UIC Final Report

REU 2004 Summer Program

The Engineering of Pt/Carbon Catalyst Preparation For application on Proton Exchange Fuel Cell Membrane

(PEFCM)

Student’s Name: Jaime O Robles Advisor: John R. Regalbuto

Catalysis Laboratory University of Illinois at Chicago

Progress Report

This paper will deal with the strong electrostatic adsorption (SEA) method used for catalyst preparation of Platinum on carbon supports for use in proton exchange fuel cell membranes (PEFCM). The main objective is to optimize dispersion and loading of Pt on carbon supports. Characterization results, of the Pt/Carbon catalyst, prepared via the SEA will suggest that the SEA is the suitable technique. Furthermore, the key variables will be identified and optimized in order to obtain the conditions at which to prepare the catalyst. Introduction The proton exchange fuel cell membrane has become the dominant fuel cell due to its advantages in power, low operating temperature, and low CO2 emission [1]. Fuel cell technology is a promising, but futuristic nonetheless. Like every innovation ever presented, fuel cells have their pros and cons. Fuel quality and storage are the main concerns that need to be addressed before commercializing this new technology. The research focuses on the two electrodes, composed of Platinum on carbon-coded film, found in the PEFCM. The Pt/carbon-coded electrodes demand a high dispersion, high weight percent of platinum on carbon to enhance its efficiency, although economically speaking this process in not sound. Due to its low operating temperature-80 degrees Celsius-such high weight percentage specifications can be used. At the catalysis laboratory, the main focus of the research is to use high and low surface area carbon supports in order to comply with the high dispersion, high weight percent demands. Table 1 shows the different carbons used, for experimentation, along with their important characteristics. Furthermore, an in depth characterization process is to be conducted using: Scanning Transmission Electron Microscope (STEM images provided by the Research Resource Center at the University of Illinois at Chicago), Extended X-ray Absorption Fine Structure imaging at Argonne National Laboratory. The characterization process will provide the results needed to understand the chemistry taking place. Ideally, the EXAFS will provide the dispersion and weight loading; STEM images will provide pictorial evidence and estimated particle size. Ultimately, the results will show that the strong electrostatic adsorption method is the supreme method to meet the demands of high dispersion high weight percent Pt/carbon catalyst.

Table 1: Carbons used for impregnation experiments.

Carbon name Company Name Type Surface Area

(m^2/g) Ketjen Black

EC 300 J Akzon Nobel cabon black 795

Ketjen Black EC 600 J Akzon Nobel cabon black 1415

Black Pearls 2000 Cabot cabon black 1475

Ensaco 350 Erachem cabon black 795

The Proton Exchange Membrane Fuel Cell (PEMFC)

How do Fuel Cells work? Fuel cells are electrochemical converter devices that convert hydrogen and oxygen into water, producing electricity and heat in the process. First hydrogen is fed to the fuel cell via a reformer or stored high pressure hydrogen. The hydrogen comes into contact with the anode where the chemistry between the two allows for the separation of the two electrons from the hydrogen atom. The electrons are conducted by the anode to an external electric circuit where electricity is produced. Meanwhile, the hydrogen cations are conducted through the electrolyte to the cathode where they meet with the oxygen gas and the free electrons to produce water and heat. Such a single procedure generates approximately .7 volts, and therefore the use of fuel stacks is in order. Fuel stacks are composed of single fuel cells combined in series or parallel. The PEMFC operates at a low temperature, usually at 176 degrees Fahrenheit (80 degrees Celsius) [2].

Figure1 shows a composite sketch of a PEMFC along with a half reaction of each electrode, which is composed of four basic elements [3].

Figure 1: Shows the four basic elements A) A) Anode B) Cathode

C) Electrolyte and D) Catalyst

The PEMFC consists of four basic elements [3]:

Anode: The cathode is the first element the Hydrogen comes into contact with. This element is the negative terminal, which serves various functions. It has the dispersed catalyst (Pt) that enables it to react with the hydrogen being fed. Once the hydrogen is disassociated from its electrons, the cathode conducts the electrons to an external circuit which produces the electricity. Cathode: The anode is the positively charged terminal, which Conducts the electrons from the external circuit to meet with the hydrogen cations and oxygen to form the water molecules. This reaction, or combination, is allowed through the dispersed platinum. Electrolyte: This element is also known as the Proton Exchange Membrane. The PEM is a special solution or specially treated material that only conducts positively charged ions. Catalyst: The catalyst is usually made of platinum particles dispersed in carbon material. It allows for the reactions between hydrogen and oxygen. The platinum catalyst always faces the PEM.

The PEMFC has had applications in the automobile industry. In the 1990s Ballard, a leader in fuel cell manufacturing, put its fuel cells into a series of prototype buses that ran on compressed hydrogen. In the late 1990s fuel cell powered buses were put onto the streets of Chicago and Vancouver. Today, fuel cells are common in space flights, transportation, portable power and large power generators, among other applications [4]. Point of zero charge (PZC) experiment - 11 point PZC measurment The point of zero charge, or PZC, can be said to be the first step in catalyst preparation. It is a very important parameter used to determine the pH range at which the impregnation step should be carried out. The point of zero charge is defined as being the pH at which the surface charge of the carbon is neutral.

The PZC experiment is carried out by contacting the different carbons with, pH adjusted, deinoized water. An eleven point PZC measurement is the most common experiment used to obtain the PZC. The eleven pH points used are 1,2,3,4,5,6,9,10,11,12,13. The carbon is weight out for each pH sample. The weight of \the carbon depends on the surface area, surface loading, and volume of sample bottle. A sample calculation is given by equation (1). Finally, the carbon is added to each sample bottle, which contains the deionized water at different pHs, and shaken for an hour. The pH is measured and recorded as final pH. Table 2 illustrates the PZC results for some of the carbons used for a surface loading of 10,000 m^2/L and a volume of 50mL bottles. The surface areas are unique to each carbon and are given in Table 2.

( )

gramsm

mL

m

AreaSurfaceVolumeLoadingSurfaceM carbon 2

32

*][

_)(*)_(

== (1)

Table 2: Point of zero charge results.

Carbon Volume (m^3) SA (m^2/g) SL (m^2/L) g of

carbon needed

PZC

Ketjen Black EC 300 J 0.050 795 10000 0.629 9.4

Ketjen Black EC 600 J 0.050 1415 10000 0.353 9.5

Black Pearls 2000 0.050 1475 10000 0.339 9.5

After the final pH is measure a strong buffering effect is evident at pHs 4,5,6 and 9, as shown by Table 3 and Figure 2 for Ketjen black EC 300J and Ketjen black EC 600J. At this final pH the PZC can be read off of Figure 3, mainly, for the 300J and 600J the PZC are 9.4, 9.5, respectively. Table 3: pH measurements for Ketjen 300J and Ketjen 600J.

KETJEN BLACK EC 300 J KETJEN BLACK EC 600 J

pH(initial) pH(final-1hr. Contact)

pH(initial) pH(final-1hr. Contact)

1.06 1.06 1.05 1.07 1.97 2.07 1.97 2.04 3.05 7.72 3.01 7.93 4.01 8.90 3.90 9.03 4.89 9.44 4.90 9.26 5.86 9.51 5.77 9.44 9.12 9.84 9.15 9.77 10.18 10.28 9.98 10.27 11.07 11.06 10.99 10.99 11.88 11.87 11.89 11.89 13.05 13.03 12.88 12.85

11-point pH PZC measerument

0123456789

1011121314

0 2 4 6 8 10 12 14pH initial

pH fi

nal

KETJEN BLACK ED 300 J

(A)

11 point PZC measurment

0123456789

1011121314

0 2 4 6 8 10 12 14pH initial

pH fi

nal

KETJEN BLACK ED 600 J

(B) Figure 3: (A) Ketjen 300J PZC measurement graph (B) Ketjen 600J PZC measurement graph.

Molecular Adsorption on Carbon Surfaces The molecular adsorption concept is adopted and used in the SEA synthesis technique. The SEA adsorption technique can be described as having a substrate and adsorbate with two modes of adsorption: physical adsorption (Physisorption) and chemical adsorption (chemisorption) [5]. The substrates are the different carbons used (see Table 1), where as the adsorbate used is hydrogen hexachloroplatinate (IV) (H2Cl6Pt obtained from Alrich chemical company). The main difference between physisorption and chemicsorption is the nature of the bonding between the molecules and surface. In physical adsorption there is no significant redistribution of electron density between the substrate (carbons) and the adsorbate (CPA). In chemical adsorption a chemical bond is formed between the adsorbate and substrate [5]. The catalyst preparation process has three main steps, CPA preparation, Impregnation, and reduction. The SEA synthesis technique is a physical adsorption process. Revised Physical Adsorption model (RPA) Before preparing a catalyst the revised physical adsorption model (RPA) important to understand. Figure 2 illustrates a simple RPA model, where an in depth theoretical RPA model was omitted for simplification purposes. Figure 4: Revised Physical Adsorption Model (Spieker and Regalbuto, CES 56, 2001, 3491, Hao and Regalbuto, JCIS, 267, 2003, 259 [6]).

OH2+

O-

OH PZC

K1

K2

[PtCl6]-2 pH<PZC

pH>PZC

[(NH3)4Pt]+2

Figure 2 shows an electrostatic interaction between the carbon support and Platinum complexes. Mainly, if the carbon surface is positively charge an anionic Pt complex is needed, i.e. CPA ([PtCl6]2-. Furthermore, if the carbon surface is negatively charge a cationic Pt complex is needed, i.e. PTA ([(NH3)4Pt]2+). For the research at hand, CPA was the target complex used for the impregnation step do to the high PZC recorded for each carbon used. Because of its characteristics, CPA has a 48 hour preparation process, which will be discuss shortly. CPA preparation In reference readings by Regalbuto and blank [7], the optimized concentration, at which to run the impregnation step, of CPA was found to be 200ppm. The preparation process begins by diluting CPA stock solution to the 200ppm needed. Next, an initial pH is needed that will shift down to the target pH of 2.8. Ideally, the initial pH for 200ppm CPA solution is 4.18, which will shift, after 48 hours, to 2.8. It is important to note that CPA solution should be exposed to light during the 48 hour shift. This is the first of three steps for the adsorption experiment. The Strong Electrostatic Adsorption Method – Wet Impregnation

The strong electrostatic adsorption (SEA) method, the second of three steps, has

become a trademark at the catalysis laboratory at the University of Illinois at Chicago lead by Professor John R. Regalbuto. The PZC data suggests that the CPA pH range should be adjusted, after 48 hours, from 1 to 7. Once the CPA pH range is obtained, the adsorption experiment is carried out. Mainly, the amount of carbon needed for each pH point is calculated using equation (1). Next, the carbon is added to a designated volume of CPA (usually 50mL bottles). The carbon support and CPA are allowed to contact for an hour, where most of the Pt uptake takes place. Before contact, a 5mL sample of the 200ppm CPA is extracted to measure its exact concentration with the Inductively Couple Plasma machine (to see whether its ~200ppm). After 1 hour contact, 5mL of the slurry is filtrated and used to measure its concentration. The difference in concentration, concentration before contact – concentration after contact, is recorded as uptake. Figure 5 suggests that a pH from 2 to 3 be used to obtain the optimum Pt uptake.

Using the data provided in Figure 5, the adsorption experiments, for the carbons given in Table 1, where conducted at pHs ranging from 2 to 3. These experiments will pin point, more accurately, a CPA pH that will yield an optimum Pt uptake. Figure 6 shows the results obtained for the SEA experiments conducted for the carbons given in Table 1. It is important to note that the results given in both Figure 5 and 6, both have carbons with high PZC values, both conducted at surface loadings of 500 m^2/L, and both with CPA concentration of 200ppm.

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15pH

Upt

ake

(ppm

SX4 1Hr pzc=7.5

SX 1Hr pzc=8.0

SX2 1Hr pzc=8.5

Vulcan 1Hrpzc=8.6Ensaco 250 1Hrpzc=8.8ensaco 350 1Hrpzc=9.8

Figure 5: pH vs. Uptake (ppm) from Xianghong Hao Thesis defense (2004). SL=500m^2/L, 200ppm CPA.

0

20

40

60

80

100

120

140

160

180

1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2pH final

Upt

ake

(ppm

)

EC 300J

EC 600J

Black Pearls

Umicore Ensaco 350

GE Ensaco 350

Figure 6: pH vs. Uptake (ppm) SL=500m^2/L, 200ppm CPA.

Figure 7 show the weight percent of each carbon used for the SEA experiments. The weight percent ranges from 20% to 30%.

Weight percent of Pt on carbon

0

5

10

15

20

25

30

35

Ketjen Black EC 300J Ketjen Black EC 600J Black Pearls Ensaco 350Carbon

Pt W

t%

Figure 7: Pt wt% of different carbons (SL=500m^2/L, 200ppm CPA). Reduction of Pt/Carbon catalyst with H2 The reduction process reduces the Pt ligands attached to the carbon surface to elemental Platinum. After the impregnation step, Platinum is found to be in its oxidation state of +4, meaning that other species are present at the surface (i.e. Cl and OH). The reduction process is started by purging the catalyst sample in He at room temperature for 30 minutes. Next, Hydrogen gas is reacted with the catalyst for 1.5 hours. At the beginning, the temperature is ramped by 5ºC per minute until reaching 200ºC. This process usually takes about 30 minutes, the rest of the time, 1 hour, the reaction takes place. Finally, after 1.5 hours of reaction, the sample is cooled to room temperature in He bath. After the reduction process, the catalyst is ready to be characterized via STEM and EXAFS. STEM characterization

STEM imaging will give a rough estimate of the Pt particle size as well as a pictorial representation of the Pt/Carbon Catalyst dispersion. From Figure 8, it can bee seen that the Pt particles are roughly 10-20 Å and highly dispersed. The EXAFS results will yield a numerical dispersion value.

Figure 8: Ketjen EC 600JC, S.A. = 1415 m^2/g, 30 wt% Pt/C, ave. particle size 10-20 Å. Extended X-ray Absorption Fine Structure The EXAFS were conducted at Argonne National Laboratory under Dr. Jeff Miller’s supervision. The results obtained are promising and can be depicted from Figure 9. Mainly, Pt-Pt bonds are close, which can be seen through the small intense peaks. Futhermore, results obtained by Dr. Miller’s fits shows that the estimated dispersion of the Pt is ~ 90%, as can be seen from Table 4.

Figure 9: Timcal Timrex Wet impregnation EXAFS results.

EXAFS Fits Pt/Carbon

June 04 Sample Scatter CN R, Å DWF (x103) Eo, eV Est. Disp.

ctmrex WI Pt-Pt 4.7 2.74 1.0 -3.8 0.9

Table 4: Timcal Timrex Wet impregnation numerical value fits.

Method S.L

m^2/L SA

(m^2/g) CPA pH

CPA concetration PZC

Point of zero charge 1000 795 - - - SEA 500 - 200 200 8 to 9

Table 5: Optimized parameter for the SEA synthesis method and PZC experiments.

0.1

0.2

0 2 4 6

Mag

nitu

de o

f FT

[k3 *C

hi(k

)]

R [Å]

Pt-Pt

Pt Higher Shells

Conclusion The research allowed for the identification of key variables shown in Table 5. The optimization of these variables is vital to the catalyst preparation steps. The ideal conditions at which to run the preparation of the catalyst are given in Table 5 as well, primarily, use high PZC carbons, prepare CPA solution at a final pH of 2.8, surface loading at 500 m^2/L, and CPA concentration of 200ppm. Using the conditions presented, one can obtain important results like those given by the STEM and EXAFS.

In closing, I would like to thank the National Science Foundation for fuding the research, the University of Illinois at Chicago, Professor John R. Regalbuto, and Dr. Jeff Miller.

References

[1] S. Giddey, F.T. Ciacchi, S.P.S. Badwal, V. Zelizko, J.H. Edwards, G.J. Duffy, Elsevier Solid State Ionics 152 (2000) 363. [2] Nice, K. (2000). How Fuel Cell Works. Retrieved July 23, 2003 from the World Wide Web: http://science.howstuffworks.com/fuel-cell.htm [3] Marshall, M. (1999). The Proton Exchange Membrane Fuel Cell. Retrieved July 23, 2004 from the World Wide Web: http://www.humboldt.edu/~serc/animation.html [4]A. B. Stambouli, E. Traversa, J. Renewable & Sustainable Energy Reviews. 6 (2002) 297-306 [5] Nix, Roger (2003). Adsorption of Molecules on Surfaces. Retrieved July 23, 2004 from the World Wide Web: http://www.chem.gmw.ac.uk/surfaces/scc [6] X. Hao, W.A. Spieker, J.R. Regalbuto, Journal of Colloid and Interface Science 267 (2003) 259