Life Cycle Greenhouse Gas Emissions of Long and Pure Carbon Nanotubes Synthesized via On-Substrate and Fluidized-Bed Chemical Vapor Deposition

Improvement in chemical vapor deposition (CVD) methods to efficiently synthesize high-quality carbon nanotubes (CNTs) is critical to realize the full potential of commercial application of CNTs. Meanwhile, methods with less environmental impact from the synthesis process and the use of material and energy are preferable for sustainable chemistry. However, they are rarely quantified in material sciences. Here, we provide a systematic investigation on the life cycle greenhouse gas (GHG) emissions of two emerging CNT synthesis methods: on-substrate and fluidized-bed CVD. Based on years-long experiments, we show the impacts of important configurations that include oxidative additives selection (CO2 or H2O), growth modes in reactors [two-dimensional (2D) flat-plate or three-dimensional (3D) spherical], catalyst deposition methods (sputtering or CVD), and carrier/purging gases (Ar or N2). We find that the life cycle GHG emissions of CNT production ranged from 28.55 (on-substrate) to 0.48 (fluidized-bed) kg CO2e/g CNTs. Considering scaling up to industrial processes, the CNT production can be as low as present carbon fiber (0.02 kg CO2e/g materials). The downstream stages of CNT are unclear as the practical usage is still in development. We therefore outline the impacts and prospects of some selected bulk, ensemble, and individual uses of CNTs in the closing remarks.