Browsing by Author "Biswas, Indro"

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Open Access
Advancement of segmented cell technology in low temperature hydrogen technologies
(2020) Biswas, Indro; Sánchez, Daniel G.; Schulze, Mathias; Mitzel, Jens; Kimmel, Benjamin; Gago, Aldo Saul; Gazdzicki, Pawel; Friedrich, K. Andreas
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.
Open Access
Crystal structure and XPS study of titanium-substituted M-type hexaferrite BaFe12−xTixO19
(2023) Mehnert, Kim-Isabelle; Häßner, Manuel; Dreer, Yanina Mariella; Biswas, Indro; Niewa, Rainer
The M-type barium hexaferrite substituted with titanium, BaFe12−xTixO19, was synthesized from sodium carbonate flux and the obtained single crystals with a maximum degree of substitution of up to about x = 0.9 were characterized. XPS measurements were carried out for the identification of side products and in particular in order to assign the valence states of the transition-metal constituents. Due to the aliovalent exchange of iron(III) with titanium(IV), an additional charge balance needs to occur. No titanium(III) was detected, while the amount of iron(II) increased in the same order of magnitude as the amount of titanium(IV); thus, the major charge balancing is attributed to the reduction of iron(III) to iron(II). According to the XPS data, the amount of titanium(IV) typically is slightly higher than that of iron(II). This is in line with a tendency to a minor formation of vacancies on the transition-metal sites becoming more important at higher substitution levels according to PXRD and WDS measurements, completing the picture of the charge-balance mechanism. XRD taken on single crystals indicates the distribution of titanium and vacancies over three of the five transition-metal sites.
Open Access
Exploring the interface of skin‐layered titanium fibers for electrochemical water splitting
(2021) Liu, Chang; Shviro, Meital; Gago, Aldo S.; Zaccarine, Sarah F.; Bender, Guido; Gazdzicki, Pawel; Morawietz, Tobias; Biswas, Indro; Rasinski, Marcin; Everwand, Andreas; Schierholz, Roland; Pfeilsticker, Jason; Müller, Martin; Lopes, Pietro P.; Eichel, Rüdiger‐A.; Pivovar, Bryan; Pylypenko, Svitlana; Friedrich, K. Andreas; Lehnert, Werner; Carmo, Marcelo
Water electrolysis is the key to a decarbonized energy system, as it enables the conversion and storage of renewably generated intermittent electricity in the form of hydrogen. However, reliability challenges arising from titanium‐based porous transport layers (PTLs) have hitherto restricted the deployment of next‐generation water‐splitting devices. Here, it is shown for the first time how PTLs can be adapted so that their interface remains well protected and resistant to corrosion across ≈4000 h under real electrolysis conditions. It is also demonstrated that the malfunctioning of unprotected PTLs is a result triggered by additional fatal degradation mechanisms over the anodic catalyst layer beyond the impacts expected from iridium oxide stability. Now, superior durability and efficiency in water electrolyzers can be achieved over extended periods of operation with less‐expensive PTLs with proper protection, which can be explained by the detailed reconstruction of the interface between the different elements, materials, layers, and components presented in this work.
Open Access
Hydrogen spillover through hydride transfer : the reaction of ZnO and ZrO2 with strong hydride donors
(2024) Benz, Michael; Bunjaku, Osman; Nowakowski, Michal; Allgaier, Alexander; Biswas, Indro; Slageren, Joris van; Bauer, Matthias; Estes, Deven P.
Hydrogen spillover, transfer of H2 from a metal surface to a support (often metal oxides), is pivotal for many heterogeneous catalytic processes, including Cu/ZnO and Cu/ZrO2 catalyzed methanol synthesis. Little is known about hydrogen spillover on ZnO or ZrO2, due to the high complexity of the metal-metal oxide interface. Here, we model hydrogen spillover on ZnO and ZrO2 by reacting them with molecular metal hydrides to see how the properties of the hydrides affect hydrogen spillover. While the good H· donors HV(CO)4dppe (1) and CpCr(CO)3H (2) do not react with the metal oxide surfaces, the strong hydride donors iBu2AlH (3), Cp2ZrHCl (4), and [HCu(PPh3)]6 (5) do reduce ZnO and ZrO2 to give defect sites with the same EPR signatures as obtained via hydrogen spillover. We also observe new M-O bonds to the surface using X-ray absorption spectroscopy (XAS). We propose that these metal oxides undergo hydrogen spillover via initial hydride transfer followed by tautomerization of the surface hydride, giving reduced sites and OH bonds. This mechanism is in contrast to the traditional spillover mechanism involving discrete proton- and electron transfer steps. We also observe that ZnO is easier to reduce than ZrO2, explaining the difficulty observing spillover on Cu/ZrO2.
Open Access
Influence of cycling profile, depth of discharge and temperature on commercial LFP/C cell ageing : post-mortem material analysis of structure, morphology and chemical composition
(2020) Simolka, Matthias; Heger, Jan-Frederik; Kaess, Hanno; Biswas, Indro; Friedrich, K. Andreas
The paper presents post-mortem analysis of commercial LiFePO4 battery cells, which are aged at 55 °C and - 20 °C using dynamic current profiles and different depth of discharges (DOD). Post-mortem analysis focuses on the structure of the electrodes using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the chemical composition changes using energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). The results show that ageing at lower DOD results in higher capacity fading compared to higher DOD cycling. The anode surface aged at 55 °C forms a dense cover on the graphite flakes, while at the anode surface aged at - 20 °C lithium plating and LiF crystals are observed. As expected, Fe dissolution from the cathode and deposition on the anode are observed for the ageing performed at 55 °C, while Fe dissolution and deposition are not observed at - 20 °C. Using atomic force microscopy (AFM), the surface conductivity is examined, which shows only minor degradation for the cathodes aged at - 20 °C. The cathodes aged at 55 °C exhibit micrometer size agglomerates of nanometer particles on the cathode surface. The results indicate that cycling at higher SOC ranges is more detrimental and low temperature cycling mainly affects the anode by the formation of plated Li.
Open Access
Investigation of the degradation phenomena of a proton exchange membrane electrolyzer stack by successive replacement of aged components in single cells
(2025) Kimmel, Benjamin; Morawietz, Tobias; Biswas, Indro; Sata, Noriko; Gazdzicki, Pawel; Gago, Aldo Saul; Friedrich, Kaspar Andreas
Due to their compactness and high flexibility to operate under dynamic conditions, proton exchange membrane water electrolyzers (PEMWEs) are ideal systems for the production of green hydrogen from renewable energy sources. For the widespread implementation of PEMWEs, an understanding of their degradation mechanism is crucial. In this work, we analyze a commercial PEMWE stack via a novel approach of breaking down from the stack to the single-cell level. Therefore, the disassembled stack components are cut to fit into single cells. Then, the aged components are successively replaced with pristine or regenerated components (cleaned and polished), and electrochemical characterizations are conducted to investigate the contributions of the individual components on performance losses. In addition, several underlying degradation phenomena are identified using different physical ex-situ analysis methods. The catalyst-coated membrane (CCM) contributes the most to performance degradation because of contamination and ionomer rearrangement. Additionally, traces of calcium, likely due to insufficient water purification used during operation or for cleaning the cell components, were found. Significant oxidation was observed on the anodic components, while the electronic conductivity on the cathode side remained unchanged. The combination of electrochemical characterization with stepwise regeneration processes and physical ex-situ analysis allows to draw conclusions about the impact of different components on degradation and to analyze the underlying aging mechanisms occurring in each component.
Open Access
Scalable fabrication of multi-layered Cu-based electrodes via solvent-free method for the selective electrochemical conversion of CO2 to C2+ products
(2024) Chen, Qinhao; Kube, Alexander; Rana, Bhawna; Biswas, Indro; Morawietz, Tobias; Kopljar, Dennis; Friedrich, Kaspar Andreas
In the research field of CO2 electroreduction, gas diffusion electrodes (GDEs) are predominantly manufactured through solvent-based processes. Meanwhile, the solvent-free method has gained heightened attention due to its potential to reduce operational and production expenses, while considering ecological aspects such as solvent evaporation, circulation, and waste treatment. Drawing from its successful applications in other fields, we have specifically developed a solvent-free manufacturing method to produce multi-layered Cu-based GDEs for CO2 electroreduction. The procedure is compatible with industrial production lines, specifically through a roll-to-roll process. By evaluating the interplay between production parameters and electrochemical performance of GDEs via various characterization methods, key factors, i.e., hydrophobicity, gas permeability, thickness, and pore size, were adjusted and applied to achieve a highly selective GDE towards C2+ products (alcohols and ethylene) at industrial relevant currents up to 300 mA cm-2 (ethylene ∼40%, ethanol ∼10%, n-propanol ∼15%).