Mechanism Of Oxygen Evolution Research Articles

Isotope-Labeling Studies in Water Oxidation Electrocatalysis

Improving understanding of the electrocatalytic mechanisms for the oxygen evolution reaction (OER) will help in designing electrode materials with lower catalytic overpotential and greater stability, increasing the efficiency and economic viability of electrolysis. Isotope-labeling can be a powerful tool for elucidating catalytic mechanisms, including in electrocatalysis. Herein, we apply 18-O labeling, utilizing a fully quantitative electrochemistry - mass spectrometry method with unprecedented sensitivity[1] and post-characterization by low-energy ion scattering spectrometry, to investigate the electrocatalytic mechanism of oxygen evolution. Nickel-iron based electrodes are used in industrial alkaline water electrolysis, but questions remain about the intrinsic activity and electrocatalytic mechanism. By performing a series of isotope-labeling experiments on a model system of mass-selected Ni-Fe nanoparticles, we show that oxygen evolution does not proceed via lattice oxygen exchange and that only the surface of the nanoparticles are active[2]. This allows us to estimate the turn-over frequency of the active sites, 6 O2 molecules per site per second, which is higher than previous estimates. We also quantify lattice oxygen exchange in IrO2 and RuO2 thin films during oxygen evolution in acidic electrolyte as a function of electrochemical roughening, and perform oxygen stripping experiments to probe OER intermediates. This talk will describe these findings and further explore the potential of isotope labeling studies in electrocatalysis. [1] D. B. Trimarco, S. B. Scott et al, Enabling real-time detection of electrochemical desorption phenomena with sub-monolayer sensitivity. Electrochimica Acta. 2018, 268, 520-530 [2] C. Roy, B. Sebok, S. B. Scott et al, Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeOxHy. Nature Catalysis. 2018, 1(11), 820–829 Figure 1

New insights into the electrochemical behaviour of porous carbon electrodes for supercapacitors

Abstract Activated carbons, with different surface chemistry and porous textures, were used to study the mechanism of electrochemical hydrogen and oxygen evolution in supercapacitor devices. Cellulose precursor materials were activated with different potassium hydroxide (KOH) ratios, and the electrochemical behaviour was studied in 6 M KOH electrolyte. In situ Raman spectra were collected to obtain the structural changes of the activated carbons under severe electrochemical oxidation and reduction conditions, and the obtained data were correlated to the cyclic voltammograms obtained at high anodic and cathodic potentials. Carbon-hydrogen bonds were detected for the materials activated at high KOH ratios, which form reversibly under cathodic conditions. The influence of the specific surface area, narrow microporosity and functional groups in the carbon electrodes on their chemical stability and hydrogen capture mechanism in supercapacitor applications has been revealed.

Photosynthetic biohydrogen production in a wastewater environment and its potential as renewable energy

Oxygen (O2) is a strong inhibitor of hydrogenase (HydA) activity and expression and altering the sulfur (S) oxidizing transitions in photosystem II (PSII) often allows algal photohydrogen production; however, this may not be practical in a wastewater environment. To counteract natural mechanisms of oxygen evolution in PSII, we utilized acetic acid and butyric acid, which are main volatile fatty acids (VFAs) found in anaerobic bacterial digestion in wastewater treatment, as oxygen regulators for photosynthetic biohydrogen production using Chlorella vulgaris. It was found that a VFA-containing synthetic wastewater promotes oxygen depletion in a photobioreactor (PBR), producing maximum hydrogen yield of 65.4 ± 0.3 μmoL H2 L−1 mM−1 acetate without artificial sulfur or chloride deprivation. Butyric acids showed no significant effect on oxygen depletion and biohydrogen production in the PBR. The measurements of both relative expression level of mRNA and specific activities of reactivate HydA revealed that repetitive algal H2 photo-evolution was possible by HydA synthesis in C. vulgaris followed by complete oxygen depletion controlled by acetic acid levels in the PBR. This emerging understanding of the role of VFAs on oxygen regulation in PSII in natural environments is expected to lead algal-driven bioenergy production technologies to the next level.

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is ...

Mechanism and Kinetics of HER and OER on NiFe LDH Films in an Alkaline Electrolyte

The mechanism and kinetics of hydrogen evolution (HER) and oxygen evolution (OER) reactions on nickel iron layered double hydroxide (NiFe LDH) in a basic electrolyte are investigated. The deposited film reported an overpotential of 247 and 245 mV at 10 mA/cm2 toward the HER and OER, respectively. A least squares procedure was performed to fit a theoretical current density model with experimental linear sweep voltammetry (LSV) results, and the chemical reaction rate constants for the OER and HER steps were identified. Electrochemical impedance spectroscopy (EIS) measurements were taken at different potentials, and the resulting kinetic model demonstrates a good agreement between theoretically calculated faradaic resistance and experimental EIS results. The HER results indicated the Heyrovsky step as rate controlling, with a dependence of reaction mechanism on potential. At low potential, the mechanism begins with a Volmer step, followed by parallel Tafel and Heyrovsky steps. At higher potential, the mechanism becomes consecutive combination of the Volmer and Heyrovsky steps. The OER data point to the formation of the adsorbed peroxide as rate controlling. The HER and OER kinetic data were combined into a model capable of predicting the electrolysis cell current-potential characteristics, which can be used for process design and optimization.

Sunlight-driven water splitting using hematite nanorod photoelectrodes.

The efficiency of nanostructures for photoelectrochemical water-splitting is fundamentally governed by the capability of the surface to sustain the reaction without electron trapping or recombination by photogenerated holes. This brief review will summarize the latest progress on hematite, designed with columnar morphology via chemical synthesis, for photoelectrochemical cell application. The columnar morphology efficiently minimizes the number of defects, grain boundaries, and surface traps normally present on the planar morphology. The major drawback related to hole diffusion through the solid/liquid interface was addressed by using high annealing temperature combined with dopant addition. A critical view and depth of understanding of these two parameters were discussed focusing on the molecular oxygen evolution mechanism from the sunlight-driven water oxidation reaction.

Reaction mechanism for oxygen evolution on RuO2, IrO2, and RuO2@IrO2 core-shell nanocatalysts

Iridium dioxide, IrO2, is second to the most active RuO2 catalyst for the oxygen evolution reaction (OER) in acid, and is used in proton exchange membrane water electrolyzers due to its high durability. To improve the activity of IrO2-based catalysts, we prepared RuO2@IrO2 core-shell nanocatalysts using carbon-supported Ru as the template. At 1.48V, the OER specific activity of RuO2@IrO2 is threefold that of IrO2. While the activity volcano plots over wide range of materials have been reported, zooming into the top region to clarify the rate limiting steps of most active catalysts is important for further activity enhancement. Here, we verified theory-proposed sequential water dissociation pathway in which the OO bond forms on a single metal site, not via coupling of two adsorbed intermediates, by fitting measured polarization curves using a kinetic equation with the free energies of adsorption and activation as the parameters. Consistent with theoretical calculations, we show that the OER activities of IrO2 and RuO2@IrO2 are limited by the formation of O adsorbed phase, while the OOH formation on the adsorbed O limits the reaction rate on RuO2.

(Invited) Nano-Scaled Perovskites for Oxygen Evolution in Alkaline Environment

Oxygen electrodes are playing a key role in electrochemical energy conversion devices such as fuel cells and water electrolyzers. In both acidic and alkaline environment, both the oxygen reduction and oxygen evolution reaction (ORR and OER), respectively, are limiting the overall energy/voltage efficiency due to its sluggish kinetics. [1, 2] Whereas in acidic environment, mainly precious metal oxides are used to catalyze the OER (e.g., IrO2), the variety of possible catalysts in alkaline electrolyte is significantly increased and many metal oxide based systems can be employed. Generally the oxygen evolution mechanism is only partly understood independent of the electrolyte environment and material used. In order to help to understand the underlying mechanism for the OER and to support the experimental results, very often computational methods are used, mainly using density functional theory (DFT) calculations. Similar approaches are also used for gaining insights into catalyst stabilities under operational conditions. In this talk, some of your recent findings on non-noble metal catalysts, mainly from the perovskite family will be presented. By employing operando X-ray absorption spectroscopy (both XANES and EXAFS), important insights could be gained with respect to the perovskite electronic structure and the local catalyst structure helping to better understand the underlying mechanism for the OER. Acknowledgement: This work is supported by the Swiss National Science Foundation, the Swiss Competence Center for Energy Research (SCCER) Heat & Electricity Storage, the Swiss Federal Office of Energy and the Competence Center Energy & Mobility Switzerland.

The open-cubane oxo-oxyl coupling mechanism dominates photosynthetic oxygen evolution: a comprehensive DFT investigation on O-O bond formation in the S4 state.

The dioxygen formation mechanism of biological water oxidation in nature has long been the focus of argument; many diverse mechanistic hypotheses have been proposed. Based on a recent breakthrough in the resolution of the electronic and structural properties of the oxygen-evolving complex in the S3 state, our density functional theory (DFT) calculations reveal that the open-cubane oxo-oxyl coupling mechanism, whose substrates preferably originate from W2 and O5 in the S2 state, emerges as the best candidate for O-O bond formation in the S4 state. This is justified by the overwhelming energetic superiority of this mechanism over alternative mechanisms in both the isomeric open and closed-cubane forms of the Mn4CaO5 cluster; spin-dependent reactivity rooted in variable magnetic couplings was found to play an essential role. Importantly, this oxygen evolution mechanism is supported by the recent discovery of femtosecond X-ray free electron lasers (XFEL), and the origin of the observed structural changes from the S1 to S3 state has been analyzed. In this view, we corroborate the proposed water binding mechanism during S2-S3 transition and correlate the theoretical models with experimental findings from aspects of substrate selectivity according to water exchange kinetics. This theoretical consequence for native metalloenzymes may serve as a significant guide for improving the design and synthesis of biomimetic materials in the field of photocatalytic water splitting.

Theoretical Study on the Mechanism of Photosynthetic Oxygen Evolution by ABEEM/MM/MD and BS-DFT

Theoretical Study on the Mechanism of Photosynthetic Oxygen Evolution by ABEEM/MM/MD and BS-DFT

Recent developments in electrochemical water oxidation

Recent developments in electrochemical water oxidation in aqueous alkaline solution at earth abundant transition metal oxyhydroxide thin film electrodes are outlined and candidate high activity materials identified. The molecular mechanism of oxygen evolution at these oxide surfaces is discussed.

Rational Ligand Design for an Efficient Biomimetic Water Splitting Complex.

Being an important biomimetic model catalyst for water oxidation, the dimanganese molecular complex [H2O(terpy)MnIII(μ-O)2MnIV(terpy)OH2]3+ (complex 1, terpy = 2,2':6',2″-terpyridine) has been investigated extensively by experimentalists. By carrying out density functional theory calculations, we explore theoretically the oxygen evolution mechanisms of complex 1. On the basis of understandings of the geometric and electronic structural features of complex 1, we explore the possibility of improving its catalytic efficiency through a rational design of ligands coordinated to the manganese ions. Recognizing that the rate-determining step of oxygen evolution is the formation of an O-O bond at a high-valent manganese center, we design a new complex, [H2O(2-bpnp)MnIII(μ-O)2MnIV(2-bpnp)OH2]3+ (complex 2, 2-bpnp = 2-([2,2'-bipyridin]-6-yl)-1,8-naphthyridine). It is verified that the proton-accepting 2-bpnp ligand leads to stabilized hydrogen bonding with surrounding water molecules, and hence, the barrier height associated with O-O bond formation is substantially reduced. Moreover, despite its larger size, the 2-bpnp ligand does not cause steric hindrance for the release of molecular oxygen. Consequently, the proposed complex 2 is expected to outperform the existing complex 1 regarding catalytic efficiency. This work highlights the potential usefulness of rational design toward reaching the high efficiency of the oxygen evolution center in photosystem II.

Oxygen Evolution at Manganite Perovskite Ruddlesden-Popper Type Particles: Trends of Activity on Structure, Valence and Covalence.

An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.

Dynamics of Proton Transfer to Internal Water during the Photosynthetic Oxygen-Evolving Cycle.

In photosynthesis, the light-driven oxidation of water is a sustainable process, which converts solar to chemical energy and produces protons and oxygen. To enable biomimetic strategies, the mechanism of photosynthetic oxygen evolution must be elucidated. Here, we provide information concerning a critical step in the oxygen-evolving, or S-state, cycle. During this S3-to-S0 transition, oxygen is produced, and substrate water binds to the manganese-calcium catalytic site. Our spectroscopic and H218O labeling experiments show that this S3-to-S0 step is associated with the protonation of an internal water cluster in a hydrogen-bonding network, which contains calcium. When compared to the protonated water cluster, formed during a preceding step, the S1-to-S2 transition, the S3-to-S0 hydronium ion is likely to be coordinated by additional water molecules. This evidence shows that internal water and the hydrogen bonding network act as a transient proton acceptor at multiple points in the oxygen-evolving cycle.

(Invited) Molecular Mechanisms of Oxygen Evolution and Water Splitting at Elevated Temperatures

High-temperature electrolysis employing oxygen-ion-conducting solid electrolytes is a promising method to synthesize fuel. However, the molecular reaction mechanisms at the gas/electrode interface is not well-understood. In recent years, a variety of in-situ characterizations and density-functional theory calculations have elucidated the reaction mechanisms. In this talk, I will present a comprehensive understanding of oxygen-evolution and water-splitting reactions on perovskite and fluorite oxides at the molecular level.

Development and characterization of new nickel coatings for application in alkaline water electrolysis

The purpose of this paper is to report on the synthesis and characterization of different electrocatalytic materials for the hydrogen and oxygen evolution reactions (HER and OER) in alkaline water electrolysis. Ni meshes surface coated with porous nickel layers have been used as substrates for electrodes. Porous nickel coatings have been modified by amorphous NiPx or Pt nanoparticles (to improve the cathode efficiency) and nickel–cobalt spinels (to improve the anode efficiency). The structure, roughness and composition of the active layers have been determined by SEM analysis, porosimetry, laser atomic emission spectroscopy (LAES), energy-dispersion analysis (EDA) and XRD analysis. Overpotentials for both HER and OER have been measured in 6 M KOH over the 0–1.0 A/cm2 current density range. Both hydrogen and oxygen overpotentials are found significantly reduced compared to smooth nickel electrodes. The hydrogen and oxygen evolution mechanisms on these catalytic surfaces are discussed.

Effect of nano-sized cerium–zirconium oxide solid solution on far-infrared emission properties of tourmaline powders

Far-infrared functional nanocomposites were prepared by the co-precipitation method using natural tourmaline [Formula: see text], where [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], or vacancy; [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], or [Formula: see text]; [Formula: see text] is [Formula: see text], [Formula: see text], [Formula: see text], or [Formula: see text]; [Formula: see text] is [Formula: see text], [Formula: see text]; and [Formula: see text] is [Formula: see text], [Formula: see text], or [Formula: see text] powders, ammonium cerium(IV) nitrate and zirconium(IV) nitrate pentahydrate as raw materials. The reference sample, tourmaline modified with ammonium cerium(IV) nitrate alone was also prepared by a similar precipitation route. The results of Fourier transform infrared spectroscopy show that tourmaline modified with Ce and Zr has a better far-infrared emission property than tourmaline modified with Ce alone. Through characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), the mechanism for oxygen evolution during the heat process in the two composite materials was systematically studied. The XPS spectra show that [Formula: see text] ratio inside tourmaline modified with Ce alone can be raised by doping Zr. Moreover, it is showed that there is a higher [Formula: see text] ratio inside the tourmaline modified with Ce and Zr than tourmaline modified with Ce alone. In addition, XRD results indicate the formation of [Formula: see text] and [Formula: see text] crystallites during the heat treatment and further TEM observations show they exist as nanoparticles on the surface of tourmaline powders. Based on these results, we attribute the improved far-infrared emission properties of Ce–Zr doped tourmaline to the enhanced unit cell shrinkage of the tourmaline arisen from much more oxidation of [Formula: see text] to [Formula: see text] inside the tourmaline caused by the change in the catalyst redox properties of [Formula: see text] brought about by doping with [Formula: see text]. In all samples, tourmaline modified with 7.14 wt.% Ce and 1.86 wt.% Zr calcined at 800[Formula: see text]C for 5 h has the best far-infrared emission property with the maximum emissivity value of 98%.

Reorganization of substrate waters between the closed and open cubane conformers during the S2 to S3 transition in the oxygen evolving complex.

A crucial step in the mechanism for oxygen evolution in the Photosystem II complex resides in the transition from the S2 state to the S3 state of Kok–Joliot’s cycle, in which an additional water molecule binds to the cluster. On the basis of computational chemistry calculations on Photosystem II models, we propose a reorganization mechanism involving a hydroxyl (W2) and a μ2-oxo bridge (O5) that is able to link the closed cubane S2B intermediate conformer to the S3 open cubane structure. This mechanism can reconcile the apparent conflict between recently reported water exchange and electron paramagnetic resonance experiments, and theoretical studies.

Native structure of photosystem II at 1.95Å resolution viewed by femtosecond X-ray pulses.

Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 ångström resolution, which revealed that the OEC is a Mn4CaO5-cluster coordinated by a well defined protein environment. However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation, and slight differences were found in the Mn-Mn distances determined by XRD, EXAFS and theoretical studies. Here we report a 'radiation-damage-free' structure of PSII from Thermosynechococcus vulcanus in the S1 state at a resolution of 1.95 ångströms using femtosecond X-ray pulses of the SPring-8 ångström compact free-electron laser (SACLA) and hundreds of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 ångströms. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.

Tailored design of boron-doped diamond electrodes for various electrochemical applications with boron-doping level and sp2 -bonded carbon impurities

The effects of sp2-bonded carbon impurities on the electrochemical properties of boron-doped diamond were investigated in moderately ([B] < 1020 cm−3) and heavily ([B] > 1021 cm−3) boron doping levels. Significant influences of sp2-bonded carbon impurities, which show glassy carbon-like electrochemical properties after anodic oxidation, were observed in heavily boron-doped diamond. This indicated that the significant effects of enhanced adsorption properties were possibly caused by surface relaxation of the strains induced by heavy boron doping and sp2-bonded carbon impurities. On the other hand, their durability was still similar to diamond electrodes rather than glassy-carbon electrodes because of the low fraction of sp2-bonded carbon impurities. Such “active” diamond electrodes are much less suitable for wastewater treatment than ordinary diamond electrodes due to a different oxygen-evolution mechanism. On the other hand, “active” BDD electrodes have a much higher efficiency for electrochemical ozone production than other BDD electrodes. The electrode properties of BDD can be designed by controlling the boron doping level and introducing the sp2-bonded carbon impurities. The guidelines proposed in this study can be used effectively to design electrodes according to their individual application, such as for use as electrochemical sensors, in wastewater treatment or electrochemical ozone production.