Durability Study on SWNT/Nanofiber Buckypaper Catalyst Support for PEMFCs

Abstract

Due to their unique microstructure, buckypaper-supported platinum ͑Pt͒ catalysts derived from carbon nanotube and carbon nanofiber have demonstrated a high Pt utilization in proton exchange membrane fuel cells ͑PEMFCs͒. ͓SWNT means singlewalled carbon nanotube.͔ The durability of a buckypaper-supported Pt catalyst was investigated using an accelerated degradation test ͑ADT͒ in a mimic cathode environment of PEMFC. Compared to commercial carbon black-supported Pt, Pt/buckypaper showed a better catalyst durability after holding at 1.2 V for 400 h; specifically, almost 80% of the Pt electrochemical surface area was lost for Pt/carbon black, with only a 43% loss for Pt/buckypaper. Transmission electron microscopy and cyclic voltammetry were used to study the Pt degradation mechanism. It was concluded that Pt coarsening and Pt detachment from buckypaper support due to carbon corrosion make the major contribution to the Pt surface area loss under this condition. The Pt loss via detachment from supports after the ADT was calculated as 18% in Pt/buckypaper, while the Pt loss was 69% in Pt/C. It is supposedly due to the higher corrosion resistance of buckypaper because of its high graphitization degree, which is indicated by a slower formation rate of surface oxides in buckypaper than in carbon black. Further durability improvement of the Pt/buckypaper is expected by improving the dispersion of Pt on the buckypaper to reduce Pt sintering.Proton exchange membrane fuel cell ͑PEMFC͒ technology has been extensively developed during the last decade and is rapidly approaching commercialization. High production cost, especially the catalyst cost due to the high price of platinum, is considered as one of the most important issues to hinder widespread application of PEMFCs. The state-of-the-art Pt utilization in a cell is 0.6 g Pt/kW with a Pt loading of 0.45 mg Pt/cm 2 , which is planned to be reduced to 0.3 g Pt/kW with a Pt loading of 0.2 mg Pt/cm 2 in a stack by 2015 to meet the requirements for transportation applications, according to the US Department of Energy's target. 1 Aside from cost, the durability of PEMFCs has been recognized recently as another critical issue for commercializing PEMFCs because 5000 h of cell operation is required for automotive applications and 40,000 h for stationary applications. Activity loss of the catalysts is believed to make a major contribution to the performance degradation of PEMFCs. 2 Conventional PEMFC catalysts are typically composed of platinum nanoparticles ranging in size from 2 to 3 nm dispersed on high surface area carbon materials, such as carbon black, to maximize the specific surface area of Pt. 3 The coarsening of Pt nanoparticles during cell operation is inherently driven by the reduction of surface energy resulting in a decrease of the electrochemical surface area ͑ECSA͒ of Pt. The catalytic activity of Pt toward the oxygen reduction reaction ͑ORR͒ decreases accordingly, leading to performance degradation of PEMFCs. The mechanisms of Ostwald ripening via a Pt dissolution and...

Degradation aspects of water formation and transport in Proton Exchange Membrane Fuel Cell: A review

“…This reported value of Pt loss in SWNT (e.g. 1.2 times) was much lower than that detected in [211] (e.g. 3.8 times).…”

“…They found the ion exchange technique to be more effective in terms of ORR and Pt utilization; the later can be further improved by using nanotube-based catalysts. This claim was supported by the measurements of [209][210][211][212] which showed high polarization performance of the cell. The good accessibility to reactant gases and the uniform distribution of Pt on the carbon structure are not the only advantages of these catalysts, but good durability as well.…”

“…After 400hrs of ADT, the loss of Pt from the active surface area using buckypaper support was 18% while the Pt loss was 69% in Pt/C. The difference between these two results could be attributed to the design aspects of the tested cells as well as to some of the operating conditions such as the level of voltage cycling [211]. On the other hand, when Pt-alumina catalyst was examined [213], Pt particles have remained almost the same size (~8 nm) after immersing it in deionised water at 80°C for one week.…”

“…The reason for this large difference in the results was due to Pt migration and ripening/aggregation which were enhanced by the carbon corrosion of the catalyst. Similarly, the Pt loss in [211] was reduced by almost half under accelerated degradation testing, e.g. 43% with single-walled buckpaper compared to 80% loss with Pt/carbon.…”

“…Carbon black-supported Pt catalysts were previously examined in [211] and [304] against new designs of single and mutli-walled carbon nanotubes (MWNT). The comparison was based on the change in Pt agglomeration and distribution at the catalyst surface after cell operation.…”