Formation Of Oxyhydroxides Research Articles

Supersaturated Doping-Induced Maximized Metal-Support Interaction for Highly Active and Durable Oxygen Evolution.

Metal-support interaction (MSI) is pivotal and ubiquitously used in the development of next-generation catalysts, offering a pathway to enhance both catalytic activity and stability. However, owing to the lattice mismatch and poor solubility, traditional catalysts often exhibit a metal-on-support heterogeneous structure with limited interfaces and interaction and, consequently, a compromised enhancement of properties. Herein, we report a universal and tunable method for supersaturated doping of transition-metal carbides via strongly nonequilibrium carbothermal shock synthesis, characterized by rapid heating and swift quenching. Our results enable ∼20 at. % Ni2FeCo doping in Mo2C, significantly surpassing the thermodynamic equilibrium limit of <3 at. %. The supersaturation ensures more catalytically active NiFeCo doping and sufficient interaction with Mo2C, resulting in the maximized MSI (Max-MSI) effect. The Max-MSI enables outstanding activity and particularly stability in alkaline oxygen evolution reaction, showing an overpotential of 284 mV at 100 mA cm-2 and stable for 700 h, while individual Ni2FeCo and Mo2C only last less than 70 and 10 h (completely dissolved), respectively. In particular, the SD-Mo2C catalyst also exhibits excellent durability at 100 mA cm-2 for up to 400 h in 7 M KOH. Such a significantly improved stability is attributed to the supersaturated doping that led to each Mo atom strongly binding with adjacent heteroatoms, thus elevating the dissolution potential and corrosion resistance of Mo2C at a high current density. Additionally, the highly dispersed NiFeCo also facilitates the formation of dense oxyhydroxide coating during reconstruction, further protecting the integrated catalysts for durable operation. Furthermore, the synthesis has been successfully scaled up to fabricate large (16 cm2) electrodes and is adaptable to nickel foam substrates, indicating promising industrial applications. Our strategy allows the general and versatile production of various highly doped transition-metal carbides, such as Ni2FeCo-doped TiC, NbC, and W2C, thus unlocking the potential of maximized or adjustable MSI for diverse catalytic applications.

Selenate oxyanion-intercalated NiFeOOH for stable water oxidation via lattice oxygen oxidation mechanism

Transition metal-based compounds can serve as pre-catalysts to obtain genuine oxygen evolution reaction (OER) electrocatalysts in the form of oxyhydroxides through electrochemical activation. However, the role and existence form of leached oxygen anions are still controversial. Herein, we selected iron selenite-wrapped hydrated nickel molybdate (denoted as NiMoO/FeSeO) as a pre-catalyst to study the oxyanion effect. It is surprising to find that SeO42− exists in the catalyst in the form of intercalation, which is different from previous studies that suggest that anions are doped with residual elements after electrochemical activation, or adsorbed on the catalyst surface. The experiment and theoretical calculations show that the existence of SeO42− intercalation effectively adjusts the electronic structure of NiFeOOH, promotes intramolecular electron transfer and O–O release, and thus lowers the reaction energy barrier. As expected, the synthesized NiFeOOH-SeO only needs 202 and 285 mV to attain 100 and 1000 mA cm−2 in 1 M KOH. Further, the anion exchange membrane water electrolyzer (AEMWE) consisting of NiFeOOH-SeO anode and Pt/C cathode can reach 1 A cm−2 at 1.70 V and no significant attenuation within 300 h. Our findings provide insights into the mechanism, by which the intercalated oxyanions enhance the OER performance of NiFeOOH, thereby facilitating large-scale hydrogen production through AEMWE.

Leptothrix ochracea genomes reveal potential for mixotrophic growth on Fe(II) and organic carbon.

Winogradsky's observations of L. ochracea led him to propose autotrophic iron oxidation as a new microbial metabolism, following his work on autotrophic sulfur-oxidizers. While much culture-based research has ensued, isolation proved elusive, so most work on L. ochracea has been based in the environment and in microcosms. Meanwhile, the autotrophic Gallionella became the model for freshwater microbial iron oxidation, while heterotrophic and mixotrophic iron oxidation is not well-studied. Ecological studies have shown that Leptothrix overtakes Gallionella when dissolved organic carbon content increases, demonstrating distinct niches. This study presents the first near-complete genomes of L. ochracea, which share some features with autotrophic iron oxidizers, while also incorporating heterotrophic metabolisms. These genome, metabolic modeling, and transcriptome results give us a detailed metabolic picture of how the organism may combine lithoautotrophy with organoheterotrophy to promote Fe oxidation and C cycling and drive many biogeochemical processes resulting from microbial growth and iron oxyhydroxide formation in wetlands.

Preparation of Oriented Nanoporous Magnetite Films by a Template-Free Solution Process from Iron Oxyhydroxide Films

Nanoporous oxide films composed of nanocrystal assemblies with an aligned crystallographic orientation are key nanostructures for efficient interfacial reactions. However, a simple, template-free method for their formation remains a challenge. Layered metal hydroxide (LMH) films are a promising precursor for spontaneous formation of nanoporous oxide films.[1-4] LMHs have a layered structure comprising 2D sheets of edge-shared octahedral hydroxide, i.e., brucite structure, with or without interlayer anions and water molecules. Calcination of LMH results in dehydration, anion elimination, and volume shrinkage, leading to the spontaneous formation of nanoporous oxides.In this study, we have prepared nanoporous and ferrimagnetic magnetite (Fe3O4) films by using layered iron hydroxides, namely, green rust (GR), as a precursor. Generally, GR has a brucite-type structure containing Fe2+ and Fe3+ ions, and is highly susceptible to oxidation by oxygen in water and air, leading to the formation of iron oxyhydroxides (FeOOH). Here, GR films were electrochemically deposited on FTO substrates in an aqueous solution containing Fe2+ and NO3 − ions at room temperature. Drying the GR films in air resulted in the formation of two-type FeOOH films, depending on the electrodeposition condition. Characterization using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and FT-IR spectroscopy revealed that one is [010]-oriented γ-FeOOH films, and the other is [001]-oriented δ-FeOOH films. The obtained FeOOH films had a mille-feuille like morphology with 500–700 nm thickness. Heat treatment of these two-type FeOOH films under vacuum at 500 °C resulted in the formation of [110]- and [111]-oriented nanoporous Fe3O4 films. The obtained Fe3O4 films showed a ferrimagnetic behavior in VSM measurements and had a nanoporous structure comprising nanoparticles with 50–100 nm diameter.Finally, a possilble interpretation for the observed orientation relationships between the FeOOH and Fe3O4 was also discussed on the basis of pyrolysis intermediates of FeOOH and similarity in their atomic arrangements. References T. Shinagawa, M. Watanabe, T. Mori, J.-i. Tani, M. Chigane and M. Izaki, Inorg. Chem., 57, 13137 (2018).T. Shinagawa, M. Chigane and M. Izaki, ACS Omega, 6, 2312 (2021).T. Shinagawa, M. Chigane and M. Takahashi, Cryst. Growth Des., 22, 4122 (2022).T. Shinagawa, N. Kotobuki and A. Ohtaka, Nanoscale Adv., 5, 96 (2023). Figure 1

Concurrent arsenic, fluoride, and hydrated silica removal from deep well water by electrocoagulation: Comparison of sacrificial anodes (Al, Fe, and Al–Fe)

Some studies have reported the removal of As (As) and fluoride (F−) using different sacrificial anodes; however, they have been tested with a synthetic solution in a batch system without hydrated silica (SiO2) interaction. Due to the above, concurrent removal of As, F−, and SiO2 from natural deep well water was evaluated (initial concentration: 35.5 μg L−1 As, 1.1 mg L−1F−, 147 mg L−1 SiO2, pH 8.6, and conductivity 1024 μS cm−1), by electrocoagulation (EC) process in continuous mode comparing three different configurations of sacrificial anodes (Al, Fe, and Al–Fe). EC was performed in a new reactor equipped with a small flow distributor and turbulence promoter at the entrance of the first channel to homogenize the flow. The best removal was found at j = 5 mA cm−2 and u = 1.3 cm s−1, obtaining arsenic residual concentrations (CAs) of 1.33, 0.45, and 0.77 μg L−1, fluoride residual concentration (CF−) of 0.221, 0.495, and 0.622 mg L−1, and hydrated silica residual concentration (CSiO2) of 21, 34, and 56 mg L−1, with costs of approximately 0.304, 0.198, and 0.228 USD m−3 for the Al, Fe and Al–Fe anodes, respectively. Al anode outperforms Fe and Al–Fe anodes in concurrently removing As, F− and SiO2. The residual concentrations of As and F− complied with the recommendations of the World Health Organization (WHO) (As < 10 μg L−1 and F− < 1 mg L−1). The spectroscopic analyses of the Al, Fe, and Al–Fe aggregates showed the formation of aluminosilicates, iron oxyhydroxides and oxides, and calcium and sodium silicates involved in removing As, F−, and SiO2. It is concluded that Al would serve as the most suitable sacrificial anode.

The role of biotite in the deterioration of pyrrhotite within concrete aggregates: Trois-Rivières case study

In Trois-Rivières, Canada, numerous commercial and residential structures suffered severe concrete degradation due to internal sulfate attack (ISA). The deterioration was attributed to the use of sulfide-bearing aggregates obtained from nearby quarries. The objective of this study was to investigate the mechanisms involving pyrrhotite oxidation and the role of biotite in this process. Detailed analysis, including reflected light optical characterization, mineral chemistry, and EPMA chemical mapping, revealed significant oxidation of pyrrhotite within damaged concrete, resulting in the formation of iron oxyhydroxides as secondary phases. Furthermore, a textural association between biotite mica and pyrrhotite was observed in fresh and damaged aggregate samples. While the chemical composition of biotite remains unchanged, it accelerates the deleterious reactions when in direct contact with pyrrhotite. As oxidation proceeds, naturally occurring biotite crystals near pyrrhotite act as pathways, facilitating the ingress of oxygen and water while simultaneously channeling the release of iron and sulfur compounds.

Synergistic Fe and Co binary single atoms based air cathodes for high performance and ultra-stable Zn-air batteries

Cost-effective highly efficient and stable air cathodes are critical for practical applications of rechargeable Zn-air batteries (ZABs). In this work, air cathodes constructed by loading carbon nanotube supported N-doped carbon anchored Fe and Co binary single atoms, f-Fe1Co1/CNT, in carbon paper composited nickel foams, NF/CP, were developed for ZABs, exhibiting outstanding bifunctional oxygen electrocatalytic activities (ΔE = 0.671 V) with a half-wave potential (E1/2) of 0.880 V (vs. RHE) for oxygen reduction reaction (ORR) and an overpotential of 321 mV at 10 mA cm−2 (η10) for oxygen evolution reaction (OER) in 0.1 M KOH, which outperformed the benchmark (Pt/C+IrO2)-based air cathode (ΔE = 0.773 V, E1/2 = 0.850 V, η10 = 393 mV). With the unique air cathode design of compositing catalyst-loading layer (nickel foam) with gas diffusion layer (carbon paper), the f-Fe1Co1/CNT@NF/CP//Zn ZAB delivered a remarkable discharge peak power density of 282.1 mW cm−2 at an ultra-high current density of 427.7 mA cm−2 and maintained an ultra-small potential gap of 0.75 V after operations at 10 mA cm−2 for 3300 cycles (1100 h), largely outperforming the (Pt/C+IrO2)@NF/CP//Zn ZAB (162.4 mW cm−2 at 247.3 mA cm−2, 0.86 V, 590 cycles). Density functional theory calculations revealed the positive long-range synergy between Fe-N4 and Co-N4 single atoms, significantly reducing the free energies of the rate-determining steps, oxygen protonation for ORR and formation of oxyhydroxides for OER at the main active site Fe-N4, to realize the much enhanced oxygen electrocatalytic activities.

Accurate construction of cobalt vacancies in Co3O4 to promote oxyhydroxide formation for water oxidation

Accurate construction of cobalt vacancies in Co3O4 to promote oxyhydroxide formation for water oxidation

Role of Water during the Early Stages of Iron Oxyhydroxide Formation by a Bacterial Iron Nucleator.

Understanding the nucleation of iron oxides and the underlying hydrolysis of aqueous iron species is still challenging, and molecular-level insights into the orchestrated response of water, especially at the hydrolysis interface, are lacking. We follow iron(III) hydrolysis in the presence of a synthetic bacterial iron nucleator, which is a magnetosome membrane specific peptide, by using a constant pH titration technique. Three distinct hydrolysis regimes were identified. Interface-selective sum frequency generation (SFG) spectroscopy was used to probe the interfacial reaction and water in direct contact with the peptide. SFG data reveal that iron(III) species react quickly with interfacial peptides while continuously enhancing water alignment into the later stages of hydrolysis. The gradually aligning water molecules are associated with initially promoted (regimes I and II) and later suppressed (regime III) hydrolysis after the saturation of water alignment has occurred until regime II. These interfacial insights are crucial for understanding the early stage of iron oxide biomineralization.

In-situ fabrication of bimetallic FeCo2O4-FeCo2S4 heterostructure for high-efficient alkaline freshwater/seawater electrolysis

Rational construction of bifunctional electrocatalysts with long-term stability and high electrocatalytic activity is of great importance, but it is challenging to obtain highly efficient non-precious metal-based catalysts for overall seawater electrolysis. Herein, a nickel foam (NF) self-supporting CoFe-layered double hydroxide (CoFe-LDH/NF) was directly converted into FeCo2O4-FeCo2S4 heterostructure via hydrothermal method in 50 mM Na2S solution, instead of FeCo2O4@FeCo2S4 core-shell structure. The FeCo2O4-FeCo2S4 heterojunction shows nanosheets structure with rough surface (the thickness of ∼ 198.9 nm), which provides rich oxide/sulfide interfaces, high electrochemical active area, a large number of active sites, as well as fast charge and mass transfer. In 1.0 M KOH solution, 1.0 M KOH + 0.5 M NaCl, and alkaline natural seawater, the FeCo2O4-FeCo2S4 heterojunction exhibits eminently electrocatalytic performance, with overpotentials of η-100 = 225 mV, η-100 = 233 mV, and η-100 = 238 mV for OER, as well as η-100 = 271 mV, η-100 = 273 mV, and η-100 = 277 mV for HER, respectively. Furthermore, self-supporting FeCo2O4-FeCo2S4 electrode (FeCo2O4-FeCo2S4/NF) as the cathode and anode of an electrolyzer exhibits a lower cell voltage of E-100 = 1.75 V in alkaline seawater than those of FeCo2S4/NF (1.77 V), CoFe-LDH/NF (1.87 V), and FeCo2O4/NF (1.91 V). Specifically, FeCo2O4-FeCo2S4 electrolyzer can stably produce hydrogen for over 48 h in alkaline freshwater/seawater electrolyte. These outstanding electrocatalytic performances and corrosion resistance to salty-water can be attributed to the surface reconstruction behavior of the FeCo2O4-FeCo2S4/NF catalyst during OER, which leads to the in-situ formation of metal oxyhydroxides. In particular, the FeCo2O4-FeCo2S4 heterojunction is also very competitive among most state-of-the-art non-noble metal-based catalysts, whether in KOH or alkaline salty-water electrolytes.

Anodic/Cathodic Properties of Ni Based Catalysts for Anion Electrolyte Membrane Water Electrolysis

Anion electrolyte membrane water electrolysis (AEM WE) using non-precious metal catalyst is one of the most prospective systems for pure hydrogen generation. The Ni based oxide shows relatively lower overpotential of oxygen evolution reaction (OER) by adding the transition metals of Co, Fe and Mn, and approaches that of precious metal of Ir based oxide. Our previous study reported that the Ni-Co based catalyst was highest OER activity at operating temperature. The crystallized Ni-Co metal-based core particles with amorphous Ni oxyhydrates of top surface (shell) was confirmed to be preferable to obtain the higher OER activity by rotating disk electrode method, transmission electron microscopy and DFT calculation. Moreover, the fused aggregated network microstructure of Ni-Co based catalyst assisted to show a metallically electronic conductivity. In this study, we evaluated both OER and hydrogen evolution reaction (HER) activity of Ni based catalysts to confirm the prospective non-precious metal catalysts for AEM WE.Each Ni based catalysts with additive of transition metals were synthesized by the flame oxide-synthesis method.1 The OER and HER activities of these catalysts were evaluated in 1 M KOH at 20 to 80 oC by use of the RDE. The Ni-Co based catalyst was highest OER activity in these Ni based catalysts obtained above. The amorphous top surface layer was correlated with a negative shift in the oxyhydroxide formation peak potential from the results of DFT calculations. 2 The HER activity of the Ni-Fe based catalyst also showed the higher OER activity. Especially, the Ni-Fe based catalyst had highest HER activity in these Ni cased catalysts above. The HER activity enhanced by the construction of well crystallized top surface in comparison with its amorphous or poorly crystalline ones. DFT calculation indicated that the defective or disordered surface was not as active for the HER. These results will provide a strategy of Ni based catalysts optimization for AEM. Acknowledgement This work was partially supported by funds for the JSPS KAKENHI (20H02839), and the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11, 5222 (2021). Figure 1

Spintronic State‐Induced Interface Reconfiguration of 2H‐Perovskite Related Oxides for Pseudocapacitance Increase

AbstractPerovskite oxides are able to realize pseudocapacitive energy storage through oxygen anion intercalation. Herein, it is demonstrated that, based on the three 2H‐perovskite related oxides Sr6Co5O15‐x, Sr5Co4O12‐x, and Sr5Co3NiO12‐x as the research objects, metal oxyhydroxides generated through surface reconfiguration during electrochemical processes can contribute extra capacity besides the oxygen‐anion intercalation. This is because the electron spin state controlled by the transition metal valence state in the pseudo trigonal prisms for the 2H‐perovskite structure will affect the Jahn–Teller (JT) distortion. The strong JT distortion of Co2+ and Ni3+ would elongate metal‐O bonds, thereby helping the formation of metal oxyhydroxides on the surface of the pristine material. Among the three 2H‐perovskite related oxides, the specific capacity of Sr5Co3NiO12‐x having the most Co2+ and Ni3+ contents showed the highest capacity increase. This study will provide a promising pathway to develop advanced perovskite‐like oxide pseudocapacitive materials.

In-situ construction of porous Fe/Ni/Co-phosphide heterostructures with electron redistribution for the efficient water oxidation reaction

Electrochemical water splitting efficiency is mainly limited by the kinetically sluggish and high overpotential oxygen evolution reaction (OER) at the anode, compared to the hydrogen evolution reaction (HER) at the cathode. Substantial efforts have been carried out to overcome the above limitations. In this work, highly active porous Fe/Ni/Co-phosphide heterostructures with electron redistribution and tailored porosity have been developed for OER catalysis application. The well-developed porosity is observed by HRTEM analysis, and the XPS analysis confirms the electronic charge redistribution. The as-synthesized catalyst exhibited an extremely low overpotential of 200 mV to reach a current density of 10 mA cm−2 and 215 mV at 50 mA cm−2 in 1 M KOH with a small Tafel slope of 42 mV dec−1 benefiting from the synergistic advantages of active heterostructures together with porous nature and regulation of the electronic structure. Also, the catalyst showed steady long-term durability during the 35 h of continuous chronopotentiometry test. The catalyst showed a high faradic efficiency of 95.8%, measured by the Rotating Ring Disk Electrode (RRDE) experiment. RRDE experiment validates the evolution of oxygen from the water oxidation process. The post-TEM analysis of the catalysts suggests that the OER catalysis goes through the formation of active species like Ni(OH)2, NiOOH, FeOOH, and CoOOH. The formation of surface oxy-hydroxides and hydroxides is responsible for the excellent OER catalytic activity.

Novel electrodeposited NiFeP/Zn bifunctional catalytic coating for alkaline water splitting

The shift towards renewable energies has promoted research into electrolysis for the obtention of pure hydrogen since this is a promising energy vector. However, electrolysis is limited due to its low efficiency. In this work, a novel NiFeP/Zn coating electrodeposited on AISI 304 steel is proposed as a bifunctional catalytic electrode for electrochemical water splitting. The leaching of Zn from the coating allows the formation of a diversified surface morphology of the electrode, obtaining an active area 1164 times greater than that of a NiFeP coating. The leaching process also allows the formation of Ni oxides, hydroxides and oxyhydroxides, which are beneficial catalytic compounds for the reactions of interest. XRD characterization indicates the presence of an amorphous structure of the coating due to the presence of P, which provides better catalytic performance for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as better corrosion behavior of the electrodes. The electrocatalytic performance of the coating shows overpotentials of 163 mV for HER, and 262 mV for OER in 1 M NaOH at |10| mAcm−2. The full cell test using the NiFeP/Zn coating as bifunctional electrode shows a remarkably low overpotential equal to 1.65 V at 10 mA cm−2 for overall water splitting.

Spinel-Derived Formation and Amorphization of Bimetallic Oxyhydroxides for Efficient Electrocatalytic Biomass Oxidation

Replacing the oxygen evolution reaction (OER) with water-assisted oxidation of organic molecules represents a promising approach for achieving sustainable electrochemical biomass utilization. Among numerous OER catalysts, spinels have received substantial attention due to their manifold compositions and valence states, yet their application in biomass conversions remains rare. Herein, a series of spinels were investigated for the selective electrooxidation of furfural and 5-hydroxymethylfurfural, two model substrates for versatile value-added chemical products. Spinel sulfides universally exhibit superior catalytic performance compared to that of spinel oxides, and further investigations show that the replacement of oxygen with sulfur led to the complete phase transition of spinel sulfides into amorphous bimetallic oxyhydroxides during electrochemical activation, serving as the active species. Excellent values of conversion rate (100%), selectivity (100%), faradaic efficiency (>95%), and stability were achieved via sulfide-derived amorphous CuCo-oxyhydroxide. Furthermore, a volcano-like correlation was established between their BEOR and OER activities based on an OER-assisted organic oxidation mechanism.

Formation mechanism of surface modified iron oxide nanoparticles using controlled hydrolysis reaction in supercritical CO2

In the synthesis of surface modified nanoparticles (NPs), supercritical CO2 is one of appealing reaction fields to realize simple and green chemistry approach without using an organic solvent. In this work, surface modified iron oxide NPs was synthesized in supercritical CO2 with various amounts of water between 5.0 and 90.0 mmol to elucidate the formation mechanism of surface modified NPs with the hydrolysis reaction. The synthesis was performed at 30.0 ± 0.9 MPa of CO2, 18 h and 100 °C, where iron(Ⅲ) acetylacetonate, water and oleic acid were used as starting materials. As a result, the smaller amount of water was revealed to be key factor to synthesize more densely modified iron oxide NPs without the aggregation and iron oxyhydroxide formation while the larger amount of water caused the sever aggregation of less modified NPs and the formation of iron oxyhydroxide. Additionally, the detailed analysis of iron oleate complex and the phase state prediction suggested that the smaller amount of water allowed more dominated formation of iron oleate and subsequent its hydrolysis reaction in uniform supercritical CO2 phase, resulting in the formation of densely modified NPs without the aggregation.

Effect of Proportions of Γ and Iron–Zinc Solid Solution Phases on the Corrosion Prevention Performance of Mixed Coating Layers on Heated Galvannealed Steel Sheets

Coating layers having different ratios of a Γ phase to an Fe–Zn solid solution phase were produced on galvannealed steel sheets by heat treatment. The corrosion prevention characteristics of these layers were then investigated using combined cyclic corrosion test, electrochemical measurements and analyses of corrosion products. A coating layer with a larger proportion of the Γ phase showed better corrosion prevention properties along with a smaller corrosion mass loss. The Γ phase exhibited a sacrificial corrosion protection effect with regard to both the Fe–Zn solid solution and the steel substrate while the Fe–Zn solid solution showed a sacrificial corrosion protection effect on the steel substrate. The corrosion products formed from the Γ phase comprised Zn5(OH)8Cl2·H2O, Zn5(CO3)2(OH)6, ZnO and various amorphous compounds, while the Fe–Zn solid solution generated Zn-containing ferrihydrite and amorphous compounds. The presence of Zn evidently stabilized the ferrihydrite and delayed the formation of crystalline iron oxyhydroxide based on comparisons with corrosion products formed on cold-rolled steel sheets. These effects improved the corrosion prevention performance of the Fe–Zn solid solution. The zinc-based corrosion products formed from the Γ phase provided even better corrosion prevention performance by delaying the formation of crystalline iron oxyhydroxide and were also themselves resistant to reduction.

Heterostructured Mo and Co-based phosphides as high-performance bifunctional electrocatalysts for overall water splitting.

Transitional metal phosphides are efficient and durable electrocatalysts for water splitting. In this work, Mo-CoP/Co2P/NF heterostructures are reported to exhibit bifunctional electrocatalyst properties in various electrolytes. The Co phosphides were found to be possessing a hydrogenase-like structure in these heterostructures with P as the proton-acceptor site and Co as the hydride-acceptor site, making them highly active during the HER process. Moreover, the electronic structure of Co ions could be modified, or the transfer of electrons could be accelerated due to the different valence states of Mo. Additionally, the Mo centers possessed superior adsorption properties toward hydrogen. Consequently, excellent performance for the electrocatalytic HER was exhibited by the Mo-CoP/Co2P/NF-300 heterostructure with small overpotentials of 86.6 mV and 48 mV at a current density of 10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 solutions, respectively. Furthermore, it also exhibited efficient OER activity in an alkaline solution with a low overpotential of 245 mV at 30 mA cm-2. Post-analysis revealed the changes in the surface and the formation of Co oxyhydroxide during the OER process, and the formation of Co-P-O during the HER process. The high HER and OER performances are attributed to these transformations of morphologies and compositions. Consequently, a two-electrode electrolyzer based on Mo-CoP/Co2P/NF-300 required voltages of 1.59 V and 1.703 V at 20 mA cm-2 and 100 mA cm-2, respectively, and maintained long-term stability.

What chemical reaction dominates the CO2 and O2 in-situ uranium leaching? Insights from a three-dimensional multicomponent reactive transport model at the field scale

The complex behavior of uranium in recovery is mostly driven by water-rock interactions following lixiviant injection into ore-bearing aquifers. Significant challenges exist in exploring the geochemical processes responsible for uranium release and mobilization. Herein this study provides an illustration of a ten-year field scale CO2 and O2in-situ leaching (ISL) process at a typical sandstone-hosted uranium deposit in northern China. We also conducte a three-dimensional (3-D) multicomponent reactive transport model to assess the effects of potential chemical reactions on uranium recovery, in particular, to focus on the role of sulfide mineral pyrite (FeS2). Numerical simulations are performed considering three potential ISL reaction pathways to determine the relative contributions to uranium release, and the results indicate that bicarbonate promotes the oxidative dissolution of uranium-bearing minerals and further accelerates the uranium leaching in a neutral geochemical system. Moreover, the presence of FeS2 exerts a strong competitive role in the uranium-bearing mineral dissolution by increasing oxygen consumption, favoring the formation of iron oxyhydroxide, and therefore causing an associated decrease in uranium recovery rates. The simulation model demonstrates that dissolution of carbonate neutralizes acidic water generated from pyrite oxidation and aqueous CO2 dissociation. In addition, the cation concentrations (i.e., Ca and Mg) are increasing in the pregnant solutions, showing that the recycling of lixiviants and kinetic dissolution of carbonate generates a larger number of dissolved Ca and Mg and inevitably triggers the secondary dolomite mineral precipitation. The findings improve our fundamental understanding of the geochemical processes in a long-term uranium ISL system and provide important environmental implications for the optimal design of uranium recovery, remediation, and risk exposure assessment.

Temperature Dependence of the Alkaline Oxygen Evolution Reaction Catalyzed By Amorphous/Crystalline Ni-Co Oxides

As the installation of renewable energy power generation systems such as solar and wind power increases, efficient energy storage devices are needed to overcome the problem of fluctuations in the availability of electrical energy. In particular, water splitting by electrolysis to produce storable hydrogen and oxygen appears to be one of the most promising approaches for large-scale energy storage. However, the efficiency of water electrolysis is greatly restricted due to the high overpotential required for the anodic oxygen evolution reaction (OER). Therefore, effective catalysts are needed to accelerate the reaction, reduce overvoltage, and increase energy conversion efficiency. Among various OER catalysts, nickel-cobalt oxides have received enormous attention due to their low cost and high OER activity in alkaline electrolyte. A better understanding of the OER kinetics is essential for accurate activity/durability predictions and the development of next generation OER catalysts. In this work, we synthesized a series of Ni-Co oxides with tunable amorphous/crystalline surface structures, with particular emphasis on the temperature dependence of the OER catalysis, as a way to elucidate the catalytic reactions, from both experimental and theoretical perspectives.The Ni-Co oxide was synthesized by the flame oxide-synthesis method.1 The as-prepared sample was treated in an oxidizing atmosphere (20% O2/N2) at elevated temperatures (from room temperature to 300 °C) without changing the Ni-Co-O atomic composition. Based on the measured composition and treatment temperature, the obtained three catalysts were denoted as Ni0.8Co0.2O-RT, Ni0.8Co0.2O-100, and Ni0.8Co0.2O-300. The OER activities were evaluated in 1 M KOH at 20 to 80 oC by use of the rotating disk electrode (RDE) technique.2 It was found that the increase in O2-treatment temperature led to increased structural order or crystallization of the oxide phase, i.e., crystallinity Ni0.8Co0.2O-RT < Ni0.8Co0.2O-100 < Ni0.8Co0.2O-300, as confirmed by XRD measurement. Figure 1a showed that the Ni-Co oxide materials presenting an amorphous or poorly crystalline oxide phase showed higher activity than their crystalline counterparts, which was correlated with a negative shift in the oxyhydroxide formation peak potential (inset of Figure 1a). The OER activation energy, extracted from temperature dependence of OER currents, also decreased with decreasing the crystalline oxide phases (Figure 1b). Thus, efficient OER was demonstrated to be associated with facilitated formation of oxyhydroxide species during electrochemical anodization. We proposed, based on density functional theory calculations, that the amorphous or disordered structure of the oxide top surface layers renders the surface geometry energetically favorable for the crucial step O + OH → OOH, which significantly accelerates the OER activity.This work was partially supported by funds for the JSPS KAKENHI (20H02839), and the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.[1] K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).[2] G. Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11 , 5222 (2021). Figure 1