Polydopamine-coated microcapsules for self-healing in proton exchange membranes

Abstract

Proton Exchange Membrane Fuel Cells (PEMFCs) are widely known for their simple design, low-temperature operation, and high-power density. Although they have numerous advantages, two major drawbacks- high cost and low durability- have hindered their extensive use and commercialization. Over use, membrane degradation leads to cell failure which adds to the high cost of this technology. ☐ The aim of this research is to improve membrane durability by introducing self-healing functionality to the membrane. This self-healing ability can be achieved by incorporating microcapsules within the membrane that are prefilled with a self-healing agent. As a result of the long-term operation, the membrane undergoes chemical and physical degradation leading to the formation of pinholes and cracks. Stress concentration at the crack tip can induce rupture of the incorporated microcapsule, thereby releasing the healing solution which will repair the cracks in-situ, thus healing the membrane and extending its lifetime. ☐ Microcapsules developed in our laboratory thus far have employed a urea formaldehyde (UF) shell. However, the UF shell is not sufficiently resilient to the strong solvents used during membrane casting, causing premature rupture of the microcapsule and loss of self-healing capability. This research is aimed at improving the stability of the microcapsule shell during membrane synthesis by coating the microcapsules with polydopamine (PDA). ☐ A procedure was developed to prepare solvent-stable microcapsules prefilled with Nafion/tributyl phosphate self-healing solution by coating their UF shells with PDA. The prepared microcapsules were characterized by SEM with the focused ion beam and optical microscopy. Optical microscopy images confirmed that cracks are healed by the self-healing solution from the microcapsule. Fuel cell performance with 6wt% PDA-coated UF microcapsules/Nafion membranes was compared with pure recast Nafion membrane. Durability testing at open circuit with relative humidity cycling confirmed that the PDA coating improved microcapsule stability and could greatly extend the durability of the fuel cell membrane