Nitrogen‐Doped Graphene Foams as Metal‐Free Counter Electrodes in High‐Performance Dye‐Sensitized Solar Cells

Abstract

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 Grätzels seminal report in 1991. [1] A typical DSSC device consists of a dye-adsorbed TiO 2 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. [2][3][4][5][6][7][8][9][10][11] In particular, carbon black, [5] carbon nanoparticles, [6] carbon nanotubes, [7,8] and graphene nanosheets [3,[9][10][11] 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. [10][11][12] 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), [10] a perfect graphene sheet may have a low charge-transfer resistance (R ct ), 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. [13] 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. [13,14] Indeed, our recent articles, along with articles of others, on nitrogen doping of carbon nanotubes and graphene [13][14][15][16][17][18] 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. [17,18]...

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

“…A DSSC with a 3D N-doped graphene foam cathode can displayed a power conversion efficiency of 7.07%, comparable to its counterpart based on a Pt cathode (7.44%) (Fig. 10c, d) [33].…”

“…A conventional DSSC is composed of a photoanode sensitized by a dye, an electrolyte containing a redox couple (e.g., triiodide/iodide), and a Pt-based counter electrode (CE) [193]. As alternatives for Pt-based catalysts, metal-free heteroatom-doped carbon catalysts have been demonstrated to be effective at catalyzing the reduction of I 3 − to I − in DSSCs [32,33]. In particular, defective graphene sheets with oxygen-contained functional groups have been demonstrated to exhibit comparable energy conversion efficiency (5.0%) to Pt (5.5%) (Fig.…”

“…A vertically Reproduced with permission from Ref. [33], Copyright 2012, Wiley-VCH. e Preparation of N,P-doped hierarchical porous carbon matrix (HPMC-NP) and the loading of iodine.…”

“…Because ORR, OER, and HER have been reviewed in several previously published articles [4,[10][11][12], the focus in this review will be on recent advances and new reactions (e.g., CO 2 RR, multifunctional electrocatalysis). A critical overview of efficient methods for developing carbon-based metal-free catalysts for various energy conversion/storage and environmental protection devices, including ORR in fuel cells [13][14][15][16][17], ORR and OER in metal-air batteries [18][19][20][21][22][23][24][25][26][27][28], OER and HER in water-splitting units [29][30][31], I − /I 3− reduction in dye-sensitized solar cells [32][33][34][35], and CO 2 RR in Li-CO 2 batteries [36][37][38][39][40][41][42] will then be provided. Finally, perspectives and challenges in this fast-growing and significant field are outlined.…”

Hydrothermal growth of MoS2/Co3S4 composites as efficient Pt-free counter electrodes for dye-sensitized solar cells

“…Dyesensitized solar cells (DSSCs), which are anticipated to be a potential alternative to silicon-based solar cells, have stood out among various photovoltaic devices as a promising renewable energy source owing to their ease of fabrication, environmental friendliness, cost-effectiveness and high efficiency [1][2][3][4][5]. After having been investigated for more than 20 years, liquid-state type DSSCs with power conversion efficiency (PCE) as high as 14.5% have been demonstrated [6]. Typically, the conventional liquid-state type DSSCs have a sandwich-type structure, consisting of a platinized fluorine-doped tin oxide (FTO) glass as the counter electrode (CE), a liquid electrolyte that traditionally contains I − /I3…”