Carbon Nitrides: New Electroactive Materials for Energy Conversion and Storage Applications

Abstract

Polymeric / graphitic carbon nitride materials (gCNMs) were first reported by Berzelius and termed ‘melon’ by Liebig in 1834.[i] Their amorphous / nanocrystalline nature has complicated the structural characterisation but, thanks to the application of new advanced techniques, the structures are now becoming understood. gCNMs consist of s-triazine or heptazine (tri-s-triazine) units linked by bridging –NH- or –N= groups to form polymeric 2D structures with different degree of condensation (Fig. 1). These structures differ from graphitic carbon or graphene in that the presence of N atoms and C-N bonds creates voids within the extended layered structure. These voids are lined with N lone pairs and / or N-H functional groups, being ideally suited for acid / base and host / guest chemistry. This includes intercalation reactions, charge storage, ionic diffusion, gas sorption and support of catalytically or electrochemically active species. Due to their extraordinary compositional and structural flexibility, a high degree of control and tuneability can be achieved. We have demonstrated that the band gap of gCNMs can be controlled by modifying the preparation experimental conditions via the extent of condensation, layer buckling and potentially charge transfer processes with guest ions.[ii]Graphitic carbon nitrides are easily prepared by condensation of C, N-containing precursors at temperatures between 550 and 650 oC in N2 (g) flow. Alternative approaches to improve crystallinity, conductivity or porosity include the use of molten eutectic mixtures, anion doping or soft / hard templates, respectively. We have recently revealed the ability of gCNMs to act as electrodes for Li intercalation.[iii] Cyclic voltammetry results showed that the gCNMs can incorporate Li+ with a peak near 1.0 V for the delithiation process (Fig. 2a). We have also demonstrated that gCNMs exhibit excellent performance as catalyst support in direct methanol fuel cells.[iv],[v] We studied gCNMs synthesised following different routes: (a) gCNM: prepared by condensation at 550 oC in N2 (g), (b) PTI-Li+Cl-: Poly(triazine imide) prepared by molten eutectic LiCl/KCl route,[vi],[vii] and (c) B-gCNM: prepared by using ionic liquid BmimBF4.[viii] They also showed improved performance when compared to commercial Vulcan (Figs. 2b and 2c). Here, we describe the differences between carbon nitrides and N-doped carbons / graphene and the promising potential of these materials for energy storage and conversion applications.[i] J. Liebig, Ann. Pharm. 1834, 10, 1.[ii] A.B. Jorge, D. J. Martin, M. T. S. Dhanoa, A. S. Rahman, N. Makwana, J. Tang, A. Sella, F. Corà, S. Firth, J. A. Darr, P. F. McMillan, J. Phys. Chem. C 2013, 117, 7178.[iii] A.B. Jorge, F. Corà, D. J. L. Brett, A. Sella, P. F. McMillan, Int. J. Nanotechnol., awaiting publication.[iv] N. Mansor, A. B. Jorge, F. Corà, C. Gibbs, R. Jervis, P. F. McMillan, X. Wang, D. J. L. Brett, ECS Trans 2013, 58, 1767.[v] N. Mansor, A. B. Jorge, F. Corà, C. Gibbs, R. Jervis, P. F. McMillan, X. Wang, D. J. L. Brett, J. Phys. Chem. C, just accepted, doi: 10.1021/jp412501j. [vi] M. J. Bojdys, J.-O. Müller, M. Antonietti, A. Thomas, Chem. Eur. .J. 2008, 14, 8177-8182.[vii] E. Wirnhier, M. Döblinger, D. Gunzelmann, J. Senker, B. V. Lotsch, W. Schnick, Chem. Eur. .J. 2011, 17, 3213.[viii] Y. Wang, J. Zhang, X. Wang, M. Antonietti, H. Li, Angew. Chem. Int. Ed. 2010, 49, 3356.