Rare-earth metal oxide hybridized PtFe nanocrystals synthesized via microfluidic process for enhanced electrochemical catalytic performance

PtFe nanocrystals hybridized by one rare-earth metal oxide (i.e., CeOx) (namely PtFe-PtxFeyCezOj) with controlled compositions and gradient microstructures are synthesized continuously in a simple programmed microfluidic process. Their electrochemical catalytic performances are evaluated by methanol oxidation reaction (MOR), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). TEM, EDS, XRD and XPS measurements are used to characterize their microstructures and compositions. Results suggest that PtFe-PtxFeyCezOj nanohybrids have overall sizes around 2 nm, showing gradient structures composed of PtFe rich inner parts and cerium oxide rich outer layers. They preserve angular shapes and rough surfaces, whose atoms have metallic states and/or multi-oxidation states. The electrochemical catalytic measurements indicate that PtFe-Pt(xF)e(y)CezO(j) nanohybrids exhibit excellent electrochemical catalytic performances in MOR, ORR and HER. Particularly, the PtFe-PtxFeyCezOj nanohybrids with the designed atom ratio of Fe: Pt: Ce = 1: 1: 1 show the best MOR performance, with a forward peak current of 1018.6 mA/mgPt, which is 5.6-fold enhancement compared to the commercial Pt/C catalysts (180.8 mA/mgPt), and the best ORR performance, with a specific activity of 3.28mA/cm(2) and mass activity of 0.73 A/mgPt, which are 6.0- and 4.6-fold enhancement compared with the commercial Pt/C catalysts (0.55 mA/cm(2) and 0.16 A/mgPt). It also gives HER a much low Tafel slope (19mV dec(-1)) comparing with those previously reported. Chronoamperometry tests suggest that PtFeCe1 catalysts can maintain 2-fold activity of commercial Pt/C catalysts at the end of 4000 s' operation. TEM images of nanohybrids before and after electrochemical catalytic tests confirm that the suitable coating of CeOx prevents the PtFe nanohybrids from intensive etching and dissolving in the strong acidic reaction electrolytes and improves the durability and microstructure stability of these nanohybrids. The greatly improved catalytic performances are fundamentally resulted not only from the synergistic interface effect between the PtFe alloy rich inner parts and the cerium oxide rich outer layers, but also from the electron orbital hybridization effects among Pt, Fe and Ce atoms. This study provides a new methodology in the microstructure design and controlled synthesis of efficient multifunctional nanocatalysts with stable microstructures by hybridizing rare-earth metal oxides to form stable thin outer-layers. These stable nanocatalysts shall preserve valuable applications in fuel cells and hydrogen production by water splitting. (C) 2019 Elsevier Ltd. All rights reserved.