The behavior of hydrothermally synthesized hematite nanorods prepared on spin coated seed layers

Herein we report on the effect of varied spin-coated seed layer concentrations of Iron (III) chloride hexahydrate (FeCl3.6H2O) on the photoelectrochemical performance of hydrothermally synthesized hematite nanorods. The seed layers were prepared from 0.05, 0.07, 0.09, 0.11, and 0.13 M concentrations of FeCl3.6H2O. The nanorods were vertically aligned with slight inclinations over the seed layers with the two lowest molar concentrations (0.05 and 0.07 M) of FeCl3.6H2O. A further increase in seed layer concentrations transformed the nanorods as they grew over others and agglomerated into clusters. Structural analysis using X-ray diffraction (XRD) and Raman spectroscopy demonstrated uniform hematite crystalline peaks for all the samples. All samples absorbed highly in the visible region within an onset absorption edge wavelength ranging from 624 to 675 nm. Overall, the nanorods synthesized over the lowest seed layer concentration of 0.05 M of FeCl3.6H2O exhibited the highest photocurrent density of 0.077 mA/cm2 at 1.5 V vs. reversible hydrogen electrode. The results obtained provide important information about the structural, optical, and photoelectrochemical properties of hematite nanorods synthesized over varied seed layer concentrations. This is a key contribution in understanding and enhancing the hematite nanorods performance for photocatalytic applications.

High Activity and Selectivity of M-N/C Catalysts toward Two-Electron O2 Electroreduction

Hydrogen Peroxide (H2O2) is a versatile and environmentally friendly chemical oxidant with a remarkably diverse range of applications, including fine chemical synthesis, first aid kits for disinfection, pulp and textile bleaching, wastewater treatment and others. Industrial production of H2O2 is based on the anthraquinone oxidation/reduction process, which consumes a lot of energy, requires complex and large-scale equipment, and mass extraction solvents, generating an enormous waste. There is a general demand for a more decentralised infrastructure, where energy conversion and chemical synthesis are conducted closer to the point of consumption. In this context, developing an electrochemical process to partially reduce O2 to H2O2 (O2 + 2H+/e- → H2O2) in an acidic medium would be an attractive strategy that could be carried out under ambient conditions using renewable energies. However, practical and economic electrocatalysts that exhibit high activity and selectivity for hydrogen peroxide production is to be developed. A series of M-N/C catalysts (M = Fe, Co, and Cu) were prepared in the present study. The performance (activity and selectivity) of these catalysts for the oxygen reduction reaction was investigated in the potential window of 0.2 V to 1.0 V vs. the Reversible Hydrogen Electrode (RHE). Electrochemical measurements demonstrated that the Co-N/C [c] electrocatalyst exhibits high ORR activity and exceptional selectivity for hydrogen peroxide production (92% at 0.5 V vs. RHE).

Fabrication of 3D hierarchical Fe2O3/SnO2 photoanode for enhanced photoelectrochemical performance

Exploring and fabricating a suitable photoanode with high catalytic activity is critical for enhancing photoelectrochemical (PEC) performance. Herein, a novel 3D hierarchical Fe2O3/SnO2 photoanode was fabricated by a hydrothermal route, combining with an annealing process. The morphology, crystal structure were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photon spectroscopy (XPS), and X-ray diffraction (XRD), respectively. The results reveal the successful preparation of Fe2O3 nanothorns on the surface of SnO2 nanosheets. The as-fabricated 3D Fe2O3/SnO2 photoanode yields obviously promoted PEC performance with a photocurrent density of approximate 5.85 mA cm-2, measured in a mixture of Na2S (0.25 M) and Na2SO3 (0.35 M) aqueous solution at 1.23 V (vs. reversible hydrogen electrode, RHE). This value of photocurrent is about 53 times higher than that of the bare SnO2 photoanode. The obvious improved PEC properties can be attributed to the 3D Fe2O3/SnO2 heterostructures that offer outstanding light harvesting ability as well as improved charge transport and separation. These results suggest that exploring a suitable 3D hierarchical photoanode is an effective approach to boost PEC performance.approach to boost PEC performance.

Au aerogel for selective CO2 electroreduction to CO: ultrafast preparation with high performance

Noble metal aerogels (NMAs) have been used in a variety of (photo-)electrocatalytic reactions, but pure Au aerogels (AG) have not been used in CO2 electroreduction to date. To explore the potential application in this direction, AG was prepared to be used as the cathode in CO2 electroreduction to CO. However, the gelation time of NMAs is usually very long, up to several weeks. Here, an excess NaBH4 and turbulence mixing-promoted gelation approach was developed by introducing magnetic stirring as an external force field, which therefore greatly shortened the formation time of Au gels to several seconds. The AG-3 (AG with Au loading of 0.003 g) exhibited a high CO Faradaic efficiency (FE) of 95.6% at an extremely low overpotential of 0.39 V, and over 91% of CO FE was reached in a wide window of -0.4 ~ -0.7 V vs. the reversible hydrogen electrode (RHE). Partial current density in CO was measured to be -19.35 mA cm-2 at -0.8 V vs. RHE under 1 atm of CO2. The excellent performance should be ascribed to its porous structure, abundant active sites, and large electrochemical active surface area. It provides a new method for preparation of AG with ultrafast gelation time and large production at room temperature, and the resulting pure AG was for the first time used in the field of CO2 electroreduction.

Atomically Dispersed Fe-Co Bimetallic Catalysts for the Promoted Electroreduction of Carbon Dioxide

The electroreduction reaction of CO2 (ECO2RR) requires high-performance catalysts to convert CO2 into useful chemicals. Transition metal-based atomically dispersed catalysts are promising for the high selectivity and activity in ECO2RR. This work presents a series of atomically dispersed Co, Fe bimetallic catalysts by carbonizing the Fe-introduced Co-zeolitic-imidazolate-framework (C–Fe–Co–ZIF) for the syngas generation from ECO2RR. The synergistic effect of the bimetallic catalyst promotes CO production. Compared to the pure C–Co–ZIF, C–Fe–Co–ZIF facilitates CO production with a CO Faradaic efficiency (FE) boost of 10%, with optimal FECO of 51.9%, FEH2 of 42.4% at − 0.55 V, and CO current density of 8.0 mA cm−2 at − 0.7 V versus reversible hydrogen electrode (RHE). The H2/CO ratio is tunable from 0.8 to 4.2 in a wide potential window of − 0.35 to − 0.8 V versus RHE. The total FECO+H2 maintains as high as 93% over 10 h. The proper adding amount of Fe could increase the number of active sites and create mild distortions for the nanoscopic environments of Co and Fe, which is essential for the enhancement of the CO production in ECO2RR. The positive impacts of Cu–Co and Ni–Co bimetallic catalysts demonstrate the versatility and potential application of the bimetallic strategy for ECO2RR.

Highly efficient ethylene production via electrocatalytic hydrogenation of acetylene under mild conditions

Renewable energy-based electrocatalytic hydrogenation of acetylene to ethylene (E-HAE) under mild conditions is an attractive substitution to the conventional energy-intensive industrial process, but is challenging due to its low Faradaic efficiency caused by competitive hydrogen evolution reaction. Herein, we report a highly efficient and selective E-HAE process at room temperature and ambient pressure over the Cu catalyst. A high Faradaic efficiency of 83.2% for ethylene with a current density of 29 mA cm−2 is reached at −0.6 V vs. the reversible hydrogen electrode. In-situ spectroscopic characterizations combined with first-principles calculations reveal that electron transfer from the Cu surface to adsorbed acetylene induces preferential adsorption and hydrogenation of the acetylene over hydrogen formation, thus enabling a highly selective E-HAE process through the electron-coupled proton transfer mechanism. This work presents a feasible route for high-efficiency ethylene production from E-HAE.

A rechargeable aqueous manganese-ion battery based on intercalation chemistry

Aqueous rechargeable metal batteries are intrinsically safe due to the utilization of low-cost and non-flammable water-based electrolyte solutions. However, the discharge voltages of these electrochemical energy storage systems are often limited, thus, resulting in unsatisfactory energy density. Therefore, it is of paramount importance to investigate alternative aqueous metal battery systems to improve the discharge voltage. Herein, we report reversible manganese-ion intercalation chemistry in an aqueous electrolyte solution, where inorganic and organic compounds act as positive electrode active materials for Mn2+ storage when coupled with a Mn/carbon composite negative electrode. In one case, the layered Mn0.18V2O5·nH2O inorganic cathode demonstrates fast and reversible Mn2+ insertion/extraction due to the large lattice spacing, thus, enabling adequate power performances and stable cycling behavior. In the other case, the tetrachloro-1,4-benzoquinone organic cathode molecules undergo enolization during charge/discharge processes, thus, contributing to achieving a stable cell discharge plateau at about 1.37 V. Interestingly, the low redox potential of the Mn/Mn2+ redox couple vs. standard hydrogen electrode (i.e., −1.19 V) enables the production of aqueous manganese metal cells with operational voltages higher than their zinc metal counterparts.

Nitrogen-incorporation activates NiFeOx catalysts for efficiently boosting oxygen evolution activity and stability of BiVO4 photoanodes

Developing low-cost and highly efficient catalysts toward the efficient oxygen evolution reaction (OER) is highly desirable for photoelectrochemical (PEC) water splitting. Herein, we demonstrated that N-incorporation could efficiently activate NiFeOx catalysts for significantly enhancing the oxygen evolution activity and stability of BiVO4 photoanodes, and the photocurrent density has been achieved up to 6.4 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G). Systematic studies indicate that the partial substitution of O sites in NiFeOx catalysts by low electronegative N atoms enriched the electron densities in both Fe and Ni sites. The electron-enriched Ni sites conversely donated electrons to V sites of BiVO4 for restraining V5+ dissolution and improving the PEC stability, while the enhanced hole-attracting ability of Fe sites significantly promotes the oxygen-evolution activity. This work provides a promising strategy for optimizing OER catalysts to construct highly efficient and stable PEC water splitting devices.

Amorphous MoSxOy/h-BNxOy Nanohybrids: Synthesis and Dye Photodegradation

Molybdenum sulfide is a very promising catalyst for the photodegradation of organic pollutants in water. Its photocatalytic activity arises from unsaturated sulfur bonds, and it increases with the introduction of structural defects and/or oxygen substitutions. Amorphous molybdenum sulfide (a-MoSxOy) with oxygen substitutions has many active sites, which create favorable conditions for enhanced catalytic activity. Here we present a new approach to the synthesis of a-MoSxOy and demonstrate its high activity in the photodegradation of the dye methylene blue (MB). The MoSxOy was deposited on hexagonal boron oxynitride (h-BNO) nanoflakes by reacting h-BNO, MoCl5, and H2S in dimethylformamide (DMF) at 250 °C. Both X-ray diffraction analysis and high-resolution TEM show the absence of crystalline order in a-MoSxOy. Based on the results of Raman and X-ray photoelectron spectroscopy, as well as analysis by the density functional theory (DFT) method, a chain structure of a-MoSxOy was proposed, consisting of MoS3 clusters with partial substitution of sulfur by oxygen. When a third of the sulfur atoms are replaced with oxygen, the band gap of a-MoSxOy is approximately 1.36 eV, and the valence and conduction bands are 0.74 eV and −0.62 eV, respectively (relative to a standard hydrogen electrode), which satisfies the conditions of photoinduced splitting of water. When illuminated with a mercury lamp, a-MoSxOy/h-BNxOy nanohybrids have a specific mass activity in MB photodegradation of approximately 5.51 mmol g−1 h−1, which is at least four times higher than so far reported values for nonmetal catalysts. The photocatalyst has been shown to be very stable and can be reused.

An Electrochemical Study of Hydrothermal Reactions at Elevated Temperatures

<p>A high temperature hydrogen electrode concentration cell based on a design published by Macdonald, Butler and Owen1, was constructed and used to study the following protolytic equilibria. Thermodynamic equilibrium constants were derived by the usual method of extrapolation to zero ionic strength. 1. The ionization of water at temperatures from 75 to 225 degrees C in 0.1, 0.3, 0.5 and 1.0 mol kg-1 KCl solution. pK degrees w = 7229.701 /T + 30.285logT - 85.007 2. The pH calibration of 0.01 and 0.05 mol kg-1 sodium tetraborate at temperatures from 75 to 250 degrees C in O.1, 0.3 and 0.5 mol kg-1 NaCl solution. 0.0l mol kg-1 Sodium Tetraborate Solution pH = -0.4830t1 + 5.5692t2 + 7.7167t3 + 8.6983 0.05 mol kg-1 Sodium Tetraborate Solution pH = -0.0455tl + 8.3987t2 + O.2123t3 8.8156 3. The second dissociation of sulphuric acid at temperatures from 75 to 225 degree C in 0.1, 0.3 and 0.5 mol kg-l KCl solution. pK degrees 2 = 5.3353t1 - 15.9518t2 - 111.4929t3 + 3.8458 pK degrees 2 = 6.1815t*1 + 12.7301t*2. + 3.0660 (up to 150 degrees C) Where the t1 to t3= and t*1 and t*2 are the Clark-Glew temperature variable terms at reference temperatures of 423.15 and 373.15 K respectively2. 4. The acid hydrolysis of K-feldspar to K-mica and quartz at a temperature of 225 degrees C. The determination of the hydrolysis equilibrium constant was limited to one temperature because of the very slow reaction rate at temperatures less than 300 degrees C. log(mK+/mH+) = 4.2 (at 225 degrees C) Where a comparison could be made, the results of this study agreed well with previously published work, with the exception of the second dissociation constant of sulphuric acid at temperatures above 150 degrees C. Accurate values for the molal dissociation constant of the KSO-4 ion pair are required at elevated temperatures before the pK degrees 2 results can be fully evaluated. This research was severely restricted by the unpredictable loss of electrical continuity between the two cell compartments at temperatures above 150 degrees C. The problem appeared to be associated with the non-wettability of the porous Teflon plug which formed the liquid junction.</p>