N.R. Elezović

Platinum nanocatalysts on metal oxide based supports for low temperature fuel cell applications

In this manuscript a survey of the contemporary research related to platinum nanocatalysts on metal oxide based supports for low temperature fuel cell applications is presented. Different carbon based supports, used as state of the art materials, are listed and discussed, as well. Although carbon based materials possess many desirable properties, such as high surface area, high conductivity and relatively low cost and easy synthesis, the large scale commercialization is limited by instability under accelerated stability testing, simulating real fuel cell operating conditions. To overcome these disadvantages of carbon supports, different metal oxide based ones have been studied and promising results are referenced. The most often used oxide based supports for low temperature fuel cell applications are presented in this review. Suitable discussion and future research related remarks are given, as well.

Pt nanoparticles on tin oxide based support as a beneficial catalyst for oxygen reduction in alkaline solutions

A platinum nanocatalyst on Sb doped tin oxide support (Sb–SnO2) was synthesized and characterized as a catalyst for oxygen reduction reaction in 0.1 mol dm−3 NaOH solution at 25 °C. Sb (5%) doped tin oxide support was synthesized by a modified hydrazine reduction procedure. The platinum nanocatalyst (20% Pt) on Sb–SnO2 support was synthesized by a borohydride reduction method. The synthesized support and catalyst were characterized by high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) and X-ray diffraction technique (XRD). X-ray photoelectron spectroscopy was applied to characterize the chemical status of elements before and after Pt-treatment. XPS spectra of Sn 3d, Pt 4f, Sb 3d and O 1s revealed that the Pt-deposition on Sb–SnO2 support induced the reduction of the Sn(4+) oxidation state to Sn(2+) and Sn(0) states, while Pt remained in the metallic state and Sb was in the (3+) oxidation state. Homogenous Pt nanoparticle distribution over the support, without pronounced particle agglomeration, was confirmed by HRTEM technique. The average Pt particle size was 2.9 nm. The electrochemically active Pt surface area of the catalyst was determined by the integration of the cyclic voltammetry curve in the potential region of underpotential deposition of hydrogen, after double layer charge correction, taking into account the reference value of 210 μC cm−2 for full monolayer coverage. This calculation gave the value of 51 m2 g−1. The kinetics of the oxygen reduction reaction with Pt/Sb–SnO2 catalyst was studied by cyclic voltammetry and linear sweep voltammetry using a rotating gold disc electrode. Two different Tafel slopes were observed: one close to 60 mV dec−1 in the low current density region, and another at ∼120 mV dec−1 in the higher current densities region, as was already referred in previous reports for the oxygen reduction reaction with polycrystalline Pt, as well as with different Pt based nanocatalysts. The specific activities for oxygen reduction, expressed in terms of kinetic current densities per electrochemically Pt active surface area, as well as per mass of Pt loaded, at the constant potential of practical interest (0.85 V and 0.90 V vs. RHE), were compared to a carbon supported (Vulcan XC-72) catalyst. The Pt/Sb–SnO2 catalyst exhibited similar catalytic activity for oxygen reduction reaction like carbon supported one. The advantages of the carbon free support application in terms of the durability and stability of the catalysts were proved by accelerated stability tests.

Fe-Mo alloy coatings as cathodes for chlorate production

The aim of this study was to gain a better understanding of the feasibility of partial replacement of dichromate, Cr(VI), with phosphate buffer, focusing on the cathode reaction selectivity for hydrogen evolution on mild steel and Fe-Mo cathodes in undivided cell for chlorate production. To evaluate the ability of phosphate and Cr(VI) additions to hinder hypochlorite and chlorate reduction, overall current efficiency (CE) measurements in laboratory cell for chlorate production on stationary electrodes were performed. The concentration of hypochlorite was determined by a conventional potentiometric titration method using 0.01 mol dm-3 As2O3 solution as a titrant. The chlorate concentration was determined by excess of 1.0 mol dm-3 As2O3 solution and excess of arsenic oxide was titrated with 0.1 mol dm-3 KBrO3 solution in a strong acidic solution. Cathodic hypochlorite and chlorate reduction were suppressed efficiently by addition of 3 g dm-3 dichromate at both cathodes, except that Fe-Mo cathode exhibited higher catalytic activity for hydrogen evolution reaction (HER). The overvoltage for the HER was around 0.17 V lower on Fe-Mo cathode than on mild steel at the current density of 3 kA m-2. It was found that a dichromate content as low as 0.1 g dm-3 is sufficient for complete suppression of cathodic hypochlorite and chlorate reduction onto Fe-Mo catalyst in phosphate buffering system (3 g dm-3 Na2HPO4 + NaH2PO4). The overall current efficiency was practically the same as in the case of the presence of 3 g dm-3 dichromate buffer (98 %). However, for the mild steel cathode, the overall current efficiency for the chlorate production was somewhat lower in the above mentioned mixed phosphate + dichromate buffering system (95%) than in the pure dichromate buffering solution (97.5%).

Electrocatalytic Activity of Nano-Sized Ebonex/Pt for Underpotential Deposition of Hydrogen

The underpotential deposition of hydrogen was studied in 0.5 mol dm-3 HClO4 solution on an electrode based on Ebonex-supported platinum electrocatalyst spread on rotation Au disk electrode (Ebonex/Pt). Pt catalyst was prepared by the impregnation method from 2-propanol solution of Pt(NH3)2(NO2)2 and Ebonex powder. Ebonex support (nonstoichiometric mixture of titanium oxides) was characterized by: X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and BET techniques. The synthesized catalyst was analyzed by TEM technique. Voltammetric profiles at the Ebonex/Pt catalyst surface in 0.5 mol dm-3 HClO4 aqueous solution obtained at different temperatures with the evaluation of the temperature effect on the reversible adsorption of the Hupd state are presented and the thermodynamic state functions for H adatom adsorption process are calculated. The chemisorptive energy strength of the Ebonex/Pt-H state is estimated in order to establish the relationship between the surface structure and the electrocatalytic activity of Ebonex/Pt electrode and compare it to the one for smooth polycrystalline Pt.