1
Polydopamine-Inspired, Dual Heteroatom-Doped Carbon Nanotubes for Highly
Efficient Overall Water Splitting
Konggang Qu, Yao Zheng, Yan Jiao,
Xianxi Zhang, Sheng Dai* and Shi-Zhang Qiao*
Dr. Konggang Qu, Yao Zheng, Yan Jiao,
Prof. Sheng Dai, Prof. Shi-Zhang Qiao
School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia
E-mail: s.dai@adelaide.edu.au;
s.qiao@adelaide.edu.au
Dr. Konggang Qu, Prof. Xianxi Zhang
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng 252059, China
Prof. Shi-Zhang Qiao
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China.
Dr. Konggang Qu and Yao Zheng contributed equally to this work.
Keywords: bifunctional electrocatalysts, nonmetallic codoping, carbon nanotube,
polydopamine, water splitting
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Abstract: Overall water splitting involved hydrogen evolution reaction (HER) and oxygen
evolution reaction (OER) is critical to renewable energy conversion and storage. Heteroatom-
doped carbon materials have been extensively employed as the efficient electrocatalysts for
independent HER or OER process, while those as the bifunctional catalysts for
simultaneously generating H
2
and O
2
by splitting water have been yet seldom reported.
Inspired by the unparalleled virtues of polydopamine (PDA), we devise the facile synthesis of
nitrogen and sulfur dual-doped carbon nanotubes (N,S-CNT) with in-situ, homogenous and
high concentration sulfur doping. The newly developed dual-doped electrocatalysts display
superb bifunctional catalytic activities for both the HER and OER in alkaline solutions,
outperforming all other reported carbon counterparts. Experimental characterizations confirm
that the excellent performance is attributed to the multiple doping together with efficient mass
and charge transfer, while theoretical computations reveal the promotion effect of secondary
sulfur dopant to enhance the spin density of dual-doped samples and consequently to form
highly electroactive sites for both HER and OER.
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1. Introduction
Photoelectrochemical water splitting has triggered huge interest as the promising technology
for efficient renewable energy conversion.
[1-3]
Currently, the most efficient catalysts to split
water are noble metals like Pt, Ir, and Ru, etc. for cathodic hydrogen evolution reaction
(HER)
[4-5]
and anodic oxygen evolution reaction (OER).
[6-7]
Extensive efforts have been
undertaken toward using earth-abundant metal materials to replace noble metals, such as the
transition-metal sulfides,
[8-9]
phosphides,
[10-12]
carbides,
[13-14]
borides
[13]
for HER and the
oxides/hydroxides of Co,
[15]
Ni,
[16]
Mn,
[17]
Fe
[18-19]
for OER. Practically, to achieve an overall
water splitting generating oxygen and hydrogen simultaneously, the coupling of HER and
OER catalysts in same electrolyte often results in incompatible integration of the catalysts and
leads to inferior overall performance. As a result, CoO
x
,
[20]
NiFeO
x
,
[21]
CoP film
[22]
and NiSe
nanowire
[23]
have been developped as efficiently bifunctinal HER and OER catalysts.
However, inherent corrosion and oxidation susceptibility of these materials in either strongly
acidic or alkaline solution largely limit their sustainable utilization, which in most cases
underperform noble-metal catalysts.
In recent years, non-metallic heteroatom-doped carbon materials have been intensively
studied for energy-related electrocatalytic reactions such as oxygen reduction reaction
(ORR),
[24-26]
OER
[27-28]
and HER
[29-32]
due to their excellent electrical conductivity, tunable
molecular structures, abundance, and strong tolerance to acidic/alkaline environments.
Impressively, engineering carbon materials by codoping two or more selected heteroatoms
greatly boosts their elctrocatalytic activities through synergistic coupling effect,
[31, 33]
but
more importantly, it can realize tailorable catalytic capabilities for specific electrocatalytic
reactions by altering doping types, sites and levels.
[34-36]
Correspondingly, several carbon
materials have been synthesized for the bifunctional ORR and OER electrocatalysts with high
activity and excellent stability.
[35, 37-38]
Although some of these have recently been
successfully developed as the HER electrocatalysts,
[29-32]
the employment of carbon-based
4
nanomaterials as the bifunctional HER and OER electrocatalysts has not been reported to date.
The possible reason is that the ORR and OER performance of carbon materials can be
comparable to or even better than metal-based catalysts, but the HER catalytic efficiency of
carbon-based materials still falls far short of that of the metallic benchmarks.
[39-41]
This
represents the current bottleneck for the development of bifunctional HER and OER carbon
electrocatalysts for overall water splitting.
Recently, we demonstrated the unparalleled virtues of polydopamine (PDA) can be
applied as an excellent platform for constructing multiple heteroatom-doped carbon materials
for electrocatalytic ORR and OER.
[38, 42]
In particular, PDA is extremely reactive to thiol
groups via Schiff-base or Michael addition reaction at room temperature without any harsh
reaction condition.
[43]
After thiol addition to PDA, it is easily to realize the N,S-codoped
carbon materials. Apart from discarding excessive N and S sources which are indispensible
for the traditional post-doping method by high temperature annealing,
[32, 44]
more crucially,
this PDA-assisted N,S co-doping strategy can effectively improve the S-doping efficiency
arising from the high grafting efficiency of thiol groups.
Guiding by this strategy, herein we report a two-step “graft-and-pyrolyze” route to
achieve N, S co-doping carbon materials which is fully different from current post-treatment
methods. PDA was first facilely deposited onto the surface of multi-walled carbon nanotubes
(CNT) and then chemically grafted with the thiol groups of S-precursors at room temperature
(see Scheme 1). After pyrolysis at high temperatures, this simple synthetic approach can
derive in situ and homogeneous N,S-codoping in the carbon framework with higher S-doping
efficiency (5.6 at. %) than most of reported methods. As a result, the obtained N,S-CNTs
exhibit superior bifunctional OER and HER eletrocatalytic activities in alkaline solutions,
better than most of reported carbon electrocatalysts. The combination of experimental
characterizations and density functional theory (DFT) calculations jointly unveil the origin of
this large activity enhancement. The results show the secondary S-doping plays a critical role
5
in the formation of catalytically active sites and enhancements of charge transfer. Considering
N and S are the most important dopants in carbon materials for electrocatalysis, this
promising synthetic method may open a new avenue to the development of carbon-based
catalysts for broader applications.
[30, 32, 38]
2. Results and Discussion
The morphologies and structures of the as-prepared N,S-CNT were investigated by the
transmission electron microscopy (TEM). As expected, the N,S-CNT well maintains its
nanotube structure (Figure 1A, Figure S1A-B), and the magnified TEM images (Figure 1B,
Figure S1C-D) indicate there is the continuous film with a thickness of approximately 2 nm
wrapping outside the intrinsic lattice fringe of CNTs, originating from the carbonization of S-
modified PDA coating. Typical elemental mapping images of N,S-CNT (Figure 1C-F)
illustrate the uniform dispersion of N, O and S elements, also confirming the homogeneous
deposition of PDA on CNTs and the following S-addition reaction on PDA. Nitrogen
adsorption confirms the N,S-CNT exhibits a large surface area of 149 m
2
g
-1
. The pore size
distribution (PSD) curve (Figure S2) further verifies the presence of mesopores with their
sizes centered at 3.0 nm and large pores with their sizes ranging from 20 to 150 nm, which
are attributable to the inner cavities of the CNT and the voids in the cross-stacking CNT
network, respectively.
[34]
The characteristic peaks of CNT-PDA at 1501 and 1608 cm
-1
in the Fourier transform
infrared (FTIR) spectra (Figure 1G) are consistent with the indole or indoline structures of
PDA,
[43]
indicating the successful deposition of PDA. For the CNT-PDA modified by sulfur
(CPS), the grafting of 2-mercaptoethanol on the CNT-PDA is evidenced by the weak band at
637 cm
-1
that corresponds to the C-S bonds.
[38]
Meanwhile, compared with CNT-PDA, the
peak at 1501 cm
-1
blue shifts to 1509 cm
-1
and the peak at 1608 cm
-1
is split into two peaks
with a major peak at 1622 cm
-1
and a shoulder peak at 1588 cm
-1
. These changes probably