Solving the fouling mechanisms in composite membranes for water purification: An advance approach.

With the increasing population worldwide more wastewater is created by human activities and discharged into the waterbodies. This is causing the contamination of aquatic bodies, thus disturbing the marine ecosystems. The rising population is also posing a challenge to meet the demands of fresh drinking water in the water-scarce regions of the world, where drinking water is made available to people by desalination process. The fouling of composite membranes remains a major challenge in water desalination. In this innovative study, we present a novel probabilistic approach to analyse and anticipate the predominant fouling mechanisms in the filtration process. Our establishment of a robust theoretical framework hinges upon the utilization of both the geometric law and the Hermia model, elucidating the concept of resistance in series (RIS). By manipulating the transmembrane pressure, we demonstrate effective management of permeate flux rate and overall product quality. Our investigations reveal a decrease in permeate flux in three distinct phases over time, with the final stage marked by a significant reduction due to the accumulation of a denser cake layer. Additionally, an increase in transmembrane pressure leads to a correlative rise in permeate flux, while also exerting negative effects such as membrane ruptures. Our study highlights the minimal immediate impact of the intermediate blocking mechanism (n = 1) on permeate flux, necessitating continuous monitoring for potential long-term effects. Additionally, we note a reduced membrane selectivity across all three fouling types (n = 0, n = 1.5, n = 2). Ultimately, our findings indicate that the membrane undergoes complete fouling with a probability of P = 0.9 in the presence of all three fouling mechanisms. This situation renders the membrane unable to produce water at its previous flow rate, resulting in a significant reduction in the desalination plant's productivity. I have demonstrated that higher pressure values notably correlate with increased permeate flux across all four membrane types. This correlation highlights the significant role of TMP in enhancing the production rate of purified water or desired substances through membrane filtration systems. Our innovative approach opens new perspectives for water desalination management and optimization, providing crucial insights into fouling mechanisms and proposing potential strategies to address associated challenges.

Electrodeposited Cobalt Nanosheets on Smooth Silver as a Bifunctional Catalyst for OER and ORR: In Situ Structural and Catalytic Characterization.

Developing earth-abundant, cost-effective, and active bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to boosting sustainable energy systems such as electrolyzers and lithium-air batteries. However, the performance of promising cobalt-based materials is impaired by the external effects of binders and carbon additives as well as inhomogeneous electrode fabrication. In this work, binder- and carbon-free flower-like Co-decorated Ag catalytic nanosheets were in situ-synthesized via a simple electrodeposition approach. The morphology, composition, and structure of Co/Ag before and after OER were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Co/Ag thin film electrodes with various Co contents exhibited a bifunctional activity toward ORR and OER due to a synergistic effect. XPS analysis suggested the formation of Co3O4 as the main active species for OER. In particular, Co (83%)/Ag surface revealed a 60 mV lower ORR overpotential than a pure Ag surface and even lower than drop-casted Co3O4 nanoparticles on Ag surface. Only 1.5% peroxide was generated, suggesting a four-electron transfer ORR. In addition, the OER onset potential on Co/Ag is 60 mV less than Co3O4. Tafel slopes of 71 and 75 mV dec-1 were obtained for ORR and OER, respectively. Importantly, the three-dimensional (3D) growth mechanism of a cobalt layer (∼1 nm) on a well-defined atomic smooth Ag surface is unraveled by in situ electrochemical scanning tunneling microscopy (EC-STM). EC-STM suggests that Co prefers to nucleate at the step edges of Ag and grows in a 3D, forming nanoparticles, where the deposition/dissolution process of the Co adlayer on Ag is reversible. This investigation may provide insights into design strategies of efficient oxygen electrocatalysts.

Antimony deposition onto Au(111) and insertion of Mg.

Magnesium-based secondary batteries have been regarded as a viable alternative to the immensely popular Li-ion systems owing to their high volumetric capacity. One of the largest challenges is the selection of Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg2+ compared with Li+. In this work, we prepared very thin films of Sb by electrodeposition on a Au(111) substrate. Monolayer and multilayer deposition (up to 20 monolayers) were characterized by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Monolayer deposition results in a characteristic row structure; the monolayer is commensurate in one dimension, but not in the other. The row structure is to some extent maintained after deposition of further layers. After dissolution of the Sb multilayers the substrate is roughened on the atomic scale due to alloy formation, as demonstrated by CV and STM. Further multilayer deposition correspondingly leads to a rough deposit with protrusions of up to 3 nm. The cyclic voltammogram for Mg insertion/de-insertion from MgCl2/AlCl3/tetraglyme (MACC/TG) electrolyte into/from a Sb-modified electrode shows a positive shift (400 mV) of the onset potential of Mg deposition compared to that of a bare Au electrode. From the charge of the Mg deposition, we find that the ratio of Mg to Sb is 1:1, which is somewhat less than expected for the Mg3Sb2 alloy.

Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer.

One of the main challenges in metal-air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c⋅u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to D2/3·c·u1/3 . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li(+) and K(+) and organic solvents that contain Li(+) , K(+) , and Mg(2+) , such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N-methyl-2-pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 µL s(-1) . This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O2 decrease as the concentration of salt in the electrolyte was increased due to a "salting out" effect.

Electrocatalytic oxidation and adsorption rate of methanol at Pt stepped single-crystal electrodes and effect of Ru step decoration: a DEMS study.

The adsorption and oxidation of methanol at Pt(331) and Ru-step-decorated Pt(331) electrodes are studied recording currents and ion currents by online differential electrochemical mass spectrometry. The CO(2) current efficiencies and the degree of surface poisoning with CO(ad) formed during methanol oxidation are independent of the flow rate, confirming the parallel pathway mechanism. The CO(2) current efficiencies decrease with increasing methanol concentration and increase with increasing potential, whereas those of methyl formate show a reverse trend. At potentials higher than 0.6 V, neither the CO(2) current efficiencies nor the methanol oxidation currents increase with increasing Ru coverage. Instead, methanol oxidation is inhibited due to blocking of the most active platinum step sites. At potentials lower than 0.6 V, however, not only the onset of methanol oxidation shifts negatively, by about 0.1 V, but also the methanol oxidation current and the CO(2) current efficiencies increase. Crucial for the use in fuel cells is the complete oxidation to CO(2), which can be achieved if the reactants first adsorb at the electrode surface along the reaction path with adsorbed CO as an intermediate. Therefore, we directly determine the methanol adsorption rates at Pt(331) as well as at Ru-step-decorated Pt(331), Pt(332), Pt(100), and Pt(11,1,1) electrodes. The methanol adsorption rate is doubled by a double step density in the case of the Pt(331) and Pt(332) electrodes, higher at higher Ru coverages, and increases by a factor of three upon increasing the potential by 0.1 V (corresponding to a Tafel slope of approximately 200 mV dec(-1)). At Pt(331) electrodes with partial step decoration, stripping of adsorbed CO (from CO gas) reveals two adsorbate states, which are also discernable when the adsorbate formed from methanol dehydrogenation is stripped.

Quantitative DEMS study of ethanol oxidation: effect of surface structure and Sn surface modification.

Using the dual thin layer flow through cell, a semi-quantitative analysis of the volatile products during the electrooxidation of adsorbed and bulk solution of 0.01 M ethanol at polycrystalline platinum, smooth, roughened and Sn modified Pt(11,1,1), Pt(311) electrodes has been done by on-line differential electrochemical mass spectroscopy (DEMS). In addition to the current efficiency of CO(2), that of acetaldehyde was determined as a function of the flow rate. At polycrystalline platinum, ethanol oxidation produces only acetaldehyde; the amount of acetaldehyde further oxidized to acetic acid is negligible due to convection conditions. For comparison and for calibration purposes, i-propanol oxidation was examined for which acetone is the only oxidation product. At Pt(11,1,1), the main oxidation product is acetaldehyde. At Pt(311), in addition to acetaldehyde, acetic acid was also formed. Surface modification with Sn did not increase the reactivity of Pt(11,1,1) instead it led to inhibition of the ethanol oxidation. In the case of Pt(311), the onset potential of oxidation was shifted negatively by 0.2 V in the presence of Sn. The results of the potentiostatic measurements showed that this shift is not associated with the production of CO(2); rather acetic acid and acetaldehyde are the main oxidation products.