Comparative Analysis of Degradation of Pristine Nafion and Catalyst Coated Membrane Subjected to Ex-Situ Fenton's Test Approach

Abstract

Polyfluorinated sulfonic –acid ionomer membranes (e.g Nafion) are extensively used today as the benchmark materials for the electrolyte membrane in PEM water electrolysis. It is broadly accepted that the state of the health of PFSA membranes are adversely impacted by the chemical degradation occured during operation of PEMWE. The crossover gases on the catalytic surface generate Hydrogen Peroxide which is homolytically or in the presence of ferrous salts produce reactive oxygen species like hydroperoxyl (HO.) and hydroxyl (HO.) radicals. These radicals attack the ionomer subsequently leading to chain scission, unzipping and loss of functional groups and release of HF in the effluent water.Other than Fe3+, several transition metal ions Cu2+, Ni2+, Ti3+, Cu+, can poison the membrane significantly. The fact that membrane degradation is accelerated owing to introduction of ferrous ions is fervently implemented in ex-situ degradation tests (known as Fentons tests) to evaluate the durability of membrane materials.This work depicts a systematic investigation of degradation of pristine Nafion and catalyst coated Nafion membrane using a Fentons accelerated aging experiment. The durability of Nafion 117 and catalyst coated Nafion membrane with various counter ions against H2O2 was explored as a degradation factor of polymer electrolyte water electrolyser. Two distinct variations of the experiments were compared : (1) Nafion and CCM were dipped in only hydrogen peroxide solution, (2) Nafion and CCM were exposed to a solution containing ferrous ions and peroxide. Accelerated aging experiments were conducted over 3-7 days. The difference in degradation phenomena with and without Fe ions were evaluated in terms of the fluoride ion release. For both cases, to quantify the effect of water activity in Nafion and CCM chemical structure on both water diffusion and interfacial transport, pulse field gradient spin echo nuclear magnetic resonance (PFGSE-NMR) technique has been employed. Furthermore the equivalent weight of the Nafion and CCM can vary in presence of aforementioned chemical stressors which can be accurately quantitatively studied by confocal Raman spectroscopy. Morphological characteristic before and after aging test was also investigated. Role of H2O2 and ferous ion in mechanical behaviour of Nafion and CCM were also analyzed. The same experiment was repeated for different transition metal ions. This work sheds light on decomposition mechanism of Nafion and CCM in more detail.