Wenzhen Li, PhD , Michigan Technological University
The sluggish oxygen reduction reaction is a long-standing scientific challenge, that dramatically reduces the efficiency of fuel cells. The objective of this ACS-PRF project is to study high performance one dimensional (1-D) M (Fe, Co, Ni, etc) core Pt Shell nanocables for fuel cell cathode catalysts. The 1-D core-shell nanowire catalysts will have improved activity due to their tuned electronic properties (optimized d-band center) and more advantageous Pt(111) facet; enhanced durability (because it can avoid nanoparticle agglomeration); and reduced cost (because only a thin layer of Pt on the catalyst surface). Success of this research will acquire fundamental understanding of controlled wet-chemistry synthesis of multi-metallic nanocatalysts at the nano and atomic scale, and deepen our insights into the structure-catalytic function relationships. This research will open a new avenue for design and preparation of efficient catalytic materials, and help our efforts to diversify the current energy supply status and reduce our dependence on foreign petroleum. Based on this ACS-PRF grant support, our research findings (9/2010-8/2011) are summarized as below:
1. We have developed a solution phase-based method to accurately prepare multi-metallic electrocatalysts. C18 surfactants (e.g. oleylamine, oleic acid, octadecenen, etc) serve as solvent, stabilizer and reducing agent (in some cases). Since metal precursors in the organic solvent have intimate contacts and closer redox potentials, better multi-metallic catalysts can be obtained. The diameter, length and morphology of these 1-D nanostructures can be controlled by tuning synthesis conditions, such as ratio of two surfactants, reduction temperature, Fe content, etc. Especially, the surface energy of metal crystallographic facets can be tuned by bonding with different surfactants, leading to controlled 1-D shapes. The surfactants can be removed through organic acid washing or electrochemical treatment.
2. We prepared ultra-thin PtxFey-nanowires (PtxFey-NWs, x, y is the atomic ratio of Pt and Fe) through this solution-phase method. With Pt amount increasing, the diameter gets larger and length gets shorter. For example, the diameter of Pt1Fe1-NW, Pt2Fe1-NW and Pt5Fe1-NW are 2.7 nm, 2.9 nm and 4.2 nm, respectively. Thicker but shorter PtFe nanorods were observed on the Pt5Fe1-NW sample. The 1-D PtFe nanowire catalysts have demonstrated higher durability than commercial Pt/C catalyst. After 1000 cycles of 0¨C1.3 V (vs RHE), the relative electrochemical surface area (ECSA) of Pt2Fe1-NW/C dropped down to 46% of its origin value, which was two times better than Pt/C catalyst (only 20%), The better durability of PtxFey-NWs may be due to 1) the very large aspect ratio as compared to Pt/C catalysts; and 2) their spin orbit coupling and the hybrization of between Fe 3d and Pt 5d states. Pt1Fe1-NWs show a mass activity (M. A.) of 47.8 mA/mg-Pt at 0.85V, while that of Pt/C was 43.5 mA/mg-Pt. After durability test, the M. A. of Pt1Fe1-NWs only decreased to 28.9 mA/mg-Pt, which is nearly twice that of Pt/C (15.8 mA/mg-Pt). The calculation of mass activity shows that PtxFey-NWs possess both a higher activity than commercial Pt/C catalysts before and after durability test.
3. We prepared novel PdFe-nanoleaves (NLs) through this solution-phase method. High-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (S/TEM) coupled with high-spatial-resolution compositional analysis clearly show this newly-developed structure is assembled from Pd-rich nanowires (NWs) surrounded by Fe-rich sheets. The Pd-NWs have a diameter in the range of 1.8-2.3 nm and length of 30-100 nm. They also have a large electrochemical surface area of >50 m2/g. Their diameter, length and morphology can be tuned by altering the nanostructure synthesis conditions and the Fe amount in the NLs. With increasing Fe content, thinner and longer sheet-enveloped nanowires can be prepared. The side surfaces of Pd-NWs observed by HR-TEM are predominantly Pd (111) facets, while the tips and ends are Pd (110) and Pd (100) facets. The Pd1-NL/C and Pd2-NL/C (PdFe-nanaleaves after organic washing and deposited on carbon black) have demonstrated a high reactivity towards electrocatalytic reduction of oxygen in a 0.1 M NaOH electrolyte, i.e. 3.0°Á increase in the specific activity and 2.7°Á increase in the mass activity compared with a commercial Pt/C catalyst (at 0 V vs. Hg/HgO). The electrocatalytic activity enhancement can be attributed to the unique nanoleaves structure that provides more Pd (111) facets, a large surface area and more resistance to Pd oxide formation.
4. We extended the solution phase catalyst synthesis method to prepare non-precious metal catalyst - Ag/C.TEM image show Ag nanoparticles with a small average diameter (5.4 nm) and narrow size distribution of 2-9 nm are well dispersed on the support of carbon black Vulcan XC-72. The intrinsic activity and reaction pathway of oxygen reduction reaction (ORR) on the Ag/C and commercial Pt/C were investigated via rotating ring disc electrode (RRDE) in 0.1 M NaOH at room temperature. The RRDE results showed the comparable activities of self-prepared Ag/C and commercial Pt/C electrocatalysts, and confirmed that the 4-electron pathway of ORR proceeded on Ag/C in alkaline electrolyte. A single H2-O2 anion exchange membrane fuel cell with the Ag/C cathode catalyst exhibits an open circuit potential of 0.98 V and a peak power density of 190 mW/cm2 at 80 oC, which is slightly lower than H2-O2 AEMFC with Pt/C cathode catalyst (247 mW/cm2).
5. We established an electrochemical probe reaction to monitor the restructuring of nanostructured catalysts under electrocatalytic reactions.
We have published two papers in Nanotechnology and Chemistry of Materials and given four presentations (in ACS and NACS conferences) that acknowledged this grant support. Three oral presentation abstracts in AIChE conference (Minneapolis, MN, October 16-21, 2011) have also acknowledged this grant. An invited chapter to a Wiley book (Advanced Materials for Low Temperature Fuel Cells) was accepted.