Oxygen Evolution Efficiency Research Articles

Mn–W Oxide Anodes Prepared by Thermal Decomposition for Oxygen Evolution in Seawater Electrolysis

Manganese-tungsten oxide electrodes have high efficiency for oxygen evolution in seawater electrolysis. Manganese oxide, tungsten oxide and manganese-tungsten oxides were coated on the IrO 2 -coated titanium (IrO 2 /Ti electrode) by thermal decomposition. The oxygen evolution efficiencies at 200 A . m -2 of the manganese oxide and tungsten oxide electrodes in 0.5 M NaCl solution at pH 8 and 30°C were 70 and 68%, respectively. The efficiency for oxygen evolution is enhanced by small amounts of mutual additions into manganese oxide or tungsten oxide. In particular, the efficiencies of α-Mn 2 O 3 -type oxide with 15 mol% W 6+ and WO 3 -type oxide with 10 mol% Mn 2+ go up to over 92 and 86%, respectively. Further increase in additives leads to the formation of MnWO 4 and to a significant decrease in the oxygen evolution efficiency.

Effects of Additional Elements on Electrocatalytic Properties of Thermally Decomposed Manganese Oxide Electrodes for Oxygen Evolution from Seawater

Manganese oxide electrodes with some additives enhance oxygen evolution efficiency in seawater electrolysis. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide (MnOX/IrO2/Ti) and oxide mixtures of manganese and iridium, ruthenium, platinum, iron, cobalt, nickel, tin, lanthanum, cerium or molybdenum ((Mn–M)OX/IrO2/Ti electrode, M: additives) were coated. The oxygen evolution efficiency of the MnOX/IrO2/Ti electrode was 68–70%. The addition of small amounts of nickel, cobalt, iron or tin enhanced the oxygen evolution efficiency. However the addition of excess amounts of these elements and additions of noble metals, cerium or lanthanum were detrimental for the oxygen evolution.Among the additives examined, molybdenum was the most effective additional element to increase the oxygen evolution efficiency. The addition of a small amount of molybdenum leads to a remarkable increase in the oxygen evolution efficiency up to 91%. The formation of a single phase Mn2O3 with molybdenum ions seems to be responsible for the high efficiency for oxygen evolution.

Surface activation of manganese oxide electrode for oxygen evolution from seawater

Utilizing the fact that the equilibrium potential of oxygen evolution is lower than that of chlorine evolution, oxygen evolution in seawater electrolysis was enhanced by decreasing the polarization potential under galvanostatic conditions through increasing the effective surface area of manganese oxide electrodes. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide was coated (MnOX/IrO2/Ti electrode). Subsequently, oxide mixtures of manganese and zinc were coated (MnOX–ZnO/MnOX/IrO2/Ti electrode). The effective surface area of the MnOX–ZnO/MnOX/IrO2/Ti electrodes was increased by selective dissolution of zinc (leaching) into hot 6M KOH. The oxygen evolution efficiency of the MnOX/IrO2/Ti electrode was 68–70%. Leaching of zinc from the MnOX–ZnO/MnOX/IrO2/Ti electrodes with 25mol% or less zinc led to a significant increase in the oxygen evolution efficiency. The maximum efficiency attained was 86% after leaching of zinc from the MnOX–25mol%ZnO/MnOX/IrO2/Ti electrode. However, large amounts of zinc addition, such as 40mol% or more are detrimental because of a decrease in the oxygen evolution efficiency. This is due to the formation of a double oxide, ZnMnO3, which is hardly dissolved in hot 6M KOH.

Growth at high CO2 affects the chloroplast number but not the photosynthetic efficiency of photoautotrophicMarchantia polymorpha culture cells

Photoautotrophic suspension cells of Marchantia polymorpha were grown at gas phase CO2 concentrations of 0.4% and 2.0%. At the higher CO2 concentration the chloroplast shape appeared to be modified and the cells had about 70% more chloroplasts per cell. Differences in chlorophyll content per cell were much less pronounced, indicating a reduction in chlorophyll content per chloroplast. Also the cell size was affected by the CO2 concentration, and our data suggest that it was about 37% lower in high CO2 grown cells than in low CO2 grown cells. The capacity and the efficiency of photosynthetic oxygen evolution on a chlorophyll basis and the photosystem II chlorophyll fluorescence parameters were almost identical in both cell types. Immunodection showed that also the ratio of light harvesting complex II antenna proteins and ribulose 1,5 bisphosphate carboxylase/oxygenase were unaltered. These data indicate that the chloroplast density within photoautotrophic culture cells may be regulated independently of their photosynthetic efficiency.

Chromatic regulation inChlamydomonas reinhardtii alters photosystem stoichiometry and improves the quantum efficiency of photosynthesis

The work addressed the adjustment of the photosystem ratio in the green algaChlamydomonas reinhardtii. It is shown that green algae, much like cyanophytes and higher plants, adjust and optimize the ratio of the two photosystems in chloroplasts in response to the quality of irradiance during growth. Such adjustments are compensation reactions and helpC. reinhardtii to retain a quantum efficiency of oxygen evolution near the theoretical maximum. Results show variable amounts of PS I and a fairly constant amount of PS II in chloroplasts and suggest that photosystem stoichiometry adjustments, occurring in response to the quality of irradiance during plant growth, are mainly an adjustment in the concentration of PS I. The work delineates chromatic effects on chlorophyll accumulation in the chloroplast ofC. reinhardtii from those pertaining to the regulation of the PS I/PS II ratio. The detection of the operation of a molecular feedback mechanism for the PS I/PS II ratio adjustment in green algae strengthens the notion of the highly conserved nature of this mechanism among probably all oxygen evolving photosynthetic organisms. Findings in this work are expected to serve as the basis of future biochemical and mutagenesis experiments for the elucidation of the photosystem ratio adjustment in oxygenic photosynthesis.

Applications of Chlorophyll Fluorescence to Forest Ecology

Functioning of photosystem II (PSII) is the most sensitive indicator of environmental stress in plants. Changes in PSII activity can be assayed rapidly and non-destructively by measurement of chlorophyll fluorescence. While there have been many laboratory studies of chlorophyll fluorescence, fluorescence techniques have seldom been applied to questions in forest ecology. Most studies have emphasised the fluorescence parameter, Fv/Fm, which is well correlated with the quantum efficiency of photosynthetic carbon dioxide assimilation or oxygen evolution. This parameter reveals information which can be related to diurnal and seasonal variation in photosynthesis, plant growth and community dynamics. Thus, fluorescence techniques provide a powerful means of linking photosynthesis with higher levels of plant functioning and have great potential for research in forest ecology.

Post-translational methylation of phycobilisomes and oxygen evolution efficiency in cyanobacteria

A post-translationally methylated asparagine residue, γ-N- methylasparagine (NMA), is found at the β-72 site in many phycobiliproteins. We have examined the effects of asparagine methylation on photosynthetic rates in the wild-type cyanobacterium Synechococcus PCC 7942 ( β-72 = NMA ) and two Synechococcus PCC 7942 methylase mutants ( β-72 = Asn ) that exhibit no detectable methylase activity. Previous studies indicate that phycobilisomes isolated from non-methylated mutants exhibit a 14% decrease in fluorescence quantum yield (Swanson, R.V. and Glazer, A.N. (1990) J. Mol. Biol. 214, 787–796). Relative rates of Photosystem II electron transfer were measured for these strains by monitoring steady state rates of oxygen evolution in whole cells. The methylase-minus mutants demonstrated lower rates of electron transfer through Photosystem II under conditions in which the phycobilisome components were preferentially illuminated with orange light at low intensity. Oxygen evolution rates were indistinguishable for the wild-type and mutant strains upon selective illumination of the chlorophyll-containing light-harvesting antennae with blue light. The observed differences in photosynthetic rates are consonant with the levels of asparagine methylation in the phycobilisomes. These results support the hypothesis that asparagine methylation which is energetically costly yet evolutionarily conserved, is associated with increased energy transfer efficiency through Photosystem II.

Induction of non-bilayer lipid phase separations in chloroplast thylakoid membranes by compatible co-solutes and its relation to therthermal stability of Photosystem II

High concentrations of compatible co-solutes such as sugars, sugar alcohols and polyols induce the phase separation of non-bilayer forming lipids in thylakoid membrane preparations. Of the co-solutes tested, sucrose, trehalose, betaine and sorbitol all led to extensive phase separation at room temperature. Glucose and glycerol were much less effective. Measurements of the temperature dependence of the fluorescence yield of chlorophyll a associated with Photosystem II (PS II), and the relative oxygen evolution efficiency of chloroplasts incubated at elevated temperatures, indicated that the presence of compatible co-solutes led to substantial increases in the threshold temperature for inhibition of PS II-mediated electron transport. The protective effect of the different co-solutes in order of efficiency, was sucrose > trehalose > sorbitol > betaine > glucose > glycerol. In general, the ability of the different co-solutes to stabilise PS II appeared to parallel their ability to induce non-bilayer lipid phase separation. Comparison of the effects of the co-solutes on isolated chloroplasts with that of heat-stress in unprotected chloroplasts suggest that there is no direct causal link between the phase-separation of non-bilayer lipids from the chloroplast membranes and the increased stability of PS II. It is concluded that the two phenomena are separate reflections of the effects of co-solutes on ordered water at lipid and protein interfaces, respectively.

Photosynthetic electron transport in genetically altered photosystem II reaction centers of chloroplasts.

Using a cotransformation system to identify chloroplast transformants in Chlamydomonas reinhardtii, we converted histidine-195 of the photosystem II reaction center D1 protein to a tyrosine residue. The mutants were characterized by a reduced quantum efficiency for photosynthetic oxygen evolution, which varied in a pH-dependent manner, a reduced capacity to oxidize artificial donors to photosystem II, and P680+ reduction kinetics (microsecond) that were essentially similar to wild type. In addition, a dark-stable radical was detected by ESR in mutant photosystem II particles but not in wild-type particles. This radical was similar in g value and lineshape to chlorophyll or carotenoid cations but could have arisen from a tyrosine-195 cation. The ability of the photosystem II trap (P680+) to oxidize tyrosine residues suggests that the mutant tyrosine residue could be used as a redox-sensitive probe to investigate the environment around the photosystem II trap.

Dissipation of excitation energy by Photosystem II particles at low pH

The effect of decreasing the pH on the properties of Photosystem II particles isolated from pea and spinach has been investigated. Between pH 6.5 and 4.0 there was a decrease in quantum efficiency of oxygen evolution that was not associated with a proportional closure of reaction centres, as determined by the fluorescence coefficient, qP. This decrease in the quantum yield of Photosystem II appeared to be due to two separate mechanisms. Between pH 6.5 and 5.0, the decrease in quantum yield was not associated with any change in fluorescence yield or in the ratio of Fv/Fm; the decrease was greater at limiting light than in saturating light and may be caused by a cyclic electron transfer that is associated with the destabilisation of the oxidised intermediates of the oxygen-evolving complex and an accelerated rate of Q−A oxidation. Below pH 5.0 the decrease in efficiency is correlated with a quenching of chlorophyll fluorescence. This quenching was light-dependent and required oxidising conditions; redox titrations indicated that a component with a midpoint oxidation reduction potential of +405 mV at pH 4.0 needed to be oxidised if the low pH-dependent quenching was to be observed. The relationship between these effects of low pH and the ‘down-regulation’ of Photosystem II observed under physiological conditions is discussed.

Modification of the Induction of Photosynthesis in Wheat by Glyphosate, an Inhibitor of Amino Acid Metabolism

Ireland, C. R., Percival, M, P. and Baker, N. R. 1986. Modification of the induction of photosynthesis in wheat by glyphosate, an inhibitor of amino acid metabolism.—J. exp. Bot. 37: 299-308. The effect of the herbicide glyphosate, an inhibitor of amino acid metabolism, upon the kinetics of induction of photosynthesis was investigated in wheat leaves. After treatment with a commercial preparation of glyphosate the induction period for both photosynthetic carbon assimilation and chlorophyll fluorescence emission was increased approximately by 50% after 2 d and by 100% after 5 d. The quenching of chlorophyll fluorescence became a slower process and the S-M transient was diminished. In the steady-state neither the rate of carbon fixation nor the level of fluorescence emission were altered. The changes in induction kinetics preceded the occurrence of visible leaf damage. The fluorescence emission was analysed by a technique which estimates the redox state of the PS2 primary electron acceptor, Q, and the photochemical and non-photochemical components of fluorescence quenching. Glyphosate produced no fundamental change in the relationship between the steady-state values of these parameters until the time when extensive leaf damage occurred. Thus there was no direct effect of glyphosate on the capacity for light capture or on the rate of photosynthetic electron transport. This was confirmed by the absence of any effect on the quantum efficiency of oxygen evolution. The slower rate of fluorescence quenching during induction is attributed to a slower generation of the trans-thylakoid A pH. It is proposed that glyphosate affects photosynthetic induction kinetics by an indirect modification of carbon metabolism which limits photosynthetic rate during this phase. The possibility of screening for rapid effects of non photosynthetic herbicides, such as glyphosate, by monitoring the kinetics of photosynthetic induction, is raised. Key words—Photosynthesis, induction, glyphosate (N-phosphonomethyl-glycine), wheat (Triticum aestivum L.). Correspondence to: Department of Biology, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ, U.K.

Changes in Hydrogen Metabolism during Greening of Pigment Mutant C-2A′ from Scenedesmus obliquus

Capacities of H2-photoproduction and photoreduction were determined in the greening pigment mutant C-2A′ of Scenedesmus obliquus. In the dark grown culture the capacity for H2-photoproduction is low and for photoreduction not detectable. Both processes increase with chloroplast development in the light. Photoreduction increases in parallel to the photosynthetic capacity using H2 instead of water as electron donor. H2 reaches optimal values during the stage of highest quantum efficiency of photosynthetic oxygen evolution, but does not increase further. Hydrogenase of adapted cells is most active in dark grown cultures and declines during greening. This indicates that hydrogenase activity is not the limiting factor in the hydrogen metabolism studied in this investigation. Studies with cells in which the formation of the reaction centers were reversibly suppressed by chloramphenicol demonstrate that hydrogen metabolism depends on intact reaction centers and can not be mediated by light harvesting chlorophylls.

Differential efficiency of photosynthetic oxygen evolution in flashing light in triazine-resistant and triazine-susceptible biotypes of Senecio vulgaris L.

Photosynthetic oxygen evolution in response to flashing light was studied in triazine-susceptible and triazine-resistant biotypes of Senecio vulgaris L. Studies were conducted to determine if the modification of the herbicide-binding site which confers s-triazine resistance also affects the oxygen-evolving system. Oxygen evolution was measured using a Joliot-type oxygen-specific electrode on broken, stroma-free chloroplasts of both biotypes. We observed abnormal patterns of oxygen evolution in resistant chloroplasts. The S′ 1 → S 2 transition is slower while the S 2 decay is faster. The S′ 2 → S 3 transition, in contrast, is slightly faster in resistant chloroplasts, while the decay of the S 3 state is the same as in susceptible chloroplasts. These altered kinetics may be due to altered Q → B (B −) electron flow in resistant chloroplasts. These results are also consistent with the hypothesis that back-reactions from the reducing (acceptor) side of Photosystem II to the oxidizing (donor) side occur with greater frequency in resistant than susceptible chloroplasts. These events are responsible for lower oxygen yield and increased ‘misses’ and ‘double hits,’ resulting in abnormal yield patterns and lower quantum yield of CO 2 fixation in resistant chloroplasts compared to the susceptible ones.

Chilling Damage to Photosynthesis in Young Zea mays

Chilling Damage to Photosynthesis in Young Zea mays

Electrodes for Alkaline Water Electrolysis

Electrodes for alkaline water electrolysis have been made by applying high specific surface area coatings of nickel or nickel‐iron alloy to steel or nickel substrates. The coatings are applied as polysilicate‐based paints containing particles of the desired metals. The coating is sintered into a porous structure which is bonded firmly to the substrate. The present electrode preparation method has been used to coat a variety of substrate forms, such as flat plates or wire screens, and is compatible with commercial alkaline electrolysis equipment. The resulting electrodes were found to be particularly effective as anodes for oxygen evolution. The efficiency of the electrodes was greatly influenced by coating microstructure. This microstructure, in turn, could be controlled by adjusting the sintering conditions. Electrochemical operating characteristics of the electrodes in at 80°C were determined. Comparable oxygen evolution efficiencies were obtained with coatings made from nickel powders, nickel flake, and nickel‐iron alloy powder.

Dichlorophenyl dimethylurea (DCMU) induced increase in chlorophyll a fluorescence intensity – An index of photosynthetic oxygen evolution in leaves, chloroplasts and algae

AbstractDichlorophenyl dimethylurea (DCMU) treatment in photosynthetic samples resulted in an increase in the level of steady state chlorophyll a fluorescence at room temperature which was directly proportional to the photosynthetic efficiency. The applicability of this method for the rapid determination of the efficiency of oxygen evolution in leaves, algae, mesophyll cells and chloroplasts has been investigated. Especially reliable values with less than 5% error were obtained if the fluorescence measurements were made under low excitation intensities with a sample chlorophyll concentration below 1.0 μg/ml.

Effects of Tin, Antimony and Molybdenum in Mn-W-O /Ir<sub>1-x-y</sub>Sn<sub>y</sub>O<sub>2+0.5y</sub>/Ti Anodes for Oxygen Evolution in Seawater Electrolysis

An attempt was made to enhance the oxygen evolution efficiency in seawater electrolysis by the addition of tin, antimony and molybdenum to Mn-W oxide electrocatalysts prepared by anodic deposition on the intermediate Ir1-x-ySnyO2+0.5y/Ti electrode. Ir1-x-ySnyO2+0.5y/Ti supported nanocrystalline γ-MnO2 type Mn-W-X-O(X=Sn,Sb,Mo) electrocatalysts with grain size of about 5-11 nm were tailored by anodic deposition. All the examined Mn-W-X(X=Sn,Sb,Mo)- O anodes showed the almost 100% oxygen evolution efficiency at 1000 A.m-2 in 0.5 M NaCl solution of pH 1 at 25°C. They guaranteed the stable anode performance of about 99.75-99.85% oxygen evolution efficiencies for more than five months. Keywords: Global warming; CO2 recycling; Oxygen evolution anode; Seawater electrolysis; Hydrogen production electrode. DOI: 10.3126/sw.v8i8.3844 Scientific World Vol.8(8) 2010 pp.39-43

Roles of Antimony and Molybdenum Additions in Mn–Sn–W–O/IrO<sub>2</sub>–SnO<sub>2</sub>–Sb<sub>2</sub>O<sub>5</sub>/Ti Anodes for Oxygen Evolution in Seawater Electrolysis

An attempt was made to enhance the oxygen evolution efficiency in seawaterelectrolysis by additions of antimony and molybdenum to manganese–tin–tungsten oxideelectrocatalyst prepared by anodic deposition on the intermediate Ir1–x–ySnxSbyO2+0.5y/Tielectrode. Ir1–x–ySnxSbyO2+0.5y/Ti supported nanocrystalline g–MnO2 type Mn–Sn–W–X(X=Sbor/and Mo)–O electrocatalysts with grain size of about 4-8 nm were tailored by anodicdeposition and these anodes showed almost 100 % oxygen evolution efficiency at currentdensity of 1000 A.m-2 in 0.5 M NaCl solution of pH 1 at 25°C. They guaranteed the stableanode performance of 99.60-99.85% oxygen evolution efficiency for more than 5 months atcurrent density of 1000 A.m-2 in 0.5 M NaCl solution of pH 1 at 25°C.Keywords: Oxygen evolution anodes, electrocatalyst, CO2 recycle, XRD, SCLM.DOI: 10.3126/jncs.v25i0.3277Journal of Nepal Chemical Society Volume 25, 2010 pp 9-18

Effects of Ir, Sn and Sb in the intermediate Ir<sub>1-x-y</sub> Sn<sub>x</sub>Sb<sub>y</sub>O<sub>2+0.5y</sub> layer of Mn-Mo-Sn-O electrocatalyst for hydrogen production

An attempt is made to reduce the use of IrO2 by the IrO2-SnO2-Sb2O5 intermediate layer which is necessary to avoid the growth of insulating titanium oxide on the titanium substrate for oxygen evolution ã-MnO2 type nanocrystalline Mn-Mo-Sn-O/Ir1-x-ySnxSbyO2+0.5y/Ti anodes in 0.5 M NaCl solution of pH 1 at 25°C. These electrodeposited Mn-Mo-Sn-based triple oxide anodes showed about 97.2-98.6 % oxygen evolution efficiency at high current density of 1000 A.m-2 in 0.5 M NaCl solution of pH 1 at 25°C. The high oxygen evolution efficiency of these triple oxide electrocatalysts maintained during prolonged electrolysis and even after the electrolysis for about two months. Key-words: Oxygen evolution efficiency; electrocatalyst; seawater electrolysis; CO2 recycling; XRD DOI: 10.3126/botor.v7i0.4387Botanica Orientalis - Journal of Plant Science (2010) 7: 79-84

The Development of the Intermediate SnO<sub>2</sub>–Sb<sub>2</sub>O<sub>5</sub> Layer on Titanium Substrate for Oxygen Evolution Anodes in Seawater Electrolysis

An attempt is made to replace the use of IrO2 by SnO2–Sb2O5 in the intermediate layer which is necessary to avoid the growth of insulating titanium oxide on the titanium substrate for oxygen evolution γ-MnO2 type Mn1-x-yMoxSnyO2+x anodes in seawater electrolysis. The manganese–molybdenum–tin triple oxides, Mn1-x-yMoxSnyO2+x, prepared by anodic deposition on the SnO2–Sb2O5-coated titanium substrate from MnSO4, Na2MoO4 and SnCl4 solutions showed around 98.6% initial oxygen evolution efficiency at a current density of 1000 Am-2 in 0.5 M NaCl of pH 1 at room temperature. In order to increase the stability of the anodes, coating at various times to form the intermediate SnO2–Sb2O5 layer with sufficient thickness on titanium substrate, was performed. The Mn1-x-yMoxSnyO2+x electrodes deposited on the intermediate layer formed from seven times coating showed about 98% oxygen evolution efficiency after 20 h electrolysis. A small addition of Sb2O5 to the intermediate layer (that is, Sb5+/Sn4+ = 0.124 in the coating solution) seems to be more effective to replace the use of IrO2 for high electronic conductivity and activity of oxygen evolution in seawater electrolysis. The formation of the double oxides of the intermediate SnO2–Sb2O5 layer after seven times coating seemed responsible for both high conductivity and stability of the Mn1-x-yMoxSnyO2+x anodes.Keywords: CO2 recycling, oxygen evolution electrode, intermediate SnO2–Sb2O5 layer, seawater electrolysis, titanium substrate.DOI: 10.3126/jncs.v23i0.2097J. Nepal Chem. Soc., Vol. 23, 2008/2009 Page: 54-64