Enhanced Electrochemical Performance of LaNiO 3 Through Mn and Co‐Doping for High‐Energy‐Density Next‐Generation Supercapacitors

Open AccessCCS ChemistryRESEARCH ARTICLE25 May 2022Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne Yuxin Liu, Yang Gao, Feng He, Yurui Xue and Yuliang Li Yuxin Liu Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yang Gao Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Feng He Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yurui Xue *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Science Center for Material Creation and Energy Conversion, School of Chemistry and Chemical Engineering, Institute of Frontier and Interdisciplinary Science, Shandong University, Jinan 250100 and Yuliang Li *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202005 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Here we report an in situ assembly growth method that controls the growth of NiTCNQ on the surface of graphdiyne (GDY). The catalytic system of donor–acceptor–donor (GDY/TCNQ/Ni) structure with multiple charge transfer (CT) was achieved by controlling the growth of NiTCNQ on the surface of GDY. Significantly, a controlled double layer interface of GDY/TCNQ/Ni was formed. This system implemented simultaneously the two elements we expected (1) an incomplete CT, and (2) the infinite distribution of active sites originating from highly asymmetric surface charge distribution. The high conductivity and typical semiconductor characteristics of the catalyst endows it with high catalytic activity. We found that an electrolytic cell consisting of the CT salt as a catalyst provided a 1.40 V ultra-small cell voltage up to 10 mA cm−2 and the outer GDY film effectively prevented the corrosion of the catalyst. Our study is the first to introduce CT complexes to a novel catalytic material platform for high selectivity of catalysts, and undoubtedly demonstrates the high selectivity, stability, and activity of such catalytic systems, which provides a new space for the development of novel conceptual catalysts. Download figure Download PowerPoint Introduction The sustainable hydrogen (H2) production through electrocatalytic overall water splitting (OWS) provides a promising method to overcome the global energy crisis and environmental problems, which is of great significance for industrial and scientific progress.1,2 In terms of hydrogen conversion, precious metal-based materials (e.g., Ru or Ir-based materials for the oxygen evolution reaction (OER); Pt-based materials for the hydrogen evolution reaction (HER)) remain the benchmark catalysts with high catalytic performances, but they are not suitable for large-scale commercialization due to their high cost, low abundance, and insufficient stability. At present, the high cost of precious metals has seriously restricted the development of this field. Transition metal-based materials, such as metal hydroxides, oxides, nitrides, phosphides, carbides, and sulfides, have been extensively studied for anodic OER and cathodic HER.3–6 However, the low catalytic activity of these systems results in sluggish kinetics of OER and HER. In addition, there is no stability advantage of these catalysts. Furthermore, to meet the requirement of industrial alkaline water splitting for hydrogen production, the catalysts should produce large current densities at low cell voltages (e.g., ∼200–400 mA cm−2 at 1.8–2.4 V) and be synthesized from earth-abundant elements to minimize the H2 production cost.7–10 Therefore, to develop catalysts with excellent comprehensive performance, we need to find a new way to produce electrocatalysts with low cost, high activity, and high stability. The unique and excellent structures and properties of graphdiyne (GDY) has triggered extensive investigations on its fundamental properties and potential applications in various research fields, such as catalysis, energy conversion and storage, optical and electrical devices, and so on.11–26 As a new sp- and sp2-cohybridized two-dimensional carbon material, GDY shows large porous structure, high intrinsic activity, excellent electron transfer ability, high electronic conductivity, and high stability.27–29 GDY is an ideal carbon material for synthesizing high-performance catalysts with determined structures and abundant active sites, and provides the opportunities for clearly understanding the relationship of structure and performance. GDY can be grown on arbitrary substrates at low temperatures and ambient pressures, which protects the parent structure and the active site structure from destruction and generates new active sites. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) is a well-known electron acceptor with high electron affinity, which easily reacts with metal atoms to form charge transfer (CT) complexes with high conductivity, high carrier density, and special electron transport properties.30 Metal-TCNQs show excellent electrical and optical properties, and unique and adjustable electron and high CT ability. CT complexes where the CT amount is not an integer is called incomplete CT; it is different from traditional inorganic complexes with an integer CT. Because of their high conductivity and semiconductor characteristics, they have attracted more and more interest in many research fields.31–34 However, until now, research on CT complexes mainly focused on controlled growth, photoelectric devices. and semiconductor properties, while basic and applied research in the field of catalysis remain silent. Recently, we took NiTCNQ–GDY as an example and explored the basis and application of NiTCNQ–GDY as an emerging catalyst in hydrogen energy conversion. In addition, we deeply understand the GDY plays the key role in the generation of efficient catalysts, and find its introduction overcomes important issues such as solvent corrosion, structural instability caused by light or electricity, and poor conductivity.35,36 In this work, we report the successful synthesis of a donor–acceptor–donor (GDY/TCNQ/–Ni) structure with multiple CT by in situ growing GDY layers on the surface of NiTCNQ nanowires (GDY–NiTCNQ). Experimental results demonstrated the incomplete CT and the infinite distribution of active sites originating from highly asymmetric surface charge distribution endow the catalyst with high conductivity, typical semiconductor characteristics, high catalytic activity, and excellent intrinsic properties. These unique and fascinating properties endow GDY–NiTCNQ with excellent catalytic activities toward OER, HER, and OWS. For instance, GDY–NiTCNQ exhibited small overpotentials of 218 and 61 mV at 10 mA cm−2 for OER and HER in 1.0 M KOH. When used as an electrolytic cell, it can drive 10 mA cm−2 at a low cell voltage of 1.40 V. This work provides a new direction toward the development of cost-effective and high-performance electrocatalysts. Experimental Methods Material TCNQ was purchased from Tokyo Chemical Industry (TCI, Shanghai, China). Acetonitrile and methanol were obtained from Concord Technology (Tianjin) Co., Ltd. Nickel foams (NFs) were treated with dilute hydrochloric acid, rinsed with deionized water, and dried with N2 flow before use. Deionized water was purified with a Millipore system. All chemicals were analytical grade and used without further purification unless otherwise specified. Preparation of NiTCNQ in acetonitrile A piece of freshly pretreated nickel film (3 cm × 1.5 cm) was immersed in 10 mM TCNQ acetonitrile solution for 10 min followed by the addition of 3% deionized water at room temperature. After reaction for 6 h, the material was removed from the solution, washed carefully with acetonitrile and water, and dried under the protection of N2. Preparation of GDY on NiTCNQ (GDY–NiTCNQ) A piece of NiTCNQ was immersed in 50 mL acetone solution of hexaethynylbenzene (HEB) at 50 °C under Ar atmosphere protected from light. After reaction for 12 h, the resulting GDY–NiTCNQ was washed with acetone, deionized water, and acetone, followed by drying at 50 °C in a vacuum oven. Characterization The morphology of the material was characterized by scanning electron microscopy (SEM; Hitachi, S 4800; Institute of Chemistry, Chinese Academy of Sciences), transmission electron microscopy (TEM, a JEOL JEM-2100F field-emission high-resolution transmission electron microscope at an accelerating voltage of 200 kV; Institute of Chemistry, Chinese Academy of Sciences), and high-resolution TEM (HRTEM). The energy-dispersive spectroscopy (EDS) mapping analysis was also obtained to characterize the elemental composition of the samples. X-ray diffraction (XRD) measurements were carried out on a PANalytical high-resolution XRD system (EMPYREAN) using Cu Kα radiation (λ = 1.540598 mm). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ECSALab 250Xi instrument with monochromatic Al Kα X-ray radiation (Institute of Chemistry, Chinese Academy of Sciences). Electrochemical measurements All electrochemical tests were carried out using an electrochemical workstation (CHI 760E) through a typical three-electrode system, in which the as-prepared samples, graphite rods, and saturated calomel electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. All the electrolytes used in the experiments were bubbled with Ar for 2 h before use. Linear sweep voltammetry (LSV) measurements were carried out at a scan rate of 2 mV s−1 in 1.0 M KOH solution. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 100 mV s−1. Electrochemical impedance spectroscopy (EIS) was obtained at the frequency range of 100 KHz to 0.1 Hz with a sampling rate of 12 points per decade, and the data obtained was fitted. All potentials were converted to the reversible hydrogen electrode (RHE) according to ERHE = ESCE + E0SCE + 0.059 × pH. X-ray absorption fine structure measurements The X-ray absorption fine structure (XAFS) spectra (Ni K-edge) were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The storage rings of BSRF were operated at 2.5 GeV with an average current of 250 mA. Using a Si(111) double-crystal monochromator, the data collection was carried out in transmission/fluorescence mode using an ionization chamber. All spectra were collected in ambient conditions. XAFS analysis and results The acquired extended XAFS (EXAFS) data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The k3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data of Ni K-edge were Fourier transformed to real (R) space using a Hanning windows (dk = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least–squares curve parameter fitting was performed using the ARTEMIS module of the IFEFFIT software packages. Results and Discussion As shown in Figures 1a and 1b, a two-step in situ growth strategy was used to selectively synthesize the GDY–NiTCNQ catalysts with D–A–D (GDY/TCNQ/Ni) structure, including the first in situ growth of NiTCNQ nanorods and the subsequent growth of the GDY film on the outer layer of the NiTCNQ nanorods that leads to the formation of GDY–NiTCNQ nanowires. Typically, with the dropwise addition of water into the acetonitrile solution of TCNQ, the Ni ions released from the substrate and then interacted with TCNQ forming the A–D (TCNQ–Ni) structure. The NiTCNQ assembled into highly ordered NiTCNQ-nanorods array on the surface of NF.37 The NiTCNQ was then immersed into the acetone solution, after which the HEB precursor was added to grow GDY films on the NiTCNQ surface with the morphology gradually changing from nanorod to nanowire. During this process, the D–A–D (GDY/TCNQ/Ni) structure was formed. Figure 1 | Schematic representation of the controlled synthesis route to the catalytic system of donor–acceptor–donor (GDY/TCNQ/Ni) structures through in situ growth of (a) NiTCNQ nanorods and (b) GDY–NiTCNQ. Download figure Download PowerPoint Figure 2a shows that the color of the samples changed from gray for NF to dark green for NiTCNQ and black for GDY–NiTCNQ during the synthetic process. SEM images clearly show that the smooth surface of 3D NF (Figures 2b–2d) was covered by a film of NiTCNQ nanorods (average diameter, ∼210 nm; Figures 2e–2h). As shown in Supporting Information Figure S1, the NiTCNQ has a flat surface and a fringe lattice spacing of 0.254 nm. The electrostatic potential (ESP) of NiTCNQ (Figure 2h) confirms the interactions between Ni ions and TCNQ with evident CT from Ni to TCNQ, which also demonstrates the formation of the A–D structure. With the growing of GDY on the NiTCNQ surface, the morphology of the nanorods of NiTCNQ changed to nanowires for GDY–NiTCNQ (Figures 2i–2l). As revealed by HRTEM images (Figures 2l and 2m), an obvious shell layer was observed outside of the NiTCNQ core, confirming the successful growth of GDY on the surface of NiTCNQ (Figure 2m). GDY–NiTCNQ exhibits a rough and porous surface, which is beneficial for enlarging the active surface area and the number of active sites. The lattice spacing of the core structure of GDY–NiTCNQ is 0.254 nm ( Supporting Information Figure S2), in accordance with the NiTCNQ, indicating the structure of NiTCNQ was well maintained during the GDY growth process. Scanning TEM (STEM) and EDS mapping images (Figures 2n–2q and Supporting Information Table S1) confirm the presence of carbon, nitrogen, and nickel elements in the GDY–NiTCNQ sample without any impurities. Ni and N are densely distributed in the inner space and sparsely distributed in the edge, and C is evenly distributed throughout the whole material. These results demonstrated the formation of the double layer interface of GDY/TCNQ/Ni, and the GDY film on the outer layer can effectively prevent the catalyst corrosion, which guarantees the long-term stability of the catalyst. Figure 2 | Morphological characterizations of GDY–NiTCNQ and NiTCNQ. (a) Photographs of (i) NF, (ii) NiTCNQ, and (iii) GDY–NiTCNQ. SEM images of (b–d) NF, (e–g) NiTCNQ, and (i–k) GDY–NiTCNQ samples. (h) Electrostatic potential of NiTCNQ. TEM images of (l and m) GDY–NiTCNQ. Elemental mapping images (n–p) of C, Ni, and N atoms in the GDY–NiTCNQ. Download figure Download PowerPoint XRD and XPS measurements were performed to characterize the compositions and structures of the samples. As shown in Figure 3a, the intensity of the peaks for GDY–NiTCNQ decreased as compared with that of NiTCNQ, which might be due to the growth of GDY on NiTCNQ. The XPS survey spectra confirm the presence of Ni, C, and N elements in accordance with the EDS result ( Supporting Information Figures S3 and S4). Compared with pure NiTCNQ (Figure 3b), the C 1s XPS spectra of GDY–NiTCNQ shifted to higher binding energies (BE) by 0.75 eV, revealing the CT from GDY to NiTCNQ. The appearance of an sp-C peak in the C 1s spectra of GDY–NiTCNQ (Figure 3c) indicates the successful growth of GDY on NiTCNQ. Moreover, an independent new peak at 291.96 eV originating from the π–π* shakeup satellite implies the interactions between NiTCNQ and GDY.7 In the Ni 2p region (Figure 3d), the peaks for GDY–NiTCNQ decreased by 1.23 eV as compared with that of NiTCNQ ( Supporting Information Figure S5), revealing the remarkable CT between NiTCNQ and GDY. For GDY–NiTCNQ, the peaks for Ni2+ 2p3/2 and Ni2+ 2p1/2 appear at 856.39 and 873.72 eV, and Ni3+ 2p3/2 and Ni3+ 2p1/2 appear at 858.30 and 875.58 eV, respectively, indicating the presence of the mixed valence states of nickel species. The N 1s (Figure 3e) and O 1s (Figure 3f) peaks of GDY–NiTCNQ located at higher BE than those of pristine NiTCNQ further reveal the presence of electron-transfer in the GDY–NiTCNQ sample.38,39 The Raman ( Supporting Information Figure S6) and Fourier transform infrared (FTIR) ( Supporting Information Figure S7) measurements of the GDY layer were conducted. As shown in Supporting Information Figure S6, pure NiTCNQ spectra features several sharp characteristic peaks. GDY–NiTCNQ exhibits characteristic peaks corresponding to the D-band, G-band, and the vibration of conjugated diine links (2189.8 and 1926.2 cm−1), while the characteristic peaks of NiTCNQ diminished. Moreover, as shown in Supporting Information Figure S7, the FTIR shows the characteristic peak corresponding to GDY, as compared with that of pure NiTCNQ. These results demonstrated the successful growth of GDY on the surface of NiTCNQ. Figure 3 | Structural characterization of GDY–NiTCNQ. (a) XRD spectra of NiTCNQ and GDY-NiTCNQ. XPS spectra for GDY–NiTCNQ and NiTCNQ of (b and c) C 1s, (d) Ni 2p, (e) N 1s, and (f) O 1s. The inset figure in (b) represents the deformation charge density of NiTCNQ. (g) The normalized Co K-edge XANES spectra and (h) the first derivative XANES of GDY–NiTCNQ along with the references. (i) EXAFS spectra of GDY–NiTCNQ at the Co K-edge. Download figure Download PowerPoint Ni K-edge X-ray absorption near-edge structure (XANES) for the samples was performed. Figure 3g shows that the GDY–NiTCNQ exhibits the higher edge energy position than those of NiCl2 and NiO, suggesting the average valence state of nickel in GDY–NiTCNQ is larger than Ni2+. This also implies the mixed valence state of nickel in GDY–NiTCNQ (Figure 3h). The Fourier transforms of the Ni K-edge EXAFS spectrum of GDY–NiTCNQ exhibited a dominant peak at 1.6 Å, which could be assigned to the nearest shell coordination of the Ni–N bond (Figure 3i). The weak peak at 2.5 Å might be attributed to the aggregation of a small number of nickel atoms. The fitted Ni–N coordination number for GDY–NiTCNQ was 4.9 with an average Ni–N bond distance of 2.05 Å ( Supporting Information Table S2), smaller than that of hexa-coordinated NiTCNQ,40 which might be ascribed to the strong interactions between the GDY and NiTCNQ species. The obviously enhanced CT ability at the double layer interfaces of GDY/TCNQ/Ni was expected to endow the catalyst with incomplete CT and the infinite distribution of active sites originating from highly asymmetric surface charge distribution. The significantly increased number of active sites would further accelerate CT and lower energy barriers of the reaction. These are all beneficial to improve the catalytic activity of the catalysts. The OER catalytic performances of the samples were studied using a three-electrode system by testing LSV in O2-saturated 1.0 M KOH solution at the scan rate of 2 mV s−1. All polarization curves were corrected by the iR-compensation. NiTCNQ, NF, and RuO2 were also tested in the same conditions for reference. As shown in Figure 4a, the anodic peak appearing in the range of 1.3–1.4 V versus RHE referred to the nickel oxidation from Ni(II) to Ni(III), and the following peak represented the OER catalytic activity. GDY–NiTCNQ showed the best OER activity with the smallest overpotential of 218 mV at 10 mA cm−2 compared with pure NiTCNQ (343 mV) and commercial RuO2 (282 mV). Moreover, GDY–NiTCNQ exhibited a lower of mV than NiTCNQ mV NF mV and RuO2 mV indicating it the reaction kinetics for OER evolution (Figure For further of the OER activity, the frequency was At the overpotential of GDY–NiTCNQ a higher of s−1 than NiTCNQ NF and of the electrocatalysts ( Supporting Information Table Figure | OER and HER (a) curves of NiTCNQ, NF and RuO2 for OER at the scan rate of 2 mV s−1. (b) of the samples for curves of GDY–NiTCNQ obtained before and after OER (d) O 1s XPS spectra of GDY–NiTCNQ after OER (e) Ni 2p XPS spectra and (f) the of Ni2+ of GDY–NiTCNQ obtained after OER (g) curves of NiTCNQ, NF, and for HER at the scan rate of 2 mV s−1. (h) of the materials for HER. (i) curves of GDY–NiTCNQ obtained before and after HER O 1s XPS Ni 2p XPS and the of Ni2+ in GDY–NiTCNQ samples. Download figure Download PowerPoint stability of the catalyst is of great significance for As shown in Supporting Information Figure GDY–NiTCNQ maintained its catalytic activity for h under at mA revealing its long-term stability ( Supporting Information Figure GDY–NiTCNQ also a in catalytic activity during the tests for (Figure The of the GDY–NiTCNQ catalysts after the stability characterized by SEM ( Supporting Information Figure and TEM ( Supporting Information Figure showed no and the nanorod morphology was well the high stability of the as-prepared catalysts. situ XPS tests were on GDY–NiTCNQ to the structural evolution during the The O 1s XPS spectra showed two new peaks at and eV lattice oxygen in metal after the tests (Figure was also found that the position of the Ni 2p peak shifted to lower BE and the of the Ni2+ increased gradually during the catalysis (Figures and the of Ni2+ at these results confirm the generation of the was with the of Ni2+ in OER which a key role in the oxygen The HER catalytic activity of GDY–NiTCNQ was further tested in 1.0 M KOH solution. Figure shows the LSV curves of GDY–NiTCNQ exhibited a small overpotential of 61 mV at 10 mA which was smaller than NiTCNQ mV) and NF and to the catalyst mV). The corresponding of GDY–NiTCNQ, NiTCNQ, NF, and were and mV (Figure The for GDY–NiTCNQ, NiTCNQ, and NF were and respectively, at the overpotential of 100 All results the obvious of HER activity. Moreover, there was a in current density after indicating the high stability (Figure The of the GDY–NiTCNQ catalysts after the HER tests ( Supporting Information Figure for Supporting Information Figure for were well the high stability of the as-prepared catalysts. To reveal the of the catalytic activity, the in situ XPS measurements were conducted. As shown in Figure new peaks corresponding to the were observed at eV in the O 1s XPS spectra as compared with the as-prepared catalysts. The of Ni2+ decreased as the HER (Figures and the of state of nickel which might be the of of HER activity. For the further of the activity of GDY–NiTCNQ, were to the CT The parameters obtained were fitted with solution CT and absorption to the (Figure GDY–NiTCNQ exhibited the of and compared with NiTCNQ = = and NF = = which the conductivity and transfer process. The electrochemical active surface area was also through layer under various scan of (Figure and Supporting Information Figure GDY–NiTCNQ showed a larger of cm−2 than NiTCNQ and NF indicating the larger surface area and higher surface assigned to the introduction of GDY. on OER and HER performance, GDY–NiTCNQ was used as and in a for OWS. The catalytic system exhibited high with a low cell voltage of 1.40 V to 10 mA lower compared with those V) (Figure and the catalysts, including [email protected] V at 10 mA [email V at 10 mA V at 10 mA and so on (Figure and Supporting Information Table S4). During the of H2 and were from the and (Figure The catalytic in in basic demonstrates the promising in applications of GDY–NiTCNQ. Figure | Electrochemical properties and performance. (a) of samples in 1.0 M KOH solution. for and the fitting is (b) The at V of scan for samples. of the (d) activities of GDY–NiTCNQ and electrocatalysts. (e) The water in a working Download figure Download PowerPoint We report an in situ assembly growth method that controls the growth of NiTCNQ on the surface of GDY, to the formation of a unique D–A–D (GDY/TCNQ/Ni) structure with multiple CT. In such catalytic systems, the amount of CT between and acceptor is that incomplete CT between and surface charge distribution the number of active sites and intrinsic activity of the catalytic system, a new for the activity of the catalytic system through incomplete CT between the The results show that the by incomplete CT demonstrates excellent electrical conductivity and typical semiconductor characteristics, to the high catalytic activity of such a catalytic system in OER, HER, and OWS. We found that an electrolytic cell consisting of the CT salt as a catalyst provided a 1.40 V ultra-small cell voltage up to 10 mA is that the outer GDY film effectively prevented the corrosion of the which is to its long-term stability. This work provides a new way to selectively novel catalysts with high selectivity, activity, and stability. Supporting Information Supporting Information is the for the of Figure and Table of is no of interest to Information This research was by a from the and of the Science of and and the of the Chinese Academy of

Supercapacitors have the characteristics of high power density, long cycle life, and fast charge and discharge rates, making them promising alternatives to traditional capacitors and batteries. The use of transition-metal compounds as electrode materials for supercapacitors has been a compelling research topic in recent years because their use can effectively enhance the electrical performance of supercapacitors. The current research on capacitor electrode materials can mainly be divided into the following three categories: carbon-based materials, metal oxides, and conductive polymers. Nickel hydroxide (Ni(OH)2) is a potential electrode material for use in supercapacitors. Depending on the preparation conditions, two crystal phases of nickel hydroxide, α and β, can be produced. When compared to α-NiOH, the structure of β-Ni(OH)2 does not experience ion intercalation. As a result, the carrier transmission rate of α-Ni(OH)2 is slower, and its specific capacitance value is smaller. Its carrier transport rate can be improved by adding conductive materials, such as graphene. β-Ni(OH)2 was chosen as an electrode material for a supercapacitor in this study. Homemade low-pressure chemical vapor deposition graphene (LPCVD-Graphene) conductive material was introduced to modify β-Ni(OH)2 in order to increase its carrier transport rate. The LPCVD method was used to grow high-quality graphene films on three-dimensional (3D) nickel foam substrates. Then, a hydrothermal synthesis method was used to grow β-Ni(OH)2 nanostructures on the 3D graphene/nickel foam substrate. In order to improve the electrical properties of the composite structure, a high-quality graphene layer was incorporated between the nickel hydroxide and the 3D nickel foam substrate. The effect of the conductive graphene layer on the growth of β-Ni(OH)2, as well as its electrical properties and electrochemical performance, was studied. When this β-Ni(OH)2/CVD-Graphene/3D-NF (nickel foam) material was used as the working electrodes of the supercapacitor under a current density of 1 A/g and 3 A/g, they exhibited a specific capacitance of 2015 F/g and 1218.9 F/g, respectively. This capacitance value is 2.62 times higher than that of the structure without modification with a graphene layer. The capacitance value remains at 99.2% even after 1000 consecutive charge and discharge cycles at a current density of 20 A/g. This value also improved compared to the structure without graphene layer modification (94.7%).

Synthesis of NiGa2S4-rGO on nickel foam as advanced electrode for flexible solid-state supercapacitor with superior energy density

Ti3C2Tx MXene, as a common two-dimensional material, has a wide range of applications in electrochemical energy storage. However, the surface forces of few-layer or monolayer Ti3C2Tx MXene lead to easy agglomeration, which hinders the demonstration of its performance due to the characteristics of layered materials. Herein, we report a facile method for preparing monolayer Ti3C2Tx MXene on nickel foam to achieve a self-supporting structure for supercapacitor electrodes under high electrostatic fields. Moreover, the specific capacitance varies with the deposition of different-concentration monolayer Ti3C2Tx MXene on nickel foam. As a result, Ti3C2Tx/NF has a high specific capacitance of 319 mF cm−2 at 2 mA cm−2 and an excellent long-term cycling stability of 94.4% after 7000 cycles. It was observed that the areal specific capacitance increases, whereas the mass specific capacitance decreases with the increasing loading mass. Attributable to the effect of the high electrostatic field, the self-supporting structure of the Ti3C2Tx/NF becomes denser as the concentration of the monolayer Ti3C2Tx MXene ink increases, ultimately affecting its electrochemical performance. This work provides a simple way to overcome the agglomeration problem of few-layer or monolayer MXene, then form a self-supporting electrode exhibiting excellent electrochemical performance.

Ti, F Codoped Sodium Manganate of Layered P2-Na _0.7 MnO _2.05 Cathode for High Capacity and Long-Life Sodium-Ion Battery

A solution strategy performed at room temperature was presented for the in situ synthesis of SnO nanoparticles. SnO nanoparticles were prepared through the following sequent procedure: (1) preparation of rod-like Cd(OH)Cl utilizing the reaction between Cd2+ ions and epoxide; (2) production of Sn21Cl16(OH)14O6 template through the cation exchange reaction between Cd(OH)Cl and Sn2+ ions; (3) formation of SnO nanoparticles on nickel foam by the in situ reaction in strong alkaline electrolyte solution before electrochemical measurement. The as-prepared SnO had very small particle size and ordered nanostructure of particulate sheet, therefore resulting in its excellent electrochemical performance including high specific capacitance and high electrochemical stability for the charge-discharge cycle. Hence, the SnO nanoparticles synthesized in this work could be considered as one promising metal oxide for the utilization as electrode material in supercapacitor.

Manganese oxide is a fascinating material for use as a thin-film electrode in supercapacitors. Herein, the consequences of copper incorporation on spray pyrolyzed manganese oxide thin films and their electrochemical performance were investigated. The Cu-incorporated manganese oxide thin films were deposited by spray pyrolysis, and their structural and electrochemical properties were thoroughly evaluated. The formation of the spinel Mn3O4 phase with effective Cu incorporation was confirmed by X-ray diffraction investigation. Through Raman studies, it was noticed that mixed phases of manganese oxide tend to form after Cu incorporation, and this result was also reflected in X-ray photoelectron spectroscopic studies. The surface morphology and roughness were also altered by the addition of copper. However, electrochemical measurements implied a reduction in the specific capacitance upon copper inclusion. The cyclic voltammetry test indicated a specific capacitance of 132 F/g for Mn3O4 electrodes, but a substantial drop for copper-incorporated samples due to the mixed manganese phase. The decremental tendency was further supported by galvanostatic charge-discharge studies and electrochemical impedance spectroscopic measurements. These results provide valuable insights into the effects of copper addition in manganese oxide thin-film-based electrodes for energy storage applications.

The demand for high-performance supercapacitors has driven extensive research into novel electrode materials with superior electrochemical properties. This study explores the supercapacitive behavior of quaternary CuNiCoZnO (CNCZO) films engineered into a three-dimensional (3D) flower-like morphology and developed on versatile substrates, including carbon cloth, stainless steel mesh, and nickel foam. The unique structural design, comprising interconnected nanosheets, enhances the electroactive surface area, facilitates ion diffusion, and improves charge storage capability. The synergistic effect of the multi-metallic composition contributes to remarkable electrochemical characteristics, including high specific capacitance, excellent rate capability, and outstanding cycling stability. Furthermore, the influence of different substrates on the electrochemical performance is systematically investigated to optimize material–substrate interactions. Electrochemical evaluations reveal outstanding specific capacitance values of 2318.5 F/g, 1993.7 F/g, and 2741.3 F/g at 2 mA/cm2 for CNCZO electrodes on stainless steel mesh, carbon cloth, and nickel foam, respectively, with capacitance retention of 77.3%, 95.7%, and 86.1% over 5000 cycles. Furthermore, a symmetric device of CNCZO@Ni exhibits a peak specific capacitance of 67.7 F/g at a current density of 4 mA/cm2, a power density of 717.4 W/kg, and an energy density of 25.6 Wh/kg, maintaining 84.5% stability over 5000 cycles. The straightforward synthesis of CNCZO on multiple substrates presents a promising route for the development of flexible, high-performance energy storage devices.

Structural and electrochemical performance of Al-substituted β-Ni(OH)2 nanosheets electrodes for nickel metal hydride battery

Facile synthesis of graphene based mixed metal sulphide nanocomposite for energy storage applications

The electrochemical performance of lithium-oxygen (Li-O2) batteries can be markedly improved through designing the architecture of cathode electrodes with sufficient spaces to facilitate the diffusion of oxygen and accommodate the discharge products, and optimizing the cathode catalyst to promote the oxygen reduction reaction and oxygen evolution reaction (OER). Herein, we report the synthesis of ruthenium (Ru) nanocrystal-decorated vertically aligned graphene nanosheets (VGNS) grown on nickel (Ni) foam. As an effective binder-free cathode catalyst for Li-O2 batteries, the Ru-decorated VGNS@Ni foam can significantly reduce the charge overpotential via the effects on the OER and achieve high specific capacity, leading to an enhanced electrochemical performance. The Ru-decorated VGNS@Ni foam electrode has demonstrated low charge overpotential of ~0.45 V and high reversible capacity of 23 864 mAh g−1 at the current density of 200 mA g−1, which can be maintained for 50 cycles under full charge and discharge testing condition in the voltage range of 2.0–4.2 V. Furthermore, Ru nanocrystal decorated VGNS@Ni foam can be cycled for more than 200 cycles with a low overpotential of 0.23 V under the capacity curtained to be 1000 mAh g−1 at a current density of 200 mA g−1. Ru-decorated VGNS@Ni foam electrodes have also achieved excellent high rate and long cyclability performance. This superior electrochemical performance should be ascribed to the unique three-dimensional porous nanoarchitecture of the VGNS@Ni foam electrodes, which provide sufficient pores for the diffusion of oxygen and storage of the discharge product (Li2O2), and the effective catalytic effect of Ru nanocrystals on the OER, respectively. Ex situ field emission scanning electron microscopy, X-ray diffraction, Raman and Fourier transform infrared measurements revealed that Ru-decorated VGNS@Ni foam can effectively decompose the discharge product Li2O2, facilitate the OER and lead to a high round-trip efficiency. Therefore, Ru-decorated VGNS@Ni foam is a promising cathode catalyst for rechargeable Li-O2 batteries with low charge overpotential, long cycle life and high specific capacity. A three-dimensional graphene-based material has an architecture that enhances lightweight lithium-oxygen batteries in multiple ways. A team led by Guoxiu Wang from the University of Technology Sydney in Australia searchs for a cathode that would alleviate some problems associated with lithium-oxygen batteries, such as their poor cycling stability and the high voltages needed to recharge them. The researchers designed a more porous type of cathode that contained vertical graphene nanosheets to prevent oxygen pathways becoming clogged by discharge products. Attaching the open-network, highly conductive carbon nanomaterials to nickel foam yielded a material containing many channels and voids for lithium by-products. Adding catalytic ruthenium nanoparticles to the graphene network resulted in a cathode that dropped recharging overpotentials significantly — from 1.5 to 0.23–V — while remaining stable for over 200 cycles. We report the synthesis of ruthenium nanocrystal-decorated vertically aligned graphene nanosheets grown on nickel foam. As an effective binder-free cathode for Li-O2 batteries, it can significantly reduce the charge overpotential via their effects on the oxygen evolution reaction and achieve high specific capacity, leading to an enhanced electrochemical performance.

Two kinds of electrode materials Ni(OH)2 and Ni(OH)2@Zn(OH)2 composite are fabricated on nickel foam. Electrochemical experiments indicate Ni(OH)2@Zn(OH)2 composite deserves further study due to high specific capacitance and good cycle stability, so that it can achieve energy storage and conversion as much as possible. When the hydrothermal time is different, the electrochemical performance of the sample is also different. Accurately, samples can obtain better electrochemical performance at 15 h, and the maximum specific capacitance of Ni(OH)2@Zn(OH)2 is 7.87 F cm−2 compared to Ni(OH)2 (0.61 F cm−2) at 5 mA cm−2. Even at 50 mA cm−2, specific capacitance is 5.24 F cm−2 and rate capability is 66.6%. Furthermore, Ni(OH)2@Zn(OH)2-15 h loses 19.8% after 1000 cycles, revealing the composite has an outstanding stable cycle. These properties also indicate Ni(OH)2@Zn(OH)2-15 h is a promising electrode material.

NiCo2O4 nanoleaf arrays (NCO NLAs) and NiCo2O4/NiCO2O4 nanofile arrays (NCO/NCO NFAs) material was fabricated on flexible nickel foam (NF) using a facile hydrothermal approach. The electrochemical performance, including the specific capacitance, charge/discharge cycles, and lifecycle of the material after the hydrothermal treatment, was assessed. The morphological and structural behaviors of the NF@NCO NLAs and NF@NCO/NCO NFAs electrodes were analyzed using a range of analysis techniques. The as-obtained nanocomposite of the NF@NCO/NCO NFAs material delivered outstanding electrochemical performance, including an ultrahigh specific capacitance (Cs) of 2312 F g−1 at a current density of 2 mA cm−2, along with excellent cycling stability (98.7% capacitance retention after 5000 cycles at 5 mA cm−2). These values were higher than those of NF@NCO NLAs (Cs of 1950 F g−1 and 96.3% retention). The enhanced specific capacitance was attributed to the large electrochemical surface area, which allows for higher electrical conductivity and rapid transport between the electrons and ions as well as a much lower charge-transfer resistance and superior rate capability. These results clearly show that a combination of two types of binary metal oxides could be favorable for improving electrochemical performance and is expected to play a major role in the future development of nanofile-like composites (NF@NCO/NCO NFAs) for supercapacitor applications.

Supercapacitors or electrochemical capacitors are receiving greater interest because of their high-power density, long life, low maintenance, and safer operation. The low energy density of such devices needs to improve. In this work, copper sulfide (CuS), a cheap and environment-friendly compound, is used along with graphene oxide (GO) to provide high energy and power densities. CuS nanoparticles and its nanocomposites with GO was synthesized using a facile method for supercapacitor applications. For the synthesis of CuS, an appropriate amount of copper acetate was dissolved in a water-butanol solution for 30 minutes followed by the addition of thiourea. The final solution was then transferred into Teflon-lined stainless-steel autoclave and heated at 180oC for 24 hours. The resulting participate of CuS was recovered using a centrifuge, washed with DI water, and dried in air for 24 hours. A similar procedure was followed for the synthesis of CuS-GO nanocomposite with the addition of appropriate amount of GO in the water-butanol solution prior to the addition of copper acetate and thiourea. Phase purity of the synthesized materials was determined by using X-ray diffraction (XRD) studies. XRD patterns of CuS showed the peaks at 2θ angle of 10.38o, 28.42o, 28.88o, 32.48o, 33.18o, 46.54o, 48.28o, and 59.68o corresponding to (100), (101), (102), (006), (110), (108), (116) planes, respectively, which indicates the covellite CuS hexagonal structure [1,2]. There were no additional peaks found in the XRD pattern of CuS-GO composite, confirming the phase purity of the synthesized materials. TEM images revealed nano-scale morphology of the synthesized particles. The particle size of CuS and CuS/GO composite were found as 95.11 nm, and 64.70 nm, respectively. All the electrochemical measurements were conducted in standard three-electrode configuration, using a platinum wire as a counter electrode and Hg/HgO as a reference electrode. CuS and its composite with graphene oxide on nickel foam were used as working electrodes. All the electrochemical measurements were performed in 3M KOH solution. Electrochemical properties of the synthesized samples were investigated using cyclic voltammetry and galvanostatic charge-discharge studies. The specific capacitance was calculated from cyclic voltammogram, obtained at various scan rate between 10 - 300 mV/s. The specific capacitance of CuS and CuS-GO nanocomposite was found to be 400 and 600 F/g, respectively at a scan rate of 10 mV/s. The detailed results on synthesis, physical, and electrochemical characterization will be presented.

Activated carbon (AC) was synthesized from rice husks using the chemical activation method with KOH, NaOH,a combination of (NaOH + Na 2 CO 3 ), and a combination of (KOH + K 2 CO 3 ) as the chemical activating reagents. The activatedcarbon with the highest surface area (around 2000 m 2 /g) and high porosity, which allows the absorption of a large number ofions, was applied as electrode material in electric double layer capacitors (EDLCs). The AC for EDLC electrodes is requiredto have a high surface area and an optimal pore size distribution; these are important to attain high specific capacitance of theEDLC electrodes. The electrodes were fabricated by compounding the rice husk activated carbons with super-P and mixed withpolyvinylidene difluoride (PVDF) at a weight ratio of 83:10:7. AC electrodes and nickel foams were assembled with potassiumhydroxide (KOH) solution as the electrolyte. Electrochemical measurements were carried out with a three electrode cell using6 M KOH as electrolyte and Hg/HgO as the reference electrode. The specific capacitance strongly depends on the porestructure; the highest specific capacitance was 179 F/g, obtained for the AC with the highest specific surface area. Additionally,different activation times, levels of heating, and chemical reagents were used to compare and determine the optimal parametersfor obtaining high surface area of the activated carbon.Key wordssupercapacitor, EDLC, activated carbons, specific capacitance, rice husk.