Interactions between electrolytes and carbon-based materials - NMR studies on electrical double-layer capacitors, lithium-ion batteries, and fuel cells

Electrical double-layer capacitors, lithium-ion batteries, and fuel cells are among the most important devices for storage and conversion of electrochemical energy. Further improvement of these systems and their single components is needed to meet the future requirements in terms of higher energy and power densities. In all of these devices, the interactions between carbonaceous materials and electrolyte components play crucial roles. In electrical double-layer capacitors and lithium-ion batteries, electrolyte ions get in direct contact with carbon electrode materials for energy storage. In fuel cells, carbon-supported electrode catalysts and polymeric membranes are among the most important components. For a directed improvement of these devices, a profound understanding of the processes taking place on the carbon-electrolyte interfaces is crucial. Especially nuclear magnetic resonance spectroscopy is qualified for the characterization of these interactions due to its chemical specificity, the local nature of NMR-relevant interactions, the possibility to investigate both amorphous and crystalline compounds, and the potential to monitor dynamic processes that occur over a wide timescale. This review addresses recent NMR spectroscopic investigations of the interactions between electrolytes and carbon materials in electrical double-layer capacitors, lithium-ion batteries, and fuel cells. Structures and properties of the most important carbon nanomaterials relevant for these devices are discussed, and the general principles of the electrochemical energy storage systems are introduced. Fundamental principles as well as hardware aspects of in situ and ex situ NMR measurements are addressed. The relationships between the properties of the carbon materials, the electrochemical characteristics, and the NMR data are explained. NMR spectroscopy as the profound analytical method is considered in combination with the operating principles of the electrochemical energy storage devices