3D aligned-carbon-nanotubes@Li2FeSiO4 arrays as high rate capability cathodes for Li-ion batteries.

3D aligned-carbon-nanotubes (ACNTs)@Li2FeSiO4 nanocomposite arrays on Al foil were developed as cathode materials for Li-ion batteries. The ACNTs were grown directly on an Al foil by a chemical vapor deposition method to achieve a 3D current collector structure for direct charge transport. Li2FeSiO4 nanoparticles were deposited on the surface of the ACNTs by a polyvinylalcohol (PVA)-assisted sol-gel method. The 3D samples showed a high degree of alignment of nanotubes with a favorable pore morphology before and after cycling. According to electrochemical measurements, the 3D sample with optimized mass ratio of ACNTs and Li2FeSiO4 (2:1) showed excellent rate capability and capacity retention, delivering a discharge specific capacity of 142 mAh g(-1) at a rate of 0.5 C (C = 160 mAg(-1)) and maintaining 99% of the initial discharge capacity after 50 cycles at 24 ° C. Up to 20 C, the delivered charge/discharge capacity was 94 mAh g(-1) after 172 cycles, which is 54% of the value obtained at C/20 (175 mAh g(-1)). In comparison, carbon coated nanoporous Li2FeSiO4 obtained under analogous conditions by a PVA-assisted sol-gel method can only deliver a capacity of 80 mAh g(-1) and showed poor rate capability. In addition, despite amorphization, dissolution and chemical composition changes occurring in the 3D samples upon extended cycling, the 3D samples showed good long-term cycling stability at a high current density (5 C), maintaining ~80% of the initial discharge capacity after 1000 cycles and ~70% after 2000 cycles.

Regulating the coordination environment of single-atom catalysts for electrocatalytic CO2 reduction.

Electrochemical CO2 reduction (ECR) through single-atom catalysts (SACs) consisting of transition metals (TMs) anchored on nitrogenated carbon (TM-N-C) has shown promise for carbon neutralization. However, high overpotentials and low selectivity are still issues. Regulating the coordination environment of anchored TM atom is important to address these problems. In this study, we evaluated nonmetal atom (NM = B, O, F, Si, P, S, Cl, As and Se) modified TM (TM = Fe, Co, Ni, Cu and Zn)@N4-C catalysts for their ECR to CO performance using density functional theory (DFT) calculations. NM dopants can induce active center distortion and tune electron structure, promoting intermediate formation. Doping heteroatoms can improve ECR to CO activity on Ni and Cu@N4 but worsen it on Co@N4 catalysts. Fe@N4-F1(I), Ni@N3-B1, Cu@N4-O1(III), and Zn@N4-Cl1(II) have excellent activity for ECR to CO, with overpotentials of 0.75, 0.49, 0.43, and 0.15 V, respectively, and improved selectivity. The catalytic performance is related to the intermediate binding strength, as evidenced by d band center, charge density difference, crystal orbital Hamilton population (COHP), and integrated COHP (ICOHP). It is expected that our work can be used as the design principle to guide the synthesis of the high-performance heteroatoms modified SACs for ECR to CO.

Sulfur-Decorated Ni-N-C Catalyst for Electrocatalytic CO2 Reduction with Near 100 % CO Selectivity.

Developing highly efficient electrocatalysts for electrochemical CO2 reduction (ECR) to value-added products is important for CO2 conversion and utilization technologies. In this work, a sulfur-doped Ni-N-C catalyst is fabricated through a facile ion-adsorption and pyrolysis treatment. The resulting Ni-NS-C catalyst exhibits higher activity in ECR to CO than S-free Ni-N-C, yielding a current density of 20.5 mA cm-2 under -0.80 V versus a reversible hydrogen electrode (vs. RHE) and a maximum CO faradaic efficiency of nearly 100 %. It also displays excellent stability with negligible activity decay after electrocatalysis for 19 h. A combination of experimental investigations and DFT calculations demonstrates that the high activity and selectivity of ECR to CO is due to a synergistic effect of the S and Ni-NX moieties. This work provides insights for the design and synthesis of nonmetal atom-decorated M-N-C-based ECR electrocatalysts.

Single transition metal atom embedded antimonene monolayers as efficient trifunctional electrocatalysts for the HER, OER and ORR: a density functional theory study.

Highly efficient, stable and cost-effective electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) have been pursued for several decades. Herein, by employing density functional theory (DFT), a wide range of transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Ir, Pt and Au) atoms anchored on antimonene (Sb monolayer) with a single Sb vacancy as single-atom catalysts (SACs) were investigated for their HER, OER and ORR performance. The results indicate that the defective Sb monolayer can be stable. Some TM@Sb monolayers show excellent stability and good electrical conductivity, beneficial for electron transfer during electrocatalytic reactions. The Ir@ and Pt@Sb monolayers exhibit excellent HER performance, both with about -0.01 eV of ΔG*H. The d band centre of the TM@Sb monolayer can be used to describe the binding strength between substrates and intermediates directly. The best OER electrocatalyst is the Pt@Sb monolayer, which shows an overpotential (ηOER) of 0.48 V. In contrast, the best ORR electrocatalyst is the Ag@Sb monolayer with an ηORR of 0.50 V, followed by Pd@, Rh@, Cd@ and Pt@Sb monolayers. Compared with pristine antimonene, only the noble metal atom could improve its OER and ORR performance effectively, and the Pt@Sb monolayer can be a trifunctional electrocatalyst for the HER/OER/ORR. Therefore, our calculations highlight a new type of SAC based on antimonene, which can be useful for energy conversion and storage.

Boosted Supercapacitive Energy with High Rate Capability of aCarbon Framework with Hierarchical Pore Structure in an Ionic Liquid.

The specific energy of a supercapacitor (SC) with an ionic liquid (IL)-based electrolyte is larger than that using an aqueous electrolyte owing to the wide operating voltage window provided by the IL. However, the wide-scale application of high-energy SCs using ILs is limited owing to a serious reduction of the energy with increasing power. The introduction of macropores to the porous material can mitigate the reduction in the gravimetric capacitance at high rates, but this lowers the volumetric capacitance. Synthetic polymers can be used to obtain macroporous frameworks with high apparent densities, but the preservation of the frameworks during activation is challenging. To simultaneously achieve high gravimetric capacitance, volumetric capacitance, and rate capability, a systematic strategy was used to synthesize a densely knitted carbon framework with a hierarchical pore structure by using a polymer. The energy of the SC using the hierarchically porous carbon was 160 Wh kg-1 and 85 Wh L-1 on an active material base at a power of 100 W kg-1 in an IL electrolyte, and 60 % of the energy was still retained at a power larger than 5000 W kg-1 . To illustrate, a full-packaged SC with the material could store/release energy comparable to a Ni-metal hydride battery (gravimetrically) and one order of magnitude higher than a commercial carbon-based SC (volumetrically), within one minute.

Coaxial carbon/metal oxide/aligned carbon nanotube arrays as high-performance anodes for lithium ion batteries.

Coaxial carbon/metal oxide/aligned carbon nanotube (ACNT) arrays over stainless-steel foil are reported as high-performance binder-free anodes for lithium ion batteries. The coaxial arrays were prepared by growth of ACNTs over stainless-steel foil followed by coating with metal oxide and carbon. The carbon/manganese oxide/ACNT arrays can deliver an initial capacity of 738 mAh g(-1) with 99.9 % capacity retention up to 100 cycles and a capacity of 374 mAh g(-1) at a high current density of 6000 mA g(-1). The external carbon layer was recognized as a key component for high performance, and the mechanism of performance enhancement was investigated by electrochemical impedance spectroscopy, electron microscopy, and X-ray diffraction analysis. The layer increases rate capability by enhancing electrical conductivity and maintaining a low mass-transfer resistance and also improves cyclic stability by avoiding aggregation of metal-oxide particles and stabilizing the solid electrolyte interface. The resultant principle of rational electrode design was applied to an iron oxide-based system, and similar improvements were found. These coaxial nanotube arrays present a promising strategy for the rational design of high-performance binder-free anodes for lithium ion batteries.

Exploring aligned-carbon-nanotubes@polyaniline arrays on household Al as supercapacitors.

Herein, we demonstrate a new approach towards the construction of supercapacitors consisting of carbon nanotubes (CNTs) and conducting polymers (ECPs) with high specific power, high specific energy, and stable cycling performance through a 3D design of a thin film of polyaniline (PANI) on an aligned small carbon nanotube (ACNT) array on household Al foils. The thin-film strategy is used to fully exploit the specific capacitance of PANI, and obtain regular pores, strong interaction between PANI and CNTs, and reduced electrical resistance for the electrodes. A facile process is developed to fabricate a highly flexible supercapacitor using this binder-free composite on household Al foil as the current collector. It exhibits high specific energy of 18.9 Wh kg(-1) , high maximum specific power of 11.3 kW kg(-1) of the active material in an aqueous electrolyte at 1.0 A g(-1) , and excellent rate performance and cycling stability. A high specific energy of 72.4 Wh kg(-1) , a high maximum specific power of 24.9 kW kg(-1) , and a good cycling performance of the active material are obtained in an organic electrolyte.