Transport phenomena associated with capacity loss of all-vanadium redox flow battery

https://doi.org/10.1016/j.ijheatmasstransfer.2019.119040Get rights and content

Highlights

  • Numerical and experimental study on the transport phenomena for VFB are conducted.

  • Analysis conducted for long-term operation.

  • Capacity loss is closely related to self-discharge reaction by ions crossover.

  • Proton transfer has significant influence on the water transfer through membrane.

  • Changes of SOC can be predicted using change of solution volume at negative electrode.

ABSTRACT

This study investigates transport of different species through the Nafion® 115 membrane in an all vanadium redox flow battery to understand transport phenomena associate with capacity loss. We consider several driving forces related to transport of vanadium ions, proton, and water molecules and examine the variations in ion concentration during charge-discharge, and long-term operation. First, variations in ion concentration are analyzed during the 3rd and the 300th charge-discharge cycles to compare the ion transport process at between early stage and late stage. The capacity loss is closely related to the self-discharge reaction caused by diffusion through the membrane, and proton transport is another important factor of capacity loss since the imbalance in proton concentration can accelerate water transport through the membrane. Furthermore, variations in solution volume and ion concentration are examined during long-term charge-discharge cycles. Since ions accumulate on the positive electrode due to self-discharge reaction and repeated electrochemical reaction, water molecules transport to the positive electrode side. Finally, the relationship between the changes in solution volume and capacity loss is examined, and an empirical equation is suggested for the prediction of capacity loss from changes in the solution volume during long-term operation. Through the results, we can predict the lifetime of VFB system by simple measurement.

Introduction

Over the last few decades, large-scale energy-storage systems (ESSs) have gained much attention as an alternative energy source of power grids. Since the power generation from renewable energy sources such as wind turbines can be intermittent, many researchers and manufacturers started to develop large-scale ESSs to supplement power supply [1,2]. Among the large-scale ESSs, all-vanadium redox flow batteries (VFBs) have garnered most interest because of their high energy efficiency, long cycle life, low maintenance cost, and flexibility for scale-up [3,4]. The VFB system was first proposed by the Skyllas-Kazacos group in 1985. Since then, many companies have succeeded in developing megawatt (MW)-class VFB systems [5,6], and some companies even tried to develop large-scale commercial VFB systems to supply power to the grid [7,8]. Although there is much competition among these companies, their understanding of the transport phenomena associated with capacity loss of VFBs remains unsatisfactory.

Many experimental and numerical studies have been conducted to study the transport phenomena that are related to capacity loss of the VFB system. Some experimental studies investigated the variations in vanadium ions concentration and changes in solution volume during operation [9], [10], [11]; the results showed that the vanadium ion concentration and volume of the electrolyte vary during self-discharge and long-term charge-discharge cycling. Other studies developed three-dimensional transient models to show the distribution of vanadium ions in a VFB by considering the crossover of vanadium ions during change and discharge [12,13]; the results showed that the distribution of vanadium ions and the capacity loss of a VFB are affected by the crossover of vanadium ions. Other researchers developed dynamic models to understand the effect of side reactions on the capacity loss during repetitive cycling [14], [15], [16]. Although most previous studies offer a basic understanding of the mechanism of capacity loss and help us to predict the amount of capacity loss during long-term operation of a VFB, they did not consider the effect of transport of protons and water molecules on capacity loss. Therefore, it is still necessary to understand relationship between the various ions transport and their associated reactions to explain the capacity loss.

The objective of this study is to examine the transport phenomena associated with capacity loss of an all-vanadium redox flow battery. The study began with the development of a transient model to understand the transport phenomena through the membrane during long-term charge-discharge cycling. Since capacity loss is closely related to the transport of ions through the membrane, the model described the transport of vanadium ions, protons, and water molecules affected by electrochemical reaction, diffusion, osmosis, hydraulic pressure difference, and self-discharge reaction. The investigation began with an examination of the variations in ion concentration and solution volume during the 3rd and the 300th charge–discharge cycles, respectively. Next, the changes in the capacity loss of the VFB was analyzed by examining the relationship between the variations in ion concentration and solution volume during long-term charge-discharge cycling. Finally, an empirical equation is suggested to predict the change in state of charge (SOC) using change in electrolyte volume.

Section snippets

Methods

The all-vanadium redox flow battery is a type of flow battery that uses vanadium as the electroactive material at the negative electrode and positive electrode, and the electrolyte circulates between the cell and the reservoir. The redox reaction takes place at electrodes in the cell, while the energy is stored chemically in the external reservoir; therefore, the power and capacity are independently determined by the cell size and electrolyte volume, respectively. The main reactions at the

Results and discussion

In this section, we will validate the VFB model by comparing the output of the model with experimental results obtained during a charge–discharge reaction. Using this model, we analyzed the variations in ion concentration of the VFB for long-term operation.

Conclusions

The transport phenomena in an all vanadium redox flow battery were analyzed to understand the relationship between changes in solution volume and capacity loss. We first examined the driving forces associated with the transport of vanadium ions, protons, and water molecules. To explain the water transport through the membrane, we developed model considering diffusion, self-discharge reaction, osmosis, and the difference in hydraulic pressure at the negative electrode and positive electrode.

Declaration of Competing Interest

None.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT (NRF-2019R1F1A1058036). Additional support was provided by basic research project funded by Korea Institute of Science and Technology Europe and `GO-KRICT' project funded by Korea Research Institute of Chemical Technology.

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