Cobalt selenide hollow nanorods array with exceptionally high electrocatalytic activity for high-efficiency quasi-solid-state dye-sensitized solar cells

doi:10.1016/j.jpowsour.2017.12.064

Journal of Power Sources

Volume 378, 28 February 2018, Pages 475-482

https://doi.org/10.1016/j.jpowsour.2017.12.064 Get rights and content

Highlights

•
Cobalt selenide hollow nanorods array as efficient electrocatalyst is proposed.
•
The ordered cobalt selenide array exhibits excellent electrocatalytic activity.
•
The ordered array structure is favorable for fast diffusion of gel electrolytes.
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Efficient and stable quasi-solid-state dye-sensitized solar cells are achieved.

Abstract

In quasi-solid-state dye-sensitized solar cells (QSDSSCs), electron transport through a random network of catalyst in the counter electrode (CE) and electrolyte diffusion therein are limited by the grain boundaries of catalyst particles, thus diminishing the electrocatalytic performance of CE and the corresponding photovoltaic performance of QSDSSCs. We demonstrate herein an ordered Co_0.85Se hollow nanorods array film as the Pt-free CE of QSDSSCs. The Co_0.85Se hollow nanorods array displays excellent electrocatalytic activity for the reduction of I₃⁻ in the quasi-solid-state electrolyte with extremely low charge transfer resistance at the CE/electrolyte interface, and the diffusion of redox species within the Co_0.85Se hollow nanorods array CE is pretty fast. The QSDSSC device with the Co_0.85Se hollow nanorods array CE produces much higher photovoltaic conversion efficiency (8.35%) than that (4.94%) with the Co_0.85Se randomly packed nanorods CE, against the control device with the Pt CE (7.75%). Moreover, the QSDSSC device based on the Co_0.85Se hollow nanorods array CE presents good long-term stability with only 4% drop of power conversion efficiency after 1086 h one-sun soaking.

Graphical abstract

Introduction

As one of the most promising photovoltaic technologies, dye-sensitized solar cells (DSSCs) have gained great attention from researchers and manufacturers all over the world due to their inherent low cost, environmental friendliness, easy fabrication procedure and relatively high power conversion efficiency [[1], [2], [3]]. A typical DSSC is composed of a photoanode (e.g. dye-sensitized TiO₂ film), an electrolyte containing the iodide/triiodide redox couple, and a counter electrode (CE) [1]. As an indispensable component of DSSCs, the CE transfers electrons from external circuit to the electrolyte by catalyzing the reduction of triiodide ions to iodide species, and meanwhile the circuit is closed. Pt is commonly used as the CE material for DSSCs due to its outstanding electrocatalytic activity for the I⁻/I₃⁻ redox reaction, but the low abundance ratio and poor long-term stability against iodine largely hinder the large-scale production of DSSCs. Therefore, developing low-cost and high-performance Pt-free electrocatalyst appears especially important [4,5].

In past years, many kinds of low-cost CE materials, such as carbon-based materials [[6], [7], [8]], conductive polymers [9,10], inorganic compounds [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] and their composites [21,22], have been studied to replace Pt. As one-dimensional (1D) materials are beneficial to electron transportation, electrocatalysts with nanorod (NR) morphology have been employed as CEs of DSSCs [13,14]. However, random packing of NRs hinders electron transfer from one NR to another NR, weakening the catalytic performance. Furthermore, the disordered nanostructure can block the electrolyte diffusion within the film, in particular for the quasi-solid-state electrolyte. By contrast, nanorod arrays (NRAs) can provide short pathway for electron transport through the NR and open channels for electrolyte diffusion. Despite these unique features, the reported NRA CEs do not show desired effect on electrocatalytic performance, and they do not show the strong points for fast electrolyte diffusion either [23,24]. Therefore, there remains a big challenge to develop highly efficient NRA based CEs, which should not only have excellent electrocatalytic activity but also be favorable for fast diffusion of redox species in quasi-solid-state DSSCs (QSDSSCs).

In this work, Co_0.85Se NRA thin films were in situ grown on the conductive glass substrate with a two-step low temperature reaction. When the Co_0.85Se NRA film is applied as the CE of QSDSSCs, it demonstrates exceptionally high electrocatalytic activity as judged from the extremely low charge transfer resistance. Moreover, the diffusion of the redox species in the quasi-solid-state electrolyte within the Co_0.85Se NRA film is much faster than that within the randomly packed NRs film. As a consequence, the QSDSSC based on the Co_0.85Se NRA CE achieves much higher power conversion efficiency than that with the Co_0.85Se randomly packed NRs CE and the Pt CE as well. More importantly, the Co_0.85Se NRA CE based QSDSSC exhibits good long-term stability under one sun soaking for more than 1000 h.

Section snippets

Materials and reagents

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99.9%), ethanol(99.7%), acetonitrile(99%), tert-butanol(99%) and chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 99%) were purchased from Sinopharm. Urea (99%), sodium borohydride (NaBH₄, 98%), selenium (Se, 99.999%), lithium perchlorate (LiClO₄, 99.99%), lithium Iodide (LiI, 99.9%), Iodine (I₂, 99.99%), 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII, 98%), 4-tertbutylpyridine (TBP, 96%), poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF–HFP) and

Characterization

The morphology of the obtained films was examined with SEM. The precursor film presents NRA morphology, as one can see in Fig. S5. After selenization of the precursor NRA, the formed Co_0.85Se film also shows NRA morphology as observed from the cross-sectional SEM image (Fig. 1a). The Co_0.85Se NR is thick (∼145 nm) at the bottom but thin (∼30 nm) at the top, and the diameter of the middle part is ∼110 nm. The thickness of the NRA film is about 3.5 μm. It is seem from Fig. 1a that some NRs are

Conclusions

In summary, ordered Co_0.85Se NRA films are grown in situ on the FTO substrates with a two-step low temperature approach and have been used as the Pt-free CE of QSDSSCs for the first time. The QSDSSC using the Co_0.85Se NRA CE achieves PCE of 8.35%, which is much higher than that (4.94%) with the Co_0.85Se randomly packed NR CE and even higher than that (7.75%) with the reference Pt CE. The excellent performance of the Co_0.85Se NRA CE is believed to originate from the rapid electron transport

Acknowledgement

We thank the financial support from the National Nature Science Foundation of China (21673049), the STCSM (168014342), the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM), and the personalized support from Fudan University for original research.

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Citation Excerpt :
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Cobalt selenide hollow nanorods array with exceptionally high electrocatalytic activity for high-efficiency quasi-solid-state dye-sensitized solar cells

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and reagents

Characterization

Conclusions

Acknowledgement

Sol. Energy Mater. Sol. Cell.

Nano Energy

Mater. Sci. Eng. B-Solid

Electrochim. Acta

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

A low-cost, high-efficiency solar cell Based on dye-sensitized colloidal TiO2 films

Nature

Photoelectrochemical cells

Nature

Porphyrin-sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency

Science

Rational screening low-cost counter electrodes for dye-sensitized solar cells

Nat. Commun.

The future of using earth-abundant elements in counter electrodes for dye-sensitized solar cells

Adv. Mater.

Highly efficient dye-sensitized solar cells based on carbon black counter electrodes

J. Electrochem. Soc.

Graphene materials and their use in dye-sensitized solar cells

Chem. Rev.

Quasi-solid dye sensitised solar cells filled with ionic liquid−increase in efficiencies by specific interaction between conductive polymers and gelators

Chem. Commun.

PEDOT nanotube arrays as high performing counter electrodes for dye sensitized solar cells. Study of the interactions among electrolytes and counter electrodes

Adv. Energy Mater.

CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells

J. Am. Chem. Soc.

A low-cost, high-efficiency solar cell Based on dye-sensitized colloidal TiO₂ films