Dynamic operation and thermoneutral load-following control of a pilot-scale solid-oxide electrolysis stack for flexible hydrogen production

Renewable hydrogen from solid-oxide electrolysis (SOE) is expected to play an important role in decarbonizing hard-to-abate sectors and linking variable renewable electricity with hydrogen production. For industrial deployment, however, SOE systems must operate flexibly under fluctuating electricity supply, and their response to dynamic operation remains insufficiently characterized. This study investigates the thermal response and apparent degradation behavior of a state-of-the-art 75-cell Topsoe SOE stack during a pressurized pilot-scale campaign comprising galvanostatic baseline operation, two load-cycling schemes lasting 48 h and 6 h, and a 72-h load-following experiment using a self-regulating thermoneutral control strategy. Baseline operation at -0.62 A/cm 2 before and after load-cycling yielded voltage degradation rates of 25.8 mV/kg H 2 and 10.4 mV/kg H 2 , respectively, while the corresponding area-specific resistance degradation rates were 16.5 mΩ⋅cm 2 /kg H 2 and 19.2 mΩ⋅cm 2 /kg H 2 . Because these changes may also reflect continued early-life stack conditioning or reversible recovery phenomena, they cannot be directly attributed to load-cycling, and no measurable degradation penalty was identified under the tested conditions. Thermal analysis showed that the transition from open-circuit voltage to electrolysis operation was the most severe transient during load-cycling, with a maximum inlet-to-outlet gas temperature difference of 11.1 °C. During the 72-h load-following experiment, the thermoneutral control strategy maintained this temperature difference within ±3 °C while operating between -0.47 A/cm 2 and -0.62 A/cm 2 , corresponding to 75-100 % of nominal load. The results demonstrate that thermoneutral load-following operation is achievable in the tested pilot configuration. However, dedicated long-term degradation studies remain necessary to assess whether this operating strategy is compatible with durable performance under application-relevant dynamic conditions.