## 5 Conclusions  
In the current work, an investigation of the potential of a fuel cell system working with O\({}_{2}\)-enriched air generated on-board was carried out. For this purpose, data regarding the sensitivity of a certain fuel cell technology with respect to the O\({}_{2}\) molar fraction in the cathode stream were extracted from the literature. Then, a performance map of a commercial O\({}_{2}\)-N\({}_{2}\) separation membrane was used to evaluate on-board O\({}_{2}\)-enriched air generation. Based on this, the required air mass flow and power consumption per active area of the fuel cell stack were quantified as a function of the O\({}_{2}\) enrichment level and the fuel cell operating conditions. Additionally, two potential sources for energy recovery, one in the H\({}_{2}\) supply line and another in the outlet of the O\({}_{2}\)-N\({}_{2}\) separation membrane, were identified and evaluated. The following conclusions were drawn from this study:  
* The main advantage from O\({}_{2}\) enrichment in terms of fuel cell stack efficiency is found at medium-to-high current intensity levels, where a higher O\({}_{2}\) concentration helps to reduce the losses related to diffusion and chemical kinetic aspects at the cathode side. In contrast, the margin for improvement is very small at low current intensity values. Additionally, the mass flow per unit of active area to be supplied to the fuel cell cathode can be reduced if the same level of excess O\({}_{2}\) ratio is applied. * Higher levels of O\({}_{2}\) enrichment imply the need to supply the O\({}_{2}\)-N\({}_{2}\) separation membrane with a mass flow that is significantly higher than the useful (permeate) O\({}_{2}\)-enriched generated stream. This reverses the reduction in cathode mass flow seen at the stack level, significantly increasing the compressor's power consumption and limiting the potential advantages of the system compared with a traditional fuel-cell system. As a consequence, the new proposed concept consumes additional power, which is mostly concentrated in the second stage of compression, driven by an electric motor. * From the sensitivity analyses performed, it was concluded that the factors that affect the most the system level efficiency are, in this order, the pressure ratio in the membrane and the compressor efficiency. Both are related to the compression work, highlighting the fact that the proposed system's potential increases as more efficient O\({}_{2}\)-N\({}_{2}\) separation membrane technologies are developed. * Regarding the cathode's excess O\({}_{2}\) ratio, a higher O\({}_{2}\) partial pressure inside the fuel cell reduces \(\lambda_{\text{O}_{2}}\) below the value of 1.5 used in the base case. Instead, the penalty induced when working with higher excess O\({}_{2}\) ratio is limited owing to the energy recovery. * In general, relatively high levels of enrichment are preferable for the technology, despite the increase in compressor power consumption, as long as it can be guaranteed that at least one-third of this consumption can be recovered from the system. Based on the results of this study, the following research areas are proposed for further investigation:
* Conduct an experimental analysis to investigate the effects of the O\({}_{2}\) molar fraction (21-40%) and excess O\({}_{2}\) ratio (1-2) in the cathode stream to ascertain the advantages of O\({}_{2}\) enrichment on cell performance at increased current densities and assess the feasibility of reducing the cathode's excess O\({}_{2}\) ratio. This research will also aid in developing a 1D fuel cell model with the ability to analyze reactive species depletion, thereby enhancing system efficiency. * Evaluate other O\({}_{2}\) enrichment separation membrane technologies with higher selectivity and analyze the potential to reduce the membrane working pressure and/or the ratio of inlet to permeate mass flows.