Unraveling the phase diagram-ion transport relationship in aqueous electrolyte solutions and correlating conductivity with concentration and temperature by semi-empirical modeling.

The relationship between structure and ion transport in liquid electrolyte solutions is not well understood over the whole concentration and temperature ranges. In this work, we have studied the ionic conductivity (κ) as a function of molar fraction (x) and Temperature (T) for aqueous solutions of salts with nitrate anion and different cations (proton, lithium, calcium, and ammonium) along with their liquid-solid phase diagrams. The connection between the known features in the phase diagrams and the ionic conductivity isotherms is established with an insight on the conductivity mechanism. Also, known isothermal (κ vs.. x) and iso-compositional (κ vs.. T) equations along with a proposed two variables semi-empirical model (κ = f (x, T)) were fitted to the collected data to validate their accuracy. The role of activation energy and free volume in controlling ionic conductivity is discussed. This work brings us closer to the development of a phenomenological model to describe the structure and transport in liquid electrolyte solutions.

Real-Time Convolutional Voltammetry Enhanced by Energetic (Hot) Electrons and Holes on a Surface Plasmon Waveguide Electrode.

Surface plasmon polaritons (SPPs) propagating along a waveguide working electrode are sensitive to changes in the local refractive index, which follow changes in the concentration of reduced and oxidized species near the working electrode. The real-time response of the output optical power from a waveguide working electrode is proportional to the time convolution of the electrochemical current density, precluding the need to compute the latter a posteriori via numerical integration. Convolutional voltammetry yields complementary results to conventional voltammetry and can be used to determine the diffusion constant, bulk concentration, and the number of transferred electrons of electroactive species. The theoretical optical response of a waveguide working electrode is derived and validated experimentally via chronoamperometry and cyclic voltammetry measurements under low power SPP excitation, for various concentrations of potassium ferricyanide in potassium nitrate electrolyte at various scan rates. Increasing the SPP power induces a regime where the SPPs no longer act solely as a probe of electrochemical activity, but also as a pump creating energetic electrons and holes via absorption in the working electrode. In this regime, the transfer of energetic carriers (electrons and holes) to the redox species dominates the electrochemical current density, which becomes significantly enhanced relative to equilibrium conditions (low SPP power). In this regime the output optical power remains proportional to the time convolution of the current density, even with the latter significantly enhanced by the transfer of energetic carriers.

Engineered interfaces between perovskite La2/3xLi3xTiO3 electrolyte and Li metal for solid-state batteries.

Perovskite La2/3xLi3xTiO3 (LLTO) materials are promising solid-state electrolytes for lithium metal batteries (LMBs) due to their intrinsic fire-resistance, high bulk ionic conductivity, and wide electrochemical window. However, their commercialization is hampered by high interfacial resistance, dendrite formation, and instability against Li metal. To address these challenges, we first prepared highly dense LLTO pellets with enhanced microstructure and high bulk ionic conductivity of 2.1 × 10 - 4 S cm-1 at room temperature. Then, the LLTO pellets were coated with three polymer-based interfacial layers, including pure (polyethylene oxide) (PEO), dry polymer electrolyte of PEO-LITFSI (lithium bis (trifluoromethanesulfonyl) imide) (PL), and gel PEO-LiTFSI-SN (succinonitrile) (PLS). It is found that each layer has impacted the interface differently; the soft PLS gel layer significantly reduced the total resistance of LLTO to a low value of 84.88 Ω cm-2. Interestingly, PLS layer has shown excellent ionic conductivity but performs inferior in symmetric Li cells. On the other hand, the PL layer significantly reduces lithium nucleation overpotential and shows a stable voltage profile after 20 cycles without any sign of Li dendrite formation. This work demonstrates that LLTO electrolytes with denser microstructure could reduce the interfacial resistance and when combined with polymeric interfaces show improved chemical stability against Li metal.

Infrared surface plasmons on a Au waveguide electrode open new redox channels associated with the transfer of energetic carriers.

Plasmonic catalysis holds promise for opening new reaction pathways inaccessible thermally or for improving the efficiency of chemical processes. We report a gold stripe waveguide along which infrared (λ0 ~ 1350 nanometers) surface plasmon polaritons (SPPs) propagate, operating simultaneously as an electrochemical working electrode. Cyclic voltammograms obtained under SPP excitation enable oxidative processes involving energetic holes to be investigated separately from reductive processes involving energetic electrons. Under SPP excitation, redox currents increase by 10×, redox potentials decrease by ~2× and split in correlation with photon energy, and the charge transfer resistance drops by ~2× as measured using electrochemical impedance spectroscopy. The temperature of the working electrode was monitored in situ, ruling out thermal effects. Chronoamperometry measurements with SPPs modulated at 600 hertz yield a commensurately modulated current response, ruling out thermally enhanced mass transport. Our observations indicate opening of optically controlled nonequilibrium redox channels associated with energetic carrier transfer to the redox species.

Demystifying the Atomistic Origin of the Electric Field Effect on Methane Oxidation.

Understanding the role of an electric field on the surface of a catalyst is crucial in tuning and promoting the catalytic activity of metals. Herein, we evaluate the oxidation of methane over a Pt surface with varying oxygen coverage using density functional theory. The latter is controlled by the electrode polarization, giving rise to the non-Faradaic modification of catalytic activity phenomenon. At -1 V, the Pt(111) surface is present, while at 1 V, α-PtO2 on Pt(111) takes over. Our results demonstrate that the alteration of the platinum oxide surface under the influence of an electrochemical potential promotes the catalytic activity of the metal oxides by lowering the activation energy barrier of the reaction.

Modification of Nickel Surfaces by Bismuth: Effect on Electrochemical Activity and Selectivity toward Glycerol.

Herein, we study the effect of adding bismuth to Ni-nanostructured catalysts (NixBi1-x, x = 100-90 at. %) for glycerol electro-oxidation in alkaline solution by combining physiochemical, electrochemical, and in situ infrared spectroscopy techniques, as well as continuous electrolysis with HPLC (high-performance liquid chromatography) product analysis. The addition of small quantities of Bi (<20 at. %) to Ni nanoparticles led to significant activity enhancement at lower overpotentials, with Ni90Bi10 displaying an over 2-fold increase compared to Ni. Small quantities of bismuth actively affected the reaction selectivity of Ni by suppressing the pathways with C-C bond cleavage, hindering the production of carbonate and formate and improving the formation of tartronate, oxalate, and glycerate. Furthermore, the effect of aging on NixBi1-x catalysts was investigated, resulting in structural modification from the Ni-Bi double shell/core structure to Bi decorated on the folded Ni sheet, thus enhancing their activity twice after 2 weeks of aging. NiBi catalysts are promising candidates for glycerol valorization to high-value-added products.

Synthesis of SrTiO3 submicron cubes with simultaneous and competitive photocatalytic activity for H2O splitting and CO2 reduction.

Single crystalline strontium titanate (SrTiO3) submicron cubes have been synthesized based on a molten salt method. The submicron cubes showed superior photocatalytic activity towards both water splitting and carbon dioxide reduction, in which methane (CH4) and hydrogen (H2) were simultaneously produced. The average production rate of methane up to 8 h is 4.39 µmol g-1 h-1 but drops to 0.46 µmol g-1 h-1. However, the average production rate of hydrogen is 14.52 before 8 h but then increases to 120.23 µmol g-1 h-1 after 8 h. The rate change of the two processes confirms the competition between the H2O splitting and CO2 reduction reactions. Band structure and surface characteristics of the SrTiO3 submicron cubes were characterized by diffuse reflective UV-Vis spectroscopy, Mott-Schottky analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results reveal that the simultaneous and competitive production of methane and hydrogen is due to a thermodynamics factor, as well as the competition between the adsorption of carbon dioxide and water molecules on the surface of the faceted SrTiO3. This work demonstrates that SrTiO3 photocatalysts are efficient in producing sustainable fuels via water splitting and carbon dioxide reduction reactions.

The utility of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) in surface and in situ studies: new data processing and presentation approach.

Infrared spectroscopy is a powerful non-destructive technique for the identification and quantification of organic molecules widely used in scientific studies. For many years, efforts have been made to adopt this technique for the in situ monitoring of reactions. From these efforts, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was developed three decades ago. Unfortunately, because of the complexity of data processing and interpretation, PM-IRRAS had been avoided in lieu of the single potential alteration infrared spectroscopy (SPAIRS) and subtractively normalized interfacial Fourier transform infrared (SNIFTIR). In this work, we present a new approach for PM-IRRAS data processing and presentation, which provides more insight into in situ and surface studies besides dramatically improving the S/N. In this new approach, we recommend three complementary methods of data treatment (eqn (7), (9) and (10)) as the new protocols for presenting PM-IRRAS data. These equations are robust in visualising the surface processes at the solid-liquid and solid-gas interphases. Eqn (7) contrasts the surface adsorbed species with respect to the isotropic background with or without the influence of the applied potential. Eqn (9) highlights the surface potential-driven changes between the sample and the reference spectra. Eqn (10) focuses on the bulk-phase (solution/gas and surface species) potential-driven changes between the sample and the reference spectra, and hence it can be used to track the production of species, which desorb from the surface upon their formation. Examples of ethanol electro-oxidation reaction are provided as a test system for in situ studies and PVP deposited on glassy carbon for thin-film studies to illustrate the utility of the new PM-IRRAS data handling protocol, which is poised to improve the understanding of the chemistry and physics of surface processes.

Atomic Layer Deposition of Pd Nanoparticles on TiO2 Nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties.

Palladium nanoparticles are grown on TiO2 nanotubes by atomic layer deposition (ALD), and the resulting three-dimensional nanostructured catalysts are studied for ethanol electrooxidation in alkaline media. The morphology, the crystal structure, and the chemical composition of the Pd particles are fully characterized using scanning and transmission electron microscopies, X-ray diffraction, and X-ray photoelectron spectroscopy. The characterization revealed that the deposition proceeds onto the entire surface of the TiO2 nanotubes leading to the formation of well-defined and highly dispersed Pd nanoparticles. The electrooxidation of ethanol on Pd clusters deposited on TiO2 nanotubes shows not only a direct correlation between the catalytic activity and the particle size but also a steep increase of the response due to the enhancement of the metal-support interaction when the crystal structure of the TiO2 nanotubes is modified by annealing at 450 °C in air.

3D-nanoarchitectured Pd/Ni catalysts prepared by atomic layer deposition for the electrooxidation of formic acid.

Three-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.

Kinetic study of electro-Fenton oxidation of azo dyes on boron-doped diamond electrode.

The present work compares electrochemical degradation of red and blue azo textile dyes in single- and two-compartment electrochemical cells in the presence of Fenton reagent (Fe2+) and using a boron-doped diamond anode. Degradation of both dyes was related to the concentration of dye, applied current density and the concentration of FeSO4 catalyst. Complete colour removal and approximately 91% of organic matter oxidation was achieved in a two-compartment electrochemical cell at an applied current density of 20 mA x cm(-2), pH of 3 and Fe(2+) ion concentration of 0.02 mM. Higher current density and reaction time were required to achieve the same removals in a one-compartment electrochemical cell. Dye degradation kinetics as well as chemical oxygen demand removal rate were successfully modelled to pseudo first-order kinetics. The apparent first-order rate constants (k(o)) for degradation of red dye with an initial concentration of 20, 40 and 60 ppm were found to be 2.67 +/- 0.16, 2.19 +/- 0.09 and 1.5 +/- 0.03 min(-1), and for blue dye at the same initial concentrations were 1.99 +/- 0.2, 0.95 +/- 0.02 and 0.71 +/- 0.030 min(-1), respectively.

Electro-oxidation of two reactive azo dyes on boron-doped diamond electrode.

Electrochemical oxidation (decolorization/degradation) of blue and red commercial reactive azo dyes was carried out on boron-doped diamond (BDD) electrode. The effect of various quantities of FeSO(4) was investigated in the electro-Fenton reaction on BDD. Progress of dyes degradation during the electrolysis and electro-Fenton reaction was monitored by UV-visible absorption and by estimation of the chemical oxygen demand (COD). Direct electrolysis showed a limiting capacity for red and blue dye removal even at high current densities, e.g. maximum red color and COD removal were 70 and 20%, respectively at 30 mA cm(-2) after 300 min. Higher red and blue color removal efficiencies were achieved by electro-Fenton oxidation. Current density of 30 mA cm(-2) in the presence of 0.05 mmol/L of FeSO(4) resulted in the red color and COD removal of 98 and 96%, respectively. The optimum FeSO(4) concentration for the electro-Fenton reaction was determined to be 0.05 mmol/L. Instantaneous current efficiency (ICE) in the presence of FeSO(4) was almost three times higher than for experiments carried out without FeSO(4).