2 2
Please cite this article in press as: Gao et al., Enabling Direct H O Production in Acidic Media through Rational Design of Transition Metal Single
Atom Catalyst, Chem (2019), https://doi.org/10.1016/j.chempr.2019.12.008
1
2
organic synthesis and effluent treatment, has an optimal pH range of 2.5–3.5.
Therefore, there is a great industrial motivation to improve H
2
O
2
catalysis in acidic
Previously, mercury-al-
nanoparticles supported on carbon, as state-of-
the-art catalysts, have been investigated for H synthesis via ORR in acidic media.
However, these catalysts contain precious noble metals and toxic mercury, thus
limiting their potential applications in H production. Although homogeneous
molecular catalysts such as cobalt macrocycles are highly selective for H produc-
tion via ORR, the low activity and poor stability hinder their possible applications.
3
,13–15
media, more specifically using PEM-type apparatus.
1
6,17
loyed platinum or palladium
2
O
2
2
O
2
2
O
2
1
8
1
9
20
Transition metals such as cobalt particles or manganese species loaded on nitro-
genated carbon materials can also be used to produce H but lack high activity.
2
O
2
Meanwhile, the non-uniform structure in these catalysts hinders their identification of
active sites, mechanistic study, and further rational optimization. In short, there is still
2 2
a lack of cost-effective electrocatalysts with high catalytic performance for H O syn-
thesis in acidic media. In recent years, single-atom catalysts (SACs) with well-defined
active centers have drawn great attention for their particularly high activity and
2
1–23
selectivity in various chemical reactions.
of H production through ORR, O–O bond breaking needs to be minimized.
Benefiting from the desirable features of SACs, in which the active sites are atomi-
cally isolated, the adsorption of O on SACs is usually of the end-on type, rather
than m-peroxo coordination, which therefore could reduce the possibility of O–O
In principle, to increase the selectivity
2
O
2
2
1
8,24,25
bond splitting.
2 2
This implies that SACs would be suitable for H O generation
via ORR. Previous studies of metal-nitrogen-carbon materials mainly focus on the
electrocatalytic activity toward four-electron ORR to H
2
O for fuel cells applica-
is rarely studied in detail.
2
6–30
tions,
whereas unfavorable two-electron ORR to H
2
O
2
Although there are few studies of their electrocatalytic activities toward two-electron
1
9,20
ORR for H
2
O
2
production,
the fundamental aspects such as active center and re-
action mechanism as well as practical electrolytic cell device aspects remain poorly
understood. Here, by combining theoretical and experimental methods, the relation
between the structure of transition metal (Mn, Fe, Co, Ni, and Cu) SACs anchored in
2 2
nitrogen-doped graphene and the catalytic performance of H O synthesis via ORR
was systematically studied. Both theoretically predicted activity-volcano relation
and experimental results show that the Co SAC possesses optimal d-band center
2 2
and can function as a highly active and selective catalyst for H O synthesis via
ORR and even slightly outperforms state-of-the-art noble-metal-based electrocata-
lysts in acidic media.
1School of Chemical and Biomedical
Engineering, Nanyang Technological University,
62 Nanyang Drive, Singapore 637459, Singapore
2Institute of Materials Science & Devices, Suzhou
University of Science and Technology, Suzhou
RESULTS AND DISCUSSION
1
6
Inspired by previous work, we first investigated the ORR process on various tran-
sition metal SACs anchored in nitrogen-doped graphene for producing H or
O along a 2 e or 4 e pathway, respectively, by DFT calculations using a compu-
215009, China
2
O
2
3Department of Physics, Southern University of
ꢀ
ꢀ
Science and Technology, No. 1088 Xueyuan
Avenue, Nanshan District, Shenzhen 518055,
China
H
2
tational hydrogen electrode model (details are given in Experimental Procedures).
ꢀ
The two-electron (2 e ) pathway to H
2
O
2
via ORR comprises two proton-coupled
4Department of Chemistry, National Taiwan
electron transfer steps with only one intermediate (*OOH):
University, Taipei 10617, Taiwan
5State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy
of Sciences, 457 Zhongshan Road, Dalian 116023,
China
+
ꢀ
ꢁ
+ O
2
+ H + e / ꢁOOH
(Equation 1)
+
ꢀ
ꢁ
OOH + H + e /H
2
O
2
+ ꢁ
(Equation 2)
6These authors contributed equally
ꢀ
where the asterisk (*) denotes the active site of the catalyst. In contrast, for the 4 e
7Lead Contact
ORR pathway, four proton-coupled electron transfer steps are included, in which O
2
is reduced to *OOH, *O, *OH, and H
Theoretically, an ideal catalyst for H
2
2
O in sequence, as displayed in Figure 1A.
synthesis should minimize the kinetic
O
2
2
Chem 6, 1–17, March 12, 2020