J. Phys. Chem. B 2004, 108, 7553-7556
7553
First Spectroscopic Observation of Peroxocarbonate/ Peroxodicarbonate in Molten Carbonate Li-Jiang Chen,† Chang-Jian Lin,*,† Juan Zuo,† Ling-Chun Song,† and Chao-Ming Huang‡ State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: June 20, 2003; In Final Form: March 31, 2004
An active oxygen species in molten carbonate is the key to elucidate the complex mechanism of the cathode reaction in MCFC. However, under acidic condition of MCFC operation (PCO2 > 1.01 × 104 Pa), the exact chemical state of the active oxygen species remains unclear and is a subject of continuous debates. In this work, we present the first experimental observation of peroxocarbonate/peroxodicarbonate species in acidic molten carbonate by using in-situ Raman spectroscopic technique. The results indicate that the predominant oxygen species (O22-) in basic lithium-rich melts became unstable with the increase of acidity level in the melts and reacted with CO2 to produce peroxocarbonate or peroxodicarbonate anion. In combination with the result of theoretical calculation, it is deduced that the peroxodicarbonate mechanism is the dominant feature in the cathode reaction path in MCFC.
Introduction The molten carbonate fuel cell (MCFC) is one of the most promising devices for conversion of chemical energy into electric energy because of its high efficiency and negligible pollution; moreover, it has been applied preliminarily to the power plant on a commercial scale. However, the relatively short cell life, mainly resulting from the dissolution of the NiO cathode, severely limits its further commercialization. Many efforts have been made to develop alternative or new composite cathodic materials to lengthen the cell life and to improve the cell performance.1-6 Apparently, it is important to elucidate the complex mechanism of the corresponding cathode reaction in MCFC.7-18 So far, three kinds of mechanisms that involve, respectively, peroxide, superoxide, and peroxocarbonate as key intermediates in the molten carbonate-based cathode reaction have been proposed,7,8 on the basis of the experimental evidence of electrochemical and ex-situ electron spin resonance (ESR) tests. However, besides O22- in basic molten carbonate,13 the direct observation or convincing in-situ spectroscopic identification of other active oxygen species (O2-/ CO42- or C2O62-) in the molten carbonate is scarce. Furthermore, the CO42- and C2O62- are also very concerned in many other fields.19-21 In this work, we present the first spectroscopic identification of peroxocarbonate species in acidic molten carbonate by using the in-situ Raman spectroscopic technique. Research efforts in this field can be tracked back to the early 1970’s, and it has been generally accepted that some kind of active oxygen species such as O22-, O2-, CO42-, or C2O62should play a very important role in the molten carbonate-based cathode reaction.7-18,22-29 Specifically, peroxide ion (O22-) was believed to be a major oxygen species in basic melts. However, under acidic condition of MCFC operation (PCO2 > 1.01 × 104 Pa), the exact chemical state(s) of the active oxygen species * Author to whom correspondence should be addressed: Fax: (+86) 592-218-9354. E-mail:
[email protected]. † State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry. ‡ Evercel, Inc., 5 Pond Park Road, Hingham, MA 02043.
remains unclear due to the relatively complicated situation, and is the subject of continuous debates.11-17 Some researchers suggested superoxide (O2-) as the predominant species in acidic molten carbonates,11-13 while others advocated the peroxocarbonate or peroxodicarbonate (CO42-/C2O62-) reaction mechanism.14-17 But both sides of the debate encountered the lack of clear-cut experimental evidence. By means of in-situ Raman spectroscopy, we recently observed in basic molten carbonate the presence of peroxide anion,18 which was produced by the following reactions:
CO32- T O2- + CO2
(1)
O2 + 2O2- T 2O22-
(2)
Stimulated by such success, we further performed an in-situ Raman spectroscopic investigation to observe the active oxygen species in acidic molten carbonate. We observed rather different spectroscopic behavior for the acidic molten carbonate compared with that of the basic molten form, and confirmed the existence of CO42-/ C2O62- in acidic molten carbonate. Experimental Section The details about in-situ Raman spectroscopic cell arrangement and measurement procedure can be found in the previous paper.18 Here the samples were mainly Li/K2CO3 (62:38) and lithium-doped NiO with thin film carbonate. Two kinds of atmospheres used in the experiments were (a) 1 atm pure oxygen and (2) 1 atm mixed gases of O2 and CO2 (1:1), respectively. The gases, often switching with each other in order to reach basic or acidic condition, flowed through the in-situ Raman cell with a rate of 5-10 mL/min. The oxygen gas was initially introduced into the cell at room temperature and flowed for about 50 min before the temperature started to rise. According to the normal operating temperature in MCFC, the in-situ Raman spectroscopic experiments were performed at 923 K. Similar to the previous work,18 the Raman signal was collected to the
10.1021/jp035749l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/08/2004
7554 J. Phys. Chem. B, Vol. 108, No. 23, 2004
Chen et al.
Figure 1. Time-dependent Raman spectra of Li/K carbonate at 923 K under 1 atm O2.
Figure 2. Raman spectra of Li/K carbonate at 923 K when the atmosphere switches to 1 atm O2 + CO2 from 1 atm O2.
CCD detector in a backscattering mode, and the excitation line was provided by a He-Ne laser at 632.8 nm. However, the Raman system was optimized this time to have a higher sensitivity than before. Results and Discussion Figure 1 shows the time-dependent Raman spectra of Li/K2CO3 under the same conditions as previous tests, 923 K and 1 atm O2 atmosphere. The bands at 1062 and 832 cm-1 were assigned to CO32- and to the O-O stretching vibration of peroxide ions, respectively.18 Along with the timely change of relative intensity of these two bands, a small peak timely appeared at 982 cm-1. This new band hints of a new species that also contains a peroxo group produced along with the consumption of peroxide anion. Noting that the CO2 content in the melt and its pressure in the atmosphere increase in a timely way as reaction (1) proceeds, we assume that the new species is either peroxocarbonate or peroxodicarbonate possibly produced by the following reactions:
CO2 + O22- T CO42-
(3)
2CO2 + O22- T C2O62-
(4)
These reactions were proposed previously by Dunks and Stelman in their electrochemical studies of molten carbonate.14 If this is true, the peak at 982 cm-1 will become more considerable in the case of the atmosphere of the molten salt turning more acidic (with higher CO2 partial pressure). The Raman spectra shown in Figure 2 confirm such an assumption. After switching to the mixed gases of CO2 and O2 (1:1) from pure O2 gas, the intensity of the several bands changed significantly. The O22- band lessened rapidly; on the contrary, the band of 982 cm-1 that is presumably assigned to the O-O stretching of CO42-/C2O62- promptly became intensive, in accordance with eq 3 or eq 4. Such assignment was also supported by the literature data. First, it was found that both the peroxodicarbonate group in CF3OC(O)OOC(O)OCF3 and the peroxocarbonate group in CF3OOC(O)OCF3 showed in Raman spectra a band around 940 cm-1, typical for a peroxide stretching mode.30 In addition, the band at 965 cm-1 in the Raman spectra of a new mononuclear iron complex was
Figure 3. Time-dependent Raman spectra of lithium-doped NiO with Li/K carbonate at 923 K under 1 atm O2.
concluded to be attributed to the O-O-C(O)O (peroxocarbonate) moiety.21 Second, the O-O stretching mode of various superoxide-like species appears essentially within the 10001200 cm-1 range in IR spectra.31 Third, when the superoxide species was added into the molten Li/K2CO3 (62:38) under acidic condition at 923 K, it was confirmed that the Raman band attributed to superoxide ions appeared at 1047 cm-1 while the CO32- band located at 1065 cm-1.32 The in-situ Ramanmeasuring system applied to our experiments has sufficient resolution to distinguish the two bands so that it is possible to know whether the superoxide ions actually exist in the melts. To simulate the actual operating situation in MCFC, a thinfilm carbonate covered with the lithium-doped NiO was investigated instead of the bulk carbonate. The Raman spectra of as-mixed molten salt were given in Figure 3 and Figure 4, features in which are essentially similar to those in Figures 1 and 2, but with different intensities. The result further demonstrates that both peroxide anions and CO42-/C2O62- are the key intermediates in the molten carbonate-based cathode reaction and the peroxide ions are inclined to form the CO42-/C2O62by reacting with CO2 as the acidity level increases in the melts. Moreover, the more intensive oxygen species band and weaker carbonate ion band in Figures 3 and 4 than in Figures 1 and 2 imply that more oxygen species exist in the vicinity of the electrode than those in the bulk melts.13
Peroxocarbonate/Peroxodicarbonate in Molten Carbonate
J. Phys. Chem. B, Vol. 108, No. 23, 2004 7555 Namely, C2O62- ions can be the main species in the melts with high acidity, while the CO42- ions were possibly the other species in the melts with low acidic level. Conclusions
Figure 4. Raman spectra of lithium-doped NiO with Li/K carbonate at 923 K when the atmosphere switches to 1 atm O2 + CO2 from 1 atm O2.
In conclusion, our in-situ Raman spectroscopic investigation demonstrated that the peroxide ion, the predominant oxygen species in basic lithium-rich melts, became unstable with the increase of acidity level in the melts and reacted with CO2 to produce peroxocarbonate or peroxodicarbonate anion. This work presented the first experimental observation of peroxocarbonate or peroxodicarbonate species in MCFC by means of in-situ Raman spectroscopy. Moreover, the theoretical calculation provided circumstantial evidence for the configurations and the thermodynamic data of the relevant reactions. It is deduced that the stability of the main oxygen species in molten carbonate is in the order: O22- < CO42- < C2O62-, with the increase of acidic level in the melts. Acknowledgment. The authors are grateful to the Nature Science Foundation of China for the financial support under Grants 1130-K16002 and 59525102. References and Notes
Figure 5. Theoretical configurations of the CO42- and C2O62- ions in molten Li/K carbonate at 923 K under 1 atm O2 + CO2 atmospheres. (a) CO42-, (b) C2O62-.
Figure 5(a) and (b) show, respectively, the theoretical configurations of CO42- and C2O62- ions in the melts system calculated by ab-initio method. Thereinto, the Raman OsO vibration corresponding to the two configurations were figured out to locate at 851 and 860 cm-1, respectively. As a reference, the O22- and CO32- were also analyzed theoretically in the same way and the calculated values were 750 and 1010 cm-1, respectively. It is found that the theoretical values of the Raman vibration assigned to these ions in melts are simultaneously lower than the experimental ones. The deviation was mainly brought about by the great difficulty to involve all influencing factors existing in the actual complex system of molten carbonate during the theoretical analysis. Apparently, it cannot be certain that the specific oxygen species in acidic molten carbonate is either CO42- or C2O62-, or both of them, only according to their similar theoretical values of Raman bands. However, C2O62- is considered as the probable main oxygen species in acidic melts containing abundant CO2 due to its higher stability than that of CO42-, as indicated by the following reactions and the relevant free-energy values from theoretical analysis:
CO2 + O22- T CO42∆G ) -368.76 kJ/mol
(3)
CO42- + CO2 T C2O62∆G ) -172.62 kJ/mol
(5)
Obviously, the peroxide ions cannot exist stably in acidic molten carbonate, and it is inclined to form the peroxocarbonatelike ions with the increase of acidic level. As for the variety of main oxygen species in acidic melts, it is deduced to be possibly dependent on the exact acidity value of molten carbonate.
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