Balancing the electron conduction and mass transfer: Effect of nickel foam thickness on the performance of an alkaline direct ethanol fuel cell (ADEFC) with 3D porous anode
Graphical abstract
The thickness of nickel foam influences both the electron conduction and mass transfer in 3D porous anode, and the optimal thickness is a trade-off between them. The 0.6 mm thickness nickel foam electrode with 1 mg cm−2 Pd loading reaches 56.3 mW cm−2 at 60 °C, which is higher than that of 2 mg cm−2 loading, proving the efficient utilization of catalyst.
Introduction
Alkaline direct ethanol fuel cell (ADEFC) is a highly efficient and environmentally friendly energy conversion device that can directly convert the chemical energy stored in ethanol into electrical energy. Compared to other alcohol fuels, ethanol is less toxic, less expensive, and more widely available. And in alkaline environments, the chemical reaction kinetics of EOR and ORR is faster, and non-precious metal catalysts can be used. With the above advantages, ADEFC has broad application prospects in the field of portable mobile power [[1], [2], [3], [4], [5]].
The core of a fuel cell is a membrane electrode assembly (MEA), the MEA includes a five-layer structure of an anode gas diffusion layer, an anode catalyst layer, a polymer electrolyte membrane, a cathode gas diffusion layer, and a cathode catalyst layer. The gas diffusion layer is used to support the catalyst layer, collect current and mass transfer [6]. The material of the gas diffusion layer that supports the catalyst layer is critical. Conventional electrode supporting materials are generally carbon paper or carbon cloth, which is complicated and expensive to produce. The use of porous metal foams such as nickel foam as anode electrode supporting materials in ADEFC has attracted researchers' attention. Metal foam has a unique three-dimensional network structure, high porosity, high conductivity, and alkali resistance [7,8]. It can be used to efficiently support catalysts and form electrodes. This electrode structure can eliminate the microporous layer and is easy to prepare [9,10]; meanwhile, porous network structure is conducive to mass transportation, and high conductivity is prone to electron conduction, which improves the cell performance [[11], [12], [13], [14]].
Until now, there are a few works reporting the porous metal foam used in fuel cell applications. Wang and Hulya et al. used the three-dimensional network structure of nickel foam to deposit Pd catalyst after introducing graphene on the surface, which greatly improved the performance of methanol and ethanol oxidation, respectively [15,16]. Li et al. prepared the CoNi/rGO@Ni foam electrode to improve the urea electro-oxidation performance [17]. Eisa et al. used hydrothermal method to in situ grow Ni/NiO nanorods on the surface of nickel foam, which provided a new strategy for the preparation of direct alcohol fuel cell electrodes [18]. Wang et al. deposited Pd catalyst directly on nickel foam by electrodeposition and found that the electrode with this structure greatly improved the catalytic activity and stability of EOR [19]. Li et al. used nickel foam integrated electrode prepared by dip-coating method to perform better on anion exchange membrane (AEM) ADEFC than conventional carbon paper electrode [20]. After that, Li et al. further improved the cell performance by improving the electrode preparation method, and the power density reached 202 mW cm−2 [21,22]. The AEM membrane can be used to conduct OH− to complete the internal circuit. At the same time, some researchers have used the Nafion membrane after alkali solution immersion treatment for ADEFC [[23], [24], [25]]. Hou et al. applied KOH-treated Nafion membrane to alkaline ethanol fuel cell, the highest power density reached 59 mW cm−2 at 90 °C, and the cell could run 473 h stably under air-breathing [26].
After analyzing the openly reported works, it is found that the effect of the physical properties of nickel foam remains unknown, while it may have a great impact on cell performance. This work mainly explores the impact of different thickness nickel foam electrodes on cell performance. The thinner the nickel foam, the better the conductivity. However, the corresponding three-dimensional space becomes narrower, which leads to partial agglomeration of the catalyst and hinders mass transfer. Therefore, there is a trade-off in the thickness of nickel foam to utilize the catalyst more efficiently, balancing the electron conduction resistance and mass transfer resistance, and improving the cell performance. To our knowledge, the different thickness of nickel foam resulting in large difference in cell performance has not been reported in ADEFC, and the conclusion in this work can lead to further improvement of fuel cell electrode in the future.
Section snippets
Experimental
In this work, Nafion membrane is used as the electrolyte membrane to conduct K+ to achieve ion conduction. The experimental principle is shown in Fig. 1.
Effect of anode nickel foam thickness
The bulk structural information of the clean nickel foam and the Pd/C-nickel foam electrode were achieved by XRD pattern as shown in Fig. 2. In Fig. 2a, the diffraction pattern of the clean nickel foam showed three peaks at 2θ values of 44.50°, 51.85°, and 76.39°, which were attributed to the (111), (200), and (220) planes of Ni face-centered cubic structure (JCPDS 04–0850). It shows that the surface of nickel foam after pretreatment is clean and free of impurities. In Fig. 2b, in addition to
Conclusion
This work focuses the effect of different thickness nickel foam anodes on the performance of alkaline direct ethanol fuel cells. The result exhibited that among the 0.3 mm, 0.6 mm and 1.0 mm thickness nickel foam electrodes loaded with 1.0 mg cm−2 Pd catalyst, the 0.6 mm thick electrode had the best cell performance, reaching a maximum power density of 56.3 mW cm−2 at 60 °C, 2.4 times higher as that of 0.3 mm and 1.8 times of that of 1.0 mm. The reason was that the 0.3 mm nickel foam was thin,
Acknowledgements
The work described in this paper was fully supported by Grants from the NSFC, China (No. 51676092, No. 21676126), a Grant from the China Postdoctoral Science Foundation (No. 2015M571685), Six-Talent-Peaks Project in Jiangsu Province (2016-XNY-015), High-Tech Research Key Laboratory of Zhenjiang City (No. SS2018002), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
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