Thermodynamic data from redox reactions at high temperatures. I. An experimental and theoretical assessment of the electrochemical method using stabilized zirconia electrolytes, with revised values for the Fe-“FeO”, Co-CoO, Ni-NiO and Cu-Cu₂O oxygen buffers, and new data for the W-WO₂ buffer

Abstract

The accuracy and reliability of high temperature thermodynamic data obtained from oxygen concentration cells with stabilized zirconia (SZ) electrolytes are assessed. The experimental apparatus and methods needed to obtain precise results are described. We then test the apparatus, firstly on symmetrical cells of the type:

$$Pt, M + M_x O|CSZ|M + M_x O, Pt$$

where M=C, Ni, Co or Fe; and then by examining all the possible permutations of heterogenous cells with these four oxygen buffers. Simultaneous linear regression of all the data (624 readings from 14 different cells) shows that the overall precision of the results is ±100 J/mol (one standard deviation). The results are in good agreement with the available calorimetric data. Although the agreement with previous electrochemical work is also good, the high precision of the present results reveals small systematic differences, and accordingly we recommend small adjustments to the existing values for these buffers. The revised equations are ($\mu _{O_2 }$ in J/mol, temperature in K, reference pressure 10⁵ Pa):

$$\begin{gathered} Cu - Cu_2 O( \pm 80): \hfill \\ - 347,705 + 246.096 T \hfill \\ - 12.9053 T In T (700< T< 1,338) \hfill \\ Ni - NiO( \pm 200): \hfill \\ - 478,967 + 248.514 T \hfill \\ - 9.7961 T In T (700< T< 1.700) \hfill \\ Co - CoO( \pm 200): \hfill \\ - 492,186 + 509.322 T - 53.284 T In T \hfill \\ + 0.02518 T^2 (700< T< 1,394) \hfill \\ - 484,276 + 235,092 T \hfill \\ - 11.3440 T In T(1,394< T< 1,760) \hfill \\ \end{gathered} $$

We also report new data for the W-WO₂ buffer. The results, which are also in very good agreement with the calorimetric data, are:

$$\begin{gathered} W - WO_2 ( \pm 300): \hfill \\ = - 569,087 + 300.479 T \hfill \\ - 15.9697 T In T(700< T< 1,700) \hfill \\ \end{gathered} $$

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