Half-Cell, Steady-State Flow-Battery Experiments. Robert M. Darling and Mike L. Perry

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1 Half-Cell, Steady-State Flow-Battery Experiments Robert M. Darling and Mike L. Perry United Technologies Research Center, East Hartford, Connecticut, 06108, USA An experimental approach designed to separately test the positive and negative sides of a redox flow battery is discussed. Circulating a single electrolyte through a cell so that oxidation occurs on one electrode and reduction occurs on the other electrode eliminates the problem of reactant crossover that occurs when different electrolytes are placed on either side of the cell. This approach allows phenomena associated with the positive and negative electrode-electrolyte combinations to be studied in isolation. Polarization curves, steady-state potential holds, and potentialcycling experiments are presented as examples of results that can be obtained with this approach. The impact of side reactions on the results is also discussed. Introduction Redox flow batteries have attributes that make them attractive for a variety of grid-scale energy-storage applications (1). A variety of positive and negative active materials have been studied in flow batteries. The all-vanadium system is attractive because mixing of the positive and negative reactants through the separator is benign and can be easily reversed (2). Nevertheless, movement of the different vanadium ions through the membrane separator complicates the study of these systems, because it causes capacity to decline and performance to decrease with repeated cycling (3). An experimental technique that eliminates crossover and allows individual electrode-electrolyte combinations to be studied separately is presented in this work. Figure 1 compares the arrangement of cell and reservoirs for a normal vanadium flow battery to the half-cell configuration used in this work. In the normal configuration, shown in figure 1a, the V(ii)/V(iii) electrolyte circulates through the negative electrode, and the V(iv)/V(v) couple circulates through the positive electrode. The various vanadium species can pass through the membrane resulting in self-discharge of the battery. A single electrolyte, the negative V(ii)/V(iii) couple is depicted in figure 1b, circulates through both electrodes in the half-cell configuration. The driving force for transport of species through the membrane approaches zero when the electrolyte circulates at open-circuit provided the pressure difference between the two sides of the membrane is small, and the driving force remains low if the stoichiometric flow is high when current is applied.

2 a) Normal configuration Cell Positive Negative Positive Membrane Negative b) Half-cell configuration Cell Negative Positive Membrane Negative Figure 1. Flow battery schematics: (a) normal and (b) half-cell configurations. Experimental Stock solutions of different oxidation states were prepared electrochemically from a starting solution of VOSO 4 in sulfuric acid. The negative electrolyte tested in this work consisted of 90% V(ii) and 10% V(iii) at a total concentration of 1.5 M in 2.6 M sulfuric acid. The positive electrolyte consisted of 90% V(v) and 10% V(iv) at a total concentration of 1.5 M in 2.6 M sulfuric acid. These solution compositions are meant to simulate a highly charged state. Other states-of-charge can be simulated by adjusting the ratios of V(ii) to V(iii) or V(iv) to V(v) in the reservoir. All cells contained Nafion 212 membranes between identical activated carbon paper electrodes. The electrodes were activated by heating them in air for 30 hours at 400 o C. The active area was 23 cm 2. The reservoir contained 90 ml of electrolyte. The reservoir was purged with nitrogen to prevent reaction with atmospheric oxygen. A peristaltic pump circulated the electrolyte at 120 ml/min, in order to minimize compositional changes over the active area.

3 Results and Discussion Polarization Figure 2 compares the polarization of identical half cells containing either V(ii)V(iii) or V(iv)/V(v). Both polarization curves pass through the origin because the electrolyte is the same on both electrodes in these two experiments. Both polarization curves are linear over the wide range of current densities tested. The slopes of the polarization curves are the same in the top right and lower left quadrants, indicating that the anodic and cathodic performances of the two electrodes are reproducible and that the relationship between the current and flow directions does not matter. The polarization of the V(ii)/V(iii) couple exceeds that of the V(iv)/V(v) couple for this particular set of materials and activation procedures. This observation is consistent with the recent findings of Aaron et al. (4) Cell voltage (V) V(ii)/V(iii) V(iv)/V(v) Currentdensity (A/cm 2 ) Figure 2. Polarization curves for 90% V(ii) plus V(iii), and 90% V(v) plus V(iv). Table I compares the measured slopes, and provides estimates for the contributions from the membrane, carbon contacts, and each electrode. Equal slopes were assigned to the anodic and cathodic reactions in both cases. Reference electrodes could be used to

4 assess the accuracy of this assumption. The areal resistance of the V(ii)/V(iii) electrode is nearly twice that of the V(iv)/V(v) electrode. TABLE I. Slopes of Polarization Curves. Slopes (m -cm 2 ) Negative, 90% V(ii) Positive, 90% V(v) Measured Membrane Contact Electrode (each) The polarizations of the electrodes contain contributions from electronic and ionic resistances and reaction kinetics. Because the electronic and ionic conductivities can be measured separately, it is possible to accurately estimate the kinetic contribution using a mathematical model. However, the results of these calculations are not presented here for the sake of brevity. Steady State and Potential Cycling The durability of electrodes and electrolytes must be established since stationary batteries must operate for years in order to be commercially successful. Studying performance degradation in vanadium flow batteries is complicated by the fact that vanadium movement through the membrane continuously changes the electrolyte composition. While the effects of crossover are recoverable and one can attempt to measure performance repeatedly at the same state-of-charge, implementing such procedures on small test articles is inconvenient. Using half cells considerably simplifies long tests because the cells can be held at essentially steady-state conditions. Figure 3 shows a portion of a constant-voltage durability test on V(iv)/V(v). The current density produced by the cell is effectively constant even though the electrolyte volume is small, because the electrolyte is not exhausted in the half-cell arrangement.

5 Current density (A/cm 2 ) Time at 250 mv (h) Figure 3. Current density versus time for a V(iv)/V(v) cell held at 250 mv. Figure 4 shows a representative portion of a potential-cycling experiment on V(iv)/V(v); potential cycling has been observed to aggravate decay in many electrochemical systems. The potential can be cycled at any rate, independent of the electrolyte volume, for any length of time in this configuration. The results of extended tests of the kind presented in figures 3 and 4 are needed in order to establish the longterm viability of different designs and electrolyte compositions.

6 Current density (A/cm 2 ) or Voltage (V) Current density Voltage Time (s) Figure 4. Potential cycling of V(iv)/V(v) system. Side Reactions Side reactions like hydrogen evolution can cause the composition of the electrolyte to change slowly with time. Hydrogen evolution causes the state-of-charge of the negative electrolyte to decrease. Figure 5 depicts how hydrogen evolution affects the potentials of the anode and cathode in a cell held at constant voltage in a solution initially containing 90% V(ii). The thermodynamically reversible potential is calculated with the Nernst equation, and equal overpotentials are assumed on the two electrodes. Hydrogen evolution causes the potentials to increase, as indicated by the arrow. This increase should be accounted for when analyzing data from such tests. Similarly, changes in the exchange current density of the main reaction with state-of-charge should be treated.

7 0-0.1 Anode Potential (V) Reversible End Start -0.4 Cathode State-of-charge Figure 5. Predicted potentials versus state-of-charge. Conclusion Half-cell experiments where a single electrolyte is circulated through both electrodes while the cell is polarized eliminate crossover, simplifying the study of selected electrode-electrolyte combinations. This approach is particularly well-suited to durability evaluations where performing frequent electrolyte maintenance and attempting to repeatedly measure performance at the same state-of-charge are time consuming and inconvenient. Acknowledgements The authors would like to thank their flow-battery project colleagues at UTRC. The work presented herein was funded, in part, by the Advanced Research Projects Agency - Energy (ARPA-E), U.S. Department of Energy (DOE) under Award Number DE- AR

8 The information, data, or work presented herein was funded in part by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References 1. A. Z. Weber, Ma. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, Q. Liu, J. Appl. Electrochem., 41, 1137 (2011). 2. M. Kazacos and M. Skyllas-Kazacos, J. Electrochem. Soc., (1989). 3. C. Sun, J. Chen, H. Zhang, X. Han, and Q. Luo, J. Power Sources, 195, 890 (2010). 4. D. Aaron, C. Sun, M. Bright, A. Papandrew, M. Mench, and T. Zawodzinski, ECS Electrochem. Lett., 2, A1 (2013).