A comparison of direct methanol fuel cell degradation under different modes of operation

https://doi.org/10.1016/j.ijhydene.2012.07.058Get rights and content

Abstract

The degradation test of direct methanol fuel cells (DMFCs) under on–off operation with variable load profile, known as a variable load on-off operation (VLOO) mode, was compared to continuous operation with a constant load, known as a constant load continuous operation (CLCO) mode. It was found that the performance loss in the VLOO mode was higher than that under the CLCO mode in terms of open circuit voltage drop (3.6 times higher) and the maximum power density reduction rate (1.23 times greater). Under both operation modes, the anode catalyst was deteriorated by Ru dissolution. However, the faster decay of cell performance under VLOO mode was mainly caused by the increase of cathode polarization due to higher methanol crossover and the decrease of Pt activity to the oxygen reduction reaction induced by Ru deposition.

Highlights

► DMFC degradation under variable load on-off (VLOO) and constant load continuous (CLCO) operation. ► Performance loss in VLOO was higher than that under CLCO. ► A major cause was the higher cathode polarization resulting from methanol crossover under VLOO mode. ► Slower Ru dissolution rate under CLCO than under VLOO mode was found.

Introduction

Direct methanol fuel cells (DMFCs) are promising power generation units for small-scale applications, such as laptop computers, due to their good energy efficiency, ease of operation at ambient pressures and good potential mobility for portable power sources [1], [2], [3]. However, a DMFC must be capable of stable operation for more than a thousand hours. This can be difficult to achieve due to many drawbacks, including methanol crossover [4], catalyst agglomeration [5], which causes the loss of catalytic active area [6], and catalyst poisoning due to accumulated intermediates or impurities [7], and deterioration of the polymer electrolyte membrane [8]. In addition, the presence of some liquids (toluene, ethylene glycol, etc.) can accelerate the sintering process of catalyst particles [9], [10], and the methanol solution is also aggressive towards Nafion® polymer electrolyte [6], leading to peeling and cracking at the contact boundary between the electrode and the membrane [11]. Therefore, achieving high long-term performance is more challenging and needs to be explored for successful application of DMFCs in next generation power sources.

Previous works have investigated the degradation of cell components during long-term operation, with a constant load applied to the cell but at different test periods from 75 h [5] to 2020 h [12]. At a steady load operation of 75 h, the results of cell performance loss indicated that agglomeration of the catalyst together with the delamination of the membrane electrode assembly (MEA) contributed to performance degradation of the DMFC. For a test period of 2020 h, PtRu nanoparticles also agglomerated and the performance of the anode degraded dramatically due to leaching of the unalloyed Ru. A few researchers have focused on the variable load conditions, but have investigated the dynamic response of the output current and voltage response for applications in vehicles [13]. None of the previous studies have focused on a combination of variable load profile with on–off operation to investigate cell performance loss. In practice, the operating condition of power supply units, such as DMFCs, is normally inconsistent and changes with the load profile of the connected electrical devices. In addition, the load is frequently turned on and off depending on the user's behaviour. Such varied load types of operation may accelerate cell performance degradation.

In this study, the degradation of DMFC performance was tested under variable load on-off (VLOO) conditions to simulate real operating conditions and to compare with constant load continuous operation (CLCO) conditions. During the durability test of 100 h, linear sweep, cyclic voltammetry, and methanol stripping experiments were performed to explore the anode catalyst's deterioration and the cathode catalyst's poisoning due to methanol crossover. A frequency response analysis was performed to investigate the cause of cell performance degradation. Moreover, an X-ray mapping technique was employed to detect the change of catalyst composition at both anode and cathode after durability tests.

Section snippets

Membrane electrode assembly (MEA) preparation

Catalyst inks were prepared in ethanol solution and a 5% Nafion® ionomer solution with 40 wt% Pt and 20 wt% Ru supported on carbon (Alfa Aesar) for the anode ink, and with 60 wt% Pt/C (Alfa Aesar) for the cathode catalyst ink. The catalyst inks were sonicated for 20 min and then sprayed on 5 cm2 of a gas diffusion layer (GDL: H2315IX53C173 (Vildona)) to loading of 1 mg cm−2 Pt catalyst. In addition, 0.5 mg cm−2 of Nafion® solution was sprayed on top of both the anode and the cathode catalyst

Durability test results

The variation in current density with test time of 100 h durability test of the DMFC under the CLCO mode is shown in Fig. 2a. The current density decreased continuously with time at the constant voltage of 300 mV from 78.2 mA cm−2 (0 h) to 26.6 mA cm−2 (100 h), although the current density was partially recovered after each intermission of measuring for cell performance degradation. This was caused by the refilling of fresh fuel solution and the surface cleaning of the anode electrode during

Conclusion

The investigation of the cell performance behaviour under the variable load on-off operation mode (VLOO) was carried out to simulate a practical condition for portable power generating devices. To identify the causes of DMFC degradation and to compare them with the cell tested under a constant load continuous operation (CLCO) mode for 100 h, the single cell was tested for its performance loss and characterized by using in-situ techniques. For 100 h of the degradation test, the VLOO mode reduced

Acknowledgements

The authors would like to acknowledge the Joint Graduate School of Energy and Environment (JGSEE), King Mongkut's University of Technology Thonburi (KMUTT) and the Royal Golden Jubilee (RGJ) program of the Thailand Research Fund (TRF) for academic scholarship and support on this work. The co-research financial support by the Higher Education Research Promotion and National Research University (NRU) Project of Thailand, Office of the Higher Education Commission is truly appreciated.

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    The significant anode overpotential for the MOR [1,2] results in a relatively low current density and a higher cathode potential during operation. It is therefore critical to distinguish the contribution of each electrode to the overall cell voltage, particularly during the study of temporary and permanent degradation of DMFC cathodes, where the former is mostly related to platinum oxide formation [3,4] and the latter to mechanisms such as Ostwald ripening and Pt dissolution [2,5–8]. Moreover, strongly heterogeneous degradation across the membrane electrode assembly (MEA) has been reported in [2], which was attributed by the authors to spatial variations in operating conditions at both anode and cathode, emphasising the need for further investigation of localised performance and degradation.

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