Comparing the Stability of Carbon-Supported Pt-Nanoparticle and Unsupported Pt-Aerogel Electrocatalysts for the Reduction of Oxygen in Polymer Electrolyte Fuel Cells

State-of-the-art electrocatalysts for the H₂-oxidation and O₂-reduction reactions (HOR and ORR, respectively) taking place inside polymer electrolyte fuel cells (PEFCs) consist of Pt‑nanoparticles supported on a high-surface area carbon black (Pt/C). This support grants the large extents of Pt-nanoparticle dispersion and reactant/product-transport required to achieve optimum PEFC-performance, but also suffers from severe corrosion during the potential cycles intrinsic to the device’s operation, compromising its reliability and commercial feasibility [1]. One approach to overcome this issue relies on the development of unsupported electrocatalysts, which can be prepared, e.g., following a nanoparticle destabilization and gelation procedure [2-6]. The resulting, highly dispersed (≈ 100 m_metal²∙g_metal^-1) aerogels can be synthesized both in mono- and bimetallic compositions that, in the case of PtPd-alloys, display a ≈ 5‑fold improvement in mass-specific ORR-activity over commercial Pt/C-catalysts [4]. Additionally, these unsupported materials also demonstrate better stability upon potential cycling, at least within the potential window of 0.5 – 1 V_RHE assessed in Ref. 4. Nevertheless, the latter study [4] did not provide any insight on the durability of these aerogels upon potential excursions up to 1.5 V_RHE concomitant to PEFC-start/stop, or on the detailed mechanisms of catalyst degradation at play in these tests.

Motivated by this lack of understanding, we have studied the stability of the monometallic Pt‑aerogel under various PEFC-relevant potential-holding and -cycling conditions. More precisely, these stability tests were performed in a three-electrode, rotating disk electrode (RDE) setup, cycling the potential between 0.5 − 1.0 V_RHE or 0.5 − 1.5 V_RHE, or holding the potential at 1.5 V_RHE for a given time. The first of these protocols is intended to mimic the cathode potential profile during high- and low-power operation, while the other two emphasize the corrosion of Pt nanoparticles and of the carbon support upon PEFC-start/stop [7]. Beyond the use of RDE‑voltammetry to assess the changes in electrochemical surface area and ORR‑activity, we performed CO-stripping voltammetry [8] and transmission electron microscopy to study the evolution of the particles’ morphology and size during these tests. Additionally, the use of a low volume (≈ 12 cm³) electrochemical cell allowed us to quantify the extent of Pt‑dissolution by analyzing the electrolyte with inductively coupled plasma/optical emission spectrometry. Considering that the Pt-aerogel consists of interconnected nanoparticles with an average diameter of 2.5 – 3 nm (Fig. 1), the results obtained with these techniques were compared to those acquired on a commercial catalyst with a similar particle size and electrochemical surface area (TKK’s 47 % Pt/C, with ≈ 80 m_Pt²∙g_Pt^-1) [9].

In summary, this contribution will provide detailed insight into the potential-induced degradation mechanisms at play on a commercially relevant Pt/C-catalyst and an unsupported Pt-aerogel, which ultimately benefits from the absence of a carbon support. Moreover, the results obtained with the latter material will serve as a benchmark for our ongoing studies on bimetallic aerogels based on the combination of Pt with other inexpensive transition metals like Ni or Fe.

Figure 1. Transmission electron micrographs of the Pt-aerogel.

Acknowledgements

Funding from the Swiss National Science Foundation (contract 20001E_151122/1), the Alexander von Humboldt Foundation, the European Research Council (ERC-2013-AdG AEROCAT) and the Deutsche Forschungsgemeinschaft (grant Nr. EY 16/10-1 and -2, RTG 1401, cfAED, and EY 16/18-1) is greatly acknowledged.

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