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Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts

Shahraei, Ali (2018):
Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8171],
[Ph.D. Thesis]

Abstract

In the context of energy transition, hydrogen is addressed as a clean future energy carrier that enables the emission-free production of energy to become independent of fossil fuels. The development of hydrogen production technologies like the electrolysis is strongly driven by the two main criteria: sustainability and economy. Over the past decades, various technological achievements resulted in a reduction of costs which has dramatically improved the economic potential of hydrogen produced by electrolysis. Especially in the field of material development, great effort was devoted to replace the precious state of the art catalyst materials with abundant cost-effective catalysts accelerating sluggish water splitting reactions. This dissertation focuses on the investigation of carbon-based cobalt catalysts with multi-heteroatom doping for the oxygen evolution reaction (OER) and hydrogen evolution reactions (HER). Within this study, two major synthesis approaches, one with metal organic framework (MOF) and another with polyaniline (PANI), were investigated in terms of structural and electrochemical characterization. Moreover, the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project. Within the MOF approach, the role of the metal species on HER activity was investigated using X-ray photoelectron spectroscopy (XPS). The discussion was further supported by density functional theory (DFT) calculations resulting in structure-activity correlations with emphasis on the importance of the nature of the metal. Besides, bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER . The PANI approach was proposed to investigate multi heteroatom doping influence on the catalytic activity and material properties. Within this approach, cobalt catalysts with variation of cobalt loading and sulfur loading in the precursors were prepared. The catalysts were highly active toward both HER and OER, though the origin of activity might be different. Several physico-chemical characterization techniques combined with post mortem analysis were carried out in order to get insight into the origin of the activity. It was found that the high HER activity of PANI-based samples is attributed to MeN4 sites, and the OER activity is originating from a hybrid cobalt complex depending on the synthesis route.

Item Type: Ph.D. Thesis
Erschienen: 2018
Creators: Shahraei, Ali
Title: Investigation of the water splitting reactions on multi-heteroatom doped cobalt-based carbon catalysts
Language: English
Abstract:

In the context of energy transition, hydrogen is addressed as a clean future energy carrier that enables the emission-free production of energy to become independent of fossil fuels. The development of hydrogen production technologies like the electrolysis is strongly driven by the two main criteria: sustainability and economy. Over the past decades, various technological achievements resulted in a reduction of costs which has dramatically improved the economic potential of hydrogen produced by electrolysis. Especially in the field of material development, great effort was devoted to replace the precious state of the art catalyst materials with abundant cost-effective catalysts accelerating sluggish water splitting reactions. This dissertation focuses on the investigation of carbon-based cobalt catalysts with multi-heteroatom doping for the oxygen evolution reaction (OER) and hydrogen evolution reactions (HER). Within this study, two major synthesis approaches, one with metal organic framework (MOF) and another with polyaniline (PANI), were investigated in terms of structural and electrochemical characterization. Moreover, the catalysts were analyzed in detail by active site identification and mechanistic understanding of the reactions within the scope of each project. Within the MOF approach, the role of the metal species on HER activity was investigated using X-ray photoelectron spectroscopy (XPS). The discussion was further supported by density functional theory (DFT) calculations resulting in structure-activity correlations with emphasis on the importance of the nature of the metal. Besides, bimetallic catalysts with optimal hydrogen binding energies were suggested as a promising active catalyst toward HER . The PANI approach was proposed to investigate multi heteroatom doping influence on the catalytic activity and material properties. Within this approach, cobalt catalysts with variation of cobalt loading and sulfur loading in the precursors were prepared. The catalysts were highly active toward both HER and OER, though the origin of activity might be different. Several physico-chemical characterization techniques combined with post mortem analysis were carried out in order to get insight into the origin of the activity. It was found that the high HER activity of PANI-based samples is attributed to MeN4 sites, and the OER activity is originating from a hybrid cobalt complex depending on the synthesis route.

Place of Publication: Darmstadt
Divisions: 07 Department of Chemistry
Date Deposited: 18 Nov 2018 20:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8171
URN: urn:nbn:de:tuda-tuprints-81712
Referees: Kramm, Prof. Dr. Ulrike I. and Suntivich, Prof. Dr. Jin
Refereed / Verteidigung / mdl. Prüfung: 18 October 2018
Alternative Abstract:
Alternative abstract Language
Wasserstoff wird im Rahmen der Energiewende als zukünftiger und sauberer Energieträger gehandelt, der die emissionsfreie Erzeugung von Energie ermöglicht um von den fossilen Brennstoffen unabhängig zu werden. Die Entwicklung von Wasserstoff-basierten Technologien wie der Elektrolyse wird hauptsächlich von zwei Faktoren angetrieben: Nachhaltigkeit und Wirtschaftlichkeit. In den letzten Jahrzenten führten verschiedene technologische Fortschritte zu einer Senkung der Kosten was das wirtschaftliche Potenzial des mittels Elektrolyse hergestellten Wasserstoffs erheblich steigerte. Im Bereich der Materialentwicklung wurden vor allem große Anstrengungen unternommen die standartmäßig eingesetzten edelmetall-basierten Katalysatoren durch kostengünstigere Katalysatoren zu ersetzten um die träge Wasserspaltungsreaktion zu beschleunigen. Im Fokus dieser Dissertation steht die Untersuchung von Kohlenstoff-basierten, heteroatom-dotierten Kobalt-Katalysatoren für die Sauerstoffentwicklungs- und die Wasserstoffentwicklungsreaktion. Im Rahmen dieser Untersuchungen wurden hauptsächlich zwei Syntheseansätze, einer basierend auf der Verwendung sogenannter Metall-Organischen Verbindungen (MOF) und ein anderer basierend auf der Verwendung von Polyanilin (PANI), hinsichtlich einer strukturellen und elektrochemischen Charakterisierung studiert. Darüber hinaus wurden die die Katalysatoren mit Blick auf die Identifizierung der aktiven Spezies und des Reaktionsmechanismus detailliert im Rahmen dieses Projekts studiert. Innerhalb des MOF-Ansatzes wurde die Rolle der Metallspezies für die HER-Aktivität mittels Röntgen-Photoelektronen Spektroskopie untersucht. Die Ergebnisse wurden zudem mittels DFT-Berechnungen (Dichtefunktionaltheorie) unterstützten und die resultierenden Struktur-Aktivitäts-Korrelationen betonen die zentrale Bedeutung der Natur des Metalls. Außerdem konnten bi-metallische Katalysatoren mit optimalen Wasserstoffbindungseigenschaften als vielversprechende aktive Katalysatoren für die HER vorgeschlagen werden. Der PANI-Ansatz wurde gewählt um den Einfluss der Heteroatom-Dotierung auf die katalytische Aktivität und die Materialeigenschaften zu studieren. Innerhalb des Ansatzes wurden Kobaltkatalysatoren unter Variation des Metall und Schwefelgehalts in der Präkursormischung hergestellt. Die resultierenden Katalysatoren waren hoch aktiv gegenüber beider, der Wasserstoff- und der Sauerstoffentwicklungsreaktion, obwohl deren katalytische Aktivität unterschiedlichen Ursprungs ist. Um den Ursprung der Aktivität zu identifizieren wurden verschiedene physiko-chemische Charakterisierungsmethoden in Kombination mit einer ausführlichen post-mortem Analyse durchgeführt. Es konnte gezeigt werden, dass die HER Aktivität der PANI-basierten Katalysatoren den Me-N4 Zentren zuzuordnen ist und die OER Aktivität auf einen von der Syntheseroute abhängigen Kobalt-Hybrid-Komplex zurückzuführen ist.German
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