Chronopotentiometric Method to Characterize the Electrochemically Active Surface Species and Area in PEM Water Electrolysis Cells and Stacks

As the demand for large-scale implementation of proton exchange membrane water electrolysis (PEMWE) grows, efficient cell diagnostics at the stack level become increasingly important. However, conventional potential-controlled diagnostics such as cyclic voltammetry (CV) are often impractical at the stack level due to a non-uniform voltage distribution when the current is applied via the bipolar plate voltage sense tabs. This study introduces chronopotentiometry (CP) as alternative for stack-level characterization. It involves applying a constant current to a stack and measuring the voltage response of each cell individually. The cell potential changes can then be converted into differential capacities, providing similar capacitance vs voltage responses as obtained by cyclic voltammetry. Here, the CP method was first validated on 5 cm² active area single-cells and then extended to a 25 cm² rainbow short-stack containing five cells with four different anode catalysts, namely IrO₂-coated TiO₂, Ir-coated TiO₂, and two different versions of partially crystalline iridium oxide coated on TiO₂. For each catalyst, the CP or CV based capacitance vs voltage profiles are essentially identical. The advantages and limitations of the CP technique are being discussed, also in view of rainbow stacks with anodes with substantially different loadings or capacitances.