Carbon Electrocatalysts for High Performance Bromine and Iodine Redox Flow Battery Electrodes

Abstract

Redox flow battery (RFB) electrodes act as bi-directional electrocatalysts, improving the kinetics of the electrolyte redox reactions of interest without being consumed in the reaction itself. This property of flow battery electrodes allows for the power and energy densities of RFBs to be uncoupled and designed separately. The power density is largely dependent on the activity of the electrocatalyst, whereas the energy density is determined by the solubility and volume of the electroactive species in the external electrolyte tank. Due to the advantages of RFBs, they have been heavily studied and implemented for applications in large-scale energy storage and are often used in tandem with renewable energy stations. Zinc-bromine flow batteries have arguably become the most promising and common flow battery technology behind that of the all-vanadium RFB, but are held back by low power densities that result from the slower Br2/Br- half-cell kinetics compared to that of the Zn2+/Zn half-cell. Fabrication of low-cost, high activity bromine electrodes will help increase the viability of this technology. Analogous zinc-iodine flow batteries have also been proposed for identical applications, as the electrolyte is much less toxic and corrosive to common battery materials than bromine, and a high solubility of electroactive species due to the formation of the triiodide ion (I3 -) (up to ~12 M). In addition, some scientific research has shifted towards flow batteries constructed from heteropolyhalides (such as I2Br- or Br2Cl-) by combining different halogen electrolytes.We investigated a variety of different carbon electrocatalysts for low-cost and durable electrodes for the I2/I-, Br2/Br, and mixed heteropolyhalide electrodes in RFB technology. A variety of carbon blacks, graphenes, and phosphorus-doped graphitic carbons (PCx) were drop-casted onto a glassy carbon electrode substrate and their electrocatalytic performance was assessed via a variety of techniques including cyclic voltammetry, rotating disc studies, and impedance spectroscopy. Phosphorus-doped graphitic carbon (PCx) was synthesized via the reaction of phosphorus chloride and benzene in different volumes ratios to obtain samples with variable amounts of phosphorus content (PC, PC3, PC5, PC8), characterized by X-ray diffraction analysis and 31P NMR.The electrochemical performance of various carbon blacks and graphenes were observed to have a strong dependence on their surface area and porosity. The electrocatalytic activity of the phosphorus-doped catalysts was seen to exhibit a volcano plot relationship with the degree of phosphorus-doping, where PC5 and PC3 exhibited higher electrocatalytic behaviour towards the halogen/halide redox reactions than PC and PC8. These observations can be explained by P31 NMR indicating an increased amount of P-O bonds with increasing amounts of doping, with the oxygen containing functional group being catalytic. Excessive doping leads to faults in the lattice structure of the P-doped graphite. The surface area of the electrocatalyst material had the largest impact overall on the electrocatalytic performance of the electrode.