Xiaoyi Cai

Recent advances in air electrodes for Zn–air batteries: electrocatalysis and structural design

Zn–air batteries have attracted significant attention because of their high energy density, environmental friendliness, safety, and low cost. The air cathode of is one of the most expensive cell components and a key factor in determining the performance of Zn–air batteries. As a fuel, O2 availability to the air cathode is determined by the level of both its dissolution and diffusion in an electrolyte, whereby electrocatalysis happens in the three-phase interface where the catalyst, electrolyte, and O2 meet. Maximizing the performance of air cathodes by rational design of the catalyst structure is of significant importance. To date, various electrocatalysts, including heteroatom-doped carbon, transition metal nitrides/oxides/sulfides, and perovskite oxides, have been developed with outstanding oxygen reduction reaction and oxygen evolution activity. More and more researchers are trying to apply electrocatalysts into Zn–air battery prototypes. The aim of this review is to afford a better understanding of air cathodes and provide guidelines to the researchers for the design and construction of high-performance, easy-to-use cathodes for metal–air batteries.

Free-standing vertically-aligned nitrogen-doped carbon nanotube arrays/graphene as air-breathing electrodes for rechargeable zinc–air batteries

Developing highly efficient air-breathing electrodes is of significant importance for various energy conversion technologies. In this work, we report the synthesis of free-standing, nitrogen (N)-doped vertically aligned carbon nanotubes (N-VA-CNTs) supported by graphene foam (GF), demonstrating excellent catalytic activity for oxygen electrochemical reactions. The presence and status of N is systematically analyzed; N-doped carbon flake fragments present on the walls of the CNTs are proposed to be responsible for the high electrocatalytic activity. A rechargeable Zn–air battery assembled from this N-VA-CNTs/GF hybrid electrode exhibits much higher power density than those of commercial Pt/C electrocatalysts. Zn–air batteries assembled from N-VA-CNTs/GF with high performance and long-term durability hold great potential in the practical application of free-standing, flexible, rechargeable metal–air batteries.

Binary metal sulfides and polypyrrole on vertically aligned carbon nanotube arrays/carbon fiber paper as high-performance electrodes

Pseudocapacitive materials, such as binary metal sulfides, show great promise as electrode candidates for energy storage devices due to their higher specific capacitance than that of mono-metal sulfides and binary metal oxides, but generally suffer from low energy densities when assembled in supercapacitor devices. To push the energy density limit of pseudocapacitive materials in devices, a new class of electrode materials with favorable architectures is strongly needed. Here, rationally designed and coaxially grown Ni–Co–S and polypyrrole on vertically aligned carbon nanotube (VA-CNT) arrays/3D carbon fiber paper (CFP) is presented as a novel freestanding electrode for energy storage devices. Our study has revealed that the catalyst preparation is the key step and the presence of an Al2O3 buffer layer is essential for the growth of VA-CNTs. The 3D hierarchical VA-CNTs/CFP allows high areal loading and high utilization efficiency of the active materials. The binder-free asymmetric energy storage devices with Ni–Co–S/VA-CNTs/CFP as the positive electrode and polypyrrole/VA-CNTs/CFP as the negative electrode, respectively, demonstrate a high energy density of 82 W h kg−1 at 200 W kg−1.

Graphene and graphene-based composites as Li-ion battery electrode materials and their application in full cells

In recent years, graphene has been considered as a potential “miracle material” that will revolutionize the Li-ion battery (LIB) field and bring a huge improvement in the performance of LIBs. However, despite the large number of publications every year, practical prototypes of graphene-based batteries are still few and no commercial products have entered large-scale production so far. In this review, we will start from a brief introduction of the working mechanism of LIBs, important concepts such as the solid–electrolyte interface (SEI), graphene and the production methods of graphene and graphene based composites followed by the review of graphene and graphene composites as electrode materials, highlighting the role graphene plays in the composite materials and the advantages and drawbacks of these materials. The previous sections laid the foundation of the focus of discussion, which is the application of graphene based materials in full cell prototypes, the difficulties they face and efforts to solve the various problems preventing the implementation of graphene based materials in practical commercial cells.

V2O5 embedded in vertically aligned carbon nanotube arrays as free-standing electrodes for flexible supercapacitors

Free-standing, three-dimensional (3D) V2O5 nanobelts coated with poly(3,4-ethylenedioxythiophene) (PEDOT) on vertically aligned CNTs (VA-CNTs)/graphene foam (GF) (PEDOT–V2O5–VA-CNTs/GF) are constructed for flexible energy storage devices. The well-aligned structure of VA-CNTs and light-weight, highly conductive GF lead to highly efficient ionic and electronic transport channels, which are of scientific and practical significance for energy storage/conversion applications. PEDOT–V2O5–VA-CNTs/GF delivers a specific capacitance of 1016 F g−1 at a current density of 1 A g−1 when tested in an aqueous electrolyte. All-solid-state asymmetric supercapacitor devices have been fabricated to demonstrate its flexibility in charging and discharging. Polypyrrole (PPy)/VA-CNTs/GF was used as the negative electrode, PEDOT–V2O5–VA-CNTs/GF as the positive electrode, and PVA/LiNO3 as the solid electrolyte. The solid-state flexible supercapacitors exhibit a high specific capacitance of 48.83 F g−1 at 1 A g−1 and an energy density of 17.34 W h kg−1 at a power density of 0.71 kW kg−1 with good cycle stability as well. This work highlights the advantages of the VA-CNTs/GF substrate in the hybrid electrode with V2O5 embedded among the VA-CNT interstitials for high-performance flexible energy storage devices.

Electrochemically Synthesis of Nickel Cobalt Sulfide for High‐Performance Flexible Asymmetric Supercapacitors

Abstract A lightweight, flexible, and highly efficient energy management strategy is highly desirable for flexible electronic devices to meet a rapidly growing demand. Herein, Ni–Co–S nanosheet array is successfully deposited on graphene foam (Ni–Co–S/GF) by a one‐step electrochemical method. The Ni–Co–S/GF composed of Ni–Co–S nanosheet array which is vertically aligned to GF and provides a large interfacial area for redox reactions with optimum interstitials facilitates the ions diffusion. The Ni–Co–S/GF electrodes have high specific capacitance values of 2918 and 2364 F g−1 at current densities of 1 and 20 A g−1, respectively. Using such hierarchical Ni–Co–S/GF as the cathode, a flexible asymmetric supercapacitor (ASC) is further fabricated with polypyrrple(PPy)/GF as the anode. The flexible asymmetric supercapacitors have maximum operation potential window of 1.65 V, and energy densities of 79.3 and 37.7 Wh kg−1 when the power densities are 825.0 and 16100 W kg−1, respectively. Its worth nothing that the ASC cells have robust flexibility with performance well maintained when the devices were bent to different angles from 180° to 15° at a duration of 5 min. The efficient electrochemical deposition method of Ni–Co–S with a preferred orientation of nanosheet arrays is applicable for the flexible energy storage devices.

Graphene-supported non-precious metal electrocatalysts for oxygen reduction reactions: the active center and catalytic mechanism

In this work, non-precious metal catalysts (FeCoN) were loaded onto the surface of N-modified graphene (N-G), graphite and activated carbon during a simple yet effective chemical process. The oxygen reduction reaction (ORR) performance of the as-prepared catalysts with different pyrolysis temperatures were evaluated in 0.5 M H2SO4. The use of N-G at a pyrolysis temperature of 750 °C yielded the highest catalyst activity, while both the metal nitrides (MeN) and pyridinic N were demonstrated as the active center for the ORR. The pyridinic-N, which sits surrounding MeN works synergistically with metal nitrides to enhance the catalytic activity of the MeN/N-G composite. A maximum power density output of 130 mW cm−2 at 0.25 V was achieved in a membrane electrode assembly (MEA) using FeCoN-N-G as a cathode catalyst. The mechanism of the ORR at the catalyst and the nature of the active center are discussed.

Correction: Recent advances in air electrodes for Zn–air batteries: electrocatalysis and structural design

Correction for ‘Recent advances in air electrodes for Zn–air batteries: electrocatalysis and structural design’ by Xiaoyi Cai et al., Mater. Horiz., 2017, 4, 945–976.

One-step coaxial electrodeposition of Co0.85Se on CoNi2S4 nanotube arrays for flexible solid-state asymmetric supercapacitors

A one-step electrochemical method is applied to coaxially deposit highly conductive Co0.85Se nanosheets composed of ultrasmall Co0.85Se nanocrystals on three-dimensional (3D) CoNi2S4 nanotube arrays supported on graphene foam (GF) (Co0.85Se@ CoNi2S4/GF) as an electrode for flexible solid-state supercapacitors. The Co0.85Se@CoNi2S4/GF electrode has an areal capacitance of 5.25 F cm−2 at 1 mA cm−2 and good rate capability (2.65 F cm−2 at 20 mA cm−2). A solid-state asymmetric supercapacitor with Co0.85Se@NiCo2S4/GF as the cathode and hollow carbon spheres (HCSs) as the anode in a PVA/KOH electrolyte was assembled which shows an energy density of 46.5 W h kg−1 at a power density of 750 W kg−1, and 89.0% capacity retention after 10 000 cycles over a potential window of up to 1.55 V. These results demonstrate that the electrodeposition method is applicable for engineering of 3D microstructured electrodes and also provides an efficient strategy for fabricating transition metal selenide based nanodevices.