Model-Based Analysis of Low Stoichiometry Operation in Proton Exchange Membrane Water Electrolysis

Proton exchange membrane water electrolysis cells are typically operated with high water flow rates in order to guarantee the feed supply for the reaction, the hydration of the ionomer phase and to homogenize the temperature distribution. However, the influence of low flow rates on the cell behavior and the cell performance cannot be fully explained. In this work, we developed a simple 1+1-dimensional mathematical model to analyze the cell polarization, current density distribution and the water flow paths inside a cell under low stoichiometry condition. The model analysis is in strong context to previous experimental findings on low water stoichiometry operations. The presented analysis shows that the low water stoichiometry can lead to dry-out at the outlet region of the anode channel, while a water splitting reaction is also present there. The simulation results show that the supply with water in this region is achieved by a net water transport from the cathode to the anode catalyst layer resulting in higher local proton resistances in the membrane and the anode catalyst laye

Degradation of proton exchange membrane (PEM) water electrolysis cells: Looking beyond the cell voltage increase

The degradation of proton exchange membrane water electrolysis cells is usually measured in a temporal increase of the cell voltage. Although this is sufficient to evaluate the stability of a system, it is less suitable for targeted material development. Thus, an overpotential-specific and temporally resolved electrochemical characterization protocol is proposed. In this the ohmic overpotential is determined with high frequency resistance measurements. These are also used in combination with polarization curves to distinguish between the kinetic and mass transport overpotentials and to determine kinetic key parameters, according to the Butler-Volmer and transition state theory. Complementary electrochemical impedance spectroscopy measurements further unravel the individual resistances. On this basis, the following statements can already be issued. The major share of the measured cell voltage increase, i.e. degradation, is of apparent nature as it is recovered once lower potentials are applied. It is suggested that this is due to changes in the oxidation states of the iridium-based catalyst. Real degradation occurs in the ohmic and mass transport overpotential mainly at higher current densities and longer operating times. The increasing kinetic overpotential with increasing operating time is primarily potential-driven. Interestingly, both the Tafel slope and the apparent exchange current density slightly increase over time. © 2019 The Author(s). Published by ECS

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

Is iridium demand a potential bottleneck in the realization of large-scale PEM water electrolysis?

Proton exchange membrane water electrolysis (PEMWE) is a key technology for future sustainable energy systems. Proton exchange membrane (PEM) electrolysis cells use iridium, one of the scarcest elements on earth, as catalyst for the oxygen evolution reaction. In the present study, the expected iridium demand and potential bottlenecks in the realization of PEMWE for hydrogen production in the targeted GW a−1 scale are assessed in a model built on three pillars: (i) an in-depth analysis of iridium reserves and mine production, (ii) technical prospects for the optimization of PEM water electrolyzers, and (iii) PEMWE installation rates for a market ramp-up and maturation model covering 50 years. As a main result, two necessary preconditions have been identified to meet the immense future iridium demand: first, the dramatic reduction of iridium catalyst loading in PEM electrolysis cells and second, the development of a recycling infrastructure for iridium catalysts with technical end-of-life recycling rates of at least 90%. © 2021 The Author(s

Modeling Overpotentials Related to Mass Transport through Porous Transport Layers of PEM Water Electrolysis Cells

Porous transport layers (PTL) are key components of proton exchange membrane water electrolysis (PEMWE) cells controlling species transport. Further optimization requires better understanding of how PTLs influence overpotentials. In this work, the data from an electrochemical overpotential breakdown is compared to a state-of-the-art model, which includes a Nernstian overpotential description, two-phase Darcian flow and advective-diffusive mass transport. Model parameters are derived from X-ray tomographic measurements, pore-scale calculations, standard models for porous materials and by transferring ex situ measurements from other materials. If the parameter set is available, model results and experimental data match well concerning PTL-related overpotentials at different current densities and operating pressures. Both experimental and modeling results suggest that mass transport through PTLs does not affect a considerable, pressure-independent share of mass transport overpotentials. Both model results and experimental findings conclude that mass transport through the cathode PTL causes overpotentials more than twice as high as through its anode counterpart. Further research opportunities regarding the relationship between PTL bulk properties and experimentally determined mass transport overpotentials are identified. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited

Evaluation of the Efficiency of an Elevated Temperature Proton Exchange Membrane Water Electrolysis System

In recent years, a significant interest has been growing in elevated temperature (ET) electrolytes for proton exchange membrane water electrolysis (PEMWE). In this study, the energy and exergy analysis developed for PEMWE has been extended to evaluate the performance of ET-PEMWE, with the model aiming to utilise the energy in the most efficient manner and also take into account potential heat losses. The latter is particularly important considering that heat losses become more pronounced with higher temperature differences. The model shows that the stack operates in autothermic mode over a considerable range of current density. Thus heating inputs to the stack and feed water become progressively unnecessary as polarization losses make up for these heating requirements. This also allows surplus heat to be utilised for secondary applications. The exergy efficiency for ET has been calculated to surpass that for low temperature (LT), with the maximum improvement reaching 3.8% points. Taking into account exergy favours higher temperature differences - a benefit which outweighs the fact that a greater quantity of thermal power is recovered in the LT system (due to higher polarization losses). This finding also shows the suitability of adopting exergy efficiency as the performance indicator for PEMWE systems. © 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited

Cost and competitiveness of green hydrogen and the effects of the European Union regulatory framework

AbstractBy 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.</jats:p

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

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