The breakdown of CNT-related publications for different battery chemistries is shown in Figure . Li-ion batteries dominate, but Zn-ion batteries represent the highest growth. In Zn-ion batteries, prominent examples of the use of CNTs are in the cathodes of aqueous Zn-ion batteries, in combination with KV3O8•0.75H2O (KVO), 177 ZnMn2O4 (ZMO), 178 M phase VO2, 179 NaxV2O5•nH2O(NVO), 180 and V2O5. In these cases, the role of the CNT is to form a high surface area, mechanically resilient, flexible, and electron-conductive scaffold that supports these materials. The goal of improving these properties is to improve the rate capability and capacity retention after repeated cycling. CNTs are also used to enable Zn metal anodes, either as a way to control Zn deposition, preventing the formation of dendrites, 182 or as a scaffold for Zn nanosheets. Research in triboelectric nanogenerators and thermoelectric generators are two other emerging applications of CNTs (represented as "energy conversion in Figure ). Triboelectric nanogenerators are devices which can generate electrical energy from many different types of motion, including human motion, or the motion of plant leaves in the wind. The origin of this electrical energy is charge separation that takes place during the mechanical interaction of two surfaces. In this application, CNTs are used to increase the surface charge density, 186-188 thereby increasing the output power of the device. CNT-containing triboelectric nanogenerators have many possible applications, including self-powered sensors, or providing power to wearable devices. Hydrogen storage shows a significantly different trend compared to the other energy topics, with generally flat or negative growth since 2010. There appear to be multiple reasons for this. One reason is the difficulty in obtaining reproducible and accurate quantitative measurements of hydrogen storage capacity, the other being that it is now thought that pure CNTs have a fairly low gravimetric storage capacity for hydrogen at room temperature, 192 below the initial US Department of Energy goal set in 2010 of 6.5 wt% for automotive H2 storage applications (later reduced in 2015 to 5.5 weight %). However, work continues on the use of modified CNTs for hydrogen storage, for example CNTs loaded with metals or heteroatoms such as N. A final application to highlight in Figure is the use of CNTs in flexible sensors and electronics. CNTs are used here primarily for their electrical properties, which can impart piezoresistive, piezocapacititve, 195 and other properties useful for sensor applications. This can involve incorporating CNTs into matrices, including hydrogels, or combining them with polymers (for example, in a polyurethane/CNT yarn with hierarchical structure 197 ). Flexible sensors can also be fabricated by printing CNT-containing ink onto flexible substrates. The major emerging application in these cases is monitoring human motion, either through wearable sensors or electronic skin, 199 as shown in Figure . The other prominent biomedical application of CNTs is drug delivery. However, publication trends in this area (Figure ) indicate this is a relatively mature field of research.