Influence of different carbon nanostructures on the electrocatalytic activity and stability of Pt supported electrocatalysts

Commercially available graphitized carbon nanofibers and multi-walled carbon nanotubes, two carbon materials with very different structure, have been functionalized in a nitric–sulfuric acid mixture. Further on, the materials have been platinized by a microwave assisted polyol method. The relative degree of graphitization has been estimated by means of Raman spectroscopy and X-ray diffraction while the relative concentration of oxygen containing groups has been estimated by X-ray photoelectron spectroscopy, which resulted in a graphitic character trend: Pt/GNF > Pt/F-GNF ? Pt/MWCNT > Pt/F-MWCNT. Transmission electron microscopy showed that the Pt particle size is around 3 nm for all samples, which was similar to the crystallite size obtained by X-ray diffraction. The activity towards electrochemical reduction of oxygen has been quantified using the thin-film rotating disk electrode, which has shown that all the samples have a better activity than the commercially available electrocatalysts. The trend obtained for the graphitic character maintained for the electrochemical activity, while the reverse trend has been obtained for the accelerated ageing test. Long-term potential cycling has demonstrated that the functionalization improves the stability for multi-walled carbon nanotubes, at the cost of decreased activity.     

Various strategies for reducing Nox emissions of biodiesel fuel used in conventional diesel engines: A review

Energy demand, decreasing fossil fuel reserves, and health-related issues about pollutants have led researchers to search for renewable alternative fuels to either partially or fully replace fossil fuels. Among many alternative fuels, biodiesel became one of the most popular choices due to similar properties to that of conventional diesel. Biodiesel produces slightly lower brake thermal efficiency compared to that of conventional biodiesel, but has an advantage of reduced emissions of CO₂, CO, HC, and smoke. However, biodiesel shows higher NOx emission which, when used in increased biodiesel market, may become a serious problem. Various strategies were attempted by different researcher to reduce NOx emissions. In this paper, various strategies, adapted for reducing NOx emissions of biodiesel fuel used in diesel engines for automobile applications, are reviewed and discussed. The strategies are grouped into three major groups, namely combustion treatments, exhaust after-treatments, and fuel treatments. Among various strategies discussed, fuel treatments, such as low temperature combustion, mixing fuel additives and reformulating fuel composition, reduce NOx emission without compromising other emission and performance characteristics and they seem to be promising for future biodiesel fuel.     

Multivariate methods using mixtures: Correspondence analysis,scaling and pattern-detection

Matrices of binary or count data are modelled under a unified statistical framework using finite mixtures to group the rows and/or columns. These likelihood-based one-mode and two-mode fuzzy clusterings provide maximum likelihood estimation of parameters and the options of using likelihood ratio tests or information criteria for model comparison. Geometric developments focused on pattern detection give likelihood-based analogues of various techniques in multivariate analysis, including multidimensional scaling, association analysis, ordination, correspondence analysis, and the construction of biplots. Illustrative examples demonstrate the effectiveness of these visualisations for identifying patterns of ecological significance (e.g. abrupt versus slow species turnover).     

Physico-chemical concept of drag reduction nature in dilute polymer solutions (the Toms effect)

The physicochemical concept of turbulent drag reduction (the Toms effect) integrates physicochemical characteristics of polymer solutions with hydrodynamic and rheological flow parameters into a generalized equation, where the increment in volumetric flow rate Q_P is a function of the external shear stress τ_w, temperature, volume of macromolecular coils with immobilized solvent V_c and a function of their volume fraction Ψ = C · [η]/(1 + C · [η]). The Q_P depends on the coil intrinsic elasticity [G] = kT/V_c as well. This model allows one: (1) to describe the Toms effect in terms of useful elastic work spent by macromolecular coils with immobilized solvent to overcome the frictional forces (i.e. the forces of intermolecular interactions), (2) to forecast the initial conditions of the Toms effect (τ_* ≈ (RT)/(M · [η])) and (3) to explain the unusual temperature dependence of the polymer solutions flow.     

Highly active and stable bi-functional NiCoO2 catalyst for oxygen reduction and oxygen evolution reactions in alkaline medium

Single step solution combustion technique was used to synthesize NiO, Co₃O₄ and NiCoO₂ mixed metal oxide with good crystallinity and uniform properties. XRD spectrum indicates the existence of cubic NiCoO₂ phase without any impurities. SEM results indicate the presence of porous structures in all the three cases, a typical characteristic of combustion synthesized samples, which is due to the evolution of gases during the synthesis process. TEM along with the phase mapping shows the presence of well dispersed elements Ni, Co and O throughout the sample. All the three catalysts were evaluated for their bifunctionality towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline medium. NiCoO₂ shows the highest number of electron transfer in the overall reaction mechanism with the maximum kinetic current density of 12.3 mA/cm². The kinetics of NiCoO₂ towards ORR and OER was analyzed using Tafel plot and compared with the mono-metal oxides. The catalytic stability was evaluated for 24 h using continuous chronoamperometric (CA) runs, where NiCoO₂ shows exceptionally stable performance without any significant decay in current. The highest activity of NiCoO₂ could be due to the presence of higher oxidation states of Ni and Co and because of the existence of the oxygen defects acting as active sites for the oxygen adsorption/desorption during the electrocatalytic reactions. Based on the activity and stability trends, NiCoO₂ is found to be a promising bifunctional oxygen electrocatalyst for long-term applications.     

Proton reduction by a Ni(II) catalyst and foot-of-the wave analysis for H2 evolution

A nickel based molecular catalyst [Ni(QCl-tpy)₂]Cl₂·7H₂O (where QCl-tpy = 2-choloro-3-(2,6-di (pyridin-2yl)pyridine-4-yl) quinoline) has been synthesized, characterized by single crystal XRD and other spectroscopic techniques. The complex [Ni^II(QCl-tpy)₂]²⁺ has also been employed for the electrocatalytic proton reduction in DMF/H₂O (95:5 v/v) using trifluoro acetic acid (TFA) as the proton source. It exhibits a reasonably efficient catalytic ability towards proton reduction under organic media. Compared to the parent [Ni^II(tpy)₂]²⁺ during the electro-catalysis, the complex [Ni^II(QCl-tpy)₂]²⁺ behaves as a better catalyst in terms of higher catalytic current and 180 mV of lower overpotential as well. It is expected due to the presence of 2-chloroqinoline moiety in the terpyridine framework. The rate of H₂ evolution was analysed with the use of Foot-of-the Wave Analysis (FOWA) method. The complex shows a TOF of 3.68 s⁻¹ as obtained from Foot-of-the Wave Analysis (FOWA) at the scan rate 100 mVs⁻¹ for 1.0 mM [Ni^II(QCl-tpy)₂]²⁺ complex. The acid base equilibria reveals the dechelation followed by protonation at one of the coordinated pyridine rings of the QCl-tpy ligand. There could be a pendant base effect towards hydrogen evolution due to dechelated pyridine ring of the coordinated QCl-tpy ligand, which acts as a proton relay. Based on the spectroscopic evidence and electrochemical studies a plausible mechanism for the reduction of proton to H₂ has been proposed.     

Spinel CoFe2O4 supported by three dimensional graphene as high-performance bi-functional electrocatalysts for oxygen reduction and evolution reaction

Spinel CoFe₂O₄ supported on three dimensional graphene (3DG) is prepared by hydrothermal reaction, which is denoted as CoFe₂O₄/3DG. The 3DG is prepared by the templated method, where coal tar pitch (CTP) and MgO are used as the carbon source and the template, respectively. The microstructure and composition of the resultant have been investigated by X-ray diffraction as well as X-ray photoelectron spectroscopy indicating the formation of spinel CoFe₂O₄ and composite of CoFe₂O₄/3DG. The multilayer structure of 3DG and CoFe₂O₄/3DG is also examined by the Raman spectra. Electrochemically, CoFe₂O₄/3DG shows high-performance half-wave potential is 0.80 V vs. RHE in O₂-saturated 0.1 M KOH, which is compared to 20 wt% Pt/C. When evaluated for OER activity, CoFe₂O₄/3DG obtains a low overpotential 1.63 V vs. RHE (at j = 10 mA cm⁻²), which is 180 mV better than 20 wt% Pt/C. Moreover, it possesses excellent durability superior to 20 wt% Pt/C.     

Enhanced electrocatalytic performance of Cu0.02S0.01/rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature

Graphene, is a carbon allotrope, which is widely used as a substrate for various catalysts due to its interesting physicochemical properties. In the present study, graphene oxide sheets were prepared from graphite, then, the graphene oxide surface was modified by a low-temperature method using sulfur and copper atoms to obtain pseudo-enzyme Cu/S/Graphene prosthetic group. The current density passing through Cu/S/Graphene catalyst was four times higher than that passing through graphite. The novel copper-based catalyst had an extraordinary performance for oxygen reduction reaction (ORR) due to the unique bio-inspired and stoichiometric structure. The results of Raman and Dispersive X-ray spectroscopy confirmed the presence of ultra-low content of copper (2%) and sulfur (1%) atoms on the graphene surface. Thermogravimetric analysis indicated a strong interaction between nanoparticles and graphene layers. The number of electrons transferred for ORR varied from 3.98 to 4.16 in a wide range of over-potentials indicating an effective 4-electron pathway form O₂ to H₂O. The Tafel slopes indicated insignificant amount of formed copper oxide on the catalyst surface. The catalyst showed excellent electrochemical durability and its half-wave potential (E_1/2) was exhibited a negative shift only 8.2 mV after 10000 cycles.