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Page 2 of 31 Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12

Keywords: Fuel cells, oxygen reduction reaction, strong metal-support interaction, stability

INTRODUCTION
The accelerated global population growth and massive consumption of fossil fuel energy have induced
imbalanced energy shortages and severe environmental disruption. Thus, the development of environment-
friendly and sustainable energy technologies has attracted widespread attention to mitigate these
[1-3]
phenomena . Benefiting from their high energy density and zero carbon emissions, proton exchange
membrane fuel cells (PEMFCs) have been considered as one of the most promising energy conversion
[4-6]
technologies in residential applications, automobile transportation, and other stationary power systems .
However, the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) is the major barrier to the
[7-9]
further scale-up of PEMFC for large-scale commercialization . Carbon-supported Pt nanoparticles (Pt/C)
have been widely employed as the ORR electrocatalysts, while the weak interaction between Pt and carbon
frequently causes the aggregation, dissolution, Ostwald ripening/coalescence, or detachment of Pt from the
carbon support, thus deteriorating the ORR activity and stability [10,11] . Furthermore, carbon supports tend to
experience severe corrosion, especially under the high potential encountered during the start-up or shut-
down stages. The strong acidic environment (pH < 1) also contributes to catalyst inactivation and short
lifetime [8,12] , which is a non-negligible factor.

To overcome such shortcomings of carbon support, different types of alternative supports have been
exploited to improve the stability of Pt-based catalysts towards ORR in highly oxidative and acidic
environments, including graphitic carbon nitride (g-C N ), transition metal oxides, carbides and nitrides
3
4
(TMOs, TMCs, and TMNs, respectively), 2D metal-organic frameworks (MOFs), covalent-organic
framework (COFs), layered double hydroxide (LDH), and so on [13-22] . Among them, TMOs, TMNs, and
TMCs are considered to be the most promising alternative supports due to their superior corrosion
resistance and strong metal-support interaction (SMSI) with Pt nanoparticles (NPs). The driving force of
SMSI is defined as minimizing the surface energy of Pt NPs by covering the mobile support suboxides,
which provide a variety of possibilities to modulate the catalytic activity, selectivity, and stability of the
active species, opening up opportunities for developing highly active and stable ORR catalysts. To be
specific, the SMSI of Pt-support usually involves interfacial electron transfer/donation and structural
reconstruction at the metal-support interface (defined as electronic and geometric effects), which has the
capacity to alter the adsorption energies of the reactants and reaction intermediates at the catalytic active
sites situated on the catalyst surface, thereby affecting the activity and stability of catalysts [23,24] . In addition,
SMSI is usually accompanied by the encapsulation of supported metal particles by the support, which
effectively stabilizes the metal particles, thus improving the stability of the catalyst. Recently, several review
articles have shed much new light on SMSI, which provides an effective method to design catalysts with
high activity and durability. For example, Wang et al. summarized several new routes to construct SMSI
involving reductive/oxidative induced SMSI, adsorbate-mediated SMSI (A-SMSI), and wet-chemistry SMSI
[25]
(wcSMSI) to improve the sinter resistance and catalytic performances of the supported metal catalysts .
Luo et al. provided an overview of the developments of SMSI and covered its applicability in both
thermocatalysis and electrocatalysis systems . Pu et al. reviewed various spectroscopic and microscopic
[26]
techniques capable of characterizing the SMSI phenomena and systematically explored the effect of SMSI on
catalytic activity/selectivity . However, these SMSI analyses were largely limited to catalytic reactions
[27]
involving CO, CH , CO , or methanol as the main reactant, lacking relevant summaries and mechanistic
4
2
elucidation in the field of ORR.