PtCo Catalyst Dissolution and Oxidation Modeling for Durability Improvement of Automotive Fuel Cell
Authors: Sekine Shinobu , Fukaya Norihiro , Kumei Hideyuki , Motiduki Jun 
Published: December 2020

Polymer electrolyte fuel cell (PEFC) is one of the candidates to replace automotive power sources using fossil fuels. For this purpose, we need to overcome three major criteria: cost, performance, and durability. One of the important topics of PEFC durability is how to keep its catalyst activity for oxygen reduction reaction (ORR). The catalyst activity is product of the electrochemical surface area (ECSA) and activity per surface area. A central issue of catalyst durability is the loss of ECSA over time. This ECSA is approximately the same as the eﬀective surface area of the catalyst. To maximize the ECSA, the radius of catalyst particles is in the order of nano meters.

In this study we focus on the ECSA loss of PtCo alloy catalysts on carbon support. These alloy catalysts have much higher activity for ORR than traditionally used Pt1. Our model of ECSA loss combines the following two mechanism:

(i) Co dissolution from the surface of the alloy catalyst,

(ii) Pt dissolution from small particles and precipitation onto large (Ostwald ripening).

The mechanism (i) changes the distribution of the alloy ratio in particle. Because of the standard dissolution potential of Co is -0.28 V vs NHE at 25◦C, Co is easy to dissolve in the aqueous media especially under low pH condition such as PEFC, and the Co ratio near surface is smaller than that of the center 2. In the case of dissolution under high potential, the amount of its depends on the Co ratio.

In the mechanism (ii), the balance between Pt and its oxides on particle surface has an important role to simulate ECSA change. While Pt is known to form stable oxide species as well as hydroxide intermediates, the mechanism and speciﬁcation still remain the subject of research 1. In the previous papers, this oxidation step is described by the nonlinear reaction rate coeﬃcient called Temkin term1, 3, 4. Though this nonlinear term is able to describe the generation of the higher oxidation such as multi-layer of PtO and PtOx (x ≥2) by only one equation3. This term is one of the reasons for extended computing time, and we cannot derive the term consistently from (non-equilibrium) thermodynamics. Here we consider the kinetics of PtCo catalyst dissolution under 0.9V/cell, which is a typical condition in vehicle usage for improving the durability5. Under this condition we need not to consider the higher oxidation process, the following simple two independent fundamental reactions1 is considered:

Pt + H2O = PtOH + H+ + e−, (1)

PtOH = PtO + H+ + e−. (2)

These consecutive reactions are based on spectroscopic results by Ishiguro et al .6 The reaction rates are determined by the ordinary Butler-Volmer equations with some parameters which depend on the temperature and the humidity, without Temkin term. Using the above model, we can calculate the evolution of ECSA loss for the long period and the results are in good agreement with experimental data [Fig. 1].

Another application of our model is to predict the necessary conditions in order to achieve both initial performance and durability. For example, if the mean radius of the catalyst is 4 nm, the deviation of this size distribution function must have about 0.7 nm or than less. The results are very severe conditions but needed to overcome for widely spread of PEFC vehicles.

Figure 1.Experiment (symbols) and modeled (solid line) ECSA change as function of square-wave potential cycle (0.1 V (3 sec) <-> 0.80, 0.85, and 0.90 V (3 sec))

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Figure 1