A Four-Electron Sulfur Electrode Hosting a Cu2+ /Cu+ Redox Charge Carrier.

The elemental sulfur electrode with Cu2+ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu2 S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g-1 based on the sulfur mass or 609 mAh g-1 based on the mass of Cu2 S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg-1 based on the mass of the Cu2 S/carbon composite cathode, and stable cycling over 110 cycles.

Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte.

Dual-ion batteries are known for anion storage in the cathode coupled to cation incorporation in the anode. We flip the sequence of the anion/cation-storage chemistries of the anode and the cathode in dual-ion batteries (DIBs) by allowing the anode to take in anions and a cation-deficient cathode to host cations, thus operating as a reverse dual-ion battery (RDIB). The anion-insertion anode is a nanocomposite having ferrocene encapsulated inside a microporous carbon, and the cathode is a Zn-insertion Prussian blue, Zn3[Fe(CN)6]2. This unique battery configuration benefits from the usage of a 30 m ZnCl2 "water-in-salt" electrolyte. This electrolyte minimizes the dissolution of ferrocene; it raises the cation-insertion potential in the cathode, and it depresses the anion-insertion potential in the anode, thus widening the full cell's voltage by 0.35 V compared with a dilute ZnCl2 electrolyte. RDIBs provide a configuration-based solution to exploit the practicality of cation-deficient cathode materials in aqueous electrolytes.

An Aqueous Dual-Ion Battery Cathode of Mn3 O4 via Reversible Insertion of Nitrate.

We report reversible electrochemical insertion of NO3 - into manganese(II, III) oxide (Mn3 O4 ) as a cathode for aqueous dual-ion batteries. Characterization by TGA, FTIR, EDX, XANES, EXAFS, and EQCM collectively provides unequivocal evidence that reversible oxidative NO3 - insertion takes place inside Mn3 O4 . Ex situ HRTEM and corresponding EDX mapping results suggest that NO3 - insertion de-crystallizes the structure of Mn3 O4 . Kinetic studies reveal fast migration of NO3 - in the Mn3 O4 structure. This finding may open a new direction for novel low-cost aqueous dual-ion batteries.

A ZnCl2 water-in-salt electrolyte for a reversible Zn metal anode.

We report a low-cost water-in-salt electrolyte, of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency (CE). In asymmetric Zn‖Zn cells with a limited mass of plated Zn as the working electrode, the ZnCl2 WiSE improves the average CE of the Zn anode to 95.4% from 73.2% in 5 m ZnCl2.

Toward Higher Capacities of Hydrocarbon Cathodes in Dual-Ion Batteries.

We report the electrochemical anion storage properties of a group of molecular solids of polycyclic aromatic hydrocarbons (PAHs): coronene, perylene, and triphenylene. We discover an interesting trend of progressively lower potentials for these three molecular solids. Our DFT calculations reveal that the inserted PF6- anions preferably bind with the edge sites of the coronene molecules as opposed to being sandwiched between two coronene molecular planes. For smaller PAHs, the more edge sites in the solids may facilitate higher capacity values. However, small PAHs do face a greater challenge of dissolution in the nonaqueous electrolyte, which affects the cycling stability.

Insights on the Proton Insertion Mechanism in the Electrode of Hexagonal Tungsten Oxide Hydrate.

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO3·0.6H2O electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO3·0.6H2O incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO3 hydrate is a three-tier process, where in the first stage 0.25 H+ is inserted per formula unit of WO3 while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked H+ is inserted, followed by the third stage with 0.17 H3O+ inserted per formula unit. Ex situ XRD reveals that protonation of the WO3 hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO3·0.6H2O exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.

Mg-Ion Battery Electrode: An Organic Solid's Herringbone Structure Squeezed upon Mg-Ion Insertion.

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg2+ and Ca2+, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalent nature of Mg2+ causes unique squeezing deformation of the electrode structure, where it contracts and expands in different crystallographic directions when hosting the inserted Mg-ions. This phenomenon is revealed experimentally by ex situ X-ray diffraction and transmission electron microscopy, and is investigated theoretically by first-principles calculations. Interestingly, hosting one Mg2+ ion requires the coordination from three PTCDA molecules in adjacent columns of stacked molecules, which rotates the columns, thus reducing the (011) spacing but increasing the (021) spacing. We demonstrate that a PTCDA Mg-ion electrode delivers a reversible capacity of 125 mA h g-1, which may include a minor contribution of hydronium storage, a good rate capability by retaining 75 mA h g-1 at 500 mA g-1 (or 3.7 C), and a stable cycle life. We also report Ca2+ storage in PTCDA, where a reversible capacity of over 80 mA h g-1 is delivered.

Photocatalyzed Reduction of Bicarbonate to Formate: Effect of ZnS Crystal Structure and Positive Hole Scavenger.

Zinc sulfide is a promising catalyst due to its abundance, low cost, low toxicity and conduction band position that enables the photoreduction of CO2 to formic acid. This study is the first to examine experimentally the photocatalytic differences between wurtzite and sphalerite under the parameters of size (micrometer and nanoscale), crystal lattice, surface area, and band gap on productivity in the photoreduction of HCO3(-). These photochemical experiments were conducted under air mass coefficient zero (AM 0) and AM 1.5 solar simulation conditions. We observed little to no formate production under AM 1.5, but found linear formate production as a function of time using AM 0 conditions. Compared to earlier reports involving bubbled CO2 in the presence of bicarbonate, our results point to bicarbonate as the species undergoing reduction. Also investigated are the effects of three hydroxylic positive hole scavengers, ethylene glycol, propan-2-ol (isopropyl alcohol, IPA) and glycerol on the reduction of HCO3(-). Glycerol, a green solvent derived from vegetable oil, greatly improved the apparent quantum efficiency of the photocatalytic reduction.