Understanding Potential Decay during OCV Hold via Dry Recovery Process

Abstract

Nearly ∼100 mV of open circuit voltage (OCV) decay is observed in the first 24 h of a typical OCV hold test for assessing the chemical durability of a polymer electrolyte. Significant recovery of this OCV loss has been achieved by subjecting the cell to so-called wet recovery process by lowering the cell potential often under over-humidified conditions, thereby confounding the effect of potential reduction and catalyst/ionomer interfacial reorganization due to humidification. In this work by applying a dry recovery protocol (30% RH) comprising 30 min of lower potential hold in the H2/N2 environment followed by the H2/Air environment, OCV recovery of 82%, 73%, 62% and 58 was achieved by holding potentials at 0.13, 0.2, 0.6, and 0.8 V, respectively. Small changes in electrochemically active surface area and hydrogen cross-over rate observed 48 h cannot explain the nearly 100 mV OCV. The wet recovery process (low potential and 100% RH) resulted in 85% recovery of ∼100 mV of lost OCV implying oxide coverage increase as the dominant factor for sharp OCV decay. Further, it is shown that cross-over hydrogen is not fully oxidized at OCV-like conditions (>0.6 V), highlighting the need to quantify potential-dependent crossover current for OCV decay models.