Page 157 - Read Online

Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12 Page 23 of 31

unique layer structures, excellent electronic properties (approximately 15,100 S/cm), and high corrosion
resistance, which facilitate various electrochemical reactions [168-170] . Among the MXene families, Ti C T
2 x
3
support is the most commonly used and expected to supply a strong adhesion for Pt NPs, benefiting from
their same elemental composition as TiC . Zhang et al. reported the presence of abundant -OH and -F
[171]
surface termination groups on Ti C T , which makes it easier to create Pt deposition sites (as shown in
3
2 x
Figure 9C). Thanks to the SMSI effect between Pt NPs and Ti C T nanosheets, the migration and
2
x
3
agglomeration of Pt NPs is effectively prevented, leading to Pt/Ti C T catalysts that exhibit superior ORR
3
2 x
activity and stability over commercial Pt/C in both acidic and alkaline environments . However, the
[168]
charge-transfer performance of the Ti C T surface with the functional groups may be limited by the
3
2 x
inherent restacking phenomenon of 2D materials. Hybridization of Ti C T with carbon nanotube (CNT) is
2 x
3
an effective strategy to prevent the agglomeration of the Ti C T support caused by strong van der Waals
2 x
3
forces and enhance electron transport between Pt-Ti C T -C. Xu et al. demonstrated that the hybridized
3
2 x
Pt/CNT-Ti C T shows excellent ORR activity (the mass activity is 3.4-fold over Pt/C) and durability (a
3 2 x
great ECSA retention of 94% with respect to that in Pt/C of 73% after 2,000 cycles accelerated stress test
[172]
-2
(AST)) . Furthermore, the Pt/CNT-Ti C T exhibited the peak power density (181 mW cm ) in the single-
3 2 x
cell MEA test compared to Pt/CNT, Pt/Ti C T , and Pt/C, attributed to the better mass transport on this
3
2 x
cathode material . In addition to Ti C T , Yang et al. revealed that V C MXene displays excellent charge
[172]
2
3
2 x
transfer kinetics and conductance . The SMSI effect enables highly dispersed Pt atoms and thin Pt films to
[173]
form on the V C, dramatically improving the stability of Pt/V C MXene towards the ORR [173,174] . Li et al.
2
2
have identified Nb CT as capable of providing a good catalytic support interaction for Pt through an
x
2
efficient method of generating reactive metal-support interactions on Pt/Nb CT catalysts at moderate
2
x
temperature . Although the use of MXene as Pt-based catalyst support has pointed out the direction to
[175]
ORR, the AST is still unsatisfactory, possibly due to the oxidative environment of the fuel cell cathodes,
which exposes MXene to oxidation over an extended period of time. Future designs could consider
incorporating nanomaterials such as carbon nanostructures into MXene to improve its oxidative stability.
Finally, while several PGM catalysts have shown impressive ORR activity in half-cell tests, few of them have
demonstrated both good activity and durability in PEMFC. This is most likely related to the intrinsic nature
of the catalyst supports. A number of novel carbon materials, including 1D carbon nanotubes, 2D graphene,
3D carbon nanocomposites, etc., have recently been investigated for PEMFC due to their promising
progress in various aspects such as corrosion resistance and porosity of the material [176-180] . However, the
small number of nucleation sites, low charge transfer for the deposition of metal NPs with carbon supports,
and the mass transfer between metal and support have been major challenges for device performance. A
series of noncarbon-based support based on their inherent properties (as shown in Figure 9D) are able to
generate SMSI effect with Pt NPs, which greatly helps to suppress the agglomeration/separation of the metal
and promote high performance and durability for PEMFC. Despite progress, there are still gaps and
challenges between the properties and practical application of Pt-based catalysts supported by TMOs,
primarily due to the poor conductivity of the TMOs supported. TMNs and TMCs, in particular TiN, TiC,
and Mo C, stand out from other materials as Pt NPs support in terms of the ORR activity and stability in
x
half-cell tests and the performance and durability in fuel cells with reported values well above those of
carbon-supported Pt-based catalysts.
PERSPECTIVE AND OUTLOOK
The development of high-stability carbon-free Pt-based catalysts is scientifically and technically essential to
facilitate their practical application in fuel cells. As discussed in this review, a series of TMOs-, TMNs-, and
TMCs-supported Pt-based catalysts provide an excellent opportunity to substitute the use of carbon at the
cathode for PEMFCs. Innovative and effective construct strategies of SMSI are essential to improve the