Resource requirements for the implementation of a global H2-powered aviation

In this paper, the resource requirements for the implementation of global H2-powered aviation are investigated to answer one of the main questions asked by many stakeholders in the aviation industry: Are there any resource limitations for the implementation of H2-powered aviation on a global scale? For this, the raw material, renewable energy and water demands for the deployment and operational phase are investigated on a global and regional perspective. It is found that the iridium demand for a global hydrogen economy could be critical as it would exceed not only the current annual production by a factor of 11 but also the current reserves about 1.7 times. The H2-powered aviation alone is not the main driver of iridium demand but could increase the limitations. With reduced specific raw material demands of further optimized electrolysis technologies and increased annual raw material production, the limitations especially for the iridium demand could be overcome. Renewable energy capacities and water availability are sufficient for demands from H2-powered aircraft on a global perspective. Nevertheless, the limited availability of renewable energy sources in some regions and regional water constraints may necessitate hydrogen import for certain airports. While water desalination is likely to overcome water constraints in regions close to the sea, for airports located in regions with detrimental availability of renewable energy sources the import of hydrogen is the only way to ensure a hydrogen supply for H2-powered aviation

Elucidating the effect of mass transport resistances on hydrogen crossover and cell performance in PEM water electrolyzers by varying the cathode ionomer content

An important challenge for polymer electrolyte membrane (PEM) water electrolysis is to reduce the permeation of the produced gases. This crossover affects the cell efficiency and causes safety issues. The crossover increases with current density, most probably due to mass transfer resistances. This work aims to investigate the influence of the cathode ionomer content on hydrogen crossover. Therefore, the ionomer content was varied between 10 and 40 wt% to clearly influence the mass transfer resistances. The best performance and lowest crossover was obtained for 10 wt% ionomer. However, within the observed ionomer range the mass transfer resistances increase with ionomer content that cause increases in hydrogen crossover and cell voltage. Both can be entirely explained by the same quantity of supersaturated dissolved hydrogen concentrations. These supersaturated concentrations cause higher cathode half-cell potentials, which explain the cell voltage increase and lead to higher concentration gradients across the membrane, which enhance the crossover. These findings highlight the importance of mass transfer resistances within catalyst layers in terms of crossover and performance. They constitute an important step in the clarification of the complex interplay between mass transport and voltage losses, enabling the development of novel electrode architectures for PEM water electrolyzers. © The Author(s) 2019

Understanding electrical under- and overshoots in proton exchange membrane water electrolysis cells

Ability of dynamic operation seems to be an important feature of proton exchange membrane water electrolyzers (PEMWE) to become a relevant part of the future energy system. However, only few fundamental analyzes of the dynamic behavior on short time scales are available in the literature. Therefore, this contribution aims to give insights into the most fundamental transient behavior of a PEMWE cell by an experimental analysis on the laboratory scale and a model based description of the ongoing phenomena. Experimental voltage and current controlled load step are carried out and analyzed by methods adapted from fuel cell characterization. The experimental analysis revealed that load steps are a combination of an instantaneous characteristic followed by dynamics of higher order dependent on activation, mass transfer and temperature effects. Potentiostatic downward steps to very low cell voltages can lead to current density reversal phenomena with highly negative peak current densities. By means of a simple prototype model analysis, these reversal processes are analyzed and the consequences of the phenomena are estimated. The simulation results indicate that a reversal of the cell current density can be attributed to a change of capacitive rather than faradaic currents, meaning that internal electrolysis processes are not involved. © The Author(s) 2019. Published by ECS

H2-powered aviation – Design and economics of green LH2 supply for airports

The economic competitiveness of hydrogen-powered aviation highly depends on the supply costs of green liquid hydrogen to enable true-zero CO2 flying. This study uses non-linear energy system optimization to analyze three main liquid hydrogen (LH2) supply pathways for five locations. Final liquid hydrogen costs at the dispenser supply costs could reach 2.04 USD/kgLH2 in a 2050 base case scenario for locations with strong renewable energy source conditions. This could lead to cost-competitive flying with hydrogen. Reflecting techno-economic uncertainties in two additional scenarios, the liquid hydrogen cost span at all five airport locations ranges between 1.37–3.48 USD/kgLH2, if hydrogen import options from larger hydrogen markets are also available. Import setups are of special importance for airports with a weaker renewable energy source situation, e.g., selected Central European airports. There, on-site supply might not only be too expensive, but space requirements for renewable energy sources could be too large for feasible implementation in densely populated regions. Furthermore, main costs for liquid hydrogen are caused by renewable energy sources, electrolysis systems, and liquefaction plants. Seven detailed design rules are derived for optimized energy systems for these and the storage components. This and the cost results should help infrastructure planners and general industry and policy players prioritize research and development needs

H2-powered aviation at airports – Design and economics of LH2 refueling systems

In this paper, the broader perspective of green hydrogen (H2) supply and refueling systems for aircraft is provided as an enabling technology brick for more climate friendly, H2-powered aviation. For this, two H2 demand scenarios at exemplary airports are determined for 2050. Then, general requirements for liquid hydrogen (LH2) refueling setups in an airport environment are derived and techno-economic models for LH2 storage, liquefaction and transportation to the aircraft are designed. Finally, a cost trade-off study is undertaken for the design of the LH2 setup including LH2 refueling trucks and a LH2 pipeline and hydrant system. It is found that for airports with less than 125 ktLH2 annual demand a LH2 refueling truck setup is the more economic choice. At airports with higher annual LH2 demands a LH2 pipeline & hydrant system can lead to slight cost reductions and enable safer and faster refueling. However, in all demand scenarios the refueling system costs only mark 3 to 4% of the total supply costs of LH2. The latter are dominated by the costs for green H2 produced offsite followed by the costs for liquefaction of H2 at an airport. While cost reducing scaling effects are likely to be achieved for H2 liquefaction plants, other component capacities would already be designed at maximum capacities for medium-sized airports. Furthermore, with annual LH2 demands of 100 ktLH2 and more, medium and larger airports could take a special H2 hub role by 2050 dominating regional H2 consumption. Finally, technology demonstrators are required to reduce uncertainty around major techno-economic parameters such as the investment costs for LH2 pipeline & hydrant systems. © 2022 The Author

Temperature and Performance Inhomogeneities in PEM Electrolysis Stacks with Industrial Scale Cells

In this work temperature inhomogeneities and their influence on PEMWE performance of industrial-scale stacks are investigated. Three temperature differences are examined: (i) between the inlet and outlet, (ii) in-between the cells of a stack, (iii) between the cell’s solid materials and the fluids. A validated stack model for temperature and performance is presented which is used to quantify the above-mentioned temperature fields and their influences on current density distribution and cell voltages. For a chosen scenario, with current densities of 2.0 A cm−2, fluid inlet temperatures of 60 °C and flow-rates of 0.15 kg s−1m−2, peak temperature differences amount to 8.2 K along-the-channel. This relates to inhomogeneities of current density of up to 10% inside a cell and deviations of cell voltage of 9 mV in-between cells in the center of the stack and outer cells. For higher current densities these differences increase further. More homogeneous temperatures allow operation at elevated average temperatures without exceeding temperature limitations and reduce the spread of degradation mechanisms. Hence, homogenous profiles lead to a more hole-some utilization of electrolysis stacks. Therefore, the ability to homogenize via alternative operation such as higher flow-rate, higher pressure and altered routing of fluid-flow is analyzed

Application and Analysis of a Salt Bridge Reference Electrode Setup for PEM Water Electrolysis: Towards an Extended Voltage Loss Break Down

Information on PEMWE performance is often obtained from full cell measurements. The level of detail of this information is, however, comparably low. This contribution analyzes kinetic parameters for anode and cathode reactions separately as a step towards an extended loss breakdown through a salt bridge reference electrode. The reference electrode setup is shown in detail, and qualitative measurements are discussed. OER and HER Tafel slopes and exchange current densities for both reactions are reported. An outlook on future use cases for the salt bridge reference electrode is given and supported by measurement data

On the correlation between the oxygen in hydrogen content and the catalytic activity of cathode catalysts in PEM water electrolysis

Altogether five platinum group metal (PGM) and PGM-free cathode catalysts were investigated in full PEM water electrolysis cells regarding their polarisation behaviour and their hydrogen and oxygen recombination properties. It was shown that the recombination activity of permeated oxygen and evolved hydrogen within the cathodic catalyst layer correlates with the activity of the oxygen reduction reaction (ORR) which was determined ex situ with linear sweep voltammetry. We found that the investigated PGM-free cathode catalysts had a low activity for the ORR resulting in higher measurable oxygen in hydrogen volume fractions compared to the PGM catalysts, which are more active for the ORR. Out of the three investigated PGM-free catalysts, only one commercially available material based on a Ti suboxide showed a similar good polarisation behaviour as the state of the art cathode catalyst platinum, while its recombination activity was the lowest of all catalysts. In addition to the recombination of hydrogen and oxygen on the electrocatalysts, we found that the prevalent carbon-based cathodic porous transport layers (PTL) also offer catalytically active recombination sites. In comparison to an inactive PTL, the measurable oxygen flux using carbon-based PTLs was lower and the recombination was enhanced by microporous coatings with high surface areas. © 2021 The Author(s)

On the Correlation between the Oxygen in Hydrogen Content and the Catalytic Activity of Cathode Catalysts in PEM Water Electrolysis

Altogether five platinum group metal (PGM) and PGM-free cathode catalysts were investigated in full PEM water electrolysis cells regarding their polarisation behaviour and their hydrogen and oxygen recombination properties. It was shown that the recombination activity of permeated oxygen and evolved hydrogen within the cathodic catalyst layer correlates with the activity of the oxygen reduction reaction (ORR) which was determined ex situ with linear sweep voltammetry. We found that the investigated PGM-free cathode catalysts had a low activity for the ORR resulting in higher measurable oxygen in hydrogen volume fractions compared to the PGM catalysts, which are more active for the ORR. Out of the three investigated PGM-free catalysts, only one commercially available material based on a Ti suboxide showed a similar good polarisation behaviour as the state of the art cathode catalyst platinum, while its recombination activity was the lowest of all catalysts. In addition to the recombination of hydrogen and oxygen on the electrocatalysts, we found that the prevalent carbon-based cathodic porous transport layers (PTL) also offer catalytically active recombination sites. In comparison to an inactive PTL, the measurable oxygen flux using carbon-based PTLs was lower and the recombination was enhanced by microporous coatings with high surface areas.BMBF, 03SF0536F, Verbundvorhaben PowerMEE: Lebensdauer- und Leistungserhöhung (POWER) von Polymerelektrolytmembranelektrolyseuren durch Hochleistungsmembranelektrodeneinheiten (MEE) - Teilprojekt: Herstellung von Membran Elektrodeneinheiten und tomographische Analys