O2 Evolution Reaction Research Articles

The Effect of MoSx Nanocoatings on the Water Electrolysis Performance Using a Nickel-Foam-Based Bifunctional Catalyst

The ability to increase the efficiency of electrochemical H2 and O2 evolution reactions in an alkaline solution using nickel-foam-based electrodes has been studied. To improve the catalytic properties of the foam, it was subjected to complex modification via sulfurizing in sulfur-containing gaseous media and the additional formation of amorphous or crystalline MoSx nanolayers. The foam was sulfidized in hydrogen sulfide or sulfur vapor at a temperature of 400°C. Amorphous MoSx films were produced via pulsed laser deposition from a MoS2 target in an H2S atmosphere. To obtain nanocrystalline catalytic MoSx layers, thin-film Mo precursors were preliminarily applied to the foam pre-sulfurized in sulfur vapor at 400°C. The deposition of precursors was carried out in vacuum at 22°C. After deposition, the foam with precursors was oxidized in air at 600°C. The modified samples were probed via scanning electron microscopy involving microanalysis and Raman spectroscopy. The effect of the precursor layer thickness on the sulfurizing efficiency and phase composition of the modified layers was established. The amorphous layer deposition was found to have a strong impact on only the O2 evolution. The crystalline layers obtained via sulfidizing of Mo-containing precursors exerted influence on both the cathodic and anodic reactions. The hydrogen overvoltage at a current density of 10 mA/cm2 was −160 mV, and the oxygen overvoltage was below 150 mV, which corresponded to the best electrocatalysts on the nickel foam after the volume modification via conventional chemical treatment (hydro- and solvothermal synthesis).

Ultrasound-accelerated synthesis of uniform SrMnO3 nanoparticles as water-oxidizing catalysts for water splitting systems.

One of the major issue in the 21st century is the humans request to green energy. The best form of green, sustainable and safe energy is hydrogen source due to its ecological and economical aspects. Herein, In order to obtain a highly water-oxidizing catalysts for water splitting systems, the sonochemical procedure applied for fabrication of practical SrMnO3 nanoparticles. Also, the influence of various green capping agents (fruit juices and vegetable wastes) was studied on the formation of uniform particles. In the present work ultrasonic probe with 60W/cm2 intensity and 18kHz frequency was used for sample synthesis. Further, catalytic behavior of these nanomaterials investigated in water splitting reaction for O2 evolution by modifying the operational variables. The best catalytic behavior observed by those nanoparticles that indicated the smallest size and the most uniform morphology (Max amount of TON=7.556). By utilizing the ultrasonic irradiation, the catalytic behavior of SrMnO3 nanoparticles improved (TON (ultrasonic bath)=8.430, TON (ultrasonic probe)=11.315). Therefore, nano-SrMnO3 was introduced as an efficient and novel nanocatalyst for O2 evolution reaction.

Fabrication of foamed zinc oxide-silica spheres coupled with Ag–AgBr for high-efficiency photo-/electrocatalytic overall water splitting

Advanced photo-/electrocatalyst is devoted for O2 and H2 evolution reactions (OER and HER) with immense quantum conversions efficiency. Herein, a novel foamed zinc oxide-silica spheres and Ag–AgBr nanocomposite (Ag–AgBr/FZSS) was engineered and passes through a complete characterization profiling. The Ag–AgBr/FZSS possesses the distinctive porous surface with open cavities. The hierarchical morphology provides many active sites and mesoporous conducting channels for visible light absorption and charge carriers transportation. The Ag–AgBr/FZSS act as outperformed photo-/electrocatalyst for OER (overpotential 348 mV @ 10 mAcm−2) and HER (potential −217 mV @ 100 mAcm−2). While under illumination conditions it reduced to 337 mV and −195 mV with high quantum conversion efficiency (37.5%). The lustrous durability further justified as a highly efficient photo-/electrocatalyst for renewable energy conversions.

2020 Roadmap on gas-involved photo- and electro- catalysis

Green reactions not only provide us chemical products without any pollution, but also offer us the viable technology to realize difficult tasks in normal conditions. Photo-, photoelectro-, and electrocatalytic reactions are indeed powerful tools to help us to embrace bright future. Especially, some gas-involved reactions are extremely useful to change our life environments from energy systems to liquid fuels and cost-effective products, such as H2 evolution (H2 production), O2 evolution/reduction, CO2 reduction, N2 reduction (or N2 fixation) reactions. We can provide fuel cells clean H2 for electric vehicles from H2 evolution reaction (HER), at the same time, we also need highly efficient O2 reduction reaction (ORR) in fuel cells for improving the reaction kinetics. Moreover, we can get the clean oxidant O2 from water through O2 evolution reaction (OER), and carry out some reactions without posing any pollution to reaction systems. Furthermore, we can translate the greenhouse gas CO2 into useful liquid fuels through CO2 reduction reaction (CRR). Last but not the least, we can get ammonia from N2 reduction reaction (NRR), which can decrease energy input compared to the traditional Hubble process. These reactions, such as HER, ORR, OER, CRR, and NRR could be realized through solar-, photoelectro- and electro-assisted ways. For them, the catalysts used play crucial roles in determining the efficiency and kinds of products, so we should consider the efficiency of catalysts. However, the cost, synthetic methods of catalysts should also be considered. Nowadays, significant progress has been achieved, however, many challenges still exist, reaction systems, catalysts underlying mechanisms, and so on. As extremely active fields, we should pay attention to them. Under the background, it has motivated us to contribute with a roadmap on ‘Gas-Involved Photo- and Electro-Catalysis’.

2020 Roadmap on two-dimensional nanomaterials for environmental catalysis

Environmental catalysis has drawn a great deal of attention due to its clean ways to produce useful chemicals or carry out some chemical processes. Photocatalysis and electrocatalysis play important roles in these fields. They can decompose and remove organic pollutants from the aqueous environment, and prepare some fine chemicals. Moreover, they also can carry out some important reactions, such as O2 reduction reaction (ORR), O2 evolution reaction (OER), H2 evolution reaction (HER), CO2 reduction reaction (CO2RR), and N2 fixation (NRR). For catalytic reactions, it is the key to develop high-performance catalysts to meet the demand for targeted reactions. In recent years, two-dimensional (2D) materials have attracted great interest in environmental catalysis due to their unique layered structures, which offer us to make use of their electronic and structural characteristics. Great progress has been made so far, including graphene, black phosphorus, oxides, layered double hydroxides (LDHs), chalcogenides, bismuth-based layered compounds, MXenes, metal organic frameworks (MOFs), covalent organic frameworks (COFs), and others. This content drives us to invite many famous groups in these fields to write the roadmap on two-dimensional nanomaterials for environmental catalysis. We hope that this roadmap can give the useful guidance to researchers in future researches, and provide the research directions.

Achievement of High Photocatalytic Performance to BaTi4O9 Toward Overall H2O Splitting

AbstractThe improvement of photocatalytic activity of BaTi4O9 to overall H2O splitting was investigated. From the evaluation of co‐catalysts, Rh0.7Cr1.3O3 was preferable. However, this co‐catalyst was degrading with the progress of the photocatalytic reaction. When CoOx was co‐loaded as the co‐catalyst for O2 evolution reaction over the surface, the steady progress of the photocatalytic reaction can be confirmed. Here, effective way and condition for loading CoOx co‐catalyst as well as effective preparation method of BaTi4O9 were demonstrated in order to improve the photocatalytic activity of BaTi4O9 with Rh0.7Cr1.3O3 and CoOx co‐catalysts to the overall H2O splitting. As the results, the photocatalyst prepared by preferable method and condition showed high photocatalytic activity to the overall H2O splitting, where the apparent quantum yield (AQY) at 313 nm reached to 41 %.

BaMnO3 nanostructures: Simple ultrasonic fabrication and novel catalytic agent toward oxygen evolution of water splitting reaction.

In the current paper, the main aim is to fabricate the BaMnO3 nanostructures via the sonochemical route. The various factor, including precursors, reaction time and power of sonication can affect the shape, size, and purity of the samples. We utilized X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray energy dispersive spectroscopy (EDS) to characterize the BaMnO3 nanostructures. The optical property of BaMnO3 nanostructures was explored by Ultraviolet-visible spectroscopy (UV-vis) and the energy gap was suitable for catalytic activity (about 2.75 eV). Changing the precursor can affect the size, nanoparticle shape, architectures, and uniformity of the samples. We employed the BaMnO3 nanostructures for O2 evolution reaction as catalysts. It can observe that increasing the homogeneity of the catalysts can increase the efficiency of the Oxygen evolution reaction. The maximum amount of the O2 evolution and the highest TOF and TON are related to nanoplate disc using barium salicylate as a precursor of barium. As a result, we can nominate the BaMnO3 nanostructures as an effective and novel catalyst for water-splitting reaction.

Photocatalytic overall water splitting by spatially-separated Rh and RhOx cocatalysts on polymeric carbon nitride nanosheets

Site-selective deposition of dual cocatalysts for H2 and O2 evolution reactions on polymeric carbon nitride (PCN) nanosheets still remains a great challenge due to the difficulty in controlling the nucleation location and growth dynamics. Herein, we report an one-step and efficient photodeposition strategy to simultaneously load Rh and RhOx nanoparticles on PCN nanosheets with desired spatial isolation. The dual cocatalysts modification can effectively accelerate the reaction kinetics and boost the separation of photoinduced charge carriers, greatly enhancing overall water photosplitting efficiency. As a result, the overall water photosplitting activity of Rh-RhOx/PCN is obviously higher than those of RhOx/PCN and Rh/PCN. This work provides a new method for loading dual cocatalysts on the PCN nanosheets with spatial isolation to achieve pure water splitting under visible light and offers promise for a wealth of innovative applications in energy conversion, catalysis and environmental protection.

Towards Electrochemical Synthesis of Cement — an Electrolyzer-Based Process for Decarbonating CaCO3 while Producing Useful Gas Streams

Cement production is currently the largest single industrial emitter of CO2, accounting for 8% (2.8 Gtons/year) of global CO2 emissions in 2015. Deep decarbonization of cement manufacturing will require remediation of both the CO2 emissions due to the decomposition of CaCO3 to CaO, and that due to combustion of fossil fuels (primarily coal) in the calcining (~900°C) and sintering (~1,450°C) processes. Here, we demonstrate an electrochemical process that uses neutral water electrolysis to produce a pH gradient in which CaCO3 is decarbonated at low pH and Ca(OH)2 is precipitated at neutral to high pH, concurrently producing a high purity O2/CO2 gas mixture (1:2 molar ratio at demonstrated stoichiometric operation) at the anode and H2 at the cathode, as shown in Figure 1. We show that the solid Ca(OH)2 product readily decomposes and reacts with SiO2 to form alite, the majority cementitious phase in Portland cement. Our electrochemical calcination approach produces concentrated gas streams from which the CO2 may be readily separated and sequestered, the H2 and/or O2 may be used to generate electric power via fuel cells or combustors, the O2 may be used as a component of oxyfuel in the cement kiln to further lower CO2 and NOx emissions, or the output gases may be used for other value-added processes including liquid fuel production. Analysis shows that in a scenario where the hydrogen produced by the reactor is combusted to heat the high temperature kiln, the electrochemical cement process can be powered solely by renewable electricity. Figure 1. Schematic of the electrolyzer-based decarbonation cell. Reactions 1a and 1b are the O2 evolution and H2 evolution half-cell reactions respectively, under near-neutral pH. Reactions 2a and 2b represent the decomposition of CaCO3 and release of CO2. Reaction 3 is the normal formation of water from its component ions. In Reaction 4, the hydroxide ions in Reaction 3 instead go towards the formation of Ca(OH)2, and the protons protonate carbonate ions (Reaction 2b). The overall reaction in which CaCO3 is converted to Ca(OH)2 with the attendant release of H2, O2 and CO2 is shown at the bottom. Figure 1

“Carbohydrate-Universal” electrolyzer for energy-saving hydrogen production with Co3FePx@NF as bifunctional electrocatalysts

Hydrogen production via solar-powered water electrolysis allows abundant but intermittent energy to be directly converted into clean fuel in a sustainable manner. However, this method is hindered by the sluggish O2 evolution reaction (OER). The carbohydrate oxidation reaction (COR) can potentially replace the OER for effective H2 production with low-energy consumption due to its more favourable thermodynamics. Herein, Co3FePx@NF is used as a “carbohydrate-universal” catalyst for four representative COR that had a potential of 1.24 V vs RHE (to 10 mA cm−2) which was 20% (310 mV) lower than commercial electrocatalysts for OER. In addition, the presence of carbohydrates had no negative effect on the high performance of Co3FePx@NF for hydrogen evolution at cathode. Motivated by the above performance improvements, a novel electrolyzer was developed that integrates H2 production and COR using Co3FePx@NF as a bifunctional electrocatalyst. A cell voltage of only 1.35 V was required to achieve a current density of 10 mA cm−2, which was much lower than commercial water splitting systems (nomarlly 1.8∼2.0 V), highlighting that the electrolyzer in this paper has great potential for use in energy-saving H2 production. Due to the improved design, the electrolyzer can be easily driven by a single perovskite solar cell delivering a solar-to-hydrogen efficiency of 13.3%, which provides a convenient and efficient way to achieve overall-sustainable H2 production. This work may serve as the foundation for further energy-saving hydrogen production technologies and carbohydrate-containing waste treatment by the rational design of electrolyzers.

Joint Theoretical and Experimental Study on the La Doping Process in In2O3: Phase Transition and Electrocatalytic Activity.

In2O3 and La3+-doped In2O3 nanostructures were synthesized through a facile and fast chemical route based on the microwave-assisted hydrothermal method combined with rapid thermal treatment in a microwave oven. The presence of the La3+ doping process modifies the size and morphology of the In2O3 nanostructures and also stabilizes the rhombohedral (rh) In2O3 phase with respect to the most stable cubic (bcc) polymorph. X-ray diffraction (XRD) patterns and Rietveld refinements, Raman, UV-vis, and energy dispersive X-ray (EDX) spectroscopies, transmission electron (TEM) and field-emission scanning electron (FE-SEM) microscopies, as well as PL emissions have been performed. To complement and rationalize the experimental results, first-principle calculations, based on density functional theory, are carried out to obtain the formation energies of the In2O3 and bcc- and rh-In2O3-doped phases, their geometry and electronic properties. Theoretical results are able to explain the relative stabilization of the rh-phase with respect to the bcc-phase based on the analysis geometry changes and the electronic redistribution induced by the La3+ doping process. In addition, Wulff construction is employed to match the theoretical and experimental morphologies of the cubic phase. The synthesized samples were applied for the O2 evolution reaction (OER). The La3+-doped In2O3 film presents superior electrocatalytic activity, with an onset potential lower than the undoped In2O3 film that can be associated with the increase in electron density caused by the La3+ doping process. This study provides a versatile strategy for obtaining In2O3 and La3+-doped In2O3 nanostructures for practical applications.

Advances and Challenges for Aprotic Lithium‐Oxygen Batteries using Redox Mediators

AbstractRechargeable aprotic Li−O2 batteries that can deliver extremely high energy density have attracted worldwide attention. However, the practical use of this promising technology is hindered by an unsatisfactory cycling life and inferior energy efficiency (high discharge/charge voltage gap), although significant progress has been made in this area in the last decade. Recent reports indicate that soluble catalysts or redox mediators dissolved in aprotic electrolytes can catalyze Li2O2 decomposition and reduce the voltage gap between O2 reduction reaction and O2 evolution reaction. Here, a specific review on a series of suitable soluble catalysts for Li−O2 batteries is provided. This review will highlight the underpinning chemistry and electrochemistry mechanisms of Li−O2 batteries, as well as the recent progress and challenges of soluble catalysts, with the aim of motivating researchers to further explore applications in Li−O2 cells and other energy storage and conversion systems.

Zn-Co electrocatalysts in lithium-O2 batteries: temperature and rotating cathode effects on the electrodeposition

In this study, we developed Zn-Co catalyst by electrodeposition process in nickel foam and steel mesh electrodes for application as a cathode in Li-O2 batteries. The electrodes were synthesized from an electrolytic bath varying the temperature (25 to 70 °C) and rotating cathode (30 to 100 rpm) by the potentiostatic method. The results of capacities in the Li-O2 cell and catalytic activity measurements showed that the Zn-Co alloy is a promising catalyst for the O2 evolution reaction (OER). Sharp surfaces composed for nanoflakes and heterogeneous surfaces with whitish agglomerates which may have increased the surface area for the housing of the discharge products was observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) evidenced Zn21Co5 composition and Raman spectra evidenced discharge product Li2O2. The maximum charge capacity of 399 mAh gc−1 for steel mesh and 182 mAh gc−1 for nickel foam electrode suggests the use of this alloy as catalysts in Li-O2 batteries.

Modulating Proton and Electron Transfer Dynamics in Molecular Electrocatalysis Using Membrane-Covered Self-Assembled Monolayers

Many reactions central to the development of renewable energy devices such as the O2 reduction reaction, the O2 evolution reaction, and the CO2 reduction reaction require the transfer of multiple electrons and protons. These reactions proceed by a variety of mechanisms depending upon the relative rates of these transfer events. We are developing electrode frameworks that enable control over proton and electron transfer kinetics to molecular catalysts such that the selectivity of catalysts can be improved. These electrodes contain a transition metal catalyst such as a Cu-triazole or an Fe-porphyrin attached via a self-assembled monolayer (SAM), which is then covered by a proton-permeable membrane. Initial studies have utilized a lipid monolayer modified with aliphatic proton carriers as the proton-permeable membrane. The kinetics of proton transfer to the catalyst are controlled by changing the quantity and identity of proton carriers in the membrane. By altering the identity of the SAM, which can be facilely done using a modular azide-alkyne click chemistry approach, the kinetics of electron transfer to the catalyst can also be controlled. One grand challenge in designing catalysts for both the O2 and CO2 reduction reactions is selectivity. We show that the selectivity of a Cu-based O2 reduction catalyst can be improved by controlling proton transfer rates such that the catalyst produces solely water. In contrast, the same Cu catalyst produces ~10% H2O2 side product when proton transfer is not regulated. We also discuss how modulating proton and electron transfer kinetics affects the selectivity of molecular CO2 reduction catalysts and the performance of Ni-based O2 evolution catalysts.

Oxygen Release Degradation in Li‐Ion Battery Cathode Materials: Mechanisms and Mitigating Approaches

AbstractWidespread application of Li‐ion batteries (LIBs) in large‐scale transportation and grid storage systems requires highly stable and safe performance of the batteries in prolonged and diverse service conditions. Oxygen release from oxygen‐containing positive electrode materials is one of the major structural degradations resulting in rapid capacity/voltage fading of the battery and triggering the parasitic thermal runaway events. Herein, the authors summarize the recent progress in understanding the mechanisms of the oxygen release phenomena and correlative structural degradations observed in four major groups of cathode materials: layered, spinel, olivine, and Li‐rich cathodes. In addition, the engineering and materials design approaches that improve the structural integrity of the cathode materials and minimize the detrimental O2 evolution reaction are summarized. The authors believe that this review can guide researchers on developing mitigation strategies for the design of next‐generation oxygen‐containing cathode materials where the oxygen release is no longer a major degradation issue.

Selecting substituent elements for LiMnPO4 cathode materials combined with density functional theory (DFT) calculations and experiments

LiMnPO4 cathode material has a high voltage platform and matches the existing electrolyte window, and thus researchers are constantly shifting their focus from LiFePO4 to LiMnPO4. However, LiMnPO4 has lower electron (ion) conductivity than LiFePO4, and besides, its delithiated phase MnPO4 will suffer thermal decomposition at lower temperatures more easily than FePO4. In order to effectively solve the above problems by elements substitution, DFT calculations are employed to screen for suitable dopants from a series of transition metals including Fe, Mg, Ni, V, Nb, Ti. Properties such as electronic structure, atomic Bader charge, O2 evolution Gibbs free energy, average voltages, and lithium ion diffusion energy barrier were evaluated. Based on the calculation, Fe is the most effective doping element because Fe doping is able to reduce the band gap of the material and improve the electronic conductivity, suppress the O2 evolution reaction of the delithiation phase and improve the thermal stability. The reason for such a situation is that Fe can form a stronger covalent bond with the surrounding O atoms to bind the escape of O. Fe doping reduces ion diffusion energy barrier to promote lithium ion diffusion. Electrochemical tests show that Fe doping can improve the electrochemical properties of LiMnPO4.

Synthesis of morphology controllable free-standing Co3O4 nanostructures and their catalytic activity for Li[sbnd]O2 cells

Free-standing Co3O4 on nickel foam is synthesized by a simple hydrothermal route, using cobalt nitrate hexahydrate as the cobalt source, and urea as the precipitator. The effects of urea on the structural and electrochemical characteristics are investigated in detail. As the urea increased, the morphology of the free-standing Co3O4 turns from hexagonal nanosheets to nanoflakes, and finally turns to nanoflakes/nanowires hybrid structure. This hybrid structure is made up of ultrathin porous nanoflakes, and short nanowires anchored at the edge of the nanoflakes, which may enhance the diffusion rate of the electrolyte and O2 and provide more active sites for both O2 reduction and evolution reactions. When applied as a free-standing cathode for the LiO2 cells, it shows better performance for the nanoflakes/nanowires hybrid structure than the others. It exhibits a high specific capacity of 5958 mA h g−1 at 0.1 mA cm−2, a long cycle life of 108 cycles with a limited capacity of 500 mA h g−1, and unique morphology of the discharge products.

Element-Specific Restructuring of Anion- and Cation-Substituted Cobalt Phosphide Nanoparticles under Electrochemical Water-Splitting Conditions

Studying the restructuring behavior of doped catalyst materials under electrochemical reaction conditions is important for understanding the structure–property relationships and for developing design principles for better catalysts. As a well-known catalyst for both the cathode (H2 evolution reaction, HER) and anode (O2 evolution reaction, OER) reactions of water electrolysis, CoP can be made even more active by cationic or anionic substitution. However, the dependence of catalytic reactivity on substitutional doping has not been sufficiently understood in the context of restructuring under working conditions. In this work, cation (Fe)- and anion (S)-substituted CoP nanoparticles are synthesized, and their surface oxidation under ambient conditions and restructuring under HER and OER conditions are investigated. For Fe0.5Co0.5P, the Fe substituents are more easily oxidized than Co in the air; they are also more difficult to reduce under HER conditions in alkaline electrolyte, and the remaining Fe–OH speci...

When Catalysts Breathe

In this issue of Joule, Yi Cui and colleagues developed a breath-mimicking structure for electrocatalysis applications where gas no longer has to be dissolved into the electrolyte. Specifically, a highly efficient three-phase contact between gas, electrolyte, and catalyst is formed, facilitating both O2 evolution (OER) and reduction (ORR) reaction processes. A low overpotential of 190 mV at 10 mA/cm2 is reported in the OER process while in the ORR process significantly higher current densities are observed.

Modulated electrochemical oxygen evolution catalyzed by MoS2 nanoflakes from atomic layer deposition

Electrochemical water splitting into H2 and O2 has attracted wide attention owing to the urgent need for clean and renewable energy sources. However, the scarcity and high-cost limit the large-scale application of noble metal catalysts such as IrO2 and RuO2. In this work, as a low-cost catalyst for the electrochemical O2 evolution reaction (OER), MoS2 nanoflakes were obtained by atomic layer deposition (ALD) using MoCl5 and H2S on carbon fiber paper surface. According to the results of electrochemical measurements, the MoS2 nanoflakes exhibit an excellent catalytic activity, and the activity can be modulated by controlling the density and the internal resistance of MoS2 nanoflakes. Moreover, the plasma treatment can further improve the activity of MoS2 nanoflakes, and the reason was discussed through the measurements of contact angle, electrochemical impedance spectroscopy, and electrochemically active surface area. The MoS2 nanoflakes obtained by ALD possess huge values for electrochemical OER as a catalyst.