Assessing Utilization Boundaries for Pt-Based Catalysts in an Operating Proton-Exchange Membrane Fuel Cell

Abstract

Octahedra (oh) PtNiX/C catalysts are notable cathode catalysts for proton-exchange membrane fuel cells due to their exceptional oxygen reduction reaction activity. Here, we investigate the degradation of oh-PtNiIr catalysts under fuel-cell conditions using operando X-ray diffraction (XRD). Employing two accelerated stress tests with different lower potential limits and XRD-coupled cyclic voltammetry on benchmark Pt and oh-PtNiIr catalysts, we find that dissolution and degradation are proportional to the extent of reduction, independent of the catalyst's nature. Our method identifies the optimal potential range for Pt-based catalysts to minimize degradation without lengthy stress tests.

Adjusting the Operational Potential Window as a Tool for Prolonging the Durability of Carbon-Supported Pt-Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts

“…In accordance with the assumptions from the results obtained using the HT-DE methodology ( Figure 3 ), the signals for both Pt and Co dissolution decrease substantially with the narrowing potential window from 0.6–0.95 V RHE ( Figure 4 a, c,e) to 0.7–0.85 V RHE ( Figure 4 b,d,f) regardless of the applied hold time at each potential. This has also been reported by previous studies, 43 , 45 , 48 , 66 , 68 , 69 but what is specific here is that both LPL and UPL are nevertheless limited to a realistic operational voltage while positively affecting the stability of both metals. What is also important to emphasize here is that dissolution during cycling when limiting both potential limits (LPL as well as UPL) is, however, not completely suppressed but only significantly reduced, i.e., minimized.…”

“…Namely, varying the LPL in the range of 0.7/0.65/0.6 V RHE , with a constant UPL (0.925 V RHE ), provided evidence that the cathodic dissolution of both Pt as well as less noble metal increases with a decrease in LPL. In line with this study, Ronovsky et al 45 provided a comprehensive XRD study on the stability of an octahedral-PtNiIr electrocatalyst during MEA square-wave accelerated degradation tests (ADTs) with a constant UPL (0.95 V) and LPL varying from 0.6 to 0.7 V. Dissolution of Pt and Ni has been tracked by observing three XRD parameters (scattering intensity, lattice parameter, and crystallite size), and a strong dependence of the stability of Pt-alloy based electrocatalyst on the LPL has been confirmed once again. The authors have observed that the extent of degradation of the analyzed electrocatalyst is proportional not only to the degree of Pt oxidation but also to the degree of reduction of the formed Pt oxide.…”

“…(ii) Narrowing the voltage window to only 0.7–0.85 V greatly inhibits both the anodic as well as cathodic dissolution of both Pt and consequently Co and thus could significantly prolong the lifetime of Pt-alloy cathodes in PEMFCs using already existing state-of-the-art Pt-alloy electrocatalysts. In the other words, limiting both potential/voltage limits to 0.7–0.85 V RHE acts advantageously through two next scenarios: (i) (s)­lower formation of Pt oxide and consequently also a lower degree of oxide place-exchange and (ii) slower kinetics of the reduction of Pt oxide or perhaps even incomplete reduction of the Pt oxides (in other words, the surface of Pt-based nanoparticles might remain partly passivated). ,, In summary, whereas the total amount of dissolved Pt (and Co) is strongly affected by the potential window and the hold time at the UPL, the mechanism of Pt dissolution (and Co dissolution) is independent of the potential window as well as the hold time at UPL. Ultimately, in accordance with previous studies, it is worth mentioning here that a similar scenario is expected for other Pt-3d transition metal alloys.…”

“…Specifically, the fuel cell electrocatalysts are exposed to very aggressive conditions, including a corrosive environment, high temperatures, humidity, frequent stops and starts, fluctuations in the operational voltage, and so forth . These nonintrinsic operational conditions, together with the intrinsic properties of the electrocatalyst (choice of M, order/disorder, dealloying/activation, type of carbon/degree of graphitization), lead to a spectrum of very complex and interconnected degradation mechanisms: (i) electrochemically induced (transient) dissolution of Pt, which is defined by the thermodynamic tendency of Pt to the formation/reduction of the Pt oxide, ,− resulting in Ostwald ripening , and/or formation of metallic Pt bands in the membrane − ; (ii) dissolution of M ,, ; and (iii) electrochemical and chemical carbon support corrosion, − leading to the agglomeration and/or detachment of whole Pt NPs. These complex instability phenomena are of crucial importance for the longevity of the PEMFCs in the case of their application in both light-duty vehicles (LDVs) and heavy-duty vehicles (HDVs). , In particular, as a plan for additional cost reduction, the target for the electrocatalyst loading for LDVs, proposed by the US Department of Energy (DoE) and expected to be achieved by 2025, is less than 0.1 mg Pt cm –2 .…”

“…Here, the gap between knowledge obtained at the rotating disk electrode (RDE) and MEA levels must be overcome. For instance, most MEA studies in the literature , usually follow the US DoE degradation protocol for fuel cells and fuel cell components . The protocol consists of 30,000 trapezoidal wave cycles in a voltage range similar to that which is expected for an automotive drive cycle (0.6–0.95 V), with a 3 s hold at each potential, at 80 °C, thus simulating close-to-real operational conditions regarding the operational lifetime of 5000 h as the current target for LDVs. , On the other hand, apart from rare studies such as the recent one presented by Imhof et al, RDE stability studies typically vary with respect to this protocol.…”