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activity and stability but also hold significant potential for use in fuel cell and metal-air battery devices.

Titanium nitride (TiN)
-1
-1
TiN as a high conducting material (119 Scm as opposed to 5 Scm for carbon black) has been recently
reported as a promising alternative candidate to the carbon-based support for Pt NPs due to its good
thermal stability, high corrosion- and structure-resistance, and electrochemically stable in fuel cell operating
[107]
conditions . Also, the SMSI effect between TiN support and Pt NPs has been confirmed to be capable of
supplying a strong adhesion to Pt NPs and accelerating electron transfer. Since Ni doping may boost the
ability of Ti atoms to transfer electrons to the adsorbed oxygen molecules and simultaneously reduce the
Ti-O strength to an appropriate level, thereby resulting in high ORR activity [33,108] . As an example, a marriage
of the high activity of the Pt shell and the low cost and superb stability of core TiN NPs was obtained by
Tian et al. via depositing several atomic layers of thick Pt shell on a binary titanium nickel nitride
[109]
nanocrystal (as shown in Figure 7A) . The TiNiN@Pt catalyst exhibited extremely high activity and
excellent durability for the ORR in the acidic solution owing to the synergetic effects of SMSI between the
ultrathin Pt skin and the ultrastable TiNiN support. However, a series of inter-particle grain boundaries
may act as electron reservoirs and traps during the electron transfer between TiN support and Pt NPs,
resulting in the size of TiN particles decreasing and thereby losing their intrinsic high electrical
conductivity. To solve this problem, Shin et al. designed a grain-boundary-free scaffold-like porous TiN
nanotube (NT) with high electrical conductivity (ca. 30-fold higher than TiN NPs) as a support for Pt NPs
[110]
(as shown in Figure 7B) . The result showed that the Pt/TiN NT exhibited a higher ORR activity and
stability compared with Pt/TiN NPs catalyst because of the unique hollow and porous, scaffold-like,
cylindrical structure of TiN NT, which allows for facilitated carrier diffusion in TiN materials, resulting in
improved electrical conduction. To date, a series of TiN modifications through nanostructure formation of
nanotubes [111-115] , nanoflakes [116,117] , nanorods , nanosphere (as shown in Figure 7C-F, respectively.) and
[118]
[119]
[122]
[121]
[120]
through doping by Nb , Mo , Cu , Co [113,118] , Cr [114,123] , and Ni were reported to modify the metal-
[124]
support interface structure, modulate the activation energy of molecular adsorption, and enhance interfacial
electron transfer and mass transfer, thereby improving the ORR activity and stability of catalysts.
Vanadium nitride (VN)
Vanadium nitride (VN), a kind of transition metal nitride, has received much attention in the field of
supercapacitors and lithium-ion batteries but less attention in ORR since most of the synthesis methods of
VN involve high-temperature calcination, which inevitably leads to agglomeration of particles, resulting in
lower specific areas. Therefore, it is necessary to seek a breakthrough from the synthesis of VN materials to
achieve the expected level of ORR activity of catalysts for Pt/VN systems. Yin et al. successfully synthesized
a VN/graphitic carbon (GC) nanocomposite for the first time, which acts as an enhanced support of Pt NPs
toward ORR . After loading 10% Pt NPs, the resulting Pt-VN/GC catalyst demonstrates higher ORR
[125]
activity than 20% Pt/C. More importantly, the electrochemically active surface area (ECSA) of 10% Pt-VN/
GC catalyst maintains 99% after 2,000 cycles, whereas Pt/C is just 75%. The excellent stability is attributed to
the synergistic and SMSI effects between VN and Pt and the stability of the GC. Recently, a high electrical
conductivity VN NFs support Pt NPs catalyst (Pt/VN) was prepared by Kim et al. (as shown in
Figure 7G) . The Pt/VN catalysts exhibited higher ORR activity and durability in acid electrolytes
[126]
compared to Pt/C. DFT calculations provided further evidence of the SMSI effect between Pt and VN,
which contributed to the excellent stability of the catalyst (as shown in Figure 7H).

Chromium nitride (CrN)
Chromium nitride (CrN) is also a viable support material and possesses several desirable properties,
including high electrical conductivity, outstanding thermal and electrochemical stability, exceptional