Two-Dimensional NH4V3O8 Nanoflakes as Efficient Energy Conversion and Storage Materials for the Hydrogen Evolution Reaction and Supercapacitors.

Herein, for the first time, we present two-dimensional (2D) NH4V3O8 nanoflakes as an excellent material for both energy conversion of the hydrogen evolution reaction and storage of supercapacitors by a simple and fast two-step synthesis, which exhibit a completely sheet-like morphology, high crystallinity, good specific surface area, and also stability, as determined by thermogravimetric analysis. The 2D-NH4V3O8 flakes show an acceptable hydrogen evolution performance in 0.5 M H2SO4 on a glassy carbon electrode (GCE) coated with 2D-NH4V3O8, which results in a low overpotential of 314 mV at -10 mA cm-2 with an excellent Tafel slope as low as 90 mV dec-1. So far, with the main focus on energy storage, 2D-NH4V3O8 nanoflakes were found to be ideal for supercapacitor electrodes. The NH4V3O8 working electrode in 1 M Na2SO4 shows an excellent electrochemical capability of 274 F g-1 at 0.5 A g-1 for a maximum energy density of 38 W h kg-1 at a power density as high as 250 W kg-1. Moreover, the crystal structure of 2D-NH4V3O8 is demonstrated by density functional theory (DFT) computational simulation using three functionals, GGA, GGA + U, and HSE06. The simple preparation, low cost, and abundance of the NH4V3O8 material provide a promising candidate for not only energy conversion but also energy-storage applications.

Introduction

With the rapidly growing renewable energy sector, the major problems of limited fossil fuel and global warming have led to an urgent need for electrochemical energy storage and conversion.[1,2] Currently, more and more energy-storage devices for supercapacitors and energy-conversion systems for the hydrogen evolution reaction have been reported.

Among various types of energy-storage devices, supercapacitors are prime candidates because of their greater safety, higher energy density, longer stability, and stronger durability in comparison with the traditional electrolytic capacitors.[3] According to the energy-storage mechanism, supercapacitors can be classified into three main types: (i) electrical double-layer capacitors using ion adsorption–desorption at the electrode/electrolyte interface; (ii) pseudocapacitors using redox reactions (oxidation/reduction reaction) at the electrode/electrolyte interface; and (iii) hybrid supercapacitors, which combine the features of electrical double-layer capacitors and pseudocapacitors.[4−7] Pseudocapacitors, also called faradic capacitors, store energy by faradic reactions at the electrode/electrolyte interface.[8] Based on the working mechanism of pseudocapacitors, various types of metal oxides, transition metal oxides, and metal–organic frameworks can be applied as electrode layers, which enhance the energy density. Currently, two-dimensional metal oxides have been studied for pseudocapacitors due to their large specific surface area, fast ionic diffusion, and high electrical conductivity.[9,10] The vanadium oxide family, with several oxidation states, is one of the promising candidates due to its cost-effectiveness, ease of synthesis, high conductivity, and durability.[11,12] Therefore, in this report, two-dimensional NH4V3O8 materials with nanoflakes in the meter scale with a high specific surface area have been studied as electrode layers in supercapacitors.

Along with the rapid development of renewable energy sources, the field of energy conversion has attracted much attention, strongly focusing on producing green fuels. Among various types of energy conversion, hydrogen energy from the water-splitting process via electrochemical reaction is considered to be a clean and economical energy with a higher energy density in comparison with chemical fuels.[13−15] Currently, research is strongly focused on hydrogen energy from the water-splitting reaction due to its stability, lack of CO emission, and high efficiency.[13] Typically, during the water-splitting reaction in an acidic medium (H2O → H2 + 1/2O2), two reactions occur under the applied power: H2 evolution reaction at the cathode (HER: 2H+ + 2e– → H2) and O2 evolution reaction at the anode (OER: 4OH– ↔ 2H2O + O2 + 4e–).[16,17] At present, noble-metal materials (Pt, Rh, Ir, etc.) are being used as catalysts to produce H2, which contribute up to 4% of the supply worldwide, but they are expensive, unstable, and less abundant on earth.[18,19] Therefore, the design of low-cost, earth-abundant, and efficient materials for stable electrochemical applications including energy conversion and storage is an issue of urgent concern in the field of renewable energies. Currently, two-dimensional (2D) materials of metals and composites serve as core materials due to (i) their large specific surface area with nanoflake layers, (ii) tunable electronic structure, and (iii) mechanical strength and high durability, which enhance the active sites for improved catalytic performances.[20,21] In addition, two-dimensional metal oxides (2D-MOs) are promising candidates due to their unique properties such as high conductivity, high mechanical strength, catalytic capability, and good electrochemical behavior.[22] In comparison with bulk metal oxide materials, 2D metal oxide materials have a higher specific surface area and good electrochemical behavior due to their better exposed active sites.[23] Moreover, the faradic reaction of 2D MO electrode materials at the electrode/electrolyte interface can enhance the electrochemical performance.[24] Among them, 2D-NH4V3O8 nanoflakes are excellent candidates for energy-storage and conversion applications because of their low cost, fast synthesis, higher specific surface area compared with other MO materials, NH4 molecular connections with VO layers to prevent the destruction of the structure, and hydrogen bonding between NH4 and VO, which improves their stability.[25−27] Accordingly, in this work, 2D-NH4V3O8 nanoflakes are introduced as cathode materials to study the hydrogen evolution reaction.

In the process of assembling the electrochemical capacitors (or supercapacitors) from 2D MO materials, the advantages of the 2D materials can be preserved well, which enhance the electron-transport kinetics via the uniformity of the electrode/electrolyte interface layer.[28] Moreover, the large surface area of 2D MO electrodes provides an excellent environment of ion pools for fast ion diffusion during the charge–discharge process.

Herein, we develop a cost-effective and simple process to prepare NH4V3O8 nanoflakes using a two-step synthesis method: (1) simple hydrothermal reaction and (2) calcination. The final NH4V3O8 product shows a good 2D structure that serves as a single material with two simultaneous applications: electrocatalysts for the hydrogen evolution reaction and electrochemical capacitors for supercapacitors. The NH4V3O8 nanoflakes are considered as good HER candidates with an exceptional overpotential value of 310 mV at 10 mA cm–2 with a good Tafel slope of 90 mV decade–1. The supercapacitor electrode slurry made from NH4V3O8 nanoflakes displays the highest specific capacitance of 274 F g–1 at 0.5 A g–1, with an excellent energy density of 38 W h kg–1. In summary, the NH4V3O8 material shows high durability with the corresponding electrochemical stability to maintain the morphology.

Experimental Section

Materials

Sodium sulfate (Na2SO4), standard 0.5 M H2SO4 (1 N) solution as the aqueous electrolyte for the HER study, acid oxalic (HO2C–CO2H), 1-methyl-2-pyrrolidone (NMP), and dimethylformamide [HCON(CH3)2, DMF] were obtained from Sigma-Aldrich. Ammonium metavanadate (NH4V3O8), ammonium hydroxide (NH4OH), and acetic acid (CH3COOH) were obtained from Alfa Aesar. The carbon paper substrate (A4 size, 0.15 mm thickness) was obtained from HOMYTECH CO., LTD., Taiwan. Ultrapure water was prepared by the Millipore Milli-Q UF system at room temperature with a resistivity of 18.2 MΩ cm.

Preparation of 2D NH4V3O8 Nanoflakes and Material Characterization

2D-NH4V3O8 was synthesized by a simple hydrothermal method. Typically, 1 g of NH4VO3 was dissolved in 20 mL of DI water to obtain a dark-yellow solution. Then, pure acid acetic was added slowly to the NH4VO3 solution with constant stirring until a pH of 6 to obtain an orange solution. This orange solution was transferred into a 100 mL Teflon-lined hydrothermal stainless steel autoclave and heated at 160 °C for 48 h before cooling down naturally to room temperature (27 °C). Finally, the 2D-NH4V3O8 target material was collected, washed with DI water, and dried at 80 °C for 12 h.

Herein, the structure and morphology of the 2D-NH4V3O8 nanoflakes were studied using various equipment, including a scanning electron microscope (SEM, SU800, Hitachi), transmission electron microscope (TEM, JEM-2100F, Joel), X-ray diffractometer (XRD, D2 Bruker) with a Cu Kα tube, Raman spectrophotometer (Jobin Yvon-Horiba, with 520 nm excitation wavelength of an Ar laser), X-ray photoelectron spectrometer (nano-Auger/ESCA electron spectroscopy vs XPS), and Fourier transform infrared (FTIR) spectrometer (PerkinElmer), and by Brunauer–Emmett–Teller (BET) analysis using Micromeritics ASAP 2020.

Electrochemical Characterizations

The catalyst ink was prepared by adding 3 mg of 2D-NH4V3O8 nanoflakes to 1 mL of DMF with constant stirring for 2 days to obtain a homogeneous solution for electrochemical studies of HER. For comparison, a commercial V2O5 product was also prepared by the same process as above.

Furthermore, the 2D-NH4V3O8 slurry for electrochemical supercapacitor studies was prepared by mixing together 8 mg of NH4V3O8 (80 wt %), 1 mg of CNT (10 wt %), 1 mg of PVDF (1 wt %), and 0.2 mL of NMP with constant stirring for 2 days to obtain a uniform active slurry. Then, this active slurry was stored for further electrochemical supercapacitor measurements.

To prepare the working electrode for the electrochemical catalyst, 2 μL of the NH4V3O8 active slurry was coated on the GCE (3 mm in diameter; 0.214 mg cm–2) and dried at 60 °C for 1 day. Finally, 2 μL of Nafion (5% in EtOH) was covered on the NH4V3O8-modified glassy carbon electrode and dried naturally. Here, a three-electrode system was studied using a carbon rod counter electrode (diameter 0.5 cm, length 20 cm), an Ag/AgCl (3 M NaCl) reference electrode, and a 0.5 M H2SO4 aqueous electrolyte. Linear sweep voltammetry (LSV) was performed in the range of 0–0.8 V vs RHE at a scan rate of 5 mV s–1. The electrochemical impedance study (EIS) data were analyzed from 100 mHz to 100 kHz at an amplitude of 5 mV. The stability test was performed following i–t curves at an overpotential point over a period of 48 h. The HER mechanism in an acidic electrolyte can be illustrated by the following equations[16,17]The Tafel slope can be calculated using the Tafel equation:where η is the overpotential, b is the Tafel slope, j is the current density, and a is a constant.

Volmer reaction of the discharge step:

Tafel reaction of the recombination–desorption step:

Heyrovsky reaction of the desorption step:

For electrochemical capacitor studies, the working electrodes were prepared by coating 20 μL of the 2D-NH4V3O8 active slurry on a carbon paper substrate (1 × 1 cm2) and drying at 80 °C for 2 days. Here, one Pt plate (1 × 2 cm2) served as the counter electrode and Ag/AgCl as the reference electrode. This three-electrode system was measured in a 1 M Na2SO4 aqueous electrolyte by CV, EIS, and galvanostatic charge–discharge (GCD) measurements. The specific capacitance was calculated by the following equation:[29,30]From the GCD curves, the energy density (E, W h kg–1) and power density (P, W kg–1) could be calculated by the following equations, respectively:[29,30]where I (A) is the discharge current, Δt is the discharge time, ΔV is the potential voltage, and mac is the weight of the active materials (including the binder).

Moreover, the specific capacitance can be calculated from CV curves by the following equation:[27]where Ei and Ef are the initial and final voltages, respectively, v is the scan rate (V s–1), m is the weight of the active material, and (Ef – Ei) is the width of the potential window.

Results and Discussion

Computational Results

Computational Details

Density functional theory (DFT) computation was performed using the Vienna ab-initio Simulation Package (VASP) code.[31−35] Electron–ion interactions and electron-exchange correlations were described by the projector augmented wave (PAW) potentials and the general gradient approximation (GGA) applying the Perdew–Burke–Ernzerhof (PBE) functional, respectively.[36,37] For comparison purposes, GGA, GGA + U, and HSE06 methods were used to decrease the self-interaction error, which selectively incorporates an energy correction to localized electron states of d or f orbitals whose self-interaction is quite large. The optimization of all atomic positions and lattice constants used the conjugate gradient scheme to relax the geometrical structure until the components of the forces on each atom were on the order of 10–4 eV Å–1. The energy cutoff was set to 400 eV to correct the accuracy of the simulated data. We used the bulk unit cell of NH4V3O8 containing 32 (N/H/V/O ∼2/8/6/16) atoms, which corresponded to two units with the NH4V3O8formula. A 2 × 2 × 2 k-point mesh for a primitive NH4V3O8 unit cell was set. Geometrical structures and electronic properties of NH4V3O8 are analyzed by means of DFT calculations.

Crystal Structure

The optimized lattice constants of NH4V3O8 obtained from our DFT calculations (Table ) are in agreement with the experimental values.[38,39] The lattice parameters calculated using the GGA + U functional for NH4V3O8 are a = 5.09855 Å, b = 8.58216 Å, c = 7.85297 Å, and β = 96.4311°, which were confirmed with respect to the reference values from JCPDS card no. 088-1473.[38] According to the results presented in Table , the GGA and GGA + U functionals provide a more accurate description of the lattice parameters than the HSE06 functional.

Table 1

Lattice Constants

NH4V3O8	a (Å)	b (Å)	c (Å)	α (deg)	β (deg)	γ (deg)
GGA	4.92066	8.28296	7.62480	90.000	94.8493	90.000
GGA + U	5.09855	8.58216	7.85297	90.000	96.4311	90.000
HSE06	4.87999	8.10927	7.48049	90.000	93.4884	90.000
experiment[8]	4.993(7)	8.423(1)	7.849(1)	90.000	96.426(3)	90.000
experiment[9]	4.975(8)	8.413(14)	7.855(6)	90.000	96.39(6)	90.000

NH4V3O8 crystallizes in the monoclinic P21/m space group. As illustrated in Figure , NH4V3O8 has a layered structure including two basic structural units of the VO6 octahedron and VO5 square pyramids, respectively. Here, NH4+ cations are located on the sites between the vanadium oxide layers. Moreover, each pair of VO5 square pyramids is connected by an edge to form a double V2O8 group, and the V2O8 groups are bridged by sharing corners to create a twisted zigzag chain. It is clearly seen that one VO6 octahedron is associated with two V2O8 sets at two edges and with two alternating V2O8 sets at two corners. These twisting and turning chains are detained together over the VO6 octahedron by sharing angles and edges in turn to form a layered structure parallel to the (001) plane. Notably, N–H–O hydrogen bonds are present between the vanadium oxide layer and the NH4+ cations. In the first V5+ site, V5+ is bonded in the five-coordinate geometry to five oxygen atoms, and the V–O bond lengths range from 1.63 to 1.93 Å. In the second V5+ site, V5+ is bonded in a six-coordinate geometry to six oxygen atoms, and the V–O bond lengths range from 1.65 to 2.35 Å. There are five inequivalent oxygen sites. In the first oxygen site, oxygen is bonded in a single-bond geometry to one V5+ atom. In the second oxygen site, it is bonded in a two-coordinate geometry to three V5+ atoms. In the third oxygen site, it is bonded in a distorted trigonal non-coplanar geometry to three V5+ atoms. In the fourth oxygen site, it is bonded in a single-bond geometry to one V5+ atom. In the fifth oxygen site, it is bonded in a bent 120-degree geometry to two V5+ atoms.

Figure 1

Crystal structure of NH4V3O8. The twisted zigzag layers of VO5 and VO6 are denoted by yellow and orange colors, respectively, whereas the NH4 groups are in gray and located in the interlayer space.

Electronic Structures

The electronic band structure and the density of states (DOS) of the NH4V3O8 crystal are calculated using three functionals, GGA, GGA+U (U = 3.5 eV), and HSE06. NH4V3O8 is a magnetic semiconductor with a band gap of 2.1 eV.[40,41] The electronic structure of V3H4NO8 calculated using the GGA functional exhibits an indirect band gap of 1.942 eV between the Γ point of the valence band and the A point of the conduction band; the direct band gap at the Γ point is considerably larger by ∼0.098 eV. Using the GGA + U (U = 3.5 eV) functional, the indirect band gap is 2.166 eV and the direct band gap is 2.318 eV. Meanwhile, using the HSE06 functional, the indirect band gap is 3.412 eV and the direct band gap is 3.618 eV. These results showed that the GGA and GGA + U functionals provide more accurate bandgap energies of NH4V3O8 compared with the HSE06 functional. The calculated band structures and DOS of V3H4NO8 are visualized in Figure a using the GGA functional and in Figure b using the GGA + U (U = 3.5 eV) functional. The lowest conduction band has the foremost V 3d. The highest occupied states are formed by the band of well-localized V 3d states, whereas the wide valence band in the range of −4 eV to the Fermi level is composed predominantly of O 2p states. In general, the DOS apparently resembles the formation of covalent V–O bonds within the anionic V3O8 structure. The relative position governing V 3d–O 2p overlaps in the valence band marks a certain flexibility of the V3O8 network for association in redox reactions and a low ability of NH4+ cations for reduction.

Figure 2

Electronic band structure and density of states of V3H4NO8 using (a) GGA and (b) GGA + U (U = 3.5 eV).

NH4V3O8 Powder Characterizations

Figure a shows the results of Raman spectroscopy of NH4V3O8 with strong clear 2θ peaks at 240, 372, 427, 673, 813, 963, and 994 cm–1, which agree with the literature data and indicate the existence of two states of VO5 and VO6 (Figure ).[42−44] The two peaks around 963 correspond to V=O stretching modes of the distorted octahedron and the peak at 994 cm–1 belongs to distorted pyramids.[43,44] The strong peak at 813 cm–1 corresponds to the V–O stretching bond.[44] The series of clear peaks at 514, 556, and 673 cm–1 are assigned to V–O–V stretching bonds.[43,44] Other peaks at frequencies below 400 cm–1 belong to V–O and N–H bending modes.[45] The phase structure of NH4V3O8 samples was studied by X-ray diffraction (XRD), and the results are depicted in Figure b with various clear peaks corresponding to the (001), (011), (100), (002), (012), (020), (022), (013), (023), and (032) planes. These diffraction peaks reveal the monoclinic form of NH4V3O8 according to JCPDS card no. 088-1473, which is in good agreement with the simulated results and previous literature.[44,46,47] No impurities were detected, indicating the appropriate method of synthesis and highly crystalline form of the NH4V3O8 sample. The functional groups of NH4V3O8 were identified by FTIR analysis and are illustrated in Figure c. There are various strong peaks at 3224, 1407, 1006, 968, 734, and 530 cm–1, which correspond to the different excitations of the NH4V3O8 sample.[44] The absorption peak at 530 cm–1 belongs to the stretching vibration of oxygen coordinates with vanadium atoms in VO5 square pyramids and the VO6 octahedron. The band at 734 cm–1 belongs to a symmetrical stretching vibration of the V–O–V bond.[48,49] The bands at 968 and 1006 cm–1 are assigned to the stretching vibration of V=O.[48,49] The band at 1407 cm–1 is attributed to NH4+ groups.[44,48,49] The clear strong peak at 3224 cm–1 shows the existence of hydrogen bonds.[44] The specific surface area and porosity were determined from the nitrogen adsorption–desorption isotherm analysis of NH4V3O8 and is depicted in Figure d. The NH4V3O8 powder exhibits a good value of 4.7 m2 g–1, which agrees with the values of 2D metal oxide materials. The increasing nitrogen adsorption at a high relative pressure illustrates the appearance of macropores in the sample, whereas the narrow open hysteresis loop from 0.9 to 0.2 of P/P0 shows the presence of mesopores.[50] As presented in the inset in Figure d, the pore size distribution shows an adsorption average pore width (4V/A by BET) of ∼22 nm. The BJH adsorption and BJH desorption average pore width (4V/A) are 29.1 and 25.3 nm, respectively.

Figure 3

(a) XRD pattern, (b) Raman spectra, (c) FTIR spectra, and (d) adsorption/desorption isotherms of N2 with the pore size distribution.

Figure shows the surface morphology of the NH4V3O8 sample by SEM and TEM measurements. It can be seen that NH4V3O8 exhibits a 2D structure of thin nanoflakes, which are composed of many sheet layers. Figures a and S1a clearly illustrate the lamellar structures with a random arrangement, which can form porous electrodes with a high active surface and high specific surface area. Moreover, the high-resolution HR-TEM images of the NH4V3O8 sample in Figures b and S1b show the perfect interlayer distance with a lattice spacing of 0.185 nm, which belongs to the NH4V3O8 nanoflakes and match well with the XRD and Raman results. Here, NH4V3O8 nanoflakes were very thin and displayed a large size and high uniformity, with a width of up to 2 μm.

Figure 4

(a) SEM, (b) HRTEM, and (c–f) TEM elemental-mapping images of NH4V3O8 nanoflakes.

Further, the successful NH4V3O8 structure is proven by TEM mapping, where N, O, and V elements are homogeneously overlapping throughout the entire sample. Note that the exemplary results obtained from the selected area TEM measurement in Figures c–f and S1c–f confirm precisely the elements of NH4V3O8 nanoflakes. In this study, to understand the growth mechanism of 2D NH4V3O8 nanoflakes, the basic reaction for the preparation of NH4V3O8 nanoflakes can be described by the following formula:[39,43]Figure shows the chemical composition and valence state of pure NH4V3O8 by the XPS analysis. Figure a illustrates the XPS survey spectra, which reveal the expected ratio of C, V, O, and N elements. The XPS peak of N 1s in Figure b shows only one centered peak at 401 eV. The V 2p peak shown in Figure c can be divided into three main peaks at around 516.7, 523.5, and 524.5 eV, which correspond to V 2p3/2, V 2p1/2 (+4), and V 2p1/2 (+5), respectively.[51,52] The O 1s spectrum in Figure d illustrates the existence of O2– at 529.6 eV and H2O at 530.6 eV.[51] In summary, it can be confirmed that NH4V3O8 with high purity can be successfully applied as the working electrode for further studies on energy storage and conversion.

Figure 5

XPS spectra of NH4V3O8 nanoflakes: (a) survey spectra, (b) N 1s, (c) V 2p, and (d) O 1s.

Hydrogen Evolution Reaction Electrocatalytic Activity of NH4V3O8

The electrocatalytic properties of NH4V3O8 were measured in standard 0.5 M H2SO4 with a three-electrode system, and the results are shown in Figure . Moreover, commercial V2O5 was also prepared for comparison in this study. As shown in Figure a, the polarization (linear sweep) curves of NH4V3O8 at the initial time and after 48 h show excellent durability, with HER onset overpotential η values of 314 and 329 mV at a current density of −10 mA cm–1, respectively. Accordingly, the HER performance of NH4V3O8 was investigated and compared with commercial V2O5 and Pt. The V2O5 displayed a value of 618 mV, whereas the commercial Pt/C has a low overpotential value of 87 mV at the same current density of −10 mA cm–1. These better results of NH4V3O8 nanoflakes in comparison with those of commercial V2O5 may be due to the good electron transfer during the electrocatalytic process, which provides more electrons for H+ reduction to H2 on the exposed edges of the NH4V3O8 nanoflakes. Another reason is the porous structure arising from the random arrangement of NH4V3O8 nanoflake layers, which provide highly exposed active sites. To investigate the HER of NH4V3O8 nanoflakes in detail, Tafel slopes were calculated following eq 4, and the results are shown in Figure b. Here, the Tafel slopes of NH4V3O8 from the LSV curves indicate that the charge-transfer kinetics at the initial time and after 48 h are 90 and 86 mV dec–1, which are much lower than those of commercial V2O5 (195 mV dec–1). These results are higher than those of Pt (86 mV dec–1) but provide promising data for further development of 2D NH4V3O8 materials. The stable Tafel slope of NH4V3O8 after 48 h of electrochemical measurement shows that this material is ideal for the HER study. Moreover, the electrode kinetics of the NH4V3O8 nanoflake material was also investigated using EIS measurements, as shown in Figure c. The equivalent series resistance (RS) and charge-transfer resistance (RCT) were 12 and 112 Ω (Figure S3 and Table S1), respectively, indicating the high electrical conductivity, highly conductive electrode–electrolyte interface, and good internal resistance of NH4V3O8 nanoflakes, which make them an excellent HER catalyst.[53] The i–t curves of NH4V3O8-modified glassy carbon electrodes at −10 mA cm–2 after 48 h of chronoamperometry measurement shown in Figure d suggest the good stability of NH4V3O8 for HER. Although the NH4V3O8-modified glassy carbon electrode shows a decrease in the slope of LSV plots and also the current density after a long-term test of 48 h due to decomposition at the electrode surface, it is still within the acceptable range for HER application. Moreover, the SEM images in Figure S2 of the NH4V3O8 material after electrochemical studies of HER demonstrate the stable 2D nanoflakes, which indicate the good structure after long electrochemical measurements.

Figure 6

Electrolytic properties of NH4V3O8: (a) LSV plots, (b) Tafel plots, (c) EIS plots, and (d) stability at 10 mA cm–2 over a period of 48 h.

Electrochemical Supercapacitor Studies

Based on the high specific surface area of transition metal oxide materials, 2D structures with highly exposed sites for HER, and good conductivity, NH4V3O8 is expected to be outstanding for electrochemical capacitor applications. The electrochemical characteristics and performance of the three-electrode system are presented in Figure . The cyclic voltammetry (CV) of the NH4V3O8 working electrode in the three-electrode system in a 1 M Na2SO4 aqueous electrolyte is shown in Figure a. It can be seen that the CV curves have strong redox and reduction peaks at low scan rates of 5 mV s–1, which indicate the redox reaction during scan progress. This could be attributed to the electrochemical capacitance behavior of the working electrode. From Figure b, a high specific capacitance of 314 F g–1 was obtained at a scan rate of 5 mV s–1. Despite the high scan rate of 100 mV s–1, an acceptable specific capacitance of 91.4 F g–1 is still obtained. The galvanostatic charge–discharge (GCD) curves at various current densities are shown in Figure c. At all current densities, the GCD curves display a nonsymmetrical triangular shape due to the redox reaction during the charge–discharge process. Here, the redox reaction is very strong at the electrode/electrolyte interface, which is attributed to the capacitance behavior. For further investigation of the contribution of the redox reaction, the Coulombic efficiency was investigated, following the Coulombic efficiency equation:[29]The ε values at various different current densities of 6, 5, 4, 3, 2, 1, and 0.5 A g–1 are 120, 107, 104, 108, 113, 124, and 170%, respectively.

Figure 7

Electrochemical properties of the three-electrode system for supercapacitor measurements: (a) CV curves, (b) specific capacitances at various scan rates, (c) GCD curves, (d) EIS plot, (e) specific capacitance at various current densities, and (f) energy density as a function of power densities.

To understand the electrical conductivity and resistance of the working electrode and aqueous electrolyte, electrochemical impedance spectroscopy (EIS) was carried out, and the results are presented in Figure d via the Nyquist plot. Here, a high-frequency intercept shows the solution resistance, with a nearly semicircular curve corresponding to the charge-transfer resistance.[54] The equivalent series resistance in this case shows a low value of 7.5 Ω, which indicates the good electrical conductivity of the working electrode and excellent ion diffusion. The equivalent series resistance is the sum of the working electrode resistance, the resistance of the electrolyte, the resistance of the electrode/electrolyte interface, and the internal resistance of the electrode material.[29,30] Further, the specific capacitance of the working electrode can be calculated from the GCD curves and is illustrated in Figure e. The specific capacitance at a high current density of 6 A g–1 is 72 F g–1. The maximum specific capacitance is 274 F g–1 at 0.5 A g–1. Further, the Ragone plot of the NH4V3O8 working electrode was calculated from the GCD curves and is illustrated in Figure f. At the highest power density of 3000 W kg–1, a good energy density of 10 W h kg–1 and a high maximum energy density of 38 W h kg–1 at a power density of 250 W kg–1 were obtained.

In this report, the NH4V3O8 nanoflakes showed excellent electrochemical properties, which served well in electrochemical capacitors and HER: (i) the uniformity and high purity of 2D NH4V3O8 nanoflakes yield high electrical conductivity, which serves well as the working electrode layer, allowing fast electron transport; (ii) 2D NH4V3O8 nanoflakes are vertically aligned on the surface, which enhances the active sites, which in turn enhance the HER performance; and (iii) the combination between the double layer and the pseudocapacitor of 2D NH4V3O8 nanoflakes during the electrochemical process improves the capacitor performance.

Conclusions

In conclusion, 2D-NH4V3O8 nanoflakes were successfully prepared by a simple and cost-effective method, which provided a high surface area of the 2D structure, good conductivity, and high purity. As a result, the NH4V3O8 nanoflakes displayed an outstanding catalytic performance via HER with a low overpotential of 314 mV vs RHE at 10 mA cm–2 and a good Tafel slope of 90 mV dec–1. Moreover, NH4V3O8 also displayed excellent stability after 48 h. Furthermore, 2D NH4V3O8 showed a high electrochemical capacitor performance of the three-electrode system of 274 F g–1 at 0.5 A g–1 and the highest energy density of 38 W h kg–1 at a power density of 250 W kg–1. This work provides a promising 2D material with two simultaneous applications as an electrocatalyst for HER as well as an electrochemical capacitor for supercapacitors.