Lifetime of Catalyst under Voltage Cycling in Polymer Electrolyte Fuel Cell Due to Platinum Oxidation and Dissolution

Round 1

Reviewer 1 Report

This work establishes a numerical model to predict the lifetime of Pt catalyst under AST, which is important for the community to realize the nature of Pt corrosion and PtO coverage. The authors show a good math background to clearly describe the reaction-diffusion process. It can be considered for publication with proper answers or revisions on the following questions. 

1. Line 50 indicates the Pt NPs are semispheres, while the volume equation used in the manuscript is for the sphere.
2. The predicted results in Table 3 are interesting. However, the lifetime predicted in this model seems to be much shorter than the experimentally determined one. For example, the lifetime of triangle AST with 0.7V-1V is only 57hours. In the reference (Sarah Stariha et al 2018 J. Electrochem. Soc. 165 F492), the Pt catalyst after 136-hrs triangle AST with 0.6V-1V still retains ~60% ECSA. Here are three questions for the authors. 1) Any experimental support for the kinetics parameters list in Table 1 and 2? 2) Do the authors fit the simulated results with experimental data? 3) Can the authors have an explanation for the difference between the simulated results and experimental results mentioned in the ref above? I don't think that the experimental results must be more reliable than the simulated ones since the experiment may be wrongly designed or may not represent the actual corrosion. Any reasonable explanation is acceptable. 

Author Response

We appreciate the reviewer’s valuable remarks.

1. We corrected "hemispheres" to "spheres" on page 2.

2. Accelerated stress tests are developing to simulate the degradation phenomena during polymer electrolyte membrane fuel cells operation. A direct comparison to experiments is hardly probable, since multiple organizations explore different voltage cycling protocols and various operating conditions (T, pH, pressure, etc). In the theoretical model we have some fitting parameters taken from the literature and used for simulation. Our theoretical results are consistent with the experimental results reported in [19] and in [17,20,21] added to the reference list. Stariha et al [19] revealed, that the Pt degradation is primary influenced by the upper potential level, by the rate of potential change (the slope). Marcu et al [17] showed that an increase in the voltage upper limit in the triangle voltage cycle leads to a significant reduction of the catalyst lifetime. In accordance with the findings of Sugawara et al [20], the Pt dissolution is accelerated when the upper potential limit is greater than 0.8 V, which confirms our results in Table 3. Takei et al [21] reported that the Pt dissolution during the load cycles occurs particularly in the cathode CL near the membrane side, as also shown in our simulation.

In the reference Stariha et al [19], "the upper potential for the triangle-wave AST was decreased from 1.0 V to 0.95 V" (see F495), then 136-hrs triangle-wave AST in Fig.6 is 0.6V-0.95V, where for 0.6V-0.95V we predict 106 hrs in Table 3. Some difference of the durability in simulations and in experiments might appear due to the error of linear extrapolation. In the revision the corresponding corrections are done on pages 2, 8 and marked in the red color.

Reviewer 2 Report

The author used mathematical modelling to predict the degradation of platinum catalysts in polymer electrolyte fuel cells. They demonstrated that the degradation could be the result of oxidation and dissolution during the cycling.  Overall, this work might be helpful for the researcher in the fuel cell area. Since the introduction seems only to focus on the theoretical studies, I am curious why didn't the authors include any relevant experimental work. Also, it would be more interesting to see how does this finding explain the experimental results. I, therefore, suggest the author improve their manuscript on these aspects.

Author Response

According to the reviewer’s remark, improvements are added on pages 2, 8 marked in the red color:

Accelerated stress tests are developing to simulate the degradation phenomena during polymer electrolyte membrane fuel cells operation. A direct comparison to experiments is hardly probable, since multiple organizations explore different voltage cycling protocols and various operating conditions (T, pH, pressure, etc). In the theoretical model we have some fitting parameters taken from the literature and used for simulation. Our theoretical results are consistent with the experimental results reported in [19] and in [17,20,21] added to the reference list. Stariha et al [19] revealed, that the Pt degradation is primary influenced by the upper potential level, by the rate of potential change (the slope). Marcu et al [17] showed that an increase in the voltage upper limit in the triangle voltage cycle leads to a significant reduction of the catalyst lifetime. In accordance with the findings of Sugawara et al [20], the Pt dissolution is accelerated when the upper potential limit is greater than 0.8 V, which confirms our results in Table 3. Takei et al [21] reported that the Pt dissolution during the load cycles occurs particularly in the cathode CL near the membrane side, as also shown in our simulation.

Reviewer 3 Report

  This manuscript entitled "Lifetime of catalyst under voltage cycling in polymer electrolyte fuel cell due to platinum oxidation and dissolution" reports novel and interesting results, by based on theoretical studying of a model of catalyst degradation.

  The text is not well followed requiring an extensive editing of English language and proper terminology.

   Moreover, authors were based on scientific assumptions, on critical steps of their methodology; however none of those assumptions is commented on either Conclusions or elsewhere in the text.

 Therefore, I can recommend the publication of this manuscript after minor revision, according to the abovementioned comments and suggestions.

Author Response

According to the reviewer’s remarks, we done the corresponding corrections marked in the red color: 

The English and terminology are improved through the text.

On page 2 we add: “The developed degradation model is one-dimensional and based on the following assumptions: the model consist of two, catalyst and membrane layers in PEMFC; the two degradation mechanisms are considered, namely, Pt dissolution and diffusion into ionomer, and formation of platinum oxides on Pt particles surface; the Pt particles are fully surrounded by ionomer on the carbon support; platinum ions diffuse through the inonomer, the diffusion into gas diffusion layer is impossible.”

Round 2

Reviewer 1 Report

The revision is clear and reasonable. I suggest publishing this work in the present form.