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Hierarchical Integration of Multi-scale Elements for Micro-Fuel Cells

Written by: 
Jeff Morse, PhD

E,F) scanning electron microscopy (SEM) images of finished Zr-BMG flow field
E,F) scanning electron microscopy (SEM) images of finished Zr-BMG flow field
Fuel cell power sources can be an efficient technology for direct conversion of various forms of hydrocarbon fuels to electrical power, and in the case of hydrogen fuel, could provide one of the greenest energy sources available. Fuel cells have the potential to impact application areas including grid scale, automotive, and portable electronics, but the predicted impact has not yet been realized as the manufacturing, cost, and reliability of fuel cell components has not matured sufficiently to be competitive with other power sources. Fuel cell optimization challenges remain due to the need to create a hierarchical materials structure at the triple-phase boundary. This facilitates  effective mass transport of reactants and byproducts with a materials region that incorporates catalyst, electrolyte, and conductor properties within a porous diffusion scaffold. Integrating these nanoscale features with microscale flow field channels requires multiple process steps and materials layers that are typical of fuel cell architectures.

Recently, Sekol et al. demonstrated the fabrication of an integrated flow field, gas diffusion layer, and catalyst in one continuous process. Exploiting the unique property of bulk metallic glass (BMG) that results in viscous softening when heated above the glass transition temperature, thermoplastic forming methods were used to essentially mold macroscale flow field channels in in one surface of a Zirconium-BMG (Zr-BMG) substrate, and subsequently mold a porous gas diffusion layer and high surface area nanowire Platinum-BMG (Pt-BMG) catalyst layer in the opposite surface. In this manner, a single component is now able to replace three components, and further, can be produced in a scalable stamping approach. The use of BMGs has further advantages including high electrical conductivity, with the ability to make mechanically robust, complex 3D structures.

H–K) SEM images of the finished Pt-BMG nanowire electrode with gas feeding through-holes
H–K) SEM images of the finished Pt-BMG nanowire electrode with gas feeding through-holes
The flow field mold was formed by anisotropic etching of silicon with nominal features of 100 µm channel width and depth. The mold was then pressed against one surface of the Zr-BMG substrate and heated with constant pressure applied. After cooling, the silicon mold was dissolved leaving the pristine flow field channels in the Zr-BMG surface. The high surface area nanowires were formed in a Pt-BMG substrate using an anodized aluminum oxide (AAO) template as the mold. After forming the array of 100nm diameter nanowires in the Pt-BMG surface, a microscale mold with silicon pillars of approximately 100µm diameter and spacing was used to create a porous path for gas to diffuse to the catalyst layer. Thus a hierarchical fabrication strategy was demonstrated to create an effective triple-phase boundary component serving several critical functions for fuel cell performance. The method can be further extended to a continuous process utilizing a monolithic substrate, and is scalable to manufacturing approaches that offer both large area and high-volume production. While this technique will find uses for a broad range of complex 3D devices and structures, the example reported by the authors demonstrated the extreme ability for tailoring both the physical and materials properties of the integrated structure via a continuous hierarchical design methodology.

Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network

Figures reprinted with permission pending from Sekol RC, Kumar G, Carmo M, Gittleson F, Hardesty-Dyck N, Mukherjee S, Schroers J, Taylor AD. 2012. Bulk metallic glass micro fuel cell. Small 9(12).

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