Abstract
Understanding water reduction towards H2 generation is crucial to overcome today's renewable energy obstacles. Previous studies have shown the superior H2 production performances of Cobalt based penta‐pyridyl (CoaPPy) and tetra‐pyridyl (CoaTPy) complexes in solution. We investigate H2 production cycles of CoaPPy and CoaTPy complexes immersed in water solution by means of Ab‐initio Molecular Dynamics and Density Functional Theory. We monitor dynamic properties of the systems, solvent response and structural changes occurring in the catalysts, by simulating all intermediate steps of the H2 production cycle. Reduction free energies and reorganization energies are calculated. Our results show that, following the first electron injection, H2 production proceeds with the singlet spin state. Following the first electron insertion, we observe a significant rearrangement of the hydrogen bonding network in the first solvation shell. The cobalt center turns out to be more accessible for the surrounding water molecules in the case of CoaTPy at all the intermediate steps, which explains its higher catalytic performance over CoaPPy. Following the first reduction reaction, a larger gain in reduction free energy is estimated for CoaTPy with respect to CoaPPy, with a difference of 0.14 eV, in line with the experiments. For the second reduction, instead, CoaPPy shows more negative reduction potential, by 0.41 eV. The water reduction reaction is a clean and sustainable way of producing hydrogen energy, however designing an effective catalyst is the other side of the coin. Our theoretical work reveals the dynamical aspect of the water reduction reaction mechanism. The relative stability of the intermediate states, the role of the solvent, the role of the coordination pocket around Co and reduction free energies are determined by modeling the different oxidation states, CoII, CoI, CoIII–H, and CoII–H.
Gurdal, Yeliz