Development of a Rotating Disk Electrode-Based Ru-Leaching AST for PEM Fuel Cell Reformate Anode Ru-Pt/C Catalysts
Authors: Diercks Justus Sebastian , Janke Elias , Bauer Florian , El-Sayed Hany A. 
Published: August 2024

Extreme heavy-duty (e.g., maritime) applications of PEM fuel cells often require liquid hydrogen carriers as fuel.[1] In the case of methanol, it is mixed with steam and converted to hydrogen and CO2 in a catalytic reformation process.[2] While a multi-step approach, including the water-gas shift reaction and selective oxidation processes, is typically undertaken to fully convert methanol to hydrogen and CO2, low ppm-level amounts of CO remain in the resulting “reformate hydrogen” gas stream.[3] This CO concentration is significantly higher than deemed non-poisonous to the platinum catalyst on the anode (i.e., < 0.2 ppm),[4] and a more CO-tolerant anode catalyst is required. To address this issue, RuPt-alloy nanoparticles supported on carbon are employed, leading to CO-tolerance of the HOR-catalysts on the anode at reformate conditions.[3] However, previous works found that during operation, Ruthenium leaches from the anode catalyst layer and travels through the membrane, subsequently depositing on the platinum nanoparticles, effectively poisoning the cathode Pt/C catalyst and significantly reducing its activity towards the oxygen reduction reaction.[3,5]

To eliminate or reduce Ru-leaching from RuPt/C catalysts while retaining its inherent CO-tolerance, efforts tuning the catalyst’s (surface) composition were made. However, as the leaching of Ru in fuel cells occurs over thousands of hours, the use of an accelerated stress test (AST) for reformate anodes is required. In one of our previous efforts, an AST for Ru-leaching in fuel cells was used to study RuPt/C degradation.[3] However, the amount of material, MEA preparation time and AST measurement time required for such a test was found to be too high for catalyst development. Additionally, fuel cell measurement capacity is limited due to its high cost and the quantification of the total amount of Ru-leached remained challenging.

In light of this, in this work, we developed an AST aimed at achieving a reliable yet short and less material consuming protocol, enabling us to quantitatively investigate the Ru-leaching of RuPt-materials during catalyst development. This was achieved by using the rotating disk electrode (RDE) configuration, a well-established electrocatalyst characterization and pre-testing method.[6] The RDE-based AST-protocol was developed using three commercial RuPt-catalysts with varying Ru:Pt ratios of 1:1, 1.5:1, and 2:1 while possessing a comparable particle size and electrochemical surface area. Additionally, the commercial catalyst with a 2:1 atomic ratio catalyst was used in a heat-treated form leading to a larger particle size and lower ECSA.

The AST-protocol was comprised of a series of voltammetric cycles aimed at aging the catalyst. During the protocol development, the number of aging cycles (100 – 1000), the scan rate (10 – 100 mV s-1), and the upper inversion potential (0.8 – 0.9 VRHE) of the cyclic voltammograms were varied to investigate their influence on catalyst degradation. This was achieved by collecting the electrolyte (Ar-saturated 0.1M H2SO4) after the AST-cycles and subsequent quantification of the dissolved Ru using inductively coupled plasma optical emission spectroscopy (ICP-OES). Furthermore, the change in the surface composition was qualitatively analyzed by comparing the CO-oxidation peak shape and position before and after the AST-cycling. Examples of the ICP-OES and CO-stripping results for the systematic investigation of the influence of cycle number and scan rate on the 1.5:1 Ru:Pt catalyst using an upper inversion potential of 0.85 VRHE are displayed in Figure 1. Based on this rigorous investigation of the parameters influencing Ru-dissolution in an RDE-environment and after the necessary validation of the results found using the finalized protocol in MEA-based testing,[5] the developed protocol could subsequently be applied to self-synthesized RuPt/C materials as well as RucorePtshell-catalysts.

In summary, this contribution showcases the development of a reliable rotating disk electrode-based accelerated stress test protocol for Ru-leaching from RuPt/C catalysts. Furthermore, its validation with commercial catalysts as well as its application to self-synthesized PEM fuel cell anode catalysts for use with reformate hydrogen, is presented.

References:

[1] L. van Biert et al., J. Power Sources
2016
327, 345-364.

[2] S.S. Ayana et al., Energies
2020, 13, 596.

[3] V. Berova et al., J. Power Sources
2023, 554, 232327.

[4] ISO 14687-2:2012: Hydrogen fuel product specification - Part 2: Proton exchange membrane fuel cell applications for road vehicles.

[5] E. Antolini, J. Solid State Electrochem.
2011, 15, 455-472.

[6] T. Lazaridis et al. Nature Catalysis
2022,
5, 363-373.

Figure 1