Tuning crystallinity to realize high energy density in supercapacitors

(Nanowerk Spotlight) Supercapacitors, a rapidly emerging type of energy storage device, hold great potential due to their interesting characteristics: high power density, fast charge and discharge rates, and long-term cycling life.
However, the use of supercapacitors is severely limited by their low energy density, which is one to two magnitudes lower in comparison with Li-ion batteries. The underlying reason for the low energy density of supercapacitors is mainly due to the energy storage mechanisms of its electrode materials.
The design of asymmetric supercapacitors that comprise high-performance pseudocapacitive cathodes and carbon anodes have been adopted as an effective strategy to address this problem.
The energy density of supercapacitors can be further increased by replacing carbon anodes with a pseudocapacitive anode with comprehensive electrochemical performance in a wide potential window.
Double-layer capacitors (carbons) and pseudocapacitors have been extensively studied, with the former presenting high power density albeit with low energy density and the latter presenting high energy density and specific capacitances.
To increase the energy density especially in aqueous electrolytes, current research has focused on widening the operating voltage window by combining high performance pseudocapacitive electrodes and carbon electrodes in asymmetric supercapacitors.
However, the attained energy density is still limited by the low capacitances of the commonly used carbon negative electrodes.
One effective way to solve the low energy density problem of supercapacitors is to replace the these carbon negative electrodes with pseudocapacitive electrodes, which are capable of providing high specific capacitances.
The difficulty with this approach is that pseudocapacitive electrodes, especially metal oxides, suffer from low conductivities making it highly challenging to achieve high capacitances, excellent rate capability, outstanding rate capability and long-term cyclability.
In new work, researchers at Wuhan University of Technology, led by Prof. Liqiang Mai, have developed a low-crystalline FeOOH nanoparticle anode with excellent comprehensive electrochemical performances at both low and high mass loadings as potential replacements for carbon negative electrodes in full supercapacitor devices. Their results have been published in Nature Communications ("Low-crystalline iron oxide hydroxide nanoparticle anode for high performance supercapacitors").
Iron oxide and hydroxide materials present the advantages of high theoretical capacitance, wide operating potential window, low cost and natural abundance. Although previous studies have been carried out to study their potential applications for supercapacitors, most of them exhibit short cycle life and poor rate performance.
The researchers’ first step was to grow small-sized highly-crystalline iron oxide (Fe2O3) nanoparticles on carbon fiber clothes substrates.
They then successfully tuned the crystallinity of the nanoparticles during the first twenty cycles. This electrochemical activation process fully converted not only the surface of the nanoparticles but the bulk as well into low-crystalline FeOOH nanoparticles, which is reported for the first time.
The schematics shows the synthesis process for the low-crystalline FeOOH nanoparticle anode. The TEM and HRTEM images before and after the first two cycles confirms the electrochemical transformation from crystalline Fe2O3 nanoparticles to low-crystalline FeOOH nanoparticles. (Image: Mai Research Group, Wuhan University of Technology) (click on image to enlarge)
This transformation process was carefully studied and confirmed by the team using various characterization techniques.
"The crystallinity phase of metal oxides affect their electrochemical performance and we observed an improved rate performance and cycling stability in the nanoparticle electrode because the low-crystalline feature endows the electrode with extraordinary high charge storage kinetics," notes Prof. Mai.
The low-crystalline nanoparticles could sustain long-term cycling up to 10,000 charge discharge cycles, coupled with good rate capability when the current density was increased 30 times. "The excellent cycle life and rate performance of our nanoparticles are on par with carbon electrodes," says Mai. "Remarkably, the capacitance of the nanoparticles is about 3.5 times the capacitances of commercial carbon electrodes in a wide potential window. Such a high capacitance makes the low-crystalline iron oxide hydroxide nanoparticles potential replacements for carbon electrodes."
Commercial supercapacitors are usually packed with a high active material mass loading (∼10 mg cm-2) to minimize the influence of the weight of the other components such as current collectors, separators and casings on the electrochemical performance.
The FeOOH anode also exhibits high capacitance at such a comparable mass loading, which would be beneficial for practical supercapacitor devices. A hybrid supercapacitor based on the nanoparticle electrode could deliver high energy and power densities.
These results suggest that the performance of supercapacitor devices can be optimized by designing high performance pseudocapacitive electrodes with suitable reaction potentials. Pseudocapacitive electrode materials are particularly interesting for achieving high volumetric capacitances owing to their high density.
In future experiments, the researchers hope to further study the potential applications of less-explored negative pseudocapacitive electrodes such as vanadium nitrides and boron nitrides.
A Nanowerk exclusive provided by International School of Materials Science and Engineering, Wuhan University of Technology

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