Cultivating crystal lattice distortion in IrO2: Via coupling with MnO2 to boost the oxygen evolution reaction with high intrinsic activity

Article abstract of DOI:10.1039/c8cc02008f

Here, we report an effective strategy to lower Ir consumption and boost the OER performance in acid by loading IrO₂ onto MnO₂, in which the IrO₂ crystals are well dispersed and undergo a so-called z-extension Jahn-Teller distortion in the octahedral structure. Compared with IrO₂, the mass activity and intrinsic activity for IrO₂/MnO₂ were largely increased.

Full text of DOI:10.1039/c8cc02008f

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Cultivating Crystal Lattice Distortion in IrO2 via Coupling with
MnO2 to Boost Oxygen Evolution Reaction with High Intrinsic
Activity
Received 00th January 20xx,
Accepted 00th January 20xx
Zhenhua Zhou, a Waqas Qamar Zaman, a Wei Sun, a Li-mei Cao, a Muhammad Tariq, a and Ji Yang a,b
DOI: 10.1039/x0xx00000x
†
www.rsc.org/
Here, we report an effective strategy to lower Ir consumption and critical advantages like load flexibility, voltage efficiency, and
boost OER performance in acid by loading IrO2 onto MnO2, in hydrogen purity. The catalysts used in acidic conditions are
which the IrO2 crystals are well dispersed, and undergo a so-called mainly based on rutile type IrO2 or RuO2.13-15 To save the
z-extension Jahn-Teller distortion in the octahedral structure. precious metals, significant researches have been reported
Comparing with IrO2, the mass activity and intrinsic activity for including introduction of dopant atoms or crystal defects,
IrO2/MnO2 were largely increased.
oxide surface leaching, and dispersion onto supports.16-18 It is
worth mentioning that the lattice distortion (axis c/a
contraction in [IrO6] octahedron) and robust catalyst/substrate
interface (IrNiOx/ATO, IrO2/ATO) were reported as two
effective approaches to simultaneously give enhancement to
catalytic activity and durability performance in acid
electrolyte.16, 17, 19 However, combining these two approaches
has yet not been reported.
Catalysts for electrochemical processes sit at the heart of key
renewable-energy technologies such as energy conversion of
intermittent solar and wind into chemical fuels (H2 via water
splitting).1 However, the anodic electrochemical processes
consume large amounts of energy, which remains a bottleneck
in the technologies.2 For these reasons, there is growing
demand to design more affordable, efficient and durable
oxygen evolution reaction (OER) catalysts, and to further
exploit their structure dependent properties. Currently Ir
based catalysts play important roles in electrochemistry, as
quantitatively an adequate proportion of Ir is being utilized for
electrochemical techniques(Figure S1).3
Mn oxides have been used as capacitor, however,
underappreciated as OER catalysts.20,
21
Birnessite MnO2
(Na0.55Mn2O4•1.5H2O, also known as δ-MnO2) has a layered
structure consisting planar edge-sharing [MnO6] octahedra,
guest cations, and H2O molecules.22 In the synthesis of
IrO2/MnO2 (Scheme S1, Fig S2, Table S1), the δ-MnO2 was
generated at room temperature (RT). Ir cations enabled to
enter the gallery space of birnessite MnO2 through ionic
exchange, which is similar to AuCl4- getting into layered double
hydroxide (LDH) as reported previously.23 Hydrothermal
reaction led to the crystallization of IrO2 nanoparticles and
exfoliation of birnessite Mn oxide. The Ir/Mn atomic ratios
revealed by EDS (Figure S3) were close to the ratios of Ir/Mn in
the starting precursors.
Strategies to design OER catalysts for acid solution show
clear difference from that for alkaline.4, 5 Normally, a wide
variety of catalysts including metal oxides, sulfides, selenides,
phosphides, and metal-free catalysts, have been developed for
alkaline electrolyte.6-9 Furthermore, the nanostructuring of
catalysts and loading onto supports largely expose the active
sites and enhance the OER performance.10, 11 These efforts
realize the goal to drive OER reaction with earth-abundant
transition metal alternatives instead of precious metals.
Unfortunately, the hydrogen commonly generates at low
efficiencies in alkaline condition.12 On contrary, acidic
electrolysis using polymer electrolyte membrane (PEM) offers
The SEM images reveal the morphology of catalysts (Fig.
1a, b). With Mn content increasing, the morphology of
particles gradually transform from nanospheres to corrugation
with observable ridges (Figure S4-6). The Transmission
electron microscopy (TEM) image of Ir0.3/Mn0.7 further
confirms the formation of ridges, as it appears spatially coarse
(Figure 1c). The ridges and interlaid flat areas may result from
the mechanically weak holding power after exfoliation. In
Figure 1d, the high-resolution TEM (HRTEM) image shows the
characteristic lattice fringes for IrO2. The EDS mapping shows
uniform distribution of Ir and Mn (Fig. S7). The TEM images in
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Figure S8 show adjoining parts of MnO2 and IrO2, implying composites were significantly decreased when lowering Ir
lattice mismatch happens.
content from IrO2 to Ir0.4/Mn0.6.
Figure 2. (a) XRD pattern of Ir/Mn composites. The black lines
at the bottom show the diffraction angle of IrO2. (b) A close
view of XRD pattern of peak (101). (c) The fitted curve of
diffraction angle of (101) for different IrO2/MnO2 composites.
For all Ir/Mn composites the Tafel slope is found to be 40
mV/dec or slightly higher, indicating that the deprotonation of
the reaction intermediate OHads controls the overall reaction
rate (Fig. S15, Table S3, Note S1). Considering that a Tafel
slope of 56 mV/dec is observed for homemade IrO2, it can be
inferred that the OER at the Ir/Mn composites is not only a
Figure 1. (a) and (b) SEM images of Ir0.3/Mn0.7 at different kinetically faster process, but it also follows a different
magnifications. (c) TEM and (d) HRTEM images of Ir0.3/Mn0.7. reaction mechanism than at bare IrO2.
The inset pattern in (d) is the inverse Fast Fourier Transform
Stability
of
the
materials
was
checked
by
pattern of (d), and the line 1, 2 and 3 is directed to the crystal chronopotentiometry measurements at a constant current
density of 100mA cm-2 (Figure 3b). As the OER proceeds,
diffraction of MnO2, IrO2 (101), and IrO2 (110), respectively.
catalysts lose their activity and need higher potential to obtain
X-ray diffraction (XRD) patterns in Fig. 2a show an IrO2 the set current density. The Ir0.3/Mn0.7 and Ir0.4/Mn0.6 well
rutile structure (JCPDS 15-0870) for all catalysts. No retained the required potentials even after 40000 s, much
characteristic peaks of birnessite (Fig. S9) or other Mn related more constant than IrO2.
peaks were observable probably as a result of exfoliation. Note
The mass activity related to the Iridium consumption is an
that marked peak shifts toward higher angle were observed important parameter, as the cost is also bottleneck issue in
for the peaks involving c-axis (mainly (101) and (211), Fig 2b, electrochemistry. The mass activity of Ir0.4/Mn0.6 is measured
-1
c). Thus, we further investigated effect of Mn on the IrO2 rutile to be 66A gIr at 1.50VRHE, showing improved mass activity
structure. Within a wide range of Mn content, the rutile compared to other Ir-containing catalysts (Fig 3c, Table S4).
structure was maintained. However, with increasing Mn The Ir mass activity of Ir0.4/Mn0.6 is 22 times than that of
content, the shifts associated to peaks (101) and (211) homemade IrO2, while the Brunauer-Emmett-Teller (BET)
presented a linear relationship, while the peaks with absent c- surface area is just 2.6 times higher, proving that the
axis (like (110) peak) had negligible variation. The observation enhancement in electrochemical performance of the
of XRD peaks shift nullified the MnIV doped IrO2 situation, composite catalysts is not limited in increasing surface area.
which has been reported previously.16 Thus we speculated that
As electrochemical surface area (ECSA) or double layer
the [IrO6] octahedron was distorted as a result of strong capacitance (Cdl) is proportional to the active sites involved in
interaction between IrO2 and MnO2. Besides the distortion electrochemical reactions, the cyclic voltammograms (CVs)
effect in the composites, the surface area and volume of pores were carried out to determine the ECSA by measuring the
are increased when increasing Mn from IrO2 to Ir0.4/Mn0.6, charging currents of double-layer capacitance at different scan
proving the positive role of Mn oxide (Fig. S11, 12).
rates or electrochemical impedance spectroscopy (EIS)
The Nyquist plots reveal that Ir0.4/Mn0.6 shows the lowest measurements (Fig S 14, 16, 17, Table S2). The Cdl of
polarization resistance, i.e. the highest OER activity among all Ir0.4/Mn0.6 at 1.25V (CV) and 1.49V (EIS) show a similar value
composites (Fig. S13, 14, Table S2). In Fig 3a and S15, the OER (about 11mF), that is about 2 times higher than that for IrO2.
polarization curves from IrO2/MnO2 composites and the By eliminating the OER current with respect to the
corresponding overpotentials at current density of 10 mA cm−2 corresponding BET surface area and ECSA at 1.49V (EIS), the
(η10) are analyzed. The onset potential for Ir0.4/Mn0.6 is normalized specific or intrinsic activity is estimated (Figure 3d,
merely 1.437V, and η10 is merely 1.516V, which are 44mV and S18). The specific activity at 1.50V of Ir0.4/Mn0.6 is 23µA
-2
57mV lower than commercial IrO2 (IrO2-c, Adamas-beta, Fig. cmBET and 4 µA cmECSA-2, which are 3.8 and 5.6 times higher
S10), respectively (Fig S16, Table S1). The η10 values of Ir/Mn than that of pure IrO2, respectively. This reveals that coupling
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of MnO2 enhances the catalytic performance of IrO2. Further, content is higher. Nevertheless, all the composites show lower
estimation of the intrinsic activities were carried on by intensities than IrO2. The decreasing in white line of Ir supports
turnover frequency (TOF), assuming that every Iridium atom is the suspicion that Ir oxidation state is slight lower than 4+.
catalytically active. The Ir0.4/Mn0.6 composite was found to From the XANES spectra of Mn in Fig. S24, it can be seen that
exhibit the highest TOF of
overpotential. We did not count Mn considering that Mn was to the presence of MnIII species.
inactive at the given OER potential region.
∼
1.7 ×10-2 s-1 at 260mV the composites possessed negative shifts, which corresponds
Figure 4. (a) The Ir LIII-edge XANES spectra for IrO2/MnO2
Figure 3. (a) Polarization curves and (b) electrochemical
stability of IrO2/MnO2 materials and homemade IrO2 (IrO2-h)
and commercial IrO2 (IrO2-c) nanoparticles. (c) Ir mass activity
and (d) oxide specific activity of OER catalysts in acidic
electrolyte. The specific Tafel plots of IrO2-h and Ir0.4/Mn0.6
are normalized to their respective BET surface area, while IrO2-
ref and RuO2-ref24 are normalized to surface area estimated
from particle size analysis from TEM images. Catalysts
Ir0.4/Mn0.6, IrO2-c and IrO2-h are from this work. The IrO2-
TiO2-245,25 IrOx/ATO,17 IrO2-150,26 IrNiOx/Meso-ATO-180,18
IrO2-ref, and RuO2-ref24 catalysts are duplicated or redrawn
from literature.
composites. The inset I is the white line spectra; (b) Fourier
transforms of k3- normalized Ir-LIII edge EXAFS. The k range is
3-14Å. The region II reflects the adsorption edge of Ir-O shell,
and the region III and IV reflect the Ir-Ir shells; (c) Enlarged
view of Ir-Ir shells. (d) The schematic diagram for Ir-Ir shells
and crystal structure of IrO2.
To further investigate the local geometrical structure,
Extended X-ray absorption fine structure (EXAFS) spectroscopy
measurements were performed (Fig. 4b and c). The peak
around 1.9 Å in the R-space Fourier transforms (FTs) spectra is
attributed to Ir-O interaction in the first coordination shell. The
peak around R=3.1 Å is related to the Ir-Ir coordination shell
(edge-sharing). As illustrated in Figure 4b and c, for a series of
composites, the edge-sharing Ir-Ir shell peaks generally shift
toward lower R as Ir/Mn decreases, displaying noticeable
distinction in comparison with that for IrO2. The shortened
distance reflects the reduction in crystal c-axis length, verifying
the distortion in [IrO6] octahedral unit.
X-ray photoemission spectrum (XPS) reveals that the
composite has plenty of surface IrIII, MnIII and hydroxyl oxygen
defects as shown in detail in Figure S19-22. For Ir4f spectrums,
the Ir0.3/Mn0.7 can be seen to have less intense peaks and
more obvious asymmetry (Figure S19, Note S2), which can be
ascribed to IrIII electronic defects, as has been observed in
amorphous IrO2 powders.13
As for the Fourier transform of EXAFS spectra of Mn K-edge
(Figure S24), the bond lengths of both Mn-O shells (near 1.8Å)
and Mn-Mn shell (near 2.8Å) of α-, β-, and δ-MnO2 are very
close. However, there is an peak at ~3.2Å of Ir0.3/Mn0.7
which may correspond to the Mn-Mn shell of α-MnO2.27 One
Mn-O shell of δ-MnO2 also shows a peak near 3.2Å, but its
intensity is relatively much lower compared to the intensity of
first Mn-O shell or Mn-Mn shell.21, 28 Thus, the Mn oxide in
Ir0.3/Mn0.7 is mixture of α-MnO2 and exfoliated δ-MnO2,
X-ray absorption measurement were further used to
explore origin of the high-energy structure (Fig.4, S23-25).
Synchrotron-based X-ray absorption near edge structure
(XANES) spectra of Ir L3 (2p3/2 to 5d) adsorption edge,
characterized by the white-line, provides information about
the oxidation state of Ir species. As shown in Figure 4a, with
decreasing Ir/Mn, the peak intensity of absorption edge
enhances, indicting a slightly higher Ir oxidation when Mn
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implying that the δ-MnO2 has partly gone through phase
transformation in hydrothermal reaction.
The distortion in crystal structure is simulated by Diamond
software (Table S5, Note S3), confirming a higher Mn content
benefits for the Jahn-Teller distortion effect. As has been
previously reported, the z-extension Jahn-Teller distortion
positively effects OER performance of IrO2. 16, 29, 30 For IrO2, the
electronic structure of Ir is 5d56s0, and 5d electrons tend to fill
the lower state t2g orbitals (i.e., dxz, dyz and dx2-y2) before they
10.
11.
12.
occupy the upper state eg doublet (i.e., dxy and dz
2) under the
octahedral crystal field. In the case of fabricated composites,
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illustrated in Figure S25, indicating partially filled eg orbitals by
5d electrons. On the basis of the d-band theory, the
electrochemical OER activity of transition metal oxide is
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d states31. Electrons in eg orbital participate in σ-bonding with
a surface-anion adsorbate and influence the bond strength of
oxygen-related intermediate species, which moderate surface-
oxygen bond strength (neither too strong nor too weak), thus
playing an important role in fostering performance and
optimization31. MnOx, as an added substrate along with
stronger interaction also provided a larger surface area for
confronting active sites of IrO2, and raised the specific activity
of IrO2. These advantages of the composite are held
responsible for the synergistic OER catalytic activity.
13.
14.
15.
16.
17.
18.
To summarize, we have provided a feasible strategy for
efficient OER electrocatalysts applied in acid media. By a
simple hydrothermal approach, IrO2 is anchored on Mn oxide.
The robust composites show superior OER performance.
Comparing with IrO2, both the specific activity and mass
activity for Ir0.4/Mn0.6 are largely increased. The IrO2 is found
to undergo a z-extension Jahn-Teller distortion and the eg
orbital is partly filled after coupling with MnO2. We believe
that this study opens up new vision for catalyst/subtract
interaction and designing for catalysts.
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Conflicts of interest
There are no conflicts to declare.
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