Alkaline Electrolyte Solution Research Articles

Development of Transition Metal Phosphates NiCoP/NF and Evaluation of Their Hydrogen Evolution Properties

Energy and the environment are one of the most important global issues in the 21st century. In order to avoid the excessive destruction of the environment and human settlements by fossil fuels, and achieve sustainable development of the Earth, clean energy carrier hydrogen shows its application value. Facing the challenges in preparing transition metal phosphides, in this study, transition metal salts and urea were used as raw materials to react under hydrothermal conditions. The transition metal basic carbonate NiCo2(CO3)1.5OH3 was grown in situ on nickel-foam (NF) substrates. After low-temperature phosphating treatment, NiCoP alloy phosphide electrocatalysts (NiCoP/NF) grown on nickel-foam substrates were obtained. The electrochemical hydrogen evolution performance was tested in 1 mol/L KOH alkaline electrolyte solution. Experiments show, NiCoP/NF heterostructure catalyst has excellent hydrogen evolution performance. In an alkaline medium, the overpotential required to obtain the catalytic current density of 10 mA/cm2 is only 93 mV, and the Tafel slope is 118 mV/dec. This is largely due to: 1) the good dispersion of NiCoP/NF nano-catalyst on the Nickel Foam substrate increased the number of active sites exposed; 2) the heterostructure promotes the electron interaction between NiCoP and NF; 3) theoretical calculations show that the construction of NiCoP/NF heterostructure can effectively reduce the dissociation barrier of water, promote the dissociation of water, the kinetic reaction process of electrocatalytic hydrogen evolution is accelerated. Therefore, the construction of NiCoP/NF nanostructured heterogeneous catalysts enriches the application of non-noble metal nanomaterials in the field of hydrogen production from water electrolysis.

How multi-level porous carbon structure affects electrocatalytic properties?

For zinc-air batteries, it is crucial to develop bifunctional electrocatalysts that exhibit exceptional activity, high stability, and low cost. Based on this, three-dimensional carbon-based catalysts with multiple pore sizes were successfully prepared by using three different combinations of soluble salts as hard templates for controlling morphology and structure. These catalysts are denoted as 3D Co/NCBMS. Because of the synergistic effect of heteroatomic doping and a multi-level porous structure, 3D Co/NCBMS exhibits better catalytic properties for OER and ORR, comparable to IrO2 and Pt/C. The half-wave potential of the 3D Co/NCBMS catalyst was 0.82 V (vs RHE) in the ORR reaction and 1.54 V in the OER reaction. 3D Co/NCBMS samples exhibit excellent stability, maintaining a charge and discharge efficiency of approximately 54 % after stable cycling for 225 h. It also demonstrates high resistance to methanol poisoning, with the current remaining at around 95 % in alkaline electrolyte solutions when assembled into zinc-air batteries.

Phase Analysis, Microhardness and Wear Resistance of Selective Plated Copper-Based Coatings Containing Silver Particles

This study investigated the composition and properties of copper-based selective electrochemical coatings containing silver particles deposited on 316L stainless steel substrates. The method of formation of the coatings, using acid and alkaline electrolyte solutions, is described. A comparative analysis of the obtained coatings with selective copper coatings without added particles was carried out. An X-ray structural analysis was performed to determine the phase composition. Mechanical properties such as microhardness, surface roughness and wear resistance were investigated. The results show that the coatings are continuous, without visible defects, and the particles are deposited in depth as well. In both types of copper coatings with added silver particles, a dominant Cu phase and the presence of Ag were registered. It was established that the type of oxides formed in the deposition process is an important factor determining the microhardness and wear resistance of the coatings. The conclusions drawn confirm the effectiveness of the selective electrochemical deposition method for obtaining copper-based coatings containing silver particles and determine their characteristic mechanical properties.

Quantitative Analysis of CO2 Evolution in an Alkaline Electrolyte Solution By Differential Electrochemical Mass Spectroscopy

CO2 is involved in various main and side reactions of electrochemical energy devices, such as carbon corrosion reactions and alcohol oxidation reactions. Differential electrochemical mass spectroscopy (DEMS) has been intensively applied in acidic electrolyte solutions to analyze these reactions. DEMS is the in-situ electrochemical measurement technique to analyze volatile species with the mass spectrometer (MS). However, the applications of DEMS to alkaline electrolyte solutions have been limited due to the high solubility of CO2 in alkaline solutions. While "conventional DEMS cells," where porous thin-film working electrodes are formed onto polytetrafluoroethylene (PTFE) membrane interface, can quantitatively analyze CO2 that has not dissolved into the electrolyte yet [1], the working electrode materials are limited to those that can be deposited on porous PTFE membrane as porous thin-films. Recently, Möller et al. have developed the DEMS system, which can detect CO2 from general planer electrodes coated by catalyst powders by combining a neutralizer and "dual thin-layer flow cell" [2]. However, due to the relatively low detection limit, quantitative analysis of evolved CO2 has not been accomplished. In this study, we attempt to use high surface area electrodes to enhance the MS signal of CO2 to enable quantitative analysis of evolved CO2. To suppress the in-plane reaction distribution in the flow cell, we integrated an anion exchange membrane into the cell and arranged the counter electrode opposing the working electrode [3] (Fig. 1 (a)).CO-stripping voltammetry was performed to evaluate the CO2 detectability of the constructed DEMS system. The DEMS cell was composed of a commercial Pt/C electrode as the working electrode, a commercial Hg/HgO electrode as the reference electrode, a Pt mesh as the reference electrode, 1 mol dm–3 KOH aq as the electrolyte solution, and 1 mol dm–3 H2SO4 aq as the neutralizer. Figure 1 (b) shows the CO-stripping voltammogram and corresponding MS signal of CO2 (m/z = 44). MS signal of CO2 accompanied by the anodic CO oxidation current is clearly observed from 0.6 to 1.2 V vs. RHE. The calibration constant, which correlates the MS signal of CO2 to partial current for CO2 evolution reaction, is successfully obtained as 2.2 × 10–10, which shows that the DEMS system established in this study can quantitatively analyze CO2. Applications to the carbon corrosion reactions are also presented at the site.

Quantitative DEMS analysis of CO2 evolution reactions in alkaline electrolyte solutions

CO2 evolution is involved in various main and side reactions of electrochemical systems, such as carbon corrosion reactions and alcohol oxidation reactions. Differential electrochemical mass spectroscopy (DEMS) has been intensively applied to analyze the partial current of the CO2 evolution reaction in acidic and neutral electrolyte solutions. However, the quantitative analysis of the CO2 evolution reaction in alkaline electrolyte solutions has not been successful when catalyst-loaded carbon electrodes were applied, due to the high solubility of CO2 in the electrolyte. In this study, we develop a new DEMS system combining microreactor and ion exchange membrane to quantitatively analyze CO2 evolution reactions of catalyst-loaded carbon electrodes in alkaline electrolyte solutions. The calibration constant, which correlates the mass signal of m/z = 44 to the partial current of the CO2 evolution reaction, is successfully obtained from the Faradaic current and the mass signal of CO stripping voltammetry. We analyze carbon corrosion reactions of Pt and metal oxide-loaded carbon electrodes to demonstrate the effectiveness of the constructed system. The corrosion behavior of the Pt-loaded carbon is similar to that in the acidic electrolytes except for the onset potential of the carbon corrosion. La0.6Ca0.4CoO3 and Ca2FeCoO5 show electrochemical carbon corrosion activity, while La0.4Sr0.6MnO3 does not show distinct electrochemical carbon corrosion activity.

Bimetallic Ni–Co-ZIF derived (Ni, Co)Se2/BiVO4 heterojunction photoelectrodes with oxygen vacancy for enhancing PEC performance

Bismuth vanadate (BiVO4) still limits its practical application due to its poor water oxidation kinetics and easy surface charge recombination. In this paper, bimetallic Ni–Co-zeolitic imidazolate frameworks (Ni–Co-ZIF) was successfully transformed into metal oxides and selenides modified on the surface of BiVO4 ((Ni, Co)Se2/BiVO4) by electrostatic adsorption and pyrolytic selenization methods to further improve the photoelectrochemistry (PEC) performance of BiVO4. In the presence of (Ni,Co)Se2 layer, the best photocurrents of (Ni, Co)Se2/BiVO4 heterojunction photoelectrodes reached to 3.30 mA/cm2, which is better than the pure BiVO4 with about 4.07 times. The reason for PEC performance enhancement of (Ni, Co)Se2/BiVO4 since that the bimetallic (Ni,Co)Se2 with more metal active sites and oxygen vacancy takes advantage of the strong conductivity of charges to improve the charge separation and transfer efficiency. Meanwhile, these (Ni, Co)Se2/BiVO4 photoelectrodes have shown more stability in alkaline electrolyte solutions.

Optimizing performance: Achieving high capacitance and cycling durability in alkaline electrolyte with SnO2/SnSe||AC/KOH-based aqueous hybrid supercapacitor

A facile wet-chemical assisted synthesis route is adapted to prepare a novel SnO2/SnSe nanocomposite for the time to construct an aqueous asymmetric hybrid supercapacitor (AAHSC). The detailed characterization reveals the appropriate formation of micro flower-like SnO2-SnSe nanocomposite-covered with SnSe network to provide a conductive support and facilitate charge transport during the electrochemical processs. Compared with pure SnO2 micro flowers and SnSe electrodes, the SnO2-SnSe nanocomposite electrode delivers a brilliant charge storage performance. A rapid charge transport pathways was accomplished due to the lowest charge transfer resistance, resulting in a high capacitance and improved charge storage properties in an aqueous alkaline electrolyte solution with incredible reversibility and rate capability. Inspired by the excellent charge storage and capacitive properties, a two-cell mode-based AAHSC was built with SnO2-SnSe nanocomposite (cathode) and activated carbon (AC) as an anode (symbolized as SnO2-SnSe||AC/KOH) displayed the highest energy of 33.4 Wh/kg at a maximum power of 4003.7 W/kg, operating in a voltage of 1.6 V with excellent cycling stability of 89.5 %.

Performance of Selective Plated Copper Coatings on Stainless Steel 316L and Nickel Alloy Ni201: Microstructure, Microhardness, Wear Resistance, and Corrosion Resistance

This study investigates the properties and performance of selective plated copper coatings on stainless steel 316L and nickel alloy Ni201 substrates, using acidic and alkaline electrolyte solutions. The analysis encompasses surface microstructure, elemental distribution, microhardness, surface roughness, wear resistance, and corrosion resistance of the coatings. The results demonstrate that the selective plated copper coatings exhibit a uniform and continuous deposition, free from visible irregularities or structural imperfections. Notably, the alkaline copper coatings exhibit significantly higher microhardness values compared to the acidic coatings, irrespective of the substrate materials. Surface roughness measurements reveal an increase in roughness after wear testing for all coatings, with the alkaline coatings showing lower initial roughness values and superior wear resistance compared to the acidic coatings. Concerning corrosion resistance, the alkaline copper coatings display a higher susceptibility to corrosion compared to the acidic coatings. Generally, the findings underscore the effectiveness of the selective plating process in producing high-quality, uniform, and defect-free copper coatings on stainless steel and nickel alloy substrates.

Effects of Electrolyte Composition on the Electrochemical Properties of an Aluminum Alloy Electrode for Al-Air Batteries

Aluminum-air batteries that use alkaline electrolytes have the advantage of a high operating voltage, but the aluminum alloy electrodes experience high corrosion rates. To address this issue, a promising solution is proposed, which involves mixing neutral electrolytes with the alkaline electrolyte. In this study, we analyzed the electrochemical characteristics of aluminum alloy electrodes in electrolyte solutions containing varying concentrations of NaCl added to a 1 M NaOH alkaline electrolyte solution to understand the effect of neutral electrolytes on the discharge performance of aluminum-air batteries. The results obtained from potentiodynamic polarization tests, electrochemical impedance spectroscopy, three-electrode discharge tests, and open-circuit potential tests confirmed that the corrosion reaction and discharge voltage of aluminum alloy electrodes are influenced by the concentration of NaCl in the alkaline electrolyte solution. The corrosion rates and discharge voltages decreased as the concentration of neutral electrolyte increased, indicating that the electrochemical properties of aluminum-air batteries are highly dependent on the electrolyte composition.

Fabrication of MIL‐101(Fe) on UiO‐66 as an Electrocatalyst for Enhanced Overall Water Splitting

AbstractThe water‐splitting process is an attractive avenue for manufacturing hydrogen gas (a green and renewable energy source). However, this process requires efficient and economical electrocatalysts to lessen the high energy barrier of water oxidation and hydrogen evolution reactions. MOF‐on‐MOF hybrid structures assembled by unifying two or more MOF units are being explored as electrocatalysts with promising results. In this study, a novel electrocatalyst nanostructure of an iron‐based MOF on a zirconium‐based MOF denoted as MIL‐101(Fe)/UiO‐66 was prepared by hydrothermal synthesis. MIL‐101(Fe) offers abundant active sites with higher surface area for catalytic activity, while UiO‐66 acts as a support to enhance stability and conductivity. It was observed that compared to the individual MOFs, hybrid (MIL‐101(Fe)/UiO‐66) exhibited significantly enhanced catalytic efficiency towards overall water‐splitting. This was evidenced by the higher current densities and lower overpotentials observed for the hybrid. The MIL‐101(Fe)/UiO‐66 showed an overpotential of 185 mV for HER and 290 mV for OER @ η10 in an alkaline electrolyte solution. Moreover, it was stable for over 24 hours with a very small decrease in activity. With MIL‐101(Fe)/UiO‐66 as the cathode and anode, the unified water‐splitting cell attained a low overall required potential of 1.58 V @ η10 demonstrating good performance of overall water splitting. It offers an appealing strategy for creating high‐performance electrocatalysts for energy‐related applications, in particular water splitting, by combining the advantages of different MOFs through facile fabrication methods.

Towards Multiphase Modeling and Simulation of Alkaline Water Electrolysis through Pore-Resolved Foam Electrodes

In today’s world, concern is growing about the future of energy. Despite very ambitious international climate goals by 2050, global energy-related carbon dioxide (CO2) emissions keep increasing. In order to tackle this problem, hydrogen (H2) might become a significant part of the solution since it is a way to produce, store, move and use energy in a clean way. However, more than 95% of the actual hydrogen production is made of grey hydrogen, e.g. H2 produced from fossil fuels, which leads to high CO2 emissions [1]. One way to decarbonise this energy vector is to use renewable energies (solar panels, wind turbines, etc.) to produce green hydrogen via water electrolysis.Among the various technologies available to perform water electrolysis, alkaline water electrolysis (AWE) is the most mature one. In general, AWE consists of two planar electrodes separated by a certain distance and operating in a liquid alkaline electrolyte solution (e.g. KOH, potassium hydroxide). Recent studies show that it is possible to significantly increase the performance of such electrolysers by substituting the traditional gap-cell design by a zero-gap cell [2,3]. To do so 3D electrodes such as foams and 3D-printed structures are used, leading to an increase of the surface area and as a result, to a higher hydrogen production rate. These 3D electrodes are incorporated between the planar current distributors at either side of a membrane or diaphragm, reducing the interelectrode distance to the thickness of the separator. In the case of traditional cells, the hydrogen bubbles will tend to evolve only from the surface of the 2D current distributor plates, while for zero-gap cells gases evolve inside the porous structure. The process can be improved even more by forcing the electrolyte flow, favouring bubble removal. Finally, a bi-layer configuration with a catalyst layer presenting high specific surface area near the separator along with a porous transport layer (PTL) allows for a very high hydrogen production in the catalyst and an easy evacuation of the bubbles through the PTL.Recent experimental results in our group validate the enhanced performance of a bi-layer foam configuration where a fine foam acts as the catalyst layer and a coarser one as the PTL. In order to have a better understanding of the electrolyte flow behaviour inside these 3D electrodes, Computational Fluid Dynamics (CFD) was used as a tool to simulate the single-phase flow numerically. The purpose of this is to determine optimal flow parameters in order to favour hydrogen bubble removal while benefiting from the high surface area of the 3D electrodes. Up to now, it appears that this kind of analysis has rarely been addressed in the literature [4].In our specific case, the foams used as the catalyst and the PTL have characteristic pore sizes of 450 µm and 3000 µm, respectively. In order to obtain an accurate geometric representation, the foams were scanned using high-resolution micro-CT (computed tomography). A computational mesh was then obtained by reconstructing the highly detailed scanned data. The incompressible Navier-Stokes equations for the single-phase flow were solved using the Finite Element Method (FEM) implemented in MigFlow software [5]. In Figure 1 we present results from a simulation with the PTL width being three times the width of the catalyst layer, and the inlet velocity 1 m/s. It appears that the electrolyte velocity in the direction perpendicular to the flow is the highest in the area close to the interface between the catalyst and the PTL, meaning that this configuration would help extracting hydrogen bubbles off the catalyst layer, towards the coarser porous region.It is important to note that the results obtained in this case were not considering the gas phase. We are currently working on the development of a multiphase pore-resolved model to see whether the assumption of optimal bubble removal is confirmed for multiphase flow. To this end, a Eulerian-Eulerian two-fluid model must be implemented for the liquid electrolyte and the gaseous bubbles. One key aspect is to correctly define closure relations for the interfacial exchange terms that appear in the momentum equations. Hydrogen bubble formation, growth and escape from the porous electrodes might also be considered. This work is a first step towards the modeling of the whole alkaline water electrolysis process through 3D porous electrodes. Figure 1

Stepwise dissolution of silica surface in alkaline solution revealed by molecular modeling

AbstractThe atomic scale solid–liquid interfacial process dominates the macro‐dissolution of silica, yet its direct experimental observation is challenging. Here we employed the reactive molecular dynamics to model this process in alkaline condition. An elevated temperature strategy under canonical ensemble (NVT) was applied to accelerate the process for sufficient sampling at temporal domain. A stepwise transformation from fully linked SiO4 network (Q3 and Q4) to reduced linkage and eventually aqueous species (Q0) was revealed. The simulated dissolution rate agrees well with the value predicted by empirical model. By tracing hydrogen atoms, we found that the dehydroxylated silica surfaces in alkaline electrolyte solutions underwent a transition from Si–O–Si bond cleavage‐dominated (mainly reacted with OH−) to hydroxylation‐dominated surface reactions. The free‐energy reconstruction of well‐tempered metadynamics reveals that 1500 K accelerates dissolution without altering the reaction pathways. These findings offer novel insights into the evolution of atomic‐scale surface configurations during the dissolution kinetics of silica.

Carbon Quantum Dots-Doped Ni3Se4/Co9Se8/Fe3O4 Multilayer Nanosheets Prepared Using the One-Step Solvothermal Method to Boost Electrocatalytic Oxygen Evolution.

Oxygen evolution reaction is a momentous part of electrochemical energy storage and conversion devices such as rechargeable metal-air batteries. It is particularly urgent to develop low-cost and efficient electrocatalysts for oxygen evolution reactions. As a potential substitute for noble metal electrocatalysts, transition metal selenides still prove challenging in improving the activity of oxygen evolution reaction and research into reaction intermediates. In this study, a simple one-step solvothermal method was used to prepare a polymetallic compound carbon matrix composite (Co9Se8/Ni3Se4/Fe3O4@C) with a multilayered nanosheets structure. It exhibited good OER activity in an alkaline electrolyte solution, with an overpotential of 268 mV at 10 mA/cm2. In addition, this catalyst also showed excellent performance in the 24 h stability test. The composite presents a multi-layer sheet structure, which effectively improves the contact between the active site and the electrolyte. The selenide formed by Ni and Co has a synergistic effect, and Fe3O4 and Co9Se8 form a heterojunction structure which can effectively improve the reaction activity by initiating the electronic coupling effect through the interface modification. In addition, carbon quantum dots have rich heteroatoms and electron transferability, which improves the electrochemical properties of the composites. This work provides a new strategy for the preparation of highly efficient OER electrocatalysts utilizing the multi-metal synergistic effect.

The morphology and phase conversion of MnO2 in g-CN@MnO2 composite with supercapacitor applications

This paper presents a method or preparing a composite with morphology and phase conversion appropriate for use in a supercapacitor. Manganese dioxide (MnO2) nanoflakes composed of MnO2 nanorods (α and γ phases) with protonated g-C3N4 (g-CN), were prepared via a hydrothermal method. As synthesized composite showed district morphology and phase conversion of MnO2. The composite was thoroughly evaluated by field emission electron microscopy (FESEM), transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Based on characterizations, morphology and phase conversion phenomenon of MnO2 in g-CN/MnO2 composite is suggested and discussed. The electrochemical performance of the composite was assessed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) and compared to the performance of bare g-CN and MnO2 nanoflakes in alkaline electrolyte solution. The composite showed more specific capacitance (148.66 F/g) at 0.5 A/g than the pristine form of its components. An asymmetric supercapacitor (g-CN@MnO2//PCNFs), assembled through a g-CN@MnO2 composite and with porous carbon nanofibers (PCNFs), achieved an energy density of 1.6 Wh/Kg and a power density of 84 W/kg. This study suggests a new avenue for the development of MnO2 based composites use as energy storage materials.

Anodic sulphite oxidation on lead electrode in a neutral environment

The purpose of the research, anodic oxidation of the sulphite ions on the lead electrode in neutral solution, is both to establish the mechanism of sulphite electrooxidation on lead electrode and to determine the optimal parameters for the anodic process. The work started from the premise that the electrodes used in the anodic electrode processes that are carried out in aqueous solutions must have catalytic properties for the oxygen evolution release reaction. Among the metals tested as possible anodes to equip the electrochemical reactors used to obtain oxygen electrochemically from alkaline and acid electrolyte solutions, platinum exhibits the highest catalytic activity. Due to both the high price and the existence of a limited quantity of this metal, researches have been focused on identifying other metals and metal alloys that have catalytic properties for anodic processes [1].The electrochemical behaviour of the sulphite ions in the test solutions was studied by cyclic and linear voltammetry. The Tafel plot method was applied for a complete characterization of the process studied at the lead electrode / neutral electrolyte interface and for the determination of kinetic parameters (current density io and anode transfer coefficient α) [2]. The chrono-electrochemical methods (chronoamperometry, chronocoulometry) were chosen to determine the optimal parameters of the studied oxidation process (current density, optimal range of specific oxidation potential for sulphite electrooxidation and degree of transformation of sulphite ions in test solutions) [3].The presented studies regarding the electrochemical behaviour of sulphite ions in neutral medium on lead electrodes, are the initial point for the development of a Na2SO3 / O2 fuel cell [3].

Designing High-Quality Electrocatalysts Based on CoO:MnO2@C Supported on Carbon Cloth Fibers as Bifunctional Air Cathodes for Application in Rechargeable Zn-Air Battery.

To achieve the requirements of rechargeable Zn-air batteries (ZABs), designing efficient, bifunctional, stable, and cost-effective electrocatalysts is vital for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which still are struggling with unsolved challenges. The present research provides a concept based on the nanoscale composites which were engineered by using MnO2@C, CoO@C, and CoO:MnO2@C bifunctional electrocatalysts for fabrication of uniform carbon cloth (CC)-based electrodes. The CoO:MnO2@C electrocatalyst represented more efficient electrochemical properties through ORR and OER processes with superior positive half-wave potential (E1/2 = 0.78 V) and better limiting current density (i = 1.10 mA cm-2) in comparison with MnO2@C (E1/2 = 0.71 V, i = 0.92 mA cm-2) and CoO@C (E1/2 = 0.69 V, i = 0.86 mA cm-2) electrocatalysts. For the rechargeable ZABs fabricated by using CoO:MnO2@C-CC as an O2-breathing cathode, the specific capacity (SC), peak power density (P), open-circuit voltage (EOCV), and gap of charge/discharge voltage resulted in values of 520 mAh gZn-1, 210.0 mW cm-2, and 1.45 and 0.45 V, respectively, that afforded greater electrochemical characters than what was obtained for ZABs based on MnO2@C-CC (410 mAh gZn-1, 195.0 mW cm-2, 1.38 and 0.44 V) and CoO@C-CC (440 mAh gZn-1, 165.0 mW cm-2, 1.15 and 0.54 V). At the same time, lower Ei=10 (= 1.45 V) implied a more efficient OER in alkaline electrolyte solution for CoO:MnO2@C than MnO2@C (Ei=10 = 1.50 V) and CoO@C (Ei=10 = 1.39 V). Based on cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and X-ray photoelectron spectroscopy (XPS) results, it could be stated that the CoO:MnO2@C catalytic surface could experience 30 and 32% lower charge transfer resistance (Rct = 13.9 Ω) than MnO2@C (Rct = 20.1 Ω) and CoO@C (Rct = 29.7 Ω), respectively, which empowers an enhancement in ORR/OER performance. Prominently, the design concept of proposed electrocatalysts could suggest clear horizon for the synthesis and development paradigms of bifunctional catalysts for energy storage materials and devices.

Effect of microarc oxidation on the properties of aluminum alloy samples

Currently, modern manufacturing industries impose special requirements on structural materials such as aluminum, titanium, and their alloys. Various methods are used to improve the physicomechanical and corrosion properties of these materials. One of the promising ways to modify the surface in order to give it multifunctional properties is the treatment of micro-arc oxidation. A distinctive feature of the process is the formation of the oxide coatings on valve metals because of exposure to micro-arc discharges. At the same time, coatings with unique properties are formed. However, the effect of the micro-arc oxidation process on the properties of the base material has been little studied. The purpose of this work is to study the effect of the micro-arc process, implemented in pulsed mode, on the properties of oxide layers, and the base material. Modification of the alloy surface was carried out in the anode mode, with small values of the duration of the anode current pulse. An alkaline electrolyte solution was used as the electrolyte. Studies of the microhardness of the oxide layer, as well as the metal layer from the interface – oxide layer /metal deep into the metal, have shown that micro-arc discharges affect not only the properties of the oxide layer but also structural changes in the thickness of the metal. It is shown that the formed oxide coating is characterized by high microhardness. The oxide coatings obtained at the duration of the anode current pulse of 100 µs – 200 µs are wear-resistant, the coatings do not collapse, and do not wear to the ground under the accepted test conditions.

Multifunctional carbon-armored Ni electrocatalyst for hydrogen evolution under high current density in alkaline electrolyte solution

Hydrogen evolution reaction (HER) electrocatalysts capable of long-term operation under high current densities are key to the industrialization of water-splitting technology. Although numerous efforts have been devoted to expose active sites sufficiently while increasing the intrinsic catalytic activity, effects of non-kinetic factors on catalytic efficiency have not yet been comprehensively investigated. Herein, multifunctional carbon-armored nickel nanoparticles (NC@NiNPs) were fabricated using an in-situ polymer encapsulation method for use as a HER electocatalyst. NC@NiNPs exhibited low overpotential (74 mV at 10 mA cm-2), low Tafel slope (85.49 mV dec-1) and excellent stability (over 260 h at 1400 mA cm-2). Surprisingly, although the intrinsic activity of NC@NiNPs was lower than that of commercial 20 % Pt/C, NC@NiNPs provided markedly greater current density than 20 % Pt/C as the operating voltage was increased. This result implied that non-kinetic factors influenced the HER process, prompting this investigation to identify these unknown factors.

Rational Placement of Catalysts for Oxygen Reduction and Evolution Reactions Based on the Reaction Sites in Porous Gas Diffusion Electrodes

Metal-air secondary batteries using alkaline electrolyte solutions are promising candidates for next-generation large-scale energy storage systems because of their potential high capacity, low cost, and high safety standard. However, large overpotential of bifunctional air electrodes hindered their wide applications. Recently, increasing research on electrocatalysts for oxygen reduction reaction (ORR) and oxygen reduction reaction (OER) has been conducted to reduce the overpotential, and many active catalysts have been developed. On the other hand, formation of reaction sites is also important since the reaction sites for ORR and OER are basically different1. In this work, we investigated the reaction sites in porous gas diffusion electrodes (GDE) and constructed a GDE with a double-layered catalyst layer based on the difference in the reaction sites for ORR and OER2.La0.6Ca0.4CoO3 (LCCO, bifunctional catalyst), La0.4Sr0.6MnO3 (LSMO, ORR catalyst) and Ca2FeCoO5 (CFCO, OER catalyst) were synthesized by the polymerized complex method. Porous gas diffusion electrodes are constructed by the hot-press method. Catalyst layers were composed of the prepared catalyst (50 wt%), graphitized carbon black (35 wt%), and poly(tetrafluoroethylene) (15 wt%). Gas diffusion layers (GDL) were commercial carbon papers (TGP-H-030H, Chemix). IR-corrected polarization curves were measured by the constant current and AC impedance measurements. Oxygen diffusion resistances for ORR and OER were estimated from the difference in the steady-state potential between pure oxygen and air atmosphere (⊿E = |E O2 – Eair|). Larger increase in ⊿E indicates larger oxygen diffusion resistances in the oxygen reaction processes3.Figure 1 (a, b) shows ⊿E-I curves of LCCO-GDE for ORR and OER in 4.0 and 8.0 dm–3 KOHaq. The oxygen permeability(D O2×C O2) in 4.0 dm–3 KOHaq is 16 times as large as that in 8.0 dm–3 KOHaq while the ion conductivities of these electrolyte solutions are at the same extent. The increase in ⊿E for ORR was larger in 8.0 dm–3 KOHaq, indicating dissolved oxygen involved in the ORR process. In contrast, the ⊿E for OER was nearly negligible in these electrolyte solutions, suggesting that oxygen was mainly transported as bubbles. We also measured ⊿E-I curves of the LCCO-GDE in 8.0 dm–3 KOHaq having different catalyst layer thicknesses. The increases in ⊿E were almost independent of the catalyst layer thicknesses for ORR and OER, suggesting that the reaction sites were concentrated in the catalyst layer. Finally, we compared polarization curves of the GDEs with single-layered catalyst layer (LSMO+CFCO|DGL-GDE) and double-layered catalyst layers (LSMO|CFCO|GDL-GDE and CFCO|LSMO|GDL-GDE) to identify the reaction sites for ORR and OER (Fig. 1 (c, d)). The smallest overpotential of CFCO|LSMO|GDL-GDE for both ORR and OER showed that the reaction sites for ORR and OER were concentrated at the electrolyte-side and gas-side, respectively. In addition, CFCO|LSMO|GDL-GDE showed higher durability than LSMO+CFCO|DGL-GDE. From these results, it can be concluded that the placement of the ORR and OER catalysts on the electrolyte-side and gas-side of the CL can improve both activity and durability of the bifunctional air electrode.

Protective Properties of Coatings Obtained via Microarc Oxidation in Alkaline Electrolyte Solutions

Protective Properties of Coatings Obtained via Microarc Oxidation in Alkaline Electrolyte Solutions