Production of carbon nanotubes from captured carbon: An ex-ante life cycle assessment case study

Carbon capture and utilization (CCU) plays a key role in reducing greenhouse gas emissions and reaching carbon neutrality goals. This study assesses the environmental impacts of producing carbon nanotubes (CNTs) via molten salt CO2 capture and electrochemical transformation (MSCC-ET) and Catalytic Chemical Vapor Deposition (CCVD), both using CO2 as feedstock. We screened and selected technologies using a parameter-based method and conducted process modeling and an ex-ante Life Cycle Assessment (LCA) to move beyond lab- scale evaluations and included the purification steps. The MSCC-ET approach showed advantages in reducing impacts related to climate change, energy, and ozone depletion impacts, specifically suggesting lower CO2 emissions. However, traditional CCVD outperformed MSCC-ET in several other impact categories. Key contributors to the environmental impacts of MSCC-ET were found in high material use in electrolysis, purification, and electricity consumption, modeled using the Belgium grid. Moreover, including the carbon capture unit in the assessment could provide a complete view of the environmental impacts regarding CCU. For every 100 tons of MWCNTs produced, a monoethanolamine (MEA)-based carbon capture unit integrated into the MSCC-ET system could roughly reduce 716.5 tons CO2-eq. Additionally, electricity consumption was found to constitute a significant portion of the environmental impacts. Therefore, a sensitivity analysis was conducted, revealing that changes in the electricity source, catalyst used, and CO2 source significantly influenced the environmental performance of both technologies. Furthermore, we summarize practical insights from this study to guide the effective application of ex-ante LCA for carbon nanomaterials. The paper concludes with actionable recommendations for early-stage technology developers on optimizing energy use, improving material efficiency, and integrating recycling to enhance sustainability. In addition, we provide recommendations for LCA practitioners on incorporating dynamic systems to transition from lab-scale to industrial contexts, thereby bridging the gap between research and practical implementation.