Atomic N-coordinated cobalt sites within nanomesh graphene as highly efficient electrocatalysts for triiodide reduction in dye-sensitized solar cells
Graphical abstract
Atomically dispersed Co-Nx-C moieties within nanomesh graphene frameworks are synthesized, yielding an excellent electrocatalytic activity and long-term stability as cathodes electrocatalysts for the reduction of triiodide in dye-sensitized solar cells.
Introduction
The aggravating modern energy shortcomings and increasing awareness of environmental issues have stimulated considerable research related to sustainable solar energy-conversion technologies. Among them, dye sensitized solar cell (DSSC) is widely recognized as a promising next generation photovoltaic technology due to its low-cost fabrication, environmental friendliness, and relatively high power conversion efficiency [1], [2], [3]. For DSSCs, the tri-iodide (I3−) reduction reaction is the cornerstone and critical process that determines the overall photovoltaic performance [4], [5]. Ideally, the electrocatalysts should simultaneously have the features of excellent catalytic activity, high electrical conductivity and superior long-term stability. Highly active electrocatalysts, traditionally the noble-metal Pt, have been employed to boost this reaction for high-performance DSSCs [6], [7], [8]. Nevertheless, the global reserve scarcity, high price and poor durability of Pt-based electrocatalysts significantly impede their widespread commercialization [9], [10], [11]. Therefore, searching for highly efficient, cost-effective and stable CE materials as alternatives to Pt electrocatalysts has been a challenging task in the past few decades.
To this end, extensive efforts have been dedicated to developing various carbon materials as Pt-free electrocatalysts due to their excellent electrical conductivity, easy availability, large surface area, metal-free nature and tunable electrochemical activity [12], [13], [14], [15], [16]. It is well established that the intrinsic electrocatalytic activities of carbon materials strongly depend on their surface nanostructures and chemical compositions [17], [18]. For instance, in our previous work, the enriched intrinsic defects within graphene scaffold have been demonstrated to be critical to the electrocatalytic activities toward the reduction of I3− [19]. In parallel, heteroatom doping, particularly nitrogen doping, has exhibited its superior capacity in altering the intrinsic catalytic properties of metal-free electrocatalysts, which is a result of the asymmetrical electron spin density and charge polarization in the sp2 carbon matrix [20], [21], [22], [23], [24]. These work highlight the significance of properly designing the active sites for highly efficient carbon-based electrocatalysts, and further investigations to identify other types of efficient active site would promote the development of the state-of-the-art electrocatalyst for DSSCs.
Very recently, the coordination between transition metal (e.g., cobalt, iron or nickel) and nitrogen (M-Nx) has been reported to have unique effects on the local electronic structures of carbon-based catalysts, thus offering high electrocatalytic activities for various electrochemical applications [25], [26], [27], [28], [29]. For instance, Belekoukia et al. prepared Co-N doped reduced graphene oxide and employed it as a cathode catalyst for the reduction of I3− in DSSCs [30]. However, in their synthetic protocol, Co in the form of nanoparticle tends to aggregate during the pyrolysis process, which leads to an inferior performance compared to that of Pt. With this knowledge, Cui et al. reported the synthesis of atomically dispersed M-Nx species incorporated graphene nanosheets via a ball milling method, and the obtained catalysts exhibited high electrocatalytic activity toward I3− reduction reaction in DSSCs [31]. Nevertheless, the uncontrollable bill milling process would inevitably damage the graphene lattice, and more seriously it is incapable of a delicate control over the nanostructure, such as desirable architecture with abundant exposed sites for M-Nx moieties attachment and appropriate porosity for mass transport of relevant reaction species. Therefore, it is of paramount importance to explore strategies that render alternative carbon-based electrocatalysts with highly efficient active sites and well-defined structures.
The current reports including our own research reveal that the hole-edge structures on graphene could facilitate easy diffusion of reactant ions to the active sites, resulting an enhanced performance in desired reactions [18], [19], [22]. Additionally, specific N-doping and metal coordination at defective edges are expected to be the most favorable active sites [32]. For example, Zhang et al. developed the direct utilization of the intrinsic defects in graphene to generate atomic cobalt active sites via defect engineering, and the as-fabricated catalysts exhibited superior activities for oxygen reduction reaction/oxygen evolution reaction [33]. Inspired by the proof-of-concepts, it is reasonable to expect that the fine control of atomic targeted active sites into a specific substrate to obtain desirable properties can enhance the catalytic activity of carbon-based electrocatalysts for the reduction of I3−, but research in this regard has rarely been reported so far.
Herein, we report a nanomesh graphene framework with atomic dispersion of Co-Nx-C moieties (Co-NMG), whereby the namomesh graphene with interconnected mesoporous structures and abundant defective edges are selected as the substrates to in situ grow and anchor the N-coordinated cobalt atoms. Inherited from the pristine namomesh graphene, the as-synthesized Co-NMG exhibits a mesoporous structure with high porosity. Such a unique structure provides maximum exposure of the active sites to the electrolyte while dramatically enhancing the active sites accessibility. Benefiting from the atomically dispersed and fully exposed highly active sites and reduced mass transport resistance of the reactants, the Co-NMG exhibits intrinsically high electrocatalytic activity and stability toward the reduction of I3−. Impressively, the DSSCs with Co-NMG CEs deliver an satisfied power conversion efficiency (9.06%) comparable with that achieved by Pt CEs (7.71%) under the same conditions. Electrochemical measurements and DFT calculations further reveal that the origin of electrocatalytic activity of Co-NMG is mainly ascribed to the nitrogen dopants, topological defects, and particularly the crucial part of Co-Nx-C moieties. Our work not only experimentally and theoretically demonstrates the great potential of Co-NMG electrocatalysts in DSSCs, but also sheds light on the rational design of highly efficient and robust metal-free catalysts that can be applied to versatile electrochemical applications.
Section snippets
Material and chemicals
All chemicals were purchased from Sigma-Aldrich Corporation (Shanghai, China) and used as received.
Synthesis of and Co, N doped nanomesh graphene and N doped nanomesh graphene frameworks
First, nanomesh graphene frameworks were synthesized via a chemical vapor deposition method with Mg(OH)2-derived MgO flakes as templates, and methane as carbon source, details can be found in our previous work [19], [34]. Then the obtained nanomesh graphene was adopted as substrates for the growth of Co, N doped nanomesh graphene (Co-NMG) frameworks. In a typical synthesis, a mixture of cobalt
Results and discussion
In this work, Mg(OH)2-derived MgO flakes are adopted as mesoporous catalysts for the templated growth of nanomesh graphene via our previously reported synthesis method [19], [34]. As expected, the obtained sample exhibits conductive interconnected mesoporous structures with considerable number of defective sites (Fig. S1). It is demonstrated that these defective sites (e.g., edges, pores) tend to adsorb and anchor metal cations coordinate with nitrogen dopants [32], [43]. Thus, the nanomesh
Conclusion
In summary, we have developed a facile and effective strategy to construct atomically dispersed N-coordinated cobalt sites within mesoporous graphene scaffold. The as-obtained Co-NMG catalysts exhibit high electrocatalytic activity and long-term stability toward the reduction of I3−. The excellent electrocatalytic performance is ascribed to the interconnected graphene structure with high surface area, good conductivity, and abundant defective edges as well as dopant species especially the Co-Nx
Acknowledgements
We gratefully thank for the China Postdoctoral Science Foundation (2017M620084), Science Foundation of China University of Petroleum, Beijing (2462017YJRC051), National Natural Science Foundation of China (Nos. 21776308, 21576289), Science Foundation of China University of Petroleum, Beijing (No. C201603) and Thousand Talents Program.
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