This study provides a direct comparison of hydrogen crossover in PEM (Nafion 117) and alkaline water electrolysis (Zirfon) at a temperature of <mml:math display="inline"><mml:mrow><mml:msup><mml:mn>60</mml:mn><mml:mo>∘</mml:mo></mml:msup><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>60∘C applying state-of-the-art separating unit materials. To this end, occurring crossover mechanisms are described first, before experimental data of the anodic hydrogen content are shown in dependence of current density, system pressure and process management strategy. The results suggest that permeation in PEM electrolyzers is mainly governed by diffusion due to a supersaturated concentration of dissolved hydrogen within the catalyst layer, showing a share of 98} of the total permeation flux at <mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi mathvariant="normal">A</mml:mi><mml:mspace width="0.16em"></mml:mspace><mml:msup><mml:mi>cm</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>1Acm−2 and atmospheric pressure. Permeation in alkaline electrolyzers also exhibits a significant influence of supersaturation, but the overall crossover is mainly influenced by mixing the electrolyte cycles, which makes up a share of 90} at <mml:math display="inline"><mml:mrow><mml:mn>0.7</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi mathvariant="normal">A</mml:mi><mml:mspace width="0.16em"></mml:mspace><mml:msup><mml:mi>cm</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>0.7Acm−2 and <mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi>bar</mml:mi></mml:mrow></mml:math>1bar . Generally it becomes evident that hydrogen permeation across the separating unit is more than one order of magnitude smaller in alkaline electrolysis, which is mainly a consequence of the significantly lower hydrogen solubility in concentrated KOH electrolyte. Finally, this study concludes with an assessment of the impact of separating unit thickness and provides mitigation strategies to reduce hydrogen crossover.

Ambitious targets to reduce emissions caused by aviation in the light of an expected ongoing rise of the air transport demand in the future drive the research of propulsion systems with lower CO₂ emissions. Regional hybrid electric aircraft (HEA) powered by conventional gas turbines and battery powered electric motors are investigated to test hybrid propulsion operation strategies. Especially the role of the battery within environmentally friendly concepts with significantly reduced carbon footprint is analyzed. Thus, a new simulation approach for HEA is introduced. The main findings underline the importance of choosing the right power-to-energy-ratio of a battery according to the flight mission. The gravimetric energy and power density of the electric storages determine the technologically feasibility of hybrid concepts. Cost competitive HEA configurations are found, but do not promise the targeted CO₂ emission savings, when the well-to-wheel system is regarded with its actual costs. Sensitivity studies are used to determine external levers that favor the profitability of HEA.

In the near future, the challenges to reduce the economic and social dependency on fossil fuels must be faced increasingly. A sustainable and efficient energy supply based on renewable energies enables large-scale applications of electro-fuels for, e.g., the transport sector. The high gravimetric energy density makes liquefied hydrogen a reasonable candidate for energy storage in a light-weight application, such as aviation. Current aircraft structures are designed to accommodate jet fuel and gas turbines allowing a limited retrofitting only. New designs, such as the blended-wing-body, enable a more flexible integration of new storage technologies and energy converters, e.g., cryogenic hydrogen tanks and fuel cells. Against this background, a tank-design model is formulated, which considers geometrical, mechanical and thermal aspects, as well as specific mission profiles while considering a power supply by a fuel cell. This design approach enables the determination of required tank mass and storage density, respectively. A new evaluation value is defined including the vented hydrogen mass throughout the flight enabling more transparent insights on mass shares. Subsequently, a systematic approach in tank partitioning leads to associated compromises regarding the tank weight. The analysis shows that cryogenic hydrogen tanks are highly competitive with kerosene tanks in terms of overall mass, which is further improved by the use of a fuel cell.

By passing the delegated acts supplementing the revised Renewable Energy Directive, the European Commission has recently set a regulatory benchmark for the classification of green hydrogen in the European Union. Controversial reactions to the restricted power purchase for electrolyser operation reflect the need for more clarity about the effects of the delegated acts on the cost and the renewable characteristics of green hydrogen. To resolve this controversy, we compare different power purchase scenarios, considering major uncertainty factors such as electricity prices and the availability of renewables in various European locations. We show that the permission for unrestricted electricity mix usage does not necessarily lead to an emission intensity increase, partially debilitating concerns by the European Commission, and could notably decrease green hydrogen production cost. Furthermore, our results indicate that the transitional regulations adopted to support a green hydrogen production ramp-up can result in similar cost reductions and ensure high renewable electricity usage.

Polymer electrolyte membrane water electrolyzers (PEMWE) are currently restricted to an operating temperature range between 50 to 80 °C. This review shows that elevated temperature (ET) above 90 °C can be advantageous with respect to i) reduced cell voltages, ii) a reduction of catalyst loading or possibly the employment of less noble electrocatalysts, and iii) a greater potential for waste heat utilization when the electrolyzer is operated in exothermal mode (when the cell voltage is higher than the thermoneutral voltage). Together with presenting an overview of the materials and components utilized in elevated temperature PEMWE under liquid and steam operation, this article summarizes the experimental and modeling performances reported to date, highlights the challenges ahead, and suggests aspects, which will need to be considered to improve the performance at elevated temperature. Key points, which arise from this work are the extensive need of re‐assessing the material selection both for the cell components and also at a system level, the effects and optimization of working with steam operation, and in the long run, the need for techno‐economic analyses to ultimately assess whether efficiency gains will truly translate to a cost‐effective technology alternative.