Following the general trend observed in the low-temperature fuel cell research to replace Pt/C catalysts by less costly and more durable compounds, as already exemplified in Pragma’s September Science Note, promising new results point to titanium dioxide.
Current polymer electrolyte fuel cells use platinum and platinum-based alloys supported on nanoporous carbon as electrodes. However, during the duty cycles of repeated start-ups and shut-downs, the fuel cell undergoes high potentials that lead to carbon and Pt degradation processes. In order to maximize catalyst utilization in the electrodes, Pt nanoparticles have been downsized to 2-3 nm. Thermodynamic size effects make them less stable than bulk Pt, which causes the dissolution/sintering into bigger agglomerates in order to restore stability. Meanwhile, the agglomeration process is accelerated by carbon corrosion in oxidative conditions: As a consequence, Pt particles are detached from their support and tend to gather together.
Unfortunately, degradation is fairly rapid under typical fuel cell conditions. Fuel cell performance and lifetime are greatly affected by these concomitant phenomena. Computational modeling shows that degradation can be significantly mitigated by increasing the particle size from 2 to 4-5 nm. Nevertheless, platinum metal is expensive and not very abundant. If current loadings are maintained in fuel cells, catalyst cost and supply will be an overwhelming obstacle for their widespread commercialization. There is an urgent need for both more robust non-platinum catalysts and non-carbon catalyst supports such as metal oxides.
Several studies have recently converged to promote titanium dioxide as alternative catalyst support in a fuel cell. Titanium dioxide is an already widely used semiconductor material with further potential applications in solar cells, biotechnology, photocatalysis, and gas sensors. Despite a high mechanical resistance and stability in acidic and oxidative environments that have raised interest for TiO2 in fuel cells for a while now, its low electronic conductivity has prevented so far any application. But a first milestone seems to have been reached lately, as shown below. This should at least address the stability issue with carbon-supported Pt catalysts.
At the Universities of Erlangen-Nürnberg (Germany) and Turku (Finnland), researchers have successfully imparted semimetal conductivity to TiO2 nanotubes through carbonization in acetylene gas atmosphere at 850°C. While carbonization forms a new carbon-containing titanium oxy-carbide compound, the nanotube structure is hardly altered. The compound has been identified as a solid solution between TiCx and TiOx rather than C-doped TiO2. It exhibits high electronic conductivity similar to metals and a much superior mechanical hardness thanks to its titanium carbide content. Together with very good electrochemical properties, these new conductive titanium oxy-carbide nanotubes show great promise especially for DMFC applications: when introduced as support for Pt and Pt-Ru anode catalysts they are claimed to increase the activity for methanol oxidation by 700%. [P. Schmuki et al., Angewandte Chemie International Edition, 48 (2009), 7236-7239].
At the University of South Carolina, the structure of titanium dioxide and the “wet” synthesis method was quite different from above, but results proved promising as well. In this study, mesoporous TiO2 was prepared via a template-assisted route where the porosity was controlled by the hydrolysis reaction of the dissolved titanium precursor in presence of a surfactant. Colloidal Pt particles were synthesized separately, and the mixture Pt(coll)+TiO2 was finally carried out in presence of a reducing agent.
The novel Pt/TiO2 catalyst was tested as cathode in a PEMFC: it showed performances comparable to or even better than Pt/C at the same loading (0.4 mg/cm²), which was attributed to improved mass transport in the thinner cathode layer. In addition, the accelerated test protocol showed an extremely high stability toward oxidation conditions: zero decrease in performance was observed after a corrosion time of 200 h. Since the Pt particle size was initially larger when supported on TiO2 than on C and the corrosion resistance of titanium dioxide is better than carbon, the decay of the active catalyst surface is strongly reduced. A strong metal support interaction between the Pt particles and the TiO2 support is also assumed to inhibit the catalyst migration and agglomeration, therefore enhancing the overall stability of the cathode. Based on these results, mesoporous TiO2 should be considered as an alternative support for Pt in fuel cells [S.-Y. Huang et al., Journal of the American Chemical Society, 131 (2009), 13898-13899].
Dr. Catherine Lepiller
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