Ultra-High Cycling Stability of 3D Flower-like Ce(COOH)3 for Supercapacitor Electrode via a Facile and Scalable Strategy

An electrode material with high performance, long durability, and low cost for supercapacitors has long been desired in academia and industry. Among all the factors that affect the electrochemical performance and cycling stability of electrode materials, the morphology and intrinsic structure characteristics are the most important. In this study, a novel 3D flower-like Ce(COOH)3 electrode material was designed by taking advantage of the Ce3+ and -COOH groups and fabricated by a one-pot microwave-assisted method. The morphology and structure characteristics of the sample were examined by SEM, EDS, TEM, XRD, FT-IR, XPS, N2 adsorption/desorption techniques, and the electrochemical behaviors were investigated in a three-electrode configuration. The Ce(COOH)3 electrode presents an excellent specific capacitance of 140 F g−1 at 1 A g−1, higher than many other previously reported Ce-based electrodes. In addition, it delivers high rate capability that retains 60% of its initial capacitance when the current density is magnified 20 times. Dramatically, the Ce(COOH)3 electrode exhibits an ultra-high cycling stability with capacitance retention of 107.9% after 60,000 cycles, which is the highest durability among reported Ce–organic compound electrodes to the best of our knowledge. The excellent electrochemical performance is ascribed to its intrinsic crystal structure and unique morphology. This work indicates that the 3D flower-like Ce(COOH)3 has significant potential for supercapacitor applications and the facile and scalable synthesis strategy can be extended to produce other metal–organic composite electrodes.


Introduction
Global climate change, environmental pollution, and energy shortages have triggered the rapid development of renewable clean energy, including solar, wind, tidal, hydrogen, and biomass energy [1][2][3]. However, due to their high volatility, they are usually unable to be directly integrated into the power grid, and, therefore, advanced energy storage devices are urgently needed to provide more flexibility and reliability for taking advantage of new energy. Among various energy storage technologies, supercapacitors have attracted significant attention from academia and industry over the past few decades due to their distinguished properties, including much higher energy density than traditional capacitors and superior power density than batteries [4]. In addition, supercapacitors have also shown great potential in many other applications, such as aerospace, electric vehicles, microsensors, and portable electronics [5][6][7].
Supercapacitors (SCs) are typically composed of two electrodes separated by an electrolyte and can be classified into two main categories: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy by the adsorption of ions capacitor electrodes using a single-step microwave-assisted method in just 10 min. The obtained Ni 0.85 Se electrode showed magnificent electrochemical properties [27]. Chen et al. synthesized a Zn-doped Ni-MOF by microwave-assisted method and hydrothermal method for comparison. The reaction time of the former was merely 6 min, much shorter than the 24 h of the latter. The sample by microwave-assisted method had a honeycomblike spherical morphology and the electrochemical performance was also much better [28].
Herein, in this work, a novel 3D flower-like Ce(COOH) 3 was designed employing the high charge radius ratio Ce 3+ ions and the well-known sturdy -COOH linkers. It was synthesized via a one-pot microwave-induced method, which provided two advantages. One thing is that the 3D flower-like Ce(COOH) 3 exhibited a superior specific capacitance of 140 F g −1 at 1 A g −1 , excellent rate capability, and ultra-high cycling stability at 10 A g −1 after 60,000 cycles (107.9% capacitance retention), which is one of the highest performances reported so far for Ce-organic compounds to our knowledge. The other is that the high purity and unique morphology of 3D flower-like Ce(COOH) 3 could be synthesized rapidly with high yields. In fact, the microwave-assisted synthesis is so facile and efficient that it can significantly reduce the cost in scalable preparation of Ce(COOH) 3 supercapacitor electrode material. The excellent electrochemical performance can be ascribed to the unique crystal structure and special morphology of the 3D flower-like Ce(COOH) 3 and the facile and scalable synthesis strategy can be extended to other electrode materials based on metal-organic compounds.

Results and Discussion
The morphologies of the samples were observed by scanning electron microscopy (SEM). As shown in Figure 1a, the as-prepared Ce(COOH) 3 particles are quite uniform and monodisperse, and the size of those ranges from 20 to 50 µm. A magnified SEM image in Figure 1b further reveals that the Ce(COOH) 3 particle presents a beautiful threedimensional (3D) flower-like shape, in which every petal of the flower is composed of numerous nanorods. EDS elemental mapping (Figure 1c-f) illustrates that Ce, C, and O were distributed throughout the prepared material. EDS analysis (Figure 1g) also verifies the element content of Ce, C, and O, which can preliminarily imply the production of Ce(COOH) 3 successfully. TEM images have been taken to further verify the sub-units of the 3D flower-like Ce(COOH) 3 structure. The magnified TEM image in Figure 1h provides the Ce(COOH) 3 Figure 2b to better know its functional moieties. The broad absorption band at~3424 cm −1 is assigned to the O-H stretching vibration of H 2 O, which could be related to the occurrence of some entrapped water. The absorption bands around 2917 cm −1 are attributed to C-H stretching vibrations. Also, two prominent bands arising at~1575 cm −1 and~1403 cm −1 are associated with the characteristics of the asymmetric and symmetric stretching vibrations of COO -, respectively. Moreover, the presence of a strong band at~777 cm −1 is the feature of the stretching vibrational mode of the Ce-O bond. In order to further confirm the formation of Ce(COOH)3 nanocrystalline, the as-prepared sample was examined by XRD and the pa erns are demonstrated in Figure  2a. All the sharp Bragg peaks at 2θ values of 17°, 24°, 29°, 34°, 41°, 45°, 48°, 51°, 57°, and 63° can be indexed to (110), (101), (300), (220), (131), (410), (321), (330), (241), and (520) la ice planes of the rhombohedral phase of Ce(COOH)3 (PDF#49-1245), respectively. No other phase was detected, confirming the high purity of the products. The FT-IR spectrum of Ce(COOH)3 is shown in Figure 2b to be er know its functional moieties. The broad absorption band at ~3424 cm −1 is assigned to the O-H stretching vibration of H2O, which could be related to the occurrence of some entrapped water. The absorption bands around 2917 cm −1 are a ributed to C-H stretching vibrations. Also, two prominent bands arising at ~1575 cm −1 and ~1403 cm −1 are associated with the characteristics of the asymmetric and symmetric stretching vibrations of COO -, respectively. Moreover, the presence of a strong band at ~777 cm −1 is the feature of the stretching vibrational mode of the Ce-O bond. Figure 2c,d exhibit the N2 adsorption/desorption isotherm and Barre -Joyner-Halenda (BJH) pore size distribution curve of the as-synthetized Ce(COOH)3. As per the IUPAC classification, the isotherm of Ce(COOH)3 can be assigned to the Langmuir type IV isotherm. The N2 adsorption isotherm that slightly creeps up in the low relative Figure 2c,d exhibit the N 2 adsorption/desorption isotherm and Barrett-Joyner-Halenda (BJH) pore size distribution curve of the as-synthetized Ce(COOH) 3 . As per the IUPAC classification, the isotherm of Ce(COOH) 3 can be assigned to the Langmuir type IV isotherm. The N 2 adsorption isotherm that slightly creeps up in the low relative pressure (P/P 0 ) range indicates the existence of micropores, while the distinct hysteresis loop in the range of 0.8-1.0 P/P 0 signifies the abundance of mesopores between the Ce(COOH) 3 nanorods. As calculated from the desorption branch of the isotherm, the pore sizes of the Ce(COOH) 3 range from 1.7 to 50 nm and mostly center around 2.7 nm. A rational hierarchical pore structure would facilitate electrolyte ion diffusion and buffer the material volume change during charge/discharge cycles, thereby proving beneficial for the electrochemical performance and cycling stability [29]. The specific surface area calculated by the Brunauer-Emmett-Teller (BET) method is 3.57 m 2 g −1 , indicating that the capacitance of Ce(COOH) 3 mainly originates from pseudo-capacitance rather than the EDLC mechanism [30].
A rational hierarchical pore structure would facilitate electrolyte ion diffusion and buffer the material volume change during charge/discharge cycles, thereby proving beneficial for the electrochemical performance and cycling stability [29]. The specific surface area calculated by the Brunauer-Emme -Teller (BET) method is 3.57 m 2 g −1 , indicating that the capacitance of Ce(COOH)3 mainly originates from pseudo-capacitance rather than the EDLC mechanism [30]. X-ray photoelectron spectroscopy (XPS) was used to study the elemental content and valence states of the Ce(COOH)3 sample. Figure 3a shows the XPS survey spectrum, revealing the existence of Ce, O, and C. Figure 3b-d present the high-resolution XPS spectra of Ce 3d, O 1s, and C 1s, respectively. The high-resolution XPS spectrum of Ce 3d can be deconvoluted to four Gaussian peaks located at 881.6 eV, 900.1 eV (Ce 3d 9 4f 2 ) and 885.4 eV, 903.9 eV (Ce 3d 9 4f 1 ). The peak positions of Ce 3d5/2 (881.6 eV, 885.4 eV) and Ce 3d3/2 (900.1 eV, 903.9 eV) are consistent with the spin-orbit features of Ce 3+ . The scan spectrum of O 1s can be deconvoluted to three peaks at 530.7 eV, 531.5 eV, and 532.1 eV, belonging to C-O, C=O, and -OH, respectively. The characteristic peaks of C 1s centered at 285 eV and 289 eV are assigned to C-O and C=O, respectively. The above results thoroughly reveal the electronic spin states of each element in the 3D flower-like Ce(COOH)3 structure and are in good agreement with previous research reports [31][32][33]. X-ray photoelectron spectroscopy (XPS) was used to study the elemental content and valence states of the Ce(COOH) 3 sample. Figure 3a shows the XPS survey spectrum, revealing the existence of Ce, O, and C. Figure 3b-d present the high-resolution XPS spectra of Ce 3d, O 1s, and C 1s, respectively. The high-resolution XPS spectrum of Ce 3d can be deconvoluted to four Gaussian peaks located at 881.6 eV, 900.1 eV (Ce 3d 9 4f 2 ) and 885.4 eV, 903.9 eV (Ce 3d 9 4f 1 ). The peak positions of Ce 3d 5/2 (881.6 eV, 885.4 eV) and Ce 3d 3/2 (900.1 eV, 903.9 eV) are consistent with the spin-orbit features of Ce 3+ . The scan spectrum of O 1s can be deconvoluted to three peaks at 530.7 eV, 531.5 eV, and 532.1 eV, belonging to C-O, C=O, and -OH, respectively. The characteristic peaks of C 1s centered at 285 eV and 289 eV are assigned to C-O and C=O, respectively. The above results thoroughly reveal the electronic spin states of each element in the 3D flower-like Ce(COOH) 3 structure and are in good agreement with previous research reports [31][32][33].  The electrochemical properties of the 3D flower-like Ce(COOH)3 were investigat in a classic three-electrode system using 3 M KOH as the electrolyte, of which a Pt f was used as the counter electrode, a commercial Hg/HgO electrode as the referen electrode, and the as-prepared Ce(COOH)3 as the working electrode. As shown in Figu 4a, the CV curves of the Ce(COOH)3 were collected at various scan rates (1-20 mV s −1 ) the potential window from 0.0 to 0.55 V. It is clearly observed that all the CV curves d viate from the rectangular shape and present obvious reversible redox peaks, reveali the pseudo-capacitance properties of Ce(COOH)3. The anodic peaks and cathodic pea center around +0.48 V and +0.38 V, respectively, corresponding to the Faradaic redox action of Ce 3+ /Ce 4+ sites. Furthermore, as the scan rates increase, the redox current den ties also increase while the profile of the CV curves is almost unchanged, indicating outstanding rate capability and reversibility for energy storing. Figure 4b presents the galvanostatic charge-discharge (GCD) curves of t Ce(COOH)3 electrode under various charge-discharge current densities (1-20 A g Despite the different current densities, all the GCD curves display similar symmetri shapes with a certain deviation from an isosceles triangle, indicating the reversible fa daic redox reaction behavior of Ce(COOH)3, which is also in good agreement with t The electrochemical properties of the 3D flower-like Ce(COOH) 3 were investigated in a classic three-electrode system using 3 M KOH as the electrolyte, of which a Pt foil was used as the counter electrode, a commercial Hg/HgO electrode as the reference electrode, and the as-prepared Ce(COOH) 3 as the working electrode. As shown in Figure 4a, the CV curves of the Ce(COOH) 3 were collected at various scan rates (1-20 mV s −1 ) in the potential window from 0.0 to 0.55 V. It is clearly observed that all the CV curves deviate from the rectangular shape and present obvious reversible redox peaks, revealing the pseudo-capacitance properties of Ce(COOH) 3 . The anodic peaks and cathodic peaks center around +0.48 V and +0.38 V, respectively, corresponding to the Faradaic redox reaction of Ce 3+ /Ce 4+ sites. Furthermore, as the scan rates increase, the redox current densities also increase while the profile of the CV curves is almost unchanged, indicating an outstanding rate capability and reversibility for energy storing.  Most importantly, the electrochemical stability is a significant property of a sup capacitor electrode. Therefore, the Ce(COOH)3 electrode was subjected to a long-te cyclic test at the constant current density of 10 A g −1 . As shown in Figure 5, the spec capacitance of the Ce(COOH)3 electrode retains 107.9% of its initial value after expe encing 60,000 cycles. At the same time, it can be seen from the inset of Figure 5 that th is no obvious change between the charge-discharge curves before and after the cycli test. This ultra-high cycling stability and remarkable reversibility of Ce(COOH)3 w first discovered in this work. In fact, as summarized in Table 1, the cycling stability a capacitance retention rate of Ce(COOH)3 are the highest among the reported Ce-orga compounds to the best of our knowledge [18,21,36,[38][39][40][41][42]. Moreover, the cycling stabil and capacitance retention rate of Ce(COOH)3 also have substantial advantages amo the other most reported metal-organic compounds known for their long cycle life, su as Ni3(HITP)2 (90% after 10,000 cycles at 2 A g −1 ) [16], Co-MOF (96% after 10,000 cycles 5 mA cm −2 ) [43], Ni/Co-MOF (89.8% after 12,000 cycles at 10 A g −1 ) [44], Cu3(HHT (79.9% after 5000 cycles at 5 A g −1 ) [45], V-MOF (92.1% after 10,000 cycles at 1 A g −1 ) [4 Cr-MOF (85% after 10,000 cycles at 0.5 A g −1 ) [47], Mn-MOF (81.18% after 10,000 cycles 10 A g −1 ) [48], and Fe-MOF (74.4% after 10,000 cycles at 1 A g −1 ) [49].
The ultra-high cycling stability and remarkable reversibility of Ce(COOH)3 can a ributed to the following aspects. The primary one is the special 3D flower-like m  . Despite the different current densities, all the GCD curves display similar symmetrical shapes with a certain deviation from an isosceles triangle, indicating the reversible faradaic redox reaction behavior of Ce(COOH) 3 , which is also in good agreement with the results of the CV curves. Then, the specific capacitances of Ce(COOH) 3 at each current density are calculated using the GCD curves and the results are shown in Figure 4c. It can be seen that the Ce(COOH) 3 electrode delivers high capacitances of 140, 129, 107.5, and 94.5 F g −1 at the current densities of 1, 2, 5, and 10 A g −1 , respectively. Even when the current density is magnified to 20 A g −1 , a capacity of 84 F g −1 is still retained (the capacitance retention rate is 60%, corresponding to the initial value at 1 A g −1 ), indicating its high rate capability. It is worth noting that the capacitive performance of Ce(COOH) 3 holds a significant advantage among the previously reported Ce-based electrodes, such as Ce(OH) 3 (75 F g −1 at 5 mV s −1 ) [34], graphene/CeO 2 (89 F g −1 at 1 A g −1 ) [35], Ce-BTC (94.8 F g −1 at 1 A g −1 ) [36], Ce(HPO 4 ) 2 .xH 2 O (114 F g −1 at 0.2 A g −1 ) [21], and carbon/CeO 2 -PVP (123.8 F g −1 at 0.25 A g −1 ) [37]. Such superior specific capacitance benefits from the unique crystal structure and special 3D flower-like morphology that offer incredibly rich Ce 3+ /Ce 4+ active sites.
Furthermore, to study the dynamic transfer process and frequency response of the charge and electrolyte ions in the Ce(COOH) 3 electrode, the electrochemical impedance spectroscopy (EIS) was measured. As depicted in Figure 4d, the corresponding Nyquist plot is obtained and the inset is the magnified part for the high-frequency region as well as the equivalent fitting circuit. The Nyquist plot consists of a quasi-semicircular part in the high-frequency range (charge transfer process) and an inclined linear part (diffusion-limited process) in the low-frequency range. In the high-frequency range, the intercept on the real (Z') axis signifies the equivalent series resistance (R s ), including the intrinsic resistance of the Ce(COOH) 3 electrode material, the ohmic resistance of the KOH electrolyte, and the interfacial resistance between the active material and the Ni foam current collector. From the Nyquist curve, the R s value is calculated to be 0.16 Ω, indicating the high conductivity of the Ce(COOH) 3 electrode material. The low R s is beneficial for the rate capability and avoiding unnecessary heat generation during the electrochemical working process. Then, the diameter of the quasi-semicircular part (or kinetic loop) in the Nyquist plot represents the charge transfer resistance (R ct ), which mainly originates from the Faradaic reactions occurring at the interface between electrode and electrolyte and of course the persistent double-layer capacitance (C dl ). According to the results of the equivalent circuit fitting, the R ct of the Ce(COOH) 3 electrode is quite low (0.63 Ω), implying a fast charge transfer at the electrode-electrolyte interface. In the mid-frequency range, the straight line with a slope of 45 • is known as the Warburg line. The Warburg resistance (W d ) reflected by the Warburg line comes from the frequency dependence of the ions' diffusion behavior from electrolyte to electrode surface. Apparently, the short Warburg line of the Nyquist plot for the Ce(COOH) 3 electrode denotes a smooth and unobstructed ion transport process, resulting in low W d . In the low-frequency range, the inclined linear part of the Nyquist plot represents the limit capacitance (C L ) and provides signals on the ideality of the Ce(COOH) 3 electrode towards supercapacitive behavior. The linear part more parallel to the imaginary (Z ) axis purports more ideal capacitive behavior. Obviously, the Ce(COOH) 3 electrode demonstrates a nearly vertical line, indicating outstanding capacitive performance. Based on the above analysis, the Ce(COOH) 3 electrode material possesses excellent impedance properties, which can also be attributed to its crystal structure and special morphology. The high crystallinity of the Ce(COOH) 3 provides a highway for the electronic transfer, reducing the internal resistance of the material. The 3D flower-like hierarchical Ce(COOH) 3 particles exhibit small and uniform size and consist of numerous nanorods orderly interconnecting each other, which can facilitate the migration of the electrolyte ions, shorten the diffusion paths to the electrode surface, and decrease the interfacial charge transfer resistance.
Most importantly, the electrochemical stability is a significant property of a supercapacitor electrode. Therefore, the Ce(COOH) 3 electrode was subjected to a long-term cyclic test at the constant current density of 10 A g −1 . As shown in Figure 5, the specific capacitance of the Ce(COOH) 3 electrode retains 107.9% of its initial value after experiencing 60,000 cycles. At the same time, it can be seen from the inset of Figure 5 that there is no obvious change between the charge-discharge curves before and after the cycling test. This ultra-high cycling stability and remarkable reversibility of Ce(COOH) 3 were first discovered in this work. In fact, as summarized in Table 1, the cycling stability and capacitance retention rate of Ce(COOH) 3 are the highest among the reported Ce-organic compounds to the best of our knowledge [18,21,36,[38][39][40][41][42]. Moreover, the cycling stability and capacitance retention rate of Ce(COOH) 3 also have substantial advantages among the other most reported metal-organic compounds known for their long cycle life, such as Ni 3 (HITP) 2 (90% after 10,000 cycles at 2 A g −1 ) [16], Co-MOF (96% after 10,000 cycles at 5 mA cm −2 ) [43], Ni/Co-MOF (89.8% after 12,000 cycles at 10 A g −1 ) [44], Cu 3 (HHTP) 2 (79.9% after 5000 cycles at 5 A g −1 ) [45], V-MOF (92.1% after 10,000 cycles at 1 A g −1 ) [46], Cr-MOF (85% after 10,000 cycles at 0.5 A g −1 ) [47], Mn-MOF (81.18% after 10,000 cycles at 10 A g −1 ) [48], and Fe-MOF (74.4% after 10,000 cycles at 1 A g −1 ) [49]. tion rate. In addition, the role of material composition design and its crystal structure cannot be ignored. According to the hard and soft acid-base (HSAB) principle, Ce 3+ coupled with high valent and small radius can bond firmly with -COOH groups. The good mechanical strength of Ce(COOH)3 can prevent it from pulverization during long-term charging and discharging. The rapid redox capability of the Ce 3+ /Ce 4+ pair and enhanced charge transfer from the high crystallinity of Ce(COOH)3 also pave the way for its ultra-high cycling stability and extraordinary reversibility for energy storing.     3 60,000/10 a 107.9% This work The ultra-high cycling stability and remarkable reversibility of Ce(COOH) 3 can be attributed to the following aspects. The primary one is the special 3D flower-like morphology. As can be seen in Figure 5, the capacitance of Ce(COOH) 3 increases in the initial 2000 cycles because the stack-up Ce(COOH) 3 petals may lead some Ce 3+ to be in a deep position where the electrolyte cannot easily reach. In pace with the charge-discharge cycles, an increasing number of Ce 3+ are activated and eventually result in a peak capacitance retention rate of 131.8%. After that, the capacitance retention value shows a slowly decreasing tendency and gradually stabilizes around 107.9% after 60,000 cycles. Similar phenomena regarding that the capacitive retention is over 100% have also been found in other researchers' works [42,50]. Generally, an electrochemical electrode dominated by a pseudocapacitance mechanism may experience large volume change during charging and discharging, leading to disintegration or partial detachment and further rapid deterioration of capacitance and service life. However, the hierarchical 3D flower-like morphology provides generous free space to buffer the volume change and shortens the diffusion paths of ions by exposing internal active sites to the electrolyte directly, thereby ensuring ultra-high long-term cycling stability and capacitance retention rate. In addition, the role of material composition design and its crystal structure cannot be ignored. According to the hard and soft acid-base (HSAB) principle, Ce 3+ coupled with high valent and small radius can bond firmly with -COOH groups. The good mechanical strength of Ce(COOH) 3 can prevent it from pulverization during long-term charging and discharging. The rapid redox capability of the Ce 3+ /Ce 4+ pair and enhanced charge transfer from the high crystallinity of Ce(COOH) 3 also pave the way for its ultra-high cycling stability and extraordinary reversibility for energy storing.

Preparation of Ce(COOH) 3
The one-pot microwave-assisted synthesis process of Ce(COOH) 3 is demonstrated in Scheme 1. In a typical run, 3.506 g Ce(NH 4 ) 2 (NO 3 ) 6 was dissolved in 12 mL ethylene glycol and stirred for 30 min to form a homogeneous solution. After adding 41.2 mL HCOOH into the above solution and continuing stirring for 3 min, the mixture was transferred into a microwave reactor (Panasonic NN-GF352 M, 2450 MHz, Osaka, Japan) under 300 W for 210 s. Then, the obtained suspension was treated by vacuum filtration processing and the precipitate was washed with ethanol and deionized water alternately until the filtrate was neutral. Finally, the collected white powder was dried in vacuum at 80 • C for 12 h.

Preparation of Ce(COOH)3
The one-pot microwave-assisted synthesis process of Ce(COOH)3 is demonstrated in Scheme 1. In a typical run, 3.506 g Ce(NH4)2(NO3)6 was dissolved in 12 mL ethylene glycol and stirred for 30 min to form a homogeneous solution. After adding 41.2 mL HCOOH into the above solution and continuing stirring for 3 min, the mixture was transferred into a microwave reactor (Panasonic NN-GF352 M, 2450 MHz, Osaka, Japan) under 300 W for 210 s. Then, the obtained suspension was treated by vacuum filtration processing and the precipitate was washed with ethanol and deionized water alternately until the filtrate was neutral. Finally, the collected white powder was dried in vacuum at 80 °C for 12 h. Scheme 1. Schematic for the synthesis of Ce(COOH)3 using a one-pot microwave-assisted method.

Characterization
The crystalline structure of the samples was characterized by X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan) using Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared spectroscopy (FTIR) was recorded on a Fourier transform infrared spectrometer (Thermo Fisher Nicolet 6700, Waltham, MA, USA). The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi SU-8100, Tokyo, Japan) a ached with an energy dispersive X-ray spectrometer (EDS, IXRF3310, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 F30, Hillsboro, OR, USA). The specific surface area was collected by Brunauer-Emme -Teller (BET) method using an Autosorb-iQ system (Quantachrome Instruments, Boynton Beach, FL, Scheme 1. Schematic for the synthesis of Ce(COOH) 3 using a one-pot microwave-assisted method.

Characterization
The crystalline structure of the samples was characterized by X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan) using Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared spectroscopy (FTIR) was recorded on a Fourier transform infrared spectrometer (Thermo Fisher Nicolet 6700, Waltham, MA, USA). The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi SU-8100, Tokyo, Japan) attached with an energy dispersive X-ray spectrometer (EDS, IXRF3310, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 F30, Hillsboro, OR, USA). The specific surface area was collected by Brunauer-Emmett-Teller (BET) method using an Autosorb-iQ system (Quantachrome Instruments, Boynton Beach, FL, USA) operated at −196 • C. The content and configuration of C, O, and Ce were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250XI, Waltham, MA, USA).

Electrochemical Measurement
The electrochemical performance of the samples was investigated using an electrochemical workstation (CHI 660E), including a classic three-electrode system. The working electrode was prepared by well grinding Ce(COOH) 3 material, acetylene black, and polyvinylidene fluoride (PVDF) in ethanol to form a homogeneous slurry. Their mass ratio was 8:1:1. The as-prepared slurry was coated onto a Ni foam (1 × 1 cm 2 ) and vacuum-dried at 80 • C overnight. Subsequently, it was pressed at 8 MPa to obtain the final working electrode. A typical mass loading of the active material was~1 mg cm −2 . A planar Pt foil and a commercial Hg/HgO electrode were used as counter and reference electrodes, respectively. The electrolyte was 3 M KOH solution. Cyclic voltammetry (CV) tests were performed in the potential range of 0-0.55 V at scan rates of 1-20 mV s −1 . Galvanostatic charge-discharge (GCD) measurements were conducted from 0 to 0.55 V at the current densities of 1-20 A g −1 . The Nyquist plots were collected via electrochemical impedance spectroscopy (EIS) at the open circuit potential with frequency ranging from 100 kHz to 1 Hz. The specific capacitances of the electrodes were calculated according to their GCD profiles using the following formula.
where i is the constant current (A) in the charging and discharging process, ∆t is the discharge time (s), ∆V is the potential window (V), and m is the mass of active material on the electrode (g).

Conclusions
In this work, a novel 3D flower-like Ce(COOH) 3 electrode material with ultra-high cycling stability was designed by taking advantage of the Ce 3+ and -COOH groups according to the HSAB principle and fabricated by a one-pot microwave-assisted method. The obtained Ce(COOH) 3 sample is well crystalline and delivers a high specific capacitance of 140 F g −1 at 1 A g −1 , which is one of the highest capacitance values reported for Ceorganic compounds. In addition, the Ce(COOH) 3 sample demonstrates high rate capability that retains 60% of its initial capacitance when the current density is magnified 20 times. Prominently, the Ce(COOH) 3 exhibits ultra-high cycling stability with capacitance retention of 107.9% after 60,000 cycles, which is also the highest among the reported Ce-organic compound electrodes and far better than most of the other reported metal-organic compounds. The excellent electrochemical performance of Ce(COOH) 3 can be attributed to its intrinsic composition and structural characteristics, including the rapid redox capability of Ce 3+ /Ce 4+ , the strong coordination bond, the high crystallinity, and, most importantly, the unique 3D flower-like morphology. These results indicate that Ce(COOH) 3 has significant potential for supercapacitor applications and the facile and scalable one-pot microwaveassisted method can be explored to produce electrode materials with outstanding cycling stability, excellent electrochemical performance, and high efficiency.