Progress of electrochemical capacitor electrode materials: A review

Silicon materials remain unused for supercapacitors due to extreme reactivity of silicon with electrolytes. However, doped silicon materials boast a low mass density, excellent conductivity, a controllably etched nanoporous structure, and combined earth abundance and technological presence appealing to diverse energy storage frameworks. Here, we demonstrate a universal route to transform porous silicon (P-Si) into stable electrodes for electrochemical devices through growth of an ultra-thin, conformal graphene coating on the P-Si surface. This graphene coating simultaneously passivates surface charge traps and provides an ideal electrode-electrolyte electrochemical interface. This leads to 10–40X improvement in energy density, and a 2X wider electrochemical window compared to identically-structured unpassivated P-Si. This work demonstrates a technique generalizable to mesoporous and nanoporous materials that decouples the engineering of electrode structure and electrochemical surface stability to engineer performance in electrochemical environments. Specifically, we demonstrate P-Si as a promising new platform for grid-scale and integrated electrochemical energy storage.

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Surface engineered porous silicon for stable, high performance electrochemical supercapacitors

Building of hierarchical core-shell hetero-structures is currently the subject of intensive research in the electrochemical field owing to its potential for making improved electrodes for high-performance micro-supercapacitors. Here we report a novel architecture design of hierarchical MnO2@silicon nanowires (MnO2@SiNWs) hetero-structures directly supported onto silicon wafer coupled with Li-ion doped 1-Methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide (PMPyrrBTA) ionic liquids as electrolyte for micro-supercapacitors. A unique 3D mesoporous MnO2@SiNWs in Li-ion doped IL electrolyte can be cycled reversibly across a voltage of 2.2 V and exhibits a high areal capacitance of 13 mFcm−2. The high conductivity of the SiNWs arrays combined with the large surface area of ultrathin MnO2 nanoflakes are responsible for the remarkable performance of these MnO2@SiNWs hetero-structures which exhibit high energy density and excellent cycling stability. This combination of hybrid electrode and hybrid electrolyte opens up a novel avenue to design electrode materials for high-performance micro-supercapacitors.

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3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li- doped ionic liquid

Conformal titanium nitride in a porous silicon matrix: A nanomaterial for in-chip supercapacitors

Highly doped silicon nanowires based electrodes for micro-electrochemical capacitor applications

In this study, we have demonstrated the possibility of using macroporous silicon electrodes in electrochemical capacitors. Macroporous silicon was used to increase the surface exchange between pore surface and electrolyte. The inherent resistivity of the porous silicon can be reduced through the use of subsequent doping and metallization processes of the macropore surface. A systematic study of the electrolyte concentration and the porous silicon depth influences was also performed. A unit cell capacitance value of 320 µF/cm2 was obtained with doped and metallized p-type macroporous silicon electrodes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations on porous silicon as electrode material in electrochemical capacitors

In this paper, we present processes to etch high thickness porous silicon layers for RF device applications. Indeed, on-chip inductors realized on bulk silicon suffer from mediocre Q-factor values mainly because of electrical losses into the substrate and capacitive coupling with the silicon appearing beyond 1 GHz. We present a detailed study of etching parameters such as the current density or the HF concentration in HF: H2O:acetic acid based electrolytes. In addition, we propose a prospective study of integrated copper inductor performances on porous silicon substrates in the range of 100 MHz to 10 GHz. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thick microporous silicon isolation layers for integrated RF inductors

A novel micro-machining technique for silicon deep anisotropic etching and isolating porous silicon structure formation is developed in this paper. This method combines aluminum thermomigration through a N type silicon wafer and silicon electrochemical etching in a HF-based solution. Using this technique, high aspect ratio trenches (1:5) and porous silicon isolating regions have been achieved through the entire thickness of the silicon wafers. Etch rate measurements have been performed varying the anodization current density. A maximum value of 22 µm.min–1 has been obtained. Moreover, the porosity behavior with the current density increase is also discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

J. Semai

Papers

Investigations on porous silicon as electrode material in electrochemical capacitors

Thick microporous silicon isolation layers for integrated RF inductors

Innovative electrochemical deep etching technique involving aluminum thermomigration