With the advances in catalyst technology has come a recognition of the importance of electrode struc- tures. Electrodes have progressed from polytetrafluoroethylene (PTFE)- bonded3 to Nafion®-impregnated PTFE-bonded4 and Nafion®-bonded.5 In PTFE-bonded electrodes, only cata- lyst particles at the membrane-catalyst layer interface were available for elec- trochemical reactions; the remaining catalyst was unusable. Introduction of ionomer such as Nafion® into the PEM Fuel Cell Catalysts: catalyst layer has improved the pro- tonic 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 sup- ports 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 cata- lysts with smaller particles.