  Operando calorimetry has previously been utilized to study degradation, side
reactions, and other electrochemical effects in electrochemical cells such as
batteries at or near room temperature. Calorimetric data can provide important
information on the lifetime and thermal properties of electrochemical cells and
can be used in practical engineering applications such as thermal management.
High temperature electrochemical cells such as solid oxide fuel cells or
electrolyzers can also benefit from operando calorimetry, although to our
knowledge no such unit has been developed commercially. Herein, we report an
operando calorimeter capable of simultaneous calorimetry and electrochemistry
at temperatures up to 1,000 {\deg}C and in both oxidizing and reducing
atmospheres. The calorimeter is constructed by modifying a commercial apparatus
originally designed to study high temperature electrochemical cells in various
gas environments. We utilize a grey-box, nonlinear system identification model
to analyze calorimetric data and achieve an electrochemical cell power
sensitivity of 16.1 +/- 11.7 mW. This operando calorimeter provides the tools
needed to study both the thermal and kinetic behavior of electrochemical cells
at elevated temperatures.
