Mesoporous carbon, vanadium pentoxide, and prussian blue for energy storage: A preliminary study

 Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
 MESOPOROUS CARBON, VANADIUM PENTOXIDE, 
 AND PRUSSIAN BLUE FOR ENERGY STORAGE: 
 A PRELIMINARY STUDY 
 Tran Van Ket, Nguyen Van Tai, Vu Thao Trang, Tran Viet Thu* 
 Le Quy Don Technical University, Hanoi, Vietnam 
 Abstract 
 In this work, we present a preliminary study on the synthesis, characterization and 
 electrochemical properties of three different nanoscale materials recently synthesized in our 
 lab: mesoporous carbon (MPC), vanadium pentoxide (VPO), and Prussian Blue (PB). 
 These materials were characterized using X-ray diffraction, scanning electron microscopy, 
 cyclic voltammetry, and galvanostatic charge-discharge techniques. The electrochemical 
 measurements revealed different behaviors for energy storage in these materials. The 
 calculated values of specific capacitance for MPC, VPO, and PB at 10 mV s 1 was 149.1, 
 71.9, and 448.6 F g 1, respectively, which are promising for electrochemical energy storage. 
 Keywords: Mesoporous carbon; vanadium pentoxide; Prussian Blue; electrochemical 
energy storage. 
1. Introduction 
 Energy storage is the vital component for constructing sustainable energy systems [1]. 
Renewable energy produced from wind turbines and photovoltaics can produce energy in 
a sustainable manner; yet their intermittent nature still inhibits them from becoming a 
principal energy carrier. To offset this problem, energy storage technologies are being 
developed to store the generated intermittent energy and make it accessible upon demand. 
Along with the energy grid applications, the transportation systems and portable 
electronic devices also set additional requirements for power sources, such as higher 
energy density, improved durability, and lower cost [2]. Currently, the dominating power 
source remains the battery, mainly the lithium-ion batteries (LiB) [3]. LiB technology 
offers numerous advantages, including high energy density and long service life. 
However, the slow charge/discharge process in LiBs results in low power density 
(ca. 10-1000 W kg-1) and short lifecycle (ca. 400-1200 cycles). Moreover, the high cost, 
limited resource, and reactive nature of lithium element have restricted their range 
of applications. 
 These disadvantages could be overcome with a different storage mechanism in 
power devices called supercapacitors [4, 5]. They are featured by high power density 
* Email: thutv@mta.edu.vn 
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Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
(typically 10 to 100 times greater than that of batteries), excellent durability, and high 
Coulombic efficiency [5]. However, supercapacitors also suffer from several 
shortcomings such as low energy density and high cost associated with the increased 
difficulty in producing high-performance materials [4, 5]. In order to bridge the gap 
between the potential and the commercial practicality of supercapacitor, recent research 
efforts have been focused on creating and improving the performance metrics of 
supercapacitor electrode materials [4]. 
 The charging mechanism, electrochemical performance, and practical applications 
of a supercapacitor are mainly determined by its electrode materials. According to their 
energy storage mechanism, supercapacitors are classified into three types: (1) 
Electrochemica double-layer capacitors (EDLCs), in which electrolyte ions are 
electrostatically absorbed/desorbed from the electrodes with large specific surface area 
such as mesoporous carbon [6]. (2) Pseudocapacitors, in which electrical charges are 
transferred through chemical adsorption/desorption or redox reactions with linear 
potential-discharge time response. Some metal oxides such as ruthenium dioxide, 
manganese dioxide, and vanadium pentoxide belong to this category [7, 8]. (3) Battery-
type supercapacitors, in which the charge storage is also faradaic in nature, but their 
charge/discharge curves display an apparent plateau feature [9]. Typical examples of these 
materials are transition metal oxides, sulfides, and Prussian Blue [10, 11]. Among these 
materials, mesoporous carbon, vanadium pentoxide, and Prussian Blue are attracting 
numerous research interests in recent years due to their promising performance [6, 10, 12]. 
However, these materials have been rarely studied by Vietnamese researchers. 
 In this work, we report on the synthesis, material characterization, and electrochemical 
properties of three different nanoscale materials recently synthesized in our laboratory: 
mesoporous carbon (MPC), vanadium pentoxide (VPO), and Prussian Blue (PB). 
Interestingly, we found that these materials exhibit different behaviors for energy storage 
corresponding to EDLC (for MPC), pseudocapacitor (for VPO), and battery-type 
supercapacitor (for PB). These materials were facilely synthesized using commercial 
chemicals and convenient laboratory facilities. 
2. Experimental 
2.1. Preparation of MSC, VPO, and PB samples 
 MSC was prepared by a two-step process. In the first step, indole (2.34 g) was 
reacted with excessive formaldehyde in a water-methanol mixture in the presence of 
 o
ZnCl2 for 24 hrs, then the dried at 105 C. In the second step, the obtained gel was 
calcinated at 450oC for 1.5 hrs in nitrogen flow, and then the temperature was increased to 
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 Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
850oC and kept for 2 hrs. The obtained solid was washed with 1M HCl solution and 
deionized water until the filtrate become neutral, and dried in vacuum for 12 hrs. 
 VPO was prepared by the hydrothermal treatment of a solution containing 
ammonium vanadate (2.34 g), citric acid (2.28 g), and 105 mL deionized water at 160oC 
for 20 hrs. The resulting solid was dried at 60oC for 10 hrs, then calcined at 400oC for 
2 hrs at a ramping rate of 2oC min 1. 
 PB was prepared by the simple co-precipitation reaction between iron(III) sulfate 
(0.16 g) and potassium hexacyanoferrate(II) (0.76 g) in 40 mL water. After dissolving and 
mixing the above two reagents in water, the mixture was magnetically stirred for 3 mins 
and then aged at room temperature for 18 hrs. The resulting mixture was filtered and 
washed with water, then dried at 60oC for 6 hrs to obtain PB sample. 
2.2. Material characterization 
 X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 Advance 
powder diffractometer with Cu Kα X-ray radiation ( = 0.15418 nm). Scanning electron 
microscopy (SEM) images of the samples were observed using a Hitachi SU70 
microscope. Both the diffractometer and the microscope are equipped at Institute of 
Chemistry, Vietnam Academy of Science and Technology. 
2.3. Electrochemical measurements 
 The electrodes were prepared by mixing active materials (MPC, VPO, or PB), 
conductive carbon, and binder (polyvinylidene fluoride) with a weight ratio of 7:2:1 in 
N-methyl-2-pyrrolidone. The resulting slurry was pasted on a Nickel foam and dried at 
100ºC for 24 hrs. The electrochemical measurements were carried out using Ag/AgCl 
reference electrode, Pt wire counter electrode, and 0.5M K2SO4 electrolytes on a 
Metrohm Autolab (PGSTAT 302N), equipped at Le Quy Don Technical University. The 
specific capacitance (Cs) was calculated from cyclic voltammogram (CV) curves using 
Cs = (∫Idt)/(m∆V), where m is the weight of active material (g), I is the current (A), and 
∆V is the potential window (V). The Cs was also calculated from galvanostatic charge-
discharge (GCD) curves at different current densities using Cs = (I∆t)/(m∆V), where I is 
the discharge current (A), and ∆t is the discharge time (sec) [13-15]. 
3. Results and Discussion 
3.1. Crystal structure 
 The XRD patterns of MPC (Fig. 1, bottom) could be well assigned to those of 
graphite (C, PDF #01-075-1621) with (002) and (100) lattice planes [14]. The XRD 
patterns of as-synthesized VPO samples (Fig. 1, middle) can be indexed to vanadium 
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Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
pentoxide (V2O5, PDF #01-076-1803) having orthorhombic crystal structure (space group 
Pmn31) with a = 11.48 Å and b = 4.36 Å. Its major diffraction peaks could be assigned to 
the (200), (010), (101), (310), (301), (020), (002) and (012) lattice planes [8]. The XRD 
patterns of PB (Fig. 1, top) show that the as-prepared PB belongs to cubic system (space 
group Fm-3m) with a = 10.13 Å (PDF #01-073-0687) and major diffraction peaks arising 
from (200), (220), (400), and (420) lattice planes of tetrairon tris(hexacyanoferrate) 
(Fe4[(Fe(CN)4]3) [11]. The absence of other peaks indicates the synthesized MPC, VPO, 
and PB were phase-pure. 
 Fig. 1. XRD patterns of MPC, VPO, and PB. 
3.2. Morphology 
 As shown in Fig. 2a, the MPC sample displays a network of carbon particles 
interconnected through open channels and mesopores of various sizes. This morphology 
is desired for the convenient exchange of electrolyte ions. The VPO sample (Fig. 2b) 
composes of one-dimensional nanorods with length of a few micrometers, which might be 
beneficial for the electronic conductivity of the electrode. The PB sample (Fig. 2c) 
exhibits the formation of aggregates, possibly formed during the ripening step. Each 
aggregate contains a large amount of primary particles with nanometer size. 
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 Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
 Fig. 2. SEM images of MPC (a), VPO (b), and PB (c). 
3.3. Electrochemical properties 
 Fig. 3 shows the CV curves for all samples recorded at a scan rate of 10 mV s 1. 
Interestingly, these samples displayed three different kinds of behavior which are typical 
for three types of supercapacitor electrode. The CV curve of PMC is closed to rectangular 
without any redox peaks, representing EDLC type within a limited potential window (-0.3 
to 0.3 V). The CV curve of VPO indicate its pseudocapacitor behavior. In the CV curve 
of PB, a pair of well-defined redox peaks is clearly observed at ca. 0.24 and 0.02 V, 
which is attributed to the proposed redox reactions involved in the intercalation of 
 III II
electrolyte (K2SO4) with PB (Fe 4[Fe (CN)6]3) as follows: 
 III II + III III
 Fe 4[Fe (CN)6]3 + xK + 3e KxFe 4[Fe (CN)6]3 
where K is the intercalated cation and K+ is its corresponded cation in the solution. Also, 
PB can undergo a redox reaction between Fe3+ and Fe2+ in its structure. This suggests the 
PB materials display battery-type behavior due to Faradaic reactions [10, 16]. The CV 
curve for PB shows a much larger internal area than those of MPC and VPO, indicating 
 1
that PB exhibits the largest Cs. The estimated Cs for MPC, VPO, and PB at 10 mV s was 
 1
149.1, 71.9, and 448.6 F g , respectively. The relatively high Cs for PB could be 
attributed to the high redox activity of the Fe(III)/Fe(II) centers in the material. 
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Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
 2.8 CVs at 10 mV s-1
 MPC
 2.1 VPO
 PB
 1.4
 )
 -1
 0.7
 0.0
 Current(A g -0.7
 -1.4
 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
 Potential (V vs. Ag/AgCl)
 Fig. 3. CV curves of MPC, VPO, and PB at a scan rate of 10 mV s 1. 
 Fig. 4 shows the GCD curves of MPC, VPO, and PB at a current density of 
1 A g 1. Their corresponding potential windows were 0.55 V (-0.3 to 0.25V), 0.9 V (0 to 
0.9 V), and 1.0 V (-0.35 to 0.65V). The discharging part of the GCD of MPC and VOP 
is nearly linear, indicating an EDLC or pseudo-capacitor behavior. The specific 
capacitance calculated from GCD result of MPC and VPO was 69.4 and 54.6 F g 1, 
respectively. While the Cs of MPC is typical for that of mesoporous carbon, the Cs of 
VPO is acceptable as compared to that of V2O5 nanobelts [8]. On the contrast, the 
discharging part of the GCD of PB clearly displayed a plateau centered at ca. 0.45 V 
 1
(vs. Ag/AgCl), indicate a battery-type behavior. The calculated Cs of PB was 70.2 F g 
at 1 A g 1, which needs to be improved for practical applications. 
 GCDs at 1 A g-1
 1.0
 MPC
 VPO
 0.8 PB
 0.6
 0.4
 Potential windowsPotential (V) 0.2
 0.0
 0 20 40 60 80 100 120 140
 Time (sec)
 Fig. 4. GCD curves of MPC, VPO, and PB at a current density of 1 A g 1. 
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 Journal of Science and Technique - N.211 (12-2020) - Le Quy Don Technical University 
4. Conclusions 
 This work provides convenient chemical routes to synthesize mesoporous carbon, 
vanadium pentoxide, and Prussian Blue for supercapacitor electrode. The as-synthesized 
materials exhibited well-defined crystal structure and morphology. They showed EDLC 
(for mesoporous carbon), pseudocapacitor (for vanadium pentoxide), and battery-type 
(for Prussian Blue) behavior for electrochemical energy storage. Although their Cs 
values were still lower than those of reported materials, there are plenty of rooms for 
improvements. Further efforts are being made including synthesis optimization, full cell 
assembly and testings. 
Acknowledgement 
 The authors acknowledge the National Foundation for Science and Technology 
Development (NAFOSTED) for financial support (Grant No. 103.02-2020.31). 
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 prospects. Advanced Science, 4(7), p. 1600539. 
10. Zhao, F., et al. (2014). Cobalt hexacyanoferrate nanoparticles as a high-rate and 
 ultra-stable supercapacitor electrode material. ACS Applied Materials Interfaces, 6(14), 
 pp. 11007-11012. 
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11. Sun, H., et al. (2014). Fe4[Fe(CN)6]3: A cathode material for sodium-ion batteries. RSC 
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12. Wu, C. and Y. Xie (2010). Promising vanadium oxide and hydroxide nanostructures: From 
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13. Mai, L.-Q., et al. (2013). Synergistic interaction between redox-active electrolyte and 
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15. Van Nguyen, T., et al. (2020). Facile synthesis of Mn-doped NiCo2O4 nanoparticles with 
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16. Yi, F., et al. (2020). Rational design of multiple prussian-blue analogues/NF composites for 
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 CACBON XỐP, VANADI(V) OXIT, VÀ PRUSSIAN BLUE TRONG 
 LƯU TRỮ NĂNG LƯỢNG: MỘT VÀI KẾT QUẢ BAN ĐẦU 
 Tóm tắt: Trong bài báo này, chúng tôi trình bày một số kết quả ban đầu về tổng hợp, xác 
định đặc trưng vật liệu và các tính chất điện hóa của ba loại vật liệu cấu trúc nano được chúng 
tôi tổng hợp gần đây: cacbon xốp (MPC), vanadi(V) oxit (VPO), và Prussian Blue (PB). Những 
vật liệu này được xác định đặc trưng bằng các kỹ thuật nhiễu xạ tia X, hiển vi điện tử quét, quét 
thế vòng tuần hoàn, và phóng nạp dòng không đổi. Các phép đo điện hóa đã cho thấy cơ chế 
lưu trữ điện tích khác nhau trong ba loại vật liệu này. Giá trị điện dung riêng tính được tại tốc 
độ quét 10 mV s 1 của MPC, VPO và PB lần lượt là 149,1; 71,9 và 448,6 F g 1. Các giá trị này 
là tương đối khả quan cho lưu trữ năng lượng điện hóa. 
 Từ khóa: Cacbon xốp; vanadi(V) oxit; Prussian Blue; lưu trữ năng lượng điện hóa. 
 Received: 14/7/2020; Revised: 23/11/2020; Accepted for publication: 26/11/2020 
  
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