...to make PEMFCs economically viable, one of the main problems to be solved is finding catalysts with sufficient activity and stability for the oxygen reduction reaction (ORR). To address this problem, a number of methods have been proposed for developing an electrocatalyst with improved performance, including alloying Pt with secondary transition metals or depositing monolayers of Pt onto other fine metal particles to create core−shell structures. Although Pt supported on carbon is still the most widely used catalyst, oxide supports as alternatives to carbon have shown improved corrosion resistance and reduced electrochemically active area (ECA) degradation rates. In addition, oxide supports are also able to affect the electrocatalytic activity of the supported noble metals due to metal−support interactions.

...Oxide suppression has the potential to improve both the ORR activity of Pt by providing a clean surface over a wide potential range and the stability of Pt since oxide species are a key intermediate in the Pt dissolution process. However, raw SnO₂ suffers from two important limitations that may preclude its use as a PEMFC cathode support. First, SnO₂ undergoes redox processes at potentials relevant to the ORR and is at least somewhat unstable. Second, SnO₂ is a wide band gap semiconductor with electrical resistivity varying from 10 to 106 Ω·cm; this high electrical resistance within the catalyst layer would be detrimental to fuel cell performance. One way to overcome these limitations to realize a high stability support is to dope Sn into a high conductivity material with good stability in acid at ORR relevant potentials, such as In₂O₃.

In this study, tin was doped into the structure of indium oxide, and the resulting tin-doped indium oxide (ITO) particles with high conductivity were used as supports for Pt.

—Liu and Mustain (2012)

They performed comparative ORR measurements using a thin film catalyst deposited onto a glassy carbon disk electrode; the Pt/ ITO loading was 9.40 μg/cm², whereas the loading was 27.6 μg/cm² for the Pt/C.

Among their findings were that the Pt/ITO catalyst exhibited a specific activity of 0.750 ± 0.04 mA/cm² at 0.9 V—3 times greater than that of Pt/C (0.235 0.01 ± mA/ cm²). After normalization to the loading amount of Pt metal, the mass activity of Pt/ITO catalyst was found to be 621 ± 31 mA/ mg_Pt, or 4 times greater than that of Pt/C (156 ± 9 mA/ mg_Pt). The value for Pt/C was nearly identical to the value accepted in the literature, thereby validating the experimental setup and approach, the researchers said.

The authors suggested that the higher specific activity of the Pt/ITO is likely a result of the synergistic effects between the surface tin of ITO and the supported Pt NPs.

It was found that the catalytic activity of Pt supported on ITO electrocatalysts for ORR can be enhanced drastically through synergistic effects between surface Sn and Pt, including preferential Pt faceting, which maximized the surface area of the Pt {111} facet that is highly active toward ORR. Although some other catalysts have shown mass activity at 0.9 V vs RHE > 1300 mA/mg_Pt, (i.e., PtIrNi), and specific activity ≈ 1.4 mA/ cm² (i.e., Pt₃Co), the stabilities of these catalysts have not been reported. Also, in both of the above cases, these catalysts have a very high platinum group metal content and are significantly more expensive than commercial Pt/C. For Pt/ITO, In and Sn, though more expensive than C, are relatively low cost materials, especially when compared to Co and Ir. In addition, high stability has already been achieved in this design, while the minor degradation is assumed to be due to the surface dissolution of Sn species.

—Liu and Mustain

Future work will focus on improving the long-term stability of Pt/ITO, and fuel cell performance using a Pt/ITO catalyst layer as the cathode will also be studied.

The authors also noted that their method for introducing desired elements as dopants in a desired lattice structure, which possesses certain preferred facets, may also be useful for the development of catalysts beyond fuel cell applications.

Resources

• Ying Liu and William E. Mustain (2012) High Stability, High Activity Pt/ITO Oxygen Reduction Electrocatalysts. Journal of the American Chemical Society doi: 10.1021/ja307635r