Single-Atom Catalyst Aggregates: Size-Matching is Critical to Electrocatalytic Performance in Sulfur Cathodes.

Electrocatalysis is critical to the performance displayed by sulfur cathodes. However, the constituent electrocatalysts and the sulfur reactants have vastly different molecular sizes, which ultimately restrict electrocatalysis efficiency and hamper device performance. Herein, the authors report that aggregates of cobalt single-atom catalysts (SACs) attached to graphene via porphyrins can overcome the challenges associated with the catalyst/reactant size mismatch. Atomic-resolution transmission electron microscopy and X-ray absorption spectroscopy measurements show that the Co atoms present in the SAC aggregates exist as single atoms with spatially resolved dimensions that are commensurate the sulfur species found in sulfur cathodes and thus fully accessible to enable 100% atomic utilization efficiency in electrocatalysis. Density functional theory calculations demonstrate that the Co SAC aggregates can interact with the sulfur species in a synergistic manner that enhances the electrocatalytic effect and promote the performance of sulfur cathodes. For example, Li-S cells prepared from the Co SAC aggregates exhibit outstanding capacity retention (i.e., 505 mA h g-1 at 0.5 C after 600 cycles) and excellent rate capability (i.e., 648 mA h g-1 at 6 C). An ultrahigh area specific capacity of 12.52 mA h cm-2 is achieved at a high sulfur loading of 11.8 mg cm-2 .