Exploring the Limitation of Molecular Water Oxidation Catalysts

Linear free energy scaling relationships (LFESRs) and volcano plots are routinely used to assess the performance of heterogeneous electrocatalysts and have only recently been concretely exploited in homogeneous catalysis. These tools efficiently compare and provide a global evaluation of catalyst performance while highlighting the limitations for a given reaction. In the framework of solid-state water oxidation, a minimal overpotential of 0.4 eV has been predicted on the basis of LFESRs. Considering the very different nature of homogeneous catalysts compared to solid-state systems, the validity of scaling relationships determined for the former cannot be assumed. To evaluate the global limitations of molecular O-2 evolution catalysts, LFESRs are established for all key intermediates for different metal (Mn, Co, Ru, Rh, Ir) and ligand (corrole and perfluoro-porphyrin) combinations assuming a mononuclear mechanism that proceeds through *=OH, *=O, and *-OOH intermediates. Our computations indicate that the LFESRs strongly depend on the choice of density functional. Using GMC-QDPT2 as a benchmark, strong scaling relationships between all intermediates are observed, but the relationships between *-OH and *=O significantly differ from those found in solid-state systems. Consequently, the shape of the molecular volcano plot changes drastically from its solid-state counterpart and shows a broad plateau at the top where the overpotential is nearly independent of the choice of catalyst. This plateau renders the performance of molecular catalysts extremely robust, but inhibits improvements by proceeding through alternative reaction mechanisms.