Multi-length scale characterization of compression on metal foam flow-field based fuel cells using X-ray computed tomography and neutron radiography

https://doi.org/10.1016/j.enconman.2020.113785Get rights and content

Highlights

  • Combined X-ray CT with Neutron imaging to characterize compression effect.

  • PEFC with medium compressed metal foam achieves the best performance.

  • Compression process decreases the pore size and narrows the PSD of metal foams.

  • Compression process facilitates effective water removal due to high pressure drop.

  • Medium compressed cell shows moderate water removal ability and parasitic power.

Abstract

The mechanical compression of metal foam flow-field based polymer electrolyte fuel cells (PEFCs) is critical in determining the interfacial contact resistance with gas diffusion layers (GDLs), reactant flow and water management. The distinct scale between the pore structure of metal foams and the entire flow-field warrant a multi-length scale characterization that combines ex-situ tests of compressed metal foam samples and in-operando analysis of operating PEFCs using X-ray computed tomography (CT) and neutron radiography. An optimal ‘medium’ compression was found to deliver a peak power density of 853 mW cm−2. The X-ray CT data indicates that the compression process significantly decreases the mean pore size and narrows the pore size distribution of metal foams. Simulation results suggest compressing metal foam increases the pressure drop and gas velocity, improving the convective liquid water removal. This is in agreement with the neutron imaging results that demonstrates an increase in the mass of accumulated liquid water with minimum compression compared to the medium and maximum compression cases. The results show that a balance between Ohmic resistance, water removal capacity and parasitic power is imperative for the optimal performance of metal foam based PEFCs.

Introduction

Flow-fields are essential components for various types of energy systems, such as fuel cells, water electrolysers and combustors, etc. [1], [2], [3], [4], [5]. In the case of polymer electrolyte fuel cells (PEFCs), flow-fields are responsible for reactant distribution and product removal across the active area which dictate the performance, efficiency and longevity of PEFCs [6], [7], [8]. Parallel and serpentine are amongst the first channel designs developed and remain the industrial standard due to their manufacturing simplicity and respectable performance they generate [9], [10], [11], [12], [13]. The pursuit for next generation flow-field design continues, however, not only circumvent issues pertaining to fuel cell performance and longevity but to improve overall system efficiency and manufacturability [14], [15], [16].

Amongst numerous flow-field designs proposed thus far [17], [18], [19], [20], [21], [22], metal foam is a promising candidate to address this problem [23]. This design could improve the flow distribution uniformity across the active area due to their favourable pore connectivity and numerous gas pathways [24]. Numerous researchers have examined the influence of flow-field separators, water management, and coating methods on the performance of metal foam based PEFCs [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. In addition, studies report that PEFCs performance is strongly affected by the compression ratios of the metal foam [35], [36], [37], [38]. A previous study showed substantially lower performance of a PEFC using uncompressed foam as flow-field than that of serpentine. However, the PEFC performance was significantly improved when the foam was compressed from 1000 µm to 150 µm [35]. Recently, a study was conducted that combines a compression testing system with X-ray computed tomography (CT) to investigate the mechanical behaviour of metal foam flow-field inside a PEFC. A 42% improvement in maximum power density was observed upon increase in foam compression ratio from 20% to 70% [36]. The authors attribute the performance improvement to the lower interfacial contact resistance between the foam and the GDL. However, the compression effect on microstructural properties and the consequent gas transport parameters across metal foam is not well understood.

An in-depth understanding of microstructural properties and gas transport parameters of compressed metal foams is indispensable for optimizing their design rules. Microstructural properties of the porous media, such as porosity, pore size distribution (PSD), tortuosity and permeability are often used to estimate the gas diffusion resistance [39], [40]. These properties can be acquired from image analysis [41], [42] and modelling of mass/heat flux on the reconstructed 3D microstructure of the porous sample acquired using X-ray CT [43], [44], [45], [46], [47]. X-ray CT has been widely employed in PEFC research, including characterizing the effect of compression [48], [49] and hot pressing [50] on the morphological structure of GDLs.

In terms of water management, we expect contrasting water removal characteristics across metal foam at different compression ratios due to change in pressure drop. Since water management is a key element dictating PEFC performance and durability [51], [52], [53], it is important to characterize the effect of compression on the water management of metal foam flow-field based PEFCs.

The ability of neutron to penetrate through metal has led to a huge breakthrough in fuel cell research, particularly in the field of water visualization [54], [55]. This method has been applied to characterize the effect of flow-field designs, compression pressures and operating conditions [56], [57], [58], [59], [60], [61] on PEFC water management. Recently, neutron radiography has been employed to compare the water management of metal foam and serpentine flow-field based PEFCs [62]. More uniform water distribution was observed for metal foam case due to the absence of a land/channel configuration. However, the metal foam flow-field suffered excess flooding at low current densities from substantially lower gas flow velocity. The work also suggested that altering foam microstructure could partly mitigate flooding in metal foam flow-field based PEFCs.

This study aims to shed light on the effect of compression on various microstructural properties, gas transport parameters and water management characteristics of metal foam flow-field based PEFCs. The difference in length scale of metal pore and the metal foam as a whole requires a sensible trade-off between image field-of-view and resolution. A multi-length scale characterization is therefore required. Here, an approach is used which combines ex-situ characterization of compressed metal foam samples with X-ray CT and in-operando analysis of operating PEFCs using neutron radiography. Pore structure extracted from X-ray CT images is employed to simulate the gas transport in metal foams under different compression ratios. Additionally, a series of measurements are performed to record the corresponding voltages and Ohmic resistances of metal foam flow-field based PEFCs. Therefore, this study fills a gap in the literature and for the first time combines experimental (X-ray CT for 3D structure, while neutron imaging for water distribution) with flow modelling to examine the effect of mechanical compression on metal foam flow-field based PEFCs.

Section snippets

Experimental setup

This section details the experimental setup used in the present work, including the fuel cell design and testing, metal foam sample preparation, X-ray CT scanning, gas flow simulation and neutron radiography.

Results and discussion

Current sweep experiments were carried out by incrementally changing current density every 1200 s at 200 mA cm−2 intervals from open circuit voltage (OCV). Each polarization was repeated twice and averaged. Fig. 2 (a) compares the cell performance under three different compression ratios (6%, 37% and 69%). The PEFC with medium compression (37%) yielded the best performance with ~2% and ~8% improvement in performance at 200 mA cm−2 with respect to high and low compression cases. A greater

Conclusion

Understanding how the extent of compression of metal foam flow-field affects the microstructural properties, gas transport parameters and water management characteristics in PEFCs is indispensable for optimizing the performance and durability. In-operando neutron radiography has been employed to visualize liquid water distribution across PEFCs with metal foams under three different compression ratios, and we present important steps made to characterize the microstructural properties of

CRediT authorship contribution statement

Y. Wu: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing – review & editing. X. Lu: Methodology, Formal analysis, Investigation, Writing – review & editing. J.I.S. Cho: Formal analysis, Investigation, Writing – review & editing. L. Rasha: Formal analysis, Investigation, Writing – review & editing. M. Whiteley: Formal analysis, Methodology, Writing – review & editing. T.P. Neville: Methodology, Investigation, Writing – review & editing. R. Ziesche:

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.

Acknowledgements

The authors gratefully acknowledge the funding from the EPSRC (EP/S018204/2, EP/P009050/1 and EP/R023581/1) for supporting Electrochemical Innovation Lab and to the China Scholarship Council, UCL Faculty of Engineering Sciences Dean’s Scholarship, STFC Futures Early Career Award (ST/R006873/1) and Sichuan Province Science and Technology Fund (2019YJ0236) for supporting Wu Yunsong.

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