Impurities In Electrolyte Research Articles

Sensitivity of Capacity Fade in Vanadium Redox Flow Battery to Electrolyte Impurity Content.

The gradual capacity decrease of vanadium redox flow battery (VRFB) over long-term charge-discharge cycling is determined by electrolyte degradation. While it was initially believed that this degradation was solely caused by crossover, recent research suggests that oxidative imbalance induced by hydrogen evolution reaction (HER) also plays a significant role. In this work by using vanadium pentoxides with different impurities content, we prepared three grades of vanadium electrolyte. By measuring electrochemical properties on carbon felt electrode in three-electrode cell and VRFB membrane-electrode assembly we evaluate the influence of impurity content on battery polarization and rate of side reactions which is indicated by the increase of average oxidation state (AOS) during charge-discharge tests and varies from 0.061 to 0.027 day-1 for electrolytes made from 99.1 and 99.9 wt % V2O5. We found that increase of AOS correlates with the increase of open-circuit voltage of VRFB in the discharged state ranging from 9.6 to 14.9 mV day-1 for highest and lowest electrolyte purity levels, respectively. While AOS increase is significant, it does not solely determine capacity fade. It is demonstrated that the presence of vanadium crossover decreases capacity fade, i. e. levels the contribution of side reactions on capacity drop.

The Effect of Electrolyte pH and Impurities on the Stability of Electrolytic Bicarbonate Conversion.

Electrolytic bicarbonate conversion holds the promise to integrate carbon capture directly with electrochemical conversion. Most research has focused on improving the faradaic efficiencies of the system, however, the stability of the system has not been thoroughly addressed. Here, we find that the bulk electrolyte pH has a large effect on the selectivity, where a higher pH results in a lower selectivity. However, the bulk electrolyte pH has no effect on the stability of the system. A decrease in CO selectivity of 30% was observed within the first three hours of operation in an optimized system with 3 M KHCO3 and gap between the membrane and electrode. Single-pass electrolyte experiments at various constant pH values (8.5, 9.0, 9.5, and 10.0), show that only at a pH of 10 the CO selectivity was stable during three hours, reaching a faradaic efficiency toward CO of only 18% as compared to an initial 55% at pH 8.5. Trace metal impurities present in the electrolyte were found to be the cause of the decrease in stability as these deposit on the electrode surface. By complexing the trace metal ions with ethylenediaminetetraacetic acid (EDTA), the metal deposition was avoided and a stable CO selectivity was obtained.

Exploring Fe Retention in Oxygen Evolution Reaction Catalysts Via Composition Engineering

The oxygen evolution reaction (OER) is an electrochemical half-reaction critical to electrolyzer and metal-air battery technologies. It is well established that Ni (oxy)hydroxides are state-of-the-art electrocatalysts for the OER in basic media when Fe impurities are present in these systems. Morphological studies have shown that these catalysts are prone to dramatic nanoscale reconstruction under electrochemical conditions, implying dissolution, migration and redeposition processes of the catalyst’s components. Processes involving the displacement of Fe are especially important to understand, as these impurities, while essential for high activity at the anode, are often associated with cathode fouling. Describing the parameters that govern Fe leaching is thus of interest for the design of materials aimed toward the maintenance of device performance and longevity. To study how the degree of restructuring relates to Fe dissolution and redeposition, we have prepared a series of electrolyte accessible Ni(z)Co(1-z)O(x)H(y) (z = 0, 0.2, 0.4, ...) thin film catalysts, whereby increased amounts of Co limit the material’s dynamic behavior. By introducing tertiary cations with trackable redox features, we map a parameter space of catalyst composition more or less capable of retaining surface active phases. Parallels with Ni-Co + Fe catalysts are then carried out in Fe-free electrolyte to disrupt absorption-desorption equilibria and connect host-structure dynamics to equilibrium concentrations of electrolyte impurities.

Evaluating the impact of iron impurities in KOH on OER performance of BaNiO3 single crystals using scanning electrochemical cell microscopy

Understanding the oxygen evolution reaction (OER) electrocatalysis requires a careful understanding of the surface and activity evolution of well-defined materials. An especially promising candidate is the BaNiO3 hexagonal perovskite, matching strides with benchmark catalysts such as Ba0.5Sr0.5Co0.8Fe0.2O3–δ. Here, we conducted a structural and electrochemical analysis of BaNiO3 single crystals to assess the influence of iron impurities in KOH electrolyte on their OER performance. We used scanning electrochemical cell microscopy (SECCM) for precise measurements on BaNiO3 single crystals in the absence of carbon additives. Cyclic voltammetry (CV) revealed that the presence of iron consistently enhanced the OER current with subsequent cycles, indicating a dynamic improvement in BaNiO3's electrochemical activity. Conversely, in iron-free KOH electrolyte, performance diminished with cycling. These findings not only underscore the critical necessity of KOH purification for BaNiO3 electrochemical studies but also showcase SECCM's unparalleled ability to offer insights into the electrochemical evolution of individual entities such as single crystals. This makes it a powerful operando tool for evaluating forthcoming single crystals, thereby aiding in the design of superior catalysts for OER reactions.

Toward High-Quality Sulfide Solid Electrolytes: A Liquid-Phase Approach Featured with an Interparticle Coupled Unification Effect.

Sulfide solid electrolytes (SSEs) are highly wanted for solid-state batteries (SSBs). While their liquid-phase synthesis is advantageous over their solid-phase strategy in scalable production, it confronts other challenges, such as low-purity products, user-unfriendly solvents, energy-inefficient solvent removal, and unsatisfactory performance. This article demonstrates that a suspension-based solvothermal method using single oxygen-free solvents can solve those problems. Experimental observations and theoretical calculations together show that the basic function of suspension-treatment is "interparticle-coupled unification", that is, even individually insoluble solid precursors can mutually adsorb and amalgamate to generate uniform composites in nonpolar solvents. This anti-intuitive concept is established when investigating the origins of impurities in SSEs electrolytes made by the conventional tetrahydrofuran-ethanol method and then searching for new solvents. Its generality is supported by four eligible alkane solvents and four types of SSEs. The electrochemical assessments on the former three SSEs show that they are competitive with their counterparts in the literature. Moreover, the synthesized SSEs presents excellent battery performance, showing great potential for practical applications.

Understanding the Effects of Transition Metal Impurities on Nickel (oxy) Hydroxide Electrocatalysts in Alkaline Media

Nickel-based materials are widely used as critical components of electrochemical energy devices, including alkaline electrolyzers, capacitors, and alkaline batteries. These materials are often preferred because of their availability and low cost compared to precious metals, which makes them excellent candidates for accomplishing the decarbonization goals and electrifying our society. However, several studies have demonstrated that the incorporation of trace impurities from commercial alkaline electrolytes induces significant changes in the intrinsic activity, pseudocapacitance, and stability of Ni-based materials, resulting in erroneous descriptions of their performance. Thus, having a better understanding of the effects of these impurities will lead to practical ways to prevent or control their impact on electrochemical energy devices.In this work, we study the incorporation of transition metals (i.e., Fe, Co, Cu) that form insoluble hydroxides in alkaline media over nickel (oxy) hydroxide electrocatalysts. We systematically examine how these trace impurities modify the OER activity, stability, and the pseudocapacitive properties of NiOOH. As depicted in Figure 1, KOH electrolytes containing trace impurities of Fe and Co induce significant changes in the OER onset potential of Ni(OH)2/NiOOH electrocatalysts, according to the irreversible charge increase measured through coulovoltammetry (QV) curves. Moreover, distinct redox peaks can be seen for each scenario. After aging in purified KOH electrolyte (Fig. 1a), the cyclic voltammogram (CV) of Ni(OH)2/NiOOH exhibits four oxidation peaks that can be ascribed to a mixture of both α and β structural phases (Fig. 1b) while aging in Fe-unpurified KOH promotes the α phase and shifts the redox peak anodically (Fig. 1c). Aging in Co-unpurified KOH promotes a more ordered β structural phase without changing the OER activity significantly (Fig. 1d), which is desirable for pseudocapacitive materials. Additional experiments examining the OER activity, stability, and capacitance provide valuable insights into these impurities, and specific strategies to prevent and minimize their effects will be discussed. Overall, this work illustrates the importance of tracking trace metal impurities in alkaline electrolytes to minimize undesirable effects in electrochemical energy devices. Figure 1

The Impact of Operando Analysis in the Understanding of Oxygen Evolution Reaction Ranging from Intrinsic to Technical Scales

The development of state of the art and cost-effective catalyst materials for the sluggish oxygen evolution reaction (OER) is of paramount importance for realizing practical water electrolysis for green hydrogen production. Crucial for this endeavor is the determination of the OER mechanism and a better understanding of all these background processes that affect catalyst activity and stability. Thus, to further elucidate all these processes operando characterization techniques performed dynamically during OER are used as powerful tools which allow monitoring of key reaction intermediates, active sites, charge transfer and material transformation coupled processes etc. In this work we summarize our recent investigations covering noble and 3d transition metal-based OER electrocatalysts by different operando techniques and we show the importance of such techniques in the unraveling catalyst activation/deactivation mechanisms. Thus, our efforts are ranging from half-cell characterization of electrocatalyst intrinsic properties to single-cell analysis of phenomena occurring during their technical operation.Operando techniques like eQCM, XAS, ICP-OES and single cell testing will be presented aiming to emphasize on the critical role of operando in the understanding of fundamental aspects of electrochemistry [1], like intrinsic catalyst properties (Fig. 1a), the effect of electrolyte impurities such as Fe (Fig. 1b-d) on Ni-based electrocatalysts in alkaline environment [2] as well as mechanistic phenomena taking place during catalyst activation (Fig. 1e-f) leading to the formation of the active sites for OER in acidic environment [3].

Dual-path glucose electrooxidation reaction on Ni(OH)2/NiOOH catalysts in alkaline media

Glucose electrooxidation has been widely studied for its application in biosensors, fuel cells and electrosynthesis of value-added products. Nickel is known to catalyze the glucose oxidation reaction (GOR) in alkaline media, but the mechanism, the nature of the active sites, and reaction products are still being debated. This study focuses on elucidating the reaction pathways of the GOR on a Ni(OH)2/NiOOH electrode through electrochemical measurements, in-situ Fourier transform infrared spectroscopy (FTIRS) and microkinetic modeling. The electrode was prepared by Ni electrodeposition on glassy carbon (NiED/GC) followed by electrochemical oxidation into Ni(OH)2/NiOOH. The shape of the cyclic voltammograms obtained during the GOR was found to be significantly influenced by the presence of Fe impurities in the NaOH electrolyte. With the help of microkinetic modeling, a dual-path mechanism was proposed for the glucose electrooxidation on the Ni(OH)2/NiOOH surface, comprised of Eley–Rideal and Langmuir-Hinshelwood pathways, with NiOOH reacting either with glucose from solution or adsorbed on Ni(OH)2 sites. In-situ FTIR measurements reveal the formation of formate at potentials as low as 1.17 V vs. RHE.

Impact of Trace Nickel in the Electrolyte on the Electrochemistry of Positive Electrodes for Lithium-Ion Batteries

This study looks into the impact of Ni2+ ions as an electrolyte impurity on the electrochemical performance of NMC811 electrodes. The results indicate that the presence of Ni impurities in the electrolyte increases the polarization, primarily at the oxide-electrolyte interface. The influence on the metallic lithium counter electrode and the electrolyte conductivity can be neglected. The Ni impurities induce the formation of high impedance CEI layers on the oxide electrode, affected by both cations and anions of the impurity and impeding the current flow and cell performance. Ni impurities also enhance electrochemical side reactions, especially oxidation of the electrolyte, diminishing both, the Coulombic efficiency during cycling and the practical specific charge. The rates of these side reactions are related to the Ni2+ ion concentration. C-rate tests confirmed that a contaminated electrolyte reduces the practical specific charge due to the increased interfacial impedance of the CEI, especially at high discharge rates. The cycling performance of NMC811 electrodes in contaminated electrolytes showed that Ni impurities, especially NiCO3, promoted capacity fading during cycling and increased cell polarization. To optimize the performance of NMC-based oxide electrodes, controlling the concentration of Ni ions in the electrolyte is critical.

The Effect of Electrolytes on the Kinetics of the Hydrogen Evolution Reaction

Amid global energy challenges, the hydrogen evolution reaction (HER) is gaining traction for green hydrogen production. While catalyst research is ongoing, recognizing electrolyte effects remains crucial for sustainable hydrogen production via renewable-powered water electrolysis. This review delves into the intricate effects of electrolytes on the kinetics of the HER. It examines key factors including the pH, cations, anions, impurities, and electrolyte concentration. This review discusses the notion that the electrolyte pH alters catalyst–electrolyte interactions and proton concentrations, thereby influencing factors such as the hydrogen binding energy, water adsorption, and overall reaction kinetics. Moreover, this review provides a briefing on the notion that electrolyte cations such as Li+ can impact the HER positively or negatively, offering opportunities for improvement based on the metal substrate. Interestingly, there is a potential that the HER can be tuned using Li+ ions to modify the M–H bond energy, demonstrating a flexibility beyond the pH levels and counter-ions. The varied adsorption energies of metal cations on metal electrodes are also found to influence the HER kinetics. The effects of electrolyte anions and impurities are also discussed, emphasizing both the positive and negative impacts on HER kinetics. Moreover, it is pointed out that the electrolyte-engineering approach enhances the HER kinetics without permanent catalyst surface modifications. This review underscores the importance of the electrolyte composition, highlighting both the challenges and potential solutions in advancing HER research for sustainable energy production.

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching.

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.

How Do Impurities in Electrolytes Affect the Electrochemical Performance of Oxides in Lithium-Ion Batteries?

The recovery and treatment of spent LIBs has already been studied extensively. On one hand, the hydrometallurgical recovery of cathode materials has aroused widespread attention because of its low energy consumption and benefit to the environment. On another hand, the purification of the materials reclaimed from spent batteries is a challenging process with high probability that, later, some remaining impurities such as transition metal carbonates and sulfates could be released into the electrolyte. In this way the impurity content will have a serious impact on the final performance of the battery. To this point different electrochemical and analytical measurements were conducted to elucidate the effects of impurities in the electrolyte formulation, specifically Li, Mn, Co, Ni, and Al carbonates, sulfates, and acetates. The base LP30 electrolyte was intentionally contaminated with a maximum concentration of selected impurity to study the effects of the contaminated electrolyte on the electrode. Different combinations and varieties were explored correspondingly. Test setups were based on both, NMC811 in half cells (vs. Li) and symmetrical Li||Li cells, and the influence of contaminations was thus exploited on the Li counter electrode and on the NMC811 in Li||NMC811 configuration. A number of standard and advanced electrochemical test techniques such as CC-CV, electrochemical impedance, rate capability tests, GITT, and CV was applied for a fully understanding of the electrochemical characteristics of standard cathode active materials such as NMC811 when electrolytes with (artificially downgraded) low purity levels are present. The presentation will focus on discussion of results regarding the presence of transition metals Ni, Co, and Mn as impurities in the electrolyte which showed a significant impact on the electrochemical performance of the tested cells. When the concentration of the impurities increases, both the overpotentials at the cathode and the rate of capacity decay during cycling increase as well. The data shown will be very helpful for the researchers to understand the impact of impurities on the performance of oxide-based LIB cathode materials.

Comparing the Lithiation and Sodiation of Hard-Carbon Using Online Electrochemical Mass Spectrometry

Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries (LIBs), especially for applications in which weight and size play a subordinate role.1 In contrast to LIBs, graphite is not a viable anode active material for SIBs since sodium cannot be inserted into graphite. So-called hard-carbons are promising anode active materials for SIBs because of their easy synthesis, high capacity, and high cycling stability.2 Major challenges are the high irreversible capacity in the first cycle and the stability of the solid electrolyte interphase (SEI).3 - 4 Online electrochemical mass spectrometry (OEMS) is a powerful tool for identifying and quantifying evolved gasses during SEI formation, giving insight into the mechanism of SEI formation. For LIB research, our group had developed a two-compartment OEMS cell design that enables the investigation of evolved gases from one of the electrodes of a battery cell without interference from the gases evolved from the other electrode, which is particularly useful for the mechanistic study of electrolyte additives or impurities without influences of the counter electrode.5–8 In this study, we investigate the gas evolution during the first lithiation/delithiation cycles of a hard-carbon anode active material using our existing two-compartment cell design. This will then be compared to the gas evolution during the first sodiation/desodiation cycles of the same hard-carbon material using a newly developed two-compartment OEMS-cell design for the study of sodium-ion batteries, which is based on a gas impermeable solid electrolyte separator composed of sodium ion conducting β-alumina. We compare the quantities of evolved gasses and look at the onset potentials of gas evolution during the first alkaline metal insertion. On the basis of gassing and impedance data,9,10 the differences in SEI formation and its properties will be discussed.Acknowledgments: We gratefully acknowledge funding by the BMBF (Federal Ministry of Education and Research, Germany) for its financial support under the auspices of the ExZellTUM III project, grant number 03XP0255

(Invited) Challenges and Opportunities of Alkaline Water Electrolysis

Alkaline water electrolysis has been operated on 100+ MW scale in the 20th century powered by hydropower. Because of this history, the technology is often regarded as a mature technology with limited improvement potential that is less suitable for flexible operation. As a result most research efforts in the field of water electrolysis focus on other technologies such as PEM, solid oxide and AEM electrolysis. This is a pity, since alkaline electrolysis is very well positioned to be deployed on large-scale for the production of green hydrogen in the coming decades. Main reason is that in contrast to the other technologies, alkaline technology does not depend on scarce and/or expensive noble or rare earth metals. Also, the use of a diaphragm instead of a membrane makes the technology more robust.Yet, to enable the large-scale deployment of alkaline electrolysis for production of green hydrogen from renewable electricity certain technical challenges need to be overcome. These include safety challenges associated with rare scenarios where hydrogen and oxygen can mix, flexibility limitations associated with gas crossover limiting the minimum load of the plant, and durability challenges resulting from frequent shutdowns of electrolyzers. In principle all these challenges can be overcome by rigorous plant design and the selection of suitable diaphragm and electrode materials, but they do require research efforts.Next to that, there is room to significantly drive down the costs of green hydrogen produced by alkaline electrolyzers. Traditionally, alkaline electrolyzers have been operated at low current densities of 0.2-0.4 A cm-2. This was related to the high ohmic drop associated with the use of thick diaphragms and non-zero-gap cell designs. Presently, advanced alkaline electrolyzers are under development that can operate at significantly higher current densities (~1 A cm-2) through the use of thinner diaphragms, zero-gap cell designs and improved electrode materials. Operation at these increased current densities increases the output of alkaline electrolyzers and in this way can effectively reduce the costs and footprint of water electrolysis plants. Yet, the use of thinner diaphragms and advanced electrodes also creates new challenges, since they can lead to increased gas crossover and vulnerability to impurities and reverse currents that occur upon the shutdown of electrolyzers.In this presentation the fundamentals of alkaline water electrolysis will be discussed. These include overpotentials and durability of electrode coatings, the role of electrolyte impurities, ohmic resistance, gas crossover through the diaphragm, the influence of operating temperature and pressure, the effect of bubbles, and the influence stray currents resulting from the manifold design. These fundamentals will be translated into a potential design for the perfect alkaline electrolyzer.Figure:alkaline electrolyzers operational in Norway in 1931. Source: Norsk Industriarbeidermuseum Figure 1

Impact of electrolyte impurities and SEI composition on battery safety

Our work highlights the impact of impurities and SEI properties on Li-ion battery safety. We could show that a thicker, inorganic SEI leads to a significant delay in thermal runaway, indicating a safer aged battery.

Failure Mechanisms at the Interfaces between Lithium Metal Electrodes and a Single-Ion Conducting Polymer Gel Electrolyte.

Polymer electrolytes have the potential to enable rechargeable lithium (Li) metal batteries. However, growth of nonuniform high surface area Li still occurs frequently and eventually leads to a short-circuit. In this study, a single-ion conducting polymer gel electrolyte is operated at room temperature in symmetric Li||Li cells. We use X-ray microtomography and electrochemical impedance spectroscopy (EIS) to study the cells. In separate experiments, cells were cycled at current densities of 0.1 and 0.3 mA cm-2 and short-circuits were obtained eventually after an average of approximately 240 cycles and 30 cycles, respectively. EIS reveals an initially decreasing interfacial resistance associated with electrodeposition of nonuniform Li protrusions and the concomitant increase in electrode surface area. X-ray microtomography images show that many of the nonuniform Li deposits at 0.1 mA cm-2 are related to the presence of impurities in both electrolyte and electrode phases. Protrusions are globular when they are close to electrolyte impurities but are moss-like when they appear near the impurities in the lithium metal. At long times, the interfacial resistance increases, perhaps due to additional impedance due to the formation of additional solid electrolyte interface (SEI) at the growing protrusions until the cells short. At 0.3 mA cm-2, large regions of the electrode-electrolyte interface are covered with mossy deposits. EIS reveals a decreasing interfacial resistance due to the increase in interfacial area up to short-circuit; the increase in interfacial impedance observed at the low current density is not observed. The results emphasize the importance of pure surfaces and materials on the microscopic scale and suggest that modification of interfaces and electrolyte may be necessary to enable uniform Li electrodeposition at high current densities.

A sustainable approach to replace bleed stream in electrical refining of copper

Despite extensive studies on electrorefining to remove impurities from copper production units, bleeding electrolytes is the most popular method for controlling impurities due to low operating costs. A controlled cooled-reheat cycle was used to remove impurities in industrial electrolytes of Khatoon Abad copper company. Phase identification and physical study were done by the XRD, SEM, TEM, and FTIR techniques. The results confirmed the possibility of the removal of 23 wt% Ni, 26 wt% Sb, 2.4 wt% As and convention of 6 wt% of As5+ to As3+ at cooled-reheat cycle.

(Invited) Designing Interphase in Rechargeable Lithium Metal Batteries Via Liquid Electrolyte Additives

The goal of reaching specific energy at 500 Wh kg-1 resurrects Li metal anode in recent years due to its high capacity (3860 mAh g-1) and low reductive potential (-3.04 V vs. standard hydrogen electrode). However, the reductive decomposition and chemical instability of the liquid electrolytes against Li metal result to unstable solid-electrolyte interphase (SEI) formation and porous Li deposition with low coulombic efficiency and severe volume expansion. Therefore, one strategy to enhance the cycle life of Li metal anode is to design robust SEI to improve the interfacial stability. Herein, we demonstrate two liquid electrolyte additives that can form and regulate desirable SEI on the Li metal anode. Acrylonitrile (AN) can be electrochemically polymerized as an additive to form a Li-ion conductive SEI during Li deposition. Electrochemical analyses, scanning electron microscopy, and X-ray photoelectron spectroscopy confirm the formation of the polymeric SEI, which induces uniform and dense Li deposition and reduced decomposition of the Li-ion electrolyte. Phosphorous pentoxide (P2O5) is also studied as an additive in the typical 1M LiPF6 electrolyte in carbonate solvents for Li metal batteries. P2O5 can effectively scavenges the active impurities in the carbonate electrolytes and form phosphorous-rich stable SEI. Significantly improved battery performance due to these two additives is demonstrated in 0.4 Ah pouch cells composed of LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode (3 mAh cm-2) and Li anode (50 mm thick) with a lean electrolyte/capacity ratio of 3 g/Ah.

Water Oxidation in the Presence of a Nickel Coordination Compound: Decomposition Products, Fe Impurity in the Electrolyte, and a Candidate as a Catalyst

Sustainable energy sources require a large-scale storage system. Water electrolysis toward hydrogen formation is a remarkable method for energy storage. However, oxygen-evolution reaction (OER) through water-oxidation reaction is a bottleneck in water-splitting systems. Different Ni-based molecular structures have been claimed to be OER catalysts. However, the impact of Ni (hydr)oxides formed by the degradation of Ni-based molecular structures on OER is a challenging issue. The effect of electrolyte impurity on OER in the presence of metal coordination compounds is rarely investigated. In this study, five important questions are considered regarding OER in the presence of a Ni coordination compound: (i) In which potential does the coordination compound start to be decomposed? (ii) After how many CVs does the decomposition occur? (iii) What is the effect of Fe impurity on OER in the presence of the Ni coordination compound? (iv) What is the product of the decomposition of the Ni coordination compound? (v) What is the true catalyst for OER in the presence of the Ni coordination compound? The roles of both the Fe impurity in the electrolyte and the Ni (hydr)oxide formed in the presence of a Ni coordination compound are investigated. The experiments showed that in the presence of the Ni coordination compound, Ni (hydr)oxide and Fe impurity in KOH formed a Ni–Fe oxide-based catalyst during OER, which is an efficient catalyst for OER. Fe impurity is a key contributor to observing OER in the presence of this Ni coordination compound.

High-voltage lithium-ion capacitors enabled by a multifunctional phosphite electrolyte additive

Combining the high energy of lithium-ion batteries with the high power and excellent cycling stability of supercapacitors, lithium-ion capacitors (LICs) stand for an emerging hybrid technology for next-generation electrochemical energy storage. Among them, the configuration composed of a battery-type cathode and a capacitor-type anode is advantageous owing to its unnecessity for electrode prelithiation but disadvantageous because of its limited operating voltage. Herein, using triphenyl phosphite (TPPi) as a powerful multifunctional electrolyte additive to not only scavenge electrolyte impurities but also form robust, uniform, and thin cathode/electrolyte interface film on cathode and solid-electrolyte interphase film on anode, we develop high-voltage LICs from a high-potential spinel LiNi0.5Mn1.5O4 cathode and an activated carbon anode. In addition to the important multifunctionalities of TPPi, we modulate the capacity ratio between the cathode and anode and composite the electrodes with carbon nanotubes (CNTs) to enhance the overall performance for our LICs. Therefore, having the synergy from the multifunctionalities of TPPi, capacity modulation between the cathode and anode, and electrode compositing with CNTs, the optimized LIC shows a high operating voltage of 3.45 V, high energy density of 61.6 Wh kg−1, high power density of 52.5 kW kg−1, and long cycle life with a capacity retention of 91.8% after 6000 cycles, outperforming all of its previously reported counterparts having a similar configuration incorporating a battery cathode and a capacitor anode. The rational strategy developed in this work is versatile and should be applicable to other electrode/electrolyte systems for advanced hybrid electrochemical energy storage.