Galvanostatic Tests Research Articles

Current collectors based on multiwalled carbon-nanotubes and few-layer graphene for enhancing the conversion process in scalable lithium-sulfur battery

We investigated herein the morphological, structural, and electrochemical features of electrodes using a sulfur (S)-super P carbon (SPC) composite (i.e., S@SPC-73), and including few-layer graphene (FLG), multiwalled carbon nanotubes (MWCNTs), or a mixture of them within the current collector design. Furthermore, we studied the effect of two different electron-conducting agents, that is, SPC and FLG, used in the slurry for the electrode preparation. The supports have high structural crystallinity, while their morphologies are dependent on the type of material used. Cyclic voltammetry (CV) shows a reversible and stable conversion reaction between Li and S with an activation process upon the first cycle leading to the decrease of cell polarization. This activation process is verified by electrochemical impedance spectroscopy (EIS) with a decrease of the resistance after the first CV scan. Furthermore, CV at increasing scan rates indicates a Li+ diffusion coefficient (D) ranging between 10−9 and 10−7 cm2·s−1 in the various states of charge of the cell, and the highest D value for the electrodes using FLG as electron-conducting agent. Galvanostatic tests performed at constant current of C/5 (1 C = 1675 mA·gS−1) show high initial specific capacity values, which decrease during the initial cycles due to a partial loss of the active material, and subsequently increase due to the activation process. All the electrodes show a Coulombic efficiency higher than 97% upon the initial cycles, and a retention strongly dependent on the electrode formulation. Therefore, this study suggests a careful control of the electrode in terms of current collector design and slurry composition to achieve good electrode morphology, mechanical stability, and promising electrochemical performance in practical Li-S cells.

Synthesis and characterization of reduced graphene oxide wrapped MoO3/TiS2 nanocomposite for high performance energy storage applications

Reduced graphene oxide (rGO) is added to MoO3 and TiS2 to develop several hybrid electrode materials, including rGO/MoO3, rGO/TiS2, and rGO/MoO3/TiS2 to enhance the electrochemical performance. SEM/EDX, TEM, XRD, XPS, and Raman techniques are used to study the resulting hybrid electrode materials. The specific capacitance and charge–discharge stability of the synthesized rGO/MoO3, rGO/TiS2, and rGO/MoO3/TiS2 electrode materials are assessed using galvanostatic and cyclic voltammetry testing. The rGO/MoO3/TiS2 exhibits higher specific capacitance of 1043.72 mA h/g at 20 mV/s as compared to rGO/TiS2 (625.61 mA h/g), rGO/MoO3 (567.54 mA h/g), and rGO (154.40 mA h/g), among the examined. All electrochemical measurements were conducted in a 3 M KOH solution. The rGO/MoO3/TiS2 electrode was found to have Rct value of 20.8 Ω.cm2. This value was much lower than that of the rGO electrode (89.8 Ω.cm2), indicating that the rGO/MoO3/TiS2 exhibits much lower resistance, which causes the improved electrochemical performance of the rGO/MoO3/TiS2. The retention of capacitance of 98.56 % after 8,000 cycles at 1 A/g shows that this rGO/MoO3/TiS2 hybrid electrode also exhibits good cycling stability. The rGO/MoO3/TiS2 composites' respective energy and power densities are determined to be 882 Wh/kg and 5773 W/kg. The good electrochemical performance of the rGO/TiS2/MoO3 composite anode is closely related to its structure, in which the TiS2/MoO3 nanoparticles are not only homogeneously anchored on the surface but also embedded in the interlayer of the rGO sheets; hence the volume change and aggregation of the TiS2/MoO3 nanoparticles can be effectively hindered. On the other hand, rGO itself is an electronic conductor; the rGO and TiS2/MoO3 nanoparticles connect closely, which offers large electrode/electrolyte contacting area, short path length for transporting of ions.

Engineering amorphous SnO2 nanoparticles integrated into porous N-doped carbon matrix as high-performance anode for lithium-ion batteries

A facile in-situ preparation strategy is proposed to anchor amorphous SnO2 nanoparticles into the porous N-doped carbon (NC) matrix to fabricate amorphous composite powders (am-SnO2@p-NC), which feature the hierarchically interconnected and well interlaced porous configurations by employing polyvinylpyrrolidone as the soft template. The morphology regulation of the porous structure is precisely realized by adjusting the content of the template and the relationship between structural evolution and electrochemical performance of composite powders is accurately described to explore the optimal template dosage. The results indicate that the am-SnO2@p-NC-50 % composite electrode can deliver the improved lithium storage capacity and cycling performance when the content of the template is controlled at 0.500 g, in which the initial discharge specific capacity is about 1557.6 mAh/g and the reversible value retains at 841.5 mAh/g after 100 cycles at 100 mA/g. Meanwhile, the discharge specific capacity of 869.8 mAh/g is exhibited for the am-SnO2@p-NC-50 % composite electrode after 60 cycles when the current density is recovered from 2000 to 100 mA/g. Moreover, the Li+ ions diffusion coefficient up to about 5.5 × 10−12 cm2/s is calculated from galvanostatic intermittent titration technique tests, which can be partly ascribed to the conductive NC substrate that provides the high electronic conductivity, and partly to the highly porous structure that shortens Li+ ions transfer pathways and guarantees the fast reaction kinetics. Therefore, the hierarchically porous engineering of carbon networks to confine amorphous transition metal oxide nanoparticles is of great significance in the development of high-performance anode materials for lithium-ion batteries.

“Win‐Win” Scenario of High Energy Density and Long Cycling Life in a Novel Na3.9MnCr0.9Zr0.1(PO4)3 Cathode

The development of high‐energy and long‐lifespan NASICON‐type cathode materials for sodium‐ion batteries has always been a research hotspot but a daunting challenge. Although Na4MnCr(PO4)3 has emerged as one of the most promising high‐energy‐density cathode materials owing to its three‐electron reactions, it still suffers from serious structural distortion upon repetitive charge/discharge processes caused by the Jahn‐Teller active Mn3+. Herein, the selective substitution of Cr by Zr in Na4MnCr(PO4)3 was explored to enhance the structural stability, due to the pinning effect of Zr ions and the ≈2.9‐electron reactions, as‐prepared Na3.9MnCr0.9Zr0.1(PO4)3/C delivers a high capacity retention of 85.94% over 500 cycles at 5 C and an ultrahigh capacity of 156.4 mAh g−1 at 0.1 C, enabling the stable energy output as high as 555.2 Wh kg−1. Moreover, during the whole charge/discharge process, a small volume change of only 6.7% was verified by in situ X‐ray diffraction, and the reversible reactions of Cr3+/Cr4+, Mn3+/Mn4+, and Mn2+/Mn3+ redox couples were identified via ex situ X‐ray photoelectron spectroscopy analyses. Galvanostatic intermittent titration technique tests and density functional theory calculations further demonstrated the fast reaction kinetics of the Na3.9MnCr0.9Zr0.1(PO4)3/C electrode. This work offers new opportunities for designing high‐energy and high‐stability NASICON cathodes by ion doping.

Molten Sodium Penetration in NaSICON Electrolytes at 0.1 A cm–2

High-conductivity solid electrolytes, such as the Na superionic conductor, NaSICON, are poised to play an increasingly important role in safe, reliable battery-based energy storage, enabling advanced sodium-based batteries. Coupled demands of increased current density (≥0.1 A cm–2) and low-temperature (<200 °C) operation, combined with increased discharge times for long-duration storage (>12 h), challenge the limitations of solid electrolytes. Here, we explore the penetration of molten sodium into NaSICON at high current densities. Previous studies of β″-alumina proposed that Poiseuille pressure-driven cracking (mode I) and recombination of ions and electrons within the solid electrolyte (mode II) are the two main mechanisms for Na penetration, but a comprehensive study of Na penetration in NaSICON is necessary, particularly at high current density. To further understand these modes, this work employs unidirectional galvanostatic testing of Na|NaSICON|Na symmetric cells at 0.1 A cm–2 over 23 h at 110 °C. While galvanostatic testing shows a relatively constant yet increasingly noisy voltage profile, electrochemical impedance spectroscopy (EIS) reveals a significant decrease in cell impedance correlated with significant sodium penetration, as observed in scanning electron microscopy (SEM). Further SEM analysis of sodium accumulation within NaSICON suggests that mode II failure may be far more prevalent than previously considered. Further, these findings suggest that total (dis)charge density (mAh cm–2), as opposed to current density (mA cm–2), may be a more critical parameter when examining solid electrolyte failure, highlighting the challenge of achieving long discharge times in batteries using solid electrolytes. Together, these results provide a better understanding of the limitations of NaSICON solid electrolytes under high current and emphasize the need for improved electrode–electrolyte interfaces.

Investigation of the Impact of High Concentration LiTFSI Electrolytes on Silicon Anodes with Reactive Force Field Simulations

The initial formation cycles are critical to the performance of a lithium-ion battery (LIB), particularly in the case of silicon anodes, where the high surface area and extreme volume expansion during cycling make silicon susceptible to detrimental side reactions with the electrolyte. The solid electrolyte interface (SEI) that is formed during these initial cycles serves to protect the surface of the anode from a continued reaction with the electrolyte, and its composition reflects the composition of the electrolyte. In this work, ReaxFF reactive force field simulations were used to investigate the interactions between ether-based electrolytes with high LiTFSI salt concentrations (up to 4 mol/L) and a silicon oxide surface. The simulation investigations were verified with galvanostatic testing and post-mortem X-ray photoelectron spectroscopy, revealing that highly concentrated electrolytes resulted in the faster formation and SEIs containing more inorganic and silicon species. This study emphasizes the importance of understanding the link between electrolyte composition and SEI formation. This ReaxFF approach demonstrates an accessible way to tune electrolyte compositions for optimized performance without costly, time-consuming experimentation.

Improved performance of the NaFePO4/Hardcarbon sodium-ion full cell

Chronoamperometry and galvanostatic procedures were applied in the LiFePO4-based ion exchange approach for the production of NaFePO4 cathodes. The materials obtained by various processes were employed as the cathode electrode in Na-ion half cells, and their structural and electrochemical properties were investigated. According to XRD patterns for both processes, it is seen that the Na-ions were successfully replaced with Li-ions. In addition of this, XPS and EDX-dot mapping analyses were supports the ion exchange procedure. The electrochemical properties of the cathode were investigated using CR2032 cells, and the diffusion coefficient and diffusion activation energy were calculated using the CV graphs at various scan rates and environmental temperatures. It was found that the Na-diffusion rate in the cell is faster at higher temperatures than at lower temperatures. The galvanostatic cycling tests were performed for C/10 and 1C rates at room temperature, 10 °C, and 50 °C and determined that the first capacities for C/10 and 1C at room temperature were 140mAh/g and 60mAh/g, respectively, with good stability. The full cells of NaFePO4/hard carbon were fabricated by pre-sodiation technique and the battery performance tests revealed that the cells have a 74 mAh/g for C/10-rate with a voltage window of 2–4 V.

Zinc Electrode Cycling in Deep Eutectic Solvent Electrolytes: An Electrochemical Study.

Among post-lithium ion battery technologies, rechargeable chemistries with Zn anodes bear notable technological promise owing to their high theoretical energy density, lower manufacturing cost, availability of raw materials and inherent safety. However, Zn anodes, when employed in aqueous electrolytes, suffer from hydrogen evolution, passivation, and shape changes. Alternative electrolytes can help tackle these issues, preserving the green and safe characteristics of aqueous-based ones. Deep eutectic solvents (DESs) are promising green and low-cost non-aqueous solvents for battery electrolytes. Specifically, the cycling of Zn anodes in DESs is expected to be reversible, chiefly owing to their dendrite-suppression capability. Nevertheless, apart from a few studies on Zn plating, insight into the cathodic-anodic electrochemistry of Zn in DESs is still very limited. In view of developing DES-based battery electrolytes, it is crucial to consider that a potential drawback might be their low ionic conductivity. Water molecules can be added to the eutectic mixtures by up to 40% to increase the diffusion coefficient of the electroactive species and lower the electrolyte viscosity without destroying the eutectic nature. In this study, we address the electrochemistry of Zn in two different hydrated DESs (ChU and ChEG with ~30% H2O). Fundamental electrokinetic and electrocrystallization studies based on cyclic voltammetry and chronoamperometry at different cathodic substrates are completed with a galvanostatic cycling test of Zn|Zn symmetric CR2032 coin cells, SEM imaging of electrodes and in situ SERS spectroscopy. This investigation concludes with the proposal of a specific DES/H2O/ZnSO4-based electrolyte that exhibits optimal functional performance, rationalized on the basis of fundamental electrochemical data, morphology evaluation and modeling of the cycling response.

Stable Cycling of Sodium Metal Anodes Enabled by a Sodium/Silica‐Gel Host

AbstractSodium metal batteries (SMBs) are innovative and promising energy storage systems. SMBs are a competitive technological paradigm compared to the state‐of‐the‐art lithium‐ion batteries thanks to the high specific capacity of sodium metal, i. e., 1166 mAh g−1, and the Na+/Na0 low redox potential of −2.71 V vs. standard hydrogen electrode. Unstable solid electrolyte interface (SEI), uncontrolled overpotentials and dendrite growth of sodium are the main drawbacks that hinder the development of SMBs. Herein, we propose a functionalized silica gel material with extended porosity (NaSGII) as an active host material in order to obtain stable cycling of sodium metal anodes. NaSGII was incorporated in composite electrodes using a conductive carbon additive and polymeric binders and the functional properties in SMBs cells demonstrated by galvanostatic tests. A Butler‐Volmer analysis demonstrated an improvement of electrokinetic parameters induced by NaSGII.

Iridium-titanium oxides for efficient oxygen evolution reaction in acidic media

Active and highly stable oxygen evolution reaction (OER) electrocatalyst for PEM-based water electrolysis are currently in high demand. Herein, we report a rutile iridium-titanium oxide solid solution (IrTiOx) through a facile one-step annealing of a Ti-based metal-organic framework precursor. The composite exhibits excellent OER activity and stability in acidic media, with a low overpotential of 296 mV at 10 mA cm−2 while the OER activity was retained during a 100-h galvanostatic stability test at a constant current of 10 mA cm−2 in 0.5 M H2SO4, outperforming the state-of-the-art IrO2-based electrocatalysts. We further demonstrate the structure evolution of iridium-titanium oxide during OER operations. In contrast to the initial uniform distribution of Ir and Ti over the entire architecture, after OER stability test, a hollow morphology is formed, in which the particle surface is covered with an IrOx-rich layer and entire particle becomes hollow. We ascribe the structure evolution to the Ir/Ti leaching and redeposition during the OER operations. We propose that the structure evolution of iridium-titanium oxide during the electrochemical process is responsible for the high OER activity and stability of IrTiOx.

Annealing‐Free Thioantimonate Argyrodites with High Li‐Ion Conductivity and Low Elastic Modulus

AbstractAlthough Li‐ion superconducting sulfides have been developed as solid electrolytes (SEs) in all‐solid‐state batteries, their high deformability, which is inherently beneficial for room‐temperature compaction, is overlooked and sacrificed. To solve this dilemmatic task, herein, highly deformable Li‐ion superconductors are reported using an annealing‐free process. The target thioantimonate, Li5.2Si0.2Sb0.8S4Br0.25I1.75, comprising bimetallic tetrahedra and bi‐halogen anions is synthesized by two‐step milling tuned for in situ crystallization, and exhibits excellent Li‐ion conductivity (σion) of 13.23 mS cm−1 (averaged) and a low elastic modulus (E) of 12.51 GPa (averaged). It has a cubic argyrodite phase of ≈57.39% crystallinity with a halogen occupancy of ≈90.67% at the 4c Wyckoff site. These increased halogen occupancy drives the Li‐ion redistribution and the formation of more Li vacancies, thus facilitating Li‐ion transport through inter‐cage pathway. Also, the facile annealing‐free process provides a unique glass‐ceramic structure advantageous for high deformability. These results represent a record‐breaking milestone from the combined viewpoint of σion and E among promising SEs. Electrochemical characterization, including galvanostatic cycling tests for 400 h, reveals that this material displays reasonable electrochemical stability and cell performance (150.82 mAh g−1 at 0.1C). These achievements shed light on the synthesis of practical SEs suffice both σion and E requirements.

Fe2O3 Nanoparticle Interfacial Reaction Redistributing Li-Ion Flux in Flexible Hierarchically Porous Membrane Electrodes for Dendrite-Free Lithium Metal Batteries

The uncontrollable formation of Li dendrites and the problems arising from it are the major obstacles in the practical application of Li metal batteries. Thereby, it is crucial to induce the uniform dispersion and deposition of Li+. To realize it, we constructed a flexible membrane electrode that facilitated Li+ dispersion by the interwoven conductive porous skeleton and simultaneously promoted Li+ deposition with doped active sites. The skeleton was composed of CNTs cross-linked with polyvinylidene fluoride (PVDF), which ensured the good conductivity and flexibility of membrane, and the −CF2 groups on PVDF chains also accelerated the migration of Li+. The doped Fe2O3 with strong lithiophilicity played a vital role in Li+ deposition. Quasi in situ X-ray photoelectron spectroscopy and in situ X-ray diffraction measurements were employed to reveal the reaction mechanism that Fe2O3 could be partially reduced by Li+ forming LixFe alloy, and reoxidized to amorphous Fe2O3. According to the galvanostatic current test of symmetric battery, the voltage had no significant changed after 1500 h, which proved that multifunctional membrane electrode effectively reduced the effect of Li dendrites. The assembled coin battery delivered an initial capacity of 125.9 mA h g–1 and still maintained at 114.9 mA h g–1 after 200 cycles at 0.5 C, with a capacity decay rate of only 0.048% per cycle. The pouch cell also exhibited excellent cycling stability with a Coulombic efficiency of >99.2% after 70 cycles at 0.5 C. This work provides a novel flexible membrane electrode design in solving the Li dendrites and developing novel electrodes for Li metal batteries.

Influence of polyethylene glycol inhibitors on properties of electroplated copper layer

Inhibitor is one of the important additive components in electrodeposited copper. As a common inhibitor, the concentration of PEG8000 has an important effect on electrodeposited copper. In this article, different concentrations of polyethylene glycol-based organic additives were added to the acidic electrolyte, and their effects on the electrochemical behavior of copper electrodeposition were investigated by galvanostatic test, cathodic polarization curve test, and cyclic voltammetry test. In addition, the surface morphology and crystallographic orientation of the obtained copper layers were analyzed by SEM and XRD. The results show that with the increase of PEG8000 concentration in the electroplating solution system, the inhibitory effect of PEG8000 on electroplating copper becomes stronger, and the copper surface density obtained by electroplating is better.

Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm−2 OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm−2 galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.

Milligrams to Kilograms - Scaling Production of Cobalt Free Lithium Excess Cathode Materials through Rotary Calcination

Both Tesla and the Government of the United States have listed the elimination of cobalt and the resilience of the battery cathode supply chain as critical issues in need of advancement 1,2. This investigation answered this call to action through scaling the production of a high-performance cobalt free lithium excess cathode material using rotary calcination. The cathode in this investigation is of the chemical series Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 (where x = 0.25), and to our knowledge this marks the first time an investigation has demonstrated the scaling of this class of materials. In order to achieve the several orders of magnitude increase in production scale the precursor feedstock for rotary calcination required comparison of sol-gel, Pechini, and solid-state precursors routes. Viability of each precursor synthesis route required comparison of synthesis time, synthesis complexity, Thermogravimetric Analysis, Dynamic Scanning Calorimetry, X-Ray Diffraction of intermediate and final materials, as well as Galvanostatic cycling of final cathode materials. Modification and controlled decomposition of sol-gel processing was ultimately used for the scaled production of precursor powders. Once the precursor feedstocks were fed through the rotary kiln we used X-Ray Diffraction, X-Ray Fluorescence, Galvanostatic Testing, and Scanning Electron Microscopy of resultant materials to optimize rotary calcination conditions to find the pareto optimal conditions that allowed for maximized material quality, and output, with minimized energy and time costs. These materials were also compared to our baseline material, which was made in a box furnace in ambient conditions over the course of 24 hours. Synthesis conditions were also modified to allow for the inclusion rapid water quenching of final scaled material to ensure material quality.Sources: F. C. for A. Batteries, National blueprint for lithium batteries 2021–2030, (2021).E. Musk and R. Denholm, UPDATES TO 2020 ANNUAL MEETING OF STOCKHOLDERS AND BATTERY DAY EVENT, (2020).

Reevaluating the Stability of the PEO-Based Solid-State Electrolytes for High Voltage Solid-State Batteries

Poly(ethylene oxide) (PEO) is a very popular polymer-based solid electrolyte material with relatively high ionic conductivity and dielectric constant1. However, many previous works mentioned that it could not be used in high voltage cells for its low oxidation potential (~3.9V vs. Li/Li+)2. However, only a handful of prior investigations evaluated the stability of the PEO-based solid-state electrolyte considering the contribution of the cathode surface chemistry to electrolyte oxidation and decomposition. Since most of the previous research exploring high voltage cathodes with PEO used LiCoO2, it could be possible the other cathode compositions will erode the PEO-based solid electrolyte in different and less extreme ways.Our work revisits the high voltage electrochemical stability of PEO-based solid state-electrolyte materials. Potentiodynamic and galvanostatic tests were performed in test cells using PEO electrolyte layers with either LiNixMnyCozO2 or LiCoO2 cathode materials. We found that the high voltage instability of PEO-based solid-state cells is profoundly affected by the instability of the cathode material used. Specifically, the LiCoO2 electrodes were observed to undergo an irreversible oxidation process where they shattered into small pieces, which then led to a rapid irreversible loss in capacity. In contrast, we found that the PEO-based solid-state electrolytes could be stably cycled with high-nickel content cathodes stably at a voltage up to 4.5V vs. Li/Li+ over many cycles with minimal capacity deterioration. K. Chrissopoulou, K. S. Andrikopoulos, S. Fotiadou, S. Bollas, C. Karageorgaki, D. Christofilos, G. A. Voyiatzis, and S. H. Anastasiadis, 44, 9710–9722 (2011).J. Qiu, X. Liu, R. Chen, Q. Li, Y. Wang, P. Chen, L. Gan, S.-J. Lee, D. Nordlund, Y. Liu, X. Yu, X. Bai, H. Li, and L. Chen, Adv. Funct. Mater., 30, 1909392 (2020).

Molten Salt Synthesis of High-Performance Cobalt Free Lithium Excess Cathodes

The cobalt free lithium excess chemistry cathode material of the compositional series Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 despite its high capacity has many shortcomings, such as voltage fade, poor materials stability, and poor rate capability 1–10. Our past investigations, among others, have highlighted the importance of the synthetic route on the on the performance of this material 6,11–13. This investigation found a novel synthesis route for Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 (x = 0.25) involving the use of molten salts during the 900 °C synthesis step would reduce the degree of voltage fade seen in materials cycling, and increase the overall stability of the material over the course of cycling. Our samples we prepared using a standard sol-gel approach where the post 500 °C precursor oxide was mixed with NaCl, Li2SO4, or LiNO3, before the final 900 °C calcination, all samples where then rapidly water quenched to stabilize the meta-stable materials phase, but also to dissolve any residual salts and wash the powders before use. These salts were selected due to their different chemistries and molten ranges. NaCl is thermodynamically and chemically stable enough where it likely functioned as an observer species, and Li2SO4 and LiNO3 could have served as additional lithiation sources. Thermogravimetric analysis was used to help determine the behaviors of these salts through range for each salts’ molten regime as well as to help determine any salt behaviors up to 900 °C; compared to the control sample NaCl sample was found to have rapid mass lost starting at 850 °C; Li2SO4 was found to cause less mass loss than the control sample; LiNO3 had rapid mass lost starting at 600 °C when compared to the control sample, suggesting a shorter window of molten-salt synthesis environment. Galvanostatic testing revealed that each molten salt sample had distinct electrochemical behaviors, and many had enhanced voltage and capacity stability. Analysis of the samples surfaces through the use of high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), suggest the formation of doped surface facets, and changes in the preferred facet orientation of samples. These methods also revealed that the molten salt synthesis conditions can lower the diffusion barrier between active material particles encouraging Ostwald ripening, as well as increased batch homogeneity.Sources: C. R. Fell et al., Chemistry of Materials, 1621–1629 (2013).Z. Lu, L. Y. Beaulieu, R. A. Donaberger, C. L. Thomas, and J. R. Dahn, Journal of The Electrochemical Society, 149, A778–A791 (2002).Y. Wu and A. Manthiram, Electrochemical and Solid-State Letters, 9, A221–A224 (2006).Z. Lu and J. R. Dahn, Journal of The Electrochemical Society, 149, A1454–A1459 (2002).N. Dupré, M. Cuisinier, E. Legall, D. War, and D. Guyomard, Journal of Power Sources, 299, 231–240 (2015).K. A. Jarvis et al., Acta Materialia, 108, 264–270 (2016).J. Wang et al., Int. J. Electrochem. Sci, 11, 333–342 (2016).M. Jiang, B. Key, Y. S. Meng, and C. P. Grey, Chemistry of Materials, 21, 2733–2745 (2009).J. Bréger et al., J Am Chem Soc, 127, 7529–7537 (2005).Y. J. Park, Y.-S. Hong, X. Wu, K. S. Ryu, and S. H. Chang, Journal of Power Sources, 129, 288–295 (2004).S. Burke and J. F. Whitacre, Journal of The Electrochemical Society, 167, 160518 (2020).S. Hy et al., Energy and Environmental Science, 9, 1931–1954 (2016).S. Laubach et al., Physical Chemistry Chemical Physics, 11, 3278–3289 (2009). Figure 1

Stability of Metal Hydroxide Organic Frameworks for Oxygen Evolution

The oxygen evolution reaction (OER) is central to storing electrical energy via chemical bonds in energy carriers and fuels through reactions such as electrochemical water splitting to produce hydrogen, CO2 reduction for CO and liquid hydrocarbons, and nitrogen to ammonia. While there has been considerable work towards the engineering of inexpensive yet highly active OER catalysts, current state-of-the-art materials are still at least an order of magnitude less active than oxygen evolving complexes found in biological systems that intricately combine inorganic metal-oxo clusters with organic ligands.1 As a result, metal organic frameworks (MOFs) have drawn considerable attention as hybrid organic-inorganic systems that can potentially mimic the unique structure of biological oxygen evolving complexes. Recently, metal hydroxide organic frameworks (MHOFs),2 a new class of MOFs that combine layered hydroxides with aromatic carboxylate linkers that stabilize the structure via π-π interactions, have been shown to display three times the tunability of layered hydroxides, offering extensive opportunities for further engineering of its electrochemical properties for numerous applications. However, the long-term stability of these material during OER is still unclear, which would be paramount to understand for further rational design of this class of material.In this study, we investigated Ni-based MHOFs with carboxylate linkers of varying π-π interaction strengths to understand how these differences affect the electrochemical stability of these materials during OER. We observed that the MHOFs all undergo activation during OER leading to two orders of magnitude increase in OER activity, where the MHOFs with weaker π-π interaction strengths tend to transform at a faster rate than the MHOFs with stronger π-π interaction strengths. We further characterized the MHOFs using a wide range of analytical techniques, including scanning transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and hard x-ray absorption spectroscopy, before and after extended OER cycling and galvanostatic tests to understand the transformed phase, which suggested that while the bulk structure largely remains unchanged, the surface undergoes significant restructuring into a Ni(OH)2-like phase during OER. Using operando UV-vis and Raman spectroscopy measurements on the MHOFs during OER to understand the factors that induce the transformation process, we found that there was a clear link between the Ni2+/3+, 4+ redox couple observed around 1.4 VRHE and a loss in the carboxylate organic linkers for the linkers with weak π-π interactions. However, for the MHOFs synthesized with linkers exhibiting strong π-π interaction strengths, there were smaller changes to the overall material during OER, suggesting that the bulk stability of these materials is largely dictated by the linker interaction strength and activation are primarily only surface transformations. These results directly demonstrate that linker selection also plays a key role in the stability MOFs under electrochemical conditions and are pertinent for rational design and understanding of the stability and activity of hybrid organic-inorganic materials as electrocatalysts.

Experimental Quantification of the Effects of Concentration and pH on Nitrate-to-Ammonia Reaction Selectivity for Copper Electrodes

Nitrates constitute more than 80% of nitrogen-contaminants in point sources of wastewater like municipal wastewater effluents, ion-exchange brines and low-level nuclear wastes. They represent an untapped source for nutrient and energy recovery, as nitrates (NO3 -) can be electrochemically reduced to ammonia (NH3), which can then be used as a fertilizer or fuel. The concentration of NO3 - and the pH of waste streams are strongly influenced by the source. For example, ion-exchange brines are highly concentrated in NO3 - and have a neutral pH as the brine results from the regeneration process of the ion-exchange resin, which does not require a high pH solution, whereas low-level nuclear wastes are highly concentrated in NO3 - and highly alkaline due to the chemical processes involved in their formation. Therefore, we have performed experiments to quantify the influences of NO3 - concentration (0.1 M – 1 M) and solution pH (8, 10 and 14) on the current-potential behavior and reaction selectivity towards NH3 production. We test the performance of Cu electrodes, as its behavior in neutral and mildly alkaline conditions—pH 8 and pH 10—has not comprehensively studied as compared to its widely established behavior in more strongly alkaline conditions, pH 12-14. Cyclic voltammograms were obtained in a standard 3-electrode cell with a copper disk (0.07 cm2) and a platinum wire as the working and counter electrodes respectively. Polarization curves from this experiment were used to inform applied potentials in the galvanostatic tests performed with a planar copper electrode (0.3 cm2) to quantify reaction selectivity to ammonia production. Electrochemical Impedance Spectroscopy (EIS) was used in situ to correlate surface changes with catalytic performance for the galvanostatic measurements. UV-Vis spectroscopy was used to identify and quantify concentrations of species present in the solution (NO3 -, NO2 -, and NH3 using the salicylate method). Our results indicate that higher currents can be reached for higher concentrations of NO3 - and higher pH solutions. The onset potentials for NO3 - to NO2 - shifted towards smaller values vs. RHE with increase in pH. Additionally, as the pH decreases from pH 14 to pH 8, new and distinct peaks appear in the cyclic voltammograms that are different from the NO3 --to-NO2 - peak. This was confirmed by running a separate measurement with only NO2 - at equivalent concentration and pH. Ammonia quantification in galavanostatic measurements points to decreasing rates of ammonia formation with pH. However, at pH 14, a smaller NO3 - concentration of 100 mM resulted in a larger NH3 selectivity compared to a 1M case, indicating the effects of competing surface reactions. EIS measurements for charge-transfer resistance and capacitance provides a useful framework to correlate observed changes in measured currents with dynamic changes to the catalytic surface. Results are further interpreted to deduce empirical correlations for the concentration and pH dependence for the exchange current density for the rate determining NO3 --to-NO2 - step on the tested Cu electrodes.

Engineering Oxygen Vacancies on Mixed-Valent Mesoporous α-MnO2 for High-Performance Asymmetric Supercapacitors.

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO2 through simply controlling the degree of redox reaction process, which could obtain a balance between Mn3+/Mn4+ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO2 including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO4 to Na2SO3. The electrode materials of α-MnO2-0.25 with an appropriate Mn3+/Mn4+ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g-1 at 0.5 A g-1, beyond most reported MnO2-based materials. The asymmetric supercapacitors formed from α-MnO2-0.25 and activated carbon can present an energy density as high as of 36.33 W h kg-1 at 200 W kg-1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics.