Efficient Li₂O₂ Formation via Aprotic Oxygen Reduction Reaction Mediated by Quinone Derivatives

Since the oxygen reduction reaction (ORR) in aprotic Li ion electrolytes accompanied by Li₂O₂ formation is a crucial reaction for the discharge of nonaqueous aprotic Li–air batteries, there is a strong demand for a reduction in the overpotential of the reaction in order to improve the discharge performance. In the present work, we investigated the effect of the addition of quinone derivatives for ORR on carbon materials in aprotic Li⁺–electrolytes. Detailed electrochemical analysis revealed that the semiquinone species catalyze the aprotic ORR, resulting in the efficient Li₂O₂ formation. Among the quinone derivatives, benzoquinone exhibited the best catalytic performance, with an overpotential for the Li₂O₂ formation of less than 100 mV

In Situ CO₂‑Emission Assisted Synthesis of Molybdenum Carbonitride Nanomaterial as Hydrogen Evolution Electrocatalyst

We reported a novel protocol to efficiently synthesize molybdenum carbonitride (MoCN) nanomaterials with dense active sites and high surface area. The key step in this protocol is the preparation of the catalyst precursor, which was obtained by polymerizing diaminopyridine in the presence of hydrogen carbonate. The abundant amino groups in the poly diaminopyridine bound numerous Mo species via coordination bonds, resulting in the formation of dense Mo active sites. The addition of hydrogen carbonate to the synthesis mixture resulted in CO₂ gas evolution as the local pH decreased during polymerization. The in situ evolved CO₂ bubbles mechanically broke down the precursor into MoCN nanomaterials with a high surface area. The synthesized MoCN materials were demonstrated as an electrocatalyst for hydrogen evolution reaction (HER). It exhibited an HER onset potential of −0.05 V (vs RHE) and a high hydrogen production rate (at −0.14 V vs RHE, −10 mA cm^–2) and is therefore one of the most efficient, low-cost HER catalysts reported to date

Efficient Bifunctional Fe/C/N Electrocatalysts for Oxygen Reduction and Evolution Reaction

Efficient electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical components of various energy conversion devices such as regenerative fuel cells and metal–air batteries. Herein, we report bifunctional transition-metal-doped carbon/nitrogen (M/C/N) materials that simultaneously electrocatalyze the ORR and OER. The OER potential of the Fe/C/N catalyst at a current density of 10 mA cm^–2 was 1.59 V_RHE, and its ORR half-wave potential was 0.83 V_RHE. Significantly, the Fe/C/N catalyst provided a potential gap of 0.76 V between the OER potential (at 10 mA cm^–2) and the ORR half-wave potential; this is the highest activity reported to date for a non-precious-metal catalyst. Two types of active center, the transition metal and a nitrogen atom, are likely responsible for the oxygen bifunctional activity

Transition Metal Complexes with Macrocyclic Ligands Serve as Efficient Electrocatalysts for Aprotic Oxygen Evolution on Li₂O₂

Since the oxygen evolution reaction (OER) in aprotic Li ion electrolytes is a crucial reaction in the charging process of nonaqueous aprotic Li–air batteries, there is a strong demand for decreasing the overpotential by developing more efficient OER catalysts. Herein, we investigated the effect of addition of transition metal complexes with macrocyclic ligands, such as porphyrins and phthalocyanines, for OER in aprotic Li ion electrolytes. Electrochemical experiments using a three-electrode system revealed that such complexes functioned as efficient OER catalysts, in which the center metal in the complex played an essential role in the catalytic process. Among the metal complexes studied, cobalt <i>tert</i>-butylphthalocyanine was found to be the best catalyst: the charging potential was lowered from 4.1 V to about 3.4 V at 1 μA/cm² by addition of 1 mM catalys

Insulative Microfiber 3D Matrix as a Host Material Minimizing Volume Change of the Anode of Li Metal Batteries

Batteries using metallic lithium (Li) as an anode have attracted a great deal of attention because they have the potential to achieve high energy density over Li-ion batteries. In order to use Li metal as a practical anode of a secondary battery, there are many problems to be overcome. A large volume change of the anode accompanying repetitive deposition and dissolution of Li is one such problem. Here we report that a 3D matrix consisting of insulative microfibers on the Li anode functions as a layer absorbing the volume change associated with the deposition/dissolution of Li as high as 10 mAh/cm². This result suggests that the use of an insulative 3D matrix layer is an effective way to minimize anode volume change under practical operating conditions

Potassium Ions Promote Solution-Route Li₂O₂ Formation in the Positive Electrode Reaction of Li–O₂ Batteries

Lithium–oxygen system has attracted much attention as a battery with high energy density that could satisfy the demands for electric vehicles. However, because lithium peroxide (Li₂O₂) is formed as an insoluble and insulative discharge product at the positive electrode, Li–O₂ batteries have poor energy capacities. Although Li₂O₂ deposition on the positive electrode can be avoided by inducing solution-route pathway using electrolytes composed of high donor number (DN) solvents, such systems generally have poor stability. Herein we report that potassium ions promote the solution-route formation of Li₂O₂. The present findings suggest that potassium or other monovalent ions have the potential to increase the volumetric energy density and life cycles of Li–O₂ batteries

True Location of Insulating Byproducts in Discharge Deposits in Li–O₂ Batteries

Lithium–oxygen batteries (LOBs) are next-generation rechargeable energy storage devices with a high theoretical gravimetric energy density. However, the expected energy density has not been fully achieved mainly because of high charging overvoltages. The inclusion of insulating byproducts in the discharge products has been suggested to be a critical factor for high overvoltages. However, these previous studies did not consider the growing/retreating fronts of the discharge deposits (i.e., the deposits/electrode interface or the deposits/electrolyte interface), potentially misleading conclusions. The aim of the present study is set to precisely determine the locations of insulating byproducts in individual discharge products in an LOB system, where the growing/retreating fronts have already been identified, thereby indicating the right direction for effectively reducing charging voltage. The analysis revealed the consistent presence of Li2CO3, a byproduct of decomposition of the electrolyte and/or positive electrode, inside the individual discharge products composed mainly of Li2O2, as expected from the growing/retreating fronts. The successful identification of the true locations of insulating byproducts in discharge deposits is pivotal because it can enhance our understanding of battery reactions, which can, in turn, pave the way for the development of design guidelines for advanced battery systems

CO Hydrogenation Promoted by Oxygen Atoms Adsorbed onto Cu(100)

The electrochemical CO2 reduction reaction (CO2RR) on Cu-based catalysts is a promising method for converting anthropogenic CO2 to valuable chemical feedstocks and fuels. Although pure CO2 gas has been widely used as a reactant in CO2RR-related research, CO2 gas collected from the atmosphere inevitably includes some amount of various impurity gases in the actual application of this method. Among such impurities, O2 gas has high reactivity and can easily contaminate the reaction environment, thereby substantially affecting the reactivity of the CO2RR. Herein, we performed first-principles calculations for the CO2RR in the presence of O2 reduction reaction intermediates on the Cu(100) surface. Specifically, we calculated the reaction and activation free energies for the hydrogenation of adsorbed CO* to CHO* on a Cu(100) surface covered with O* or OH*. When the coverage of O* reached 25%, the initial state of CO hydrogenation became destabilized to a greater extent than the transition state, which decreased the reaction and activation free energies by 0.27 and 0.16 eV, respectively. The projected density of states analyses revealed that O* weakens the interaction between CO* and the Cu surface, whereas OH* less strongly affects CO hydrogenation

Electrocatalytic Reduction of Nitrate to Nitrous Oxide by a Copper-Modified Covalent Triazine Framework

It was found that copper-modified covalent triazine frameworks (Cu-CTF) efficiently catalyze the electrochemical reduction of nitrate and promote N–N bond formation of nitrous oxide (N₂O), a key intermediate for N₂ formation (denitrification). A Cu-CTF electrode exhibited an onset potential of −50 mV versus RHE for the electrochemical nitrate reduction reaction (NRR). The faradaic efficiency for N₂O formation by Cu-CTF reached 18% at −200 mV versus RHE, whereas that for Cu metal was negligible. On the basis of density functional calculations for Cu-CTF, both solvated and surface-bound nitric oxide (NO) were generated by the NRR due to the moderate adsorption strength of Cu atoms for NO, a property that facilitated the effective dimerization of NO through an Eley–Rideal-type mechanism

Improved Energy Capacity of Aprotic Li–O₂ Batteries by Forming Cl-Incorporated Li₂O₂ as the Discharge Product

Aprotic lithium–oxygen (Li–O₂) batteries are promising devices for use in sustainable energy management systems as they have the potential to achieve significantly higher energy densities than current state-of-the-art Li-ion batteries. However, the low electrical conductivity of the main discharge product, lithium peroxide (Li₂O₂), which forms on the positive electrode, gradually suppresses the electrochemical reactions involved in the discharge process, thereby lowering the energy capacity of these systems. Herein, we demonstrate that the energy capacity of Li–O₂ batteries can be significantly improved by simply adding chloride ions to the electrolyte. Scanning electron microscopy analysis revealed that thick chloride (Cl)-incorporated Li₂O₂ films formed on the positive electrode as the discharge product. Using conductive atomic force microscopy, the Cl-incorporated Li₂O₂ films were shown to exhibit much higher electric conductivity than pristine Li₂O₂. Taken together, the present findings suggest that modulation of the electrical conductivity of the discharge product by the incorporation of heteroatoms is an effective approach for constructing Li–O₂ batteries with high volumetric energy density