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atoms, including Pt, Pd, Ir, etc. atoms, over Mo C by bonding with its surface Mo atoms to obtain
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thermodynamically stable single-atom catalysts (SACs) and investigated their corresponding ORR activity
-
[148]
and selectivity based on DFT calculations . It is found that only Pt@Mo C exhibits extraordinary 4e ORR
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activity with an overpotential of only 0.33 V, exceeding the state-of-the-art Pt (111) catalyst (as shown in
Figure 8D). Zhang et al. reported a β-Mo C support quasi-paired Pt single atoms catalyst (Pt /Mo C), that
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quasi
is, two closely neighboring and yet non-contiguous Pt sites exhibit synergistic interactions while remaining
[149]
“single” (as shown in Figure 8E (i)) . Similar to the isolated Pt single atoms, there is no crystalline Pt phase
in the X-ray diffraction (XRD) pattern, and no Pt-Pt bond is present (as shown in Figure 8E (ii-iv)). The
Pt /Mo C catalyst showed higher stability as compared to Pt/C, with a current density retention of more
quasi
2
than 85% after 20,000s (as shown in Figure 8E (v)), which was attributed to the synergistic interaction from
the two quasi-paired Pt atom sites in modulating the binding mode of reaction intermediates as well as the
SMSI between Pt and Mo C. Nowadays, Mo C-supported Pt single-atom catalysts are at the forefront of
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research and promising for practical applications in PEMFCs.
Besides TiC and Mo C, a series of carbides (TaC , WC [151,152] , NbC [153,154] , ZrC [13,155] , etc.)-supported Pt-based
[150]
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catalysts have also been reported to exhibit excellent ORR performance due to SMSI effect. Some recently
published literature on the ORR performance of Pt/TMCs is summarized in Table 4. In addition, similar to
TMNs, it is possible that these carbides may act as catalytic centers themselves. This outcome would be
desirable because the parent metals of most TMC are orders of magnitude more abundant in the earth’s
crust and less expensive than Pt-based metals . However, the carbide particle structure itself and the
[156]
surface area make the system even more complex. In addition, the nucleation and growth mechanism of
catalyst particles on the carbide surfaces is not fully understood at present, and the effects of surface
functional groups, surface defects, and relative catalyst particle distribution still need further investigation.
Moreover, carbon corrosion has been a drawback for the commercial application of Pt-based catalysts
supported by TMCs. Future work should pay more attention to the durability of the TMCs-supported
catalysts, especially in a realistic PEMFC cathode environment under working conditions, and elucidate the
corrosion mechanisms and durability behavior.

Other non-carbon supporting materials
Transition metal sulfide
Presently, 2D layered transition metal chalcogenides (TMS), especially MoS , have been used as potential
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supports for Pt-based catalysts because of their unique layer structures, abundant defects, and edge
locations [157,158] . Bothra et al. have systematically explored the ORR activity of different size (Pt) clusters
n
(n = 1-12) supported MoS by first-principles density functional theory. This scaling relationship gives rise
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to a model volcano curve (as shown in Figure 9A), indicating that Pt /MoS is the best electrocatalyst for
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7
ORR with a minimum overpotential value of 0.33 V, which is highly dependent on the binding strength of
the oxygenated species and can be correlated with electron transfer between Pt and MoS 2 [159] . Therefore, the
restacking behavior of the MoS layer reduces the anchoring sites for Pt deposition and does not provide
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high catalytic activity for PEMFC applications. To solve this issue, Anwar et al. introduced MoS into a
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graphene with a mesh shape and electronic conductivity to prepare a hybrid support material (MoS -rGO)
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for a Pt-based catalyst. The results show that the abundantly exposed edges of the MoS NPs constitute a
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homogeneous dispersion of Pt NPs, the mesh structure of graphene prevents the leaching of the Pt NPs, and
the outstanding electronic conductivity of the r-GO cooperatively leads to the higher electrochemical
performance . Subsequently, a flowerlike MoS /N-doped reduced graphene oxide supported ultrafine Pt
[160]
2
[158]
NPs catalyst (Pt@MoS /NrGO) was successfully synthesized by Logeshwaran et al. . The purpose of
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incorporating N atoms into r-GO is to enhance its conductivity and catalytic activity owing to the high-
electron transport kinetics and the capability to inhibit Pt NPs from agglomerating on the support.