Ultrafast Synthesis of Oxygen Vacancy Rich Pt-Doped La0.6Sr0.4CoO3 with Enhanced Oxygen Evolution Activities

In this work, a hard‐template‐based nano‐replication process is employed to prepare mesoporous MnO2 materials. We adopted four silica materials, namely, SBA‐15, KIT‐6, MCM‐41, and MCM‐48 as templates to prepare MnO2 materials by using a nanocasting route. The structural characteristics of the resultant MnO2 materials were thoroughly investigated by using XRD, BET, FESEM, HR‐TEM, XPS, and Raman techniques. All studies confirmed the replication of porous MnO2 nanostructures from silica supports in β‐crystallographic form. By employing X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS), we identified the existence of Mn and oxygen vacancies in the developed MnO2 material. The hydrodynamic linear sweep voltammetric technique was employed to demonstrate the efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activity of the catalyst in 0.1 M KOH solution. The KIT‐6‐derived MnO2 nanostructure displayed equal activity to the benchmark catalyst RuO2. This work lays a foundation to prepare and employ ordered mesoporous MnO2 materials as OER/ORR catalysts, which have potential applications in electrochemical energy conversion and storage devices.

Interaction of photosystem II proteins with non-aggregated membranes constituted of phosphatidylglycerol and the electrically neutral phosphatidylcholine enhances the oxygen-evolving activity

The oxygen evolution reaction is a key reaction in water splitting. The common approach in the development of oxygen evolution catalysts is to search for catalytic materials with new and optimized chemical compositions and structures. Here we report an orthogonal approach to improve the activity of catalysts without alternating their compositions or structures. Specifically, liquid phase exfoliation is applied to enhance the oxygen evolution activity of layered double hydroxides. The exfoliated single-layer nanosheets exhibit significantly higher oxygen evolution activity than the corresponding bulk layered double hydroxides in alkaline conditions. The nanosheets from nickel iron and nickel cobalt layered double hydroxides outperform a commercial iridium dioxide catalyst in both activity and stability. The exfoliation creates more active sites and improves the electronic conductivity. This work demonstrates the promising catalytic activity of single-layered double hydroxides for the oxygen evolution reaction.

IntroductionWe have been studying more than a quarter of century for supply of renewable energy in the form of methane via electrolytic hydrogen generation using carbon dioxide as the feedstock. Although we created cathodes and anodes for direct seawater electrolysis, the energy efficiency was not sufficient. For immediate industrialization, we are concentrating our current effort on alkaline water electrolysis, creating efficient cathodes and anodes. The present work aimed to determine the composition of active Ni-Fe-Co-C alloy anodes prepared by electrodeposition and to examine the durability for continuous electrolysis at a high current density of 6000 Am-2 in 4.5 M KOH at 90°C. Experimental Procedures The substrate for electrodeposition was a chemically etched Ni plate. Electrodeposition was carried out at a current density of 300 Am-2 for 10 min in 1.14 M NiSO4-0.189 M NiCl2-0.49 M H3BO3-0.104 mM CH3(CH2)11OSO3Na solutions containing various concentrations of FeSO4, CoSO4 and lysine, the pH of which was adjusted at 1.5 by the addition of 18 M H2SO4. Lysine was added as the carbon source. The composition of metallic elements in the electrodes was analyzed by electron probe microanalysis, and carbon was chemically analyzed by combustion infrared absorption method. The solution used for oxygen evolution was 4.5 M KOH at 90oC. The activity of the anodes was evaluated from galvanostatic polarization curves measured by the current density rise every 1 min. The potential rise due to the solution resistance at high current densities was corrected by the current interruption technique. Potentiodynamic polarization curves were also measured at a potential sweep rate of 0.52mV-1 for comparison. Results and Discussion As-deposited alloys were used for examination of oxygen evolution activity. Ni was a stable anode but less active for industrial electrolysis. The addition of Co ion and lysine to the deposition electrolyte containing Ni ion enhanced the oxygen evolution activity, but the activity of Ni-Co-C alloys was not yet sufficiently high. Ni-Fe-Co-C alloys deposited showed significantly high activity for oxygen evolution. An increase in Fe content in the alloy was more effective than the increase in Co content for enhancement of oxygen evolution activity. Thus, an increase in Co concentration in deposition electrolytes did not largely change the activity because the increase in Co content in the alloy resulted in the decrease in the content of Fe. The addition of lysine to the deposition electrolytes containing Ni, Fe and Co ions enhanced the oxygen evolution activity. The highest activity was attained at 0.2 M lysine in the deposition solution, but further increase in lysine concentration was not effective. An increase in lysine concentration led to asymptotic increase in the C content in the Ni-Fe-Co-C alloys up to about 6 at%. The deposition electrolyte to form the most active Ni-Fe-Co-C alloy for oxygen evolution was 1.33M Ni-0.072M Fe-0.018M Co-0.2M lysine solution. When the Ni-Fe-Co-C alloy prepared from this electrolyte was used for oxygen evolution at a current density of 6000 Am-2 for 800 h, the mass loss was about 5 % of the original mass. The galvanostatic polarization potential of as-deposited Ni-Fe-Co-C alloy measured by the current density rise every 1 min was in the active region at lower current densities, but the potential shifed to the oxygen evolution region at higher current densities. Once the specimen was polarized in the oxygen evolution region, the potential was stayed in the oxygen evolution region even when the current density was lowered. After oxygen evolution at 6000 Am-2 for long period of time, the potential stayed in the oxygen evolution region regardless of current densities. In fact, the dissolution current density estimated from the mass loss by polarization at 6000 Am-2 for 800 h even on the assumption of Fe dissolution in the form of Fe6+ as FeO4 2- was only 4.5 x 10-3 Am-2 that is less than one-millionth of oxygen evolution current. Consequently, the active Ni-Fe-Co-C alloy anode thus prepared is sufficiently stable maintaining the passive state in the severe environment for oxygen evolution at 6000 Am-2 in a concentrated KOH solution at 90°C for long period of time. Conclusion The active oxygen evolution anode in alkaline water electrolysis for hydrogen production was created. The addition of Fe, Co and C to Ni enhanced the oxygen evolution efficiency in 4.5 M KOH solution at 90°C. The Ni-Fe-Co-C alloy thus prepared kept the passive state during electrolytic oxygen evolution at 6000 Am-2 for a long period of time.

Bipolar performance of the electroplated iron–nickel deposits for water electrolysis

The functional role of cytochrome (cyt) b(559) in photosystem II (PSII) was investigated in H22K alpha and Y18S alpha cyt b(559) mutants of the cyanobacterium Synechocystis sp. PCC6803. H22K alpha and Y18S alpha cyt b(559) mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b(559) in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b(559) in H22K alpha mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b(559) in Y18S alpha mutant PSII core complexes was shifted to 561 nm. Y18S alpha and H22K alpha mutant PSII core complexes contained predominately the low potential form of cyt b(559). The findings lend support to the concept that the redox properties of cyt b(559) are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b(559) mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b(559) in protection of PSII under photoinhibition conditions in vivo.

Phosphatidylglycerol effect on oxygen-evolving activity in Ca2+-depleted photosystem II

Oxygen reduction/evolution bi-functional air electrodes with high activity are required for constructing high-performance metal–air secondary batteries. Conventionally, carbon-supported electrocatalysts have been used as the air electrode materials. However, these types of air electrodes have a problem that the carbon black is corroded to water-soluble organic compounds during oxygen evolution reaction. Therefore, we have investigated reduced graphene oxides as an alternative to the conventional carbon black. Cyclic voltammetry in the range of the potential in which the oxygen evolution reaction occurs revealed that the reduced graphene oxide is stable against corrosion during the oxygen evolution reaction. This result indicates that the reduced graphene oxide is a promising candidate electrode material for bi-functional air electrodes. To improve the oxygen reduction and evolution activities of reduced graphene oxides, doping of nitrogen species and loading of perovskite-type oxide catalysts to reduced graphene oxide were examined. It was found that doping of nitrogen species was effective for improving both the oxygen reduction and evolution activity. LaMnO3 and LaNiO3 catalysts were effective for improving the oxygen reduction and evolution activities, respectively. The best oxygen reduction and evolution activity was obtained by the LaMnO3-loaded nitrogen-doped reduced graphene oxide and LaNiO3-loaded nitrogen-doped reduced graphene oxide, respectively.

INTRODUCTIONIn the future, hydrogen production will be performed by direct electrolysis of seawater. However, every time hydrogen is produced, it is necessary to avoid generating chlorine because it has a negative impact on the ecosystem. We have been developing an oxygen evolution anode for seawater electrolysis that produces only oxygen without chlorine. It has been found that anodically deposited g-MnO2-type Mn1-xMoxSnyO2+xanodes showed the oxygen evolution efficiency for more than 4000 hours in the electrolysis of 0.5 M NaCl [1]. The anodic deposition method has required many steps and further cleaning of the electrolytic diaphragm. For these reasons, in order to commercialize oxygen evolution anodes for seawater electrolysis, it is essential to establish a simple manufacturing method. We are considering fabricating Mn oxide as an active material on an IrO2 intermediate layer formed on a titanium substrate using calcination method.In this study, Mn oxide electrodes were fabricated by coating various concentrations of manganese nitrate butanol solutions on the IrO2intermediate layer and at various calcination temperatures. We have investigated the crystal structure and Mn oxide weight that would have the highest activity for oxygen evolution efficiency.EXPERIMENTALTitanium metal substrates were immersed to roughen enhancing the anchor effect of the substrate on the electrocatalyst layer in concentrated H2SO4. IrO2 intermediate layer was formed by coating on one side of the titanium substrate in 0.26 M Chloroiridic acid with a brush, dried at 363 K for 10 min in air. The other side was coated in the same procedure and dried. And then these substrates was calcined at 723 K for 10 min in air. This procedure was repeated three times but the final calcination of the specimen was continued for 60 min at 723 K in air for calcination. Mn oxide electrocatalyst for oxygen evolution was formed by coating on the IrO2 / Ti in 0.073～0.52 M Mn(NO3)2 butanolsolution with a brush and drying at 353 K for 90 min in the same way as when forming the IrO2 intermediate layer. Calcination was performed at 473, 573, and 723 K. Impurities formed without becoming manganese oxides were electrolytically cleaned in 0.5 M NaCl until no dissolution of the impurities was observed.The performance of the electrode was examined by electrolysis of 0.5 M NaCl solution at 1000Am-2. The oxygen evolution efficiency was estimated by the difference between the total charge passed during electrolysis and the formation charge of chlorine analyzed by iodometric titration. Polarization curves were measured galvanostatically. Correction for IR drop was made with the electrochemical impedance spectroscopy (EIS) method.The characterization of the electrode was carried out by XRD and EPMA.RESULTS AND DISCUSSIONXRD diffraction clarified that Mn₂O₃ were consisted at 723 K, Ramsdellite-type MnO₂ at 673 K, and IrO2 at 573 K.Figure 1 shows the relationship between Mn oxide weight and oxygen evolution efficiency. The oxygen evolution efficiency of the anode consisted of only IrO2 was about 8%. It found that the oxygen evolution efficiency was different due to the difference in calcination temperature, that is, the difference in crystal structure to Mn oxide weight of about 2.5 mgcm-2. The oxygen evolution efficiency of the anode with Ramsdellite-type MnO₂ formed at 573 K was about 57 % at 1.8 mgcm-2. The oxygen evolution efficiency of anodes formed at 473 and 723 K with the equivalent weight was about 52 and 32 % respectively. Previous research has shown that anodes containing the γ-MnO2 formed by anodic deposition have high catalytic activity for oxygen evolution [2]. The γ-MnO2 contains stacking faults in which β-MnO2 is mixed into the ramsdellite-MnO2[3]. The electrode with 4.5 mgcm-2 of Mn₂O₃ at 723 K shows the maximum oxygen generation efficiency, which is about 83%.A future challenge is to add substances such as molybdenum as additional elements to obtain higher oxygen evolution efficiency. In conventional electrode fabrication using anodic deposition, the addition of molybdenum significantly has improved oxygen generation efficiency.REFERENSE[1]Z. Kato, J. Bhattarai , N. Kumagai, K. Izumiya, and K. Hashimoto, Applied. Surface. Science. 257 (2011) 8230.[2] A. A. El-Moneim, N. Kumagai, K. Asami, and K. Hashimoto , Materials Transactions, Vol. 46, No. 2 (2005) pp. 309[3] Jian-Bao LI, K. Koumoto and H. Yanagida, Journal of the Ceramic Society of Japan, 96 [1] (1988)74 Figure 1

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.

The relative photocatalytic activity of a series of lead-IVB group oxides (PbCrO4, PbMoO4, and PbWO4) was studied for hydrogen evolution from aqueous methanol solution and for oxygen evolution from aqueous silver nitrate solution. Among the compounds, only PbMoO4 and platinized PbMoO4 powders acted as photocatalysts for oxygen and hydrogen evolution reactions from aqueous solutions, with an activity for oxygen evolution on PbMoO4 comparable to that observed on TiO2.

Nickel oxide thin films were deposited by reactive sputtering in a 20% oxygen/argon atmosphere for use as oxygen evolution catalysts in the photoelectrochemical production of hydrogen. The optical properties of the films were also characterized to evaluate their application as window layers. The polycrystalline films deposited at residual gas pressures of 6 or 10 mTorr exhibited moderate activity for oxygen evolution in 1 N KOH and pronounced coloration and bleaching during alternating anodic/cathodic bias. Properties of these films were not sensitive to growth rate over the range studied, 0.5 to 4 Å/s. In contrast, films deposited at 2 mTorr exhibited poor activity for oxygen evolution and severely limited electrochromic behavior which we attribute to marked changes in the morphology and crystallinity in the low‐pressure films. The films grown at 6 mTorr and higher tended to be more oriented, to have a higher degree of crystallinity, and higher oxygen content. Strong linkages between the electrochemical and optical behaviors observed in this work provide new insights into the processes involved in oxygen evolution reaction catalysis and electrochromism in reactively sputtered films. The results presented indicate that reactive sites located on or near grain boundaries are responsible for both behaviors.

Advances demonstrate that the incorporation of phosphorous into the network of nitrogen, sulfur, or fluorine‐doped carbon materials can remarkably enhance their oxygen and hydrogen evolution activities. However, the electrocatalytic behaviors of pristine phosphorous single‐doped carbon catalysts toward the oxygen and hydrogen evolution reactions (OER and HER) are rarely investigated and their corresponding active species are not yet explored. To clearly ascertain the effects of phosphorous doping on the OER and HER and identify the active sites, herein, phosphorous unitary‐doped graphite layers with different phosphorous species distributions are prepared and the correlations between the oxygen or hydrogen evolution activity and different phosphorous species are investigated, respectively. Results indicate that phosphorous single‐doped graphite layers show a superior oxygen evolution activity to most of the reported OER catalysts and the commercial IrO2 in alkaline medium, and comparable hydrogen evolution activity to most reported carbon catalysts in acidic medium. Moreover, the relevancies unveil that the COP species are the main OER active species, and the defects derived from the decomposition of C3P = O species are the main active sites for HER, as evidenced by density functional theory calculations, showing a new perspective for the design of more effective phosphorous‐containing water‐splitting catalysts.

The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni‐Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides.

Direct detection of self-reconstruction-accelerated oxygen evolution activity in MoCoNi hydroxides