As shown in Figure , another emerging application for CNTs is in the electrochemical reduction of CO2. This can be used as an approach for carbon capture that simultaneously generates a variety of useful chemical products. CNTs can be used in several ways related to catalyzing this process, including as a scaffold for supporting single-molecule metal based catalysts, covalent organic frameworks (COFs), and pipet-like bismuth nanorods. CNTs, particularly when they are doped with nitrogen or other atoms, can also catalyze CO2 reduction, acting as metal-free electrocatalysts. To better understand the use of CNTs in composites, we have broken down the types of composites by the matrix material (left), application (center), and the most commonly used matrix substances (extreme right) and showed their co-occurrences in a Sankey graph (Figure ). This data reflects the common use of polymers in CNT composites, for a variety of applications. Among these, the use of conductive polymers including PANI, polypyrrole, and others in energy storage and sensor applications is especially prominent. Examples include the use of a PANI/SWCTN composite in Al-ion batteries, 209 a combination of polypyrrole and CNTs to enable the use of black phosphorus in Li-ion battery anodes, 210 and the use of CNTs and polypyrrole in aqueous Zn-ion battery cathodes. A similar breakdown of matrix material (left), application (center), and commonly used matrix substances (extreme right) with respect to patent publications can be found in the Supplementary Information.