Search

Understanding of the protonic defect transport mechanism in Ba(Ce,Zr)O3 perovskite oxides and defining the appropriate temperature range for ionic conductivity are fundamental to material design and application as electrolytes for solid oxide fuels and electrolyzer cells, isotopic separation membranes, and hydrogen sensors. Structural features of the material, and lattice distortions and proton diffusion are key factors that define the protonic conduction. The crystal structure of protonated and deuterated BaCe0.4Zr0.4Y0.2O3-? (BCZY) perovskite and its correlation with protonic transport was studied by in situ neutron powder diffraction (NPD) and complementary thermogravimetry (TG), quasi-elastic neutron scattering (QENS) and isotope exchange depth profiling (IEDP) techniques. A second order phase transition from rhombohedral to cubic symmetry takes place at intermediate temperatures (400-600 °C). Dynamic measurements of NPD allowed the detection of the temperature of the phase transition for BCZY at around 520 °C. Crystallographic and microstructural parameters, including deuterium occupancy and anisotropic thermal parameters, were determined from high resolution NPD data. The deuterium (and oxygen) occupancy for pre-hydrated BCZY was at its maximum at low temperature and decreased at temperatures greater than 400 °C, even through the phase transition and beyond 600 °C. By contrast, proton diffusion increased with temperature above the phase transition. The combination of both effects, i.e., deuterium content and diffusion coefficient, explains the previous results showing that the proton conductivity dominates the ionic conductivity over the oxygen vacancy mechanism until 600 °C. The phase transition is mainly related to oxygen sublattice relaxation and does not impact the protonic transport mechanism.

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) employing a phosphoric acid H3PO4-doped membrane are considered to be promising sustainable electrochemical energy storage. The high-temperature operation has several advantages, such as a higher tolerance to CO poisoning, allowing coupling of HT-PEMFCs with reformers [1-3], as well the possibility for heat and electric energy co-generation [1,2]. However, during operation, phosphorus oxo-acids (e.g.: H3PO3) are generated on the anode. These impurities adsorb on the Pt catalyst [4-6], thus possibly negatively affecting the HT-PEMFCs performance. A detailed understanding of the H3PO3-catalyst (Pt) interaction is hence necessary for further HT-PEMFC optimization. However, besides an investigation of the H3PO3 adsorption behavior on Pt [6,7], literature on the behavior of the H3PO3 in contact with Pt (with/without polarization) is scarce. In this work, the oxidation mechanism of H3PO3 was investigated using a combination of in situ x-ray spectroscopy techniques that directly probe the H3PO3/Pt interface interaction, complemented by ex situ x-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography (IEC). IEC gave insights into the effect of Pt on the stability of deaerated aqueous H3PO3 solutions. XPS was conducted on H3PO3/support structures (including Au and Pt supports) to determine to what extent the support affects the H3PO3 oxidation. Furthermore, in-situ dip and pull near-ambient pressure (NAP-)XPS was conducted to investigate the state of H3PO3 at the H3PO3/Pt interface and in solution bulk. It was observed that at the H3PO3/Pt interface, H3PO3 was chemically oxidized to H3PO4, while in the bulk solution it remains stable, as shown in Figure 1. Moreover, in situ x-ray absorption spectroscopy at the P K-edge was conducted at different concentrations of H3PO3 in aqueous solutions (i.e., different amounts of H2O) in contact with Pt, to determine the role of H2O in the oxidation of H3PO3. A higher degree of oxidation was observed for the less concentrated H3PO3, implying that H2O participates in the oxidation mechanism of H3PO3 to H3PO4. References: [1] Chandan et al., A.. J. Power Sources 2013, 231, 264–278. [2] Asensio, et al. Chem. Soc. Rev. 2010, 39 (8), 3210. [3] Q. Li et al, J. Electrochem. Soc. 2003, 150 (12), A1599. [4] Sugishima et al, J. Electrochem. Soc. 1994, 141 (12), 3332. [5] Doh et al. ChemElectroChem. 2014, 1 (1), 180–186. [6] Prokop et al, Electrochimica Acta 2015, 160, 214–218. [7] Prokop et al, Electrochimica Acta 2016, 212, 465–472. Figure 1