Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Supercapacitors have drawn considerable attention in recent years due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the effect of nanostructures on the properties of supercapacitors including specific capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design.

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Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions

Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g–1 and 82.2% capacity retention over 400 charge–discharge cycles between 1.2 and 0.01 V (vs Na+/Na) at a constant current of 50 mA g–1 (0.2 C). Excellent cycling stability is also observed at an even higher charge–discharge rate. A high reversible capacity of 149 mAh g–1 also can be obtained at a current rate of 500 mA g–1 (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.

Sodium ion insertion in hollow carbon nanowires for battery applications.

We introduced a facile method to construct hierarchical nanocomposites by combining one-dimensional (1D) conducting polyaniline (PANI) nanowires with 2D graphene oxide (GO) nanosheets. PANI nanowire arrays are aligned vertically on GO substrate. The morphologies of PANI nanowires can be controlled by adjusting the ratios of aniline to GO, which are attributed to different nucleation processes. The hierarchical nanocomposite structures of PANI−GO were further proved by UV−vis, FTIR, and XRD measurements. The hierarchical nanocomposite possessed higher electrochemical capacitance and better stability than each individual component as supercapacitor electrode materials, showing a synergistic effect of PANI and GO. This study will further guide the preparation of functional nanocomposites by combining different dimensional nanomaterials.

Hierarchical Nanocomposites of Polyaniline Nanowire Arrays on Graphene Oxide Sheets with Synergistic Effect for Energy Storage

The decomposition of carbon materials and organic binders in Li–air batteries has been reported repeatedly in recent literature. The decomposition of carbon can harm the batteries’ cyclability further by catalyzing electrolyte degrading. Therefore, there is a critical need to exploit a new catalyst support substituting carbon and develop a binder free cathode preparation strategy for Li–air batteries. Herein, TiO2 nanotube arrays growing on Ti foam are used as the catalyst support to construct carbon and binder free oxygen diffusion electrodes. After being coated with Pt nanoparticles by a cool sputtering approach, the TiO2 nanotube arrays are used as cathodes of Li–O2 batteries. Benefiting from the stability of TiO2 in the discharge/charge processes, the Li–O2 batteries realize enhanced cyclability at high current densities (for instance, more than 140 cycles at 1 or 5 A g–1), within wide discharge/charge voltage windows (for instance, 1.5–4.5 V). X-ray photoelectron spectra and a scanning electron micro...

Enhanced Cyclability of Li–O2 Batteries Based on TiO2 Supported Cathodes with No Carbon or Binder

Template preparation of Pt-Ru and Pt nanowire array electrodes on a Ti/Si substrate for methanol electro-oxidation

A porous MoO3 film is prepared by electrodeposition on Ni foam substrates and exhibits a capacity of 650 mAh g−1 at a current density of 3 A g−1 as anodes for lithium ion batteries Electrochemical measurements demonstrate that the outstanding rate performances are due to the improved Li+ diffusion kinetics

Electrochemical preparation of porous MoO3 film with a high rate performance as anode for lithium ion batteries

Preparation of polyaniline nanowire arrayed electrodes for electrochemical supercapacitors

Bimetal catalysts are good alternatives for non-enzymatic glucose sensors owing to their low cost, high activity, good conductivity, and ease of fabrication. In the present study, a self-supported CuNi/C electrode prepared by electrodepositing Cu nanoparticles on a Ni-based metal–organic framework (MOF) derivate was used as a non-enzymatic glucose sensor. The porous construction and carbon scaffold inherited from the Ni-MOF guarantee good kinetics of the electrode process in electrochemical glucose detection. Furthermore, Cu nanoparticles disturb the array structure of MOF derived films and evidently enhance their electrochemical performances in glucose detection. Electrochemical measurements indicate that the CuNi/C electrode possesses a high sensitivity of 17.12 mA mM−1 cm−2, a low detection limit of 66.67 nM, and a wider linearity range from 0.20 to 2.72 mM. Additionally, the electrode exhibits good reusability, reproducibility, and stability, thereby catering to the practical use of glucose sensors. Similar values of glucose concentrations in human blood serum samples are detected with our electrode and with the method involving glucose-6-phosphate dehydrogenase; the results further demonstrate the practical feasibility of our electrode.

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Guangyu Zhao

Papers

Enhanced Cyclability of Li–O2 Batteries Based on TiO2 Supported Cathodes with No Carbon or Binder

Template preparation of Pt-Ru and Pt nanowire array electrodes on a Ti/Si substrate for methanol electro-oxidation

Electrochemical preparation of porous MoO3 film with a high rate performance as anode for lithium ion batteries

Preparation of polyaniline nanowire arrayed electrodes for electrochemical supercapacitors

A CuNi/C Nanosheet Array Based on a Metal–Organic Framework Derivate as a Supersensitive Non-Enzymatic Glucose Sensor