Non-aqueous Systems Research Articles

Introducing Artificial Solid Electrolyte Interphase onto the Anode of Aqueous Lithium Energy Storage Systems.

Aqueous lithium energy storage systems (ALESSs) offer several advantages over the commercially available nonaqueous systems, and the most noteworthy is that ALESSs have higher ionic conductivity, can be used safely, and are environmental-friendly in nature. The ALESS, however, exhibits faster capacity fading than their nonaqueous counterparts after repeated cycles of charge and discharge, thus limiting their wide-range applications. Excessive corrosion of metallic anodes in the aqueous electrolyte and accelerated growth of dendrites during the charge-discharge process are found to be the main reasons that severely impact the life span of ALESSs. Here, we introduce ultrathin graphene films as an artificial solid electrolyte interface (G-SEI) on the surface of a zinc anode to improve the cycling stability of an aqueous lithium battery system. The G-SEI is fabricated at different thicknesses and areas ranging from ∼1 to 100 nm and ∼1 to 10 cm2, respectively, via a Langmuir-Blodgett trough method and deposited onto the surface of the zinc anode. Electrochemical characterizations show a significant reduction in corrosion current density (0.033 mA cm-2 vs 1.046 mA cm-2 for the control), suppression of dendritic growth (∼50%), and reduction in charge-transfer resistance (222 Ω vs 563 Ω for the control) when the G-SEI is utilized. The aqueous battery system with the G-SEI (100 nm thickness) on the anode exhibits ∼17% improvement in cycling stability (82% capacity retention after 300 cycles) compared to the control system. Comprehensive microscopy and spectroscopy characterizations reveal that the G-SEI not only controls the ion transport between the electrolyte and the anode surface (lower corrosion) but also promotes a uniform deposition (less dendritic growth) of zinc on the anode.

Quantitative Nano-Structure–Property Relationships for the Nanoporous Carbon: Predicting the Performance of Energy Storage Materials

Nanoporous carbon-based energy storage is a fast-growing research field thanks to high energy densities of carbon electrodes with nanoporous amorphous texture. To support the developments on electrical double-layer based ultracapacitors, it is necessary to improve understanding about relationships between the porous structure and energy storage behavior of carbon materials. This can be facilitated by the analysis of complex data sets and the development of corresponding descriptive and predictive models. Related to that, this work presents an in silico regression model to predict the suitability of various carbon materials for energy storage, thus being probably the first time a quantitative nanostructure–property relationship (QnSPR) approach is applied to the nanoporous carbon materials. With this study, which is based on the experimental data of 100 carbide-derived carbon materials, it has been shown that the electrical double layer capacitance of carbon electrode in a nonaqueous electrolyte can be predicted using experimentally determined specific surface area, a volume of certain pore size fraction of carbon and a bulk density of carbon electrode. The three-parameter QnSPR model for volumetric cathodic capacitance of carbon in triethylmethylammonium tetrafluoroborate/propylene carbonate electrolyte, CV,NEG = f(SBET, Vd<1.14, Del), comprising the above-mentioned parameters and characterized by R2 = 0.94 and s2 = 8.7, confirms the important role of carbon pore size for the double layer capacitance. It was shown that carbon pores with a size below 1.1 nm have the most significance for achieving high energy densities in the nonaqueous electrochemical systems studied. Putting the results of this research into wider perspective, it has been shown that the QnSPR approach provides a useful tool for describing and predicting the variable performance-related physical properties of nanoporous carbon and nanomaterial properties in general. The models are available in the QsarDB repository (http://dx.doi.org/10.15152/QDB.205).

(Invited) Process Development and Analysis of Si Electrodeposition in Ionic Liquid

Si thin films and micro/nano structures have been widely used for various applications such as semiconductor and MEMS devices, solar cells, battery electrodes, etc. While dry processes such as chemical vapor deposition have been conventionally applied for their fabrication, electrochemical approaches attract attention for their capability to form uniform and precise structures to wide and non-flat surfaces, with high productivity. In this paper we describe the results of our investigation on Si electrodeposition, mainly focusing upon the process development as well as mechanistic understanding of the deposition process through experimental and theoretical approaches. Since the redox potential of Si species is very negative, nonaqueous systems such as organic solvents, molten salts or ionic liquids have been applied for the electrodeposition of Si [1-3]. We have mainly used low temperature (up to 40°C) ionic liquid (IL), trimethylhexyl ammonium (TMHA+) bis-trifluorosulfonyl imide (TFSI-), as electrolyte [4]. The TMHA-TFSI is hydrophobic IL in which SiCl4, the precursor of Si, could be sufficiently dissolved. The electrodeposition was carried out potentiostatically using a three electrode sell. Au/n-Si or FTO substrate, Pt wire, and Ag/Ag+ were used as working, counter, and reference electrodes, respectively. By optimizing the deposition parameters, uniform Si films were obtained. The results of structural and compositional analyses indicated that the as-deposited films were amorphous and contained impurities such as C, while their amount could be reduced by annealing. It was also confirmed that the electric properties of the deposited films could be controlled by adding dopant species such as AlCl3 in the bath for the electrodeposition [5]. In order to investigate the mechanism of the deposition process, we applied electrochemical quartz crystal microbalance (EQCM) and the results indicated that the reduction of SiCl4 to Si proceeded via intermediate species such as Si (III) and Si (II) [4, 6]. In situ X-ray reflection measurements were also employed using synchrotron radiation lightsource in combination with density functional theory (DFT) calculation for nanoscopic analysis of the structure of IL-electrode interface during the deposition [7]. It was indicated that layered structure consisting of Si multimer such as Si2Cl6 dimer was formed at the surface of electrode prior to the deposition of elemental Si, suggesting formation of the intermediate states during the initial deposition stage of Si. The authors would like to thank Dr. J. Komadina, Dr. A. Mehta, Mr. M. Nishida, and Mr. H. Takai for their experimental help and valuable discussion. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. [1] T. Munisamy, A. J. Bard, Electrochim. Acta, 55, 3797 (2010). [2] M. Bechelany, J. Elias, P. Brodard, J. Michler, L. Philippe, Thin Solid Films, 520, 1895 (2012). [3] F. Endres, A.P. Abbott, D.R. MacFarlane, Electrodeposition from Ionic Liquids, Wiley-VCH, 2008. [4] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100, 236 (2013). [5] Y. Tsuyuki, H. Takai, Y. Fukunaka, T. Homma, Jpn. J. Appl. Phys., in press. [6] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183, 49 (2015). [7] Y. Tsuyuki, T. Fujimura, M. Kunimoto, Y. Fukunaka, P. Pianetta, T. Homma, J. Electrochem. Soc., 164, D994 (2017).

Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO· Radicals and Implication for Catalytic Coantioxidant Systems.

We report a novel coantioxidant system based on TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) that, in biologically relevant model systems, rapidly converts chain-carrying alkylperoxyl radicals to HOO·. Extremely efficient quenching of HOO· by TEMPO blocks the oxidative chain. Rate constants in chlorobenzene were measured to be 1.1 × 109 M-1 s-1 for the reductive reaction TEMPO + HOO· → TEMPOH + O2 and 5.0 × 107 M-1 s-1 for the oxidative reaction TEMPOH + HOO· → TEMPO + H2O2. These rate constants are significantly higher than that associated with the reaction of HOO· with α-tocopherol, Nature's best lipid soluble antioxidant ( k = 1.6 × 106 M-1 s-1). These data show that in the presence of ROO·-to-HOO· chain-transfer agents, which are common in lipophilic environments, the TEMPO/TEMPOH couple protects organic molecules from oxidation by establishing an efficient reductive catalytic cycle. This catalytic cycle provides a new understanding of the efficacy of the antioxidant capability of TEMPO in nonaqueous systems and its potential to act as a chemoprotective against radical damage.

Separation application of superhydrophobic Cu gauze to a non-aqueous system: Biodiesel collection from glycerol/FAME two-phase mixture

In this study, a superhydrophobic Cu gauze (SCG) was prepared and used for the phase separation of non-aqueous FAME-glycerol mixtures. The separation of three FAME/glycerol mixing scenarios in biodiesel production, blending, emulsion, and floating FAME, was conducted in batches. Results confirmed glycerol-repellence and FAME-attraction of the obtained SCG. The separation efficiency of FAME/glycerol blending mainly depended on impurities that affected glycerol solubility in FAME. Emulsion separation was determined by the relative size between the dispersed glycerol droplets and SCG mesh. Floating FAME on the glycerol surface was completely collected with a sponge enwrapped by SCG with 99.12% recovery. The continuous separation of FAME/glycerol mixture with SCG produced qualified biodiesel product with 0.0166 wt% glycerol content and phase separation efficiency reached 98.26%. This work broadens the possible application of superhydrophobic surfaces to non-aqueous systems.

Aqueous/Nonaqueous Hybrid Electrolyte for Sodium-Ion Batteries

Here, we report an aqueous/nonaqueous hybrid electrolyte based on sodium trifluoromethanesulfonate with an expanded electrochemical window up to 2.8 V and high conductivity (∼25 mS cm–1 at 20 °C). The hybrid electrolyte inherits the safety characteristic of aqueous electrolytes and the electrochemical stability of nonaqueous systems, enabling stable and reversible operation of the Na3V2(PO4)3/NaTi2(PO4)3 sodium-ion battery.

Emerging Nonaqueous Aluminum-Ion Batteries: Challenges, Status, and Perspectives.

Aluminum-ion batteries (AIBs) are regarded as viable alternatives to lithium-ion technology because of their high volumetric capacity, their low cost, and the rich abundance of aluminum. However, several serious drawbacks of aqueous systems (passive film formation, hydrogen evolution, anode corrosion, etc.) hinder the large-scale application of these systems. Thus, nonaqueous AIBs show incomparable advantages for progress in large-scale electrical energy storage. However, nonaqueous aluminum battery systems are still nascent, and various technical and scientific obstacles to designing AIBs with high capacity and long cycling life have not been resolved until now. Moreover, the aluminum cell is a complex device whose energy density is determined by various parameters, most of which are often ignored, resulting in failure to achieve the maximum performance of the cell. The purpose here is to discuss how to further develop reliable nonaqueous AIBs. First, the current status of nonaqueous AIBs is reviewed based on statistical data from the literature. The influence of parameters on energy density is analyzed, and the current situation and existing problems are summarized. Furthermore, possible solutions and concerns regarding the construction of reliable nonaqueous AIBs are comprehensively discussed. Finally, future research directions and prospects in the aluminum battery field are proposed.

A Mechanistic Investigation of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

Na2V6O16·3H2O Barnesite Nanorod: An Open Door to Display a Stable and High Energy for Aqueous Rechargeable Zn-Ion Batteries as Cathodes.

Owing to their safety and low cost, aqueous rechargeable Zn-ion batteries (ARZIBs) are currently more feasible for grid-scale applications, as compared to their alkali counterparts such as lithium- and sodium-ion batteries (LIBs and SIBs), for both aqueous and nonaqueous systems. However, the materials used in ARZIBs have a poor rate capability and inadequate cycle lifespan, serving as a major handicap for long-term storage applications. Here, we report vanadium-based Na2V6O16·3H2O nanorods employed as a positive electrode for ARZIBs, which display superior electrochemical Zn storage properties. A reversible Zn2+-ion (de)intercalation reaction describing the storage mechanism is revealed using the in situ synchrotron X-ray diffraction technique. This cathode material delivers a very high rate capability and high capacity retention of more than 80% over 1000 cycles, at a current rate of 40C (1C = 361 mA g-1). The battery offers a specific energy of 90 W h kg-1 at a specific power of 15.8 KW kg-1, enlightening the material advantages for an eco-friendly atmosphere.

Hybrid Aqueous/Non-aqueous Electrolyte for Safe and High-Energy Li-Ion Batteries

Summary Recent breakthroughs in aqueous electrolytes made highly safe 3.0 V class aqueous Li-ion batteries possible. However, the formed solid-electrolyte interphase therein still cannot effectively support the desired energy-dense anode and cathode materials. In this work, we report a new class of electrolytes, by hybridizing aqueous with non-aqueous solvents, that inherits the non-flammability and non-toxicity characteristics from aqueous and better electrochemical stability from non-aqueous systems. The secondary interphasial ingredient (alkylcarbonate) introduced by non-aqueous component helps to expand the electrochemical window of the hybridized electrolyte to 4.1 V, which supports the operation of a 3.2 V aqueous Li-ion battery based on Li 4 Ti 5 O 12 and LiNi 0.5 Mn 1.5 O 4 to deliver a high energy density of 165 Wh/kg for >1,000 cycles. The understanding of how a better interphase could be tailored by regulating the inner-Helmholtz interfacial structures of the hybridized electrolyte provides important guidelines for designing future electrolytes and interphases for new battery chemistries.

Pseudocapacitive layered birnessite sodium manganese dioxide for high-rate non-aqueous sodium ion capacitors

Layered birnessite sodium manganese dioxide exhibits a highly pseudocapacitive sodium storage behavior with non-phase changes in non-aqueous systems, leading to high rate and long-life performance.

Hunting mermaids in real space: known knowns, known unknowns and unknown unknowns.

We review efforts to realise so-called mermaid (or short-ranged attraction/long ranged repulsion) interactions in 3d real space. The repulsive and attractive contributions to these interactions in charged colloids and colloid-polymer mixtures, may be accurately realised, by comparing particle-resolved studies with colloids to computer simulation. However, when we review work where these interactions have been combined, despite early indications of behaviour consistent with predictions, closer analysis reveals that in the non-aqueous systems used for particle-resolved studies, the idea of summing the attractive and repulsive components leads to wild deviations with experiment. We suggest that the origin lies in the weak ion dissociation in these systems with low dielectric constant solvents. Ultimately this leads even to non-centro-symmetric interactions and a new level of complexity in these systems.

Novel, Non-aqueous Bioconversion Systems Using Fungal Spores.

Two novel types of non-aqueous bioconversion systems using fungal spores, either adsorbed on the surface of a filter pad or entrapped in calcium alginate beads, were constructed and applied for a model reaction: reduction of benzil to benzoin by Aspergillus sojae NBRC 32074. The spores adsorbed on a filter pad catalyzed the reduction in some toxic organic solvents, such as methylcyclohexane (log P: 3.61) and din-butyl ether (3.21). For the relationship between the reduction activity and the log P value of the organic solvent, a highly positive correlation (R2: 0.815) was observed. Surprisingly, the reduction proceeded in the more hydrophilic and toxic tert-butyl acetate (log P: 1.76). Glycerol was selected as the best hydride source. The higher the glycerol content, the more the benzoin was produced. While the production of benzil by spores was lower than that by mycelia in harmless di-n-hexyl ether (log P: 5.12), mycelia could not catalyze the reduction in the toxic tert-butyl acetate. In contrast, the spores entrapped in the calcium alginate beads could catalyze the reduction. Although the reduction by alginate-entrapped spores could be stably repeated 5 times in di-n-hexyl ether without a decline in the reduction activity, it was observed that the reduction activity of the spores gradually decreased after repeated reduction in tert-butyl acetate.

Photoinduced DNA Solubilization in Chloroform Solution by Complexing with Photoionizable Malachite Green Derivative

Direct solubilization of DNA in organic solvents has attracted growing interest owing to its ability to improve the properties and versatility of DNA in nonaqueous systems, especially when irradiated. In this study, we demonstrate photoinduced DNA solubilization in a chloroform solution containing photoionizable malachite green leuconitrile carrying a long alkyl chain (MGL). Ultraviolet (UV) irradiation of MGL afforded a cationic surfactant, which then interacted with oppositely charged DNA. DNA solubilization in a chloroform/1-hexanol mixture suggested the formation of a DNA–ionized MGL complex. Fluorescence spectroscopy was used to obtain the evidence for the DNA–ionized MGL complex.

High-Voltage Aqueous Magnesium Ion Batteries.

Nonaqueous rechargeable magnesium (Mg) batteries suffer from the complicated and moisture-sensitive electrolyte chemistry. Besides electrolytes, the practicality of a Mg battery is also confined by the absence of high-performance electrode materials due to the intrinsically slow Mg2+ diffusion in the solids. In this work, we demonstrated a rechargeable aqueous magnesium ion battery (AMIB) concept of high energy density, fast kinetics, and reversibility. Using a superconcentration approach we expanded the electrochemical stability window of the aqueous electrolyte to 2.0 V. More importantly, two new Mg ion host materials, Li superconcentration approach we expanded the electrochemical stability window of the aqueous electrolyte to 2.0 V. More importantly, two new Mg ion host materials, Li3V2(PO4)3 and poly pyromellitic dianhydride, were developed and employed as cathode and anode electrodes, respectively. Based on comparisons of the aqueous and nonaqueous systems, the role of water is identified to be critical in the Mg ion mobility in the intercalation host but remaining little detrimental to its non-diffusion controlled process. Compared with the previously reported Mg ion cell delivers an unprecedented high power density of 6400 W kg ion cell delivers an unprecedented high power density of 6400 W kg while retaining 92% of the initial capacity after 6000 cycles, pushing the Mg ion cell to a brand new stage.

Highly Porous N-Doped Graphitic Carbon from Bio-Waste As Bifunctional Elactrocatalyst for Hybrid Na-Air Battery

Rechargeable metal-air batteries have emerged as one of the most promising candidates for sustainable energy storage in the near future due to its high energy density [1,2]. In such a context, Lithium-air battery with a very high theoretical energy density (5200 Wh kg-1) received some very first attentions, but researchers soon realized that low Li abundance on Earth, poor cyclic stability and high operation overpotential make them not only high costs but also unrealistic in practical applications. More recently, Sodium-air batteries (Na-air) have risen as an effective alternate to Li-air batteries, given the facts that Na-air batteries are much safer and low cost. Na-air battery has several other advantages including low overpotential and high power density, arising from much higher ionic conductivity of Na-ion based solid electrolyte (e.g. Na3Zr2Si2PO12) and high solubility of discharge generated product (i.e. NaOH rather than LiOH) [3,4]. Based on cell design and electrolyte type, Na-air batteries can be classified into two major types: Non-aqueous and Aqueous (or) hybrid batteries [5]. Recent studies have shown that aqueous Na-air batteries hold some edges over non-aqueous systems, including low operation overpotential, higher rate capability, improved energy efficiency, and higher power density. Bifunctional catalysts are prominent to attain high capacity, maximum energy efficiency and long cycle-life for aqueous rechargeable Na-air batteries [6,7]. In this work, we report the synthesis of bifunctional noble metal free, highly porous N-doped graphitic carbon from bio-waste. The highly porous carbon showed higher specific surface area of 994 m2 g-1. Electrocatalytic property of the carbon based material was evaluated using linear sweep voltammetry with rotating ring disk electrode (RRDE). The N-doped graphitic carbon nanostructures showed better oxygen evolution activity compared to Pt/C catalyst. Highly porous N-doped graphitic carbon was investigated as a potential electrocatalyst for rechargeable hybrid Na-air battery for the first time. The fabricated hybrid Na-air battery with the N-doped graphitic carbon catalyst as air-cathode delivered low overpotential and its round trip energy efficiency reached above ~85 %. The hybrid Na-air battery exhibited stable cycle performance up to 20 cycles. Acknowledgement Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship.

Toward Soluble, Stable Organic Electroactive Materials for Non-Aqueous Redox Flow Batteries

Commercial redox flow batteries (RFBs) contain highly acidic and corrosive electrolytes, a cause for concern with widespread use to due concerns about safety and environmental contamination. Replacing these electrolytes with non-aqueous equivalents allows for a safer storage medium. Additionally, utilization of organic electroactive materials may lead to more scalable technologies that do not rely on mined materials such as vanadium and lithium. Furthermore, non-aqueous electrolytes could enable a 2- to 3-fold increase in operating voltage due to the wider operating voltage of non-aqueous systems. Despite the promise for a safer, scalable, higher energy system, the number of organic compounds reported in non-aqueous flow battery systems has been limited to a few classes of compounds, many of which suffer from instability and/or insolubility, especially in charged states. Our recent efforts have focused on the development of organic electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability and solubility in all relevant states of oxidation while keeping syntheses short, high yielding, and scalable. In particular, we highlight the design, synthesis, and electrochemical analysis of new phenothiazine and napthoquinone derivatives designed to serve as one- or two-electron donors. We show that tactical placement of substituents leads to improved stability and increased solubility. Additionally, a new approach to modification of redox potentials using strategic substituent placement will be presented.

Nonprecious Electrocatalysts for Li-Air and Zn-Air batteries: Fundamentals and recent advances

Over their 150-year history, new twists on metal-air batteries have continued to emerge up to the present day. In this article, we present the fundamentals and recent advances in nonprecious electrocatalysts for metal-air batteries. The catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium (Li)-air and zinc (Zn)-air batteries, including transition-metal oxides, inorganic-organic hybrid materials, and metal-free materials, are summarized. Both aqueous and nonaqueous systems are also considered. We briefly introduce the mechanism and challenges of oxygen electrochemistry and provide a perspective on the design and development of nonprecious electrocatalysts.

Mechanistic Aspects in the Formation, Growth and Surface Functionalization of Metal Oxide Nanoparticles in Organic Solvents.

The synthesis of metal oxide nanoparticles in organic solvents, so-called nonaqueous (or nonhydrolytic) processes represent powerful alternatives to aqueous approaches and have become an independent research field. 10 Years ago, when we published our first review on organic reaction pathways in nonaqueous sol-gel approaches, the number of examples was relatively limited. Nowadays, it is almost impossible to provide an exhaustive overview. Here we review the development of the last few years, without neglecting pioneering examples, which help to follow the historical development. The importance of a profound understanding of mechanistic aspects of nanoparticle crystallization and formation mechanisms can't be overestimated, when it comes to the design of rational synthesis concepts under minimization of trial-and-error experiments. The main reason for the progress in mechanistic understanding lies in the availability of characterization tools that make it possible to monitor chemical reactions from the dissolution of the precursor to the nucleation and growth of the nanoparticles, by ex situ methods involving sampling after different reaction times, but more and more also by in situ studies. After a short introduction to experimental aspects of nonaqueous sol-gel routes to metal oxide nanoparticles, we provide an overview of the main and basic organic reaction pathways in these approaches. Afterwards, we summarize the main characterization methods to study formation mechanisms, and then we discuss in great depth the chemical formation mechanisms of many different types of metal oxide nanoparticles. The review concludes with a paragraph on selected crystallization mechanisms reported for nonaqueous systems and a few illustrative examples of nonaqueous sol-gel concepts applied to surface chemistry.

Metal–Air Batteries: Will They Be the Future Electrochemical Energy Storage Device of Choice?

Metal–air batteries have a theoretical energy density that is much higher than that of lithium-ion batteries and are frequently advocated as a solution toward next-generation electrochemical energy storage for applications including electric vehicles or grid energy storage. However, they have not fulfilled their full potential because of challenges associated with the metal anode, air cathode, and electrolyte. These challenges will have to be properly resolved before metal–air batteries can become a practical reality and be deployed on a large scale. Here we survey the current status and latest advances in metal–air battery research for both aqueous (e.g., Zn–air) and nonaqueous (e.g., Li–air) systems. An overview of the general technical issues confronting their development is presented, and our perspective on possible solutions is offered.