Bifunctionality and Reversibility of Colloidal Pt-Ir Anode Catalyst Material for Hydrogen Starvation
Authors: Ioselli Giulia , Seidel Philipp , Yoon Sooyoung , Blaseio Sonja , Hasché Frédéric , Oezaslan Mehtap 
Published: December 2024

Cell reversal events are based on the hydrogen starvation, that takes place during to fast load changes, blockage of flow field channels or catalyst layer pores by liquid water. During the hydrogen starvation, protons and electrons continue to be supplied to the cathode, which is why carbon oxidation reaction (COR) sets in at the anode.[1] To mitigate the carbon oxidation reaction (COR), various catalyst strategies have been reported in the literature. For instance, the addition of iridium-based co-catalysts such as IrOx promote the oxygen evolution reaction (OER) instead of the carbon oxidation reaction (COR). [1-5] Although iridium is very costly and scarce, its use can be further increased by atomically mixing it with platinum to form a Pt-Ir alloy.In this work, we have prepared atomically mixed Pt-Ir alloy catalysts that combine both functionalities (hydrogen oxidation reaction and OER) in one nanoparticle (NP) in a unique matter. A colloidal route was chosen to control the particle size and composition of Pt-Ir NPs. The adopted synthesis method is based on the “Co4Cat” concept in methanol. [6] Afterwards, the colloidal NPs with controlled size of 1 – 2 nm and atomic Pt:Ir ratio of 1:1 and 3:1 were deposited on Vulcan XC72 Carbon and characterized by TEM, XPS and XRF techniques.The electrochemical experiments were performed in a three-electrode arrangement using a thin film rotating disc electrode (TF-RDE) technique. 0.1 M HClO4 was used as electrolyte solution. The ECSA was determined by underpotential deposited hydrogen (Hupd). Linear sweep voltammetry (LSV) measurements were performed to establish the HOR and OER activity. Thereby, the electrolyte was saturated with H2 and Ar for HOR and OER measurements, respectively.The PtIr/V and Pt3Ir/V catalysts with 1 – 2 nm show values of electrochemically active surface area (ECSA) of 70 ± 2 m2 g-1
PtIr and 73 ± 3 m2 g-1
PtIr. The HOR kinetics on platinum – iridium surfaces is very fast in acidic media. Therefore, we could only compare the measured HOR polarization curves with the theoretical diffusion limiting current. Since these are always on top of each other, we can conclude that the Pt-Ir catalysts are very active for HOR. In addition, the OER polarization curves were analyzed and showed an Ir-based mass activity of 70 ± 6 A g-1
Ir for PtIr/V and 104 ± 6 A g-1
Ir for Pt3Ir/V at iR-corrected potential of 1.50 VRHE. As a comparison, commercial IrOx shows a mass activity of 47 ± 6 A g-1
Ir at 1.50 VRHE.To investigate the electrochemical reversibility of Pt-Ir alloy catalysts, we alternated between the LSV measurements in the region of HOR and OER at least five times. One cycle includes three LSV measurements under each HOR and OER conditions. The HOR and OER activities for PtIr/V and Pt3Ir/V continuously decrease within the 5 cycles between HOR and OER. In other words, after the 5th cycle the OER mass activity dropped to 28 ± 3 A g-1
Ir and 32 ± 4 A g-1
Ir for PtIr/V and Pt3Ir/V catalysts, respectively. However, the ECSA values mainly retain and are 68 ± 2 m2 g-1
PtIr for PtIr/V and 70 ± 2 m2 g-1
PtIr for Pt3Ir/V.We can sum up that the colloidal Pt-Ir catalysts show bifunctional properties towards HOR and OER and reversible behaviour, which can be helpful to improve the cell reversal tolerance during the H2 starvation.
Literature:
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