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In conventional luminol electrochemiluminescence (ECL) systems, hydrogen peroxide and dissolved oxygen are employed as typical co‐reactants to produce reactive oxygen species (ROS) for efficient ECL emission. However, the self‐decomposition of hydrogen peroxide and limited solubility of oxygen in water inevitably restrict the detection accuracy and luminous efficiency of luminol ECL system. Inspired by ROS‐mediated ECL mechanism, for the first time, we used cobalt‐iron layered double hydroxide as co‐reaction accelerator to efficiently activate water to generate ROS for enhancing luminol emission. Experimental investigations verify the formation of hydroxyl and superoxide radicals in the process of electrochemical water oxidation, which subsequently react with luminol anion radicals to trigger strong ECL signals. Finally, the detection of alkaline phosphatase has been successfully achieved with impressive sensitivity and reproducibility for practical sample analysis. [ABSTRACT FROM AUTHOR]

Herein, we present a stable water‐soluble cobalt complex supported by a dianionic 2,2′‐([2,2′‐bipyridine]‐6,6′‐diyl)bis(propan‐2‐ol) ligand scaffold, which is a rare example of a high‐oxidation species, as demonstrated by structural, spectroscopic and theoretical data. Electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements revealed that the CoIV center of the mononuclear complex in the solid state resides in the high spin state (sextet, S=5/2). The complex can effectively catalyze water oxidation via a single‐site water nucleophilic attack pathway with an overpotential of only 360 mV in a phosphate buffer with a pH of 6. The key intermediate toward water oxidation was speculated based on theoretical calculations and was identified by in situ spectroelectrochemical experiments. The results are important regarding the accessibility of high‐oxidation state metal species in synthetic models for achieving robust and reactive oxidation catalysis. [ABSTRACT FROM AUTHOR]

The understanding of O-O bond formation is of great importance for revealing the mechanism of water oxidation in photosynthesis and for developing efficient catalysts for water oxidation in artificial photosynthesis. The chemical oxidation of the RuII 2(OH)(OH2) core with the vicinal OH and OH2 ligands was spectroscopically and theoretically investigated to provide a mechanistic insight into the O-O bond formation in the core. We demonstrate O-O bond formation at the low-valent RuIII2(OH) core with the vicinal OH ligands to form the RuII2(µ-OOH) core with a µ-OOH bridge. The O-O bond formation is induced by deprotonation of one of the OH ligands of RuIII2(OH)2 via intramolecular coupling of the OH and deprotonated O2 ligands, conjugated with two-electron transfer from two RuIII centers to their ligands. The intersystem crossing between singlet and triple states of RuIII 2(µ-OOH) is easily switched by exchange of H+ between the µ-OOH bridge and the auxiliary backbone ligand. [ABSTRACT FROM AUTHOR]

Developing efficient hole transport layers and water oxidation electrocatalysts is vital for improving the performance of photoelectrochemical (PEC) devices for water-splitting. In this work, we synthesized Co 3 O 4 nanoparticles/Ti 3 C 2 -MXene hybrid material and utilized it as a dual-function material to transport the photogenerated holes and catalyze the water oxidation reaction. It was found that the modification of BiVO 4 with Co 3 O 4 /Ti 3 C 2 -MXene prolonged the lifetime of the photogenerated holes by 13.7-fold (i.e. from 0.26 s for BiVO 4 to 3.56 s for Co 3 O 4 /Ti 3 C 2 /BiVO 4) and significantly reduced the charge transfer resistance. The modification of BiVO 4 with only Ti 3 C 2 -MXene resulted in significant surface recombination more likely due to the high concentration of the accumulated holes in the Mxene layer because of its low electrocatalytic activity toward water oxidation reaction. Employing the Co 3 O 4 /Ti 3 C 2 -MXene hybrid material significantly reduced the rate constant of surface recombination and dramatically enhanced the charge transfer efficiency (i.e. from 0.17% for BiVO 4 to 14.0% for Co 3 O 4 /Ti 3 C 2 /BiVO 4 at 0.65 V RHE). Under optimized conditions, the Co 3 O 4 /Ti 3 C 2 /BiVO 4 photoanode was able to deliver 5.05 mA cm−2 at 1.23 V RHE and outperforms Ti 3 C 2 /BiVO 4 and Co 3 O 4 /BiVO 4 photoanodes. Moreover, it showed high stability for over 50 hours and reached a Faradaic efficiency of 90%. An applied bias photo-to-current efficiency (ABPE) value of 1.71% at 0.62 V Pt was obtained for the overall water splitting. This work provides a facile method to reduce surface recombination and enhance the stability and PEC efficiency of BiVO 4 photoanodes via the modification with dual-function oxygen evolution catalyst/hole transport layer hybrid material. [Display omitted] [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • POMs were converted to CoWO 4 nanoparticles by annealing. • POM-derived a-CoWO 4 exhibited excellent catalytic activity for water oxidation. • Its catalytic activity was closely related to the formation of oxygen vacancies. • It was readily integrated on a desired surface by adsorption and annealing of POMs. • This enabled simple preparation of efficient (photo)anodes for water oxidation. Abstract Highly efficient water-oxidation catalysts (WOCs) were readily prepared through the simple heat treatment of cobalt-containing polyoxometalate [Co 4 (H 2 O) 2 (PW 9 O 34) 2 ]10− (POM). The annealing of soluble POM molecules at high temperatures in air led to the formation of insoluble nanoparticles, of which the crystal structure and catalytic activity can be controlled by the annealing temperature. POMs were converted to amorphous and crystalline CoWO 4 nanoparticles when annealed at 400 and 500 °C, respectively. Interestingly, amorphous CoWO 4 nanoparticles exhibited excellent catalytic activity near the neutral pH of pH 8.0, making them superior to both pristine POM and POM-derived crystalline CoWO 4 nanoparticles. X-ray absorption and photoelectron spectroscopies combined with density functional theory (DFT) calculations revealed that their outstanding performance was resulted from the generation of large amounts of oxygen vacancies upon annealing, leading to the optimum distance between the nearest Co ions for the Langmuir-Hinshelwood (LH) mechanism. Based on these findings, we could readily immobilize CoWO 4 -based WOCs on the surfaces of various electrodes for efficient electrochemical and photoelectrochemical water oxidation through the annealing of POMs pre-adsorbed onto the desired electrode surface. This study may provide insights not only for the synthesis of efficient electrocatalysts derived from POMs but also for their immobilization onto the desired electrode surface for practical applications. [ABSTRACT FROM AUTHOR]

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area.

Physical vapor deposition (PVD) techniques are proposed as high performing and easily tunable methods to synthesize mixed-metal oxides thin films as water oxidation catalysts (WOC). These techniques allow us to have a good control on the catalyst film surface morphology and crystallinity, with good adhesion of the catalysts on electrodes. Pure and binary metal oxides thin films based on Fe, Co and Ni were synthesized using electron beam deposition (EBD). WOC films are systematically studied through SEM microscopy, Raman spectroscopy and XPS analysis. Electrochemical characterization has been carried out to investigate the relevance of synergistic effects towards the water oxidation reaction. Among all the mixed oxides Ni–Fe oxide showed the best catalytic results. [ABSTRACT FROM AUTHOR]

The application of the recently discovered oxofluoride solid solution (CoxNi1−x)3Sb4O6F6 as a catalyst for water oxidation is demonstrated. The phase exhibits a cubic arrangement of the active metal that forms oxo bridges to the metalloid with possible catalytic participation. The Co3Sb4O6F6 compound proved to be capable of catalyzing 2H2O→O2+4H++4e− at 0.33 V electrochemical and ≤0.39 V chemical overpotential with a TOF of 4.4⋅10−3, whereas Ni3Sb4O6F6 needs a higher overpotential. Relatively large crystal cubes (0.3-0.5 mm) are easily synthesized and readily handled as they demonstrate both chemical resistance to wear after repeated in situ tests under experimental conditions, and have a mechanical hardness of 270 V0.1 using Vickers indentation. The combined properties of this compound offer a potential technical advantage for incorporation to a catalytic interface in future sustainable fuel production. [ABSTRACT FROM AUTHOR]

The understanding of O-O bond formation is of great importance for revealing the mechanism of water oxidation in photosynthesis and for developing efficient catalysts for water oxidation in artificial photosynthesis. The chemical oxidation of the Ru II 2 (OH)(OH 2 ) core with the vicinal OH and OH 2 ligands was spectroscopically and theoretically investigated to provide a mechanistic insight into the O-O bond formation in the core. We demonstrate O-O bond formation at the low-valent Ru III 2 (OH) core with the vicinal OH ligands to form the Ru II 2 (μ-OOH) core with a μ-OOH bridge. The O-O bond formation is induced by deprotonation of one of the OH ligands of Ru III 2 (OH) 2 via intramolecular coupling of the OH and deprotonated O - ligands, conjugated with two-electron transfer from two Ru III centers to their ligands. The intersystem crossing between singlet and triple states of Ru II 2 (μ-OOH) is easily switched by exchange of H + between the μ-OOH bridge and the auxiliary backbone ligand., Competing Interests: The authors declare no competing interest.

Water oxidation catalysts (WOCs) play an important role in performance improvement for TiO 2 photoanode-based photoelectrochemical (PEC) water splitting, but their promotion effect is greatly restricted by the deep valence band of TiO 2 and great challenges to maximize their electrocatalytic water oxidation activity and synergistic interaction with TiO 2. Herein, we report direct growth of ultra-small, uniformly dispersed, and highly dense CoO x nanoparticles (NPs) on TiO 2 nanowire arrays (NWAs) via polymer-mediated self-assembly, in which the polymer serves simultaneously as a linker, stabilizer, and reductant. The obtained nanohybrid arrays show a photocurrent density of 2.09 mA cm−2 and a photoconversion efficiency of 1.01%, which are 3.17 and 3.37 times, respectively higher than that (0.66 mA cm−2, 0.30%) of pristine TiO 2 NWAs, and are among the largest reported for WOC-assembled TiO 2 photoanodes. Further, their photocurrent density decreases only by ~11.81% while that of TiO 2 NWAs decays by ~54.40% after 6900 s. The remarkable promotion effect of CoO x NPs is ascribed to their high intrinsic catalytic activity, ultra-small size, uniform dispersion, and high density significantly enhancing charge separation and hole injection and accelerating charge transfer and transport. This work not only creates a novel CoO x NPs/TiO 2 NWA photoelectrode to significantly promote the PEC water oxidation performance of TiO 2 NWAs and provides scientific insights into the enhancement mechanism, but also offers a general strategy for self-assembly of WOCs on various photoelectrodes with complex geometries for PEC water splitting. Image 1 • CoO x NPs were directly grown on TiO 2 NWAs via polymer-mediated self-assembly. • PVP could interact with TiO 2 , stabilize CoO x and its precursors, and reduce Co2+. • CoO x NPs/TiO 2 NWAs show much higher water oxidation performance than TiO 2 NWAs. • The activity is among the largest reported for WOC-assembled TiO 2 photoanodes. • CoO x NPs enhance charge separation, transfer, and transport and hole injection. [ABSTRACT FROM AUTHOR]

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area. [ABSTRACT FROM AUTHOR]

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO 4 modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO 4 photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO 4 due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

The unusually high tolerance toward chemical functional groups of the copper(I)-catalyzed Huisgen-Sharpless-Meldal 1,3-dipolar cycloaddition of azides and alkynes protocol (the CuAAC or "click" reaction) associated with its mild conditions and high yields has been explored in the present methodology to successfully prepare water oxidation catalyst iridium oxide nanoparticles decorated with organic dyes. The "click reaction" has proven to be an excellent synthetic tool to overcome the incompatible solubility of the hydrophilic iridium oxide nanoparticles and the hydrophobic dyes. A complex artificial photosynthetic model designed to mimic the photoinduced redox processes occurring in photosystem II is used as a hydrophobic dye to highlight the efficiency and selectiveness of the method.

There is an urgent demand to develop earth-abundant electrocatalysts for efficient and durable water oxidation under mild conditions. A nickel-substituted cobalt-borate nanowire array is developed on carbon cloth (Ni-Co-Bi/CC) via oxidative polarization of NiCo 2 S 4 nanoarray in potassium borate (K-Bi). As a bimetallic electrocatalyst for water oxidation, such Ni-Co-Bi/CC is superior in catalytic activity and durability in 0.1 m K-Bi (pH: 9.2), with a turnover frequency of 0.33 mol O 2 s -1 at the overpotential of 500 mV and nearly 100% Faradaic efficiency. To drive a geometrical catalytic current density of 10 mA cm -2 , it only needs overpotential of 388 mV, 34 mV less than that for Co-Bi/CC, outperforming reported non-noble-metal catalysts operating under benign conditions. Notably, its activity is maintained over 80 000 s. Density functional theory calculations suggest that the O* to OOH* conversion is the rate-determining step and Ni substitution decreases the free energy on Co-Bi from 2.092 to 1.986 eV., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A novel 3D WO 3 /BiVO 4 /cobalt phosphate composite inverse opal is designed for photoeletrochemical (PEC) water splitting, yielding a significantly improved PEC performance., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)