Electrode Reaction Research Articles

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System.

Water desalination mediated by electrochemical reactions to directly capture and release salt at electrode materials offers a low-voltage method for producing freshwater. Developing new system designs has allowed electrode materials to maximize their capacity for salt separation, especially when a multichannel system is used to introduce a separate electrode rinse solution. Here, we show that the use of an additive can provide a new strategy for improving electrode capacity and, hence desalination performance, which so far has been limited to increasing the electrolyte concentration. A custom-built, 2/2-channel flow cell divided by two cation exchange membranes and an anion exchange membrane was fed with 50 mM NaCl as the feed (two inner channels) and 0.5 M NaCl containing up to 0.1 M phosphate as the electrode rinse (two outer channels). Using manganese oxide electrodes with phosphate buffer-mediated redox reactions exhibited an improved desalination capacity of 68.0 ± 5.2 mg g-1 (0.55 mA cm-2) and a rate of 5.6 ± 1.3 mg g-1 min-1 (0.96 mA cm-2). The improvement was attributed to the buffer that served as a proton donor for promoting the H+ insertion reaction of amorphous or poorly crystalline MnO2. Additionally, the buffering capacity against acidification and the creation of insoluble manganese phosphate on the electrode surface prevented the dissolution of Mn2+, which could otherwise occur at the anode due to a decrease in the local pH upon H+ deinsertion. Thus, the use of manganese oxide electrodes coupled with phosphate provides a new strategy of increasing electrode capacity for water desalination.

How Relevant Are SEI Studies from Half Cell Samples to Understand Aging Mechanisms in Potassium Batteries? An in-Depth Photoelectron Spectroscopy Study

Owing to a shared common technological basis, monovalent K-ion battery (KIB) systems follow in the footsteps of Li-ion batteries (LIBs) with the aim to develop a broader portfolio of available battery technologies and to put them on a more sustainable material basis. For example, both cations intercalate into a graphite host structure, which can be used in combination with carbonate-based electrolytes as negative electrode. Especially in half cells, the degradation of graphite electrodes in KIBs is significantly accelerated in comparison to LIBs. Previous surface studies demonstrated that the formation of a durable and protective solid electrolyte interphase (SEI), that is paramount for long cycle life in alkali-ion batteries, is a major bottleneck for KIBs because of less benign SEI properties[1][2].Recently reported crosstalk phenomena in half cell configurations due to the highly reactive potassium counter electrode make the analysis of surface layers a particular challenge [2][3]. In this presentation, a detailed characterization of the SEI formation on graphite was performed in both half and full cell configurations by in-house and synchrotron-based photoelectron spectroscopy (PES) in order to demonstrate fundamental differences in surface layer composition under the influence of a reactive counter electrode. While it is generally acknowledged that PES results show differences in SEI composition and thickness between both configurations, our results clearly demonstrate that in case of KIBs the changes become severe. This challenges interpretations of SEI layer properties made in half cells, as they are distinctly different from those in full cells. More importantly, this also affects interpretations on the major electrolyte degradation pathways in these systems. Using PES depth profiling and complementing results with gas chromatography, as well as DFT calculations, delivers new insights on the irreversible processes in KIBs. Reference s : [1] Naylor, et al., ACS Appl. Mater. Interfaces. 11 (2019) 45636–45645.[2] Allgayer et al., ACS Appl. Energy Mater. 5 (2022) 1136–1148.[3] Hosaka et al., ACS Energy Lett. 6 (2021) 3643–3649.

Green Chemistry of the Future: Co-Electrosynthesis of Biobased Platform Chemicals and Hydrogen

The European Green Deal sends a strong signal for a climate-friendly and CO2-neutral economy. An important aspect to support this is a sustainable and reduced use of resources as well as efficient and green conversion technologies for renewable value-added products. For this reason, electrosynthetic approaches become increasingly important, as electrochemistry fulfils several criteria of green chemistry and offers the possibility to directly use current from renewable energy sources [3]. However, electrolysis and electrosynthesis currently often require the use of platinum group metals (PGM) and need separators, which are mostly based on perfluorinated and polyfluorinated alkyl substances (PFSA). Classical water electrolysis for production of green hydrogen also requires high overvoltage’s, which is due to the oxygen evolution reaction (OER). This is where coupling systems come in, in which the sluggish OER is replaced by a thermodynamically more efficient reaction, such as the electrooxidation of organic substances (EOO). An example of this is the electrooxidation of biobased 5-HMF or various sugars (xylose, glucose) to platform chemicals. When coupled with the hydrogen evolution reaction (HER), the EOO of 5-HMF to 2,5-furandicarboxylicacid (FDCA) requires a theoretical cell voltage of 0.3 V. In contrast, theoretical cell voltages of 1.23 V are required for the OER/HER [5]. With the focus on sustainable and energy-efficient systems, the simultaneous production of platform chemicals and green H2 via paired electrolysis offers high potential. [1-7]Previous studies have mainly focused on electrode materials, morphology and its electrocatalytic activity in divided batch cells. A strongly alkaline environment is considered advantageous for the electro-organic oxidation reaction, while the HER is reported to be kinetically slower in alkaline than in acidic conditions. Overall, the coupled electrolysis of H2 and platform chemicals is described as a promising technology in the development stage. [5-7]In this context, the present research explores paired electrosynthesis of platform chemicals and H2 to advance sustainable and efficient energy sources. A central goal is the prototype development of an undivided flow reactor in TRL4. Particular attention will be paid to mild operating conditions (such as a mild-alkaline environment, low operating pressures and low temperatures), PGM-free catalysts and the elimination of PFSA materials. To achieve this goal, various milestones have been defined. Among them is the development of 3D electrodes with suitable and selective catalysts. These are to be electrochemically tested in a flow-through test cell on a laboratory scale. In order to create a basis for analysis, the electrode reactions were first investigated electrochemically in a typical divided H-cell and reference measurements were carried out on nickel-based materials. In this work, NiOOH sheet and foam electrodes were electrochemically produced and subsequently investigated by cyclic voltammetry (CV) followed by bulk electrolysis. On this basis, suitable analytics (HPLC, UV-VIS) were also determined to quantify and classify the derivatives of the electrosynthesis process. These pre-experiments form the basis for the ongoing development of initial concept ideas for a modular test cell that allows various materials and structures to be electrochemically investigated.Overall, this research contributes to advancing sustainable and efficient energy sources and enabling the production of platform chemicals from renewable raw materials. Acknowledgments The authors thank the European Union’s Horizon Europe research and innovation programme under grant agreement N° 101070856 ELOBIO (Electrolysis of Biomass) for funding.

(Invited) Electrochemical and Postmortem Analyses of 18650-Type Li-Ion Secondary Battery Degraded at High and Low Temperature Environments

Lithium-ion secondary batteries have been widely used as small-size and lightweight energy sources for applications of laptops, robots and electric vehicles. For realizing a high performance, the energy sources are required to have high energy density with high reliability. The batteries are charged and discharged repeatedly based on anode and cathode reactions, which both accompany side reactions to a greater or less extent. The battery degradation phenomena are well explained by kinetics of chemical reactions evaluated by non-destructive examination.1 To understand the battery degradation mechanism for developing a durable one, it is indispensable to clarify the side reactions occurred to cause the degradation. For that purpose, differential capacity curves (dQ/dV vs. V) and electrochemical impedance spectroscopy (EIS) were employed as electrochemical measurements.2 To support the mechanism estimated by the measurements, postmortem analyses were conducted using SEM, XPS and XRD. These investigations were performed to the commercially available 18650 cell consisting of Li(Ni, M)O2 (M: metal elements to partially replace Ni) as the cathode material and graphite as the anode material. The employed loads were calendar deterioration and charge/discharge cycle deterioration.3,4 Firstly, the calendar deterioration behavior of the 18650-type battery was evaluated by storing them at 80 °C for different durations. The results indicate that the battery capacity decreased with the increasing number of storage days in a high-temperature environment. From the differential capacity curves, we found that the change in the overvoltage, which was caused by the electrode reaction, that appeared at approximately 4.2 V was most significant. To analyze this phenomenon, EIS of the deteriorated battery was measured at 4.2 V, and it was confirmed that the cathode-based resistance of the battery increased significantly at this potential. Furthermore, postmortem analysis revealed that the cathode active material of the tested batteries clearly deteriorated; however, no significant change in the active material in the anode was observed. Therefore, it is considered that the cathode materials of nickel-based lithium-ion secondary batteries deteriorate by storing at 80 °C, thereby resulting in reduced battery capacity and increased cathode component resistance after storing at high temperatures.Secondly, cycle deterioration behaviors of the lithium-ion batteries in different operating temperature were studied. Compared with the deterioration in the high-temperature range, the degree of decrease in battery capacity at the low temperature was greater, and the more severe the deterioration was shown at temperatures far from the safe temperature range. From the differential capacity analysis, the peaks corresponding to the anode was significantly reduced at the voltage of the initial charging stage after cycle deterioration at low temperature. This means that the lithium ions intercalated into/deintercalated from the graphite layer decreased. In addition, EIS of the batteries before and after cycle deterioration was measured at each deteriorated temperature. Based on these, postmortem analysis was conducted with a major focus on the anode after 20-cycle deterioration at low temperature. From the SEM observation, the anode can be known to grow thicker with generating cracks and gas pockets. According to the XPS measurements, it is found that Li2CO3 and LiF are generated by decomposition of electrolytic solution at the anode surface.Overall, it was clarified that the deterioration at high temperature affected the cathode and that at low temperature affected the anode.

Electrode Reactions on Conducting Polymers

The electrocatalysis behind the direct chemical-to-electrical energy interconversion is one of key facets in the development of sustainable economy. This results in a technological demand for a variety of applications such as electrical transportation, electrified synthesis and the grid balancing. Here are three examples of problematized electrocatalytic processes. (A) The sluggish kinetics and multistep character of oxygen reduction reaction (ORR) are causing losses in electrical energy conversion and poor selectivity, which limit the wide implementation of oxygen-associated energy conversion technologies. (B) The water electrolysis yielding the hydrogen evolution reaction (HER), the key process in technologies of green hydrogen, provides only 4% of worldwide hydrogen production due to the high catalyst cost. (C) The control of the proton-coupled electron transfers on organic redox molecules such as benzenediols. These three illustrate the stimulus of the intensive research on noble-metal-free electrocatalysts. Conducting polymers, the organic materials synthesised from abundant element, constitute a distinct class of molecular electrocatalysts [1, 2] attributed with behaviour of mixed ion-electron conductors (MIEC). Firstly, the landscape of ORR phenomena happening on p- and n-type conducting polymers at the mechanistic and device levels are discussed [1-3]. Secondly, the effect of proton supply is rationalized at both mechanistic and device levels for HER on PEDOT-triflate [4]. Thirdly, the significant effect of ionic transport on the rate of the proton-coupled electron transfers was observed and conceptualized as a ion-selective electrocatalysis (ISEC) [5].[1] E. Mitraka, M. Gryszel, M. Vagin, M.J. Jafar, A. Singh, M. Warczak, M. Mitrakas, M. Berggren, T. Ederth, I. Zozoulenko, X. Crispin, E.D. Głowacki, Electrocatalytic Production of Hydrogen Peroxide with Poly(3,4‐ethylenedioxythiophene) Electrodes, Advanced Sustainable Systems, 3 (2019) 1800110.[2] Z.X. Wu, P.H. Ding, V. Gueskine, R. Boyd, E.D. Glowacki, M. Oden, X. Crispin, M. Berggren, E.M. Bjork, M. Vagin, Conducting Polymer-Based e-Refinery for Sustainable Hydrogen Peroxide Production, Energy & Environmental Materials.[3] M. Vagin, V. Gueskine, E. Mitraka, S.H. Wang, A. Singh, I. Zozoulenko, M. Berggren, S. Fabiano, X. Crispin, Negatively-Doped Conducting Polymers for Oxygen Reduction Reaction, Advanced Energy Materials, 11 (2021) 2002664.[4] R. Valiollahi, M. Vagin, V. Gueskine, A. Singh, S.A. Grigoriev, A.S. Pushkarev, I.V. Pushkareva, M. Fahlman, X.J. Liu, Z. Khan, M. Berggren, I. Zozoulenko, X. Crispin, Electrochemical hydrogen production on a metal-free polymer, Sustainable Energy & Fuels, 3 (2019) 3387-3398.[5] M. Vagin, C.Y. Che, V. Gueskine, M. Berggren, X. Crispin, Ion-Selective Electrocatalysis on Conducting Polymer Electrodes - Improving the Performance of Redox Flow Batteries, Advanced Functional Materials, 30 (2020) 2007009.

Revealing Phase Transition in Ni-Rich Cathodes Via Nondestructive Entropymetry Method

With the expanding requirements of recent energy regulations and economic interest in high-performance batteries, the need to improve battery energy density and safety has gained prominence. High-energy-density lithium batteries, employed in next-generation energy storage devices, rely on nickel-rich cathode materials. Since these have extremely high charge/discharge capacity, high operating voltage, prolonged cycle life, and low cost, nickel-rich cathode materials such as Ni-rich NCM (LiNix>0.8CoyMnzO2) and Ni-rich NCA (LiNix>0.8CoyAlzO2) are of particular interest to researchers. Several in situ characterization methodologies are currently used to understand lithium-ion battery electrode response and deterioration better. Nevertheless, in many contexts, these measurement methodologies must be combined with specially designed cells and electrode materials with distinct forms, which is sometimes inconvenient. As an alternative, thermothermo-voltametric dynamic characterization may be utilized to describe the thermal internal characteristics of various electrode materials, such as the structural changes and electrode reactions that occur during charging and discharging. In this paper, a non-destructive entropy measurement method demonstrates that phase change occur for NCM (LiNi0.83Co0.12Mn0.05O2) and NCA (LiNi0.88Co0.09Al0.03O2) at 40-30% of SOC and 90-80% of SOC, respectively. This is confirmed by ex-situ X-ray diffraction measurements for these highly popular cathodes.

How Reference Electrodes Improve Our Understanding of Degradation Processes in Half and Full-Cell KIB Setups

For the last decade, the search for suitable alternatives for Li-ion batteries (LIBs) has steeply increased. While elements such as lithium and cobalt, are in high demand for LIBs, they could soon be replaced by less costly and more abundant elements1. One attractive alternative is potassium-ion batteries (KIBs) because of the abundance of potassium resources, and the lower standard electrode potential of K/K+ compared to lithium2.For characterization of KIB materials can be used same approaches as for LIB. An exception to that is describing electrochemical behavior, especially in a half-cell. One of the most important challenges for KIBs is a reliable 3-el setup with a stable and reproducible reference electrode (RE). Without a RE, the only possibility to independently quantify the anode and/or cathode impedance is the disassembly of multiple cells and the recombination of the identical anode or cathode pairs in symmetrical cells3. While using a metallic reference electrode is a practical solution for LIBs (using Li metal), the high reactivity of K renders the metal unsuitable as a reference electrode in KIBs.In this study, we outline our research on stable and reliable reference electrodes and compare several established approaches, specifically K-metal, a partially discharged Prussian white (K2Fe[Fe(CN)6]) and a more classic Ag/AgCl redox couple. Except for battery research, Ag|AgCIsat REs are commonly used in non-aqueous media in electrochemistry and offer the advantage of comparatively high chemical inertness, unlike K-metal or cathode-type RE. It was found that the evolution of degradation products from K-metal also induced significant drifts on a partly charged cathode RE (Fig. a), thus excluding both as reliable RE systems.In our revised cell design, the Ag|AgCIsat reference electrode was integrated into a thin-layer battery compartment. With this 3-electrode configuration half and full cell experiments, previous 2-electrode tests were revisited. So, the only way to compare the behavior of electrodes in half in full is to have stable and chemically passive RE which is Ag/AgCl. With this setup we were able to study the impact of electrolyte additives on the electrode reactions in more detail and pinpoint cut-off limits in graphite/ K2Fe[Fe(CN)6]. For example, in our experiments it was observed how graphite electrodes degrade under the influence of the DTD (1,3,2-Dioxathiolane 2,2-Dioxide) additive(Fig b).

Electrochemical Direct Air Capture of Carbon Dioxide by a Redox-Mediated Salt Splitting Process

Direct air capture of carbon dioxide is an important emerging direction for electrochemical separation technology. Carbon dioxide removal has a critical role to play in the coming century as a complement to deep decarbonization efforts, because removal may offset emissions we cannot avoid, such as agriculture for food security. Direct air capture might even be used to decrease the atmospheric carbon dioxide level if the value at which it stabilizes is deemed too high. Electrochemical methods allow us to power a carbon removal device directly by carbon-free electricity. Here, we present a salt splitting process, based on bipolar membrane electrodialysis, which begins with a neutral electrolyte (e.g. KCl), and creates a strong base (e.g., KOH) and a strong acid (e.g., HCl). The base may then be used to absorb carbon dioxide from ambient air; the acid is then used to extract the carbon dioxide by recombining with the base, reversing the salt splitting process. The process is driven by iron(II/III) hexacyanide redox on carbon cloth electrodes, which is kinetically facile and contributes relatively little to the cell voltage.Energy required for redox-mediated salt splitting depends on contributions from water dissociation in the bipolar membrane, ohmic resistances in ion exchange membranes and electrolyte layers, and electrode reactions. We show that the largest contribution to the energy required comes from the bipolar membrane, motivating further research on bipolar membrane development. We also discuss the role of ion exchange membranes in this system: these membranes must provide high ion conductivity while providing selectivity against leakage of acid and base products. Several commercial membranes were screened for conductivity and water content, and their electrodialysis performance is compared. Finally, we use technoeconomic modeling to assess how electrodialysis operating conditions and electrolyte composition affect energy, land, and water demands of a continuously operating direct air capture process, with redox-mediated salt splitting as the centerpiece. Figure 1

Uniform Zinc Plating Enabled By Polyimide Nanofiber Network for Long-Life Aqueous Zinc Metal Batteries

The rapid development of portable electronics and emerging electric vehicles has created a pressing need for electrical energy storage systems. Among these systems, zinc metal batteries (ZMBs) have shown great potential due to their high theoretical capacity density, low reduction potential, and low-cost anodes. However, the large-scale application of rechargeable ZMBs has been limited by the well-known issues of dendrite growth and adverse side reactions in zinc metal electrodes[1].Various strategies have been proposed to overcome the challenges associated with zinc dendrite growth, including substrate structure optimization, surface modification of zinc metal, and design of novel electrolytes. Among these approaches, constructing a protective layer on the zinc anode surface is a promising strategy due to its simplicity and cost-effectiveness. A dense polyamide coating layer, prepared by the doctor blading method, has been developed to regulate the aqueous Zn deposition behavior by elevating the nucleation barrier and restricting the 2D diffusion of Zn2+ ions [2]. Additionally, an Al2O3 coating layer has been developed using atomic layer deposition to improve the rechargeability of Zn anodes for zinc-ion batteries (ZIBs)[3]. However, these dense layers face a significant challenge to ionic conductivity due to the high energy barrier that hinders the diffusion of Zn2+ ions, leading to poor battery performance, especially under high current densities.Herein, we report utilizing an electrospun polyimide network (noted as PI-Zn hereafter) to resolve this dilemma. The polyimide (PI) chain has numerous carbonyl oxygen atoms that function as electron donors, supported by theoretical calculations (Fig.1). These atoms can establish stable bonds with Zn ions, thereby reducing the activation energy for de-solvation and enhancing the kinetics of Zn ion deposition. Unlike traditional coating methods, our electrospinning polymers form a fiber-like network structure that provides ion channels, thereby enhancing ion transportation. From the Nyquist plots of symmetrical batteries (Fig. 2), the PI-Zn electrode exhibits smaller charge transfer resistance and lower activation energy compared to the bare Zn electrode. The Transmission X-Ray Microscope results (Fig.3) reveal that the deposition on the bare Zn is heterogeneous and dendritic. In contrast, the PI-Zn electrode presented a smooth surface and dendrite-free morphology. The result demonstrates that the charge transfer ability, de-solvation, and deposition kinetics have been improved by PI-Zn, which further increases the plating uniformity.The stability of the Zn anode was evaluated by long-term galvanostatic cycling of a symmetrical Zn cell (Fig. 4). The cell with bare Zn failed after cycling for 100 h at a current density of 1 mA cm-2 resulting from the short circuit. In comparison, the cell with PI-Zn exhibits a stable polarization voltage and an ultralong cycling life of over 1200 hours, indicating the extremely reversible plating/stripping enabled by the PI network. Further interrogation of the PI-Zn was carried out by stringent deep-discharge tests in symmetrical Zn cells under extremely high current density and capacity of 20 mA cm-2 and 10 mA h cm-2. The result shows that the cell with PI-Zn can cycle for more than 200 hours, two times longer than that with bare Zn, proving the advantages of the unique fiber-like network structure, which provides fast and homogeneous Zn ion transportation. Moreover, the fiber network effectively suppresses the hydrogen evolution reaction, verified by the corrosion test, despite the existence of numerous ion channels.Full prototype cells were assembled to prove the practical applications of our strategy. Two promising cathodes for aqueous ZIBs, MnVOH and MnO2, in two different electrolyte systems (Zn(OTf)2 and ZnSO4) were coupled with bare Zn or PI-Zn. For both systems, the capacity retention and Coulombic efficiency are both significantly improved with the help of PI network. The results are consistent with the lifespan difference of symmetrical Zn cells, as expected due to the same plating and stripping processes on Zn. These findings demonstrate that the strategy presented here stands to both dramatically extend battery cycle life and boost battery performance, a promising approach to solve the anode issues in advanced Zn batteries. Cao, Z., et al., Advanced Energy Materials, 2020. 10(30): p. 2001599. Zhao, Z., et al., Energy & Environmental Science, 2019. 12(6): p. 1938-1949. He, H., et al., Journal of materials chemistry A, 2020. 8(16): p. 7836-7846. Figure 1

Effect of Electrolyte on Sodium-Ion Storage Behavior into Non-Graphitizable Carbon Negative Electrode

Introduction Sodium-ion batteries (SIBs) are expected to be an alternative power source to lithium-ion batteries (LIBs) because their abundant resources reduce the cost of SIBs. As the negative electrode of SIBs, non-graphitizable carbon recieves a lot of attention because it can store ions not only in the graphene interlayer but also in its internal pores.[1] By controlling the pores, it is possible to achieve larger reversible capacity of more than 400 mAh g−1.[2] While many researchers have studied non-graphitizable carbon from the viewpoint of thermodynamics, there are a few reports that pay attention to kinetic viewpoint. The kinetic viewpoint is a very important factor because it affects the rate performance of the battery.We have focused on the interfacial reactions of non-graphitizable carbon electrodes and found that the charge-transfer resistance and the activation energy of the charge-transfer reaction in SIBs were larger than those in LIBs.[3] This result and the Lewis acidity indicated that the desolvation process was not identified as the rate determining step in the charge-transfer reaction. One of the significant factors responsible for the increased resistance is the effect of the solid electrolyte interphase (SEI). Objective In this work, we used six types of electrolytes and formed SEI on the surface of non-graphitizable carbon. The interfacial reaction of non-graphitizable carbon with SEI derived from these electrolytes was investigated.In this paper, we report about the effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon electrode. Experimental Non-graphitizable carbon heat-treated at 2273 K (HC-2000) was used as the negative electrode material. A three-electrode cell was used for electrochemical measurements. HC-2000 composite electrode, natural graphite composite electrode, and sodium metal were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolytes were prepared using sodium bis(trifluoromethanesulfonyl) amide (NaTFSA), sodium bis(fluorosulfonyl) amide (NaFSA), and NaPF6 as sodium salts and ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 by volume) and fluoroethylene carbonate (FEC) as solvents.Cyclic voltammetry (CV) was performed to form SEI on HC-2000. The scan rate and range were 0.1 mV s−1 and 0–2.5 V, respectively. After CV, electrochemical impedance spectroscopy (EIS) was performed with an electrode potential of 0.2 V and an AC amplitude of 10 mV in the frequency range of 100 kHz–10 mHz. In addition, X-ray photoelectron spectroscopy (XPS) was used to analyze the electrode after CV measurements with respect to SEI. Results and discussion Figure 1 shows the Nyquist plots obtained from the EIS analyses. The semicircles in the low frequency region were attributed to the charge-transfer resistances. Among EC+DEC solvents, the system of NaFSA had the largest charge-transfer resistance. For NaTFSA and NaPF6, the charge-transfer resistances increased in the FEC solvent, while no significant differences in the charge-transfer resistance were observed for the NaFSA system in the EC+DEC and FEC solvents.The SEI components of the HC-2000 composite electrode in each electrolyte were determined by XPS analysis after CV measurements. In NaFSA/EC+DEC system, the amount of NaF was remarkably large, while the amount of NaF was relatively small in NaTFSA/EC+DEC and NaPF6/EC+DEC systems. On the other hand, in FEC solvents, the amount of NaF increased in NaTFSA and NaPF6 systems. The systems with higher NaF contents had the larger charge-transfer resistances. This result indicates that NaF was involved in the charge-transfer resistance.Finally, EIS analyses were performed at different temperatures. The activation energies were calculated using Arrhenius plots and the Arrhenius equation. The values of the activation energies were in the range of 60–70 kJ mol−1. There was a slight variation in the values, but they were within the error range and were not significantly different for all systems. These results showed that NaF affects the frequency factor rather than the activation energy. Conclusion The effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon were investigated. The systems with the large charge-transfer resistance had a large amount of NaF. NaF did not affect the activation energy of charge-transfer resistance but did affect the frequency factor.

Poor Cycling Performance of Rechargeable Lithium–Oxygen Batteries Under Lean-Electrolyte and High-Areal-Capacity Conditions: Role of Carbon Electrode Decomposition

There has been considerable interest in developing rechargeable lithium oxygen batteries (LOBs) that have the potential to significantly exceed the energy density limit of a Li ion battery (approximately 300 Wh/kg). Although 500 Wh/kg LOB has been demonstrated and exhibited stable discharge/charge cycle at room temperature condition1, their cyclability remains less than 10 cycle. For realizing the LOB with practically high energy density and long cycle life, the deep understanding of complicated reaction in LOBs has been highly demanded. Especially, the problems specifically associated with the limited electrolyte (“lean-electrolyte”) conditions need to be taken into account in LOBs.Although the importance of investigating negative lithium electrodes in lean-electrolyte systems has been recognised in recent years2,3, specific problems associated with the negative lithium electrode in a LOB has not been investigated so far. In particular, chemical crossover from the positive electrode to the negative electrode is a crucial factor that needs to be taken into consideration. Actually, recent studies have shown that LiOH is formed on the surface of a lithium-metal electrode through reaction with H2O in the electrolyte4,5. This H2O is generated by the decomposition of the tetraethylene glycol dimethyl ether (TEGDME) solvent at the positive oxygen electrode, which then migrates to the negative electrode side to react with the lithium-metal electrode. Consequently, problems related to chemical crossover are prominent in lean-electrolyte LOBs, where the effective water concentration in the electrolyte is much higher than that in an excess electrolyte system.Despite the importance of investigating the reaction of negative lithium electrodes in LOB with appropriate technological parameters, mechanistic details remains unclear due to the technical difficulty for applying non-destructive analytical methods. In the present study, we investigated the reaction of a negative lithium electrode in a lean-electrolyte LOB operating under high areal capacity conditions. The use of advanced non-destructive analytical methods, including three-electrode electrochemical techniques and an in-situ analytical setup revealed that the reaction efficiency of the lithium electrode mostly decreases through chemical crossover from the positive oxygen electrode6. In particular, the H2O generated as a side-product at the positive oxygen electrode crosses over to the negative electrode side and reacts with the lithium-metal electrode to form a porous LiOH/Li2CO3 layer on the electrode surface. LiOH and Li2CO3 accumulates, and the porous layer thickens through repeated discharge/charge cycling, which supresses efficient Li-ion transport through the layer.Furthermore, using a solid-state ceramic-based separator to protect the lithium-metal negative electrode, the side reaction with the negative electrode was considerably suppressed, which helped exclusively assess the degradation phenomena occurring in the oxygen positive electrode. By series of in-situ and ex-situ analysis revealed the following points: (i) Although the electrolyte degradation intensified with increasing repeated discharge/charge cycling, the scarcity of the electrolyte minimally influenced the cycle life of the LOBs. (ii) Most of the Li2CO3-like solid-state side products decomposed during charging at voltages higher than 3.8 V. (iii) Severe decomposition of the carbon electrode occurred during cycling, and an equivalent amount of CO2 was generated during charging at voltages higher than 3.8 V. Because the carbon electrode used in these experiments merely decomposed via simple electrochemical oxidation at voltages up to 4.5 V, the carbon electrode was considered to deteriorate through the decomposition of its solid-state side products. The findings of this study are anticipated to guide future investigations on the advancement of LOBs. In particular, detailed understanding of the reaction at the interface between the carbon electrode and solid-state products is crucial for realizing practical high-energy-density LOBs with long cycle life.

Real-Time Mass Spectrometry for Simultaneous Monitoring of Electrode Dissolution and Reaction Product Distribution during Organic Electrosynthesis

An organic electrosynthesis is an emerging tool for modern chemical manufacturing fitting the environmental and sustainability agenda. It utilizes polarized electrode surfaces as “traceless” oxidants or reductants, whereas its heterogeneous nature and exceptional tunability enable easy access to radical intermediates and novel mechanistic pathways. In the line of further energy input improvement, there is a growing demand for appropriate catalytic materials devoted to organic molecule transformations, for which the activity and selectivity have so far been most studied. Although the investigation of electrodes’ and electrocatalysts’ stability in the field of electrochemical energy conversion is well-established, there is still a lack of attention to the symmetrical studies for organic electrosynthesis. Nevertheless, electrode material losses shorten the reactor’s lifetime and can cause metal cathodic re-depositions, severely altering reaction product selectivity and final product contaminations. Although some reports on the ex-cell metal dissolution analysis are available, no information about its mechanism can be gained in such a way. To highlight the issue numerically, an analytical method to characterize rates and mechanisms of catalyst dissolution within a real-time response-enabled potential-, temperature-, or time-resolved parametrization of the electrocatalyst is highly demanded.Herein we present a system to study organic electrosynthetic protocols under dynamic operations to quantitatively monitor the electrode dissolution aligned with reaction products during organic molecules transformations. The instrument comprises an electrochemical flow cell (EFC) with exchangeable working electrodes, the outlet of which is interfaced with an online quadrupole mass spectrometer (OQMS) for the analysis of extracted by solid membrane gaseous products and an inductively coupled plasma mass spectrometer (ICP-MS) for the detection of dissolved metals. The system is exemplified by a benchmark Kolbe electrolysis over platinum Pt anode, commonly considered a stable and inert electrode material. Based on the literature, methanol has been used as a solvent of choice, while partially neutralized acetic acids with different bases were studied as objects. The methodology shed some light on understanding the role of platinum oxide formation on the electrode surface and can be used to study time- and potential-resolved catalyst stability and the possibility for dissolved transition metal-catalyzed electroorganic transformations.

Investigating the Effect of Alkali Metal Cations on Kolbe Electrolysis

Carboxylic acids, a major fraction of pyrolysis oil can be transformed electrochemically in alkanes and alcohols by Kolbe and Hofer Moest reaction. The selectivity of electrochemical decarboxylation depends on various factors, i.e. electrode material and reaction conditions (supporting electrolyte, pH and current densities) which are well explained in the literature1–3. Alkali metal cations (Li+, Na+, K+, Cs+) are widely considered inert,4 nevertheless in recent studies, a significant influence on different electrochemical reactions such as methanol5,6, ethanol7, and formic acid8 oxidation is reported. It is assumed that oxygenated species present on the electrode surface form (strong/weak) non-covalent interaction with the alkali metal cations.In this work, we studied the electrochemical oxidation of acetic acid (Kolbe electrolysis) and explored the influence of monovalent alkali metal cations (Li+, Na+, K+ and Cs+). It is observed that the product distribution and overall electrochemical activity are influenced by the presence of cations with K+ being most suitable for Kolbe electrolysis (order of activity Li+<Na+<K+~Cs+). The trend was observed irrespectively of the electrolyte conditions throughout the entire pH range at low current densities (25 mA/cm2), i.e. at pH 3 where OER is dominating, pH 5 where OER transitioned to Kolbe electrolysis and pH 9 where Hofer Moest reaction occurs. With increasing current densities (>50 mA/cm2), the influence of cations on electrooxidation diminishes showing the limitations of the effect of non-covalent interactions.To confirm the surface coverage and transition of OER to Kolbe reaction with increasing potential, we probed reaction selectivity in the inflection zone potential regime by means of RRDE. The analysis confirmed that the OER is suppressed completely in the presence of K+ cations after reaching potentials of ~2.5VRHE (inflection zone), while for Li+, OER still proceeds as confirmed by the ORR reaction at the ring electrode. Because surface coverage reflects the high activity of Kolbe electrolysis, we confirmed that the smaller cations interfere more strongly with the electrode surface adsorbed oxygenates due to strong non-covalent interaction and thus affecting the reaction. References F. J. Holzhäuser, J. B. Mensah, and R. Palkovits, Green Chem., 22, 286–301 (2020).C. Stang and F. Harnisch, ChemSusChem, 9, 50–60 (2016).T. Ashraf, A. P. Rodriguez, B. Mei, and G. Mul, Faraday Discuss. (2023)H. J. Schäfer, in Comprehensive Organic Synthesis, B. M. Trost and I. Fleming, Editors, p. 633–658, Pergamon, Oxford (1991)K. Silambarasan, J. Joseph, and S. Mayavan, Applied Surface Science, 489, 149–153 (2019).D. Strmcnik et al., Nature Chem, 1, 466–472 (2009).C. Han et al., ACS Appl. Mater. Interfaces, 14, 5318–5327 (2022).B. A. F. Previdello, E. G. Machado, and H. Varela, RSC Advances, 4, 15271–15275 (2014). Figure 1

(General Student Poster Award Winner, 3rd Place) Corrosion Electrochemistry in Molten Flinak Salts

An existing knowledge gap in the field of molten salt (MS) corrosion is the lack of in situ, diagnostical and instantaneous measurements of the underlying corrosion system. Numerous investigations comparing the corrosion behavior of commercial alloys with complex microstructures and compositions in molten fluoride salts have been conducted during the past five years. A few of these have been expanded from exposure studies to those using electrochemical methods[1,2]. In essence, electrochemical techniques offer an in situ and diagnostical way to analyze the corrosion behavior of potential materials in real-time, either in open-circuit (static immersion) or polarized conditions. Investigations on MS corrosion have found few mechanistic insights that elucidate the fundamentals of electrode reactions and characteristics of the electrode-salt interface. However, fate of elements oxidized are not tracked. Nonetheless can be difficult to understand the electrochemical corrosion behavior of structural alloys in non-aqueous, aggressive media, such as molten LiF-NaF-KF (FLiNaK) salts, as they may be influenced by a wide range of parameters (such as microstructure, impurity, and potential).Understanding the mechanism of an alloy's most susceptible alloying constituent's dissolution and describing the rate-determining steps (RDS) of this process using the principles of thermodynamics, kinetics, and electrochemistry are fundamental initial steps in understanding the complexities of alloy corrosion. Chromium (Cr) is frequently referred to in molten fluorides research as an essential yet harmful alloying element in candidate molten salt reactor structural alloys (i.e.Ni-based superalloys and stainless steels) due to its high thermodynamic driving force to dissolve relative to other alloying elements. Because of that Cr is a crucial material for research because it serves as the basis for work on Fe-Cr, Ni-Cr, and Fe-Ni-Cr alloys as well as other alloys. However, published research does not consistently describe the mechanism relating to Cr corrosion in FLiNaK.The objective of this work was to explore the electrochemical corrosion behavior of pure Cr in molten FLiNaK salts at 600oC using electrochemical methods. In this study, we describe and clarify the regimes of Cr corrosion in FLiNaK at 600 oC that are potential-dependent, rate-limiting charge-transfer and mass-transport regulated.A range of measurements based on electrochemistry and material characterization, such as linear, cyclic, and potentiostatic polarization techniques, is presented. A key finding in this work was the attempt to establish a connection between the reactivity of Cr in an aggressive, non-aqueous electrolyte such as molten FLiNaK to electrochemical potentials and corrosion morphology predicted thermodynamically. It was found, that the corrosion mechanism of Cr in FLiNaK was dependent upon mixed potentials established by the anodic Cr/Cr(II) and/or Cr(II) /Cr(III) oxidations coupled with cathodic reactions by multiple oxidizer candidates whose predominance is a function of salt impurity level and Cr exposure times. Results suggest that the early stage of Cr dissolution is regulated by charge-transfer (or activation-controlled) in FLiNaK at 600oC. This type of mechanism can occur at low overpotential (i.e. lower driving force) and can be interrupted by the oversaturation of Cr at the surface. Its presence may be eliminated if the salt film is present. The electrochemical potential in which mass transport regime is rate-limiting falls within the phase stability field where CrF6 3- species dominates, and leading to the formation of a K3CrF6 salt film. The post-exposure SEM analysis revealed that Cr dissolution in this regime involves volume transport within each grain, resulting in the revelation of surface facets containing crystallographic features of cubic crystals.The application of electrochemistry enables a more straightforward way to identify potential RDS that control the rate of Cr corrosion and offers a temporal description of the corrosion mechanism. Such fundamental understanding, especially on the nature of cathodic and anodic reactions of Cr, can provide a baseline to understand the corrosion mechanism of complex alloys. The post-corrosion microstructure investigation, the XRD of the solidified salt combination, and the thermodynamic analysis of the stable phases present in each corrosion regime all corroborated these conclusions. Acknowledgments Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Wang, Y. L., et al. Corrosion Science 109, 43–49 (2016).Chan, H.L., et al. NPJ Materials Degradation 6(1), 46 (2022).

Non-Aqueous Rechargeable Rb-O2 Batteries

Li-O2 batteries have attracted increasing attention because their theoretical energy density exceeds that of current lithium-ion batteries.[1] Generally, the positive electrode reaction in nonaqueous alkali metal-O2 batteries proceeds through oxygen reduction and corresponding reoxidation, forming alkali metal superoxide (MO2) as the discharge product. It is known that the stability of the superoxide (MO2) depends on the relationship between the ionic radius of the alkali metal ion (M+) and the superoxide ion (O2 −).[2] For example, LiO2 consists of small Li+ (ionic radius in octahedral coordination: 0.76 Å) and relatively large O2 − (1.49 Å), resulting in its short lifetime and rapid disproportionation reaction; 2LiO2 → Li2O2+O2. As the two-electron reduction product, Li2O2, exhibits a large overpotential to re-oxidize it, Li-O2 cells are known to have low reversibility.[1] On the other hand, NaO2 is relatively stable because the ionic radius of Na+ (1.02 Å) is relatively large compared to that of Li+. Thus, Na-O2 battery is also promising due to the low charge overpotential of the one-electron reduction product, NaO2. However, NaO2 is highly reactive with a trace amount of moisture, CO2, and electrolyte components, partially forming Na2O2 and/or its hydrates.[3] Further stabilization was observed by using K+ (1.38 Å) to form KO2, thus K-O2 cell is known to have high stability and reversibility as well as low overpotential.[4] Following these trends, in this study, we focused on Rb+ (1.52 Å), which has a similar ionic radius to O2 − (1.49 Å), and investigated the electrochemistry of oxygen reduction and evolution reaction in Rb-containing non-aqueous electrolytes.The working electrode is prepared by mixing Ketjen black (KB) and polytetrafluoroethylene (PTFE) in a weight ratio of 80:20, followed by coating it on carbon paper. The counter electrode consists of activated carbon, KB, and PTFE mixed with an 80:10:10 weight ratio. Ag+/Ag reference electrode and rubidium bis(trifluoromethanesulfonyl)amide (RbTFSA)/ triglyme (G3) electrolytes were also used to fabricate three-electrode O2-cell. The cells were first assembled in an Ar-filled glove box, followed by purging dry-O2 gas to supply an adequate amount of O2 in the cell. Note that Rb metal was not used for safety reasons.Figure (a) presents the charge-discharge curves obtained with 1 mol/dm3 RbTFSA/G3 electrolyte under a dry-O2 and dry-Ar atmosphere. While no capacity was observed under the Ar atmosphere, distinct plateaus appeared at 2.57 and 2.67 V (vs. Rb+/Rb) at discharge and charge sequence, respectively, under the O2 atmosphere. A discharge capacity of ~0.6 mAh cm−2 and a relatively small overpotential of 100 mV were achieved, as well as high Coulombic efficiency at the initial cycle of 71 %. Figure (b) shows XRD patterns of pristine and fully discharged electrodes, where all of the new peaks that appeared through discharge reaction can be assignable to RbO2. Therefore, the discharge product of the Rb-O2 cell is confirmed to be RbO2, similar to Na-O2 and K-O2 cells. The reaction mechanism, reversibility, and stability of the O2/RbO2 electrode will be presented and discussed.Reference:[1] P. Bruce, J-M. Tarascon, et al., Nat. Mater., 11, 19-29 (2012).[2] L. Qin, Y. Wu, et al., J. Am. Chem Soc., 142, 11629 (2020).[3] P. Hartmann, P. Adelhelm, et al., Nat. Mater., 12, 228 (2013).[4] X. Ren, Y. Wu, J. Am. Chem. Soc., 2, 2923 (2013). Figure 1

(Keynote) Competition of Oxygen and Chlorine Evolution on Oxide Surfaces – a Soft X-Ray and Dems Approach to Identification of Active Site

Anodic gas evolving reactions, e.g. oxygen evolution and chlorine evolution, are of fundamental importance both in energy storage as well as in electrolytic production of chemicals. Both reactions are of electrocatalytic nature, proceed in the same potential range and are catalyzed by the same types of electrode materials. While the oxygen evolution is thermodynamically favored the two electron chlorine evolution is kinetically preferred, namely at acid and neutral pH. This kinetic preference is industrially explored in chlor-alkali processes as well in chlorate electrolysis. The growing interest in electrolytic hydrogen production fueled by the popularity of the hydrogen as the likely replacement of fossil fuels brings up the question of the tolerance of the industrial anodes to presence of chlorides in the water intended for electrolytic hydrogen production. Such a tolerance can be best achieved by steering the selectivity of the anode between oxygen and chlorine evolution aiming eventually at the possibility of the sea water use in hydrogen producing water electrolysis. So far existing attempts at control the anode selectivity in presence of chlorides explore two principal approaches – pH control and surface engineering. While the pH related approach is rationalized by different pH sensitivity of each of the competing reaction the surface engineering related activities so far rely on trial and error approach. Qualitatively the selectivity may be controlled by either selectively promoting/suppressing each of the contributing reactions. While the literature lists attempts when the overall selectivity towards OER is apparently achieved by selective suppression of CER [1] more recent reports suggest that the actual selectivity is in fact controlled by the ability of the catalysts to support a lattice oxygen activating mechanism of the oxygen evolution process [2]. While these assignments are based on correlation of selectivity measurements with local structure and DFT modelling and a direct evidence of such a behavior is so far missing. This paper will document here the capability of the real-time and operando soft X-ray absorption spectroscopy [3] to provide this missing evidence. Anodic behavior of two type of ruthenium oxide electrodes in chloride free and chloride containing solutions will be documented by comparison of the nature and potential dependent population of the oxygen related intermediates formed at anode surface in the potential range of oxygen and chlorine evolution in acid media (pH=1) and combined with DEMS data to identify the active sites for each contributing electrode reaction.

Distribution of Relaxation Times of Fuel Electrodes for Reversible Solid Oxide Cells Fabricated Under Various Conditions

Introduction Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can switch reversibly between power generation as a solid oxide fuel cell (SOFC) and hydrogen production as a solid oxide electrolysis cell (SOEC). It is important to systematically understand the electrode reaction processes and degradation factors in these two modes to tailor high-performance and durable r-SOC electrodes. The dependence of polarization resistance on cell fabrication conditions and operating conditions has been individually investigated for SOFC and SOEC by impedance measurement and distribution of relaxation times (DRT) analysis [1-4], but it is necessary to evaluate r-SOC as a whole [5]. Here in this study, we aim to systematically clarify the similarities and differences in the electrode reaction processes at the fuel electrodes of both SOFC and SOEC modes of r-SOCs, with respect to fuel electrode fabrication conditions. Experimental Several types of cells were fabricated with different fuel electrode constituent materials, thicknesses, and sintering temperatures. Three types of fuel electrode materials were used: Ni-GDC cermet, a composite of Ni and mixed ionic-electronic conductor Gd0.9Ce0.1O3 (GDC); Ni-YSZ cermet, a composite of Ni and pure ionic conductor YSZ (8 mol% Y2O3 - stabilized ZrO2); and Ni-GDC co-impregnated electrode with LST-GDC as a backbone support (LST: La0.1Sr0.9TiO3). Scandia-stabilized zirconia (ScSZ) was used as the electrolyte plate, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was used as the air electrode. GDC buffer layer was prepared between the electrolyte and the air electrode to suppress the formation of interfacial insulting layers.Electrochemical impedance measurements were performed for the fuel electrode of the fabricated cell under SOFC and SOEC modes by supplying H2-H2O mixture and varying operating temperature, fuel humidification, and current density. The DRT analysis was conducted to separate various polarization resistance components of the fuel electrode with different relaxation times. The area of each DRT peak corresponds to the resistance of each electrode reaction process. In this study, activation energy was derived based on the assumption that the electrical conductance is of an Arrhenius-type thermally activated character. Its dependence on the fuel electrode fabrication conditions was investigated. Furthermore, electrode microstructure was observed by using focused-ion beam scanning electron microscopy (FIB-SEM). Results and Discussion Figure 1 shows typical DRT peaks of the Ni-GDC cermet fuel electrode in (a) SOFC and (b) SOEC modes, measured at different temperatures. In the range of 10-1 to 106 Hz where the frequency response was measured, three DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all with a DRT peak area decreasing with increasing operating temperature. This indicates that there are at least three electrode reaction processes with different relaxation times, which are thermally active processes. Activation energies of 0.1-0.3 eV (SOFC mode) and 0.2-0.5 eV (SOEC mode) were obtained for P1, and 0.9-1.0 eV (SOFC mode) and 1.1-1.3 eV (SOEC mode) for P2. The activation energies in SOEC mode were higher than those in SOFC mode for both P1 and P2. In the presentation, the dependence of activation energies on the fuel electrode constituent materials and related electrode reaction processes will be discussed.

A Tetramethylurea-Electrolyte-Based Lithium-Oxygen Battery with High Performance and Alternative Mechanism

The electrolyte is a critical component in electrochemical energy storage devices as it creates the reaction environment for the electrode process. In lithium–oxygen (Li–O2) batteries, the oxygen electrode reaction involves a complex multiphase transition (e.g., gas, liquid, solid) and a variety of intermediates (e.g., O2 –, O2 2–, 1O2), making the role of the electrolyte system more prominent. As an ideal electrolyte for Li–O2 batteries, it should not only resist the attack of superoxide intermediate, but also provide a kinetically favorable O2 redox reaction mechanism. However, currently, no electrolyte solvent is perfectly suitable for Li–O2 batteries, which greatly limited their practical applications. Here, we present a new electrolyte solvent, 1,1,3,3-tetramethylurea, for Li–O2 batteries that exhibits high performance and an alternative mechanism. Specifically, it enables a kinetically favorable one-electron Li2O2 oxidation pathway upon charging. Furthermore, due to the absence of alpha-hydrogen atoms in its molecular structure, it also exhibits good stability in the presence of superoxide intermediates, significantly suppressing the related hydrogen extraction side reactions. Consequently, Li–O2 batteries utilizing this electrolyte show a much lower charge overpotential and a four-fold longer cycle life compared to those with the benchmark electrolyte. Acknowledgments X.L. acknowledges financial support from the Marie Skłodowska-Curie Actions (grant agreement No. 101064286) for his postdoctoral fellowship.

Thermodynamic analysis of Ti, As-containing systems based on E-pH diagrams and partial pressures

Abstract&#x0D; The removal of arsenic from the technological circulation of metallurgical enterprises is a technologically necessary process, leading from year to year to the formation of new volumes of arsenic-containing objects. One of the forms of extracting arsenic from the production stage is arsenates of various metals, which are difficult and poorly soluble compounds. This approach also does not solve environmental issues in general. Thus, titanium arsenates under storage and burial conditions are subject to hydrolysis, which leads to gradual pollution and poisoning of the environment with arsenic-containing compounds. The practical value of titanium and its compounds in economic activities forces us to look for ways to rationally use arsenic-containing titanium compounds.&#x0D; The article is the first to carry out a thermodynamic analysis of the behavior of arsenic and titanium based on potential-pH diagrams and partial pressures. The course and directions of chemical reactions in titanium- and arsenic-containing systems, the conditions for the stability of their constituent phases, the thermodynamic parameters of the behavior of the participating components, their compounds, the range of their stability, and the redox processes of the formation of chemical products have been studied.&#x0D; The regions of existence of titanium arsenate are determined, and chemical and electrode reactions for the production of titanium arsenate from titanium and arsenic compounds are considered. The results of the work confirm both the effectiveness of using titanium compounds to remove arsenic from solutions in the form of poorly soluble manganese arsenate, and the possibility of regenerating titanium in the form of oxides and bases.&#x0D; Key words: thermodynamic systems, diagrams, arsenic compounds, titanium compounds, titanium arsenate.

Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte and the Study of Ion Crossover in Zn-Ce Redox Flow Battery

The development of a highly efficient and inexpensive energy storage technology is central to the operation of a sustainable smart grid and widespread utilization of intermittent renewable energy sources. An emerging and promising grid-scale technology for electrochemical energy storage is the redox flow battery (RFB). A unique and particularly valuable aspect of the design of RFBs is that their energy and power capacities can be scaled independently. Some of their other advantages include short response time, long life-cycles and no detrimental effects associated with being fully charged or discharged [1]. The Zn-Ce RFB, first developed by Plurion Inc (UK) in 2005 [2], has an open-circuit voltage as high as 2.4 V which is one of the highest among the various RBFs.To further improve the efficiency of Zn-Ce RBFs, the effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce RFB has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid and pure methanesulfonic acid has been assessed and compared on the basis of the cyclic voltammetry response for the Ce(III)/Ce(IV) redox couple and galvanic charge-discharge of a bench-scale Zn-Ce RFB. We have observed the Ce(III)/Ce(IV) reaction to exhibit faster kinetics and the battery to attain higher coulombic efficiency and long-term performance over ~40 charge-discharge cycles in the mixed 2 mol/L MSA–0.5 mol/L H2SO4 electrolyte compared to that achieved in the commonly used 4 mol/L pure MSA electrolyte due to lower H+ crossover and the prevention of CeO2 precipitation. The coulombic efficiency fade rate in the mixed MSA-H2SO4 electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation [3]. Our results show that a mixed MSA–H2SO4 acid electrolyte may be a better option for the positive side of a Zn-Ce RFB as a large-scale energy storage device.At the same time, undesired ion crossover is a practical issue that cannot be neglected since it decreases the concentrations of electroactive species in both negative and positive electrolytes and can cause battery self-discharge and thereby lower battery efficiency and longevity. The concentrations of electroactive species in both the positive and negative electrolytes after each charge-discharge cycle have been monitored as a function of cycle number. We have found that as much as 48.6% of the Zn(II) ions are transported through the Nafion 117 membrane from the negative to the positive electrolyte and 11.1% of Ce(III) ions are transported from the positive to the negative electrolyte by the end of 30 cycles (~55 hours of battery operation). A simple and efficient method to mitigate this problem is to add a certain amount of Zn(II) to the positive electrolyte and Ce(III) to the negative electrolyte at the outset of operation in order to reduce the concentration gradients between the two sides. From our cyclic voltammetry results, the addition of Zn(II) ions into the positive electrolyte has no deleterious effect on the Ce(III)/Ce(IV) redox couple and in fact appears to slightly reduce the separation between the reduction and oxidation peaks, suggesting better reversibility of the positive electrode reaction. In addition, experiments involving 30 cycles of battery charge-discharge have been conducted using Zn(II)-containing pure MSA positive electrolyte as well as Zn(II)-containing MSA-H2SO4 mixed acid electrolytes to analyze battery efficiency. Our results show that both the coulombic and voltage efficiencies are increased compared to that attained using the original electrolytes without Zn(II) initially added to the positive electrolytes. The energy efficiency increases by 19.7% in the case of the Zn(II)-containing pure MSA electrolyte and 6.1% for the Zn(II)-containing H2SO4-MSA mixed acid electrolyte (see Figure 1). Another benefit of adding Zn(II) ions to the MSA pure positive electrolyte is that it prevents the precipitation of CeO2 routinely observed to occur in MSA-only electrolytes after 10 – 15 charge-discharge cycles, which further enhances the energy density of the Zn-Ce RFB.References Nguyen, T. and R.F. Savinell, Flow Batteries. Electrochemical Society Interface, 2010. 19(3), 54.Clarke, R., B. Dougherty, S. Harrison, P. Millington, and S. Mohanta, US Patent 2004/0202925 A1. Cerium Batteries, 2004.Yu, H., M. Pritzker, and J. Gostick, Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte in Zn-Ce Redox Flow Battery. J. Electrochemical Society, 2023. 170(2), 020536. Figure 1