Tuning metal single atoms embedded in NxCy moieties toward high-performance electrocatalysis

Abstract

Noble nanoparticle (NP)-sized electrocatalysts have been exploited for diverse electrochemical reactions, in particular, for an eco-friendly hydrogen economy such as water splitting. Recently, minimal amounts of single atoms (SAs) are exploited to maximize the active surface area and to tune the catalytic activity by coordinating the SAs in defect sites of N-doped graphene (G_N). For the hydrogen evolution reaction (HER) and oxygen evolution/reduction reactions (OER/ORR), we show high-performance 3d–5d transition metal (TM) SA catalysts using density functional theory (DFT) along with machine learning (ML)-based descriptors. We explore the stability and activity of TM–G_N from the view of structure/coordination, formation energy, structural/electrochemical stability, electronic properties, electrical conductivity, and reaction mechanism, which have not been seriously explored yet. Among various –N_nC_m moieties, the –N₂C₂ moieties tend to be more easily formed and show higher electrochemical catalytic performance and longer durability (without aggregation/dissolution) compared with the widely studied pure –C₄/C₃ and –N₄/N₃ moieties. We found that some TM(SA)s favor a new OER/ORR mechanism, completely different from any known mechanism. The ML-based descriptors showing super HER/OER/ORR performances better than those of bench-mark noble metal catalysts are assessed. In the N₂C₂ templates, Ni/Ru/Rh/Pt show low HER overpotentials. Here, the H adsorption sites are shared by both the metal and C (not N), which was undiscussed in most of the previous literature where the H is attached on top of a metal atom. Low OER overpotentials are found for Pt/Ni–N₂C₂, Ni/Pd–C₄, and Rh–N₄, while low ORR overpotentials are found for Ir/Rh-N₄, Pd–C₄, Ru–N₃C₁ and Ni/Pd/Pt–N₁C₃. The present findings should help in designing high-performance SA catalysts for other various electrocatalytic reactions such as the ammonia evolution reaction.