Oxygen Reduction at Carbon Supported Pt and PtRu Catalysts in Alkaline Solutions

In order to establish a baseline for non-precious metal catalysts for oxygen reduction reaction (ORR) in alkaline environment, we investigated the ORR kinetics, including the ORR pathway and the apparent activation energy of ORR on Pt/C in an alkaline solution. The stability of Pt/C and PtRu/C was studied by potentiostatic and also potentiodynamic measurements in an O₂-saturated 0.1 M NaOH solution.

In 0.1 M NaOH solutions, the onset potential for the ORR using Pt/C is similar (about 0.05 V vs. HgO/Hg) as what was observed using PtRu/C.  At an overpotential of 0.350 V, the kinetic ORR current (i_k) on the PtRu/C is 3 times lower than on the Pt/C. RRDE measurements show that the H₂O₂ yield on both catalysts are negligible at potentials higher than -0.3 V, and less than 4% for maximum, indicating that the ORR mechanism on the PtRu/C is the same as on the Pt/C, i.e., 4 e^- transfer is the predominant pathway.
The activation energy of the ORR on the Pt and PtRu is similar, which is about 25 kJ mol^-1 at low current densities and about 15 kJ mol^-1 at high current densities. The ORR activation energy is independent of the rotation rate. For the Pt/C, the net oxidation/reduction current is less cathodic than would be expected if the EOR and ORR occurred without interaction at potentials lower than -0.05 V vs. HgO/Hg. The evidence suggests that the ORR is suppressed by ethanol adsorbates when both ethanol and oxygen exist in solution. At potentials higher than -0.05 V, the net oxidation/reduction current is more cathodic than expected if the reactions (EOR and ORR) occur without interaction, indicating the EOR is suppressed by the ORR. The high EOR oxidation current on Pt/C cathode would produce mixed potential and degrade the fuel cell performance. For the PtRu/C, the EOR only has little influence on the ORR. The stability tests indicate that PtRu catalysts are stable under fuel cell cathode working conditions in alkaline solutions