{"id":{"repo_id":"tu-berlin","oai_identifier":"oai:depositonce.tu-berlin.de:11303/23490"},"canonical_url":"https://search.dev.ndltd.org/etd/tu-berlin/oai:depositonce.tu-berlin.de:11303/23490","repository":{"repo_id":"tu-berlin","name":"Technische Universität Berlin","base_url":"https://api-depositonce.tu-berlin.de/server/oai/request"},"display":{"title":"s-, p-, and f-block metals-mediated novel iron-based compounds as precatalysts for efficient alkaline OER","abstract":"Background: Generating green hydrogen (H2) through alkaline water electrolysis (AWE) powered by renewable resources is crucial for decarbonizing our society. A significant challenge in AWE is kinetically sluggish anodic oxygen evolution reaction (OER). Currently, the anodes used in industrial AWE systems incorporate OER catalysts based on non-noble transition metals (TMs). These catalysts, however, only achieve moderate performance. As a result, substantial research has focused on developing a new generation of cost-effective and more efficient TM-based catalysts. Among these, iron (Fe)-based catalysts are particularly promising due to Fe's low cost, high abundance in the earth's crust, and its ability to exist in various redox states. Challenge: However, very few pure Fe-based compounds perform well for alkaline OER. During the reaction, these compounds primarily act as precatalysts, inevitably reconstructing into (oxy)hydroxides (FeOxHy) that exhibit low or non-conductivity and easily dissolve Fe atoms. Additionally, the reconstructed FeOxHy suffers from insufficient exposure of surface active sites and inadequate mass transport and bubble detachment capabilities. Moreover, although many Fe-based compounds are claimed to possess intrinsically \"outstanding OER capabilities,\" their performances are often mis/overestimated due to unintentional interference from nickel (Ni) or cobalt (Co) (e.g., from the Ni foam substrate). Consequently, universally feasible strategies to optimize pure Fe-based compounds as efficient alkaline OER precatalysts for broader-scale applications have not yet been demonstrated. Solutions: Developing new Fe-based compounds that incorporate carefully chosen s-, p-, and f-block metals (spfMs) could effectively address the challenges mentioned above. spfMs typically exhibit two distinct behaviors during alkaline OER: ⅰ) dissolving into the aqueous electrolyte and ⅱ) remaining at the anode as new, insoluble, oxidized phases or as residual precatalysts. In the former scenario, the structural reconstruction of the targeted Fe-based precatalysts can facilitate the in-situ formation of FeOxHy, potentially increasing surface porosity, reducing crystallinity, and decreasing particle size. In the latter scenario, the remaining spfM oxides or precatalysts can enhance charge transfer, stabilize the active phase (sites), and optimize the binding and activation abilities toward reactants, intermediates, and products. As a result, the alkaline OER activity and stability can be significantly enhanced. Results: By incorporating the representative s-block metals lithium (Li) and calcium (Ca), p-block elements boron (B) and germanium (Ge), and f-block metal cerium (Ce), three unique novel bulk Fe-based compounds as precatalysts were developed, constituted by ⅰ) all leaching (LiFeBPO), ⅱ) both leaching and non-leaching (CaFe6Ge6), and ⅲ) all non-leaching (CeFe2) spfMs, respectively. We unequivocally determined that all of them underwent distinctive OER-driven reconstructions mediated by their respective SPFMs. Eventually, the precisely modified α-FeOOH in-situ evolved as the real active phase. Specifically, i) the drastic/complete loss of spfMs induced a deep/thorough reconstruction of LiFeBPO into ultrasmall low-crystalline α-FeOOH nanoparticles forming a 3D open porous skeleton. This structure increased the exposure and accessibility of active sites, leading to excellent bulk activity. Mass transport and bubble detachment were also improved. ⅱ) Regrading CaFe6Ge6, the partial loss of spfMs resulted in a porous heteroshell where α-FeOOH was coupled with CaCO3, promoting adsorption and transport of reactants, as well as activating more Fe sites for catalysis. The higher intrinsic activity of α-FeOOH was achieved, and the residual CaFe6Ge6 intermetallic core facilitated charge migration. ⅲ) All spfMs were preserved, mediating the surface conversion of CeFe2 into α-FeOOH nanodomains heterogeneously coupled with CeO2. The pseudo‐periodic confinement of insoluble CeO2 enhanced the performance of α-FeOOH by preventing active site blockage and dissolution while optimizing the electronic structure and intermediate adsorption energies. Meanwhile, the retained CeFe2 elevated the overall charge transfer. As a result, the reconstructed α-FeOOH from these three compounds exhibited boosted alkaline OER performances, surpassing those of directly synthesized FeOxHy and even comparable to or better than benchmark NiFeOxHy under identical operational conditions. Additionally, when deposited on a Ni foam substrate, they demonstrated impressive alkaline OER activity and stability, outperforming most documented TM-based counterparts. Conclusion & Perspective: This dissertation explicitly correlates the relationship between composition, reconstruction, active structure, and OER properties of the studied Fe-based compounds while avoiding interferences from Ni and Co. It illustrates that incorporating suitable spfMs is an effective strategy to optimize pure Fe-based compounds for efficient alkaline OER catalysis, enhancing the performance of anodes for practical AWE. Furthermore, exemplified by LiFeBPO, the practicality and versatility of spfMs-mediated Fe-based compounds were validated through the efficient and selective electrooxidation of various alcohols. This illustrates the broad perspective of this new concept.","abstract_html":"Background: Generating green hydrogen (H2) through alkaline water electrolysis (AWE) powered by renewable resources is crucial for decarbonizing our society. A significant challenge in AWE is kinetically sluggish anodic oxygen evolution reaction (OER). Currently, the anodes used in industrial AWE systems incorporate OER catalysts based on non-noble transition metals (TMs). These catalysts, however, only achieve moderate performance. As a result, substantial research has focused on developing a new generation of cost-effective and more efficient TM-based catalysts. Among these, iron (Fe)-based catalysts are particularly promising due to Fe's low cost, high abundance in the earth's crust, and its ability to exist in various redox states. Challenge: However, very few pure Fe-based compounds perform well for alkaline OER. During the reaction, these compounds primarily act as precatalysts, inevitably reconstructing into (oxy)hydroxides (FeOxHy) that exhibit low or non-conductivity and easily dissolve Fe atoms. Additionally, the reconstructed FeOxHy suffers from insufficient exposure of surface active sites and inadequate mass transport and bubble detachment capabilities. Moreover, although many Fe-based compounds are claimed to possess intrinsically "outstanding OER capabilities," their performances are often mis/overestimated due to unintentional interference from nickel (Ni) or cobalt (Co) (e.g., from the Ni foam substrate). Consequently, universally feasible strategies to optimize pure Fe-based compounds as efficient alkaline OER precatalysts for broader-scale applications have not yet been demonstrated. Solutions: Developing new Fe-based compounds that incorporate carefully chosen s-, p-, and f-block metals (spfMs) could effectively address the challenges mentioned above. spfMs typically exhibit two distinct behaviors during alkaline OER: ⅰ) dissolving into the aqueous electrolyte and ⅱ) remaining at the anode as new, insoluble, oxidized phases or as residual precatalysts. In the former scenario, the structural reconstruction of the targeted Fe-based precatalysts can facilitate the in-situ formation of FeOxHy, potentially increasing surface porosity, reducing crystallinity, and decreasing particle size. In the latter scenario, the remaining spfM oxides or precatalysts can enhance charge transfer, stabilize the active phase (sites), and optimize the binding and activation abilities toward reactants, intermediates, and products. As a result, the alkaline OER activity and stability can be significantly enhanced. Results: By incorporating the representative s-block metals lithium (Li) and calcium (Ca), p-block elements boron (B) and germanium (Ge), and f-block metal cerium (Ce), three unique novel bulk Fe-based compounds as precatalysts were developed, constituted by ⅰ) all leaching (LiFeBPO), ⅱ) both leaching and non-leaching (CaFe6Ge6), and ⅲ) all non-leaching (CeFe2) spfMs, respectively. We unequivocally determined that all of them underwent distinctive OER-driven reconstructions mediated by their respective SPFMs. Eventually, the precisely modified α-FeOOH in-situ evolved as the real active phase. Specifically, i) the drastic/complete loss of spfMs induced a deep/thorough reconstruction of LiFeBPO into ultrasmall low-crystalline α-FeOOH nanoparticles forming a 3D open porous skeleton. This structure increased the exposure and accessibility of active sites, leading to excellent bulk activity. Mass transport and bubble detachment were also improved. ⅱ) Regrading CaFe6Ge6, the partial loss of spfMs resulted in a porous heteroshell where α-FeOOH was coupled with CaCO3, promoting adsorption and transport of reactants, as well as activating more Fe sites for catalysis. The higher intrinsic activity of α-FeOOH was achieved, and the residual CaFe6Ge6 intermetallic core facilitated charge migration. ⅲ) All spfMs were preserved, mediating the surface conversion of CeFe2 into α-FeOOH nanodomains heterogeneously coupled with CeO2. The pseudo‐periodic confinement of insoluble CeO2 enhanced the performance of α-FeOOH by preventing active site blockage and dissolution while optimizing the electronic structure and intermediate adsorption energies. Meanwhile, the retained CeFe2 elevated the overall charge transfer. As a result, the reconstructed α-FeOOH from these three compounds exhibited boosted alkaline OER performances, surpassing those of directly synthesized FeOxHy and even comparable to or better than benchmark NiFeOxHy under identical operational conditions. Additionally, when deposited on a Ni foam substrate, they demonstrated impressive alkaline OER activity and stability, outperforming most documented TM-based counterparts. Conclusion & Perspective: This dissertation explicitly correlates the relationship between composition, reconstruction, active structure, and OER properties of the studied Fe-based compounds while avoiding interferences from Ni and Co. It illustrates that incorporating suitable spfMs is an effective strategy to optimize pure Fe-based compounds for efficient alkaline OER catalysis, enhancing the performance of anodes for practical AWE. Furthermore, exemplified by LiFeBPO, the practicality and versatility of spfMs-mediated Fe-based compounds were validated through the efficient and selective electrooxidation of various alcohols. This illustrates the broad perspective of this new concept.","abstract_has_math":false,"creators":["Yang, Hongyuan"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Driess , Matthias"],"committee_chairs":[],"committee_members":[],"year":2024,"date_issued":"2024","date_published":"2024","updated_at":"2026-07-27T21:28:57Z","subjects":[],"languages":["en"],"rights":[],"rights_urls":["https://creativecommons.org/licenses/by-nc/4.0/"],"identifier_entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://doi.org/10.14279/depositonce-22304"],"render_values":[{"text":"https://doi.org/10.14279/depositonce-22304","href":"https://doi.org/10.14279/depositonce-22304","code":true}]}]},"links":{"outbound_url":"https://depositonce.tu-berlin.de/handle/11303/23490","outbound_label":"Repository record","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Driess , Matthias"]},{"key":"dc:creator","label":"Author","values":["Yang, Hongyuan"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2024-12-09T07:59:06Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2024-12-09T07:59:06Z"]},{"key":"dc:date.issued","label":"Date","values":["2024"]},{"key":"dc:type","label":"Dc Type","values":["Doctoral Thesis"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights.uri","label":"Rights URI","values":["https://creativecommons.org/licenses/by-nc/4.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://depositonce.tu-berlin.de/handle/11303/23490","https://doi.org/10.14279/depositonce-22304"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Background: Generating green hydrogen (H2) through alkaline water electrolysis (AWE) powered by renewable resources is crucial for decarbonizing our society. A significant challenge in AWE is kinetically sluggish anodic oxygen evolution reaction (OER). Currently, the anodes used in industrial AWE systems incorporate OER catalysts based on non-noble transition metals (TMs). These catalysts, however, only achieve moderate performance. As a result, substantial research has focused on developing a new generation of cost-effective and more efficient TM-based catalysts. Among these, iron (Fe)-based catalysts are particularly promising due to Fe's low cost, high abundance in the earth's crust, and its ability to exist in various redox states. Challenge: However, very few pure Fe-based compounds perform well for alkaline OER. During the reaction, these compounds primarily act as precatalysts, inevitably reconstructing into (oxy)hydroxides (FeOxHy) that exhibit low or non-conductivity and easily dissolve Fe atoms. Additionally, the reconstructed FeOxHy suffers from insufficient exposure of surface active sites and inadequate mass transport and bubble detachment capabilities. Moreover, although many Fe-based compounds are claimed to possess intrinsically \"outstanding OER capabilities,\" their performances are often mis/overestimated due to unintentional interference from nickel (Ni) or cobalt (Co) (e.g., from the Ni foam substrate). Consequently, universally feasible strategies to optimize pure Fe-based compounds as efficient alkaline OER precatalysts for broader-scale applications have not yet been demonstrated. Solutions: Developing new Fe-based compounds that incorporate carefully chosen s-, p-, and f-block metals (spfMs) could effectively address the challenges mentioned above. spfMs typically exhibit two distinct behaviors during alkaline OER: ⅰ) dissolving into the aqueous electrolyte and ⅱ) remaining at the anode as new, insoluble, oxidized phases or as residual precatalysts. In the former scenario, the structural reconstruction of the targeted Fe-based precatalysts can facilitate the in-situ formation of FeOxHy, potentially increasing surface porosity, reducing crystallinity, and decreasing particle size. In the latter scenario, the remaining spfM oxides or precatalysts can enhance charge transfer, stabilize the active phase (sites), and optimize the binding and activation abilities toward reactants, intermediates, and products. As a result, the alkaline OER activity and stability can be significantly enhanced. Results: By incorporating the representative s-block metals lithium (Li) and calcium (Ca), p-block elements boron (B) and germanium (Ge), and f-block metal cerium (Ce), three unique novel bulk Fe-based compounds as precatalysts were developed, constituted by ⅰ) all leaching (LiFeBPO), ⅱ) both leaching and non-leaching (CaFe6Ge6), and ⅲ) all non-leaching (CeFe2) spfMs, respectively. We unequivocally determined that all of them underwent distinctive OER-driven reconstructions mediated by their respective SPFMs. Eventually, the precisely modified α-FeOOH in-situ evolved as the real active phase. Specifically, i) the drastic/complete loss of spfMs induced a deep/thorough reconstruction of LiFeBPO into ultrasmall low-crystalline α-FeOOH nanoparticles forming a 3D open porous skeleton. This structure increased the exposure and accessibility of active sites, leading to excellent bulk activity. Mass transport and bubble detachment were also improved. ⅱ) Regrading CaFe6Ge6, the partial loss of spfMs resulted in a porous heteroshell where α-FeOOH was coupled with CaCO3, promoting adsorption and transport of reactants, as well as activating more Fe sites for catalysis. The higher intrinsic activity of α-FeOOH was achieved, and the residual CaFe6Ge6 intermetallic core facilitated charge migration. ⅲ) All spfMs were preserved, mediating the surface conversion of CeFe2 into α-FeOOH nanodomains heterogeneously coupled with CeO2. The pseudo‐periodic confinement of insoluble CeO2 enhanced the performance of α-FeOOH by preventing active site blockage and dissolution while optimizing the electronic structure and intermediate adsorption energies. Meanwhile, the retained CeFe2 elevated the overall charge transfer. As a result, the reconstructed α-FeOOH from these three compounds exhibited boosted alkaline OER performances, surpassing those of directly synthesized FeOxHy and even comparable to or better than benchmark NiFeOxHy under identical operational conditions. Additionally, when deposited on a Ni foam substrate, they demonstrated impressive alkaline OER activity and stability, outperforming most documented TM-based counterparts. Conclusion & Perspective: This dissertation explicitly correlates the relationship between composition, reconstruction, active structure, and OER properties of the studied Fe-based compounds while avoiding interferences from Ni and Co. It illustrates that incorporating suitable spfMs is an effective strategy to optimize pure Fe-based compounds for efficient alkaline OER catalysis, enhancing the performance of anodes for practical AWE. Furthermore, exemplified by LiFeBPO, the practicality and versatility of spfMs-mediated Fe-based compounds were validated through the efficient and selective electrooxidation of various alcohols. This illustrates the broad perspective of this new concept.","Hintergrund: Die Erzeugung von grünem Wasserstoff (H2) durch alkalische Wasserelektrolyse (AWE = alkaline water electrolysis), die mit erneuerbaren Ressourcen betrieben wird, ist entscheidend für die Dekarbonisierung unsere Gesellschaft. Eine große Herausforderung bei der AWE ist die anodische Sauerstoffentwicklungsreaktion (OER = oxygen evolution reaction), die durch ihre komplexe Kinetik nur gehemmt abläuft. Derzeit werden für die Anoden industrieller AWE-Systeme OER-Katalysatoren auf der Basis unedler Übergangsmetalle (TMs = transition metals) verwendet, welche jedoch nur eine mäßige Aktivität aufweisen. Daher wurde intensiv an der Entwicklung einer neuen Generation von kostengünstigen und effizienten Katalysatoren auf TM-Basis geforscht. Unter diesen sind Katalysatoren auf der Fe-Basis besonders vielversprechend, da Fe kostengünstig ist, reichlich in der Erdkruste vorkommt und in verschiedenen Redoxzuständen vorliegen kann. Herausforderung: Es gibt jedoch nur sehr wenige reine Fe-Verbindungen, die sich gut für alkalische OER eignen. Während der Reaktion fungieren diese Verbindungen in erster Linie als Präkatalysatoren, die unweigerlich in (Oxy)hydroxide (FeOxHy) umgewandelt werden, welche dann eine geringe oder keine Leitfähigkeit aufweisen und anfällig für Zersetzungsprozesse sind. Darüber hinaus verfügt das rekonstruierte FeOxHy nur über eine geringe Oberfläche und damit oberflächenaktiven Zentren, sowie schlechteren Massentransport-eigenschaften und gehemmter Desorption der erzeugten Sauerstoffgasblasen. Obwohl vielen Verbindungen auf Fe-Basis \"hervorragende OER-Aktivität\" nachgesagt werden, wird ihre Leistung aufgrund unbeabsichtigter Einflüsse durch Nickel (Ni) oder Kobalt (Co) (z. B. durch das Ni-Schaum-Substrat) häufig falsch eingeschätzt. Folglich gibt es keine allgemein anwendbaren Strategien zur Optimierung reiner Eisenverbindungen als effiziente OER-Präkatalysatoren im Alkalischen. Lösungen: Um die oben genannten Herausforderungen erfolgreich angehen zu können, bedarf es der Entwicklung neuer Verbindungen auf Fe-Basis, die auch s-, p- und f-Block-Metalle (spfMs = s-, p-, and f-block metals) enthalten. spfMs zeigen typischerweise zwei unterschiedliche Verhaltensweisen während der alkalischen OER auf: ⅰ) sie werden zersetzt und lösen sich in dem wässrigen Elektrolyten, und ⅱ) sie verbleiben an der Anode als neue, unlösliche, oxidierte Phasen oder in Spuren als Präkatalysator. Im ersten Szenario kann die strukturelle Rekonstruktion der angestrebten Fe-basierten Präkatalysatoren die in-situ Bildung von FeOxHy erleichtern, was die Oberflächenporosität erhöhen, die Kristallinität verringern und die Partikelgröße reduzieren kann. Im letztgenannten Szenario können die verbleibenden spfM-Oxide oder -Präkatalysatoren den Ladungstransfer verbessern, die aktive Phase (Zentren) stabilisieren und die Bindungs- und Aktivierungsfähigkeiten gegenüber Reaktanten, Zwischenprodukten und Produkten optimieren. Infolgedessen können die Aktivität und Stabilität der alkalischen OER erheblich verbessert werden. Ergebnisse: Durch die Einbindung der s-Block-Metalle wie Lithium (Li) und Calcium (Ca), der p-Block-Elemente wie Bor (B) und Germanium (Ge) und des f-Block-Metalls Cer (Ce), wurden drei einzigartige, neuartige, auf Fe-basierende Verbindungen als Prekatalysatoren hergestellt, die sich durch eine ⅰ) vollständig auslaugende (LiFeBPO), ⅱ) sowohl auslaugende als auch nicht auslaugende (CaFe6Ge6) bzw. ⅲ) nicht auslaugende (CeFe2) Struktur der spfMs auszeichnen. Wir haben eindeutig festgestellt, dass alle von ihnen durch ihre jeweiligen spfMs unterschiedliche OER-getriebene Rekonstruktionen durchlaufen haben. Letztendlich entstand in-situ das präzise modifizierte α-FeOOH, welches als die eigentliche aktive Phase identifiziert wurde. Spezifisch, im Fall i) LiFeBPO führte der drastische/vollständige Verlust der spfMs zu einer Rekonstruktion in ultrakleine, niedrigkristalline α-FeOOH-Nanopartikel, die ein offenporiges 3D-Skelett bildeten. Durch diese Struktur wurde die Exposition und Zugänglichkeit der aktiven Zentren erheblich verbessert, was zu einer ausgezeichneten Volumenaktivität führte. Auch der Massentransport und die Desorption der gebildeten Gasblasen wurden deutlich verbessert. ii) Bei CaFe6Ge6 führte der Teilverlust der spfMs zu einer hochporösen Hetero-Schale, in der α-FeOOH stark mit CaCO3 gekoppelt war. Dies förderte die Adsorption und den Transport von Reaktanten und aktivierte mehr Fe-Zentren für die Katalyse. Dadurch wurde eine Steigerung der intrinsischen Aktivität von α-FeOOH erreicht, und der verbleibende intermetallische CaFe6Ge6-Kern erleichterte die Ladungsmigration. iii) Alle spfMs blieben erhalten. Dies regulierte die Rekonstruktion der Oberflächen von CeFe2 in α-FeOOH-Nanodomänen, die heterogen mit CeO2 gekoppelt waren. Der pseudo-periodische Einschluss von unlöslichem CeO2 verbesserte die Leistung von α-FeOOH, indem er die Blockierung der aktiven Stelle und die Auflösung verhinderte und gleichzeitig die elektronische Struktur und die Adsorptionsenergien optimierte. Gleichzeitig erhöhte das erhaltene CeFe2 den gesamten Ladungstransfer. Infolgedessen zeigten die rekonstruierten α-FeOOH aus diesen drei Verbindungen verbesserte alkalische OER-Leistungen, die diejenigen von direkt synthetisiertem FeOxHy übertrafen und sogar mit denen von NiFeOxHy unter identischen Betriebsbedingungen vergleichbar oder besser waren. Darüber hinaus zeigten sie, wenn sie auf einem Ni-Schaum-Substrat abgeschieden wurden, eine beeindruckende alkalische OER-Aktivität und Stabilität, die die meisten dokumentierten Gegenstücke auf TM-Basis übertrafen. Zusammenfassung und Ausblick: In dieser Dissertation wird die Beziehung zwischen Zusammensetzung, Rekonstruktion, aktiver Struktur und OER-Eigenschaften der untersuchten Fe-basierten Verbindungen untersucht, wobei Einflüsse durch Ni und Co vermieden werden. Es zeigt, dass die Einbeziehung geeigneter spfMs eine wirksame Strategie zur Optimierung reiner Fe-basierter Verbindungen für eine effiziente alkalische OER-Katalyse ist, die die Leistung von Anoden für praktische AWE verbessert. Darüber hinaus wurden am Beispiel von LiFeBPO die Praktikabilität und Vielseitigkeit Fe-basierten Verbindungen gepaart mit spfMs demonstriert, indem zusätzlich eine effiziente und selektive Elektrooxidation verschiedener Alkohole nachgewiesen werden konnte. Dies veranschaulicht das breite Anwendungsspektrum dieses neuen Konzepts."]},{"key":"dc:title","label":"Title","values":["s-, p-, and f-block metals-mediated novel iron-based compounds as precatalysts for efficient alkaline OER"]}]}],"canonical_facts":{"dc:contributor.advisor":["Driess , Matthias"],"dc:creator":["Yang, Hongyuan"],"dc:date.accessioned":["2024-12-09T07:59:06Z"],"dc:date.available":["2024-12-09T07:59:06Z"],"dc:date.issued":["2024"],"dc:description.abstract":["Background: Generating green hydrogen (H2) through alkaline water electrolysis (AWE) powered by renewable resources is crucial for decarbonizing our society. A significant challenge in AWE is kinetically sluggish anodic oxygen evolution reaction (OER). Currently, the anodes used in industrial AWE systems incorporate OER catalysts based on non-noble transition metals (TMs). These catalysts, however, only achieve moderate performance. As a result, substantial research has focused on developing a new generation of cost-effective and more efficient TM-based catalysts. Among these, iron (Fe)-based catalysts are particularly promising due to Fe's low cost, high abundance in the earth's crust, and its ability to exist in various redox states. Challenge: However, very few pure Fe-based compounds perform well for alkaline OER. During the reaction, these compounds primarily act as precatalysts, inevitably reconstructing into (oxy)hydroxides (FeOxHy) that exhibit low or non-conductivity and easily dissolve Fe atoms. Additionally, the reconstructed FeOxHy suffers from insufficient exposure of surface active sites and inadequate mass transport and bubble detachment capabilities. Moreover, although many Fe-based compounds are claimed to possess intrinsically \"outstanding OER capabilities,\" their performances are often mis/overestimated due to unintentional interference from nickel (Ni) or cobalt (Co) (e.g., from the Ni foam substrate). Consequently, universally feasible strategies to optimize pure Fe-based compounds as efficient alkaline OER precatalysts for broader-scale applications have not yet been demonstrated. Solutions: Developing new Fe-based compounds that incorporate carefully chosen s-, p-, and f-block metals (spfMs) could effectively address the challenges mentioned above. spfMs typically exhibit two distinct behaviors during alkaline OER: ⅰ) dissolving into the aqueous electrolyte and ⅱ) remaining at the anode as new, insoluble, oxidized phases or as residual precatalysts. In the former scenario, the structural reconstruction of the targeted Fe-based precatalysts can facilitate the in-situ formation of FeOxHy, potentially increasing surface porosity, reducing crystallinity, and decreasing particle size. In the latter scenario, the remaining spfM oxides or precatalysts can enhance charge transfer, stabilize the active phase (sites), and optimize the binding and activation abilities toward reactants, intermediates, and products. As a result, the alkaline OER activity and stability can be significantly enhanced. Results: By incorporating the representative s-block metals lithium (Li) and calcium (Ca), p-block elements boron (B) and germanium (Ge), and f-block metal cerium (Ce), three unique novel bulk Fe-based compounds as precatalysts were developed, constituted by ⅰ) all leaching (LiFeBPO), ⅱ) both leaching and non-leaching (CaFe6Ge6), and ⅲ) all non-leaching (CeFe2) spfMs, respectively. We unequivocally determined that all of them underwent distinctive OER-driven reconstructions mediated by their respective SPFMs. Eventually, the precisely modified α-FeOOH in-situ evolved as the real active phase. Specifically, i) the drastic/complete loss of spfMs induced a deep/thorough reconstruction of LiFeBPO into ultrasmall low-crystalline α-FeOOH nanoparticles forming a 3D open porous skeleton. This structure increased the exposure and accessibility of active sites, leading to excellent bulk activity. Mass transport and bubble detachment were also improved. ⅱ) Regrading CaFe6Ge6, the partial loss of spfMs resulted in a porous heteroshell where α-FeOOH was coupled with CaCO3, promoting adsorption and transport of reactants, as well as activating more Fe sites for catalysis. The higher intrinsic activity of α-FeOOH was achieved, and the residual CaFe6Ge6 intermetallic core facilitated charge migration. ⅲ) All spfMs were preserved, mediating the surface conversion of CeFe2 into α-FeOOH nanodomains heterogeneously coupled with CeO2. The pseudo‐periodic confinement of insoluble CeO2 enhanced the performance of α-FeOOH by preventing active site blockage and dissolution while optimizing the electronic structure and intermediate adsorption energies. Meanwhile, the retained CeFe2 elevated the overall charge transfer. As a result, the reconstructed α-FeOOH from these three compounds exhibited boosted alkaline OER performances, surpassing those of directly synthesized FeOxHy and even comparable to or better than benchmark NiFeOxHy under identical operational conditions. Additionally, when deposited on a Ni foam substrate, they demonstrated impressive alkaline OER activity and stability, outperforming most documented TM-based counterparts. Conclusion & Perspective: This dissertation explicitly correlates the relationship between composition, reconstruction, active structure, and OER properties of the studied Fe-based compounds while avoiding interferences from Ni and Co. It illustrates that incorporating suitable spfMs is an effective strategy to optimize pure Fe-based compounds for efficient alkaline OER catalysis, enhancing the performance of anodes for practical AWE. Furthermore, exemplified by LiFeBPO, the practicality and versatility of spfMs-mediated Fe-based compounds were validated through the efficient and selective electrooxidation of various alcohols. This illustrates the broad perspective of this new concept.","Hintergrund: Die Erzeugung von grünem Wasserstoff (H2) durch alkalische Wasserelektrolyse (AWE = alkaline water electrolysis), die mit erneuerbaren Ressourcen betrieben wird, ist entscheidend für die Dekarbonisierung unsere Gesellschaft. Eine große Herausforderung bei der AWE ist die anodische Sauerstoffentwicklungsreaktion (OER = oxygen evolution reaction), die durch ihre komplexe Kinetik nur gehemmt abläuft. Derzeit werden für die Anoden industrieller AWE-Systeme OER-Katalysatoren auf der Basis unedler Übergangsmetalle (TMs = transition metals) verwendet, welche jedoch nur eine mäßige Aktivität aufweisen. Daher wurde intensiv an der Entwicklung einer neuen Generation von kostengünstigen und effizienten Katalysatoren auf TM-Basis geforscht. Unter diesen sind Katalysatoren auf der Fe-Basis besonders vielversprechend, da Fe kostengünstig ist, reichlich in der Erdkruste vorkommt und in verschiedenen Redoxzuständen vorliegen kann. Herausforderung: Es gibt jedoch nur sehr wenige reine Fe-Verbindungen, die sich gut für alkalische OER eignen. Während der Reaktion fungieren diese Verbindungen in erster Linie als Präkatalysatoren, die unweigerlich in (Oxy)hydroxide (FeOxHy) umgewandelt werden, welche dann eine geringe oder keine Leitfähigkeit aufweisen und anfällig für Zersetzungsprozesse sind. Darüber hinaus verfügt das rekonstruierte FeOxHy nur über eine geringe Oberfläche und damit oberflächenaktiven Zentren, sowie schlechteren Massentransport-eigenschaften und gehemmter Desorption der erzeugten Sauerstoffgasblasen. Obwohl vielen Verbindungen auf Fe-Basis \"hervorragende OER-Aktivität\" nachgesagt werden, wird ihre Leistung aufgrund unbeabsichtigter Einflüsse durch Nickel (Ni) oder Kobalt (Co) (z. B. durch das Ni-Schaum-Substrat) häufig falsch eingeschätzt. Folglich gibt es keine allgemein anwendbaren Strategien zur Optimierung reiner Eisenverbindungen als effiziente OER-Präkatalysatoren im Alkalischen. Lösungen: Um die oben genannten Herausforderungen erfolgreich angehen zu können, bedarf es der Entwicklung neuer Verbindungen auf Fe-Basis, die auch s-, p- und f-Block-Metalle (spfMs = s-, p-, and f-block metals) enthalten. spfMs zeigen typischerweise zwei unterschiedliche Verhaltensweisen während der alkalischen OER auf: ⅰ) sie werden zersetzt und lösen sich in dem wässrigen Elektrolyten, und ⅱ) sie verbleiben an der Anode als neue, unlösliche, oxidierte Phasen oder in Spuren als Präkatalysator. Im ersten Szenario kann die strukturelle Rekonstruktion der angestrebten Fe-basierten Präkatalysatoren die in-situ Bildung von FeOxHy erleichtern, was die Oberflächenporosität erhöhen, die Kristallinität verringern und die Partikelgröße reduzieren kann. Im letztgenannten Szenario können die verbleibenden spfM-Oxide oder -Präkatalysatoren den Ladungstransfer verbessern, die aktive Phase (Zentren) stabilisieren und die Bindungs- und Aktivierungsfähigkeiten gegenüber Reaktanten, Zwischenprodukten und Produkten optimieren. Infolgedessen können die Aktivität und Stabilität der alkalischen OER erheblich verbessert werden. Ergebnisse: Durch die Einbindung der s-Block-Metalle wie Lithium (Li) und Calcium (Ca), der p-Block-Elemente wie Bor (B) und Germanium (Ge) und des f-Block-Metalls Cer (Ce), wurden drei einzigartige, neuartige, auf Fe-basierende Verbindungen als Prekatalysatoren hergestellt, die sich durch eine ⅰ) vollständig auslaugende (LiFeBPO), ⅱ) sowohl auslaugende als auch nicht auslaugende (CaFe6Ge6) bzw. ⅲ) nicht auslaugende (CeFe2) Struktur der spfMs auszeichnen. Wir haben eindeutig festgestellt, dass alle von ihnen durch ihre jeweiligen spfMs unterschiedliche OER-getriebene Rekonstruktionen durchlaufen haben. Letztendlich entstand in-situ das präzise modifizierte α-FeOOH, welches als die eigentliche aktive Phase identifiziert wurde. Spezifisch, im Fall i) LiFeBPO führte der drastische/vollständige Verlust der spfMs zu einer Rekonstruktion in ultrakleine, niedrigkristalline α-FeOOH-Nanopartikel, die ein offenporiges 3D-Skelett bildeten. Durch diese Struktur wurde die Exposition und Zugänglichkeit der aktiven Zentren erheblich verbessert, was zu einer ausgezeichneten Volumenaktivität führte. Auch der Massentransport und die Desorption der gebildeten Gasblasen wurden deutlich verbessert. ii) Bei CaFe6Ge6 führte der Teilverlust der spfMs zu einer hochporösen Hetero-Schale, in der α-FeOOH stark mit CaCO3 gekoppelt war. Dies förderte die Adsorption und den Transport von Reaktanten und aktivierte mehr Fe-Zentren für die Katalyse. Dadurch wurde eine Steigerung der intrinsischen Aktivität von α-FeOOH erreicht, und der verbleibende intermetallische CaFe6Ge6-Kern erleichterte die Ladungsmigration. iii) Alle spfMs blieben erhalten. Dies regulierte die Rekonstruktion der Oberflächen von CeFe2 in α-FeOOH-Nanodomänen, die heterogen mit CeO2 gekoppelt waren. Der pseudo-periodische Einschluss von unlöslichem CeO2 verbesserte die Leistung von α-FeOOH, indem er die Blockierung der aktiven Stelle und die Auflösung verhinderte und gleichzeitig die elektronische Struktur und die Adsorptionsenergien optimierte. Gleichzeitig erhöhte das erhaltene CeFe2 den gesamten Ladungstransfer. Infolgedessen zeigten die rekonstruierten α-FeOOH aus diesen drei Verbindungen verbesserte alkalische OER-Leistungen, die diejenigen von direkt synthetisiertem FeOxHy übertrafen und sogar mit denen von NiFeOxHy unter identischen Betriebsbedingungen vergleichbar oder besser waren. Darüber hinaus zeigten sie, wenn sie auf einem Ni-Schaum-Substrat abgeschieden wurden, eine beeindruckende alkalische OER-Aktivität und Stabilität, die die meisten dokumentierten Gegenstücke auf TM-Basis übertrafen. Zusammenfassung und Ausblick: In dieser Dissertation wird die Beziehung zwischen Zusammensetzung, Rekonstruktion, aktiver Struktur und OER-Eigenschaften der untersuchten Fe-basierten Verbindungen untersucht, wobei Einflüsse durch Ni und Co vermieden werden. Es zeigt, dass die Einbeziehung geeigneter spfMs eine wirksame Strategie zur Optimierung reiner Fe-basierter Verbindungen für eine effiziente alkalische OER-Katalyse ist, die die Leistung von Anoden für praktische AWE verbessert. Darüber hinaus wurden am Beispiel von LiFeBPO die Praktikabilität und Vielseitigkeit Fe-basierten Verbindungen gepaart mit spfMs demonstriert, indem zusätzlich eine effiziente und selektive Elektrooxidation verschiedener Alkohole nachgewiesen werden konnte. Dies veranschaulicht das breite Anwendungsspektrum dieses neuen Konzepts."],"dc:identifier.uri":["https://depositonce.tu-berlin.de/handle/11303/23490","https://doi.org/10.14279/depositonce-22304"],"dc:language.iso":["en"],"dc:rights.uri":["https://creativecommons.org/licenses/by-nc/4.0/"],"dc:title":["s-, p-, and f-block metals-mediated novel iron-based compounds as precatalysts for efficient alkaline OER"],"dc:type":["Doctoral Thesis"]},"updated_at":"2026-07-27T21:28:57Z"}