Microwave-Enabled Incorporation of Single Atomic Cu Catalytic Sites in Holey Graphene: Unifying Structural Requirements of a Carbon Matrix for Simultaneous Achievement of High Activity and Long-Term Durability

Abstract

This work reports our discoveries from the first exploration in microwave pyrolysis of a metal−organic framework. A time-and energy-efficient approach was developed for direct fabrication of electrochemical single-atom catalysts (E-SACs) without the requirement of post-treatment. The most unique structure of the fabricated E-SAC is that the Cu catalytic sites were not in the amorphous carbon matrix as those achieved via traditional pyrolysis but in the basal planes of pristine holey graphene nanoplatelets. The as-prepared Cu-E-SAC exhibits excellent catalytic activity and selectivity in reducing oxygen to water in both acidic and alkaline media. The desired direct 4e − pathway is more favorable in acidic versus alkaline media, which is different from all the Cu-E-SACs reported so far and most transition-metal-based E-SACs. The superior performance is attributed to the unique structure of the catalytic sites. The large graphene domains in the holey graphene materials provide higher delocalized electron-rich π band and increase the d-orbital energy level of the Cu centers. Consequently, their binding strength for molecular oxygen is largely enhanced, improving the oxygen reduction reaction and likely promoting a direct 4e − pathway with minimized generation of a peroxide byproduct. Considering the high conductivity and excellent stability against oxidation of the holey graphene material, this work, for the first time, suggests that the contradictory structural requirement of a carbon matrix for high catalytic activity and long-term durability can be unified and simultaneously satisfied. Combined with the merits of simplicity and rapidness for fabricating both holey graphene and E-SACs, this work provides a possible strategy to address the critical challenges of precious metal-free singleatom catalysts.

One‐dimensional nanomaterial supported metal single‐atom electrocatalysts: Synthesis, characterization, and applications

“…Reproduced with permission. [104] Copyright 2020, American Chemical Society to elemental analysis with a small energy difference. In addition, EELS can also analyze the chemical bond types and elemental valence states of the samples, which are beyond the capability of EDS.…”

“…These single dispersed metal atoms were identified to be Cu, which were coordinated with N with a Cu-to-N ratio of around 1:4 by EELS spectrum (Figure 7I). [104]…”

Rapid synthesis of active Pt single atoms and Ru clusters on carbon black via a highly efficient microwave strategy for the hydrogen evolution reaction in acidic and alkaline media

“…Notably, in recent years, researchers have even successfully synthesized a series of single-atom dispersed metals (e.g., Co, Ni, Cu) by a rapid microwave-heating strategy. [31][32][33] Drawing inspiration from this, we propose a novel process for the preparation of DACs/SACCs based on an efficient microwave reduction strategy. The idea behind this method is to rapidly reduce two representative metal salt solutions using microwave instantaneous heating technology, taking advantage of its immediacy and efficiency to prevent the long-distance diffusion and mass transfer of metal atoms or clusters during the reduction process.…”

Ultrafast synthetic strategies under extreme heating conditions toward single-atom catalysts

“…Specifically, the ultrashort thermal shock innovatively utilizes high temperatures to synthesize SACs while the heating period is kept so short as to avoid the metal atom aggregation and destruction of the substrates, thereby maintaining the stability of the single-atom dispersion. As schematically shown in figure 1, different ultrashort thermal shock methods have been developed for the synthesis of SACs, including microwave heating [44][45][46][47][48][49][50], Joule heating [26,[51][52][53][54][55], laser heating [28] and so on. The ultrafast thermal shock processes provide several unique characteristics that are not possible with the conventional prolonged heating treatment: (a) fast ignition, rapid heating, ultrahigh transient temperature and instant quenching process to hinder the atom diffusion and agglomeration; (b) induction heating that ensures the uniform heat distribution throughout the whole material; (c) high energy efficiency and high cost-effectiveness for controllable and scalable manufacturing.…”