$^{17}$O Solid-State NMR Spectroscopy of Functional Oxides for Energy Conversion

The main aim of this thesis is the development of ¹⁷O solid-state nuclear magnetic resonance (NMR) spectroscopic techniques to study the local structure and ion dynamics of functional oxide materials for applications in energy conversion, in particular as electrodes and electrolytes in solid oxide fuel cells (SOFCs). Broadly, the work comprises two related areas: (1) application of a combined experimental and computational methodology to enable the first ¹⁷O solid-state NMR studies of paramagnetic oxides, in particular a class of perovskite-derived structures used as mixed ionic-electronic conductors (MIECs) for SOFC cathodes, and (2) further uses of multinuclear variable-temperature NMR spectroscopy, with emphasis on ¹⁷O NMR results, to elucidate mechanistic details of oxide-ion motion and sublattice exchange in a novel family of promising SOFC electrolyte materials based on δ-Bi₂O₃. In the first section, ¹⁷O magic-angle spinning (MAS) NMR spectra of the paramagnetic MIEC, La₂NiO_4+δ, are presented and rationalized with the aid of periodic DFT calculations. Advanced NMR pulse programming and quadrupolar filtering techniques are coupled to extract high-resolution spectra. In particular, these data reveal local structural distortions in La₂NiO_4+δ that arise from incorporation of interstitial oxide defects. Moreover, variable-temperature spectra indicate the onset of oxide-ion motion involving the interstitials at 130 °C, which is linked to an orthorhombic$-$tetragonal phase transition. By analyzing the ion dynamics on the spectral timescale, specific motional mechanisms are elucidated that prove relevant to understanding the functionality and conductivity of this phase. Next, a similar methodology is applied to the Sr-doped analogues, La_2-xSr_xNiO_4+δ, in an exploration of the defect chemistry and electronic structure of these phases (0 $\leq {x} \leq$ 1). By following the doping-induced evolution of spectral features assigned to interstitial and equatorial oxygen environments, changes in the ionic and electronic conductivity, respectively, are rationalized. This approach has been extended to the acquisition and assignment of ¹⁷O NMR spectra of isostructural Sm_2-xSr_xNiO_4+δ and Pr_2-xSr_xNiO_4+δ phases, promising SOFC cathode materials that exhibit paramagnetism on the A site (A = Sm, Pr). The final section details the characterization of oxide-ion motion in the fluorite-type phases Bi_1-xV_xO_1.5+x and Bi_1-xP_xO_1.5+x ($x$ = 0.087 and 0.148) developed as SOFC electrolytes. Variable-temperature NMR experiments between room temperature and 923 K reveal two distinct mechanisms. For the V-doped phases, an oxide-ion conduction mechanism is observed that involves oxygen exchange between the Bi-O sublattice and rapidly rotating VO₄ tetrahedral units. The more poorly conducting P-doped phase exhibits only vacancy conduction with no evidence of sublattice exchange, a result ascribed to the differing propensities of the dopants to undergo variable oxygen coordination. These initial insights suggest chemical design rules to improve the next generation of oxide-ion conducting materials.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2018

Author dc:creator

• Halat, David Michael

Advisor dc:contributor.advisor

• Grey, Clare Philomena

• solid-state NMR spectroscopy
• energy conversion
• materials chemistry

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/0ecc6218-206a-4c93-bd9e-c1296be229b2/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

Author Identifier

0000-0002-0919-1689
0000-0001-5572-192X

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/270723