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Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12 Page 3 of 31

In this review, particular attention is paid to the recent progress in the design, construction, and emerging
applications of SMSI in ORR catalysts. Particularly, we start by optimizing the intrinsic activity of TMOs,
TMNs, and TMCs supports. By elaborately selecting their compositions, precisely modulating the synthesis
methods, and further adjusting the metal-support interaction, we aim to improve the ORR activity and
stability of the catalysts. This distinguishes our view from the previous review articles [27,28] . Firstly, the basic
principles of the ORR, some descriptors of the computational activity, and electrochemical activity are
briefly introduced. Secondly, detailed strategies of manipulating SMSI through rational design of catalyst
structures and different atmosphere treatments in recent years are summarized. In addition, the precise
synthetic methods and effective improvement strategies of several TMO-, TMN-, and TMC-supported Pt-
based catalysts applications for ORR are discussed. Finally, the prospects and challenges of SMSI for ORR
catalysts are provided.

BASIC PRINCIPLES OF THE ORR
Mechanisms of the ORR
The ORR process generally consists of four steps as follows: (1) diffusion and adsorption of O on the
2
electrocatalyst surface; (2) electron migration from the electrode to adsorbed O molecules; (3) weakening
2
[29]
and splitting of O=O bonds; and (4) removal of the generated species to the electrolyte . Typically, O can
2
be reduced to H O or H O through two different electron transfer pathways: the direct four-electron
2
2
2
transfer pathway (Eq. 1) and the indirect two-electron transfer pathway (Eq. 2). Four-electron transfer
occurs to reduce molecular oxygen to water, facilitating the ORR process. However, it is always
accompanied by the reduction of the two-electron pathway, resulting in the partial reduction of oxygen to
hydrogen peroxide products, which reduces the electrocatalytic selectivity. Both four- and two-electron
transfer pathways involve various oxygen-containing intermediates such as *O, *OH, or *OOH (*represents
the active catalytic site) [30-32] . More specifically, there are three possible reaction mechanisms: dissociative,
associative, and peroxo mechanisms, as shown in Figure 1A. For the dissociative and associative
mechanisms of the four-electron transfer pathway, their difference depends on whether they involve the
[33]
formation of *OOH intermediate .
O + 4H + 4e → 2H O, E = 1.229 V vs. RHE (1)
-
0
+
2
2
O + 2H + 2e → 2H O , E = 0.695 V vs. RHE (2)
0
+
-
2
2
2
where RHE represents the reversible hydrogen electrode.
Linear scaling relationships
Density functional theory (DFT) calculations have been extensively used to understand the free energies of
those intermediates and further estimate the ORR performance. By extending the calculations to various
close-packed metal surfaces, the scaling relationship between ORR activity and oxygen adsorption energy
(ΔE ) have been eventually plotted in ‘‘volcano plots’’ with Pt located at the extreme tip of the linear
o
currently (as shown in Figure 1B) . Generally, the ΔE of Pt-based catalysts is determined by the position of
[34]
o
the d-band center relative to the Femi level, while the shift of the d-band center can be modulated by alloy
with other elements (ligand effect), such as Ni, Co, Fe, etc. [33,35] . An ideal candidate metal should have a
moderate affinity for oxygen. More specifically, if the metals bind oxygen too strong, the ORR will be
retarded due to the difficulty in removing intermediates (O* or OH*) formed by proton-coupled electron
transfer, whereas the metals bind oxygen too weak, the O adsorption and later dissociation to form O* will
2
be constrained. Additionally, a series of catalysts with the SMSI effect, compared to PtM/C catalysts, can
rapidly accelerate the electron transfer in the metal-support interface and precisely regulate the d-band