Electroceramic Materials for High-Purity Hydrogen Extraction from Liquid Hydrocarbon Fuels: Fundamental Investigations of Coupled Electrochemical and Catalytic Phenomena

Reports: DNI10 49007-DNI10: Electroceramic Materials for High-Purity Hydrogen Extraction from Liquid Hydrocarbon Fuels: Fundamental Investigations of Coupled Electrochemical and Catalytic Phenomena

Benjamin A. Wilhite, University of Connecticut

Cobalt-doped barium cerate-zirconate perovskites (BaCo_xCe_yZr_1-x-yO_3-a) displaying both catalytic and mixed-conducting properties for high-purity hydrogen extraction from methanol have been synthesized and analyzed to verify crystal phase, structure, surface morphology and composition. During the first funding period of this effort, homogeneous BaCe_0.25Zr_0.47Co_0.13O_3-d powders were successfully synthesized via oxylate co-precipitation. These powders demonstrated high catalytic activity and selectivity for methanol partial oxidation to hydrogen and carbon dioxide. Sintered pellets of these powders demonstrated electrochemical activity under inert gas environments (verifying p-type semiconduction) and under moist reducing environments (suggesting additional protonic conduction). These findings, summarized in our previous research report, support continued investigations of coupled catalytic hydrogen production and electrochemical purification using these novel materials.

During the past 12-month funding period, we have begun investigations into the influence of precursor composition (Co doping levels, A:B cation ratio) upon calcined powder homogeneity, sintered pellet structures and decomposition pathways. This information provides a basis for relating catalytic and transport properties of cobalt-doped barium cerate-zirconate perovskites to microstructure, as we proceed with additional catalytic and electrochemical studies.

Material Synthesis

BCZC was synthesized via solid-state reaction from oxide powders in order to directly control the A:B cation ratio and Co doping levels. Milled powder mixtures were calcined in air at 1550^oC for 4 hours to achieve uniform calcination of the precursor powder. Target material composition was initially selected as BaCe_0.25Zr_0.47Co_0.13O_3-d, with an A: B ratio 1.18, to match compositions achieved using oxylate co-precipitation methods (Suresh et al., 2010). Additional powders were synthesized across a range of cobalt doping (0% - 15%) and A:B ratios of 0.85 – 1.2 to investigate the influence of precursor composition upon calcined powder homogeneity.

Materials Analysis of Calcined Powders

X-ray diffraction (XRD) was used to screen all calcined powders to confirm phase(s) present. As summarized in Table X, XRD analysis indicated either (i) a single cubic phase attributed to BaCe_0.25Zr_XCo_YO₃, or (ii) two cubic phases with the second phase attributed to BaCoO_2.5. Scanning transmission electron microscopy (STEM) with X-ray energy dispersive spectroscopy (XEDS) was employed to validate homogeneity of BaCe_0.25Zr_0.47Co_0.13O_3-d (13% Co, A:B = 1.18). Results confirmed a homogeneous cubic-phase powder achieved upon calcination.

Figure 1: STEM and STEM-XEDS elemental maps for sintered BaCe_0.25Zr_0.47Co_0.13O_3-dpowders; (a) as-sintered material demonstrates exolution of cobalt resulting in a BaO/CoO rich intergranular region; (b) sintered material after reduction at 925^oC indicates hydrolysis of BaO-rich intergranular region combined with agglomeration of Co.

Materials Analysis of Sintered Pellets

The homogeneous calcined powders of BaCe_0.25Zr_0.47Co_0.13O_3-d were compacted at 100MPa and sintered at 1550^oC for 12 hours with heating and cooling rates of 5^oC/min. Cross-sections of the as-sintered pellet were obtained using focused-ion beam (FIB) milling and analyzed by STEM-EDS. Imaging indicates a re-distribution of elements between grain volumes and the integranular region, with barium and cobalt being the primary constituents of the latter (Figure 1a). Analysis thus indicates that a Ba-Co-O3 phase forms upon sintering. The high metal content of the intergranular region results in low grain-boundary electrical resistances relative, as shown by electrochemical impedence spectroscopy at 400^oC in nitrogen environment.

Sintered pellets were also exposed to moist reducing environment (10% H₂, 4% H₂O) at 600^oC and 925^oC for 24 hours to investigate materials stability. After exposure, cross-section analysis via FIB and STEM-XEDS was performed to observe microstructure and local composition. Exposure at 600^oC resulted in near-complete loss of the intergranular region; this is attributed to hydrolysis of BaO to Ba(OH)₂ in the BaO/CoO rich region. Grains demonstrated a mottled appearance, indicating a uniform hydrolysis decomposition of Ba(CeZr)O₃ to BaO (which can subsequently dissolve under moist conditions) and a nano-porous CeO₂/ZrO₂ grain. During the remainder of the funding effort, we will explore the possibility of exploiting these decomposition shapes to synthesize nanoporous Co-CeO₂/ZrO₂ catalysts via this hydrothermal decomposition route.

Exposure at 925^oC resulted in a near-complete exolution of Co from the grain region and formation of Co agglomerates in the intergranular region (Figure 1b). Grains demonstrated an un-mottled appearance with uniform distributions of Ba, Ce and Zr; thus exposure at 925^oC did not result in grain decomposition; likewise, degradation of the intergranular region was also reduced. These findings are in agreement with published observations that stability of BaCeZrO₃ against hydrolysis increases with temperature. These findings indicate that employing excess A:B cation ratios enables the synthesis of super-saturated mixtures of CoO-BaCeZrCoO₃. Sintering of pressed powders with excess A-cation (Ba) results in (i) exolution of excess Co to the intergranular region, and (ii) decomposition of the BaO-rich intergranular region. We hypothesize that reduction in both doping levels and A:B cation ratios will result in reduction in both the volume and susceptibility of the intergranular region, improving stability of electroceramic films. Conversely, we hypothesize that high doping levels and excess A cation may be exploited to produce nanoporous reforming catalysts via combination of pore formation by hydrolysis and exolution of Co atoms to the pore surface. During the remainder of the funding period, analysis of sintered pellets formed from powders of varying dopant levels and A:B cation ratios will be performed to confirm these hypotheses and to identify optimal compositions for materials stability.