Electrocatalysis for Energy Storage: Screening, Understanding and Improving Hydrogen Evolution Electrocatalysts in H2-Br2 Flow Batteries
In a transition from a society powered by greenhouse gas-emitting fossil fuels to one powered by renewable energy, energy storage can play a key role. Of the many technology options, one of the most promising is flow batteries, especially the hydrogen-bromine flow battery, which is the focus of this dissertation. To investigate the economic feasibility of a hydrogen-bromine battery as an energy storage device, the levelized cost of energy was calculated, and a sensitivity analysis indicated that the largest improvements to the cost of energy storage will come from improving the system lifetime and efficiency. The key scientific challenges to doing so require creating stable and efficient electrocatalysts. By electrochemically and chemically screening hundreds of metal sulfide materials selected based on our best chemical knowledge, ruthenium and rhodium based metal sulfides were determined to have sufficient stability to operate as hydrogen-bromine electrocatalysts, and exhibit promising activity for hydrogen evolution and oxidation. Incorporating cobalt and nickel into ruthenium sulfide greatly increased the electrocatalyst activity, which we came to understand through combined efforts of theory and gas-phase measurements. The increased activity is most likely due to increased rates of charge-transfer in the hydrogen evolution and oxidation reaction. However, even with incorporation of dopant atoms, the ruthenium sulfide compounds had relatively low hydrogen oxidation activity, possibly due to its semiconducting properties. Rhodium sulfide showed higher activity than even the best ruthenium sulfide materials, but still lower than platinum, although with much improved stability over platinum. Through selective synthesis of different rhodium sulfide phases, as well as poisoning experiments coupled with spectroscopy and density functional theory calculations, the activity of rhodium sulfide was determined to come from the metallic phases Rh17S15 and Rh3S4, in particular the metal sites on these compounds (rather than on sulfur atoms). By selectively forming these phases, the rhodium sulfide showed the highest activity, with the Rh2S3 and RhS2 phases showing low activity. Efforts to improve the rhodium sulfide by incorporation of dopant atoms were not as effective as for the ruthenium sulfide compounds, as transition metals such as Fe, Co, Ni and Cu caused the formation of an inactive rhodium thiospinel phase, and platinum group metal dopants showed no improvement in the rhodium sulfide on a metal sulfide-area basis. The greatest improvements in the activity of the electrocatalyst come from smaller particle sizes of Rh17S15 and Rh3S4 (increased dispersion), and minimization of inactive rhodium sulfide phases.