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 PtCoO_x 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 PtCoO_x 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 PtCoO_x alloy core. Thus, the CoO_x species are highly stable during the electrochemical dealloying in acidic media. The ORR mass activity (0.56 ± 0.14 A mg_Pt^-1 at 0.9 V_RHE) of the PtCoO_x core-shell catalyst is 2.5-times higher, whereas the ORR specific activity (592 ± 171 µA cm_Pt^-2 at 0.9 V_RHE) is 3-times higher than that for commercial Pt/C (0.24 ± 0.05 A mg_Pt^-1, 187 ± 29 µA cm_Pt^‑2). The stability of the CoO_x 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 PtCoO_x 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 PtCoO_x 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.