Can CO-Tolerant Anodes be Economically Viable for PEMFC Applications with Reformates?

Recent advances in removing carbon monoxide (CO) from about 1% to ~10 ppm level via preferential oxidation (PROX) of CO in hydrogen feeds^[1] renewed the interest in developing CO-tolerant catalysts for using inexpensive reformates as the fuel other than pure hydrogen. The savings would be considerable since the 2015 price target for hydrogen produced by reforming natural gas is $2 per kg H₂, viz., ~40% cheaper than pure hydrogen generated by water electrolysis at $3.5 per kg H₂^[2].

RuPt alloy and Ru-Pt core-shell nanocatalysts have been studied as CO-tolerant anode catalysts for PEM fuel cells. However, dissolution of Ru occurs at occasional high potentials experienced at the anode, causing performance decay. To address this issue, we developed a surfactant-free and scalable synthesis method to produce ordered Ru(core)-Pt(shell) nanoparticles with tunable Pt shell thickness of 1 to 3 atomic layers. Commonly found disordering in Ru cores was attributed to defect-induced partial alloying, and was eliminated by annealing Ru cores before coating Pt shells. The single crystalline particles with a sharp Ru-Pt boundary and ordered transition from Ru’s hexagonal close-packed lattice to Pt’s face-centered cubic structure were confirmed by atomically resolved scanning transmission electron microscopy images coupled with density functional theory calculations^[3].

We shall mainly show that a complete Pt bilayer shell protects Ru from dissolution during potential cycles up to 0.95 V while exhibiting enhanced CO-tolerance. The results demonstrate a promising anode with less than 0.1 mg cm^-2 total metal loadings for operating PEM fuel cells with reformates. In order to find optimal Pt shell thickness and metal to carbon weight ratio, we developed an electrochemical testing method using standalone, gas-diffusion electrodes for evaluating the performance without and with CO in hydrogen. The findings will be discussed in comparison with the results of membrane electrode assembly tests under realistic conditions, shedding light on causes of deactivation effects and ways to alleviate them.

 Acknowledgements

The work at Brookhaven National Laboratory was supported by U.S. Department of Energy under contract DE-AC02-98CH10886 and Power Bridge NY.

References

[1]        K. Liu, A. Wang, T. Zhang, ACS Catalysis 2012, 2, 1165-1178.

[2]        Satyapal, S. http://www.hydrogen.energy.gov/pdfs/htac_apr13_1_satyapal.pdf, Fuel cell            technologies update. page 17 (2013).

 [3]       Y.-C. Hsieh, Y. Zhang, D. Su, V. Volkov, R. Si, L. Wu, Y. Zhu, W. An, P. Liu, P. He, S. Ye, R. R. Adzic, J. X. Wang, Nat Commun 2013, 4, 2466.