Mehtap Oezaslan

The transportation and storage of renewable energy remain two of the biggest challenges along of the energy transition. Here esmajor technologiesHydrogen-connected technologies offer promising solutions to tackle them. In this talk, we show how various electrochemical energy conversion and storage technologies can be integrated to an integrated physical network at megawatt scale. Hydrogen Terminal Braunschweig is a demonstration facility established by Steinbeis-Innovationszentrum (siz) energieplus and Technical Electrocatalysis Laboratory, Universität Hamburg and some institutes from the Technische Universität Braunschweig.

The efficient energy storage devices are required to overcome the problem of fluctuations in the availability of electric energy. The water electrolysis to produce storable hydrogen and oxygen is one of the most expecting approaches for large-scale energy storage. The proton exchange membrane water electrolysis (PEMWE) has a potential to produce the high purified hydrogen under the operation at high current densities. However, the reducing the Ir loading at the OER electrode is essential to realize large-scale applications of PEMWE. This requires the development of highly active OER catalysts to accelerate the reaction or reduce the overpotential is the key issues toward the widespread applications. Herein, we report a remarkably active OER catalyst of Ir oxide with nanorod-like structure supported on niobium-doped tin oxide (Nb-SnO₂), showing an order of magnitude increase in Ir mass-specific activity than a commercial IrO_x catalyst at 1.5 V vs. RHE. We also investigated the temperature dependence of the OER activity, as a way to elucidate the mechanism of enhanced catalysis, from both experimental and theoretical perspectives.

Hydrogen production plays a crucial role in achieving a carbon-neutral energy sector. In the current stage,^[1] anion exchange membrane water electrolyzer (AEMWE) offer the promising advantages of combining solid membranes with an alkaline environment. This allows for easier handling, operation at high current densities and lower material costs due to the less-corrosive alkaline conditions.^[2-3] However, the hydrogen evolution reaction (HER) activity of platinum-group metal (PGM) electrocatalysts is dramatically reduced with increasing pH values.^[4-5] Latest research works on HER catalysts suggest a positive influence of alloying Pt with transition metals. In particularly, Co, Ni and Ru^[6-8] have been found to show the most promising effect due to a lowering of the d-band center caused by lattice compression of Pt-Pt^[9-10] and a higher presence of adsorbed oxygen species on the more oxophilic transition metal.^[6-7]

One of the key challenges to maintain efficient hydrogen production for alkaline water electrolysis (AWE) is the reliable design of electrolytic cells and stacks in combination with appropriate feed solution composition. [1] Iron (Fe) and its oxygen compounds can enhance the kinetics of both half-cell reactions, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), for nickel(Ni)-based catalyst materials. [2,3] On the other hand, iron contamination in the electrolyte caused by corrosion of the system components and fittings can lead to dramatic performance losses of AWE over a longer operational period. The redox chemistry of iron species in highly concentrated alkaline electrolyte and their effects on the AWE cell performance are poorly understood to date. Therefore, systematic investigations with controlled amounts of iron contamination at single cell level are necessary to correlate the changes in the kinetic and ohmic losses in order to accurately predict the performance of AWE technology.

Proton exchange membrane water electrolysis (PEMWE) is a key technology for production of hydrogen at high current density and dynamic operation. However, long-term durability of PEMWE is limited by the sensitivity of perfluorosulfonic acid (PFSA) membranes to impurities in the water feed.[1,2] Even small amounts of metal cations alter membrane properties such as conductivity, water uptake, and transport phenomena due to proton-cation exchange processes on the sulfonic groups.[3,4] These cation-exchange processes not only reduce system efficiency but also influence water management, which is highly critical for membrane hydration. Despite their practical relevance, a fundamental relationship between cation contamination and water transport in PEMWE is not fully understood to date.

Raman spectroscopy reveals low oxygen coverage on Pt sites, correlating with enhanced ORR activity. In line with DFT calculations, a clear weakening by ∼38 cm ⁻¹ ≈0.48 kJ mol ⁻¹ of Ir–O–Pt compared to the Ir–O–Ir bond results in lower OER activity.

Physical and chemical parameters, such as temperature, water/hydrogen/oxygen partial pressures, are distributed inhomogeneous inside a polymer electrolyte fuel cell during the operation and have a large influence on its performance and durability. In this study, the oxygen partial pressure (p(O₂)) is visualized in real‐time/space using an oxygen‐sensitive dye on the surface of the gas diffusion layer (GDL) during power generation at temperatures of 80, 100, and 110 °C using a 20 mm × 20 mm single cell with ten straight gas flow channels. p(O₂) on the surface of the GDL is visualized for the first time at temperatures higher than 100 °C, desired especially for heavy‐duty vehicle application, due to advantages such as less susceptibility to catalyst poisoning and the option to use smaller and lighter radiators. The oxygen partial pressure on the surface of the GDLs is monitored to be higher than the values expected from a simple model and decreased only slightly along the gas flow channel with increasing current densities. The work shows that high p(O₂) on the surface of the GDL is due to the short gas flow channels and accumulating water/vapor inside the GDL and the catalyst layer limiting the gas diffusion.

Recently, impinging jet flow cells have gained significant interest to combine electrochemical experiments with chemical analysis in a unique manner. One potential use of these flow cells is the measurement of square working electrodes, which is typically not possible with the well-known rotating disk electrode technique. Computational fluid dynamics (CFD) simulations are usually required to accurately predict mass transport rates and limiting currents of these electrochemical reactions, because analytical solutions are only available for specific cases. In this work, we present a universal analytical model for the mass transport processes in impinging jet electrodes, derived from the mathematical superimposition of three established equations from the literature. Our universal model contains four logical cases depending on the input parameters of the cell geometry, volume flow rate and reaction. A direct comparison with numerical and experimental data for the oxidation of ferro(II)cyanide shows excellent agreement with average relative errors of below 10 %. Altogether, our universal analytical model is able to replace time- and resource-intensive CFD simulations to predict experimental results obtained from the impinging jet electrodes in the near future.

Electrospinning has emerged as a promising approach to prepare porous cathode catalyst layers (CCLs) with low loading of platinum group metals (PGM) for PEM fuel cells. In particular, electrospun nanometer-sized fibers can be used as a backbone to increase the utilization and accessibility of catalytically active sites at low PGM loading as well as improve water management at high current density. [1, 2] Beside the low PGM loading, the absence of external humidifier is important for practical applications, because it can further decrease the parasitic power losses as well as complexity, weight, volume, and cost of the PEMFC system. [3] In this context, electrospun nanofiber CCLs show a pronounced humidity-dependent performance loss, which hinders their operation range compared to conventional CCLs prepared by decal transfer process. For example, Brodt et al. show a performance decrease for electrospun CCL of around 75 % at relative humidity (RH) of 40 % compared to 100 % RH. [4] In contrast, the electrosprayed CCL exhibits only a loss of around 30 % under the same conditions. [4] Therefore, the development of highly active and robust CCLs operating at high current density under low humidity remain a great challenge to date. In this work, we systematically investigated the effects of the humidity on the performance of the nanofiber-based CCLs. To prepare the CCLs with a geometric surface area of 50 cm², a nozzle-free electrospinning machine by Elmarco S.R.O. (Liberec, Czech Republic) was employed. The ink containing Pt/C catalyst, ionomer, and polyacrylic acid (PAA) was mixed using a disperser. Thereby, several highly homogeneous CCLs deposited on a gas diffusion layer (GDL) with controlled platinum loading of up to 0.1 mg cm^-2 _geo were prepared and characterized using micro X-ray fluorescence spectroscopy (μXRF). The morphology and chemical distribution of the as-prepared CCLs were evaluated from the scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). We then correlated the structural information (Pt and Nafion distribution, size of the nanofibers, etc.) of the CCLs with the electrochemical performance (U-I curve, ECSA, HFR, proton conductivity, etc.) at single cell level. Additionally, the impact of the humidity variations (20 – 100 % RH) for the nanofiber-based CCLs was compared to the conventional CCLs prepared by decal-transfer method. Moreover, high resolution (S)TEM-EDX was employed to uncover the distribution of the PFSA ionomer, Pt/C catalyst and PAA. These data are correlated with the ECSA and indicate the utilization and accessibility of Pt nanoparticles linked with the spatial distribution of PFSA ionomer. Altogether, we provide fundamental insights into the relationship between humidity and performance for nanofiber-based PEMFC CCLs prepared by electrospinning. New strategies will be presented to overcome the structure-humidity-sensitivity behavior of these novel three-dimensional nanofiber-based CCLs. [5] References [1] Brodt M, Wycisk R, Pintauro P N: Nanofiber Electrodes for High Power PEM Fuel Cells. J. Electrochem. Soc. 2013, 160 (8), F744-F749. DOI: https://doi.org/10.1149/2.008308jes. [2] Zhang W, Pintauro P N: High-Performance Nanofiber Fuel Cell Electrodes. ChemSusChem 2011, 4 (12), 1753-1757. DOI: https://doi.org/10.1002/cssc.201100245. [3] Ren G, Qu Z, Wang X et al.: Electrospun fabrication and experimental characterization of highly porous microporous layers for PEM fuel cells. International Journal of Hydrogen Energy 2024, 55, 455 - 463. DOI: https://doi.org/10.1016/j.ijhydene.2023.11.226. [4] Brodt M, Han T, Dale N et al.: Fabrication, In-Situ Performance, and Durability of Nanofiber Fuel Cell Electrodes. J Electrochem Soc 2015, 162, F84-F91. DOI: https://doi.org/10.1149/2.0651501jes. [5] Kallina V, Hasché F, Oezaslan M: Advanced Design of Electrospun Nanofiber Cathode Catalyst Layers for PEM Fuel Cells at Low Humidity. Current Opinion in Electrochemistry 2024, submitted.