Modifications in coordination structure of Mg[TFSA]2-based supporting salts for high-voltage magnesium rechargeable batteries.

To achieve a sustainable-energy society in the future, next-generation highly efficient energy storage technologies, particularly those based on multivalent metal negative electrodes, are urgently required to be developed. Magnesium rechargeable batteries (MRBs) are promising options owing to the many advantageous chemical and electrochemical properties of magnesium. However, the substantially low working voltage of sulfur-based positive electrodes may hinder MRBs in becoming alternatives to current Li-ion batteries. We proposed halide-free noncorrosive ionic liquid-based electrolytes incorporating Mg[TFSA]2 for high-voltage MRB applications. Upon the complexation of Mg[TFSA]2 with tetraglyme (G4) and strict control of the liquid states, the electrolytes achieved excellent anodic stability up to 4.1 V vs. Mg2+/Mg even at 100 °C. The modest electrochemical activities for magnesium deposition/dissolution in the [Mg(G4)][TFSA]2/ionic liquid electrolyte can be improved by certain modifications to the coordination state of [TFSA]-. Dialkyl sulfone was found to be effective in changing the coordination state of [TFSA]- from associated to isolated (free). This coordination change successfully promoted magnesium deposition/dissolution reactions, particularly in the coexistence of ether ligand. By contrast, the coordination of Mg2+ by strongly donating agents such as dimethyl sulfoxide and alkylimidazole led to the complexes inactive electrochemically, suggesting that interaction between Mg2+ and coordination agents predominates the fundamental electrochemical activity. We also demonstrated that an enhancement in the electrochemical activity of electrolytes contributed to improvements in the cycling ability of magnesium batteries with 2.5 V-class MgMn2O4 positive electrodes.

Atomically mixed Fe-group nanoalloys: catalyst design for the selective electrooxidation of ethylene glycol to oxalic acid.

We demonstrate electric power generation via the electrooxidation of ethylene glycol (EG) on a series of Fe-group nanoalloy (NA) catalysts in alkaline media. A series of Fe-group binary NA catalysts supported on carbon (FeCo/C, FeNi/C, and CoNi/C) and monometallic analogues (Fe/C, Co/C, and Ni/C) were synthesized. Catalytic activities and product distributions on the prepared Fe-group NA catalysts in the EG electrooxidation were investigated by cyclic voltammetry and chronoamperometry, and compared with those of the previously reported FeCoNi/C, which clarified the contributory factors of the metal components for the EG electrooxidation activity, C2 product selectivity, and catalyst durability. The Co-containing catalysts, such as Co/C, FeCo/C, and FeCoNi/C, exhibit relatively high catalytic activities for EG electrooxidation, whereas the catalytic performances of Ni-containing catalysts are relatively low. However, we found that the inclusion of Ni is a requisite for the prevention of rapid degradation due to surface modification of the catalyst. Notably, FeCoNi/C shows the highest selectivity for oxalic acid production without CO2 generation at 0.4 V vs. the reversible hydrogen electrode (RHE), resulting from the synergetic contribution of all of the component elements. Finally, we performed power generation using the direct EG alkaline fuel cell in the presence of the Fe-group catalysts. The power density obtained on each catalyst directly reflected the catalytic performances elucidated in the electrochemical experiments for the corresponding catalyst. The catalytic roles and alloying effects disclosed herein provide information on the design of highly efficient electrocatalysts containing Fe-group metals.

Layered perovskite oxide: a reversible air electrode for oxygen evolution/reduction in rechargeable metal-air batteries.

For the development of a rechargeable metal-air battery, which is expected to become one of the most widely used batteries in the future, slow kinetics of discharging and charging reactions at the air electrode, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, are the most critical problems. Here we report that Ruddlesden-Popper-type layered perovskite, RP-LaSr3Fe3O10 (n = 3), functions as a reversible air electrode catalyst for both ORR and OER at an equilibrium potential of 1.23 V with almost no overpotentials. The function of RP-LaSr3Fe3O10 as an ORR catalyst was confirmed by using an alkaline fuel cell composed of Pd/LaSr3Fe3O10-2x(OH)2x·H2O/RP-LaSr3Fe3O10 as an open circuit voltage (OCV) of 1.23 V was obtained. RP-LaSr3Fe3O10 also catalyzed OER at an equilibrium potential of 1.23 V with almost no overpotentials. Reversible ORR and OER are achieved because of the easily removable oxygen present in RP-LaSr3Fe3O10. Thus, RP-LaSr3Fe3O10 minimizes efficiency losses caused by reactions during charging and discharging at the air electrode and can be considered to be the ORR/OER electrocatalyst for rechargeable metal-air batteries.

Ultrahigh pressure-induced modification of morphology and performance of MOF-derived Cu@C electrocatalysts.

We report the pyrolysis of copper-containing metal-organic frameworks under high pressure and the effect of the applied pressure on the morphology and electrocatalytic performance toward the oxygen-related reactions of the products. The high-pressure and high-temperature (HPHT) syntheses were performed under 5, 2.5, 1, and 0.5 GPa, and the Cu@C products were obtained except for the 2.5 GPa experiment. Copper formed a shell-like nanostructure on the carbon matrices during the 0.5 GPa experiment, whereas copper formed sub-nanometer sized particles in the carbon matrices with the increasing pressure. It is considered that the transportation of copper atoms by outgassing during the pyrolysis affects the morphology. Electrochemical measurements revealed that all samples exhibited activity for the oxygen reduction reaction (ORR). The 0.5 GPa-treated product also exhibited the oxygen evolution reaction (OER). The overall ORR/OER performance of this product was excellent among Cu-based bifunctional materials even though it did not contain cocatalysts such as nitrogen-doped carbon or other metal elements. The Cu(iii) species in the nano-thick copper shell structure provided the active sites for the OER.

Evidence of nonelectrochemical shift reaction on a CO-tolerant high-entropy state Pt-Ru anode catalyst for reliable and efficient residential fuel cell systems.

A randomly mixed monodispersed nanosized Pt-Ru catalyst, an ultimate catalyst for CO oxidation reaction, was prepared by the rapid quenching method. The mechanism of CO oxidation reaction on the Pt-Ru anode catalyst was elucidated by investigating the relation between the rate of CO oxidation reaction and the current density. The rate of CO oxidation reaction increased with an increase in unoccupied sites kinetically formed by hydrogen oxidation reaction, and the rate was independent of anode potential. Results of extended X-ray absorption fine structure spectroscopy showed the combination of N(Pt-Ru)/(N(Pt-Ru) + N(Pt-Pt)) ≑ M(Ru)/(M(Pt) + M(Ru)) and N(Ru-Pt)/(N(Ru-Pt) + N(Ru-Ru)) ≑ M(Pt)/(M(Ru) + M(Pt)), where N(Pt-Ru)(N(Ru-Pt)), N(Pt-Pt)(N(Ru-Ru)), M(Pt), and M(Ru) are the coordination numbers from Pt(Ru) to Ru(Pt) and Pt (Ru) to Pt (Ru) and the molar ratios of Pt and Ru, respectively. This indicates that Pt and Ru were mixed with a completely random distribution. A high-entropy state of dispersion of Pt and Ru could be maintained by rapid quenching from a high temperature. It is concluded that a nonelectrochemical shift reaction on a randomly mixed Pt-Ru catalyst is important to enhance the efficiency of residential fuel cell systems under operation conditions.

Inexpensive gram scale synthesis of porous Ti4O7 for high performance polymer electrolyte fuel cell electrodes.

As a fuel cell catalyst support, more than 2 g of Magnéli phase Ti4O7 fine-particles were synthesized in a single reaction via an inexpensive route. The single-cell performance reached that of commercial carbon-supported platinum, with an excellent load cycle durability, one of the highest ever reported for oxide-supported platinum catalysts.

Synthesis of catalysts with fine platinum particles supported by high-surface-area activated carbons and optimization of their catalytic activities for polymer electrolyte fuel cells.

Herein, we demonstrated that carbon-supported platinum (Pt/C) is a low-cost and high-performance electrocatalyst for polymer electrolyte fuel cells (PEFCs). The ethanol reduction method was used to prepare the Pt/C catalyst, which was realized by an effective matching of the carbon support and optimization of the Pt content for preparing a membrane electrode assembly (MEA). For this, the synthesis of Pt/C catalysts with different Pt loadings was performed on two different carbons (KB1600 and KB800) as new support materials. Analysis of the XRD pattern and TEM images showed that the Pt nanoparticles (NPs) with an average diameter of ca. 1.5 nm were uniformly dispersed on the carbon surface. To further confirm the size of the NPs, the coordination numbers of Pt derived from X-ray absorption fine structure (XAFS) data were used. These results suggest that the NP size is almost identical, irrespective of Pt loading. Nitrogen adsorption-desorption analysis indicated the presence of mesopores in each carbon. The BET surface area was found to increase with increasing Pt loading, and the value of the BET surface area was as high as 1286 m2 gcarbon -1. However, after 40 wt% Pt loading on both carbons, the BET surface area was decreased due to pore blockage by Pt NPs. The oxidation reduction reaction (ORR) activity for Pt/KB1600, Pt/KB800 and commercial Pt/C was evaluated by Koutecky-Levich (K-L) analysis, and the results showed first-order kinetics with ORR. The favourable surface properties of carbon produced Pt NPs with increased density, uniformity and small size, which led to a higher electrochemical surface area (ECSA). The ECSA value of the 35 wt% Pt/KB1600 catalyst was 155.0 m2 gpt -1 higher than that of the Pt/KB800 and commercial Pt/C (36.7 wt%) catalysts. A Higher ECSA indicates more available active sites for catalyst particles. The single cell test with MEA revealed that the cell voltage in the high current density regions depends on the BET surface area, and the durability of the 35 wt% Pt/KB1600 catalyst was superior to that of the 30 wt% Pt/KB800 and commercial Pt/C (46.2 wt%) catalysts. This suggests that an optimal ratio of Pt to Pt/KB1600 catalyst provides adequate reaction sites and mass transport, which is crucial to the PEFC's high performance.

Superionic Conduction in Co-Vacant P2-Nax CoO2 Created by Hydrogen Reductive Elimination.

The layered P2-Nax MO2 (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2-Nax CoO2 created by hydrogen reductive elimination lead to an ionic conductivity of 0.045 S cm(-1) at 25 °C. Using in situ synchrotron X-ray powder diffraction and Raman spectroscopy, the composition of the superionic conduction phase is evaluated to be Na0.61 (H3 O)0.18 Co0.93 O2 . Electromotive force measurements as well as molecular dynamics simulations indicate that the ion conducting species is proton rather than hydroxide ion. The fact that the Co-stoichiometric compound Nax (H3 O)y CoO2 does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity.

CO2-free power generation on an iron group nanoalloy catalyst via selective oxidation of ethylene glycol to oxalic acid in alkaline media.

An Fe group ternary nanoalloy (NA) catalyst enabled selective electrocatalysis towards CO2-free power generation from highly deliverable ethylene glycol (EG). A solid-solution-type FeCoNi NA catalyst supported on carbon was prepared by a two-step reduction method. High-resolution electron microscopy techniques identified atomic-level mixing of constituent elements in the nanoalloy. We examined the distribution of oxidised species, including CO2, produced on the FeCoNi nanoalloy catalyst in the EG electrooxidation under alkaline conditions. The FeCoNi nanoalloy catalyst exhibited the highest selectivities toward the formation of C2 products and to oxalic acid, i.e., 99 and 60%, respectively, at 0.4 V vs. the reversible hydrogen electrode (RHE), without CO2 generation. We successfully generated power by a direct EG alkaline fuel cell employing the FeCoNi nanoalloy catalyst and a solid-oxide electrolyte with oxygen reduction ability, i.e., a completely precious-metal-free system.