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Wasserstoff kann als Bindeglied zwischen elektrischer Energie aus erneuerbaren Quellen und dem nicht-elektrischen Energiebedarf der verschiedenen Sektoren des Energiesystems einen Beitrag zur Reduzierung der Treibhausgas-Emissionen leisten. Die konventionelle Wasserstoffproduktion basiert vornehmlich auf fossilen Ressourcen wie Erdgas oder Kohle und ist mit hohen Treibhausgas-Emissionen verbunden. Neben der Erweiterung der konventionellen Erzeugungsverfahren durch eine CO2-Abscheidung und Speicherung (CCS) werden neue Erzeugungsverfahren zur Etablierung einer nachhaltigen Wasserstoffwirtschaft benötigt. In dieser Arbeit werden die Wasserstofferzeugung mittels thermischer Zerlegung von Methan (TDM) und Wasserelektrolyse in Hinblick auf die Energie- und Exergieeffizienz verglichen. Die Festoxid-Elektrolyse (SOEL) wird als potentiell effizienteste Elektrolyse-Technologie verwendet. Die Systeme werden durch eine Wasserstoffaufbereitung und Verdichtung zur Einhaltung geltender Normen für den Einsatz als Kraftstoff in Brennstoffzellen und zur Erreichung eines Austrittsdruckes von 700 bar ergänzt. In zwei experimentellen Studien werden die nicht-katalytische TDM-Reaktion und die SOEL im Labormaßstab untersucht. Die TDM-Reaktion wird in Hinblick auf die entstehende Kohlenstofferscheinungsart und der Reaktionskinetik unter Anwesenheit von Wasserstoff und Kohlenstoffdioxid untersucht. Raman-Spektroskopie-Aufnahmen des Produktkohlenstoffs ergeben im Vergleich zu Graphit eine stark ungeordnete Mikrostruktur. Die Auswirkungen, einer von Graphit abweichenden Kohlenstofferscheinungsart, werden durch Berechnung des Reaktionsgleichgewichtes der TDM-Reaktion für in der Literatur beobachtete Kohlenstofferscheinungsarten evaluiert. In einer elektrochemischen Charakterisierung von SOEL-Zellen, wie sie in den großskaligen SOEL-Systemen eingesetzt werden, wird eine kommerzielle elektrolytgestützte Zelle mit modifizierten Zellen verglichen. Die Modifizierung erfolgt durch eine Bearbeitung, Hydrogen, as an essential link between electrical energy from renewable sources and the non-electrical energy demand in the various sectors of the energy system, can contribute to reducing the greenhouse gas (GHG) emissions. Conventional hydrogen production is mostly based on fossil fuels, such as natural gas or coal, and causes significant greenhouse gas emissions. In addition to implementing carbon capture and storage (CCS) technologies at conventional production sites, new production processes are required if we are to establish a sustainable hydrogen economy. This thesis compares hydrogen production by thermal decomposition of methane (TDM) and hydrogen production by water electrolysis in terms of energy and exergy efficiency. Solid oxide electrolysis (SOEL), as potentially the most efficient water electrolysis technology, is used. The processes are supplemented by a hydrogen purification and compression system in order to fulfill the standards for the use of hydrogen as fuel in fuel cells and deliver a hydrogen pressure of 700 bar. Two experimental studies investigate the non-catalytic TDM reaction and the SOEL in laboratory scale. The non-catalytic TDM reaction is investigated regarding the carbon type produced and the reaction kinetics in the presence of hydrogen and carbon dioxide. Raman spectroscopy measurements of the carbon produced revealed a highly disordered microstructure compared to graphite. The impact of the carbon type produced is evaluated by calculating the equilibrium of the TDM-reaction for a variation of the most relevant carbon types observed in literature. In a comprehensive electrochemical characterization of solid oxide cells, as used in large scale SOEL-systems, a commercially available electrolyte-supported cell is compared with two femtosecond laser treaded cells. The effect of the treatment on the electrochemical performance is analyzed by means of impedance spectroscopy and a subsequent DRT-analysis. Compared to the reference cell, t

Electrolyte-supported solid oxide cells are often used for steam electrolysis. Advantages are high mechanical stability and a low degradation rate. The aim of this proof of concept study was to use a femtosecond laser to process the electrolyte of an electrolyte-supported solid oxide cell and evaluate the effect of this laser treatment on the electrochemical performance. The femtosecond laser treatment induces a macroscopic and a superimposed microscopic structure. It can be proven that the electrolyte remains gas tight and the electrochemical performance increases independently of the laser parameters. The initial area-specific resistance degradation during a constant current measurement of 200 h was reduced from 7.9% for a non-treated reference cell to 3.2% for one of the laser-treated cells. Based on electrochemical impedance measurements, it was found that the high frequency resistance of the laser-treated cells was reduced by up to 20% with respect to the reference cell. The impedance spectra were evaluated by calculating the distribution of relaxation times, and in advance, a novel approach was used to approximate the gas concentration resistance, which was related to the test setup and not to the cell. It was found that the low frequency polarization resistance was increased for the laser-treated cells. In total, the area-specific resistance of the laser-treated cells was reduced by up to 14%.

The calculation of the entropy production rate within an operational high temperature solid oxide fuel cell (SOFC) is necessary to design and improve heating and cooling strategies. However, due to a lack of information, most of the studies are limited to empirical relations, which are not in line with the more general approach given by non-equilibrium thermodynamics (NET). The SOFC 1D-model presented in this study is based on non-equilibrium thermodynamics and we parameterize it with experimental data and data from molecular dynamics (MD). The validation of the model shows that it can effectively describe the behavior of a SOFC at 1300 K. Moreover, we show that the highest entropy production is present in the electrolyte and the catalyst layers, and that the Peltier heat transfer is considerable for the calculation of the heat flux in the electrolyte and cannot be neglected. To our knowledge, this is the first validated model of a SOFC based on non-equilibrium thermodynamics and this study can be extended to analyze SOFCs with other solid oxide electrolytes, with perovskites electrolytes or even other electrochemical systems like solid oxide electrolysis cells (SOECs).