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

Gamma-Aminobutyric Acid and Glutamate Concentrations in the Striatum and Anterior Cingulate Cortex Not Found to Be Associated with Cognitive Flexibility

Behavioral flexibility and goal-directed behavior heavily depend on fronto-striatal networks. Within these circuits, gamma-aminobutyric acid (GABA) and glutamate play an important role in (motor) response inhibition, but it has remained largely unclear whether they are also relevant for cognitive inhibition. We hence investigated the functional role of these transmitters for cognitive inhibition during cognitive flexibility. Healthy young adults performed two paradigms assessing different aspects of cognitive flexibility. Magnetic resonance spectroscopy (MRS) was used to quantify GABA+ and total glutamate/glutamine (Glx) levels in the striatum and anterior cingulate cortex (ACC) referenced to N-acetylaspartate (NAA). We observed typical task switching and backward inhibition effects, but striatal and ACC concentrations of GABA+/NAA and Glx/NAA were not associated with cognitive flexibility in a functionally relevant manner. The assumption of null effects was underpinned by Bayesian testing. These findings suggest that behavioral and cognitive inhibition are functionally distinct faculties, that depend on (at least partly) different brain structures and neurotransmitter systems. While previous studies consistently demonstrated that motor response inhibition is modulated by ACC and striatal GABA levels, our results suggest that the functionally distinct cognitive inhibition required for successful switching is not, or at least to a much lesser degree, modulated by these factors