Protons Crossing Triple Phase Boundaries based on Pd and Barium Zirconate: A Density Functional Theory Study
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Protons Crossing Triple Phase Boundaries based on Pd and Barium Zirconate: A Density Functional Theory Study Massimo Malagoli1 and Angelo Bongiorno1 1
School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 303320400, U.S.A.
ABSTRACT Density functional theory calculations are used to address the energetics of protons crossing “triple phase boundaries” based on Pd and barium zirconate. Our calculations show that the proton transfer reaction at these triple phase boundaries is controlled by the terminal layer of the electrolyte in contact with the metallic catalyst and gas phase. Hydrogen spilling onto the electrolyte surface is energetically favored at peripherical sites of the metal-electrolyte interface, and proton incorporation into the sub-surface region of the electrolyte involves energies of the order of 1 eV. At the triple phase boundary, the energy cost associated with the proton transfer reaction is controlled by both the nature of chemical contact and the Schottky barrier at the metal-electrolyte interface. INTRODUCTION Triple phase boundaries (TPBs) correspond to electrochemical “hot spots” in heterogeneous porous electrodes of fuel cells [1]. The concept of TPB holds that key reaction steps governing fuel cell electrodes occur at confined spatial sites – called “triple phase boundaries” – where metal catalyst, electrolyte, and gas phase enter into contact [2, 3]. The future of the fuel cell technology relies on new materials and solutions enhancing electrochemical performances. To this end, a basic understanding of TPBs needs to be achieved [2, 3]. Here, we use density functional theory (DFT) calculations to investigate TPBs based on Pd and a proton conducting electrolyte, mimicking TPBs in the anode of proton ceramic fuel cells (PCFC) [4, 5]. The current understanding of materials surfaces and interfaces, TPBs, and elementary processes in fuel cells is limited and a matter of significant interest [6–9]. Among the various efforts, computational modeling is becoming a useful means for gaining insight on such complex systems. So far, the most notable computational efforts undertaken to study TPBs and go beyond phenomenological pictures are those of Shishkin et al. [10, 11] and Cucinotta et al. [12]. These DFT studies considered TPBs based on Ni and yttria-stabilized zirconia (YSZ), and focused on reaction mechanisms leading to the formation of water molecules near these TPBs [10, 12]. In the anode of PCFCs, hydrogen or hydrocarbons split on the surface of a metallic catalyst. Hydrogen adsorbed on the metal surface migrates toward the TPBs, incorporates into the proton conducting ceramic, diffuses across the electrolyte, and finally reaches the cathode where it reacts with oxygen ions to form water molecules [4, 5]. In this work, we focus on TPBs in the anode of PCFCs. In particular, we consider TPBs formed by Pd and barium zirconate (BZ), a proton conducting electrolyte forming a class of novel materials for
new generation of SOFCs, for hydrogen separation and purification, and
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