PEM Fuel Cell Catalysts

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With the advances in catalyst technology has come a recognition of the importance of electrode structures. Electrodes have progressed from polytetrafluoroethylene (PTFE)- bonded3 to Nafion®-impregnated PTFE-bonded4 and Nafion®-bonded.5 In PTFE-bonded electrodes, only catalyst particles at the membrane-catalyst layer interface were available for electrochemical reactions; the remaining catalyst was unusable. Introduction of ionomer such as Nafion® into the PEM Fuel Cell Catalysts: catalyst layer has improved the protonic conductivity in the electrode layer, extending the reaction zone and Cost, Performance, and Durability increasing the catalyst utilization. The use of Nafion®-bonded electrodes has by Chunzhi He, Sanket Desai, Garth Brown, and Srinivas Bollepalli been a key step in the development of electrode structures. roton exchange membrane (PEM) fuel cells are typi- While significant technical advances have been made in many aspects, chal- cally classified as methanol-based or hydrogen-based lenges remain toward commercializa- Pdepending on the fuel used to convert chemical energy tion of fuel cells. To be competitive into electricity. Although direct methanol fuel cells have with traditional power sources, fuel cells must improve in cost, perfor- advantages of fuel availability and storage, long unsolved mance, and durability. [Editor’s note: problems of poor anode kinetics and high methanol cross- See article by Mathias et al. on p. 24 of over limit their potential use mainly to applications with this issue.] low power requirements, such as portable appliances. Cost Hydrogen fuel cells, on the other hand, are targeted toward A key factor toward commercial- applications that emphasize energy efficiency and require ization of the fuel cell technology high power densities, and may potentially replace the is its cost competitiveness. The pre- internal combustion engine in automobiles. dominant cost driver is the amount of precious metal used. Researchers are striving to reduce the precious metal The electrochemical reactions in a densities. A great deal of research has requirement in the cell by design- hydrogen fuel cell are simple (Eq. 1-3). been focused on improving the oxy- ing better materials and improving Hydrogen is oxidized at the anode into gen reduction kinetics. the overall system engineering. The protons and oxygen is reduced at the Oxygen reduction catalysts have move from platinum black to carbon cathode to produce water. Both reac- evolved from platinum black to carbon supported platinum catalysts has sig- tions can be catalyzed by platinum. supported platinum nificantly cut platinum requirements. catalysts. Platinum Typical loadings in the electrode today Anode H → 2H+ + 2e- [1] 2 black catalysts are are about 0.4-0.8 mg platinum/cm2, + - Cathode ½ O2 + 2H + 2e → 2H2O [2] not economically which is significantly lower than 25 feasible due to their mg/cm2 with early platinum black cat- Overall H2 + ½ O2 → H2O [3] low surface areas, alysts. The US Department of Energy E = 1.229 V at 25 °C requiring the use (DOE) has set targets of 0.3 mg/cm2 for of higher plati- 2010 and 0.2 mg/cm2 for 2015. While hydrogen oxidation over num loadings per unit area in order Due to the fast kinetics of hydrogen platinum is intrinsically very fast, oxy- to attain reasonable performance. oxidation, anode loadings could be gen reduction over platinum is very Carbon-supported platinum (Pt/C) lowered from 0.40 to 0.05 mg/cm2 slow. Exchange current density is one catalysts have higher active surface without significant loss in perfor- measure of kinetics of electrochemi- areas and are the materials of choice mance. Oxygen reduction is slow, and cal reactions. The exchange current in today’s fuel cells. Pt/C catalysts reducing the cathode loading results in density of hydrogen oxidation on are now available in loadings from a performance loss. Lowering the cath- platinum is almost three orders of around 10% to over 50% platinum. ode loading fourfold, which is required magnitude higher than that of oxygen Developments in catalyst synthesis to meet the DOE target, would lead 2 -3 reduction, e.g., 1 mA/cm (H2) vs. 10 technology have allowed for produc- to a loss of about 40 mV. To achieve 2 1,2 mA/cm (O2). The polarization loss tion of catalysts containing over 50% this target, and maintain today’s state at the anode under practical operating platinum with very small, stable plati- of-the-art performance, alternative current densities such as at 0.4 A/cm2 num particles. For example, catalysts catalysts may have to be developed is about 10 mV, while that at the cath- with 55% platinum by weight can be that are four times more active than ode is over 400 mV at similar current made with an average platinum par- (continued on next page) ticle size of approximately 2.1 nm.

The Electrochemical Society Interface • Fall 2005 41 He et al. While smaller particles are therefore Water, formed at the cathode and (continued from previous page) generally desirable, the final perfor- essential to the efficient fuel cell opera- mance of the catalyst in the operating tion, can be a hindrance to the free traditional Pt/C. One alternative cur- environment is a result of many fac- flow of the gases. Water is a great con- rently being explored is the use of Pt- tors. Several studies have shown that ductor of protons through the cell, but X/C alloy catalysts (X = Co, Fe, Ni, or the oxygen reduction reaction exhibits accumulation of water in the electrode other transition metals). Pt-Co/C, for a particle size effect in the range of 1-5 layer may lead to cell flooding that instance, has shown a twofold activity nm, finding that an optimal particle blocks access of the gases to platinum. 6,7 enhancement for oxygen reduction, size exists for high catalytic activity. The water that is produced must be allowing a reduction in platinum This effect has been reviewed in the appropriately eliminated from the cell. 2 loading from 0.40 to 0.20 mg/cm . A literature by Kinoshita,9,10 Stonehart,11 The catalyst layer must allow for an further twofold activity enhancement and Mukerjee;12 and several theories optimal balance of water to maintain at the cathode is needed to meet the have been proposed to explain this an efficient operation. above mentioned target. According to effect - absence of faceting in platinum a report by Technology Tracking,8 uti- The physical and chemical proper- particles in that size regime, instability ties of the carbon substrate can alter lizing nanostructured carbons such as of small particles, and poor platinum nanofibers, nanotubes, etc. as supports the nature of water-catalyst interac- dispersion on such catalysts. While tion. Porous supports typically exhibit may help realize the target. Several results differ on the optimal particle novel approaches being pursued in the higher water sorption. Depending on size for highest catalytic activity, they the pore size distribution in the car- US and abroad hold promise to boost suggest that the catalyst must be catalytic activity and may potentially bon, water may or may not accumu- designed with a specific particle size to late within the catalyst layer. Presence reduce the loading to as low as 0.1 maintain optimal catalytic activity. mg/cm2. of chemical functionalities on the The platinum particle size can be carbon support can alter the catalyst derived from an X-Ray diffraction hydrophilicity and hydrophobocity, Performance (XRD) pattern, which also provides thereby affecting the water balance. Cost reductions in fuel cells neces- information on crystal phases in the In fact, the catalyst support can be sitate improvements in catalyst catalyst. The XRD technique is gener- selectively treated by specifically intro- performance. Platinum particle size, ally complemented by cyclic voltam- ducing hydrophilic and hydrophobic crystallite geometry, metal dispersion metry (CV) to measure the catalytically modifications to design a material on the carbon support, conductivity, active surface area. The active area with good water balance. and porosity of the catalyst layer are depends on the metal particle size and Mass transport losses can also be important features that determine on the uniformity of metal dispersion minimized by reducing the thick- the catalytic behavior in the operat- on the carbon support. Uniform cata- ness of the catalyst layers, attainable ing environment. These features can lyst dispersion, observable by transmis- by using catalysts more concentrated be controlled by proper design of the sion electron microscopy (TEM) (Fig. in platinum. For a specific platinum catalyst synthesis process. Appropriate 1), is essential for good platinum utili- loading, e.g., 0.3 mg Pt/cm2, a catalyst choice and modification of reagents zation and for resistance to platinum with 60% platinum forms a thinner and the carbon support can further sintering. layer than a similar catalyst with 40% influence the catalyst properties. A good catalyst must have optimal platinum. Results show that catalysts In a Pt/C catalyst, small platinum mass transport characteristics. During with higher platinum concentrations particles are preferentially anchored cell operation, the reactant gases offer performance advantages at high to high energy sites on the carbon — hydrogen and oxygen — traverse current densities where mass transport surface. The density of such sites on the porous electrode layer to reach phenomena usually dominate. For a the support influences the platinum active platinum sites. Resistance to the given support, catalysts with higher crystallite size in the resulting catalyst. transport of gases must be minimized. platinum loadings, however, also tend Carbon blacks typically used as supports for catalysts have surface areas between 250 and 1200 m2/g. Supports with higher surface areas tend to have a higher number of active sites for platinum deposition and lead to catalysts with smaller particles. Specific treatments can be applied to the support to increase or decrease the number of active centers available on the surface. Carbon blacks can be chemi- cally modified to introduce functional groups that act as sites for platinum anchoring. Besides the nature of the carbon support, the platinum particle size is influenced by the catalyst preparation process, post treatment, and handling of the catalyst. FIG. 1. Transmission electron microscopy (TEM) image of DURA-lystTM-1140 (40% Catalysts with smaller platinum par- Pt/C catalyst). ticles have higher active surface area.

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to have larger platinum particles, and hence lower active areas. The balance of surface area losses and mass trans- Proton Conduction port losses must be weighed depending on the application under consider- H+O S- H+O S- ation. 3 3 During cell operation, electrons and protons generated or consumed Black Black

at platinum sites migrate through the N N electrode layer. The electrode layer H H Carbon must exhibit good electronic and Carbon protonic conductivities to minimize resistive losses in the cell. Commonly n used carbon supports (Conductex® 975 and Vulcan® XC-72) are good conduc- tors of electrons. The catalyst by itself, Electron Conduction however, cannot conduct protons. Presence of ionomer in the catalyst layer is necessary to conduct the pro- FIG. 2. Catalysts with surface modified carbon blacks show tons. Therefore proton conduction enhanced proton and electron conduction. may depend dramatically on the for- mulation of the catalyst layer. Recent advances in catalyst design have begun 1.00 to address this limitation. As with electron conduction, thinner catalyst 0.95 layers help reduce resistive losses due to proton conduction. Further, the sur- 0.90 face chemistry of the catalyst support 0.85 can be tailored appropriately, such as illustrated in Fig. 2, to impart protonic 0.80 conductivity to the catalyst. However, such changes must be carefully per- 0.75 Cell Voltage (V) Voltage Cell formed to minimize any adverse impact on other important features of 0.70 the catalyst. 0.65 The performance of the catalyst in a fuel cell is a net effect of the features 0.60 discussed above. It is typically tested 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 in a laboratory by a polarization curve, Current Density (A/cm2) which measures the voltage produced TM by a cell for a range of current densi- FIG. 3. Polarization curve of a H2/air fuel cell (DURA-lyst -2255 ® ties (Fig. 3). Pt/C, Nafion -112, air = 2.5 stoic, constant H2 flow rate, ambient pressure, 70°C). Durability

In addition to cost and performance, platinum, and corrosion of the carbon decreases the mobility of the metal long term stability of the catalyst support. particles. The nitrogen moiety in the is important. Often-quoted lifetime Sintering of platinum particles on polymer acts as a Lewis base that can targets for fuel cells are 5,000 h for anchor the platinum particles.13 automotive and 40,000 h for station- the carbon support decreases catalyti- ary applications. The membrane and cally active surface areas. Small metal Dissolution of platinum from the the catalyst must withstand these particles of the catalyst may dissolve carbon support is less favorable under durations without significant changes into the acidic operating environ- neutral pH or zero potential. In the in performance. The membrane may ment, precipitating onto larger metal acidic operating environment in the degrade over time due to attack by particles leading to particle growth; fuel cell, or in the presence of a poten- peroxide radicals which can form at or the particles may directly coalesce tial field, platinum can dissolve from the cathode. Presence of contaminants with each other due to movement on the catalyst. The dissolved metal cat- in the cell can accelerate the rate of the carbon surface. Both mechanisms ions can travel through the system and peroxide generation. Contaminants occur to some degree, with dissolu- contaminate the polymer membrane, such as chloride ions also may poison tion-precipitation being more preva- thus decreasing protonic conductiv- the platinum catalyst. Purity of fuel lent when load cycling occurs. The sin- ity and affecting cell performance. cell components is critical to stabil- tering of the catalyst may be reduced Platinum alloy catalysts show greater ity, although high purity sometimes by strengthening the metal-support resistance to dissolution and sinter- 14 results in higher cost. The catalyst interaction. For example, modification ing, although care must taken to also may lose stability due to sintering of the carbon support, such as graft- ensure that the alloying element itself of platinum particles, dissolution of ing polyaniline to the support surface, (continued on next page)

The Electrochemical Society Interface • Fall 2005 43 He et al. may also be modified by graphitiza- in automotive applications. Concerns (continued from previous page) tion. When traditional carbon blacks of platinum dissolution and carbon are heated to high temperatures corrosion must be addressed to realize does not leach into the cell. (e.g., ~2,000 °C), lattice rearrange- the DOE targets for cell durability. Corrosion of the carbon support also ments occur, increasing the graphitic At Columbian Chemicals Company, 15 may lead to performance loss. When nature of the material and decreasing we are investigating alternative carbon corrodes, the relative percent- the number of active surface sites. technologies, such as tailored mor- age of conductive material in the cata- Graphitization produces a material phologies and surface modification lyst layer decreases. The resistance of which is highly resistant to oxidation of carbon nanomaterials, nanotubes, the remaining dielectric material then and carbon corrosion. However, with and nanofibers for improved catalyst dominates the cell resistance. Further, fewer active sites, metal deposition on performance and durability. Electrodes as the carbon support oxidizes, the such supports is more difficult. composed of catalysts with tailored thickness of the catalyst layer decreas- supports have longer lifetimes, greater es, decreasing electrical contact with Conclusion permeability of reactants, and higher the current collector and increasing Continued developments in catalyst protonic conductivity for enhanced the cell resistance. Carbon corrosion catalysis. These novel carbon supports also decreases the number of sites materials are critical to the commer- cialization of PEM fuel cells. Catalyst and alloy catalysts are focused to bring available to anchor platinum, resulting the technology closer toward realizing in metal sintering. loadings have been reduced more than 50-fold in the last few decades, from the DOE targets. The extent of carbon corrosion in early 25 mg/cm2 to current loadings the cell depends on the operating of 0.5 mg/cm2 or less. The use of 50% Note conditions and the specific chemistry or higher Pt/C catalysts in recent years DURA-lyst™ and Conductex® are of the support used. Higher operating has allowed for better gas transport voltage increases the degradation rate. registered trademarks of Columbian and improved catalyst utilization. Chemicals Company; Nafion® is a The surface area of the carbon sup- However, challenges remain to make port also influences the rate of carbon registered trademark of E.I. Dupont fuel cells competitive with traditional de Nemours and Company; Vulcan® corrosion. The higher the surface area power sources. Cathode losses with of the carbon, the faster is the rate at is a registered trademark of Cabot current materials are still too high Corporation.  which it corrodes. A catalyst maker to meet DOE performance targets. may attempt to lower the available Stability is of particular concern for support surface area by selecting an dynamic operating conditions, such as alternative carbon. The carbon support

References 5. X. Ren, M. S. Wilson, and S. Gottesfeld, J. 11. P. Stonehart, Ber. Bunsen-Ges. Phys. Chem., Electrochem. Soc., 143, L12 (1996). 94, 913 (1990). 1. N. M. Markovic, Handbook of Fuel Cells 6. S. S. Kocha and H. A. Gasteiger, Paper 12. S. Mukerjee, J. Appl. Electrochem., 20, 537 – Fundamentals, Technology and Applications, presented at Fuel Cell Seminar 2004, San (1990). W. Vielstich, A. Lamm, and H. A. Gasteiger, Antonio, TX. 13. S. C. Roy, A. W. Harding, A. E. Russell, and Editors, Vol. 2, p. 374, John Wiley & Sons 7. H. A. Gasteiger, S. S. Kocha, B. Sompalli, K. M. Thomas, J. Electrochem. Soc., 144, Ltd., New York (2003). and F. T.Wagner, Appl. Catal. B: 2323 (1997). 2. A. Hamnett, Handbook of Fuel Cells Environmental, 56, 9 (2005). 14. J. S. Buchanan, L. Keck, J. Lee, G. A. Hards, – Fundamentals, Technology and Applications, 8. Report by Technology-Tracking, Institute of and N. Scholey, in Proceedings of the First W. Vielstich, A. Lamm, and H. A. Gasteiger, Physics Publishing (2005). International Fuel Cell Workshop (1989). Editors, Vol. 1, p. 34, John Wiley & Sons Ltd., New York (2003). 9. K. Kinoshita, J. Electrochem. Soc., 137, 845 15. H. A. Gasteiger, Keynote Lecture presented (1990). at 19th North American Catalysis Society 3. A. J. Appleby and E. B. Yeager, Energy Meeting, Philadelphia, PA (2005). (Oxford), 11, 137 (1986). 10. K. Kinoshita, Modern Aspects of Electrochemistry, Vol. 14, p. 557, J. O'M. 4. J. Kim, S.-M. Lee, and S. Srinivasan, J. Bockris, B. E. Conway, and R. E. White, Electrochem. Soc., 142, 2670 (1995). Editors, Plenum, New York (1982).

About the Authors His research focuses on fuel cell catalyst of copolymer/metal nanocomposites development and reaction modeling in deposited from supercritical ﬂuids. His CHUNZHI HE is a fuel cell scientist heterogeneous systems. He has a BS in e-mail address is gbbrown@phelpsdodge. at Columbian Chemicals Company, chemical engineering from the University com. Marietta, GA. Before he joined of Bombay (India), and a PhD from the Columbian, he had worked for H Power SRINIVAS BOLLEPLLI is a Senior Scientist University of Virginia. He can be reached Corp. and Giner, Inc. on H /air fuel leading the Surface Modiﬁcation 2 by e-mail at [email protected]. cells and direct methanol fuel cells. His Technology and the Next Generation research work includes PEM fuel cell GARTH BROWN is currently working at Fuel Cell Catalysts Development at MEA fabrication, catalyst synthesis and Columbian Chemicals Company as a Phelps Dodge. Earlier, he was a DuPont characterization, and GDL development. fuel cell scientist working on development Post Doctoral Fellow at the Department He has a PhD in chemical engineering of new PEM fuel cell catalysts. Before of Polymer Science and Engineering, from the University of Connecticut. working within the Fuel Cell group, he University of Southern Mississippi, He can be reached by e-mail at worked within the industrial lab working Hattiesburg, MS. He has a PhD from [email protected]. with carbon black and plastics. He Indian Institute of Technology, Madras obtained his PhD from the University and MS and BS from University of SANKET DESAI is a Senior Scientist and of Massachusetts in polymer science Madras, India. He can be reached by e- Catalyst Group Leader at Phelps Dodge. and engineering working on preparation mail at [email protected].

44 The Electrochemical Society Interface • Fall 2005