Short Communication
Specific approaches to dramatic reduction in stack activation time and perfect long-term storage for high-performance air-breathing polymer electrolyte membrane fuel cell

https://doi.org/10.1016/j.ijhydene.2017.05.119Get rights and content

Highlights

  • Efficient stack activation and long-term storage for air-breathing PEMFC were investigated.

  • The pre-activation using DI water significantly reduces the whole activation time within 1 h.

  • The humidified N2 gas minimizes the Pt oxidation and dying of polymer membrane for 60 days.

Abstract

Air-breathing polymer electrolyte membrane fuel cell (PEMFC) systems without the humidifier and air blower have been developed to overcome the cost and complexity of balance of plants (BOPs). Until now, there has been no specific way to improve the stack's initial performance through the specific activation protocol and maintain the initial performance for a very long time. Herein, we studied a technique for finishing the total activation within 1 h by using a pre-activation process (i.e., soaking the stack in a DI-water reservoir) and applying current at 0.65 V. The pre-activation procedure significantly increased the swelling of the polymer membrane and the Nafion binder in the catalyst layer, reducing the total activation time. Also, we showed that long-term storage using humidified N2 gas in a closed box did not hinder the electrocatalytic activity of the Pt and the drying of the polymer membrane for 60 days.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) have been considered to be one of the most promising energy conversion devices for small-scale portable power generators because of their high power density, quick response to changes in current demand, and relatively low operating temperature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Until now, however, the high cost and complexities of the balance of plants (BOPs), such as humidifiers, air blowers, and valves, have posed a great challenge to the widespread use of small-scale portable PEMFC systems. For these reasons, air-breathing PEMFC systems without humidifiers and air blowers have been developed to overcome the cost and complexity of the balance of plants [6], [15], [16], [17], [18], [19], [20]. In air-breathing systems, the external fan is installed at the open-cathode side as a substitute for an air blower and a cooling system, and this decreases the cost, power consumption, and volume of the entire system.

However, the structural problems associated with open-cathode type air-breathing systems have a major impact on the stack's activation process and long-term performance because of i) the severe drying of the Nafion polymer membrane by poor water management and ii) the low oxygen reduction reaction (ORR) by poor transportation of oxygen to the entire surface area of the active catalyst [16]. Specifically, the low humidified air supplied by an external fan is not able to sufficiently increase the water content in the polymer membrane as compared to a PEMFC stack with the closed-cathode type. In addition, a very low ORR induced on a Pt catalyst has a negative effect on the production of water in the cathode electrode, resulting in severe drying of the membrane during the activation process. Thus, the stack spent much more time on the activation procedure than the cathode-closed PEMFC system. Furthermore, the air-breathing PEMFC stack frequently was kept at atmospheric conditions for several day or months until the power supply was operating normally, resulting in a significant decrease in the initial performance of the stack after activation primarily due to the severe drying of the polymer membrane and the oxidation of the Pt. Eventually, both the low water content in the polymer membrane and the severe oxidation of the Pt become critical factors that have a strong negative influence on the activation of an air-breathing PEMFC stack and its long-term storage.

In the previous our work, we injected the DI water into the cathode-closed PEMF stack and stored it at high temperature at over than 65 °C [21]. However, we applied the pre-activation protocol by soaking the air-breathing PEMFC stack in a DI water reservior at room temperature to significantly increase the stack performance. The pre-activation procedure consists of storing the air-breathing PEMFC stack in distilled water at room temperature for 24 h to increase the apparent phase separation between the hydrophobic and hydrophilic regions in the polymer membrane and in the Nafion binder in the catalyst layer. This method improve the air-breathing PEMFC stack in spite of the its structural problem and decrease the activation time within just 1 h as compared to 24 h by conventional activation protocol. In addition to the development of a rapid procedure for activating the stack, we also suggest the use of an effective long-term storage method to maintain the initial performance for 60 days after the stack has been activated. Specifically, the air-breathing stack was kept in a closed box with humidified nitrogen (N2) for 60 days to minimize the decrease in the stack's performance. In this work, we found that long-term storage in humidified N2 did not affect the cell's performance significantly.

Section snippets

Experimental

The commercial Gore PRIMEA 57 membrane-electrode-assembly (MEA) with an active area of 25 cm2 was used to conduct new activation of a 10-cell, air-breathing PEMFC stack. The thickness of the membrane was 18 μm, and the Pt loadings on the anode and cathode sides were 0.1 and 0.4 mgPt cm−2, respectively. A 10-cell stack with graphite plates that were in-house designed was used in this work. Dry hydrogen was fed to the anode using a graphite plate with a 3-channel serpentine flow field. The fuel

Results and discussion

In the total activation procedures, there were three important observations, i.e., i) the increase in the proton conductivity through the phase separation between the hydrophilic and hydrophobic regions due to the swelling of the polymer membrane; ii) the increase in oxygen transport due to phase separation in the Nafion ionomer that was near the Pt electrocatalyst; and iii) the decrease in the oxidation of Pt for enhancing ORR activity. The air-breathing PEMFC stack potentially could be

Conclusions

A specific activation process for an air-breathing PEMFC stack is required to increase its initial performance for a very short time. However, the structural problem of an open cathode-type air-breathing stack has a major impact on insufficient swelling of polymer membrane and binder in the catalyst layer as well as the decrease in Pt oxidation. In this study, we showed that the pre-activation of stack in a reservoir of DI water significantly increased the phase separation between the

Acknowledgment

This work was conducted under the framework of the research and development program of the Korea Institute of Energy Research (B7-2451-14).

References (31)

Cited by (7)

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  • Performance improvement of air-breathing proton exchange membrane fuel cell stacks by thermal management

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  • Experimental and numerical investigation on effects of cathode flow field configurations in an air-breathing high-temperature PEMFC

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    Due to the strict requirement of volume and weight for portable power applications, air-breathing PEMFCs that utilize natural convection and diffusion of air have been proposed [14–17]. Since the cathode directly exposed to the ambient air, the use of blowers or compressors is not necessary and the PEMFC system complexity is significantly reduced [18]. However, the performance of air-breathing PEMFCs is significantly affected by the cathode open area and flow field configurations since air supply at the cathode completely relies on buoyancy-induced natural convection due to temperature and concentration gradients [19–21].

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These authors contributed equally to this work.

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