Characterization of Pt-Fe nanowires for PEM Fuel Cell catalyst application


Nitin Tyagi

Material Science Department
 University of Rochester


Introduction
Prof. Li's novel solution 
Sputtering
FESEM
TEM
S-TEM
EDS
Image Colorization using Photoshop
Electron Flight Simulation
References
Acknowledgement


Introduction

       Polymer Electrolyte Membrane(PEM) fuel cells are very promising high power suppliers.  They produce electric power by transforming hydrogen  and oxygen into water at a low temperature with a high efficiency.  Platinum is the major catalyst in proton-exchange-membrane  fuel cells  (PEMFC).


GM Fuel Cell Concept Car at Rochester
    Figure 1: GM Fuel Cell Concept Car at Rochester

The high cost  of Pt is the primary  problem which is making commercialization difficult for Fuel cells

Solution:

1.Nanotechnology-  Catalyst is usually made into nanoparticles to maximize the surface area to weight ratio. 
2.Alloying-  Alloying Pt with cheaper and readily available metals like Fe, Ni etc to reduce the amount of Pt required.



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Prof. Li’s novel solution

Dr.  Li's lab in the Mechanical Engineering department  has has come up with a novel solution to solve this problem. Using the electrospinning fabrication technique  his group has made one dimensional nanostructures of Pt and Pt-Fe alloys. They have the following advantages:


   The seven EM techniques used to characterize the Nanowires are:

1.Gold Sputtering
2. FESEM- Secondary electron (InLense)
3. TEM bright field
4. STEM
5. X-ray (Both STEM EDS and SEM-EDAX)
6. Image colorization using Photoshop
7. Electron flight simulation
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Sputtering
Sputtering system was used for putting a Au layer on the nanowire sample for better SEM characterization.
Current~15-20 mA
Time~ 40 sec
Coating Thickness~ 3-4 nm
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FESEM

The nanowires were characterized using the Zeiss Supra 40VP FESEM.

In      InLense detector gave the best images. The working distance was kept very short( ~2cm)for InLense imaging.  The accelerating voltage used was 20 KV. Both porous and solid wires were characterized. However in the SEM they do not show any difference.  Four different types of samples were imaged:

1.Pt nanowires
2.Pt-Fe nanowires
3.Pt-Fe3   nanowires
4.Pt-Fe7  nanowires

F      From the images we can see that the wires appear uniform phase and there are no discernable Fe or Pt regions. Thus  Pt and Fe are a single phase and it seems to be a homogeneous alloy.

Pt nanowire  Pt-Fe nanowire

    Figure 2: Pt nanowire                                                                                                                        Figure 3: Pt-Fe nanowire


The length of the NWs is in microns while the diameter is in tens of nm (10-50nm).

Pt-Fe3 nanowire Pt-Fe7 nanowire
    Figure4: Pt-Fe3 nanowire                                                                                                                Figure 5: Pt-Fe7 nanowire

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TEM

Bright field TEM imaging of PtFe3 Nanowires(NW) was done on FEI Tecnai F20 TEM. The sample was dissolved in ethanol and ultrasonicated to disperse it well it the solution.  Sonication is important as otherwise all the nanowires agglomerate together and it is very difficult to find a single NW to image.

HRTEM  of PtFe3 Solid Nanowires  Low Magnification TEM image of PtFe3  Solid nanowire

Figure 6: HRTEM  of PtFe3 Solid Nanowires;                                                             Figure 7: Low magnification TEM image of Ptfe3 Solid nanowire;                                  Dark area are Pt rich area, Lighter area are Fe rich                                Polycrystalline  in nature




The porous nanowires appear to be more polycrytalline with the small crystallites more loosely bound than compared to the Solid NW. Care must betaken not to sonicate for too long (not more than 2 minutes) as that could break apart the porous NWs. Solid wires are more robust compared to Porous wires.
Dark Field Imaging of the same samples did not show any significant change in the image as compared to bright field.


: Low magnification TEM image of PtFe3 Porous Nanowires  HRTEM  of PtFe3 Porous Nanowires;

Figure 8: Low magnification TEM image of PtFe3 Porous Nanowires                        Figure 9 : HRTEM of PtFe3 nanowires; Dark areas are Pt rich, Lighter are Fe rich

Scanning- Transmission Electron Microscopy ( S-TEM)

HAADF detector was used for taking STEM micrograph of PtFe3 nanowires. The image quality is not very good. This could be because the beam was being focused on a single NW.  Since the diameter of the NW is only 10 nm, the interaction volume is very small for the electron beam. This leads to a very small signal for the STEM detector

S-TEM


Figure 10: S-TEM


EDS

X-ray characterization was done using HAADF- EDX detector in the STEM. In the SEM , EDAX was used. The sample characterized was PtFe3  Solid nanowire and the  results are as expected. The atomic ratio of Pt:Fe was 1:3

STEM EDS for PtFe3  Solid nanowire

Figure 11: STEM EDS for PtFe3  Solid nanowire

SEM-EDAX for PtFe3  Solid nanowire confirming the 1:3 atomic ratio of Pt:Fe

            Figure 12: SEM-EDAX for PtFe3  Solid nanowire confirming the 1:3 atomic ratio of Pt:Fe

Image Colorization using Photoshop

  Image colorization was done using Photoshop. Multiple layers were used to get the desired picture.  For the SEM image one layer was used to paint the NWs blue while another layer was used to color the background NWs green. For the HRTEM image the Pt rich area has been colored dark blue while Fe rich region has been colored light blue .The background was colored black. Choice of color was very tricky because the natural color of Pt is grey and of Iron is also blackish-grey.  However some Pt alloys do have a bluish tinge and so it was decided to use various shades of blue.

Colorization of SEM micrograph using Photoshop

                   

            Figure 13: Colorization of SEM micrograph using Photoshop

    

Photoshop Colorization of HRTEM image

            Figure 14: Photoshop Colorization of HRTEM image; Pt rich area has been colored dark blue; 

            Fe rich region has been colored light blue

Electron Flight Simulation

Winxray-1.3” is freely available electron flight simulation software which was used to simulate the electron-sample interaction for the PtFe3  sample in the SEM. The accelerating voltage was set as 20KV as most of the micrographs were taking using this value. The atomic ratios of Pt:Fe was set as 1:3. The simulation calculated a maximum depth of penetration of the electrons as ~ 830nm. The width of penetration  was calculated as ~ 600nm.

X-Y Plane (the plane of the sample)

            Figure 15: X-Y Plane (the plane of the sample):

            Width of penetration ~ 600nm

X-Z Plane

            Figure 16: X-Z Plane:

            Depth of Penetration~ 830nm

References

(1) ,P.J. Ferreira, G.J. La, O.Y. Shao-Horn, D. Morgan, R. Makharia, S.Kocha, H.A. Gasteiger,“Detachment of Pt and/or Pt alloy nanoparticles from the carbon support” J. Electrochem. Soc. 152, A2256-A2271, (2005).
(2) J.L. Shui, James C.M. Li “Effects of Solution and Processing Variables on the Morphology of Poly(vinyl pyrrolidone)/H2PtCl6 Composite Nanofibers made by ElectrospinningNanoletters 9(4)(2009)1307-1314.

Acknowledgement

I would like to thank Brian McIntyre, our course Instructor for his guidance and help with the project. I would also like thank  Dr. Jianglan Shui, graduate student from Dr. Li's group in Mechanical Engineering Department for help with the samples and the interpretation of results.

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