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Electrocatalytic water oxidation is a key transformation in many strategies designed to harness solar energy and store it as chemical fuels. Half of the overall water-splitting reaction, it is kinetically slow and requires significant overpotentials to match the current efficiency of hydrogen electrodes. Understanding the mechanisms of the best electrocatalysts for water oxidation has been a fundamental chemical challenge for decades, and progress has been made using in situ methodologies. Here, we quantitate evolved dioxygen isotopologue composition via gas-phase EPR spectroscopy to elucidate the mechanisms of water oxidation on metal oxide electrocatalysts with high precision. Isotope fractionation is paired with computational and kinetic modelling, showing that this technique is sensitive enough to differentiate O-O bond forming steps. Strong agreement between experiment and theory indicates that for the layered double hydroxide—one of the best earth-abundant electrocatalysts to be studied—water oxidation proceeds via a dioxo coupling mechanism rather than a hydroxide attack. EPR detection of O2 provides new information about perhaps the most important chemical reaction for sustaining a liveable planet.In addition to the work with water oxidation and O2 EPR, I have worked on multiple collaborations by assisting various research groups with CW EPR and pulsed EPR. I herein present published work done with T. Don Tilley’s group at UC Berkeley, characterizing an intermediate in a silicide compound transformation. I also present unpublished work done with Elizabeth Nolan’s group at MIT, studying the effects of adding Ca to a Cu centered calprotectin and how it affects the number of His ligands coordinating to the Cu center. I also present submitted work done with Chris Chang’s group at UC Berkeley, characterizing Cu uptake in RAD23B.

Oxygen powers aerobic life. Its production on Earth relies on the cellular process of photosynthesis, in which the energy of sunlight is converted into an electrochemical proton gradient, driving the synthesis of biomass and plant growth. At the heart of photosynthesis lies photosystem II, an enzyme which catalyzes the oxidation of water to molecular oxygen. Following photon absorption, chlorophylls funnel light energy to the reaction center, initiating charge separation. This triggers rapid electron transfers, ultimately resulting in the reduction of quinone and the oxidation of water to molecular oxygen. The molecular principles of photosystem II are investigated in this thesis by combining atomistic molecular dynamics with hybrid quantum/classical simulations. We identify a regulatory role of bicarbonate in preventing the formation of harmful singlet oxygen, elucidate proton transfer pathways and their dependency on S state dynamics, and characterize water networks essential for efficient proton translocation. Additionally, our work on far-red light-adapted photosystem II highlights how specific chlorophyll substitutions expand the spectral range of photosynthesis, facilitating efficient light absorption and energy transfer under scarce light conditions.

Inspired by the function of crucial components in photosystem II (PSII), electrochemical and dyesensitized photoelectrochemical (DSPEC) water oxidation devices were constructed by the selfassembly of well-designed amphipathic Ru(bda)-based catalysts (bda = 2,2'-bipyrdine-6,6'-dicarbonoxyl acid) and aliphatic chain decorated electrode surfaces, forming lipid bilayer membrane (LBM)-like structures. The Ru(bda) catalysts on electrode-supported LBM films demonstrated remarkable water oxidation performance with different O-O formation mechanisms. However, compared to the slow charge transfer process, the O-O formation pathways did not determine the PEC water oxidation efficiency of the dyesensitized photoanodes, and the different reaction rates for similar catalysts with different catalytic paths did not determine the PEC performance of the DSPECs. Instead, charge transfer plays a decisive role in the PEC water oxidation rate. When an indolo[3,2-b] carbazole derivative was introduced between the Ru (bda) catalysts and aliphatic chain-modified photosensitizer in LBM films, serving as a charge transfer mediator for the tyrosine-histidine pair in PSII, the PEC water oxidation performance of the corresponding photoanodes was dramatically enhanced., These authors contributed equally to this work: Yingzheng Li, Shaoqi Zhan

Semiconductor metal oxides are intensively studied in electrodes for photoelectrochemical (PEC) water splitting. On a series of nanoparticulate TiO2 photoanodes, we analyze specific fabrication variables by means of data fitting. First, the experimental outcome is gathered using PEC characterization techniques, mostly cyclic voltammetry and transient photocurrent measurements. Subsequently, we apply models to gain insights into the involved charge trapping and transfer phenomena. We find that capacitance coefficients and the switch-on transient kinetics depend on the TiO2 layer thickness, respectively indicating surface mechanisms and stationary regimes that are mediated by light accessibility. On the contrary, exponential factors of capacitance are independent of thickness, but reflect changes in the density of electron states with different sintering atmospheres. Also, the transfer resistance in the electrolyte side is indirectly influenced by sintering. Through meticulous quantitative analysis of trends, we stablish simple mathematical relationships that connect thickness-dependent parameters. This knowledge delves into fundamental mechanisms governing the TiO2 photoelectrode behaviour, and aims to facilitate further improvements in the efficiency of materials and electrodes for green hydrogen production.

Graphene oxide has been prepared from an industrial coke-like waste. The morphology of this material (GO-CW) is similar to that of standard graphene oxide (GO-G), although its surface has more defects. Both materials were used to prepare hybrid NHC-Ir(I)/graphene materials, consisting of molecular Ir(NHC) complexes covalently anchored to the graphene surface through the NHC moiety, following two different synthetic routes: (a) direct graphene electrografting of the previously synthesized aniline-functionalized imidazole-2-ylidene-Ir(I) complex, and (b) a two-step sequence comprising initial electrografting of aniline-imidazolium salts and subsequent chemical reaction with Ir(I) precursor anchoring imidazole-2-ylidene-Ir(I) molecular complexes. The synthesized NHC-Ir/graphene hybrid catalysts are active in the oxygen evolution reaction (OER) resulting in current densities in a similar range to those of other NHC-iridium(I) catalysts supported on GO. The highest activity corresponds to hybrid catalysts prepared by the two-step route, with even higher activity and stability when graphene oxide from industrial waste is used. EXAFS spectra of the materials prepared from both synthesis routes before oxidation catalysis reveal the local Ir coordination shell and a structural interaction between Ir and graphene. Both XANES and EXAFS spectra after electrocatalysis point to more oxidized species in which the molecular nature of the iridium catalysts is preserved.

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV−visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

In recent years, there has been a growing awareness of the need to transition to renewable energy to mitigate the effects of climate change and reduce greenhouse gas emissions. Governments, businesses, and individuals are investing in renewable energy infrastructure and technologies to create a more sustainable energy future. This thesis explores transition metal-based molecular complexes for water oxidation catalysis and their integration into water-splitting devices. Water oxidation (WO) is a key reaction in all processes intended to produce chemical fuels from water. When water splitting is driven by solar light, it is also referred to as artificial photosynthesis. The work focuses on a molecular copper catalyst with a tetra-amidate macrocyclic ligand (TAML), called [CuMac]2−, which can be isolated as a (Me4N)2[CuMac] salt. Derivatives of the organic ligand scaffold bearing either two ester or two carboxylic acid functionalities were synthesized to enable heterogenization of [CuMac]2−. The compounds were evaluated for their oxygen evolution reaction (OER) activity at different operational pH. However, introducing the carboxylic moieties appeared to be kinetically detrimental to the OER activity compared to that of the parent [CuMac]2−. A study on identifying possible short-lived intermediates in the catalytic cycle of [CuMac]2−is presented. UV-visible transient absorption spectroscopy was used to deduce the electron transfer mechanism and kinetics of catalyst oxidation in an attempt to understand the catalytic cycle's early steps. Using [Ru(bpy)3]Cl2 and persulfate as the photosensitizer and the oxidative quencher, respectively, flash-photolysis was employed to gather evidence of the one-electron oxidized catalyst and to follow intermediates that could previously not be resolved temporally by electrochemical means. Printed electrodes (PEs) were fabricated using graphite as the conductive material and a polymer binder P(MMA-s-HEMA). The PEs were then covalently fu

The development of photo/electroactive catalysts sustainably producing hydrogen from water splitting and selectively hydrogen peroxide is of paramount importance to alleviate climate change effects. Herein, an anionic cobalt porphyrin (CoP) derivative is electrostatically interfaced with a positively charged modified molybdenum disulfide (MoS2), forming CoP/MoS2, which is accordingly employed as nonprecious photo/electrocatalyst for water oxidation reaction (WOR) and selective H2O2 production. According to the results, CoP/MoS2 shows remarkable bifunctional photo/electrocatalytic performance for WOR and 2e− pathway O2 reduction reaction (ORR) in alkaline electrolyte. Upon visible light irradiation, electrochemical measurements on a fluorine-doped tin oxide (FTO) coated glass electrode reveal an onset potential of 0.595 mV (ORR) and 1.575 mV (WOR) vs. reversible hydrogen electrode, being improved by approximately 80 mV, in both cases, compared to the dark conditions. Notably, the use of the FTO set-up not only enabled us to evaluate the photo/electrocatalytic activity of the CoP/MoS2 nanoensemble but also mimics the practical conditions in photo/electrochemical devices. The outstanding bifunctional photo/electrocatalytic performance of CoP/MoS2 is attributed to (a) the use of CoP as versatile single-atom molecular catalyst and photosensitizer (b) the strong ion-pair interactions between cationic modified MoS2 and the anionic CoP derivative, which prevent aggregation, ensuring better accessibility of the reactants to cobalt active sites, and (c) the co-existence of 1T and 2H phase at modified MoS2, offering improved electrical conductivity and intrinsic electrocatalytic activity along with enhanced intraensemble electronic interactions upon illumination. This work is expected to inspire the design of advanced and low-cost materials for the sustainable production of renewable fuels.

Energy shortage and environmental pollution limit the sustainable development of human beings. Water, as a clean and renewable resource, provides a solution for sustainable energy conversion from water oxidation catalysis. Lessons should be learned from nature to explore efficient artificial catalysts. In this chapter, we will review recent major progress in natural photosynthesis, from the structure and functions to the mechanisms for the water-oxidizing center of the biological enzyme. Later, the development of molecular and material water oxidation catalysts is discussed, with a focus on the mechanisms and rational catalyst design., Part of ISBN 9780128231531QC 20230721

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the ‘water wheel’ to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proto

Efficient catalysts for the oxygen evolution reaction (OER) are essential for advancing renewable energy technologies. This study focuses on the synthesis and characterization of CoxPy, CoO, CoP, NiCoP, and NiCo/C nanoparticles as catalysts for the oxygen evolution reaction (OER). The CoxPy nanoparticles exhibited a sea-urchin-like morphology, providing a high surface area for electrocatalytic activity. Other catalysts exhibited distinct morphologies, including spherical particles and porous structures. Characterization techniques, such as transmission electron microscopy and X-ray diffraction, confirmed the morphology and composition of the synthesized nanoparticles. The electrocatalytic performance of the catalysts was evaluated through polarization curve measurements, and a microkinetic steady-state model based on the Electrochemical Oxide Path was developed to understand the OER mechanism. The impact of adventitious Fe on catalyst activity was also investigated, revealing the high sensitivity of CoxPy to Fe. The findings suggest the dominance of the Electrochemical Oxide Path in the OER mechanism for all catalysts, with NiCoP showing the lowest onset potential.

Climate change, due to the continued emissions of carbon dioxide into our atmosphere and subsequent warming of the planet, is an existential crisis facing humanity. The rising temperature is are resulting in the melting of our ice caps and glaciers, influencing the weather patterns, are making many parts of the world unliveable and are driving many species into extinction. If the current trend is continued, catastrophic and irreversible effects to our planet is almost guaranteed. To tackle this issue, we need to immediately find a viable replacement for the carbon emitting fossil fuels currently used in our energy, transportation and chemical infrastructures. Moving to renewable energy sources like wind and solar energy is already proving to be the answer to this problem. However, in-order to scale the use of these non-emitting sustainable energy sources rapidly, some of the issues associated with these sources like the intermittency and the heavy reliance of fossil fuels in the hard to electrify sectors need to be dealt with...., ChemE/Materials for Energy Conversion and Storage

The CQDs + Ni(OH)2/BiVO4 photoanode was fabricated by introducing CQDs with fluorescence and optical properties. The CQDs + Ni(OH)2/BiVO4 photoanode exhibited a low water oxidation onset potential (∼0.2 V vs. RHE) and a photocurrent density of 3.05 mA cm−2 at a voltage of 1.23 V vs. RHE, which is about 2.5 times than that of the unmodified BiVO4 electrode, and the charge injection efficiency is 65%. Surface dynamics test showed that CQDs + Ni(OH)2/BiVO4 photoanode had higher charge transfer rate and lower charge recombination rate than BiVO4 electrode. As a cheap and stable catalyst for water oxidation, Ni(OH)2 combined with CQDs can effectively improve the migration of photogenerated electrons and holes, thereby improving the photoelectrochemical (PEC) water oxidation activity of the photoanode.

The reaction kinetics of spin-polarized oxygen evolution reaction (OER) can be enhanced by ferromagnetic (FM) catalysts under an external magnetic field. However, applying a magnetic field necessitates additional energy consumption and creates design difficulties for OER. Herein, we demonstrate that a single-domain FM catalyst without external magnetic fields exhibits a similar OER increment to its magnetized multi-domain one. The evidence is given by comparing the pH-dependent increment of OER on multi- and single-domain FM catalysts with or without a magnetic field. The intrinsic activity of a single-domain catalyst is higher than that of a multi-domain counterpart. The latter can be promoted to approach the former by the magnetization effect. Reducing the FM catalyst size into the single-domain region, the spin-polarized OER performance can be achieved without a magnetic field, illustrating an external magnetic field is not a requirement to reap the benefits of magnetic catalysts.

The development of high current density anodes for the oxygen evolution reaction (OER) is fundamental to manufacturing practical and reliable electrochemical cells. In this work, we have developed a bimetallic electrocatalyst based on cobalt–iron oxyhydroxide that shows outstanding performance for water oxidation. Such a catalyst is obtained from cobalt–iron phosphide nanorods that serve as sacrificial structures for the formation of a bimetallic oxyhydroxide through phosphorous loss concomitantly to oxygen/hydroxide incorporation. CoFeP nanorods are synthesized using a scalable method using triphenyl phosphite as a phosphorous precursor. They are deposited without the use of binders on nickel foam to enable fast electron transport, a highly effective surface area, and a high density of active sites. The morphological and chemical transformation of the CoFeP nanoparticles is analyzed and compared with the monometallic cobalt phosphide in alkaline media and under anodic potentials. The resulting bimetallic electrode presents a Tafel slope as low as 42 mV dec–1 and low overpotentials for OER. For the first time, an anion exchange membrane electrolysis device with an integrated CoFeP-based anode was tested at a high current density of 1 A cm–2, demonstrating excellent stability and Faradaic efficiency near 100%. This work opens up a way for using metal phosphide-based anodes for practical fuel electrosynthesis devices.

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the ‘water wheel’ to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proto, Peer Reviewed

Artificial photosynthesis aims to produce fuels and chemicals from simple building blocks (i.e. water and carbon dioxide) using sunlight as energy source. Achieving effective photocatalytic systems necessitates a comprehensive understanding of the underlying mechanisms and factors that control the reactivity. This review underscores the growing interest in utilizing bioinspired artificial vesicles to develop compartmentalized photocatalytic systems. Herein, we summarize different scaffolds employed to develop artificial vesicles, and discuss recent examples where such systems are used to study pivotal processes of artificial photosynthesis, including light harvesting, charge transfer, and fuel production. These systems offer valuable lessons regarding the appropriate choice of membrane scaffolds, reaction partners and spatial arrangement to enhance photocatalytic activity, selectivity and efficiency. These studies highlight the pivotal role of the membrane to increase the stability of the immobilized reaction partners, generate a suitable local environment, and force proximity between electron donor and acceptor molecules (or catalysts and photosensitizers) to increase electron transfer rates. Overall, these findings pave the way for further development of bioinspired photocatalytic systems for compartmentalized artificial photosynthesis.

Photosystem II (PSII) enables global-scale, light-driven water oxidation. Genetic manipulation of PSII from the mesophilic cyanobacterium Synechocystis sp. PCC 6803 has provided insights into the mechanism of water oxidation; however, the lack of a high-resolution structure of oxygen-evolving PSII from this organism has limited the interpretation of biophysical data to models based on structures of thermophilic cyanobacterial PSII. Here, we report the cryo-electron microscopy structure of PSII from Synechocystis sp. PCC 6803 at 1.93-Å resolution. A number of differences are observed relative to thermophilic PSII structures, including the following: the extrinsic subunit PsbQ is maintained, the C terminus of the D1 subunit is flexible, some waters near the active site are partially occupied, and differences in the PsbV subunit block the Large (O1) water channel. These features strongly influence the structural picture of PSII, especially as it pertains to the mechanism of water oxidation.

The addition of an oxygen evolution electrocatalyst to an anode catalyst layer is demonstrated as a practical approach to address the voltage reversal of proton exchange membrane fuel cells (PEMFCs). In this work, IrOXnanoparticles uniformly deposited on niobium-doped TiO2(IrOX/Nb0.1Ti0.9O2) by a sol-gel method are employed as reversal tolerant anodes (RTAs) for PEMFCs. The membrane electrode assembly (MEA) with this RTA possesses a reversal tolerance time of 29.1 min, which is the best result among all the as-prepared MEAs. Meanwhile, the decrease in the cell voltage of this MEA after reversal testing is only 61 mV at a current density of 1.2 A cm-2with a degradation rate of 11.1%. The improved reversal tolerance is attributed, in part, to the uniformly distributed IrOXnanoparticles, boosting the oxygen evolution reaction and, in part, to avoid the carbon oxidation reaction by the application of a noncarbon support. This result can lay a fundamental basis for the application of RTA based on a noncarbon support to improve the anode reversal tolerance for PEMFCs. © 2022 American Chemical Society. All rights reserved.

Metal-organic frameworks (MOFs) are a class of hybrid materials that offer a promising platform for a range of catalytic reactions. Due to their complex structure, MOFs offer unique opportunities to serve as novel catalysts, or as host to improve the properties of previously studied species. However, while other catalytic approaches have been studied for many decades, the recency of their discovery means that significant work is still needed to develop MOFs as a viable option for large scale application. Herein, we aimed to advance the field of MOFs as both novel catalysts, and as host platforms for other catalytic species. To this end, we studied synthetic pathways to produce favorable MOF properties such as higher porosity and active site concentration through introduction of defects and macromorphological control, as well as utilization of molecular catalysts imbedded in the MOF structure for multicomponent, light driven reactivity. Chapter 1 introduces the history MOFs and the pursuit of the stable structures commonly associated with MOF chemistry. The synthesis process for zirconium-based MOFs will be discussed, with specific attention given to the modulated synthesis process which can harnessed to change MOF properties and improve catalysis. Two specific reactions will be introduced which serve as a basis for study in this work. First, the hydrolysis of organophosphate nerve agents by MOFs acting as novel catalysts will be introduced. The mechanism of reaction, as well as previous work in this field will be discussed. Finally, water oxidation as part of artificial photosynthesis through incorporated molecular catalysts will be introduced. Chapter 2 presents a modulator screening study on a zirconium-based MOF, UiO-66. One of the most commonly studied MOFs, UiO-66 provides an excellent platform for synthetic modulation. Particle size and defect level were measured of 26 synthetic variations and synthetic conditions were found to isolate changes in defect level

The low stability of the electrocatalysts at water oxidation (WO) conditions and the use of expensive noble metals have obstructed large-scale H2 production from water. Herein, we report the electrocatalytic WO activity of a cobalt-containing, water-soluble molecular WO electrocatalyst [CoII(mcbp)(OH2)] (1) [mcbp2−=2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine] in homogeneous conditions (overpotential of 510 mV at pH 7 phosphate buffer) and after anchoring it on pyridine-modified fluorine-doped carbon cloth (PFCC). The formation of cobalt phosphate was identified only after 4 h continuous oxygen evolution in homogeneous conditions. Interestingly, a significant enhancement of the stability and WO activity (current density of 5.4 mA/cm2 at 1.75 V) was observed for 1 after anchoring onto PFCC, resulting in a turnover (TO) of >3.6×103 and average TOF of 0.05 s−1 at 1.55 V (pH 7) over 20 h. A total TO of >21×103 over 8 days was calculated. The electrode allowed regeneration of∼ 85 % of the WO activity electrochemically after 36 h of continuous oxygen evolution.

Artificial photosynthesis requires catalysts for efficient and selective conversions of small molecules. Molecular catalysts are advantageous to use in these instances as they offer precise control over chemical reactivity. They are synthetically tuneable, and their catalytic mechanisms are often well documented and more readily understood than those of solid-state catalysts. In this thesis, the synthetic incorporation of molecular catalysts into heterogenised molecular anodes for water oxidation are evaluated. The catalysts are incorporated as structural linkers into porous metal-organic framework (MOF) structures, and as coordination oligomers stacked onto graphitic surfaces. The preparation of MOF/molecular catalyst hybrid materials of two topologies, UiO (UiO = Universitet i Oslo) and NU-1000 (NU = Northwestern University), were investigated. Multiple synthetic methods for the incorporation of molecular ruthenium-based catalysts into MOFs were examined in papers I and II. In paper III of this thesis, a Ru-bda type molecular complex was successfully used in the solvothermal synthesis of a new MOF. The resulting material is the first of its kind that is built exclusively from molecular water oxidation catalyst linkers. It is shown that MOF incorporation greatly enhances the structural stability of the catalyst linker in chemical water oxidation experiments, giving rise to higher turnover numbers compared to that of a homogenous reference system. Finally, paper IV describes a stable and inert molecular ruthenium complex, which possesses a flexible adaptative multidentate equatorial (FAME) type equatorial ligand with a carbanion on the equatorial ligand that forms a C-Ru bond. This molecular complex is studied in homogeneous phase, and subsequently incorporated into a coordination oligomer, which can be activated for water oxidation catalysis. This finding broadens the field of molecular catalysis significantly, and proves that supramolecular interactions can be us

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.

Water oxidation is an important reaction for multiple renewable energy conversion and storage-related devices and technologies. High-performance and stable electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Bimetallic (oxy)hydroxides have been widely used in alkaline OER as electrocatalysts, but their activity is still not satisfactory due to insufficient active sites. In this research, A unique and efficient approach of sacrificial W to prepare CoFe (oxy)hydroxides with abundant active species for OER is presented. Multiple ex situ and operando/in situ characterizations have validated the self-reconstruction of the as-prepared CoFeW sulfides to CoFe (oxy)hydroxides in alkaline OER with synchronous W etching. Experiments and theoretical calculations show that the sacrificial W in this process induces metal cation vacancies, which facilitates the in situ transformation of the intermediate metal hydroxide to CoFe-OOH with more high-valence Co(III), thus creating abundant active species for OER. The Co(III)-rich environment endows the in situ formed CoFe oxyhydroxide with high catalytic activity for OER on a simple flat glassy carbon electrode, outperforming those not treated by the sacrificial W procedure. This research demonstrates the influence of etching W on the electrocatalytic performance, and provides a low-cost means to improve the active sites of the in situ self-reconstructed bimetallic oxyhydroxides for OER.

Isolation of RuIII-bda (17-electron specie) complex with an aqua ligand (2-electron donor) is challenging due to violation of the 18-electron rule. Although considerable efforts have been dedicated to mechanistic studies of water oxidation by the Ru-bda family, the structure and initial formation of the RuIII-bda aqua complex are still controversial. Herein, we challenge this often overlooked step by designing a pocket-shape Ru-based complex 1. The computational studies showed that 1 possesses the crucial hydrophobicity at the RuV(O) state as well as similar probability of access of terminal O to solvent water molecules when compared with classic Ru-bda catalysts. Through characterization of single-crystal structures at the RuII and RuIII states, a pseudo seven-coordinate “ready-to-go” aqua ligand with RuIII...O distance of 3.62 Å was observed. This aqua ligand was also found to be part of a formed hydrogen-bonding network, providing a good indication of how the RuIII-OH2 complex is formed., QC 20220811

Photosynthetic water oxidation is catalyzed by a manganese–calcium oxide cluster, which experiences five “S-states” during a light-driven reaction cycle. The unique “distorted chair”-like geometry of the Mn4CaO5(6) cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S1 to S3 states. The isomers are interconvertible in the S1 and S2 states, while in the S3 state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yz state to the S3nYz• → S4+Yz step, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yz state, is fully recovered in the ensuing S3nYz• state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese–ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II., Swedish Research Council 2020-03809

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda (bda=2,2 '-bipyridine-6,6 '-dicarboxylate) based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved.

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO2 photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO+˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm−2 in the presence of Na2SO3 and a highly stable photocurrent density of >0.5 mA cm−2 at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability., QC 20230206

A low-cost and easy-to-construct dye-sensitized photoanode for water oxidation was prepared by co-loading a new triphenylamine donor-acceptor organic chromophore (D1) and a Co4O4 cobalt cubane-based water oxidation catalyst (CoF) on TiO2 thin films. As the first example of a molecular catalyst-based, noble metal-free, dye-sensitized photoanode, the electrode, TiO2|D1,CoF, produces a higher photocurrent than other noble metal-free, dye-sensitized photoanodes with a relatively high faradaic efficiency for O2 production. The success of the dyad system lies in retarded interfacial charge recombination and extended light absorption both of which take advantage of the pyridine anchor-functionalized chromophore. In addition, the CoF catalyst, decorated with hydrophobic aliphatic chains, serves as a protective layer for stabilizing the organic dyes on electrode surfaces. Transient absorption measurement suggests the accumulation of oxidative equivalents on CoF by electron transfer on the TiO2 surface. The results and design principles presented here demonstrate the use of molecular engineering as a powerful tool for the exploration of low-cost solar water-splitting devices., QC 20230124

The aim of this review is to elucidate geometric structures of the catalytic CaMn4Ox (x = 5, 6) cluster in the Kok cycle for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII) based on the high-resolution (HR) X-ray diffraction (XRD) and serial femtosecond crystallography (SFX) experiments using the X-ray free-electron laser (XFEL). Quantum mechanics (QM) and QM/molecular mechanics (MM) computations are performed to elucidate the electronic and spin structures of the CaMn4Ox (x = 5, 6) cluster in five states S-i (i = 0 similar to 4) on the basis of the X-ray spectroscopy, electron paramagnetic resonance (EPR) and related experiments. Interplay between the experiments and theoretical computations has been effective to elucidate the coordination structures of the CaMn4Ox (x = 5, 6) cluster ligated by amino acid residues of the protein matrix of PSII, valence states of the four Mn ions and total spin states by their exchange-couplings, and proton-shifted isomers of the CaMn4Ox (x = 5, 6) cluster. The HR XRD and SFX XFEL experiments have also elucidated the biomolecular systems structure of OEC of PSII and the hydrogen bonding networks consisting of water molecules, chloride anions, etc., for water inlet and proton release pathways in PSII. Large-scale QM/MM computations have been performed for elucidation of the hydrogen bonding distances and angles by adding invisible hydrogen atoms to the HR XRD structure. Full geometry optimizations by the QM and QM/MM methods have been effective for elucidation of the molecular systems structure around the CaMn4Ox (x = 5, 6) cluster in OEC. DLPNO-CCSD(T-0) method has been applied to elucidate relative energies of possible intermediates in each state of the Kok cycle for water oxidation. Implications of these results are discussed in relation to the blueprint for developments of artificial catalysts for water oxidation.

ZnO-based photoanodes absorb a limited spectrum of light, and their photogenerated electrons and holes combine easily. Such features limit their photoelectrochemical activity. Herein, we report the synthesis of ternary heterostructures comprising ZnO nanorods (NRs) and ZnS nanoparticles wrapped in MoS2 nanosheets (ZnO/ZnS/MoS2, ZSM) directly grown on a substrate by a low-cost hydrothermal route and their performance as anodes for photoelectrochemical water splitting. The ZSM heterostructures exhibit a sixfold photocurrent density of 0.72 mA cm-2, a fourfold maximum applied bias photon-to-current efficiency of 0.28% compared to bare ZnO NRs, and excellent photostability. These improvements in the overall photoelectrochemical activity are due to enhanced light absorption in the visible light range, higher surface active sites, and efficient charge separation enabled by the introduction of a ZnS/MOS2 coating. Our study demonstrates an alternative architecture design of ternary heterostructures with increased photoelectrochemical activity. In general, this approach of constructing a double heterojunction can also be extended to other materials with similar architectures.

Manganese dioxide compounds are widely used in electrochemical applications e.g. as electrode materials or photocatalysts. One of the most used polymorphs is gamma-MnO2 which is a disordered intergrowth of pyrolusite beta-MnO2 and ramsdellite R-MnO2. The presence of intergrowth defects alters the material properties, however, they are difficult to characterise using standard X-ray diffraction due to anisotropic broadening of Bragg reflections. We here propose a characterisation method for intergrown structures by modelling of X-ray diffraction patterns and pair distribution functions (PDF) using gamma-MnO2 as an example. Firstly, we present a fast peak-fitting analysis approach, where features in experimental diffraction patterns and PDFs are matched to simulated patterns from intergrowth structures, allowing quick characterisation of defect densities. Secondly, we present a structure-mining-based analysis using simulated gamma-MnO2 superstructures which are compared to our experimental data to extract trends on defect densities with synthesis conditions. We applied the methodology to a series of gamma-MnO2 samples synthesised by a hydrothermal route. Our results show that with synthesis time, the intergrowth structure reorders from a R-like to a beta-like structure, with the beta-MnO2 fraction ranging from ca. 27 to 82% in the samples investigated here. Further analysis of the structure-mining results using machine learning can enable extraction of more nanostructural information such as the distribution and size of intergrown domains in the structure. Using this analysis, we observe segregation of R- and beta-MnO2 domains in the manganese oxide nanoparticles. While R-MnO2 domains keep a constant size of ca. 1-2 nm, the beta-MnO2 domains grow with synthesis time.

Catalytic carbon dioxide transformation to low valence carbon molecules such as carbon monoxide, formic acid, methanol and methane is a sustainable way to produce fuels and chemicals. Molecular catalysts can be designed to selectively transform CO2 at mild conditions, but a solvent medium is required. Instead, the molecular catalysts can be immobilized on solid supports to facilitate the continuous flow procedure and the separation of the products to recycle the catalytic systems for a more sustainable process. Photosensitive supports may also favor the light-absorbing steps and electron transfer processes. In this article, the recent results obtained in the photocatalytic CO2 reduction using catalytic systems formed by molecular compounds anchored on solids are reviewed.

In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.

Understanding the seven coordination and O–O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2′-bi(nicotinic acid)-6,6′-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda (H2bda = 2,2ʹ-bipyridine-6,6ʹ-dicarboxylic acid), Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid), and Ru-biqa (H2biqa = (1,1ʹ-biisoquinoline)-3,3ʹ-dicarboxylic acid), we emphasized that seven coordination clearly determines presence of RuV[dbnd]O with high spin density on the ORuV[dbnd]O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O–O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV[dbnd]O···O[dbnd]RuV], which may be significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure–activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O–O bond formation., QC 20210204

Numerous aspects of the water oxidation mechanism in photosystem II have not been fully elucidated, especially the O[sbnd]O bond formation pathway. However, a body of experimental evidences have identified the O5 and W2 ligands of the oxygen-evolving complex as the highly probable substrate candidates. In this work, we studied O[sbnd]O bond formation between O5 and W2 based on the native Mn4Ca cluster by density functional calculations. Structural rearrangements before the formation of the S4 state were found as a prerequisite for O[sbnd]O bond formation between O5 and W2, regardless if the suggested pathways involving the typical Mn1(IV)-O[rad] species or the recently proposed Mn4(VII)(O)2 species. Possible alternatives for the S2 → S3 and S3 → S4 transitions accounting for such required rearrangements are discussed. These findings reflect that the structural flexibility of the Mn4Ca cluster is essential to allow structural rearrangements during the catalytic cycle., QC 20210603

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm- 2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

Herein, we report on a new hybrid compound (NH4){[Cu(dien)(H2O)2]2[β-VMo7O26]}·1.5H2O (1), where dien = diethylenetriamine, containing an extremely rare mixed-metal pseudo-octamolybdate cluster. An ex situ EPR spectroscopy provided insights into the formation of paramagnetic species in reaction mixture and in solution of 1. The magneto-structural correlations revealed weak antiferromagnetic exchange interactions between the [Cu(dien)]2+ cations transmitted by intermolecular pathways. The cyclic voltammetry showed the one-electron process associated with the Cu3+/Cu2+ oxidation followed by the multi-electron catalytic wave due to water oxidation with a faradaic yield of 86%. The title compound was thus employed in homogeneous water oxidation catalysis using tris(bipyridine)ruthenium photosensitizer. At pH 8.0, efficiency of the catalytic system attained 0.19 turnovers per second supported by the relatively mild water oxidation overpotential of 0.54 V.

The efficiency of electrolytic hydrogen production is limited by the slow reaction kinetics of oxygen evolution reaction (OER). Surface-reconstructed ferromagnetic (FM) catalysts with a spin-pinning effect at the FM/oxyhydroxide interface could enhance the spin-dependent OER kinetics. However, in real-life applications, electrolyzers are operated at elevated temperature, which may disrupt the spin orientations of FM catalysts and limit their performance. In this study, we prepared surface-reconstructed SmCo5/CoOxHy, which possesses polarized spins at the FM/oxyhydroxide interface that lead to excellent OER activity. These interfacial polarized spins could be further aligned through a magnetization process, which further enhanced the OER performance. Moreover, the operation temperature was elevated to mimic the practical operation conditions of water electrolyzers. It was found that the OER activity enhancement of the magnetized SmCo5/CoOxHy catalyst could be preserved up to 60 °C. © 2021 Wiley-VCH GmbH

Rational design of active oxygen evolution reaction (OER) catalysts is critical for the overall efficiency of water electrolysis. The differing spin states of the OER reactants and products is one of the factors that slows OER kinetics. Thus, spin conservation plays a crucial role in enhancing OER performance. In this work, ferromagnetic (FM)–antiferromagnetic (AFM) Fe3O4@Ni(OH)2 core–shell catalysts are designed. The interfacial FM–AFM coupling of these catalysts facilitates selective removal of electrons with spin direction opposing the magnetic moment of FM core, improving OER kinetics. The shell thickness is found critical in retaining the coupling effect for OER enhancement. The magnetic domain structure of the FM core also plays a critical role. With a multiple domain core, the applied magnetic field aligns the magnetic domains, optimizing the electron transport process. A significant enhancement of OER activity is observed for the multiple domain core catalysts. With a single-domain FM core with ordered magnetic dipoles, the spin-selective electron transport with minimal scattering is facilitated even without an applied magnetic field. A magnetism/OER activity model therefore hypothesizes that depends on two main parameters: interfacial spin coupling and domain structure. These findings provide new design principles for active OER catalysts. © 2021 Wiley-VCH GmbH.

We developed a photoanode consisting of Au-Ag alloy nanoparticles (NPs), a TiO2 thin film and a Au film (AATA) under modal strong coupling conditions with a large splitting energy of 520 meV, which can be categorized into the ultrastrong coupling regime. We fabricated a photoanode under ultrastrong coupling conditions to verify the relationship between the coupling strength and photoelectric conversion efficiency and successfully performed efficient photochemical reactions. The AATA photoanode showed a 4.0 % maximum incident photon-to-current efficiency (IPCE), obtained at 580 nm, and the internal quantum efficiency (IQE) was 4.1 %. These results were attributed to the high hot-electron injection efficiency due to the larger near-field enhancement and relatively negative potential distribution of the hot electrons. Furthermore, hybrid mode-induced water oxidation using AATA structures was performed, with a Faraday efficiency of more than 70 % for O-2 evolution.

Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.

The development of new procedures for synthesizing new, low-cost, and stable metal oxides for the oxygen-evolution reaction (OER) by water oxidation is critical. Since the 2000s, there has been a rapid rise in the use of first-row transition metal (hydr)oxides for the OER. However, there is still a need to design and synthesize an efficient and stable catalyst for the OER. The present paper aims to design and synthesize an OER catalyst based on "a look at nature" strategy. In a simple and chemical evolution-like experiment, for the first time, a solution composed of various perchlorate cations, namely, Li(I), Mg(II), Ca(II), Al(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ba(II), and Na2SiO3, at a concentration of 1.0 mM at pH=4 and a potential of 1.40 V vs. NHE, which was under stirring for 15 days, was investigated. The stable OER catalyst on the electrode was characterized by X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Based on the extrapolation of the Tafel plot, the onset of the OER in the presence of this catalyst was 130 mV lower than a bare electrode without it. This approach could be a roadmap to design and synthesize new and stable catalysts.

In this work, we present the preparation and performance of screen-printed anodes for electrochemical water splitting in neutral media. With the combination of printed electrodes and molecular water oxidation catalysts, we successfully take advantage of a low-cost and up-scalable fabrication method of graphitic electrodes with the outstanding catalytic activity and stability of oligomeric ruthenium-based molecular water oxidation catalysts, offering a promising electroanode for water oxidation applications.

Electrocatalytic water oxidation requires efficient catalysts to reduce the overpotential and accelerate the sluggish kinetics of oxygen formation. Here, a promising NiFeBa material was prepared by the co-electrodeposition of Ba2+, Ni2+, and Fe3+ as an efficient catalyst for electrocatalytic water oxidation. NiFeBa showed enhanced water oxidation performance compared with NiFe layered double hydroxide and NiFe oxide, delivering a current density of 10 mA cm−2 at an overpotential of 180 mV. Doped Ba ions played a key role in stabilizing the electrogenerated Ni3+ species, producing more octahedral Ni−O structures for lattice oxygen-based water oxidation, adjusting the catalytic mechanism, and finally leading to an enhancement of catalytic efficiency., QC 20220314

The rationalization of chemical-physical proprieties of transition metal complexes is fundamental in order to understand and tune their reactivity. In this thesis, a systematic investigation of the geometrical and electronic properties of [M(Py5OH)Cl]+ complexes (M= Mn, Fe, Co, Ni) has been performed, and their ability to act as molecular water oxidation catalysts has been probed. Through this scientific journey, new insights into their chemical and physical properties have been revealed. The spin crossover behavior of the ferrous chloride complex ([Fe(Py5OH)Cl]PF6) is the first example of a molecular Fe(II) complex coordinated to a weak-field ligand that can be thermodynamically stable in a low-spin electron configuration (Chapter 3). The spin state also dictates the electrochemical proprieties of the one-electron oxidized state of all the metal complexes investigated in our study (Chapter 4). The atypical rhombicity of the manganese complex ([Mn(Py5OH)Cl]PF6) gives an unusual anisotropic EPR signal for a Mn(II, S = 5/2) complex. This is compared with the analog [Mn(Py5OMe)Cl]PF6 complex providing, in combination with DFT calculations, insight into how the magnetic parameters (i.e., zero field splitting) are affected by small structural changes (Chapter 5). Finally, I investigated the role of water as substrate for water oxidation catalysis with the [M(Py5OH)Cl]+ complexes. The addition of small amounts of water into a non-aqueous medium allowed trapping possible water-bound intermediates for the Fe complex in the M(III) oxidation state but not for the other complexes. Nevertheless, all Py5OH-metal complexes are not particularly active catalysts with a maximum turnover number (TON) of 2. By introducing two methoxy functional groups, we obtained [Fe(Py5OMe)Cl]+ that turns out to facilitate water oxidation catalysis with a TON = 133 in a light-driven experiment. Further electrochemical experiments and post-catalytic solution analysis reveals that the oxygen evolution

This work reports the hybridization of patronite (VS4) sheets with reduced graphene oxide and functionalized carbon nanotubes (RGO/FCNT/VS4) through a hydrothermal method. The synergistic effect divulged by the individual components, i.e., RGO, FCNT, and VS4, significantly improves the efficiency of the ternary (RGO/FCNT/VS4) hybrid toward the oxygen evolution reaction (OER). The ternary composite exhibits an impressive electrocatalytic OER performance in 1 M KOH and requires only 230 mV overpotential to reach the state-of-the-art current density (10 mA cm–2). Additionally, the hybrid shows an appreciable Tafel slope with a higher Faradaic efficiency (97.55 ± 2.3%) at an overpotential of 230 mV. Further, these experimental findings are corroborated by the state-of-the-art density functional theory by presenting adsorption configurations, the density of states, and the overpotential of these hybrid structures. Interestingly, the theoretical overpotential follows the qualitative trend RGO/FCNT/VS4 < FCNT/VS4 < RGO/VS4, supporting the experimental findings.