Die vorliegende Arbeit entstand im Zeitraum von 05/2014 bis 03/2017 in Kooperation mit Umicore AG & Co. KG und ist thematisch direkt mit dem BMWi-Projekt „NeoKarII“ mit der Suche nach neuartigen, oxidischen Elektrodenmaterialien für Polymerelektrolytbrennstoffzellen (PEM-FC) verknüpft. In dieser Arbeit wurden verschiedene oxidische Materialien über Sol-Gel- und Imprägnationsverfahren hergestellt und deren Eignung für eine Brennstoffzellenanwendung geprüft. Neben umfassenden röntgenographischen Untersuchungen zu den Stoffsystemen IrO2-TiO2, IrO2-SnO2 und Cr2O3-TiO2 sind in dieser Arbeit auch zusammensetzungsabhängige Änderungen der elektrischen Leitfähigkeit gezeigt. Zu diesem Zweck wurde ein neuer Leitfähigkeitsversuchsstand mit einer 2-Punkt-Durchgangswiderstandsmessmethode entwickelt und in Betrieb genommen. Der halbautomatische Versuchsstand ermöglicht eine temperatur- und druckprogrammierte Vermessung pulverförmiger Werkstoffe. / This thesis was developed between 05/2014 and 03/2017 together with Umicore AG & Co. KG and is related to the BMWi project 'NeoKarII' with the search for novel, oxidic electrode materials for polymer electrolyte fuel cells (PEM-FC). In this work, various oxidic materials were prepared by sol-gel and impregnation processes and their suitability for a fuel cell application was tested. In addition to comprehensive X-ray diffraction examinations of the materials systems IrO2-TiO2, IrO2-SnO2 and Cr2O3-TiO2, the composition dependence of electrical conductivity is also shown. For this purpose, a new measurement setup with a 2-point resistance measurement method was developed and brought into services. The semi-automatic setup allows a temperature- and pressure-programmed measurement of powdered materials.

Reine und dotierte SnO2 Feinstpulver und Einkristalle wurden mit verschiedenen Methoden hergestellt. Die Abhängigkeit der Eigenschaften von der Dotierungsart und der Dotierungskonzentration wurde untersucht. Die Dotierung mit fünfwertigen Elementen (Sb, Nb) führt zur Erniedrigung des spezifischen elektrischen Widerstandes und die Dotierung mit dreiwertigen Elementen (z.B. In) zu seiner Erhöhung. An den dotierten Materialien kann mittels XPS eine Segregationsschicht nachgewiesen werden. Der Umfang dieser Schicht ist abhängig sowohl von dem Dotierungselement, als auch von den Herstellungsbedingungen (z.B. Temperatur und Temperungszeit). Für die Pulver wird, im Gegensatz zu den Einkristallen, kein thermodynamisches Gleichgewicht für Segregationsprozess im untersuchten Zeitfenster gefunden. In der vorliegenden Arbeit wird ein Model vorgestellt, dass es erlaubt, die Dicke der Segregationsschicht, als auch der verbleibenden Volumenkonzentration der Dotierungselement im SnO2 Kristallit zu berechnen. Die Volumenkonzentration beträgt in Abhängigkeit von der Dotierungsart und Temperatur bis zu 70% der gesamten Dotierungskonzentration. Die sich ausbildende Segregationsschicht erreicht einen Bedeckungsgrad von bis zu einer Monolage. Die Aktivierungsenergie der Diffusion, sowie die freie Enthalpie des Segregationsprozesses können für die hier untersuchten Dotierungselemente in SnO2 bestimmt werden.

Die vorliegende Arbeit entstand im Zeitraum 12/2014 bis 02/2018 im erweiterten Rahmen des BMWi-Projekts „NeoKarII“ in Kooperation mit der Umicore AG & Co. KG. Das Projekt befasste sich mit der Suche nach neuartigen, oxidischen Elektrodenmaterialien für Polymerelektrolytmembranbrennstoffzellen (PEM-FC). Im Rahmen dieser Arbeit wurden Niob- und Tantal-dotierte Zinnoxide mit verschiedenen Dotiergraden (hauptsächlich 1 bis 10 %) über Sol-Gel Synthesen, Imprägnierungen und Co-Fällung hergestellt. Für die Co-Fällungen wurde eine MicroJet-Reaktor Anlage entwickelt und aufgebaut. Die Materialien wurden anschließend röntgenografisch untersucht und auf ihre Eignung für die Anwendung als Katalysatorträgermaterial in PEM-FC geprüft. Als Zielgrößen dienten die BET-Oberfläche und spezifische Leitfähigkeit, welche mit einem eigens entwickelten Leitfähigkeitsmessstand ermittelt wurde.:1 Einleitung 1.1 Brennstoffzellen 1.2 Methoden zur Synthese der Mischoxide 1.3 Eigenschaften von Zinnoxiden 1.4 Zielsetzung 2 Ergebnisse und Diskussion 2.1 Synthesen und Beobachtungen 2.2 Röntgenfluoreszenzanalyse 2.3 Pulver-Röntgendiffraktometrie 2.4 BET-Oberfläche 2.5 Leitfähigkeit 2.6 Röntgenphotoelektronenspektroskopie 3 Experimenteller Teil 3.1 Synthesen 3.2 Analytische Methoden 4 Zusammenfassung und Ausblick 4.1 Zusammenfassung 4.2 Ausblick 5 Anhang / This thesis was developed between 12/2014 and 02/2018 in an extended framework of BMWi project „NeoKarII “ in cooperation with Umicore AG & Co. KG. The project was concerned with the search for novel oxidic electrode materials for polymer electrolyte fuel cells (PEM-FC). In this work we prepared niobium- and tantalum-doped tin oxides with different doping levels (mainly 1 to 10 %) by sol-gel synthesis, impregnation and co-precipitation. For co-precipitation we developed and built a MicroJet reactor plant. We analysed the materials by X-ray diffraction and tested them for suitability for use as catalyst support material in PEM-FC. Target values were BET surface area and specific conductivity, which was determined using a specially developed conductivity measurement setup.:1 Einleitung 1.1 Brennstoffzellen 1.2 Methoden zur Synthese der Mischoxide 1.3 Eigenschaften von Zinnoxiden 1.4 Zielsetzung 2 Ergebnisse und Diskussion 2.1 Synthesen und Beobachtungen 2.2 Röntgenfluoreszenzanalyse 2.3 Pulver-Röntgendiffraktometrie 2.4 BET-Oberfläche 2.5 Leitfähigkeit 2.6 Röntgenphotoelektronenspektroskopie 3 Experimenteller Teil 3.1 Synthesen 3.2 Analytische Methoden 4 Zusammenfassung und Ausblick 4.1 Zusammenfassung 4.2 Ausblick 5 Anhang

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.