What is the true nature of conducting proton in perovskite ceramic membrane: hydroxyl ion or interstitial proton ?
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What is the true nature of conducting proton in perovskite ceramic membrane: hydroxyl ion or interstitial proton ? Aneta Slodczyk1, Philippe Colomban1, Oumaya Zaafrani1, Olivier Lacroix2, Johan Loricourt3, Frederic Grasset2 and Beatrice Sala2 1
LADIR, UMR7075 CNRS & UPMC, Thiais, 94320, France. AREVA NP, Université Montpellier 2, Montpellier, 34095, France. 3 SCT, Bazet, 65460, France. 2
ABSTRACT The proton conducting perovskites are widely investigated due to their high potential as electrolyte membranes of fuel cells, water steam electrolysers and CO2/syngas converters. Our comprehensive spectroscopic (Raman, IR, neutron), thermogravimetric, elastic and quasi-elastic neutron diffusion as well as conductivity studies performed on Ln/RE- modified zirconate ceramics with controlled densification (90-99% of theoretical density) reveal the important differences between the surface and bulk protonic species. The results clearly show that trivialization of the protonation process complexity can favorite the adsorption of the surface protonic species (hydroxide, hydrocarbonates, etc), prohibit the incorporation of bulk protons, i.e. species responsible for the proton conduction and confuse the understanding of fundamental aspects concerning the proton conductors such as the true nature of conducting species. Our studies reveal that OH- ions are located at the surface of poor densified ceramic and the bulk conducting protons exhibit an ionic, free of covalent-bonded nature. INTRODUCTION Recently, in the twilight of the fossil fuel economy and in front of global warming problem, hydrogen appears as an alternative energy vector for a sustainable modern world. Previously however two bottlenecks have to be successfully overcame: i) its low cost and environment friendly (CO2 free) production using the steam water electrolysers, ii) development of long life time high efficiency fuel cells and CO2/ syngas converters [1-4]. Such devices require the association of particular materials for the electrodes and the electrolyte. Perovskite type oxides exhibiting the significant conduction in a medium temperature range (400-600°C) [2-4] offer the advantage of good compromise between thermodynamic (noble metal electrodes not necessary) and engineering requirements (expansive alloys corrosion-resist not necessary). Consequently, different perovskite proton conductors A(Sr, Ba)B(Zr, Ce, Ti, Sn, Ta, Nb)O3 modified by incorporation of Lanthanides (Ln) or Rare Earths (RE) in order to create the oxygen vacancies and then protonated/hydrated (the proton content is not intrinsic to the pristine structure) have been intensively studied [2-17]. The first electrolysers [17] were tested. We have recently demonstrated the big potential of RE/Ln - substituted strontium zirconate ceramic membranes for hydrogen production at medium temperatures (~500-600°C) under significant water pressure [18]. However, the following problems need clarification: i) the systematic lack of differentiation between the bulk protons and protonic moieties adsorbed on the ma
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