Silicon Nanowires and Nanotrees Electrodes for Pseudocapacitors

Supercapacitors integration in micro-electronic circuit should improve portable devices efficiency [1] and be easier with silicon (Si) electrodes. In our previous work we have realized highly doped Si nanowires (SiNWs) micro-supercapacitor (µ-SC) electrodes showing quasi ideal capacitive behavior [2, 3]. This works studies the elaboration and characterization of highly doped Si nanotrees (SiNTrs) µ-SC and SiNWs micro-pseudocapacitors electrodes. It focuses on our recent advances on SiNTrs growth and use in µ-SC.

SiNWs and SiNTrs are grown by CVD on highly doped Si via localized gold catalysis. SiNTrs are grown either by two CVD growths separated by a new gold deposition, or by a single growth in which an annealing (gold pours along NWs) separates trunks and branches growths [4, 5]. Electrochemical properties are evaluated in organic electrolyte (1M, NEt₄BF₄, PC) and ionic liquid (EMI-TFSI) by EIS, cyclic voltametry and galvanostatic charge/discharge. For PC PEDOT films was electrodeposited on SiNWs.

As seen previously, electrode capacity increases with the electrode developed surface, i.e. SiNWs length, density and SiNTrs branches. SiNTrs are tuned by fitting HCl gas ratio during trunk growth, annealing time and temperature. Doping gas/silane ratio monitors their doping level. Highly doped, dense, hyperbranched (L ≈ 3 µm, Ø ≈ 30 nm), 40 µm long SiNTrs electrodes have a 950 µF.cm^-2 capacity, i.e. about 158 fold bulk Si one and twice 50µm long SiNWs electrode one. SiNTrs/SiNTrs µ-SCs show highly stable cycle efficiency (97 %) and capacity over at least 100 000 cycles for current densities ranging from 5 to 500 µA.cm^-2. Using ionic liquid as electrolyte improves devices voltage and thus performances.

[1] J.R. Miller, P. Simon, Science, 2008, 321, 651

[2] F. Thissandier, and al, Electrochem. Comm., 2012, 25, 109

[3] F. Thissandier, and al, Nanoscale Res. Lett., 2012, accepted

[4] P. Gentile, and al, Nanotechnol., 2008, 19, 125608

[5] R. Maboudian, and al, J. Mater. Chem., 2008, 18, 5376