Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels
Introduction
The increasing energy consumption, existing nature of traditional energy sources, and raising awareness for climate change alleviation have propelled researchers to investigate renewable and sustainable energy as alternatives. Among various newly proposed renewable energy devices like wind turbines [1], photovoltaic panels [2], solar thermal systems [3] and so on, hydrogen fuel cells have become an ideal candidate as energy-conversion devices for generating portable, automotive, and distributed power applications [4,5]. One of the most widely used fuel cell types is polymer electrolyte membrane fuel cells (PEMFCs) as they have advantages including relatively low operation temperature (60–90°C), quick start-up procedure, and simpler design [6,7]. Nevertheless, liquid water removal from PEMFCs should be carefully monitored because water management plays an important role in maintaining the optimal performance and durability of PEMFCs [8,9]. Whilst the membrane needs to be sufficiently hydrated to ensure the practical ionic conductivity, excessive liquid water accumulated in the gas diffusion layer (GDL) and flow channels could block the pathways for reactants thus resulting in performance degradation of PEMFCs. Liquid accumulation and transport in the PEMFC components have been extensively investigated to facilitate proper water management [[10], [11], [12]].
Understanding the liquid dynamics inside the flow channel greatly benefits the water management of PEMFCs, and several strategies have been proposed to overcome the operational deficiency caused by excessive water in diffusion media and flow channels. Firstly, water removal efficiency is significantly influenced by the material properties of GDL, which is typically made by carbon fibre or carbon cloth with high porosity and electrical conductivity [13,14]. PTFE (polytetrafluoroethylene) or FEP (fluorinated ethylene propylene) is usually applied on GDL to increase the hydrophobicity of the material, and therefore to assist the water drainage process during the operation of PEMFCs. Studies have shown that the wettability condition of GDL has significant influences on droplet detachment size [15] and water drainage efficiency [16]. Applying PTFE coating can improve the hydrophobicity of GDL material, and the liquid removal efficiency is enhanced by increasing PTFE loading where the droplets are found to detach with smaller diameters. Some researchers have managed to manipulate the spatial distribution of wettability in GDL to facilitate the liquid removal and reactant delivery in PEMFCs [17,18]. In addition to the wettability of GDL material, the operational conditions of PEMFCs also strongly affect the water accumulation and distribution, in which the airflow rate [[19], [20], [21]], operation temperature [22,23], operating pressure [24] and reactants humidity [25] are investigated. Finally, various geometrical configurations of the flow fields have been proposed by researchers to mitigate the water accumulation, including pin-type [26,27], parallel/straight [28,29], serpentine [30,31], integrated [32], interdigitated [33], and so on.
However, to the best of authors’ knowledge, only few studies have been conducted to investigate the effects of GDL surface roughness on the efficiency of water removal inside PEMFCs [[34], [35], [36], [37], [38]]. The heterogeneity of GDL material is revealed by SEM images as shown in Fig. 1. The complex and nonuniform structures of GDL serve as the bottom substrate of flow channel, and the rough surface can significantly influence the liquid water transport and distribution in PEMFCs. Some researchers reconstructed GDL surface structures with patterned elements to represent the actual roughness, and they investigated the effects of surface roughness on water removal in PEMFC flow channel based on numerical models [[34], [35], [36], [37]]. For example, He et al. [34] generated the rough surfaces by rectangle and triangle elements, and the roughness is defined as the ratio between the unit element and channel height. The results highlighted that increasing roughness element height facilitates the water drainage on hydrophilic surface, and the triangle elements perform better than rectangle elements in terms of water removal efficiency. Chen et al. [36] generated rough GDL surface by arrays of cubic columns, and roughness is characterised by column size and spacing. An analytical model is developed to predict the detachment of emerging droplets based on the balance between detaching and retention forces. The results underlined that the droplet removal time and coverage ratio decrease with the increase of surface roughness. Chen et al. [37] further examined the droplet transport inside flow channel with different layouts of GDL carbon fibre such as crisscross distributions, parallel distributions, directional distributions, and orthogonal distributions. It turned out that the surface microstructures influence the liquid water removal, and the directional distributions of carbon fibres best facilitate the liquid transport and reduce flooding. Ding et al. [35] investigated the formation and growth of liquid emerging from inlet pores of GDL with different number and diameter of inlet pores are simulated, and found the surface microstructures assist the liquid removal process especially for highly hydrophilic or hydrophobic cases. In addition, Ashrafi and Shams [38] modelled 2D rough surfaces characterised by different RMS roughness and roughness density values, showing that increasing and lead to the increase of pressure drop, droplet elongation and water retention, but decrease the amount of water adhered to the top wall.
Nevertheless, the above-mentioned studies either simplified the surface roughness of GDL material by modelling patterned elements which leads to insufficient representation of surface roughness, or the dynamic processes of droplet emerging from the GDL layer to flow channel are not fully incorporated. Experimental observations serve as powerful tools to reveal the droplet dynamics inside the PEMFCs [[39], [40], [41], [42]]. However, there are certain limitations for both in-situ and ex-situ techniques due to the differences in time and length scales between experimental setup and realistic operation conditions. Therefore, the computational fluid dynamics (CFD) methods are applied to provide valuable solutions to the gas-liquid interaction problems in PEMFCs [36,38,[43], [44], [45]]. CFD models are capable of handling strong deformation of the droplet, as well as reproducing different flow regimes such as droplet flow, film flow, and slug flow. In this study, a 2D Volume of Fluid (VOF) model is applied to investigate the liquid removal efficiency on rough surfaces considering the droplet emergence, growth, detachment and removal. Rough surfaces characterised by RMS roughness and roughness wavelength are constructed to match realistic GDL material roughness properties. The effects of GDL surface roughness and airflow rate on liquid removal efficiency are examined by comparing droplet detachment time and elongation, and the different detachment regimes and flow patterns are identified according to droplet breakup location and detachment ratio.
Section snippets
Numerical model
In this section, we will introduce the 2-dimensional Volume of Fluid (VOF) method to study the gas-liquid interaction inside the flow channel of PEMFC. The flow channels are modelled with a rough GDL, with variable roughness features to perform sensitivity studies. It is noteworthy that 2D computational models were selected in this work to access meaningful statistical data for multiple cases over rough surfaces with random features. Thus, the overall computational cost with over 160 cases
Results and discussion
The movement and geometrical shape of a droplet emerging from a GDL pore are mainly influenced by a combination of several forces: (1) pressure force that arising from the pressure drop at different sides of the droplet; (2) shear force resulted by the airflow passing above the droplet; (3) adhesive force which is related to the contact area between the droplet and rough surface; (4) surface tension force due to the pressure difference at the water and air surface; and (5) other forces such as
Conclusion
In this study, the effects of airflow rate and GDL surface roughness on the droplet detachment and removal efficiency are investigated using the VOF method. The rough surfaces are characterised by RMS roughness and roughness wavelength containing both lateral and vertical roughness information, and the dynamics of droplet from emergence to removal are fully incorporated. Rolling, lifting, and breakup are identified as three different droplet movement modes on rough surface subject to
Acknowledgements
This work was financially supported by Australian Research Council (Projects DP170102886) and The University of Sydney SOAR Fellowship. This research was undertaken with the assistance of the HPC service at The University of Sydney.
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2022, Chemical Engineering ScienceCitation Excerpt :This flow process has been studied widely (Niblett, 2020; Jeon and Kim, 2015; Xu et al., 2017), especially regarding the impact of surface roughness on the wetting and detachment of the droplets adhered to GDLs. Specifically, the roughness can alter the apparent surface wettability, i.e., the rougher the surface is, the more likely that the droplet touches GDLs in the Cassie mode (only the top is wetted) rather than the Wenzel mode (the porous zone is also wetted) and therefore promoting detachment (Bao and Gan, 2020); additionally, the roughness may affect the moving trail of a droplet resulting in a deviation from the channel center and touching the channel walls (Hou et al., 2020). On the other hand, the microstructure of GDLs can be artificially designed to assist the droplet motion (Chen et al., 2013).