Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety.

A cobalt phthalocyanine having an electron-poor CoN4 (+δ) in its phthalocyanine moiety was presented as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR). We suggested that hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allows CoN4 to have the highly HPOR-active monovalent oxidation state and facilitates HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co─OOH- encourages an axially coordinated cobalt oxo complex (O═CoN4 ) to form, the O═CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction guaranteeing low-voltage hydrogen production is successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4 . Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V is obtained. Also, the techno-economic benefit of hydrogen peroxide as a hydrogen carrier is evaluated by comparing hydrogen peroxide with other hydrogen carriers such as ammonia and liquid organic hydrogen carriers.

Selectively Enhanced Electrocatalytic Oxygen Evolution within Nanoscopic Channels Fitting a Specific Reaction Intermediate for Seawater Splitting.

Abundant availability of seawater grants economic and resource-rich benefits to water electrolysis technology requiring high-purity water if undesired reactions such as chlorine evolution reaction (CER) competitive to oxygen evolution reaction (OER) are suppressed. Inspired by a conceptual computational work suggesting that OER is kinetically improved via a double activation within 7 Å-gap nanochannels, RuO2 catalysts are realized to have nanoscopic channels at 7, 11, and 14 Å gap in average (dgap ), and preferential activity improvement of OER over CER in seawater by using nanochanneled RuO2 is demonstrated. When the channels are developed to have 7 Å gap, the OER current is maximized with the overpotential required for triggering OER minimized. The gap value guaranteeing the highest OER activity is identical to the value expected from the computational work. The improved OER activity significantly increases the selectivity of OER over CER in seawater since the double activation by the 7 Å-nanoconfined environments to allow an OER intermediate (*OOH) to be doubly anchored to Ru and O active sites does not work on the CER intermediate (*Cl). Successful operation of direct seawater electrolysis with improved hydrogen production is demonstrated by employing the 7 Å-nanochanneled RuO2 as the OER electrocatalyst.

Guaiacol as an Organic Superoxide Dismutase Mimics for Anti-ageing a Ru-based Li-rich Layered Oxide Cathode.

High-capacity Li-rich layered oxides using oxygen redox as well as transition metal redox suffer from its structural instability due to lattice oxygen escaped from its structure during oxygen redox and the following electrolyte decomposition by the reactive oxygen species. Herein, we rescued a Li-rich layered oxide based on 4d transition metal by employing an organic superoxide dismutase mimics as a homogeneous electrolyte additive. Guaiacol scavenged superoxide radicals via dismutation or disproportionation to convert two superoxide molecules to peroxide and dioxygen after absorbing lithium superoxide on its partially negative oxygen of methoxy and hydroxyl groups. Additionally, guaiacol was decomposed to form a thin and stable cathode-electrolyte interphase (CEI) layer, endowing the cathode with the interfacial stability.

Breathable Artificial Interphase for Dendrite-Free and Chemo-Resistive Lithium Metal Anode.

A dendrite-free and chemically stabilized lithium metal anode is required for extending battery life and for the application of high energy density coupled with various cathode systems. However, uneven Li metal growth and the active surface in nature accelerate electrolyte dissipation and surface corrosion, resulting in poor cycle efficiency and various safety issues. Here, the authors suggest a thin artificial interphase using a multifunctional poly(styrene-b-butadiene-b-styrene) (SBS) copolymer to inhibit the electrochemical/chemical side reaction during cycling. Based on the physical features, hardness, adhesion, and flexibility, the optimized chemical structure of SBS facilitates durable mechanical strength and interphase integrity against repeated Li electrodeposition/dissolution. The effectiveness of the thin polymer film enables high cycle efficiency through the realization of a dendrite-free structure and a chemo-resistive surface of Li metal. The versatile anode demonstrates an improvement in the electrochemical properties, paired with diverse cathodes of high-capacity lithium cobalt oxide (3.5 mAh cm-2 ) and oxygen for advanced Li metal batteries with high energy density.

Sphere-to-Multipod Transmorphic Change of Nanoconfined Pt Electrocatalyst during Oxygen Reduction Reaction.

An oxygen reduction reaction (ORR) catalyst/support system is designed to have Pt nanoparticles nanoconfined in a nanodimensionally limited space. Holey crumpled reduced graphene oxide plates (hCR-rGO) are used as a carbon support for Pt loading. As expected from interparticular Pt-to-Pt distance of Pt-loaded hCR-rGO longer than that of Pt/C (Pt-loaded carbon black as a practical Pt catalyst), the durability of ORR electroactivity along cycles is improved by replacing the widely used carbon black with hCR-rGO. Unexpected morphological changes of Pt are electrochemically induced during repeated ORR processes. Spherical multifaceted Pt particles are evolved to {110}-dominant dendritic multipods. Nanoconfinement of a limited number of Pt within a nanodimensionally limited space is responsible for the morphological changes. The improved durability observed from Pt-loaded hCR-rGO originates from 1) dendritic pod structure of Pt exposing more active sites to reactants and 2) highly ORR-active Pt {110} planes dominant on the surface.

Production of germanium nanoparticles via laser pyrolysis for anode materials of lithium-ion batteries and sodium-ion batteries.

Germanium nanoparticles were synthesized and subjected to study as anode materials for lithium ion batteries and sodium ion batteries. Laser pyrolysis of GeH4 was used to produce germanium nanoparticles and the average diameter of these nanoparticles was easily controlled by regulating sensitizer gas flow rates during the process. 60 and 10 nm diameter nanoparticles were synthesized and micron-size powder was purchased and these three pure germanium powder samples were tested as the anode materials of lithium ion batteries and sodium ion batteries in terms of cycle retention, long term cycles and the kinetics of reactions. Experimental results showed that the smallest powder sample which is synthesized, average 10 nm, exhibited excellent performances in both kinds of batteries. According to the results, the characteristics of batteries improved as the size of germanium powder decreased consistently. Pure germanium was thoroughly investigated as an anode of metal-ion batteries with regard to its powder size. The experimental data and synthesis approach of germanium nanoparticles suggested in this research would be a good example for the utilization of elemental germanium in high performance batteries.

Mesoporous Germanium Anode Materials for Lithium-Ion Battery with Exceptional Cycling Stability in Wide Temperature Range.

Porous structured materials have unique architectures and are promising for lithium-ion batteries to enhance performances. In particular, mesoporous materials have many advantages including a high surface area and large void spaces which can increase reactivity and accessibility of lithium ions. This study reports a synthesis of newly developed mesoporous germanium (Ge) particles prepared by a zincothermic reduction at a mild temperature for high performance lithium-ion batteries which can operate in a wide temperature range. The optimized Ge battery anodes with the mesoporous structure exhibit outstanding electrochemical properties in a wide temperature ranging from -20 to 60 °C. Ge anodes exhibit a stable cycling retention at various temperatures (capacity retention of 99% after 100 cycles at 25 °C, 84% after 300 cycles at 60 °C, and 50% after 50 cycles at -20 °C). Furthermore, full cells consisting of the mesoporous Ge anode and an LiFePO4 cathode show an excellent cyclability at -20 and 25 °C. Mesoporous Ge materials synthesized by the zincothermic reduction can be potentially applied as high performance anode materials for practical lithium-ion batteries.

ZnO decorated germanium nanoparticles as anode materials in Li-ion batteries.

Germanium exhibits high charge capacity and high lithium diffusivity, both are the key requirements for electrode materials in high performance lithium ion batteries (LIBs). However, high volume expansion and segregation from the electrode during charge-discharge cycling have limited use of germanium in LIBs. Here, we demonstrate that ZnO decorated Ge nanoparticles (Ge@ZnO NPs) can overcome these limitations of Ge as an LIB anode material. We produced Ge NPs at high rates by laser pyrolysis of GeH4, then coated them with solution phase synthesized ZnO NPs. Half-cell tests revealed dramatically enhanced cycling stability and higher rate capability of Ge@ZnO NPs compared to Ge NPs. Enhancements arise from the core-shell structure of Ge@ZnO NPs as well as production of metallic Zn from the ZnO layer. These findings not only demonstrate a new surface treatment for Ge NPs, but also provide a new opportunity for development of high-rate LIBs.

LiFePO4 Nanostructures Fabricated from Iron(III) Phosphate (FePO4 x 2H2O) by Hydrothermal Method.

Electrode materials having nanometer scale dimensions are expected to have property enhancements due to enhanced surface area and mass/charge transport kinetics. This is particularly relevant to intrinsically low electronically conductive materials such as lithium iron phosphate (LiFePO4), which is of recent research interest as a high performance intercalation electrode material for Li-ion batteries. Many of the reported works on LiFePO4 synthesis are unattractive either due to the high cost of raw materials or due to the complex synthesis technique. In this direction, synthesis of LiFePO4 directly from inexpensive FePO4 shows promise.The present study reports LiFePO4 nanostructures prepared from iron (III) phosphate (FePO4 x 2H2O) by precipitation-hydrothermal method. The sintered powder was characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Inductive coupled plasma-optical emission spectroscopy (ICP-OES), and Electron microscopy (SEM and TEM). Two synthesis methods, viz. bulk synthesis and anodized aluminum oxide (AAO) template-assisted synthesis are reported. By bulk synthesis, micro-sized particles having peculiar surface nanostructuring were formed at precipitation pH of 6.0 to 7.5 whereas typical nanosized LiFePO4 resulted at pH ≥ 8.0. An in-situ precipitation strategy inside the pores of AAO utilizing the spin coating was utilized for the AAO-template-assisted synthesis. The template with pores filled with the precipitate was subsequently subjected to hydrothermal process and high temperature sintering to fabricate compact rod-like structures.

Conductivity-Dependent Completion of Oxygen Reduction on Oxide Catalysts.

The electric conductivity-dependence of the number of electrons transferred during the oxygen reduction reaction is presented. Intensive properties, such as the number of electrons transferred, are difficult to be considered conductivity-dependent. Four different perovskite oxide catalysts of different conductivities were investigated with varying carbon contents. More conductive environments surrounding active sites, achieved by more conductive catalysts (providing internal electric pathways) or higher carbon content (providing external electric pathways), resulted in higher number of electrons transferred toward more complete 4e reduction of oxygen, and also changed the rate-determining steps from two-step 2e process to a single-step 1e process. Experimental evidence of the conductivity dependency was described by a microscopic ohmic polarization model based on effective potential localized nearby the active sites.

An inter-tangled network of redox-active and conducting polymers as a cathode for ultrafast rechargeable batteries.

A 1D organic redox-active material is combined with another 1D conductive material for rechargeable batteries. Poly(vinyl carbazole) (or PVK) and poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (or PEDOT:PSS) are used as the redox-active and conductive 1D materials, respectively. Due to their extremely anisotropic geometry, the two polymers are expected to be inter-tangled with each other, showing a kinetically ideal model system in which each redox-active moiety of PVK is supposed to be directly connected with the conducting pathways of PEDOT:PSS. In addition to its role as a conductive agent providing kinetic benefits, PEDOT:PSS works as an efficient binder that guarantees enhanced electrochemical performances with only a tenth of the amount of a conventional binder (polyvinylidene fluoride or PVdF). The benefit of gravimetric energy density gain obtained using the conductive binder comes mainly from efficient spatial coverage of binding volume due to the low density of PEDOT:PSS. Towards realizing flexible all-polymer batteries, a quasi-all-polymer battery half-cell is designed using the PVK/PEDOT:PSS composite with a polymer gel electrolyte.

Doubling the capacity of lithium manganese oxide spinel by a flexible skinny graphitic layer.

By coating nanoparticular lithium manganese oxide (LMO) spinel with a few layers of graphitic basal planes, the capacity of the material reached up to 220 mA h g(-1) at a cutoff voltage of 2.5 V. The graphitic layers 1) provided a facile electron-transfer highway without hindering ion access and, more interestingly, 2) stabilized the structural distortion at the 3 V region reaction. The gain was won by a simple method in which microsized LMO was ball-milled in the presence of graphite with high energy. Vibratory ball milling pulverized the LMO into the nanoscale, exfoliated graphite of less than 10 layers and combined them together with an extremely intimate contact. Ab initio calculations show that the intrinsically very low electrical conductivity of the tetragonal phase of the LMO is responsible for the poor electrochemical performance in the 3 V region and could be overcome by the graphitic skin strategy proposed.

Formulating Electron Beam-Induced Covalent Linkages for Stable and High-Energy-Density Silicon Microparticle Anode.

High-capacity silicon (Si) materials hold a position at the forefront of advanced lithium-ion batteries. The inherent potential offers considerable advantages for substantially increasing the energy density in batteries, capable of maximizing the benefit by changing the paradigm from nano- to micron-sized Si particles. Nevertheless, intrinsic structural instability remains a significant barrier to its practical application, especially for larger Si particles. Here, a covalently interconnected system is reported employing Si microparticles (5 µm) and a highly elastic gel polymer electrolyte (GPE) through electron beam irradiation. The integrated system mitigates the substantial volumetric expansion of pure Si, enhancing overall stability, while accelerating charge carrier kinetics due to the high ionic conductivity. Through the cost-effective but practical approach of electron beam technology, the resulting 500 mAh-pouch cell showed exceptional stability and high gravimetric/volumetric energy densities of 413 Wh kg-1, 1022 Wh L-1, highlighting the feasibility even in current battery production lines.

Influence of the Lithium-Ion Concentration in Electrolytes on the Performance of Dye-Sensitized Photorechargeable Batteries.

Dye-sensitized photorechargeable batteries (DSPBs) have recently gained attention for realizing energy recycling systems under dim light conditions. However, their performance under high storage efficiency (i.e., the capacity charged within a limited time) for practical application remains to be evaluated. Herein, we varied the lithium (Li)-ion concentration, which plays a dual role as energy charging and storage components, to obtain the optimized energy density of DSPBs. Electrochemical studies showed that the Li-ion concentration strongly affected the resistance characteristics of DSPBs. In particular, increasing the Li-ion concentration improved the output capacity and decreased the output voltage. Consequently, the energy density of the finely optimized DSPB improved from 8.73 to 12.64 mWh/cm3 when irradiated by a 1000-lx indoor light-emitting-diode lamp. These findings on the effects of Li-ion concentrations in electrolytes on the performance of DSPBs represent a step forward in realizing the practical application of DSPBs.

Shifting Target Reaction from Oxygen Reduction to Superoxide Disproportionation by Tuning Isomeric Configuration of Quinone Derivative as Redox Mediator for Lithium-Oxygen Batteries.

Quinones having a fully conjugated cyclic dione structure have been used as redox mediators in electrochemistry. 2,5-Ditert-butyl-1,4-benzoquinone (DBBQ or DB-p-BQ) as a para-quinone derivative is one of the representative discharge redox mediators for facilitating the oxygen reduction reaction (ORR) kinetics in lithium-oxygen batteries (LOBs). Herein, we presented that the redox activity of DB-p-BQ for electron mediation was possibly used for facilitating superoxide disproportionation reaction (SODR) by tuning the isomeric configuration of the carbonyl groups of the substituted quinone to change its reduction potentials. First, we expected a molecule having its reduction potential between oxygen/superoxide at 2.75 V versus Li/Li+ and superoxide/peroxide at 3.17 V to play a role of the SODR catalyst by transferring an electron from one superoxide (O2-) to another superoxide to generate dioxygen (O2) and peroxide (O22-). By changing the isomeric configuration from para (DB-p-BQ) to ortho (DB-o-BQ), the reduction potential of the first electron transfer (Q/Q-) of the ditert-butyl benzoquinone shifted positively to the potential range of the SODR catalyst. The electrocatalytic SODR-promoting functionality of DB-o-BQ kept the reactive superoxide concentration below a harmful level to suppress superoxide-triggered side reaction, improving the cycling durability of LOBs, which was not achieved by the para form. The second electron transfer process (Q-/ Q2-) of the DB-o-BQ, even if the same process of the para form was not used for facilitating ORR, played a role of mediating electrons between electrode and oxygen like the Q/Q- process of the para form. The ORR-promoting functionality of the ortho form increased the LOB discharge capacity and reduced the ORR overpotential.

Critical Void Dimension of Carbon Frameworks to Accommodate Insoluble Products of Lithium-Oxygen Batteries.

High-energy density lithium-oxygen batteries (LOBs) seriously suffer from poor rate capability and cyclability due to the slow oxygen-related electrochemistry and uncontrollable formation of lithium peroxide (Li2O2) as an insoluble discharge product. In this work, we accommodated the discharge product in macro-scale voids of a carbon-framed architecture with meso-dimensional channels on the carbon frame and open holes connecting the neighboring voids. More importantly, we found that a specific dimension of the voids guaranteed high capacity and cycling durability of LOBs. The best LOB performances were achieved by employing the carbon-framed architecture having voids of 0.8 µm size as the cathode of the LOB when compared with the cathodes having voids of 0.3 and 1.4 µm size. The optimized void size of 0.8 µm allowed only a monolithic integrity of lithium peroxide deposit within a void during discharging. The deposit was grown to be a yarn ball-looking sphere exactly fitting the shape and size of the void. The good electric contact allowed the discharge product to be completely decomposed during charging. On the other hand, the void space was not fully utilized due to the mass transfer pathway blockage at the sub-optimized 0.3 µm and the formation of multiple deposit integrities within a void at the sur-optimized 1.4 µm. Consequently, the critical void dimension at 0.8 µm was superior to other dimensions in terms of the void space utilization efficiency and the lithium peroxide decomposition efficiency, disallowing empty space and side reactions during discharging.

Amphi-Active Superoxide-Solvating Charge Redox Mediator for Highly Stable Lithium-Oxygen Batteries.

A multifunctional electrolyte additive for lithium oxygen batteries (LOBs) was designed to have (1) a redox-active moiety to mediate decomposition of lithium peroxide (Li2O2 as the final discharge product) during charging and (2) a solvent moiety to solvate and stabilize lithium superoxide (LiO2 as the intermediate discharge product) in electrolyte during discharging. 4-Acetamido-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) or AAT was employed as the additive working for both charge and discharge processes (amphi-active). The redox-active moiety was rooted in TEMPO, while the acetamido (AA) functional group inherited the high donor number (DN) of N,N-dimethylacetamide (DMAc). Integrating two functional moieties (TEMPO and AA) into a single molecule resulted in the bifunctionality of AAT (1) facilitating Li2O2 decomposition by the TEMPO moiety and (2) encouraging the solvent mechanism of Li2O2 formation by the high-DN AA moiety. Significantly improved LOB performances were achieved by the superoxide-solvating charge redox mediator, which were not obtained by a simple cocktail of TEMPO and DMAc.

One-dimensional (1D) nanostructured and nanocomposited LiFePO4: its perspective advantages for cathode materials of lithium ion batteries.

Nanostructured materials have attracted recent research interest as battery materials due to their expected enhancement of properties. The characteristic nanoscale dimension and its structuring guarantees improved charge and mass transfer during charge/discharge processes. Among the potential cathode materials investigated as a substitute to LiCoO(2), one of the most promising materials is LiFePO(4) with olivine structure (LFP). In this perspective article, the current research and development in the synthesis and electrochemical studies of nanostructured LFP are reviewed with a special emphasis on one-dimensional (1D) nanostructures and nanocompositing with 1D conductive materials. In addition to various examples of 1D LFP with detailed synthetic methods, why 1D nanostructures could be meaningful is discussed in terms of a geometric point of view and the anisotropic lithiation/de-lithiation mechanism of LFP.

Metal-Ion Chelating Gel Polymer Electrolyte for Ni-Rich Layered Cathode Materials at a High Voltage and an Elevated Temperature.

Nickel-rich layered oxides (LiNi1-x-yCoxMnyO2; (1 - x - y) ≥ 0.6), the high-energy-density cathode materials of lithium-ion batteries (LIBs), are seriously unstable at voltages higher than 4.5 V versus Li/Li+ and temperatures higher than 50 °C. Herein, we demonstrated that the failure mechanism of a nickel-rich layered oxide (LiNi0.6Co0.2Mn0.2O2) behind the instability was successfully suppressed by employing cyanoethyl poly(vinyl alcohol) having pyrrolidone moieties (Pyrd-PVA-CN) as a metal-ion-chelating gel polymer electrolyte (GPE). The metal-ion-chelating GPE blocked the plating of transition-metal ions dissolved from the cathode by capturing the ions (anode protection). High-concentration metal-ion environments developed around the cathode surface by the GPE suppressed the irreversible phase transition of the cathode material from the layered structure to the rock-salt structure (cathode protection). Resultantly, the capacity retention was significantly improved at a high voltage and a high temperature. Capacity retention and coulombic efficiency of a full-cell configuration of a nickel-rich layered oxide with graphite were significantly improved in the presence of the GPE especially at a high cutoff voltage (4.4 V) and an elevated temperature (55 °C).

A Three-Dimensional Nano-web Scaffold of Ferroelectric Beta-PVDF Fibers for Lithium Metal Plating and Stripping.

Lithium metal has been considered as an anode material to improve energy densities of lithium chemistry-based rechargeable batteries (that is to say, lithium metal batteries or LMBs). Higher capacities and cell voltages are ensured by replacing practically used anode materials such as graphite with lithium metal. However, lithium metal as the LMB anode material has been challenged by its dendritic growth, electrolyte decomposition on its fresh surface, and its serious volumetric change. To address the problems of lithium metal anodes, herein, we guided and facilitated lithium ion transport along a spontaneously polarized and highly dielectric material. A three-dimensional web of nanodiameter fibers of ferroelectric beta-phase polyvinylidene fluoride (beta-PVDF) was loaded on a copper foil by electrospinning (PVDF#Cu). The electric field applied between the nozzle and target copper foil forced the dipoles of PVDF to be oriented centro-asymmetrically and then the beta structure induced ferroelectric polarization. Three-fold benefits of the ferroelectric nano-web architecture guaranteed the plating/stripping reversibility especially at high rates: (1) three-dimensional scaffold to accommodate the volume change of lithium metal during plating and stripping, (2) electrolyte channels between fibers to allow lithium ions to move, and (3) ferroelectrically polarized or negatively charged surface of beta-PVDF fibers to encourage lithium ion hopping along the surface. Resultantly, the beta-PVDF web architecture drove dense and integrated growth of lithium metal within its structure. The kinetic benefit expected from the ferroelectric lithium ion transport of beta-PVDF as well as the porous architecture of PVDF#Cu was realized in a cell of LFP as a cathode and lithium-plated PVDF#Cu as an anode. Excellent plating/stripping reversibility along repeated cycles was successfully demonstrated in the cell even at a high current such as 2.3 mA cm-2, which was not obtained by the nonferroelectric polymer layer.