Limited Current Density Research Articles

Li Ion Hopping Conduction in Highly Concentrated Liquid Electrolytes

We demonstrate here that Li+ ion hopping conduction emerges in highly concentrated liquid electrolytes composed of LiBF4 and sulfolane (SL). Self-diffusion coefficients of Li+ (D Li), BF4 − (D BF4), and SL (D SL) were measured with pulsed field gradient (PFG) NMR. In the concentrated electrolytes with molar ratios of SL/LiBF4 ≤ 3, the ratios of diffusion coefficients, D SL/D Li and D BF4/D Li, become lower than unity, suggesting faster diffusion of Li+ than SL and BF4 −. X-ray crystallographic analysis of a single crystal of LiBF4:SL (1:1) solvate revealed that the two oxygen atoms of the sulfone group are involved in bridging coordination of two different Li+ ions. In addition, the BF4 − anion also participates in bridging coordination of Li+. Raman spectroscopy revealed that Li+ ions are bridged by SL and BF4 − even in the highly concentrated LiBF4/SL solution. The fact that D Li is larger than D SL and D BF4 in the highly concentrated electrolytes strongly suggests that Li+ exchanges between the coordination sites formed by the ligands (SL and BF4 −) and moves forward leaving behind the ligands. The SL and BF4 − provide closely placed coordination sites in the highly concentrated electrolytes. The spatial proximity of coordination sites is assumed to enable Li+ hopping conduction. Furthermore, we found that Li+ hopping conduction occurs in the highly concentrated SL-based electrolytes containing other Li salts, such as LiClO4 and Li(NSO2F)2. The higher diffusion coefficient of Li+ than that of anion results in the higher Li+ ion transference number (> 0.5) in the electrolytes. The higher Li+ ion transference number was effective in suppressing the concentration polarization in a lithium battery, leading to increased limiting current density and improved rate capability compared to the conventional concentration electrolyte. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368 and 18H03926) from the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). Reference K. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi, M. Watanabe, J. Phys. Chem. B, 122, 10736-10745 (2018).

Complexing‐Coprecipitation Method to Synthesize Catalysts of Cobalt, Nitrogen‐Doped Carbon, and CeO2 Nanosheets for Highly Efficient Oxygen Reduction

AbstractThe oxygen reduction reaction (ORR) is the key cathode reaction in renewable‐energy technologies. Developing highly efficient, low‐cost, methanol‐tolerant and durable ORR catalysts is highly needed. In this research, we reported a simple complexing‐coprecipitation method to synthesize CeO2 nanosheets encapsulated by a nitrogen‐doped carbon (NC) layer with uniformly dispersed Co nanoparticles catalysts on the surface. In particular, the 0.5Co‐NC‐CeO2 sample exhibits high ORR catalytic activity with an onset potential of 875 mV (vs. RHE), a diffusion‐limited current density of 4.72 mA/cm2 and a positive half‐wave potential of 817 mV (vs RHE), while 20% Pt/C presents an onset potential of 934 mV (vs. RHE), a diffusion limited current density of 5.8 mA/cm2 and half‐wave potential of 850 mV (vs. RHE). In addition, the catalyst 0.5Co‐NC‐CeO2 also possesses a robust durability, that is the half‐wave potential exhibits a negative shift of only 17 mV after 1000 cycles. The enhanced ORR performance can be attributed to the synergistic effect among Co, the nitrogen‐doped carbon layer and CeO2. The results demonstrated here have implications in designing non‐Pt ORR catalysts.

Space Charge Effects on Ion Mobility Spectrometry.

We study the space charge limited maximal current density j″ of mobility-selected ions that can be transmitted in ion mobility spectrometry (IMS). Theory and experiments focus on differential mobility analyzers (DMAs), but are readily generalizable to other IMS devices. Repulsion between the ions in the cloud leads to beam spreading, with significant broadening once the ion number density n becomes comparable to the space charge saturation limit nsat = εoEo/(eΔ). Δ is the distance traversed by the ions in the direction of an externally imposed electric field Eo, and e is the charge on each ion. For ions of electrical mobility Z, j″ is then limited below j″sat = ZEoensat = ZεoEo2/Δ. A theory including diffusion and space charge effects is developed that reduces to Burgers' exactly solvable equation. The theory is tested in experiments with room temperature electrosprays (ES) of 100mM [ethyl3N+-formate-] in methanol. This spray produces primarily a single ionic species at very high initial concentration n, which may be tuned above or below nsat by varying the distance from the ES emitter to the inlet slit of the DMA. Mobility-selected ion densities n > 3.108 ions/cm3 are achieved, with n~nsat, and with drastically broadened mobility peak shapes having the approximate top hat-predicted shapes. However, the largest n values approaching nsat are not quantitatively measurable because the densest sprays do not fill the outlet slit length. Graphical Abstract .

Pore network modeling of liquid water and oxygen transport through the porosity-graded bilayer gas diffusion layer of polymer electrolyte membrane fuel cells

In polymer electrolyte membrane fuel cells (PEMFCs), understanding the effect of graded porosity on liquid water distribution and oxygen diffusion inside the porous components is essential for the improvement of water management strategy. In this work, a regular three-dimensional pore network model is constructed to represent the bilayer gas diffusion layer (GDL) consisting of a porosity-graded micro-porous layer (GMPL) and a gas diffusion backing layer (GDBL). Based on this model, the liquid water distribution at breakthrough, relative oxygen effective diffusivity and limiting current density as a function of water saturation are obtained using invasion percolation algorithm and resistor-network theory, respectively. Parametric studies are also conducted to elucidate the effects of several structural factors, such as the graded porosity value of the GMPL, the GMPL thickness and the number of sub-layers in GMPL. The simulated results illustrate that the introduction of graded porosity can relieve the extent of the flooding phenomenon to a certain extent, and the underlying mechanism can be explained from the perspective of the pore scale. Furthermore, an optimum graded porosity value where the material has the preferred oxygen transfer capacity and high limit current density under partially saturated condition (water saturation is between 0.025 and 0.5) can be found.

High-Performing PGM-Free AEMFC Cathodes from Carbon-Supported Cobalt Ferrite Nanoparticles

Efficient and durable non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, cobalt ferrite (CF) nanoparticles supported on Vulcan XC-72 carbon (CF-VC) were created through a facile, scalable solvothermal method. The nano-sized CF particles were spherical with a narrow particle size distribution. The CF-VC catalyst showed good ORR activity, possessing a half-wave potential of 0.71 V. Although the intrinsic activity of the CF-VC catalyst was not as high as some other platinum group metal (PGM)-free catalysts in the literature, where this catalyst really shined was in operating AEMFCs. When used as the cathode in a single cell 5 cm−2 AEMFC, the CF-VC containing electrode was able to achieve a peak power density of 1350 mW cm−2 (iR-corrected: 1660 mW cm−2) and a mass transport limited current density of more than 4 A cm−2 operating on H2/O2. Operating on H2/Air (CO2-free), the same cathode was able to achieve a peak power density of 670 mW cm−2 (iR-corrected: 730 mW cm−2) and a mass transport limited current density of more than 2 A cm−2. These peak power and achievable current densities are among the highest reported values in the literature to date.

A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser cathodes by using different precipitants

Abstract In order to optimize the synthesis process of barium-calcium-aluminate (BCA) for the application of impregnated dispenser cathodes, the influence of two precipitants, ammonium carbonate (AC) and ammonium hydrogen carbonate (AHC), on the chemical composition, phase transformation, thermal behaviors and morphology of the precursors was systematically investigated by means of XRD, TG-DSC, FESEM, and EDS mapping analysis. The results revealed that AC precipitant facilities to form NH4Al (OH)2CO3 (AACH) phase and barium calcium carbonate in the precursor, while the product precipitated by AHC comprises amorphous γ-AlOOH phase and the carbonate. Both precursors precipitated by the AC and AHC undergo various stages of phase transformations and finally converted to an emission-active material Ba3CaAl2O7 phase after sintering at 1200 °C. DC emission current values have been measured and the maximum space charge limited current densities are 8.0 ± 0.3 and 6.5 ± 0.4 A/cm2 at 1130 °Cb for the testing cathodes impregnated with the AC and AHC precursor aluminates, respectively. The difference in the emission capacity originates from the barium content in the aluminates prepared with different precipitants.

Porous NiCo2S4/Co9S8 Microcubes Templated by Sacrificial ZnO Spheres as an Efficient Bifunctional Oxygen Electrocatalyst

AbstractInnovative nano/microarchitectures, owing to their promising catalytic features, such as abundant active sites, high surface area, and enhanced ion transport, are ideal for oxygen electrocatalysis. Herein, an inventive approach is put forward to fabricate ternary metal sulfide hollow microcubes by a hydrothermal method between spherical porous ZnO with transition metal cations in the presence of urea followed by etching and sulfurization. The significant electrocatalytic activity toward oxygen evolution reaction (OER) with onset potential of 1.48 V (vs reversible hydrogen electrode (RHE)) is obsereved. The objective material shows an overpotential of 320 mV, to achieve a current density of 10 mA cm−2 for OER and excellent robustness with slight variation in potential after long‐term chronopotentiometry test for 12 h. As expected, the material also exhibits oxygen reduction reaction (ORR) activity with a half wave potential of 0.8 V (vs RHE) and limited current density of 5.1 mA cm−2. Meanwhile, further investigations confirm the near four‐electron transport pathways for ORR. Such up‐bottom etching followed by sulfurization method provide a new concept for engineering porous electrocatalysts.

Porous Co-N-C ORR catalysts of high performance synthesized with ZIF-67 templates

Oxygen reduction reaction (ORR) is a multiphase reaction, involving electron transfer and ion diffusion. A high-performance ORR catalyst must have high activity and good-pore characteristics. In this study, we developed a new strategy to synthesize porous Co-N-C catalysts based on zeolitic imidazolate frameworks (ZIF-67) as both templates and nitrogen sources. During high-temperature annealing treatment of ZIF-67/ZIF-8/graphene oxide, ZIF-8 and ZIF-67 were decomposed to Co-N-C and ZnO. Then ZnO was reduced to volatile zinc at 920 °C, resulting in porous Co-N-C catalysts. Those Co-N-C catalysts showed a high limit current density of 6.23 mA cm−2, which was higher than the Co-N-C arising from ZIF-67/graphene oxide and N-doped carbon from ZIF-8/graphene oxide. The improved performance can be ascribed to the presence of Co-N-C and high specific surface area in Co-N-C from ZIF-67/ZIF-8/graphene oxide. The presence route may provide a new insight for synthesis and application of zeolitic imidazolate frameworks in energy fields.

Design of spiral-wound electrodialysis modules

Spiral-wound electrodialysis (ED) modules are of interest because, in a parallel flow configuration where both the diluate and concentrate streams flow from the inner electrode to the outer electrode along a spiral path, the applied current density decreases as the concentration in the diluate stream and associated limiting current density (LCD) decrease. By matching the applied current density as closely as possible to the LCD at any given location in a stack, the required amount of membrane area is minimized, reducing capital cost. This work presents an analytical model for a spiral-wound ED module and experimental validation of that model using a prototype stack with two cell pairs and four revolutions. A constant voltage was applied and the total current and stream conductivities at mid-stack and the output were recorded. Experimental results agreed with the model for all parameters to within 15%. The model was used to explore the most cost-effective spiral stack designs for desalting brackish groundwater, examining both a standard Archimedean spiral (as is common for spiral-wound RO modules), and a novel ideal spiral. The ideal spiral shape was found to reduce total cost by 21% and capital cost by 39% with respect to an Archimedean spiral.

Bimetallic Covalent Organic Frameworks for Constructing Multifunctional Electrocatalyst.

Covalent organic frameworks (COFs) are a new class of crystalline porous polymers comprised mainly of carbon atoms, and are versatile for the integration of heteroatoms such as B, O, and N into the skeletons. The designable structure and abundant composition render COFs useful as precursors for heteroatom-doped porous carbons for energy storage and conversion. Herein, we describe a multifunctional electrochemical catalyst obtained through pyrolysis of a bimetallic COF. The catalyst possesses hierarchical pores and abundant iron and cobalt nanoparticles embedded with standing carbon layers. By integrating these features, the catalyst exhibits excellent electrochemical catalytic activity in the oxygen reduction reaction (ORR), with a 50 mV positive half-wave potential, a higher limited diffusion current density, and a much smaller Tafel slope than a Pt-C catalyst. Moreover, the catalyst displays superior electrochemical performance toward the hydrogen evolution reaction (HER), with overpotentials of -0.26 V and -0.33 V in acidic and alkaline aqueous solution, respectively, at a current density of 10 mA cm-2 . The overpotential in the catalysis of the oxygen evolution reaction (OER) was 1.59 V at the same current density.

Nitrogen‐doped Porous Carbon with Brain‐like Structure Derived from Quaternary Bipyridinium‐type Framework for Efficient Oxygen Reduction Electrocatalysis and Supercapacitors

Nitrogen-doped porous carbon materials have now become one of the most important candidates for efficient oxygen reduction electrocatalysis and supercapacitors because of their low cost and high stability. In the present study, a novel triazine-based, quaternary bipyridinium-type framework (TBPF) is synthesized by polycondensation of 4,4′-bipyridine with cyanuric chloride. The as-synthesized TBPF is then subject to pyrolysis at 900 °C, yielding N-doped porous carbon (N/C-900). The N/C-900 material possesses brain-like architecture, high N-containing functionalities (5.40 at.% N), and relatively high specific surface area (684 m2 g−1), which can act as a truly metal-free catalyst toward ORR and an electrode material for supercapacitors. Due to the synergistic effects, the N/C-900 material shows outstanding ORR electrocatalytic activity and stability in alkaline media with an onset potential of 0.969 V (vs RHE), a half-wave potential of 0.843 V (vs RHE), a limited current density of 5.05 mA cm−2, and a 3 mV negative shift of half-wave potential after 3000 cycles. Furthermore, the N/C-900-based supercapacitor electrode also displays relatively high specific capacitance (217 F g−1 at 0.5 A g−1) and excellent stability (96 % capacitance retention after 10000 cycles) in 6 M KOH electrolyte.

Separation of metals from electroplating wastewater using electrodialysis

ABSTRACTIn this study, the separation of metals from electroplating wastewater using an electrodialysis system was investigated. The separation efficiency of the metal ions increased as the applied voltage was increased from 6 to 18 V. However, at 18 V white sediments formed due to the high current density. The limiting current density (LCD) in the tested wastewater was calculated to be 22.5 mA/cm2 and the optimum applied voltage was estimated to be 12 V. The electrical conductivity of the diluate was reduced by 96.8%, and the conductivity of the concentrate increased. The separation efficiency of copper and nickel nearly identical, and after 25 min of operation, the removal efficiency of both metals was ˃99%. The final concentrations of Cu2+ and Ni2+ in the circulating concentrate were 1,000 and 1,200 mg/L with recovery rates of 90.7% and 90.2%, respectively. Thus, it was suggested that electrodialysis could effectively treat electroplating wastewater and recover ˃90% of the metals.

Achieving Janus Ag@N-doped carbon for oxygen reduction reaction from eccentric encapsulated Ag@polypyrrole

Taking full advantage of both silver and nitrogen-doped carbon (N-C) with finely designed nanostructure is a great challenge in achieving excellent electrocatalysts for oxygen reduction reaction (ORR). The Janus nanostructure composed of Ag nanoparticles (NPs) and N-C with individual exposed surface meets this purpose quite well. This can be achieved via pyrolyzing the core-shell Ag@polypyrrole (Ag@PPy) with eccentric encapsulated structure, where the AgNPs locate away from the center of the PPy spheres. The as-synthesized Janus nanostructured Ag@N-C demonstrates comparable ORR activity to commercial Pt/C catalyst with an onset and half-wave potential of 0.955 V and 0.821 V vs. RHE, respectively, and a limited current density of 4.976 mA cm−2 at 0.365 V, and it also illustrates better stability and stronger methanol tolerance. The excellent ORR activity is largely attributed to the synergistic effect originating from the interaction between AgNPs and carbonized PPy half-shell, where the former one leads to a higher disordered degree in the carbon layer and the latter results in the positive shift of Ag 3d binding energy to facilitate the active oxygen adsorption and keeps the AgNPs from aggregation to avoid activity losing.

Nitrogen-doped braided-looking mesoporous carbonaceous nanotubes as an advanced oxygen reduction electrocatalyst

Exploring low-cost, high-active, durable electrocatalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries is highly desirable but remains a great challenge. In this work, with the application of a newly developed metal-free nanocasting method, an in-situ mesoporous nitrogen-doped braided-looking carbon nanotube (MNCNT) catalyst was fabricated. It was examined to possess a high specific surface area (828 m2g-1), and pores with the tunable size. The catalyst shows superb ORR activity with a half-wave potential (E1/2) of 0.77 V vs. RHE in alkaline medium, inferior to that of commercial 20 wt% Pt/C, but superior to those of 5 wt% Pt/C and most other metal-free catalysts reported in previous literature. Besides, a high limited current density of 3.9 mA cm−2 at 0.2 V vs. RHE is yielded, and strong methanol tolerance as well as good stability of such catalyst is observed. According to the comparable study, the high ORR activity can be attributed to the porous feature of such nanotubes, as it may engender more active sites for ORR (porous nature & more edges), and boost reactivity of the active sites (nitrogen-doping), compared with the general carbon nanotubes. This work may intrigue the inspiration to explore highly efficient metal-free and carbon-based electrocatalysts toward boosting ORR.

N, P dual-doped multi-wrinkled nanosheets prepared from the egg crude lecithin as the efficient metal-free electrocatalyst for oxygen reduction reaction

N, P dual-doped carbon nanosheets was prepared by an in-situ foaming technology. The crude lecithin extracted from egg yolk was explored to prepare this novel carbon material. In the synthesis process, the crude lecithin molecular is as the Gemini surfactant and the graphitic carbon nitride (g-C3N4) plays the role of self-sacrificing template. At the high temperature, the sandwich-mixture “lecithin/C3N4/lecithin” expanded and carbonized with the decomposition of g-C3N4. This induces the formation of many multi-wrinkled nanosheets and synchronously constitute a multi-doped carbon foam. This unique carbon material exhibits a super-efficient catalytic activity towards oxygen reduction reaction, with the limited current density of 6.7 mA cm−2 tested at 1600 rpm in 0.1 M KOH solution, which is far larger than that of Pt/C (20%) catalyst (5.5 mA cm−2). It was demonstrated that this novel catalyst also possesses great catalytic stability. After about 9 h continuous test, the current density of this catalyst remained above 91%. These results demonstrate that the N, P dual-doped multi-wrinkled carbon nanosheets can be used as the high-efficient metal-free electrocatalyst towards oxygen reduction reaction.

Passivation of Zinc Anodes in Alkaline Electrolyte: Part II. Influence of Operation Parameters

On the basis of a previously developed measurement method [M. Bockelmann, M. Becker, L. Reining, U. Kunz, and T. Turek, J. Electrochem. Soc., 165 (13), A3048 (2018)] the influence of current load interruptions, KOH electrolyte composition, temperature, and forced electrolyte convection on the anodic passivation of zinc was investigated in this study. Our aim was to find appropriate experimental conditions which could allow a long-term usability of the zinc anode without formation of passive films. We found out that interruptions lasting several minutes during galvanostatic dissolution of zinc, as well as increasing temperature and raising concentration of OH− ions in the electrolyte, effectively prolonged the service life of the electrode, but could not fully prevent it from passivation. On the contrary, increasing concentration of zinc oxide in the electrolyte enhanced the direct oxidation of zinc. However, application of sufficiently strong electrolyte convection and electrode overpotentials below a limiting value of about 0.1 V, allowed for electrode dissolution times of more than 1000 minutes without any indication of passive film formation. Therefore, the passivation of zinc anodes can be effectively avoided in cells with forced electrolyte convection and limited current densities.

Fe,N-Codoped Graphdiyne Displaying Efficient Oxygen Reduction Reaction Activity.

On account of the high-cost of platinum, researchers are working to develop a new catalyst that is cheaper and has a catalytic effect equivalent to platinum. Herein, owing to the unique acetylenic bonds in graphdiyne, iron, nitrogen co-doped graphdiyne (Fe-N-GDY) is a promising nonprecious metal catalyst, which has been developed with just a small amount of iron precursor with the plan to substitute it for Pt-based catalysts. The as-synthesized Fe-N-GDY composited catalyst shows excellent catalytic performance with the onset potential of 0.94 V versus reversible hydrogen electrode and limited current density of 5.4 mA cm-2 . Moreover, it shows excellent resistance to methanol poisoning and stability in both acidic and alkaline electrolytes, which makes potentially applicable in the oxygen reduction reaction field.

A novel electrocatalyst based on Fe2Ni1 nanoparticles anchored nitrogen doped graphene nanosheets towards efficient oxygen reduction reaction

The fabrication of efficient and durable novel electrocatalysts using a facile and cheap way is a potential approach for real energy conversion technology. In this research, a novel nanohybrid based on iron nickel nanoparticles well attached on nitrogen doped graphene nanosheets (Fe2Ni1 NPs/N-Gr) was synthesized through a simple refluxing strategy followed by a calcination process. It was discovered that the material displayed good catalytic behavior for oxygen reduction reaction (ORR) in alkaline environment. The ORR kinetics of the material primarily occurred through a 4-electron transferred pathway consistent with high diffusion limit current density, excellent onset potential (−0.013 V) as well as half-wave potential (−0.065 V). Furthermore, the material showed good stability and outstanding methanol tolerance as compared to a commercial Pt/C catalyst. The resulting catalytic performance of the material was assumed to the synergistic effect from bimetallic Fe2Ni1 NPs and thin nitrogen doped graphene, which improved electroactive site numbers, promoted charge transfer ability, and adjusted adsorption energy for oxygen molecules. The results imply that such novel materials may be an effective and low-cost catalyst for ORR in energy conversion technology.

Uniform WMoSx nanoparticles attached graphene nanosheets as highly effective electrocatalyst for oxygen reduction reaction in alkaline medium

Abstract Small and uniform bimetallic WMoSx nanoparticles well distributed on surface of thin graphene nanosheets (WMoSx-Gr) was prepared and investigated as an effective electrocatalysts for oxygen reduction reaction (ORR) in alkaline electrolyte. This material exhibited highly catalytic activity relative to a commercial Pt/C product. In this context, the material showed a large limit current density, positive onset potential (−0.05 V) as well as half wave potential (−0.13 V). In particularly, the superior durability and methanol tolerance of the as-synthesized material were also pronounced, as compared to Pt/C. The high ORR activity of the WMoSx-Gr material was assumed to the synergistic effect produced from a unique nanostructure of the WMoSx and the high surface area graphene with its excellent conductivity. The obtained results imply that such hybrid material can open a new type of highly efficient non-Pt catalyst for ORR in alkaline environment.

In-situ fabrication of nitrogen-doped carbon nanosheets containing highly dispersed single iron atoms for oxygen reduction reaction

Iron and nitrogen co-doped carbons show great potential for high-performance electrochemical oxygen reduction reaction. However, the rational design of atomically dispersed iron over nitrogen-doped carbons with activity comparable to that of Pt-C is still challenging. Herein, we develop a new approach that enables the direct formation of intrinsically nitrogen-functionalized two-dimensional sheet-like carbons containing a high concentration of single Fe atoms. This strategy only involves one-step pyrolysis of both, guanine and iron nitrate, without using any guiding agent and sacrificial template. The electrochemistry tests demonstrate an excellent ORR performance of the prepared Fe-Nx-C catalyst with a half-wave potential of 0.85 V and a limited current density of −6.5 mA/cm2 in alkaline medium, outperforming the commercial Pt-C and most of previously reported Fe-Nx-C catalysts. We believe that the emergence of superior ORR performance is mostly attributed to the uniform dispersion of single Fe atoms at the molecular level and the formation of abundant coordinated Fe-Nx sites. In addition, the high surface area, optimal porosity and defective structure (particularly the defects at the edge) of the two-dimensional carbons are also beneficial for the improved ORR activity.