# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 5 Life Cycle Assessment
### Carbon Nanotubes  
Climate change provoked by excessive worldwide CO\({}_{2}\) emissions into the atmosphere has recently attracted global concern. The actual large-scale methods generally utilize monoethanolamine for the capture of CO\({}_{2}\). Nonetheless, CO\({}_{2}\) regeneration requires high temperature values, and every adsorption/desorption cycle is an energy-demanding pro cess, as it also entails material losses. The amines with enhanced capture capability are currently implemented as adsorbents to overcome existing limitations. Nevertheless, the EI caused by the adsorbents were not often discussed in the past. Wu et al. [175] determined the EI of new CO\({}_{2}\) capture adsorbents by synthesizing CNTs with adsorbed and covalently bonded polyethyleneimine and compared them with those associated with the conventional monoethanolamine capturer using the LCA technique. Carbon payback periods were also studied to achieve an improved knowledge on the scalability of the evaluated new materials. The obtained results suggested that the use of CNTs contributed to most of the EI for CNTs with polyethyleneimine. The corresponding carbon payback periods are over 40 times longer than when applying monoethanolamine during the production stage. Nonetheless, the energy consumption of the adsorbed polyethyleneimine may be reduced by up to 60% in comparison to the monoethanolamine in each adsorption/desorption cycle due to its lower heat transfer capability. Additionally, the rate of CO\({}_{2}\) remission for CNT-polyethyleneimine is two-fold greater than that associated with monoethanolamine, revealing its potential for application in large-scale CO\({}_{2}\) captures. In essence, the study indicated that solid amine is a promising option for CO\({}_{2}\) capture but nevertheless requires several further ameliorations. Future investigation works should be focused on reducing the initial manufacturing of the nanomaterials and increasing the product lifespan based on their environmental tradeoffs. Moreover, the authors highlighted that the emphasis on only the CO\({}_{2}\) capture effectiveness by the innovative materials may not be totally adequate, and more extensive studies should be performed based on the comparison during the nanomaterial LCA. The life cycle environmental and health impacts were modeled using SimaPro software with Ecoinvent and United States Life Cycle Inventory databases. Among others, the midpoint categories that were examined were ozone depletion (kg CFC-11 eq. ), global warming (kg CO\({}_{2}\) eq. ), carcinogenic (CTUh), respiratory impact (kg PM2.5 eq. ), ecotoxicity (CTUe), and fossil fuel depletion (M) surplus). The researchers argued that the energy penalty, functional units, and scalability among other topics are the main limitations and challenges that are overlooked in the capture and storage of carbon studies [176]. The obtained results suggested that carbon payback depends appreciably on the lifetime of the CNT-polyethyleneimine. Many published works revealed a consistent recovery up to 100 cycles; however, the findings of this work indicated that a total of more than 900 adsorption/desorption cycles are needed to achieve a breakeven CO\({}_{2}\) payback for physically adsorbed CNT-polyethyleneimine. Moreover, to optimize fabrication and improve the CO\({}_{2}\) adsorption capability, further studies should also be carried out to ensure the maximum lifetime capability of the CNT-polyethyleneimine under extreme conditions to receive more payback outcomes. The researchers have simplified the analysis by assuming no extra environmental burden is induced by the degradants of the monoethanolamine. The monoethanolamine degradant dilution may decrease their CO\({}_{2}\) capture capability. Such practical limitations may reduce the effectiveness of the liquid phase capturing solutions and highlight the advantages of the solid amine for CO\({}_{2}\) remission.