Sulfated metal oxides as additive in Nafion membranes for solid polymer electrolyte electrolyzer applications

Transcription

1 2 nd EURO-MEDITERRANEAN HYDROGEN TECHNOLOGIES CONFERENCE EmHyTeC2014, 9-12 December 2014, Taormina, Italy Sulfated metal oxides as additive in Nafion membranes for solid polymer electrolyte electrolyzer applications M.A. Navarra 1, S. Siracusano 2, V. Baglio 2, A.S. Aricò 2 and S. Panero 1 mariassunta.navarra@uniroma1.it 1 Department of Chemistry, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185, Rome, Italy 2 CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, Messina, Italy Consiglio Nazionale delle Ricerche

2 INTRODUCTION Proton exchange membrane (PEM) water electrolysis is considered the most promising method to produce hydrogen with a high degree of purity from renewable energy resources! The process is characterized by high efficiencies and suitable current density even at moderate temperatures. - Ecological cleanliness - small mass-volume characteristics - no corrosive electrolyte - low 298 K: E rev = 1.23 V E th = 1.48 V - compressed gases directly from the electrolyser at an increased level of safety A. S. Arico`, S. Siracusano, N. Briguglio, V. Baglio, A. Di Blasi, V. Antonucci, J Appl Electrochem 43 (2013)

3 INTRODUCTION The main disadvantages of PEM electrolysis are high cost: - noble metal catalysts - perfluorinated membranes - titanium current collectors energy loss: - oxygen evolution electrode overpotential - polymer electrolyte membrane resistance It is therefore crucial to develop optimal oxygen-evolving electro-catalysts and highly conductive, mechanically robust membranes in order to minimize energy loss and enhance system durability. A. S. Arico`, S. Siracusano, N. Briguglio, V. Baglio, A. Di Blasi, V. Antonucci, J Appl Electrochem 43 (2013)

4 INTRODUCTION THE GOAL: Improvement of the proton conducting membrane higher temperature (80 C<T<120 C) lower gas permeability improved mechanical properties THE STRATEGIES: alternative new polymer systems modification of the conventional perfluorosulfonic-based polymer system (i.e., Nafion membranes) M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hhydrogen En 38 (2013)

5 INTRODUCTION Nafion-membrane structure: the random network model K.D. Kreuer, J. Membrane Sci., 185 (2001) 2 K. Mauritz, R. Moore, Chem. Rev., 104 (2004) 4535 B. Smitha et al., J. Membrane Sci., 259 (2005) 10 S.M. Haile, Acta Materialia, 51 (2004) 5981

6 INTRODUCTION NANOCOMPOSITE MEMBRANES: incorporation of various hygroscopic inorganic particles in the host polymer Malhotra and Datta first proposed the addition of INORGANIC SOLID ACIDS in conventional membranes such as Nafion. S. Malhotra, R. Datta, J. Electrochem. Soc., 144 (1997) 23 Why? IMPROVING WATER RETENTION PROVIDING ADDITIONAL ACID SITES REDUCING GAS CROSS-OVER ENHANCING MECHANICAL PROPERTIES Our choice: Superacidic sulfated metal oxide

7 INTRODUCTION Sulfated zirconia (SZrO 2 ) S. Hara, M. Miyayama, Sol. State Ionics, 168 (2004) 111 It is recognized as one of the strongest acid among all those known solid! Our investigations on the use of SZrO 2 as additive in Nafion membranes: -M.A. Navarra, F. Croce, B. Scrosati, J. Mater. Chem., 17 (2007) M.A. Navarra, C. Abbati, B. Scrosati, J. Power Sources, 183 (2008) 109 -M.A. Navarra, C. Abbati, F Croce, B. Scrosati, Fuel Cells, 9 (2009) 222 -A. D'Epifanio, M.A. Navarra, F.C. Weise, B. Mecheri, J. Farrington, S. Licoccia, S. Greenbaum, Chemistry of Materials, 22 (2010) 813 -S. Siracusano, V. Baglio, M.A. Navarra, S. Panero, V. Antonucci, A.S. Aricò, Int. J. Electrochem. Sci., 7 (2012) 1532

8 INTRODUCTION The acid strength of a surface is the ability of the surface to convert an adsorbed neutral base B into its conjugate acid BH + (AB) and is expressed in terms of Hammet acidity function, Ho, which is defined by the following equations: Brønsted acid: Ho log a H f f B BH pk a log B BH Lewis acid: Ho log a A f f B AB pk a log B AB

9 INTRODUCTION Aim of this work: to investigate the properties of other sulfated metal oxides and their effect as additives by comparing the behavior of various composite SMO 2 -added Nafion membranes with that of additive-free Nafion systems. S-SnO 2 S. Brutti, R. Scipioni, M.A. Navarra, S. Panero, V. Allodi, M. Giarola, G. Mariotto, International J. Nanotechnology 11 (2014) : SnO 2 -Nafion nanocomposite polymer electrolytes for fuel cell applications R. Scipioni, D. Gazzoli, F. Teocoli, O. Palumbo, A. Paolone, N. Ibris, S. Brutti, M. A. Navarra, Membranes (2014), volume 4, pp : Preparation and characterization of nanocomposite polymer membranes containing functionalized SnO 2 additives S-TiO 2 M. Sgambetterra, S. Panero, J. Hassoun, M.A. Navarra, Ionics, (2013), volume 19, pp : Hybrid membranes based on sulfated titania nanoparticles as low cost proton conductors

10 Intensity (a.u.) S-TiO 2 1-step sol-gel synthesis XRD Anatase PDF # TiO 2 -b JCPDS # O O O O 400 C air, 3h degree) Crystallite size 10.1 ± 0.6 nm Krishnakumar B, Velmurugan R, Swaminathan M, Catal. Commun 2011, 12, 375 BET specific surface area 150 m 2 /g

11 S-TiO 2 TEM

12 S-TiO 2 FT-IR S-TiO 2 TiO 2 O-H stretching 3385 cm -1 O-H bending 1630 cm -1 Ti-O bending 650 cm Wave number (cm -1 ) [1] Nakamoto K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 1986, Wiley and Sons, New York.

13 Weight (%) Derivative Weight (s -1 ) S-TiO 2 TGA D-TGA 100,0 97,5 S-TiO 2 S-TiO 2 hydrolized 0,00 S-TiO 2 S-TiO 2 hydrolized 95,0 92,5 2% -0,03 90,0 8% -0,06 87, Temperature ( C) Temperature ( C)

14 S-TiO 2 / Nafion composite membranes Nafion solution EW 1100 g N,N-dimethylacetamide (DMAc) Inorganic compound magnetic stirring casting thermal treatment (100 C over night) hot-pressing activation procedure (H 2 O 2, H 2 SO 4, H 2 O) Sample Filler content (w/w %) Thickness( m) M0-80 M M M7 7 90

15 (mol H 2 O / acid site) S-TiO 2 / Nafion composite membranes values at controlled relative humidity (RH) M5 20 RH = 30% RH = 100% M0 M2 M5 M7

16 S-TiO 2 / Nafion composite membranes Electrolysis tests IrRuO 2 / membrane / Pt-C Catalyst loading: 1.5 mg/cm 2 ANODE (CCM) 0.5 mg/cm 2 CATHODE Single Cell fixture: 5cm 2 active area Water flow rate to the anode: 4 ml/min P = 1 atm Nafion / S-TiO 2 5% Membrane-electrode assembly (MEA) hot-pressing conditions: 135 C 0.2 atm 1.5 min Ti grid was used as anode backing layer CATHODE-SIDE VIEW

17 S-TiO 2 / Nafion composite membranes Electrolysis tests T = 80 C T = 100 C

18 S-TiO 2 / Nafion composite membranes in situ 1.5 V 20 khz 0,1 Hz amplitude of the sinusoidal signal: 10 mv T = 80 C T = 100 C

19 CONCLUSIONS Nano-metric sulfated titania particles, with highly homogeneous morphology, have been obtained by a fast, 1-step synthesis. It has been demonstrated the role of the sulfated titania in: - promoting higher hydration level - reducing electrolyte resistance - controlling membrane-electrode interface contact The presence of the inorganic compound allowed the achievement of higher electrolysis performance at 100 C, under practical operating conditions corresponding to suitable current density.

20 ACKNOWLEDGMENTS Dr. Sergio Brutti, Università della Basilicata, Potenza, Italy Dr. Mirko Sgambetterra, Sapienza University of Rome, Italy Prof. Bruno Scrosati, Italian Institute of Technology, Genova, Italy The results of this work have been obtained in the framework of project titled NAMED-PEM Advanced nanocomposite membranes and innovative electrocatalysts for durable polymer electrolyte membrane fuel cells, funded by the Italian Ministry of Education and Research, MIUR (PRIN , prot. 2010CYTWAW_001) GRAZIE per l attenzione!