Comparison of Galvanostatic and Potentiostatic Accelerated Stress Tests of Bifunctional Pt-Ir Alloy Anode Catalysts for H2 Starvation Using Rde Technique
Authors: Seidel Philipp , Ioselli Giulia , Yoon Sooyoung , Blaseio Sonja , Mintel Florian , Hasché Frédéric , Oezaslan Mehtap 
Published: December 2024

One of the main challenges to overcome for the wide implementation of PEMFCs is the improvement of their long-term durability. For instance, the blockage of the diffusion pathway by liquid water or rapid changes of load can lead to hydrogen starvation.[1] Due to the hydrogen starvation, the anode potential increases and accelerates the carbon oxidation reaction (COR) to further supply electrons and protons for the cathode ORR half-cell reaction, also referred to as cell reversal event.[2] Aggregation and particle detachment associated with the carbon corrosion occur, resulting in a drastic loss of the PEMFC performance.[3] Therefore, material innovation solutions are needed to overcome the cell reversal events. In this context, fast and simple accelerated stress tests and set-ups are needed. Currently, galvanostatic accelerated stress tests (G-AST) at single cell level are often used to evaluate the cell reversal tolerance (CRT) of catalyst materials, which is extremely time-consuming, costly and slow.[4] On the other hand, the rotating disc electrode (RDE) technique is widely used, simple, less expensive and fast for catalyst screening. However, the electrochemical measurements of activity and durability are usually carried out in the potentiostatic mode. The research question arises whether the RDE technique is capable of evaluating the CRT behaviour of novel catalyst materials using galvanostatic AST.
This systematic work compared both galvanostatic and potentiostatic accelerated stress tests (G-AST and P-AST) protocols to evaluate the degradation behaviour of novel bifunctional Pt-Ir alloy catalyst materials for hydrogen starvation. The Pt-Ir alloy catalysts with 1:1 and 3:1 ratios were prepared by wet-impregnation route. Here, the PtIr and Pt3Ir catalysts supported on Vulcan XC72 show a mean particle size of 3-5 nm and total metal loading of 35-50 wt.%Pt+Ir obtained from TEM, TGA and EDX data. In a RDE set-up equipped with three-electrode configuration, all electrochemical measurements were performed in 0.1 M HClO4 and room temperature. At begin-of-life (BoL), the performance of the Pt-Ir catalysts was evaluated by measuring the ECSA via Hupd and CO stripping methods, HOR and OER polarization curves. It is noted that the HOR polarization curves were only fitted with the diffusion limiting current due to the fast kinetics of the HOR on platinum and iridium surfaces in acidic media. For the G-AST protocol, a current density of 0.5 mA/cm2
geo was held for 10h with a cut off at 2 VRHE. The time it takes to reach 2 VRHE is referred to as the fail time (FT) and signifies the loss of the protection mechanism of catalyst materials, namely the OER activity. Based on the results from the G-AST protocol, the same FT was used to perform the chronoamperometric measurements by holding the potential at 1.6 VRHE during the P-AST protocol.
At the BoL, the Pt-Ir catalysts with 3-4 nm size show sufficient ECSA values (45-50 m2/gPt+Ir via Hupd) and improved OER activity (PtIr: 23±6 A/gPt+Ir and Pt3Ir: 8±1 A/gPt+Ir @1.5 VRHE) compared to the commercial Pt/V catalyst (79±4 m2/gPt via Hupd, 6±1 A/gPt @1.5 VRHE) and IrOx (47±6 A/gIr). During the G-AST, we observed that the FT increases with higher Ir content. In addition, the Pt3Ir/V catalyst shows a significant improvement of FT compared to the Pt/V. More precisely, during the G-AST protocol, the potential of 2 VRHE by applying a constant current density of 0.5 mA/cmgeo was already reached after 15min, while for the Pt3Ir/V catalyst this took at least 1 hour. Furthermore, our data shows that the G-AST protocol is more aggressive compared to the P-AST to evaluate the catalyst aging processes.
Altogether, we showed the influence of galvanostatic and potentiostatic ASTs for rapid benchmarking of novel bifunctional cell reversal tolerant Pt-Ir alloy catalysts using the three-electrode RDE technique. This study can be helped to improve the electrochemical results obtained from the RDE to the catalyst coated membranes.
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