A major contribution to the energy loss in fuel cells originates from poor kinetics of the oxygen reduction reaction (ORR) at the cathode. The ORR mechanism has been understood in descriptor-based approaches, which reveal an activity volcano with a significant overpotential of at least 0.4 V. This energy loss is directly linked to the scaling relation between the binding energy of the ORR intermediates, OH and the OOH. It has become apparent that new catalyst designs are necessary in order to circumvent this scaling relation.
One strategy is to stabilize the OOH intermediate in a dissociated state on two active sites, as an O + OH intermediate. Here we demonstrate the feasibility of this strategy in a systematic study of diporphyrin molecular catalysts. This class of catalysts contains two metal sites, whose catalytic chemistry can be influenced by ligands. Using density functional theory (DFT), we study the ORR activity as a function of intermetallic distance, metals, and ligands. Several diporphyrin catalysts are identified with a theoretical overpotenial of less than 0.3 V. The enhanced catalytic activity is understood as a combination of a geometric effect from the diporphyrin structure and an electronic effect from the choice of metal center and ligand. We propose a strategy to reduce the energy loss and climb the 3D volcano by appropriate design of the geometric and the electronic effects.
Hao Wan, Thomas Mandal Østergaard, Logi Arnarson and Jan Rossmeisl
ACS Sustainable Chemistry & Engineering
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