• Energy Research

The aim of this study is to provide a better understanding of performance degrading mechanisms occurring when a proton exchange membrane water electrolyzer (PEM-WE) is coupled with renewable energies, where times of operation and idle periods alternate. An accelerated stress test (AST) is proposed, mimicking a fluctuating power supply by operating the electrolyzer cell between high (3 A cm(geo)(-2)) and low current densities (0.1 A cm(geo)(-2)), alternating with idle periods during which no current is supplied and the cell rests at open circuit voltage (OCV). Polarization curves, periodically recorded during the OCV-AST, reveal an initial increase in activity (approximate to 50 mV after 10 cycles) followed by a significant decrease in performance during prolonged OCV cycling due to an increasing high frequency resistance (HFR) (approximate to 1.6-fold after 718 cycles). These performance changes can clearly be related to the OCV periods, since they are not observed in a reference experiment where the OCV period is replaced by a potential hold at 1.3 V. The origin of the phenomena, which are responsible for the initial performance gain as well as the subsequent decay are analyzed via detailed electrochemical and physical characterization of the MEAs, and an operating strategy to prevent performance degradation is proposed. (c) The Author(s) 2019. Published by ECS.

In this work, ≈25 μm thin titanium microporous layers (MPLs) with ≈2 μm small pores and low surface roughness were coated and sintered on top of ≈260 μm thick commercial titanium-powder-sinter sheets with ≈16 μm pores, maintaining a porosity of ≈40% in both layers. Serving as porous transport layers (PTLs) on the anode side in proton exchange membrane water electrolyzers (PEMWEs), these pore-graded, two-layer sheets (“PTL/MPL”) are compared to single-layer PTLs in single-cell PEMWEs. The PTL/MPL samples prepared here give a 3–6 mΩ cm2 lower high-frequency resistance (HFR) compared to the as-received single-layer PTL, which is attributed to a partial reduction of the TiO2 surface passivation layer during the MPL sintering process. For ≈1 μm thin anodes with an iridium loading of ≈0.2 mgIr cm−2, the use of an MPL leads to a ≈24 mV improvement in HFR-free cell voltage at 6 A cm−2. As no such benefit is observed for ≈9 μm thick anodes with ≈2.0 mgIr cm−2, mass transport resistances within the PTL/MPL play a minor role. Possible reasons for the higher catalyst utilization in ultra-thin electrodes when using an MPL are discussed. Furthermore, an MPL provides superior mechanical membrane support, which is particularly relevant for thin membranes.

Abstract Neutron depth profiling (NDP) is a non-destructive, isotope-sensitive profiling technique to monitor concentration profiles in almost any material matrix. Since NDP is sensitive to 6Li and lithium is widely used for different material science applications such as ceramics, optical waveguides or energy-storage systems, NDP offers answers to a broad spectrum of research questions. In the present work, the recently developed instrument N4DP at MLZ is used to address two research questions which are hardly accessible by conventional analytical techniques. First, the homogeneity of lithium formations within lithium niobate thin films for optical waveguide applications is investigated. Afterwards, the accumulation of inactive lithium in the solid-electrolyte-interphase (SEI) of silicon-graphite electrodes for lithium-ion batteries is studied ex situ. Since the material mass loading differs considerably between the two applications, a new analytical technique is introduced which mathematically separates the different particle signals and thus allows to investigate samples with high mass loadings.

The gas evolution during the formation of graphite electrodes is quantified by On-line Electrochemical Mass Spectrometry (OEMS) for dry electrolyte (< 20 ppm H2O) and 4000 ppm H2O containing electrolyte to mimic the effect of trace water during the formation process. While the formation in dry electrolyte mainly shows ethylene (C2H4) from the reduction of ethylene carbonate (EC) and small amounts of hydrogen (H2), the formation in water-containing electrolyte yields large amounts of H2 and considerable amounts of CO2 in addition to the expected C2H4 evolution. We could show that a protective solid-electrolyte interphase (SEI) layer formed by pre-cycling the graphite electrode in 2% vinylene carbonate (VC) containing electrolyte can reduce the H2 evolution in water-containing electrolyte by a factor of 7.5 compared to a pristine graphite electrode. Consequently, the ability of graphite electrodes to form an SEI prevents excessive gassing from trace water, which, e.g., is observed for non-SEI forming lithium titanate (LTO) electrodes.

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The impact of the electronic resistance of platinum-group-metal-free cathode catalyst layers (PGM-free CCLs) for proton-exchange-membrane fuel cells (PEMFCs) was systematically investigated. Here we selected two different PGM-free catalysts (having high and low electronic conductivity) and integrated them into CCLs without and with conductive carbon fiber additives. To investigate the impact of the electronic resistivity ( ρ e − ) of PGM-free catalysts and CCLs, their through-plane ρ e − values were quantified by an in situ electrochemical impedance spectroscopy (EIS) approach based on a one-dimensional transmission line model. The results indicate that the electronic conductivity of PGM-free CCLs can be increased by adding carbon additive, resulting in a significant improvement of the fuel cell performance (by ∼60 mV at 1 A cm−2 in H2/O2 configuration). Ex situ four-point probe measurements of the in-plane ρ e − values of some of the CCLs were found to differ vastly from the through-plane ρ e − values. This difference is attributed to the anisotropic morphology of the CCLs, caused by preferential fiber orientation and/or cracks in the CCLs. In the end, we suggest guidelines for the design and evaluation of PGM-free CCLs and for assessing and improving their electronic resistance.