Buckypaper Supported Pt Monolayers Catalyst for PEM Applications
Authors: Abdelhafiz Ali , Gupta Kishor , Kumar Satish , Alamgir Faisal M 
Published: February 2020

Buckypaper supported Pt monolayers catalyst for PEM applications Low temperature fuel cells such as polymer exchange membrane (PEM) or solid-acid fuel cells (SAFCs) typically utilize Pt materials as the catalyst of choice. Lowering the loading of Pt while ensuring its service life remain two major goals for lowering the cost barriers to the commercialization of these fuel cells. In addition, compacting the design of fuel cells for non-stationary applications (e.g. vehicles), while increase the output power density is a desirable achievement for a more efficient fuel cell. Conductive carbon is the current low-cost option as the catalyst support but carbon corrosion continues to be an impediment to the long-term stability of the catalyst systems in fuel cells. Carbon Nanotubes (CNT) possess high specific surface area, mechanical robustness, good tunable electrical conductivity and inertness against most of chemicals, especially acids. Over the past decade, there has been a growing interest in trying to incorporate CNT-based materials as catalyst supports especially in cases where scale-up to mass production is viable. Buckypaper, as an ultrathin mesh of aggregated CNTs can be produced in a roll-to-roll fashion for several meters, while maintaining a highly porous structure with specific surface area of hundreds meter squares per gram of CNT. In addition, their chemistry can be changed from pristine to super hydrophobic to hydrophilic through chemical functionalization of CNT. The inherent properties of CNT, its catalytic applications, high surface area and facile fabrication of buckypaper inspired us to explore the efficacy of using a few monolayers of Pt supported on buckypaper as a catalyst architecture for fuel cells.

Herein, following a Surface Limited Redox Replacement (SLRR) few monolayers (MLs) of Pt are grown electrochemically on top of a stand-alone buckypaper. Pt thickness is controlled systematically through under potential deposition iterations and the growth was monitored by cyclic voltammetry (CV) scans. Pt MLs wet the CNT ribbons of the buckypaper, while avoiding agglomeration. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis show that surface-utilization is efficient, as Pt is successively deposited not only on the near surface CNTs, but down to the bulk of buckypaper. In addition, XPS analysis shows that the Pt_MLs are in their metallic state, thus confirming successful SLRR rather than eletroless deposition (where some cationic Pt would be expected).

Catalyst longevity under processing conditions is a crucial factor to determine the lifetime of fuel cell and its price, which affects its commercialization directly. Department of Energy (DOE) sets a testing protocol to test catalyst activity loss under electrochemical testing. DOE protocols’ target to save 60% of the catalyst activity by 30,000 testing cycles. Our Pt_MLs/buckypaper catalysts show promising results, while saving 50% of the catalyst activity as shown through linear sweep voltammetry (LSV) currents for oxygen reduction reaction (ORR). Raman analysis shows no alteration of CNT G and 2D bands after 30,000 testing cycles, in comparison to Raman spectra of the same sample set before testing, which indicates no carbon corrosion.

 In conclusion, no carbon corrosion is observed for CNT after long testing (i.e. 30,000 cycles). In addition, power density of Pt MLs catalysts is enhanced by wetting-ability of Pt to the buckypaper throughout its internal 3D porous structure. Moreover, buckypaper shows high susceptibility to Pt interfacial adhesion, and the interfacial interaction between Pt and CNT surface helps to retain Pt atoms as 2D film without ripening. Catalyst activity retained after 30,000 testing cycles is ~ 50%. This initial result promises with more enhancement for power density and catalyst lifetime with chemical functionalization of CNTs in our future work.