Abstract
A self-supported Pt-CoO alloy catalyst has recently been reported as a new concept for Pt-based catalysts combining high surface area with high ORR activity. [1] Very recently, the presence of cobalt oxide species within Pt-Co catalyst after electrochemical dealloying in acidic media has also been reported by Weber et al. [2] However, the elemental distribution particularly for light elements like oxygen as well as the influence of the Co oxide on the ORR activity are still unclear to date.We prepared a disordered PtCoOx alloy catalyst using wet-impregnation - freeze-drying - thermal annealing method. [3] After electrochemical activation by dealloying, the less noble metal is dissolved from the nanoparticle surface and the remaining Pt surface atoms are forming a protective particle shell referred to as core-shell catalyst. [2, 3] Using high resolution scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) we were able to explore the detailed structure of the activated PtCoOx catalyst with a Pt-enriched shell. Based on the EELS elemental maps of Pt, Co and O, we observed that oxygen is mainly located at the interface between the Pt-enriched shell and the PtCoOx alloy core. Thus, the CoOx species are highly stable during the electrochemical dealloying in acidic media. The ORR mass activity (0.56 ± 0.14 A mgPt -1 at 0.9 VRHE) of the PtCoOx core-shell catalyst is 2.5-times higher, whereas the ORR specific activity (592 ± 171 µA cmPt -2 at 0.9 VRHE) is 3-times higher than that for commercial Pt/C (0.24 ± 0.05 A mgPt -1, 187 ± 29 µA cmPt -2). The stability of the CoOx species and the electrochemical catalyst durability were tested by using an accelerated stress test (AST, 10,000 cycles from 0.5 to 1.0 VRHE) in acidic media. Here, the PtCoOx core-shell catalyst showed an improved electrochemical durability compared to Pt/C and maintains 85% of the initial ECSA, 54% of the initial ORR mass activity and 68% of the initial ORR specific activity, respectively. From the STEM-EELS and XPS measurements, we revealed an increase of the thickness of the Pt-enriched shell of several monolayers after the AST protocol. Very surprisingly, the cobalt oxide in the sub-surface layers still remains, but it is less narrowly distributed than before the AST experiment.Thus, we suggest that the Co oxide species in PtCoOx alloy catalyst might have a positive effect on the ORR performance and durability and could even be a yet undiscovered alternative to metallic cobalt.Reference:[1] G.W. Sievers et al., Nat. Mater., 2021, 20, 208-213;[2] D.J. Weber et al., J. Mater. Chem. A, 2021, 9, 15415-15431;[3] M. Oezaslan et al., J. Electrochem. Soc., 2012, 159, B394-B405.
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