Water Oxidation Activity Research Articles

Tailoring of ZnSe/GCN/MoS2 ternary heterojunction with enhanced photoelectrochemical activity for water oxidation

This research endeavor aims to fabricate a novel ternary heterojunction interface to enhance photoelectrochemical water Oxidation. ZnSe/GCN/MoS2 heterostructure series is prepared via the ultrasonication method. Confirmation of as-prepared heterostructures was done through FTIR, XRD, SEM, EDS, XPS, Raman, UV-DRS, and PL techniques. The best photoresponse among the ternary series was found for 1% ZnSe/GCN/MoS2 (1ZGM), showing the highest photocurrent density of 1.73 mA during linear sweep voltammetric studies. This is an increase of 7 times in the photoresponse than the pure GCN. These outcomes are reinforced by the EIS results, showing the smallest semicircle for 1ZGM in a Nyquist plot and an enhanced lifetime of 100.7 ms in a bode plot. The smallest Tafel slope, higher donor density, and reduced electron-hole recombination, suggest that a ternary heterojunction is successfully fabricated for PEC water oxidation.

Introducing High-Valence Iridium Single Atoms into Bimetal Phosphides toward High-Efficiency Oxygen Evolution and Overall Water Splitting.

Single atoms are superior electrocatalysts having high atomic utilization and amazing activity for water oxidation and splitting. Herein, this work reports a thermal reduction method to introduce high-valence iridium (Ir) single atoms into bimetal phosphide (FeNiP) nanoparticles toward high-efficiency oxygen evolution reaction (OER) and overall water splitting. The presence of high-valence single Ir atoms (Ir4+ ) and their synergistic interaction with Ni3+ species as well as the disproportionation of Ni3+ assisted by Fe collectively contribute to the exceptional OER performance. In specific, at appropriate Ir/Ni and Fe/Ni ratios, the as-prepared Ir-doped FeNiP (Ir25 -Fe16 Ni100 P64 ) nanoparticles at a mass loading of only 35µg cm-2 show the overpotential as low as 232mV at 10mA cm-2 and activity as high as 1.86 A mg-1 at 1.5V versus RHE for OER in 1.0m KOH. Computational simulations confirm the vital role of high-valence Ir to weaken the adsorption of OER intermediates, favorable for accelerating OER kinetics. Impressively, a Pt/C||Ir25 -Fe16 Ni100 P64 two-electrode alkaline electrolyzer affords a current density of 10mA cm-2 at a low cell voltage of 1.42V, along with satisfied stability. An AA battery with a nominal voltage of 1.5V can drive overall water splitting with obvious bubbles released.

Operando Condition Reaction Modeling Shows Highly Dynamic Attachment of Oligomeric Ruthenium Catalysts

To increase the stability and current density of molecular-catalyst-based electroanodes for water oxidation, immobilization of the catalysts at the electrode surface is a common strategy. A prominent example is the oligomerized Ru(tda) molecular catalyst, which showed outstanding current densities even at neutral pH values. One of the most challenging aspects of immobilized catalysts is to understand the interaction between the catalyst and the surface under operando conditions. Experiments are often performed under model conditions, and computational methods to study reaction steps are typically limited to a few hundred atoms. In this study, we combined three computational methods, density functional theory electronic structure computations, molecular dynamics for large-scale simulations of the catalyst–solid interaction, and empirical valence bond for reaction modeling the catalyst at the interface of a large carbon support and a phosphate water buffer. These techniques allowed us to explore the combined effects of solvent, hydrophobic directionality, and electric field on the attachment and reactivity of a Ru(tda) pentamer at a graphene surface. Our simulations have a perfect agreement with the experimental characterization under model conditions. However, we find that under operando conditions, where the catalyst is oxidized to the active RuV state, with a phosphate-containing electrolyte and an applied electric field, the attachment is completely reversed compared to the model conditions with RuII and organic solvents. This reversed attachment leads to a water-excluded region close to the active RuV═O center. The EVB reaction modeling showed that the reaction could still proceed to form an O–O bond via an oxide relay mechanism, where a dangling carboxylate reacts with the oxo via nucleophilic attack. We find that the activation energies are identical in water solution and at the electrode surface, showing how this mechanism is key to highly active molecular water oxidation catalysts immobilized on surfaces. Since attachment to surfaces could have a strong, and often negative, influence on the reactions, this study provides a guideline on how to model reactions without compromising the complexity of the electrode environment.

MnCo2O4 decorating porous PbO2 composite with enhanced activity and durability for acidic water oxidation

Developing a promising catalyst with both improved activity and robust durability for the oxygen evolution (OER) in an acidic electrolyte is extremely desirable. Here we constructed a novel MnCo2O4 decorating PbO2 electrode (3D-Ti/α-PbO2/β-PbO2-MnCo2O4) with a sandwich-like structure via a successive co-precipitation and co-electrodeposition onto porous titanium (3D-Ti) support. The experimental results indicated that the introduction of porous titanium is essential for achieving a unique porous structure architecture. In addition, the adoption of MnCo2O4 increased the contact area with electrolyte due to the large BET surface area of 144.06 cm2 g−1. Consequently, the synthesized 3D-Ti/α-PbO2/β-PbO2-MnCo2O4 composite electrode exhibited a low overpotential of 379 and 453 mV at 10 and 50 mA cm−2, respectively, as well as a minor Tafel slope of 133 mV per decade in 0.5 M H2SO4 solution. Furthermore, 3D-Ti/α-PbO2/β-PbO2-MnCo2O4 presents robust durability, keeping steady for more than 140 h at 50 mA cm−2 and without observable performance degradation, which is superior to many reported state-of-the-art non-precious OER catalysts. This route is for creating and synthesizing non-precious and superior performance catalysts for electrolysis and beyond.

One dimensional nickel phosphide polymorphic heterostructure as carbon-free functional support loading single-atom iridium for promoted electrocatalytic water oxidation

Although conducting materials such as carbon nanotube and carbon fiber paper (CFP) have been extensively employed as support of electrocatalytic active sites, most of them are of poor catalytic functionality by themselves and undesirable stability during strong acid/alkaline environments or oxidation process. Here we report a novel one-dimensional (1D) nickel phosphide polymorphic heterostructure (denoted as NPPH) to work as one effective carbon-free functional support for loading of single-atom Ir water oxidation electrocatalyst. Specifically, the NPPH composed of both Ni12P5 and Ni2P phases is not only active for robust alkaline water oxidation but also is of good stability and hydrophilicity for favorable loading of single-atom dispersed iridium. The NPPH supported single-atom Ir electrocatalyst (Ir/NPPH) is found to exhibit remarkably superior water oxidation activity with respect to the NPPH itself or CFP supported single-atom Ir catalyst (Ir/CFP), demonstrating the synergetic promotion effect between NPPH and single-atom Ir catalyst. Furthermore, the NPPH supported single-atom Ir catalyst can bear alkaline water oxidation for over 120 h at current density of 50 mA cm−2. The NPPH developed here is expected as functional support to composite with other water oxidation catalysts, as may be an alternative strategy of developing highly efficient carbon-free electrocatalysts.

Accelerating water oxidation on BiVO4 photoanodes via surface modification with Co dopants

On the BiVO4 photoanode surface, oxygen vacancies modified by cobalt incorporation have a remarkable impact on the water oxidation activity and charge injection efficiency.

Atomically dispersed Ir catalysts exhibit support-dependent water oxidation kinetics during photocatalysis.

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Recent research has suggested that the reaction mechanisms are sensitive to the hole density at the active sites. However, these previous results were obtained on catalysts of different materials featuring distinct active sites, making it difficult to discriminate between competing explanations. Here, a comparison study based on heterogenized dinuclear Ir catalysts (Ir-DHC), which feature the same type of active site on different supports, is reported. The prototypical reaction was water oxidation triggered by pulsed irradiation of suspensions containing a light sensitizer, Ru(bpy)32+, and a sacrificial electron scavenger, S2O82-. It was found that at relatively low temperatures (288-298 K), the water oxidation activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports were comparable within the studied range of fluences (62-151 mW cm-2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibited a ca. 100% higher water oxidation activity than on CeO2. The divergent activities were attributed to the distinct abilities of the supporting substrates in redistributing holes. The differences were only apparent at relatively high temperatures when hole redistribution to the active site became a limiting factor. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Intrinsically robust cubic MnCoOx solid solution: achieving high activity for sustainable acidic water oxidation

The MnCoOx solid solution catalysts show excellent catalytic efficiency and stability in the acidic OER. The investigation and DFT studies highlight the essential role of Co3+ and Mn4+ cations and oxygen vacancies in the OER mechanism.

CeO2 supported high-valence Fe oxide for highly active and stable water oxidation

The high-valence Fe ions are stabilized on a CeO2 support and achieved a record low overpotential of 219 mV to reach the current density of 50 mA cm−2.

Engineering a molecular ruthenium catalyst into three-dimensional metal covalent organic frameworks for efficient water oxidation.

The water oxidation reaction plays an important role in clean energy conversion, utilization, and storage, but mimicking the oxygen-evolving complex of photosystem II for designing active and stable water oxidation catalysts (WOCs) is still an appealing challenge. Here, we innovatively engineered a molecular ruthenium WOC as a metal complex building unit to construct a series of three-dimensional metal covalent organic frameworks (3D MCOFs) for realizing efficient oxidation catalysis. The resultant MCOFs possessed rare 3D interlocking structures with inclined interpenetration of two-dimensional covalent rhombic nets, and the Ru sites were periodically arranged in the crystalline porous frameworks. Impressively, these MCOFs showed excellent performance towards water oxidation (the O2 evolution rate is as high as 2830 nmol g-1 s-1) via the water nucleophilic attack pathway. Besides, the MCOFs were also reactive for oxidizing organic substrates. This work highlights the potential of MCOFs as a designable platform in integrating molecular catalysts for various applications.

CeO2/CuO/NiO hybrid nanostructures loaded on N-doped reduced graphene oxide nanosheets as an efficient electrocatalyst for water oxidation and non-enzymatic glucose detection.

In this work, the three-component heterostructure of CeO2/CuO/NiO was synthesized by a co-precipitation procedure and heating at a temperature of 750 °C. Then, CeO2/CuO/NiO nanoparticles were successfully supported on N-doped reduced graphene oxide (N-rGO) by a hydrothermal method. The obtained nanomaterials were used as effective electrocatalysts for the oxygen evolution reaction and glucose sensing in an alkaline medium. The results indicated that when CeO2/CuO/NiO is anchored on N-rGO nanosheets, active catalytic sites increase. On the other hand, N-doped rGO enhances electrical conductivity and electron transfer for water or glucose oxidation. CeO2/CuO/NiO@N-rGO has a large electrochemically active surface area and more active catalytic positions, and thus exhibits high activity for the OER with a low overpotential of 290 mV, a suitable Tafel slope of 110 mV dec-1, and superior stability and durability for at least 10 hours. CeO2/CuO/NiO@N-rGO can also detect glucose with a high sensitivity of 912.7 μA mM-1 cm-2, a low detection limit of 0.053 μM, a wide linear range between 0.001 and 24 mM, and a short response time of about 2.9 s. Moreover, the high selectivity and stability of this electrode for glucose sensing show its potential for clinical applications.

Regulation of bulk reconstruction of FeNiMoO4via NH3 treatment for high performance water oxidation

A Fe-doped NiMoO4 pre-catalyst with lattice defects induced by NH3 treatment shows bulk and rapid reconstruction to distorted γ-Ni(Fe)OOH with improved intrinsic activity for water oxidation.

Electrochemical generation of high-valent oxo-manganese complexes featuring an anionic N5 ligand and their role in O-O bond formation.

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help in understanding the mechanism of O-O bond formation presumably occurring in the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscore the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species [(dpaq)MnIV(O)]+ (2) through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one electron to a formal manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital, whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from the metal and the ligand, leaving a single electron in the quinoline-dominant orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal-ligand (quinoline)-based oxidation is proposed to generate a formal Mn(VI) species (4), a non-heme analogue of the species 'compound I', formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O-O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide.

Iron(III) ion-assisted transformation of ZIF-67 to a self-supported FexCo-layered double hydroxide for improved water oxidation.

Herein, we have demonstrated Lewis acid Fe(III)-assisted hydroxylation of ZIF-67 to form FexCo-layered double hydroxide (LDH) nanosheets. The catalyst Fe0.4Co-LDH produced an excellent water oxidation activity to reach a current density of 20 mA cm-2 at only 190 mV overpotential, superior to that of hydrothermally synthesized LDH with a similar composition.

Heterointerface engineering of NiFe (oxy)hydroxides/CNTs by in situ anchoring of sub-nano Au for efficient water oxidation.

Heterointerface engineering of NiFe (oxy)hydroxides is a prospective way of improving OER activity by the pre-catalysis of metal hydroxides accompanying the modulation of defects, but enhancement of the kinetics is controversial. Herein, in situ phase transformation of NiFe hydroxides was proposed and heterointerface engineering was optimized by sub-nano Au anchoring in simultaneously formed cation vacancies. Controllable size and concentrations of anchored sub-nano Au in the cation vacancies resulted in the modulation of the electronic structure at the heterointerface, and improved water oxidation activity was ascribed to the enhanced intrinsic activity and charge transfer rate. Here, Au/NiFe (oxy)hydroxide/CNTs with an Fe/Au molar ratio of 24 exhibited an overpotential of ∼236.3 mV at 10 mA cm-2 in 1.0 M KOH under simulated solar light irradiation, which is ∼19.8 mV lower than that without the consumption of solar energy. Spectroscopic studies reveal that the photo-responsive FeOOH in these hybrids and modulation of sub-nano Au anchoring in cation vacancies are favorable in improving solar energy conversion and suppressing photo-induced charge recombination.

Engineering a non-noble plasmonic center in MOF-derived Z-scheme heterojunctions for enhanced photoelectrochemical water splitting

A plasmonic Z-scheme catalyst is designed by the in situ derivation of NH2-MIL-125 and oxygen-vacancy engineering. The catalyst exhibits superior water oxidation activity in the PEC water splitting applications.

4f-2p-3d orbital overlap in a metal-organic framework-derived CeO2/CeCo-LDH heterostructure promotes water oxidation.

Herein, we have demonstrated a facile method for the synthesis of CeO2/Ce-Co-LDH heterostructures using zeolitic imidazolate framework-67 as the precursor. The Ce-incorporation in Co-LDH results in 4f-2p-3d orbital overlap to tune the electronic structure whereas the oxygen-deficient CeO2 controls the interface charge transfer. This results in excellent water oxidation activity to attain 500 mA cm-2 current density at 320 overpotential.

Homogeneously Nitrogen Doped BaLa4Ti4O15Extending Visible Light Responsive Range for Photocatalytic Oxygen Evolution

Homogeneous nitrogen doping is a promising strategy for narrowing the band gap of layered metal oxides without altering the crystal structures and microstructures of precursors. In this work, BaLa4Ti4O15, a (111) plane-type layered perovskite metal oxide with a band gap of about 3.77 eV, was doped with nitrogen by a thermal treatment in an ammonia atmosphere. The nitrogen dopants are homogeneously distributed in the layered perovskite framework, resulting in an extended band-to-band visible light absorption edge up to 600 nm, corresponding to a band gap of 2.07 eV. It is worth noting that the nitrogen doping process has trivial impact on the crystal structure, microstructure, and chemical states of the BaLa4Ti4O15, which minimizes the side effect of photogenerated carrier separation and surface chemical reaction. BaLa4Ti4O15–xNy exhibited efficient photocatalytic water oxidation activity under visible light. This work highlights the usefulness of homogeneous doping for tuning wide band gap layered metal oxides.

Alloy Metal Nanocluster: A Robust and Stable Photosensitizer for Steering Solar Water Oxidation.

Metal nanoclusters (NCs) have been unleashed as an emerging category of metal materials by virtue of integrated merits including the unusual atom-stacking mode, quantum confinement effect, and fruitful catalytically active sites. Nonetheless, development of metal NCs as photosensitizers is blocked by light-induced instability and ultrashort carrier lifespan, which remarkably retards the design of metal NC-involved photosystems, hence resulting in the decreased photoactivities. To solve these obstacles, herein, we conceptually probed the charge transfer characteristics of the BiVO4 photoanode photosensitized by atomically precise alloy metal NCs, wherein tailor-made l-glutathione-capped gold-silver bimetallic (AuAg) NCs were controllably self-assembled on the BiVO4 substrate. It was uncovered that alien Ag atom doping is able to effectively stabilize the alloy AuAg NCs and simultaneously photosensitize the BiVO4 photoanode, significantly boosting the photoelectrochemical (PEC) water oxidation performances. The reasons for the robust and stable PEC water oxidation activities of the AuAg NCs/BiVO4 composite photoanode were unambiguously unleashed. We ascertain that Ag atom doping in the staple motif of Aux NCs efficaciously protects the NCs from rapid oxidation, enhancing the photostability, boosting the photosensitization efficiency, and thus leading to the considerably improved PEC water splitting activities compared with the homometallic counterpart. This work could afford a new strategy to judiciously tackle the inherent detrimental instability of metal NCs for solar energy conversion.

Characterization of Reaction Intermediates Involved in the Water Oxidation Reaction of a Molecular Cobalt Complex

Molecular cobalt(III) complexes of bis-amidate-bis-alkoxide ligands, (Me4N)[CoIII(L1)] (1) and (Me4N)[CoIII(L2)] (2), are synthesized and assessed through a range of characterization techniques. Electrocatalytic water oxidation activity of the Co complexes in a 0.1 M phosphate buffer solution revealed a ligand-centered 2e-/1H+ transfer event at 0.99 V followed by catalytic water oxidation (WO) at an onset overpotential of 450 mV. By contrast, 2 reveals a ligand-based oxidation event at 0.9 V and a WO onset overpotential of 430 mV. Constant potential electrolysis study and rinse test experiments confirm the homogeneous nature of the Co complexes during WO. The mechanistic investigation further shows a pH-dependent change in the reaction pathway. On the one hand, below pH 7.5, two consecutive ligand-based oxidation events result in the formation of a CoIII(L2-)(OH) species, which, followed by a proton-coupled electron transfer reaction, generates a CoIV(L2-)(O) species that undergoes water nucleophilic attack to form the O-O bond. On the other hand, at higher pH, two ligand-based oxidation processes merge together and result in the formation of a CoIII(L2-)(OH) complex, which reacts with OH- to yield the O-O bond. The ligand-coordinated reaction intermediates involved in the WO reaction are thoroughly studied through an array of spectroscopic techniques, including UV-vis absorption spectroscopy, electron paramagnetic resonance, and X-ray absorption spectroscopy. A mononuclear CoIII(OH) complex supported by the one-electron oxidized ligand, [CoIII(L3-)(OH)]-, a formal CoIV(OH) complex, has been characterized, and the compound was shown to participate in the hydroxide rebound reaction, which is a functional mimic of Compound II of Cytochrome P450.