Ag and Ag-Mn nanowire catalysts for alkaline fuel cells

摘要

Low loadings of Ag and Ag-Mn nanowire catalysts were applied to the surface of a CNT-base electrode. The catalyzed electrodes had a 60 mV larger onset potential and promoted the ORR via the direct 4 electron pathway. The Ag/CNT, Ag-Mn/CNT, and CNT samples produced a Tafel slope of about 70 mV/decade which confirmed the ORR activation was limited by the migration of oxygen molecules to active surface sites. The catalytic performance of the Ag and Ag-Mn nanowires was also comparable to that of a bulk catalyst but at a much lower loading. Electrochemical test results showed that the Ag and Ag-Mn catalysts exhibited similar performance. The Ag-Mn nanowire catalysts were synthesized using a unique electroless deposition procedure to co-deposit Ag and Mn. ICP confirmed that 2 to 9 at% Mn was present in the nanowires. XPS and XRD analysis showed that the Ag-Mn nanowires were composed of Mn in solid solution with Ag and a thin surface layer containing MnO and MnO_2. The Ag-Mn nanowires were expected to be the most active. The equivalent performance between Ag and Ag-Mn samples was attributed to the presence of inactive MnO and low concentrations of MnO_2 in the nanowires. Although MnO_2 is known to be active towards the ORR, the dominant Mn species in the nanowires was MnO.