6 ANÁLISIS ECONÓMICO
6.3 COSTO TOTAL DEL PROYECTO
Besides the catalyst particles, also the catalyst support material can suffer from degradation with ongoing fuel cell operation.[3,54] The turbostratic carbon used as catalyst support material can get oxidized which leads to a detachment of catalyst particles and therefore to a reduced efficiency. Although the oxidation of carbon is kinetically hindered to a great extent, high temperatures and potentials can still lead to carbon corrosion.[55,56] A sign for carbon corrosion is a detectable amount of CO2 in the exhaust gases.[57] Due to this type of degradation non-carbon based materials are investigated as possible candidates to replace HSAC. Several requirements have to be fulfilled by an adequate electrode material. A high electrical conductivity is needed to ensure optimal transport of the electrons to the electrodes and away from them. Furthermore a stable anchoring of the catalyst to the electrode is important to prevent catalyst movement, agglomeration or loss. Besides a high corrosion resistance, a high surface area of the applied electrode material is beneficial to obtain good long-term efficiency. Alternatives to standard carbon material that are currently under investigation are electrodes based on e.g. titanium oxide, silicon oxide and tungsten oxide.[58-60]
Tungsten is a transition element that can form a variety of oxides with different valence states (from −1 to +6). This flexibility makes tungsten suitable for photochromic, electrochromic, photocatalytic, gas sensor and fuel cell applications.[61] Stoichiometric WO3 is a 5d-transition-metal-oxide which exhibits a slightly distorted ABO3 perovskite crystal structure. The W ions occupy the octahedral B cation sites and the A site is vacant.[62,63] This monoclinic crystal structure is formed below 180 °C.[64] Stoichiometric WO3 is an insulator due to the unoccupied 5d-orbitals of the W6+.[60,63] Since a high conductivity is vital for a fuel cell electrode material, tungsten suboxides WO3−x are used.[65] Due to oxygen vacancies pentavalent tungsten atoms are present in the WO3−x which positively influences the electric conductivity of the material.[65-68] In that case, the 5d-orbitals host delocalized electrons which makes WO3−x an n-type semiconductor with a band gap of 2.6–2.8 eV. Further advantages of this material are its high corrosion resistance, high melting point (1700 °C), high tolerance against CO and excellent stability against the electrochemical conditions in a fuel cell. It can be used as catalyst, co-catalyst or catalyst support in fuel cell applications.[58,60,68-71] Additionally, WO3−x can also support the catalyst reaction due to hydrogen spill over when it is used as electrode material at the anode. The phenomenon of hydrogen spill over is related to the formation of tungsten bronzes as first described by Wöhler in 1824.[72] During this process the protons, formed from hydrogen directly at the catalyst, can transfer to the support material WO3−x and form oxyhydroxides as WO3−x(OH)y. Hence the active catalyst sites can receive the next reactant faster. Via the Grotthuss mechanism the adsorbed protons can travel through the anode material to the electrolyte.[73] This mechanism was proven by Tseung et al. in 1997.[74]
2.6
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