The Impact of Potential Cycling on PEMFC Durability

Abstract

Voltage cycling is one of the most damaging stressors for automotive PEMFC. Understanding of the effects of voltage cycling on performance degradation is crucial to improve PEMFC durability for automotive applications. This study focuses on the interaction between upper potential limit (UPL) and lower potential limit (LPL) on the stability of PEMFC. A well-defined peak of degradation rate is observed when the LPL is ∼0.8 V with UPL of 1.35 V. A mathematical model was developed to understand the observed relationship between degradation rate and lower potential. Modeling results suggest that when cycling to a lower potential of ∼0.8 V, almost all dissolved Pt migrate from the catalyst layer to the membrane with negligible re-deposition, resulting in a peak of degradation rate at ∼0.8 V. The amount of Pt in the membrane (PITM) measured at end of life (EOL) samples correlates with degradation rates and is in agreement with modeling results.An automotive fuel cell needs to withstand thousands of operational hours. During normal operation of the vehicle, while load cycles occur, there will be fast dynamic change of cathode potential in the range of ∼0.6-0.95 V. During startup and shutdown, the cathode potential can be higher than 1.35 V. 1 It is well known that Pt is electrochemically unstable during voltage transitions, especially when the potential goes above 1.0 V. 2,3 The reaction of Pt to form ions, Eq. 1, is a typical failure mode during voltage cycling and it has been extensively studied by many groups. 4-10 Some researchers have shown that Pt will go into ion form during an anodic potential sweep. 11-13 It is also known that Pt can be oxidized to PtO or PtO 2 at the potentials above 1.15 V. [11][12][13] The reactions are shown as below Eq. 2 and 3.During the cathodic sweeps, Pt oxide can be electrochemically dissolved by Eq. 4 or chemically dissolved by Eq. 5. 11,14,15 Repeated anodic and cathodic voltage sweeps will accelerate Pt dissolution. As a result, permanent Pt loss could occur and ultimately lead to the fuel cell stack decay. The dissolution of Pt is further exacerbated with higher upper potentials. Imai et al. carried out in-situ measurements of Pt oxide at different potentials in aqueous media. 16 They observed place exchange phenomenon when the potential was higher than 1.4 V, suggesting Pt could suffer more severe dissolution at such upper potentials. Therefore, it is of great interest to study the impact of parameters in voltage cycling on Pt durability.The Pt durability can be dependent on many factors, including voltage window (upper potential, lower potential), operation conditions (relative humidity (RH), temperature, gas pressure, etc). The impact of RH and temperature has been investigated and discussed by some publications from Bi et al. 17,18 The importance of upper potential on platinum dissolution during voltage cycles has also been studied by many groups. Wang et al. conducted experiments to measure the equilibrium concentration of dissolved Pt at different upper potentials in acidic s...

Facile synthesis of Pt3Ni alloy nanourchins by temperature modulation and their enhanced electrocatalytic properties

“…The multi-branched structure can prevent the aggregation of PtNi alloy nanourchins. Furthermore, it has been proved that the rod morphology does not undergo significant ripening [57][58][59][60][61]. Therefore, we believe that the loss of ESA on the PtNi alloy nanourchins could be largely attributed to the electrochemical dissolution.…”

“…However, the dissolution of PtNi alloy nanourchins should be significantly suppressed, because the PtNi alloy nanourchins have higher onset potential for Pt oxidation in Fig. 4A, which contributes positively to less dissolution of Pt via the oxidation state [61]. …”

Pt Catalyst Degradation in Aqueous and Fuel Cell Environments studied via In-Operando Anomalous Small-Angle X-ray Scattering

“…The particle size dependence of Pt dissolution has been attributed to the Gibbs-Thomson effect which induces an effectively higher potential on smaller particles as compared to larger ones and a Pt dissolution equilibrium and rate which increase with increasing anodic potential above 0.7 V. [10,11,20,28,38]. [27,54] Re-deposition will occur when metallic sites are present on the particle surfaces [53] and when the concentration of dissolved Pt surrounding the particles is higher than the equilibrium dissolved Pt concentration, which is a function of Pt particle size. [55] The extent of GSA loss for the experiments in this study can be interpreted in the context of the proposed mechanism.…”

“…There are several modeling efforts of Pt electrocatalyst ECA loss in MEAs induced by various potential cycling protocols and temperatures. The Pt dissolution rates utilized in the modeling efforts of Darling and Meyers, [51] Bi et al, [52] and Zhang et al [53] are shown in Table 2. The cycling conditions which effect Pt dissolution rates were different in these efforts and differ from what were used in this study, but in general the rates range from 1 x 10 -10 to 2 x 10 -9 g/cm 2 -cycle.…”

Platinum Oxide Growth on Pt/C Fuel Cell Catalysts Using Asymmetric Scan Electrochemical Quartz Crystal Nanogravimetry

“…The dominant decay mechanisms are Pt dissolution with or without redeposition [26][27][28][29][30][31][32], migration/ coalescence [33], catalyst nanoparticle size distribution change [34,35], and detachment from the support due to carbon corrosion [36]. The factors influencing the transient potential profiles include transition rates, the upper and lower potential limits (range between 0.6 and 1.0 V), and time at either idle (∼1.0 V) or peak (∼0.6 V) power [37]. The transition rates allow for the extent to oxide formation and reduction that control surface reorganization [25].…”