Rational design of ultrahigh loading metal single-atoms (Co, Ni, Mo) anchored on in-situ pre-crosslinked guar gum derived N-doped carbon aerogel for efficient overall water splitting

Rationally designing single-atom catalysts (SACs) is a potential strategy to reach efficient energy conversion as they combine the merits of both maximizing atomic utilization and minimizing the cost of the heterogeneous catalysts. However, when increasing the loading capacity of single-atoms, how to effectively prevent the aggregation of metal single atoms to achieve higher atom utilization efficiency is an urgent issue. Herein, we develop an in-situ pre-crosslinking method to prepare a series of metal single-atoms anchored on nitrogen-doped carbon aerogel (M-SA@NCA, M = Co, Ni, Mo) as high-efficiency bifunctional catalysts for hydrogen/oxygen evolution reactions (HER/OER). Thanks to the intrinsic chemical crosslinking ability between the metal ions and the substrate functional groups, these metal single-atoms can be effectively and stably anchored in the three-dimensional network as crosslinked point of the substrate, thus endowing M-SA@NCA with ultrahigh single-atom loading (Co, ~27.30 wt%; Ni, ~18.35 wt%; Mo, ~18.16 wt%), and therefore display more active sites, and excellent stability (up to 100 h for water-splitting in 1.0 M KOH). Furthermore, the systematic X-ray absorption fine structure (XAFS), in-situ attenuated total internal reflection (in-situ ATR spectra) and density functional theory (DFT) calculation are used to acquire deep insight on the bonding modes between metal single-atoms and substrates, and the influence of different configuration on the catalytic performance. The combined results reveal that Co atoms facilitate the formation of pyrr-N, and the optimal configuration is CoN4 in HER process with smallest onset potential of 12.5 mV, closing to the Pt/C (20 wt%); while the best OER catalyst is Ni-SA@NCA with NiC2N2-o-5 moieties, outperforming the benchmarked IrO2. This work provides a solution for investigating the interaction between single-atom-substrate configuration and catalytic performance, and represents a critical step towards the rational design and synthesis of SACs with extremely high single-atom loading, maximum atomic utilization efficiency and thus superior catalytic activity.