Self-stabilized Pt–Rh bimetallic nanoclusters as durable electrocatalysts for dioxygen reduction in PEM fuel cells
Self-stabilized Pt–Rh nanoclusters (NCs) were prepared by using a surfactant-free chemical reduction method with formic acid as the reducing agent. The elemental composition was determined by EDX analysis. The synthesized cluster was used as a supportless (SL) electrocatalyst for the reduction of oxygen (ORR) in acid medium. The composition of Pt–Rh bimetal NCs, in terms of atomic weight percentage, was optimized based on the available electrochemical surface area. Hydrodynamic linear scan voltammetric profiles show that the onset potential for oxygen reduction is 0.78 V vs. RHE at the electrode rotation rate of 2400 rpm with 17.8 μg cm−2 loading of the SL Pt3Rh exhibiting the limiting current density of 3.5 mA cm−2. The durability of the electrocatalysts was investigated by performing the accelerated durability test (ADT): the electrochemical surface area (ECSA) for SL Pt3Rh increased by nearly 9.2% while retaining nearly 85% of its initial limiting current density after 15 000 potential cycles. For comparison Vulcan-carbon-supported Pt3Rh was synthesized under identical conditions and subjected to electrochemical investigations. Both supportless and VC-supported Pt3Rh NC electrocatalysts were found to use a direct 4-electron transfer mechanism. In order to improve the activity, SL Pt@Pt3Rh NC was synthesized and used as the catalyst. At 0.9 V, the mass activity (0.085 mA μg−1) of the Pt@Pt3Rh NC was found to be nearly 34 times greater than that of SL Pt3Rh NC (0.0025 mA μg−1). We conclude that the SL Pt3Rh NC could potentially be used as an electrocatalyst for ORR in a sulfuric acid medium since it possesses good stability compared to Pt-based ORR catalysts reported in the literature.