Enhanced Redox Stability Research Articles

Rigid Fe(III) Chelate with Phosphonate Pendants: A Stable and Effective Extracellular MRI Contrast Agent.

A novel Fe(III) complex, Fe-tBPCDTA, was synthesized and explored as a potential contrast agent for MRI. Compared to established agents like Fe-EDTA and Fe-tCDTA, Fe-tBPCDTA exhibited moderate relaxivity (r1 = 1.17 s-1·mmol-1) due to its enhanced second-sphere mechanism. It also displayed improved kinetic inertness, lower cytotoxicity, and enhanced redox stability. In vivo studies demonstrated its function as an extracellular fluid agent, providing tumor contrast comparable to that of Gd-DTPA at a higher dosage. Complete renal clearance occurred within 24 h. These findings suggest Fe-tBPCDTA as a promising candidate for further development as a safe and effective extracellular MRI contrast agent.

Enhanced redox performance of LaFeO3 perovskite through in-situ exsolution of iridium nanoparticles for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) can produce high-purity hydrogen without additional separation units. However, the low reactivity of LaFeO3 oxygen carrier towards methane restricts the overall process performance. To enhance the redox activity of widely used LaFeO3 perovskite, a small amount of iridium was introduced to LaFeO3. The presence of iridium significantly enhanced both particle reduction and oxidation, and the particle preparation pathways of oxygen carriers greatly affected the long-term performance. The overall enhancement from the incorporation of iridium was demonstrated by density functional theory calculations. Despite these beneficial effects of iridium addition, surface iridium species prepared by an impregnation method were severely aggregated during successive redox cycling, leading to deactivation of the particles. In contrast, B-site incorporation of iridium leads to the exsolution of IrOx nanoparticles anchored on the perovskite host as well as to enhanced redox stability, which is essential for the long-term operation of the chemical looping process.

Mechanistic Insight on the Solid Electrolyte Interphase (SEI) Formed By a Superconcentrated [Li][TFSI] in Acetonitrile Electrolyte Near Lithium Metal

Superconcentrated salt solutions are promising alternatives to conventional electrolytes in Li-ion batteries because of their enhanced redox stability and tendency to form a stable and rigid solid electrolyte interphase (SEI). Herein, we performed density functional theory-based molecular dynamics (DFT-MD) simulations to understand the initial electrode reactions between the Li metal and superconcentrated electrolyte solution of Li bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) in acetonitrile (AN). We thoroughly analyzed the structure and composition of the SEI formed near the Li metal. We show that reductive decomposition of the TFSI anions contributes majorly in the initial SEI formation and the AN molecules are relatively stable against the redox processes occurring near the Li metal. The atomized C, O, F, and S atoms contributed by the TFSI anions forms the stable inorganic layer leading to highly structured and inhomogeneous SEI. The AN molecules and undissociated N–SO2–CF3 fragments are mostly aligned away from the Li layers. Figure 1

Enhanced performance of copper ore oxygen carrier by red mud modification for chemical looping combustion

Copper ore has been considered as a promising low-cost oxygen carrier candidate for chemical looping combustion (CLC) due to its high reactivity, but pure Cu ore suffers from serious deactivation during long term redox cycling. The present work proposes to combine Cu ore with red mud, an abundant polluting waste of aluminum industry, to improve the reactivity and stability of Cu ore for CLC of methane. Characterizations, redox experiment and kinetic analysis are performed on the mixed oxygen carriers to study the relationship between the physiochemical properties and the redox activity. The results show that the addition of a suitable proportion of red mud (30 wt%) significantly improves the reactivity of Cu ore. Specifically, the average CH4 conversion increases from 67% to 86%, and the actual oxygen carrying capacity rises from 8.2% to 13.8%. In addition, the activation energy for the reduction of Cu ore oxygen carrier by methane decreases from 76.6 kJ/mol to 58.5 kJ/mol after modification. The synergy among CuO in copper ore, Fe2O3 and other components (e.g., Na+, Ca2+, SiO2 and Al2O3) in red mud play an important role in this enhancement. The interaction between CuO and Fe2O3 promotes the reducibility of oxygen carrier via forming a more active oxide (CuFe2O4), and the presence of foreign ions (Na+ and Ca2+) and inert oxides (Al2O3 and SiO2) contributes to the enhanced redox stability of oxygen carrier via acting as supports or additives.

Enhanced redox stability and optical contrast of electrochromic copolymers from selenophene and 3-methylthiophene

A series of poly(selenophene-co-3-methylthiophene) films were synthesized by electrochemical copolymerization of selenophene and 3-methylthiophene under different feed ratios in tetrahydrofuran–boron trifluoride diethyl etherate (THF–BFEE) mixture. Structural interpretation, surface morphology, and electrochemical and electrochromic properties of the resultant copolymer films were determined and comparatively discussed. These copolymer films displayed reversible color changes from sandy-brown in the neutral state to deep blue and black in the oxidized state, clearly different from their parent homopolymers polyselenophene and poly(3-methylthiophene). The copolymers also showed more favorable electrochromic kinetic parameters in comparison with both of their homopolymers, including higher optical contrast, faster switching time, more efficient coloration efficiency, and better optical memory. Under the monomer feed ratio of selenophene/3-methylthiophene in 1:2, the copolymer film exhibited the highest optical contrast of 70% at 1080 nm. Furthermore, excellent redox stability was observed for all the copolymers after secondary electrochemistry doping by cyclic voltammetry in monomer-free BFEE.

Kinetics of hydrogen reduction of titanium-doped molybdenum dioxide

Ti-doped MoO2 was synthesized to broaden the oxygen-to-carbon ratio operating range of MoO2 for partial oxidation of long-chain hydrocarbons by increasing the redox stability. The structure modification causes the hydrogen reduction mechanism to change from three-dimensional nuclei growth with an activation energy of 61.3 kJ mol−1 to a three-dimensional hydrogen diffusion limited model with an activation energy of 317.9 kJ mol−1. Because of the enhanced redox stability, Ti-doped MoO2 has potential as an alternative anode in direct liquid-fed solid oxide fuel cells.

Tuning the optoelectronic properties of polyfuran by design of furan-EDOT monomers and free-standing films with enhanced redox stability and electrochromic performances

Most recently, conjugated oligo-/polymers containing furan have regained attention due to their unique properties and promising application in organic electronics. Herein, to acquire a thorough fundamental understanding of the electrosynthesis and properties of furan-EDOT copolymers from different initial monomers, the synthesis and electropolymerization performances of furan-EDOT monomers, namely 5-(furan-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin (EDOT-Fu), 5,7-di(furan-2-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin (Fu-EDOT-Fu), and 2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)furan (EDOT-Fu-EDOT), were comprehensively reported and the effect of different monomers on the structure and properties of the resulting polymers obtained under optimized electrical conditions were systematically evaluated. The monomers exhibit good blue-green photoluminescence with quantum yields ranging from 0.5 to 40%, which may be used as building blocks for rational design of fluorescent conjugated systems. The onset oxidation potential ranged from 0.78 V-0.45V with the incorporation of EDOT unit in monomer chain, thus leading to the facile electrodeposition of free-standing films with improved optoelectronic properties in comparison with polyfuran. The obtained copolymers featured the advantageous combination of polyfuran and PEDOT, such as higher fluorescence and better planarity of polyfuran, transparency and excellent redox stability of PEDOT. Structure characterization and properties of the as-formed copolymer films from different initiative monomers, including FT-IR, UV-vis, TG, fluorescence, surface morphology and electrochromic properties, etc., were systematically investigated and comparatively discussed.

Effect of support on redox stability of iron oxide for chemical looping conversion of methane

The chemical looping processes utilize lattice oxygen in oxygen carriers to convert carbonaceous fuels in a cyclic redox mode while capturing CO2. Typical oxygen carriers are composed of a primary oxide for active lattice oxygen storage and a ceramic support for enhanced redox stability and activity. Among the various primary oxides reported to date, iron oxide represents a promising option due to its low cost and natural abundance. The current work investigates the effect of support on the cyclic redox performance of iron oxides as well as the underlying mechanisms. Three ceramic supports with varying physical and chemical properties, i.e. perovskite-structured Ca0.8Sr0.2Ti0.8Ni0.2O3, fluorite-structured CeO2, and spinel-structured MgAl2O4, are investigated. The results indicate that the redox properties of the oxygen carrier, e.g. activity and long-term stability, are significantly affected by support and iron oxide interactions. The perovskite supported oxygen carrier exhibits high activity and stability compared to oxygen carriers with ceria support, which deactivate by as much as 75% within 10 redox cycles. The high stability of perovskite supported oxygen carrier is attributable to its high mixed ionic–electronic conductivity. Deactivation of ceria supported samples results from solid-state migration of iron cations and subsequent enrichment on the oxygen carrier surface. This leads to agglomeration and lowered lattice oxygen accessibility. Activity of MgAl2O4 supported oxygen carrier is found to increase during redox cycles in methane. The activity increase is a consequence of surface area increase caused by filamentous carbon formation and oxygen carrier fragmentation. While higher redox activity is desired for chemical looping processes, physical degradation of oxygen carriers can be detrimental.

Pseudocapacitive Hausmannite Nanoparticles with (101) Facets: Synthesis, Characterization, and Charge‐Transfer Mechanism

Hausmannite Mn3 O4 octahedral nanoparticles of 18.3 ± 7.0 nm with (101) facets have been prepared by an oxygen-mediated growth. The electrochemical properties of the Mn3 O4 particles as pseudocapacitive cathode materials were characterized both in half-cells and in button-cells. The Mn3 O4 nanoparticles exhibited a high mass-specific capacitance of 261 F g(-1), which was calculated from cyclic voltammetry analyses, and a capacitive retention of 78% after 10,000 galvanostatic charge-discharge cycles. The charge-transfer mechanisms of the Mn3 O4 nanoparticles were further studied by using synchrotron-based in situ X-ray absorption near edge spectroscopy and XRD. Both measurements showed concurrently that throughout the potential window of 0-1.2 V (vs. Ag/AgCl), a stable spinel structure of Mn3 O4 remained, and a reversible electrochemical conversion between tetrahedral [Mn(II) O4 ] and octahedral [Mn(III) O6 ] units accounted for the redox activity. Density functional theory calculations further corroborated this mechanism by confirming the enhanced redox stability afforded by the abundant and exposed (101) facets of Mn3 O4 octahedra.

Redox stability and electrical conductivity of Fe2.3Mg0.7O4±δ spinel prepared by mechanochemical activation

This paper addresses the potential of mechanochemical activation of MgO and α-Fe2O3 precursor powders to obtain Fe2.3Mg0.7O4 ceramics with enhanced redox stability and electrical conductivity. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest the initial formation of the spinel phase after 5 h of high-energy milling in inert gas, but after 10 h of mechanoactivation, the precursor still comprised hematite as a major phase with minor amounts of magnesiowustite as by-product. The activated mixtures can be nearly completely converted to spinel solid solution by heating to 1173 K, whereas single-phase, dense spinel ceramics can be prepared by sintering at 1773 K in inert atmosphere. These ceramics demonstrated redox stability under mildly reducing conditions (p(O2) ∼ 10 Pa), as confirmed by XRD, thermogravimetry and electrical measurements. The electrical conductivity of Fe2.3Mg0.7O4 at this oxygen partial pressure is lower compared to magnetite, but it is still as high as 60 S/cm at 1073 K and 15 S/cm at room temperature. Cooling below 1473 K in air results in a drop of conductivity due to segregation of hematite phase at the grain boundaries. However, the phase separation is kinetically stagnated at 1073 K, and, after slight initial degradation, the retained electrical conductivity is more than 3 orders of magnitude higher compared to hematite and MgFe2O4 spinel.

Emitting electrode coatings with redox-switchable conductivity: incorporation of ruthenium(ii)-2,6-di(quinolin-8-yl)pyridine complexes into polythiophene by electropolymerization

Polythiophenes doped with ruthenium(II)-2,6-di(quinolin-8-yl)pyridine complexes are prepared via an electrochemical polymerization approach. The influence of the ruthenium(II)–thiophene ratios and different complex assemblies on the electrochemical, conductivity, and optical properties of the polymer are studied. The polymers feature an enhanced redox stability with increasing ruthenium(II) content and reversibly switchable conductivities (up to 10−5 S cm−1), combined with the characteristic emission of the complexes at around 750 nm under ambient conditions.

Enhanced redox stability and electrochromic properties of aromatic polyamides based on N,N‐bis(4‐carboxyphenyl)‐N′,N′‐bis(4‐tert‐butylphenyl)‐1,4‐phenylenediamine

AbstractA new bis(triphenylamine)‐type dicarboxylic acid monomer, N,N‐bis(4‐carboxyphenyl)‐N′,N′‐bis(4‐tert‐butylphenyl)‐1,4‐phenylenediamine, was prepared by a well‐established procedure and led to a new family of redox‐active aromatic polyamides with di‐tert‐butyl‐substituted N,N,N′,N′‐tetraphenylphenylenediamine (TPPA) segments. The resulting polyamides were amorphous with good solubility in many organic solvents, and most of them could be solution cast into flexible polymer films. The polyamides exhibited high thermal stability with glass‐transition temperatures in the range of 247–293 °C and 10% weight‐loss temperatures in excess of 500 °C. They showed well‐defined and reversible redox couples during oxidative scanning, with a strong color change from a colorless or pale yellowish neutral form to green and blue oxidized forms. They had enhanced redox stability and electrochromic performance when compared with the corresponding analogs without tert‐butyl substituents on the TPPA unit. The polyamide with TPPA units in both the diacid and diamine components shows multicolored electrochromic behavior. A polyamide containing both the cathodic coloring anthraquinone chromophore and the anodic coloring TPPA chromophore has the ability to show red, green, and blue states, toward single‐component RGB electrochromics. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Self-Doped Polyaniline Nanoparticle Dispersions Based on Boronic Acid−Phosphate Complexation

Poly(anilineboronic acid)/phosphate nanoparticle dispersions are produced in high yields using the reactivity of the boronic acid moiety with phosphate in the presence of fluoride. The poly(anilineboronic acid)/phosphate dispersions have been characterized using spectroscopic, microscopic and electrochemical techniques. According to 11 B NMR studies, the formation of anionic tetrahedral boronate group in phosphoric acid in the presence of fluoride forms the basis of self-doped, stabilized PABA nanoparticle dispersion. Transmission electron microscope images show that 25-50 nm diameter PABA nanoparticles are formed under these conditions. UV-vis, FT-IR-ATR spectroscopic and cyclic voltammetric results confirm the formation of the conducting form of PABA. Films produced from these particles exhibit enhanced redox stability and potential dependant conductivity under neutral and basic pH conditions due to the formation of a boron- phosphate complex containing fluoride, which results in a self-doped form of the polymer.

Conducting Poly(anilineboronic acid) Nanostructures: Controlled Synthesis and Characterization

AbstractSelf‐doped poly(anilineboronic acid) (PABA) nanostructures have been prepared by chemical polymerization in an aqueous acid solution and aliphatic alcohols. In aliphatic alcohols, PABA is soluble under the polymerization conditions. According to 11B NMR studies, the formation of anionic tetrahedral boronate ester in aliphatic alcohols in the presence of fluoride forms the basis of self‐doped, soluble PABA. Transmission electron microscope images show the formation of nanostructures with different shapes and forms in different solvents after precipitation of the polymer as a result of ion exchange in the presence of 0.5 M HCl. The spectroscopic and cyclic voltammetric results confirm the formation of conducting PABA nanostructures in high yield. The conductivity of PABA thin films made from the nanostructures prepared in aqueous acid, methanol, ethanol and 1‐propanol is approximately 15 and 3.0, 2.1 and 1.9 S · cm−1, respectively. The conducting PABA nanostructures are processable since they are easily re‐dispersed in the various solvents up to approximately 5 mg · mL−1. UV‐vis kinetic measurements, XPS and 11B NMR results suggest that the formation of various nanostructures is influenced by the polymerization rate, the degree of self‐doping versus external doping, and the polarity of the solvents used. The PABA nanostructured films produced from these structures exhibit enhanced redox stability in a wider potential window in non‐aqueous media compared to polyaniline due to formation of six‐member heterocyclic complexes containing a boron‐imine dative bond resulting in a self‐doped self‐cross‐linked polymer. The degree of crosslinking depends on the nature of nanostructures. In non‐aqueous media, the self‐doped, self‐cross‐linked PABA nanostructured films are much less susceptible to cathodic and anodic degradation at extreme potentials, overcoming a major limitation of more common conducting polymers.magnified image

Enhanced electrochemical stability of polyaniline in ionic liquids

The electrochemical performance of polyaniline thin films on platinum electrode substrates has been investigated using cyclic voltammetry in two aprotic ionic liquid electrolyte systems, 1-butyl-3-methyl imidazolium hexafluorophosphate (BMI PF6−) or 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfon)imide (EMI TFSI). Enhanced redox stability of the polyaniline was observed in both electrolyte systems. Initial redox cycles in both ILs exhibited increasing current responses, over the first 10–20 cycles, which was attributed to the electrochemically induced ion exchange processes occurring within polyaniline film. Extended cycling after the redoping process was observed to result in a stable and reversible electrochemistry.