Effects of composition and nanostructuring of palladium selenide phases, Pd4Se, Pd7Se4 and Pd17Se15, on ORR activity and their use in Mg-air batteriesElectronic supplementary information (ESI) available. See DOI: 10.1039/c7ta00253j

The increasing demand for conversion and storage of energy has been the driving force for scientific pursuits towards developing novel, highly active and selective materials. In the present study, different phases of palladium selenides (Pd 4 Se, Pd 7 Se 4 and Pd 17 Se 15 ) are synthesized and explored as electrocatalysts for the oxygen reduction reaction (ORR). Various palladium selenide phases are synthesized using a facile and cost effective method of thermolysis of metal-didecylselenium precursor complexes. The effect of the composition of different phases on catalytic activity towards the ORR is studied. Among the three phases, Pd 4 Se is observed to show good catalytic properties with fast kinetics. A four-electron reduction pathway is deciphered in the case of the palladium rich Pd 4 Se phase while the reaction terminates at the peroxide stage when Pd 17 Se 15 is used as the catalyst. Possible reasons based on differences in the environment around Pd for the three phases are indicated. However, nanostructuring improves the catalytic activity of the palladium deficient phase, Pd 17 Se 15 , and a four-electron pathway is observed in the case of nanorods. The use of various conducting supports affects the catalytic activities and reduced graphene oxide (rGO) is found to improve the activity of the bulk Pd 17 Se 15 phase. The selenide-based materials are employed as ORR catalysts in air-breathing Mg-air primary batteries with Pd 4 Se showing high specific discharge capacity. The increasing demand for conversion and storage of energy has been the driving force for scientific pursuits towards developing novel, highly active and selective materials.