Formation of Sheet Electrodes Composed of Highly Dispersed Ultralong Carbon Nanotubes and Their Application to the Electrochemical Reduction of Carbon Dioxide

Ultralong carbon nanotubes (UL-CNTs), which have recently been successfully synthesized with lengths of several centimeters, are highly attractive as electrode materials because they can be processed into sheets of sufficient strength through simple procedures. The key to achieving high electrode performance lies in preparing a highly dispersed suspension of the UL-CNTs. However, this is a challenging task due to the very strong cohesive forces. Here, we demonstrate that the UL-CNTs can be effectively debundled and dispersed in water by applying ultrasonic irradiation at low temperatures in a dilute aqueous isopropanol solution containing an appropriate concentration of sodium dodecyl sulfate. By simply subjecting this well-dispersed mixture to suction filtration and water washing, sheet (UL-CNTS) electrodes with a clean surface and sufficient strength were produced without the use of a binder. Furthermore, we investigated their application to the electrochemical CO2 reduction reaction (eCO2RR), which is of critical importance from the perspectives of resources, energy, and the environment. The electrodes were prepared by electrodepositing Sn particles onto UL-CNTs (UL-CNT-Sn) and, for comparison, onto commercially available carbon paper, commonly used as a carbon electrode (CP-Sn). The UL-CNTS-Sn electrode produces HCOOH via eCO2RR at a rate of 132 mmol g–1 h–1, which is more than an order of magnitude higher than that of the CP-Sn electrode. This excellent performance of UL-CNTS-Sn was primarily attributed to the increase in effective reaction surface area with the high dispersion of UL-CNTs and the improvement in electrical conductivity resulting from the relaxation of strain in the UL-CNTs.