(6ef) Highly Active and Durable Extended Surface Electrocatalysts

Extended surface nanostructures have been developed as catalysts for a variety of electrochemical processes. The majority of efforts in this field have focused on oxygen reducing electrocatalysts for proton exchange membrane fuel cells and oxygen evolving electrocatalysts for acidic electrolyzers. Preliminary advanced geometry materials, however, have also been developed for alcohol oxidation and anion exchange membrane fuel cells (oxygen reduction, hydrogen oxidation, and alcohol oxidation). The formation of these nanomaterials has included various gas phase (sputtering, atomic layer deposition, and chemical vapor deposition) and liquid phase (direct deposition and spontaneous galvanic displacement) processes. In recent years, spontaneous galvanic displacement has become a focus due to a unique combination of high site-specific activity and surface area commonly found in materials produced by the method.

The principal application of interest with this catalyst type has historically been proton exchange membrane fuel cells, where the amount of platinum in the catalyst layer accounts for a significant portion of fuel cell cost and limits the commercial deployment of the device.[1] Extended surface nanomaterials have previously been developed as electrocatalysts for fuel cells and offer key advantages to nanoparticles: an order of magnitude higher specific activity; long range conductivity; and long term durability.[2] Extended nanostructures, however, are traditionally limited by low surface area. Spontaneous galvanic displacement occurs when a less noble metal template contacts a more noble metal cation and combines aspects of corrosion and electrodeposition. Although catalyst development has included a variety of approaches, catalysts formed by galvanic displacement are ideally situated, being able to take advantage of the specific activities generally associated with the catalyst type while significantly improving upon the surface area.[3]

Efforts in this field initially used silver and copper templates as a learning tool, but have since moved to nickel and cobalt since these metal have lower redox potentials and will not plate the anode during fuel cell operation. Recent developments in platinum-nickel nanowires have produced materials with surface areas in excess of 90 m² g_Pt^‒1.[4] Post-synthesis processing has created nanostructures with oxygen reduction mass activities 7 times greater than platinum nanoparticles and 5 times greater than the U.S. Department of Energy membrane electrode assembly target in rotating disk electrode half-cells. Following accelerated durability testing (30,000 cycles, 0.6‒1.0 V, rotating disk electrode), these materials lose 3% oxygen reduction mass activity and less than 0.5% catalyst mass to dissolution. Remaining barriers, including synthesis scalability and electrode fabrication, are being addressed. Recent improvements in membrane electrode assembly performance suggest that the promise of extended surface catalysts in half-cells can be realized in the device.

[1] D. Papageorgopoulos, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review14/fc000_papageorgopoulos_2014_o.pdf, 2014.

[2] M. Debe, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review09/fc_17_debe.pdf, 2009.

[3] S.M. Alia, Y.S. Yan, B.S. Pivovar, Catalysis Science & Technology, 4 (2014) 3589-3600.

[4] S.M. Alia, B.A. Larsen, S. Pylypenko, D.A. Cullen, D.R. Diercks, K.C. Neyerlin, S.S. Kocha, B.S. Pivovar, ACS Catalysis, 4 (2014) 1114-1119.