Increasing KOH Concentration Research Articles

Poly(Arylene Alkylene)s with Tetrazole Pendants for Alkaline Ion-Solvating Polymer Electrolytes.

Alkaline ion-solvating membranes derived from a tetrazole functionalized poly(arylene alkylene) are prepared, characterized and evaluated as electrode separators in alkaline water electrolysis. The base polymer, poly[[1,1'-biphenyl]-4,4'-diyl(1,1,1-trifluoropropan-2-yl)], is synthesized by superacid catalyzed polyhydroxyalkylation and subsequently functionalized with tetrazole pendants. After equilibration in aqueous KOH, the relatively acidic tetrazole pendants are deprotonated to form the corresponding potassium tetrazolides. The room temperature ion conductivity is found to peak at 19 mS cm-1 in 5 wt. % KOH, and slightly declines with increasing KOH concentration to 13 mS cm-1 in 30 wt. % KOH. Based on an overall assessment of the mechanical properties, conductivity and electrode activity, 30 wt. % KOH is applied for alkaline electrolysis cell tests. Current densities of up to 1000 mA cm-2 were reached with uncatalyzed Ni-foam electrodes at a cell voltage of less than 2.6 V, with improved gas barrier characteristics compared to that of the several times thicker Zirfon separator.

Impact of the alkalinity on the stability and catalytic activity of Ni/Nb2O5 composite electrodes for the hydrogen evolution reaction in NaCl solutions

This study investigates the stability and catalytic activity of a Ni/Nb2O5 composite catalyst containing 13 wt.% Nb2O5 in 3.43 wt.% NaCl electrolytes with different concentrations of KOH (5.6, 15, and 30 wt.%). Results show that the addition of KOH significantly enhances the catalyst's stability, compared to NaCl alone. Furthermore, increasing KOH concentration decreases catalytic activity for HER, indicating that the electrolyte's alkalinization is a trade-off between stability and activity. The decline in catalytic activity is suggested to be due to a change in the reaction mechanism, from generating Cl2 to O2 as KOH concentration increases, favoring water decomposition over oxidation. Raman microscopy and UV–vis absorbance analysis show a decrease in surface oxidation with increasing KOH concentration. Using 3.43 wt.% NaCl with the addition of 5.6 or 15 wt.% KOH is feasible to increase electrode activity while maintaining electrocatalytic activity after aging in full hydrogen evolution, making it a good alternative electrolyte to replace KOH solutions. The findings have important implications for optimizing the synthesis of catalytic materials for H2 production and selecting electrolytes for long-term electrochemical cell operation.

HER Kinetics of Pt and Ni in Alkaline Medium at Elevated Temperatures (25-200 ºC) and Pressures (1-50 bar)

The hydrogen evolution reaction (HER) in alkaline medium is of great importance for the economic production of sustainable hydrogen. Nevertheless, the origin of the sluggish reaction kinetics observed in alkaline compared to acidic environment, remains a point of debate. [1-5] In order to promote understanding, and to extend the discussion at conditions closer to industrial operation, we have undertaken a systematic determination of the HER kinetics on Pt and Ni within a broad range of temperatures (25-200 ⁰C) and pressures (1-50 bar) in alkaline environment using 1 M or 10 M aqueous KOH as electrolyte.At present, the study of HER at high temperatures and pressures (HTP) is limited by the interim stability and lifetime of the available reference electrodes [6, 7]. We have overcome this limitation by employing a novel design for a Palladium hydride (PdH) and a reversible hydrogen (RHE) reference electrodes that enable facile and stable utilization at HTP [8]. A constant electrochemically driven supply of H2 on the PdH surface ensures preservation of the mixed (α+β)-phase in the PdH layer and thereby stable potential at HTP conditions for prolonged periods. Furthermore, the potential of the PdH reference electrode has been calibrated as a function of temperature and pressure with respect to the reversible hydrogen electrode (RHE). The potential of the PdH electrode in the mixed (α+β)-phase is ~55 mV vs. the RHE at 25 ⁰C, 1 bar, and observed to be independent of temperature and pressure.As shown in Figure 1, the HER kinetics are faster on Pt relative to Ni and become more sluggish on both metal surfaces upon increasing KOH concentration from 1 to 10 M at 25 ⁰C, 1 bar. A more pronounced thermal activation is observed in 10 M versus 1 M KOH though, resulting in a reversal of the pH trend at 100 ⁰C, 1 bar. References Sheng, M. Myint, J. G. Chen, Y. Yan, Energy Environ. Sci., 6, 1509–1512 (2013).Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, M. T. M. Koper, Nature Energy, 2, 17031–17037 (2017).Rossmeisl, K. Chan, E. Skúlason, M. E. Björketun, V. Tripkovic, Catalysis Today, 262, 36–40 (2016).Subbaraman, D. Tripkovic, D. Strmcnik, K. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N. M. Markovic, Science, 334, 1256–1261 (2011).Liu, J. Li, L. Jiao, H. T. T. Doan, Z. Liu, Z. Zhao, Y. Huang, K. M. Abraham, S. Mukerjee, Qingying Jia, J. Am. Chem. Soc., 141, 3232−3239 (2019).H. Miles, G. Kissel, P. W. T. Lu, S. Srinivasan, J. Electrochem. Soc., 123, 332–336 (1976).J. Appleby, G. Crepy, J. Jacquelin, Int. J. Hydrog. Energy, 3, 21–37 (1978).P. Leuaa, C. Chatzichristodoulou, J. Electrochem. Soc., 169, 054534 (2022). Figure 1: (a) HER overpotential versus current density (normalized by the electrochemically active surface area) on Pt and Ni at 25 ⁰C, 1 bar, in 1 and 10 M KOH, and (b) temperature evolution of the HER overpotential at 10 mA/cm2 (normalized by the electrochemically active surface area) for Pt and Ni at 25-100 ⁰C, 1 bar, in 1 and 10 M KOH. Figure 1

Effect of KOH Concentration on the Performance of HHO Generator at Varying Plate Surface Textures

In an effort to improve the performance of the HHO generator, the surface texture of the electrode plate has been introduced through this study. Optimization of the HHO generator’s performance has been developed continuously in many parts of the world. Therefore, the effect of KOH concentration on the performance of the HHO generator at different plate surface textures was investigated in this study. The HHO generator used in this study combines wet and dry cells. Generally, the results showed an increase in output current, HHO production rate, and output temperature, and conversely, a decrease in operating time due to the effect of increasing KOH concentration and different surface textures. The output current exhibits a significant increase of over 200% for all types of plate surfaces. For the output temperature, the smallest increase of about 9.7% is achieved by the linear plate surface. Meanwhile, the best HHO gas production rate is 267.26 L/min with an operating time of only 112.25 s, which occurred on the surface of the linear plate. Briefly, increasing the KOH concentration followed by modification of the plate surface has yielded better results compared to the plain surface. As for the surface texture, it causes a change in the plate surface area, which has a positive impact on the performance of the HHO generator.

Effect of KOH concentration on corrosion behavior and surface morphology of stainless steel 316L for HHO generator application

Hydrogen production could be enhanced by increasing the potassium hydroxide (KOH) concentration, but higher KOH concentrations result in higher corrosion rates. Therefore, a deep investigation of the electrochemical behavior of stainless steel (SS 316L) in the KOH solution is needed. This study investigates the influence of KOH concentrations on the electrochemical behavior, surface morphology, structure, and sample phases of SS 316L. The investigations were conducted by some electrochemical techniques, UV-vis, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The corrosion rate was found to increase, and solution resistance to decrease with increasing KOH concentration. Samples tested in 5, 30, and 50 g l-1 of KOH showed corrosion rates of 0.457, 2.362, and 5.613 µm year-1, respectively. A wide passive region and the noblest pitting potential were noticed for the sample with 5 g l-1 of KOH. Moreover, Mott-Schottky plots and characteristic wavelengths of UV-Vis suggest the formation of iron and chromium oxides by the passivation of samples. The SEM analysis showed a dynamic change of surface morphology from the lowest to the highest concentration with the intergranular corrosion found at the grain boundaries area. In conclusion, concentrations < 50 g l-1 KOH could be recommended since they would support the optimum remaining life of SS 316 L plates in HHO generators.

KOH Activation with Microwave Irradiation and its Effect on the Physical Properties of Orange Peel Activated Carbon

Chemical activation with assisted microwave irradiation was used to produced activated carbon from orange peel waste. The activating agent was potassium hydroxide (KOH) with concentrations of 2 M, 3 M, and 4 M. The microwave irradiation was done for 15 minutes with a 630 Watt output power. KOH concentration affected the physical properties of OP-ACxM. With increasing KOH concentration, the interlayer spacing (d002 and d100 ) grew, stack height (Lc ) increased, and stack width (La) dropped. The number of pores on the surface of OP-ACxM increased after the chemical activation process. In OP-ACxM, FTIR analysis reveals the presence of O-H, C-H, C-C, C=O, C=C, and C-O.

Introduction of surface defects in NiO with effective removal of adsorbed catalyst poisons for improved electrochemical urea oxidation

The electrochemical urea oxidation reaction (UOR) is a promising low cost method to replace water oxidation reaction for low energy electrochemical hydrogen production. The important factors to be considered in order to attain better UOR efficiency are electrocatalyst design by tuning defects and surface area and effective removal of CO2 poisoning on the catalyst surface. In this study, we have prepared surface oxidised NiO (e-NiO) with surface defects by electron beam bombardment and shown it to oxidize urea more effectively than pristine NiO with an onset potential of 0.33 V vs Ag/AgCl(sat.KCl) and Tafel slope of 34.69 mV/dec for 0.4 M urea in 1 M KOH electrolyte. The higher UOR current density exhibited by e-NiO is associated with the higher number of Ni3+ ions on the surface as revealed by X-ray photoelectron spectroscopy, leading to enhanced formation of NiO(OH) active species. A combination of voltammetry and impedance studies are presented to understand the active species formation and surface poisoning by released CO2 species. The dominant surface poisoning on e-NiO resulting from enhanced defects can be removed by increasing KOH concentration and the impedance spectra discloses effective removal of CO2 in 6 M KOH and 0.4 M urea. This is reflected in the UOR performance of e-NiO as drastic reduction in Tafel slope (22.29 mV/dec), lower onset potential (0.24 V vs Ag/AgCl(sat.KCl)) and high UOR current maxima of 20 mA/cm2 at the applied potential of 0.35 V vs Ag/AgCl(sat.KCl). The correlation of indirect and direct electrochemical urea oxidation reactions in pristine and e-NiO catalysts can be explained in terms of the strength of urea adsorption on surface sites derived from voltammograms and fitted impedance spectra.

The Sol-hydrothermal Synthesis of Potassium Bismuth Titanate Nanoparticles

Aims: To propose a method of KBT synthesis at a lower temperature to solve the volatility of the components. Background: Lead-based perovskite materials have long been employed in electroceramic industries due to their excellent piezoelectric, ferroelectric, and dielectric properties. The high toxicity of lead, however, leads to the replacement of the use of lead-based perovskite in devices with more environmentally friendly materials. KBT powders are traditionally prepared by solid-state reaction through the calcinations of K2CO3, Bi2O3 and TiO2 at high temperature. The high-temperature calcination process leads to serious particle agglomeration, grain growth and small surface area, which decrease the activity of the KBT powder. Instability of the KBT ceramic according to the high volatility of its component ions at elevated temperatures was the main concern for the application feasibility. Objective: This work was aimed to present the simplified method called “sol-hydrothermal” for the synthesis of KBT nanoparticles. Microstructure and phase evolution of the nanoparticles were investigated in detail. Method: The sol-hydrothermal method was performed in potassium hydroxide (KOH) solution at 140-200°C for 2-24 h. Result: The result showed that increasing hydrothermal temperatures from 140°C to 200°C, the crystal structure was changed from pseudo-cubic to tetragonal. At 200°C, phase separation was observed. Suitable hydrothermal time was found to be between 6-12 h, above which phase separation was also observed. Increasing the KOH solution concentration from 10 to 12 or 15 and finally, 20 M gave rise to greater KBT peak intensity, suggesting a more complete crystallization process when the concentration was increased. Tetragonal KBT nanoparticles with c/a ratio of 1.0620 were obtained under the synthesis condition of 180°C for 12 h in 20 M KOH solution. Sinterability of the synthesized KBT nano-particles was further investigated by varying the sintering temperatures from 1000°C to 1080°C; the highest relative density of 97% was obtained in the sample sintered at 1050°C. However, at this sintering temperature and beyond, the sublimation of the K-containing component occurred as evident by the appearance of Bi2O3 and Bi4Ti3O12 phases. Conclusion: In summary, KBT nanoparticles have been successfully prepared by the simple solhydrothermal method in a basic solution at low temperatures. Synthesis temperature, time and KOH concentration were found to affect the powder characteristics greatly. Increasing synthesis temperature was found to affect the phase development while increasing synthesis time resulted in the development of crystallinity of the KBT powder obtained. Increasing KOH concentration from 10 M to 20 M gave rise to different particle growth and agglomeration degrees. The optimum synthesis conditions were at 180°C for 24 h in 10 M KOH solution. At this condition, KBT powder with a uniform particle size distribution and tetragonal structure could be obtained. The synthesis powder showed excellent sinterability. Sintering at only 1020°C for 2 h gave rise to fine grain ceramics with 95% relative density. However, as potassium was prone to sublime, increasing sintering temperature to 1050°C and beyond resulted in K-deficient phases. Sintering of the KBT should be done in K-saturating atmosphere to suppress this sublimation.

Polysulfone-polyvinylpyrrolidone blend membranes as electrolytes in alkaline water electrolysis

Development of thin, dense and robust alkaline polymer membranes with high hydroxide ion conductivity is key to advanced alkaline electrolysis as it can enable operation at higher current density and/or efficiency, while improving the dynamic response of the electrolyzer. In this work, a homogeneous blend membrane system based on poly(arylene ether sulfone) (PSU) and poly(vinylpyrrolidone) (PVP) is explored as an alkaline ion-solvating polymer matrix. Increasing PVP content in the blend drastically increases electrolyte uptake, and at PVP contents higher than 45 wt%, the membrane can support ion conductivity in a technologically relevant range of 10–100 mS cm-1 or even higher when equilibrated in 20 wt% aqueous KOH. The membrane system is extensively characterized throughout the full composition range and the down-selected composition composed of 25% PSU and 75% PVP is employed in a single cell lab-scale water electrolyzer, showing excellent performance and stability during the course of one week at 500 mA cm-2 at 60 °C in 20 wt% KOH. Good performance stability was demonstrated for more than 700 h at 80 °C, but the gradually increasing KOH concentration due to evaporative loss of water resulted in membrane degradation.

Investigation on entrance mechanism of calcium and magnesium into micro-arc oxidation coatings developed on Ti-6Al-4V alloys

Micro-arc oxidation (MAO) coatings containing both calcium (Ca) and magnesium (Mg) elements and possessing a proper coating porosity are desirable for surface treatment of medical Ti-6Al-4V alloys. The influences of Na2CaY, Na2MgY, KOH, and final voltage on the Ca content, Mg content, and porosity of MAO coatings developed on Ti-6Al-4V alloys were investigated by an orthogonal experiment under constant voltage control mode. The results indicate that Ca and Mg separately enter into MAO coatings primarily via diffusion, and the competition between these two elements is relatively insignificant. The increasing KOH concentration can increase both Ca and Mg amounts in MAO coatings. High solution pH, which increases with increasing KOH, Na2CaY or Na2MgY concentrations, favors formation of MAO coatings. At the same processing condition, the Ca amount in MAO coatings is higher than the Mg amount, which may be mainly attributed to the smaller solubility product constant of Ca3(PO4)2 than that of Mg3(PO4)2. Final voltage is beneficial to coating formation and has a small impact on Ca or Mg contents but is the most important factor governing coating porosity. With the increasing final voltage, the coating porosity firstly increases from 300 to 350 V, but exhibits a decreasing trend after 350 V.

Effect of KOH as Deacetylation Agent on Physicochemical Properties of Glucomannan

Glucomannan is one of the high viscosity polysaccharides with high water absorbing capacity as well as solubility. These properties limit glucomannan applications. Hence the property needs to be modified. Glucomannan has 5-10% acetyl groups which conferred its physicochemical properties. Replacing the acetyl with alkali, transforms the physicochemical characterisation of the glucomannan. This study aimed to determine the effect of deacetylation conditions on the physicochemical properties of glucomannan of Amorphophallus oncophyllus. The effects of concentration of KOH (0.1, 0.25 and 0.5 M) and reaction time (2, 8, 16 and 24 h) on the degree of deacetylation (DD), solubility, and viscosity of the glucomannan were studied. The concentration of KOH and reaction time showed a positive impact on the DD of the glucomannan. Solubility and viscosity decreased with increasing KOH concentration and extending the duration of the deacetylation. It was suggested that the changes in the physicochemical properties were due to the replacement of the acetyl group with the alkali during deacetylation. However, the effect of this replacement was not linear with the variables. This nonlinearity could be due to the various removal distributions of the acetyl groups during the deacetylation.

Synthesis and Characterisation of Anodized Powder Metallurgy Mg-Ca Alloy

The Mg-1wt.%Ca alloy was fabricated using powder metallurgy method. The anodizing process were done by using different voltage (5V, 15V, 25V) and concentration of KOH (0.1M, 0.5M, 1.0M). The surface changes on PM Mg/1wt.%Ca resulted by anodizing was analyzed using SEM-EDX and XRD pattern. Meanwhile, surface hardness was measured by micro-Vickers hardness machine. The experiment found different XRD pattern between all non-anodized and anodized samples. The study found that increasing the voltage will increase the hardness while increasing KOH concentration reduced the hardness. The relation of the hardness and oxide film formation can be analyzed using SEM-EDX and XRD pattern. The optimum value for voltage, KOH concentration and hardness are 25V, 0.1M and 27.2 HV. The XRD detect the changes in PM Mg/1wt.% Ca indicates the oxide film formation.

(Invited) Electrochemical Formation of Nanoporous Indium Phosphide in KOH Electrolytes

Anodization of highly doped n-InP in KOH can result in a nanoporous sub-surface layer. Pores originate from surface pits and an isolated porous domain is initially formed beneath each pit. Domains are separated from the surface by a thin non-porous layer and each is connected to the electrolyte by its pit. Pores emanate along the <111>A directions to form truncated tetrahedral domains. We propose a three-step model of electrochemical pore formation: (1) hole generation at pore tips, (2) hole diffusion and (3) electrochemical oxidation of the semiconductor to form etch products. Step 1 determines the overall etch rate. However, if the kinetics of Step 3 are slow relative to Step 2, then etching can occur at preferred crystallographic sites leading to pore propagation in preferential directions. Pore width decreases with increasing anodization temperature and with increasing KOH concentration up to 9 mol dm-3, above which it decreases. At low current densities pores have sharp tips and triangular cross-sections; at higher current densities, the pore tips and cross-sections become more rounded while the pore width decreases. These observations are explained by the three-step model.

High-Surface-Area Mesoporous Activated Carbon from Hemp Bast Fiber Using Hydrothermal Processing

Synthesis of activated carbon from waste biomass is of current interest towards sustainability. The properties of biomass-derived activated carbon largely depend on the carbonization process. This study reports the preparation of mesoporous activated carbon with extremely high surface area from hemp bast fiber using hydrothermal processing. Hot water processing (390–500 °C) followed by activation using KOH and NaOH was investigated at different mass ratios. The described approach was found to enhance the mesoporosity (centered at 3.0 to 4.5 nm) of the hemp-derived activated carbon (HAC) from activation [confirmed by BJH (Barrett-Joyner-Halenda) pore size distribution and TEM (transmission electron microscopy) imaging]. BET (Brunauer-Emmett-Teller) results showed that the product has an extremely high surface area (2425 m2/g) while the surface functional groups (–OH, –COOH, C=C/C–C) were confirmed by FTIR (Fourier transform infrared spectroscopy) and further quantified by XPS (X-ray photoelectron spectroscopy). Increasing KOH concentration was found to enhance the surface area with a maximum biochar-to-KOH (g/g) ratio of 1:3. The crystallite domain size of HAC was determined using Raman spectroscopy of different wavelengths. The procedure described in this study is an environmentally friendly scalable route for the mass production of activated carbon using hemp fiber.

Influence of electrolyte concentration on the electrochemical characteristics of LaGaO3 perovskite oxide as novel anode material for Ni/MH batteries

Abstract Recently, the ABO 3 -type perovskite oxides have been proposed as a promising novel anode for (Ni/MH) rechargeable battery due to the best electrochemical performances. In this work, LaGaO 3 perovskite was synthesized by a sol-gel technique and their physico-chemical properties for Ni/MH batteries application are tested in details under various KOH electrolyte concentrations (1 M, 3 M and 7 M). The electrochemical measurements showed that all the electrochemical parameters, such as the activation cycle number, the maximum capacity ( C max ), the hydrogen diffusion ( D H ) and the exchange current density ( I 0 ), have been greatly influenced by the variation of KOH concentration. It was found that with increasing electrolyte concentration, the activation process of this oxide was improved leading to an increase of the highest electrochemical capacity from 10 mAh g −1 (1 M) to 220 mAh g −1 (7 M). Also, the hydrogen diffusion and the current density increase with increasing KOH, which implies that increasing KOH concentration is beneficial to hydrogen diffusion process and charge transfer reaction. All the measurements revealed that the best electrochemical performances of the negative electrode are obtained at KOH 7 M.

The effect of KOH concentration on chemical activation of porous carbon sorbents for carbon dioxide uptake and carbon dioxide–methane selectivity: the relative formation of micro- (&lt;2 nm) versus meso- (&gt;2 nm) porosity

Porous carbon (PC) sorbents are synthesized from polymer precursors mixed with a chemical activation reagent and pyrolised (>500 °C). KOH is known to be the best activator for a wide range of precursors, as it creates PCs with a large surface area (1200–4000 m2 g−1). In order to determine the optimum KOH : polymer ratio for both CO2 adsorption and CO2/CH4 selectivity, we prepared a set of five S-containing porous carbon (SPC) samples from polythiophene (PTh) with increasing KOH : PTh ratio (1 to 5), and investigated CO2 and CH4 uptake measurements on carbonaceous SPC samples up to a pressure limit of 30 bar. The SPCs have been characterised by XPS, SEM, TEM and BET surface area analysis. Although the apparent surface area and total pore volume increased with increasing KOH concentration, the maximum CO2 uptake (5–30 bar) was demonstrated for samples with KOH : PTh = 3. This equates to SPC samples with a surface area and total pore volume of ∼2700 m2 g−1 and 1.5 cm3 g−1, respectively. Greater values for either parameter do not enhance the CO2 uptake, showing that it is not total porosity that is important. SPC samples formed with KOH : PTh = 3 show both a maximum C composition (85%), and a maximum fraction of micro-pores ( 2 nm). KOH : PTh = 3 is also the synthetic condition to maximise CH4 uptake (5–30 bar); however, the optimum CO2/CH4 selectivity occurred with KOH : PTh = 2. This correlates with a different surface area (2200 m2 g−1) and total pore volume (1.2 cm3 g−1) that are required for optimum CO2 uptake. These results suggest that process conditions that lead to high relative microporosity need to be considered rather than the total surface area or pore volume.

Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru-PNP Pincer Complexes.

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(C2H4Pi-Pr2)2)) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H2 and CO2. Herein, we describe an investigation that combines experiment, spectroscopy, and theory to provide a mechanistic rationale for this process. During catalysis, the presence of two anionic resting states was revealed, Ru-dihydride (3-) and Ru-monohydride (4-) that are deprotonated at nitrogen in the pincer ligand backbone. DFT calculations showed that O- and CH- coordination modes of methoxide to ruthenium compete, and form complexes 4- and 3-, respectively. Not only does the reaction rate increase with increasing KOH, but the ratio of 3-/4- increases, demonstrating that the "inner-sphere" C-H cleavage, via C-H coordination of methoxide to Ru, is promoted by base. Protonation of 3- liberates H2 gas and formaldehyde, the latter of which is rapidly consumed by KOH to give the corresponding gem-diolate and provides the overall driving force for the reaction. Full MeOH reforming is achieved through the corresponding steps that start from the gem-diolate and formate. Theoretical studies into the mechanism of the catalyst Me-1a (N-methylated 1a) revealed that C-H coordination to Ru sets-up C-H cleavage and hydride delivery; a process that is also promoted by base, as observed experimentally. However, in this case, Ru-dihydride Me-3 is much more stable to protonation and can even be observed under neutral conditions. The greater stability of Me-3 rationalizes the lower rates of Me-1a compared to 1a, and also explains why the reaction rate then drops with increasing KOH concentration.

Characterization of Ceramic Oxide Layer Produced on Commercial Al Alloy by Plasma Electrolytic Oxidation in Various KOH Concentrations

Plasma electrolytic oxidation (PEO) is a promising coating process to produce ceramic oxide on valve metals such as Al, Mg and Ti. The PEO coating is carried out with a dilute alkaline electrolyte solution using a similar technique to conventional anodizing. The coating process involves multiple process parameters which can influence the surface properties of the resultant coating, including power mode, electrolyte solution, substrate, and process time. In this study, ceramic oxide coatings were prepared on commercial Al alloy in electrolytes with different KOH concentrations (0.5 ~ 4 g/L) by plasma electrolytic oxidation. Microstructural and electrochemical characterization were conducted to investigate the effects of electrolyte concentration on the microstructure and electrochemical characteristics of PEO coating. It was revealed that KOH concentration exert a great influence not only on voltage-time responses during PEO process but also on surface morphology of the coating. In the voltage-time response, the dielectric breakdown voltage tended to decrease with increasing KOH concentration, possibly due to difference in solution conductivity. The surface morphology was pancake-like with lower KOH concentration, while a mixed form of reticulate and pancake structures was observed for higher KOH concentration. The KOH concentration was found to have little effect on the electrochemical characteristics of coating, although PEO treatment improved the corrosion resistance of the substrate material significantly.

Effect of KOH concentration on the electrochemical behavior of coatings formed by pulsed DC micro-arc oxidation (MAO) on AZ31B Mg alloy

In this work, KOH, Na2SiO3 and Na2B4O7 were used as electrolytes for pulsed DC micro-arc oxidation (MAO) process. We studied the effect of KOH concentration of these electrolytes on the microstructure, chemical composition, and electrochemical behavior of pulsed DC MAO coatings formed on AZ31B Mg alloy. The surface characteristics of the coatings have been investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). All electrochemical measurements revealed that pulsed DC MAO process significantly improved the electrochemical behavior of AZ31B Mg alloy. Moreover, these measurements revealed that the sample 3 (1.5 M KOH) exhibited the best electrochemical behavior which resulted in high corrosion resistance compared to that of the other MAO treated samples. This behavior is attributed directly to the pore size, porosity percentage and micro-cracks. According to SEM micrographs, with increasing KOH concentration from 0.5 M to 1.5 M, the average pore size and porosity percentage decrease. However, increasing the concentration from 1.5 M to 2.0 M causes deleterious influence on the average pore size and micro-cracks.

Effects of duty cycle and electrolyte concentration on the microstructure and biocompatibility of plasma electrolytic oxidation treatment on zirconium metal

Recently, the plasma electrolytic oxidation (PEO) process has been widely studied and applied in the industrial setting due to its ability to create functional oxide layers on Al, Ti, Mg, and Zr alloys. In this work, a pulsed direct current (DC) power supply was adopted to grow the zirconia coating on pure Zr metal by PEO treatment. A fixed frequency of 1000Hz and constant current of 2A were used to fabricate all zirconia coatings. Duty cycle values of 25%, 75%, and 100% were used and 0.1M K3PO4 aqueous solution containing three different concentrations of KOH, 0.01, 0.05 and 0.1M, was also used in fabrication. The plasma breakdown voltage decreased with increasing KOH concentration due to its higher electrolyte conductivity. The PEO oxide coating consisted of a thin continuous barrier layer and a thick porous outermost layer, which consisted of mainly monoclinic and minor tetragonal ZrO2 phases. The PEO treatment of Zr metal provided excellent corrosion resistance in Hank's solution and good biocompatibility for 3T3 and MG63 cells. These results suggest that PEO coatings having potential applications in the biomedical field were confirmed in this study.