622
Alcohol Oxidation on Porous PtCu Catalyst
Direct alcohol fuel cells (DAFCs) are promising power conversion systems developed to overcome the restrictions of hydrogen fuel in proton exchange membrane fuel cells. Liquid fuels, such as methanol, ethanol and formic acid, are easily handled, stored and transported, compared to gaseous fuels, such as H2 and natural gas1. However, DAFCs typically have lower performances than fuel cells that utilize H2. Here, we report a PtCu/C catalyst that exhibits higher activity and lower onset potential compared to Pt/C. We also utilize the ratio of the forward anodic peak current (If) over the reverse anodic peak current (Ib), If/Ib, to evaluate the CO poisoning resistance of catalysts. Our data shows that PtCu/C catalyst has higher activity and higher If/Ib value for methanol and ethanol oxidation reaction compared to commercial Pt/C catalyst in both acidic and basic medium.
Experimental
A nanoporous Cu support was formed by etching Al from an in-house prepared CuAl alloy, with composition ranging between 20 to 50 wt% Cu. Upon etching, the nanoporous support provides a high surface area of 10-15 m2/g. Pt was deposited onto the support via a galvanic displacement reaction at a constant rotation rate. Alcohol oxidation activity was evaluated using a rotating ring disc electrode (PINE Instruments) as working electrode, with a reversible hydrogen electrode as reference and a Pt mesh as a counter electrode. All experiments are performed in 0.1M HClO4 + 0.5M MeOH (or 0.5M EtOH) and 0.1M KOH + 0.5M MeOH (or 0.5M EtOH) at room temperature.
Results and Discussion
Figure 1 compares the CV curves of PtCu catalyst with commercial Pt/C catalyst in a 0.5M MeOH + 0.1M HClO4. Both CVs were obtained at 1600 rpm and sweep rate of 10mV/s. The If/Ib values of the forward peaks were calculated and used to suggest poisoning resistance of the catalysts with a larger If/Ib ratio implying greater resistance to electrode poisoning. Our data shows that the If/Ib value of PtCu (~1.5) is much higher than Pt/C (~0.5). Our data indicates that smaller overall Pt surface area gave higher If/Ib ratio (>1), as the deposited Pt surface area reaches a value closer to the Pt/C, the If/Ib ratio starts to ressemble those of Pt/C (at ca. 0.7). Finally, Figure 3 exhibits the bar graph of If/Ib for methanol and ethanol oxidation vs. Pt deposition time in both acidic and alkaline conditions. Alkaline conditions were performed in 0.1M KOH + 0.5M alcohol and acidic conditions were in 0.1M HClO4 + 0.5M alcohol. CVs were collected at 1600 rpm and 100 mV/s. Figure 2 shows that the PtCu catalysts consistently gave higher If/Ib ratios compared to Pt/C. Figure 3 also illustrates that the 2.5min Pt deposition shows the greatest resistance to poisoning during methanol oxidation and the 0.5min Pt deposition catalysts is the most resistant to carbonaceous poisoning during ethanol oxidation in 0.1M KOH. In acidic conditions, both methanol and ethanol oxidation gave the least poisoning characteristic for 0.25min Pt deposition. In this paper we will discuss PtCu as a promising catalyst for alcohol oxidation reactions in DAFCs.