2.5. General Synthetic Methods for SAECs with High Metal Loadings on Practical Scale

SAECs exhibit tremendous potential in electrocatalysis. However, the scalable synthesis of SAECs with high density of active sites is of great challenge, due to the balance between single-atomic dispersion and loading in the formation of M-N-C sites at high temperatures. For practical applications, the synthesis of SAECs on a large scale with generality is highly required. In very recent years, great progress has been made [104]. For example, the loading of isolated metal atoms has been as high as 18 wt% [105], the general method has been extended to synthesize SAECs for more than 34 types [102], and the production of SAECs on a kilogram scale has been realized [106]. Finally, the introduction of the deep learning algorithm together with big data technology will greatly speed up the screening process and start up a new direction of rational design and modification for complicated SAECs with expected electrochemical catalytic performance.

As shown in Figure 5(a), a cascade anchoring strategy was developed for the mass production of a series of M-N-C SAECs (M=Mn, Fe, Co, Ni, Cu, Mo, Pt) with a metal loading up to 12.1 wt% [107]. Firstly, glucose molecules chelate with metal ions and bind to O-rich carbon support. Excessive glucoses were used to isolate glucose-metal complexes on the carbon substrate. Then, the chelated metal complexes release metal atoms in the pyrolysis process at high temperatures, which were captured by decomposed CN_x species from melamine to generate M-N_x moieties and embed into the carbon matrix to form SAECs. By reacting Ir(CO)₂(acac) with O-containing groups on the reduced graphene aerogel (rGA), atomically isolated iridium complexes could be immobilized on rGA. The rGA substrate provides highly effective bonding sites for metal anchoring superior to those of metal oxides, due to the merits of uniformity and high density. This approach could prepare a single-atom Ir catalyst with remarkably high Ir loading up to 14.8 wt% [109]. With carbon cloth- (CC-) supported NiO (NiO/CC) as the support, Wang et al. prepared a single-atom Ir catalysts with the Ir loading as high as 18 wt% [105]. Firstly, a piece of NiO/CC was immersed into a chloroiridic acid ethanol solution for 10 min and then dried at 80°C. Subsequently, the dried sample was calcined at 350°C for 2 h in air, cooled down, and washed with water. The Ir-NiO/CC catalyst was obtained after dried in air. It was observed that atomically dispersed Ir-atoms are anchored at the outermost surface of NiO and are stabilized by covalent Ir-O bonding, which induces the isolated Ir atoms to form a favorable Ir(IV) oxidation state.

General synthetic methods for the synthesis of SAECs on practical scale. (a) The preparation of M-NC SACs with the cascade anchoring strategy. Reproduced from [107]. (b) Preparation of Pd1/ZnO on kilogram scale. Reproduced from [106]. (c) Synthesis of single-atom Pt/CNT_IL_SiO₂. Reproduced from [108].

A general precursor dilution strategy was developed to prepare SAECs. Firstly, metalloporphyrin (MTPP) containing target metals copolymerized with tetraphenylporphyrin (TPP). The TPP molecules were used as diluents to separate metal atoms. After pyrolysis at high temperatures, carbon-supported single-atom catalysts (M₁/N-C) were obtained [83]. By using this method, twenty-four kinds of single-atom catalysts including noble and nonnoble metals (such as Pt, Pd, Ru, Rh, Au, Ag, Ir, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Zr, Mo, W, Cd, In, Sn, Er, and Bi) were successfully prepared. This method could also be extended to prepare bimetallic Pt₁-Sn₁/N-C single-atom catalysts. Furthermore, a general electrochemical deposition strategy applicable to a wide range of metals and supports was developed to prepare SAECs. In a standard three-electrode device, a glassy carbon electrode loaded with Co(OH)₂ nanosheets were used as the working electrode and a diluted H₂IrCl₆ solution was used as the metal precursor and the electrolyte. The depositing process started from 0.10 to -0.40 V in cathodic deposition and from 1.10 to 1.80 V in anodic deposition. The scanning cycle was repeated for three times to obtain A-Ir₁/Co(OH)₂ from the anode and ten times to obtain C-Ir₁/Co(OH)₂ from the cathode. More than 30 kinds of different SAECs (Fe, Ni, Co, Zn, Cu, Cr, V, Ag, Mn, Ru, Ir, Pd, Rh, Pt, Au, etc.) are prepared from anodic or cathodic deposition by changing supports and metal precursors [46].

With carbon black as the support, 1,10-phenanthroline (Phen) as the N-containing ligand, and transition metal salts as the metal precursor, a general ligand-mediated strategy was developed to prepare transition metal-based SAECs on a large scale [110]. Firstly, nickel(II) acetate coordinated with 1,10-phenanthroline in ethanol to form Ni-Phen complexes. Then, Ni-Phen complexes were adsorbed onto carbon black support. After pyrolysis under argon atmosphere, single-atom Ni-SAC catalyst was obtained. The Ni loading could be as high as 5.3 wt%. By changing the metal precursors, Mn-, Fe-, Co-, Zn-, Cr-, Cu-, Ru-, and Pt-based SAECs and Fe/Co-, Ru/Fe-, Ru/Co-, and Ru/Ni-based binary SAECs were also successfully prepared. To be highlighted, this synthetic approach could be enlarged to produce carbon-based SAECs on a kilogram scale. By ball-milling iron(II) acetate, 1,10-phenanthroline (Phen), and ZIF-8, Sun et al. prepared a FePhenMOF precursor [39]. After thermal treatment of FePhenMOF under Ar and NH₃ atmosphere, a single-atom FePhenMOF-ArNH₃ could be prepared. This ball-milling method could be easily scaled up to synthesize gram-level SAECs in one pot depending on the size of furnace and ball-milling machine. Furthermore, this method was extended to prepare ZnO- and CuO-supported Pd-based single-atom catalysts on a kilogram scale [106]. As shown in Figure 5(b), firstly, the mixture of Pd(acac)₂ and Zn(acac)₂ with a weight ratio of 1 : 400 was thoroughly grounded. After calcination at 400°C for 2 h in air, the single-atom Pd₁/ZnO catalyst was obtained. With the same method, ZnO-supported Rh- and Ru-based single-atom catalysts were also prepared on a kilogram scale.

With the high-speed development of artificial intelligence, the deep learning algorithm attracted more and more attention in a broad research field. Sun et al. for the first time used the deep learning algorithm to develop graphdiyne-supported SAECs with zero-valenced central metal atoms [111]. By quantifying the electron transfer ability and zero-valence stability between metals and graphdiyne support, it was found that among all transition metals, Co, Pd, and Pt showed exceptional stability of zero-valence SAECs based on the evident energy barrier difference between losing electrons and gaining electrons. This novel deep learning algorithm together with big data technology starts up a new direction of rational design and modification for complicated SAECs with expected electrochemical catalytic performance.