Identification of Active Surface Species in Molten Carbonates Using in situ Raman Spectroscopy

Here we report the results of a study on active surface species of a pristine and modified (Li-Na)2CO3 eutectic using in situ Raman spectroscopy technique. The effects of gas compositions, temperature, time, and alkaline earth have been systematically studied. The species of CO42–, HCO4–, and C2O52– are identified as the three major active species on the surface of (Li-Na)2CO3 eutectic by a combined Raman spectroscopy and theoretical density functional theory calculations. The results further reveal that CO42–, HCO4–, and C2O52– are preferably formed in the presence of O2, H2O, and high CO2 concentration. With the addition of Ba to the pristine (Li-Na)2CO3 eutectic, the Raman CO42–/HCO4– shifts become more pronounced.


INTRODUCTION
Molten carbonate fuel cells (MCFCs) are a class of energy-efficient and low-emission power generators (Morita et al., 2002;Watanabe et al., 2006;Kawase, 2017). One of their unique features is the use of CO 2 and O 2 as the cathode gas. The mixture of CO 2 and O 2 is also a major component in flue gas produced by either coal-fired or natural-gas (NG)-fired power plants. A natural and logical question is if the flue gas can be directly utilized as the oxidant by MCFCs to produce additional power while mitigating the CO 2 emission.
The concentration of CO 2 emitted by coal-fired power plants is more than twice that emitted by NG-fired power plants, e.g., 10 vs. 4%. The current benchmark MCFCs can operate normally with a CO 2 concentration higher than 10%; at lower CO 2 concentrations, their performance is significantly limited by diffusion-related concentration polarization. To generate power from MCFCs using low CO 2 concentration flue gas produced from NG-fired power plants, the existing MCFC cell/stack technology needs to be further improved to minimize the concentration polarization and meet the performance requirements. In addition, there is a significant amount of H 2 O, e.g., 10%, in NG flue gas. The effect of H 2 O on the MCFC performance is not well understood from early studies.
A recent result has confirmed that MCFCs exhibit a high concentration-polarization at low CO 2 concentrations and surprisingly enhanced performance in the presence of H 2 O (Rosen et al., 2020). An electrochemical model based on active species peroxide-ion (O 2 2− ) and superoxide (O 2 − ) has been proposed with satisfactory agreement with the experimental data (Cassir et al., 1993). Indeed, an independent Raman spectroscopy study has confirmed the existence of O 2 2− on the surface of a Li 2 /K 2 CO 3 eutectic (Chen et al., 2004).
A parallel conduction mechanism is also proposed for OH − as a working ion when H 2 O is present in the oxidant feed. From the water mass collected at both cathode and anode sides, it appears to suggest that the conduction mechanism of OH − in a MC matrix is real (Rosen et al., 2020). However, more experimental evidence related to active species to support water transport is needed.
Raman spectroscopy is a very sensitive technique to detect vibrational modes of C-O-C, C-O, and C = O bonds that are abundant in C-containing species such as CO 3 2− and CO 4 2− and are of interest to this research (Itoh et al., 2004;Mendoza et al., 2004). We have previously used Raman spectroscopy to probe the surface chemistry of an Ag-MC (Molten Carbonate) membrane operated under a flue gas condition containing O 2 and CO 2 to facilitate our understanding of why enhanced oxygen permeation was observed . We concluded from both experimental and theoretical data that LiCO 4 − was the active species on the Ag-MC membrane surface under simulated flue gas conditions and the extra oxygen in CO 4 2− ligand relative to CO 3 2− is the reason for the increased oxygen transport. The in situ Raman spectra of the Ag-MC membrane were collected by a LabRam/HR confocal Raman system (LabRam Invers, Horiba Jobin-Yvon) equipped with a 632.8 nm He-Ne laser and hot stage (Linkam TS1500, 0-1,500 • C). From this work, we were able to identify that CO 4 2− is an active species on the surface of an atomic layer deposition Al 2 O 3 -coated Ag-MC composite membrane and explain the phenomenon of the enhanced oxygen flux based on the percarbonate mechanism. In a similar work (Mendoza et al., 2004), we were also able to identify the active surface species as C 2 O 5 2− with in situ Raman spectroscopy when a MC melt was exposed to a pure CO 2 atmosphere.
These prior works demonstrate the versatility and capability of in situ Raman spectroscopy in probing active species on the surface of MC. Therefore, in this study, the in situ Raman spectroscopy technology was selected to identify the species of molten carbonate exposed to varies atmospheres at MCFC operating temperatures.

Sample Preparation
The molten carbonate compositions under this study were Li 2 CO 3 -Na 2 CO 3 eutectic with a ratio of 52:48 mol% (MC), and 10 mol%-SrCO 3 added MC (Sr-MC) and 10 mol%-BaCO 3 added MC (Ba-MC). The reason for selecting Li 2 CO 3 -Na 2 CO 3 eutectic was primarily based on the consideration of its higher ionic conductivity than other alkali carbonates eutectics. To prevent MC overflow at high temperatures, an Ag porous matrix was used to contain all MCs. The Ag porous matrix was prepared in the same way as in our previous study (Zhang et al., 2016). Briefly, Ag powders (99.9% metal basis, Alfa Aesar) were mixed with carbon black as a pore former in a volume ratio of 1:1 in ethanol. The dried power mixture was then pressed into pellets under 70 MPa, followed by sintering at 650 • C for 2 h in air to remove the carbon pore former and achieve good mechanical strength. Then the porous Ag pellet was soaked into a MC at 650 • C for 2 h in air, forming Ag/MC composite.

Characterization
The Raman system used for this study is a Horiba Jobin-Yvon LabRAM HR800. It features a focal length of 800 mm, spectral length of 100-4,000 cm −1 with a spectral resolution up to 1.5 cm −1 , accuracy of wavenumber of 1 cm −1 and spatial resolution up to 1 µm. We use the following operating parameters for Raman spectroscopy study on the surface of a MC: • Wavelength: 300-1,800 cm −1 • Light source: 632 nm red • Acquisition time: 20 s • Accumulation: 1 • Hole: 500 • Filter: 100%.
The system is also adaptable to high-temperature subsystem for in situ measurements. The high temperature unit (or herein called hot stage) is model Linkam TS1500. The hotstage is connected to a water-cooling system driven by a water circulation pump. The actual temperature was calibrated with the melting point of prepared MC (490 • C), and temperature effect was studied at 575 and 625 • C. Since the atmosphere effect is a focus of the study, we systematically varied the gas compositions over the MC at a temperature. Table 1 lists all the gas mixtures studied. The total gas flow was fixed at 50 sccm for all measurements.

DFT Calculations
The equilibrium geometries of species were fully optimized at the B3LYP/6-311G * level using the Gaussian 09 program package (Hehre, 2002;Frisch et al., 2009). To better understand the vibrational modes and figure out fingerprints of these species, Raman spectra were simulated using the B3LYP/6-311G * basis sets under the harmonic approximation at the same level of geometry optimization. Vibrational analysis showed that all the structures were at minima in the potential energy surface (no imaginary frequencies). Based on the Raman activity obtained from density functional theory (DFT) calculations, the Raman intensity was determined by the GaussSum software (O'Boyle et al., 2008).

RESULTS AND DISCUSSION
Comparison of Raman Spectra in Dry Gas Figure 1 compares the Raman spectra at 575 • C in pure N 2 (gas #1), 2% CO 2 (gas #2), and 10% CO 2 (gas #3) atmospheres. The lower intensity of the Raman spectra in 10% CO 2 is due to the presence of Ag in the Ag-MC sample used to prevent the overflow of the MC. In the following studies, all Raman spectra were collected with the Ag-MC sample. As shown in the Figure 1, the main band at 1,072 cm −1 is assigned to symmetric stretching ( (Bates et al., 1972;Chen et al., 2002;Zhang et al., 2013). Clearly, there is no difference in these Raman shifts, implying that dry N 2 and CO 2 conditions do not invoke new species other than CO 3 2− . Similar phenomenon was also observed in our previous study, where there is no new peak in the dry gas conditions with Ag-MC sample .
The temperature and time effect on the Raman spectra of the Ag-MC samples in dry 10% CO 2 atmosphere (gas #3) are compared in Figure 2. Figures 2A,B show the Raman spectra of the Ag-MC sample at 575 and 625 • C, respectively. These results confirm that there is no difference in spectrum in dry gas at both temperatures.

CO 2 Concentration Effect in Wet Gas
To investigate why steam promotes the MCFC performance, we also conducted the Raman spectroscopy of the Ag-MC sample in wet gas (10% H 2 O) with different CO 2 concentrations. For an easy comparison, the Raman spectra in dry N 2 is also shown in Figure 3. Some new shift between 400 and 607 cm −1 appear in wet CO 2 atmospheres. To identify these new shifts, DFT calculations were performed. From the DFT calculations, the weak but new shifts at 409 and 567 cm −1 seem to relate to HCO 4 − , while the weak but new shifts at 607 cm −1 can be assigned to CO 4 2− . These two species are observed in CO 2 concentration gases from 2 to 45% CO 2 , while at high CO 2 -concentraiton, e.g., 90% CO 2 , C 2 O 5 2− species appears at 455 cm −1 . The observation of C 2 O 5 2− species at high CO 2 concentration is reasonable given the fact that CO 2 dissolution into MC to form C 2 O 5 2− by CO 2 +CO 3 2− = C 2 O 5 2− (Claes et al., 1996). When the gas atmosphere changed from dry N 2 to wet 2% CO 2 , the weak but new bands at 567 and 607 cm −1 seem to emerge, which correspond to HCO 4 − and CO 4 2− , respectively. (Note: the wet 2% CO 2 gas also contains 9% O 2 , see Table 1, gas #4). Then the intensity of the new bands slightly increased by increasing CO 2 concentration to 9 and 45%. The formation mechanisms for the observed HCO 4 − and CO 4 2− are (Zhang et al., 2014;Tong et al., 2016): (1) From these reactions, H 2 O is essential to form HCO 4 − , but not for CO 4 2− species. However, it is interesting to see that CO 4 2− appears in wet flue gas but not in dry flue gas (i.e., ∼10% CO 2 -10% O 2 ), see Figure 2B.  FIGURE 3 | Raman spectra, along with DFT calculations, of the Ag-MC sample collected at 625 • C in different CO 2 concentration gas conditions: dry N 2 (gas #1), 2% CO 2 (gas #4), 9% CO 2 (gas #5), 45% CO 2 (gas #6), 90% CO 2 (gas #7).
Frontiers in Energy Research | www.frontiersin.org further increasing CO 2 concentration to 90% (no O 2 ), which is consistent with our previous study (Zhang et al., 2013).

The Effect of Alkaline Earth-Addition to MC
To further confirm that the formation of HCO 4 − and CO 4 2− is highly influenced by the solubility of molecular oxygen, and therefore, the basicity in MCs. We measured the Raman spectra of alkaline earth-addition MC, since it has been reported that the alkaline earth metals can enhance the oxygen solubility of the molten carbonate (Scaccia and Frangini, 2009). At first, the Raman spectra of Ba-and Sr-added MC under in 45% CO 2 -10% O 2 -10% H 2 O-N 2 atmosphere (gas #6) were collected and are shown in Figure 4. It seems that Sr-doped MC sample show similar new bands with the pristine MC sample. However, the intensities of HCO 4 − and CO 4 2− bands of Ba-added MC samples are increased appreciably. In addition, a couple of more pronounced new peaks at ∼360 and 770 cm −1 are observed of the Ba-added sample. Compared to DFT-calculated Raman bands, it is determined that the new shift is related to CO 4 2− . These results further suggest that high oxygen solubility promotes the formation of CO 4 − . Since both the H 2 O and alkaline earth metals increase the oxygen solubility of MCs, we also conducted Raman study of the Ba-MC sample under dry gas condition. Figure 5 shows that the Ba-MC sample does exhibit new surface species under CO 2 -free, H 2 O-free and wet gas conditions. Similar Raman spectra were obtained under the dry flue gas and wet 45% CO 2 atmosphere, confirming that the oxygen solubility highly influences the formation of CO 4 2− and HCO 4 − . Figure 5 also shows significant appearance of new shifts at 403 and 803 cm −1 under dry 10% O 2 condition. DFT calculations suggest that it is related to BaCO 3 . This finding implies that Ba-MC is unstable under dry 10% O 2 -N 2 , precipitating out BaCO 3 on the MC surface. However, in the presence of both H 2 O and CO 2 , BaCO 3 re-dissolves back into MC, but with a pronounced shift at ∼600 cm −1 relating to species HCO 4 − and CO 4 2− . In addition, the new shift at 360 cm −1 is attributed to CO 4 2− according to the DFT calculation, further indicating that the Ba-added MC enhances oxygen solubility. For convenience, we show in Figure 6 a schematic of the BaCO 3 precipitation and re-dissolution mechanisms in dry O 2 and CO 2 -H 2 O-O 2 -containing gas mixtures, respectively.
Overall, based on the above Raman spectroscopic results, we propose the following mechanisms for CO 2 and H 2 O transport in MCFCs with a high-basicity MC as the electrolyte, H 2 as the anode gas and CO 2 /H 2 O/O 2 as the cathode gas: At Cathode: Parallel reaction (1) : At Anode: Main reaction : Since HCO 4 − is virtually an intermediary species, the experimentally observed H 2 O and CO 2 transport in MCFC are carried out by OH − and CO 3 2− .  FIGURE 6 | Surface species of Ba-added MC exposed to different gas mixtures.

CONCLUSION
In summary, the surface chemistry of pristine and Ba-or Srmodified Li-Na eutectic under seven gas mixtures has been studied by in situ Raman spectroscopy. Compared to the pristine MC under dry N 2 , all the CO 2 -containing dry gas conditions do not change Raman spectra of the MC. Under wet gas conditions, relatively weak new shifts appear in 300-700 cm −1 and their intensities increase with CO 2 concentration. Assisted by DFT calculations, the new shifts are identified to be CO 4 2− /HCO 4 − at low CO 2 concentrations and C 2 O 5 2− at high CO 2 concentrations. With the addition of Ba to the pristine MC, the CO 4 2− /HCO 4 − shifts become more pronounced due to enhanced oxygen solubility in MC. It was also discovered that BaCO 3 can precipitate out under CO 2 -free dry gas environment. Based on the found surface intermediate species, a mechanism concerning H 2 O and CO 2 co-transport is proposed.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

FUNDING
Financial supports from National Science Foundation (Award # 1924095) and ExxonMobil are greatly appreciated.