Water Oxidation System Research Articles

Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach.

For the first time, the manganese gallide (MnGa4) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnOx‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ‐MnO2, feitknechtite β‐MnOOH, and hausmannite α‐Mn3O4. The abundance and intrinsic stabilization of MnIII/MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm−2 with a durability of more than five days.

Reasonable regulation of kinetics over BiVO4 photoanode by Fe–CoP catalysts for boosting photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using earth-abundant semiconductor offer a promising strategy to produce the sustainable clean energy. Herein, we successfully engineered BiVO4 photoanode with Fe-doped CoP oxygen evolution catalysts (BiVO4–Fe/CoP) for the first time. Fe/CoP catalysts could significantly break the kinetic limitations of BiVO4, contributing to the enhanced charge injection efficiencies and charge carrier density. The rational heterostructure has boosted the photocurrent density and incident photon-to-current conversion efficiency (IPCE) to 2.16 mA/cm2 and 43% (7 times than that of the bare BiVO4), respectively. Meanwhile, the BiVO4–Fe/CoP photoanode also exhibited the desirable long-term stability during PEC water oxidation for 4 h. The results prove the feasibility of BiVO4–Fe/CoP configuration, which further provided a novel approach towards the development of efficient PEC water oxidation system.

Combined effects of protective film and oxidant on the performance of the supercritical water oxidation system with a film protective reactor: A simulation study

Supercritical water oxidation (SCWO) is a promising technology for treating organic waste and recovering energy. Water film protective reactor (WFPR), which is also referred to as transpiring wall reactor, provides a solution to corrosion and salt plugging issues. Three novel film protective reactors (FPRs) using air, nitrogen, and hydrogen peroxide as protective fluids were proposed to improve the corrosion and cost issues in WFPRs. Detailed optimized process parameters and flow field characteristics within the FPRs were obtained using Aspen Plus and computational fluid dynamics, respectively. The anti-corrosion performance and prevention of salt plugging in the FPRs were analyzed, and economic analyses for SCWO systems with different FPRs were conducted. Results indicate that the WFPR and hydrogen peroxide FPR present good solubility for inorganic salts but a high probability of corrosion. An SCWO system with the WFPR or nitrogen FPR is more competitive for industrial plants compared with other SCWO systems.

Experimental investigation and simulation optimization of a pilot-scale supercritical water oxidation system

Supercritical water oxidation (SCWO) technology has an excellent performance in treating high concentration and refractory organic wastes. While the high treatment cost is one of the key drawbacks currently restricting its large-scale application. In this study, a SCWO pilot system with a novel jet reactor (treatment capacity of 200 kg/h) was experimentally investigated, and the process model was developed to optimize the system by Aspen Plus. The energy and exergy efficiency of the system was evaluated, and the wastewater treatment cost in different operating conditions was estimated. The results show that chemical oxygen demand (COD) removal efficiency could be increased with the increase of the temperature, pressure, and the excess oxygen coefficient. By locating the electric heater 1 (H1) before the heat exchanger 2 (H2), and improving the outlet temperature of the oxygen compressor (MP1), the wastewater treatment cost could be reduced to 53.5 $/t, and the energy and exergy efficiency of the SCWO system could be improved to 81.61% and 8.91%, which is 26.28% and 6.23% higher than that of the current system, respectively. Both the reaction temperature and COD concentration of the wastewater have positive effects on the wastewater treatment cost, e.g. the wastewater treatment cost could decrease from 80.88 $/t to 59.27 $/t when the reaction temperature increases from 380 °C to 420 °C, and the SCWO system could achieve the energy self-sufficient operation when the COD concentration is over 60,000 mg/L.

The Importance of the Interfacial Contact: Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)Catalytic Water Oxidation?

Optimizing interfacial contact between graphene and a semiconductor has often been proposed as essential for improving their charge interactions. Herein, we fabricated bismuth vanadate-reduced graphene oxide (BiVO4/rGO) composites with tailored interfacial contact extents and revealed their disparate behavior in photoelectrochemical (PEC) and powder suspension (PS) water oxidation systems. BiVO4/rGO with a high rGO coverage on the BiVO4 surface (BiVO4/rGO HC) exhibited an 8-fold enhancement in the PEC photocurrent density with respect to neat BiVO4 at 0 V versus Ag/AgCl, while BiVO4/rGO with a low rGO coverage (BiVO4/rGO LC) gave a lesser 3-fold enhancement. In contrast, BiVO4/rGO HC delivered a detrimental effect, while BiVO4/rGO LC exhibited an enhanced performance for oxygen evolution in the PS system. The phenomenon is attributed to changes in the hydrophobicity of the BiVO4/rGO composite in conjunction with the interfacial contact configuration. A better BiVO4/rGO interfacial contact was found to improve the charge separation efficiency and charge transfer ability of the composite material, explaining the superior PEC performance of BiVO4/rGO HC. Additionally, optimization of the interfacial contact extent was revealed to further improve the energetics of the composite material, as evidenced by a Fermi level shift to a more negative potential. However, the high hydrophobicity of BiVO4/rGO HC arising from the higher rGO reduction extent triggered poor water miscibility, reducing the surface wettability and therefore hampering the photocatalytic O2 evolution activity of the sample. The study underlines water miscibility as a governing issue in the PS system.

Modeling the Catalyst Activation Step in a Metal–Ligand Radical Mechanism Based Water Oxidation System

Designing catalysts for water oxidation (WOCs) that operate at low overpotentials plays an important role in developing sustainable energy conversion schemes. Recently, a mononuclear ruthenium WOC that operates via metal–ligand radical coupling pathway was reported, with a very low barrier for O–O bond formation, that is usually the rate-determining step in most WOCs. A detailed mechanistic understanding of this mechanism is crucial to design highly active oxygen evolution catalysts. Here, we use density functional theory based molecular dynamics (DFT-MD) with an explicit description of the solvent to investigate the catalyst activation step for the [Ru(bpy) 2 (bpy–NO)] 2 + complex, that is considered to be the rate-limiting step in the metal–ligand radical coupling pathway. We find that a realistic description of the solvent environment, including explicit solvent molecules and thermal motion, is crucial for an accurate description of the catalyst activation step, and for the estimation of the activation barriers.

Enhanced photocatalytic activity of BiVO4 coupled with iron-based complexes for water oxidation under visible light irradiation.

[Fe2(TPA)2(μ-O)Cl2]2+ (TPA = tris(2-pyridylmethyl)amine) was investigated as a pre-catalyst, and greatly enhanced the photocatalytic water oxidation activity of BiVO4. An extremely high oxygen yield of 99.1% and apparent quantum yield of 44.3% were obtained in the BiVO4-NaIO3 photocatalytic water oxidation system.

Energy Consumption and Economic Analyses of a Supercritical Water Oxidation System with Oxygen Recovery

Oxygen consumption is one of the factors that contributes to the high treatment cost of a supercritical water oxidation (SCWO) system. In this work, we proposed an oxygen recovery (OR) process for an SCWO system based on the solubility difference between oxygen and CO2 in high-pressure water. A two-stage gas–liquid separation process was established using Aspen Plus software to obtain the optimized separation parameters. Accordingly, energy consumption and economic analyses were conducted for the SCWO process with and without OR. Electricity, depreciation, and oxygen costs contribute to the major cost of the SCWO system without OR, accounting for 46.18, 30.24, and 18.01 $·t−1, respectively. When OR was introduced, the total treatment cost decreased from 56.80 $·t−1 to 46.17 $·t−1, with a reduction of 18.82%. Operating cost can be significantly reduced at higher values of the stoichiometric oxygen excess for the SCWO system with OR. Moreover, the treatment cost for the SCWO system with OR decreases with increasing feed concentration for more reaction heat and oxygen recovery.

Oxidation of Water by Cerium(IV) Compounds in the Presence of Trinuclear Ruthenium Complexes

The oxidation of water by Ce(IV) compounds in the presence of ruthenium complex (NH3)5Ru–O–Ru(NH3)4–O–Ru(NH3)5Cl6 is studied. It is shown that the complex is a catalyst of water oxidation if one-electron oxidizing species Ce4+ are used. Four units of the indicated ruthenium complex then organize themselves into a reactive dodecanuclear cluster. It is established that the formation of bi-, tetra-, and polynuclear cerium clusters in the reacting system is accompanied by the four oxygen atoms in the catalyst molecule being replaced with 4NH3 ligands, resulting in a different catalytic system for biomimetic water oxidation.

Photocatalytic water oxidation over BiVO4 with interface energetics engineered by Co and Ni-metallated dicyanamides

Photocatalytic water oxidation based on semiconductors usually suffers from poor charge transfer from the bulk to the interface, which is necessary for oxygen generation. Here, we construct a hybrid artificial photosynthesis system for photocatalytic water oxidation. The system consists of BiVO4 as the light harvester, a transitional metal complex (M(dca)2, M = Co, Ni, dca: dicyanamide) as the water oxidation catalyst, and S2O82− as a sacrificial electron acceptor. The system exhibits enhanced oxygen evolution activity when M(dca)2 is introduced. The BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems exhibit excellent oxygen evolution rates of 508.1 and 297.7 μmol/(h·g) compared to the pure BiVO4 which shows a photocatalytic oxygen evolution rate of 252.2 μmol/(h·g) during 6 h of photocatalytic reaction. Co(dca)2 is found to be more effective than Ni(dca)2 as a water oxidation catalyst. The enhanced photocatalytic performance is ascribed to the M(dca)2-engineered BiVO4/electrolyte interface energetics, and to the M(dca)2-catalyzed surface water oxidation. These two factors lead to a decrease in the energy barrier for hole transfer from the bulk to the surface of BiVO4, which promotes the water oxidation kinetics.

Artificial Metalloproteins Containing Co4O4 Cubane Active Sites.

Artificial metalloproteins (ArMs) containing Co4O4 cubane active sites were constructed via biotin-streptavidin technology. Stabilized by hydrogen bonds (H-bonds), terminal and cofacial CoIII-OH2 moieties are observed crystallographically in a series of immobilized cubane sites. Solution electrochemistry provided correlations of oxidation potential and pH. For variants containing Ser and Phe adjacent to the metallocofactor, 1e-/1H+ chemistry predominates until pH 8, above which the oxidation becomes pH-independent. Installation of Tyr proximal to the Co4O4 active site provided a single H-bond to one of a set of cofacial CoIII-OH2 groups. With this variant, multi-e-/multi-H+ chemistry is observed, along with a change in mechanism at pH 9.5 that is consistent with Tyr deprotonation. With structural similarities to both the oxygen-evolving complex of photosystem II (H-bonded Tyr) and to thin film water oxidation catalysts (Co4O4 core), these findings bridge synthetic and biological systems for water oxidation, highlighting the importance of secondary sphere interactions in mediating multi-e-/multi-H+ reactivity.

Metal–Organic Framework Derived Narrow Bandgap Cobalt Carbide Sensitized Titanium Dioxide Nanocage for Superior Photo‐Electrochemical Water Oxidation Performance

AbstractDespite recent progress in photo‐electrochemical (PEC) water oxidation systems for TiO2‐based photoanodes, PEC performance improvement is still seriously hampered due to poor carrier transport efficiency and sluggish surface water oxidation kinetics of pristine TiO2. Herein, for the first time a brand new metal–organic framework (MOF)‐derived Co3C nanosheet with narrow bandgap energy is demonstrated, to effectively sensitize TiO2 hollow cages as a heterostructure photoanode for PEC water oxidation. It is found that MOF‐derived Co3C nanosheet with narrow bandgap characteristic can simultaneously accelerate the surface water oxidation kinetics and extend the light harvesting range of pristine TiO2. Meanwhile, a uniquely matched type‐II heterojunction constructed between MOF‐derived Co3C and TiO2 results in an evidently spontaneous e−/h+ separation. MOF‐derived Co3C/TiO2 heterostructure photoanodes bring about drastically improved PEC water oxidation performance. Specifically, MOF‐derived Co3C‐3/TiO2 photoanode with an optimized content of Co3C achieves the highest photocurrent density and charge separation efficiency of 2.6 mA cm−2 and 92.6% at 1.23 V versus reversible hydrogen electrode, corresponding to 201% and 152% improvement compared with pristine TiO2 nanocages. The ingeniously prepared MOF‐derived Co3C carbide with narrow bandgap energy as a cocatalyst paves new way to construct potentially high performance solar‐energy conversion system.

Highly self-diffused Sn doping in α-Fe2O3 nanorod photoanodes initiated from β-FeOOH nanorod/FTO by hydrogen treatment for solar water oxidation.

In this study, we present an advanced strategy of low-temperature hydrogen annealing combined with high- temperature quenching in air for activating α-Fe2O3 nanorod photoanodes to boost the photoelectrochemical performance. We report that various low-temperature annealing conditions (340, 360, 380, and 400 °C) under hydrogen gas flow convert β-FeOOH into magnetite (Fe3O4) as well as introduce Sn4+ diffusion from FTO substrates to its surface. Furthermore, high-temperature quenching (800 °C) resulted in the phase change of magnetite (Fe3O4) into hematite (α-Fe2O3) and self Sn4+ doping into the hematite lattice. Thus, the hydrogen-assisted thermally activated hematite photoanode achieved a photocurrent density of 1.35 mA cm-2 at 1.23 V vs. RHE and 1.91 mA cm-2 at 1.4 V vs. RHE, which is 70% and 80% higher than that of directly quenched hematite at 800 °C. These combined two step strategies provide new insight into high Sn-self doping for α-Fe2O3 photoanodes and allow for further development of more efficient solar water oxidation systems.

Cobalt manganese spinel as an effective cocatalyst for photocatalytic water oxidation

Efficient co-catalysts are crucial for photocatalytic water oxidation towards conversion of solar energy into chemical energy. Herein, we develop a sustainable and effective water oxidation system using graphitic carbon nitride (g-C3N4) and cubic cobalt manganese spinel (c-CoMn2O4) as light transducer and water oxidation co-catalyst, respectively. The surface modified g-C3N4 with c-CoMn2O4 not only accelerates the interface transfer rate of charge carriers but also reduces the excessive energy barrier for OO formation, leading to an enhanced photocatalytic activity of water oxidation. Benefiting from the well surface engineering of g-C3N4, the g-C3N4-CoMn2O4 (CN-CM) composites exhibit an enhanced performance of photocatalytic water oxidation. The oxygen evolution rate (OER) of CN-CM is 4 times higher than that of pristine g-C3N4. It is a highly active in oxidation of water, with an apparent quantum yield (AQY) of ca. 1% at 380nm with AgNO3 as sacrificial agent. This improvement is mainly due to the mixed-valence Co and Mn cations contained in c-CoMn2O4 spinel.

Rational Design Rules for Molecular Water Oxidation Catalysts based on Scaling Relationships.

Lowering the overpotential required for water oxidation is of paramount importance for the efficient production of carbon-neutral fuels. This article highlights the intrinsic influence of the water oxidation mechanism used by molecular catalysts on the theoretically achievable minimal overpotential, based on scaling relationships typically used for heterogeneous catalysts. Due to such scaling relationships, catalysts that operate through the water nucleophilic attack mechanism have a fundamental minimal overpotential of about 0.3 V, whereas those that follow the dinuclear radical oxo coupling mechanism should in principle be able to operate with a lower overpotential. Therefore, it is recommended to design catalysts operating through the latter mechanism to achieve very efficient water oxidation systems.

Synthesis of Well-Defined, Surfactant-Free Co3O4 Nanoparticles: The Impact of Size and Manganese Promotion on Co3O4 Reduction and Water Oxidation Activity

A surfactant-free synthetic route has been developed to produce size-controlled, cube-like cobalt oxide nanoparticles of three different sizes in high yields. It was found that by using sodium nitrite as salt-mediating agent, near-quantitative yields could be obtained. The size of the nanoparticles could be altered from 11 to 22 nm by changing the cobalt concentration and reaction time. These surfactant-free nanoparticles form ideal substrates for facile deposition of further elements such as manganese. The effect of size of the cobalt oxide nanoparticles and the presence of manganese on the reducibility of cobalt oxide to metallic cobalt was investigated. Similarly, the effect of these parameters was investigated with a visible light promoted water oxidation system with cobalt oxide as catalyst, together with [Ru(bpy)3]2+ light harvester dye and an electron acceptor. A novel surfactant-free synthetic route has been developed to produce size-controlled, cube shaped cobalt oxide nanoparticles in high yields.

Development of a ruthenium multi-pyridine complex as photosensitizer for highly efficient light driven water oxidation

As we all know, the photosensitizer is the vital role in photocatalytic water oxidation system. In this paper, a novel [Ru(bpy)2(tpphz)](PF6)2 complex (PS1) was synthesized and characterized fully. And, it displayed a higher photoactivity (TON, 110) than [Ru(bpy)3](PF6)2 (PS2) (TON, 62) and [Ru(bpy)2(dcb)](PF6)2 (PS3) (TON, 82) for water oxidation in a three-component system with Ru(bda)(isoq)2 as catalyst. The main reason for the enhancement of photocatalytic activity should be that the π-π stacking supramolecular interaction between the π-tpphz ligand of the PS1 and the π-isoq ligand of the catalyst increased the intermolecular interaction between PS1 and catalyst, and promoted the molecular aggregation and accelerated valid electronic transmission. Nevertheless, a modest driving force of PS1 for photocatalysis water oxidation should be another contribution.

Toward non-precious nanocomposite photocatalyst: An efficient ternary photoanode TiO2 nanotube/Co9S8/polyoxometalate for photoelectrochemical water splitting

At present the photoelectrochemical (PEC) water splitting achieves a very high efficiency, but the development of the efficient and inexpensive photoanode materials is still desired for practical application. Herein, we fabricate a composite photoanode consisting of TiO2 nanotube arrays modified with polyoxometalate (POM) and Co9S8, in which POM aims to facilitate charge separation and Co9S8 acts as good electron transport mediator. It is the first introduction of Co9S8 into PEC water oxidation system. Although these three components are all non-precious inorganic materials, the photocurrent density of the TiO2/Co9S8/POM composite photoanode reaches up to 1.12mA/cm2 at 1.23V vs RHE under simulated solar light illumination, which is among the highest PEC water oxidation photoanodes. Particularly, the TiO2/Co9S8/POM composite photoanode shows an 8.5-fold improvement in photocurrent density compared to the pristine TiO2 nanotube array photoanode. The measurements of EIS and Mott-Schottky plots revealed that the TiO2/Co9S8/POM composite photoanode held the best efficiency of charge separation and transport at the interface as well as the highest carrier density. The aforementioned results demonstrate a feasible approach to improving the PEC water splitting efficiency by dual modification of TiO2 nanotube arrays with low-cost materials.

MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS2 and WS2 can oxidize water to O2 under incident light. Thin films of solution-processed MoS2 and WS2 nanosheets display n-type positive photocurrent densities of 0.45 mA cm–2 and O2 evolution under simulated solar irradiation. WS2 is significantly more efficient than MoS2; however, bulk heterojunctions (B-HJs) of MoS2 and WS2 nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS2 and WS2 nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production.

Energy consumption and exergy analyses of a supercritical water oxidation system with a transpiring wall reactor

A supercritical water oxidation system with a transpiring wall reactor was simulated by Aspen Plus, and simulation model was validated by comparisons with experimental reactor outlet temperatures and product properties (total organic carbon and CO). Energy and exergy analyses were conducted to reduce energy consumption and exergy loss of the system. It is indicated that the system’s energy efficiency and exergy efficiency are 97.73% and 13.28% at typical operating conditions, respectively. The exergy loss of electric heater, heat exchanger and reactor account for 39.89%, 26.64%, and 17.23% of the system’s total exergy loss, respectively. The process optimization is conducted by preheating the middle branch of transpiring water with the heat of the reaction products to reduce energy consumption, and the net electric cost is reduced from 7.43 ¥/h to 5.96 ¥/h. It can be observed that when the split coefficient for split 2 equals to 0.6, the minimum electricity input is required for the system. When the feed concentration is increased from 2wt.% to 10wt.%, net electric cost per COD significantly decreases from 14.05 ¥/kg to 1.31 ¥/kg, which indicates that higher feed concentrations are beneficial for reducing the cost of energy consumption. What is more, when feed flow rate increases from 6kg/h to 16kg/h, net electric cost per COD increases from 3.21 ¥/kg to 4.61¥/kg, which shows that higher energy consumption costs will be required at higher feed flow rates.