Browsing by Author "Pötzsch, Daniel"

Now showing 1 - 1 of 1

Open Access
Mixed-conducting (Ba,Sr)(Co,Fe,Zn)O3-δ as cathode material for proton-conducting ceramic fuel cells : defect chemistry and oxygen reduction mechanism
(2014) Pötzsch, Daniel; Maier, Joachim (Prof. Dr.)
In the present study mixed-conducting solid oxides with perovskite structure are investigated regarding their applicability as oxygen electrode (cathode) in solid oxide fuel cells based on proton-conducting electrolyte membranes. This type of high temperature fuel cells is a promising device as it may allow one to decrease the operating temperature. At reduced temperatures the overpotential of the cathode dominants over all contributions to the performance of the fuel cell. Hence, it is mandatory to systematically and purposefully enhance the catalytic activity of the cathode, for which a fundamental understanding of the electrochemical properties of the materials is key. These properties are inter alia determined by the defect chemistry of the material's mobile charge carriers. In humid, oxidizing atmosphere at elevated temperature three defects have to be considered: Oxygen vacancies, interstitial protonic defects associated with a regular oxygen site and electron holes. A mixed proton/hole conductivity in the cathode is highly desired as to make the whole electrode surface catalytically active for water formation. The bulk thermodynamic and transport behavior regarding three mobile charge carriers are significantly more complex than for systems with only'' two mobile defects. Numerical simulations are necessary to describe and understand the bulk thermodynamic and transport properties. The proton concentration of mixed-conducting perovskites was ex-situ determined by Karl-Fischer titration and thermogravimetry analyzing the mass spectrometer signal, and in-situ by dynamic, thermogravimetric relaxation experiments upon step-wise changes in the water partial pressure. BSFZ was found to be the most promising candidate of the four and, therefore, selected for further investigations. From a thermodynamic point of view two limiting possibilities incorporating protons are identified: Incorporating a water molecule occupying an oxygen vacancy and forming two protonic defects (acid-base thermodynamics) and taking up water releasing simultaneously oxygen, i.e. hydration-deoxygenation, formally equivalent to pure hydrogen incorporation (redox thermodynamics). Depending on temperature, oxygen and water partial pressure any combination of both mechanisms is possible. With the help of numerically simulating the transport behavior at different conditions, measuring the mass relaxation upon water partial pressure changes at two different oxygen partial pressures and determining the thermodynamic properties in dry conditions, the proton concentration could be calculated applying the thermodynamic model. For BSFZ the mass relaxation transients upon pH2O change were measured for two different p2O values. Interestingly, the mechanism of proton uptake was found to change from predominantly acid-base water uptake to predominantly redox hydrogen uptake. The transients could be fitted through the known solution of Fick's second law of one-dimensional diffusion into a plane sheet (with sufficiently fast surface equilibration) obtaining chemical diffusivities. The proton conductivity is calculated using its concentration and diffusivity. The obtained values are up to one and a half orders of magnitude below the proton conductivity of 15% Y-doped BaZrO3 being one of the best known high temperature proton conductors. Nevertheless, even the estimate of the lower limit of proton conductivity in BSFZ is orders of magnitude larger than a required minimum to make the whole electrode surface catalytically active. This is to the best of my knowledge the first study providing quantitative values for the proton conductivity in those mixed-conducting perovskites typically used as cathode material in solid oxide fuel cells. The electrochemical activity of BSFZ and BSCF was investigated by impedance spectroscopy. For microelectrodes the low frequency contribution typically dominates the overall impedance, and its resistance is inversely proportional to the reaction rate determining the overall oxygen to water reaction. The inverse dependence of the surface reaction resistance to the area of the microelectrode confirms that the whole electrode surface is catalytically active. Its dependency on oxygen and water partial pressure provides important information about the oxygen reduction mechanism. The exponents of the oxygen and water partial pressure dependency indicate that molecular oxygen and oxygen vacancies are participating in the rate determining step of the oxygen reduction reaction.