Sustained Water Oxidation Research Articles

A robust strategy to boost the proton transfer of heterogeneous catalysts for efficient and sustainable water oxidation towards practical applications

A facile yet robust strategy was developed for accelerating the proton transfer of metal hydroxides via incorporating Lewis basic sites, thereby achieving sustainable water oxidation towards practical applications.

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.

Electrochemical Reduction of Flue Gas CO2 in a Dual Methanol/Water Electrolysis System for the Synthesis of Methyl Formate

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of the nonaqueous solvent methanol allows for higher CO2 solubility and novel products through the incorporation of methanol as an intermediate. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with a faradaic efficiency for methyl formate as high as 75% measured in pH < 2. Furthermore, strategies to limit the concentration of formic acid and maintain the surface oxide of the catalyst have been shown to be effective at improving the steady-state durability of the CO2-to-methyl formate process. Tests with simulated flue gas as the feedstock have shown a high level of tolerance for the Pb-catalyzed electrolysis to SO2, NO, and dilute O2 contaminants. Finally, a comparative technoeconomic analysis will be discussed to highlight target performance metrics and the viability of alternate pathways for electrochemical conversion of CO2 to methyl formate.

Unveiling the Effect of Flue Gas Contaminants on Electrochemical Reduction of CO2 to Methyl Formate in Dual Methanol/Water Electrolysis System

Exploiting renewable energy sources to drive CO2 reduction electrochemically into value-added products has gained tremendous attention towards reducing the greenhouse effect. Upstream processes for capturing and purifying the CO2 raise the overall cost of the electrolysis system. Direct reduction of flue gas into value-added C2+ products has attained much consideration in recent years. However, the sensitivity of the cathode catalyst performance in the presence of flue gas contaminants, particularly O2, and strong competition from the hydrogen evolution reaction in aqueous electrolyte are major challenges for such an approach. Herein, the influence of flue gas contaminants on electrochemical reduction of CO2 in acidic nonaqueous methanol to a methyl formate product has been investigated on a Pb-catalyzed electrode in conjunction with aqueous anolyte for the promotion of a sustainable water oxidation half-reaction. The presence of 4% O2, 0.05% SO2, and 0.05% NO is shown to have minimal effect on the total faradaic efficiency of CO2 reduction products. CO2 concentration-dependent measurements reveal declining CO2 reduction faradaic efficiencies corresponding to the decrease in partial pressure, which is attributed to CO2 mass transfer limitations to the electrode surface. XPS analysis displays the relative stability of the Pb working electrode before and after the electrochemical operation during exposure to flue gas components, which further highlights the promising tolerance of this system for direct flue gas conversion. Figure 1

Interface engineering breaks both stability and activity limits of RuO2 for sustainable water oxidation

Designing catalytic materials with enhanced stability and activity is crucial for sustainable electrochemical energy technologies. RuO2 is the most active material for oxygen evolution reaction (OER) in electrolysers aiming at producing ‘green’ hydrogen, however it encounters critical electrochemical oxidation and dissolution issues during reaction. It remains a grand challenge to achieve stable and active RuO2 electrocatalyst as the current strategies usually enhance one of the two properties at the expense of the other. Here, we report breaking the stability and activity limits of RuO2 in neutral and alkaline environments by constructing a RuO2/CoOx interface. We demonstrate that RuO2 can be greatly stabilized on the CoOx substrate to exceed the Pourbaix stability limit of bulk RuO2. This is realized by the preferential oxidation of CoOx during OER and the electron gain of RuO2 through the interface. Besides, a highly active Ru/Co dual-atom site can be generated around the RuO2/CoOx interface to synergistically adsorb the oxygen intermediates, leading to a favourable reaction path. The as-designed RuO2/CoOx catalyst provides an avenue to achieve stable and active materials for sustainable electrochemical energy technologies.

The effect of flue gas contaminants on electrochemical reduction of CO2 to methyl formate in a dual methanol/water electrolysis system

CO 2 electroreduction to value-added products has promise as a scalable technique for mitigating climate change, but CO 2 purification requirements raise the overall process cost. Direct reduction of flue gas is thus an attractive approach, but the sensitivity of the catalyst activity to flue gas contaminants and increased hydrogen evolution with diluted CO 2 have been major challenges. Herein, flue gas electroreduction in a methyl formate synthesis route has been investigated on a Pb-catalyzed electrode in acidic methanol catholyte with an aqueous anolyte for promotion of a sustainable water oxidation half-reaction. Contaminant concentrations of 50 ppm SO 2 and NO each had a minimal effect on the product faradaic efficiencies, whereas 4% O 2 led to a notable improvement in partial current density for methyl formate attributed to the improved durability of the catalyst surface oxide. Decreased CO 2 concentration showed a corresponding decline in current density attributed to CO 2 mass transfer limitations. • Study of the effects of flue gas contaminants and CO 2 concentration • Pb-catalyzed methyl formate production from CO 2 and methanol • High-performance tolerance to SO 2 and NO impurities • Improved performance with dilute O 2 Increasing atmospheric CO 2 is contributing to anthropogenic climate change. Industrial utilization of CO 2 can add economic value to this waste product and provide a route to decreased greenhouse gas emissions. Direct electroreduction of post-combustion flue gas would bypass expensive and energy-intensive carbon capture technologies and improve the overall economics of the process. However, the contaminants in flue gas and the lower partial pressure of CO 2 can deleteriously affect the electrolyzer performance. Herein, the influence of flue gas components and combined simulated flue gas feedstock on the electrochemical reduction of CO 2 in a methyl formate synthesis route has been investigated on a Pb-catalyzed cathode in acidic methanol in conjunction with water at the anode for a sustainable reaction. The results show promise for a viable route to produce a chemical not reported in aqueous CO 2 electroreduction directly from the output of a fossil fuel utility or similar industrial process. The feasibility of electrochemical reduction of waste carbon dioxide from industrial flue gas was investigated in an uncommon methanol/water dual electrolyte approach targeting a methyl formate product. The work demonstrates a pathway to produce a chemical not formed in aqueous CO 2 reduction that is largely compatible with direct operation from flue gas.

Photoelectrochemical Water Oxidation over Novel Semiconducting Zinc-Based Metal–Thiolate Framework

Designing an efficient catalyst for a sustainable photoelectrochemical water oxidation reaction is very challenging in the context of renewable energy research. Here, we have introduced a new semiconducting porous zinc-thiolate framework via successful stitching of an "N" donor linker with a triazine-based tristhiolate secondary building unit in the overall architecture. The introduction of both linker and tristhiolate ligand synergistically modifies the architecture by making it a rigid, crystalline, three-dimensional, thermally stable, and porous framework. Our novel zinc-thiolate framework is used as an n-type semiconductor as revealed from the solid-state UV-vis DRS spectroscopic analysis, ac and dc conductivity analysis, and Mott-Schottky plot. This n-type semiconductor-based zinc-thiolate framework is utilized in the photoelectrochemical water oxidation reaction. It displayed a very high efficiency for a visible-light-driven oxygen evolution reaction (OER) in a KOH medium using standard Ag/AgCl as the reference electrode. The superiority of this material was further revealed from the low onset potential (0.822 mV vs RHE), high photocurrent density (0.204 mA cm-2), good stability, and high O2 evolution rate (77 μmol g-1 of oxygen evolution within 2 h), and a good efficiency (ABPE 0.42%, IPCE 29.6% and APCE 34.5%). Furthermore, the porosity in the overall framework seems to be a blessing to the photoelectrochemical performance due to better mass diffusion of the electrolyte. A detailed mechanism for the OER reaction was analyzed through density functional theory analysis suggesting the potential future of this Zn-thiolate framework for achieving a high efficiency in the sustainable water oxidation reaction.

Electrochemical Reduction of Flue Gas CO2 in a Dual Methanol/Water Electrolysis System for the Synthesis of Methyl Formate

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of the nonaqueous solvent methanol allows for higher CO2 solubility and novel products through the incorporation of methanol as an intermediate. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with a faradaic efficiency for methyl formate as high as 75% measured in pH < 2. Tests with simulated flue gas as the feedstock have shown a high level of tolerance for the Pb-catalyzed electrolysis to SO2, NO, and dilute O2 contaminants.

Stabilizing single-atomic ruthenium by ferrous ion doped NiFe-LDH towards highly efficient and sustained water oxidation

Single-atom catalysts (SAC) possesses the potentials of developing cost-effective water splitting devices; however, the active sites tend to form high valence state and dissolve into electrolyte at the anode during long-term oxidation process, leading to severe stability issue. Herein, we anchored single atomic Ru on Fe2+ ions doped NiFe layered double hydroxides, affording a strong electronic interaction between Ru and Fe2+, which lowered the oxidation state of Ru and meanwhile enhanced the Fe-O bond strength, thus resulting in significantly improved activity and stability of both the Ru sites and NiFe-LDH substrates. With 1.32 wt% Ru loading, the Ru/NiFe2+Fe-LDH catalyst showed a low overpotential of 194 mV at 10 mA / cm2 and a small Tafel slope of 36 mV / dec, which outperformed most of the Ru and NiFe based alkaline OER catalysts reported so far. In addition, the catalyst did not show obvious performance decay and dissolution after 100 h stability test at 100 mA / cm2, attributing to the strong electronic coupling between the single atomic Ru and the NiFe2+Fe-LDH, as evidenced by the X-ray absorption spectroscopy and density function theory calculation results that Fe2+ transferred 1.54 e- to Ru.

Facile, sustainable and unassisted plain water oxidation on Au/Ce0.9Ti0.1O2 nanorods in direct sunlight

Facile, sustainable and unassisted plain water oxidation on Au/Ce0.9Ti0.1O2 nanorods in direct sunlight

The pH and Potential Dependence of Pb-Catalyzed Electrochemical CO2 Reduction to Methyl Formate in a Dual Methanol/Water Electrolyte.

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of non-aqueous solvents like methanol allows for higher CO2 availability and novel products. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with higher pH conditions leading to formate and low pH favoring methyl formate. The potential dependence of the product distribution in acidic catholyte was also investigated, with a faradaic efficiency for methyl formate as high as 75 % measured at -2.0 V vs. Ag/AgCl.

Sustained Water Oxidation with Surface and Interface Engineered Wo3/Bivo4 Heterojunction Photoanodes

Sustained Water Oxidation with Surface and Interface Engineered Wo3/Bivo4 Heterojunction Photoanodes

Visible-Light-Driven Water Oxidation on Self-Assembled Metal-Free Organic@Carbon Junctions at Neutral pH.

Sustainable water oxidation requires low-cost, stable, and efficient redox couples, photosensitizers, and catalysts. Here, we introduce the in situ self-assembly of metal-atom-free organic-based semiconductive structures on the surface of carbon supports. The resulting TTF/TTF•+@carbon junction (TTF = tetrathiafulvalene) acts as an all-in-one highly stable redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven water oxidation reaction (WOR) at neutral pH, eliminating the need for metallic or organometallic catalysts and sacrificial electron acceptors. A water/butyronitrile emulsion was used to physically separate the photoproducts of the WOR, H+ and TTF, allowing the extraction and subsequent reduction of protons in water, and the in situ electrochemical oxidation of TTF to TTF•+ on carbon in butyronitrile by constant anode potential electrolysis. During 100 h, no decomposition of TTF was observed and O2 was generated from the emulsion while H2 was constantly produced in the aqueous phase. This work opens new perspectives for a new generation of metal-atom-free, low-cost, redox-driven water-splitting strategies.

Ultrathin Cobalt Oxide Interlayer Facilitated Hole Storage for Sustained Water Oxidation over Composited Tantalum Nitride Photoanodes

The hole-storage layer (HSL) strategy has been demonstrated as an efficient interfacial modification method to overcome the instability of tantalum nitride (Ta3N5) photoanodes and further boost high performance in photoelectrochemical (PEC) water oxidation reaction. Herein, we report that the CoOx/Ni(OH)x bilayer as a typical HSL could effectively extract and store photogenerated holes from Ta3N5, resulting in a decent photocurrent enhancement and stable water oxidation for at least 30 h. Most strikingly, the reversible formation of Co(IV) species inside the ultrathin CoOx layer during PEC water oxidation is found to regulate the hole-storage process, leading to facilitated photogenerated hole extraction capacity and suppressed charge recombination. Furthermore, upon the insertion of the CoOx/Ni(OH)x bilayer for the Ta3N5/CoPi photoanode, the photocurrent could be evidently increased, emphasizing the general applicability of the HSL strategy in promoting water oxidation reaction.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Selective Electrochemical Alkaline Seawater Oxidation Catalyzed by Cobalt Carbonate Hydroxide Nanorod Arrays with Sequential Proton-Electron Transfer Properties

Seawater oxygen evolution is one of the promising energy conversion technologies for large-scale renewable energy storage. It requires efficient catalysts to accelerate the oxygen evolution reaction (OER) for sustained water oxidation, avoiding chlorine evolution under acidic conditions or hypochlorite formation in alkaline solutions. Conventional metal oxide-based OER catalysts follow the adsorbate evolution mechanism that involves concerted proton-electron transfer steps at the active sites. Thus, on the scale of reversible hydrogen electrode, their catalytic activity is independent of the pH of electrolytes. In the present study, nanostructured cobalt carbonate hydroxide (CoCH) with sequential proton-electron transfer properties was tested as a catalyst for seawater oxygen evolution. CoCH exhibited pH-dependent water oxidation activities, thereby providing larger potential and current operating windows for selective water oxidation compared to the catalysts with pH-independent OER activities. The operating window can be further expanded by increasing the pH of the electrolyte.

In situ hydrothermal growth of SnS/Ni foam for electrochemical energy storage and conversion

SnS/Ni foam electrode via in situ hydrothermal growth as multifunctional catalyst for both electrochemical energy storage and conversion was explored. Half cell fabrication was achieved with usage of binder free in situ grown SnS/Ni foam working electrode in alkaline medium for supercapacitor and water oxidation. Higher specific capacitance of 686F/g at 10 mV/s and 145F/g at 5 A/g was achieved for SnS/Ni foam electrode from cyclic voltammogram (CV) and galvanostatic charging discharging (GCD) profiles. The higher oxygen evolution reaction (OER) activity of 419 mA/g with 286 mV overpotential was attained from linear sweep voltammogram (LSV) study with very low charge transfer resistance (Rct) of 12.9 Ω. Excellent and robust OER stability of 99% was achieved over 10 h of sustainable water oxidation. This result would give an appreciable impact on in situ growth of metal chalcogenides for electrochemical energy applications.

Redox-Active Ligand Assisted Catalytic Water Oxidation by a RuIV =O Intermediate.

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. RuV =O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with RuIV =(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of RuIV =O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Redox‐Active Ligand Assisted Catalytic Water Oxidation by a RuIV=O Intermediate

AbstractWater splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O−O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru‐based catalyst was reported in 1982. RuV=O is considered a key intermediate to initiate the O−O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru‐based catalyst that displays water oxidation reactivity with RuIV=(O) with the help of a redox‐active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of RuIV=O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Synthesis of an amorphous Geobacter-manganese oxide biohybrid as an efficient water oxidation catalyst

Self-decorated Mn2O3 nanocrystals on Geobacter sulfurreducens were synthesized as sustainable and efficient water oxidation catalysts.