Electrode Materials for Enhancing the Performance of a Zinc Iodide Flow Battery at High Current Density

Abstract

The zinc-iodide flow battery (ZIFB) has a high energy density and uses a benign electrolyte material [1]. However, the energy density and cyclability the ZIFB is hindered by zinc dendrite growth and electrode polarization in this battery [2]. In our previous work a new flow field design was used to mitigate dendrite formation and enhance the performance of the ZIFB [3]. Despite the enhancement achieved, capacity fade and limited cyclability at current densities greater than 20 mA cm- 2 remain a challenge for ZIFBs. In this study, the use of a high electrical conductivity material for the negative (zinc) electrode, combined with an electrocatalyst on the positive (iodide) electrode was investigated, in order to improve the cyclability and energy density of the ZIFB at high current densities.The use of a high electrical conductivity electrode has been shown to enable rapid transfer of surface charge, and maintain a uniform interfacial electric field [4]. A uniform interfacial electric field can reduce surface charge accumulation and enable a more uniform zinc deposition [5]. This leads to uniform zinc deposition and high rechargeability of the zinc electrode. In this study a copper foil was used as a high conductivity electrode material at the negative (zinc) side of the ZIFB. Scanning electron microscopy (SEM) images of the zinc deposit show that epitaxially layered deposition was obtained on the electrode surface. For all the experiments, charge/discharge of a Zn-I battery were carried out using a 3.0 M ZnI2 electrolyte on both sides of the battery, the charge and discharge current density was 40 mA cm-2, and charging was carried out to 60% state of charge. A Nafion 117 membrane was used to separate the electrolyte compartments.Nitrogen doped carbon obtained by carbonizing polyaniline (PANI) is a promising electrocatalyst material for the next generation energy storage technologies [6]. A carbonized PANI-graphite felt composite was used as the electrode material on the positive (iodide) side of the ZIFB. PANI was uniformly coated on graphite felt using brush, followed by heating to 850 °C for 2 hours under the N2 atmosphere. The use of the carbonized PANI modified graphite felt electrode material in the positive side of the ZIFB (with a graphite felt negative electrode), led to an enhancement of the energy efficiency of around 4% (from 67 % to 71%) at a current density of 40 mA cm-2. The carbonized-PANI modified electrode was characterized using X-ray diffraction, Raman spectroscopy, nitrogen adsorption, and SEM. SEM results indicated that agglomerated nano-grains of carbonized PANI were distributed on the graphite felt fibers. Nitrogen adsorption analysis confirmed that the carbonized PANI-graphite felt electrode had a higher specific surface area than the pristine graphite felt.The performance of a ZIFB using a carbonized PANI-graphite felt positive electrode, and a copper foil negative electrode was evaluated by cycling for over 100 cycles. Stable cycling was observed, with a coulombic efficiency of 99%. When compared with using pristine graphite felt on the negative and positive electrodes, a significant improvement in the charge-discharge energy efficiency of around 10 % (from 67 % to 76%) was obtained.