A nanowire photoanode SrTaO 2 N, a semiconductor suitable for overall water-splitting with a band gap of 2.3 eV, was coated with functional overlayers to yield a core−shell structure while maintaining its one-dimensional morphology. The nanowires were grown hydrothermally on tantalum, and the perovskite-related oxynitride structure was obtained by nitridation. Three functional overlayers have been deposited on the nanowires to enhance the efficiency of photoelectrochemical (PEC) water oxidation. The deposition of TiO x protects the oxynitride from photocorrosion and suppresses charge-carrier recombination at the surface. Ni(OH) x acts a hole-storage layer and decreases the dark-current contribution. This leads to a significantly improved extraction of photogenerated holes to the electrode−electrolyte surface. The photocurrents can be increased by the deposition of a cobalt phosphate (CoP i ) layer as a cocatalyst. The heterojunction nanowire photoanode generates a current density of 0.27 mA cm −2 at 1.23 V vs the reversible hydrogen electrode (RHE) under simulated sunlight (AM 1.5G). Simultaneously, the dark-current contribution, a common problem for oxynitride photoanodes grown on metallic substrates, is almost completely minimized. This is the first report of a quaternary oxynitride nanowire photoanode in core− shell geometry containing functional overlayers for synergetic hole extraction and an electrocatalyst.

Intermediate term discharge experiments were performed for Si-air full cells using As-, Sb-and B-doped Si-wafer anodes, with 100 and 111 orientations for each type. Discharge characteristics were analyzed in the range of 0.05 to 0.5 mA/cm 2 during 20 h runs, corrosion rates were determined via the mass-change method and surface morphologies after discharge were observed by laser scanning microscopy and atomic force microscopy. Corresponding to these experiments, potentiodynamic polarization curves were recorded and analyzed with respect to current-potential characteristics and corrosion rates. Both, discharge and potentiodynamic experiments, confirmed that the most pronounced influence of potentials -and thus on performance -results from the dopant type. Most important, the corrosion rates calculated from the potentiodynamic experiments severely underestimate the fraction of anode material consumed in reactions that do not contribute to the conversion of anode mass to electrical energy. With respect to materials selection, the estimates of performance from intermediate term discharge and polarization experiments lead to the same conclusions, favoring 100 and 111 As-doped Si-wafer anodes. However, the losses in the 111 As-doped Si-anodes are by 20% lower, so considering the mass conversion efficiency this type of anode is most suitable for application in non-aqueous Si-air batteries. One line of development in technologies for electrical energy storage is metal-air batteries, which provide high specific energies and -when referring to Zn, Al, Fe, or Si -are at the same time resource effective with respect to the availability and price of the anode materials. The theoretical specific energy of a Si-air cell, related to the anode mass only, is 8470 Wh/kg. Using Si material in aqueous alkaline solutions, however, results in a severe corrosion reaction which is accompanied by intense hydrogen evolution.1-3 Despite the corrosion reaction, it is still feasible to build an alkaline Si-air cell at a discharge potential around 1.1 V, however, with sacrifice of huge amount of Si anode to corrosion. [4][5][6] Therefore, new approaches to establish batteries on silicon materials have been put forward using ionic liquid electrolytes. One of the possible approaches is the usage of EMIm(HF) 2.3 F electrolyte which possesses high conductivity, low viscosity and chemical stability in air.7-10 The proof of concept, that substantial discharge was possible when using EMIm(HF) 2.3 F electrolyte, was proposed in 2010 according to the following reactions: Additionally, a screening of several anode materials -As-, Sband B-doped Si wafers -was performed, in which the cell potential at intermediate current densities as determined from potentiodynamic polarization measurements, was set as major criterion. The corrosion current densities as obtained by the Tafel fits from the polarization experiments for the different wafer types were also considered for the material selection. However, owing to the low corrosion rates, it played a minor ro...

Potassium‐oxygen batteries (KOB) are a promising energy storage technology with high theoretical energy of 935 Wh/kg and long cycle life. Potential applications require the development of affordable cathode materials with high practical capacity. In this article, we show that low‐cost polytetrafluoroethylene (PTFE) treatment increases the discharge performance of a commercial carbon paper cathode. Cross‐sectional scanning electron microscopy reveals that PTFE alleviates mass transport limitations and facilitates homogeneous deposition of the discharge product potassium superoxide (KO2) within the cathode pore structure. Using electrochemical impedance spectroscopy, we found that PTFE, in combination with the appropriate electrolyte volume, can prevent pore flooding by the electrolyte. Free volume permits fast, gaseous oxygen transport throughout the cathode, which lowers mass transfer resistances and improves the rate capability. Moreover, PTFE enables high pore volume filling by KO2 and, in turn, high discharge capacity. Our results demonstrate that controlling the mass transport is essential for high‐performance cathodes for KOB.

LiNi0.5Mn1.5O4 (LNMO) spinel has been extensively investigated as one of the most promising high-voltage cathode candidates for lithium-ion batteries. The electrochemical performance of LNMO, especially its rate performance, seems to be governed by its crystallographic structure, which is strongly influenced by the preparation methods. Conventionally, LNMO materials are prepared via solid-state reactions, which typically lead to microscaled particles with only limited control over the particle size and morphology. In this work, we prepared Ni-doped LiMn2O4 (LMO) spinel via the polyol method. The cycling stability and rate capability of the synthesized material are found to be comparable to the ones reported in literature. Furthermore, its electronic charge transport properties were investigated by local electrical transport measurements on individual particles by means of a nanorobotics setup in a scanning electron microscope, as well as by performing DFT calculations. We found that the scarcity of Mn3+ in the LNMO leads to a significant decrease in electronic conductivity as compared to undoped LMO, which had no obvious effect on the rate capability of the two materials. Our results suggest that the rate capability of LNMO and LMO materials is not limited by the electronic conductivity of the fully lithiated materials.