Electrochemical characterization of manganese oxides as a water oxidation catalyst in proton exchange membrane electrolysers.

The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO2/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.

Mechanistic Investigation of Water Oxidation Catalyzed by Uniform, Assembled MnO Nanoparticles.

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.

A structured three-dimensional polymer electrolyte with enlarged active reaction zone for Li-O2 batteries.

The application of conventional solid polymer electrolyte (SPE) to lithium-oxygen (Li-O2) batteries has suffered from a limited active reaction zone due to thick SPE and subsequent lack of O2 gas diffusion route in the positive electrode. Here we present a new design for a three-dimensional (3-D) SPE structure, incorporating a carbon nanotube (CNT) electrode, adapted for a gas-based energy storage system. The void spaces in the porous CNT/SPE film allow an increased depth of diffusion of O2 gas, providing an enlarged active reaction zone where Li(+) ions, O2 gas, and electrons can interact. Furthermore, the thin SPE layer along the CNT, forming the core/shell nanostructure, aids in the smooth electron transfer when O2 gas approaches the CNT surface. Therefore, the 3-D CNT/SPE electrode structure enhances the capacity in the SPE-based Li-O2 cell. However, intrinsic instability of poly(ethylene oxide) (PEO) of the SPE matrix to superoxide (O2(·-)) and high voltage gives rise to severe side reactions, convincing us of the need for development of a more stable electrolyte for use in this CNT/SPE design.

A 3.5 V lithium-iodine hybrid redox battery with vertically aligned carbon nanotube current collector.

A lithium-iodine (Li-I2) cell using the triiodide/iodide (I3(-)/I(-)) redox couple in an aqueous cathode has superior gravimetric and volumetric energy densities (∼ 330 W h kg(-1) and ∼ 650 W h L(-1), respectively, from saturated I2 in an aqueous cathode) to the reported aqueous Li-ion batteries and aqueous cathode-type batteries, which provides an opportunity to construct cost-effective and high-performance energy storage. To apply this I3(-)/I(-) aqueous cathode for a portable and compact 3.5 V battery, unlike for grid-scale storage as general target of redox flow batteries, we use a three-dimensional and millimeter thick carbon nanotube current collector for the I3(-)/I(-) redox reaction, which can shorten the diffusion length of the redox couple and provide rapid electron transport. These endeavors allow the Li-I2 battery to enlarge its specific capacity, cycling retention, and maintain a stable potential, thereby demonstrating a promising candidate for an environmentally benign and reusable portable battery.