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Page 2 of 21 Chen et al. J Mater Inf 2022;2:19 https://dx.doi.org/10.20517/jmi.2022.23

INTRODUCTION
The widespread prevalence of catalysts in industry determines human living standards, as more than 85% of
[1-3]
chemicals are made through catalytic reactions . In their inception phase, with the growing understanding
of catalysis, a series of catalysts with high catalytic efficiency have been developed and designed, resulting in
[4-6]
the rapid development of catalysis . The development of catalysts has been mainly focused on the two
strategies of simplification and complication. The former involves reducing the dimensions (i.e., 3D → 2D
→ 1D → 0D) or decreasing the particle size of the catalysts. Regarding complication, single-atom catalysts
(SACs) take this strategy to the extreme and have become the most popular catalysts due to their high
[7,8]
catalytic performance . The large-scale production of SACs, however, still presents an enormous
challenge. Researchers have attempted to synthesize complex morphologies or structures to improve
catalytic performance. Alloying has been proven to be one of the most effective methods by various research
groups through the design of different alloy systems, including binary, ternary and quaternary alloys, with
excellent catalytic performance [9-12] .

As opposed to SACs, which have the simplest active site, high-entropy alloy (HEA) catalysts have more
[13]
complex active sites due to their large compositional space and diverse atomic arrangements . HEAs are
composed of five or more elemental components in near-equimolar ratios and were first reported by Cantor
and Yeh in 2004 [14,15] . Due to their rich compositional and configurational spaces, some novel HEAs with
specific mechanical properties have been designed and synthesized by direct current magnetron
[16,17]
co-sputtering . As shown in Figure 1, significant exponential growth has been witnessed in the field of
HEAs as the number of publications grew from 137 publications in 2011 to 2684 publications in 2021.
During the development of HEAs in the last two decades, researchers have systematically investigated and
summarized the properties of HEAs, including high entropy, sluggish diffusion, lattice distortion and
[18,19]
cocktail effects . These properties have shown their potential to improve the stability and activity of HEA
catalysts [20-23] . Moreover, considering the high abundance and low cost of Fe, Co, Ni and Cu, Li et al.
synthesized HEA Pt Ni Fe Co Cu nanoparticles for catalyzing the hydrogen evolution reaction (HER)
18 26 15 14 27 [24]
and methanol oxidation reaction (MOR) . This shows that utilizing HEAs as catalysts could provide a
strategy to reduce their cost by alloying non-noble metals. Overall, HEA catalysts exhibit significant
[13]
potential due to their huge compositional space . Although the research into HEA catalysts is gradually
increasing, as shown in the inset of Figure 1, the application of HEAs in catalysis was only ~3% based on
publications (82/2684) in 2021.

Although HEAs have demonstrated excellent catalytic performance for various catalytic reactions, such as
the HER, oxygen reduction reaction (ORR), oxygen evolution reaction (OER), CO reduction reaction (CO
2 2
RR), MOR and ammonia decomposition reaction, their structure-property-performance relationships are
still ambiguous due to the complex active sites of HEA catalysts. A HEA with a face-centered cubic (FCC)
10
structure containing five elements should have 5 = 9,765,625 active sites on its (111) surface when only the
top site (one atom) and the nearest neighbor atoms (nine atoms) are considered active centers, while it
15 18 19
should be 5 and 5 -5 for bridge (two active atoms + 13 neighbor atoms) and hollow (hollow-FCC: three
active atoms + 15 neighbor atoms; hollow-hexagonal-closed packed (HCP): three active atoms + 16
neighbor atoms) sites, respectively. Herein, symmetry was not considered for calculating the number of
active sites since there is no symmetry on HEA surfaces when considering second nearest neighbor atoms.
Such vast possibilities make it impossible for them to be investigated experimentally or computationally.
Until now, the development of HEA catalysts has been sluggish and random, limited by the traditional
[25]
“trial-and-error” methods or “directed research” approaches . The available data are limited for
understanding the nature of HEA catalysts. Consequently, the greatest challenge is identifying the active
center in order to explore the structure-property-performance relationships of HEA catalysts. Only by fully