NiO Solubility in Mixed Alkali/Alkaline Earth Carbonates

Abstract

Nickel oxide false(normalNiOfalse) cathode solubility in the molten carbonate fuel cell (MCFC) electrolyte has been identified as one of the major technical obstacles facing fuel cell commercialization. The solubility of transition metal oxides is thought to be a function of the acid/base chemistry of the melt, controlled by the CO2 partial pressure of the oxidant gas. In this study, equilibrium nickel oxide solubility experiments were performed in a molten Li2CO3‐K2CO3 mix under various gas compositions to identify the conditions of minimum solubility. An analytical procedure is presented in which the dissolved nickel ions are isolated from the frozen electrolyte and spectrometrically analyzed to a detection limit of 0.5 ppm (μg Ni/g electrolyte). Under current MCFC operating conditions, the CO2 partial pressure is such that normalNiO cathode solubility is found to occur under acidic dissolution conditions. For a given gas atmosphere, it is shown that normalNiO solubility can be reduced by the addition of small amounts of basic alkaline earth oxides to the acidic electrolyte. The addition of 1 weight percent normalSrO resulted in a fifteenfold decrease in the dissolved normalNiO concentration.

Complex Formation during Dissolution of Metal Oxides in Molten Alkali Carbonates

“…For the acidic dissolution at p CO 2 higher than 0.1 atm, the slopes of the log(x NiO )-log (p CO 2 ) are claimed to be around unity by Ota et al 7 and Doyon et al 10 as well. For PbO from the present work, however, a linear regression of log(x NiO ) vs. log(p CO 2 ) gives the acidic slopes between 0.88 and 0.91 for three compositions of the melt.…”

“…6 In binary mixtures of Li-K, Li-Na, and Na-K carbonates, both acidic and basic dissolution of NiO has been observed. Orfield and Shores 6 reported that the Li 2 CO 3 -rich mixtures (higher than 50% mol Li 2 CO 3 at 910ЊC) showed only the basic dissolution, while the results from Ota et al 7 and Doyon et al 10 for the Li/K ϭ 62/38 melt showed a well-defined acidic dissolution in the temperature range from 650 to 750ЊC. The temperature difference may partly account for this by considering the fact that the free energy of dissociation of an alkali carbonate depends upon the temperature of the melt and the dissolution of metal oxides depends on the basicity.…”

“…Furthermore Doyon et al 10 and Plomp et al 11 demonstrated that the addition of basic oxides (MgO and SrO) into the Li/K ϭ 62/38 melt led to a decrease in solubility of NiO. At different compositions of the Li/K binary carbonates, the NiO solubility was found to decrease with an increase in the content of lithium salt.…”

“…At different compositions of the Li/K binary carbonates, the NiO solubility was found to decrease with an increase in the content of lithium salt. The similarity of the lithium effect to that of MgO and SrO led some workers 6,7,10 to conclude that Li 2 CO 3 is the most basic salt among the three alkali carbonates (basic Li 2 CO 3 > Na 2 CO 3 > K 2 CO 3 acidic).…”

“…Several investigations have been carried out to measure the solubility of NiO in individual alkali carbonates, 3 binary mixtures, [4][5][6][7] ternary mixtures, 8,9 and alkali/alkaline earth carbonate mixtures. 10,11 In order to obtain other materials as possible substitutes for the lithiated NiO cathode, extensive research has been conducted. Among the numerous candidates of pervoskite-type compounds and mixed metal oxides, the chemistry of CuO, 12,13 ZnO, 12,13 LiFeO 2 , [12][13][14][15] (Li,K) 2 [16][17][18][19] and La 2 ZrO 3 20 has been investigated in the alkali carbonate mixtures in the temperature range between 550 and 950ЊC.…”