Initial Current Density Research Articles

Spherical CeO2 synthesized by molten-salt with unique fast variable valence properties for lithium-ion battery anode

As an important rare earth oxide, fluorite CeO2 is widely used in lithium ion batteries because it can quickly and directly change the oxidation state of cerium from Ce3+ to Ce4+. In this work, the ball milling method and the molten salt approach were both used to successfully produce amorphous carbon-coated CeO2 composite. The CeO2@C composite has excellent rate characteristics, high reversibility, and cycling stability. In contrast to the pure CeO2 electrode, the reversible capacity of CeO2@C remains at 264.8 mAh/g after 100 cycles at 100 mA g−1, with a capacity retention rate of 79.3%. The CeO2@C specific capacity is returned to 257.2 mAh/g, accounting for 89% of the initial capacity, after large current fluctuations and returning to the initial current density.

Ex situ/Operando X‐Ray Absorption Spectroscopy on Fe0.07Zr0.93O2‐δ/C vs. Fe−N−C as Pt‐Group‐Metal‐Free Oxygen Reduction Reaction Catalysts in Proton Exchange Membrane Fuel Cells

AbstractIn this study, ex situ and operando X‐ray absorption spectroscopy (XAS) is employed to shed light on structure and degradation mechanism of Fe‐based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Ex situ XAS on pristine Fe0.07Zr0.93O2‐δ/C catalyst confirms the incorporation of Fe3+ in the ZrO2 structure and clearly exclude any significant presence of Fe−N−C‐type structures. The edge shift in data on in‐house aged samples demonstrates a mixed oxidation state of Fe (Fe3+ and Fe2+), consistent with Fe demetalation from the ZrO2 structure. Furthermore, a more symmetric coordination in the pre‐edge shape points towards the formation of oxidic Fe clusters upon aging. Fe demetalation is inferred also from the edge shift to higher energy (presence of Fe3+) in operando XAS data at 0.3 V, due to Fe phases not electrically polarizable/reducible at the applied voltage. Electrochemical data exclude any correlation between extent of aging and type of test, also for a commercial Fe−N−C catalyst by Pajarito Powder. The observed faster aging for Fe0.07Zr0.93O2‐δ compared to Fe−N−C is attributed to an improved mass transport to/from active sites, manifest also in very similar initial current densities at 0.3 V, despite much higher catalyst activity for Fe−N−C.

An economical electrocoagulation process of a hazardous anionic azo dye wastewater with the combination of recycled electrodes and solar energy.

The energy and electrode costs are the restrictions of applying electrocoagulation (EC) in wastewater treatment and many attempts have been made to decrease these costs. In this study, an economical EC was investigated to treat a hazardous anionic azo dye wastewater (DW) that threatens the environment and human health. Firstly, an electrode for EC process was produced from recycled aluminum cans (RACs) by remelting in an induction melting furnace. The performance of the RAC electrodes in the EC was evaluated for COD, color removal, and the EC operating parameters such as initial pH, current density (CD), and electrolysis time. Response surface methodology which is based on central composite design (RSM-CCD) was used for the optimization of the process parameters which were found to be pH 3.96, CD 15mA/cm2, and electrolysis time 45min. The maximum COD and color removal values were determined as 98.87% and 99.07%, respectively. The characterization of electrodes and the EC sludge was conducted by XRD, SEM, and EDS analyses for the optimum variables. In addition, the corrosion test was conducted to determine the theoretical lifetime of the electrodes. The results showed that the RAC electrodes show an extended lifetime as compared to their counterparts. Secondly, the energy cost required to treat DW in the EC was aimed to decrease by using solar panels (PV), and the optimum number of PV for the EC was determined by the MATLAB/Simulink. Consequently, the EC with low treatment cost was proposed for the treatment of DW. An economical and efficient EC process for waste management and energy policies was investigated in the present study which will be instrumental in the emergence of new understandings.

NiS Nanosheets Decorated on Hollow Carbon Spheres from Liquefied Wood for Supercapacitors

Carbon-based supercapacitors with high performance have a wide foreground among various energy storage devices. In this work, wood-based hollow carbon spheres (WHCS) were prepared from liquefied wood through the processes of emulsification, curing, carbonization, and activation. Then, the hydrodeposition method was used to introduce nickel sulfide (NiS) to the surface of the microspheres, obtaining NiS/WHCS as the supercapacitor electrode. The results show that NiS/WHCS microspheres exhibited a core-shell structure and flower-like morphology with a specific surface (307.55 m2 g-1) and a large total pore volume (0.14 cm3 g-1). Also, the capacitance could be up to 1533.6 F g-1 at a current density of 1 A g-1. In addition, after 1000 charge/discharge cycles, the specific capacitance remained at 72.8% at the initial current density of 5 A g-1. Hence, NiS/WHCS with excellent durability and high specific capacitance is a potential candidate for electrode materials.

Novel hierarchical porous carbons from waste printed circuit boards for oxygen reduction reaction

BackgroundThe non-metallic carbon catalyst in oxygen reduction reaction (ORR) is mainly hindered by their low catalytic activity and high cost. MethodsNovel hierarchical porous carbons (HPCs) with N, S codoping were synthesized from WPCBs based on the crab shell templates accompanied by different KOH activation temperatures and the modification of IL. Significant findingsOwing to the ordered hierarchical porous structure, HPCs-850 have large SBET (2209 m2/g) and high Vtot (1.389 cm3/g). After IL modification, N and S elements are successfully introduced into the carbon network of HPCs-850 to synthesize LHPCs-850. In an alkaline medium, the reduction potential, initial voltage, half-wave potential, and limiting current density of LHPCs-850 are 0.68 V, 1.05 V, 0.78 V, and 4.9 mA/cm2, demonstrating LHPCs-850 have excellent ORR catalytic performance. After 18,000 s durable measurement, the relative current density of LHPCs-850 still keeps 92% of its initial current density, which is more stable than that of HPCs-850 (87%). DFT calculation confirms that N, S codoping can improve catalytic activity by providing additional ORR active sites number. The proposed route to prepare WPCBs-based LHPCs broadens a new field of large-scale synthesis of electrocatalytic materials from industrial wastes.

Metal-organic frameworks stabilized Mo and W binary single-atom catalysts as high performance bifunctional electrocatalysts for water electrolysis

Single atom catalysts (SACs) are commonly composed of single atoms (SAs) anchored on relatively inert supports containing surface heteroatoms, leading to low specific catalytic efficiency. A new SAC design strategy, anchoring SAs on catalytically active porous supports, was proposed and demonstrated with NiFe-based metal-organic frameworks (MOFs) stabilized Mo and W binary SACs, MOF-MoSAWSA, as highly efficient and stable bifunctional electrocatalysts for electrolytic water splitting. MOF-MoSAWSA delivered current densities of 10 and 500 mA cm−2 at ultralow overpotentials of 199 and 250 mV for oxygen evolution reaction and at decent overpotentials of 57 and 297 mV for hydrogen evolution reaction, respectively. For overall water splitting, the MOF-MoSAWSA//MOF-MoSAWSA couple achieved ultralow cell voltages of 1.501 and 1.780 V to deliver current densities of 10 and 500 mA cm−2, respectively, and exhibited excellent stability with only 2% decay in current densities after a 56-hour operation at an ultrahigh initial current density of 515 mA cm−2. The success was attributed to the positive synergy between the SAs and MOF and between Mo and W SAs, as revealed with X-ray absorption and photoelectron spectroscopic analyses. The present development offers a new route for fabrication of advanced SA based catalysts.

Pseudocapacitive sodium-ion storage in one-dimensionally structured anatase TiO2 nanofiber anode for high performance sodium-ion batteries

Nanofibers provide high surface area to volume ratio, vectoral conduction, fast ionic diffusion, and short pathways for sodium ions. Titania nanofibers exhibit excellent electrochemical performance when used for sodium-ion storage. One-dimensional titania nanofiber was synthesized by an electrospinning process as an anode material for high capacity sodium-ion batteries. The electrochemical kinetics of titania nanofiber for sodium-ion storage were examined. Titania nanofiber exhibited pseudocapacitive capacitive properties with high rate performance, giving capacity of 97 mA h g−1 after 1000 cycles with a current density of 1000 mA g−1. Even when current rate returned to the initial current density of 50 mA g−1, titania nanofiber electrode capacity climbed to 206 mA h g−1 again without capacity fading, which indicated good rate performance, superior sodium insertion/extraction reaction, and high reversible capacity.

Vesicular AuPd alloy nanowires for enhanced electrocatalytic activity

Developing excellent fuel cells requires complex design and precise tuning of catalyst structure and composition. This paper presents a simple approach to fabricate AuPd vesicular nanowires (VNWs) with controlled composition. The method uses homogeneous Te nanowires (NWs) as templates and reducing agents in the replacement reaction and NaClO as etching agent. Bimetallic synergism and rich catalytic sites in the vesicular nanowires give AuPd catalysts the excellent electrocatalytic performance for methanol oxidation. In particular, the Au28Pd71 VNWs showed the highest electrocatalytic activity for methanol oxidation, and its highest mass current density was1.278 A mg−1, which was 1.22 and 1.65 times that of Pt VNWs and commercial Pt/C, separately. Furthermore, Au28Pd71 VNWs exhibited good stability after 2000 cycles, maintaining an initial mass current density of 85.2%. As the optimal component ratio, Au28Pd71 exhibited superior electrocatalytic performance for methanol oxidation. This discovery demonstrated a new approach for the programming and fabrication of senior methanol fuel cells electrocatalysts.

NiMo-MOF-Derived Carbon-Armored Ni4Mo Alloy of an Interwoven Nanosheet Structure as an Outstanding pH-Universal Catalyst for Hydrogen Evolution Reaction at High Current Densities

Development of highly efficient and stable non-precious metal-based pH-universal catalysts for hydrogen evolution reaction (HER) at high current densities remains challenging for water electrolysis-based green hydrogen production. Herein, a simple solvothermal process was developed to synthesize a NiMo metal-organic framework (MOF), from which a carbon-armored Ni4Mo alloy of an interwoven nanosheet structure was derived with a two-stage thermal treatment, to serve as a high-performance pH-universal HER catalyst. It requires low overpotentials of 22, 48, and 98 mV to achieve a current density of -10 mA cm-2 and 192, 267, and 360 mV to deliver an ultrahigh current density of -500 mA cm-2 in alkaline, acidic, and neutral media, respectively, and exhibits remarkable operational stability at an ultrahigh initial current density of -500 mA cm-2 for over 50 h, making it promising for applications in large-scale green hydrogen production. The success can be attributed to the unique catalyst design of a carbon-armored, composition-optimized NiMo alloy of an advantageous nanostructure of interwoven nanosheets for enhanced utilization of active sites and mass transfer of electrolytes and gaseous products, made possible with a MOF-derivation fabrication approach.

Pluronic-123 Assisted Synthesis of Cobalt Vanadate Microparticles (µ-CoV MPs) for Durable Electrochemical Oxygen Evolution Reaction in Seawater and Connate Water

Exploring different catalytic material paradigms could drive the search for the best oxygen evolution reaction (OER) catalyst to achieve industrially-feasible hydrogen fuel from water. Cobalt-based materials are considered good choices in this regard. Herein, we synthesized Pluronic-123 (P-123)-stabilized, unique, rough, globular-shaped cobalt vanadate microparticles (µ-CoV MPs) using an ultrasonic-assisted solvothermal method. The as-synthesized µ-CoV MPs were subjected to high-temperature annealing to improve the crystallinity and the surface polymer moieties were pyrolyzed. Conventional SEM, XRD, FTIR, and BET analyses evaluated the morphological and structural features. The temperature-controlled crystalline phase led to extensive OER performance in SW electrolytes. The OER onset potential (VOER) was observed at 1.557 V@10 mA/cm2 in seawater (SW) for µ-CoV MPs annealed at 400 °C compared to the VOER of 1.632 V of non-annealed µ-CoV MPs. The current density showed a steep increase beyond 1.557 V, confirming the excellent electrokinetics OER behavior of the µ-CoV MPs-deposited electrode. The chronoamperometric (I–t) OER stability comparison in SW and connate water (CW) electrolytes indicated only a <20% initial current density decrease after 8 h in the case of the SW electrolyte. However, the CW electrolyte posed serious challenges to the electrode and activity was completely lost after <2 h. The electrolytic comparison indicated that SW is highly suitable for µ-CoV MPs electrodes.

Adjusting the Near-Surface Composition of Pt–Zn Intermetallic Nanoparticles to Enhance Methanol Oxidation Activity and Stability

Platinum-based materials are promising catalysts for methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs), but it is challenging to balance activity and stability in actual application. Thus, in our work, the graphitized porous carbon (PC) with a high specific surface area and hierarchical porous structure was successfully prepared as the supported material. Disordered A1-PtZn NPs supported on PC was synthesized via one-step coreduction method at 300 °C and H2 atmosphere. Afterward, an ordered PtZn intermetallic compound (L10-PtZn/PC) was acquired at a higher annealing treatment of 600 °C for 12 h, and L10-PtZn/PC-acid with Pt-skin was also obtained via acid etching. The smaller PtZn nanoparticles (ca. < 5 nm) can be synthesized and distributed on the PC homogeneously. The L10-PtZn/PC-acid catalysts display a high order degree and stronger electronic interaction between Pt and Zn atoms, which is favorable for withstanding CO poisoning in MOR. Thus, the L10-PtZn/PC-acid catalyst has a mass activity of 1130 mA mgPt–1 and a specific activity of 2.23 mA cm–2, which are 3.0 and 2.2 times higher than commercial Pt/C. Additionally, the Pt-skin of L10-PtZn/PC-acid can provide a protection for subsurface Zn and keep the structural stability effectively. Therefore, after 2000 cycles of cyclic voltammetry, the L10-PtZn/PC-acid sample remains 86.62% of the initial peak current density, while commercial Pt/C only retains 47.65%. In situ FTIR studies have demonstrated that the OHads species easily absorbed on the Zn atoms and facilitates a direct pathway without CO in the MOR on the intermetallic PtZn. This work provides a novel strategy to develop an ideal near-surface composition Pt-based nanocatalyst with high activity and stability for direct methanol fuel cells.

Hexavalent Chromium Removal from Water and Wastewaters by Electrochemical Processes: Review

Hexavalent chromium (Cr(VI)) is a toxic, mutagenic, teratogenic, and carcinogenic species. Its origin is in industrial activities. Therefore, its effective control is realized on a source basis. Although chemical methods proved effective in removing Cr(VI) from wastewaters, more economic solutions with a minimum sludge production have been sought. Among them, the use of electrochemical processes has emerged as a viable solution to the problem. Much research was conducted in this area. The aim of this review paper is to make a critical evaluation of the literature on Cr(VI) removal by electrochemical methods, particularly electrocoagulation with sacrificial electrodes, and to assess the present data as well as to point out the areas that need further elaboration. Following the review of the theoretical concepts of electrochemical processes, the literature on the electrochemical removal of Cr(VI) was evaluated on the basis of important elements of the system. Among them are initial pH, initial Cr(VI) concentration, current density, type and concentration of supporting electrolyte, and the material of electrodes and their operating characteristics and process kinetics. Dimensionally stable electrodes that realize the reduction process without producing any sludge were evaluated separately. Applications of electrochemical methods to a wide spectrum of industrial effluents were also assessed.

Electropolishing Stainless Steel Optimization Using Surface Quality, Dimensional Accuracy, and Electrical Consumption Criteria

Electropolishing (EP) processes require high electrical consumption that must be optimized to minimize production costs without sacrificing the objectives of surface quality and dimensional accuracy. The aim of the present paper was to analyze the effects of the interelectrode gap, initial surface roughness, electrolyte temperature, current density, and EP time on aspects of the EP process applied to AISI 316L stainless steel, which have not been examined in the literature, such as polishing rate, final surface roughness, dimensional accuracy, and electrical consumption cost. In addition, the paper aimed to obtain optimum individual and multi-objective considering criteria of surface quality, dimensional accuracy, and electrical consumption cost. The results showed that the electrode gap was not significant on the surface finish or current density, and the EP time was the parameter having the greatest effect on all criteria analyzed, with a temperature of 35 °C showing the best electrolyte performance. The initial surface texture with the lowest roughness Ra10 (0.5 ≤ Ra ≤ 0.8 μm) obtained the best results with a maximum polishing rate of ~90% and minimum final roughness (Ra) of ~0.035 μm. The response surface methodology showed the EP parameter effects and the optimum individual objective. The desirability function obtained the best global multi-objective optimum, while the overlapping contour plot provided optimum individual and simultaneous per polishing range.

Efficient Removal Of Bromate From Aqueous Solution Through Electrocoagualtion Using Aluminum Electrodes

In this study the removal efficiency of bromate from aqueous solution using the electrocoagulation process was investigated. The effects of operational parameters such as initial pH, initial bromate concentration, current density, type electrode, salt concentration, and temperature on the removal efficiency have been studied. With an initial pH of 3.7, an initial bromate concentration of 100 mg/L, (current density 22.7 mA/cm2 using Al electrodes, salt concentration of 1.5 g/L NaCl and temperature of 25° C, The results showed that the removal percentage for bromate was 97% using Al/Al electrodes at 10 min. It can be concluded that electrocoagulation process by Al electrode is very efficient and clean process for bromate removal from Aqueous solution.

Modeling transport of membrane water in PEMFC and energy loss analysis at −20 °C based on mathematical boundedness

This work analyzes why the classic cold start-up model cannot match experimental data well in high initial membrane water content due to the unboundedness of membrane water content conservation equation, and a mathematical framework to ensure the boundedness of membrane water content is developed to solve these issues. Based on vehicle mechanism and Grotthuss mechanism, a “traffic jam” model is applied to describe the transport of membrane water reaching its maximum content. To keep the membrane water content within the upper bound, a new governing equation is proposed and the corresponding boundary conditions and discretization schemes are set. This model has been proved in eleven cases by comparing with experimental data including different initial membrane water content and different current density. Some of these cases are considered impossible to exist in classical cold start-up theory. The results illustrate that the boundedness model can show a clearer physical process and obtain more accurate and reasonable calculation results.

Understanding the key role of the surface structure of L11-ordered PtCu intermetallic electrocatalyst toward methanol oxidation reaction by dealloying methods

An accurate understanding of the surface structure–activity relationship is essential to rational designing highly efficient electrocatalysts for methanol oxidation reaction (MOR). Here, L11-ordered PtCu/C with rough and smooth Pt shells were obtained by electrochemically (ED) and chemically dealloying (CD) methods to investigate the difference in their MOR performances. The rough Pt shells (PtCu/C-700-ED) exhibited mass activity (MA) of 1625.2 mA mgPt−1 at peak potential, 2.19 and 2.74 times higher than smooth Pt shells (PtCu/C-700-CD) (743.1 mA mgPt−1) and commercial Pt/C (593.5 mA mgPt−1), respectively. DFT calculations indicate that PtCu with rough surfaces has a weaker CO binding energy than that with smooth surfaces, leading to better CO resistance and MOR activity. However, accelerated durability tests exhibit that the PtCu/C-700-CD catalysts obtained activity above the initial current density after 2000 cycles compared to PtCu/C-700-ED (36.48% loss) and commercial Pt/C (67.35% loss), indicating that smooth Pt shells have prominent structural stability. In addition, similar results were found simultaneously for other ordered Pt-based binary catalysts.

Bi-Phase NiCo2S4-NiS2/CFP Nanocomposites as a Highly Active Catalyst for Oxygen Evolution Reaction

Pursuing oxygen evolution reaction (OER) catalysts with high activity and stability is attracting many researchers. Here, we first designed and synthesized a biphasic three-dimensional structured catalyst of nickel–cobalt sulfide NiCo2S4-NiS2/CFP, which is a nickel–cobalt sulfide composite decorated on carbon fiber paper (CFP) by a one-step hydrothermal method. It exhibited an overpotential of about 165 mV at a current density of 10 mA cm−2 and a small Tafel slope of 81.54 mV dec−1. Furthermore, the long-term stability of the NiCo2S4-NiS2/CFP nanocomposite was 90.0% of the initial current density even after 12 h. The excellent catalytic performance of the NiCo2S4-NiS2/CFP nanocomposite can be attributed to several aspects. Firstly, the petal-like morphology of the NiCo2S4-NiS2 nanocomposite exposes more active sites. Secondly, the stability of the composite catalyst is significantly enhanced by its firm anchoring on the CFP. Thirdly, the catalytic performance was significantly improved by the addition of mixed valence of Ni or Co on the {111} plane of spinel NiCo2S4. Finally, the three-dimensional CFP substrate provides an efficient pathway and a stable integrated structure for the transmission of electrons and ions. Our one-step hydrothermal synthesis method provides a simple and economical way to obtain high-performance and robust OER catalysts.

Decolorization of reactive blue-2 dye in aqueous solution by electrocoagulation process using aluminum and steel electrodes

In this study we have applied electrocoagulation process for the decolorization of synthetic wastewater containing Reactive Blue-2 (RB-2). Electrocoagulation is eco-friendly, cost-effective and low maintenance technique which is best wastewater technology to remove total suspended solids (TSS), soluble organic compounds and other contaminants from water. Experiments for decolorization of RB-2 solutions were performed by varying the types of electrode Aluminum (Al) and steel connected with DC power supply. Color removal of RB-2 solution was determined by double beam UV spectrophotometer. Significance of the data was determined by applying Two-way ANOVA. Synthetic wastewater samples were prepared in laboratory using RB-2 dye by dissolving its known concentrations in distilled water. In this study we had studied the effect of four experimental variables initial pH, electrolysis time and current density and dye concentration on efficiency of color removal of RB-2 dye. Optimum experimental conditions for RB-2 dye was found at 95% color removal efficiency i.e. electrolysis time = 6 min, initial pH = 3 and at current density = 16.6 mA/cm2 and dye concentration = 5 ppm using steel electrodes. Among both the electrodes, steel showed best results at all experimental variables. Chlorine quenching experiments reveals that active chlorine species had also significantly affects the color removal. It was confirmed from the ANOVA that the decolorization of RB-2 solutions were significantly affected by varying the types of electrodes and other experimental parameters. From overall results it can be concluded that this system can be apply for decolorization of real wastewater.

On the cone-to-jet transition region and its significance in electrospray propulsion

A novel method for identifying the cone-to-jet transition region is proposed to characterize the electrospray dynamics, and thus the Taylor cone, a fundamental principle of ion extraction from the tip of the Taylor cone in an electric field to produce μN thrust for nanosatellites' application. A numerical model is developed and validated against the droplet's diameter with a proper characterization of the cone-to-jet transition region based on the hydrodynamic pressure gradient at the cone-jet axis. The results show that the cone-to-jet transition region is reduced when the electric potential increases and the flow rate decreases. Moreover, the current density is found to be higher for the lower flow rate and large electric potential. Interestingly, the current density at the jet axis increases significantly at the flow rate of 1 cc/hr. This implies that below a certain droplet diameter, the current density carried by the jet interface is completely migrated into the jet, and the substantial change in the current density occurs within the cone-to-jet transition region. When comparing the current density along the jet axis, the small flow rate and large electric potential carry the maximum current density at the jet interface. The identification of cone-to-jet transition region is central in modeling the ion beam trajectory via initial current density profile required for particle-in-cell calculation for assuring the performance of a field emission electric propulsion thruster.

Cr(OH)3 nanosheets@ZIF67 electrocatalysts prepared by electrodeposition method for efficient oxygen evolution reaction

In this paper, a Cr(OH)3 NSs@ZIF67 (NSs = nanosheets) electrocatalyst is prepared on foam Ni via a simple and rapid electrochemical deposition method. Excellent electrocatalytic activity of Cr(OH)3 NSs@ZIF67 is demonstrated. It can use the overpotential of 281 mV and 390 mV respectively to drive 10 mA cm−2 and 50 mA cm−2. It is observed that the Cr(OH)3 NSs@ZIF67 electrode has the highest initial current density at 1.57 V compared with the other two monomer electrodes and shows excellent stability at the end of 60 000 s. It has the largest electrochemical activity specific surface and lowest charge-transfer resistance, and M–O bonds (M = Co, Cr) and shifting of binding energy peaks at the interface lead to more active sites and more efficient electron transfer for oxygen evolution reaction. This work highlights the construction of highly efficient composite electrocatalysts composted of low-dimensional non-precious transition metal compounds and metalorganic frameworks, promoting the development of low-cost non-noble metal composites in energy chemistry.