Efficient Water Oxidation Catalysts Research Articles

{Co4O4} Cubanes in a conducting polymer matrix as bio-inspired molecular oxygen evolution catalysts

Exploration of efficient molecular water oxidation catalysts for long-term application remains a key challenge for the conversion of renewable energy sources into fuels. Cuboidal {Co4O4} complexes keep attracting interest as molecular water oxidation catalysts as they combine features of both heterogeneous and homogeneous catalysis with bio-inspired motifs. However, the application of many cluster-based catalysts for the oxygen evolution reaction still requires new stabilization strategies. Drawing inspiration from the stabilizing effects of natural polymers, we introduce a conductive polymer-hybrid approach to covalently immobilize {Co4O4} cubane oxo clusters as oxygen evolution catalysts. Polypyrrole is applied as an efficient p-type conducting polymer that promotes hole transfer during the oxygen evolution reaction, resulting in higher turnover frequency compared to the pristine {Co4O4} oxo cluster and heterogeneous Co-oxide benchmarks. The asymmetric coordination of {Co4O4} not only mitigates catalyst decomposition pathways, but also increases the catalytic efficiency by exposing a directed cofacial dihydroxide motif during catalysis.

A Proton-Conductive Co(II)-Polyoxometalate Acts as a Precatalyst for Efficient Electrocatalytic Water Oxidation.

A cobalt(II)-containing polyoxometalate, [H3O]5[{Co(H2O)4}3{Na(H2O)4}W12O42]·3H2O (Co-POM), has been isolated in a one-step facile aqueous synthesis and characterized unambiguously using single-crystal X-ray crystallography along with routine spectral analysis. The paratungstate cluster anion [W12O42]12- coordinates with {CoII(H2O)4}2+ and {Na(H2O)4}+ complex cations resulting in the formation of the water-insoluble Co-POM compound having three-dimensional (3-D) extended structure. Motivated by the protonated water molecules existing as the counter cations in Co-POM, herein, we demonstrate the detailed proton conductivity studies of the Co-POM, reaching a value of 1.04 × 10-2 S cm-1 at 80 °C and 98% relative humidity (RH). The temperature- and humidity-dependent proton conductivity in Co-POM is governed by Grotthus mechanism with Ea = 0.25 eV. In addition, we examined the electrochemical behavior of Co-POM, in an alkaline borate buffer where it is found to be electrochemically unstable and acts as a precatalyst (and not a true catalyst) for oxygen evolution reaction (OER). We also discuss the "post-mortem" analysis of the postelectrolysis sample to identify the active species which turns out to be a cobalt oxide material (Co3O4) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).

Construction of Ag/AgCl@MIL-53(Fe): Achieving Efficient Photocatalytic Water Oxidation via the Plasma-Phase Silver and Heterojunction Synergistic Effect

The development of highly efficient water oxidation catalysts is a bottleneck in achieving artificial photosynthesis, as water oxidation is a complex process involving multiple electron and proton transfers. To further improve the photocatalytic water oxidation performance of MIL-53(Fe), a series of Ag/AgCl@MIL-53(Fe) samples with different Fe:Ag ratios were synthesized by hydrothermal methods and used in the photocatalytic water oxidation reactions. XRD characterization showed the successful preparation of Ag/AgCl@MIL-53(Fe) heterostructure catalysts. The reaction results showed that sample AAM-2 (Fe:Ag=5:1) had the best photocatalytic water oxidation performance, with the highest TOF value of 0.14 mmol/(g·s) and quantum efficiency of 39.0 % under the conditions of catalyst mass of 1 mg and pH=9.0 for boric acid-borax buffer solution. Measurements of photocurrent indicated that the AAM-2 samples doped with Ag/AgCl had higher photocurrent densities, thus improving the photocatalytic performance. This provides new insights for constructing highly efficient porous MOFs catalysts.

Bioinspired octanuclear copper cubane complex as an efficient molecular electrocatalyst for water oxidation

Water oxidation catalysts (WOCs) with excellent activity and stability are highly desired. Numerous efforts have been dedicated to develop WOCs mimicking the cubic structure and activity of Mn4CaO5 cluster in oxygen-evolving center (OEC) of photosynthesis system II (PSII). Herein, a octanuclear Cu cluster [Cu8(dpy{OH}O)8(OAc)4]4+ (1, dpy{OH}O− = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone) that contains two Cu4O4 cubic fragments with bioinspired structure mimicking the Mn4CaO5 cluster of OEC of natural PSII is developed as efficient homogeneous catalyst for electrochemical water oxidation under near neutral condition for the first time. It exhibits outstanding water oxidation catalytic activity via accelerating oxygen evolution with onset overpotential of 730 mV and appreciable turnover frequency of 20.1 s−1. Collective experiments together confirmed the homogeneity and stability of cluster 1 under long-term water oxidation catalytic cycle. Kinetic study reveals the water nucleophilic attack (WNA) catalytic mechanism occurs on the coordination unsaturated Cu center of the cluster. This work provides the inspiration of the development of robust bioinspired catalyst for electrochemical water oxidation.

Synergistic doping of Iron and selenium on nickel hydroxide nitrate nanoarrays-derived as an efficient electrocatalyst for overall water splitting

The fabrication of highly efficient and stable catalysts for water oxidation is important to promote the development of the hydrogen fuel industry. In this study, dual-element-doped two-dimensional nickel hydroxide nitrate nanoarrays (NiHN) self-supported on nickel foam were fabricated via a facile electrodeposition process. The results demonstrated that Fe and Se were incorporated into the nickel hydroxide structure. The binder-free FeSe-NiHN catalyst exhibited low overpotentials of 129 and 250 mV at 10 mA cm−2 for the hydrogen evolution reaction and at 100 mA cm−2 for the oxygen evolution reaction in 1 M KOH media, respectively. Furthermore, a durability test conducted at 70 mA cm−2 for 28 h showed no evident degradation of the catalyst activity, verifying its exceptional stability. Thus, FeSe-NiHN is a potential water splitting electrocatalyst for inexpensive hydrogen production.

Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO2 Photoanode for Water Oxidation in Acid.

We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na8K8[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (Co9POM) and nanoporous n-type TiO2 photoelectrodes (henceforth "TiO2-Co9POM") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO2-Co9POM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO2 in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound Co9POM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that Co9POM serves as an efficient WOC, extracting photogenerated holes from TiO2 on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation.

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

An eco-friendly route for green synthesis of ZnO-CoFe2O4 nanoparticles from cardamom and ginger extract as an efficient electrochemical catalyst for water oxidation

Water splitting has been one of the potential techniques as a clean and renewable energy resource for the fulfillment of world energy demands. One of the major aspects of this procedure is the exploitation of efficient and inexpensive electrocatalysts due to the fact that the water oxidation procedure is accompanied by a delayed reaction. In this research, ZnO-CoFe2O4 nanostructure was successfully synthesized via the green method and green resources from cardamom seeds and ginger peels for oxygen evolution reaction (OER). The modified Glassy carbon electrode (GCE) with ZnO-CoFe2O4 is effective for the electrochemical water oxidation interaction since it has sufficient electrical strength and excellent catalytic performance. The creation of rice-like and small granular structures of ZnO-CoFe2O4 nano-catalysts was confirmed by characterization methods such as XRD, FESEM, EDS and MAP. According to the achieved results, in the electrolysis of water, with in-cell voltage of 1.40 V and 50 mA cm–2 for current density in a 0.1 M KOH electrolyte and OER only has 170 mV overpotentials.

Catalytic Water Oxidation by Ruthenium Complexes with Pyridine Alkoxide Ligands

AbstractLigand design is crucial for the design of robust and efficient molecular water oxidation catalysts (WOCs) based on metal complexes. Herein we report the water oxidation studies of a series of Ru WOCs bearing bipyridine dialkoxide ligands with different substituents. Combined with density functional theory (DFT) calculations, the proposed water oxidation mechanism is bimolecular coupling (I2M) of two seven‐coordinate (CN7) Ru=O species. The use of bulky phenyl groups on the planar alkoxide ligand helps to stabilize the high oxidation CN7 intermediates, without loss of ligand flexibility. This strategy may be useful for the design of other active WOCs.

Heterojunction and co-catalyst engineering of bismuth vanadate by nickel molybdenum oxide and carbon quantum dot decorations for photoelectrochemical water splitting

Improving the charge diffusion length and water oxidation kinetics of bismuth vanadate (BiVO4) is essential to achieve better photoelectrochemical catalytic ability for water oxidation. Establishing heterojunctions and incorporating co-catalysts with BiVO4 create facile charge transfer paths and generate more water oxidation reactions. In this work, nickel molybdenum oxide (NM) and carbon quantum dot (CQD) are firstly deposited on BiVO4 by hydrothermal and soaking processes to design an efficient co-catalyst/photocatalyst system for water oxidation. The depositing amount of CQD is optimized to balance contributions of CQD and NM as the hole sink and the light-to-electron generator, respectively. The highest photocurrent densities of 2.2 and 3.9 mA/cm2 are achieved for BiVO4/NM/CQD in the electrolyte without and with hole scavenger, respectively. The BiVO4 electrode only shows a photocurrent density of 1.2 mA/cm2 in the electrolyte without hole scavenger. The best photoelectrochemical catalytic ability of BiVO4/NM/CQD is approached by the reduced charge-transfer resistance and charge recombination as well as the enhancement on carrier densities. The excellent long-term stability with the photocurrent retention of 90% under illumination for 24 h is also obtained for BiVO4/NM/CQD. This study opens a blueprint on designing efficient catalysts for water oxidation by incorporating heterojunction and co-catalyst materials simultaneously.

A cauliflower-like Ni metal-organic phosphate modified carbon paste electrode as an efficient catalyst for electrochemical water oxidation in neutral media

We fabricated the cauliflower-like Ni metal-organic phosphates (Ni-MOPh) with different synthesis time (Ni-MOPh-Ts, Ts = Times) as a carbon paste electrode material (Ni-MOPh-Ts/C) for oxygen evolution reaction (OER) in neutral media. The as-synthesized Ni-MOPh-12 h/C shows a better OER ability than that of Ni-MOPh-6 h/C and Ni-MOPh-2 h/C, which also shows conspicuous performance with the overpotential of 420 mV to obtain 10 mA cm−2. Reaction kinetics studies exhibit that Ni-MOPh-12 h/C can provide higher electrochemical ability, mainly due to the small charge transfer resistance, and a high number of catalytic active sites.

Observation of Support-Dependent Water Oxidation Kinetics on Molecularly-Derived Heterogeneous Ir-Oxide Catalysts

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Understanding the material properties that determine water oxidation activity is critically important for the design of efficient and durable water oxidation catalysts. The electronic structure and surface chemistry of the catalyst influence the accumulation of oxidative charge (holes) on catalytically active sites. Hole distribution dynamics and molecular structure of the active sites are generally accepted to jointly determine activity. However, these properties are typically inherent to materials; it has been difficult to independently tune them on most heterogeneous catalysts, hindering the development of an understanding of their relative influence. Herein, heterogenized dinuclear Ir catalysts (Ir-DHC) supported on different oxides were employed to overcome this challenge. It was found that at low temperatures (288-298 K), the photocatalytic activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports are comparable within the studied range of fluences (62-151 mW/cm2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibits a ∼100% higher water oxidation activity than on CeO2. The divergent activities under these conditions were attributed to the distinct abilities of the supporting substrates to redistribute surface holes. Specifically, ITO is relatively efficient in distributing photogenerated holes between Ir-DHC active sites, whereas CeO2 exhibits sluggish hole redistribution because of its reducibility, thereby limiting hole accumulation at Ir-DHC active sites. The temperature dependence was explained by a temperature induced switch in the rate-determining step of the reaction. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation.

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

Coupling ferromagnetic ordering electron transfer channels and surface reconstructed active species for spintronic electrocatalysis of water oxidation

Sluggish reaction kinetics of oxygen evolution reaction (OER), resulting from multistep proton-coupled electron transfer and spin constriction, limits overall efficiency for most reported catalysts. Herein, using modeled ZnFe2−xNixO4 (0 ≤ x ≤ 0.4) spinel oxides, we aim to develop better OER electrocatalyst through combining the construction of ferromagnetic (FM) ordering channels and generation of highly active reconstructed species. The number of symmetry-breaking Fe–O–Ni structure links to the formation of FM ordering electron transfer channels. Meanwhile, as the number of Ni3+ increases, more ligand holes are formed, beneficial for redirecting surface reconstruction. The electro-activated ZnFe1.6Ni0.4O4 shows the highest specific activity, which is 13 and 2.5 times higher than that of ZnFe2O4 and unactivated ZnFe1.6Ni0.4O4, and even superior to the benchmark IrO2 under the overpotential of 350 mV. Applying external magnetic field can make electron spin more aligned, and the activity can be further improved to 39 times of ZnFe2O4. We propose that intriguing FM exchange-field interaction at FM/paramagnetic interfaces can penetrate FM ordering channels into reconstructed oxyhydroxide layers, thereby activating oxyhydroxide layers as spin-filter to accelerate spin-selective electron transfer. This work provides a new guideline to develop highly efficient spintronic catalysts for water oxidation and other spin-forbidden reactions.

Constructing the oxygen diffusion paths for promoting the stability of acidic water oxidation catalysts

Anchoring Ir onto metal-oxide support can effectively address the scarcity and insufficient activity problems of Ir-based acidic water oxidation catalysts by triggering the lattice-oxygen-mediated mechanism (LOM). However, the stability of these catalysts is usually sacrificed due to support overoxidation and reconstruction during the LOM pathway. Here, we propose constructing oxygen diffusion paths at a mixed-metal oxide support (ZrOx and TaOx) interface for a stable LOM. The new synthesized Ir-ZrTaOx catalyst exhibits two orders higher mass activity than IrO2 and retains 1,000-h electrolysis. It demonstrates low degradation (4.7 μV h−1) during the 200-h operation in a proton-exchange membrane (PEM) electrolyzer using only 1/4 Ir loading. Operando spectroscopic evidence combined with theory studies suggests that the oxygen diffusion path at the ZrOx/TaOx interface inhibits the overoxidation and detrimental reconstruction of TaOx. The strategy of stabilizing the LOM will benefit the future design of stable and efficient water oxidation catalysts.

Activation stainless steel with electrochemical etching-hydrothermal as an efficient and stable alkaline water oxidation electrode

Stainless steel (SS) has great potential to become an efficient oxygen evolution reaction (OER) catalytic material, but an effective activation method is still lacking. Here, an electrochemical etching-hydrothermal activation strategy is proposed to turn SS into an efficient and stable OER electrode, which can deliver high current density of 300 mA cm−2 with only 314 mV, as well as offers a low Tafel slope of 30 mV dec−1. The catalytic phase formed by the activation has a low surface Ni/Fe ratio and completely disobeys the traditional “Fe-effect” activity trend. The performed O–K edge X-ray adsorption spectra revealed that the high valence states of Ni sites with modified electronic structure may be responsible for the enhanced OER performance. More importantly, the assembled overall water-splitting cell showed that it can withstand long-term durability of 100 h at an ultra-high current density of 2 A cm−2. The strategy presented here could yield an opportunity to convert SS into an efficient and robust catalyst for alkaline water oxidation and is expected to lead to more promising ones due to the diverse composition of SS.

Effect of mixed-valence of manganese on water oxidation activity of La1-xCaxMnO3 (0 ≤ x ≤ 1) solid solutions

The water oxidation reaction is one of the crucial process involves 4e- and 4H+ transfer, which causes a high energy barrier and makes the water-splitting process a slow kinetic reaction. Herein, we have systematically observed the water oxidation activity of La1-xCaxMnO3 (0≤ x ≤ 1) solid solutions by tuning the valence of Mn ion through selective substitution of La3+ with Ca2+. With increasing Ca concentration in La1-xCaxMnO3 solid solutions, the water oxidation activity increases up to x = 0.5 and then decreases. Mixed-valence of Mn generated due to substitution of La3+ with Ca2+, which leads to the double exchange mechanism. This could results in high electrical conduction and a faster electron transfer rate because of the rapid charge exchange between Mn3+ and Mn4+ ions, which pinpoints the reason behind the enhanced oxygen evolution activity. Our findings will be a promising way to relate the Mn mixed-valence with its catalytic activity to design and development of efficient water oxidation catalysts.

One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe2+ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s-1). The observed overpotential to achieve a current density of 10 mA cm-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.

Molecular engineering metal-organic frameworks as efficient electrochemical catalysis for water oxidation.

Metal-organic framework (MOF) solids with their variable functionalities are relevant for energy conversion technologies. However, the development of electroactive and stable MOFs for electrocatalysis still faces challenges. Here, we design a molecularly engineered MOF system featuring a 2D coordination network based on mercaptan-metal links (e.g., nickel, as for Ni(DMBD)-MOF). The crystal structure is solved from microcrystals by continuous rotation electron diffraction (cRED) technique. Computational results indicate a metallic electronic structure of Ni(DMBD)-MOF due to the Ni-S coordination, highlighting the effective design of the thiol ligand for enhancing electro-conductivity. Additionally, both experimental and theoretical studies indicate that (DMBD)-MOF offers advantages in the electrocatalytic oxygen evolution reaction (OER) over non-thiol (e.g., 1,4-benzene dicarboxylic acid) analog (BDC)-MOF, because it poses fewer energy barriers during the rate-limiting *O intermediate formation step. Iron-substituted NiFe(DMBD)-MOF achieves a current density of 100 mA·cm-2 at a small overpotential of 280mV, indicating a new MOF platform for efficient OER catalysis. This article is protected by copyright. All rights reserved.

Mononuclear ruthenium (II) complex covalently anchored on melem and g-C3N4 as efficient heterogeneous catalysts for chemical water oxidation

Ru-melem and Ru-C3N4 were synthesized by a simple and facile strategy to construct a novel covalently anchoring by introducing easily synthesized amide bond as a bridge connecting the Ru-terpy and melem or g-C3N4, respectively. The covalent anchoring of Ru complex on melem or C3N4 not only makes these materials exhibit water oxidation activity under CeIV-driven (CeIV = Ce(NH4)2(NO3)6) reaction condition, but also makes the obtained heterogeneous catalysts show higher catalytic activity than the corresponding homogeneous catalysts, which reveals that the covalent anchoring strategy of Ru complex is beneficial to improve the catalytic activity of homogeneous Ru catalysts. The synthetic method of hybrid catalysts offers an insightful strategy for enhancing water oxidation activity of molecular catalysts.