Aprotic lithium-oxygen (Li-O2 ) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen ((1) Δg ) is formed upon Li2 O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time- and voltage-resolved in operando EPR spectroscopy in a purpose-built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O2 cells.

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Time-resolved formation of micro-structured mossy/dendritic lithium is investigated during battery cycling byoperandoEPR spectroscopy, using a novel electrochemical cell design.

Aprotic lithium-oxygen (Li-O 2 )b atteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density.Am ajor challenge is their poor reversibility caused by degradation reactions,which mainly occur during battery charge and are still poorly understood. Herein, we showt hat singlet oxygen ( 1 D g )i s formed upon Li 2 O 2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through areaction with aspin trap to form as table radical that was observed by time-and voltageresolved in operando EPR spectroscopyi nap urpose-built spectroelectrochemical cell. According to our estimate,alower limit of approximately 0.5 %o ft he evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O 2 cells.

There is now a significant body of literature which reports that stripes form in the ligand shell of suitably functionalised Au nanoparticles. This stripe morphology has been proposed to strongly affect the physicochemical and biochemical properties of the particles. We critique the published evidence for striped nanoparticles in detail, with a particular focus on the interpretation of scanning tunnelling microscopy (STM) data (as this is the only technique which ostensibly provides direct evidence for the presence of stripes). Through a combination of an exhaustive re-analysis of the original data, in addition to new experimental measurements of a simple control sample comprising entirely unfunctionalised particles, we show that all of the STM evidence for striped nanoparticles published to date can instead be explained by a combination of well-known instrumental artefacts, or by issues with data acquisition/analysis protocols. We also critically re-examine the evidence for the presence of ligand stripes which has been claimed to have been found from transmission electron microscopy, nuclear magnetic resonance spectroscopy, small angle neutron scattering experiments, and computer simulations. Although these data can indeed be interpreted in terms of stripe formation, we show that the reported results can alternatively be explained as arising from a combination of instrumental artefacts and inadequate data analysis techniques.

A time-of-flight imaging technique is introduced to visualize fluid flow and dispersion through porous media using NMR. As the fluid flows through a sample, the nuclear spin magnetization is modulated by RF pulses and magnetic field gradients to encode the spatial coordinates of the fluid. When the fluid leaves the sample, its magnetization is recorded by a second RF coil. This scheme not only facilitates a time-dependent imaging of fluid flow, it also allows a separate optimization of encoding and detection subsystems to enhance overall sensitivity. The technique is demonstrated by imaging gas flow through a porous rock.

We have used nuclear magnetic resonance (NMR) to obtain spatially and temporally resolved profiles of gas flow in microfluidic devices. Remote detection of the NMR signal both overcomes the sensitivity limitation of NMR and enables time-of-flight measurement in addition to spatially resolved imaging. Thus, detailed insight is gained into the effects of flow, diffusion, and mixing in specific geometries. The ability for noninvasive measurement of microfluidic flow, without the introduction of foreign tracer particles, is unique to this approach and is important for the design and operation of microfluidic devices. Although here we demonstrate an application to gas flow, extension to liquids, which have higher density, is implicit.hyperpolarization ͉ xenon ͉ magnetic resonance imaging M iniaturized fluid-handling devices have attracted considerable interest recently in many areas of science (1). Such microfluidic chips perform a variety of functions ranging from analysis of biological macromolecules (2, 3) to catalysis of reactions and sensing in the gas phase (4,5). To enable precise fluid handling, accurate knowledge of the flow properties within these devices is important. Because of low Reynolds numbers, laminar flow is usually assumed. However, either by design or unintentionally, the flow characteristic in small channels is often altered (e.g., by surface interactions, viscous and diffusional effects, or electrical potentials). Therefore, its prediction is not always straightforward (6-8). Currently, most microfluidic flow measurements rely on optical detection of markers (9, 10), requiring the injection of tracers and transparent devices. Here we show profiles of microfluidic gas flow in capillaries and chip devices obtained by NMR in the remote-detection modality (11,12). Through the transient measurement of dispersion (13), NMR is well adaptable for noninvasive yet sensitive determination of the flow field and provides a potentially more powerful tool to profile flow in capillaries and miniaturized flow devices.NMR remote detection separates the encoding of NMR information from the detection of the actual signal. Information about a stationary object of interest is encoded into spin polarization of a mobile sensor by using radiofrequency (rf) pulses and field gradients. The spin sensor is then physically transferred to a different location for detection, which leads to a decisive enhancement of signal whenever geometrical constraints prevent the use of a sensitive NMR coil for detection directly at the sample site (11,12). In the present work, we studied gas flow in model microfluidic devices by using hyperpolarized 129 Xe as a spin-carrying nucleus (14). Experimental ProceduresImage information was encoded with an rf coil that completely encompassed the microfluidic device (Fig. 1). Such a coil arrangement ideally allows NMR-image information to be obtained from the entire microfluidic device. For measuring flow, it provides a decisive advantage over localized detection, which is achievable, for example,...

Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.