Elsevier

Journal of Power Sources

Volume 196, Issue 8, 15 April 2011, Pages 3743-3749
Journal of Power Sources

Effects of operating conditions on durability of polymer electrolyte membrane fuel cell Pt cathode catalyst layer

https://doi.org/10.1016/j.jpowsour.2010.12.068Get rights and content

Abstract

In this study, we investigated the effects of humidity and oxygen reduction on the degradation of the catalyst of a polymer electrolyte membrane fuel cell (PEMFC) in a voltage cycling test. To elucidate the effect of humidity on the voltage cycling corrosion of a carbon-supported Pt catalyst with 3 nm Pt particles, voltage cycling tests based on 10,000 cycles were conducted using 100% relative humidity (RH) hydrogen as anode gas and nitrogen of varying humidities as cathode gas. The degradation rate of an electrochemical surface area (ECSA) was almost 50% under 189% RH nitrogen atmosphere and the Pt average particle diameter after 10,000 cycles under these conditions was about 2.3 times that of a particle of fresh catalyst because of the agglomeration of Pt particles.

The oxygen reduction reaction (ORR) that facilitated Pt catalyst agglomeration when oxygen was employed as the cathode gas also demonstrated that Pt agglomeration was prominent in higher concentrations of oxygen. The ECSA degradation figure in 100% RH oxygen was similar to that in 189% RH nitrogen. It was concluded that liquid water, which was dropped under a supersaturated condition or generated by ORR, accelerated Pt agglomeration. In this paper, we suggest that the Pt agglomeration degradation occurs in a flooding area in a cell plane.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are expected to have applications as a clean power source for vehicles. At present, PEMFCs developed thus far work on a practical scale. However, further improvements are necessary to enhance their durability in order to make operation on a commercial scale viable. For vehicles, operation modes, such as running, stopping, idling, and parking modes, all impact on catalyst degradation to varying degrees. In order to improve PEMFC durability it is important to understand how start/stop operations and load changing influence the degradation mechanism to. One of the major deterioration factors considered is changing the electrochemical potential of the catalyst layer with load changing. Platinum corrosion rates in fuel cells have been observed to increase when the catalyst is exposed to high voltage [1], [2], [3], [4], particularly when the cell voltage is cycled [5], [6].

It is known that voltage cycling of carbon supported platinum (Pt/C) in aqueous acids at room temperature leads to accelerated platinum dissolution rates compared to extended holds at constant potentials [5], [7]. Johnson et al. reported that Pt(II) is produced when an oxidized platinum electrode is reduced [8], and it is assumed that a greater number of oxide formation–reduction cycles per unit time leads to a greater instability of Pt. [3], [8]. Ross [9] conceptualizes platinum coarsening and area loss at two different length scales, viz. (i) a nanometer-scale Ostwald-ripening process, where smaller platinum particles dissolve in the ionomer phase and redeposit on larger platinum particles that are separated by a few nanometers; and (ii) a micrometer-scale diffusion process, where dissolved platinum ions diffuse toward the matrix and undergo reduction on the anode side at low potentials. Paik et al. investigated the effect of time periods and lower voltage level as cycling parameters on catalyst stability. Their cycling parameters have been found to impact the rate of decay in platinum electrochemical area (ECSA) at the same cumulative time of exposure to the high voltage level (1.3 VRHE). A higher frequency cycling of 0.5–1.3 VRHE was found to accelerate the decay rate of Pt ECSA. ECSA loss is due to the growth in platinum particle size. When the lower voltage was set above the Pt oxide reduction potential, the rate of ECSA loss was found to decrease. It suggests that Pt dissolution rates decelerate due to a partially remaining passive film [10].

Platinum corrosion rates in fuel cells have been observed to increase when the catalyst is exposed to high humidity in the same voltage cycling tests [11]. H2–N2 PEM systems were carried out to investigate the effects of potential and humidity for electrocatalyst corrosion in previous reports [1], [10], [11], [12]. Although these methods reasonably accounted for the degradation mechanism of the electrocatalyst layer in voltage cycling test, it may not be possible to extrapolate this mechanism to an actual fuel cell system. In the cell plane, especially the cathode side of the actual fuel cell stack, the proportion of water is increases from the gas inlet to the exit as a result of an oxygen reduction reaction. The condensed water will be generated as the result of high current density operation for automotive fuel cell. Previous reports verified the effect of humidity below 100% RH, but did not focus on the influence of condensed liquid water on electrocatalyst corrosion. Although N2 cathode gas in the voltage cycling test would clarify the effect of voltage or humidity without ORR, the oxygen reduction reaction still occurs at the cathode in normal fuel cell operations. Because water is generated by the ORR, it is not obvious whether the electrocatalyst corrosion is accelerated by ORR or by high humidity conditions resulting in ORR. It is important to clarify the contribution of ORR to electrocatalyst corrosion.

Herein, we examined the effect of cell operating humidity conditions, especially condensed liquid water on the durability of a PEMFC cathode catalyst layer in the load changing operation mode of vehicles using a single cell. At the same time, we attempted to consider the effect of oxygen reduction reaction (ORR) and the water generated by ORR on the durability of PEMFC.

Section snippets

Preparation

The membrane electrode assembly (MEA) was prepared by a decal method by hot pressing Nafion N112 between two catalyst layer decal sheets at 140 °C. These decal sheets were prepared by printing catalyst ink, mixing an ionomer (Du Pont 20 wt% Nafion solution) and a commercial carbon-supported Pt electrocatalyst with a Pt weight ratio of about 50 wt% on a PTFE substrate. Porous carbon papers (SGL Carbon GDL24BC) were used as gas diffusion media and current collectors. The Pt loading in the catalyst

Results and discussion

The voltage cycling test using the humidified N2 as cathode gas to evaluate the effect of humidity is described first. We then present a discussion regarding the voltage cycling test using various oxygen gas concentrations to evaluate the effect of ORR on the degradation of the catalyst.

Conclusion

The effects of humidity, especially that because of condensed liquid water, on the durability of PEMFC cathode catalyst layer in the voltage cycling test were investigated. It was shown that ECSA decreased more rapidly under conditions of higher humidity and also decreased markedly in a supersaturated atmosphere, namely, in the presence of liquid water. We concluded that Pt agglomeration, which causes ECSA degradation, in a supersaturated atmosphere caused a significant reduction in cell

Acknowledgments

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan, and was conducted jointly with the Daido Institute of Technology. The authors thank Prof. Hori (Daido Institute of Technology) for technical support.

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