390
P. Kumar
silicon (Desplobain et al. 2007; Thissandier et al. 2012; Thissandier et al. 2013).
Carbon-based nanostructured materials have been used for supercapacitor applications along with the development of new architectures of meso-and nanoporous
materials because of their inherent good electrical conductivity and electrochemical stability (Jha et al. 2012; Zhu et al. 2012). However, volumetric performance
is often ignored due to inhomogeneity in sample thickness despite the industrial
importance of volumetric storage characteristics when assessing performance for
technological applications (Gogotsi and Simon 2011). This highlights a structural
benefit of controllable porous materials, such as porous silicon that is formed by
electrochemical etching process and the porous active material that can be varied by
etching condition can decide the volumetric energy storage properties. PS has been
a material of interest due to its high energy density and easy synthesis. However,
due to high resistance and high reactivity of PS it results in low power density and
poor stability. Coating of thin layer of materials on PS has been possible solution to
increase the stability and lessen the resistance (Oakes et al. 2013; Rowlands et al.
1999), but even for coated electrodes the performance has still been many orders
of magnitudes lower that of carbon-based supercapacitors. However, the resistance
of the complex Si nanostructure has limited the power density, therefore, in addition to the stability the coating also needs to provide high conductivity. Graphene
coating on PS is an alternative to provide both stability as well as high conductivity.
Graphene a 2 D material has shown a wide range of properties that includes strength,
elasticity, mechanical stiffness, electrical and thermal conductivity, it is chemically
inert, impermeable to gases, optically active and flexible. Graphene, being a highly
inert material can act as a corrosion barrier against most gases, water, and oxygen
diffusion.
Functionalization of porous silicon surface with graphene was done by Oakes et al.
for stable and high-performance electrochemical supercapacitor. In this study highly
doped (0.01–0.02 cm) Si wafer has been used to make porous silicon in HF-based
electrolyte using electrochemical etching. As shown in Fig. 3 by changing etching
conditions that include HF concentration in electrolyte, etching current density, etching time, type and doping level of Si, the physical properties of PS can be varied. An
optimized PS of 75% porosity and 4 µm thick sample has been used for graphene
coating to check its stability and energy storage capacity. For graphene coating, the
PS sample has been treated with C 2 H 2 /H 2 /Ar gas mixtures over a temperature ramp
from 650 to 850 °C for 20 min. With this treatment mono or few layer graphene has
been coated on PS surface. The coating of graphene on PS has enhanced conductivity
as well as provides a stable electrode–electrolyte interface that is critical to attain
good energy storage characteristics. With the same porous structure, graphene-coated
PS devices compared to pristine PS has leaded much greater charge storage capacity
as illustrated in Fig. 5a. FESEM images of pristine PS and graphene-coated PS shown
in Fig. 5b and c respectively revealed nanoscale structure provides high surface areas
that enable this material architecture for electrochemical supercapacitor electrodes
reported by Oakes et al. in scientific reports (Oakes et al. 2013). They have shown
that the graphene-coated PS has outstanding electrochemical properties compared
to uncoated PS in every measure and by using electrochemical device testing data, a
P. Kumar
silicon (Desplobain et al. 2007; Thissandier et al. 2012; Thissandier et al. 2013).
Carbon-based nanostructured materials have been used for supercapacitor applications along with the development of new architectures of meso-and nanoporous
materials because of their inherent good electrical conductivity and electrochemical stability (Jha et al. 2012; Zhu et al. 2012). However, volumetric performance
is often ignored due to inhomogeneity in sample thickness despite the industrial
importance of volumetric storage characteristics when assessing performance for
technological applications (Gogotsi and Simon 2011). This highlights a structural
benefit of controllable porous materials, such as porous silicon that is formed by
electrochemical etching process and the porous active material that can be varied by
etching condition can decide the volumetric energy storage properties. PS has been
a material of interest due to its high energy density and easy synthesis. However,
due to high resistance and high reactivity of PS it results in low power density and
poor stability. Coating of thin layer of materials on PS has been possible solution to
increase the stability and lessen the resistance (Oakes et al. 2013; Rowlands et al.
1999), but even for coated electrodes the performance has still been many orders
of magnitudes lower that of carbon-based supercapacitors. However, the resistance
of the complex Si nanostructure has limited the power density, therefore, in addition to the stability the coating also needs to provide high conductivity. Graphene
coating on PS is an alternative to provide both stability as well as high conductivity.
Graphene a 2 D material has shown a wide range of properties that includes strength,
elasticity, mechanical stiffness, electrical and thermal conductivity, it is chemically
inert, impermeable to gases, optically active and flexible. Graphene, being a highly
inert material can act as a corrosion barrier against most gases, water, and oxygen
diffusion.
Functionalization of porous silicon surface with graphene was done by Oakes et al.
for stable and high-performance electrochemical supercapacitor. In this study highly
doped (0.01–0.02 cm) Si wafer has been used to make porous silicon in HF-based
electrolyte using electrochemical etching. As shown in Fig. 3 by changing etching
conditions that include HF concentration in electrolyte, etching current density, etching time, type and doping level of Si, the physical properties of PS can be varied. An
optimized PS of 75% porosity and 4 µm thick sample has been used for graphene
coating to check its stability and energy storage capacity. For graphene coating, the
PS sample has been treated with C 2 H 2 /H 2 /Ar gas mixtures over a temperature ramp
from 650 to 850 °C for 20 min. With this treatment mono or few layer graphene has
been coated on PS surface. The coating of graphene on PS has enhanced conductivity
as well as provides a stable electrode–electrolyte interface that is critical to attain
good energy storage characteristics. With the same porous structure, graphene-coated
PS devices compared to pristine PS has leaded much greater charge storage capacity
as illustrated in Fig. 5a. FESEM images of pristine PS and graphene-coated PS shown
in Fig. 5b and c respectively revealed nanoscale structure provides high surface areas
that enable this material architecture for electrochemical supercapacitor electrodes
reported by Oakes et al. in scientific reports (Oakes et al. 2013). They have shown
that the graphene-coated PS has outstanding electrochemical properties compared
to uncoated PS in every measure and by using electrochemical device testing data, a