Renewable Energy Global Innovations features: One-pot synthesis of hierarchical MnO2-modified diatomites for electrochemical capacitor electrodes

Significance Statement

The hierarchical and porous MnO₂-modified diatomite structures are prepared for the first time by a one pot hydrothermal method. We also demonstrate the synthesis of size- and shape-controlled MnO₂ nanostructures by replica molding from diatom silica structures for high-performance supercapacitors. The results show that birnessite-type MnO₂ nanosheets are observed to grow vertically on the purified diatomite, thus building hierarchical architecture. Three types of hierarchical hollow MnO₂ patterns with different three-dimensional (3D) structures, shapes and large surface areas were successfully prepared from three diatom species by a template-assisted hydrothermal process. The extraordinary precision and nano-scale resolution of 3D replications of complex biological architecture from diatoms to artificial MnO₂ structures are confirmed. The electrochemical results demonstrate that the MnO₂-modified diatomite electrode exhibits highly reversible features, good rate abilities, and good cycle stability (95.92% over 5000 cycles) demonstrating the suitability of the low-cost MnO₂-modified diatomite structure as a potential electrode material for supercapacitors. 

About The Author

Dr. Yu Xin Zhang received his B. Eng. and M. Eng. in Chemical Engineering from Tianjin University in 2000 and 2003, respectively. He received his Ph.D degree in Chemical and Biomolecular Engineering from the National University of Singapore (NUS) in 2008, and continued to work as a research fellow in Prof. Hua Chun Zeng’s group at NUS till 2009. Now Dr. Zhang is a full professor of College of Materials Science and Engineering in Chongqing University. Dr. Zhang’s research interest is self-assembled nanostructures for energy storage materials and photocatalysts. 

Journal Reference

Journal of Power Sources. Volume 246, 2014, Pages 449-456.

Yu Xin Zhang ^1,2,*, Ming Huang¹, Fei Li¹ , their collaborators

Show Affiliations

1. College of Material Science and Engineering, Chongqing University, Chongqing 400044, P.R. China

2. National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, P.R. China

Abstract

The hierarchical and porous MnO₂-modified diatomite structures are prepared for the first time by a one-pot hydrothermal method. The morphology and structure of MnO₂-modified diatomite hierarchical structures are examined by focus ion beam scanning electron microscopy (FIB/SEM) and X-ray diffraction spectroscopy (XRD). The results show that Birnessite-type MnO₂ nanosheets are observed to grow vertically on the purified diatomite, thus building hierarchical architecture. Furthermore, the electrochemical properties of the MnO₂-modified diatomite electrodes are elucidated by cyclic voltammograms, galvanostatic charge/discharge tests and electrochemical impedance spectroscopy in 1 M Na₂SO₄ electrolyte. The electrochemical results demonstrate that the MnO₂-modified diatomite electrode exhibits highly reversible features and good rate abilities, respectively. Significantly, it exhibits the specific capacitance of 202.6 F g^-1 for the MnO₂-modified diatomite and 297.8 F g^-1 for the MnO₂ nanostructures after etching the diatomite. The capacitance retention of 95.92% over 5000 cycles further indicates the suitability of the low-cost MnO₂-modified diatomite structure as a potential electrode material for supercapacitors.

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Renewable Energy Global Innovations features: Self-Assembly of Mesoporous Nanotubes Assembled from Interwoven Ultrathin Birnessite-type MnO2 Nanosheets for Asymmetric Supercapacitors

Significance Statement

 Here, we develop a simple and cost-effective approach to prepare CuO@MnO₂ core-shell nanostructures without any surfactants and ultrathin MnO₂ nanosheets-built nanotubes have been fabricated via a large-scale chemical etching method. An asymmetric supercapacitor with CuO@MnO₂ core-shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode yields an energy density of 22.1 Wh kg^-1 and a maximum power density of 85.6 kW kg^-1; the device shows a long-term cycling stability which retains 101.5% of its initial capacitance even after 10000 cycles. The MnO₂ nanotubes in a three-electrode system display much high specific capacitance (377.5 F g^-1 at current density of 0.25 A g^-1), good rate performance.

Moreover, an asymmetric supercapacitor on the basis of MnO₂ nanotubes as the positive electrode and activated graphenes (AG) as the negative electrode produced an energy density of 22.68 Wh kg^-1 and a maximum power density of 4.5 kW kg^-1. Such a facile strategy to fabricate the hierarchical CuO@MnO2 core-shell nanostructure and MnO₂ nanotubes with significantly improved functionalities opens up a novel avenue to design electrode materials on demand for high-performance supercapacitor applications. 

About The Author

Dr. Yu Xin Zhang received his B. Eng. and M. Eng. in Chemical Engineering from Tianjin University in 2000 and 2003, respectively. He received his Ph.D degree in Chemical and Biomolecular Engineering from the National University of Singapore (NUS) in 2008, and continued to work as a research fellow in Prof. Hua Chun Zeng’s group at NUS till 2009. Now Dr. Zhang is a full professor of College of Materials Science and Engineering in Chongqing University. Dr. Zhang’s research interest is self-assembled nanostructures for energy storage materials and photocatalysts. 

Journal Reference

Scientific Reports. Volume 4:4518, 2014.

Ming Huang¹, Yuxin Zhang^1,2, Fei Li¹, Lili Zhang³, Rodney S. Ruoff⁴, Zhiyu Wen² , Qing Liu¹

Show Affiliations

1. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China,

2. National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, P.R. China,

3. Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island 627833, Singapore, Department of
4. Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, One University Station C2200, Austin, Texas 78712, United States.

Abstract

Porous nanotubes comprised of MnO2 nanosheets were fabricated with a one-pot hydrothermal method using polycarbonate membrane as the template. The diameter and thickness of nanotubes can be controlled by choice of the membrane pore size and the chemistry. The porous MnO2 nanotubes were used as a supercapacitor electrode. The specific capacitance in a three-electrode system was 365 F g21 at a current density of 0.25 A g21 with capacitance retention of 90.4% after 3000 cycles. An asymmetric supercapacitor with porous MnO2 nanotubes as the positive electrode and activated graphene as the negative electrode yielded an energy density of 22.5 Wh kg21 and a maximum power density of 146.2 kW kg21; these values exceeded those reported for other MnO2 nanostructures. The supercapacitor performance was correlated with the hierarchical structure of the porous MnO2 nanotubes.

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Renewable Energy Global Innovations features: Optimization of the CeO2/CeCl3 cycle by cerium IV oxide reductive dissolution catalysis

Journal Reference

International Journal of Hydrogen Energy, Volume 40, Issue 39, 2015, Pages 13272–13280. 

Florent Lemont, Alisée Barbier, Samuel Resin

Commissariat à l’Energie Atomique (French Atomic Energy Commission) – DEN/DTCD/SCDV/LPIC, France

Abstract

While thermochemical cycles can be a way to produce hydrogen, physiochemical studies show that implementing them is often difficult for reactivity reasons. Most of the cycles actually involve solid–gas type systems with limited reactivity due to interface passivation processes. To overcome this difficulty, studies have shown that using the CeO₂/CeCl₃ pair, in which the cerium undergoes a reversible oxidation–reduction cycle, has enormous potential since it may partially be carried out in aqueous phase by reductive dissolution from cerium oxides (IV) to cerium chloride (III).

If the first reaction of the cycle is well known for industrial application, its second and its third reaction still need some investigation. Thus, this article primarily describes the work done on the second reaction to assess the possibility of carrying out cerium reduction in aqueous phase. The extremely positive results have highlighted the possibility of achieving 100% reaction efficiency in systems catalyzed by fluoride ions. Conducting the reaction with in-line distillation of the excess water also helps significantly reduce reaction time which offers good potential for the next stage. A ratio of 8 ml of a 20 w% HCl solution per gram of CeO₂ containing 6w% of CaF₂ leads to ensure a total reaction in a few minutes at 108.6 °C (boiling temperature of the H₂O–HCl azeotrope).

The work presented herein also describes a brief feasibility study for the third reaction cycle which could be carried out by spraying the solution from the second reaction, in a hot column whose temperature will be determined by further work. These results have allowed upgrading the first flowsheet proposed in a previous publication.

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