Complementary Effects of External Strain and Functionalization Towards Enhanced HER Activity on FePS₃ Basal Plane

In the quest for inexpensive photo- or electro-catalytic materials, layered transition metal tri-chalcogenides (TMTC) have recently emerged as one of the most promising candidates for electrocatalytic Hydrogen Evolution Reaction (HER). Earlier experiments showed that among a wide range of investigated MPX₃ compounds, FePS₃, CoPS₃, NiPS₃, FePSe₃ and MnPSe₃ can exhibit excellent HER activity for electrocatalytic water splitting in both acidic and alkaline medium. However, most of the exciting results involved investigations of the activity on their bulk or few-layer forms where the activity enhancement is mainly attained through quantum confinement either by reducing the layer numbers or using substrate effect. In a couple of recent works through first principles simulations we discovered that the basal plane of these TMTC monolayers are not at all active, it is the chalcogen or P edge sites that facilitates the HER process. While this microscopic knowledge of catalytically active sites is a definite progress, the particular knowledge that the active sites are at the edges is only partly positive. It would be more beneficial to have a material with active sites on the basal planes because in a layered material, there will be larger number of basal plane sites than edge sites. Hence in this current work, we have tried to envisage if any of these TMTC materials can be engineered to have active sites on their basal planes. And as a test material we selected FePS₃ monolayer where a three-fold strategy has been considered to tune the HER activity on the basal plane. We investigate how functionalization and vacancy induced defect in pristine FePS₃ monolayer can influence its electrochemical activity. Our results suggest that N substitution for S creates catalytically active sites that are not present in the pristine material. On the other hand, although C- and P-substitution induces a significant improvement in the activity but it is not as good as the nitrogen case. Therefore a further strain engineering has been implemented in these two systems which results in very promising HER activity. In addition, to explore the hydrogen coverage dependency, we have determined the activity in both low and high H⁺ concentration condition. To the best of our knowledge, this is the first work employing detailed first principles calculations showing how a synergy between functionalization through doping and strain engineering can enhance HER catalytic activity on the basal planes of FePS₃. We strongly believe that this work will be an interesting addition to the body of work on this important class of materials