Nickel sulfide counter electrode modified with
polypyrrole nanoparticles to enhance catalytic
ability for flexible dye-sensitized solar cells
Gentian Yue,
*
a
Peng Li,
*
b
Fumin Li
a
and Chong Chen
a
An efficient counter electrode (CE) of nickel sulfide/polypyrrole/titanium foil (NiS/PPy/Ti) with excellent
electrochemical catalytic ability was prepared and used as a platinum (Pt)-free CE for flexible dye-
sensitized solar cells (FDSSCs). The surface morphology of the NiS/PPy/Ti CE was characterized by
scanning electron microscopy. The electrochemical performance was also characterized according to
a series of electrochemical tests which indicated that the NiS/PPy/Ti CE showed great electrocatalytic
ability and low charge transfer resistance compared to the Pt/Ti electrode in an iodide/triiodide
electrolyte. As a result, the FDSSC based on the NiS/PPy/Ti CE exhibited a high light–electric conversion
efficiency of 7.66% under 100 mW cm
2
illumination, comparable to that of the Pt/Ti-based FDSSC
(7.31%). Furthermore, the FDSSC based on the NiS/PPy/Ti counter electrode had increased short-circuit
current density and is very stable.
1. Introduction
Dye-sensitized solar cells (DSSCs), consisting of one TiO
2
pho-
toanode adsorbed dye, one redox mediator of iodide/triiodide
(I
/I
3
) and one platinum (Pt) catalyst to complete the redox
reaction,
1
have attracted great attention for their feasibility, low
cost and high efficiency since 1991. Though Pt has a good
catalytic activity and provides over 13% light-electric conversion
efficiencies for DSSCs,
2
it is an expensive metal, hence,
researchers are seeking economic, stable, highly electrically
conductive and superior electrocatalytic counter electrode (CE)
materials. A range of alternative materials to Pt including
carbon materials, conducting polymers, transition metal
suldes, nitrides, carbides and alloys have oen been re-
ported.
3–10
Among them, nickel sulde (NiS) displays reasonable
electrocatalytic activity for the redox couples, comparable to
that of the Pt CE.
11–13
Lin et al.
14
prepared a highly transparent
NiS CE on polyimide as a Pt- and transparent conductive oxide
(TCO)-free exible CE for DSSCs by using an electrodeposition
technique and presented a good photovoltaic performance for
thiolate/disulde mediated DSSCs. Therefore, NiS is a strong
potential candidate for an efficient CE in electronics, optoelec-
tronics, and memory devices. Also, polypyrrole (PPy), due to its
superior catalytic ability and economic, stable, and simple
fabrication process, is considered to be one of the most
promising conducting polymers for efficient CE materials in
optoelectronic devices.
15–18
Besides, exible DSSCs (FDSSCs) are
the inevitable trend for DSSC practical development since their
shape or surface can be devised and constructed as needed
using the techniques of large-scale roll-to-roll processing and
rapid coating, which could be a promising solution for many
impending energy and environmental issues.
Thus, in this work, it is envisaged that titanium foil (Ti) as
the exible substrate will improve the conductive performance,
and the NiS/Ti CE with PPy nanoparticles decorated will serve as
a Pt-free CE in FDSSCs. The NiS/PPy/Ti CE showed enhanced
electrocatalytic activity and low charge transfer resistance,
demonstrated by the results of the cyclic voltammetry (CV) and
electrochemical impedance spectroscopy (EIS) measurements.
The FDSSC fabricated with the NiS/PPy/Ti CE exhibited a greatly
improved light-electric conversion performance of 7.66% under
100 mW cm
2
irradiation.
2. Experimental
2.1 Preparation of NiS/PPy/Ti CE
The NiS/PPy/Ti CE was prepared by using a two-step
electrochemical/chemical process as follows. Firstly, cleaned
Ti foil was immersed in 0.05 M NiCl
2
and 1.0 M TU polymeri-
zation solution to carry out the electrodeposition at a constant
potential of 1.2 V vs. Ag/AgCl at room temperature (about
25
C) with an exposed area of 0.8 0.8 cm
2
. Secondly, the
obtained NiS/Ti CE was immersed in a freshly prepared solution
consisting of [Py] : [Fe
3+
] ¼ 1 : 1, and then 1 ml of 25.1 g l
1
(NH
4
)
2
S
2
O
8
solution was fast added into it, and the solution was
kept for 4 hours at 4
C, resulting in an in situ polymerization
a
Key Laboratory of Photovoltaic Materials of Henan, School of Physics & Electronics,
Henan University, No. 1, Jinming Rd., Kaifeng, 475004, China. E-mail: yuegentian@
126.com; Tel: +86 371 23881602
b
College of Mechanical Engineering and Automation, Huaqiao University, No. 668,
Jimei Rd., Xiamen, 361021, China. E-mail: lipengcad@163.com
Cite this: RSC Adv.,2016,6,61278
Received 24th April 2016
Accepted 16th June 2016
DOI: 10.1039/c6ra10585h
www.rsc.org/advances
61278 | RSC Adv.,2016,6,61278–61283 This journal is © The Royal Society of Chemistry 2016
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