Water Oxidation System Research Articles

Promoting Water Oxidation by Proton Acceptable Groups Surrounding Catalyst on Electrode Surface.

Large-scale hydrogen production through water splitting represents an optimal approach for storing sustainable but intermittent energy sources. However, water oxidation, a complex and sluggish reaction, poses a significant bottleneck for water splitting efficiency. The impact of outer chemical environments on the reaction kinetics of water oxidation catalytic centers remains unexplored. Herein, chemical environment impacts were integrated by featuring methylpyridinium cation group (Py+) around the classic Ru(bpy)(tpy) (bpy = 2,2'-bipyridine, tpy = 2,2':6',2''-terpyridine) water oxidation catalyst on the electrode surface via electrochemical co-polymerization. The presence of Py+ groups could significantly enhance the turnover frequencies of Ru(bpy)(tpy), surpassing the performance of typical proton acceptors such as pyridine and benzoic acid anchored around the catalyst. Mechanistic investigations reveal that the flexible internal proton acceptor anions induced by Py+ around Ru(bpy)(tpy) are more effective than conventionally anchored proton acceptors, which promoted the rate-determining proton transfer process and enhanced the rate of water nucleophilic attack during O-O bond formation. This study may provide a novel perspective on achieving efficient water oxidation systems by integrating cations into the outer chemical environments of catalytic centers.

Evaluation of Ce-MOFs as Photoanode Materials for the Water Oxidation Reaction: The Effect of Doping with [Ru(bpy)(dcbpy)(H2O)2]2+ Catalyst.

Artificial photosynthesis stands out as a highly effective method for harnessing sunlight to produce clean and renewable energy. The light-absorbing properties, chemical stability, and high redox activity of Ce-based metal-organic frameworks (MOFs) make them attractive materials for visible-light-driven water splitting. Currently, Ce-based MOFs remain a relatively underexplored system for photocatalytic water oxidation in acidic media. In this study, we synthesized a Ce-MOF with different linkers (1,4-benzenedicarboxylic acid, tetrafluoroterephthalic acid, 2-nitroterephthalic acid, 2,2'-bipyridine-5,5'-dicarboxylic acid, and 4,4'-biphenyldicarboxylic acid), which exhibit light-absorbing capability. Ce-based MOFs doped with [Ru(bpy)(dcbpy)(H2O)2]2+ (MOF-1 and MOF-2) water oxidation catalyst showed an enhanced photoelectrocatalytic current of ∼10-4 A·cm-2 at pH = 1, which is comparable with the [Ru(bpy)(dcbpy)(H2O)2]2+-doped MIL-126 Fe-based MOF. We also demonstrated the long-term durability of Ru-doped Ce-MOFs for photoelectrocatalytic water oxidation under acidic conditions. The as-synthesized MOFs were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-visible diffuse reflectance spectroscopy, scanning electron microscopy (SEM), and electric conductivity measurements. This study contributes to the development of cost-effective materials for sustainable photocatalytic water splitting processes.

Bifunctional NiCo-LDH cocatalyst with hole extraction and high catalytic activity for boosting photoelectrochemical water oxidation of Nd doped BiVO4 photoanode

The slow charge transfer kinetics and water oxidation dynamics hinder the efficient photoelectrochemical (PEC) water splitting of BiVO4. Here, Nd:BiVO4@NiCo-LDH photoanode was successfully prepared by introducing rare earth element Nd with a special 4 f energy level as a dopant and loading a bifunctional NiCo-LDH cocatalyst. The Nd:BiVO4@NiCo-LDH photoanode exhibits good PEC performance. The photocurrent density was 4.1 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE), which is about 5.1 times more than that of the unmodified BiVO4 electrode, and the onset potential was negatively shifted by 399 mV. Various physical characterizations and photoelectrochemical tests indicate that the Nd:BiVO4@NiCo-LDH photoanode has higher charge transfer rate and lower charge recombination rate than that of BiVO4. The NiCo-LDH catalyst serves as a bimetallic oxygen evolution reaction (OER) cocatalyst, where Co cation has hole extraction ability to reduce carrier recombination, and Ni addition increases the active site. The synergistic effect of the two significantly boosts the PEC water oxidation activity of the photoanode. This work has broad prospects for the construction of high-performance water oxidation systems in various photoanodes.

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.

Facilitating photogenerated carrier transfer and water oxidation kinetics of BiVO4 photoanode via novel Zn@Co3C functional layer

Currently, the sluggish charge transfer kinetics and water oxidation kinetics are still the bottlenecks faced by BiVO4 photoanodes at present. Herein, an excellent Zn@Co3C functional layer is reported for the first time to tackle these problems of BiVO4 photoanode. Zn@Co3C was prepared using a facile electrochemical method with 1,2-dimethylimidazole as a carbon source. In the BiVO4/Zn@Co3C photoanode, Co3C acts as a novel oxygen evolution co-catalyst to significantly enhance water oxidation kinetics. An electron depletion layer formed around Zn species provides a strong driving force for the migration of photogenerated holes from BiVO4 to Co3C catalytic sites. Furthermore, the alternating distribution of Co3C and Zn species on the surface of BiVO4 photoanodes increases the utilization efficiency of Co3C co-catalyst. The BiVO4/Zn@Co3C system constructed in this work not only demonstrates good photoelectrochemical performance in electrolytes (4.6 mA cm−2 at 1.23 V vs. RHE without any hole scavengers), but also enables water splitting in natural seawater. This work demonstrates a novel functional layer that simultaneously facilitates photogenerated carrier transfer and water oxidation kinetics, which holds great promise for constructing high-performance water oxidation systems in various photoanodes.

Hydrophilic MnO2 nanowires coating with o-fluoroaniline for electrocatalytic water oxidation

In the active Mn4CaO5 cluster of the natural water oxidation system, one of the dangling Mn atoms is surrounded by water molecules through coordination and hydrogen bonding. It means that the enrichment of water molecules around the Mn catalysts has a positive effect on the water oxidation catalysis. Inspired by nature photosynthesis, the hydrophilic surfaces are believed to be able to capture water molecules to assist water oxidation. Herein, we report MnO2@PFANI nanowires with improved mass transfer efficiency and conductivity for electrocatalytic water oxidation. The as-prepared nanowires are synthesized by coating hydrophilic polymers on MnO2 nanowires. The hydrophilic surface captures water molecule and promotes mass transfer processes, and the conductivity of the polymer can also increase charge transfer processes. To reach a current density of 10 mA cm−2, a low overpotential of 440 mV is required in a 1.0 M KOH solution for the MnO2@PFANI nanowires. The hydrophilicity and conductivity are systematically investigated by both physical and electrochemical measurements. This study provides a surface engineering idea for the fabrication of efficient electrocatalysts.

A p‐n WO3/SnSe2 Heterojunction for Efficient Photo‐assisted Electrocatalysis of the Oxygen Evolution Reaction

Water splitting is important to the conversion and storage of renewable energy, but slow kinetics of the oxygen evolution reaction (OER) greatly limits its utility. Here, under visible light illumination, the p‐n WO3/SnSe2 (WS) heterojunction significantly activates OER catalysis of CoFe‐layered double hydroxide (CF)/carbon nanotubes (CNTs). Specifically, the catalyst achieves an overpotential of 224 mV at 10 mA cm−2 and a small Tafel slope of 47 mV dec−1, superior to RuO2 and most previously reported transition metal‐based OER catalysts. The p‐n WS heterojunction shows strong light absorption to produce photogenerated carriers. The photogenerated holes are trapped by CF to suppresses the charge recombination and facilitate charge transfer, which accelerates OER kinetics and boost the activity for the OER. This work highlights the possibility of using heterojunctions to activate OER catalysis and advances the design of energy‐efficient catalysts for water oxidation systems using solar energy.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures.

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Effect of hydrothermal flame generation methods on energy consumption and economic performance of supercritical water oxidation systems

Hydrothermal flames are critical to solving preheating problems and enhancing feed degradation in supercritical water oxidation (SCWO) systems. In this work, different hydrothermal flame generation methods including hot water injection, evaporation concentration, and addition of auxiliary fuel are proposed for SCWO of cutting fluids. The proposed systems are simulated using Aspen Plus 8.2, and the simulation model is validated by comparisons with experimental temperature profiles and product properties. A low ratio between the heat source flow and the feed flow (FR) requires a high heat source temperature to keep the hydrothermal flame stable and results in a high electricity consumption, and FR = 2 is chosen as the optimum condition for the SCWO system with hot water injection (SCWOH). The increase in concentration ratio and fuel concentration can effectively reduce the energy consumption for the SCWO systems with evaporation concentration (SCWOE) and addition of auxiliary fuel (SCWOF), respectively. The exergy analysis results indicate that most of exergy inputs are destructed inside the system, and the exergy distribution coefficients of the reactor exceed 80% in the SCWOE and SCWOF. Economic analysis results show the SCWOE has the lowest treatment cost of 95.9 USD/t.

Self‐assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light‐Driven Water Oxidation in Pure Water

AbstractWater‐soluble multinuclear complexes based on ruthenium 2,2′‐bipyridine‐6,6′‐dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three‐component visible light‐driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co‐solvents and with increasing oligomer length. In‐depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self‐assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s−1) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)‐based photocatalytic water oxidation systems.

Self-assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light-Driven Water Oxidation in Pure Water.

Water-soluble multinuclear complexes based on ruthenium 2,2'-bipyridine-6,6'-dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three-component visible light-driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co-solvents and with increasing oligomer length. In-depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self-assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s-1 ) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)-based photocatalytic water oxidation systems.

BiVO4/Rh–Ci/HCO3− hetero-/homogeneous dual co-catalyst-decorated photoanode system for photoelectrochemical water oxidation

Bismuth vanadate is one of the most efficient photoanode materials for solar water-splitting. However, its performance is limited by two main factors: poor charge separation and slow water oxidation kinetics. Herein, Rh–Ci/HCO3− hetero-/homogenous dual co-catalysts were introduced on the BiVO4 surface to improve the water oxidation performance. This Rh–Ci/HCO3− dual co-catalyst system improved the water oxidation activity of BiVO4 by enhancing the charge injection efficiency and stabilizing the photoelectrochemical water oxidation kinetics. BiVO4 decorated with Rh–Ci/HCO3− exhibited a higher photocurrent (2.011 mA∙cm−2) at 1.23 VRHE than bare BiVO4 (1.217 mA∙cm−2).

Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface.

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6'-dicarboxylato-2,2'-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na2S2O8). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S2O82- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS+ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS+ to catalyst is rather fast in homogeneous solution (kobs > 1 × 104 s-1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (kobs ≈ 17 s-1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Supercritical water co-oxidation behavior in the different monohydric alcohol-ammonia reaction environment

The degradation of ammonia is a key rate-limiting step during the supercritical water oxidation of nitrogen-containing organics. This paper studied the co-oxidation behavior between different ammonia-alcohol environments, including the influence of reaction parameters and the co-oxidation mechanism. The results showed that increasing temperature, oxidation coefficient, residence time, and alcohol concentration significantly promoted the degradation of NH3–N and TOC, while rising the ammonia concentration enhanced the NH3–N destruction but inhibited the TOC degradation. Alcohols were oxidized first in the co-oxidation system to produce more OH* and HO2* radicals. Ethanol generated the highest concentration of HO2* in the shortest time, leading to more significant ammonia removal than isopropanol and methanol; however, the produced intermediate products like aldehydes and ketones reacted with residual ammonia to generate a small amount of organics at lower temperatures, inhibiting the degradation of alcohols slightly, and combined catalyst or nitrate in the batch reactor or used continuous supercritical water oxidation or supercritical hydrothermal combustion system without controlling the exotherm of fuels could improve this.

Ag2O modified CuO nanosheets as efficient difunctional water oxidation catalysts

Semiconductors have been given immense attention for years regarding the use of photocatalytic water oxidation reaction (WOR) to reduce the potential barrier. However, some known semiconductors, such as TiO2 and CuO, exhibit outstanding water oxidation activity in the UV light region. Therefore, it is important to explore the methods to expand the usage of these catalysts in the visible as well as IR regions. This study introduces a simple and facile chemical co-precipitation method to fabricate a sort of Ag2O-modified CuO nanosheets with excellent catalytic properties. The as-prepared Ag2O/CuO nanosheets exhibit a robust catalytic efficiency towards the visible-light-driven WOR. Due to the addition of Ag2O, the Ag2O/CuO nanosheets display photocatalytic activity. The total turnover number (TON) was 8.20 and when the effect of chemocatalytic activities was deducted, the TON, quantum yield (QY), and corresponding photonic efficiency (ζp) are 2.92, 0.65 %, and 0.33 %, respectively, which shows that Ag2O/CuO nanosheets possess water oxidation sites as well as light-absorption centers. With the synergy effect between the Ag2O and CuO, the catalyst efficiently reduced the recombination probability of photoexcited electrons and holes. Intriguingly, Ag2O/CuO nanosheets expanded the utility in the visible region as long as 765 nm, which evolved 95.89 % oxygen compared to that of the light cutting off at 420 nm. The insight into the WOR pathway is that surficial –OH groups of Ag2O/CuO nanosheets play a key role in this catalytic reaction. The mechanism of Ag2O/CuO nanosheets in the persulfate/NaOH system for water oxidation was consequently proposed according to electron spin resonance measurements and two radical scavengers (methanol and tert-butanol alcohol). This study offers useful guidance for developing difunctional water oxidation catalysts without photosensitizers.

Formation and evolution characteristics of hydrothermal flames inside a transpiring wall reactor: A transient numerical investigation

Transpiring-wall reactors with hydrothermal flames are effective in preventing corrosion and salt deposition in supercritical water oxidation systems. To uncover the not-well-understood evolution characteristics of the hydrothermal flame surrounded by a transpiring water film, a 2-D unsteady simulation is firstly conducted in this study. The transient results show that the variations of flow fields and flame shapes are attributed to the competition between inertial forces and buoyant forces. And the quasi-steady results present that high feed concentration may damage transpiring wall and delay the evolution of hydrothermal flames. In addition, a low feed temperature or a big flow area of jet can lead to open hydrothermal flames, which adversely affect the reactor. Furthermore, Froude Number, Fr, shows effectiveness in predicting quasi-steady flame shapes. For large-Fr conditions, the inertial force outweighs the buoyant force and hence closed flames are developed; while for low-Fr cases, open flames are developed.

Front Cover: Electrochemical Polymerization of a Carbazole‐Tethered Cobalt Phthalocyanine for Electrocatalytic Water Oxidation (ChemNanoMat 6/2022)

The four-electron oxidation of water is critical for achieving artificial photosynthesis. Inspired by a natural system for water oxidation, a function-integrated catalytic system, which has both catalytic centers and charge transporting units, has been developed. A newly designed carbazole-tethered cobalt phthalocyanine was electrochemically polymerized to afford polymer-based material. The obtained polymer exhibited high charge transfer ability and catalyzed water oxidation reaction effectively. More information can be found in the Research Article by Mio Kondo, and Shigeyuki Masaoka et al.

Boosting Efficiency in Light‐Driven Water Splitting by Dynamic Irradiation through Synchronizing Reaction and Transport Processes**

This work elaborates the effect of dynamic irradiation on light‐driven molecular water oxidation to counteract deactivation. It highlights the importance of overall reaction engineering to overcome limiting factors in artificial photosynthesis reactions. Systematic investigation of a homogeneous three‐component ruthenium‐based water oxidation system revealed significant potential to enhance the overall catalytic efficiency by synchronizing the timescales of photoreaction and mass transport in a capillary flow reactor. The overall activity could be improved by a factor of more than 10 with respect to the turnover number and a factor of 31 referring to the external energy efficiency by controlling the local availability of photons. Detailed insights into the mechanism of light driven water oxidation could be obtained using complementary methods of investigation like Raman, IR, and UV/Vis/emission spectroscopy, unraveling the importance of avoiding high concentrations of excited photosensitizers.

Front Cover: Fabrication of a Function‐Integrated Water Oxidation Catalyst through the Electrochemical Polymerization of Ruthenium Complexes (ChemElectroChem 2/2022)

The Front Cover illustrates the fabrication of a function-integrated catalytic system for water oxidation. By the electrochemical polymerization of ruthenium complexes bearing carbazole moieties, two distinct functions, catalytic centers (represented by skunk cabbages) and charge transporting sites (fairies moving around), are successfully incorporated into one material. As a result, dramatic enhancement of catalytic activity was achieved. More information can be found in the Research Article by H. Iwami, M. Kondo and S. Masaoka.

Fabrication of a Function‐Integrated Water Oxidation Catalyst through the Electrochemical Polymerization of Ruthenium Complexes

AbstractWater oxidation (2H2O→O2+4H++4e−) is an essential reaction for artificial photosynthesis. In nature, photosystem II efficiently catalyzes this reaction through the synergistic effect of the catalytic center and charge transporters. However, the development of artificial catalytic systems for water oxidation in which the catalytic center is surrounded by charger transporters remains challenging. Herein, a function‐integrated water oxidation catalyst was fabricated by the electrochemical polymerization of mononuclear ruthenium complexes bearing carbazole moieties. A new precursor complex, Ru(bda)(czpy)2 (2; bda2−=2,2′‐bypyridine‐6,6′‐dicarboxylate, czpy=9‐[2‐(4‐pyridinyl)ethyl]carbazole), was successfully synthesized and characterized by 1H NMR spectroscopy, single‐crystal X‐ray structural analysis, and elemental analysis. Under electrochemical conditions, the carbazole moieties of 2 dimerized to form a polymeric material, Poly‐2. Poly‐2 exhibited an excellent charge‐transporting ability and catalyzed electrochemical water oxidation with high efficiency. Notably, nonpolymeric systems were not catalytically active, indicating that synergism between the catalytic center and charge transporters enhanced the catalytic performance of Poly‐2.