Influence of loading, metallic surface state and surface protection in precious group metal hydrogen electrocatalyst for H₂/Br₂ redox-flow batteries

Performance, durability, and abundance/cost of electrocatalytic materials are fundamental parameters in for large electrochemical storage solutions like redox-flow batteries (RFB). The acidic environment in Hydrogen–Bromine RFB (HBRFB), which targets tens of thousands of hours in durability, makes the challenge even more acute. Continuous effort to find the most effective and stable catalyst can promote HBRFB goal to become sustainable for high power storage systems. Herein, we explore the lower limits in catalyst loading for the two most active precious group metals (PGMs) – platinum and iridium (individually and in a bimetallic catalyst). The catalyst has been structurally characterized and lab-scale redox-flow cells have been cycled with a decreasing loading of PGM. Carbon support and polymeric coating on Pt catalyst shows a significant increase in the utilization of the catalyst. It enables low platinum loadings (down to 0.36 mgPt/cm²) while maintaining a high discharge power of 0.56 W/cm² and surface activity (peak power normalization to the electrochemical active area) of 63 W/m². On the other side, Ir catalyst shows lower discharge power of 0.34 W/cm², which cannot be improved even at high loading. Finally, bimetallic PtIr reaches a good compromise of high power 0.54 W/cm² and durability for more than 700 cycles. Each catalyst shows high PGM utilization with discharge power above 1.5 W/mg_PGM.