A numerical study of electrode thickness and porosity effects in all vanadium redox flow batteries
Introduction
Flow batteries are the most promising medium-to-large scale electrochemical energy storage systems due to its phenomenal advantages, such as high energy efficiency, fast response, scalability, and the most significant of which are the long-life cycles and expandable features [1], [2], [3], [4], [5], [6], [7], [8], [9]. Due to the large energy storage capacity and long discharge time, Vanadium redox flow battery (VRFB) is very attractive when coupling with the renewable energy sources. For a sustainable and clean future, renewable energies such as solar, wind and tidal have been the primary center of research and development efforts [3,[10], [11], [12], [13], [14], [15]]. In the mid of 1980s, Skyllas-Kazacos invented VRFBs which have been comprehensively scrutinized [16], [17], [18], [19], [20], [21], [22]. The performance of redox flow battery has been significantly improved by investigating the key materials, such as electrode [23], [24], [25], [26], [27], [28], membrane [29,30] and electrolyte [31], [32], [33]. The oxidation/reduction (Redox) reactions of the vanadium species occurs at the positive and negative electrodes for the charging/discharging of the VRFB. This leads to an excellent electrochemical reversibility [34], [35], [36], [37], [38], [39]. Among these scientific works, computational numerical simulation is a significant tool to reveal the nature of the electrochemical reaction of VRFB, and to save time, cost and tremendous experimental work. Yang et al. [40] numerically scrutinized the effect of operating parameters on the performance of VRFB during discharging process. They concluded that the discharging cell voltage was increased with the increment of electrolyte flow rate, initial acid concentration and initial vanadium concentration. Xu et al. [41] numerically investigated the performance of VRFB by considering the flow fields. They concluded that the VRFBs with flow fields showed 5% higher energy efficiency than the VRFBs without flow fields. Xu et al. [42] proposed that the VRFBs with serpentine flow fields offered higher performance than parallel flow field fields in terms of power based and round-trip efficiencies. Khazaeli et al. [43] numerically investigated the effect of electrolyte flow rate on the performance of VRFB. They concluded that the increment of electrolyte flow rate from 10 to 70 ml/min boost 50 mV more cell voltage. Zeng et al. [44] proposed a hierarchical interdigitated flow field design and a conventional interdigitated flow field design for the performance of redox flow batteries. They concluded that the hierarchical interdigitated flow field design can significantly increase the pump-based voltage efficiency by 4.2% and reduce the pumping loss by 65.9% at the flow rate of 3.0 mL min−1 cm−2 and current density of 240 mA cm−2 compared with the conventional interdigitated flow field design. Park et al. [45] experimentally investigated the effects of compressed carbon felt electrodes on the performance of VRFB. They found that the porosity and specific resistance of the electrodes decreases with the increment of percentage of electrode compression. In addition, the energy efficiency of the cell initially increases as the percentage of electrode compression rises up to 20%, and then the energy efficiency of the cell decreases with further increase in percentage of electrode compression. Shah et al. [46] presented a two-dimensional transient model for VRFB based on the conservation laws of charge, momentum and mass. The model is used to discover the impacts of variations of electrolyte flow rate, electrode porosity and ion concentration on the battery performance. You et al. [47] proposed a two-dimensional steady state model to scrutinize the effects of electrode porosity and applied current density on the charging and discharging performance of the battery. They found that a high electrode porosity decreases the depletion rate of reactant concentration. Chen et al. [48] studied the effects of electrode thickness and porosity on the performance of VRFB. They found that there is an optimum electrode thickness in order to obtain more cell efficiency and saturated cell capacity, and the rise of porosity increase the cell capacity while it reduce the system efficiency. Lee et al. [49] numerically studied the power-based efficiency of VRFB for different serpentine channel size and electrolyte flow rate. They found the maximum power-based efficiency of 96.6% for the channel size of 1.9 mm at 60 ml/min. moreover, they concluded that the increased electrolyte flow rate improves the electrochemical performance of the battery.
In this study, a three-dimensional model based on the computational fluid dynamics (CFD) and the electrochemical reactions is developed. We present a numerical evaluation of the effects of electrode thickness and porosity on the performance of VRFB using three-dimensional numerical simulation. The three-dimensional numerical simulations were carried out under different parametric conditions applied on the electrode. This study contributes to the fundamental understanding of various electrode thickness sizes on the electrochemical performance and power-based efficiency which should be very helpful to optimizing the VRFB designs near the future.
Section snippets
Model building approach
The schematic diagram of a VRFB is shown in Fig. 1, which is composed of two electrodes, current collectors, a proton exchange membrane, two pumps for circulating the electrolyte solution and two electrolyte solution storage tanks. The electrolyte solution consists of vanadium ions of different valences, V4+/V5+ for the positive electrode and V2+/V3+ for the negative electrode. The electrolyte solutions are stored in the electrolyte tanks and circulated by liquid pumps. The redox reactions of
Results and discussions
The numerical model used in this study was validated by comparing the simulation and experimental results. The effects of electrode thickness, porosity and electrolyte flow rate on the performance of VRFB were numerically investigated.
Conclusions
In this study, a three-dimensional numerical model has been successfully developed and applied to the study of electrode thickness sizes and electrode porosity for a VRFB. The effects of electrode thickness, electrode porosity, electrolyte flow rate and concentration on the power-based efficiency and electrochemical performance of VRFB has been numerically investigated. The results reveal that the increased electrode thickness significantly improves the uniformity of vanadium concentration,
CRediT authorship contribution statement
Ehtesham Ali: Methodology, Software, Writing - original draft. Hwabhin Kwon: Software, Data curation. Jaehun Choi: Formal analysis. Jonghyeon Lee: Software. Jungmyung Kim: Validation. Heesung Park: Supervision, Funding acquisition.
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.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) [No. NRF-2019R1A2C1002212] and also NRF grant funded by Korea government (MSIP) [2018R1A5A6075959].
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