σ total (S/cm) Exerimental σ total (S/cm) 1. Al (S 4 ) 3 -- ~ 9.0x x x10-9. AlNH 4 (S 4 ) 1.3x x x x FeS x x x10-7 The results obtained by transference number measurement for dehydrated salts are indicative of the fact that they can sustain roton (H + or H - ) mobility in hydrogen ambient at elevated temerature ~ ºC. Therefore, it is also exected that the hydrogen diffusion coefficient would be high at these temeratures. To confirm the facts in-situ IR sectral studies have been carried out at 50 ºC under - dehydrated salts ket in vacuum at 50 ºC, and - dehydrated salts ket in hydrogen at 50 ºC for sufficiently long time. The IR sectra of dehydrated salts in vacuum and hydrogen atmoshere at 50 ºC are shown in Fig

8 K. Pandey et al. / Port. Electrochim. Acta 7 (009) FeS 4 In Vaccum In Hydrogen Transmittance In hydrogen AlNH 4 ( S 4 ) In vaccum Al ( S 4 ) 3 In hydrogen In vaccum Wave number (cm ) Figure 3. Infrared sectra of Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4. The sectra in vacuum and hydrogen are qualitatively similar in all resect. The closer look of H bond region shows a ossible formation of new H bonding due to the diffusion of external hydrogen in lattice. The region of strong-h eak in the sectra is cm -1. Ideally, in the fully dehydrated salt H eak at ~ 300 cm -1 region is not exected. However, we do see a residual H absortion eak even in the hydrated samle laced in vacuum. This may be ossible due to the ineffective N urging of the instrument (and/or some residual H bands in KBr or the samle). The imortant oint of our resent interest is to study the change (if any) in H eak when the IR sectrum is recorded in hydrogen ambient. A comarison of the area under H eak in vacuum, and in hydrogen ambient shows an enhancement (~1.5 times). It is ossibly due to the formation of new H bonds by the hydrogen diffusing in the salt lattice. The electrical conductivity of dehydrated salts at temerature range o C in various environments was measured by comlex imedance lot. Enhancement of conductivity in H ambient In dehydrated salts the conductivity increases after inserting hydrogen into the chamber. The material changes their conductivity from non-ionic to ionic when laced in the H ambient. However, the utake of hydrogen in dehydrated salts is slow. It takes some time (few minutes to hour) to reach the steady state of conductivity value (σ hydrogen ). The variation of conductivity with time after the 106

9 K. Pandey et al. / Port. Electrochim. Acta 7 (009) insertion of hydrogen is shown in Fig. 4. It is clear that the equilibration is faster (30-40 min) for Al (S 4 ) 3 and AlNH 4 (S 4 ), while it takes a very long time (>4 h) in the case of FeS 4. It is interesting to note that there is a correlation between the number of water molecules removed during dehydration and the equilibration time (σ value). The equilibration time of σ in hydrogen ambient for five dehydrated salts [i.e., CaS 4, FeS 4, Fe (S 4 ) 3, AlNH 4 (S 4 ) and Al (S 4 ) 3 ], as a function of the number of water molecules, was shown in Fig. 5. It seems that more voids are created in those lattices from which more water molecules have been removed and hence they follow faster reaction kinetics to equilibrate the conductivity in hydrogen ambient. However, this is only an emirical observation T=00 C AlNH (S ) Conductivity (S/cm) Al( S 4) 3 x FeS 4 x Min. Hours Time Figure 4. Variation of electrical conductivity of Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4 with time after insertion of hydrogen (the conductivity of t=0 min is σ vacuum ). = σ vs. 1/T measurement The log σ vs. 1/T lots are shown in Fig. 6 for samles laced in vacuum, air and hydrogen. Sufficient time was always allowed for equilibration whenever ambient was changed. The log σ vs. 1/T lots is linear and it can be reresented by the Arrhenius tye exression: σ = σ ex E KT a 0 (13) where σ 0 is the re-exonential factor and E a is the activation energy. The calculated values of σ 0 and E a are given in Table

10 K. Pandey et al. / Port. Electrochim. Acta 7 (009) CaS.H 4 FeS.7H 0 4 Tim e (m in.) Fe (S ) 9H 4 3. AlNH (S ) 1H Al (S ) 16H No. of water molecules in the samle Figure 5. Time taken for equilibration of conductivity in hydrogen ambient for dehydrated salts (salts having different amount of water of crystallization). Figure 6. Variation of electrical conductivity of Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4. with temerature in three environments ( i.e., air, vacuum and hydrogen). 108

11 K. Pandey et al. / Port. Electrochim. Acta 7 (009) Table 3. Value of σ 0 and E a of the dehydrated salts. S.No. Electrolyte material Atmoshere σ 0 (S.cm -1 ) E a (ev) 1. Air 1.6 x Al (S 4 ) 3 Vacuum 1.5 x Hydrogen 1.0 x Air 3.8 x AlNH 4 (S 4 ) Vacuum 3.1 x Hydrogen 5.8 x Air 3.5 x FeS 4 Vacuum. x Hydrogen.7 x In Al (S 4 ) 3, the σ vs. 1/T shows a deviation from Arrhenius behavior at T> 350 ºC. In the thermal study of aluminum sulhate, it is reorted that there is a hase transition (amorhous to crystalline) around this temerature. The non ionic conductivity (i.e. in air and vacuum) increases with a faster rate at temerature >350 ºC while in hydrogen ambient the conductivity (ionic in nature) starts decreasing. The ossible reason is that the crystalline nature is not suitable for the ionic conductivity enhancement. In AlNH 4 (S 4 ) there is only a linear region in the entire temerature range studied but in FeS 4 the deviation from linear behavior starts at ~ 340 C in air and hydrogen. This effect is more rominent in case of hydrogen ambient. It is reorted that FeS 4 has a dissociation temerature of ~ 480 C. The resent trend is ossibly a reflection of a re-dissociation effect. The dissociation seems to be assisted by hydrogen ambient. Electrochemical emf measurement For the electrochemical emf measurement, we use two tyes (symmetrical and asymmetrical) of concentration cell, using the dehydrated salt as an electrolyte: Symmetrical cell Asymmetrical cell Air Dehydrated salt with electrode Air Humid air Dehydrated salt with electrode Dry air or Hydrogen Dehydrated salt with electrode Dry air The ossible chemical reactions at the different electrodes are given below: In the symmetrical cell wet air/dehydrated salt/air cell 109

12 K. Pandey et al. / Port. Electrochim. Acta 7 (009) and H H e H e H Similarly in the asymmetrical cell H / dehydrated salt / air or wet air / dehydrated salt / air H H + + e and 1 H + + e H + Electrochemical emf generated in the two concentration cells is exressed as follows: in symmetrical concentration cell H ( I) E = ln F ( II) H in asymmetrical concentration cell (14) E = E0 ( I) H ln F H ( II) (15) where P H, P, and P H, are artial ressures of hydrogen, oxygen or water, resectively, R is the gas constant, F is the Faraday constant, and E is the standard electrode otential. The electrochemical emf generated for different cells with dehydrated salts as electrolyte (Al (S 4 ) 3, AlNH 4 (S 4 ) and FeS 4 ) are given in Table 4. In this study a construction of symmetrical concentration cell was urosely assembled to check that the electrochemical emf develoed in asymmetrical cells is genuinely due to the resence of higher activity of rotonic secies on one side. The symmetrical cell gives nearly zero otential as exected, since the electrochemical activity on the two sides is the same. However, the asymmetrical cell gives genuine electrochemical emf due to the activity of rotonic secies on the electrodes. This is an indication of ossible motilities of gaseous ionic secies in the electrolyte. Table 4. Electrochemical emf of the dehydrated salts at 00 ºC and 300 ºC. S. No. Electrolyte materials Cell tye Wet air // Dry air Hydrogen //Air ( H0 ) 00 ºC 300 ºC 00 ºC 300 ºC 1. Al (S 4 ) 3 1 mv 3 mv mv 190 mv. AlNH 4 (S 4 ) 1. mv 8.5 mv 51.5 mv 30 mv 3. FeS mv 3.1 mv 80 mv 180 mv 110

13 K. Pandey et al. / Port. Electrochim. Acta 7 (009) Conclusion Transference number, electrochemical emf measurement and electrical conductivity measurements (in the hydrogen ambient, the magnitude of the conductivity increases ~ three times with resect to air/vacuum) confirm that these dehydrated salts are rimarily non ionic in air, but show a good rotonic behavior in the hydrogen ambient. This study rooses an alternative set of materials (i.e., dehydrated salts) as ossible electrolyte for the intermediate temerature fuel cell. References 1. K. Korsesch, G. Simader, Fuel Cell and their Alications, VCH Venhenium, R.C.T. Slade, N. Singh, Solid State Ionics 46 (1991) S. Chandra, Suerionic Solids- Princiles and Alication, North- Holland, Amesterdam, S. Chandra, in: S. Chandra, A.L. Laskar (Eds.) Suerionic Solids and Solid Electrolytes- Recent Trends, Academic Press, New York, 1989, Nakamura, T. Kodama, I. gino, Y. Miyake, Chem. Lett. 1 (1979) Y. Liu, J.Y. Lee, L. Houng, Journal of Power Source 19 (004) L.S. Ng and A.A. Mohamad, Journal of Power Source 163(1) (006) H. Iwahara, H. Uchida and N. Meada, Solid State Ionics 3/4 (1981) H. Iwahara, H. Uchida, K. Morimoto, J. Electrochem. Soc. 137 (1990) K. Pandey, M.M. Dwivedi, Science Letter 5(3&4) (00) Ph. Colomban, Proton Conductors: Solid, Membranes and Gels-Materials and Devices, Cambridge University Press, K.D. Kreuer, in: B.V.R. Chaudhari et al. (Eds.), Solid State Ionics: Science and Technology, World Scientific Publication Co. 1998, K. Pandey, Ph. D. Thesis, B.H.U. Varanasi, C. Duval, Inorganic Thermogravimetric Analysis, Elsevier, Publishing Co., New York (1953). 15. R.W.G. Wyckoff, Crystal Structure Vol. 3 (1960). 16. J.B. Wagner Jr., C. Wagner, J. Chem. Phys. 6 (1957) B. Ilshner, J. Chem. Phys. 8 (1958) W. Suksamai, I.S. Metcalfe, Solid State Ionics 178 (007) Y. Nigara, K. Kawamura, T. Kawada, J. Mizusaki, M. Ishigame in: B.V.R. Chaudhari et al. (Eds.), Solid State Ionics: Science and Technology, World Scientific Publication Co., 1998, T. Scherban, A.S. Nowick, Solid State Ionics 35 (1989) H. Iwahara, Solid State Ionics (1996)