Performance improvement of proton exchange membrane fuel cells with compressed nickel foam as flow field structure

https://doi.org/10.1016/j.ijhydene.2020.04.238Get rights and content

Highlights

  • Compression ratio of porous nickel foam used in the flow field is determined.

  • Catalytic activity and gas diffusion is improved by adopting nickel foam flow field.

  • Optimum operating parameters are obtained using the experimental method.

  • Fuel cell with compressed nickel foam exhibits a peak power density of 1.89 W/cm2.

Abstract

In order to improve the performance of proton exchange membrane fuel cell (PEMFC), the compressed nickel foam as flow field structure was applied to the fuel cell. The fuel cell test system was built and the performance of fuel cells with nickel foam flow field with different thicknesses were tested and analyzed by electrochemical active surface area (EASA), electrochemical impedance and polarization curve. And its operating parameters were optimized to improve the performance of PEMFC. Our results show that the membrane electrode assembly (MEA) can show a larger catalytic active area and uniformity of gas diffusion can be improved by using the nickel foam flow field instead of the conventional graphite serpentine flow field, and the impedance characteristic of 110PPI nickel foam can be improved by increasing the compression ratio of the original material. What's more, the polarization characteristic and power output performance of PEMFC with nickel foam flow field were improved by optimizing the operating parameters. Using the optimized operating parameters (cell temperature = 80 °C; humidification temperature = 75 °C; stoichiometric ratio = 2; back pressure = 0.23 Map), a peak power density of 1.89 W cm−2 was obtained with an output voltage of 0.46 V.

Introduction

The proton exchange membrane fuel cell has a broad application prospect in the field of new energy vehicles due to its outstanding advantages of low start-up temperature, zero emission, high energy conversion efficiency, and large energy density [[1], [2], [3], [4]]. Bipolar plate, as an important component of fuel cell, has the functions of conducting electrons, distributing reactant gas and water-heat management. Especially, the flow field fabricated on the bipolar plate can guide the reactant gas to permeate through the gas diffusion layer (GDL) and reach the catalyst layer where the chemical reaction takes place. A reasonable flow field design can optimize the effect of gas diffusion and improve the power density of fuel cell. On the contrary, unreasonable design of flow field will lead to an uneven distribution of gas concentration and pressure as well as liquid water aggregation [5]. Furthermore, the water aggregation will block the gas channels of flow field and the micro-pores of GDL. The fuel cell operating in the state of reactant starvation for a long time will accelerate the loss of the catalyst and lead to the corrosion of the carbon carrier. Therefore, the design of bipolar plate has a serious impact on the performance and durability of fuel cells [6,7].

Recently, the optimal design of the bipolar plate reported in literatures is mostly related to the shape of flow field [8], such as the fractal structure inspired by the natural structure of the roots and veins of plant as well as the lung trachea and blood vessels of human [6,9,10]. Besides, the structures of flow field with grid-shape, spiral-shape and Z-shape have also received wide attention [2,8]. Recently, Toyota disclosed their bipolar plate with 3D mesh flow field. The staggered wedge-shaped baffles can ensure that the reactant gas has a good diffusion effect in both the through-plane direction and the in-plane direction [1]. However, considering the manufacturing difficulty and cost, the conventional serpentine, parallel and interdigitated flow fields with rectangular straight channels are still the most widely applied. Among them, serpentine and interdigitated flow fields have the ability to provide appropriate flow distribution, but they may cause large pressure drop [5,7,11,12]. The pressure drop caused by parallel flow field is small, but the flow distribution perpendicular to the flow direction may be uneven if the flow field is not well designed [[13], [14], [15], [16], [17]]. In order to solve the problems mentioned above, a lot of optimum design works were carried out for the configuration of the inlet and outlet, the cross-section shape and depth to width ratio of flow channel and so on [13,15,16,[18], [19], [20]]. In addition, the corrugated, tapered or even local blocked baffle structure fabricated on the bottom or side wall of the channel can also effectively improve the distribution characteristics of reactant gas and contribute to water management [2,11,21,22].

Although the performance of mass transfer in conventional flow fields has been improved by the above optimization work, the uneven gas distribution and the low utilization of catalyst still existed because of the ribs covering MEA plane, followed by uneven distribution of temperature and water [[23], [24], [25]]. Recently the porous flow field with three-dimensional interconnected pore structure has been introduced into the design of bipolar plate, and proved to be feasible in enhancing the ability of reactants to be exposed to the catalytic electrode without increasing the platinum load [26]. The porous flow fields reported in literatures include nickel foam, porous nickel base alloy, carbon foam, aluminum foam, stainless steel fiber sintered sheet, and copper fiber sintered sheet [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. At the same time, some researches have proved that surface coating technology could be used to solve the corrosive problems of porous metal materials in fuel cells [28,29]. The coating can not only keep the metal material from rapid oxidation under the conditions of high temperature and high humidity, but also enhance the conductivity of flow field. For example, Tseng et al. improved the peak power density of a single cell with nickel foam flow field from 1.1 W cm−2 to 1.5 W cm−2 after coating Au on the surface of the nickel foam.

Some researchers also focused on the evaluation and optimization of flow behavior inside the porous flow field. Wu et al. used in-operando neutron radiography to provide an evidence of how and where water is generated, accumulated and removed in the PEMFC with metal foam flow field, and correlated the water ‘maps’ with electrochemical performance and durability [35,37,38]. Fly et al. designed a visual model to observe how the water flow in the porous flow field, and the dyed water was used to replaced reactant gas. The flow and pressure distribution was improved by optimizing the inlet configurations of flow field and pressing grooves on the surface of porous foam [39]. Tsai et al. introduced a physical separator structure into the design of metal foam flow field and divided the complete flow field into multiple regions, which combined with multiple inlets was used to effectively improve the flow distribution [40,41]. Kariya et al. studied that adapting a flow field fabricated by spherical alloy powder to improve the diffusion ability of oxygen and the power density of fuel cell. However, the large flow resistance forced them to fabricate different channel structures on the surface of flow field to reduce the pressure drop [42]. In addition, the results from Shin et al. also indicated that both the diffusion area and the contact area should be taken into account when using the metal Ni foam as flow field. The diffusion uniformity and pressure drop in the flow field was affected by the diffusion area, while the electrical impedance of the cell was determined by the contact area of porous foam field [27].

Above introductions indicated that porous foam as a flow field of fuel cell have received extensive attention. However, there are very few studies focused on the impact of basic structural parameters such as porosity and compression ratio of nickel foam on the electrochemical performance of fuel cell. In this study, the fuel cells with compressed nickel foam flow field were fabricated and a fuel cell test system was built to study the electrochemical performance of nickel foam flow field with different structural parameters. And the effect of operating parameters including the humidification temperature, cell temperature, back pressure and stoichiometric ratio on the performance of PEMFC were systematically analyzed and optimized.

Section snippets

Porous nickel foam flow field

The commercial nickel foam (110PPI,JSD, Suzhou, China) used to replace the conventional graphite serpentine flow field, was fabricated from polyurethane soft foam as matrix by the following steps: pretreatment, chemical deposition, electrodeposition, incineration and thermal reduction. The porosity of nickel foam is up to 95%, even 98%. The structural characteristics of nickel foam are usually determined by thickness and average number of pores per inch (PPI). However, due to the limitation of

Optimization mechanism analysis of porous foam flow field

Fig. 4 shows the mass transfer schematic diagram of fuel cell with nickel foam flow field. In the conventional “rib - channel” structure, the channels distribute reactants and remove products, while ribs are needed to provide adequate support, compression, and conduction of heat and current [50]. As we know, the platinum-carbon catalyst layer coating on the membrane by electrostatic spraying ensured the uniform dispersion of catalyst in the whole MEA plane [46]. However, only the platinum

Conclusion

In order to improve the performance of PEMFC, the fuel cells with compressed nickel foam as flow field structure were assembled. The fuel cell test system was designed to characterize the electrochemical performance of novel fuel cell. Meanwhile, the influence of the structural parameters of nickel foam and operating parameters on the performance of fuel cells was systematically tested and analyzed. And the following conclusions were obtained:

  • (1)

    Porous nickel foam has the potential to replace the

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51922092), and the Natural Science Foundation of Fujian Province of China (No.2017J06015). In addition, the supports from the Pre-research Project of Equipment in 13th Five-Year of China (Nos. 41421020103 and 41421020501) are also acknowledged.

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