Development of Continuous and Ultrathin Pt Electrocatalysts Using Plasma-Assisted ALD

High cost of Pt prevents wide-spread application of fuel cell vehicles. Fine nanoparticles (3-5nm in diameter) are traditionally used in order to increase electrochemically active surface area.  A continuous Pt film is more stable and has a 5-10 fold higher area-specific activity than Pt nanoparticles.¹  This is only economically viable when the Pt film is less than ~3 nm thick.

Pt ALD usually yields Pt nanoparticles during initial film growth.  We have shown earlier in work on flat substrates that tungsten, with a higher surface energy than platinum (3.5 J/m² vs 2.5 J/m²), can serve as an adhesion layer to achieve Pt ALD films that are dense and continuous at thicknesses of ~1.5 nm.^2,3

To fabricate a practical electrocatalyst we developed a plasma-assisted, open-ended rotary reactor to deposit W and Pt ALD films on the high surface area metal oxide particles. We also deposited the layers on 3M NSTF substrate, an oriented organic crystalline substrate.

Transmission electron microscopy (TEM) showed uniform and continuous ~3-5 nm Pt films on the TiO₂ particles.  The W ALD and Pt ALD films deposited on the 3M NSTF substrate were characterized using TEM with electron energy loss spectroscopy (EELS).  The TEM-EELS analysis revealed W ALD and Pt ALD films with thicknesses of ~3 nm that were continuous and conformal on the high aspect ratio NSTF substrates.  The production of these thin film on high surface area substrates demonstrates that it is possible to further reduce cost of fuel cell catalyst. 

Acknowledgements

The authors thank Mark K. Debe, Edward M. Fischer, and Andrew Steinbach of 3M for providing non-platinized NSTF substrate. Thanks are also extended to Cristin Keary, Tom Moylan, and Peter Harvey for engineering support.

References

1. H. A. Gasteiger et al., Appl. Cat. B: Env. 56, 9 (2005).
2. L. Baker et al., Appl. Phys. Lett. 101, 111601 (2012).
3. US Patent 8,647,723.