## 1 Introduction
This technology, currently used for industrial N\({}_{2}\) production, consumes less energy than other techniques when intermediate levels of O\({}_{2}\) purity are required [36]. Such polymeric membranes are based on a hollow fiber system, where air passes through small channels, enabling the selective permeation of O\({}_{2}\) molecules while blocking other gases [37]. However, to the best of the authors' knowledge, no previous work has analyzed the potential of such membranes in the context of fuel-cell system operation. In order to determine the potential of on-board O\({}_{2}\) enrichment with polymeric membranes for fuel-cell systems, it is crucial to consider the energy consumption associated with the separation of O\({}_{2}\) from atmospheric air. In this sense, in order to achieve a proper N\({}_{2}\)-O\({}_{2}\) separation, such membranes need to maintain a minimum pressure ratio between the inlet of the membrane and the permeate (i.e., the O\({}_{2}\)-enriched stream) flow. Consequently, this entails the generation of significantly higher pressure upstream of the separation membrane than the one arriving the fuel cell stack. Moreover, a significantly higher inlet mass flow rate must be supplied to the membrane compared to the desired O\({}_{2}\)-enriched stream, with the excess mass flow primarily comprising N\({}_{2}\), not directly useful for the current application. The ratio between the permeate stream mass flow and the inlet air mass flow is defined as the stage cut ratio. Both of these factors, related to membrane selectivity, contribute to an increase in compressor power consumption. Therefore, in order to assess the real potential of on-board O\({}_{2}\) enrichment for fuel cell systems, it is necessary to carefully study the gas management systems to reduce net gas consumption and improve gas integration in the overall balance of plant. In this study, an evaluation of the potential benefits to the fuel cell balance of plant including on-board O\({}_{2}\)-enriched generation was performed. For this purpose, the performance of a fuel cell stack was estimated using data from the literature, while the energy associated to N\({}_{2}\)/O\({}_{2}\) separation was extracted using a working map from a commercially available polymeric membrane. Then, two sources for energy recovery, one in the hydrogen supply and another in the reentate flow at the separation membrane outlet, were designed and evaluated. The performance of this system in terms of the balance of plant efficiency was assessed against an atmospheric fuel cell fed with standard air. With these considerations, the main objectives of the current work were to:  
* Quantify the effect of O\({}_{2}\) enrichment on a PEMFC stack efficiency and mass flow with constant O\({}_{2}\) and H\({}_{2}\) excess ratio. * Evaluate the power consumption linked to the operation of a polymeric membrane for O\({}_{2}\) and N\({}_{2}\) separation. * Propose a novel balance for a plant layout for on-board O\({}_{2}\) enrichment and identify the potential sources for energy recovery. * Analyze the sensitivity of the new system efficiency regarding its main design and operational parameters to evaluate its potential with respect to a standard architecture. The paper structure is divided into five different sections. Section 2 describes the proposed architecture of the balance of a plant with O\({}_{2}\) enrichment compared to a standard system from a current vehicle application and summarizes the main hypothesis and equations involved in the evaluation of the different components, with particular attention to the fuel cell stack and the separation membrane. Section 3 describes the contributions from the fuel cell stack, the power consumption of the compressors, and the energy recovery to the overall balance of plant efficiency as a function of the level of O\({}_{2}\) enrichment.