Insights into the degradation of metallic core-shell nanoparticles under fuel cell conditions by 3D identical location STEM

Abstract text (incl. figure legends and references)

Proton exchange membrane fuel cells (PEMFCs) are electrochemical devices capable of generating electricity by oxidizing H2, reformate (H2 rich gas with carbon monoxide (CO) impurities) or other fuels. In recent times, metallic core-shell nanoparticles (NPs) (M@Pt, M=Ru, Rh…) have attracted a big interest as anode catalysts of reformate fed PEMFCs [1]. The high catalytic activity of Pt towards the hydrogen oxidation reaction, together with the CO poisoning tolerance introduced by the accompanying metal make them ideal for heavy-duty applications. Furthermore, since in M@Pt NPs the less stable metal (Rh or Ru) is not directly exposed to the electrolyte, their stability is expected to be higher than in the corresponding alloyed NPs, which commonly suffer from dissolution and dealloying [2]. However, M@Pt can still suffer from degradation under fuel cell conditions by processes that are yet not fully understood, which hinders the design of more stable and durable catalysts.

We investigated the degradation behavior of Rh@Pt NPs by means of identical location-scanning transmission electron microscopy (IL-STEM). This quasi in-situ technique allows to overcome the limitations of the ex-situ techniques, in which only statistical general insights are possible, since in IL-STEM the changes of individual particles are tracked between potential cycles. In particular, we characterized the Rh@Pt NPs after 0, 1000, 4000 and 10000 potential cycles (0.06-0.8V, 0.1V/s). Furthermore, since many of the degradation phenomena take place in 3D (e.g., particle migration and corresponding aggregation), selected regions were reconstructed in 3D by means of electron tomography (Figure 1).

We observed particle migration on the carbon support in all the stages of the potential cycling. However, no widespread particle aggregation was observed, even after 10000 potential cycles. A slight Rh dissolution (up to 5 at.%) during the cycles was detected, which decreased as the number of cycles increased. Even though some small particles dissolved during the first 1000 cycles, the main degradation mechanism responsible for the loss of electrochemically active surface area was found to be particle detachment.

Our results indicate that the investigated Rh@Pt NPs present a remarkable stability, and show how IL-STEM can be used for studying the degradation of catalyst NPs.

[1] A. Garzón Manjón, M. Vega Paredes, V. Berova, T. Gänsler, T. Schwarz, N. Rivas, K. Hengge, T. Jurzinsky, C. Scheu, Insights into the performance and degradation of Ru@Pt core-shell catalysts for fuel cells by advanced (scanning) transmission electron microscopy, [Submitted].

[2] M. Vega Paredes, A. Garzón Manjón, B. Hill, T. Schwarz, N. Rivas, T. Jurzinsky, K. Hengge, F. Mack, C. Scheu, Evaluation of functional layers thinning of high temperature polymer electrolyte membrane fuel cells after long term operation, Nanoscale. (2022). https://doi.org/10.1039/D2NR02892A.