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

Abstract

Redox flow battery (RFB) electrodes act as bi-functional electrocatalysts, improving the kinetics of the redox reactions of interest without being consumed in the reaction itself. This property of flow batteries allows them to be fully discharged without damaging the electrode, and also 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 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 sources. Zinc-bromine hybrid 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. Analogous zinc-iodine based flow batteries have recently become the subject of much scientific interest as well, as the electrolyte is less toxic and corrosive to common battery materials compared to bromine, and a high concentration of electroactive species can be achieved due to the formation of the triiodide ion (I3 -) (up to ~12 M). In addition, some scientific research has recently shifted towards flow batteries constructed from heteropolyhalides (such as I2Br-or Br2Cl-) by combining different halogen-based electrolytes. Flow batteries constructed with Br2/Br-and I2/I-half-cells have been able to achieve some of the highest energy densities reported for aqueous flow batteries to date, but improvements still need to be made to the electrode materials.We investigated a variety of different carbon electrocatalysts for fabrication of low-cost and durable electrodes for the I2/I-, Br2/Br-, and heteropolyhalide redox couples in RFB technology. A variety of carbon blacks 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 electrode studies, and electrochemical impedance spectroscopy. Phosphorus-doped graphitic carbon 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 electrocatalytic activity of various carbon blacks had a strong dependence on their surface area and porosity. The electrocatalytic activity of the phosphorus-doped graphitic carbons exhibit a bell curve 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 31PNMR 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.