Highly-dispersed and high-metal-density electrocatalysts on carbon supports for the oxygen reduction reaction: from nanoparticles to atomic-level architectures

Abstract

Electrocatalyst for oxygen reduction reaction (ORR) is crucial for a variety of renewable energy applications (e.g., proton exchange membrane fuel cells, PEMFCs). The synthesis of highly-dispersed and high-metal-density ORR electrocatalysts...

Emerging Ultrahigh‐Density Single‐Atom Catalysts for Versatile Heterogeneous Catalysis Applications: Redefinition, Recent Progress, and Challenges

“…At present, the hotspot research directions of SACs include the following: coordination environment (including the first coordination and adjacent environment), [17,18] molecular engineering (involving precursor molecular design and molecular catalyst grafting), [19,20] support engineering (for example, nanostructure design and defect structure regulation), [21,22] thermodynamic stability (such as thermal atomization in preparation and stability in application), [23,24] dynamic catalytic structures (dynamic change and plasticity of metal atoms in catalytic reaction process), [25,26] batch preparation (such as gram-level and kilogram-level preparation), [27,28] and ultrahigh content (limit loading on differentiated supports and site density/distance effect of metal atoms) [29,30] (see Figure 2 left for details). It is obvious that the SACs can be considered for mass production (batch preparation in industrial conditions) and practical applications when the foregoing five points are fully studied and optimized.…”

“…Facing the complexities in the design of atomic-level heterogeneous catalysts, one can solve above bottleneck problems with SAC's libraries with widely adjustable metal loading (especially the ultrahigh-loading AFCs equipped by "dense site effects"). [29,30] In general, with the increase of metal content (that is shortening of site distance synchronously), the catalytic reactivity of metal atom catalysts is greatly enhanced (i.e., the specific reactivity depending on site density), which can further optimize the surface density of the metal atom (i.e., the design of AFCs with dense site effect) based on specific practical applications. [29] Taking the atomically dispersed metal sites with controllable surface density as a research object, researchers have deeply studied and revealed the origin of the enhancement of the activity of carbon-supported SACs at the sub-nano level with the increase of the distance between atom sites.…”

“…[29,30] In general, with the increase of metal content (that is shortening of site distance synchronously), the catalytic reactivity of metal atom catalysts is greatly enhanced (i.e., the specific reactivity depending on site density), which can further optimize the surface density of the metal atom (i.e., the design of AFCs with dense site effect) based on specific practical applications. [29] Taking the atomically dispersed metal sites with controllable surface density as a research object, researchers have deeply studied and revealed the origin of the enhancement of the activity of carbon-supported SACs at the sub-nano level with the increase of the distance between atom sites. [30] Understanding the sitedistance effect is of great significance to elucidate the catalytic mechanism of adjacent atom active sites and helps to release the potential (e.g., accurate pathway selectivity, high catalytic reactivity, and high thermodynamic stability) of AFCs materials with dense-site 3D atomic construction.…”

“…[223] In recent years, transition metal (such as Fe and Co) single-atom electrocatalysts have flourished because of their excellent catalytic activity, low cost, and excellent stability. [29] At present, single-atom electrocatalysts have made great achievements in ORR; especially, the synchronous increase in active site density and intrinsic ORR activity of single-atom electrocatalysts is an attractive progress (see Figure 22 and Table 4 for details). [46,47,61] Single-atom Fe catalysts have emerged as an attractive candidate catalyst for promoting ORR reactions, but the synthesis of high-density Fe SACs remains a significant challenge.…”

“…For some special electrocatalytic oxidation or reduction reactions, the catalytic activation of reactants often requires the synergistic action of multiple metal active sites (e.g., the four-electron oxygen reduction requires two consecutive atoms as an active site). [29] In addition, [47] Copyright 2020, Springer Nature. E-H) Reproduced with permission.…”