NiO as an Efficient Counter Electrode Catalyst for Dye-Sensitized Solar Cells
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- Wang, H., Wei, W. & Hu, Y.H. Top Catal (2014) 57: 607. doi:10.1007/s11244-013-0218-8
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NiO is an important heterogeneous catalyst employed in chemical processes. However, it is a new topic to explore NiO as a counter electrode catalyst for dye-sensitized solar cells (DSSCs). In this paper, NiO with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was demonstrated an efficient DSSC counter electrode with a maximum power conversion efficiency of 7.58 %. Furthermore, electrochemical impedance spectroscopy and cyclic voltammetry measurements revealed that the excellent photovoltaic performance is due to the combination between the high catalytic activity of NiO and the superior electrical conductivity of PEDOT:PSS. The optimum weight ratio of NiO to PEDOT:PSS is 48.
KeywordsNickel oxide (NiO)Electrode catalystsDye-sensitized solar cells (DSSC)
As a representative of the third-generation solar cell, dye-sensitized solar cells (DSSCs) have generated intense interest due to their easy fabrication, low cost, and high energy conversion efficiency [1–4]. In a DSSC, electrons transfer from an excited dye sensitizer into the semiconductor conduction band, then pass through the transparent conductive electrode to the external circuit, and finally to the counter electrode. The oxidized dye could be regenerated by a redox electrolyte, and the oxidized electrolyte could diffuse to the counter electrode and be reduced by electrons [5, 6]. As one of the most important components, an ideal counter electrode would possess both excellent catalytic activity for the reduction of oxidized electrolyte and high conductivity for electron transfer from the external circuit to the electrolyte [7–11].
Nickel oxide (NiO) is widely employed as a heterogeneous catalyst used in chemical and petrochemical processes, such as syngas production [12–15]. Furthermore, as a p-type semiconductor, NiO-based photocathode was combined with n-type TiO2-based photoanode to form a tandem DSSC, yielding an open-circuit voltage of 0.73 V . Many efforts, such as morphology control of NiO film  and modification of NiO preparation , were attempted to achieve higher performance for tandem DSSCs, but their power conversion efficiencies are stills very low (<1 %) [17–20]. On the other hand, NiO was also exploited for DSSC counter electrodes. Guai et al.  showed that a DSSC with the counter electrode consisting of sulfur-doped NiO exhibited a high power conversion efficiency up to 5.04 %. Bajpai et al.  deposited NiO nanoparticles on graphene platelets as electrocatalyst for DSSCs, yielding 3.06 % efficiency. Okumura et al.  reached a high efficiency of 5.11 % with the DSSC using NiO hybridized carbon film as a cathode. However, the combination between NiO and a conductive polymer as a DSSC counter electrode has not been explored.
As a counter electrode material for DSSC, the ionic feature of NiO would promise its strong interaction with the ions of iodine-based electrolytes, leading to excellent catalytic activity. However, because the poor electrical conductivity of NiO, its combination with a conductive component (such as conductive polymer) is necessary. For this reason, we combined NiO with a conductive polymer—(poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as a composite counter electrode and revealed its properties in this work. The excellent performances of DSSCs with the NiO/PEDOT:PSS counter electrodes were demonstrated.
2.1 Counter Electrode Preparation and Characterization
NiO powder (Aldrich) was mixed with 2.5 % PEDOT:PSS aqueous solution with stirring for 5 min to prepare a homogeneous paste with a selected weight ratio (16:1, 48:1, 64:1, or 96:1) of NiO to PEDOT:PSS. The paste was deposited on fluorine-doped tin oxide (FTO) coated glass to form a composite film by doctor blading method. The NiO-based counter electrode was obtained by heating the NiO/polymer coated FTO glass at 80 °C for 4 h. For comparison, pure PEDOT:PSS and NiO counter electrodes were also prepared using the approach, in which aqueous pastes of PEDOT:PSS and NiO were coated on FTO glasses with doctor blading method, respectively, followed by heating at 80 °C for 4 h. The X-ray diffraction spectrometer (Scintag XDS2000 Powder Diffractometer) was employed to characterize the crystalline structure of NiO, PEDOT:PSS, and NiO/PEDOT:PSS films.
2.2 Photoelectrode Preparation
Fluorine-doped tin oxide (FTO) coated glasses were cleaned and treated by 0.4 mM TiCl4 (70 °C) for 30 min. TiO2 paste (ethanol as solvent) was deposited on the FTO glass by doctor blading method to form a TiO2 nanocrystalline film. The obtained TiO2 photoelectrode was heated at 325, 375, 450, and 500 °C for 5, 5, 15, and 15 min, respectively, followed by TiCl4 treatment again and then heat treatment at 500 °C for another 30 min. The treated TiO2 photoelectrode was immersed in a cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bis-tetrabutylammonium (N719, 0.3 mM in ethanol) dye solution for 24 h to achieve dye sensitization.
2.3 Fabrication and Characterization of DSSCs
The sandwich DSSCs were assembled using the N719-sensitized TiO2 photoelectrode and NiO/PEDOT:PSS composite counter electrode with liquid electrolyte (I−/I3− redox couple) between them. The electrolyte consists of 0.025 M LiI, 0.04 M I2, 0.28 M tert-butyl pyridine (TBP), 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), and 0.05 M guanidinium thiocyanate in acetonitrile/valeronitrile solvent with 85/15 volume ratio. The photovoltaic performance of the obtained NiO-based DSSCs was evaluated by Kithley 2400 under illumination of AM 1.5 simulate sunlight (1sun, 100 mW cm−2) using a Newport solar simulator. Electrochemical impedance spectroscopy (EIS) data were collected at a potential equal to the open-circuit voltage of a DSSC and 10 mV amplitude over the frequency range of 0.1–100 kHz by CHI 600D electrochemical workstation in dark condition. Cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation (EG&G Princeton Applied Research) with a three-electrode system (NiO/PEDOT:PSS composite as a working electrode, Pt wire as a counter electrode, and Ag/AgCl as a reference electrode) containing acetonitrile solution of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4.
3 Results and Discussion
The photovoltaic performance and electrochemical characteristics of the DSSCs with different materials as CEs
Jsc (mA cm−2)
NiO is an excellent catalyst for DSSC counter electrode. The DSSC with a NiO/PEDOT:PSS counter electrode reached a maximum power conversion efficiency of 7.58 %. Furthermore, the effect of NiO content on the energy conversion efficiency was demonstrated. As NiO content increased, the energy conversion efficiency increased to the maximum value and then decreased. Such a variation of conversion efficiency with NiO content is dependent on the current density (Jsc), which is determined by combination between the catalytic activity of NiO and the conductivity of PEDOT:PSS.
This work was supported by the U.S. National Science Foundation (NSF-CBET-0931587) and the ACS Petroleum Research Fund (PRF-51799-ND10). Hu also thanks Charles and Carroll McArthur for their great support.