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At a low depth of discharge, the performance of rechargeable alkaline Zn/MnO2 batteries is determined by the concomitant processes of hydrogen ion insertion and electro-reduction in the solid phase of γ-MnO2. Ab initio computational methods based on density functional theory (DFT) were applied to study the mechanism of hydrogen ion insertion into the pyrolusite (β), ramsdellite (R), and nsutite (γ)MnO2 polymorphs. Itwas found that hydrogen ion insertion induced significant distortion in the crystal structures ofMnO2 polymorphs. Calculations demonstrated that the hydrogen ions inserted into γ-MnO2 initially occupied the larger 2×1 ramsdellite tunnels. The protonated form of γ-MnO2 was found to be stable over the discharge range during which up to two hydrogen ions were inserted into each 2×1 tunnel. At the same time, the study showed that the insertion of hydrogen ions into the 1×1 pyrolusite tunnels of γ-MnO2 created instability leading to the structural breakdown of γ-MnO2. The results of this study explain the presence of groutite (α-MnOOH) and the absence of manganite (γ-MnOOH) among the reaction products of partially reduced γ-MnO2. [ABSTRACT FROM AUTHOR]

Rechargeable alkaline Zn/MnO2 batteries are being developed for use as cost-effective grid-scale energy storage devices. Previous studies have shown that limiting the depth of discharge (DOD) of the MnO2 cathode extends cell lifetime while still providing a cost-effective battery system. Herein, a comprehensive study of triethanolamine (TEA) as an additive in Zn/MnO2 limited DOD batteries is provided by examining the effect of TEA in full cells as well as independently on the cathode, anode, separator, and electrolyte. Improvement in cycle-ability of the cathode (on average, 80% of cycled capacity remains after 191 cycles without TEA, 568 cycles with TEA) and a decrease in ionic zinc mobility across Celgard 3501 (7.91 x 10-5 cm²/min without TEA, 3.56 x 10-5 cm²/min with TEA) and Cellophane 350P00 (3.26 x 10-5 cm²/min without TEA, 4.74 x 10-6 cm²/min with TEA) separators upon the addition of TEA are demonstrated. However, TEA increased both the reduction potential of Zn (-0.68 V vs. Hg/HgO without TEA, -0.76 V with TEA) and the solubility of Zn2+ (0.813 M without TEA, 1.023 M with TEA). Overall, the addition of TEA extended the lifetime of limited DOD cells on average by 297%. [ABSTRACT FROM AUTHOR]

An anodic stripping voltammetry (ASV) sensing platform which provides real time determination of zincate diffusion through membrane separators in alkaline electrolyte with total experimental times on the order of hours is presented. Advanced separators are essential for future rechargeable alkaline zinc batteries. In order to be screened, these membranes need to be evaluated for their zincate blocking performance. Current complexometric titration and elemental analysis methods for zincate membrane diffusion characterization can take days to weeks to obtain results as well as include large sample dilution factors and require additional sample processing. The anodic stripping voltammetry (ASV) sensing platform presented here provides real time determination of zincate diffusion in alkaline electrolyte with total experimental times on the order of hours. This method eliminates the need for sample dilution and post experiment sample processing. This technique significantly increases the throughput for the screening of advanced alkaline battery separators resulting in rapid turnaround times in the analysis of these vital membranes. To evaluate the utility of this method, zincate diffusion through commercial Celgard 3501 and Cellophane 350P00 was monitored using both ASV and inductively coupled plasma-mass spectrometry (ICP-MS) methods. The obtained zincate diffusion coefficients for both techniques were found to compare favorably. [ABSTRACT FROM AUTHOR]

A wide range of metal oxides have been studied as pseudocapitors, with the goal of achieving higher power than traditional batteries and higher energy than traditional capacitors. However, most metal oxides have relatively low conductivity, and the few exceptions, like RuO2, are prohibitively expensive. Mixed metal oxides provided an opportunity to incorporate small amounts of expensive materials to enhance the performance of a less expensive, poorer performing material. Here, by homogeneously co-depositing a small amount of energy dense and conductive RuO2 into MnO2 nanowires, we demonstrate an improvement in specific capacitance. Importantly, we also demonstrate that this improvement is not primarily provided by redox activity of RuO2, but rather by improvement of the composite conductivity. A series of RuO2–MnO2 composite nanowires with different RuO2 loading percentages have been synthesized by performing co-electrodeposition in a porous alumina template. The structure of these RuO2–MnO2 nanowires is characterized by TEM and SEM. EDS mapping shows that RuO2 is well distributed in MnO2 matrix nanowires. The chemical constituents and the phase of these composite nanowires are confirmed by X-ray photoelectron and Raman spectroscopy. The amount of RuO2 is controlled by varying the concentrations of RuCl3 and MnAc2 in the deposition solution. The precise masses of MnO2 and RuO2 are determined by ICP-AES elemental analysis. MnO2 nanowires with 6.70 wt% RuO2 demonstrate a specific capacitance of 302 F g−1 at 20 mV s−1, compared to 210 F g−1 for pristine MnO2 nanowires. Investigation of the RuO2 loading amount effect was conducted by electrochemical impedance spectroscopy (EIS) and deconvolution of capacitances, using methods previously reported by both Dunn and Transsiti. The RuO2–MnO2 nanowires studied here demonstrate a simple, straighforward method to overcome the intrinsically poor conductivity of MnO2, and clarify the source of RuO2's contribution to the improved performance. [ABSTRACT FROM AUTHOR]

Magnesium batteries have been considered to be one of the promising beyond lithium ion technologies due to magnesium's abundance, safety, and high volumetric capacity. However, very few materials show reversible performance as a cathode in magnesium ion systems. We present herein the best reported cycling performances of MnO2 as a magnesium battery cathode material. We show that the previously reported poor Mg2+ insertion/deinsertion capacities in MnO2 can be greatly improved by synthesizing self-standing nanowires and introducing a small amount of water molecules into the electrolyte. Electrochemical and elemental analysis results revealed that the magnitude of Mg2+ insertion into MnO2 highly depends on the ratio between water molecules and Mg2+ ions present in the electrolyte and the highest Mg2+ insertion capacity was observed at a ratio of 6H2O/Mg2+ in the electrolyte. We demonstrate for the first time, that MnO2 nanowire electrode can be “activated” for Mg2+ insertion/deinsertion by cycling in water containing electrolyte resulting in enhanced reversible Mg2+ insertion/deinsertion even with the absence of water molecules. The MnO2 nanowire electrode cycled in dry Mg electrolyte after activation in water-containing electrolyte showed an initial capacity of 120 mA h g−1 at a rate of 0.4 C and maintained 72% of its initial capacity after 100 cycles. [ABSTRACT FROM AUTHOR]

A redox exchange mechanism between potassium perruthenate (KRuO4) and the functional groups of selected polymers is used here to induce RuO2 into and onto conductive polymer nanowires by simply soaking the polymer nanowire arrays in KRuO4 solution. Conductive polymer nanowire arrays of polypyrrole (PPY) and poly(3,4-ethylenedioxythiophene) (PEDOT) were studied in this work. SEM and TEM results show that the RuO2 material was distributed differently in the PPY and PEDOT nanowire matrices. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were used to confirm the dispersion and formation of RuO2 materials in these polymer nanowires. Cyclic voltammetry and galvanostatic charge–discharge experiments were used to characterize their electrochemical performance. RuO2–polymer samples prepared with a 6 min soaking time in 10 mM KRuO4 solution show a high specific capacitance of 371 F g−1 and 500 F g−1 for PEDOT-based and PPY-based composite nanowires, respectively. This is attributed to the high exposure area of the conductive RuO2 and the good conductivity of the polymer matrix. This work demonstrates a simple method to synthesize heterogeneous polymer based-materials through the redox reaction between conductive polymers and high oxidation state transition metal oxide ions. Different heterogeneous nanocomposites were obtained depending on the polymer properties, and high energy storage performance of the metal oxides can be achieved within these heterogeneous nanostructures. [ABSTRACT FROM AUTHOR]

Hybrid palladium oxide/poly(3,4-ethylenedioxythiophene) (PdO x /PEDOT) films were prepared through a spontaneous reaction between aqueous PdCl 4 2- ions and a nanostructured film of electropolymerized PEDOT. Spectroscopic and electrochemical characterization indicate the presence of mixed-valence Pd species as-deposited (19 ± 7 at% Pd 0 , 64 ± 3 at% Pd 2+ , and 18 ± 4 at% Pd 4+ by X-ray photoelectron spectroscopy) and the formation of stable, electrochemically reversible Pd 0 /α-PdO x active species in alkaline electrolyte and furthermore in the presence of oxygen. The elucidation of the Pd speciation as-deposited and in solution provides insight into the mechanism of electroless deposition in neutral aqueous conditions and the electrocatalytically active species during oxygen reduction in alkaline electrolyte. The PdO x /PEDOT film catalyses 4e - oxygen reduction ( n = 3.97) in alkaline electrolyte at low overpotential (0.98 V vs. RHE, onset potential), with mass- and surface area-based specific activities competitive with, or superior to, commercial 20% Pt/C and state-of-the-art Pd- and PEDOT-based nanostructured catalysts. The high activity of the nanostructured hybrid PdO x /PEDOT film is attributed to effective dispersion of accessible, stable Pd active sites in the PEDOT matrix., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Carbon-coated α-MnO 2 nanowires (C-MnO 2 NWs) were prepared from α-MnO 2 NWs by a two-step sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (<5 nm, ≤1.2 wt % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly 5 orders of magnitude (α-MnO 2 , 3.2 × 10 -6 S cm -1 ; C-MnO 2 , 0.52 S cm -1 ) and an increase in surface Mn 3+ (average oxidation state: α-MnO 2 , 3.88; C-MnO 2 , 3.66) while suppressing a phase change to Mn 3 O 4 at high temperature. The enhanced physical and electronic properties of the C-MnO 2 NWs-enriched surface Mn 3+ and high conductivity-are manifested in the electrocatalytic activity toward the oxygen reduction reaction (ORR), where a 13-fold increase in specific activity (α-MnO 2 , 0.13 A m -2 ; C-MnO 2 , 1.70 A m -2 ) and 6-fold decrease in charge transfer resistance (α-MnO 2 , 6.2 kΩ; C-MnO 2 , 0.9 kΩ) were observed relative to the precursor α-MnO 2 NWs. The C-MnO 2 NWs, composed of ∼99 wt % MnO 2 and ∼1 wt % carbon coating, also demonstrated an ORR onset potential within 20 mV of commercial 20% Pt/C and a chronoamperometric current/stability equal to or greater than 20% Pt/C at high overpotential (0.4 V vs RHE) and high temperature (60 °C) with no additional conductive carbon.

A wide range of metal oxides have been studied as pseudocapitors, with the goal of achieving higher power than traditional batteries and higher energy than traditional capacitors. However, most metal oxides have relatively low conductivity, and the few exceptions, like RuO2, are prohibitively expensive. Mixed metal oxides provided an opportunity to incorporate small amounts of expensive materials to enhance the performance of a less expensive, poorer performing material. Here, by homogeneously co-depositing a small amount of energy dense and conductive RuO2 into MnO2 nanowires, we demonstrate an improvement in specific capacitance. Importantly, we also demonstrate that this improvement is not primarily provided by redox activity of RuO2, but rather by improvement of the composite conductivity. A series of RuO2-MnO2 composite nanowires with different RuO2 loading percentages have been synthesized by performing co-electrodeposition in a porous alumina template. The structure of these RuO2-MnO2 nanowires is characterized by TEM and SEM. EDS mapping shows that RuO2 is well distributed in MnO2 matrix nanowires. The chemical constituents and the phase of these composite nanowires are confirmed by X-ray photoelectron and Raman spectroscopy. The amount of RuO2 is controlled by varying the concentrations of RuCl3 and MnAc2 in the deposition solution. The precise masses of MnO2 and RuO2 are determined by ICP-AES elemental analysis. MnO2 nanowires with 6.70 wt% RuO2 demonstrate a specific capacitance of 302 F g(-1) at 20 mV s(-1), compared to 210 F g(-1) for pristine MnO2 nanowires. Investigation of the RuO2 loading amount effect was conducted by electrochemical impedance spectroscopy (EIS) and deconvolution of capacitances, using methods previously reported by both Dunn and Transsiti. The RuO2-MnO2 nanowires studied here demonstrate a simple, straighforward method to overcome the intrinsically poor conductivity of MnO2, and clarify the source of RuO2's contribution to the improved performance.