Operating sensitivities for steam versus co-electrolysis on a commercial solid-oxide electrolysis stack with Power-to-X implications

Same-hardware, stack-scale evidence for PtX syngas routes is scarce. We quantify, on identical solid-oxide hardware, how operating conditions affect stack voltage and specific electricity demand to guide selection between steam electrolysis, paired downstream with RWGS, and direct co-electrolysis. We report new stack-level steam-electrolysis data and benchmark them against our previously published co-electrolysis dataset on the same five-cell electrolyte-supported stack and test rig. Measurements are non-overlapping with aligned operating windows and a single analysis pipeline. A structured design-of-experiments with regression yields sensitivities to current density, fuel-side composition/flow, and temperature. Electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT), and in-plane temperature mapping provide mechanistic context for ohmic, charge-transfer, and transport contributions. Within the tested window, co-electrolysis shows stronger voltage sensitivity to current density and the air-outlet setpoint. Differences in specific electricity demand are modest per stack yet material at plant scale. Holding hardware and analysis constant, we deliver a reproducible benchmark that supports route selection and informs thermal and current-density control in commercial PtX plants. • DoE-based quantification of parameter effects on stack voltage (steam EC). • EIS, DRT, and in-plane temperature diagnostics on a 5-cell commercial stack. • Same-hardware benchmark: co-electrolysis shows larger per-unit sensitivities. • At matched production, co-electrolysis requires 2 % higher DC power and kWh Nm − 3 .