New trends in PEM water electrolysis systems development open up new technology gaps and requirements that have not been discussed before with respect to PEM water electrolysis. For example, hydrogen is considered as one of the best solutions for large-scale energy storage that comes from renewable and intermittent power sources such as wind and solar electricity [1], therefore, new megawatt PEM electrolysers are required. These trends are evident through new large-scale recent installations, especially for Power-to-Gas projects in Europe and plan to treat contaminated water at Fukushima Nuclear Power Plant. Even though PEM electrolysis has in fact been used for quite a few years now without undergoing substantial improvements over those years, however, with the new focus on hydrogen as the energy carrier there is much more interest in low-cost and high-efficiency H2 production. There are two main ways to lower the cost of hydrogen production via PEM water electrolysis: to lower the capital expenses (CAPEX) and/or to lower the operating expenses (OPEX). 3M has recently demonstrated a very effective way to address reducing the high CAPEX by increasing the range of current densities where electrolyzers can operate from a maximum of about state-of-the-art 2.0 A/cm2, to as much as 20 A/cm2 by employing a novel 3M’s proprietary Nano Structured Thin Film (NSTF) catalysts and more conductive 3M PFSA based electrolytes in the electrolyzer MEA [2]. What the cited work does not however addresses is an inherent limitation associated with 3M’s Ir-NSTF based anode catalysts, namely their near complete inertness to HOR. An additional tradeoff between performance and gas-crossover also exists when thinner PEM membranes are used. Resulting high hydrogen cross-over creates a gap in the otherwise complete list of requirements for catalyst coated membrane (CCM) that has to be met to become successfully employed in PEM water electrolyzer [3-4]. This was plainly demonstrated in the early 3M catalyst trials where fuel cell derived Pt alloy based NSTF catalysts were used for water electrolysis. The reported durability of such electrolyzer catalysts was excellent, with performance however leaving room for improvement [5]. The performance aspect of 3M electrolyzer catalysts has since been addressed by switching to electrolyzer specific (and Ir based) catalyst compositions. Such approach has left a utility gap by eliminating hydrogen-crossover mitigating components out of the catalyst. To address this problem alternative means for mitigation strategies have to be devised. In this work we will present some data, such as permeability (gas cross-over) of oxygen and hydrogen as a function of current density and other operational variables, aimed at establishing baselines for un-mitigated hydrogen cross-over of 3M electrolyzer MEAs based on 3M NSTF low-PGM loading catalyst and several types of PerFluoro Sulfonic Acid (PFSA) based PEM membranes, both widely commercially available (such as Nafion™ membranes) and their counterparts made by 3M. Dependence of such unmitigated hydrogen cross-over on applied pressure, pressure differentials, and temperature will be discussed and analyzed with reference to existing models of gas crossover [6-8](. In addition, we will also discuss challenges of in-situ and ex-situ gas –cross over measurements and also intend to present results of our initial attempts to employ alternative (to alloying Ir with Pt) mitigation strategies and gauge their respective effectiveness at various electrolyzer operating conditions and time frames. All proposed X-over mitigation strategies are selected with strong emphasis on compatibility of these candidate solutions with high speed/low cost roll-to-roll manufacturing process that will not be negatively affected by their properties and/or needed modifications. In: D. Bessarabov, H. Wang, H. Li, and N. Zhao (Eds): “PEM Electrolysis for Hydrogen Production: Principles and Applications”, CRC Press., 2015. K. A. Lewinski, S. M. Luopa, “High Power Water Electrolysis as a New Paradigm for Operation of PEM Electrolyzer”, Spring ECS Meeting, Chicago, IL, May 2015. K. A. Lewinski, et al., “NSTF Advances for PEM Electrolysis - the Effect of Alloying on Activity of NSTF Electrolyzer Catalysts and Performance of NSTF Based PEM Electrolyzers”, Fall ECS Meeting, Phoenix, AZ, Oct 2015. K. A. Lewinski, et al., ECS Transactions, 69 (17) , p. 893-917 (2015). M.K. Debe, et al., Journal of The Electrochemical Society, 159 (6), (2012), p. K165-K176. C. Mittelsteadt, M. Umbrell, 207th ECS Meeting, Abstract #770 D. Bessarabov, “Gas Permeability of Proton Exchange Membranes”, Chapter 21, in: PEM Fuel Cell Diagnostic Tools , Editor(s): H. Wang, Xiao-Zi Yuan, Hui Li, CRC Press, Pages: 443-473, 2011 M. Schalenbach at al., J. Phys. Chem. C 2015, 119, 25156−25169 Figure 1