Research paper
Study on a drasitically hydrogen consumption saving conditioning method for Polymer electrolyte membrane fuel cell

https://doi.org/10.1016/j.est.2021.103338Get rights and content

Highlights

  • A cost-effective method for fuel cell conditioning is proposed.

  • Conditioning processing time is significantly shortened.

  • The consumption of hydrogen gas is significantly reduced.

  • Role of conditioning as a means of expanding triple-phase boundary is investigated.

  • Improved performance in terms of limiting current density is achieved.

Abstract

In this study, a cost-effective fuel cell conditioning method is proposed to significantly shorten the processing time and reduce the consumption of hydrogen gas. This method, known as Air braking (AB) promotes triple-phase boundary (TPB) relocation by controlling air supply. Commonly used conditioning methods, including Constant Current (CC) and Constant Voltage (CV), are compared, and the results indicate that the limiting current densities of Air braking, Constant Voltage, and Constant current are 2.0616, 2.0072, and 1.9256 A/cm2, respectively. Using Air braking, peak performance is achieved within 2 h, which is faster than that of Constant current and Constant Voltage at 4 and 21 h, respectively. Additionally, scanning electron micrograph (SEM) images of the membrane-electrode assembly (MEA) cross sections are captured after the Constant Current, Constant Voltage, and Air braking processes, and the thickness of the cross sections are measured. The cathode and anode catalyst layers are thickest when the Air braking method is used. This means that using Air braking conditioning activates the catalyst penetration into the membrane, expands the triple phase boundary, and consequently leads to performance improvement. According to our results, MEA conditioning with the Air braking method shortens the conditioning time significantly and achieves maximal MEA performance.

Introduction

As the importance of the green energy increases, fuel cells have become a candidate substitute for the combustion engine. Among the various fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) is a particularly promising device for the transportation sectors due to its low operating temperature, compactness, and high power density. A process that is called break-in, conditioning, or activation [1] is required during PEMFC manufacturing. Because a newly manufactured PEMFC is not able to attain optimal performance, the conditioning process is necessary before actual PEMFC operation. However, the conditioning process may take hours or even days depending on the method of conditioning, and the fuel cell may not reach the maximum performance due to an improper conditioning procedure [2]. Therefore, it is important to choose an appropriate conditioning method to minimize the conditioning time and the hydrogen consumption [3].

Understanding the fundamentals of the conditioning process helps in establishing manufacturing procedures that permit activation of the PEMFC. The following are theories that explain the conditioning mechanism.

  • (i)

    The conditioning of the fuel cell has advantageous effects on the catalyst, e.g., the removal of impurities introduced during the process of manufacturing the membrane-electrode assembly (MEA), the activation of a catalyst that does not participate in the reaction, and the creation of a transfer passage for reactants to the catalyst [4].

  • (ii)

    The membranes are initially dry, hindering stack performance until the membranes hydrate during the incubation period [5].

  • (iii)

    The initial performance of a new MEA usually improves during the conditioning period, as the electrolyte requires hydration to ensure the passage of hydrogen ions [6].

Theories about PEMFC conditioning are understood, though theories that support the conditioning mechanism have not yet been accurately identified. Thus, an additional analysis to determine the optimal conditioning method is required. We focused on the role of conditioning as a means of improving the ionic conductivity of the catalyst layer [5,6]. That is, the conditioning process should activate the electrodes of the MEA, increasing the triple phase boundary (TPB) where the electrochemical reaction has occurred. A schematic of the TPB of the MEA is shown in Fig. 1. During conditioning, the TPB expands; this is explained by the theory of the introduction of Nafion electrolytes (hereafter, Nafion) into the catalyst layer [5,6] and by the carbon oxidation reaction (COR) of the carbon support. This Nafion intrusion into the catalyst layer is thought to result from the hydration of the membrane. Swelled Nafion surrounds Pt particles, so that the TPB increases. The TPB increase is also related to the COR of the carbon support consisting of Nafion. In some cases, the carbon support has a defect site consisting of C+, which is prone to reversible electrochemical oxidation and to forming Csingle bondOH groups. Through this reaction, carbon surface oxides (COsurf) can form and become oxidized to CO2, a process that is listed in Table 1 [7,8]. In this conversion process, the carbon support is lost, and the TPB is increased at the catalyst-ionomer-gas conversion point. COR is likely to occur at a relatively high potential (> 0.6 V) [9], and the increase in the TPB enhances the performance of the PEM fuel cell. The performance reaches a maximum value and plateaus after a certain amount of time passes.

Several studies have been carried out on PEMFC conditioning methods. Yuan et al. investigated the Constant Voltage (CV) conditioning method with a voltage of 0.6 V, and the effect of operating temperature, cycling steps, and frequencies on MEA performance. They found that a high temperature produces better performance; however high cycling steps are not effective for MEA conditioning [10]. Zhiani et al. compared different MEA conditioning methods including the CV method, constant low current method, and the United States Fuel Cell Council (USFCC) activation protocol. They showed that the use of CV achieved steady state performance after 6 h, while using constant current required 16 h, and the USFCC protocol reached a steady state after 19 h [11]. Silva and Rouboa applied a conditioning procedure consisting of six load cycles. They discovered that the MEA performance improved significantly during the conditioning procedure due to the expansion of the TPB at the electrode/electrolyte interface [12]. Furthermore, the conventional conditioning methods are listed in Table 2.

In this study, a conditioning method called Air braking (AB), which was suggested by Hyundai Motors, was researched [13]. Eickes et al. used this method to recover the performance of a DMFC. This study only explained about the AB method process, not verified the before and after AB method performance comparison [14]. Air braking is operated by the following mechanism. When Air supply is stopped while hydrogen is supplied to anode, the air supply stopping creates a restoration atmosphere due to hydrogen diffusion in the cathode. So that the oxide film formed on the platinum catalyst layer of MEA is removed to activate the platinum catalyst. This method can shorten the conditioning time considerably and save cost in PEFMC development. First, among the conventional conditioning methods that is listed in Table 2, the methods that operate with a CV at 0.6 V and a constant current at 0.25 A/cm2 are used in a performance comparison with AB and to verify its effectiveness. Second, the activation loss and ohmic loss of the MEAs using CC, CV, and AB were analyzed to determine the effect of each conditioning method on MEA over-voltage. Additionally, scanning electron micrograph (SEM) images of cross sections of the MEAs using CC, CV, and AB were captured, and the thicknesses of the cross sections were analyzed to investigate the structural change during the conditioning process. Finally, the amount of time required for and hydrogen consumption of the conditioning methods were measured. The conditioning method proposed in this study lowers the required time and hydrogen consumption and accelerates the use of hydrogen energy.

Section snippets

Experimental

In this section, we introduce the experimental equipment and conditioning procedures for CC, CV, and AB. Also, the methods for analyzing the over-voltage and the SEM images are explained.

Results and discussion

In this section, the performance of the MEAs using the CC, CV, and AB methods is presented. Furthermore, the over-voltages using the CC, CV, and AB methods are compared to demonstrate the effectiveness of AB method. Next, the thicknesses of cathode, anode, and membrane using the CC, CV, and AB methods are analyzed to observe the TPB replacement. Lastly, the time required for conditioning and hydrogen consumption are compared.

Conclusions

A new conditioning method called AB, which controls air supply, was developed, and its performance was measured by comparing it with the conventional conditioning methods CC and CV. The conditioning time required to achieve maximal performance was 2 h for AB, 4 h for CV, and 21 h for CC. From this result, it was verified that of the three kinds of conditioning process, AB shortens the conditioning time remarkably. After the conditioning process was finished, the limiting current densities of

CRediT authorship contribution statement

Joo hee Song: Conceptualization, Investigation, Resources, Supervision, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. Min soo Kim: Data curation, Writing – review & editing. Ye rim Kang: Supervision, Resources. Dong kyu Kim: Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1F1A105803613), supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20206810100030) and conducted under the framework of research and development program of the Korea Institute of Energy Research (KIER) (C1-2416).

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