Economically viable V2O3@activated carbon composite materials as counter electrodes for dye sensitized solar cells by single step reduction
Graphical abstract
Three V2O3@C CE-based DSSCs demonstrated PCE of 4.94, 5.55 and 5.32% were obtained for AC:NH4VO3 mass ratios of 1:3, 2:3 and 4:3 respectively, which were superior to that of Pt (4.54%), AC (2.10%) and V2O3 (3.33%) CE-based DSSCs from J-V curves.
Introduction
Solar energy is the largest natural energy resource which can be explored and utilized much more generously. Dye sensitized solar cells (DSSCs) are designed to be a class of competitive devices that directly convert solar energy into electricity. DSSCs consist of a dye-sensitized photoanode, an electrolyte with redox couple, and a counter electrode (CE) [1,2]. The chief concern in the commercialization of DSSCs is cutting down the manufacturing cost further, while keeping up decent efficiencies and lifetime. In a DSSCs, a 10 μm thick film composed of a three-dimensional (3D) network of randomly dispersed spherical TiO2 nanoparticles is typically employed as a photoanode [3] To obtain effective photoanodes, a variety of film preparation techniques have been developed and applied for crafting a diverse assortment of nanostructured semiconductor photoanodes. Lin et al. [4] report hierarchically structured TiO2 nanotube arrays composed of an under layer of highly ordered anatase TiO2 nanotubes with small rutile nanocrystal emerged on the tube walls and a top layer of flower shaped structures containing dispersedly grown short rutile TiO2 nanorods. Such nanotubes yield power conversion efficiency (PCE) of 7.24%. Lin et al. also embedded Graphitic thin films with highly dispersed TiO2 nanoparticles [5]. Arrays of TiO2 dots embedded in carbon matrix were fabricated via UV-stabilization of polystyrene-block-poly(4-vinylpyridine) films containing TiO2 precursors followed by direct carbonization. Photoanode containing carbon/TiO2 thin layers at both sides of pristine TiO2 layer, an increase of 40.6% in overall PCE was achieved compared with neat TiO2-based DSSCs.
The CE is an important component of DSSCs and plays an important role in collecting electrons from an external circuit and in catalyzing the regeneration of the redox couple at the CE/electrolyte interface [6,7]. Generally, Pt is the most-common CE material for I3−/I− based DSSCs owning to its excellent electrocatalytic activity, superior conductivity and stability [8,9]. However, Pt is not only expensive and rare but can also be readily corroded by the I3−/I− electrolyte. Development of alternative Pt-free electrocatalytic materials is considered to be one of the crucial steps toward improved energy conversion efficiency and low-cost alternatives of DSSCs [10,11]. Over the past few years, chalcogen compounds of transition metal include oxides (TMOs) and Sulfides (TMSs), as a representative family of functional materials was of particular interest owing to its unique excellent electrocatalytic activity, low cost, abundant resources, and potential applications as compared to their bulk counterparts [12]. Because of TMOs and TMSs have similar electronic structures with Pt, it can present Pt-like electrocatalytic activity and have great potential in DSSC applications [13]. Up to now, various types of TMOs have been synthesized, such as Nb2O5, NbO2 [14], HfO2 [15], TaO [16], RuO2 [17], a-Fe2O3 [18], Cr2O3, MoO2 [19,20], WO3 and WO2 [21], etc. TMSs such as TiS2 [22], CoS [23], MoS2, WS2 [24,25], Ag2S [26], CuSn [27], Sb2S3 [28], VS2 [29], SnS2 [30], etc. The performance of single transition metal compounds catalysts can be further improved through conductive carbon and carbon derivative materials (activated carbon, multiwall carbon nanotubes, graphite, nanotubes, graphene and C60). Carbon composites have attracted research attention to enhance the value of products and to reduce production costs, which has become increasingly prominent. Previously, Qiu et al. [31] report nitrogen-doped carbon nanowires (NCWs) through combining oxidation polymerization from p-phenylenediamine with carbonization process. The NCWs exhibit a superior response to the I3− reduction in DSSCs with a high PCE of 8.90%. Active-site-enriched selenium-doped graphene (SeG) was crafted by ball-milling followed by high-temperature annealing to yield abundant edge sites and fully activated basal planes. The SeG exhibited a superior response to the I3− reduction with a PCE of 8.42% [32]. Qiu et al. [33] also report a facile yet effective strategy for engineering sulfur-doped porous graphene (SPG) using sulfur powder as the sulfur source and pore-forming agent. The as-made SPG as the CE for DSSCs achieves a PCE of 8.67%. Choi [34] have synthesized the hybrids of various nanoparticles (NPs) and reduced graphene oxide (RGO) by dry plasma reduction. The NiO-NPs/RGO hybrid exhibited a PCE of 7.42%. RuO2/RGO was the similar synthesized and applied as CE that resulted in PCE of 8.32%, which is higher than pure RuO2 NPs (2.36%) [35].
As shown in our previous work [9,14,19,21,24,[36], [37], [38]], we had synthesized a series of transition metal oxides (TiO2, ZrO2, Cr2O3, WO2 WO3, Nb2O5 V2O5 and MoO2)，which showed a certain catalytic activity toward the iodide redox couples. In TMOs family, vanadium based oxides are of great interest in the field of energy storage technologies such as batteries [[38], [39], [40]] and super capacitors [41], because of their diverse chemical motifs, layered structure, high energy capacity and moderate work functions. Jun et al. [42] have fabricated DSSCs by using graphene modified vanadium pentoxide nanobelts (GVNBs, V2O5) as a cost-effective counter electrodes (CEs), which yielded PCE of 6.17%. GVNBs were synthesized by facile one step hydrothermal method without using any reducing agent and harmful solvents. V6O13 has been implemented for the first time as a CE in DSSC devices. The photovoltaic performance of 0.57% is achieved [43]. In this paper, three proportions of V2O3@activated carbon (V2O3@C) based on activated carbon (AC) and ammonium vanadate (NH4VO3) with mass ratios of 1:3, 2:3 and 4:3 were synthesized by a facile one-step pyrolysis method and applied as CE in DSSCs. V2O3 and the corresponding composites might be essential in various electrochemical devices, due to their great flexibility in structure and morphology, convenience in synthesis, and high electrocatalytic activity toward the electrolyte. V2O3 was easy to obtain by the high temperature pyrolysis method under nitrogen protection. AC as one of carbon materials has high specific surface areas, suitable pore size distributions, low cost, nontoxicity and easy processability [44]. AC is used as a reducing agent to restore NH4VO3 to V2O3 under high temperature, and simultaneously act as a carrier and catalyst [45,46]. It is the V2O3 is loaded onto AC that can increase the dispersion of the catalyst and reduce the resistance. The coupling action of the activated carbon‑vanadium oxide hybrid composites exhibits the optimum performance by the appropriate ratio. We have carried out detailed characterization of synthesized product and investigated the photovoltaic performance for V2O3@C CEs in DSSCs.
Section snippets
Synthesis of V2O3 and V2O3@C composites
All chemicals were purchased from Aladdin and used without further purification. The V2O3 was directly prepared by thermal processing the ammonium vanadate (NH4VO3, purity >99%) at 1100 °C for 1 h under a high-purity N2 atmosphere. AC and NH4VO3 (V) with mass ratios of 1:3, 2:3 and 4:3, respectively, milled for 0.5 h and were mixed in distilled water with 0.5 h stirring at boiling temperature. After drying the mixture at 150 °C, the V2O3@C composite materials were prepared by thermal processing the
Materials characterization
The phase characteristics at room temperature for AC, pure V2O3, and their composite materials V2O3@C are identified from XRD patterns as presented in Fig. 1. It is obvious that AC displays a weak and broad diffraction peak at 25.8° referred to the (200) facet reflection, indicating the poor ordering and non-crystalline nature of the AC [47]. The most intensive diffraction peaks values of pure V2O3 at 33.0, 36.2, 37.7, 41.2, 43.7, 49.8, 53.9, 58.3, 63.5, 65.2, 70.8, 76.3, 78.4, 80.4, 82.1 and
Conclusions
To improve the catalytic activity of V2O3, three proportions composites catalysts of V2O3@C were synthesized with a simple in situ method and then introduced the composite into DSSCs as a counter electrode catalyst. Based on the analysis of the cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel-polarization curve measurements, the catalytic activity of V2O3@C composite for the regeneration of iodide redox couples of is indeed enhanced significantly as compared with pure V2O3,
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 21473048 and 21303039), the Natural Science Foundation of Hebei Province (Nos. B2015205163 and B2016205161); Science and Technology Bureau of Hebei Province (No. 16211117); Natural Science Foundation of Hebei Education Department (No. QN2017087).
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2022, Surfaces and InterfacesCitation Excerpt :The development of CEs with excellent electro-catalytic activity, high stability, high conductivity and accessibility to replace the traditional Pt CE is a crucial route to realize the large-scale commercial application of DSSCs [3]. To resolve this issue, a variety of efficient, stable and low-cost Pt alternative materials have been developed, such as carbonaceous materials [4–9], conductive polymers [10–15], transition metal compounds (TMCs) [16–23], alloys [24–27] and metal organic frameworks (MOF) [28–30], and were applied to CEs in DSSCs in order to obtain ideal power conversion efficiency (PCE). At present, the research of CEs materials mainly focuses on the improvement of preparation methods, composition adjustment or composite, morphology control, surface doping or modification, and other routes, so as to promote the catalytic performance of the materials.