Kinetic effect of the ionomer on the oxygen reduction in carbon-supported Pt electrocatalysts

The mechanism of the oxygen reduction reaction (ORR) on nanoparticulated Pt/C-Nafion electrodes prepared in one step has been studied to simulate the reaction in the cathode of a Polymer Electrolyte Fuel Cell (PEFC). The kinetic parameters have been obtained by hydrodynamic polarization in O₂-saturated 0.01–1.00 M H₂SO₄ and temperatures in the range 25.0–50.0 °C. The ORR current density was maximum and practically independent of the ionomer fraction in the rage 10–55 wt% Nafion. The poorer proton conductivity for lower Nafion fractions and the formation of catalyst areas completely surrounded by Nafion together with adsorption of Pt sites by sulfonate groups for higher Nafion fractions, explain the minor ORR activity in these conditions. The ionomer influence on the O₂ diffusion at high overpotentials for Pt/C-Nafion was negligible when the Nafion content was smaller than 20 wt%. The higher kinetic current density for Pt/C-Nafion (100 mA cm⁻²) with respect to smooth Pt-Nafion (40 mA cm⁻²), together with the smaller activation energy of the former (25 ± 4 kJ mol⁻¹) with respect to the latter (42 ± 5 kJ mol⁻¹) highlighted the better properties attained by the nanosize effect. A remarkable novel result is that the reaction order of H⁺ in HClO₄ is close to unity, whereas in sulfuric acid it is significantly smaller and changes with potential, what has been related to the sulfate adsorption. The anomalous dependence of the charge transfer coefficient with temperature was then explained by the thermal change of the double layer structure and the variation of the coverage of adsorbed species on Pt. The more sensitive effect for Pt/C-Nafion than for smooth Pt-Nafion was ascribed to the stronger interaction between the components when the nanoparticles are involved.     

The influence of resorcinol on the kinetics of underpotentially deposited hydrogen,cathodic hydrogen and anodic oxygen evolution reactions,examined at polycrystalline Pt electrode in 0.1 M NaOH solution

This work presents ac. impedance spectroscopy and cyclic voltammetry electrochemical examinations on the influence of surface-adsorbed resorcinol (RC) or its oxidation/reduction derivatives on the kinetics of underpotential deposition of hydrogen (UPD of H), cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) processes, examined at polycrystalline Pt electrode in 0.1 M NaOH supporting electrolyte. The recorded data provided evidence on the beneficial role of Pt-electrosorbed resorcinol molecules on the kinetics of UPD of H. On the other hand, the effect of surface-adsorbed RC species (or its electro-oxidation intermediates) on the OER's kinetics was purely detrimental, whereas the resorcinol effect on the cathodic HER was hardly discernible. The above was all elucidated through evaluation of the ac. Impedance-associated parameters and cyclic voltammetry profiles recorded for RC-free and resorcinol-modified NaOH electrolytes.     

Carbon paper supported Pt/Au catalysts prepared via Cu underpotential deposition-redox replacement and investigation of their electrocatalytic activity for methanol oxidation and oxygen reduction reactions

Through a simple and rapid method, carbon papers (CPs) were coated with Au and the resulting Au/CP substrates were used for the preparation of Pt/Au/CP by Cu underpotential deposition (Cu UPD) and redox replacement technique. A series of Pt_n/Au/CP catalysts (where n = number of UPD-redox replacement cycles) were synthesized and their electrochemical properties for methanol oxidation reaction (MOR), and oxygen reduction reaction (ORR) were investigated by electrochemical measurements. The Pt_n/Au/CP electrodes show higher electrocatalytic activity and enhanced poison tolerance for the MOR as compared to a commercial Pt/C on CP (Pt/C/CP). The highest mass specific activity and Pt utilization efficiency for MOR was observed on Pt₁/Au/CP with a thickness close to a monatomic Pt layer. Chronoamperometric tests in methanol solution revealed that Pt_n/Au/CPs have much higher CO tolerance compared to Pt/C/CP. Among the Pt_n/Au/CPs, CO tolerance decreases with increasing the amount of deposited Pt, indicating that the exposed Au atoms in close proximity to Pt plays a positive role against CO poisoning. Compared with the Pt/C/CP, all the Pt_n/Au/CP electrodes show more positive onset potentials and lower overpotentials for ORR. For instance, the onset potential of ORR is 150 mV more positive and the overpotential is ∼140 mV lower on Pt₄/Au/CP with respect to Pt/C/CP.     

The effect of heteroatoms doping for the Pt supported graphene hollow spheres on electrocatalytic properties towards oxygen reduction and hydrogen evolution reaction

Noble metal Pt is the acknowledged efficient catalyst for oxygen reduction (ORR) and hydrogen evolution reaction (HER) in commercial applications. However, due to its high price and limited reserves, its large-scale application is limited. In order to overcome this defect, the loaded Pt nanoparticles (NPs) should be small and dispersed efficiently through the design of electrode materials, so as to improve the utilization efficiency of Pt. In addition, the introduction of non-noble metal active sites can reduce the consumption of Pt efficiently. In this work, hollow graphene spheres are used as the carrier and the heteroatoms (N, Fe and Co) are introduced. The results show that the introduction of Fe and Co can form very effective heteroatom active sites (carbon encapsulated Fe/Co metals and FeCo alloy, and/or metal nitrides Fe/Co-N_x-C) in the substrate material, which improve the catalytic activity of the electrode material effectively and the utilization efficiency of Pt. In addition, the generation of Fe/Co-N_x-C active sites and the loading of Pt are also closely related to the doped N atoms. The onset potential, limiting current density (J_L), half-wave potential (E_1/2) and Tafel slope of sample FeCo-N_xHGSs/Pt (10 wt%) can exceed or comparable to those of commercial catalysts Pt/C (20 wt%) towards ORR both in acid and alkaline electrolyte. Moreover, the values of η₁₀₀ and the Tafel slope for FeCo-N_xHGSs/Pt towards HER can also exceed the commercial catalysts Pt/C (20 wt%) in acid and alkaline electrolytes. The purpose of reducing the usage amount of precious metals without reducing the catalytic performance is realized. The relationship between the ORR and HER performance of the resultant electrode catalyst and the doped heteroatoms, such as nitrogen (N), iron (Fe) and cobalt (Co) atoms, was studied and discussed in detail.     

One-step electroless deposition of Pd/Pt bimetallic microstructures by galvanic replacement on copper substrate and investigation of its performance for the hydrogen evolution reaction

A facile and one-step method for fabrication of Pd/Pt bimetallic microstructure using galvanic replacement reaction is presented. This electroless deposition was performed without any additive reagent via simple immersion of the copper sheet in cation aqueous solution of Pd and Pt. The as-prepared electrode was characterized by using the techniques of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and cyclic voltammetry and tested for the hydrogen evolution reaction (HER) in the acidic media. Comparison of the HER on the Pd/Pt bimetallic catalysts with different Pd:Pt percentage compositions indicated that the Pd₆₀Pt₄₀ catalyst had the highest HER activity among all the Pd/Pt catalysts and a better performance than the pure Pt. The effects of galvanic replacement time and concentration of H₂SO₄ on the catalytic activity of as-prepared electrode for HER were comparatively investigated.     

Tailoring the hydrophilic and hydrophobic reaction fields of the electrode interface on single crystal Pt electrodes for hydrogen evolution/oxidation reactions

Interfacial hydrophobic/hydrophilic reaction fields significantly affect various reactions at the electrode surface. The hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR) have been investigated on single crystal Pt electrodes modified with hydrophobic/hydrophilic cations and anion-exchange copolymers in alkaline solutions. In alkali metal hydroxide solutions, Pt (110) exhibits the highest HER/HOR activity in the low-index planes of Pt. On the low-index planes of Pt, the hydrophilicity of the alkali metal cation in the supporting electrolyte activates the HER/HOR depending on its hydration energy. Hydrophilic cations at the interface facilitate the extraction of hydrogen from the hydrated water. The modification of anion-exchange copolymers with a hydrophobic skeleton on Pt (110) further enhanced the HER/HOR activity. The hydrogen bonding network formed around the hydrophobic species facilitated the mobility of water molecules and the OH⁻ as the reactant/product of the HER/HOR. Appropriately forming hydrophilic and hydrophobic reaction fields at the interface improved the HER/HOR activity.     

Preparation of a CO-tolerant PtRuxSny/C electrocatalyst with an optimal Ru/Sn ratio by selective Sn-deposition on the surfaces of Pt and Ru

A CO-tolerant PtRu_xSn_y/C electrocatalyst, with an optimal x/y ratio of 0.8/0.2, was prepared by selectively depositing Sn on the metallic surface of PtRu_0.8/C for use as the anode in a polymer electrolyte membrane fuel cell. The CO tolerance of the catalyst was greater when Sn was added by chemical vapor deposition (CVD) than by a conventional precipitation method because most of the Sn added by CVD was located in the vicinity of Pt and Ru surfaces, on which CO molecules were strongly adsorbed. Accordingly, the bi-functional mechanism of CO oxidation, which involved the migration of oxygenated species from the Sn to the adsorbed CO, was expected to be promoted to greater extents in the catalysts prepared by Sn-CVD than those prepared by Sn-precipitation. On the other hand, the ligand-effect mechanism of CO oxidation, which was facilitated by the Pt-Ru alloy formation, was not much affected by the added Sn because the Pt-Ru alloy remained unchanged, particularly when y ≤ 0.2. Among PtRu_xSn_y/C catalysts prepared by Sn-CVD, one prepared by partially substituting Sn for Ru in the PtRu_1.0/C catalyst, e.g., PtRu_0.8Sn_0.2/C, showed higher CO tolerance than one prepared by simply adding Sn to the PtRu_1.0/C catalyst, e.g., PtRu_1.0Sn_0.2/C, which demonstrated the importance of an optimum x/y ratio in the design of the ternary PtRu_xSn_y/C catalysts.     

Study of the electrocatalytic properties of Ir4(CO)12 for the oxygen reduction and hydrogen oxidation reactions,in the absence and presence of fuel cell contaminants

Carbonyl cluster compounds are promising potential substitutes for platinum as catalysts in fuel cells. In this work, we have used Ir₄(CO)₁₂ as a new electrocatalyst to study its activity for the Oxygen Reduction Reaction (ORR) in the absence and presence of methanol in different concentrations (1.0 and 2.0 mol L⁻¹), and the Hydrogen Oxidation Reaction (HOR) with pure hydrogen and hydrogen/carbon monoxide mixtures ([CO] = 100 ppm and 0.5%). This iridium cluster can perform both the ORR and HOR, and it exhibits the important property of being tolerant to the aforementioned contaminants to an important extent, in contrast to platinum, which is easily deactivated even by traces of CO or methanol. The compound was electrochemically studied by rotating disk electrode (RDE) measurements, using the cyclic and linear sweep voltammetry (CV and LSV) techniques. The ORR temperature dependence (293 K–333 K) was studied and kinetic parameters such as the Tafel slope, charge transfer coefficient, exchange current density and activation energy were calculated from the LSV polarization curves. This carbonyl complex is one of the few transition metal clusters whose catalytic activity for these reactions has been reported.