Aplicación de un diseño experimental factorial en el estudio de la adsorción de fenol y nitrofenoles con nanofibras de carbono

El objetivo principal de la presente investigación fue el estudio de la adsorción de fenol, 2-nitrofenol, 4-nitrofenol y 2,4-dinitrofenol con nanofibras de carbono (CNF) como adsorbente mediante el uso de un diseño experimental aplicando el modelo factorial de Box Wilson. El uso de este modelo permitió estudiar el efecto en conjunto de diferentes variables (pH, fuerza iónica y concentración inicial del adsorbato) y encontrar las condiciones óptimas para la adsorción. Las CNF fueron sintetizadas por el método CVD utilizando una mezcla de etileno e hidrógeno en un reactor tubular de lecho fijo empleando un catalizador de Ni/SiO2 por 4h a 873K. El material adsorbente (CNF) fue caracterizado mediante diferentes técnicas instrumentales: ATR, FTIR, DRX, SEM, EDX, sorción de N2, titulación Boehm y pHPZC. El análisis por DRX permitió la determinación de las fases cristalográficas en la estructura y la naturaleza grafítica del material. Se encontraron dos picos representativos de planos grafíticos (d101, 44,52° y d002, 25,76°). La titulación Boehm permitió determinar los grupos ácidos superficiales: fenólicos 12,22±0,28 mmolH+/gCNF, lactónicos 6,47±0,12 mmolH+/gCNF y carboxílicos 0,89±0,17 mmolH+/gCNF. La espectroscopia infrarroja (FTIR) permitió identificar los grupos funcionales. El análisis por SEM mostró que las nanofibras se encuentran aglomeradas, sin un ordenamiento aparente, relacionado posiblemente con la alta temperatura empleada en la síntesis (600°C). El análisis elemental realizado (EDX) indica la presencia de solo dos elementos: C (96,4%) y Ni (3,36%), éste último relacionado con el catalizador empleado en la síntesis. Mediante la sorción de N2 se determinó que las CNF tenían un área superficial de 120m2.g-1. Las CNF resultaron ser mayoritariamente micro-mesoporosas, lo que podría favorecer la adsorción de moléculas grandes como el fenol y los nitrofenoles. La isoterma de adsorción de N2 fue de tipo IV, según la clasificación IUPAC, típica de materiales carbonosos. El punto de carga cero encontrado fue de 6,5. El estudio de la cinética de adsorción permitió determinar los tiempos de equilibrio que fueron: 250min para fenol, 300min para 4-nitrofenol y 2,4-dinitrofenol y 400min para 2-nitrofenol. Se aplicó el diseño experimental en base al modelo factorial de Box-Wilson con dos variables (pH y fuerza iónica) y tres variables (pH, fuerza iónica y concentración inicial de adsorbato). A partir de los ensayos propuestos por el diseño factorial se obtuvieron ecuaciones de regresión (funciones de respuesta) de dos y tres variables utilizando el software estadístico JMP® 7.0.1. En base al análisis matemático de estas funciones, se determinó que las condiciones de máxima adsorción para todos los adsorbentes fueron pH = 1 y 20%NaCl. Las condiciones medias fueron pH = 7 y 10%NaCl y las condiciones menos favorables fueron pH = 13 y 0%NaCl. A partir de estas condiciones se realizaron las isotermas de adsorción. Utilizando el software de química computacional HyperChemTM 8.0.3 se realizaron los cálculos de densidad electrónica para los adsorbatos de estudio bajo diferentes condiciones de pH. Se encontró que, para todas las especies adsorbidas, las condiciones ácidas aumentan la electrofilicidad del anillo aromático por una disminución de la densidad electrónica en él mismo. Además, la sustitución de un grupo nitro (NO2) en la estructura lleva a un aumento de la electrofilicidad (disminución de la densidad de carga en el anillo) por su carácter atractor, lo que se tradujo en un aumento de la adsorción. Así mismo, para elucidar el posible mecanismo de adsorción se determinaron las isotermas de adsorción. Los resultados experimentales se correlacionaron con seis modelos de isotermas: Freundlich, Langmuir, Elovich, Temkin, Redlich-Peterson, Dubinin-Radushkevich. En general, se encontró que la adsorción se produce en centros activos con una superficie mixta y con una distribución homogénea de energía. El orden descendente obtenido en relación con a la capacidad de adsorción promedio fue: 2,4-dinitrofenol > 4-nitrofenol > 2-nitrofenol > fenol. Este orden se explica mediante el efecto del pH y la fuerza iónica en la adsorción. El pH tiene el efecto de protonar/desprotonar al adsorbato y este hace variar la electrofilicidad que es determinante en la adsorción. Por otro lado, la fuerza iónica está relacionada con el efecto de “salting out”, que hace cambiar la solubilidad de los adsorbatos debido a la presencia de electrólitos en solución.Tesi

Stannites – a new promising class of durable electrocatalysts for efficient water oxidation

The oxygen evolution reaction (OER) through water oxidation is a key process for multiple energy storage technologies required for a sustainable energy economy such as the formation of the fuel hydrogen from water and electricity, or metal‐air batteries. Herein, we investigate the suitability of Cu2FeSnS4 for the OER and demonstrate its superiority over iron sulfide, iron (oxy)hydroxides and benchmark noble‐metal catalysts in alkaline media. Electrodeposited Cu2FeSnS4 yields the current densities of 10 and 1000 mA/cm2 at overpotentials of merely 228 and 330 mV, respectively. State‐of‐the‐art analytical methods are applied before and after electrocatalysis to uncover the fate of the Cu2FeSnS4 precatalyst under OER conditions and to deduce structure‐activity relationships. Cu2FeSnS4 is the first compound reported for OER among the broad class of stannite structure type materials containing multiple members with highly active earth‐abundant transition‐metals for OER.DFG, 390540038, EXC 2008: Cluster of Excellence UniSysCatTU Berlin, Open-Access-Mittel - 201

Boosting water oxidation through in situ electroconversion of manganese gallide: an intermetallic precursor approach

For the first time, the manganese gallide (MnGa4) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnOx‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ‐MnO2, feitknechtite β‐MnOOH, and hausmannite α‐Mn3O4. The abundance and intrinsic stabilization of MnIII/MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm−2 with a durability of more than five days.DFG, 390540038, EXC 2008: UniSysCatTU Berlin, Open-Access-Mittel - 201

Enabling Iron‐Based Highly Effective Electrochemical Water‐Splitting and Selective Oxygenation of Organic Substrates through In Situ Surface Modification of Intermetallic Iron Stannide Precatalyst

A strategy to overcome the unsatisfying catalytic performance and the durability of monometallic iron‐based materials for the electrochemical oxygen evolution reaction (OER) is provided by heterobimetallic iron–metal systems. Monometallic Fe catalysts show limited performance mostly due to poor conductivity and stability. Here, by taking advantage of the structurally ordered and highly conducting FeSn2 nanostructure, for the first time, an intermetallic iron material is employed as an efficient anode for the alkaline OER, overall water‐splitting, and also for selective oxygenation of organic substrates. The electrophoretically deposited FeSn2 on nickel foam (NF) and fluorine‐doped tin oxide (FTO) electrodes displays remarkable OER activity and durability with substantially low overpotentials of 197 and 273 mV at 10 mA cm−2, respectively, which outperform most of the benchmarking NiFe‐based catalysts. The resulting superior activity is attributed to the in situ generation of α‐FeO(OH)@FeSn2 where α‐FeO(OH) acts as the active site while FeSn2 remains the conductive core. When the FeSn2 anode is coupled with a Pt cathode for overall alkaline water‐splitting, a reduced cell potential (1.53 V) is attained outperforming that of noble metal‐based catalysts. FeSn2 is further applied as an anode to produce value‐added products through selective oxygenation reactions of organic substrates.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat"TU Berlin, Open-Access-Mittel – 202

Crystalline Copper Selenide as a Reliable Non‐Noble Electro(pre)catalyst for Overall Water Splitting

Electrochemical water splitting remains a frontier research topic in the quest to develop artificial photosynthetic systems by using noble metal‐free and sustainable catalysts. Herein, a highly crystalline CuSe has been employed as active electrodes for overall water splitting (OWS) in alkaline media. The pure‐phase klockmannite CuSe deposited on highly conducting nickel foam (NF) electrodes by electrophoretic deposition (EPD) displayed an overpotential of merely 297 mV for the reaction of oxygen evolution (OER) at a current density of 10 mA cm−2 whereas an overpotential of 162 mV was attained for the hydrogen evolution reaction (HER) at the same current density, superseding the Cu‐based as well as the state‐of‐the‐art RuO2 and IrO2 catalysts. The bifunctional behavior of the catalyst has successfully been utilized to fabricate an overall water‐splitting device, which exhibits a low cell voltage (1.68 V) with long‐term stability. Post‐catalytic analyses of the catalyst by ex‐situ microscopic, spectroscopic, and analytical methods confirm that under both OER and HER conditions, the crystalline and conductive CuSe behaves as an electro(pre)catalyst forming a highly reactive in situ crystalline Cu(OH)2 overlayer (electro(post)catalyst), which facilitates oxygen (O2) evolution, and an amorphous Cu(OH)2/CuOx active surface for hydrogen (H2) evolution. The present study demonstrates a distinct approach to produce highly active copper‐based catalysts starting from copper chalcogenides and could be used as a basis to enhance the performance in durable bifunctional overall water splitting.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat"TU Berlin, Open-Access-Mittel – 202

Synthese von molekular abgeleiteten Übergangsmetall-Pniktiden für eine effiziente Wasserspaltung

The generation of renewable and sustainable electricity can be achieved through fuel cells by the reaction hydrogen (H2) with oxygen (O2). In order to meet the current energy demand with fuel cells, a large feedstock of H2 is necessary. Alkaline water electrolysis is the only mature energy conversion technology for the renewable production of H2 through the hydrogen evolution reaction (HER). The efficiency of water-splitting is limited by its other half reaction, the oxygen evolution reaction (OER), due to the multiple proton-coupled electron-transfer (PCET) steps involved in it. Currently, ruthenium- and iridium-based catalysts and elemental platinum still represent the benchmark catalysts for practical applications of OER and HER, respectively. Nonetheless, their high cost and scarce availability limit their use in large-scale applications. Transition metal pnictides have become an attractive choice as OER and HER electrocatalysts due to their low hydrogen adsorption energies, faster reactions kinetics, and electroconductivity. Common synthesis of transition metal pnictides are based in solid-state methods, which lead to materials with uncontrolled aggregation, heterogeneous morphology, and random composition. The design of nanostructured electrocatalysts of specific size, stoichiometry, and morphology can be achieved by the molecular decomposition of single-source precursors (SSP). The tuning of the experimental conditions (i.e., decomposition method, reaction time, temperature, solvent) leads to nanostructures of different characteristics. In this work, a molecular synthetic approach has been used to access transition metal phosphides (TM-Ps) and arsenides (TM-As). Deeper investigations have been carried out to understand the kinetics and electronic processes during catalysis and relate them to the chemical and electronic transformations of the materials. Amorphous materials are known for their large electrochemically active surface (ECSA), which usually allows higher electrocatalytic activities in comparison to their crystalline counterparts. Keeping this in mind, amorphous and crystalline cobalt phosphide (CoP) have been synthesized by the thermolysis (hot-injection and pyrolysis) of a SSP. Amorphous CoP notably displayed higher activity for all tested electrocatalytic reactions in comparison to the crystalline CoP. During OER, the electrochemical corrosion and oxidation of phosphorus led to the formation on both materials of a CoIII (oxy)hydroxide shell. The presence of CoIV OER active sites in the surface, in combination with CoIII led to distorted CoO6 octahedra (Jahn-Teller distortion) that facilitated the absorption of oxygen intermediates. During HER, a complete loss of surface phosphorus and oxidation of the surface led to the formation of a Co-enriched (oxy)hydroxide phase, followed by an in situ reduction to generate Co0 active sites for HER. The difference in activity was related to unique electronic properties and surface characteristics of the high ECSA of amorphous materials, that offers a larger number of active centers and larger flexibility upon catalysis. TM-As have received little attention in water-splitting applications due to the high toxicity of As. However, their high electrical conductivity render these materials active for OER electrocatalysis. Crystalline FeAs nanoparticles were accessed through the hot-injection of a SSP. The electrochemical exploration showed high OER catalytic activity and stability in comparison to the other Fe-based reference electrodes and Fe-based electrocatalysts reported in the literature. During OER, the material underwent corrosion, which caused the complete oxidation and dissolution of As into the electrolyte. Ex-situ characterizations, as well as quasi in situ XAS, revealed the total electroconversion of the FeAs to a highly porous nanocrystalline iron (oxy)hydroxide phase, the 2-line ferrihydrite. The semiconductor nature of this phase enabled faster electron transport. Moreover, the generated phase contained Jahn-Teller distorted FeIII atoms that behaved as active centers for OER. Importantly, the dissolved As was successfully recaptured at the cathode making the complete process sustainable and energy-efficient. The influence of the non-metal (pnictogen) in the activity was studied by preparing transition metal pnictides with equal metal content, morphology, and composition. A room temperature salt-metathesis was used to access amorphous NiAs and NiP. Electrochemical experiments revealed a higher OER activity of NiP in comparison to NiAs and other Ni-based materials reported in the literature. The post-operando analysis revealed oxidation and dissolution of the pnictogen during OER and generation of a γ-NiIIIOOH phase. This phase contained sheets of edge-sharing NiO6 octahedra separated by a large interlayer distance (7 Å), which provided the space for the intercalation of water and anions which were exposed to the NiIII active centers. The difference in activity was attributed to the higher loss of the pnictogen element in NiP (86 %) in comparison to the NiAs (45 %), which resulted in a larger structural transformation of the NiP to a γ-NiIIIOOH phase. This work illuminates that easy preparation routes towards nanoscaled TM-As and TM-Ps of different morphology and composition which could ensure the physical and electronic properties necessary for OER, HER and OWS. The studied materials showed electrocatalytic activities under alkaline conditions and comparable to the ones of precious-metal-based catalysts. Moreover, the developed methods are flexible and shows a wide range of attainable products depending on the preparation route. Therefore, it opens the route to the facile preparation of transition metal pnictides and their further application in other areas.Die Erzeugung von erneuerbarem und nachhaltigem Strom kann mittels Brennstoffzellen durch die Reaktion von Wasserstoff (H2) mit Sauerstoff (O2) erreicht werden. Um den aktuellen Energiebedarf mit Brennstoffzellen abzudecken, werden große Mengen H2 benötigt. Die alkalische Wasserelektrolyse ist die einzige ausgereifte Energieumwandlungstechnologie für die nachhaltige Erzeugung von H2 durch die Wasserstoffentwicklungsreaktion (HER). Die Effizienz der Wasserspaltung wird durch die andere dazugehörige Halbreaktion, die Sauerstoffentwicklungsreaktion (OER), aufgrund der damit verbundenen mehrfachen Protonen gekoppelten Elektronentransferschritte (PCET) begrenzt. Derzeit sind Katalysatoren auf Ruthenium- und Iridiumbasis sowie Platin immer noch die Benchmark-Katalysatoren für praktische Anwendungen von OER und HER. Ihre hohen Kosten und ihre geringe Verfügbarkeit schränken jedoch ihre Verwendung im benötigten industriellen Maßstab ein. Pniktide von Übergangsmetallen (TM) sind aufgrund ihrer geringen Wasserstoffadsorptionsenergien, der schnelleren Reaktionskinetik und der elektrischen Leitfähigkeit zu einer attraktiven Wahl als OER- und HER-Elektrokatalysatoren geworden. Gängige Synthesen für Übergangsmetallpniktide basieren auf Festkörpermethoden, die zu Materialien mit unkontrollierter Aggregation, heterogener Morphologie und zufälliger Zusammensetzung führen. Das Design von nanostrukturierten Elektrokatalysatoren mit spezifischer Größe, Stöchiometrie und Morphologie kann durch die molekulare Zersetzung von Single-Source-Vorläufern (SSP) erreicht werden. Die Abstimmung der experimentellen Bedingungen (z. B. Zersetzungsverfahren, Reaktionszeit, Temperatur, Lösungsmittel) führt zu Nanostrukturen mit unterschiedlichen Eigenschaften. In dieser Arbeit wurde ein molekularer Syntheseansatz verwendet, um auf Übergangsmetallphosphide (TM-Ps) und Arsenide (TM-As) zuzugreifen. Es wurden umfangreiche Untersuchungen durchgeführt, um die Kinetik und die elektronischen Prozesse während der Katalyse zu verstehen und diese mit den chemischen und elektronischen Umwandlungen der Materialien in Zusammenhang zu bringen. Amorphe Materialien sind bekannt für ihre große elektrochemisch aktive Oberfläche (ECSA), die im Vergleich zu ihren kristallinen Gegenstücken normalerweise höhere elektrokatalytische Aktivitäten ermöglicht. Vor diesem Hintergrund wurden amorphes und kristallines Kobaltphosphid (CoP) durch Thermolyse (Hot-Injection und Pyrolyse) eines SSP synthetisiert. Amorphes CoP zeigte bei allen getesteten, elektrokatalytischen Reaktionen im Vergleich zum kristallinen CoP eine deutlich höhere Aktivität. Während der OER führten die elektrochemische Korrosion und Oxidation von Phosphor zur Bildung einer CoIII-(oxy)hydroxidhülle auf beiden Materialien. Das Vorhandensein von aktiven CoIV OER-Zentren an der Oberfläche in Kombination mit CoIII führte zu verzerrten CoO6-Oktaedern (Jahn-Teller-Effekt), die die Absorption von Sauerstoffzwischenprodukten erleichterten. Während der HER führten ein vollständiger Verlust an Oberflächenphosphor und eine Oxidation der Oberfläche zur Bildung einer Co-angereicherten-(oxy)hydroxidphase, gefolgt von einer in situ-Reduktion zur Erzeugung von Co0-aktiven Zentren für die HER. Der Unterschied in der Aktivität hing mit den einzigartigen elektronischen Eigenschaften und der hohen ECSA von amorphen Materialien zusammen, die eine größere Anzahl aktiver Zentren und eine größere Flexibilität bei der Katalyse bietet. TM-As haben aufgrund der hohen Toxizität von As bei Wasserspaltungsanwendungen wenig Beachtung gefunden. Ihre hohe elektrische Leitfähigkeit macht diese Materialien jedoch für die OER-Elektrokatalyse aktiv. Kristalline FeAs-Nanopartikeln wurden durch Hot-Injection eines SSP hergestellt. Die elektrochemischen Untersuchungen zeigten eine hohe katalytische OER-Aktivität und Stabilität im Vergleich zu den anderen in der Literatur angegebenen Referenzelektroden auf Fe-Basis. Während der OER korrodierte das Material, was zur vollständigen Oxidation und Auflösung von As in der Elektrolytlösung führte. Ex-situ-Charakterisierungen sowie quasi in situ-XAS zeigten die vollständige Elektrokonversion des FeAs in eine hochporöse, nanokristalline Eisen(oxy)hydroxidphase, das 2-Linien-Ferrihydrit. Die Halbleiternatur dieser Phase ermöglichte einen schnelleren Elektronentransport. Darüber hinaus enthielt die erzeugte Phase Jahn-Teller-verzerrte FeIII-Atome als aktive Zentren für die OER. Wichtig ist, dass das gelöste Arsen erfolgreich an der Kathode zurückgewonnen wurde, wodurch der gesamte Prozess nachhaltig und energieeffizient ist. Der Einfluss des Nichtmetalls (Pniktogen) auf die Aktivität wurde untersucht, indem TM pniktide mit gleichem Metallgehalt, Morphologie und Zusammensetzung hergestellt wurden. Eine Salzmetathese bei Raumtemperatur wurde verwendet, um auf amorphes NiAs und NiP zuzugreifen. Elektrochemische Experimente zeigten eine höhere OER-Aktivität von NiP im Vergleich zu NiAs und anderen in der Literatur angegebenen Materialien auf Ni-Basis. Die post-operando-Analyse ergab eine Oxidation und Auflösung des Pniktogens während der OER und die Erzeugung einer γ-NiIIIOOH-Phase. Diese Phase enthielt Schichten von kantenverknüpften NiO6-Oktaedern, die durch einen großen Zwischenschichtabstand (7 Å) voneinander getrennt waren, was die Interkalation von Wasser und Anionen ermöglichte, die somit Zugang zu den aktiven NiIII-Zentren hatten. Der Aktivitätsunterschied wurde auf den höheren Pniktogenverlust in NiP (86%) im Vergleich zu NiAs (45%) zurückgeführt, was zu einer größeren strukturellen Umwandlung des NiP in eine γ-NiIIIOOH-Phase führte. Diese Arbeit beleuchtet, dass einfache Herstellungswege zu nanoskaligen TM-As und TM-Ps unterschiedlicher Morphologie und Zusammensetzung führen können, die die für OER, HER und die gesamte Wasserspaltung erforderlichen physikalischen und elektronischen Eigenschaften besitzen. Die untersuchten Materialien zeigten unter alkalischen Bedingungen elektrokatalytische Aktivitäten, die mit denen von Katalysatoren auf Edelmetallbasis vergleichbar sind. Darüber hinaus sind die entwickelten Methoden flexibel und zeigen je nach Herstellungsweg eine breite Palette erreichbarer Produkte. Daher eröffnet diese Arbeit auch den Weg zur einfachen Herstellung von Übergangsmetallpniktiden und ihrer weiteren Anwendung in anderen Bereichen

Amorphous outperforms crystalline nanomaterials: surface modifications of molecularly derived CoP electro(pre)catalysts for efficient water-splitting

The single source precursor (SSP) approach was used to prepare highly active CoP bifunctional electro(pre)catalysts for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and overall water splitting (OWS) reaction starting from a molecular β-diketiminato Co(I) cyclo-P4 complex. Crystalline or amorphous CoP particles were attained depending on the preparation route. Notably, the amorphous CoP displayed higher activity compared to the crystalline CoP on nickel foam (NF) and fluorinated tin oxide (FTO) substrates due to its unique electronic properties and surface characteristics. During the OER, severe oxidation to Co-oxy(hydroxides)/oxides by the loss of P was found to be crucial to increase the concentration of CoOx active sites. Interestingly, complete leaching of surface P from CoP and surface Co enrichment occurred during the HER. Finally, an OWS device was fabricated where the amorphous CoP outperformed the crystalline CoP with respect to low OWS cell voltage (with a difference of 130 mV) and enhanced stability for 5 days.DFG, 390540038, EXC 2008: UniSysCatTU Berlin, Open-Access-Mittel - 201

A Low-Temperature Molecular Precursor Approach to Copper-Based Nano-Sized Digenite Mineral for Efficient Electrocatalytic Oxygen Evolution Reaction

In the urge of designing noble metal-free and sustainable electrocatalysts for oxygen evolution reaction (OER), herein, a mineral Digenite Cu$_{9}$S$_{5}$ has been prepared from a molecular copper(I) precursor, [{(PyHS) $_{2}$Cu$^{l}$ (PyHS)} $_{2}$] (OTf)$_{2}$ (1), and utilized as an anode material in electrocatalytic OER for the first time. A hot injection of 1 yielded a pure phase and highly crystalline Cu$_{9}$S$_{5}$, which was then electrophoretically deposited (EPD) on a highly conducting nickel foam (NF) substrate. When assessed as an electrode for OER, the Cu$_{9}$S$_{5}$/NF displayed an overpotential of merely 298±3 mV at a current density of 10 mAcm$^{-2}$ in alkaline media. The overpotential recorded here supersedes the value obtained for the best reported Cu-based as well as the benchmark precious-metal-based RuO$_{2}$ and IrO$_{2}$ electrocatalysts. In addition, the choronoamperometric OER indicated the superior stability of Cu$_{9}$S$_{5}$/NF, rendering its suitability as the sustainable anode material for practical feasibility. The excellent catalytic activity of Cu$_{9}$S$_{5}$ can be attributed to the formation of a crystalline CuO overlayer on the conductive Cu$_{9}$S$_{5}$ that behaves as active species to facilitate OER. This study delivers a distinct molecular precursor approach to produce highly active copper-based catalysts that could be used as an efficient and durable OER electro(pre)catalysts relying on non-precious metals