Sub‐1 nm High‐Entropy Materials for Electrochemical Applications

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

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

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

First-principles and experimental insight of high-entropy materials as electrocatalysts for energy-related applications: Hydrogen evolution, oxygen evolution, and oxygen reduction reactions

Black phosphorus quantum dots (BPQDs) are a very promising zero-dimensional nanomaterial that has attracted considerable interest due to its exceptional characteristics, including high carrier mobility and excellent optical properties with tunable bandgap. BPQDs are ideal for potential applications in optoelectronics and energy storage devices. They are used in solar cells, photodetectors, supercapacitors, and lithium and sodium ion batteries. This paper discusses various BPQD synthesis routes, from scalability to size control, focusing on their potential applications in energy storage devices and optoelectronics. The study primarily focuses on integration of BPQDs with diverse materials, including graphene, carbon nanotubes, polymers, and metal oxides. Addressing issues of stability, scalability and conductivity will pave the way for their wider practical application. Furthermore, this review discusses the future outlook for BPQD composites in developing the next generation of technologies, emphasising their potential to enhance the efficiency, flexibility and sustainability of energy and optoelectronic systems. • BPQDs are an important class of zero dimensional nanomaterials for energy storage and optoelectronic devices. • Scalable synthesis enables precise control of BPQD size and properties. • BPQD based composites increase charge transport, cycling stability and storage capability in energy storage devices. • Engineered BPQDs materials boost efficiency in batteries, supercapacitor and solar cells. • Hybrid BPQD based systems enable high performance, flexible, wearable, and light-responsive devices.

There is a high demand for alternative clean energy sources as the world faces serious environmental and energy crisis. Renewable energy sources such as solar and wind are highly intermittent and diffuse which leaves the prime option of storing the clean energy in form of chemical bonds and converting it back efficiently when required. Such techniques must be carried out using electrochemical methods in particular water splitting and rechargeable metal air batteries (rechargeable MAB). Water electrolysis has captured high attention as a green method to produce hydrogen which is the principal constituent behind a promising ‘hydrogen economy’. MABs, particularly zinc-air batteries (ZAB) are considered as potential candidates for post-lithium batteries as they exhibit much higher theoretical energy density. 
\nThe water splitting electrolyser carries oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode, meanwhile, in a rechargeable ZAB cathode oxygen reduction reaction (ORR) occurs during the discharging process and OER during the recharging process. These reactions determine the efficiency of these electrochemical techniques. Since these reactions are thermodynamically uphill, they require highly active electrocatalysts to enhance the overall efficiency. Currently, the state-of-the-art electrocatalysts for OER, HER, and ORR are precious metal‐based materials. The high cost, poor stability, and scarcity of such electrocatalysts limit the industrial usage of these techniques. Since cobalt is one of the most abundant transition metals on the earth and has attracted much interest as a non-precious electrocatalyst towards all HER, OER, and ORR, they have to be analyzed for their bifunctional activities electrocatalysts (towards ORR/OER or HER/OER). Moreover, fabrication and operational cost factors can be also reduced by design bifunctional electrocatalysts. The hypothesis of our work is that the bifunctional electrocatalytic activities of Co-based materials can be enhanced through proper morphology engineering, integration of metal components with carbon supports, stabilization of single atoms, and structural engineering approaches. 
\nTo realize our hypothesis, initially, earth abundant metals-based phosphide compound (NiCo2Px) with unprecedented surface morphology and shape tuning was synthesized via facile hydrothermal treatments. One of the NiCo2Px was designed to have active sharp edges (spiked) on a hollow spherical surface while the other was spherical with a smooth surface. The highly exposed, branched spikes-covered hollow structure of NiCo2Px shows remarkable performance enhancement for HER and OER in a wide range of pH solutions. The active site density and synergistic effects were tailored according to the surface morphological features of this catalyst. An alkaline electrolyzer assembled using the optimized catalyst produced 10 mAcm-2 of current density at 1.62 V without almost any decrease in this value even after the continuous run for 50 hours. 
\nTo understand the effect of integrating metal components with carbon supports, the second work elaborate engineering earth abundant bimetal/metal oxide nanoparticles encapsulated in a mesoporous carbon framework for ORR and OER activity. The Fe, Co, and CoO containing electrocatalyst was developed by direct annealing of N enriched ZIF superstructures in the N2 environment. Owing to the factors like high surface area carbon framework with uniform dopant distribution, sufficient mesopore density, and presence of metal-Nx/C structures, this electrocatalyst had a comparable performance with precious metal based electrocatalysts. Moreover, this work also revealed the significance of annealing temperature in tuning the bifunctional activity of such materials. However, it was found that the optimized material was not much stable during the electrocatalytic process.
\nTo enhance the stability and activity of ZIF derived materials, in the third part, isolated single atomic sites of Fe and Co coordinated with nitrogen on carbon support (Fe,Co-SA/CS) was designed and deployed. The (Fe,Co-SA/CS) was prepared by taking the advantage of unique structure and pore characteristics of ZIF and molecular size of Ferrocene, a Fe-containing species. Fe single atomic sites neighbouring Co sites facilitated relatively easy reactant adsorption and charge transfer on Co active sites, which enhanced bifunctional activity of this material towards ORR and OER in alkaline electrolyte. This resulted in the Fe,Co-SA/CS attaining a similar oxygen electrode activity with that of commercial electrocatalysts. 
\nFinally, a fast and facile method for improving the activity through the structural engineering approach is elaborated when compared with the complex synthesis processes of previous works. Here, a cheap oxygen reduction electrocatalyst based on metal boride - N doped carbon heterointerfaces (CoB-Nx/C) was developed and analyzed. Transition metal borides are known to show superior OER performance when compared with their counterparts, however, their ORR performance was unexplored due to severe oxidation effects. Meanwhile, as prepared CoB-Nx/C outperformed the Pt/C electrocatalyst towards ORR due to its nanocrystalline-nanosheet nature, kinetic enhancement due to charge redistribution at heterointerfaces, and incorporation of the N doped carbon. Also, annealing temperature as well as the amount of carbon and nitrogen sources was optimized to tune the activity of this material. At last, a ZAB was assembled using CoB-Nx/C structures that provided an open-circuit voltage of 1.50 V with excellent cycling stability.

The demanding all-in-one electrocatalyst system for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in zinc-air batteries or water splitting requires elaborate material manufacturing, which is usually complicated and time-consuming. Efficient interface engineering between MXene and highly active electrocatalytic species (CoS2) is, herein, achieved by an in situ hydrothermal growth and facile sulfurization process. The CoS2@MXene electrocatalyst is composed by one-dimensional CoS2 nanowires and two-dimensional MXene nanosheets, which lead to a hierarchical structure (large specific surface area and abundant active sites), a spatial electron redistribution (high intrinsic activity), and high anchoring strength (superior performance stability). Therefore, the electrocatalyst achieves enhanced catalytic activity and long-time stability for ORR (a half-wave potential of 0.80 V), OER (an overpotential of 270 mV at 10 mA cm−2, i.e., η10 = 270 mV) and HER (η10 = 175 mV). Furthermore, the asymmetry water splitting system based on the CoS2@MXene composites delivers a low overall voltage of 1.63 V at 10 mA cm−2. The solid-state zinc-air batteries using CoS2@MXene as the air cathode display a small charge-discharge voltage gap (0.53 V at 1 mA cm−2) and superior stability (60 circles and 20-h continuous test). The energy interconversion between the chemical energy and electricity can be achieved by a self-powered system via integrating the water splitting system and quasi-solid-state zinc-air batteries. Supported by in situ Raman analyses, the formation of cobalt oxyhydroxide species provides the active sites for water oxidation. This study paves a promising avenue for the design and application of multifunctional nanocatalysts.

We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-en...

The development of low‐cost multifunctional electrocatalysts with high activity for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is critical for the advancement of sophisticated energy conversion and storage devices. Herein, a trifunctional Ni(S0.51Se0.49)2@NC catalyst is designed and fabricated using a dianionic regulation strategy. Synchrotron radiation X‐ray absorption spectroscopy and density functional theory calculations reveal that simultaneous sulfidation and selenization can induce the electronic delocalization of Ni(S0.51Se0.49)2 active sites to enhance the adsorption of *OOH/*OH intermediate for ORR/OER and H* intermediate for HER. The OER and HER mechanisms are revealed by in situ Raman spectroscopy. The Ni(S0.51Se0.49)2@NC exhibits trifunctional catalytic activity for the HER (111 mV at 10 mA cm−2), OER (320 mV at 10 mA cm−2), and ORR (half‐wave potential of 0.83 V). The rechargeable zinc–air batteries (ZABs) exhibit an open‐circuit voltage of 1.46 V, a specific capacity of 799.1 mAh g−1, and excellent stability for 1000 cycles. The water electrolytic cell using Ni(S0.51Se0.49)2@NC electrodes delivers a current density of 10 mA cm−2 at a cell voltage of 1.59 V, and it can be powered using the constructed ZABs. These findings contribute to developing low‐cost and efficient non‐noble metal multifunctional catalysts.

Environmental concerns and rising demand for portable electronics and electric vehicles have stimulated the development of energy storage devices such as batteries and supercapacitors, towards higher energy density and power density, which significantly depend on the advancement of new materials used in these devices. Most studies in the literature utilize noble metals as catalysts, complex fabrication procedures, which are not easily scalable. Moreover, these techniques fabricate powder-based materials, which have to be blended with electrically insulating polymeric binders and coated onto conductive substrates to be utilized in a commercial system. Thus, development of nanostructured advanced energy storage materials with high stability, optimum pore structure/morphology, binder-free characteristic, and high catalytic activity is challenging and essential. This dissertation focuses on synthesis and understanding process-structure-performance correlation of binder-free carbon nanofiber-based advanced electrodes for applications in various energy storage devices. Carbon nanofibers (CNFs) are excellent candidates for application as electrodes in electrochemical energy storage (EES) devices because of their unique properties such as high mechanical strength, high electrical and thermal conductivities, high chemical stability, flexibility, and high specific surface area. Electrospinning is a simple and versatile fiber formation technique using a strong electric field to pull or thin out a polymer solution or melt jet forming ultrathin fibers with diameters in the range of 50-500 nm. This continuous fiber-formation technique inherently forms a free-standing non-woven fiber mat, thus, potentially allowing their direct application without the addition of any binders. Carbon nanofiber electrodes fabricated in this work are free-standing with a continuous interconnected network providing fast ion-diffusion as well as fast transport of electrons within the network, a characteristic essential for efficient energy storage, also allowing it to be directly used in any EES system without any further processing. Advanced carbon nanofibers with controlled pore architectures and enhanced functionalities were fabricated and characterized as cost-effective and performance-effective electrodes for supercapacitors, lithium-sulfur, and lithium-air batteries. Each of these EES systems is at a different stage of development and has different requirements of the materials used in it. Thus, studies were conducted focused on the issues to be addressed in each of these areas and varied carbon nanofiber based electrodes were synthesized and studied for each application. Chapter 1 provides the introduction to electrochemical energy storage systems, their working operation, and challenges associated with them, the motivation of using carbon nanofibers as electrodes in these systems and a summary of the dissertation. Chapter 2 discusses a novel technique towards introducing pseudocapacitive functionalities on carbon nanofibers using a low-cost material, sodium chloride, for application in supercapacitors. Chapter 3 includes the study on free-standing carbon nanofibers with controlled morphologies as an interlayer in lithium-sulfur cells, demonstrating improved discharge capacity and cycle life. Chapter 4 comprises of the investigation of cobalt nanoparticles embedded porous carbon nanofibers as efficient bifunctional catalysts for both oxygen reduction and oxygen evolution reactions (ORR/OER) and as an efficient cathode for lithium-oxygen batteries.

Aminouracil-assisted synthesis of CoFe decorated bougainvillea-like N-doped carbon nanoflowers for boosting Zn–air battery and water electrolysis

Rechargeable zinc-air battery (RZAB) represents one of the ‘beyond-the-lithium-ion’ battery technologies with great potential for renewable energy storage. It is safe, environmentally benign, and excellent potential for affordable applications in resource-limited countries, ranging from residential and industrial electricity supply, transport (e.g., electric vehicles) to mobile and consumer electronics markets. RZABs possess high theoretical specific energy density of 1086 Wh/kg, which is 5 times greater than that of the conventional lithium-ion battery (LIB). The key challenge that conspires against the widespread commercialization of RZAB is the sluggish oxygen reaction kinetics that impedes reversibility of the system. Thus, it has become quite critical to develop low-cost and high-performance bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [1,2].High entropy materials (HEMs) have emerged as electrocatalysts for ORR and OER. HEMs contain five or more metals in equal proportions. Their unique conformational entropy and physico-chemical properties (including lattice distortion, synergistic effects amongst the different metals, and rich defect chemistries) promise to improve the kinetics of ORR / OER and electrochemical cycling stability. In this work, the high entropy spinel oxide, (CoCuFeMnNi)3O4 supported on conductive carbon has been synthesized and characterised using XRD, XPS, HRTEM, SEM and others. Preliminary electrochemistry shows improved ORR/OER kinetics. This presentation will discuss the performance of the initial lab-based RZAB using this electrocatalyst. References AB Haruna and KI Ozoemena, Manganese-based bifunctional electrocatalysts for zinc-air batteries, Opin. Electrochem. 2020, 21, 219-224AK Ipadeola, AB Haruna, L Gaolatlhe, AK Lebechi, J Meng, QQ Pang, K Eid, AM Abdullah, and KI Ozoemena, Efforts at Enhancing Bifunctional Electrocatalysis and Related Events for Rechargeable Zinc-Air Batteries; ChemElectroChem 2021, 8, 3998-4018

Trifunctional iridium-based electrocatalysts for overall water splitting and Zn-air batteries

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Alkali-ion batteries, including potassium-ion batteries, lithium-ion batteries, and sodium-ion batteries are important energy storage devices; however, with the cation size increased, there exists ...