DOI: 10.1002/cphc.201400083 Magnetic Resonance Imaging of Electrochemical Cells Containing Bulk Metal Melanie M. Britton* [a] 1. Introduction In the drive to improve many electrochemical technologies, such as energy storage, metal electrofinishing and corrosion prevention, better understanding of the electrochemical reac- tions, transport and concentration gradients within these sys- tems during operation is necessary. However, there are few methods that are able to visualise and quantify these spatially, in situ and in real time. Magnetic resonance imaging (MRI), which is a technique typically associated with medical research and diagnosis, is an excellent tool for non-invasively studying complex, spatially heterogeneous chemical systems in materi- als, engineering and chemical research. [2] Although MRI has enormous potential for in situ investigation of the spatial distri- bution, speciation and mobility of molecules and ions in elec- trochemical systems, there are currently very few examples of MRI being used to probe them. There are a number of reasons for this. Firstly, care must be taken when performing magnetic resonance experiments on the electrically conductive electro- lytes that are typically found in electrochemical cells, which can lead to experimental difficulties related to radiofrequency (RF) losses and heating of the electrolyte. [3] Other experimental challenges are associated with setting up electrochemical cells containing bulk metals inside a strong magnetic field and the artefacts caused by the presence of these metals that lead to undesirable variations in the RF and magnetic fields across the sample. [1, 4] Some of these challenges have been addressed for in situ nuclear magnetic resonance (NMR) spectroscopy studies of batteries, fuel cells and supercapacitors. [5] However, a number of recent papers have shown that such issues can also be overcome for MRI experiments and that it is possible to collect viable, quantitative and spatially-resolved in situ data. [6] Rotating-frame imaging has been performed on cylin- drically symmetric fuel cells and lithium batteries, by using toroid cavity detectors, [7] enabling the acquisition of spatially resolved spectra and one-dimensional (1D) profiles. [6a,b, 8] This concept article, however, focuses on recent MRI studies of elec- trochemical cells that have employed magnetic field gradient imaging methods. [6c–f] Although all but one of the reviewed studies have focused on lithium or zinc batteries, the MRI methods employed could be applied to many other electro- chemical systems containing bulk metals and involve the elec- trodissolution/deposition of these metals and the movement and concentration gradients of electroactive species, such as those found in corrosion or electroplating/polishing systems. 2. Magnetic Resonance Imaging MRI is a technique based on NMR spectroscopy and is able to map the spatial distribution and concentration of molecules and ions containing NMR-active nuclei [2] (e.g. 1 H, 19 F, 7 Li). The structure and environment of chemical species can be identi- fied from their chemical shift or NMR relaxation properties and their transport behaviour can be probed by using pulsed field gradient (PFG) NMR methods. [2] By coupling these NMR param- eters with spatial encoding methods, it is possible to map chemical composition, molecular environment and transport behaviour for a range of chemical species. MRI relies on the application of magnetic field gradients (G r ) to spatially locate nuclei [Eq. (1)] by making their precessional frequency [w(r)] dependent on position (r): wðrÞ¼ gB 0 þ gGr ð1Þ in which g is the magnetogyric ratio and B 0 is the static mag- netic field. Position can be encoded directly through the spa- tially dependent frequency of nuclei (frequency encoding) during signal acquisition or through a magnetisation phase shift induced by the application of a magnetic field gradient prior to signal acquisition (phase encoding). [9] If 1D profiles are required, frequency encoding is typically employed because it [a] Dr. M. M. Britton School of Chemistry, University of Birmingham Edgbaston, Birmingham, B15 2TT (UK) E-mail : m.m.britton@bham.ac.uk The development of improved energy-storage devices, as well as corrosion prevention and metal-electrofinishing technolo- gies, requires knowledge of local composition and transport behaviour in electrolytes near bulk metals, in situ and in real time. It remains a challenge to acquire such data and new ana- lytical methods are required. Recent work shows that magnetic resonance imaging (MRI) is able to map concentration gradi- ents and visualise electrochemical processes in electrochemical cells containing bulk metals. This recent work, along with the challenges, and solutions, associated with MRI of these electro- chemical cells are reviewed.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00,1–7 &1& These are not the final page numbers! ÞÞ CHEMPHYSCHEM CONCEPTS