Chemical Engineering Journal 431 (2022) 134072
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Defective RuO2/TiO2 nano-heterostructure advances hydrogen production
by electrochemical water splitting
Wenqiang Li a, 1, Heng Zhang a, b, 1, Manzhou Hong a, b, Lilei Zhang a, Xun Feng a, *, Mengfei Shi a,
Wenxuan Hu a, Shichun Mu c, d, *
a
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, PR China
College of Chemistry, Zhengzhou University, Zhengzhou 450001, PR China
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
d
Foshan Xianhu Laboratory, Foshan 528200, PR China
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Defective RuO2
TiO2
Hydrogen evolution reaction
Oxygen evolution reaction
DFT calculation
Exploring highly active and stable catalysts toward hydrogen evolution reactions and oxygen evolution reactions
(HER/OER) is the key for electrochemical water splitting. Herein, density functional theory (DFT) calculation
results forecast that the defect-rich RuO2 and TiO2 nano-heterostructures can effectively adjust the electron
structure of RuO2, and accelerate the water electrocatalysis, consequently reinforcing the intrinsic activity of the
catalyst. Experimentally, to form an integrated nano-heterostructure, a facile approach is designed for in situ
fabrication of TiO2 on Ti mesh (TM), simultaneously combined with defective RuO2 (D-RuO2) nanoparticles.
Benefiting from the rich active sites, the D-RuO2/TiO2/TM nano-heterostructure formed provides current den­
sities of 50 mA/cm2 at 71 mV for HER and 10 mA/cm2 at 296 mV for OER in alkaline media. For overall water
splitting, the electrolyzer assembled with D-RuO2/TiO2/TM electrode can reach 10 mA/cm2 with a voltage of
only 1.59 V. Moreover, under a fixed current density, such an electrolyzer also achieves an outstanding stability.
1. Introduction
Hydrogen is a promising energy carrier due to its superior calorific
value and environmental friendliness. Among the different methods of
hydrogen production, electrochemical water splitting, consisting of
hydrogen evolution reaction (HER) and oxygen evolution reaction
(OER), is an effective and green approach for producing hydrogen [1–5].
Hitherto, precious platinum (Pt)- and iridium (Ir)-based electrocatalysts
exhibit the best HER and OER performance separately for water split­
ting. However, they undergo high cost, scarcity, and only single catalytic
activity for large-scaled H2 generation [6–8]. Compared to Pt and Ir
metals, Ru possesses lower cost, but it lags far behind in the activity and
stability. Hence, in order to accelerate the practical applications of
hydrogen production by full water splitting, it is important to explore a
new and simple approach to prepare bifunctional Ru-based electro­
catalysts with excellent OER and HER activity and stability on a large
scale.
Due to advantages of the interfacial effect in adjusting the electronic
structure of electrode materials, setting up a heterointerface structure
becomes one of valid methods to enhance the catalytic activity of cat­
alysts toward water splitting [9–12]. In addition, through the strong
interaction between metal with supports, the active noble metal could
be evenly dispersed on the supports, achieving outstanding catalytic
efficiency and stability [13–16]. For example, Mu’s group reported that
ultralow Ru nanoparticles loaded on transition metal phosphides only
required a low overpotential for HER [17]. And Ru/reduced TiO2 pre­
pared by Chen et al., exhibited a relatively low overpotential because of
the weak OH adsorption of reduced TiO2 in the HER process [18].
Moreover, compared to use organic additives to fasten catalysts on
electrode surfaces, if active components are directly grown on substrates
with the high conductivity, it also can show the fast electron transport
and reaction kinetics [19–21]. For example, Ni/NiMoN nanowire arrays
and Co@NC/Ti exhibit vectorial electron transport characteristics and
high electrochemical surface areas [22–24].
Metal doping of transition-metal oxides and defects/vacancies in
ultrasmall nanoparticles are considered as effective strategy to improve
the electrocatalytic activity due to the upsurge of active sites [25]. In
comparison with RuO2/ non-defective CeO2, the mass activity of RuO2/
* Corresponding authors.
E-mail addresses: fengx@lynu.edu.cn (X. Feng), msc@whut.edu.cn (S. Mu).
1
These authors contributed equally.
https://doi.org/10.1016/j.cej.2021.134072
Received 23 October 2021; Received in revised form 29 November 2021; Accepted 6 December 2021
Available online 16 December 2021
1385-8947/© 2021 Elsevier B.V. All rights reserved.
W. Li et al.
Chemical Engineering Journal 431 (2022) 134072
defective CeO2 increased to 2.45 times at an overpotential of 0.4 V [26].
Ultrafine defective RuO2 nanoparticles on carbon cloth presented a
lower overpotential in 0.5 M H2SO4 [27]. Herein, TiO2 as supports in
Ru/TiO2-based nanomaterials can promote the water dissociation and
weaken OH adsorption, which promote the conversion of water to H2
[28–30]. Toward this end, to form the heterostructure the combination
of TiO2 and defective RuO2 would be a high efficient catalyst for water
splitting.
Here, we first predict the total water splitting activity of the defective
RuO2/TiO2 heterostructure catalyst through density functional theory
(DFT) calculations, which shows that setting up heterostructures be­
tween TiO2 and defective RuO2 is conducive to the OER and HER pro­
cess. With the encouragement of theoretical predictions, we then design
and prepare a defective RuO2/TiO2 nano-heterostructure catalyst on Ti
mesh (D-RuO2/TiO2/TM) by impregnating the Ru-containing precursor
over Ti mesh, followed by a thermal-oxidative treatment at high tem­
peratures. Due to the rich active sites and the excellent intrinsic activity
created by defective RuO2, the resultant defective RuO2/TiO2/TM nanoheterostructure reveals superior catalytic activity for both OER, HER
and overall water splitting in 1 M KOH solutions.
experiments.
2.2. Synthesis of D-RuO2/TiO2/TM catalysts
1.20 g RuCl3⋅xH2O and 0.5 g MgCl2 were dissolved in 10 mL of
deionized water under magnetic stirring. The precursor solution was
stored in the dark for further use. A piece of Ti mesh (2 cm × 3 cm) was
immersed in the precursor solution and sonicated for about 30 s. After
drying at 60 ℃ in oven, the Ti mesh containing precursor was annealed
at 450 ◦ C for 3 h in the air to obtained Mg-RuO2/TiO2/TM. After cooled
to room temperature, Mg-RuO2/TiO2/TM was collected. Last, the final
product D-RuO2/TiO2/TM can be obtained by removing the Mg element
in Mg-RuO2/TiO2/TM through acid tread for 18 h.
RuO2/TiO2/TM and TiO2/TM can be obtained as the same process as
D-RuO2/TiO2/TM except the absence of MgCl2 or RuCl3⋅xH2O in the
precursor solutions. To explore the effect of interaction between TiO2
and RuO2 on activity, two other control samples were also prepared
using the same synthetic method of D-RuO2/TiO2/TM apart from the
pyrolysis temperature. The pyrolysis temperature of samples was 350 ◦ C
and 550 ◦ C, which were named D-RuO2/TiO2/TM-350 and D-RuO2/
TiO2/TM-550, respectively. The related Material characterization,
Electrochemical measurements and Theoretical calculation details
are present in Support Information.
2. Experimental section
2.1. Materials and reagent
3. 3.Results and discussion
Ruthenium chloride hydrate (RuCl3⋅xH2O), magnesium chloride
(MgCl2) were purchased from Aladdin Ltd (Shanghai, China). Com­
mercial IrO2 was purchased from Sigma-Aldrich. Hydrochloric acid
(36% wt) and sulfuric acid (68% wt) were purchased from Sinopharm
Ltd (Shanghai, China). The Ti mesh (2 cm × 3 cm) was carefully soni­
cated in 0.5 M H2SO4 and acetone for 20 min to remove impurity of
surface. Then the surface was cleaned for several times by deionized
water and ethanol. Deionized water was used throughout all
3.1. Theoretical prediction
DFT calculations were first performed to understand the catalytic
activity of defect-rich RuO2 and TiO2 heterogenous nanoparticles to
water splitting (Fig. 1A) [27]. As shown in Figure S1, electrons are
transferred from the bottom TiO2(1 1 1) layer to the upper RuO2 layer,
and cause a charge-rich state on the Ru, beneficial to the formation of
Fig. 1. Atomic and electronic structures of D-RuO2/TiO2 nano-heterostructure and the corresponding free energy profiles for HER. (A) Side views of D-RuO2/TiO2.
(B) Electron localization function analysis mapped for the first atomic layer in D-RuO2/TiO2 slab. (C)ΔGH profiles for the HER of TiO2, RuO2/TiO2, Pt/C and D-RuO2/
TiO2, (D) Density of states of Ti in TiO2, RuO2/TiO2 and D-RuO2/TiO2. (E) Partial electronic density of states of Ru d orbital in RuO2, RuO2/TiO2 and D-RuO2/TiO2.
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Chemical Engineering Journal 431 (2022) 134072
abundant surface catalytic active centers. In Fig. 1B, the defect is created
by removing one Ru atom in the first layer in RuO2/TiO2. Based on the
electron localization function analysis map, we can find that the sur­
rounding Ru atom has more positive electrons due to the vacancy caused
by Ru defects, conducive to water splitting [17]. To unveil how the
interface between defects RuO2 and TiO2 influences the free energy
profile of HER, the hydrogen adsorption Gibbs free energies (ΔGH) of
pure TiO2, RuO2, RuO2/TiO2, D-RuO2/TiO2 were calculated (Fig. 1C).
The ΔGH value of TiO2 is 0.32 eV, suggesting that hydrogen absorption
on TiO2 is suppressed. When the RuO2 and TiO2 surface is combined to
form the RuO2/TiO2 heterostructure, hydrogen atoms trend to be
adsorbed on Ru sites. And the ΔGH of RuO2/TiO2 is − 0.17 eV, unsuitable
for HER, due to the strong binding energy will restrains the desorption of
H2. With introduction of defects, the ΔGH is − 0.12 eV, very close to the
Pt (-0.1 eV), indicating that D-RuO2/TiO2 has a Pt-like adsorption en­
ergy and a fast hydrogen release rates in the HER process. Furthermore,
the significant decreased ΔGH* value demonstrates that the hetero­
interface effect between defective RuO2 and TiO2 not only decreases the
hydrogen adsorption energy, but also speeds up the hydrogen release
rates in the HER process, allowing the D-RuO2/TiO2 catalyst with
highest HER activity [31].
Unlike TiO2 with a wide band gap (Fig. 1D), RuO2/TiO2 and D-RuO2/
TiO2 are metallic in nature, indicating a high electrical conductivity. As
a result, constructing the RuO2/TiO2 hybrid interface and introducing
defects in RuO2 could modulate the electronic structure of Ru. There­
fore, compared with RuO2, the d-band center (εd) of Ru d orbital in
defective RuO2/TiO2 moves to low energy level (Fig. 1E), indicating that
the interaction between adsorbed oxygen species and Ru sites is weak­
ened, conducive to the OER performances [15,32,33]. Generally, the
defective RuO2/TiO2 hybrid interface constructed can adjust the elec­
tronic state of RuO2, and then optimize the adsorption energy of in­
termediates in the HER key steps, speeding up the whole OER kinetic
process, advantageous to the whole water splitting.
atomic radius of Mg (1.6 Å) than that of Ru (1.89 Å), Mg atoms can be
easily inserted into the RuO2 nanoparticles, creating some oxygen va­
cancies and reducing the crystallinity of RuO2. After removing Mg atoms
in Mg-RuO2/TiO2 by acid etching, D-RuO2/TiO2/TM with some metal
defects can be achieved.
The XRD pattern (Figure S2) displays the diffraction peaks at
38.4◦ ,40.2◦ , 53.0◦ , 63.0◦ , 70.7◦ , 76.2◦ and 77.4◦ , which correspond to
(0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) lattice planes of Ti
substrate (JCPDF #44–1294), respectively. Except for the Ti peaks, only
two diffraction peaks at 37.3◦ and 35.1◦ appear due to the poor crys­
tallinity of samples, corresponding to the (1 0 1) crystal plane of RuO2
(JCPDF #02–1365) and the (3 0 1) plane of TiO2 (JCPDF #34–0180).
As displayed in the scanning electron microscopy (SEM) images
(Fig. 3A-B), lots of defective RuO2/TiO2 nanoparticles were distributed
on Ti mesh, quite different from the pure Ti mesh (Figure S3). The
transmission electron microscopy (TEM) image displays (Fig. 3C) that
the average grain size of D-RuO2/TiO2 is about 11.5 nm. Such small
nanoparticles are beneficial for exposing more catalytically active sites
for water splitting. The high-resolution TEM (HRTEM) image (Fig. 3D)
display a clear interface structure. The lattice fringe is 0.347 nm,
assigning to the (1 0 1) crystal plane of TiO2. At the same time, the lattice
fringe spacing of 0.255 and 0.316 nm, in good consistence with (1 0 1)
and (1 1 0) planes of RuO2, respectively, [35] basically consistent with
the XRD results. Additionally, the elemental mapping images (Fig. 3E-H)
make clear that the Ru, Ti and O elements in D-RuO2/TiO2 is uniform
distribution. Based on the content of Ru, Ti, O elements (Figure S4), the
atom ratio of Ru + Ti: O is about 1:2, indicating the successful synthesis
of D-RuO2/TiO2 nano-heterostructure catalysts.
X-ray photoelectron spectroscopy (XPS) spectrum was used to
investigate the binding states and quantitative chemical compositions of
the TiO2/TM, TiO2/RuO2/TM and D-RuO2/TiO2/TM. As exhibited in
Fig. 4A, the Ru, Ti, O elements can be observed in samples. In Fig. 4B,
the binding energies of 464.2 and 458.6 eV in Ti4+ region of D-RuO2/
TiO2/TM samples correspond to Ti 2p1/2 and Ti 2p3/2, respectively.
And the spin–orbit splitting is 5.6 eV(464.2–458.6 = 5.6), which is very
consistent with the value of the Ti4+ oxidation state in TiO2-based
nanocomposites [36]. Between Ti 2p3/2 and Ti 2p1/2 (Fig. 4B), the
peak at about 462.0 eV is attributed to the Ru 3p3/2, while Ru 3p3/2 is
absent in TiO2/TM (Figure S5). Two peaks of Ru in D-RuO2/TiO2/TM at
280.6 and 284.3 eV are coincident with the Ru 3d5/2 and Ru 3d3/2 of
RuO2 (Fig. 4C), confirming the presence of Ru (IV) in RuO2 in the
composite [35]. While the peak located at 284.8 eV can be attributed to
C 1 s [15,37]. Compared to TiO2, the Ti peaks in RuO2/TiO2 shift more
3.2. Structure of D-RuO2/TiO2/TM
The synthetic route of D-RuO2/TiO2/TM is shown in Fig. 2. First, a
piece of Ti mesh was immersed into an aqueous solution including
magnesium chloride (MgCl2) and ruthenium chloride hydrate
(RuCl3⋅H2O). After drying in the oven, the Ti precursor was annealed in
air at high temperatures for a certain time, then Mg-RuO2/TiO2/TM can
be obtained. At high temperatures (above 450℃) in the air atmosphere,
the surface Ti in Ti mesh was oxidized to TiO2 [34]. Owing to the smaller
Fig. 2. Schematic fabrication of D-RuO2/TiO2 nano-heterostructures on Ti mesh (D-RuO2/TiO2/TM).
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Chemical Engineering Journal 431 (2022) 134072
Fig. 3. (A,B) SEM images of D-RuO2/TiO2 on Ti mesh. (C,D) TEM and HRTEM image of D-RuO2/TiO2. (E-H) EDS elemental mapping images for D-RuO2/TiO2. The
sample was stripped from D-RuO2/TiO2/TM using ultrasonic treatment.
negatively due to a stronger electron transfer from TiO2 to Ru
(Figure S5). Compared to RuO2/TiO2/TM, the Ti peaks in D-RuO2/TiO2/
TM shift positively, indicates the significant electron transfer between
TiO2 and defective RuO2 due to the strong metal-support interaction,
resulting in the decreasing the chemical state of Ru. Thus, the Ru 3p
peaks for D-RuO2/TiO2 shift negatively (Figure S6), also confirming
that oxygen defects were successfully introduced into RuO2 [18,29]. For
D-RuO2/TiO2/TM, the O 1 s spectra is composed of three oxygen peaks
(Fig. 4E). Among them, the O1(529.5 eV), O2 (531.1 eV) and O3 (532.5
eV) are assigned to the metal–oxygen bonds, the surface chemisorbed
oxygen such as O22– or O-(belonging to defectoxide and hydroxyl-like
group), and water molecules, respectively [38,39]. Meanwhile, the
ratio of O2/O1 (peak area) of D-RuO2/TiO2 is 1.4, larger than that of
RuO2/TiO2 (0.43) (Fig. 4F), proving a larger amount of surface oxygen
vacancies on the D-RuO2/TiO2 [40–42]. Furthermore, electron spin
resonance (ESR) spectrum was performed to directly evidence the
presence of vacancies in RuO2/TiO2/TM and D-RuO2/TiO2/TM
(Fig. 4F). D-RuO2/TiO2/TM reveals a much stronger oxygen vacancy
related ESR signal than RuO2/TiO2/TM, indicating that extensive oxy­
gen vacancies formed on the surface of D-RuO2/TiO2 [43,44]. The ox­
ygen vacancies on the surface of RuO2 can form more electrochemically
active sites, thereby improving the electrocatalytic activity of catalysts
[45,46].
has big influences on the RuO2 activity. At current densities of 10, 100,
150, and 200 mA/cm2, the overpotentials are 10, 114, 147, and 165 mV,
respectively, proving the extremely high HER activity of D-RuO2/TiO2/
TM in alkaline media. In addition, it is worth noting that at high current
densities above 150 mA/cm2, the HER activity of D-RuO2/TiO2/TM
even surpasses that of commercial Pt/C on TM (Fig. 5A). Noting that, the
excellent HER performance of D-RuO2/TiO2/TM in alkaline media is far
ahead of the reported RuO2-based materials, and can be compared with
the best Ru-based electrocatalysts as reported, such as RuO2/N-C(Ƞ10 =
40 mV) [47], RuO2-300Ar(Ƞ10 = 17 mV) [48], CoOx-RuO2(Ƞ10 = 24
mV) [49] as well as Pd3Ru /C (Ƞ10 = 42 mV) [50] (Table S1).
In addition, to study the reaction kinetics during HER processes, the
Tafel curves of different catalysts were measured (Fig. 5C). Obviously,
the Tafel slopes of D-RuO2/TiO2/TM is 73.5 mV dec-1, close to the Tafel
slopes (97.5 mV/dec) obtaining from the true steady-state polarization
curve (Figure S7) [51], smaller than that of RuO2/TiO2/TM (105.3 mV
dec-1), TiO2/TM (206.6 mV dec-1) and Bare TM (237.5 mV dec-1). Ac­
cording to the obtained Tafel value, the HER process on the D-RuO2/
TiO2/TM electrode should be the Volmer- Heyrovsky pathway [25].
Moreover, the Nyquist plots demonstrate that the D-RuO2/TiO2/TM
electrode has the smallest charge transfer resistance (Rct) (Fig. 5D),
suggesting it has an excellent electron transfer. Above all, the higher
HER activity of the D-RuO2/TiO2/TM electrode can be explained by the
smallest Tafel slope and minimal Rct. Furthermore, the exchange cur­
rent densities (j0) of D-RuO2/TiO2/TM, Pt/C on TM, RuO2/TiO2/TM and
TiO2/TM are 6.9, 5.4, 5.01 and 0.38 mA cm− 2, respectively (Figure S8),
manifesting that D-RuO2/TiO2/TM holds better intrinsic catalytic ac­
tivity than RuO2/TiO2/TM and TiO2/TM.
Durability is another important factor in assessing the practical ap­
plications of an HER catalyst. After 1000 CV cycles, the LSV curves
(Fig. 5E) show a negligible change. Furthermore, to further evaluate the
durability, we performed the electrolysis measurement under the fixed
current continuously for 18 h. As shown in Fig. 5F, D-RuO2/TiO2 only
emerges nearly unchanged in the overpotential after electrolysis,
proving the high catalytic durability.
After the durability test, from the XRD (Figure.S9A), it shows the
presence of RuO2 and TiO2 in the composite, and no significant change
3.3. Electrochemical evaluation for HER
To understand the effect of constructing defect-rich nano-heter­
structures on the electrochemical activity, the HER activity of D-RuO2/
TiO2/TM was explored in 1 M KOH solutions at 5 mV s− 1. As depicted in
Fig. 5A, we applied the solution resistance to achieve the iR compen­
sated polarization curves for TM, TiO2/TM, RuO2/TiO2/TM and DRuO2/TiO2/TM. Obviously, it can be seen that the performance was
enhanced after generating defects and integrating TiO2 with RuO2. This
can also be observed from the comparison of overpotentials (Fig. 5B), in
which the overpotentials at 50 mA/ cm2 (Ƞ50) of TiO2/TM and RuO2/
TiO2/TM are 407 mV and 114 mV, respectively. While D-RuO2/TiO2/
TM shows a low Ƞ50 of 71 mV, suggesting the defect-rich heterogenous
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Fig. 4. (A) XPS spectra for TiO2/TM, RuO2/TiO2/TM and D-RuO2/TiO2/TM; Ti 2p (B) and Ru 3d spectra(C)for D-RuO2/TiO2/TM; O1S spectra of RuO2/TiO2/TM(D)
and D-RuO2/TiO2 /TM(E), respectively; (F) ESR spectra of RuO2/TiO2/TM and D-RuO2/TiO2/TM.
can be observed in the chemical state with initial one, indicating the
outstanding durability of D-RuO2-TiO2/TM for HER. In addition, the
XPS spectra also illustrate that the composition of D-RuO2-TiO2/TM
does not change significantly (Figure. S9B-D). Moreover, the HRTEM
image of D-RuO2/TiO2 after long-term HER in 1 M KOH certifies the
crystal structure without distinct change (Figure. S10). In general, in
alkaline media, the performance of D-RuO2-TiO2/TM is better than most
HER catalysts as previously reported.
TM were obtained. As shown in Fig. 6A, the OER performance of bare
TM is limited, while D-RuO2/TiO2/TM exhibits relatively high OER
activities. It is worth noting that the LSV curves show the order for OER
activities: D-RuO2/TiO2/TM < IrO2 on TM < RuO2/TiO2/TM < TiO2/
TM. As presented in Fig. 6B, the Ƞ10 of D-RuO2/TiO2/TM (296 mV) is
54, 75 and 226 mV, smaller than that of IrO2 on TM (350 mV), RuO2/
TiO2/TM (371 mV) and TiO2/TM (522 mV), respectively. Overall, the DRuO2/TiO2/TM electrode displays the highest OER catalytic activity,
representing that the defects and interfaces can create more active sites
to promote the catalytic performance. Furthermore, as shown in
Table S2, it also confirms that the activity of D-RuO2/TiO2/TM is su­
perior to some reported Ru-based anode electrocatalysts in alkaline
media.
In order to further understand the OER kinetics, the Tafel curves of
3.4. Electrochemical evaluation for OER
Next, we further probed the OER catalytic activity of D-RuO2/TiO2 in
1 M KOH solutions. The polarization curves with iR 100% compensated
for bare TM, TiO2/TM, RuO2/TiO2/TM, D-RuO2/TiO2/TM and IrO2 on
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Fig. 5. HER polarization curves for bare TM, Pt/C on TM, TiO2/TM, RuO2/TiO2/TM and D-RuO2/TiO2/TM in 1 M KOH recorded at 5 mV s− 1. (B) Corresponding
overpotentials at j = 50 mA/cm2. (C) Corresponding Tafel plots. (D) Nyquist plots of TiO2/TM, RuO2/TiO2/TM and D-RuO2/TiO2/TM measured at overpotential of
120 mV. (E) Polarization curves recorded initial and after 1000 CV cycles for D-RuO2/TiO2/TM. (F) Time-dependent overpotentials curve for D-RuO2/TiO2/TM under
a static current density for 18 h.
the catalysts were plotted. The Tafel slopes of D-RuO2/TiO2TM, IrO2 on
TM, RuO2/TiO2/TM and TiO2/TM were measured to be 40.6, 70.6, 71.7
and 171.6 mV dec-1, respectively (Fig. 6C). Especially, the Tafel slope of
D-RuO2/TiO2TM obtained from the OER polarization curves with 100%
IR drop is 40.6 mV/dec, close to that (72.2 mV/dec) of D-RuO2/TiO2TM
from the true steady-state polarization curve (Figure S11). The smallest
Tafel slope of D-RuO2/TiO2/TM in all control samples suggests it per­
forms the best OER catalyst in all catalysts. Electrochemical impedance
spectroscopy (EIS) of samples (Fig. 6D) exhibits that compared with
RuO2/TiO2/TM, both TiO2/TM and D-RuO2/TiO2/TM possess remark­
ably low charge-transfer resistance, suggesting an expeditious charge
transfer.
After 1000 CV cycles, the OER polarization curve of D-RuO2/TiO2/
TM nearly overlaps with the initial one, demonstrating it almost keeps
unchanged the OER activity (Fig. 6E). In addition, electrolytic mea­
surements were performed continuously for 20 h at a fixed current
density to further evaluate durability. As displayed in Fig. 6F, for DRuO2/TiO2 it changes slightly in overpotentials with time, also proving
the excellent catalytic stability. In addition, the XRD spectra of D-RuO2/
TiO2 after durability tests show that its physical structure is well pre­
served (Figure. S12A). XPS analysis of the samples after OER durability
tests demonstrates that the intensity of Ru 3p test decreases slightly
compared with the initial one, indicating the surface passivation during
the OER process (Figure. S12 B-D).
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Fig. 6. (A) OER polarization curves for Bare TM, Pt/C on TM, TiO2/TM, RuO2/TiO2/TM and D-RuO2/TiO2/TM in 1 M KOH recorded at 5 mV s− 1. (B) Corresponding
overpotentials at j = 10 mA/cm2. (C) Corresponding Tafel plots. (D) Nyquist plots of the Bare TM, Pt/C on TM, TiO2/TM, RuO2/TiO2/TM and D-RuO2/TiO2/TM
measured at overpotential of 350 mV. (E) Polarization curves recorded initial and after 1000 CV cycles for D-RuO2/TiO2/TM. (F) Time-dependent overpotential curve
for D-RuO2/TiO2/TM under a static current density for 20 h.
3.5. Electrochemical evaluation for water splitting
shown in Fig. 7C, the LSV after 1000 CV cycles remains almost the same
as the initial one. Figure S13 displays that the D-RuO2/TiO2/TM||DRuO2/TiO2/TM electrolyzer has outstanding durability and almost
negligible decay of current densities after long-time stability test for 20 h
under the fixed potential 1.8 V. What’s more, the chronopotentiometry
curve of D-RuO2/TiO2/TM||D-RuO2/TiO2/TM were conducted under
250 mA/cm2 for 72 h (Fig. 7D). The real-time potential displays a slight
change, also confirming the high durability of D-RuO2/TiO2/TM for
water splitting.
The H2 and O2 gases generated from the alkaline electrolyzer were
quantitatively collected by the water drainage method. The vol­
ume–time curve (Figure S14) reveals a volume ratio of 2.1:1 for the
collected H2 to O2, approaching to the theoretical 2:1 for water
Inspired by the outstanding OER and HER catalytic performances of
D-RuO2/TiO2/TM in alkaline solutions, an electrolyzer was assembled
by employing D-RuO2/TiO2/TM as bifunctional catalyst for overall
water splitting. Fig. 7A-B manifest that to reach 10 mA/cm2 the
assembled device only needs a cell voltage of 1.59 V, significantly less
than those of Pt/C||IrO2 (1.63 V at 10 mA/cm2) and RuO2/TiO2/||
RuO2/TiO2 couples (1.64 V at 10 mA/cm2). This activity even exceeds
than those reported rare precious metal materials in 1.0 M KOH
(Table S3), for instance Ni@Ru/CNS-10% (1.612 V at 10 mA/cm2), [52]
Ru1Co2 NP (1.59 V at 10 mA/cm2), [53] CoFeRu@C (1.62 V at 10 mA/
cm2), [54] and PdP2@CB (1.72 V at 10 mA/cm2) [55]. In addition, as
7
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Chemical Engineering Journal 431 (2022) 134072
Fig. 7. Polarization curves without iR-compensation of RuO2/TiO2/TM|| RuO2/TiO2/TM, Pt/C on TM||IrO2 on TM and D- RuO2/TiO2/TM||D- RuO2/TiO2/TM
toward overall water splitting in 1 M KOH recorded at 5 mV s− 1. (B) Corresponding overpotentials at j = 10 mA/cm2. (C) Polarization curves recorded initial and
after 1000 CV cycles for D-RuO2/TiO2/TM||D-RuO2/TiO2/TM (D) Potential–time response curve of D-RuO2/TiO2/TM||D-RuO2/TiO2/TM at 250 mA/cm2 in 1 M KOH
for 72 h.
electrolysis. This demonstrate the almost 100% faradic efficiency for full
water splitting. In conclusion, D-RuO2/TiO2/TM has an high efficiency
and durability for hydrogen production from electrochemical water
splitting.
be drawn that the pyrolysis at 350 ◦ C is not enough to obtain the TiO2
phase, while it is the optimal at 450 ◦ C. As demonstrated by the more
electrochemical tests (Figures S19-S20), the superior OER and HER
activity of D-RuO2/TiO2/TM is derived from the lowest charge transfer
resistance and the highest ECSA. In short, the appropriate pyrolyzed
temperature can optimize the interfacial synergistic effects and the
overall catalytic activity of materials.
Based on the above experimental and analytic results, the perfect
activities of the D-RuO2/TiO2/TM heterogeneous materials toward
overall water splitting could be attributable to the following aspects: (i)
DFT calculation reveals that setting up a hybrid interface between
defective RuO2 and TiO2 can optimize the adsorption energy of in­
termediates in HER, speeding up the OER overall kinetic process,
conducive to the whole water splitting. (ii) The defects in D-RuO2/TiO2/
TM change the electronic structures of existing Ru, and generate more
active sites, promoting the intrinsic catalytic activity. (iii) The doublelayer capacitance tests demonstrate that the D-RuO2/TiO2/TM has
large electrocatalytic active surface area and more catalytic active sites,
thereby speeding up the electrochemical process. (IV) The synergistic
effect between defective RuO2 and TiO2 phases can significantly
enhance the electrocatalytic activity toward OER together. (V) The
Nyquist plots indicate that defective D-RuO2/TiO2 has a faster charge
transfer capacity during water splitting processes than other contrast
samples, owing to the synergistic effects between TiO2 and defective
RuO2.
3.6. Mechanism analysis
The double-layer capacitance (Cdl) used to estimate the electro­
catalytic active surface area (ECSA) (Figure S15A–C) can effectively
explain the inherent activities of the catalytic materials [56].
Figure S15D reveals the Cdl value of D-RuO2/TiO2/TM is 48.8 mF cm− 2,
about 1.35 and 25.6-fold larger than that of RuO2/TiO2/TM (36.1 mF
cm− 2) and TiO2/TM (1.9 mF cm− 2), respectively. Such a Cdl value cer­
tifies that D-RuO2/TiO2/TM represents a larger ECSA (813 cm2) than
RuO2/TiO2/TM (601 cm2) TiO2/TM (31.6 cm2). Obviously, CV mea­
surements reveal that D-RuO2/TiO2/TM owns a larger ECSA, exposing
extensive active sites, and leading to superior activity than control
samples. As shown in Figure S16, the superior ECSA normalized OER
and HER activities further reveal that D-RuO2/TiO2/TM possesses
higher intrinsic catalytic activity than TiO2/TM and RuO2/TiO2/TM.
To explore the effect of interaction between TiO2 and defective RuO2
on activity, another two materials (namely D-RuO2/TiO2/TM/TM-350
and D-RuO2/TiO2/ TM-550) were further synthesized by changing the
pyrolysis temperature to 350 ◦ C and 550 ◦ C. As illustrated by the XRD
patterns (Figure S17) and polarization curves (Figure S18), the pyro­
lyzed temperature of precursors shows an important influence on the
crystal structure and electrocatalytic performance. One conclusion can
8
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Chemical Engineering Journal 431 (2022) 134072
4. Conclusion
[10] Y. Hu, X. Luo, G. Wu, T. Chao, Z. Li, Y. Qu, H. Li, Y. Wu, B. Jiang, X. Hong,
Engineering the atomic layer of RuO2 on PdO nanosheets boosts oxygen evolution
catalysis, ACS Appl. Mater. Inter. 11 (45) (2019) 42298–42304, https://doi.org/
10.1021/acsami.9b1649210.1021/acsami.9b16492.s001.
[11] S. Ajmal, H.T.D. Bui, V.Q. Bui, T. Yang, X. Shao, A. Kumar, S.-G. Kim, H. Lee,
H Accelerating water reduction towards hydrogen generation via cluster size
adjustment in Ru-incorporated carbon nitride, Chem. Eng. J. 429 (2022) 132282,
https://doi.org/10.1016/j.cej.2021.132282.
[12] Y.Z. Wang, M. Yang, Y.M. Ding, N.W. Li, L. Yu, Recent advances in complex hollow
electrocatalysts for water splitting, Adv. Funct. Mater. (2021) 2108681, https://
doi.org/10.1002/adfm.202108681.
[13] J. Villanueva-Cab, S.R. Jang, A.F. Halverson, K. Zhu, A.J. Frank, Trap-free
transport in ordered and disordered TiO2 nanostructures, Nano Lett. 14 (5) (2014)
2305–2309, https://doi.org/10.1021/nl4046087.
[14] K. Li, Y. Li, Y. Wang, J. Ge, C. Liu, W. Xing, Enhanced electrocatalytic performance
for the hydrogen evolution reaction through surface enrichment of platinum
nanoclusters alloying with ruthenium in situ embedded in carbon, Energy Environ.
Sci. 11 (5) (2018) 1232–1239.
[15] G. Zhang, B. Wang, L.u. Li, S. Yang, J. Liu, S. Yang, Tailoring the electronic
structure by constructing the heterointerface of RuO2-NiO for overall water
splitting with ultralow overpotential and extra-long lifetime, J Mater. Chem. A 8
(36) (2020) 18945–18954.
[16] H. Zhang, W. Li, X. Feng, N. Chen, H. Zhang, X. Zhao, L. Wang, Z. Li, Interfacial
FeOOH/CoO nanowires array improves electrocatalytic water splitting, J Solid.
State. Chem. 298 (2021), 122156.
[17] D. Chen, Z. Pu, R. Lu, P. Ji, P. Wang, J. Zhu, C. Lin, H.-W. Li, X. Zhou, Z. Hu, F. Xia,
J. Wu, S. Mu, Ultralow Ru loading transition metal phosphides as high-efficient
bifunctional electrocatalyst for a solar-to-hydrogen generation system, Adv. Energy
Mater. 10 (28) (2020) 2000814, https://doi.org/10.1002/aenm.v10.2810.1002/
aenm.202000814.
[18] L.-N. Chen, S.-H. Wang, P.-Y. Zhang, Z.-X. Chen, X. Lin, H.-J. Yang, T. Sheng, W.F. Lin, N.a. Tian, S.-G. Sun, Z.-Y. Zhou, Ru nanoparticles supported on partially
reduced TiO2 as highly efficient catalyst for hydrogen evolution, Nano Energy 88
(2021) 106211, https://doi.org/10.1016/j.nanoen.2021.106211.
[19] J. Lim, S. Yang, C. Kim, C.-W. Roh, Y. Kwon, Y.-T. Kim, H. Lee, Shaped Ir-Ni
bimetallic nanoparticles for minimizing Ir utilization in oxygen evolution reaction,
Chem Commun 52 (32) (2016) 5641–5644.
[20] J. Sun, S.E. Lowe, L. Zhang, Y. Wang, K. Pang, Y. Wang, Y. Zhong, P. Liu, K. Zhao,
Z. Tang, H. Zhao, Ultrathin nitrogen-doped holey carbon@graphene bifunctional
electrocatalyst for oxygen reduction and evolution reactions in alkaline and acidic
media, Angew. Chem. Int. Edit. 57 (50) (2018) 16511–16515, https://doi.org/
10.1002/anie.201811573.
[21] B.W. Xue, C.H. Zhang, Y.Z. Wang, W.W. Xie, N.-W. Li, L.e. Yu, Recent progress of
Ni–Fe layered double hydroxide and beyond towards electrochemical water
splitting, Nanoscale Advances 2 (12) (2020) 5555–5566.
[22] C. Zhou, Y. Zhang, Y. Li, J. Liu, Construction of high-capacitance 3D CoO@
polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor,
Nano Lett. 13 (5) (2013) 2078–2085, https://doi.org/10.1021/nl400378j.
[23] W. Zhou, Y. Zhou, L. Yang, J. Huang, Y. Ke, K. Zhou, L. Li, S. Chen, N-doped
carbon-coated cobalt nanorod arrays supported on a titanium mesh as highly active
electrocatalysts for the hydrogen evolution reaction, J Mater. Chem. A 3 (5) (2015)
1915–1919.
[24] W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H. Zhang, Onestep synthesis of Ni3S2 nanorod@Ni(OH)(2) nanosheet core-shell nanostructures
on a three-dimensional graphene network for high-performance supercapacitors,
Energy Environ. Sci. 6 (7) (2013) 2216–2221.
[25] B. Mao, P. Sun, Y. Jiang, T. Meng, D. Guo, J. Qin, M. Cao, Identifying the transfer
kinetics of adsorbed hydroxyl as a descriptor of alkaline hydrogen evolution
reaction, Angew. Chem. Int. Edit. 59 (35) (2020) 15232–15237, https://doi.org/
10.1002/anie.202006722.
[26] F. Liang, Y. Yu, W. Zhou, X. Xu, Z. Zhu, Highly defective CeO2 as a promoter for
efficient and stable water oxidation, J Mater. Chem. A 3 (2) (2015) 634–640.
[27] R. Ge, L. Li, J. Su, Y. Lin, Z. Tian, L. Chen, Ultrafine defective RuO2 electrocatayst
integrated on carbon cloth for robust water oxidation in acidic media, Adv. Energy
Mater. 9 (35) (2019) 1901313, https://doi.org/10.1002/aenm.v9.3510.1002/
aenm.201901313.
[28] Z. Wei, Z. Zhao, J. Wang, Q. Zhou, C. Zhao, Z. Yao, J. Wang, Oxygen-deficient TiO2
and carbon coupling synergistically boost the activity of Ru nanoparticles for the
alkaline hydrogen evolution reaction, J Mater. Chem. A 9 (16) (2021)
10160–10168.
[29] Y. Wang, Q. Zhu, T. Xie, Y. Peng, S. Liu, J. Wang, Promoted alkaline hydrogen
evolution reaction performance of Ru/C by introducing TiO2 nanoparticles,
ChemElectroChem 7 (5) (2020) 1182–1186, https://doi.org/10.1002/
celc.201902170.
[30] A. Martínez-Séptimo, M.A. Valenzuela, P. Del Angel, R.d.G. González-Huerta,
IrRuOx/TiO2 a stable electrocatalyst for the oxygen evolution reaction in acidic
media, Int. J Hydrogen Energ. 46 (2021) 25918-25928. https://doi.org/10.1016/j.
ijhydene.2021.04.040.
[31] P. Wang, J. Zhu, Z. Pu, R. Qin, C. Zhang, D. Chen, Q. Liu, D. Wu, W. Li, S. Liu,
Interfacial engineering of Co nanoparticles/Co2C nanowires boosts overall water
splitting kinetics, Appl Catal B: Environ 296 (2021), 120334.
[32] D.i. Zhao, K. Sun, W.-C. Cheong, L. Zheng, C. Zhang, S. Liu, X. Cao, K. Wu, Y. Pan,
Z. Zhuang, B. Hu, D. Wang, Q. Peng, C. Chen, Y. Li, Synergistically interactive
pyridinic-N-MoP sites: identified active centers for enhanced hydrogen evolution in
alkaline solution, Angew. Chem. Int. Edit. 59 (23) (2020) 8982–8990, https://doi.
org/10.1002/anie.201908760.
In summary, we reported a simple but effective strategy to synthesize
the bi-functional defective RuO2/TiO2 nano-heterostructure catalyst.
Importantly, as predicted by DFT calculations, the Ru d-band center in
defective RuO2/TiO2 can shift to a low-energy level due to the strong
interface interaction, and weakens the interaction between adsorbed
oxygen species on Ru sites, benefiting for the water splitting. As ex­
pected, the D-RuO2/TiO2/TM catalyst exhibited excellent performance
for both OER and HER in 1 M KOH. For overall water splitting device
assembled here, it merely required 1.59 V to achieve a current density of
10 mA/cm2. The defective RuO2/TiO2/TM also reveals superior dura­
bility. Overall, owing to low-cost, facile syntheses and outstanding cat­
alytic activity, the defect-rich RuO2/TiO2/TM is a promising electrode
toward high-efficiency hydrogen production via electrocatalytic water
splitting.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the Natural Science Foundation of China
(Nos. 21671114, 22075223, and U1804131), and the Tackle Key
Problem of Science and Technology Project of Henan Province, China
(No. 202102210245), and Program for Science & Technology Innova­
tion Talents in Universities of Henan Province (No. 21IRTSTHN004),
Natural Science Foundation of Henan Province (No.202300410288).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.cej.2021.134072.
References
[1] M.S. Faber, S. Jin, Earth-abundant inorganic electrocatalysts and their
nanostructures for energy conversion applications, Energy Environ. Sci. 7 (11)
(2014) 3519–3542.
[2] F. Yu, H.Q. Zhou, Y.F. Huang, J.Y. Sun, F. Qin, J.M. Bao, W.A. Goddardiii, S. Chen,
Z.F. Ren, High-performance bifunctional porous non-noble metal phosphide
catalyst for overall water splitting, Nat. Commun. 9 (2018) 2551, https://doi.org/
10.1038/s41467-018-04746-z.
[3] L.e. Yu, J.F. Yang, B.Y. Guan, Y. Lu, X.W.D. Lou, Hierarchical hollow nanoprisms
based on ultrathin ni-fe layered double hydroxide nanosheets with enhanced
electrocatalytic activity towards oxygen evolution, Angew. Chem. Int. Ed. Engl. 57
(1) (2018) 172–176, https://doi.org/10.1002/anie.201710877.
[4] P.Y. Wang, Z.H. Pu, W.Q. Li, J.W. Zhu, C.T. Zhang, Y.F. Zhao, S.C. Mu, Coupling
NiSe2-Ni2P heterostructure nanowrinkles for highly efficient overall water
splitting, J Catal. 377 (2019) 600–608, https://doi.org/10.1016/j.
jcat.2019.08.005.
[5] O. Kasian, S. Geiger, T. Li, J.-P. Grote, K. Schweinar, S. Zhang, C. Scheu, D. Raabe,
S. Cherevko, B. Gault, K.J.J. Mayrhofer, Degradation of iridium oxides via oxygen
evolution from the lattice: correlating atomic scale structure with reaction
mechanisms, Energy Environ. Sci. 12 (12) (2019) 3548–3555.
[6] Y. Qiao, P. Yuan, C.-W. Pao, Y. Cheng, Z. Pu, Q. Xu, S. Mu, J. Zhang, Boron-rich
environment boosting ruthenium boride on B, N doped carbon outperforms
platinum for hydrogen evolution reaction in a universal pH range, Nano Energy 75
(2020) 104881, https://doi.org/10.1016/j.nanoen.2020.104881.
[7] W.Q. Li, Z.Y. Hu, Z.W. Zhang, P. Wei, J.A. Zhang, Z.H. Pu, J.W. Zhu, D.P. He, S.
C. Mu, G. Van Tendeloo, Nano-single crystal coalesced PtCu nanospheres as robust
bifunctional catalyst for hydrogen evolution and oxygen reduction reactions,
J Catal. 375 (2019) 164–170, https://doi.org/10.1016/j.jcat.2019.05.031.
[8] Y.-Z. Wang, Y.-M. Ding, C.-H. Zhang, B.-W. Xue, N.-W. Li, L.e. Yu, Formation of
hierarchical Co-decorated Mo2C hollow spheres for enhanced hydrogen evolution,
Rare Metals 40 (10) (2021) 2785–2792, https://doi.org/10.1007/s12598-02101765-6.
[9] S.F. Sun, X.L. Jin, B.W. Cong, X. Zhou, W.Z. Hong, G. Chen, Construction of porous
nanoscale NiO/NiCo2O4 heterostructure for highly enhanced electrocatalytic
oxygen evolution activity, J Catal. 379 (2019) 1–9, https://doi.org/10.1016/j.
jcat.2019.09.010.
9
W. Li et al.
Chemical Engineering Journal 431 (2022) 134072
[45] Q. Yao, B. Huang, Y. Xu, L. Li, Q.i. Shao, X. Huang, A chemical etching strategy to
improve and stabilize RuO2-based nanoassemblies for acidic oxygen evolution,
Nano Energy 84 (2021) 105909, https://doi.org/10.1016/j.nanoen.2021.105909.
[46] Y. Gao, C. Liu, W. Zhou, S. Lu, B. Zhang, Anion Vacancy Engineering in
Electrocatalytic Water Splitting, ChemNanoMat 7 (2) (2021) 102–109, https://doi.
org/10.1002/cnma.202000550.
[47] C.-Z. Yuan, Y.-F. Jiang, Z.-W. Zhao, S.-J. Zhao, X. Zhou, T.-Y. Cheang, A.-W. Xu,
Molecule-Assisted Synthesis of Highly Dispersed Ultrasmall RuO2 Nanoparticles on
Nitrogen-Doped Carbon Matrix as Ultraefficient Bifunctional Electrocatalysts for
Overall Water Splitting, ACS Sustain. Chem. Eng. 6 (9) (2018) 11529–11535,
https://doi.org/10.1021/acssuschemeng.8b0170910.1021/
acssuschemeng.8b01709.s001.
[48] Y. Dang, T. Wu, H. Tan, J. Wang, C. Cui, P. Kerns, W. Zhao, L. Posada, L. Wen, S.
L. Suib, Partially reduced Ru/RuO2 composites as efficient and pH-universal
electrocatalysts for hydrogen evolution, Energy Environ. Sci. 14 (10) (2021)
5433–5443.
[49] T. Yu, Q. Xu, G. Qian, J. Chen, H. Zhang, L. Luo, S. Yin, Amorphous CoOxDecorated Crystalline RuO2 Nanosheets as Bifunctional Catalysts for Boosting
Overall Water Splitting at Large Current Density, ACS Sustain. Chem. Eng. 8 (47)
(2020) 17520–17526, https://doi.org/10.1021/acssuschemeng.0c0678210.1021/
acssuschemeng.0c06782.s001.
[50] X. Qin, L. Zhang, G.-L. Xu, S. Zhu, Q.i. Wang, M. Gu, X. Zhang, C. Sun, P.
B. Balbuena, K. Amine, M. Shao, The Role of Ru in Improving the Activity of Pd
toward Hydrogen Evolution and Oxidation Reactions in Alkaline Solutions, Acs
Catal 9 (10) (2019) 9614–9621, https://doi.org/10.1021/
acscatal.9b0174410.1021/acscatal.9b01744.s001.
[51] S. Anantharaj, S. Noda, M. Driess, P.W. Menezes, The Pitfalls of Using
Potentiodynamic Polarization Curves for Tafel Analysis in Electrocatalytic Water
Splitting, ACS Energy Lett. (2021) 1607–1611, https://doi.org/10.1021/
acsenergylett.1c00608.
[52] W. Wu, Y. Wu, D. Zheng, K. Wang, Z.H. Tang, Ni@Ru core-shell nanoparticles on
flower-like carbon nanosheets for hydrogen evolution reaction at All-pH values,
oxygen evolution reaction and overall water splitting in alkaline solution,
Electrochim Acta 320 (2019) 11, https://doi.org/10.1016/j.
electacta.2019.134568.
[53] Y. Bao, J. Dai, J. Zhao, Y. Wu, C. Li, L. Ji, X. Zhang, F. Yang, Modulation in
Ruthenium-Cobalt Electronic Structure for Highly Efficient Overall Water Splitting,
Acs Applied Energy Materials 3 (2) (2020) 1869–1874, https://doi.org/10.1021/
acsaem.9b0231510.1021/acsaem.9b02315.s001.
[54] M.X. Yang, T.L. Feng, Y.X. Chen, J.J. Liu, X.H. Zhao, B. Yang, Synchronously
integration of Co, Fe dual-metal doping in Ru@C and CDs for boosted water
splitting performances in alkaline media, Appl Catal B: Environ 267 (2020) 9,
https://doi.org/10.1016/j.apcatb.2020.118657.
[55] F. Luo, Q. Zhang, X. Yu, S. Xiao, Y. Ling, H. Hu, L. Guo, Z. Yang, L. Huang, W. Cai,
H. Cheng, Palladium Phosphide as a Stable and Efficient Electrocatalyst for Overall
Water Splitting, Angew Chem Int Edit 57 (45) (2018) 14862–14867, https://doi.
org/10.1002/anie.201810102.
[56] J. Li, H.e. Huang, X. Cao, H.-H. Wu, K. Pan, Q. Zhang, N. Wu, X. Liu, Template-free
fabrication of MoP nanoparticles encapsulated in N-doped hollow carbon spheres
for efficient alkaline hydrogen evolution, Chem. Eng. J. 416 (2021) 127677,
https://doi.org/10.1016/j.cej.2020.127677.
[33] D.W. Wang, Q. Li, C. Han, Q.Q. Lu, Z.C. Xing, X.R. Yang, Atomic and electronic
modulation of self-supported nickel-vanadium layered double hydroxide to
accelerate water splitting kinetics, Nat Commun. 10 (2019) 3899, https://doi.org/
10.1038/s41467-019-11765-x.
[34] X. Zhang, D. Li, J. Wan, X. Yu, Preparation of Ti mesh supported N-S-C-tridoped
TiO2 nanosheets to achieve high utilization of optical energy for photocatalytic
degradation of norfloxacin, Rsc Advances 6 (22) (2016) 17906–17912.
[35] M.T. Zhang, J.X. Chen, H. Li, P.W. Cai, Y. Li, Z.H. Wen, Ru-RuO2/CNT hybrids as
high-activity pH-universal electrocatalysts for water splitting within 0.73 V in an
asymmetric-electrolyte electrolyzer, Nano Energy 61 (2019) 576–583, https://doi.
org/10.1016/j.nanoen.2019.04.050.
[36] S. Nong, W. Dong, J. Yin, B. Dong, Y. Lu, X. Yuan, X. Wang, K. Bu, M. Chen,
S. Jiang, L.-M. Liu, M. Sui, F. Huang, Well-dispersed ruthenium in mesoporous
crystal TiO2 as an advanced electrocatalyst for hydrogen evolution reaction, J Am.
Chem. Soc. 140 (17) (2018) 5719–5727, https://doi.org/10.1021/
jacs.7b1373610.1021/jacs.7b13736.s001.
[37] G. Zhou, M. Li, Y. Li, H. Dong, D. Sun, X. Liu, L. Xu, Z. Tian, Y. Tang, Regulating the
electronic structure of CoP nanosheets by O incorporation for high-efficiency
electrochemical overall water splitting, Adv Funct. Mater. 30 (7) (2020) 1905252,
https://doi.org/10.1002/adfm.v30.710.1002/adfm.201905252.
[38] H. Idriss, On the wrong assignment of the XPS O1s signal at 531–532 eV attributed
to oxygen vacancies in photo- and electro-catalysts for water splitting and other
materials applications, Surf Sci 712 (2021) 121894, https://doi.org/10.1016/j.
susc.2021.121894.
[39] W. Hu, Y. Liu, R.L. Withers, T.J. Frankcombe, L. Norén, A. Snashall, M. Kitchin,
P. Smith, B. Gong, H. Chen, J. Schiemer, F. Brink, J. Wong-Leung, Electron-pinned
defect-dipoles for high-performance colossal permittivity materials, Nat. Mater. 12
(9) (2013) 821–826, https://doi.org/10.1038/nmat3691.
[40] L. Xu, Q.Q. Jiang, Z.H. Xiao, X.Y. Li, J. Huo, S.Y. Wang, L.M. Dai, Plasma-Engraved
Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen
Evolution Reaction, Angew Chem Int Edit 55 (17) (2016) 5277–5281, https://doi.
org/10.1002/anie.201600687.
[41] D.A. Kuznetsov, M.A. Naeem, P.V. Kumar, P.M. Abdala, A. Fedorov, C.R. Müller,
Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen
Evolution Activity, J Am. Chem. Soc. 142 (17) (2020) 7883–7888, https://doi.org/
10.1021/jacs.0c0113510.1021/jacs.0c01135.s001.
[42] Z. Xiao, Y.-C. Huang, C.-L. Dong, C. Xie, Z. Liu, S. Du, W. Chen, D. Yan, L.i. Tao,
Z. Shu, G. Zhang, H. Duan, Y. Wang, Y. Zou, R.u. Chen, S. Wang, Operando
Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co3O4 for Oxygen
Evolution Reaction, J Am. Chem. Soc. 142 (28) (2020) 12087–12095, https://doi.
org/10.1021/jacs.0c0025710.1021/jacs.0c00257.s001.
[43] F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan, Y.i. Xie, Oxygen Vacancies Confined
in Ultrathin Indium Oxide Porous Sheets for Promoted Visible-Light Water
Splitting, J Am. Chem. Soc. 136 (19) (2014) 6826–6829, https://doi.org/10.1021/
ja501866r.
[44] G. Liu, L. Xu, Y. Li, D. Guo, N. Wu, C. Yuan, A. Qin, A. Cao, X. Liu, Metal-organic
frameworks derived anatase/rutile heterostructures with enhanced reaction
kinetics for lithium and sodium storage, Chem Eng J 430 (2022) 132689, https://
doi.org/10.1016/j.cej.2021.132689.
10