Experimental and Model-based Investigations on Gas Crossover in Polymer Electrolyte Membrane Water Electrolyzers

Gas crossover of the product gases, hydrogen and oxygen, through the thin membranes of polymer electrolyte membrane (PEM) water electrolyzers is a major challenge for its further commercialization. It causes safety issues, efficiency losses and triggers degradation mechanisms. In particular, the e ects on gas crossover during PEM water electrolysis are not fully understood yet. In context of the present work, these effects will be investigated experimentally and model-based. In the first part of the dissertation the infuences of different operating conditions: pressure, temperature as well as current density and changes of the electrode structure on gas crossover are experimentally investigated. It is shown that both hydrogen and oxygen crossover increase strongly with current density. However, an increase of the cathode pressure shows no significant infuence on the qualitative extend of this correlation. Thus it is assumed that the underlying mechanisms for this crossover increase are also independent of pressure. This finding stands in contrast to the common explanation in the literature. It is commonly assumed that the crossover increases due to local pressure enhancements. However, since gas transport in general is strongly dependent on pressure this approach contradicts the experimental findings. An alternative explanatory approach is discussed within this work, in which the focus is on the transport of dissolved gases from the catalyst particles through the ionomer to the pore space. Transport limitations on this path, which are independent of pressure, leads to supersaturated dissolved gas concentrations. These concentrations increase with current density, which leads to higher concentration gradients across the membrane and thus to gas crossover increases. The experimental variation of the cathode ionomer content supports this explanation approach. Higher ionomer contents lead to signi cant steeper crossover increases, which can be explained by the increase of the transport resistances due to thicker ionomer films. The investigation of the cell voltage reveals a direct correlation of the increased crossover and mass transport based voltage losses. In the second part, a comprehensive one-dimensional model is formulated to investigate the experimental findings in more detail. The focus is on the previously described theory of supersaturated dissolved gas concentration within the catalyst layers. The simulation results based on literature parameters strengthen this theory. The local profiles reveal that the supersaturated concentrations occur directly at the membrane/catalyst layer interfaces, where the local gas formation is maximal. Furthermore, the complex interactions between ohmic, kinetic and mass transport losses of the catalyst layers are investigated. Finally, the gas crossover is studied by a system consideration with regard to safety and efficiency

Hydrogen Crossover in PEM Water Electrolysis at Current Densities up to 10 A cm−2

Hydrogen crossover poses a critical issue in terms of the safe and efficient operation in polymer electrolyte membrane water electrolysis (PEMWE). The impact of key operating parameters such as temperature and pressure on crossover was investigated in the past. However, many recent studies suggest that the relation between the hydrogen crossover flux and the current density is not fully resolved. This study investigates the hydrogen crossover of PEMWE cells using a thin Nafion 212 membrane at current densities up to 10 A cm−2 and cathode pressures up to 10 bar, by analysing the anode product gas with gas chromatography. The results show that the hydrogen crossover flux generally increases over the entire current density range. However, the fluxes pass through regions with varying slopes and flatten in the high current regime. Only considering hydrogen diffusion as the single transport mechanism is insufficient to explain these data. Under the prevailing conditions, it is concluded that the electro-osmotic drag of water containing dissolved hydrogen should be considered additionally as a hydrogen transport mechanism. The drag of water acts opposite to hydrogen diffusion and has an attenuating effect on the hydrogen crossover in PEMWE cells with increasing current densities

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

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

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

Communication - Proving the Importance of Pt-Interlayer Position in PEMWE Membranes for the Effective Reduction of the Anodic Hydrogen Content

Gas crossover through the membrane poses a significant challenge to proton exchange membrane water electrolysers. This work investigates the influence of the position of platinum-based recombination interlayers integrated in the membrane on the anodic hydrogen in oxygen content. The results show that all interlayer positions reduce the anodic hydrogen content without performance losses compared to the reference without interlayer. However, an interlayer positioned closer to the anode is more effective than closer to the cathode. Further, the effect of the interlayer is more pronounced with increasing anode pressure. © 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited

Effect of Recombination Catalyst Loading in PEMWE Membranes on Anodic Hydrogen Content Reduction

Integrating platinum-based recombination catalysts into proton exchange membrane water electrolysis systems effectively reduces the anodic hydrogen content. We studied the effect of the platinum loading of an interlayer close to the anode within the membrane on the anodic hydrogen in oxygen content. For the investigated Pt-loadings between 1 μgPt cm−2 and 140 μgPt cm−2, the results revealed that for a 110 μm membrane, 7 μgPt cm−2 were sufficient to allow a safe operation at cathode pressures up to 10 bar. A further increase of the Pt-loading did not significantly improve the reduction of the anodic hydrogen in oxygen content

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)

Compact silicon-based attenuated total reflection (ATR) sensor module for liquid analysis

Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids and solids, and a variety of ATR accessories for laboratory FTIR spectrometers are available. However, ATR spectroscopy is rarely found in industrial processes, where compact, robust, and cost-effective sensors for continuous operation are required. Here, narrowband photometers are more appropriate than FTIR instruments. We show the concept and implementation of a compact Si-based ATR module with a four-channel microelectromechanical systems (MEMS) detector. Measurements of liquid mixtures demonstrate the suitability for applications in the chemical industry. Apart from sapphire (for wavelengths below 5 µm) and diamond (extending to the far-infrared region), most materials for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, or aggressive cleaning agents. However, using diamond coatings on Si improves the stability of the sensor surface. In addition, by proper choice of incidence angle and coating thickness, an enhancement of the ATR absorbance is theoretically expected and demonstrated by first experiments using a compact sensor module with a diamond-coated Si ATR element.</p