A Ni_x:V_y–Se nanoparticle decorated hierarchical porous Zn–Co–P nanowire array electrode for high energy density asymmetric supercapacitors†

JiuYi Dai^a, Soram Bobby Singh^a, Manoj Bollu^a, Nam Hoon Kim*^a and Joong Hee Lee*^ab
aDepartment of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. E-mail: jhl@jbnu.ac.kr; nhk@jbnu.ac.kr
^bCarbon Composite Research Centre, Department of Polymer-Nano Science and Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea

First published on 27th August 2024

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Introduction

With the rapid rise of the energy crisis, environmental pollution and fossil fuel consumption, research on green and renewable energy is intensifying.^1–5 However, the variable nature of power generation from renewable resources, such as tidal, wind, and solar energy, is a critical challenge to maintaining a constant power supply; hence, a viable strategy to develop high-efficiency energy conversion/storage technology linked to variable energy sources to store and supply power on demand is much needed.^6–12 Among the high-efficiency energy storage/conversion technologies, supercapacitors (SCs) have many desirable energy characteristics, such as flexible power, high charge/discharge efficiency, long durability, low maintenance costs, and eco-friendly operation.^13–18 Nevertheless, the poor energy density of SCs dramatically limits the practicality of SCs in high-energy-demanding devices. Recently, battery-type pseudocapacitors governed by fast reversible multi-electron faradaic redox mechanisms have gained a lot of attention, owing to their advantages, which include excellent energy density, high specific capacity, and high-rate capability.^19–25 However, two main factors limit the practicality of the battery-type pseudocapacitive materials for high-performance SCs: (i) the use of a binder, which results in poorer electrochemical properties and poor cycling performance of the electrode, and (ii) the fast structural degradation of pseudocapacitive materials, owing to rapid faradaic redox reactions.²⁶ One of the strategies to overcome the above limitations is the development of a hierarchical 3D nanostructure using the direct growth technique, where active pseudocapacitive materials are grown over the current collector enhancing the electron transfer between the active material and the current collector and also improving the structural/electrochemical stability of the active materials.^13,27–30

In recent years, zinc-based metal phosphides have emerged as an exciting class of low-cost electrochemical active materials for high-performance SCs, due to their terrestrial abundance, high chemical stability, electrical conductivity, and mechanical strength.^31–34 Wenjing Chu et al.³⁵ reported the synthesis of Zn_0.33Co_0.67P by a MOF derivatization method and phosphating. Although the reported Zn_0.33Co_0.67P yields a large specific capacitance of 2115.5 F g⁻¹ at 1 A g⁻¹, the electrode shows poor cycling stability with 80% retention after only 7000 cycles. The poor cycle durability of zinc-based electrodes limits their application in real SC application devices that require long-term cycling stability.^31–34 For practical applications, improving both the specific capacitance and the cycle durability is an important task. Among the reported strategies to improve the cycling stability, an efficient approach is to construct a rationally designed composite heterostructure electrode by integrating high electrochemical performance materials. The rationally designed composite heterostructure inherits the advantages of the two components with ideal chemistry/physical properties, together with faster rates of charge transfer, increased structural stability, and shortened ionic diffusion distances, as a result of the modulated internal electric fields at the heterointerfaces.^36–42 Compared with single-structure electrode materials, composite porous electrode materials have more abundant faradaic reactions and higher capacity, due to the increased electrochemically active sites.^34,43,44

Lately, transition metal selenide (TMS) materials have gained much attention as battery-type electrodes for high-energy-density supercapacitors. Vanadium selenides are considered promising energy storage candidates among the reported transition metal selenide electrodes, due to their catalytic activity and electrical conductivity that result from their structural integrity and synergistic effects.^11,45–48 However, the electrochemical activity of vanadium selenides is still unsatisfactory, in particular their low cycling stability and poor rate capability, which ultimately hinder their potential use.^14,49–52 Recently, transition metal element doping or constructing a bimetallic has been studied as a competent approach to increase the electrochemical properties/rate capacity or structural firmness of electrode materials,^53,54 for example, Ni_xCo_1−xSe₂ (ref. 55–57), Ni–Mn–Se,⁵⁸ Co–Cd–Se,⁵⁹ Fe_xCo_3−xSe₄,⁶⁰ and NiFe₂Se₄ (ref. ⁶¹ and ⁶²). Compared to single metal selenides, they have more abundant electroactive sites, multiple valence states, and higher electrical conductivity. They have thus attracted particular interest as promising battery-type electrode materials. As reported earlier, we have exploited the bimetallic synergy effect on tuning the morphology/electrochemical performance of the Ni–Mo–Se shell by changing the ratio of Ni and Mo to improve the SC performance.⁶³ Among the transition metal dopants, bimetallic Ni is the most attractive material, owing to its natural richness, cost-effectiveness, multiple oxidation states, and good stability in alkaline medium. Also, the introduction of the Ni element may lead to structural defects/deformation of the host electrode materials, allowing for more redox reaction active sites and ionic/electronic transport pathways, thereby likely reducing the internal resistance, facilitating electron collection, and improving electrode durability. Considering the advantages of Ni, the construction of a nickel-doped vanadium selenide nanoparticle as a composite material might be an interesting method to advance the structural/electrochemical properties of vanadium selenides.

Herein, we design and develop a cost-effective and forthright method for the controlled growth of Ni_x:V_y–Se nanoparticle (NP) decorated Zn–Co–P nanowire arrays (NWAs) with a sole hierarchical nanoporous composite heterostructure. Owing to the synergistic effects between the Ni_x:V_y–Se NP material and the porous structure of Zn–Co–P core materials, the as-prepared Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode achieved a high specific capacity of 690.65 mA h g⁻¹ (areal capacity ∼1.73 mA h cm⁻²) at 4 mA cm⁻². To demonstrate the practicability of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode, a solid-state asymmetric supercapacitor (SS-ASC) is fabricated with Fe₂O₃@CNF/N-rGO as the negative electrode and PVA-based KOH-gel as an electrolyte. The assembled Ni_0.5V_0.5–Se/Zn–Co–P//Fe₂O₃@CNF/N-rGO SS-ASC device delivered a working voltage window of 1.7 V and supplied a high energy density of 123.6 W h kg⁻¹ at ∼1050.2 W kg⁻¹ power density.

Experimental

Preparation of the Ni_x:V_y–Se/Zn–Co–P NWA electrode

Results and discussion

Fabrication, structure, and morphology of Ni_x:V_y–Se/Zn–Co–P NWAs

Recently, the construction of a rational porous hierarchical structure for energy storage electrode materials has been considered an effective method to improve the energy storage capability of SC devices. Fig. 1 shows the scheme, which highlights the synthetic procedure of the hierarchical Ni_x:V_y–Se/Zn–Co–P NWA structure electrodes. The electrode is successfully constructed using a simple and scalable three-step method involving hydrothermal treatment, phosphorization, and potentiostatic deposition and is used as the ASC cathode. First, the nanoporous Zn–Co–P NWs are uniformly grown on porous NF via a two-step process: (i) a one-step hydrothermal treatment to deposit a uniform Zn–Co precursor on the NF and (ii) an effective phosphorus ion-exchange by using the chemical vapor deposition (CVD) method. Notably, the CVD method has been used as a general strategy for the phosphating process. Second, the Ni–V–Se NPs are deposited uniformly over the Zn–Co–P NWs through a simple electrodeposition process to obtain a compact hierarchical nanoporous composite structure of the Ni_x:V_y–Se nanoparticle decorated Zn–Co–P NWA electrode. It is expected that the developed composite electrode will improve the electronic transport kinetics between the Ni:V–Se/ZnCoP and the nickel foam, thereby reducing contact resistance, which enhances the electrochemical properties of the active materials.

Fig. S1, ESI,† 2a and b show the surface morphology photographs of the Zn–Co precursor and Zn–Co–P NWs at various magnifications. Fig S1, ESI,† shows the grass-like nanowire morphology of the Zn–Co precursor; meanwhile, the as-synthesized Zn–Co–P NWs maintain their ultrathin uniform thickness of 40−70 nm and sharply tipped grass-like nanowire morphology after the phosphorization reaction and grow uniformly over the entire nickel foam substrate. The intrinsic structural morphology of the Zn–Co–P NWs was further studied using the TEM technique and is highlighted in Fig. 2c–e. The TEM study reveals that the diameter of the Zn–Co–P NW is around 62 nm; it consists of nano-crystallites and is highly porous. Furthermore, the HR-TEM photographs of the Zn–Co–P NW (Fig. 2f) display lattice fringes with spacings of 0.475, 0.282, and 0.279 nm, respectively, which match with the (101), (011), and (002) crystal planes, respectively, of the Zn–Co–P NWs. The observed porous nanostructure of the Zn–Co–P NWs may be formed after the Zn–Co hydroxide precursor phosphorization. During the phosphating process, the phosphorus element vapor replaced the oxygen element. In addition, the HR-TEM images reveal that the Zn–Co–P nanowires consist of many nanoparticles with highly porous nano-network structures, which might be advantageous for electrolyte ion penetration and transportation during energy conversion and storage, as well as offering more electrochemically active sites for the subsequent electrodeposition. Furthermore, as highlighted in Fig. 2d (inset), the SAED pattern of the Zn–Co–P NWs shows noticeable diffraction rings, indicating the nanocrystalline nature of the Zn–Co–P nanowires. The EDS color mapping photograph of the Zn–Co–P NW further reveals the uniform dispersal of Co, Zn, and P elements. Further, we have studied the porosity of Zn–Co–P by the BET method. Fig. S2a and b, ESI,† highlight the adsorption–desorption isotherms and pore size distribution of the Zn–Co–P NWA electrode.

To construct the hierarchical porous Ni_x:V_y–Se/Zn–Co–P NWA electrode, Ni-doped vanadium selenide (Ni_x:V_y–Se) nanoparticles were electrodeposited over the Zn–Co–P NWs. Various compositions of Ni-doped vanadium selenide (Ni_x:V_y–Se) NPs were electrodeposited over the Zn–Co–P NWs to investigate the structural and coactive effects of Ni-doping in the Ni_x:V_y–Se NPs, and to determine the best Ni:V ratio. In particular, the electrodeposition method delivers an uncomplicated technique for the cost-effective and large-scale manufacture of energy conversion and storage electrodes. The microstructures of the various compositions of the Ni_x:V_y–Se/Zn–Co–P NWAs were determined using the FE-SEM technique. Fig. 3a, b and S3, ESI,† show the FE-SEM images of the Ni_0.5V_0.5–Se, Ni_0.25V_0.75–Se, Ni_0.75V_0.25–Se, Ni–Se, and V–Se samples at different magnifications, respectively. As demonstrated in the FE-SEM images, all the compositions of Ni_x:V_y–Se NPs have similar structures and consist of many nanoparticles. The FE-SEM images highlight that the surface view of Ni-doped Ni_x:V_y–Se presents other details of nanostructures with various compositions of Ni:V, in which the pure Ni–Se shell shows a densely packed layered structure of fine nanoparticles. The pure V–Se shells exhibit nanoparticle layered structures, accompanied by a few larger spherical nanoparticles. The composition of Ni_0.25V_0.75–Se and Ni_0.75V_0.25–Se both show densely packed layered structures of nanoparticles. The Ni_0.25V_0.75–Se shell is accompanied by a few larger spherical nanoparticles, the same as the pure V–Se material. The Ni_0.5V_0.5–Se shell presents a layered structure of fine nanoparticles packed and accompanied by larger nanoparticles homogeneously decorated on the surface of the nanowire array. These results further indicate that the nanostructure of Ni_x:V_y–Se materials is tunable by adjusting the composition of the transition metal atoms. Among the samples, the Ni_0.5V_0.5–Se/Zn–Co–P electrode demonstrated the best hierarchical nanostructure array, which is anticipated to show more promising electrochemical performance. Various surface morphologies of the Ni_x:V_y–Se NP materials with the diverse molar ratios of Ni and V in the Ni_x:V_y–Se/Zn–Co–P NWA hybrid electrode are expected to reveal different electrochemical performances.

Fig. 3a and b highlight the surface views of the Ni_0.5V_0.5–Se/Zn–Co–P NWA hybrid electrode at various magnifications. The low- and high-magnification FE-SEM photographs (Fig. 3a and b) demonstrate the uniform deposition of Ni_0.5V_0.5–Se NPs over the Zn–Co–P NWs, thereby forming a hierarchical nanostructure composite electrode. It is anticipated that such a composite NWA electrode will augment faradaic redox sites, consequently enhancing the rapid transportation of ions and electrons between collectors and the electrochemically active material. Fig. S4, ESI,† highlights the elemental mapping image of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs. The internal morphology and structure of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode were further examined through TEM analysis, as depicted in Fig. 3c–f. The HR-TEM photographs of the Ni_0.5V_0.5–Se/Zn–Co–P NW demonstrate that the nanoporous structure is composed of both nanowires and nanoparticles, corresponding well to the surface view of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs (Fig. 3b). Furthermore, the HR-TEM image of the Ni_0.5V_0.5–Se nanoparticles (Fig. 3e–f) exhibits lattice fringes with a spacing of 0.316, 0.192, and 0.203 nm, corresponding to the (100), (106), and (102) crystal planes, respectively, of Ni–V–Se. Furthermore, the clear diffraction rings observed in the SAED pattern indicate the polycrystalline nature of the electrodeposited Ni_0.5V_0.5–Se NPs (as revealed in the inset of Fig. 3d). Fig. 3g highlights the elemental color mapping of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs. The EDS results confirm that the Zn, Co, P, Ni, V, and Se elements are homogeneously distributed in the Ni_0.5V_0.5–Se/Zn–Co–P NWAs, indicating that Zn, Co, P, Ni, V, and Se coexist in the composite material, further confirming the successful construction of the hierarchical composite NWAs. Further, the elemental composition of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs was confirmed by inductively coupled plasma mass spectrometry (ICP-Ms), as reported in ESI, Table S1.†

The crystal structure of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs was characterized by the XRD method. Fig. 3h shows the XRD peaks of the Zn–Co–P NWs and Ni_0.5V_0.5–Se/Zn–Co–P NWAs. The diffraction peaks of Zn–Co–P matched JCPDS card no. 00-029-0497 and JCPDS card no. 00-038-1354, which correspond to CoP and ZnP₄, respectively. The diffraction peaks observed at 23.6, 31.6, 36.3, 46.2, 56.1, 76.3, and 83.8° are assigned to the (101), (011), (111), (112), (020), (114), and (321) crystal planes, respectively, of CoP, while the diffraction peaks observed at 40.6, 44.4, 68.6, and 83.7° are assigned to the (211), (213), (322), and (414) crystal planes, respectively, of ZnP₄. In addition, the XRD diffraction of the Ni_0.5V_0.5–Se NP peaks matched JCPDS card no. 01-073-1478 and JCPDS card no. 01-075-0610, which correspond to NiV₂Se₄ and NiSe, respectively. The peaks observed at the 2θ values of 33.83, 48.68, 51.96, 57.16, 80.28, and 83.54° correspond to the (112), (−205), (310), (206), (−209), and (510) crystal planes, respectively, of NiV₂Se₄, while the XRD peaks detected at 32.8, 33.6, and 80.0° match the (101), (002), and (210) crystal planes, respectively, of NiSe. Moreover, the three XRD peaks detected at 44.4, 51.6, and 76.1° correspond to the NF substrates (JCPDS card no. 20-078).

Furthermore, the valence state and chemical components of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs were studied using XPS characterization. Fig. S5, ESI,† illustrates the full survey XPS spectra of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs, indicating the existence of Co, Zn, P, Ni, V, and Se as basic elements, consistent with the EDX analysis (Fig. S5, ESI†) and conforming to the EDS color mapping (Fig. 3g). Fig, S4, ESI,† demonstrates a strong O 1s peak arising from the surface oxidation of the material, consistent with the EDX analysis results (Fig. S5, ESI†). Fig. 4a–f highlight the Gaussian-fitted, high-resolution XPS spectra of Co 2p, Zn 2p, P 2p, Ni 2p, V 2p, and Se 3d, respectively. As highlighted in the high-resolution XPS spectrum of Co 2p (Fig. 4a), two twin-peaks are depicted centered at 795.3 and 779.8 eV, which match Co 2p_1/2 and Co 2p_3/2, respectively. Two satellite peaks are also observed at binding energies (B.E.) of 783.4 and 802.4 eV, respectively. The presence of two pairs of doublets also indicates the existence of Co²⁺ and Co³⁺ oxidation states. Fig. 4b shows the Zn 2p XPS spectrum, which displays two peaks positioned at 1022.2 and 1045.3 eV, which correspond to Zn 2p_3/2 and Zn 2p_1/2 (ΔeV = 23.1).^15,64,65 As highlighted in Fig. 4c, the P 2p XPS spectrum presents one satellite peak and one main peak. The main peak of P 2p at 129.4 eV shows two deconvoluted peaks with the B.E. of 129.18 and 129.98 eV (ΔeV = 0.8), corresponding to P 2p_1/2 and P 2p_3/2 states, respectively. The peak at a higher B.E. of 133.38 eV can be attributed to oxygen–phosphorus (P–O) bonds due to the partial oxidation of the Zn–Co–P NWs (Fig. 4c).^35,65–67 A broad peak was observed at a higher B.E. of 138.1. eV, corresponding to Zn 3s (Fig. 4c).^35,65,68,69 The fitted V 2p high-resolution spectrum (Fig. 4d) exhibits two main peaks centered at 524.2 and 516.7 eV that correspond to V 2p_1/2 and V 2p_3/2, which could be additionally deconvoluted into four peaks located at 524.5, 523.6, 516.9, and 516.4 eV, respectively. The deconvoluted peaks of V 2p, corresponding to the electronic states V⁴⁺ 2p_1/2, V²⁺ 2p_1/2, V⁴⁺ 2p_3/2, and V²⁺ 3p_3/2, are shown in Fig. 4d. Fig. 4e highlights the fitted high-resolution spectrum of Ni 2p, which shows two twin peaks centered at 872.4 and 854.8 eV, corresponding to Ni 2p_1/2 and Ni 2p_3/2. Meanwhile, these Ni 2p twin peaks also indicate the presence of Ni³⁺ and Ni²⁺ oxidation states. Also, two satellite peaks are observed at the B.E. of 878.8 and 861.4 eV, respectively. In addition, the fitted spectra of Se 3d display two peaks, a prominent peak at 55 eV and a satellite peak at 60.0 eV (Fig. 4f). Further, the deconvolution of the Se 3d peak at 55 eV reveals two peaks with B.E. of 54.7 and 55.6 eV (ΔeV = 0.9), which correspond to the Se 3d_5/2 and Se 3d_3/2 states, respectively, of Se 3d. The presence of the SeO_x satellite peak at 60.0 eV indicates the partial oxidation of the materials in the environment.

Electrochemical performances of Ni_x:V_y–Se/Zn–Co–P

Initially, we examine the electrochemical performances of the different compositions of Ni_x:V_y–Se/Zn–Co–P NWA electrodes using a 3-electrode system. Fig. S6a, ESI,† presents the comparative CV profiles of the Zn–Co–P NWs, in comparison with various compositions of the Ni_x:V_y–Se/Zn–Co–P NWAs (denoted as V–Se/Zn–Co–P, Ni–Se/Zn–Co–P, Ni_0.5V_0.5–Se/Zn–Co–P, Ni_0.25V_0.75–Se/Zn–Co–P, and Ni_0.75V_0.25–Se/Zn–Co–P) electrodes, measured at a scan rate of 15 mV s⁻¹ in the working potential range of −0.3 to 0.9 V. It is noteworthy that all the sample composition samples displayed a pair of strong redox peaks, representing quasi battery-like features, i.e., the dominance of the fast-redox reaction process contribution to the charge storage mechanism. Surprisingly, the Ni_0.5V_0.5–Se/Zn–Co–P/Ni electrode demonstrates a high peak current and a large CV loop area, signifying a large energy storage capability, as compared to other compositions. Furthermore, the as-constructed hierarchical NWA composite materials deliver better energy storage performances than the bare Zn–Co–P NWs, suggesting that the rational strategy and fabrication of hierarchical composite nanostructure materials can improve the capacity of the electrode. In addition, the potential gap between the cathodic peak and anodic peak of Ni_0.5V_0.5–Se/Zn–Co–P/NF (0.646 V) is larger than those of Zn–Co–P/NF (0.537 V), Ni–Se/Zn–Co–P/NF (0.587 V), and V–Se/Zn–Co–P/NF ( 0.572 V), respectively, indicating its improving reaction kinetics and altered electrochemical activity owing to the strong interaction between the Ni_x:V_y–Se NPs and Zn–Co–P NWs in the composite heterointerface electrode structure.^13,27,29,70 The quasi-reversible electrochemical reactions for the Ni_x:V_y–Se/Zn–Co–P NWA electrode can be defined as

NiVSe + 2OH− ↔ NiSeOH + VSeOH + 2e−	(1)

VSeOH + OH− ↔ VSeO + H2O + e−	(2)

NiSeOH + OH− ↔ NiSeO + e−	(3)

ZnCoP + OH− ↔ ZnCoPOH + e−	(4)

ZnCoPOH + OH− ↔ ZnCoPO + H2O + e−	(5)

Fig. S6b, ESI,† displays the GCD plots of all the different compositions of the Ni_x:V_y–Se/Zn–Co–P NWAs, which are recorded at 4 mA cm⁻². All the GCD profiles, recorded in the voltage range of −0.1 to 0.45 V, exhibited quasi-symmetric charged/discharged features in addition to an insignificant IR drop, suggesting good electronic and ionic transfer/diffusion in the as-fabricated electrode materials. Significantly, the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode exhibits a longer discharge time than those of the other composition materials, suggesting more promising energy storage performance. Considering the electrochemical performance, the composition with a Ni to V molar ratio of 0.5:0.5, i.e., V (0.5):Ni (0.5), is studied as the optimized composition. Fig. 5a depicts the CV profiles of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs that were observed at various scan rates, from 5 to 50 mV s⁻¹. The CV profiles of all samples show a pair of redox peaks and maintain their characteristic CV profile even at a high scan rate, indicating a typical quasi-battery-like faradaic process, good rate capability, and good electrochemical reversibility. In addition, characteristic reduction and oxidation peak shifts nearer higher −ve and +ve voltages are observed with the increase in the sweep rate. Although the various compositions of Ni_x:V_y–Se/Zn–Co–P materials demonstrate similar profiles at the same sweep rates, the positions of redox peaks are different, which might be due to the difference in the electroactivity and nanostructure of the materials. To gain more insight into the charge storage contribution, the electrode reaction kinetics of the Zn–Co–P, Ni–Se/Zn–Co–P, V–Se/Zn–Co–P, and Ni_0.5V_0.5–Se/Zn–Co–P electrodes from CV profiles were studied. The nature of the energy storage mechanism in the quasi-battery type electrode is controlled by two different mechanisms, i.e., capacitive and diffusion charge storage. The energy storage process of the electrode is examined through the correlation between the scan rate and peak current, which can be described as per the power-law equation, as shown in eqn (6) and (7):

i = avb	(6)

log(ip) = log(a) + blog(v)	(7)

where “i_p” (mA) is the redox peak current and “v” (mV s⁻¹) is the sweep rate; “a” and “b” are two constants and “b” is the slope.⁷⁰ Typically, for the capacitive behavior electrode, the slope of the function is close to 1, while a slope of the function close to 0.5 indicates battery-type behavior, where the diffusion mechanism is dominant.⁷⁰ As revealed in Fig. 5b and S7, ESI,† the b values of the Zn–Co–P, Ni–Se/Zn–Co–P, V–Se/Zn–Co–P, and Ni_0.5V_0.5–Se/Zn–Co–P electrodes are 0.613, 0.596, 0.598, and 0.553, calculated from the anodic peak, while 0.583, 0.577, 0.578, and 0.528 are obtained from the cathodic peak, respectively, signifying that all the materials exhibited both capacitive and battery-type behaviors. The calculated b value of the Ni_0.5V_0.5–Se/Zn–Co–P/NF electrode is found to be closer to 0.5, signifying the battery-type features of Ni_0.5V_0.5–Se/Zn–Co–P/NF, which might originate from the synergistic effect of the battery-like materials of Ni–Se, V–Se, and Zn–Co–P. To further study the capacity input of the diffusion and surface-controlled processes, the capacity contribution can be calculated via the following eqn (8) and (9):⁷¹

i(V) = k1v + k2v1/2	(8)

i(V)/v1/2 = k1v1/2 + k2	(9)

where k₁ and k₂ are two constants; “k₁v” and “k₂v^1/2” represent the contributions from the surface-controlled and the diffusion-controlled process, respectively. Using eqn (9), we can fit the functional relationship of “v^1/2” and “i(V)/v^1/2”, and the constants k₁ and k₂ can be calculated at sweep rates from 5 to 50 mV s⁻¹. At a sweep rate of 15 mV s⁻¹, the overall diffusion-controlled contribution capacity of the Zn–Co–P, Ni–Se/Zn–Co–P, V–Se/Zn–Co–P, and Ni_0.5V_0.5–Se/Zn–Co–P electrodes is determined to be 79.4, 83.11, 82.99, and 90.26%, respectively (Fig. 5c and S8, ESI†). As demonstrated in Fig. 5d, as the sweep rate increases, the contribution of the surface-controlled process for the Ni_0.5V_0.5–Se/Zn–Co–P electrode also increases from 5.86 to 16.46%, indicating its highly efficient charge transfer kinetics at high scan rates. Similar features can also be found in Fig. S8, ESI,† for the Zn–Co–P, Ni–Se/Zn–Co–P, and V–Se/Zn–Co–P electrodes.

Fig. 5d illustrates the GCD profiles of the Ni_0.5V_0.5–Se/Zn–Co–P NWAs at various applied current densities with a working potential range of −0.1 to 0.45 V. The non-linear charge/discharge characteristics of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode with an evident voltage platform at different applied currents from 4 to 40 mA cm⁻² demonstrate a quasi-battery type charging/discharging mechanism, in accordance with the CV profile and capacity contribution analysis. The areal and specific capacities of the Ni_x:V_y–Se/Zn–Co–P NWA electrodes are calculated from the discharge profiles. The calculated areal capacities of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode are found to be 1.73, 1.64, 1.56, 1.48, 1.30, 1.15, 1.02, 0.90, and 0.67 mA h cm⁻² at 4, 6, 8, 10, 15, 20, 25, 30, and 40 mA cm⁻² current densities, respectively (Fig. 5e), with the corresponding specific capacities of 690.65, 654.73, 623.62, 592.19, 521.42, 461.20, 407.47, 360.52, and 268.46 mA h g⁻¹. The electrode displays an attractive capacitive retentivity of 66.2% at 20 mA cm⁻² current density. The electrode still shows a retention of about 38.7% of the original capacity, even at a high discharged current loading, 40 mA cm⁻², indicating outstanding rate capability. The electrochemical performance of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode shows considerably more promise than the previously reported TMC and other composition materials, as highlighted in Table S2, ESI†.

The diffusion of the electrolyte and the electronic/ionic transfer kinetics of the electrolyte–electrode inter-surface are investigated by EIS measurements. Fig. S9, ESI,† highlights the EIS profiles of the various compositions of the Ni_x:V_y–Se/Zn–Co–P NWA electrode. As depicted in the EIS profile, the point where the graph intersects the x-axis in the high-frequency region represents the internal resistance (R_s). As the inset of Fig. S9, ESI,† illustrates, the Ni_0.5V_0.5–Se/Zn–Co–P NWAs demonstrate a lower R_s value of ∼1.12 Ω than the Zn–Co–P NWAs, V–Se/Zn–Co–P NWAs, Ni–Se/Zn–Co–P NWAs, Ni_0.25V_0.75–Se/Zn–Co–P NWAs, and Ni_0.75V_0.25–Se/Zn–Co–P NWAs with R_s values of ∼1.68, 1.37, 1.67, 1.35, and ∼1.82 Ω, respectively. Moreover, the diameter of the semicircle of the EIS profiles represents the interface charge transfer resistance (R_ct). As shown in the inset of Fig. S9, ESI,† the Ni_0.5V_0.5–Se/Zn–Co–P NWAs possess a more minute semicircle than the other Ni_x:V_y–Se/Zn–Co–P electrodes in a high-frequency region. As predicted, the Ni_0.5V_0.5–Se/Zn–Co–P NWAs exhibit an R_ct of ∼0.30 Ω, which is lower than that(R_ct ∼0.55 Ω) of Zn–Co–P electrodes, suggesting higher electronic transfer kinetics. Further, the EIS profile of Ni_0.5V_0.5–Se/Zn–Co–P exhibits more vertical lines than those of the other electrode materials in the low-frequency region, demonstrating better electrolyte diffusion properties. The results of the EIS measurement further demonstrate the better electrical/ionic conductivity and small charge-transfer resistance of the Ni_x:V_y–Se/Zn–Co–P NWA electrode, which might be attributed to the following reasons: (i) the synergistic effect of Ni and V in the ternary Ni–V–Se NPs; (ii) the unique hierarchical NP/NW composite nanoporous network structure, which might supply multiple transport paths for electronic/ionic diffusion; and (iii) the highly integrated constructed composite nanoporous architecture of the current collector.

Furthermore, to evaluate the cycle durability, the bare Zn–Co–P NW and Ni_0.5V_0.5–Se/Zn–Co–P NWA electrodes were subjected to 10000 continuous cycles of GCD testing performed at a current density of 40 mA cm⁻². As shown in Fig. 5f, the Ni_0.5V_0.5–Se/Zn–Co–P electrode delivered a high-capacity retention of 96.3%, even after the 10000 GCD tests, which is far better than the bare Zn–Co–P NWs (a capacity retention of ∼90.8%). In addition, the inset of Fig. 5f displays the first and last ten GCD cycles. The GCD profiles retain their initial shapes after the long-term cycling test, suggesting the excellent long-term cycling performance of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode. These excellent electrochemical energy storage performances can be ascribed to the following features: (i) the Ni_0.5V_0.5–Se NPs decorated on the Zn–Co–P NW framework might supply multiple transport paths for electronic/ionic diffusion that allow fast electronic/ionic transport on the inter-surface of the active materials and current collector and also induce a strong synergistic effect between the Ni–V–Se NPs and Zn–Co–P NWs; (ii) the synergistic effects of Ni and V in the ternary Ni–V–Se NPs suggest a decrease in the resistance of internal charge transfer, as well as inducing structural deformation of Ni–V–Se during the electrodeposition process, supplying abundant electrochemically active sites and electron/ion transport routes; the Ni–V–Se NPs might also improve the steadiness of the materials; and (iii) the well-distributed Zn–Co–P NW conductive skeleton with the nanoporous structure on the current collector minimizes the pathways and offers suitable channels for the rapid movement of ions/electrons, leading to boosted rate capability;⁷¹ the incorporation of Se and P elements into the internal structure of the Ni_0.5V_0.5–Se/Zn–Co–P plays a crucial role in enhancing both electrical conductivity and electrochemical performance and stability.

Electrochemical performances of the Ni_0.5V_0.5–Se/Zn–Co–P//Fe₂O₃@CNFs/N-rGO asymmetric supercapacitors

The rational design and construction of ASCs represent an effective strategy to develop high-energy-density supercapacitors. ASCs exhibit high operating potential windows and high energy densities by combining negative and positive electrode electrochemical performance. The choice of the negative electrode material plays a crucial role in enhancing the energy storage capacity of the supercapacitor. Herein, we synthesized an Fe₂O₃@CNFs/N-rGO electrode as a negative material following our previously reported work⁷² to construct ASCs with a Ni_0.5V_0.5–Se/Zn–Co–P electrode. As suggested in our previous work, the Fe₂O₃@CNFs/N-rGO electrode might offer various advantages such as a high surface area and excellent electrical conductivity owing to the presence of CNFs in the composite electrode, and also the presence of N-rGO might further enhance electrical conductivity and provide additional active sites for electrochemical reactions due to nitrogen doping. In addition, the combination of Fe₂O₃, CNFs, and N-rGO might lead to a synergistic effect that could improve the overall electrochemical performance of the electrode. The CNFs and N-rGO form a conductive network that supports the Fe₂O₃ nanoparticles, preventing their agglomeration and ensuring their uniform distribution. Fig. S10 and S11, ESI,† demonstrate the morphology and BET measurement of the Fe₂O₃@CNFs/N-rGO electrode. A detailed BET analysis is reported in the ESI.† Fig. S12, ESI,† highlights the electrochemical performance of the Fe₂O₃@CNFs/N-rGO electrode. To evaluate the potential application of the Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode for the high energy density supercapacitor, an all-solid-state asymmetric supercapacitor (SS-ASC) was constructed using Ni_0.5V_0.5–Se/Zn–Co–P and Fe₂O₃@CNFs/N–rGO as the positive and negative electrodes, respectively, with solid gel-like KOH/PVA as the electrolyte, and was termed the Ni_0.5V_0.5–Se/Zn–Co–P//Fe₂O₃@CNFs/N–rGO ASC device, or later, simply the SS-ASC device. In this study, the electrochemical performance of the as-fabricated device was investigated by performing CV, GCD, and EIS in a two-electrode system. To improve ASC energy storage performance, the selection of the potential window of the operating cell plays a crucial role. Fig. S13a, ESI,† depicts the CV profiles of the Ni_0.5V_0.5–Se/Zn–Co–P (−0.3 to 0.9 V) and Fe₂O₃@CNFs/N-rGO (−1.4 to 0 V) electrodes, measured in a 3-electrode system at 15 mV s⁻¹. Fig. S13b, ESI,† represents the CV profiles of the SS-ASC device obtained at a scan rate of 50 mV s⁻¹ for different voltage ranges/windows. When SS-ASC was operated in the voltage window of (0–0.6) to (0–1.2) V, the respective CV profiles showed a quasi-rectangular feature, indicating an incomplete faradaic redox process of SS-ASC within the applied potential range. When the operating potential was up to ∼1.4–1.9 V, a more perfect faradaic redox process was revealed in the CV profiles, indicating battery-type pseudocapacitive behavior. It is observed that when the operating voltage reaches 2.0 V, completion of the redox reaction is noticeable in the CV profiles, which is ascribed to the occurrence of the completed oxidation and reduction reactions. Fig. 6a reveals the CV profiles of the SS-ASC device tested in the operating potential of 0–2.0 V at various scan rates, from 10 to 50 mV s⁻¹. All CV plots of SS-ASC demonstrate the combined electrochemical behavior of the battery-type characteristics and EDLC features. Remarkably, the CV profile of the solid-state device measured at a scan rate of 10 mV s⁻¹ shows 4 redox peaks, which include one oxidation peak (observed at 1.423 V potential) and three reduction peaks (observed at 0.831, 1.115, and 1.416 V potentials, respectively). As the scan rate increases, the respective oxidation peaks and the reduction peaks move towards higher positive potentials and negative potentials, respectively. When the scan rate is around 15–25 mV s⁻¹, only three redox peaks are observed from the CV profile, one oxidation peak in a potential range of 1.46–1.61 V and two reduction peaks in a potential range of 0.68–1.11 V. When the scan rate reaches 30 mV s⁻¹, the high potential reduction peak disappears, and only a pair of redox peaks can be observed, which is due to the increase of the current density at high scan rates; the buildup of charges results in electrode polarization, and at the same time, the faradaic reaction rate increases and the ion diffusion rate cannot meet the requirements for a complete redox reaction, which leads to the high-potential reduction peak disappearing, and the shift of the redox peak to high potential.^{3,27,29,30,70} To further study the energy storage kinetics of the SS-ASC device, following eqn (7), we acquired the b value from the CV profile (Fig. 6a). As shown in Fig. 6b, the b value for SS-ASC is 0.736 calculated from the reduction peaks, while 0.728 is calculated from the oxidation peaks, respectively, indicating that the SS-ASC device possesses both pseudocapacitance and battery behavior.

Moreover, the contribution of the surface and diffusion-controlled characteristics to the overall capacity of the SS-ASC device is determined through eqn (6), by fitting the linear relation of v^1/2 and i(V)/v^1/2 of the redox peak; the k₁ and k₂ values could be calculated from 10 to 50 mV s⁻¹ scan rates. As shown in Fig. 6c, as the sweep rate increases from 10 to 50 mV s⁻¹, the contribution of the surface-controlled mechanism increases from 33.61 to 53.09%, revealing its high charge transfer efficiency at high sweep rates.

Furthermore, the charged/discharged profile of the solid-state device in various voltage ranges is also performed at 15 mA cm⁻² to ensure the working voltage of the SS-ASC device (Fig. S13c, ESI†). Fig. 6d highlights the charged/discharged characteristic profiles of the SS-ASC device tested in the operating voltage window of 1.7 V at different current densities. The SS-ASC device exhibits quasi-symmetric GCD characteristics, owing to the excellent pseudocapacitive features of the SS-ASC device. As shown in the GCD profiles, a well-defined plateau of ∼1.14 V in the discharge profile at lower current densities (up to 8 mA g⁻¹) was noticed, which demonstrates that both positive and negative electrodes exhibit battery-type behavior.^27,29,70 The specific capacities of the SS-ASC device calculated from the GCD profiles at the current densities of 8, 12, 15, 20, 30, 40, 60, and 80 mA cm⁻² are evaluated to be 145.5, 114.2, 111.8, 107.2, 100.2, 95.7, 90.0, and 85.2 mA h g⁻¹, respectively (Fig. S13e, ESI†). It is claimed that the SS-ASC device exhibits a significantly higher capacity of ∼145.5 mA h g⁻¹ at 8 mA cm⁻². Moreover, the SS-ASC device also displays an excellent rate capability, the device exhibits a high-capacity retention of ∼65.77% when the current density increases to 40 mA cm⁻²; even when the applied current density is promoted to 80 mA cm⁻², the device exhibits a capacity retention of ∼55.89% (Fig. S13e, ESI†). The superb charge storage capacity of the as-fabricated SS-ASC device might be attributed to the combined outstanding energy conversion/storage properties of both the positive electrode (Ni_0.5V_0.5–Se/Zn–Co–P) and the negative electrode (Fe₂O₃@CNFs@N-rGO), as highlighted in the earlier section of the electrochemical test.

In addition, a small IR drop of 0.035 V (at 8 mA cm⁻²) in the GCD profile was observed, which is explained by the equivalent series resistance (ESR) of SS-ASC. Moreover, the reduction in specific capacity with increasing applied current could be attributed to the increase in IR drop, stemming from the intrinsic equivalent series resistance (ESR) of the SS-ASC device, along with incomplete faradaic reactions at high current densities. To further study the ESR and energy storage kinetics, the Nyquist plots of SS-ASC were obtained using the EIS test in the frequency range from 100 kHz to 0.01 Hz (Fig. S13f, ESI†). The Nyquist plots of SSASC have an almost vertical line in the low-frequency region, indicating excellent capacitive characteristics. In the higher frequency region, the SS-ASC device demonstrates an ultra-low R_s and R_ct of 1.98 and 2.92 Ω, respectively, which indicate a small device resistance and high electronic/ion mobility between the solid–gel electrolyte and the active material. Cycling stability is one of the critical factors in evaluating the practical application of ASC devices. The stability of the SS-ASC device was examined by testing 10000 successive GCD cycles at an operating current density of 50 mA cm⁻² (Fig. 6e; the inset shows the first and last ten cycles of the GCD profiles). The SS-ASC device showed a loss of 3.6% in the initial capacity after 5000 cycles, and a loss of 4.2% after 10000 cycles, indicating its superior long-term stability (∼95.8% retention of its initial specific capacity). To further evaluate the energy-storage properties of the as-constructed devices, we calculate the energy density and power density of SS-ASC from the GCD profile via eqn (3) and S4.† Fig. 6f shows the graph of energy density vs. power density (namely, the Ragone plot) of the SS-ASC device, along with those of previously reported ASC devices. As demonstrated in the Ragone plot, SS-ASC exhibits an outstanding energy conversion/storage performance of ∼123.6 W h kg⁻¹ (energy density) at ∼1050.2 W kg⁻¹ (power density). Even at a high power density (∼8934.3 W kg⁻¹), the SS-ASC device demonstrates superb energy storage performance (∼72.4 W h kg⁻¹), indicating the outstanding rate capability of the assembled SS-ASC device. It should be noted that the energy and power densities of the as-prepared ASCs are evaluated to exceed those of earlier reported TMC-based ASC devices, such as the Zn_0.33Co_0.67P//Bi₂O₃ ASC (83.05 W h kg⁻¹ at 775.02 W kg⁻¹),³⁵ ZNCP-NF//AC ASC (37.59 W h kg⁻¹ at 856.52 W kg⁻¹),⁶⁶ ZNCO@Co–Ni-LDH-2//NGH ASC (63.28 W h kg⁻¹ at 796.53 W kg⁻¹),⁷³ ZnNiAlCoO//RGO ASC (72.4 W h kg⁻¹ at 533.0 W kg⁻¹),⁷⁴ and Ni–Co–P/PO_x/C/NF//RGO/NF ASC (37.59 W h kg⁻¹ at 800 W kg⁻¹).⁷⁵ Table S3, ESI,† compares the power/energy densities of previously reported ASC devices with those of our device. Practical applications of the as-fabricated SS-ASC devices are additionally tested with two of the as-fabricated ASC devices connected in series, as demonstrated in Fig. 6f. The two SS-ASC devices can power a blue LED when the group of SS-ASC charges to ∼3.2 V. These results highlight that the as-constructed Ni_0.5V_0.5–Se/Zn–Co–P//Fe₂O₃@CNFs/N-rGO ASC device, with cost-effectiveness and high energy storage performance, has admirable potential for the application of the high energy-demand supercapacitor in energy-consuming devices, such as the smart electric grid and new energy vehicles.

Conclusions

We designed a low-cost and straightforward method for the development of a Ni_x:V_y–Se NP decorated Zn–Co–P NWA composite heterostructure electrode. The synergetic effect and tunable morphology of the Ni_x:V_y–Se/Zn–Co–P NWA electrode material, and the effects of its controllable composition on electrochemical activity, have been studied and discussed. The optimized Ni_0.5V_0.5–Se/Zn–Co–P NWA electrode provides an excellent energy storage performance with an areal capacity of ∼1.73 mA h cm⁻², corresponding to a superb specific capacity of 693.03 mA h g⁻¹ at a current density of 2 mA cm⁻². In addition, the NWA electrode also exhibits superior cycling stability (96.4% capacity retention after 10000 cycles). Also, an SS-ASC device fabricated with Ni_0.5V_0.5–Se/Zn–Co–P NWAs and Fe₂O₃@CNFs/N-rGO as the positive and negative electrodes, respectively, exhibits excellent energy storage capability, which features a high operating voltage range of ∼1.7 V, a high energy density of 123.6 W h kg⁻¹ at ∼1050.2 W kg⁻¹ (power density) and superb cycling stability, with ∼95.8% capacity retention after 10000 cycles. These findings suggest that the produced hierarchical Ni_x:V_y–Se/Zn–Co–P NWA composite electrode could serve as a promising candidate for the development of next-generation high-energy-density supercapacitors, suitable for applications in equipment with high energy requirements.

Data availability

The data are available on request from the authors (E-mail: jhl@jbnu.ac.kr).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Regional Leading Research Center Program (2019R1A5A8080326) through the National Research Foundation funded by the Ministry of Science and ICT of the Republic of Korea. Also was supported by the H2KOREA funded by the Ministry of Education (2024Hydrogen Industry-002, Innovative Human Resources Development Project for Hydrogen Industry).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03570d

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