Application In Water Oxidation Research Articles

Immobilization of Iridium Triazolylidene Complexes into Polymer Scaffolds and Their Application in Water Oxidation

AbstractA triazolylidene irdium complex was postmodified with simple methods to introduce two alcohol groups in the triazolylidene backbone. The reaction of this difunctionalized iridium triazolylidene unit with terephthalic acid chloride allowed for integrating these iridium complexes into a polymeric assembly. Both the monomeric complexes as well as the polymerized systems showed activity in water oxidation driven by cerium ammonium nitrate as a chemical oxidant with comparable catalytic performance. Post‐reaction analysis of the aqueous reaction solution by ICP MS showed a partial loss of iridium from the polymer into the aqueous phase under catalytic conditions, indicating a need for more robust polymer supports for this type of application.

Electrodeposition of defect-rich high entropy ZIF and its application in water oxidation

High-entropy materials has achieved much attention in the recent decade years. However, there is few reports on high-entropy MOFs due to the large difference of coordination ability for different metal ions. In this work, high-entropy ZIFs are successfully electrodeposited on the nickel foam substrate under high overpotential, and the effects of potential, electrodeposition time and metal precursor concentration on the morphologies and catalytic activity of deposits are examined. The results indicate that the prepared HE-ZIF is made of nanoparticles with a low crystalline structure, and it shows a high electrocatalytic activities for oxygen evolution reaction (OER) as it possesses an abundance of defects such as vacancy defects, lattice distortions and amorphous structure. The overpotential at the current density of 100 mA cm−2 is 290 mV with good durability in 100 h continuous test for OER in 1 mol L−1 KOH.

WO3 layer sensitized with BiVO4 and MIL-101(Fe) as photoanode for photoelectrochemical water oxidation

Thick tungsten oxide layers were prepared electrophoretically in order to be used as photoanodes in photoelectrochemical water oxidation applications. The highest photocurrent density was obtained for a WO3 layer with thickness of ∼900 nm. Additionally, WO3/BiVO4 and WO3/BiVO4/MIL-101(Fe) heterojunctions have been fabricated using WO3 layer as substrate. WO3/BiVO4 shows an increased value of the electrochemical active surface area indicating that more sites of this photoanode are activated through the formation of the heterojunction. The small V5+ signals observed in the V 2p XPS spectra of these heterojunctions were attributed to the substitution of V5+ atoms with W6+ atoms on the surface of BiVO4. Excessive W doping of the BiVO4 film determined the decrease of the photoelectrochemical performance of WO3/BiVO4 photoanode. The significant improvement of the photoconversion efficiency of the sample decorated with MIL-101(Fe) indicated that this co-catalyst provides sites more efficiently in the photoelectrochemical process. This performance was correlated with the reduced value of the charge transfer resistance at electrode/electrolyte interface obtained from electrochemical impedance spectroscopy investigation.

Electrochemically synthesized Cd doped ZnO nanorods and integrated atomic Cd-S bridged Cd:ZnO/CdS heterostructure photoanode for enhanced visible light responsive water oxidation applications

Cd-doped ZnO and CdS-decorated ZnO:Cd/CdS heterostructure nanorod arrays (NRA) were successfully fabricated and integrated in this study. The severe surface charge recombination and weak visible light responsivity of ZnO are barriers to the PEC application. Herein, electrochemically synthesized Cd doped ZnO nanorod arrays caged with visible light responsive CdS semiconductor catalyst rehabilitated the photoelectrocatalytic characteristics of the photoanode. The chemically modified Cd:ZnO/CdS photoanode shows a notable photocurrent density of 3.6 mA/cm2(at 0.2 V vs Ag/AgCl) and a prominent onset potential that is negatively shifted by 324 mV relative to the pure ZnOphotoelectrode. The 1D CdS cage around ZnO nanorod improves the visible light responsive photo electrochemical performance with low −1.064 V vs Ag/AgCl onset potential, 740 Ohm/cm2 charge transfer resistance and 0.24% of solar to hydrogen conversion efficiency with high built-in potential (515 mV). The realized result caused by the Cd doping and heterostructure of the ZnO:Cd/CdS photoanode with an exceptional nanostructure photoanode could be a pioneering technique for cost-effective applications in the photoelectrochemistry field.

Enhanced Electrocatalysts Fabricated via Quenched Ultrafast Sintering: Physicochemical Properties and Water Oxidation Applications

AbstractThe synthesis of transition metal oxides is typically time‐ and energy‐consuming. Recently, fast sintering methods have demonstrated great potential in reducing ceramic sintering time and energy use, improving the commercial prospects of these materials. In this article, a quenched ultrafast high‐temperature sintering (qUHS) technique is developed to sinter metastable brownmillerite SrCoO2.5 (SCO) in less than a minute. Surprisingly, SCO fabricated by qUHS shows higher activity for the oxygen evolution reaction (OER) than solid‐state‐reaction‐synthesized SCO. Comparing samples produced by these two techniques, the increased OER performance of SCO qUHS is likely due to the synergistic combination of surface Co chemical state, higher mesoporosity, and enhanced hydroxyl ion (OH–) adsorption. This work demonstrates the potential of qUHS for producing high‐performance electrocatalysts and provides detailed insights into the impact of ultrafast sintering on the materials’ physical properties and electrocatalytic activity.

Cobalt, Iron, and Manganese Metallocorroles in Catalytic Oxidation of Water. An Overview of the Synthesis, Selected Redox and Electronic Properties, and Catalytic Activities

AbstractMetallocorroles have emerged as interesting and versatile catalysts for a variety of redox reactions. These include clean‐energy related small‐molecule activation reactions such as proton, carbon dioxide and dioxygen reductions, as well as water oxidation, for which their ability to undergo several oxidation processes and reach high formal metal oxidation states makes them attractive catalysts. The diversity of substituent and central metal variations in metallocorroles opens up innumerable possibilities to tailor the properties and reactivity of this family of metallomacrocycles, and the richness of the (electro)chemistry and spectroscopy of metallocorroles allows for an array of instrumental techniques to be applied to the study of various (electro)catalytic processes. In this paper, we will succinctly summarize the synthesis of several cobalt, manganese, and iron metallocorroles with different apical ligands, discuss their redox properties and metal versus ligand centered oxidations through selected examples, and review their application in water oxidation (electro)catalysis. We will discuss some of the factors which affect the catalytic activity, some of the proposed mechanistic features and point out what we consider to be open questions and future perspectives in this field.

TiO2 Nanotubes Synthesis with Dominant {001} Exposed Facets for Efficient Photocatalytic and Photoelectrochemical Water Oxidation Applications

The photoreactivity of TiO2 nanotube arrays (TiNT) is mainly related to the light absorption and photogenerated charge carriers separation efficiencies. Recently, engineering of the crystal facets has gained increasing attention because of its exceptional ability to extend the photoresponse toward visible light and to separate photogenerated charges. Here, TiNT with high crystallinity and percentage of exposed {001} facets was synthesized using a two-step anodization process in ethylene glycol based electrolyte containing 2 wt% of water and 0.27 M NH4F. Decreasing the potential of the second anodization (from 50 to 20 V) resulted in an increase in crystallinity, crystallite size, and percentage of exposed {001} facets. Because trap states limit the photoreactivity performance of TiNT by facilitating the recombination of photogenerated electron/hole pairs, correlation between crystallinity, preferential crystalline orientation, gap states characteristics (density and location), and photoreactivity performance was investigated. TiNT with a high percentage of {001} exposed facets, consisting mainly of shallow gap states with low density, significantly improve photoelectrochemical water oxidation and photocatalytic efficiencies.

Development and characterisation of photoelectrochemical MIS structures with RuO2/TiO2 gate stacs for water oxidation

Abstract This paper is dedicated to preparation and analysis of metal insulator semiconductor (MIS) photoanode with a metal organic chemical vapor deposited RuO2 layer and TiO2 protection layer for photoelectrochemical water splitting. It is shown that utilization of TiO2 layers of 2, 4, and 6 nm thickness preserve the catalytic activity of underlying RuO2. The origin of increased overpotential and decreased photovoltage of the photoanode upon the increase of TiO2 layer thickness is discussed in the paper. Results revealed that utilization of TiO2 layer in the MIS concept is suitable for photoelectrochemical water oxidation applications.

Effect of film morphology on water oxidation enhancement in NiFeCo modified hematite photoanodes

Hematite (α-Fe2O3) is a promising candidate for water oxidation applications due to its abundance in the earth crust and its suitable bandgap. However, hematite performance is severely limited by electron-hole recombination at its interface with the electrolyte; something that can be addressed using electrocatalysts. In this report, we evaluate the influence of a ternary NiFeCo co-catalyst to enhance the water oxidation performance of hematite photoanodes. Thus, NiFeCo co-catalyst is optimized for hematite thin films with i) dense and ii) porous (nano-rod) morphologies. Both hematite films are prepared using electron beam evaporation method followed by an annealing step and NiFeCo co-catalyst is processed using electrodeposition. The results demonstrate optimal thickness of NiFeCo co-catalyst is thinner for porous hematite layers relative to the dense ones indicating that the optimal condition of the co-catalyst is affected by hematite morphology. The optimized dense NiCoFe/hematite photoanode shows nearly than 2.8-fold enhancement relative to the pristine hematite photoanode, whereas the optimized porous NiCoFe/hematite photoanode shows 2.2-fold enhancements. Transient photocurrent measurements confirm that NiFeCo facilitates hole injection from hematite into the electrolyte. These findings have important implications for optimizing electrocatalysts for photoelectrochemical water splitting photoanodes and taking into account the effect of surface morphology.

Orthorhombic tantalum pentoxide nanorods for electrochemical applications

Water splitting is an important method for hydrogen production. Notably, tantalum oxide has the potential to employ extensive variety appliances because of its outstanding electrical and optical properties. Tantalum pentoxide (Ta2O5) nanopowders were produced using the sol–gel process. The effect of calcination time plays a main role on Ta2O5 crystal structure configuration. Transmission electron microscope images explored obtained Ta2O5 nanorod formation and investigated by electrochemical studies for its use in electrochemical water-splitting applications. The calculated specific capacitance values of Ta2O5 electrodes at different temperature conditions were 146.4, 191.7, and 221.7 F/g. Fabricated Ta2O5 electrodes showed overpotential of 304, 278, and 267 mV. Current densities of Ta2O5 electrodes at different calcinations times were 353, 419, and 461 mA/g. Ta2O5 powder calcined for 6 h revealed high specific capacitance and low overpotential, indicating better electrochemical reactivity suitable for water oxidation applications.

Electrodeposition of (hydro)oxides for an oxygen evolution electrode.

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the electrolyzers that are currently available do not have anodic electrodes that are robust enough and highly active for the oxygen evolution reaction (OER). Electrodeposition provides a feasible route for preparing freestanding OER electrodes with high active site utilization, fast mass transport and a simple fabrication process, which is highly attractive from both academic and commercial points of view. This minireview focuses on the recent electrodeposition strategies for metal (hydro)oxide design and water oxidation applications. First, the intrinsic advantages of electrodeposition in comparison with traditional technologies are introduced. Then, the unique properties and underlying principles of electrodeposited metal (hydro)oxides in the OER are unveiled. In parallel, illustrative examples of the latest advances in materials structural design, controllable synthesis, and mechanism understanding through the electrochemical synthesis of (hydro)oxides are presented. Finally, the latest representative OER mechanism and electrodeposition routes for OER catalysts are briefly overviewed. Such observations provide new insights into freestanding (hydro)oxides electrodes prepared via electrodeposition, which show significant practical application potential in water splitting devices. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.

Electrocatalytic Water Oxidation with Surface Anchored Mononuclear Manganese (II) ‐ Polypyridine Complexes

AbstractCatalytic oxidation of water by manganese complexes is of great interest because it can serve as a model for the natural oxygen evolving complex, Mn4CaO5. Complex compounds are highly active and tunable, but their limited stability and solubility restrict them for practical water oxidation applications. In the present article we have prepared surface anchored Mn(II)‐complexes, [MnII(H2O)2(X)2(dcbpy)] (where dcbpy= 4,4′‐dicarboxy‐2,2′‐bipyridine; X=acetate, sulphate, chloride ions) on gold electrode surface using self‐assembly process and applied for the electrocatalytic oxidation of water in neutral pH. The modified electrode was characterized by using different spectral, microscopic and electrochemical techniques. The Mn(II)‐acetate containing gold electrode (Mn(II)‐dcbpy‐Au) electrocatalytically oxidized water at a current density of 1.6 mA cm−2 with low overpotential ∼ 310 mV and a Tafel slope of 56 mV/decade. The modified electrode showed high electrochemical stability during controlled potential electrolysis. It was also observed that the Mn(II)‐acetate immobilized on dcbpy‐Au electrode showed higher electrocatalytic activity in oxidation of water than Mn(II)‐chloride or Mn(II)‐sulfate containing dcbpy‐Au electrode. The newly in‐situ generated Mn(II)‐complex modified electrode is inexpensive and highly efficient for the catalytic oxidation of water than the surface bound Mn‐based electrocatalysts reported previously.

Nanoflakes-assembled 3D flower-like nickel hydroxidenitrate as a highly efficient electrocatalyst for water oxidation

Abstract This paper reports a nanoflakes-assembled 3D flower-like nickel hydroxidenitrate (NiNH) and its application in water oxidation. It is demonstrated for the first time that the NiNH is an efficient oxygen evolution reaction (OER) electrocatalyst in alkaline solution. Specifically, it can drive a current density of 10 mA cm−2 at a fairly low overpotential of 280 mV and a correspondent small Tafel slope of 39.6 mV dec−1. The impressive OER catalytic activity of the as-prepared NiNH can be ascribed to its favorable 3D hierachical microstructure. The present research not only provide us a prospective and cost-effective electrocatalyst for water splitting but also affords us an inspiration for designing effective transition-metal based electocatalysts through microstructure regulation.

The IV period transition metal modified carbon@TiO2 nanoflower with high photo-electrochemical water oxidation performance under solar irradiation

Improving the photo-electrochemical water oxidation performance (PEWOP) of pristine TiO2 has been seriously hampered by its poor carrier transport efficiency and sluggish surface water oxidation kinetics. In this work, we prepared a novel IV period transition metal modified carbon layer-coated TiO2 (M-C@TiO2) nanoflower via a facile hydrothermal process followed by annealing at 800 °C. The as-prepared M-C@TiO2 nanoflower is uniform and stable with the size < 600 nm. The PEWOP of different IV period transition metal Zn, Cu, Ni, Co, Fe, Mn and Cr modified M-C@TiO2 nanoflowers are systematically investigated. Among them, the Cu, Ni, and Mn metals show high sensitivity to the current response in the modified nanoflowers. The current density of as-prepared Cu0.1-C@TiO2, Ni0.5-C@TiO2, and Mn1.0-C@TiO2 nanoflowers are increased to 4.6, 6.7 and 6.2 times higher than that of pristine TiO2, respectively, implying a significant enhancement to the PEWOP of pristine TiO2. This work provides an effective way to significantly enhance the PEWOP of pristine TiO2. Furthermore, it may arouse new interest and insight for the designing and preparing of M-C@TiO2 nanostructures for large-scale and practical water oxidation applications.

Progress in understanding hematite electrochemistry through computational modeling

Hematite (α-Fe2O3) is one of the most widely studied materials for the application of water oxidation in the field of renewable energy. In order to push hematite toward maximal efficiency, a better understanding of the intrinsic limitations is essential. In this review, we cover our latest results on identifying and finding ways to overcome bottlenecks by modelling the electrochemical processes in hematite at three regimes: 1. at the bulk, the calculated hole effective mass is unfavorably high and conductivity can be improved through doping and alloying, 2. at the surface, catalysis is slow and involves long-lived surface states, but the catalytic overpotential can be reduced by adding a doping gradient or overlayers without strain, and 3. at the back contact the losses can be improved by selecting a proper metal contact. We provide unique analysis approaches that reveal fundamental understanding of electrochemistry with hematite. These models have made an impact on understanding experimental observations and have influenced subsequent theoretical investigations.

Kinetics and Faradaic Efficiency of Oxygen Evolution on Reduced HxWO3 Photoelectrodes

Tungsten oxide (WO3) electrodes were synthesized by spin-coating an ammonium metatungstate sol. Instability in photocurrent during water oxidation applications has previously been attributed to for...

Facile synthesis of MOF-Derived Co@CoNx/bamboo-like carbon tubes for efficient electrocatalytic water oxidation

The development of economical noble metal-free electrocatalysts for oxygen evolution reaction (OER) with high catalytic activity and durability is crucial for the practical applications of many energy storage and conversion technologies. Here, we report a facile and economical synthesis of Co@CoNx/N-doped carbon tubes hybrid (Co@CoNx/NCT hybrid) via one-step pyrolysis of ZIF-67 at a low temperature of 650 °C with the introduction of urea, which functions as both nitrogen source and promoter of carbon tube growth. The resultant Co@CoNx/NCT hybrid exhibits bamboo-like structure with numerous active sites originated from abundant CoNx species, high nitrogen doping level and topological defects, rendering excellent catalytic activities towards OER in alkaline medium. As a result, the as-obtained Co@CoNx/NCT hybrid shows an overpotential of as low as 290 mV at a current density of 10 mA cm−2 with a small Tafel slope of 60 mV dec−1, which outperforms most noble metal-free electrocatalysts and even commercial IrO2. Therefore, the Co@CoNx/NCT hybrid holds great promise for efficient electrocatalytic water oxidation applications.

Thickness-dependent photoelectrochemical properties of a semitransparent Co3O4 photocathode

Co3O4 has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel Co3O4 electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co3O4 samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent Co3O4 samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co3O4 related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two Co3O4 photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h−1 cm−1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h−1·cm−1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co3O4 photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.

Preparation of a novel Fe2O3-MoS2-CdS ternary composite film and its photoelectrocatalytic performance

A novel Fe2O3-MoS2-CdS film was fabricated by electrodeposition, hydrothermal reaction and chemical bath deposition. Its photoelectrochemical (PEC) performance for water oxidation and pollutants removal under visible light were studied. The results indicated that it is an effective strategy to fabricate the ternary heterojunction by Fe2O3, MoS2 and CdS. The photocurrent density of Fe2O3-MoS2-CdS composite increases by 40 times relative to pure Fe2O3 at 0.45 V vs. Ag/AgCl in 0.1 M NaOH aqueous solution under visible light irradiation. Among the tested films, Fe2O3-MoS2-CdS exhibited the highest PEC activity for phenol degradation. The highest degradation efficiency (90.47%), rate constant (0.0076 min−1) and electrochemical enhancement value (97.37%) were achieved under the optimized conditions. Hydroxyl radicals and superoxide radicals were considered as main active species. The excellent PEC performance was attributed to the ternary heterojunction structure formed by Fe2O3, MoS2 and CdS, which efficiently reduced charge transfer resistance, facilitated charge transfer, and improved photoelectric conversion efficiency. Fe2O3-MoS2-CdS is a promising composite material for PEC application in water oxidation and pollutants removal.

Corrigendum: An Oxofluoride Catalyst Comprised of Transition Metals and a Metalloid for Application in Water Oxidation.

The authors wish to correct a mistake in the selection of figures in the manuscript. Figure 6a and the left side of Figure S8 are incorrect and actually show a crystal after three oxidation experiments instead of showing a fresh crystal before the start of oxidation experiments. The correct micrograph for Figure 6a was found, and is shown below. The two micrographs are very similar and do not impose any technical difference, thus the results and conclusions remain unchanged in the article. a) Image of the crystal before catalytic testing A replacement for Figure S8 in the supplementary materials was not available, so that figure has been removed and a corrected version of the Supporting Information has been uploaded. The authors wish to apologize for this error and for any inconvenience caused.