DMFC electrocatalysts set the catalytic efficiency that dictates the DMFC per­formance and establish a large component of DMFC cost. Here, we discuss some recent developments in DMFC electrocatalysts and the materials used to support them.

Gonzalez et al. evaluated Pt and PtRu supported on single-wall carbon nano­tubes (SWNTs) and multiwall carbon nanotubes (MWNTs) as electrocatalysts [36]. The materials are integrated into electrodes and hot pressing with Nafion 115 to form MEAs. Half-cell experiments show the PtRu/C electrocatalysts have an earlier onset of methanol oxidation (i. e., lower potential) than the Pt/C coun­terparts. An MEA made with PtRu/MWNT showed the highest activity. When the electrocatalysts are evaluated in DMFCs, it was found the PtRu/C electrocat­alysts performed better than their Pt/C counterparts. The sequence of electroac­tivity being PtRu/MWNT > PtRu/C > PtRu/SWNT. Maximum power densities greater than 100 mW cm-2 are obtained with PtRu catalyst loadings of 0.4 mg cm-2.

A modified alcohol reduction method was used by Hwang et al. to produced nanosized PtRu/C electrocatalysts [53]. Various electrocatalyst preparations are compared to a commercial PtRu/C electrocatalyst, E-Tek 40 for morphology and effectiveness as an electrocatalyst. Transmission electron microscopy (TEM) show in-house preparations similarly well-dispersed as the commercial electro­catalyst. Metal particle size can be tailored by selection of the concentrations of preparation components. Activity of the electrocatalysts are compared under various methanol conditions in half-cell measurements made in a three-electrode cell. The results are mixed. Under realistic operating conditions (e. g., 40°C, 0.4 V, and [MeOH] ~ 15%) one of the in-house electrocatalysts outperforms the commercial catalyst. However, at higher potentials, the commercial catalyst per­forms better. This same trend holds true at [MeOH] = 35% and 50%, where the in-house catalyst performs better than the commercial at 0.4 V, but the commercial performs better at higher potentials. AC impedance data for this in-house catalyst suggest it has a lower resistance at all potentials and for all concentrations of methanol considered in the study.

An alternative to the costly Ru often employed in Pt-based bimetallic elec­trocatalysts of DMFCs may be Sn. The impact of introducing Sn to the Pt/C anode electrocatalyst of direct alcohol fuel cells (i. e., methanol and ethanol) was evaluated by Zhou et al. [47]. Cyclic voltammograms recorded vs. SCE showed the Sn-bearing electrocatalyst, PtSn/C, had a more favorable onset of oxidation potential (20 mV) than Pt/C (250 mV) and PtRu/C (110 mV), however, the peak oxidation potential of PtSn/C was intermediate (640 mV) to Pt/C (700) and PtRu/C (500 mV). For further comparison, three single-cell DMFCs were pre­pared. The maximum power densities of the three cell tracked with the peak oxidation potentials of the cyclic voltammetric experiments: PtRu/C (136 mW cm-2) > PtSn/C (55 mW cm-2) > Pt/C (17 mW cm-2). The work points to the possibility that adding relatively inexpensive Sn to the Pt/C anode may signifi­cantly improve DMFC catalysts. The effectiveness of this approach is difficult to assess as the performance of the Pt/C blank is so poor, however, the performance trends warrant further research.

One strategy to limit the effects of methanol crossover in DMFCs is to develop cathode electrocatalysts active toward the oxygen reduction reaction (ORR) but inactive toward the methanol oxidation reaction (MOR). Pt is active toward both reactions whereas Pd is active only toward ORR. In a short com­munication, Sun et al. use voltammetry and chronoamperometry to demonstrate a Pd:Pt alloy in the ratio of 3:1 supported on carbon is effective toward the ORR and ineffective toward the MOR [49]. The group goes on to compare the performance of DMFCs impregnated with either the Pd3Pt1/C electrocatalyst or Pt/C control in the cathode. The cell using the PdPt alloy had better overall performance with a maximum power density, ~40% higher than that of the Pt/C control. The authors think that once the ratio of Pd to Pt rises above a certain point, the active sites of the Pt become isolated and overall activity of the alloy toward MOR drops precipitously.

The incorporation of the oxygen storage material CeO2 into the carbon sup­ported Pt electrocatalysts of the cathode was found to enhance the performance of DMFCs when run on air [46]. The ceria compound is known to act as an oxygen storage buffer and helps to maintain the local oxygen pressure, but the effect only appears to be beneficial when using air. Yu et. al. found that when run on pure oxygen, the presence of ceria oxide in the cathode diminished the maximum power of a DMFC by ~20%. The most pronounced enhancements on performance are when operating the cell at low air-flow rates (250 sccm), but the effect becomes minimal at higher flow rates (1250 sccm). Impedance spectro­scopy was used to determine the polarization resistance of the ceria-doped and nondoped Pt/C cathodes under air and O2. The resistance of the ceria-doped cathode was lower than the control when under an air atmosphere, but the ceria — doped cathode had a higher resistance under O2. An optimized cathode compo­sition for use in air was found to be 1 wt % CeO2 and 40 wt % Pt/C.

The performance of PEFCs run on H2 and O2 has been shown to improve upon the incorporation of TiO2 into the carbon-supported Pt electrocatalyst layer [54]. Manthiram and Xiong tested the same modification of the DMFC electro­catalyst layer [48]. A number of deposition methods were tried as well as a series of heat treatments under a reducing atmosphere. The treatments result in an array of different sized particles (3.8 to 25.4 nm) that do not necessarily correlate with the electrochemically active surfaces areas (2.59 to 21.87 m2 g-1) of the particles. Some of the Pt/TiO2/C electrocatalysts showed higher activity toward ORR in sulfuric acid at room temperature than the Pt/C control. Also, a number of the TiO2-doped electrocatalysts have lower charge transfer resistances, as measured by AC impedance, than the Pt/C control. Cyclic voltammetry shows the hydrogen desorption potential decreases and the potential for reduction of platinum oxide increases in the presence of the added TiO2. When the performance parameters of the various Pt/TiO2/C electrocatalysts are compared, it is found the Pt/TiO2/C electrocatalyst prepared by depositing hydrated TiO2 onto a Pt/C substrate and subsequently heat treating at 500°C performed best. When integrated into the cathode of a DMFC, the TiO2 doped electrocatalyst shows higher tolerance to methanol crossover than the Pt/C control. The tolerance becomes more pro­nounced as the methanol concentration of the fuel stream is raised above 1.0 M. The performance of this TiO2-doped electrocatalyst, used either in the heat-treated
or as prepared form in the cathode of a DMFC was higher than an equivalent cell made with a Pt/C cathode.

There is independent evidence that MWNTs are a carbon support for DMFCs superior to the ubiquitous Vulcan XC-72, and that alloying Pt with other transition metals such as Ni, Fe, and Co produces catalysts with higher activity toward ORR than Pt alone. In a brief and preliminary paper, Li et al. bring these two notions together and test a PtFe alloy supported upon MWNTs for use as an electrocatalyst in DMFC cathodes [50]. A modified polyol strategy was used to prepare the electrocatalyst. Characterization of the material showed that the Pt:Fe ratio was ~1:1; however, the Pt and Fe did not form a stable alloy. The electro­chemical performance of the material was tested subsequent to introduction into the cathodes of DMFCs. It was found the mass activity (current per mg Pt) of the PtFe/MWNT cell held at 600 mV (the activation-controlled cell potential region) was ~40 % higher (4.7 vs. 3.3 mA mg-1 Pt) than the Pt/MWNT control cell. At a current density of 300 mA cm-2, the PtFe/MWNT cell held a cell potential of 210 mV versus 151 mV for the Pt/MWNT control. Though the mean particle size for Pt in the PtFe/MWNT material was larger than that of the Pt/MWNT material, the specific activity of the PtFe/MWNT electrocatalyst was more than 2-fold higher at 117 mA m-2 Pt versus 50 mA m-2 Pt for the Pt/MWNT control. The authors propose the presence of Fe in the material as being respon­sible for the enhanced ORR activity and DMFC performance and that detailed work be conducted to elucidate the Fe and Pt interaction resulting in the enhanced performance.