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Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method
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Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method Bhupendra Singh a,b , Ji-Hye Kim a , Jun-Young Park c , Sun-Ju Song a,b,∗ a
Ionics Lab, School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwang-Ju 500-757, Republic of Korea b Research Institute for Catalysis, Chonnam National University, Republic of Korea c Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Received 31 March 2014; received in revised form 12 April 2014; accepted 14 April 2014
Abstract Mn2+ -doped Sn1−x Mnx P2 O7 (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 ◦ C, as all the reaction by-products are vaporizable at <150 ◦ C. The dopant Mn2+ acts as a sintering aid and leads to dense Sn1−x Mnx P2 O7 samples on sintering at 1100 ◦ C. Though conductivity of Sn1−x Mnx P2 O7 samples in the ambient atmosphere is 10−9 –10−6 S cm−1 in 300–550 ◦ C range, it increases significantly in humidified (water vapor pressure, pH2 O = 0.12 atm) atmosphere and reaches >10−3 S cm−1 in 100–200 ◦ C range. The maximum conductivity is shown by Sn0.88 Mn0.12 P2 O7 with 9.79 × 10−6 S cm−1 at 550 ◦ C in ambient air and 2.29 × 10−3 S cm−1 at 190 ◦ C in humidified air. It is observed that the humidification of Sn1−x Mnx P2 O7 samples is a slow process and its rate increases at higher temperature. The stability of Sn1−x Mnx P2 O7 samples is analyzed. © 2014 Elsevier Ltd. All rights reserved. Keywords: Tin pyrophosphate; Proton conductivity; Proton-conducting ceramic-electrolyte fuel cells; Sintering-aid; Co-precipitation method
1. Introduction Lowering of operating temperature of fuel cells has been one of the important goals in fuel cell research and the development of electrolytes with high ionic conductivity and stability is a crucial requirement for the realization of this goal.1–3 Recently, a number of proton-conducting tetravalent metal pyrophosphates (MP2 O7 , where M = Sn, Ge, Zr, Si, Ce, Ti) have been reported as potential electrolytes for proton-conducting ceramic-electrolyte fuel cells (PCFCs) in the temperature range of 150–400 ◦ C.2–7 Among various MP2 O7 acceptor doped tin pyrophosphates have shown maximum ionic conductivity.2,8–11 Conventionally undoped/doped-SnP2 O7 are prepared by mixing a Sn containing salt or oxide with phosphoric acid and calcining the mixture at high temperatures to form the SnP2 O7 .2,8,12 This processes mostly results in impurity of ∗
Corresponding author at: School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwang-Ju 500757, Republic of Korea. Tel.: +82 62 530 1706; fax: +82 62 530 1699. E-mail address: [email protected] (S.-J. Song).
precursor oxide, if P/Sn ratio is low, and it is difficult to control the P/Sn ratio. The solution phase synthesis methods can be useful in removing this drawback as well as in obtaining better control on particle size.13–16 However, the solution phase synthesis method reported so far either involves expensive precursor or have multi-step processing routes and have high crystallization temperature.13–16 Therefore, it is necessary to look for new synthesis method which can be advantageous for the enhancing the properties of material as well as be economically beneficial. Due to the poor sinterability of SnP2 O7 , the fabrication of dense electrolyte samples have been one of the major changes toward their application as electrolytes in PCFCs. Often, it requires sintering at ≥1400 ◦ C to get fairly dense tin pyrophosphate samples.2,8,17 However, sintering at such a high temperature leads to the loss of phosphate phase due to the evaporation at high temperatures, which severely lowers their proton conductivity, as the phosphate phase helps generating additional proton incorporation sites as well as provide additional proton migration path.7,12,18 A number of strategies have been proposed for the fabrication of dense metal pyrophosphate samples.17,19 Phadke et al.17 have proposed the sintering of metal
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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pyrophosphates specimen at 1400 ◦ C in a sealed crucible in presence of some sacrificial powder of metal pyrophosphate, and Sato et al.19 have fabricated dense metal oxide-metal pyrophosphate composites (such as SnO2 –SnP2 O7 , TiO2 –TiP2 O7 , so on) by reacting porous metal oxide substrate with excess phosphoric acid solution at ∼600 ◦ C. However, both methods have some complications and have some special requirements. Therefore, it is imperative to look for new alternatives for the fabrication of dense tin pyrophosphate electrolyte specimens. Recent reports have shown that some dopants not only enhance the ionic conductivity of tetravalent metal pyrophosphates but also act as sintering aid and help getting dense electrolyte samples at significantly lower temperatures.20,21 In this work, we have synthesized Mn-doped tin pyrophosphates (Sn1−x Mnx P2 O7 , x = 0, 0.05, 0.1, 0.12, 0.15, 0.2) using a new solution-precipitation process which leads to crystalline product at calcination temperatures as low as 300 ◦ C, and dense electrolyte samples on sintering at 1100 ◦ C. Therefore, objective of this study is to prepare dense Sn1−x Mnx P2 O7 (SMP) samples by combining the advantages of co-precipitation method of powder preparation with the potential sintering-aid effect of Mn2+ dopant. Various SMP samples were characterized by XRD, SEM, energy dispersive X-ray analysis (EDX), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and their electrical properties were studied using EIS technique for their potential application as solid electrolyte in PCFCs in 100–220 ◦ C range.
Table 1 Details of various Sn1−x Mnx P2 O7 compositions. Lattice parameter is calculated from the Rietveld analysis of XRD data.
2. Materials and methods
The crystalline phases in heat-treated powders and the powders from crushed sintered disks were examined using an X-ray diffractometer (XRD-7000, Shimadzu) equipped with a Cu˚ and operated at 40 kV and K␣ radiation source (1.5406 A) 30 mA at a scan rate of 1◦ /min between scanning angles (2θ) of 10–80◦ . The microstructure of the fractured section of the sintered pellets was analyzed using a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi). The energy dispersive X-ray analysis (EDX) was performed by an EDX analyzer (EMAX, HORIBA) integrated with the SEM analysis system. The thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) was performed using a thermal analyzer (NETSZCH SDT Q600 V8.3 Build 101) at a heating rate of 5 ◦ C/min in air from room temperature to 1000 ◦ C range.
2.1. Powder synthesis and dense sample fabrication Mn2+ -doped tin(IV) pyrophosphates, Sn1−x Mnx P2 O7 (x = 0–0.2), were synthesized by a co-precipitation method using tin(II)oxalate as tin precursor. Tin(II)oxalate (98%, Aldrich), nitric acid (60%, DaeJung), Mn(II)acetate tetrahydrate (≥99%, Sigma-Aldrich) and phosphoric acid (85%, DaeJung) were used as the starting materials. Firstly, 0.1 moles of {(1 − x)Sn + xMn} were dissolved into nitric acid solution. For this, Sn(II) oxalate was dispersed into 50 ml water in 500 ml glass beaker and then nitric acid was slowly added to the dispersion with constant stirring. The insoluble Sn(II) oxidizes, initially slowly and then vigorously, to soluble Sn(IV) with the increase in the amount of nitric acid.22 A slight excess of nitric acid was added to the solution to prevent any reduction of Sn(IV). The dopant precursor Mn(II)acetate tetrahydrate was added to Sn(IV) solution with stirring. The appropriate amount of phosphoric acid was separately mixed with water to make a 200 ml solution in 500 ml beaker. The solution containing metal precursors was then added drop-wise to the phosphoric acid solution with constant stirring. It is important to mention that if phosphoric acid is added to the metal precursor solution, it leads to instant precipitation, which may cause inhomogeneous particle growth. After 30 min the temperature of mixture was slowly raised to 60–70 ◦ C while stirring, which leads to the appearance of a gel-like precipitation in the solution and then temperature was raised to ∼85 ◦ C to dry out the solution.
Sample composition
Sample ID
Lattice parameter (Å)
Linear shrinkage on sintering (%)
SnP2 O7 Sn0.95 Mn0.05 P2 O7 Sn0.9 Mn0.1 P2 O7 Sn0.88 Mn0.12 P2 O7 Sn0.85 Mn0.15 P2 O7 Sn0.8 Mn0.2 P2 O7
SnP SMP50 SMP100 SMP120 SMP150 SMP200
7.9641 7.9676 7.9703 7.9817 7.9751 7.9713
6.1 19.1 16.1 15.7 14.3 13.2
The gel-like precipitate was completely dried in hot-air oven at ∼140 ◦ C. The obtained solid was ground with mortar and pestle, and was kept in covered alumina crucible for calcination at 300 ◦ C in air for 8 h. The as-calcined powder was ground with mortar and pestle, and the obtained powder was used for further characterization. For the pellet preparation, 550 ◦ C heat-treated powders were sieved through a 38 m mesh test sieve and molded into 1.5–2.0 mm thick and 12 mm diameter disks. The disks were cold isostatically pressed at 200 MPa and were sintered at 1000–1200 ◦ C in covered alumina crucible for 10 h in air. Various Sn1−x Mnx P2 O7 (x = 0–0.2), samples are compiled in Table 1 for the ease of identification. 2.2. Physical characterization
2.3. Electrochemical characterization The sintered CGP disk was polished with sandpaper, painted with colloidal gold (Pelco® colloidal gold paint) on both sides and dried in oven at 200 ◦ C for 2 h. A platinum mesh connected to a platinum wire was separately placed in contact with each sides of painted pellet and pressed against mica sheets to ensure proper electrical contact. The impedance data were recorded in two-electrode configuration at zero dc-bias over the frequency range of 1–107 Hz using a frequency response analyzer (S1260A, Solartron Analytical). The perturbation voltage was kept at 50 mV. The sample was placed in an alumina chamber and shielded by a copper tube to minimize the induction effect from the heating element whose temperature was
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controlled by a heating furnace and monitored by an inserted K-type thermocouple placed closely to the sample. The ionic conductivity was measured at the ambient atmosphere and in humidified air with water vapor pressure (pH2 O) of 0.12 atm. The desired pH2 O was maintained by regulating the humidity of alumina chamber by an upstream humidifier and the flow of air in the chamber was maintained at a rate of 200 cc/min. The supply lines from the upstream humidifier to the alumina chamber were kept at ∼70 ◦ C using heating tape to avoid any condensation of moisture inside the supply line. For the calculations of ionic conductivity from impedance data, the equivalent circuit modeling was performed using equivalent circuit simulation software ZView-Impedance software. 3. Results and discussion 3.1. Phase composition and microstructure of Sn1−x Mnx P2 O7 The solution phase synthesis methods, previously proposed for undoped and doped tin pyrophosphates, involve tin chloride as tin precursor and (NH4 )2 PO4 or K4 P2 O7 as precipitating agent. Therefore, as-prepared powders contain by-products, such as NH4 Cl, and crystallize only at fairly high temperatures 650 ◦ C.13–15 In the present method, however, no such by-products are formed while using tin(II)oxalate as tin source and phosphoric acid as precipitating agent, as during the oxidation of tin(II) by nitric acid to tin(IV), the oxalate component decomposes to CO2 and O2 in the presence of nitric acid,23 and nitric acid and acetic acids vaporize during heating at 80–140 ◦ C. As a result, the as-prepared powders are expected to contain only amorphous phases of tin pyrophosphate and excess phosphates and therefore the most probable chemical composition of the as-prepared powder can considered as SnP2 O7 /Sn(HPO4 )2 ·xHPO4 ·yH2 O. Fig. 1 shows the TGA/DSC of as-prepared powders of SnP and SMP100. As can be seen, there are four prominent regions of weight loss in the TGA graph. The initial weight loss around ∼100 ◦ C in region 1 may be due to the loss of water presumably incorporated during synthesis and weight loss around ∼200 ◦ C in region 2 can be attributed to the partial dehydroxylation of the as-prepared powder. The rapid weight loss around ∼300 ◦ C in region 3 is mainly due to the conversion of Sn(HPO4 )2 ·yHPO4 into SnP2 O7 ·yHPO4 . The appearance of an exotherm in the DSC graph around 300 ◦ C indicates the crystallization of SnP2 O7 . The significant weight loss in region 4 can be attributed to the partial loss of excess phosphate phase due to the conversion of yHPO4 phase into amorphous phosphate phases, (collectively represented as Pm On , where m and n are arbitrary numbers). There was no weight loss in 600–1000 ◦ C range, but the DSC curve tends to form an exotherm >1000 ◦ C, which could be due to the phase transition at the higher temperature.13 It is reported that crystalline SnP2 O7 has very complicated structure. Though it exhibits cubic structure with space group Pa3¯ (2 0 5), a pseudocubic 3 × 3 × 3 superlattice is also reported.24 When SnP2 O7 is heated at ≥1000 ◦ C, the pseudo-cubic superlattice changes to orthorhombic superlattice.13 The comparison of DSC graphs in
Fig. 1. (a) TGA-DSC of as-prepared (a) SnP and (b) SMP100 powder with initial P/(Sn + Mn) ratio of 3.
Fig. 1, however, shows that such phase transition is suppressed on the doping of Mn2+ . Fig. 2 shows XRD of various SMP powders calcined at 300 ◦ C in air for 8 h. As can be seen, powders are well-crystalline and all diffraction peaks can be assigned to cubic lattice with space group Pa3¯ [JCPDS No. 291352]. Unlike the previous
Fig. 2. XRD of Mn2+ -doped SnP2 O7 powders calcined at 300 ◦ C in air for 8 h.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 3. Rietveld analysis of XRD pattern of various SMP powder prepared with P/(Ce + Mn) molar ratio = 3 and calcined at 300 ◦ C in air for 8 h.
reports of tin pyrophosphates synthesized by co-precipitation methods, pseudo-cubic 3 × 3 × 3 superlattice is not clearly visible in the present case, however, some unknown peaks start appearing in SMP150 and SMP200, indicating that the solubility limit of Mn2+ is ∼0.12 mole. To obtain the structural parameters, we have performed Rietveld analyses of the XRD data applying MS modeling program. The peak shape was assumed to be Tomandl pseudo-Voigt functions. The background of each profile was approximated by a 20-parameter polynomial. With a weighted residual factor (Rwp ) of ∼17%, the best refinement fit results of various SMP samples are given in Fig. 3. The lattice parameter calculated from the profile matching of XRD data is given in Table 1 and, as can be seen, the lattice parameter increases with increase in doping level from x = 0–0.12 but decreases beyond x > 0.12. The decrease in lattice parameter beyond x > 0.12 may be due to the superlattice formation or due
to the appearance of impurities.13 The ionic radius of Sn4+ is ˚ while that of Mn2+ in high and low spin states is 0.83 0.69 A ˚ 25 and therefore the expansion in SnP2 O7 on Mn2+ and 0.67 A, doping indicates that Mn2+ mostly exists in high spin state in Mn2+ -doped SnP2 O7 . Fig. 4 shows SEM image of fractured section of various Sn1−x Mnx P2 O7 (x = 0–0.2) pellets sintered at 1100 ◦ C for 10 h. As can be seen, the undoped SnP2 O7 sample is highly porous and no grain growth is visible in it. However, the porosity of SMP50 sample is significantly reduced and well-developed grains are clearly visible, indicating that the dopant Mn2+ is acting as a sintering aid. The appearance of bright edges in individual grains in SEM of SMP50 indicates that the excess amorphous phosphate is possibly located at the surface of the grains, thereby forming a core-shell type morphology.15 The enhanced densification is also consistent with the total linear shrinkage measured
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Fig. 4. SEM images of fractured section of SMP pellets sintered at 1100 ◦ C in air for 10 h. SMP powders synthesized with initial P/(Sn + Mn) ratio of 3 and calcined at 550 ◦ C were used for pellet preparation.
Fig. 5. SEM images of fractured section of SMPs pellets sintered at (1) 1000 ◦ C, (2) 1100 ◦ C, and (3) 1200 ◦ C.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 7. Variation of ionic conductivity of 1100 ◦ C sintered SMP pellets vs. temperature in ambient air.
ducting, they are corrosive in nature and can cause corrosion of Pt, the commonly used electrodes during the fuel cell application of such materials. Therefore, the composite with localized distribution of higher concentrations of phosphate phase, such as SnP2 O7 –SnO2 composites, can be mechanically less stable during fuel cell operation than the composite with uniform distribution of phosphate phase, such as SMPs in the present study. Fig. 6. Line-scan EDS profiles of the elements in fractured section of SMP100 synthesized with initial P/(Sn + Mn) = 3 and sintered at 1200 ◦ C, (a) Phosphorous and (b) Tin.
in this study. The total linear shrinkage (s), {s = (lo − l)/lo }, where lo and l are the diameters of SMP pellets before and after the sintering, of pellets increases dramatically in Mn2+ -doped samples. For undoped SnP it was only 6.2% but for SMP50 it increases to 19.1%, which along with SEM data clearly supports the sintering-aid effect of Mn2+ -doping. In the SEM image of SMP120 step-formation across the fractures clearly is visible, indicating that the fracture is across the grains and sample is highly dense. However, in SEMs of SMP150 and SMP200 grain size becomes smaller and step-formations are less visible, which could be due to the influence of the unknown secondary phases present in these samples. Fig. 5 shows SEM images of fractured section of SMP100 and SMP120 pellets heat-treated at 1000–1200 ◦ C in air for 10 h. As can be seen, 1000 ◦ C heat-treated samples are poorly sintered but 1100 ◦ C and 1200 ◦ C heat-treated samples are well-sintered and step-formations on the fractured surface increases with the increase in temperature. Fig. 6 shows line-scan EDS profiles of tin and phosphorous in fractured section of SMP120 synthesized with initial P/(Sn + Mn) = 3 and heat-treated at 1100 ◦ C. As can be seen, the distribution of phosphorous and tin almost overlaps each other, indicating that excess phosphate phase is homogeneously distributed in SMPs. This observation is clearly different from the dense SnP2 O7 –SnO2 composites proposed by Sato et al.,19,26 where phosphate phase is formed only in the pores of the SnO2 substrate and tin and phosphorous phase hardly overlaps. Though amorphous phosphates are highly proton con-
3.2. Ionic conductivity of Sn1−x Mnx P2 O7 The ionic conductivity of SMPs was measured in ambient atmosphere and in humidified air with water vapor pressure (pH2 O) = 0.12 atm. The EIS response of SnP and SMP50 was very poor at ≤350 and <300 ◦ C, respectively, and data acquisition was not possible. Therefore, the ionic conductivity of SnP and all SMP samples was measured at temperatures ≥400 ◦ C and ≥300 ◦ C, respectively. Fig. 7 shows variation of ionic conductivity of SMPs with temperature at the ambient atmosphere in 300–550 ◦ C range. As can be seen, conductivity of SMPs shows Arrhenius type behavior and more than 1 order of magnitude increase is observed with increasing temperature in 300–550 ◦ C range. Also, the conductivity of SMPs shows dependence on dopant concentration and, in general, maximum conductivity is shown by SMP120, which is consistence with the solubility limit of Mn2+ in tin pyrophosphate. The conductivity of SMP120 and SMP150 was 9.79 × 10−6 and 8.96 × 10−6 S cm−1 at 550 ◦ C. Fig. 8 shows the comparison of ionic conductivity of SMPs with some of previously reported tin pyrophosphate based materials in unhumidified/ambient atmosphere.13,17,19,27 As can be seen, the conductivity of SMP120 is higher than that of other tin pyrophosphate based materials, except 8% Sm-doped SnP2 O7 –SnO2 composite. This can be attributed to the nature and concentration of dopants as well as the presence of residual amorphous phosphate phase. Doped tin pyrophosphates have been reported to have co-ionic conduction of oxide ions, holes and protons,21,28 which can be created by a number of ways, as represented by Eqs. (1)–(6), given below: SnO2
••
MnO−→MnSn + O× o + Vo
(1)
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 8. Comparison of ionic conductivity of SMP pellets with literature data.
1/2O2 + Vo •• → Oo x + 2h•
(2)
H2 O(g) + 2h• + 2Oo x → 2OHo • + 1/2O2(g)
(3)
••
•
(P2 O6 )P2 O7 + (P2 O7 )× P2 O7 + H2 O(g) → 2(HP2 O7 )P2 O7 , ••
••
where (P2 O6 )P2 O7 ≡ Vo
(4)
•
//
3(P2 O7 )× P2 O7 + H2 O(g) 2(HP2 O7 )P2 O7 + (2PO4 )P2 O7
(5)
It is reported that hydration/dehydration of crystalline pyrophosphate bulk is a slow process,4,11,29,30 therefore, proton incorporation in tin pyrophosphates via Eqs. (3)–(5) is expected to be minimal, unless they are sufficiently exposed to highly humidified conditions for a prolonged time period. However, in tin pyrophosphates having excess amorphous phosphate phase (represented as Pm On ), additional proton incorporation can occur as //
• 3(Pm On )× Pm On + H2 O(g) 2(HPm On )Pm On + (Pm On+1 )Pm On
(6)
Fig. 9. Variation of ionic conductivity of 1100 ◦ C sintered SMP50 pellet vs. time during humidification in air (pH2 O = 0.12 atm) at 100 ◦ C.
Fig. 10. (a) Variation of ionic conductivity of 1100 ◦ C sintered SMP50 pellet vs. time in humidified air (pH2 O = 0.12 atm) during stepwise temperature change; (a) 100–120 ◦ C and (b) 160–190 ◦ C.
Given the highly hygroscopic nature of amorphous phosphate phase, formation of protonic species due to exposure to the ambient moisture during the material processing, via Eq. (6), cannot be ruled out. Therefore, higher conductivity of SMP120, compared to Sn0.9 In0.1 P2 O7 17 and Sn0.9 Ce0.1 P2 O7 ,27 partly could be due to the lesser loss of excess phosphate phase during sintering at lower temperature. Similarly, higher conductivity of SMP120, compared to Sn0.92 In0.08 P2 O7 13 could be due to the use of excess phosphate employed in the synthesis of SMP120. Similar argument can be used to explain the difference in conductivity of SMP120 and 8% Sm-doped SnP2 O7 –SnO2 composite,19 as already discussed that SnP2 O7 –SnO2 composite contains very little amount of pyrophosphate phase in it and amorphous phosphate phase can be considered as the major proton conducting phase, therefore its electrical behavior can be markedly different from that of pyrophosphate-rich composites. This observation is further supported by the fact that all pyrophosphate phase rich compositions in Fig. 8 show strong temperature dependent Arrhenius type behavior in 300–550 ◦ C range but SnP2 O7 –SnO2 composite show very poor temperature dependence in this range. Given the fact of slow hydration of pyrophosphate bulk,4,11,29,30 humidification of SMP samples was initially performed at 100 ◦ C in air (pH2 O = 0.12 atm). Fig. 9 shows variation
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 12. Variation of ionic conductivity of 1100 ◦ C sintered SMPs pellets vs. temperature in humidified air (pH2 O = 0.12 atm).
Fig. 11. (a) Variation of ionic conductivity of 1100 ◦ C sintered SMP120 pellets vs. time in humidified air (pH2 O = 0.12 atm) (a) during humidification at 160 ◦ C, and during stepwise temperature change; (b) 160–190 ◦ C, and (c) 190–220 ◦ C.
of ionic conductivity of SMP50 sample with time during humidification and, as can be seen, initially conductivity increases rapidly which is followed by a slower increase in conductivity. The ionic conductivity of SMP50, which was <10−8 S cm−1 in ambient atmosphere, increases to >10−5 S cm−1 in humidified atmosphere within 2 h of humidification, then it further increases gradually to 1.35 × 10−4 S cm−1 after prolonged humidification. Though the overall increase in ionic conductivity of SMP50 can be attributed to the effect of proton incorporation during humidification process via Eqs. (3)–(6), the rapid hydrolysis of amorphous phosphate phase via Eq. (6) has major contribution in the initial rapid increase in conductivity. Similarly, slow increase during prolonged time duration can be attributed mainly to the proton incorporation in the pyrophosphate bulk via Eqs. (3)–(5).
Fig. 10 shows the variation of ionic conductivity of SMP50 with temperature in 100–190 ◦ C range after humidification at 100 ◦ C. As can be seen, there is an overall increase in conductivity with the increasing temperature and it reaches to 3.52 × 10−4 S cm−1 at 190 ◦ C. However, the conductivity decreases on increasing temperature >190 ◦ C and it has a value of 1.82 × 10−5 S cm−1 at 220 ◦ C. Fig. 11 shows variation of ionic conductivity of SMP150 sample with time during humidification at 160 ◦ C and during temperature change in 160–220 ◦ C range. As can be seen, the conductivity follows similar trend as observed for humidification of SMP50 at 100 ◦ C, except that the overall humidification process is rapid compared to the latter one. This could be attributed to the increase in rate of hydration of pyrophosphate bulk with the increase in temperature.29 Fig. 12 shows variation of ionic conductivity of various SMP samples in humidified air in 100–220 ◦ C range. SMP samples show variation in conductivity with dopant concentration and maximum conductivity is shown by SMP120, which is 2.29 × 10−3 S cm−1 at 190 ◦ C. The ionic conductivity shown by SMP samples is >1 order of magnitude higher than that reported for dense Sn0.91 Zn0.09 P2 O7 ,21 but it is nearly 1 order of magnitude lower than those reported for un-sintered pellets of doped tin pyrophosphate–phosphate composites7 and dense SnP2 O7 –SnO2 composites,19,26 which can be attributed mainly to the difference in amount of excess phosphate phase among various compositions, as discussed earlier. Furthermore, comparison of ionic conductivity of SMPs in ambient air (Fig. 7) and humidified air (Fig. 12) shows that a difference of >4 order of magnitude in ionic conductivity is observed in two conditions, which is starkly different from the observations in case of un-sintered pellets of doped tin pyrophosphate-phosphate composites6 and dense SnP2 O7 –SnO2 composites,19,26 where only ≤1 order of magnitude difference in conductivity is observed in two conditions. The reason for this difference can be traced to the processing history of samples used in conductivity measurement; e.g., dense SnP2 O7 –SnO2 composite was washed in water for longer duration and was dried at 100 ◦ C for 1 h only, thus leaving SnP2 O7 –SnO2 composite amply hydrated even in unhumidified atmosphere.
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Fig. 13. SEM images of fractured section of SMP50 and SMP150 pellets after long term (>8 days) conductivity measurement in humidified air (pH2 O = 0.12 atm).
The diameter of SMP pellets used for the conductivity measurement in humidified atmosphere was compared before and after the measurement. There was a slight increase in pellet diameter after the conductivity measured in humidified air for 8 h. This could be due to the volume expansion because of hydrolysis of P O bonds to P O H. In order to analyze the effect of volume expansion on the microstructure of dense pellets, SEM of pellets was obtained after the conductivity measurement. Fig. 13 shows SEM images of fractured section of SMP50 and SMP150 pellets after long term (>8 days) conductivity measurement in humidified air (pH2 O = 0.12 atm). As can be seen, no clear change in microstructure, compared to SEM in Fig. 4, was visible, indicating overall stability of dense SMP pellets in humidified air. 4. Conclusions Mn2+ -doped Sn1−x Mnx P2 O7 (x = 0–0.2) were synthesized by a new co-precipitation method, using tin(II)oxalate and Mn(II)acetate as tin(IV) and manganese(II) precursors. XRD analysis shows that pure crystalline tin pyrophosphate can be obtained at 300 ◦ C, and the doping limit of Mn2+ is at least x = 0.12. The dopant Mn2+ acts as a sintering aid and leads to dense Sn1−x Mnx P2 O7 samples on sintering at 1100 ◦ C, and the sinterability and grain-growth of Sn1−x Mnx P2 O7 samples increases with the increase in Mn2+ doping level. Also, it was found that, in the ambient air, the conductivity of Sn1−x Mnx P2 O7 samples was 10−8 –10−6 S cm−1 in 300–550 ◦ C range, but it increased significantly in humidified air atmosphere to >10−3 S cm−1 in 100–200 ◦ C range. Sn0.88 Mn0.12 P2 O7 showed maximum conductivity of 9.79 × 10−6 at 550 ◦ C in ambient air and 2.29 × 10−3 S cm−1 at 190 ◦ C in humidified air. The ionic conductivity Sn1−x Mnx P2 O7 samples was compared with some of previously reported compositions of tin pyrophosphate-based materials and the difference in conductivity was discussed in terms of phosphate content and processing history of materials. The stability of Sn1−x Mnx P2 O7 samples in humidified atmosphere is analyzed and, after 8 days of continuous conductivity measurement in humid atmosphere, though there was an increase in pellet diameter due to the hydrolysis of P O bond, no change in microstructure of pellets was observed, thus indicating the stability of dense Sn1−x Mnx P2 O7 .
Acknowledgement This research was supported by the Fusion Research Program for Green Technologies through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2011-0019303).
References 1. Wachsman ED, Lee KD. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935–9. 2. Paschos O, Kunze J, Stimming U, Maglia F. A review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells. J Phys: Condens Matter 2011;23:234110 [26 pp]. 3. Jin YC, Lee B, Hibino T. Development and application of SnP2 O7 -based proton conductors to intermediate-temperature fuel cells. J Jpn Pet Inst 2010;53:12–23. 4. Singh B, Im HN, Park JY, Song SJ. Studies on ionic conductivity of Sr2+ -doped CeP2 O7 electrolyte in humid atmosphere. J Phys Chem C 2013;117(6):2653–61. 5. Singh B, Im HN, Park JY, Song SJ. Electrical behavior of CeP2 O7 for the application in low-temperature proton conducting ceramic fuel cells. J Electrochem Soc 2012;159(12):F819–25. 6. Nagao M, Kamiya T, Heo P, Tomita A, Hibino T, Sano M. Proton conduction in In3+ -doped SnP2 O7 at intermediate temperatures. J Electrochem Soc 2006;153:A1604–9. 7. Nalini V, Sorby M, Amezawa K, Haugsrud R, Norby T. Structure, water uptake, and electrical conductivity of TiP2 O7 . J Am Ceram Soc 2011;94:1514–22. 8. Scott K, Xu C, Wu X. Intermediate temperature proton-conducting membrane electrolytes for fuel cells. WIREs Energy Environ 2014;3:24–41. 9. Jin YC, Shen YB, Hibino T. Proton conduction in metal pyrophosphates (MP2 O7 ) at intermediate temperatures. J Mater Chem 2010;20:6214–7. 10. Nagao M, Takeuchi A, Heo P, Hibino T, Sano M, Tomita A. A protonconducting In3+ -doped SnP2 O7 electrolyte for intermediate-temperature fuel cells. Electrochem Solid-State Lett 2006;9:A105–9. 11. Singh B, Im HN, Park JY, Song SJ. Electrical conductivity of M2+ -doped (M = Mg, Ca, Sr, Ba) cerium pyrophosphate-based composite electrolyte for low-temperature proton conducting electrolyte fuel cells. J Alloys Compd 2013;578:279–85. 12. Tomita A, Kajiyama N, Kamiya T, Nagao M, Hibino T. Intermediatetemperature proton conduction in Al3+ -doped SnP2 O7 . J Electrochem Soc 2007;154:B1265–9. 13. Tao S. Conductivity of SnP2 O7 and In-doped SnP2 O7 prepared by an aqueous solution method. Solid State Ionics 2009;180:148–53. 14. Einsla ML, Mukundan R, Brosha E, Garzon FH. New synthesis routes for indium-doped tin pyrophosphate proton conductors. ECS Trans 2008;16:2165–70.
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15. Park CJ, Kim DH, Kim JK, Lee JS, Song SJ. Synthesis of proton-conducting, In-doped SnP2 O7 core-shell-structured nanoparticles by coprecipitation. J Electrochem Soc 2009;156:E23–5. 16. Hibino T, Kabayashi K, Fujita S. Design of proton-conducting Sn0.95 Al0.05 P2 O7 with a mesoporous structure. J Mater Chem A 2013;1: 13082–8. 17. Phadke SR, Bowers CR, Wachsman ED, Nino JC. Proton conduction in acceptor doped SnP2 O7 . Solid State Ionics 2011;83:26–31. 18. Singh B, Jeon SY, Kim JH, Park JY, Bae C, Song SJ. Ionic conductivity of Gd3+ doped cerium pyrophosphate electrolytes with core–shell structure. J Electrochem Soc 2014;161(4):F464–72. 19. Sato Y, Shen Y, Nishida M, Kanematsu W, Hibino T. Proton conduction in non-doped and acceptor-doped metal pyrophosphate (MP2 O7 ) composite ceramics at intermediate temperatures. J Mater Chem 2012;22:3973–81. 20. Singh B, Kim JH, Jeon SY, Park JY, Song SJ. Mn2+ -doped CeP2 O7 composite electrolytes for application in low temperature proton-conducting ceramic electrolyte fuel cells. J Electrochem Soc 2014;161(1):F133–8. 21. Xiao J, Zhang H, Yang Z, Wang H, Ma G, Zhou Z. Proton and oxide-ion conduction in ZnO doped SnP2 O7 ceramics. J Alloys Compds 2012;521:106–11. 22. Parra R, Ramajo LA, Góes MS, Varela JA, Castro MS. From tin oxalate to (Fe, Co, Nb)-doped SnO2 : sintering behavior, microstructural and electrical features. Mater Res Bull 2008;43:3202–11.
23. Kubota M. Decomposition of oxalic acid with nitric acid. J Radioanal Nucl Chem 1982;75:39–49. 24. Gover RKB, Withers ND, Allen S, Withers RL, Evans JSO. Structure and phase transitions of SnP2 O7 . J Solid State Chem 2002;166:42–8. 25. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chaleogenides. Acta Cryst 1976;A32:751–67. 26. Shen YB, Nishida M, Kanematsu W, Hibino T. Synthesis and characterization of dense SnP2 O7 –SnO2 composite ceramics as intermediatetemperature proton conductors. J Mater Chem 2011;21:663–70. 27. Singh B, Kim JH, Park JY, Song SJ. Effect of partial substitution of Sn4+ by Ce4+ on sinterability and ionic conductivity of SnP2 O7 . ECS Trans 2014, under review. 28. Chen X, Wang C, Payzant EA, Xia C, Chu D. An oxide ion and proton co-ion conducting Sn0.9 In0.1 P2 O7 electrolyte for intermediate-temperature fuel cells. J Electrochem Soc 2008;155:B1264–9. 29. Alberti G, Casciola M, Marmottini F, Vivani R. Preparation of mesoporous zirconium phosphate-pyrophosphate with a large amount of thermally stable acid groups on the pore surface. J Porous Mater 1999;6:299–305. 30. Alberti G, Casciola M, Cavalaglio S, Vivani R. Proton conductivity of mesoporous zirconium phosphate pyrophosphate. Solid State Ionics 1999;125:91–7.
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Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method Bhupendra Singh a,b , Ji-Hye Kim a , Jun-Young Park c , Sun-Ju Song a,b,∗ a
Ionics Lab, School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwang-Ju 500-757, Republic of Korea b Research Institute for Catalysis, Chonnam National University, Republic of Korea c Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Received 31 March 2014; received in revised form 12 April 2014; accepted 14 April 2014
Abstract Mn2+ -doped Sn1−x Mnx P2 O7 (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 ◦ C, as all the reaction by-products are vaporizable at <150 ◦ C. The dopant Mn2+ acts as a sintering aid and leads to dense Sn1−x Mnx P2 O7 samples on sintering at 1100 ◦ C. Though conductivity of Sn1−x Mnx P2 O7 samples in the ambient atmosphere is 10−9 –10−6 S cm−1 in 300–550 ◦ C range, it increases significantly in humidified (water vapor pressure, pH2 O = 0.12 atm) atmosphere and reaches >10−3 S cm−1 in 100–200 ◦ C range. The maximum conductivity is shown by Sn0.88 Mn0.12 P2 O7 with 9.79 × 10−6 S cm−1 at 550 ◦ C in ambient air and 2.29 × 10−3 S cm−1 at 190 ◦ C in humidified air. It is observed that the humidification of Sn1−x Mnx P2 O7 samples is a slow process and its rate increases at higher temperature. The stability of Sn1−x Mnx P2 O7 samples is analyzed. © 2014 Elsevier Ltd. All rights reserved. Keywords: Tin pyrophosphate; Proton conductivity; Proton-conducting ceramic-electrolyte fuel cells; Sintering-aid; Co-precipitation method
1. Introduction Lowering of operating temperature of fuel cells has been one of the important goals in fuel cell research and the development of electrolytes with high ionic conductivity and stability is a crucial requirement for the realization of this goal.1–3 Recently, a number of proton-conducting tetravalent metal pyrophosphates (MP2 O7 , where M = Sn, Ge, Zr, Si, Ce, Ti) have been reported as potential electrolytes for proton-conducting ceramic-electrolyte fuel cells (PCFCs) in the temperature range of 150–400 ◦ C.2–7 Among various MP2 O7 acceptor doped tin pyrophosphates have shown maximum ionic conductivity.2,8–11 Conventionally undoped/doped-SnP2 O7 are prepared by mixing a Sn containing salt or oxide with phosphoric acid and calcining the mixture at high temperatures to form the SnP2 O7 .2,8,12 This processes mostly results in impurity of ∗
Corresponding author at: School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwang-Ju 500757, Republic of Korea. Tel.: +82 62 530 1706; fax: +82 62 530 1699. E-mail address: [email protected] (S.-J. Song).
precursor oxide, if P/Sn ratio is low, and it is difficult to control the P/Sn ratio. The solution phase synthesis methods can be useful in removing this drawback as well as in obtaining better control on particle size.13–16 However, the solution phase synthesis method reported so far either involves expensive precursor or have multi-step processing routes and have high crystallization temperature.13–16 Therefore, it is necessary to look for new synthesis method which can be advantageous for the enhancing the properties of material as well as be economically beneficial. Due to the poor sinterability of SnP2 O7 , the fabrication of dense electrolyte samples have been one of the major changes toward their application as electrolytes in PCFCs. Often, it requires sintering at ≥1400 ◦ C to get fairly dense tin pyrophosphate samples.2,8,17 However, sintering at such a high temperature leads to the loss of phosphate phase due to the evaporation at high temperatures, which severely lowers their proton conductivity, as the phosphate phase helps generating additional proton incorporation sites as well as provide additional proton migration path.7,12,18 A number of strategies have been proposed for the fabrication of dense metal pyrophosphate samples.17,19 Phadke et al.17 have proposed the sintering of metal
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pyrophosphates specimen at 1400 ◦ C in a sealed crucible in presence of some sacrificial powder of metal pyrophosphate, and Sato et al.19 have fabricated dense metal oxide-metal pyrophosphate composites (such as SnO2 –SnP2 O7 , TiO2 –TiP2 O7 , so on) by reacting porous metal oxide substrate with excess phosphoric acid solution at ∼600 ◦ C. However, both methods have some complications and have some special requirements. Therefore, it is imperative to look for new alternatives for the fabrication of dense tin pyrophosphate electrolyte specimens. Recent reports have shown that some dopants not only enhance the ionic conductivity of tetravalent metal pyrophosphates but also act as sintering aid and help getting dense electrolyte samples at significantly lower temperatures.20,21 In this work, we have synthesized Mn-doped tin pyrophosphates (Sn1−x Mnx P2 O7 , x = 0, 0.05, 0.1, 0.12, 0.15, 0.2) using a new solution-precipitation process which leads to crystalline product at calcination temperatures as low as 300 ◦ C, and dense electrolyte samples on sintering at 1100 ◦ C. Therefore, objective of this study is to prepare dense Sn1−x Mnx P2 O7 (SMP) samples by combining the advantages of co-precipitation method of powder preparation with the potential sintering-aid effect of Mn2+ dopant. Various SMP samples were characterized by XRD, SEM, energy dispersive X-ray analysis (EDX), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and their electrical properties were studied using EIS technique for their potential application as solid electrolyte in PCFCs in 100–220 ◦ C range.
Table 1 Details of various Sn1−x Mnx P2 O7 compositions. Lattice parameter is calculated from the Rietveld analysis of XRD data.
2. Materials and methods
The crystalline phases in heat-treated powders and the powders from crushed sintered disks were examined using an X-ray diffractometer (XRD-7000, Shimadzu) equipped with a Cu˚ and operated at 40 kV and K␣ radiation source (1.5406 A) 30 mA at a scan rate of 1◦ /min between scanning angles (2θ) of 10–80◦ . The microstructure of the fractured section of the sintered pellets was analyzed using a field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi). The energy dispersive X-ray analysis (EDX) was performed by an EDX analyzer (EMAX, HORIBA) integrated with the SEM analysis system. The thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) was performed using a thermal analyzer (NETSZCH SDT Q600 V8.3 Build 101) at a heating rate of 5 ◦ C/min in air from room temperature to 1000 ◦ C range.
2.1. Powder synthesis and dense sample fabrication Mn2+ -doped tin(IV) pyrophosphates, Sn1−x Mnx P2 O7 (x = 0–0.2), were synthesized by a co-precipitation method using tin(II)oxalate as tin precursor. Tin(II)oxalate (98%, Aldrich), nitric acid (60%, DaeJung), Mn(II)acetate tetrahydrate (≥99%, Sigma-Aldrich) and phosphoric acid (85%, DaeJung) were used as the starting materials. Firstly, 0.1 moles of {(1 − x)Sn + xMn} were dissolved into nitric acid solution. For this, Sn(II) oxalate was dispersed into 50 ml water in 500 ml glass beaker and then nitric acid was slowly added to the dispersion with constant stirring. The insoluble Sn(II) oxidizes, initially slowly and then vigorously, to soluble Sn(IV) with the increase in the amount of nitric acid.22 A slight excess of nitric acid was added to the solution to prevent any reduction of Sn(IV). The dopant precursor Mn(II)acetate tetrahydrate was added to Sn(IV) solution with stirring. The appropriate amount of phosphoric acid was separately mixed with water to make a 200 ml solution in 500 ml beaker. The solution containing metal precursors was then added drop-wise to the phosphoric acid solution with constant stirring. It is important to mention that if phosphoric acid is added to the metal precursor solution, it leads to instant precipitation, which may cause inhomogeneous particle growth. After 30 min the temperature of mixture was slowly raised to 60–70 ◦ C while stirring, which leads to the appearance of a gel-like precipitation in the solution and then temperature was raised to ∼85 ◦ C to dry out the solution.
Sample composition
Sample ID
Lattice parameter (Å)
Linear shrinkage on sintering (%)
SnP2 O7 Sn0.95 Mn0.05 P2 O7 Sn0.9 Mn0.1 P2 O7 Sn0.88 Mn0.12 P2 O7 Sn0.85 Mn0.15 P2 O7 Sn0.8 Mn0.2 P2 O7
SnP SMP50 SMP100 SMP120 SMP150 SMP200
7.9641 7.9676 7.9703 7.9817 7.9751 7.9713
6.1 19.1 16.1 15.7 14.3 13.2
The gel-like precipitate was completely dried in hot-air oven at ∼140 ◦ C. The obtained solid was ground with mortar and pestle, and was kept in covered alumina crucible for calcination at 300 ◦ C in air for 8 h. The as-calcined powder was ground with mortar and pestle, and the obtained powder was used for further characterization. For the pellet preparation, 550 ◦ C heat-treated powders were sieved through a 38 m mesh test sieve and molded into 1.5–2.0 mm thick and 12 mm diameter disks. The disks were cold isostatically pressed at 200 MPa and were sintered at 1000–1200 ◦ C in covered alumina crucible for 10 h in air. Various Sn1−x Mnx P2 O7 (x = 0–0.2), samples are compiled in Table 1 for the ease of identification. 2.2. Physical characterization
2.3. Electrochemical characterization The sintered CGP disk was polished with sandpaper, painted with colloidal gold (Pelco® colloidal gold paint) on both sides and dried in oven at 200 ◦ C for 2 h. A platinum mesh connected to a platinum wire was separately placed in contact with each sides of painted pellet and pressed against mica sheets to ensure proper electrical contact. The impedance data were recorded in two-electrode configuration at zero dc-bias over the frequency range of 1–107 Hz using a frequency response analyzer (S1260A, Solartron Analytical). The perturbation voltage was kept at 50 mV. The sample was placed in an alumina chamber and shielded by a copper tube to minimize the induction effect from the heating element whose temperature was
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controlled by a heating furnace and monitored by an inserted K-type thermocouple placed closely to the sample. The ionic conductivity was measured at the ambient atmosphere and in humidified air with water vapor pressure (pH2 O) of 0.12 atm. The desired pH2 O was maintained by regulating the humidity of alumina chamber by an upstream humidifier and the flow of air in the chamber was maintained at a rate of 200 cc/min. The supply lines from the upstream humidifier to the alumina chamber were kept at ∼70 ◦ C using heating tape to avoid any condensation of moisture inside the supply line. For the calculations of ionic conductivity from impedance data, the equivalent circuit modeling was performed using equivalent circuit simulation software ZView-Impedance software. 3. Results and discussion 3.1. Phase composition and microstructure of Sn1−x Mnx P2 O7 The solution phase synthesis methods, previously proposed for undoped and doped tin pyrophosphates, involve tin chloride as tin precursor and (NH4 )2 PO4 or K4 P2 O7 as precipitating agent. Therefore, as-prepared powders contain by-products, such as NH4 Cl, and crystallize only at fairly high temperatures 650 ◦ C.13–15 In the present method, however, no such by-products are formed while using tin(II)oxalate as tin source and phosphoric acid as precipitating agent, as during the oxidation of tin(II) by nitric acid to tin(IV), the oxalate component decomposes to CO2 and O2 in the presence of nitric acid,23 and nitric acid and acetic acids vaporize during heating at 80–140 ◦ C. As a result, the as-prepared powders are expected to contain only amorphous phases of tin pyrophosphate and excess phosphates and therefore the most probable chemical composition of the as-prepared powder can considered as SnP2 O7 /Sn(HPO4 )2 ·xHPO4 ·yH2 O. Fig. 1 shows the TGA/DSC of as-prepared powders of SnP and SMP100. As can be seen, there are four prominent regions of weight loss in the TGA graph. The initial weight loss around ∼100 ◦ C in region 1 may be due to the loss of water presumably incorporated during synthesis and weight loss around ∼200 ◦ C in region 2 can be attributed to the partial dehydroxylation of the as-prepared powder. The rapid weight loss around ∼300 ◦ C in region 3 is mainly due to the conversion of Sn(HPO4 )2 ·yHPO4 into SnP2 O7 ·yHPO4 . The appearance of an exotherm in the DSC graph around 300 ◦ C indicates the crystallization of SnP2 O7 . The significant weight loss in region 4 can be attributed to the partial loss of excess phosphate phase due to the conversion of yHPO4 phase into amorphous phosphate phases, (collectively represented as Pm On , where m and n are arbitrary numbers). There was no weight loss in 600–1000 ◦ C range, but the DSC curve tends to form an exotherm >1000 ◦ C, which could be due to the phase transition at the higher temperature.13 It is reported that crystalline SnP2 O7 has very complicated structure. Though it exhibits cubic structure with space group Pa3¯ (2 0 5), a pseudocubic 3 × 3 × 3 superlattice is also reported.24 When SnP2 O7 is heated at ≥1000 ◦ C, the pseudo-cubic superlattice changes to orthorhombic superlattice.13 The comparison of DSC graphs in
Fig. 1. (a) TGA-DSC of as-prepared (a) SnP and (b) SMP100 powder with initial P/(Sn + Mn) ratio of 3.
Fig. 1, however, shows that such phase transition is suppressed on the doping of Mn2+ . Fig. 2 shows XRD of various SMP powders calcined at 300 ◦ C in air for 8 h. As can be seen, powders are well-crystalline and all diffraction peaks can be assigned to cubic lattice with space group Pa3¯ [JCPDS No. 291352]. Unlike the previous
Fig. 2. XRD of Mn2+ -doped SnP2 O7 powders calcined at 300 ◦ C in air for 8 h.
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Fig. 3. Rietveld analysis of XRD pattern of various SMP powder prepared with P/(Ce + Mn) molar ratio = 3 and calcined at 300 ◦ C in air for 8 h.
reports of tin pyrophosphates synthesized by co-precipitation methods, pseudo-cubic 3 × 3 × 3 superlattice is not clearly visible in the present case, however, some unknown peaks start appearing in SMP150 and SMP200, indicating that the solubility limit of Mn2+ is ∼0.12 mole. To obtain the structural parameters, we have performed Rietveld analyses of the XRD data applying MS modeling program. The peak shape was assumed to be Tomandl pseudo-Voigt functions. The background of each profile was approximated by a 20-parameter polynomial. With a weighted residual factor (Rwp ) of ∼17%, the best refinement fit results of various SMP samples are given in Fig. 3. The lattice parameter calculated from the profile matching of XRD data is given in Table 1 and, as can be seen, the lattice parameter increases with increase in doping level from x = 0–0.12 but decreases beyond x > 0.12. The decrease in lattice parameter beyond x > 0.12 may be due to the superlattice formation or due
to the appearance of impurities.13 The ionic radius of Sn4+ is ˚ while that of Mn2+ in high and low spin states is 0.83 0.69 A ˚ 25 and therefore the expansion in SnP2 O7 on Mn2+ and 0.67 A, doping indicates that Mn2+ mostly exists in high spin state in Mn2+ -doped SnP2 O7 . Fig. 4 shows SEM image of fractured section of various Sn1−x Mnx P2 O7 (x = 0–0.2) pellets sintered at 1100 ◦ C for 10 h. As can be seen, the undoped SnP2 O7 sample is highly porous and no grain growth is visible in it. However, the porosity of SMP50 sample is significantly reduced and well-developed grains are clearly visible, indicating that the dopant Mn2+ is acting as a sintering aid. The appearance of bright edges in individual grains in SEM of SMP50 indicates that the excess amorphous phosphate is possibly located at the surface of the grains, thereby forming a core-shell type morphology.15 The enhanced densification is also consistent with the total linear shrinkage measured
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Fig. 4. SEM images of fractured section of SMP pellets sintered at 1100 ◦ C in air for 10 h. SMP powders synthesized with initial P/(Sn + Mn) ratio of 3 and calcined at 550 ◦ C were used for pellet preparation.
Fig. 5. SEM images of fractured section of SMPs pellets sintered at (1) 1000 ◦ C, (2) 1100 ◦ C, and (3) 1200 ◦ C.
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Fig. 7. Variation of ionic conductivity of 1100 ◦ C sintered SMP pellets vs. temperature in ambient air.
ducting, they are corrosive in nature and can cause corrosion of Pt, the commonly used electrodes during the fuel cell application of such materials. Therefore, the composite with localized distribution of higher concentrations of phosphate phase, such as SnP2 O7 –SnO2 composites, can be mechanically less stable during fuel cell operation than the composite with uniform distribution of phosphate phase, such as SMPs in the present study. Fig. 6. Line-scan EDS profiles of the elements in fractured section of SMP100 synthesized with initial P/(Sn + Mn) = 3 and sintered at 1200 ◦ C, (a) Phosphorous and (b) Tin.
in this study. The total linear shrinkage (s), {s = (lo − l)/lo }, where lo and l are the diameters of SMP pellets before and after the sintering, of pellets increases dramatically in Mn2+ -doped samples. For undoped SnP it was only 6.2% but for SMP50 it increases to 19.1%, which along with SEM data clearly supports the sintering-aid effect of Mn2+ -doping. In the SEM image of SMP120 step-formation across the fractures clearly is visible, indicating that the fracture is across the grains and sample is highly dense. However, in SEMs of SMP150 and SMP200 grain size becomes smaller and step-formations are less visible, which could be due to the influence of the unknown secondary phases present in these samples. Fig. 5 shows SEM images of fractured section of SMP100 and SMP120 pellets heat-treated at 1000–1200 ◦ C in air for 10 h. As can be seen, 1000 ◦ C heat-treated samples are poorly sintered but 1100 ◦ C and 1200 ◦ C heat-treated samples are well-sintered and step-formations on the fractured surface increases with the increase in temperature. Fig. 6 shows line-scan EDS profiles of tin and phosphorous in fractured section of SMP120 synthesized with initial P/(Sn + Mn) = 3 and heat-treated at 1100 ◦ C. As can be seen, the distribution of phosphorous and tin almost overlaps each other, indicating that excess phosphate phase is homogeneously distributed in SMPs. This observation is clearly different from the dense SnP2 O7 –SnO2 composites proposed by Sato et al.,19,26 where phosphate phase is formed only in the pores of the SnO2 substrate and tin and phosphorous phase hardly overlaps. Though amorphous phosphates are highly proton con-
3.2. Ionic conductivity of Sn1−x Mnx P2 O7 The ionic conductivity of SMPs was measured in ambient atmosphere and in humidified air with water vapor pressure (pH2 O) = 0.12 atm. The EIS response of SnP and SMP50 was very poor at ≤350 and <300 ◦ C, respectively, and data acquisition was not possible. Therefore, the ionic conductivity of SnP and all SMP samples was measured at temperatures ≥400 ◦ C and ≥300 ◦ C, respectively. Fig. 7 shows variation of ionic conductivity of SMPs with temperature at the ambient atmosphere in 300–550 ◦ C range. As can be seen, conductivity of SMPs shows Arrhenius type behavior and more than 1 order of magnitude increase is observed with increasing temperature in 300–550 ◦ C range. Also, the conductivity of SMPs shows dependence on dopant concentration and, in general, maximum conductivity is shown by SMP120, which is consistence with the solubility limit of Mn2+ in tin pyrophosphate. The conductivity of SMP120 and SMP150 was 9.79 × 10−6 and 8.96 × 10−6 S cm−1 at 550 ◦ C. Fig. 8 shows the comparison of ionic conductivity of SMPs with some of previously reported tin pyrophosphate based materials in unhumidified/ambient atmosphere.13,17,19,27 As can be seen, the conductivity of SMP120 is higher than that of other tin pyrophosphate based materials, except 8% Sm-doped SnP2 O7 –SnO2 composite. This can be attributed to the nature and concentration of dopants as well as the presence of residual amorphous phosphate phase. Doped tin pyrophosphates have been reported to have co-ionic conduction of oxide ions, holes and protons,21,28 which can be created by a number of ways, as represented by Eqs. (1)–(6), given below: SnO2
••
MnO−→MnSn + O× o + Vo
(1)
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 8. Comparison of ionic conductivity of SMP pellets with literature data.
1/2O2 + Vo •• → Oo x + 2h•
(2)
H2 O(g) + 2h• + 2Oo x → 2OHo • + 1/2O2(g)
(3)
••
•
(P2 O6 )P2 O7 + (P2 O7 )× P2 O7 + H2 O(g) → 2(HP2 O7 )P2 O7 , ••
••
where (P2 O6 )P2 O7 ≡ Vo
(4)
•
//
3(P2 O7 )× P2 O7 + H2 O(g) 2(HP2 O7 )P2 O7 + (2PO4 )P2 O7
(5)
It is reported that hydration/dehydration of crystalline pyrophosphate bulk is a slow process,4,11,29,30 therefore, proton incorporation in tin pyrophosphates via Eqs. (3)–(5) is expected to be minimal, unless they are sufficiently exposed to highly humidified conditions for a prolonged time period. However, in tin pyrophosphates having excess amorphous phosphate phase (represented as Pm On ), additional proton incorporation can occur as //
• 3(Pm On )× Pm On + H2 O(g) 2(HPm On )Pm On + (Pm On+1 )Pm On
(6)
Fig. 9. Variation of ionic conductivity of 1100 ◦ C sintered SMP50 pellet vs. time during humidification in air (pH2 O = 0.12 atm) at 100 ◦ C.
Fig. 10. (a) Variation of ionic conductivity of 1100 ◦ C sintered SMP50 pellet vs. time in humidified air (pH2 O = 0.12 atm) during stepwise temperature change; (a) 100–120 ◦ C and (b) 160–190 ◦ C.
Given the highly hygroscopic nature of amorphous phosphate phase, formation of protonic species due to exposure to the ambient moisture during the material processing, via Eq. (6), cannot be ruled out. Therefore, higher conductivity of SMP120, compared to Sn0.9 In0.1 P2 O7 17 and Sn0.9 Ce0.1 P2 O7 ,27 partly could be due to the lesser loss of excess phosphate phase during sintering at lower temperature. Similarly, higher conductivity of SMP120, compared to Sn0.92 In0.08 P2 O7 13 could be due to the use of excess phosphate employed in the synthesis of SMP120. Similar argument can be used to explain the difference in conductivity of SMP120 and 8% Sm-doped SnP2 O7 –SnO2 composite,19 as already discussed that SnP2 O7 –SnO2 composite contains very little amount of pyrophosphate phase in it and amorphous phosphate phase can be considered as the major proton conducting phase, therefore its electrical behavior can be markedly different from that of pyrophosphate-rich composites. This observation is further supported by the fact that all pyrophosphate phase rich compositions in Fig. 8 show strong temperature dependent Arrhenius type behavior in 300–550 ◦ C range but SnP2 O7 –SnO2 composite show very poor temperature dependence in this range. Given the fact of slow hydration of pyrophosphate bulk,4,11,29,30 humidification of SMP samples was initially performed at 100 ◦ C in air (pH2 O = 0.12 atm). Fig. 9 shows variation
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 12. Variation of ionic conductivity of 1100 ◦ C sintered SMPs pellets vs. temperature in humidified air (pH2 O = 0.12 atm).
Fig. 11. (a) Variation of ionic conductivity of 1100 ◦ C sintered SMP120 pellets vs. time in humidified air (pH2 O = 0.12 atm) (a) during humidification at 160 ◦ C, and during stepwise temperature change; (b) 160–190 ◦ C, and (c) 190–220 ◦ C.
of ionic conductivity of SMP50 sample with time during humidification and, as can be seen, initially conductivity increases rapidly which is followed by a slower increase in conductivity. The ionic conductivity of SMP50, which was <10−8 S cm−1 in ambient atmosphere, increases to >10−5 S cm−1 in humidified atmosphere within 2 h of humidification, then it further increases gradually to 1.35 × 10−4 S cm−1 after prolonged humidification. Though the overall increase in ionic conductivity of SMP50 can be attributed to the effect of proton incorporation during humidification process via Eqs. (3)–(6), the rapid hydrolysis of amorphous phosphate phase via Eq. (6) has major contribution in the initial rapid increase in conductivity. Similarly, slow increase during prolonged time duration can be attributed mainly to the proton incorporation in the pyrophosphate bulk via Eqs. (3)–(5).
Fig. 10 shows the variation of ionic conductivity of SMP50 with temperature in 100–190 ◦ C range after humidification at 100 ◦ C. As can be seen, there is an overall increase in conductivity with the increasing temperature and it reaches to 3.52 × 10−4 S cm−1 at 190 ◦ C. However, the conductivity decreases on increasing temperature >190 ◦ C and it has a value of 1.82 × 10−5 S cm−1 at 220 ◦ C. Fig. 11 shows variation of ionic conductivity of SMP150 sample with time during humidification at 160 ◦ C and during temperature change in 160–220 ◦ C range. As can be seen, the conductivity follows similar trend as observed for humidification of SMP50 at 100 ◦ C, except that the overall humidification process is rapid compared to the latter one. This could be attributed to the increase in rate of hydration of pyrophosphate bulk with the increase in temperature.29 Fig. 12 shows variation of ionic conductivity of various SMP samples in humidified air in 100–220 ◦ C range. SMP samples show variation in conductivity with dopant concentration and maximum conductivity is shown by SMP120, which is 2.29 × 10−3 S cm−1 at 190 ◦ C. The ionic conductivity shown by SMP samples is >1 order of magnitude higher than that reported for dense Sn0.91 Zn0.09 P2 O7 ,21 but it is nearly 1 order of magnitude lower than those reported for un-sintered pellets of doped tin pyrophosphate–phosphate composites7 and dense SnP2 O7 –SnO2 composites,19,26 which can be attributed mainly to the difference in amount of excess phosphate phase among various compositions, as discussed earlier. Furthermore, comparison of ionic conductivity of SMPs in ambient air (Fig. 7) and humidified air (Fig. 12) shows that a difference of >4 order of magnitude in ionic conductivity is observed in two conditions, which is starkly different from the observations in case of un-sintered pellets of doped tin pyrophosphate-phosphate composites6 and dense SnP2 O7 –SnO2 composites,19,26 where only ≤1 order of magnitude difference in conductivity is observed in two conditions. The reason for this difference can be traced to the processing history of samples used in conductivity measurement; e.g., dense SnP2 O7 –SnO2 composite was washed in water for longer duration and was dried at 100 ◦ C for 1 h only, thus leaving SnP2 O7 –SnO2 composite amply hydrated even in unhumidified atmosphere.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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Fig. 13. SEM images of fractured section of SMP50 and SMP150 pellets after long term (>8 days) conductivity measurement in humidified air (pH2 O = 0.12 atm).
The diameter of SMP pellets used for the conductivity measurement in humidified atmosphere was compared before and after the measurement. There was a slight increase in pellet diameter after the conductivity measured in humidified air for 8 h. This could be due to the volume expansion because of hydrolysis of P O bonds to P O H. In order to analyze the effect of volume expansion on the microstructure of dense pellets, SEM of pellets was obtained after the conductivity measurement. Fig. 13 shows SEM images of fractured section of SMP50 and SMP150 pellets after long term (>8 days) conductivity measurement in humidified air (pH2 O = 0.12 atm). As can be seen, no clear change in microstructure, compared to SEM in Fig. 4, was visible, indicating overall stability of dense SMP pellets in humidified air. 4. Conclusions Mn2+ -doped Sn1−x Mnx P2 O7 (x = 0–0.2) were synthesized by a new co-precipitation method, using tin(II)oxalate and Mn(II)acetate as tin(IV) and manganese(II) precursors. XRD analysis shows that pure crystalline tin pyrophosphate can be obtained at 300 ◦ C, and the doping limit of Mn2+ is at least x = 0.12. The dopant Mn2+ acts as a sintering aid and leads to dense Sn1−x Mnx P2 O7 samples on sintering at 1100 ◦ C, and the sinterability and grain-growth of Sn1−x Mnx P2 O7 samples increases with the increase in Mn2+ doping level. Also, it was found that, in the ambient air, the conductivity of Sn1−x Mnx P2 O7 samples was 10−8 –10−6 S cm−1 in 300–550 ◦ C range, but it increased significantly in humidified air atmosphere to >10−3 S cm−1 in 100–200 ◦ C range. Sn0.88 Mn0.12 P2 O7 showed maximum conductivity of 9.79 × 10−6 at 550 ◦ C in ambient air and 2.29 × 10−3 S cm−1 at 190 ◦ C in humidified air. The ionic conductivity Sn1−x Mnx P2 O7 samples was compared with some of previously reported compositions of tin pyrophosphate-based materials and the difference in conductivity was discussed in terms of phosphate content and processing history of materials. The stability of Sn1−x Mnx P2 O7 samples in humidified atmosphere is analyzed and, after 8 days of continuous conductivity measurement in humid atmosphere, though there was an increase in pellet diameter due to the hydrolysis of P O bond, no change in microstructure of pellets was observed, thus indicating the stability of dense Sn1−x Mnx P2 O7 .
Acknowledgement This research was supported by the Fusion Research Program for Green Technologies through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2011-0019303).
References 1. Wachsman ED, Lee KD. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935–9. 2. Paschos O, Kunze J, Stimming U, Maglia F. A review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells. J Phys: Condens Matter 2011;23:234110 [26 pp]. 3. Jin YC, Lee B, Hibino T. Development and application of SnP2 O7 -based proton conductors to intermediate-temperature fuel cells. J Jpn Pet Inst 2010;53:12–23. 4. Singh B, Im HN, Park JY, Song SJ. Studies on ionic conductivity of Sr2+ -doped CeP2 O7 electrolyte in humid atmosphere. J Phys Chem C 2013;117(6):2653–61. 5. Singh B, Im HN, Park JY, Song SJ. Electrical behavior of CeP2 O7 for the application in low-temperature proton conducting ceramic fuel cells. J Electrochem Soc 2012;159(12):F819–25. 6. Nagao M, Kamiya T, Heo P, Tomita A, Hibino T, Sano M. Proton conduction in In3+ -doped SnP2 O7 at intermediate temperatures. J Electrochem Soc 2006;153:A1604–9. 7. Nalini V, Sorby M, Amezawa K, Haugsrud R, Norby T. Structure, water uptake, and electrical conductivity of TiP2 O7 . J Am Ceram Soc 2011;94:1514–22. 8. Scott K, Xu C, Wu X. Intermediate temperature proton-conducting membrane electrolytes for fuel cells. WIREs Energy Environ 2014;3:24–41. 9. Jin YC, Shen YB, Hibino T. Proton conduction in metal pyrophosphates (MP2 O7 ) at intermediate temperatures. J Mater Chem 2010;20:6214–7. 10. Nagao M, Takeuchi A, Heo P, Hibino T, Sano M, Tomita A. A protonconducting In3+ -doped SnP2 O7 electrolyte for intermediate-temperature fuel cells. Electrochem Solid-State Lett 2006;9:A105–9. 11. Singh B, Im HN, Park JY, Song SJ. Electrical conductivity of M2+ -doped (M = Mg, Ca, Sr, Ba) cerium pyrophosphate-based composite electrolyte for low-temperature proton conducting electrolyte fuel cells. J Alloys Compd 2013;578:279–85. 12. Tomita A, Kajiyama N, Kamiya T, Nagao M, Hibino T. Intermediatetemperature proton conduction in Al3+ -doped SnP2 O7 . J Electrochem Soc 2007;154:B1265–9. 13. Tao S. Conductivity of SnP2 O7 and In-doped SnP2 O7 prepared by an aqueous solution method. Solid State Ionics 2009;180:148–53. 14. Einsla ML, Mukundan R, Brosha E, Garzon FH. New synthesis routes for indium-doped tin pyrophosphate proton conductors. ECS Trans 2008;16:2165–70.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024
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15. Park CJ, Kim DH, Kim JK, Lee JS, Song SJ. Synthesis of proton-conducting, In-doped SnP2 O7 core-shell-structured nanoparticles by coprecipitation. J Electrochem Soc 2009;156:E23–5. 16. Hibino T, Kabayashi K, Fujita S. Design of proton-conducting Sn0.95 Al0.05 P2 O7 with a mesoporous structure. J Mater Chem A 2013;1: 13082–8. 17. Phadke SR, Bowers CR, Wachsman ED, Nino JC. Proton conduction in acceptor doped SnP2 O7 . Solid State Ionics 2011;83:26–31. 18. Singh B, Jeon SY, Kim JH, Park JY, Bae C, Song SJ. Ionic conductivity of Gd3+ doped cerium pyrophosphate electrolytes with core–shell structure. J Electrochem Soc 2014;161(4):F464–72. 19. Sato Y, Shen Y, Nishida M, Kanematsu W, Hibino T. Proton conduction in non-doped and acceptor-doped metal pyrophosphate (MP2 O7 ) composite ceramics at intermediate temperatures. J Mater Chem 2012;22:3973–81. 20. Singh B, Kim JH, Jeon SY, Park JY, Song SJ. Mn2+ -doped CeP2 O7 composite electrolytes for application in low temperature proton-conducting ceramic electrolyte fuel cells. J Electrochem Soc 2014;161(1):F133–8. 21. Xiao J, Zhang H, Yang Z, Wang H, Ma G, Zhou Z. Proton and oxide-ion conduction in ZnO doped SnP2 O7 ceramics. J Alloys Compds 2012;521:106–11. 22. Parra R, Ramajo LA, Góes MS, Varela JA, Castro MS. From tin oxalate to (Fe, Co, Nb)-doped SnO2 : sintering behavior, microstructural and electrical features. Mater Res Bull 2008;43:3202–11.
23. Kubota M. Decomposition of oxalic acid with nitric acid. J Radioanal Nucl Chem 1982;75:39–49. 24. Gover RKB, Withers ND, Allen S, Withers RL, Evans JSO. Structure and phase transitions of SnP2 O7 . J Solid State Chem 2002;166:42–8. 25. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chaleogenides. Acta Cryst 1976;A32:751–67. 26. Shen YB, Nishida M, Kanematsu W, Hibino T. Synthesis and characterization of dense SnP2 O7 –SnO2 composite ceramics as intermediatetemperature proton conductors. J Mater Chem 2011;21:663–70. 27. Singh B, Kim JH, Park JY, Song SJ. Effect of partial substitution of Sn4+ by Ce4+ on sinterability and ionic conductivity of SnP2 O7 . ECS Trans 2014, under review. 28. Chen X, Wang C, Payzant EA, Xia C, Chu D. An oxide ion and proton co-ion conducting Sn0.9 In0.1 P2 O7 electrolyte for intermediate-temperature fuel cells. J Electrochem Soc 2008;155:B1264–9. 29. Alberti G, Casciola M, Marmottini F, Vivani R. Preparation of mesoporous zirconium phosphate-pyrophosphate with a large amount of thermally stable acid groups on the pore surface. J Porous Mater 1999;6:299–305. 30. Alberti G, Casciola M, Cavalaglio S, Vivani R. Proton conductivity of mesoporous zirconium phosphate pyrophosphate. Solid State Ionics 1999;125:91–7.
Please cite this article in press as: Singh B, et al. Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.04.024