Self-stacked multilayer FeOCl supported on a cellulose-derived carbon aerogel: a new and high-performance anode material for supercapacitors

Article abstract of DOI:10.1039/c8ta12261j

To build high-energy density asymmetric supercapacitors (ASCs), current studies are always directed towards cathode materials; however, anode materials are paid much less attention. Here we for the first time demonstrate that orthorhombic FeOCl with a sel

Full text of DOI:10.1039/c8ta12261j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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DOI: 10.1039/C8TA12261J
Table of Contents Entry
We for the first time demonstrate that orthorhombic FeOCl with a self-stacked
laminated structure suits to be a high-performance anode material for supercapacitors
since its unique laminated structure can provide abundant active sites for migration and
intercalation reactions of electrolyte ions. By introducing a highly conductive and
porous cellulose-derived carbon aerogel matrix, the mechanical stability and charge-
storage kinetics of FeOCl are significantly heightened. Thus, FeOCl@CDCA delivers
an ultra-high elecrochemical activity. The findings open a new pathway to the design
of high-energy density of energy-storage systems using the FeOCl-based anode.

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DOI: 10.1039/C8TA12261J
Self-stacked multilayer FeOCl supported on a cellulose-derived carbon
aerogel: A new and high-performance anode material for supercapacitors
Caichao Wan,*a Yue Jiao,b Wenhui Bao,b He Gao,b Yiqiang Wu,*a Jian Li*b
aCollege of Materials Science and Engineering, Central South University of Forestry and
Technology, Changsha 410004, P. R. China
bMaterial Science and Engineering College, Northeast Forestry University, Harbin 150040, P.
R. China
*Corresponding authors. E-mails: wancaichaojy@163.com (C. Wan); wuyq0506@126.com
(Y. Wu); nefulijian@163.com (J. Li)
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Abstract: To build high-energy density of asymmetric supercapacitors (ASC), curDrOenI: 1t0s.1t0u3d9/iCe8sTA12261J
are always directed towards cathode materials; however, anode materials are paid much less
attention. Here we for the first time demonstrate that orthorhombic FeOCl with a self-stacked
laminated structure suits to be a high-performance anode material for supercapacitors since its
unique laminated structure can provide abundant active sites for migration and intercalation
reactions of electrolyte ions. By introducing a highly conductive and porous cellulose-derived
carbon aerogel (CDCA) matrix, the mechanical stability and charge-storage kinetics of FeOCl
are significantly heightened. FeOCl@CDCA delivers an ultra-high areal specific capacitance
of 1618 mF cm‒2 (647 F g‒1) at 2 mA cm‒2 and outstanding cycle stability with no more than
10% capacitance loss after 10000 cycles in 1 M Na2SO4 between ‒1 and 0 V vs. Ag/AgCl. An
ASC operating at 0‒1.8 V was fabricated using a FeOCl@CDCA anode and a cheap MnO2
cathode. The ASC displays a highly competitive energy/power density (289 ꢀμW h cm−2 at 1.8
mW cm−2) and excellent rate capability and cycle stability. These findings may open a new
pathway to the design of high-energy density of energy-storage systems using FeOCl-based
anodes.
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DOI: 10.1039/C8TA12261J
Introduction
Supercapacitors are always considered as one of the most promising candidates for next-
generation energy storage systems due to their distinct merits like fast charging/discharging
rate, high power density and long cycle life.[1-3] Compared to another electrochemical energy
storage device (namely batteries) with a bigger market at present, supercapacitors are capable
to deliver hundred to many thousand times higher power in the same volume, but they cannot
store the same amount of charge (usually 3–30 times lower).[2] Therefore, the major challenge
for supercapacitors is to increase energy density and concurrently maintain high power density
and cycle stability. Electrochemical activity of electrodes plays a key role in determining the
property of supercapacitors. Electrode materials can be classified into two categories based on
working potential window, i.e., cathode materials (>0 V vs. SCE) and anode materials (<0 V
vs. SCE). Up to now, cathode materials have been paid more attention and already achieved
brilliant progress,[4-9] while the studies on performance enhancements of anode materials are
distinctly fewer. In addition, based on the difference in charge storage mechanisms, electrode
materials can also be divided into pseudocapacitive materials (e.g., transition-metal oxides or
hydroxides and conjugated polymers) and electrical double-layer capacitor (EDLC) materials
(like carbon materials).[1, 10] Carbon materials have already worked as anode materials for the
assembly of asymmetric supercapacitors (ASCs) due to their wider working potential window,
higher electrical conductivity and more outstanding cycle stability when compared to those of
pseudocapacitive anode materials (like Bi2O3,[11] V2O5,[12] MoO2[13] and FeOx[14]); unfortunately,
they often suffer from lower capacitance and energy density, which are exactly the advantages
of pseudocapacitive materials. Consequently, an effective integration of these two classes of
active substances is believed to helpfully develop novel high-performance anode materials.[15,
16]
Experimental researches have demonstrated that iron oxides with variable oxidation states are
promising candidates as anodes in ASCs.[14, 17-19] However, iron oxychloride (FeOCl) has not
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been paid enough attention, especially for supercapacitors. From the viewpoint of crystalline
DOI: 10.1039/C8TA12261J
structure, orthorhombic FeOCl can be regarded that the “OH” of γ-FeOOH is substituted with
“Cl”. Therefore, FeOCl has the similar structure as MOOH-typed compounds with weak
interaction between layers (i.e., Van der Waals' force) and strong binding within the layered
planes.[20] The so-called “van der Waals layer” of FeOCl, as illustrated in Figure 1a, suits for
intercalation reactions by transferring charges between intercalated compounds and the FeOCl
matrix.[21] By calculation, we find that the interlayer spacing of FeOCl (d010=0.783 nm, Figure
S1) is enough for the entrance and migration of many electrolyte ions (like Na+ with a hydrated
ionic radius of 0.358 nm).[22] Along with the movement of Na+, the intercalated Na+ participates
in the reduction reaction of FeOCl as an anode. Obviously, the laminated structure of FeOCl
is able to provide abundant channels and active sites for the intercalation reactions and is thus
expected to display an ideal electrochemical activity.
In this work, we for the first time report the application of FeOCl as a novel anode material
for supercapacitors. Its self-stacked laminated structure is in favour of intercalation reactions
of electrolyte ions. After the integration of a conductive matrix (i.e., cellulose-derived carbon
aerogel, CDCA), the mechanical stability and ionic and charge-storage kinetics of FeOCl are
heightened. As a result, FeOCl@CDCA displays an ultra-high areal specific capacitance of
1618 mF cm‒2 (647 F g‒1) at 2 mA cm‒2 and outstanding cycle stability (ca. 91% after 10000
cycles). We assembled an asymmetrical supercapacitor (ASC) using a FeOCl@CDCA anode
and an inexpensive MnO2 cathode, which possesses highly competitive energy/power density
(289 ꢀμW h cm−2 at 1.8 mW cm−2) amongest current state-of-the-art ASCs.
Results and discussion
The FeOCl, which was synthesized via a simple chemical-vapor-transport technique (details
are available in SI), exhibits a 2D sheet-like structure with a size of several microns and the
sheets stack into a laminated structure (as shown in Figure 1c and d). To address the issue of
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poor electrical conductivity, the FeOCl was combined with a conductive CDCA matrix by
DOI: 10.1039/C8TA12261J
ball-milling (Figure 1b) and the laminated structure of FeOCl was well maintained before and
after the ball-milling (green frames, Figure 1e). From the higher-magnification SEM image in
Figure 1f, it is clear that the feature structure of CDCA adheres to the surface of FeOCl sheets.
In addition, the Raman spectrum also demonstrates the presence of CDCA with typical D and
G-bands (Figure S2), reflecting a good mixture between the two components. The G-band of
FeOCl@CDCA slightly upshifts as compared to that of CDCA due to charge-transfer effects
between them,[23] which is another evidence for their co-presence. Moreover, from the TEM
image of FeOCl@CDCA, an obvious lamellar structure can be identified (as directed by green
circles).
From X-ray diffraction (XRD) patterns (Figure 1f), the FeOCl and FeOCl@CDCA both show
the characteristic peaks of orthorhombic FeOCl (JCPDS no. 24-1005), while the ball-milling
results in the broadening and decrease in the peak intensity, indicative of the grain refining of
FeOCl by the ball-milling. X-ray photoelectron spectroscopy (XPS) was adopted to identify
the elemental compositions and chemical state of Fe in the composite. As shown in Figure 1i,
the signals of O (KLL), Fe 2S, Fe 2P, Fe (LVV), O 1s, C 1s, Cl 2s and Cl 2p were detected. In
addition, for the Fe 2p core-level XPS spectrum (Figure 1j), the Fe 2p peaks at 711.6 eV and
725.4 eV correspond to Fe 2p3/2 and Fe 2p1/2 spin−orbit peaks of FeOCl, suggesting that the
Fe element appears as Fe(III) state.[24] In addition, the Fe 2p3/2 peak is accompanied by a
shake-up satellite situated at 719.3 eV, further representative of Fe (III). It is well-known that
surface area and pore size of active substances play important roles in electrochemical
reactions.[25, 26] CDCA is a typical 3D porous material with a large surface area (157 m2 g‒1,
its isotherms and morphology are available in Figure S3 and S4) and has been widely used to
support active substances for multifarious applications (such as catalysts,[27] sensors[28] and
selective adsorption[28, 29]). The surface area of self-stacked multilayer FeOCl is about 40 m2
g‒1. The incorporation of CDCA results in a remarkable increase by 40% to ~56 m2 g‒1 for
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FeOCl@CDCA. The larger surface area is able to provide more reaction sites DaOnI:d10p.10o3r9o/Cu8sTA12261J
CDCA can serve as a reservoir to store electrolyte and facilitate ion transfer. The isotherms of
FeOCl@CDCA show a hysteresis loop (Figure 1k), which reflects the existence of mesopores
(2‒50 nm). The adsorption does not reach a plateau near the P/P0 of 1.0, revealing the
presence of macropores (>50 nm). The speculations are proved by the pore size distribution
(1.8‒260 nm, calculated by the Barret-Joyner-Halenda method). Such a pore size distribution
helps for electrolyte penetration, allowing reaction species to quickly access the electrode. [30]
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Figure 1. Micromorphology, synthesis and characterizations of FeOCl@CDCA: (a) crystalline structure of
FeOCl; (b) schematic diagram of the ball-milling process; SEM images of (c, d) the FeOCl and (e, f)
FeOCl@CDCA (d and f are the enlarged images of areas within the red frames of c and e), and the insets of c
and e show the element mappings; (g) TEM images of FeOCl@CDCA; (h) XRD pattern; (i) XPS survey
spectrum and (j) Fe 2p core-level XPS spectrum of FeOCl@CDCA; (k) N2 adsorption‒desorption isotherms, and
the inset shows the pore size distribution.
To evaluate electrochemical performances of FeOCl@CDCA as a supercapacitor electrode,
its cyclic voltammetry (CV) and galvanostatic charge‒discharge (GCD) curves, cycle stability,
and electrochemical impedance spectra (EIS) were measured and compared to those of CDCA
and FeOCl electrodes in a conventional three-electrode system, as shown in Figure 2a. Figure
2c compares the CV curves of CDCA, FeOCl and FeOCl@CDCA electrodes at the scan rate
of 5 mV s−1. The contribution of Ni foam to the capacitance is negligible (Figure S5). Besides,
the area enclosed within the CV curve of pseudocapacitive FeOCl (Figure S6), corresponding
to energy storage capability, is much larger than that of CDCA with an EDLC characteristic
(Figure S7). Moreover, the combination of two materials results in a larger area of CV curve
for FeOCl@CDCA composite, reflecting its stronger charge storage ability. The CV curves of
FeOCl@CDCA at different scan rates are presented in Figure 2d, where a pair of redox peaks
are identified (see also Figure 2c) and represent a redox insertion reaction which is derived
from the 3D absorption of electroactive species into the electrode material. Besides, the XPS
spectrum of FeOCl@CDCA electrode after electrochemical tests suggests the transformation
of Fe (III) to Fe (II) (Figure S8). Therefore, the possible charge storage process of FeOCl in
the Na2SO4 electrolyte is described as:[31]
Fe(III)ClO+Na +e  NaFe(II)ClO
(1)
To strengthen the validation of the proposed Na+-intercalation mechanism, the CV curve of
FeOCl@CDCA was also measured in 1 M MgSO4 electrolyte. As the hydrated ionic diameter
of Mg2+ (0.856 nm)[22] is larger than that of the interlayer distance of FeOCl (0.783 nm), a
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notably decreased capacitance measured in the MgSO4 electrolyte will help to demDoOnI:s1t0r.a10te39/tCh8eTA12261J
intercalation mechanism. As expected, the area enclosed within the CV curve measured in the
solution of MgSO4 is much smaller than that measured in the solution of Na2SO4 (Figure S9a),
indicating a sharp decline in capacitance when tested in MgSO4 electrolyte possibly due to the
reduction of intercalation reaction area.
Charge-storage kinetics of electrode materials towards Na+ is evaluated by CV analysis. The
analysis is performed regarding the behavior of peak currents by assuming that the current (i)
obeys a power-law relationship with the scan rate (ν):[32]
i  avb
(2)
where both a and b are adjustable parameters. The b-value can be determined by the slope of
log(v)–log(i) plots. In particular, the b-value of 0.5 represents a total diffusion‒controlled
behavior, whereas 1.0 reveals a capacitive process. Figure 2e shows the log(v)–log(i) plots for
CDCA, FeOCl and FeOCl@CDCA electrodes. The b value of CDCA is calculated to be 0.935
for anodic currents, indicating that the majority of currents at the peak potential is capacitive.
The result demonstrates that the capacitive behavior dominates the reaction of CDCA with
fast kinetics. By contrast, the FeOCl has the b value of 0.579 (close to 0.5), suggesting that
the kinetic behavior of FeOCl is mainly controlled by diffusion process. Moreover, it is clear
that the integration of CDCA increases the b value of FeOCl@CDCA to 0.622, reflecing the
positive contribution of CDCA to the charge-storage kinetics of FeOCl@CDCA.
To study the capacity contribution, the surface and insertion capacities of FeOCl@CDCA at
different scan rates are calculated. The current response i at a fixed potential V is described as
the sum of two contributions from surface and intercalation capacities, as discussed below:[33]
i(V )  k1v  k2v1/2
(3)
(4)
i(V )/v1/2  k1v1/2  k2
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where v represents the scan rate, while k1v and k2v1/2 denote the current contributions from the
DOI: 10.1039/C8TA12261J
surface capacity (surface pseudocapacity and double-layer capacity) and insertion processes
(insertion pseudocapacity), respectively. By determining k1 and k2, we can distinguish the
fractions of current arising from the surface capacity and insertion processes at a specific
potential. At a scan rate of 5 mV s−1 (Figure 2f), ~9% of the total current, i.e., the capacity, is
capacitive in nature. Similarly, the contribution ratios between the two different processes at
other scan rates are calculated. The quantified results show that the capacitive contribution
gradually improves with the increasing scan rate. The reason may be the fact that higher scan
rates restrict the ion diffusion process, which prevents the ions from accessing the laminated
structure. Therefore, at high scan rates, the contribution of surface capacity of the electrode
becomes more remarkable. In addition, at scan rates of 5–100 mV s−1, the fraction of insertion
capacity is higher (68%‒91%), demonstrating that the kinetic behavior of FeOCl@CDCA is
mainly controlled by diffusion process. The result is consistent with that of b-value analysis.
For further investigating ionic transport process, apparent diffusion coefficients of the FeOCl
and FeOCl@CDCA electrodes are calculated based on the Randles–Sevick equation:[34]
1/2
NFvD
RT


IP  0.4463n FAC
 [269000n3/2 AD1/2C]v1/2
(5)




where Ip is the peak current, n is the number of electrons transferred per molecule during the
electrochemical reactions (1 in this case), A is the exposed area of the electrode (1 cm2), C is
the concentration of sodium ions in the electrolyte (1 M in the present system), D is the ionic
diffusion coefficient (including the diffusion of both sodium and hydroxyl ions, and v is the
scan rate. From Figure S10, the calculated ionic diffusion coefficient of FeOCl@CDCA is
8.21 × 10−7 cm2 s−1, which is almost 11.9 times as high as that of the FeOCl (6.92 × 10−8 cm2
s−1). These results indicate that FeOCl@CDCA exhibits a faster ionic diffusion rate, resulting
in enhanced storage capacity.
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Figure 2. Kinetic analysis of the electrochemical behavior of FeOCl@CDCA electrode: schematic diagrams of
(a) the electrochemical testing apparatus using a conventional three-electrode system and (b) the plausible
mechanism of electrochemical reactions; (c) CV curves at the scan rate of 5 mV s−1; (d) CV curves of
FeOCl@CDCA at scan rates of 5‒100 mV s−1; (e) determination of the b value using the relationship between
the peak current and the scan rate according to the voltammograms in d; (f) 푖(V)/푣1/2 vs. 푣1/2 plot using the anodic
current at a potential of ‒0.15 V, and the inset shows surface and insertion capacity contributions at different
scan rates.
GCD is a reliable method for determining capacitances of supercapacitors. As seen in Figure
3a, the non-linear shape reflects the pseudocapacitance feature of FeOCl@CDCA, consistent
with the result of CV analysis. The good symmetry between the charge and discharge curves
demonstrates the good reversibility and coulombic efficiency (namely the ratio of discharging
and charging capacitances). Based on these GCD curves, FeOCl@CDCA achieves the highest
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areal specific capacitance of about 1618 mF cm‒2 (647 F g‒1) at a discharge current of 2 mA
DOI: 10.1039/C8TA12261J
cm‒2 (0.8 A g‒1) (Figure 3b), which is around 1.3 and 14.7 times higher than that of the FeOCl
and CDCA electrodes (ca. 696 and 103 mF cm‒2. Details on GCD plots of CDCA and FeOCl
electrodes are available in Figure S11). When the current density notably increases from 2 to
50 mA cm‒2, FeOCl@CDCA remains a high capacitance retention of 54% (875 mF cm‒2/350
F g‒1), indicating its excellent rate capability. Such high capacitances of FeOCl@CDCA are
more superior to numerous results reported for anodes, e.g., γ-Fe2O3/graphene (76 F g‒1),[35]
mesoporous MoO2 (146 F g‒1),[36] polypyrrole@V2O5 (308 F g‒1),[12] Ti‒Fe2O3@PEDOT (311.6
F g‒1),[37] highly functionalized activated carbons (525 F g‒1)[38] and iron nanosheets/graphene
(717 F g‒1)[39], as listed in Table S1. We also measured the GCD curve of FeOCl@CDCA in
the solution of MgSO4 at 5 mA cm‒2. A capacitance value of 759 mA cm‒2 is obtained (Figure
S9b), lower than half of that measured in the NaSO4 solution (1552 mA cm‒2), which further
demonstrates the intercalation mechanism and reduced intercalation reaction sites in MgSO4
solution. Moreover, it is known that the crystal structure of FeOCl is similar to that of FeOOH.
By comparing their capacitances, the areal specific capacitance of FeOCl@CDCA electrode
(1.62 ꢀF cm−2, 0.8 Aꢀg−1) is comparable to or even higher than that of FeOOH-based electrodes,
such as FeOOH nanoparticles[40] (1.71 F cm−2, 1 Aꢀg−1), nano-sized columned β-FeOOH[41]
(1.16 F cm−2, 0.5 A g−1) and FeOOH nanorod/carbon tube network (0.79 F cm−2, 0.5 A g−1)[42]
Because of the high mass loading (2.5 mg cm−2), the corresponding gravimetric specific
capacitance of FeOCl@CDCA is 647 F g‒1, lower than that of FeOOH nanoparticles (1066 F
g−1, 1.6 mg cm−2), but higher that that of nano-sized columned β-FeOOH (116 F g−1, 10 mg
cm−2) and FeOOH nanorod/carbon tube network (396 F g−1, 2 mg cm−2). In addition, it is
known that FeOOH is thermally unstable and is quantitatively converted to the topotactically
related γ-Fe2O3 when the isostructural γ-FeOOH is treated with organic bases at temperatures
of 120‒140 °C.[43] Also, according to the report by Tang et al., α-Fe2O3 nanorods are prepared
.
by calcining α-FeOOH at 250 °C for 2 h[44]. By contrast, the thermal stability of FeOCl is
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more superior because FeOCl is obtained by calcining the mixture of α-Fe2O3 aDnOdI: F10e.1C03l93/Ca8tTA12261J
380°C. In consequence, FeOCl also has own unique advantages when acting as supercapacitor
electrodes, as compared with FeOOH.
The capacitance contribution from CDCA and FeOCl components to the total capacitance of
FeOCl@CDCA can be estimated based on the above b-value analysis (Figure 2e). CDCA has
a capacitance behavior-dominant charge-storage kinetics while the kinetic behavior of FeOCl
is mainly controlled by diffusion process. Therefore, the capacitance contribution of CDCA
and FeOCl compositions to the total capacitance of FeOCl@CDCA can be distinguished
roughly as surface capacitance and intercalation capacitance, respectively. By virtue of the
capacitance contribution analysis, the fraction of insertion capacitance of FeOCl@CDCA is
much higher (68%‒91%) than that of surface capacitance (9%‒32%) at scan rates of 5–100
mV s−1. Thus, we can roughly speculate that capacitance contribution from the FeOCl should
account for more than 60% while the contribution from CDCA should account for less than
40%. Besides, the increase of scan rates will increase the contribution proportion from CDCA
and decrease the contribution proportion from the FeOCl.
Also, FeOCl@CDCA shows excellent cycle stability with no more than 10% capacitance loss
(ca. 9%) after 10000 cycles (Figure 3c), lower than that of the FeOCl (ca. 25% loss) (Figure
S12). The dominant cycle behavior of FeOCl@CDCA is attributed to contributions of CDCA
to the stability of FeOCl, possibly including two points: (1) 3D interconnected CDCA might
act as buffers to alleviate strains of FeOCl in the process of intercalation and de-intercalation
of ions;[45] (2) good mixture between CDCA and FeOCl effectively prevent the aggregation of
FeOCl. As presented in the inset of Figure 3c, the SEM image of FeOCl@CDCA after 10000
cycles demonstrates that the laminated structure is well maintained and the laminated FeOCl
surrounded with CDCA does not contact with others, indicating its good shape stability and
dispersion. Similar results can be found for other composites composed of carbon materials
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and metallic oxides/hydroxides.[46, 47] Moreover, a high coulombic efficiency of 96%‒100% is
DOI: 10.1039/C8TA12261J
achieved during the cycles.
EIS was measured to quantify the resistance at the electrode/electrolyte interface and analyze
causes for performance differences of CDCA, FeOCl and FeOCl@CDCA (Figure 3d). The
equivalent circuit diagram used for the fitting of EIS data is shown in the inset of Figure 3d.
By comparing Nyquist plots, we can see that integrating the FeOCl with CDCA decreases the
diameter of semicircles (an indicator for charge transfer resistance, Rc) from 1 Ω (FeOCl) to
0.4 Ω (FeOCl@CDCA), because the highly conductive CDCA constructs dense conducting
networks and provides plentiful mobile charge carriers and conductive channels. This result
agrees well with that of electrical conductivity measurements showing a significant increase
from outrange (<10‒5 S cm‒1) for the FeOCl to 0.26 S cm‒1 for FeOCl@CDCA. This value is
close to that of some conjugated polymers (0.01–400 S cm‒1).[48] In addition, the projected
length of the 45° Warburg region on the real impedance axis (Z’) reflects the ion penetration
process. Obviously, the length of FeOCl@CDCA is shorter than that of CDCA and FeOCl,
indicative of a shorter ion-diffusion path. Therefore, the EIS suggests that FeOCl@CDCA has
better ionic/electronic transport ability. In the low-frequency region, FeOCl@CDCA shows a
more vertical line along the imaginary axis as compared to that of the FeOCl, demonstrating
its more ideally capacitive behavior because of the introduction of non-Faradic charge storage
process.
Based on these analyses, the more superior electrochemical properties of FeOCl@CDCA are
possibly attributed to synergistic functions of CDCA and FeOCl, which can be summarized as
follows: (1) the large surface area of FeOCl consisting of laminated microsheets has the ability
to provide abundant space and sites for the electrochemical reactions; (2) FeOCl has a high
theoretical capacitance value of ~899 F g‒1 (the calculation is available in SI) and the
laminated structure of FeOCl supported on the CDCA shows an excellent mechanical stability,
helpful for achieving long service life; (3) the porous CDCA with a large surface area can act
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as an electrolyte storage and shorten diffusion path of ions between ions and laminated FeOCl
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(Figure 3e). The highly conductive CDCA is able to supply abundant mobile charge carriers
and conductive channels during the reactions.
Figure 3. Electrochemical properties of FeOCl@CDCA electrode: (a) GCD curves of FeOCl@CDCA at current
densities of 2‒50 mA cm−2; (b) rate performance; (c) cycle stability and coulombic efficiency of FeOCl@CDCA
at 50 mA cm−2 and the inset shows its SEM image after 10000 cycles; (d) Nyquist plots and insets show the
enlarged image at the high-frequency region and the equivalent circuit used for EIS data fitting; (e) schematic
diagrams of functions of CDCA and FeOCl components on the electrochemical reactions.
An ASC was assembled to evaluate the potential of FeOCl@CDCA for practical applications.
The schematic diagram (Figure 4a) depicts the fabrication of ASC with FeOCl@CDCA as an
anode and a common cheap Ni foam-supported MnO2 as a cathode. The MnO2 cathode shows
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a moderate areal specific capacitance of 318 mF cm‒2 (ca. 127 F g‒1) at 2 mA cm and good
rate capability (the detailed electrochemical evaluation is available in Figure S13). Considering
that FeOCl@CDCA was tested within a stable potential window from ‒1 to 0 V (vs. Ag/AgCl)
while that of the MnO2 electrode was measured within the window of 0~0.8 V (vs. Ag/AgCl),
such two electrodes combination as ASC may afford a device with 1.8 V operation voltages
in 1ꢀM Na2SO4 as electrolyte. As illustrated in Figure 4b, the stable electrochemical voltage of
the ASC can be extended from 0.8 to 1.8 V. In addition, the CV curves of the ASC with a non-
rectangular shape at different scan rates in a 1.8ꢀV potential window (Figure 4c) reveals that
the capacitance primarily comes from redox reactions.
The GCD curves of the ASC at different current densities were tested. As seen in Figure 4d,
both charge and discharge curves keep a good symmetry at such a high operation voltage of
1.8 V, verifying its outstanding reversibility and good coulombic efficiency. In addition, the
ASC delivers an areal specific capacitance of 641 and 372 mFꢀcm‒2 (128 and 74 F g‒1) at a
current density of 2 and 50 mAꢀcm‒2, respectively. A superior rate capability with ~58% of
the capacitance is retained in that current density range (Figure 4e). From the Ragone plot
(Figure 4f), the ASC exhibits a maximum areal energy density of 289 ꢀμW h cm−2 at a power
density of 1.8 mW cm−2 and an areal energy density of 168 μW h cm−2 at a maximum power
density of 45 mW cm−2. The attained energy densities of our ASC are superior to those of
many recently reported ASCs with a pseudocapacitive storage mechanism (see Figure 4f and
Table S2).[49-54] Moreover, the ASC remains at 88.7% of the initial capacitance after 10000
cycles (Figure 4g), reflecting its good cycle stability. In addition, based on previous researches,
possible reasons for the decrease of the capacitance with cycle number is possibly summarized
into three points: (1) a small quantity of structural alternation/disintegration around the active
sites, by virtue of repeated ingression and depletion of ions in the electrode materials during
the Faradaic reactions;[55] (2) dissolution/detachment of active materials (like a small portion
of Fe dissolution) leading to reduction of electrochemical active sites;[56, 57] (3) consumption
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of electrolyte caused by tiny minority of irreversible side-reactions and interface reactions.
DOI: 10.1039/C8TA12261J
59]
Also, the device delivers a high coulombic efficiency (>97%) during the cycles, agreeing
well with the speculation from the GCD analysis. The inset of Figure 4g presents the Nyquist
plots of the ASC before and after the cycles. Obviously, the solution resistance (Rs) did not
change much (~3 Ω) before and after tests. The corresponding charge transfer resistances Rc
are 1.0 and 2.4 Ω, respectively. The slightly enlarged charge transfer resistance after the cycle
tests is possibly related to the destruction and dissolution of a part of electroactive materials
suffering from plentiful harsh redox reactions. This slight increment further indicates that the
ASC has favorable cycle stability. Furthermore, these low Rc and Rs values are expected to
enhance its power density. These outstanding electrochemical performances of the
MnO2//FeOCl@CDCA ASC demonstrate that FeOCl@CDCA suits to be a cost-effective and
high-performance anode material for supercapacitors.
Figure 4. Electrochemical properties of the MnO2//FeOCl@CDCA ASC: (a) schematic diagram of the
assembled ASC device in a two-electrode system; (b) CV curves in different potential windows at the scan rate
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DOI: 10.1039/C8TA12261J
of 100ꢀmVꢀs ; (c) CV curves at scan rates of 5‒100 mV s ; (d) GCD curves at current densities of 2‒50 mA
cm−2; (e) rate performance; (f) Ragone plot of the ASC compared with data of some other ASCs in the literature;
(g) cycle stability and coulombic efficiency at the current density of 50 mA cm−2, and the inset shows the
Nyquist plots before and after the cycles.
Conclusion
In summary, we demonstrate that the self-stacked and multilayer FeOCl supported on CDCA
is suitable as a novel and high-performance anode which delivers an ultra-high areal specific
capacitance of 1618 mF cm‒2 (647 F g‒1) at 2 mA cm‒2 and outstanding cycle stability with no
more than 10% capacitance loss (ca. 9%) after 10000 cycles. The laminated structure of the
hierarchical FeOCl with a superior mechanical stability (strengthened by CDCA) provides
abundant channels and active sites for the migration and intercalation reactions of Na+, which
plays a key role in the high electrochemical activity. The assembled MnO2//FeOCl@CDCA
ASC shows a highly competitive energy/power density (289 ꢀμW h cm−2 at 1.8 mW cm−2) and
cycle stability among many ever-reported ASCs storing energy by Faradaic reactions. More
importantly, the cost-effective and easily synthesized FeOCl@CDCA anode with outstanding
electrochemical properties should be applicable to assembly of diversified high-performance
energy-storage devices (e.g., Li/Na-ion batteries) that are not limited to supercapacitors.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (grant nos.
31530009 and 31470584), the Youth Scientific Research Foundation, Central South
University of Forestry and Technology (grant no. QJ2018002A), and the Fundamental
Research Funds for the Central Universities (grant no. 2572018AB09).
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