Instability and Degradation Mechanism of Platinum-Group Metal (PGM)-Based Carbon Supported Electrocatalysts in Alkaline Medium

Abstract

Direct liquid alkaline fuel cells (DLAFC) are considered high-energy density generators and arouse interest because (i) their energy density can be higher than for closed systems like Li-ion batteries, and (ii) they do not suffer from issues related to H 2 compression, storage and delivery as for proton-exchange membrane fuel cells (PEMFC). Moreover, electrocatalyst commonly used for PEMFC suffer from a poor durability in operation [1]. In the case of alkaline fuel cells, despite few studies, the common belief supposes a larger stability of electrocatalysts in alkaline medium compared to acidic medium, owing to the better thermodynamic stability of many metals and oxides at high pH, as predicted by Pourbaix [2]. In reality, some recent studies showed that the state-of-the-art carbon-supported platinum nanocatalysts are subjected to severe degradations upon a mild potential cyclic procedure in 0.1 M NaOH at 25°C [3]. The loss of electrochemical surface area (ECSA) reaches 60 % after 150 cycles in the range 0.1 - 1.23 V vs. RHE at 100 mV s -1 . Herein, the same degradation procedure has been performed with Pd/C and PtRu/C: non-negligible ECSA loss ( ca. 35 %) is monitored for Pd/C, whereas very harsh degradation is noted for PtRu/C. Identical-location transmission electron microscopy studies (ILTEM) revealed that the main degradation mechanism is, whatever the electrocatalyst, the detachment of the nanoparticles from the carbon substrate (Figure 1 D-E-F). It is suspected that this detachment follows the rupture of the binding between the PGM nanoparticles and the carbon support, triggered by the formation of carbonates originating from PGM-assisted corrosion of the carbon support. The CO-stripping experiments performed in NaOH 0.1 M (Figure 1 A-B-C) confirm that the ability of each electrocatalysts to form CO 2 is linked to the observed degradations. This work was partly supported by an ONR-global grant (project ONR N62909-16-1-2137). References [1] L. Dubau, L. Castanheira, F. Maillard et al., WIREs Energy and Environment 2014, 540, 3. [2] M. Pourbaix, National Association of Corrosion Engineers 1979, 453. [3] A. Zadick, L. Dubau, N. Sergent, G. Berthomé, M. Chatenet, ACS Catalysis 2015, 4819, 5. Figure 1