Multifactor performance analysis of reversible solid oxide cells based on proton-conducting electrolytes

Reversible solid oxide cells (rSOCs) based on proton-conducting electrolytes represent a relatively new and cost-effective possibility for carrying out chemical-to-electrical energy conversion in direct and reverse directions with very high efficiency and low environmental impact. Here we report our findings regarding a modernised approach of rSOC testing, which differs from the traditional characterisation of electrochemical cells, consisting in volt-ampere measurements and impedance spectroscopy analysis under open circuit voltage (OCV) conditions. Expanding the bias range from 0.4 to 1.6 V, the designed rSOC was studied in different (fuel cell, OCV, electrolysis cell) modes and its performance was successfully correlated with ohmic and electrode electrochemical responses depending on the measurement temperature and water vapour partial pressure in oxidant gas. On the basis of this approach, the following new results can be formulated: (i) the ohmic resistance of the proton-conducting electrolytes is a variable parameter depending on the bias in contrast to the convenient oxygen-ionic conductors, for which it is assumed to be a constant; (ii) the electrolyte exhibits predominating proton transport with an activation energy of ∼0.3 eV over the whole bias range; (iii) the output parameters should be correlated with the ohmic and polarisation resistances determined at certain biases (a voltage corresponding to the maximal power density realisation or a thermoneutral voltage) instead of those measured under OCV mode. Concluding, this approach allows the main external factors affecting the rSOC’s performance to be disclosed along with proposed means for its future optimisation in the applied direction.