• Energy Research

Abstract In this paper we report an approach to improve water management of commercial GDLs by introducing hydrophobicity patterns. Specifically, line and grid patterns have been created in the MPL side by laser radiation. For an in-depth investigation of these modified GDLs the current density distribution was monitored during fuel cell operation. Additionally, the physical properties of these materials were investigated by a number of ex situ methods such as Fourier transform infrared microscopy, electrochemical impedance spectroscopy and water vapor sorption. Furthermore, a comparison of the physical properties of the patterned GDLs with chemically modified GDLs (treated in H2SO4 and H2O2) is provided. Our results show a clearly improved homogeneity of current density distribution of the patterned GDLs compared to untreated GDLs. This observation is likely due to a reduced local hydrophobicity which facilitates water diffusion along the flow field of the fuel cell. However, performance of the fuel cell was not affected by the MPL irradiation. Graphical Abstract

The durability and performance of electrochemical energy converters, such as fuel cells and electrolysers, are not only dependent on the properties and the quality of the used materials. They strongly depend on the operational conditions. Variations in external parameters, such as flow, pressure, temperature and, obviously, load, can lead to significant local changes in current density, even local transients. Segmented cell technology was developed with the purpose to gain insight into the local operational conditions in electrochemical cells during operation. The operando measurement of the local current density and temperature distribution allows effective improvement of operation conditions, mitigation of potentially critical events and assessment of the performance of new materials. The segmented cell, which can replace a regular bipolar plate in the current state of the technology, can be used as a monitoring tool and for targeted developments. This article gives an overview of the development and applications of this technology, such as for water management or fault recognition. Recent advancements towards locally resolved monitoring of humidity and to current distributions in electrolysers are outlined.

Abstract This work provides single cell durability tests of membrane electrode assemblies in dynamic operation regularly interrupted by recovery procedures for the removal of reversible voltage losses. Degradation rates at different loads in one single test can be determined from these tests. Hence, it is possible to report degradation rates versus current density instead of a single degradation rate value. A clear discrimination between reversible and irreversible voltage loss rates is provided. The irreversible degradation rate can be described by a linear regression of voltage values after the recovery steps. Using voltage values before refresh is less adequate due to possible impacts of reversible effects. The reversible contribution to the voltage decay is dominated by an exponential decay after restart, eventually turning into a linear one. A linear-exponential function is proposed to fit the reversible voltage degradation. Due to this function, the degradation behavior of an automotive fuel cell can be described correctly during the first hours after restart. The fit parameters decay constant, exponential amplitude and linear slope are evaluated. Eventually, the reasons for the voltage recovery during shutdown are analyzed showing that ionomer effects in the catalyst layer and/or membrane seem to be the key factor in this process.

Abstract The durability of polymer electrolyte membrane fuel cells (PEMFCs) needs to be further improved to cope with application requirements and economic competitiveness. This paper highlights the challenges in the reliable determination of degradation rates and lifetime. The reliable evaluation of performance degradation rates is fundamental to quantify and benchmark durability and to allow comparisons between PEMFC durability tests performed using different materials or in different laboratories. The use of efficient recovery procedures enables the discrimination of reversible and irreversible voltage losses and facilitates the understanding of recovery mechanisms. In the end, recent contributions about lifetime diagnoses and prediction are presented, which are promising to be implemented in PEMFC applications.

Abstract In this study, the effects of temperature, pressure and relative humidity (RH) on hydrogen crossover rate from anode to cathode of a PEMFC is investigated. Segmented cells are used to measure the local hydrogen crossover current density ( j H 2 cross ) distribution. The results present approximate linear increase of the hydrogen crossover rate with increasing temperature and hydrogen back pressure with rates of 0.038 mA cm−2 K−1 and 3.33 mA cm−2 bar−1, respectively. Generally, slightly increased H2 crossover is observed in gas inlet areas than cell average. Unlike the approximate linear relationship between temperature or pressure with hydrogen crossover, the effect of relative humidification on hydrogen crossover is more complex with different increasing rate before fully humidification and dramatic decline at excessive humidification. It is demonstrated that segmented cells can be advantageously applied to study local H2 crossover of intact MEAs.

Abstract To both extend system lifetime and enable reliable performance benchmarking, recovery procedures are of great importance to distinguish and mitigate the reversible and irreversible performance degradation of polymer electrolyte membrane fuel cells. In this work, three common recovery procedures available in the literature (JRC-based protocol, DOE-based protocol, and overnight rest) being part of a load cycling durability test are characterized by polarization curves and electrochemical impedance spectroscopy. Such direct comparison using same conditions and material has not been published so far. To compare the relative recovery of the three recovery procedures the recovery related to the last operation period, to the beginning of the whole test, and the non-recovered performance loss within each operation period are assessed. Moreover, the mechanisms leading to the various recovery effects are analyzed. Generally, with the contribution of gas purging to water removal, all three recovery procedures reduce greatly mass transfer resistance and recover most of the performance loss in the high current density range. At lower current density, the three procedures differ substantially. In the case of JRC-based protocol, the kinetic losses are recovered by the reduction of Pt oxides and structure change of ionomer by reducing the cathode potential and fuel cell temperature, respectively. The overnight rest results in similar recovery of the performance loss. The DOE-based protocol leads to relatively low recovery of losses in the kinetic region of the polarization curves. Additionally, the effect of the cell operating history is considered.