In comparison to renewable sources of energy such as wind and solar, redox flow batteries have been seen to provide a promising solution for on-demand operations. In addition to low cost, the fast kinetics of the H₂-Br₂ redox reactions also adds to the advantages of these types of redox flow batteries. However, halide adsorption on the state-of-the-art Pt/C catalyst (for hydrogen oxidation reaction-HOR) has led to significant decreases in the performance as well as the stability of the overall system, thus driving the overall cost. However, in 2015, Northeastern University Center of Renewable Energy Technology (NUCRET) and TVN Systems reported a H₂-Br₂ battery that uses newly synthesized membrane, gas diffusion electrode and HOR catalyst, with several fold higher immunity towards bromine/bromide absorption. At NUCRET, we focus on optimizing that specialized HOR catalyst to further improve its resistance to Br₂/Br ^-. We have successfully synthesized and studied the Hydrogen Oxidation Reaction (HOR) and Hydrogen Evolution Reaction (HER) of the Pt-N_x/C, Ir-N_x/C and Pt-Ir-N_x/C in 1M HBr+1mM Br₂ solution (mimic of H₂-Br₂ battery cross over condition) and compared its activity to the purchased Pt/C, Ir/C and Pt-Ir (1:1) /C. The rotating disk electrode (RDE) results from the pre soak, tested in 1 M HBr solution and the post soak, tested after soaking the catalyst for 24h in 1M HBr+1mM Br₂ solution, suggest two important discoveries. The first regarding the Pt-Ir-N_x/C catalyst is that it was found to have the best resistance toward halide poisoning, compared to Pt-N_x/C, Ir-N_x/C, Pt/C, Ir/C, Pt-Ir (1:1)/C. We concluded that alloying Pt-Ir as well as doping nitrogen into the catalyst matrix is the key to blocking Br₂ absorption. Interestingly, as we tune the Pt:Ir ratio, we discovered that increased Ir content results in better resistance to poisoning. For example, Pt:Ir = 0.5:0.6 has higher losses in HOR/HER performance than Pt:Ir=0.5:0.8. This is explained by the second discovery, which was Ir/C was 3 times more resistant to Pt/C. For the first time, we report these observations and insight on its structure property relationship using XPS, in-situ Raman and DFT. Firstly, Ir/C, Pt-Ir/C, Pt-Ir-N_x/C, Pt-N_x/C, Ir-N_x/C models are built using DFT, as well as simulation with H₂ vs. Br₂ adsorption. In summary, this presentation successfully explains the role of nitrogen and Ir in the catalyst matrix at a fundamental level, as well as optimization of the best HOR catalyst for H₂-B₂redox flow batteries. This research opens the door for clean and renewable energy future, where this low cost and practical battery could be used for grid scale energy applications.

Acknowledgement: The authors would like to extend their gratitude toward Tufts University and WITec. for their guidance and providing with Raman instrument, as well as University of New Mexico for their XPS analysis.