Full Length ArticlePhosphorus-doped porous graphene nanosheet as metal-free electrocatalyst for triiodide reduction reaction in dye-sensitized solar cell
Graphical abstract
Introduction
Dye-sensitized solar cells (DSSCs), with high power conversion efficiency (PCE) over 13%, have drawn enormous attention in the past two decades and are considered as one of the most competitive candidates for low-cost, efficient and eco-friendly energy conversion devices [1], [2], [3]. Generally, a DSSC is composed of the following three components: a dye-sensitized TiO2 photoanode, an electrolyte with iodide/triiodide (I−/I3−) as redox shuttle and a counter electrode (CE). The CE plays a pivotal role in the implement of the light-electricity conversion circuit by catalysing the reduction of the redox couple which was used as a mediator to regenerate dye after electron injection [4], [5], [6]. To meet these requirements, an ideal CE should simultaneously have the features of excellent conductivity to transfer charges and superior catalytic activity for the reduction of triiodide [7], [8]. Thus, platinum (Pt) has been commonly regarded as the benchmark CE material for DSSC. However, Pt, one of the most expensive noble metals, enormously raises the cost of DSSCs, thereby impeding their commercial production [9]. To this end, massive efforts have been devoted to exploring efficient, cheap and abundant CE materials for DSSCs.
Pt-free alternative materials, previously proposed as CEs in DSSCs, include carbon materials, transition metal compounds, conductive polymers, hybrids and so forth, which have been systematically summarised [1], [7]. Among them, carbon materials, such as carbon black [10], mesoporous carbon [11], [12], carbon nanotubes [13] and graphene [14], [15], have shown great potential for practical use by virtue of their low cost, earth-abundance, excellent conductivity and high resistance against electrolyte corrosion [16]. Unfortunately, the catalytic activity of pure carbon materials is comparatively poor [17], [18]. Wu et al. compared nine kinds of carbon material CEs and achieved PCEs of 2.80–7.50%, with mesoporous carbon achieving comparable performance in comparison to Pt [11]. However, the competitive performance was achieved by increasing the thickness of CE (25 um) to increase the amount of active sites. Compared to improving their intrinsic catalytic activities, this strategy inevitably results in bulky and opaque devices, thus hindering their widespread applications [1], [11]. Evidently, the relatively inert nature of carbon materials has become a bottleneck problem for their practical use in DSSCs.
As validated, the catalytic activity of carbon materials is derived from defects, functional groups and the charge polarization induced by the heteroatoms in the carbon framework [6], [19]. Recently, aiming to boost the performance of carbon materials in DSSCs, heteroatom doping, such as boron (B) [6], [16], nitrogen (N) [20], [21], sulphur (S) [22], [23], phosphorus (P) [24], [25], halogen [26] and silicon [27], [28] has been demonstrated as an efficient approach to tailor intrinsic catalytic activities of carbon materials due to the charge polarization stemmed from the electronegativity difference between carbon and heteroatoms (e.g. C: 2.55, N: 3.04, B: 2.04, P: 2.19, etc.) [29], [30], [31], [32]. Among them, phosphorus, with a lower electronegativity and larger atom size, is expected to pose a unique effect on the electrocatalytic activity of carbon based material in terms of inducing changes in the local charge density and creating more structural disorders [33], [34], [35], which is unfortunately rarely investigated, particularly as CEs in DSSCs. Wang et al. first utilized P-doped reduced graphene oxide (rGO) as CEs in DSSCs, with a relatively low PCE of 6.04% [24]. This might be caused by the inevitable aggregation and restacking between rGO, which significantly reduces the surface area and active sites [36]. In a very recent study, co-doping strategy was first introduced by Yu et al., the as fabricated N, P co-doped graphene based DSSCs achieved an encouraging efficiency although it suffered from thick CEs (10 μm) to some extent [37]. Meanwhile, it should be noted that rational structure design is another important strategy to further boost the material performance [38]. As validated that the porous structure with high surface area provides favorable paths for mass transport, in which more active sites are exposed to the redox couple in the electrolyte, thus facilitating the ingress of I3−, the reduction of I3− and finally the diffusion of I− [39], [40]. Unfortunately, the potential of phosphorus doped graphene with rationally designed nanostructure in DSSCs remains to be explored to date.
Herein, we synthesized P-doped porous graphene nanosheet architectures (P-G) with a large specific surface area (SSA) of 1627.8 m2 g−1 using a vertical CVD system, and applied it as CE for DSSC. Due to abundant catalytic sites stemmed from the enriched open edges within the unique porous nanosheet architectures, associated with P atoms doping to further enhance its intrinsic catalytic activity, DSSC with P-G CE at a relatively thin thickness of 5 μm yielded a high PCE of 7.08%, which was comparable to that of DSSC with Pt as CE (7.19%). Our findings demonstrate P-G with proper nanostructure has great potential to replace Pt CE in DSSCs.
Section snippets
Synthesis of porous graphene (G) and P-doped porous graphene (P-G)
In the case of P-G, MgO powder was first mixed with deionized water under ultrasonic agitation for 1 h. Then the mixture solution was boiled for 24 h with reflux condensation. After vacuum filtration and drying, the resultant powder was calcined at 500 °C for 30 min to remove water thus forming porous MgO template. Successively, the porous MgO was ultrasonically dispersed in (NH4)3PO4·6H2O solution for 1 h and dried at 80 °C for 24 h, followed by calcination at 600 °C for 1 h to remove NH3, and finally
Results and discussion
As illustrated in Fig. 1, porous Mg3(PO4)2 is adopted as mesoporous catalysts for the templated CVD growth of P-G nanosheets, and it should be noted that with the vertical fluidized-bed CVD reactor, P-G can be readily produced at kilogram-scale, which is vital for its practical use in DSSCs. The morphology and structure is investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). According to SEM images
Conclusion
In summary, we have successfully synthesized P-doped porous graphene nanosheet architectures (P-G) with a large specific surface area of 1627.8 m2 g−1. Moreover, benefiting from P heteroatom doping further enhancing its intrinsic catalytic activity, along with abundant catalytic sites originated from the unique porous architecture, the DSSC with P-G as CE approaches 98.5% that of Pt based counterpart (7.19%), exhibiting great potential in the replacement of Pt CEs in DSSCs. More importantly, our
Acknowledgements
We acknowledge support from the National Natural Science Foundation of China (Nos. 21576289, 21322609, 11274362), Science Foundation of China University of Petroleum, Beijing (No. C201603), Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (2462014QZDX01) and Thousand Talents Program.
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These authors contributed equally to this work.