Cinthia Alegre

Towards new generation fuel cell electrocatalysts based on xerogel–nanofiber carbon composites

Xerogel–nanofiber carbon composites (XNCCs) have been easily synthesized by using a Ni catalyst supported on carbon xerogel (CXG), growing randomly oriented carbon nanofibers (CNFs) within the coralline-like structure of the xerogel (CXG). This novel composite combines the advantages of xerogel and fiber nanostructures. The interactions between these phases as well as their effect as a support on Pt electrocatalysts for the oxygen reduction reaction (ORR) have been investigated. Platinum catalysts supported on different XNCCs (varying in terms of CXG and CNF contents) as well as on bare CXG and CNFs have been synthesized using a microemulsion route. They have been characterized in terms of structure, morphology and porosity and investigated for the ORR in a half-cell configuration. The catalyst supported on the XNCC with a 44% CNF content shows the best electrochemical behavior. This catalyst formulation leads to a catalytic activity 5 times higher than that obtained on a Vulcan-based catalyst at low overpotential and 2.5 times higher at large overpotential. Accelerated degradation tests also show better stability for the composite support-based catalyst. Compared to bare CNF and CXG supports, a stabilization effect is envisaged by the presence of highly graphitic CNFs within the composite structure.

Investigation of Supported Pd-Based Electrocatalysts for the Oxygen Reduction Reaction: Performance, Durability and Methanol Tolerance

Next generation cathode catalysts for direct methanol fuel cells (DMFCs) must have high catalytic activity for the oxygen reduction reaction (ORR), a lower cost than benchmark Pt catalysts, and high stability and high tolerance to permeated methanol. In this study, palladium catalysts supported on titanium suboxides (Pd/TinO2n–1) were prepared by the sulphite complex route. The aim was to improve methanol tolerance and lower the cost associated with the noble metal while enhancing the stability through the use of titanium-based support; 30% Pd/Ketjenblack (Pd/KB) and 30% Pd/Vulcan (Pd/Vul) were also synthesized for comparison, using the same methodology. The catalysts were ex-situ characterized by physico-chemical analysis and investigated for the ORR to evaluate their activity, stability, and methanol tolerance properties. The Pd/KB catalyst showed the highest activity towards the ORR in perchloric acid solution. All Pd-based catalysts showed suitable tolerance to methanol poisoning, leading to higher ORR activity than a benchmark Pt/C catalyst in the presence of low methanol concentration. Among them, the Pd/TinO2n–1 catalyst showed a very promising stability compared to carbon-supported Pd samples in an accelerated degradation test of 1000 potential cycles. These results indicate good perspectives for the application of Pd/TinO2n–1 catalysts in DMFC cathodes.

Investigation of the activity and stability of Pd-based catalysts towards the oxygen reduction (ORR) and evolution reactions (OER) in iron–air batteries

Pd-based catalysts supported on commercial carbon supports (Ketjenblack and Vulcan) have been studied for both the ORR and the OER. Pd proved to be a suitable bi-functional catalyst for the ORR and the OER, with Vulcan as the carbon support providing the highest stability during accelerated degradation tests.

Platinum Ruthenium Catalysts Supported on Carbon Xerogel for Methanol Electro‐Oxidation: Influence of the Catalyst Synthesis Method

A high surface area, highly mesoporous carbon xerogel was synthesised and used as a support in the preparation of platinum–ruthenium catalysts by different synthetic routes. The platinum–ruthenium carbon xerogel catalysts were physico‐chemically characterised and used for the chemical electro‐oxidation of methanol. The synthetic routes pursued included: 1) impregnation with metal chloride precursors and reduction with two different reducing agents: sodium borohydride and formic acid; 2) a microemulsion‐based method and 3) a sulfite complex method, which led to catalysts with different physico‐chemical features that strongly influence their catalytic behaviour towards methanol oxidation. The electro‐oxidation of methanol was found to depend on both the crystal size and the extent of active phase reduction as well as on the platinum concentration on the catalyst surface, which were maximised for the impregnation method and reduction with formic acid.

N-Doped Carbon Xerogels as Pt Support for the Electro-Reduction of Oxygen

Durability and limited catalytic activity are key impediments to the commercialization of polymer electrolyte fuel cells. Carbon materials employed as catalyst support can be doped with different heteroatoms, like nitrogen, to improve both catalytic activity and durability. Carbon xerogels are nanoporous carbons that can be easily synthesized in order to obtain N-doped materials. In the present work, we introduced melamine as a carbon xerogel precursor together with resorcinol for an effective in-situ N doping (3–4 wt % N). Pt nanoparticles were supported on nitrogen-doped carbon xerogels and their activity for the oxygen reduction reaction (ORR) was evaluated in acid media along with their stability. Results provide new evidences of the type of N groups aiding the activity of Pt for the ORR and of a remarkable stability for N-doped carbon-supported Pt catalysts, providing appropriate physico-chemical features.

Nitrogen Doped and Functionalized Carbon Materials as Supports for Catalysts in Electro-Oxidation of Methanol

The objective of this work is to study the behavior of Nitrogen-doped carbons as supports of catalysts for the electro-oxidation of methanol. Two carbon materials have been considered: a) carbon xerogels (CXG), highly mesoporous, whose porosity and pore size distribution are easily performed during the synthesis method; b) carbon nanofibers (CNF), which have a high electrical conductivity, good behavior in high temperature conditions and resistance to acid/basic media. Meanwhile, a commercial carbon black (Vulcan XC72R) which is commonly used in manufacturing of electrocatalysts fuel cells was used for comparison. Nitrogen was introduced into the CXG during the synthesis process, what is commonly referred as doping, by including melamine as a reactant. In contrast, N-groups were created over CNF by post-treatment with: ammonia (25%), urea (98%), melamine (99%) and ethylenediamine (99.5%), with a carbon: nitrogen molar ratio 1:0.6. N-containing carbon materials were characterized by elemental analysis, nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), SEM-EDX and TEM to determinate the amount and forms of nitrogen introduced. Pt-catalysts were prepared by the microemulsion method. The influence of the nitrogen doping and functionalization on the catalytic behavior in the electrochemical oxidation of methanol was evaluated by different physicochemical and electrochemical analysis.

Influence of Synthesis pH on Textural Properties of Carbon Xerogels as Supports for Pt/CXs Catalysts for Direct Methanol Fuel Cells

Carbon xerogels (CXs) have been prepared by polycondensation of resorcinol and formaldehyde. Two synthesis pHs were studied in order to evaluate its influence on the electrochemical behaviour of Pt catalysts supported on previous carbon xerogels, synthesized by conventional impregnation method. Catalysts were also synthesized over a commercial carbon black (Vulcan-XC-72R) for comparison purposes. Characterization techniques included nitrogen physisorption, scanning electron microscopy, and X-ray diffraction. Catalysts electrochemical activity towards the oxidation of carbon monoxide and methanol was studied by cyclic voltammetry and chronoamperometry to establish the effect of the carbon support on the catalysts performance. Commercial Pt/C catalyst (E-TEK) was analyzed for comparison purposes. It was observed that the more developed and mesopore-enriched porous structure of the carbon xerogel synthesized at a higher initial pH resulted in an optimal utilization of the active phase and in an enhanced and promising catalytic activity in the electrooxidation of methanol, in comparison with commercial catalysts.