## 6 Conclusions and Outlook  
Carbon capture on ships during maritime operation is an emerging measure in order to achieve the necessary CO\({}_{2}\) emissions reduction in a key carbon-intensive sector. The constraints imposed by the ship environment such as the available volume, energy supply and most importantly, on-board safety, constitute a difficult to tackle combination of process limitations that need to be countered using compact but efficient solutions. Post-combustion CO\({}_{2}\) capture using modular membrane contactors offers a number of advantages together with the necessary technological maturity for direct on-board application. However, the true bottleneck of such a system remains the choice of the solvent for reasons directly associated with both process efficiency as well as on-board safety. In this work, a comprehensive review of the various classes of CO\({}_{2}\) capture solvents, based on their operational, thermodynamic, and kinetic characteristics, was carried out, utilizing appropriately selected KPIs targeted to the shipping industry's demands. A qualitative analysis of solvent performance, performed on the basis of the selected KPIs in order to assess their compliance with the maritime industry requirements, revealed that presently, no solvent is able to fully satisfy all the conflicting objectives of an \"ideal\" on-board operation. Benchmark solvents such as primary and secondary amines appear to adequately serve the shipping needs, showing good compatibility with the majority of the selected KPIs and moreover have the advantage of maturity, resulting from their extensive on-shore operation. On the other hand, recently developed solvents, such as phase change solvents and ionic liquids, which excel in operational performance, simply lack the maturity to be safely considered as CO\({}_{2}\) capture candidates in ships. A multi-objective analysis among the solvents with respect to the most crucial KPIs showed exactly this; under present operating conditions and protocols, secondary amines showed an overall superior performance being the only solvent type that outperformed two other solvent classes. However, if the bottleneck of maturity is removed, ionic liquids clearly prevailed by exceling in all considered KPIs over almost all other types of solvents. The latter directly points toward the need to accelerate research on new and on-demand solvents such as ILs in order to allow their efficient penetration in large-scale industrial applications like the maritime environment. Safe results regarding a direct and detailed quantitative ranking are difficult to extract based on the present knowledge due to the lack of experience from real-time operation of on-board solvent-based membrane CO\({}_{2}\) capture systems. Furthermore, the large catalogue of solvents presented here has been so far used mainly in configurations with separation columns. As such, data on the potential interactions of the chemical solvent with the employed membrane and their effect on the operation on the compact contactor system need to be enriched through targeted solvent-membrane pair studies. Finally, the present study focused on using KPIs which are mostly ship-oriented. However, the evaluation of the overall system performance has to account for indices that equally comprise the effects of both solvent characteristics (e.g., thermodynamic, thermochemical, and hydrodynamic performance) together with the prerequisites for efficient on-board operation. To this end, future studies that target the formulation of appropriate indices correlating the solvent properties with the desired on-board operating characteristics may greatly facilitate the assessment and subsequent penetration of solvent-based membrane CO\({}_{2}\) capture systems on ships. Conceptualization, T.D., A.A., D.K. and G.S. ; methodology, T.D., A.A., D.K., G.S. and C.G. ; data curation, T.D., A.A., D.K., C.G., L.N., K.B., S.E., C.P., H.R., U.L., D.H. and G.P. ; writing--original draft preparation, T.D., A.A., D.K. and C.G.