Nitrogen-doped graphene foams as metal-free counter electrodes in high-performance dye-sensitized solar cells.

Owing to their low-cost production, simple fabrication, and high energy conversion efficiency, dye-sensitized solar cells (DSSCs) have attracted much attention since Oregan and Gr tzel s seminal report in 1991. A typical DSSC device consists of a dye-adsorbed TiO2 photoanode, counter electrode, and iodide electrolyte. The counter (cathode) electrode plays a key role in regulating the DSSC device performance by catalyzing the reduction of the iodide–triiodide redox species used as a mediator to regenerate the sensitizer after electron injection. The ideal counter electrode material should possess a low sheet resistance, high reduction catalytic activity, good chemical stability, and low production costs. Because of its excellent electrocatalytic activity for the iodine reduction, high conductivity, and good chemical stability, platinum has been widely used as a counter electrode in DSSCs. However, the high costs of Pt and its limited reserves in nature have been a major concern for the energy community. Recently, much effort has been made to reduce or replace Pt-based electrodes in DSSCs. In particular, carbon black, carbon nanoparticles, carbon nanotubes, and graphene nanosheets have been studied as the counter electrode in DSSCs. However, their electrical conductivities and reduction catalytic activities still cannot match up to those of platinum. To improve the device performance for DSSCs with a carbon-based counter electrode, it is important to balance its electrical conductivity and the electrocatalytic activity. Since the electrocatalytic activity of graphene for the triiodide reduction often increases with increasing number of defect sites (e.g., oxygen-containing functional groups in reduced graphene oxide), a perfect graphene sheet may have a low charge-transfer resistance (Rct), but a limited number of active sites for catalyzing the triiodide reduction. Unlike chemical functionalization of graphene to introduce electrocatalytic active sites by damaging the conjugated structure in the graphitic basal plan with a concomitant decrease in the electrical conductivity, doping the carbon network with heteroatoms (e.g., N, B, and P) can introduce electrocatalytic active sites with a minimized change of the conjugation length. Furthermore, heteroatom doping has also been demonstrated to enhance the electrical conductivity and surface hydrophilicity to facilitate charge-transfer and electrolyte–electrode interactions, respectively, and even impart electrocatalytic activities. Indeed, our recent articles, along with articles of others, on nitrogen doping of carbon nanotubes and graphene have clearly shown that nitrogen-doped carbon nanomaterials can act as metal-free electrodes to show even higher electrocatalytic activities, better long-term operation stability, and more tolerance to crossover/poisoning effects relative to a platinum electrode used for oxygen reduction in fuel cells. The newly discovered electrocatalytic reduction activities, together with the doping-enhanced electrical conductivities and surface hydrophilicity, made N-doped carbon nanomaterials ideal as low-cost, but very effective, counter electrodes in DSSCs. To our best knowledge, however, the possibility for N-doped carbon nanomaterials to be used as metal-free electrocatalysts at the counter electrode for triiodide reduction in DSSCs has not been exploited. In the present study, we prepared three-dimensional (3D) N-doped graphene foams (N-GFs) with a nitrogen content as high as 7.6% by annealing the freeze-dried graphene oxide foams (GOFs) in ammonia, and used the resultant 3D N-GFs supported by fluorine-doped tin oxide (FTO) glass substrates as the counter electrode in DSSCs. We found that the resultant DSSCs with the foamlike N-doped graphene counter electrode showed a power conversion efficiency as high as 7.07%, a value which is among the highest efficiencies reported for DSSCs with a metal-free carbon-based counter electrode and is comparable to that of DSSCs with a Pt counter electrode (7.44%) constructed under the same condition. The observed superb performance of DSSCs with the newly developed 3D N-GF metal-free counter electrode can be attributed to the heteroatom doping-induced high [*] Dr. Y. Xue, Dr. H. Chen, Dr. J. Qu, Prof. L. Dai Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology and Optometry, Wenzhou Medical College 270 Xueyuan Xi Road, Wenzhou, Zhejiang 325027 (China) E-mail: jia.qu@163.com