Résumé
The attack by radical intermediates like hydroxy and hydroperoxy (∙OH, ∙OOH) radicals on the polymer chain is the current generally accepted mechanism of chemical degradation of perfluorosulfonic acid membranes [1]. The formation of the reactive oxygen species (ROS) occurs through decomposition of hydrogen peroxide in the presence of trace metal ions originating from the corrosion of cell components, including the cathode platinum catalys [2,3]. Different approaches have been adapted to introduce the radical scavenger into membrane electrode assembly (MEA) such as [4] preparation of electrode with the addition of hydrogen peroxide decomposition catalyst or development of composite Nafion® membrane with dispersed inorganic hydroxyl radical scavenger in nanoparticles or ionic form. It has been demonstrated that incorporation of radical scavenger lowers the polymer degradation rate [5, 6]. We will report the development of a nanofiber-network material enriched with cerium oxide nanoparticles as radical trap at the membrane electrode interface. The mitigation properties of such modified MEAs were verified in in situ OCV hold test conditions at low relative humidity 50 % and high temperature 90 ºC, with the radical scavenger interlayer oriented preferentially to the anode or to the cathode side. Reference membranes with an additional nanofiber layer but no cerium component were also examined to ensure that observable effects are due to the presence of radical scavenger. The result of OCV hold testing under these conditions show that whereas MEAs integrating non-modified Nafion®-212, or Nafion®-212 modified by an interlayer of nanofibre PFSA (no cerium oxide) only, show a marked drop in OCV with time, and end of life at <200 hours, an MEA comprising a cerium oxide nanofiber interlayer at the anode side gave very stable open circuit voltage and a significantly longer lifetime. Post mortem analysis of the MEAs and analysis of eluent water by liquid chromatography were combined to better understand the overall degradation process occurring in cerium enriched and cerium free MEAs, including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM) analyses. D.E. Curtin, R.D. Lousenberg, T.J. Henry, P.C. Tangeman, and M.E. Tisack Journal of Power Sources, 131, 41-48 (2004). J. Peron, Y. Nedellec, D.J. Jones, and J. Roziere, Journal of Power Sources, 185,1209-1217 (2008). Y. Nosaka, K. Ohtaka, N. Ohguri, and A.Y. Nosaka, Journal of The Electrochemical Society, 158, B430-B433 (2011). F.D. Coms, H. Liu, and J.E. Owejan, ECS Transactions, 16, 1735-1747 (2008). P. Trogadas, J. Parrondo, and V. Ramani, Electrochemical and Solid-State Letters, 11, B113-B116 (2008) P. Trogadas, J. Parrondo, and V. Ramani, ACS Applied Materials & Interfaces, 4, 5098-5102 (2012)