Composite Nafion membranes with nano-crystalline YSZ for PEFC applications in drastic conditions

INTRODUCTION Nano-crystalline Yttria-stabilized zirconia (YSZ) oxide has been extensively studied as inorganic filler for composite membranes in Polymer Electrolyte Fuel Cell (PEFC) applications. Membranes with YSZ, usually stabilized with an 8 mol.% of Yttrium Oxide (Y2O3) [1-2], are able to operate at higher cell temperatures and reduced relative humidity levels than pure polymer membranes due to their improved proton conductivity, water retention ability and mechanical characteristics. In the present work, on basis of a previous study [3], a synthesised nano-crystalline c-ZrO2 powder was stabilized varying the Y2O3 concentration in order to verify the doping influence on membranes characteristics, particularly investigating the proton conductivity trend and PEFC electrochemical behaviour in drastic conditions (I-V curves and accelerated degradation tests-ADT@Tc=95°C; RH=50%). EXPERIMENTAL As elsewhere reported [2-3], synYSZ was prepared using a pure ZrO2 nH2O, synthesised per basic hydrolysis from ZrOCl2 [4], and a c-Y2O3, obtained per calcination from the precursor Y2(CO3)3, and used as a cubic phase inductor. Four different molar concentrations (4.8, 8, 12 and 15mol. %) of Y2O3 were used to prepare the corresponding fillers. A standardised [2] filler content (5wt.%) was used to prepare the following composite Nafion membranes: NYSZ4.8, NYSZ8, NYSZ12, NYSZ15, following the procedure elsewhere reported [5]. A pristine recast Nafion® membrane (N-recast) was used as a reference. As already reported [3], the membranes were characterised from a chemical-physical (water retention, swelling, IEC), structural-morphological (XRD, TG, DMA, cross-section SEM) point of view and in terms of proton conductivity at fully humidification. In order to define the final application, further, the proton conductivity with reduced relative humidity levels and the electrochemical performance (I-V curves, accelerated degradation tests, cross-over measurements) in drastic conditions (Tc=95°C; RH=50%) were studied. For ADT, the samples were stressed cycling the current between 0.1-0.4 A cm-2 for 30 min. at each current value (each cycle lasted 60 min.) up to the performance failure or significant performance decrease. RESULTS AND DISCUSSION Starting form the previous study [3], a particular behaviour in terms of proton conductivity at 100%RH was found: two different activation energies trends were individuating into the T ranges 30-80°C and 80-120°C, attributable to an intrinsic proton conductivity of the filler (presence of proton charge carriers) [2]. In Table 1, such results in terms of activation energies @100%RH are reported: Table 1 Eact.@100%RH into the entire investigated temperature range. MembraneEact. @30-80°C(KJ/molK)Eact. @80-120°C(KJ/molK) N-recast 11.52 8.56 NYSZ4.8 12.60 6.90 NYSZ8 13.30 5.09 NYSZ12 12.70 6.83 NYSZ15 13.10 8.41 SynYSZ filler (soft H+ conductor) probably gives its contribution to the proton conduction at higher cell temperatures, as already confirmed from IEC data [3]. The proton conductivity measurements performed at reduced relative humidity supplied high values at higher cell temperatures (100-120°C) for membranes containing the highest amount of Y2O3 into the filler (12 and 15 mol.%) compared to pristine membrane confirming the filler intrinsic conductivity able to favour the proton conduction mechanism. Concerning ADT results, MEAs realised with composite membranes provided a higher endurance with respect to pristine membrane. NYSZ12 and NYSZ15 with the highest amount of Y2O3 in the filler recorded a cycles number greater than 120 with a reduced increase of H2 cross-over. Particularly, NYSZ15 supplied the lowest cross-over value and the best performance at the end of ADT. CONCLUSION Composite Nafion membranes containing nano-crystalline YSZ with different Y2O3 loadings were prepared and tested in H2-PEFC drastic conditions. The filler introduction preserves the membranes hydrophilic properties, improves the proton conductivity and electrochemical performance. Proton conductivity measured at reduced relative humidity levels supplied encouraged results as well as ADT results, above all for membranes with the higher loadings of Y2O3 in the filler (12, 15 mol.%). REFERENCES 1. T.Weissbach et al, J. Membr. Sci. 498, (2016) 2. A. Saccà et al, Polymer Testing 65, 322 (2018) 3. A.Saccà et al, ANM 2017, Aveiro (Portugal), July 2017 4. N.A. Comelli et al, J. Chil. Chem. Soc. 49, 3 (2004) 5. A. Alvarez et al, Int. J. Hydrogen Energy, 36 (2011).