No description available.

No description available.

No description available.

No description available.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal- yse numerous graphene-sheet based materials in an attempt to study their effects on the performance of polymer electrolyte membrane fuel cell (PEM-FC) mate- rials. It has been noted in the literature that fuel cells which incorporated these materials (e.g. functionalized graphene, doped carbon nanotubes (CNTs), etc.) displayed increased performance over a wider range of environmental conditions, chiefly temperature and relative humidity. The inter-material interactions behind this phenomenon are poorly described at best. Due to its extreme site speci ficity and sensitivity to minute differences in nuclear electromagnetic environments, ss- NMR is an ideal tool for investigating the complicated interactions at work in these systems. While the electronically conductive, amorphous, non-stoichiometric, and low spin-density nature of these materials presented challenges to the collection of NMR spectra, the results presented here display the remarkable utility of this method in the study of analogues and derivatives of graphene. Graphene Oxide (GO), a derivative of graphene, has intrinsic proton conduc- tivity which is similar to Na fon, the most popular proton exchange membrane material currently used in fuel cells. Research into acid-functionalized graphene oxides and determining the role of acidic groups in increasing proton conductivity will help to improve polymer electrolyte membrane performance in fuel cell sys- tems. Multinuclear solid-state NMR (ssNMR) spectroscopy was used to analyse the structure and dynamics of GO and a number of sulfonic acid derivatives of GO, both novel and previously reported. 13C spectra showed the disappearance of surface-based oxygen groups upon GO functionalization, and can be used to identify linker group carbon sites in previously synthesized and novel functional- ized GO samples with high speci city. Dehydration of these samples allows the collection of 1H spectra with resolved acidic proton and water peaks. The effect of dehydration on the proton spectrum is partially reversible through rehydration. Deuteration of the acidic groups in high temperature and acidic conditions was virtually unsuccessful, indicating that only the surface and not the intercalated functional groups play a role in the enhanced proton conductivity of ionomer / functionalized GO composites. Increased surface area and increased delamination of functionalized GO is suggested to be important to improved PEM-FC perfor- mance. This synthesis and method of analysis proves the utility of ssNMR in the study of structure and dynamics in industrially relevant amorphous carbon ma- terials, despite the obvious di culties caused by naturally broad signals and low sensitivity. Graphene and carbon nanotubes (CNTs) have been investigated closely in re- cent years due to their apparent positive effect on the electrochemical performance of new fuel cell and battery systems as catalyst stabilizers, supports, or as metal- free catalysts. This is particularly true for doped graphene and CNTs, where only a small amount of doping with nitrogen and/or phosphorus can have a re- markable effect on materials performance. A direct link between structure and function in these materials is, as of yet, unclear. Doped graphene and CNTs were synthesized using varied chemical vapour deposition (CVD)-based methods, and ssNMR was used to unambiguously identify dopant atom sites, revealing that these particular synthesis methods result in highly homogeneous populations of installed phosphorus and nitrogen atoms. We present the first experimental 15N spectrum for graphitic nitrogen in N-doped graphene. 15N-labeled nitrogen doped graphene synthesized as reported here produces mainly graphitic nitrogen sites located on the edges of sheets and around defect sites. 1H-1H and 1H-15N corre- lations were also used to probe dopant nitrogen sites in labelled and unlabelled N-doped graphene. A nearly homogeneous population of phosphorus in P-doped graphene is found, with an overwhelming majority of graphitic phosphorus and a small amount of phosphate oligomer. Similar findings are noted for the phos- phorus sites in phosphorus and nitrogen co-doped CNTs with a minor change in chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs versus graphene sheets) with signifi cantly different electronic structures. Ionomeric sulfonated polyether ether ketone (SPEEK) membranes were doped with functionalized graphenes, and the proton conductivities of these composite membranes was measured at fuel cell operational temperatures and percent relative humidities (%RH). The differences in proton conductivity between pure SPEEK membranes and composites with different dopants and doping levels at varied conditions were investigated through high-fi eld 1H ssNMR. It was found that high- speed MAS was able to dehydrate membranes under water-saturated conditions, and so lower %RH conditions were better able to produce reliable ssNMR results. The addition of graphitic dopants appeared to have an overall detrimental effect on the bulk proton conductivity of membranes, while concurrently these doped membranes had a broadened operational temperature window. In an attempt to explore the positive influence of nitrogen doping on the effec- tive lifetime of carbon-supported platinum catalysts used in automotive hydrogen fuel cell systems, solid-state NMR was employed to probe the difference (if any) between doped catalyst supports made from different carbon and nitrogen sources. 1H spectroscopy showed a variety of sites present in the doped samples; some likely from residual starting material but others from novel sites within the doped cat- alyst supports. Double-quantum and 2D 1H experiments were used to examine the structure of these catalysts, while 13C CPMG experiments (see Chapter 2) revealed subtle differences in the nuclear relaxation rates of these materials, poten- tially related to their electronic conductivity. The results of the ssNMR analysis were insuffcient to provide an unambiguous picture of the dopant sites within these carbon black samples; this was due in equal parts to the lack of isotopically labelled dopants, the effects of electronic induction and ring current shifts on data acquisition and analysis, and the broad array of different 13C chemical shift en- vironments present in the carbon black itself. While the data is still interesting spectroscopically, suggestions are made at the end of this chapter to expand upon the lessons learned through this study to produce more useful results from similar samples in the future. / Thesis / Doctor of Philosophy (PhD) / Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal- yse numerous graphene-sheet based materials in an attempt to study their effects on the performance of polymer electrolyte membrane fuel cell (PEM-FC) materials. It has been noted in the literature that fuel cells which incorporated these materials (e.g. functionalized graphene / graphite, doped carbon nanotubes (CNTs), etc.) displayed increased performance over a wider range of environmental conditions, chiefly temperature and relative humidity. The inter-material interactions behind this phenomenon are poorly described at best. Due to its extreme site specifi city and sensitivity to minute differences in nuclear electromagnetic environments, ss- NMR is an ideal tool for investigating the complicated interactions at work in these systems. While the electronically conductive, amorphous, non-stoichiometric, and low spin-density nature of these materials presented challenges to the collection of NMR spectra, the results presented here display the remarkable utility of this method in the study of analogues and derivatives of graphene. Covalently functionalized graphene / graphite was synthesized, and the struc- tures of several derivatives were recorded with remarkable resolution, such that functional group carbons were resolvable. The proton dynamics of this material were remarkably slow, and so improvements in composite PEM ion conductiv- ity were proposed to be caused by surface interactions between dopant and poly- mer. The proton dynamics of ionomer graphene composites were also investigated through ssNMR. A number of graphene and CNT samples doped with phosphorus and 15N-labelled nitrogen were also analysed, and the synthesis methods employed were found to produce chemically homogeneous dopant sites with few by-products. Absent isotopic labelling, nitrogen dopant sites in carbon black samples were found to affect the relaxation properties of protons within nitrogen doped carbon black.

No description available.

In this work, X-ray and neutron powder diffraction experiments and complementary solid-state NMR spectroscopy are used to characterise NaNbO₃-based perovskite phases. Samples of NaNbO₃, KₓNa₁₋ₓNbO₃ and LiₓNa₁₋ₓNbO₃ are synthesised using a variety of techniques and subsequently characterised. For NaNbO₃, it is observed that at least two room temperature perovskite phases can co-exist, P and Q, and that each phase can be formed exclusively by manipulating the synthetic approach utilised. Phase Q can also be formed by the substitution of a small amount of K⁺ or Li⁺ for Na⁺. The room temperature phases of these materials are also analysed using NMR spectroscopy and X-ray diffraction. It is found that, for KₓNa₁₋ₓNbO₃, preferential A-site substitution of K⁺ for Na⁺ may occur, and this observation is supported using a range of NMR techniques and density functional theory calculations. The high-temperature phase behaviour of NaNbO₃ and KₓNa₁₋ₓNbO₃ (x = 0.03 to 0.08) is analysed using high-resolution neutron and X-ray powder diffraction to determine when phase changes occur and to characterise each phase. Characterisation of these materials is supported used complementary symmetry mode analysis. For the LiₓNa₁₋ₓNbO₃ perovskite system, complex phase behaviour is observed at room temperature. High-resolution neutron powder diffraction data shows that, over the range 0.08 < x < 0.20, phase Q may co-exist with a rhombohedral phase, with the proportions of the two highly dependent upon the synthetic conditions used. Furthermore, using X-ray diffraction and NMR spectroscopy, phase Q is shown to undergo a crystal-to-crystal transition to the rhombohedral phase. For higher values of x, two compositionally-distinct rhombohedral phases are formed, termed Na-R3c and Li-R3c, as determined from neutron powder diffraction data.

Depuis 1949, date à laquelle Bloembergen en introduisit le concept, la diffusion de spin nucléaire suscite un vif intérêt en résonance magnétique. La diffusion de spin, qui peut être définie comme le transfert de polarisation de spin induit par l’interaction dipolaire, est un mécanisme omniprésent dans les solides. Les mesures expérimentales de ce phénomène contiennent des informations sur la structure du système étudié. La diffusion de spin est cependant un problème quantique à N corps, ce qui rend sa description ab initio relativement difficile. L’objectif principal de cette thèse est d’obtenir une description quantitative et ab initio de la diffusion de spin, en modélisant de manière adéquate la dynamique à N corps sous-jacente. Tout d’abord, nous exploitons une approche existante, reposant sur l’utilisation d’une équation maître pour les polarisations, dans le cas de la diffusion de spin entre carbones permise par les protons (PDSD). Ensuite, nous introduisons une méthode permettant de simuler l’évolution temporelle d’un ensemble d’observables pour un système de spins nucléaires fortement couplés, en utilisant les corrélations de petit ordre dans l’espace de Liouville (LCL). Le modèle LCL fournit une description précise du transfert de polarisation pour les systèmes polycristallins soumis à la rotation à l’angle magique. Après avoir décrit le modèle, nous analysons plus en détail la réduction de l’espace de Liouville pour les solides, afin d’identifier les conditions dans lesquelles l’approximation LCL est valide. Enfin, nous effectuons des simulations de la diffusion de spin entre pro- tons (PSD) et entre carbones (PDSD), à partir de la structure des cristaux étudiés et sans aucun paramètre libre, et nous constatons pour des solides organiques polycristallins que leur accord avec les mesures expérimentales est excellent. / Since its introduction by Bloembergen in 1949, nuclear spin diffusion has been a topic of significant interest in magnetic resonance. Spin diffusion, which can be defined as the transfer of spin polarisation induced by the dipolar interaction, is a ubiquitous transport mechanism in solids. Experimental observations of spin diffusion contain structural information. However, the many-body nature of the problem makes it difficult to describe from first principles. The central goal of this thesis is to obtain a quantitative description of the spin diffusion phenomenon from first-principles, through the development of suitable models of the underlying many-body dynamics. To that end we first consider an extension of an existing approach that relies on a master equation to describe the polarisations, for the case of proton-driven carbon-13 spin diffusion (PDSD). Second, a novel approach is introduced for the simulation of the time evolution of selected observables for large strongly coupled nuclear spin systems, using low-order correlations in Liouville space (LCL). Following the introduction of this new simulation method, Liouville-space reduction in solids is analysed in more detail, in order to identify the conditions under which the LCL approximation is valid. Finally, using the LCL model, simulations of proton spin diffusion (PSD) and PDSD are performed, directly from crystal geometry and with no adjustable parameters, and are found to be in excellent agreement with experimental measurements for polycrystalline organic solids.

Esta tese baseia-se no estudo da dinâmica e conformação de polímeros no estado sólido utilizando-se técnicas modernas de RMN. Para o estudo quantitativo da dinâmica molecular lenta dos grupos laterais em uma série de poli (alqui1 metacrilato)s que apresentam diferentes tamanhos para os seus grupos laterais foram utilizadas duas novas técnicas de exchange: centerband-only detection of exchange (CODEX) e pure exchange (PUREX). Rotações dos grupos ésteres de 180&#176 flip) acopladas a movimentos de pequenos ângulos em torno da cadeia principal (&#60 20&#176), que são associados a relaxação &#946 nestes polímeros, são observados distintamente. As porcentagens de grupos laterais que realizam os movimentos de flip foram obtidas com 3% de precisão. Esses valores decrescem com o tamanho do grupo lateral, indo de 34% (PMMA) até cerca de 10% (PcHMA) à temperatura ambiente. No PMAA nenhum movimento lento dos grupos laterais é detectado. A fração de grupos laterais que realizam o flip se mantém constante com o aumento da temperatura para o PMMA até próximo da sua transição vítrea (Tg), enquanto que para o PEMA, o PiBMA e o PcHMA essa fração aumenta continuamente até próximo da Tg (de 31% para 80% para o PEMA). Movimentos independentes de pequena amplitude (&#60 5&#176) que cooperam para a acomodação dos grupos laterais durante o flip das cadeias vizinhas também foram verificados. O monitoramento do sinal atribuído ao grupo CH2 através da técnica CODEX confirma os resultados obtidos para o movimento da cadeia principal. Além disso, observa-se em alguns poli(acri1atos &#945-substituídos) que o tamanho dos grupos laterais a também têm influência na fração dos grupos ésteres que sofrem reorientação a 25&#176C . Para o estudo da conformação de polímeros foram implementados experimentos Double Quantum, que determinam ângulos de torção entre pares 13C-13C na cadeia polimérica. Foram realizados experimentos para dois polímeros, poli(óxido etileno) - (POE) e poli(eti1eno teraftalato) - (PET), enriquecidos em 13C com e sem desacoplamento homonuclear (13C-13C) em um espectrômetro Variam Inova 400. / This work is based on the study of dynamics and conformation of solid-state polymers, using NMR modern techniques. For the quantitative study of slow sidegroup dynamics in a series of poly(alkyl methacrylate)s and other of poly(&#945-substituted acrylate)s with varying sidegroup sizes two new 13C exchange techniques were used: centerband-only detection of exchange (CODEX) and pure exchange (PUREX). Flips and small-angle motions of the ester groups associated with the p-relaxation are observed distinctly, and the fraction of slowly flipping groups has been measured with 3% precision. A decreasing in these values occurs growing the sidegroup size. In PMMA, 34% of sidegroups flip, while the fraction is c.a. 10% in PcHMA around room temperature. In PMAA, no slow sidegroup flips are detected. In PMMA, the flipping fraction is temperature-independent between 25&#176C and 96&#176C, while in PEMA, PiBMA, and PcHMA it increases continuously between room temperature and Tg (from 31 to 80% for PEMA). Backbone CH2 CODEX data confirm the results obtained for the main chain movements. Flip-independent small amplitude motions (&#60 5&#176) are also observed. Besides, it is noticed that in some poly(&#945 -substituted acrylate)s the a sidegroup size has influence in the fraction of slowly flipping ester groups at 25&#176C. For the study of polymers conformation Double Quantum NMR experiments were used, for torsion angles determination between 13C-13C pairs through the polymer chain. Experiments for two standards 13C labeled samples: poly(ethy1ene oxide) - (PEO) and poly(ethy1ene teraftalate) - (PET) were performed, with and without homonuclear decoupling (13C-13C), in a Varian Inova 400 spectrometer.

Compostos intercalados de lítio sac considerados materiais apropriados para eletrodos de baterias recarregáveis de lítio. O conhecimento das taxas de difusão, assim como os mecanismos relacionados com a mobilidade dos íons nos espaços interlaminares são portanto pertinentes para a caracterização das propriedades deste tipo de compostos. Investigações por ressonância magnética nuclear tem um papel ativo no estudo do comportamento dinâmico e estrutural destes compostos de lítio sólidos. Por essa razão este trabalho e baseado nos espectros de RMN obtidos entre 150K e 400K em um campo de 2T. As larguras de linha do 1H e do Li+ foram medidas a partir da transformada de Fourier dos FIDs resultantes da excitação da amostra por um pulse &#960/2. O desacoplamento de 1H foi utilizado, quando necessário, para excluir a contribuição dipolar magnética a destes núcleos a largura de linha do 7Li. Os tempos de relaxação spin-rede para os núcleos 7Li e 1H foram obtidos utilizando-se o método de inversão-recuperação. A partir destas medidas, diversos parâmetros dinâmicos e estruturais destes compostos foram encontrados como: energias de ativação, tempos de correlação e coeficientes de difusão dos íons lítio. / Lithium intercalation compounds are considered suitable materials for electrodes in rechargeable lithium batteries. The knowledge of diffusion rates as well as of mechanistic features related with the mobility of ions in the interlamellar spaces is therefore relevant for the regulation of the properties of this kind of compounds. Nuclear magnetic resonance investigations have played an active role in the study of the dynamic and structural behavior of these solid lithium compounds. For this reason this work is based on NMR spectra recorded between 150K and 400K at 2T. The 1H and Li+ linewidths were measured from the Fourier transform of the FIDs following a &#960/2 excitation. IH decoupling was used, when necessary, to exclude the magnetic dipolar contribution of this nucleus to the 7Li linewidth. The 7Li and 1H spin-lattice relaxation time T1 were measured using the inversion-recovery method. Several dynamic and structural NMR parameters were obtained from these NMR measurements like activation energies, correlation times and diffusion coefficients.