Tianyi Kou

Paper-Based Electrodes for Flexible Energy Storage Devices

Paper‐based materials are emerging as a new category of advanced electrodes for flexible energy storage devices, including supercapacitors, Li‐ion batteries, Li‐S batteries, Li‐oxygen batteries. This review summarizes recent advances in the synthesis of paper‐based electrodes, including paper‐supported electrodes and paper‐like electrodes. Their structural features, electrochemical performances and implementation as electrodes for flexible energy storage devices including supercapacitors and batteries are highlighted and compared. Finally, we also discuss the challenges and opportunity of paper‐based electrodes and energy storage devices.

Ultrathin mesoporous NiO nanosheet-anchored 3D nickel foam as an advanced electrode for supercapacitors

As promising electrode materials for electrochemical supercapacitors, pseudocapacitive transition metal oxides such as NiO possess high theoretical specific capacitance, environmental benignity and good abundance. However their areal capacitance and cycling stability are greatly restricted by their poor electronic conductivity (NiO, 10−2 to 10−3 S cm−1). Here we propose an in situ growth strategy in combination with nanoscale design to construct ultrathin mesoporous NiO nanosheets on a 3D network of nickel foam. The hybrid structures show well enhanced conductivity and ion transfer, giving rise to an ultrahigh specific capacitance of 2504.3 F g−1 which is close to the theoretical value of NiO. The electrodes also exhibit remarkable cycling stability (no degradation of the overall capacitance after 45 000 cycles). The amazing electrochemical performance of such hybrid structures makes them potential electrodes in supercapacitors. The present strategy could be popularized in other transition metal oxides like Co3O4, MnO2, etc. to create electrodes with desirable nanostructures.

Hierarchically porous carbon foams for electric double layer capacitors

The growing demand for portable electronic devices means that lightweight power sources are increasingly sought after. Electric double layer capacitors (EDLCs) are promising candidates for use in lightweight power sources due to their high power densities and outstanding charge/discharge cycling stabilities. Three-dimensional (3D) self-supporting carbon-based materials have been extensively studied for use in lightweight EDLCs. Yet, a major challenge for 3D carbon electrodes is the limited ion diffusion rate in their internal spaces. To address this limitation, hierarchically porous 3D structures that provide additional channels for internal ion diffusion have been proposed. Herein, we report a new chemical method for the synthesis of an ultralight (9.92 mg/cm3) 3D porous carbon foam (PCF) involving carbonization of a glutaraldehydecross-linked chitosan aerogel in the presence of potassium carbonate. Electron microscopy images reveal that the carbon foam is an interconnected network of carbon sheets containing uniformly dispersed macropores. In addition, Brunauer–Emmett–Teller measurements confirm the hierarchically porous structure. Electrochemical data show that the PCF electrode can achieve an outstanding gravimetric capacitance of 246.5 F/g at a current density of 0.5 A/g, and a remarkable capacity retention of 67.5% was observed when the current density was increased from 0.5 to 100 A/g. A quasi-solid-state symmetric supercapacitor was fabricated via assembly of two pieces of the new PCF and was found to deliver an ultra-high power density of 25 kW/kg at an energy density of 2.8 Wh/kg. This study demonstrates the synthesis of an ultralight and hierarchically porous carbon foam with high capacitive performance.

Amorphous Mixed-Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability

Previous studies show that vanadium oxides suffer from severe capacity loss during cycling in the liquid electrolyte, which has hindered their applications in electrochemical energy storage. The electrochemical instability is mainly due to chemical dissolution and structural pulverization of vanadium oxides during charge/discharge cyclings. In this study the authors demonstrate that amorphous mixed-valence vanadium oxide deposited on exfoliated carbon cloth (CC) can address these two limitations simultaneously. The results suggest that tuning the V4+ /V5+ ratio of vanadium oxide can efficiently suppress the dissolution of the active materials. The oxygen-functionalized carbon shell on exfoliated CC can bind strongly with VO x via the formation of COV bonding, which retains the electrode integrity and suppresses the structural degradation of the oxide during charging/discharging. The uptake of structural water during charging and discharging processes also plays an important role in activating the electrode material. The amorphous mixed-valence vanadium oxide without any protective coating exhibits record-high cycling stability in the aqueous electrolyte with no capacitive decay in 100 000 cycles. This work provides new insights on stabilizing vanadium oxide, which is critical for the development of vanadium oxide based energy storage devices.

Recent advances in chemical methods for activating carbon and metal oxide based electrodes for supercapacitors

Carbon and metal oxides are considered as the two most important categories of electrode materials for supercapacitors. To date, significant progress has been made in the synthesis of different carbon and metal oxide based electrodes. However, the limited ion accessible area of many carbon materials, poor electrical conductivity of most metal oxides, and sluggish ion diffusion in the bulk carbon or metal oxide materials are major barriers for these electrodes to achieve high energy density and power density simultaneously. In the past decade, numerous activation methods have been demonstrated to overcome these limitations. In this review, we summarize the recent advances in chemical methods for activating carbon and metal oxide electrodes for supercapacitors, including heteroatom doping, increasing porosity, introducing oxygen vacancies, and chemical exfoliation. The fundamentals of these methods are discussed and exemplified. We also discuss the challenges and opportunities of different activation methods.

Tri-layered graphite foil for electrochemical capacitors

Free-standing carbon structures are promising electrode materials for electrochemical capacitors. However, these electrodes usually have small mass that limits the amount of energy that can be stored. Increasing the electrode mass typically leads to reduction of gravimetric capacitance and rate capability due to the sluggish mass transfer kinetics and increased internal resistance. It has been a challenge to improve both specific capacitance and rate capability of an electrode with high mass. Here we demonstrate a new method to convert graphite foil (8.5 mg cm−2) with a compact layered structure into a unique tri-layered structure that consists of a top layer of partially exfoliated graphene sheets, a middle layer of intercalated graphite sheets and a bottom layer of graphite. This unique structure shows enhanced ion accessible surface area and pseudocapacitance. The seamless connection between the three layers ensures efficient electron transport across the electrode. The tri-layered graphite foil electrode delivers an excellent capacitance of 820 mF cm−2 at 5 mA cm−2 (corresponding to 96.5 F g−1), which is 400 times higher than the untreated foil. Moreover, it retains 75% capacitance when the current density is increased from 5 to 100 mA cm−2. These values are among the best values reported for carbon-based electrodes with comparable mass.

Eutectic-directed self-templating synthesis of PtNi nanoporous nanowires with superior electrocatalytic performance towards the oxygen reduction reaction: experiment and DFT calculation

One-dimensional (1D) nanostructures have been receiving significant attention due to their unique properties and potential applications. However, their low-cost, highly efficient synthesis remains a great challenge. Herein, we proposed a eutectic-directed self-templating strategy to synthesize PtNi nanoporous nanowires (NPNWs) through the combination of rapid solidification and dealloying. The eutectic-induced phase confinement and dealloying inheritance effect jointly favored the formation of PtNi NPNWs. The PtNi NPNWs exhibit superior electrocatalytic activity (5-fold enhancement in the specific activity) and enhanced durability towards oxygen reduction reaction, as benchmarked with commercial PtC. Moreover, the mechanisms for the activity enhancement have been rationalized on the basis of adsorption energy, d-band center, thermodynamics, and kinetics through density functional theory calculations.

‘Casting’ nanoporous nanowires: revitalizing the ancient process for designing advanced catalysts

Eutectic reaction frequently occurs in metallurgical solidification of alloy melts, and has been widely studied for traditional structural and engineering applications for decades. Here, we inject new blood into this ancient process and develop its novel application in the general preparation of advanced nanostructured materials. Based upon solidification control and dealloying inheritance effect, we ‘cast’ advanced M (M = Pt, Pd or PdPt)–Ni nanoporous nanowires (NPNWs) via a novel eutectic-directed self-templating strategy. The PdPtNi NPNWs exhibit impressively enhanced electrocatalytic performance towards ethylene glycol and glycerol electrooxidation in an alkaline electrolyte compared to commercial Pt/C (40 wt%), as evidenced by enhanced electrocatalytic activity, better anti-poisoning property and stability. The superior electrocatalytic performance of PdPtNi NPNWs originates from its unique nanoporous nanowire network structure, alloying effect of Ni, modified electronic states of Pd/Pt and synergistic interactions between Pt, Pd and Ni.