DETERMINING THE OPERATING CONDITIONS OF ALL-VANADIUM REDOX FLOW BATTERY

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00650 DETERMINING THE OPERATING CONDITIONS OF ALL-VANADIUM REDOX FLOW BATTERY Jungmyoung Kim, Heesung Park * Changwon National University, 20 Changwondaehak-ro, Changwon city, 51140, Republic of Korea Presenting Author: jungmyoungkim@changwon.ac.kr * Corresponding Author: heesungpark@changwon.ac.kr ABSTRACT The All-Vanadium Redox Flow Battery (V-RFB) are used as large-scale energy storage systems for inernittent power sources. The V-RFB performance is determined in accordance with various conditions, such as the material properties of the cell and the composition ratio of the electrolyte or the driving conditions. In this study, the active area of the 25cm 2 scale in the V-RFB cell in the low current density range depending on the temperature and electrolytic solution flow rate changes against V-RFB discharge have been measured experimentally, through the i-v curve was studied the performance of the V-RFB system according to the driving condition. The experimental data, obtained in the low current density range is used for the determined variables in the V-RFB system performance analysis model that describes the i-v curve. The analytical model is the configuration of the two basic losses experienced by a V-RFB, such as activation and ohmic losses. The Tafel constants(exchange current density and electron transfer coefficient) and ohmic resistance is estimated through regression. The exchange current density and the coefficient of the electron transfer, the losses are plotted against temperature and the related parameters are estimated. The performance of the active area 25cm 2 V-RFB cells used in this study has been increased in the electrolyte temperature in the range of 278~328K. KEYWORDS: All-vanadium redox flow battery, Tafel constants, Electric losses, Electrolyte temperature, cell performance 1. INTRODUCTION The All-vanadium redox flow batteries (V-RFBs) technology has considerable potential in large-scale energy storage system (LSESS) to meet requisite of several applications. The energy sources to provide a significant amount of energy, such as the solar and wind turbines. Irregular renewable energy sources has prompted the requirements for energy storage system. [1] The volume of the electrolyte can be independently increased to support energy capacity of several size. The V-RFB builds upon this regular feature. [1-3] The performance of a V-RFB is shown by a graph of its cell voltage (V) versus current density (i). This graph is referred to as the polarization or i-v curve, is the important special feature of a V-RFB. [4] For a several V-RFB parameters, such as ambient temperature, electrolytes flow rate and state of charge, theoretically can calculate the electrical energy. Wherein calculating electrical energy is larger than the actual measurements, because it did not consider the irreversible loss (also called overpotentials). These losses are changed in accordance with the driving conditions. The irreversible losses occurs according to the charge transfer due to the electric resistance and mass transfer of a electrode or kinetics of the electrochemical reactions on the cell component. Therefore, the V- RFB cell voltage can be described by the following equation (1). [5] (1) 1

2 Where E cell is the actual voltage measurement; E rev is the reversible open circuit voltage (OCV); E act is the activation overpotential on the electrodes; E ohm is the ohmic overpotential due to the electrical resistance; and E con is the concentration losses on the activation volume, due to mass transport. The reversible OCV, E rev is calculated from the Nernst equation as follows. [6] (2) where E 0 pos and E 0 neg are standard reduction potentials for the reactions at the electrodes, respectively; R is the universal gas constant; T is the electrolyte absolute temperature; and F is Farady constant; and c a is the molar concentration of vanadium species a. The activation overpotential, E act is approximated by the Butler-Volmer equation; (3) where Eq(3) is known as the Tafel equation, which is equivalent to the Butler-Volmer equation. The α is the Tafel slope; and i is the current density; and i 0 is the exchange current density. The parameters α and i 0 in Eq.(3) are determined by measured polarization curves on curve fitting. Eq.(3) is valid only for i > i 0. The ohmic overpotential caused by internal resistance of the V-RFB and generally expressed as; (4) where A cell is the activation area; and R cell is the component resistance and contact resistance. The concentration overpotential is due to the electrolyte flow channels and activation area and change in the concentrations of both side electrode in the electrolytes and they are expressed in a general form; (5) where i M is the maximum current density. In this paper, the experimentally obtained i-v curve data of a 25cm 2 activation area V-RFB cell in the low current density range depending on the temperature and electrolytic solution flow rate changes against V-RFB discharge have been measured. The V-RFB of the above study a model predicting the parameters of the theoretical equation. In addition, the effect of temperature on activation overpotential, ohmic overpotential studied. 2.1 EXPERIMENTAL SETUP 2. EXPERIMENTAL A schematic of the experimental setup in Fig. 1. The cell of the V-RFB was constructed using PTFE outer plates (each mm). Copper current collector ( mm) were placed using PTFE O-rings (diameter 5mm), and the graphite ( mm) is using to flexible. The gaskets ( mm, activation area of mm) was constructed using PTFE. Electrolyte was circulated through each activation volume through the peristaltic pump circuit and glass reservoir (1000ml, with a nitrogen filling). The separator was a proton exchange membrane (Nafion 117, DuPont). The electrodes ( mm) were composed of graphite felt. A flexible graphite foil was placed between the graphite plate and the current collector in Fig. 2. 2

3 The electrolyte was prepared by dissolving 1.6mol dm 1 VOSO4 in a 4.0mol dm 1 H2SO4 solutions. The volumetric flow rate was in the range m 3 s 1. The electrolyte was in the range K, and discharge current density was in the range A m 2. Fig. 1 Schematic of the experimental setup of the Vanadium redox flow battery (V-RFB) system. Fig. 2 General V-RFB cell components. 3

4 2.2 EXPERIMENTAL RESULTS Fig. 3 is shown the i-v curves of the discharging V-RFB single cell at different electrolytes temperature. For the same operating conditions, that is represented the V-RFB better performance during the high temperature. Fig. 3 i-v curves of measured cell voltage at different electrolyte temperature. 3. CONCLUSIONS In this study, a uniquely designed experiment was conducted to determine the Tafel slope and intercepts of a V-RFB. Therefore, the concentration of the electrolyte solution was uniformly supplied, using four tanks formed the noncirculating device. The i-v curves data obtained from the V-RFB of 25cm 2 activation area operated at different temperature with constant state of charge and flow velocity are fitted to an analytical model, where the measuring voltage is a function of the temperature and flow rate. The single cell has Tafel slope and intercepts, ohmic overpotential and activation loss are found by multiple linear regression analysis. The state of charge overpotential is calculated based on Nernst equation. Meanwhile, linear regression results in a better fit in terms of the reasonability of the fitted Tafel parameters. ACKNOWLEDGMENT This research was financially supported by the Ministry of Education and National Research Foundation of korea through the Human Resource Training Project for Regional Innovation (No. 2015H1C1A ) and was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1D1A3A ). NOMENCLATURE Acell activation area (m 2 ) a vanadium species ( - ) ca molar concentration (m 3 /dm) Eact activation overpotential (V) Ecell cell voltage (V) Econ concentration overpotential (V) 4

5 Eohm ohmic overpotential (V) Erev reversible open circuit voltage (V) E 0 neg negative electrode potential (V) E 0 pos positive electrode potential (V) F Farady constant (C/mol) i current density (A/m 2 ) i0 exchange current density (A/m 2 ) im maximum current density (A/m 2 ) R molar gas constant (J/(Kmol)) Rcell cell resistance (Ω) T temperature (K) α Tafel slope ( - ) REFERENCE [1] T. Nguyen and R. F. Savinell, "Flow batteries", Interface, vol. 19, pp , [2] C. Ponce de León, A. Frías-Ferrer, J. González-García, D. A. Szánto, and F. C. Walsh, "Redox flow cells for energy conversion," Journal of Power Sources, vol. 160, pp , [3] A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, "Redox flow batteries: a review," Journal of Applied Electrochemistry, vol. 41, pp , [4] H. Shaker, "Analytical modeling of PEM fuel cell i-v curve", Renewable energy, vol. 36, pp , [5] A. A. Shah, R. Tangirala, R. Singh, R. G. A. Wills, F. C. Walsh, "Dynamic unit cell model for the all-vanadium flow battery", J. Electrochem. Soc, Vol. 158, A , [6] J. Newman, Electrochemical Systems, Prentice Hall, Englewood Cliffs, NJ,