Enhanced O2 solubility by RE2O3 (RE = La, Gd) additions in molten carbonate electrolytes for MCFC

https://doi.org/10.1016/j.molliq.2007.09.006Get rights and content

Abstract

The solubilization of O2 reactant gas in mixtures of molten alkali carbonate, which are to be used as electrolytes in the Molten Carbonate Fuel Cells, has been investigated as a function of melt composition, temperature and O2–CO2 gas atmosphere. The baseline content of dissolved oxygen in (52 + 48) mol% Li2CO3 + Na2CO3 and (62 + 38) mol% Li2CO3 + K2CO3 is markedly increased by the additions of 0.5 mol% rare-earth (RE = La and Gd) oxides to the melts. By a curve-fitting method the mechanisms through which oxygen dissolves into the melts are predicted. At 923 K the oxygen dissolution mechanism passes from a superoxide path, which is dominant in the additive-free melts, to either a prevalent peroxide path in the Li–Na melt or a mixture of superoxide and peroxide ions in the Li–K electrolyte. At lower temperature (873 K) RE oxide additives are effective in modifying the oxygen dissolution mechanism only for the Li–Na melts.

Introduction

Molten Carbonate Fuel Cells (MCFCs) are now approaching the pre-commercialization step as many sub-megawatt power plants have been under demonstration worldwide for several years [1], [2].Well-established manufacturing processes are nowadays employed in MCFC technology [3], [4]. However, the research continues to be geared towards the construction of more stable components in the corrosive electrolyte (a molten alkali metal carbonate mixture) in order to reduce costs and reach longer cell-lifetime. The NiO cathode dissolution in the standard molten Li–K carbonate still remains the major barrier to long-term operation of the cell. Since the work of Baumgartner, who was the first to investigate alternative cathodes as solution to the problem individuating LaNiO3 and LaSrCoO3 as two potentially alternatives [5], many further efforts have been made to substitute NiO with another more stable cathode into the electrolyte, for example by mixing with elements that have lower solubility than nickel oxide [6], [7], [8], [9], [10], [11]. However, at this writing any attempt to substitute the cathode material has failed because of manufacturing difficulties and poor cell performances. Thus, the design of milder corrosive conditions through electrolyte compositional modifications has been a recently revived strategy to control the problem of the cathode dissolution.

In the late nineties Argonne laboratories proposed a new electrolyte based on a mixture of lithium and sodium carbonates [12]. The addition in equimolar proportion of barium and calcium carbonate to the lithium-rich-sodium carbonate system resulted in an eutectoide composition combining a low melting point and an absence of cation segregation phenomenon [13]. Their study rely upon most early investigations on carbonate properties that indicated several advantages of lithium–sodium eutectic carbonate over the standard lithium–potassium carbonate in terms of higher electrochemical performance [14] and electrical conductance [15], lower volatility of sodium compared to potassium [16], and lower NiO solubility [17].

The employment of rare-earth (RE) elements as additives to the Li–Na eutectic carbonate melt in order to decrease the NiO cathode solubility has been recently proposed [18], [19]. The solubility of the NiO in the La-additive Li–Na eutectic carbonate melt has been found surprisingly low (6 mol ppm compared to the 15 mol ppm in La-additive free-Li–Na eutectic system) [20], [21]. Amelioraments of the catalytic NiO properties on the O2 reduction reaction (ORR) have been reported when RE elements like La and Ce [22] or mischmetal are incorporated into the cathode [23]. Because the ORR occurs at the triple phase boundary (electrolyte–electrode–oxygen), it is of primary importance to know the oxygen content in the electrolyte and, above all, the form of dissolved oxygen species actually taking part to ORR. In authors' recent work it has been found that the addition of La2O3 to the Li–Na eutectic molten carbonate gives rise to a ten-fold increase of the total oxygen content into the electrolyte, which is also accompanied by a change in the mechanism of oxygen dissolution from superoxide (SOP) to peroxide (POP) path [24]. Improvements of ORR kinetics have been found by Ota et al. [25] when La2O3 is incorporated in the Li–Na eutectic electrolyte, which appears to be consistent with our experimental data.

The objective of the present paper is to complement oxygen solubility data reporting the effect of RE (La and Gd) additions on the oxygen dissolution behaviour in the (52 + 48) mol% (Li–Na)CO3 and (62 + 38) mol% (Li–K)CO3 eutectic electrolytes. The oxygen solubility has been measured as a function of melt composition, temperature and O2–CO2 gas atmosphere through the precise and accurate method previously described [26].

Section snippets

Experimental

The apparatus and the methodology for oxygen measurements have been previously described in [26]. In brief, commercial powders of analytical reagent grade of Li2CO3 (Carlo Erba), Na2CO3 (Carlo Erba), K2CO3 (Carlo Erba), La2O3 (Rudi-Pont) and Gd2O3 (Aldrich) were previously dried at 573 K for 24 h in air. Then the carbonate salts were exactly weighted, transferred to 50-cm3 polyethylene bottles together with glass-balls and mixed in a jar mill for 24 h in a dry-room (RH < 0.2%). The eutectics were

Results and discussion

We have proposed an accurate and a precise analytical method to obtain oxygen solubility data based on the indirect spectrometric determination of Cr6+ produced by chemical reaction of Cr3+ with the oxygen dissolved into the melts [27]. The proposed method improved the early suggested spectrophotometric method's detection limit [28] as a more sensitive analytical instrumentation was used, which was coupled to an analyte pre-concentration step. This method allows to analyze samples, which

Conclusions

The incorporation of 0.5 mol% of La2O3 and Gd2O3 into both the alternative (52 + 48)  mol% Li2CO3 + Na2CO3 and the standard (62 + 38) mol% Li2CO3 + K2CO3 eutectic melts provokes a noticeable enhancement of O2 gas dissolution into the melt at 873 and 923 K. This tendency is however less marked when these RE elements are added to Li–K melt. The RE presence into the melts gives rise to a change in the oxygen dissolution mechanism with evidence of only peroxide ions in the Li–Na melt. The situation is more

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