(375e) Layered Double Hydroxides As An Effective Additive in Polymer Gelled Electrolyte Based Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSC) continue to draw a great deal of research attention as a promising alternative clean energy device, because of the simplicity and low cost involved in the manufacturing process and the reasonable power conversion efficiency (PCE). The boost in PCE to 12.3 % reported by Yella et al. [1] in 2012 makes DSSCs even more competitive. One of the key issues yet to be fully resolved is the volatility, leakage, and poor long term stability associated with the use of liquid electrolytes in DSSCs. Intensive and extensive research efforts have been devoted to the development of gel or solid electrolytes to tackle the issue. For solid electrolytes, the poor contact at the electrolyte/electrode interface often hampers the boost in PCE. Gel electrolytes, on the other hand, possess the advantages of low volatility and superior long term stability as compared with liquid electrolytes, and of good electrolyte/electrode contacts as compared with solid electrolytes. In the past few years, use of polymer gelled electrolytes containing performance boosting inorganic additives has been a promising approach. Particularly, clay materials, because of their high chemical stability, ion exchange ability, and natural abundance, have been a popular candidate as the inorganic additive [2]. Ho and co-workers added exfoliated montmorillonites to polymer gelled electrolytes to reduce the electrolyte/electrode interfacial impedances and to enhance the reduction rate of I₃^- at the cathode [3,4]. Geng et al. found that addition of montmorillonites to poly(ethylene oxide) (PEO) gelled electrolytes can inhibit the charge recombination at the anode [5]. Lai et al. used polyvonydiene fluoride-cohexafluoro propylene (PVDF-HFP) gelled electrolytes with mica nanoparticles as the additive to reduce the crystallinity of the polymer, leading to higher PCEs [6].

The above mentioned layer-structured materials, montmorillonites and mica, belong to the class of cationic clays, carrying negative charges on their hydroxide layers counter-balanced by positive inter-layer ions. The popular electrolyte redox couple for DSSCs, I^-/I₃^-, however, contains anions. The use of anionic clay materials, containing inter-layer cations, may prove advantageous for DSSCs [2]. In this work, layered double hydroxides (LDH), a class of anionic clay materials, were developed as the inorganic additive for PVDF-HFP gelled electrolytes. ZnAl-CO₃-LDH and ZnAl-Cl-LDH were prepared with a separated nucleation and aging method and low supersaturation co-precipitation method, respectively. The addition of the two LDHs, in terms of PCEs, significantly improved over the plain PVDF-HFP gelled electrolyte and competed favorably with the liquid electrolyte, 8.13% for the liquid electrolyte, 7.48% for the PVDF-HFP gelled electrolyte, 8.11% for the ZnAl-CO₃-LDH/PVDF-HFP gelled electrolyte, and 8.23% for the ZnAl-Cl-LDH/PVDF-HFP gelled electrolyte based DSSCs. The good performance in PCE achieved by the LDH based DSSCs was mainly attributed to the significant boost in open circuit voltages (Voc), from 0.74 V for both the liquid electrolyte and PVDF-HFP gelled electrolyte based DSSCs to 0.79V for the ZnAl-CO₃-LDH/PVDF-HFP gelled electrolyte based DSSC and 0.84V for the ZnAl-Cl-LDH/PVDF-HFP gelled electrolyte based DSSC. The boost in Voc was found to come from the positive shift in the redox potential of the electrolyte redox couple as revealed from a cyclic voltammetry analysis. As for the long term stability, a PCE retention rate of 89% after 624 h was achieved with the ZnAl-CO₃-LDH/PVDF-HFP gelled electrolyte based DSSC, much better than that of the liquid electrolyte based one.
References:

1. A. Yella, H.-W. Lee, H.-N. Tsao, C. Yi, A.-K. Chandiran, M.-K. Nazeeruddin,  Eric W.-G. Diau, C.-Y. Yeh, S.-M. Zakeeruddin, M. Grätzel, Science 334, (2011)
2. X.-Wang, S.-A. Kulkarni, B.-I. Ito, S.-K. Batabyal, K. Nonomura, C.-C. Wong, M. Gratzel, S.-G. Mhaisalkar, S. Uchida, ACS Appl. Mater. Interfaces, 5(2013)
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