STUDY OF THE (ELECTRO)CHEMICAL EQUILIBRIA IN MOLTEN-CARBONATE UNDER MCFC CATHODE GAS CONDITIONS - PART II - NONEQUILIBRIUM STUDY OF (ELECTRO)CHEMICAL REACTIONS INVOLVED IN OXYGEN REDUCTION IN MOLTEN-CARBONATE

Assuming fast CO2 dissolution into the melt, the recombination reactio n rate of O2- With CO2 was determined at 8.1 +/- 0.1 x 10(-2) atm(-1) s(-1) with oxide addition experiments. This is a few times higher than the value of 1.7 x 10(-2) atm(-1) s(-1) determined by Lu and Selman u sing the same experimental method [1]. Both values are six orders of m agnitude smaller than the value of 7.6 x 10(4) atm(-1) s(-1) obtained by Nishina et al. from CV measurements [2]. A slow CO2 dissolution rat e, though unlikely, was discussed as possible explanation for this dis crepancy. For the fast and slow recombination reaction rates an acidic and basic melt was calculated respectively in the cathode in an opera ting MCFC. It was found here that the predominant reduced oxygen speci es in molten Li/K carbonates changes from peroxide in basic melts to p robably superoxide in acidic melts, which is in accordance with most o f the recent literature [2-10]. In the former case effects from autoca talytic reactions on the performance of the MCFC cathode as suggested by Adanuvor et al. are likely [11]. Non-equilibrium experiments showed that the cyclic voltammetry (CV) response of the reversible reduction of O-2(2-) to O2- in basic melts is characterised by the parameter ga mma = c(O2-) root DO2-/cO(2)(2-) root DO22- which was determined here at gamma = 2.9 +/- 0.2/root pO(2) atm(-1/2). The equilibrium constants for 1/2O(2) + O2- double left right arrow O-2(2-) and CO32- double le ft right arrow CO2 + O2- were determined at 0.55 +/- 0.2 atm(-1/2) and 1.5 x 10(-8) atm respectively. The diffusion constant of peroxide at 4 +/- 2 x 10(-6) cm(2) s(-1). These values are in reasonable accordanc e with literature data [7, 8, 12]. (C) 1998 Elsevier Science Ltd. All rights reserved.