Development of a Low-Cost and High-Efficiency 500 W Portable PEMFC System

Project Objective: The proposed project is based on our recent technical breakthrough on catalyst usage and life, and cell power density by using the mixed carbon nanotubes (CNT) and carbon nanofibers (CNF) free-standing papers (buckypaper) as supporting network of catalytic electrodes. The project will explore and demonstrate effective and scalable techniques to fabricate affordable large size Pt/buckypaper electrode with gradient-structured ready for fuel cell stack. We will systematically investigate the effect of electrode microstructure on the performance of resultant fuel cells and optimize the electrode structure to meet or exceed the DOE's 2015 PGM and cell performance targets. At the end of the project, we will demonstrate a product prototype of a 500 W fuel cell stack used for portable power applications.

In our previous work, CNT and CNF derived buckypaper materials were used as catalyst support for PEMFC [1-3]. High catalyst efficiency for buckypaper-supported Pt was achieved due to the maximization of the three-phase boundary (TPB) in the electrode by depositing Pt nanoparticles on the outermost surface area of an established porous CNT/CNF network. In our new method, a free-standing buckypaper consisting of CNT was constructed; the Pt nanoparticles were then deposited on the buckypaper in a liquid solution and were not uniformly distributed on the CNT surface. The advantages of a catalytic electrode made with this new method include: (1) its unique microstructured well-connected nanotubes network that ensures a pathway for electrons; (2) the Pt is not uniformly distributed CNT surface, in fact it is electrodeposited on the most accessible sites in the buckypaper and is not covered by either CNT or binder materials; (3) the open vacancies (pores) mostly in the meso- to macro-size range can be effectively covered by Nafion electrolytes, which facilitates the maximization of the three-phase boundary where the electrochemical reaction takes place; (4) the high porosity of buckypaper also benefits the mass transfer process within the catalyst layer resulting in further improvement for Pt utilization. Furthermore, the buckypaper can be made with the porosity grading in such a manner that the high porosity at fuel and air side for anode and cathode catalytic electrodes, respectively, that benefits the mass transfer; and the low porosity at membrane side that allows high Pt loading density at or near membrane interface; and (5) the Pt/buckypaper cathode also showed greater durability in electrochemical oxidation than the Pt/C cathode, which is 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. Combining these advantages, the-state-of-art performances achieved by applying Pt deposited gradient-structured buckypaper electrode are approaching 2015 DOE technical targets. For example, by using a double-layered buckypaper consisting of a single wall carbon nanotube (SWNT)/CNF mixture (1:3 wt./wt.) sublayer and a CNF sublayer and a buckypaper-supported Pt catalyst (Pt loading 0.11 mg/cm2) as the cathode catalyst layer, remarkable Pt utilization (0.18 gPt/kW) was achieved with a rated power of 880 mW/cm2 at 0.65 V as shown in Fig. 1.


Fig. 1 Cell potentials and power densities as a function of current density for the 5 cm2 MEAs using Pt/(SWNT)/CNT.

Fig. 2 Left: schematic illustration of stack design developed by BIE; Right: photo of a 20-cell stack prototype.

References

  1. W. Zhu, J.P. Zheng, R. Liang, B. Wang, C. Zhang, G. Au, and E. J. Plichta, "Durability Study of Carbon Nanotube/Nanofiber Buckypaper Catalyst Support for PEMFCs", J. Electrochem. Soc. 156, B1099 (2009).
  2. W. Zhu, D. Ku, J.P. Zheng, R. Liang, B. Wang, C. Zhang, S. Walsh, G. Au, and E. J. Plichta, "Buckypaper-Based Catalytic Electrodes for Improving Platinum Utilization and PEMFC's Performance", Electrochimica Acta, 55, 2555 (2010).
  3. W. Zhu, J.P. Zheng, R. Liang, B. Wang, C. Zhang, G. Au, and E. J. Plichta, "Ultra-Low Platinum Loading High-Performance PEMFCs using Buckypaper Supported Electrodes", Electrochemistry Communications, 12, 1654 (2010).

Contact

Sponsor