In Silico Optimization of Organic-Inorganic Hybrid Perovskites for Photocatalytic Hydrogen Evolution Reaction in Acidic Solution

We previously reported the atomistic reaction mechanism for the photocatalytic hydrogen evolution reaction (HER) on the CH₃NH₃PbI₃ organic-inorganic hybrid perovskites based on quantum mechanics calculations of the transition-state barriers, including several layers of explicit acidic solvent. Here, we extend these studies using in silico optimization to discover additional promising photocatalysts. We consider replacing (i) Pb with Sn, (ii) I with Br, and (iii) CH₃NH₃ cation with several organic cations, including NH₂(CH)NH₂ cation as the photocatalyst for HER. We compared the activation barriers and reaction energies for each case. In our previous studies, we found that both H atoms of the H₂ product are extracted from surface organic cations with protons from the solution migrating along Grotthuss water chains to replace the H of the organic cations. This two-step reaction mechanism involves formation of an intermediate lead hydride bond, with the lead atoms and the surface organic cations both playing essential roles. Among the perovskites investigated here, we predict that NH₂(CH)NH₂PbI₃ exhibits the best HER performance with a predicted 10-fold improvement in the reaction rate compared to CH₃NH₃PbI₃. We also suggest that the lead-free tin iodide perovskites might exhibit a rate comparable to that of lead iodide perovskites with the same organic cations. However, replacing iodine by bromine significantly increases the activation barrier. We find for these lead iodide perovskites, the increased proton affinity of the surface organic cations enhances the photocatalytic efficiency, with NH₂(CH)NH₂ the best case examined.