Studies of Iron-Ligand Complexes for an All-Iron Flow Battery Application

Seven organic ligands were investigated for use to coordinate reactive ions in the positive electrolyte of an all-iron flow battery. Exchange current densities, diffusion coefficients, and open circuit potentials of the ligand complexed ferric/ferrous redox couple are presented on a glassy carbon electrode. Results in a flow cell configuration suggest an ohmically controlled cell voltage and a highly distributed current distribution; the contributions from charge transfer and mass transfer are minor. The open circuit potential becomes a major factor in the selection of the complexed redox couple for determining which ligand to use in a flow battery. The iron-glycine complex was further investigated as a function of the ratio of glycine to ferric/ferrous ions and the pH of the solution. Results suggest a 1:1 glycine to iron ion electrolyte will be soluble up to 0.5 M ferric ion at a pH of 2 with a reaction potential of 468 mV vs. Ag/AgCl (0.690 vs SHE), suitable for use as a positive redox couple in the all-iron flow battery.