Continuous health monitoring of solid oxide electrolyzers using pulse-width modulation current interrupt analysis

Abstract

As solid oxide electrolyzer (SOE) technology moves from button cells to commercial stacks, there is a growing need for fast and inexpensive methods to track the stack’s state-of-health during operation. Electrochemical impedance spectroscopy (EIS) is the standard diagnostic at the lab scale, but the cost, current limitations, and test duration of frequency-response analyzers make routine EIS measurements impractical for large stacks/systems. To address this problem, pulse-width modulation is combined with current-interrupt analysis (PWM-CI) to achieve continuous, in-situ monitoring of SOE health. The proposed PWM-CI method utilizes the “off” portion of the PWM cycle (when the applied voltage to the cell is removed) to perform rapid current interrupt measurements and analyzes the resulting voltage transients using a developed state-space model that explicitly accounts for cable inductance, switch capacitance, and oscilloscope properties. A fitting framework is developed that extracts ohmic, polarization, and total resistances from individual interrupts using only 50 ms of voltage decay, while avoiding the multi-minute interruption of hydrogen production required for EIS. PWM-CI tracking of button cells over 500 h generates ohmic resistances and time constants in agreement with EIS and distribution-of-relaxation-times analysis respectively, and accurately captures the degradation trends. The PWM-CI method is further demonstrated on a 45-cell commercial technology demonstrator stack, where inductive and capacitive effects cause oscillatory transients that are accounted for using the state-space model. After calibration of the electronic parasitics, PWM-CI yields stable resistance and time-constant estimates over 957 h of stack operation. The developed PWM-CI approach provides a practical pathway toward continuous health monitoring of SOE stacks under realistic dynamic operating conditions.