Electrospinning is a simple method for producing nanoscale or microscale fibers from a wide variety of materials. Intrinsically conductive polymers (ICPs), such as polyaniline (PANI), show higher conductivities with the use of secondary dopants like m-cresol. However, due to the low volatility of most secondary dopants, it has not been possible to electrospin secondary doped ICP fibers. In this work, the concept of secondary doping has been applied for the first time to electrospun fibers. Using a novel design for rotating drum electrospinning, fibers were efficiently and reliably produced from a mixture of low- and high-volatility solvents. The conductivity of electrospun PANI–poly­(ethylene oxide) (PEO) fibers prepared was 1.73 S/cm, two orders of magnitude higher than the average value reported in the literature. These conductive fibers were tested as electrodes for supercapacitors and were shown to have a specific capacitance as high as 3121 F/g at 0.1 A/g, the highest value reported, thus far, for PANI–PEO electrospun fibers.

Bifunctional catalysts capable of catalyzing both oxygen reduction reaction (ORR) and oxygen evolution (OER) reaction are extremely valuable for oxygen-based energy conversion devices such as regenerative fuel cells and metal−air batteries. However, the underlying property of such catalysts that gives rise to their bifunctionality is not yet known nor explored. With the first use of near-infrared photoluminescence spectroscopy for tracking the changes in the individual metal cation valence states during electrocatalysis in combination with in situ gravimetric and resistance measurements, we show the underlying correlation between catalytic activity, potential-dependent resistance, and nature of reaction intermediates on various bifunctional and nonbifunctional surfaces. Our results show that bifunctional MnO x reversibly switches electrical polarity from p-type to n-type along with the formation of high-valent cationic Mn 4+ active sites as well as low-valent cationic Mn 2+ active sites during OER and ORR, respectively, which is absent in nonbifunctional NiO and Co 3 O 4 . Results also show that other key process steps such as lattice hydration/dehydration occur during polarization. These results are rationalized in terms of a band structure framework which correlates electrochemical activity with the formation energy of various metal cation intermediates.

A new design strategy for the development of bifunctional electrocatalysts capable of catalyzing both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is proposed. In this strategy, the MnO x lattice is doped with either electropositive (Sr, Ba) or electronegative (Bi, Pb) elements that results in the coincorporation of electron-rich donor (Mn2+) and electron-poor acceptor (Mn4+) defects in the same parent (Mn3+) lattice. These defects effectively catalyze the reduction (ORR) and oxidation (OER) processes on the same electrode surface. This study is based on the results of a previous study on Mn2O3 that showed Mn2+ and Mn4+ as the active sites for ORR and OER processes, respectively. Our results show that BiMnO x is the most promising bifunctional catalyst with OER/ORR activities that are comparable to the individual activities of state-of-the-art commercial Pt or RuO2 catalysts. Stability tests show the catalyst to be stable for more than 3 h of continuous OER or ORR polarization. This work provides a pathway for the individual tuning of defects to control electrocatalytic activities, which opens up new possibilities for the rational design of many perovskite-based oxides.

Birnessite, the closest naturally occurring analog of the Mn 4 CaO 5 cluster of photosystem II, is an important model compound in the development of bio-inspired electrocatalysts for the water oxidation reaction. The present work reports the formation mechanism of the key Mn III intermediate realized through the study of the effects of several electrolyte anions and cations on the catalytic efficiency of birnessite. In situ spectroelectrochemical measurements show that the activity is controlled by a dynamic dissolution-oxidation process, wherein Mn III is formed through the oxidation of labile uncomplexed Mn II that reversibly shuttles between the birnessite and the electrolyte in a manner similar to the photoactivation in photosystem II. The role of electrolyte cations of different ionic radii and hydration strengths is to control the interlayer spacing, whereas electrolyte anions control the extent of deprotonation of complexed Mn II in the lattice. Both in turn govern the shuttling efficiency of uncomplexed Mn II and its subsequent electro-oxidation to Mn III .

The effects of ion intercalation in transition metal chalcogenides like MoS2 has been well studied, although the nature of this interaction is not clearly known. In this Article, we show that defect-ion interaction is one of the key parameters that control many of the electrical, optical, structural, and electrocatalytic properties of MoS2. The results show for the first time that modulation of the concentration of intrinsic defects in MoS2 containing an excess of ‘S’ atoms in the lattice through Li+ insertion can lead to a new type of semiconductor-to-insulator-to-metal electronic phase transition with a concomitant change in the electrical conductivity from p-type to n-type, and a reversible 1T → 2H → 1T structural phase transformation. Using near-infrared photoluminescence and X-ray photoelectron spectroscopy measurements to directly monitor defect-ion interactions, it is shown that the observed changes are a direct result of changes in the electronic structure resulting from passivation of S-excess defects by Li+ and subsequently from the formation of electrochemically induced S-deficient vacancy defects in MoS2. The effects of these broad range modulation on the catalytic rates of oxygen and hydrogen evolution reactions are shown. The structure–property–activity correlation shown here has important implications for chalcogenides-based semiconductors in general.