In-situ segmented characterizations of a solid oxide electrolysis cell operating under steady and dynamic conditions

• Segmented electrode method with five thermocouples is developed for a SOEC. • Effects of H 2 pressure and fuel flow rates on local electrolysis are examined. • Performance responses under various dynamic voltage loadings are studied. • A temperature rise rate limit is identified and influence of heat radiation is analyzed. • A fast-cycling strategy is explored to stabilize current and temperature. This study employs a segmented electrode technique to resolve the spatiotemporal electro-thermal coupling characteristics of a solid oxide electrolysis cell (SOEC) under steady-state and dynamic operations. By integrating a quadripartite air electrode and an X-pattern thermocouple array, the current and temperature distributions are systematically mapped under varying hydrogen partial pressures and fuel flow rates. Steady-state analysis demonstrates that fuel utilization exceeding 20 % induces current density reduction in downstream regions due to concentration polarization, with distribution of relaxation times and electrochemical impedance spectroscopy identifying low-frequency mass transport (1–50 Hz) as the dominant limitation. Upstream reactant depletion and product accumulation consequently induce downstream concentration polarization and overpotential reduction, ultimately resulting in current inhomogeneity. During dynamic voltage loading (linear/step/cyclic modes), 0.1 V steps in high-bias regimes reach 73 % of the maximum temperature change rate observed during abrupt voltage termination, with both responses constrained by thermal inertia. Maximum thermal gradients of 0.5 K/cm emerge near the thermoneutral voltage during step loading, attributed to asymmetric heat radiation from the cavity-clamp fixture. Notably, current responses outpace mass transfer and thermal transients, enabling high-frequency voltage cycling (<10 s) to suppress current overshoot by compensating for fuel replenishment hysteresis, achieving stabilized current and temperature profiles. The widely assumed thermoneutral switch between open circuit voltage (OCV) and 1.3 V fails to achieve isothermal operation due to unavoidable transient endothermic effects. This work establishes critical thresholds for fuel utilization and electro-thermal regulation strategies, providing theoretical foundations for integrating SOECs into renewable energy systems with fluctuating power inputs.