Abstract
Fuel cell (FC) is considered to be one of the most promising energy conversion devices that could possibly be a good alternative replacing the fossil-fuel based technologies in the future. One of the greatest shortcomings of the low-temperature FCs is the poor kinetics of the oxygen reduction reaction (ORR) happening at the cathode side. As it is widely known, the most extensively used cathode catalyst for FCs is Pt or its alloys. But many research groups all over the world study nitrogen-doped metal-free catalyst materials that could possibly be used in FCs as well [1].
In recent years, the conversion of biomass into more valuable carbon nanomaterials has been a topic of great interest [2]. The conversion of biomass to carbon nanostructures paves the way for a new avenue of nanomaterial synthesis and could potentially solve scale-up issues with traditional methods of graphene production. Biomass is abundant, the global production of agricultural lignocellulosic wastes is around ~200 billion per year [3].
In this research we demonstrate a two-step synthesis method of producing nitrogen-doped graphene-like material from biomass that shows better electrochemical activity towards the ORR than commercially produced nitrogen-doped graphene oxide. A novel and sustainable method was established to prepare nitrogen doped graphene from the renewable, secondary raw material with potential application in the catalysis of ORR. Current research is also proposing a good alternative solution to the increasing need of synthetic graphite, graphene nanoplatelets and graphene by the various industries, including fuel cells and batteries.
In this work, alder wood char was used as a precursor material for producing a graphene-like carbon catalyst for the ORR. In the first step, activated carbon (AC) was prepared using chemical activation with NaOH, the ratio of activator vs wood char was 3:1 and the process was carried out in the flowing argon atmosphere. The activation process is important to achieve higher specific surface area. The second step was the doping of AC with nitrogen using dicyandiamide (DCDA) as a nitrogen source, the mass ratio of carbon material/DCDA was 1:20. The graphene-like structure was determined using transmission electron microscopy and Raman spectroscopy. The successful doping of nitrogen was confirmed with using X-ray photoelectron spectroscopy. The most abundant nitrogen component was pyridinic-N (42%), followed by amine (37%) and pyrrolic-N (21%). Electrochemical characterisation was carried out in 0.1 M KOH using rotating disc electrode (RDE) method. Glassy carbon (GC) electrodes were modified with the catalyst ink (4 mg ml−1). The initial loading of the catalyst material on the GC electrode was 400 μg cm−2. Electrochemical measurements showed that the ORR activity of the N-doped wood-based graphene-like catalyst (N-AWC) is superior compared to commercial N-doped graphene (N-GO) in the alkaline media (Figure 1a). Onset potential of the ORR was shifted to positive direction by 75 mV in comparison to the commercially available nitrogen-doped graphene and half-wave potential was shifted even by the magnitude of 1.5. For comparison the graphene oxide (GO) polarisation curve can also be seen (Figure 1). In the studied range, the number of electrons transferred per O2 molecule was 4, indicating that the process proceeds via 4-electron pathway.
Usually the conversion of biomass may include many steps, including an additional pre-treatment step, but our two-step approach has a potential to also be integrated in the larger scale synthesis processes.
[1] Q. Wei, X. Tong, G. Zhang, J. Qiao, Q. Gong, S. Sun, Catalysts 2015, 5, 1574.
[2] M. Borghei, J. Lehtonen, L. Liu, O. J. Rojas, Adv. Mater. 2017, 1703691.
[3] J. K. Saini, R. Saini, L. Tewari, 3 Biotech 2015, 5, 337.
Figure 1