Durable Oxide-Based Catalysts for Application as Cathode Materials in Polymer Electrolyte Fuel Cells (PEFCs)

Polymer electrolyte fuel cells (PEFCs) have been the subject of extensive investigations in the last decade as they represent an alternative power source to conventional engines and secondary batteries. At present, one of the main drawbacks hindering PEFCs widespread commercialization is the slow kinetics of the oxygen reduction reaction (ORR) at the cathode and its corrosion stability. Most of the PEFC cathode catalysts are based on Pt or Pt-alloy nanoparticles supported on high-surface area carbons. Despite the good performance as cathode catalyst, Pt-supported on carbon suffers from corrosion stability, being fuel cell lifetime determining. Indeed, in real fuel cell operation, the cathode can reach potentials as high as 1.5 V, which can lead to severe oxidation of the carbon support. Carbon corrosion results in loss of carbon surface area and detachment of the Pt-nanoparticles, and, thus, in loss of the electrode integrity with rapid failure of the fuel cell.[1] Therefore, a growing interest is raising towards alternative, more stable support materials to carbon, such as carbides and oxides. The most research on carbides has been oriented towards tungsten carbide, due to its high electronic conductivity and thermal stability. However, the corrosion stability of, e.g., WC is still controversial, since oxidation of tungsten carbide at high potentials has been recently reported. [2] On the other hand, the use of metal oxides in their highest oxidation state should lead to more stable supports, and based on this idea several investigations on oxide-based catalysts have been reported in the literature in the last years. Thermochemically calculated pHpotential diagrams have shown that only few oxides are stable at pH=0, 80 °C, and with an applied potential of 1.0 V vs. standard hydrogen electrode. [3] In this contribution, theoretical ab-initio DFT calculations have been also performed to calculate the stability potential windows, theoretical Pourbaix diagrams and dynamic modeling of the metal oxide surface changes upon application of different potentials. Among the stable oxides, we have selected tin-based compounds as a novel, durable support for PEMFC catalysts since, besides their stability, they can also achieve high conductivity and they are relatively low-cost materials. Model thin-film electrodes of doped-tin oxide have been fabricated by reactive magnetron sputtering on glassy carbon substrates. A systematical investigation using X-ray photoelectron spectroscopy (XPS) has been carried out to correlate the discharge power and the oxygen content in the plasma to the oxidation state of the tin-based films, which is essential to achieve a stable oxide support in its highest oxidation state. The model thin film electrodes have been also characterized by X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). The electrochemical characterization of the model electrodes was carried out both on pure oxide films and on Pt-supported on the oxides. Cyclic voltammetry and rotating disc electrode measurements were performed in liquid electrolyte to evaluate the electrochemical stability and activity of the oxide-based model electrodes. In addition to thin film model electrodes, doped tin oxide nanopowders have been synthesized by a modified sol gel method. Figure 1 shows the XRD pattern of 5 at% Bi-doped tin oxide powder prepared from aqueous solution containing tin citrate and bismuth nitrate. Chelation of cations is achieved by adding citric acid to the solution and using ethylene glycol as polymerization agent. Once the gel was achieved, it was dried at 150 °C overnight and then calcined at 600 °C for 2 h to obtain single phase oxides. Figure 2 shows a typical SEM micrograph of the calcined Bi-doped tin oxide nanopowder. The oxide powder electrodes were also investigated by XPS in order to correlate the powder processing to the dopant distribution (surface segregation). Finally cyclic voltammetry and rotating disc electrode measurements were performed in liquid electrolyte for both pure oxides and Pt/oxides catalysts.