A Unique 3D Nitrogen-Doped Carbon Composite as High-Performance Oxygen Reduction Catalyst

The synthesis and properties of an oxygen reduction catalyst based on a unique 3-dimensional (3D) nitrogen doped (N-doped) carbon composite are described. The composite material is synthesised via a two-step hydrothermal and pyrolysis method using bio-source low-cost materials of galactose and melamine. Firstly, the use of iron salts and galactose to hydrothermally produceiron oxide (Fe2O3) magnetic nanoparticle clusters embedded carbon spheres. Secondly, magnetic nanoparticles diffused out of the carbon sphere when pyrolysed in the presence of melamine as nitrogen precursor. Interestingly, many of these nanoparticles, as catalyst-grown carbon nanotubes (CNTs), resulted in the formation of N-doped CNTs and N-doped carbon spheres under the decomposition of carbon and a nitrogen environment. The composite material consists of integrated N-doped carbon microspheres and CNTs show high ORR activity through a predominantly four-electron pathway.

: EDX analysis on FeNP diffused out of sphere on GAL-Fe-N.

Raman and XRD characterisation
Raman spectroscopy performed on GAL-Fe-A and GAL-Fe-N ( Figure S3A) showed characteristic peaks at 1338 cm -1 (D band) and 1585 cm -1 (G band) corresponds to the amorphous disordered carbon associated with the defects in the edges and vibrational bands of the highly ordered graphitic carbon domains, respectively [1,2]. The broad peak at 2684 cm -1 corresponding to the 2D peak indicates a few layers of the graphitic carbon structure [3][4]. The ID/IG ratio calculated to find the disorder associated with these samples showed 1.05 and 1.27 for GAL-Fe-A and GAL-Fe-N, respectively. Most notably the intensity ratio of the N-doped samples in the presence of iron (GAL-Fe-N), was higher than GAL-Fe-A. This can be ascribed to the incorporation of N and Fe in the graphene lattice which are distorting the hexagonally bonded carbon atoms in the graphitic framework and creating defects [5][6]. To determine the crystal structure of the prepared composites, XRD was performed on GAL-Fe-A and GAL-Fe-N and is shown in Figure S3B. The graph showed three characteristic peaks at 25.85, 42.21, 44.36 °C corresponding to (002), (110), and (101) facets, respectively for the amorphous graphitic carbon [7][8].

Surface area analysis of GAL-Fe-A and GAL-Fe-N
The nitrogen sorption isotherm and pore size distribution of GAL-Fe-A and GAL-Fe-N is shown in Fig. S4. The characteristic type IV isotherm and H4 hysteresis loop shows the presence of mesoporous slip-like pores [9]. The Barret-Joyner-Halenda (BJH) analysis measured a surface area of 311.03 and 101.76 m 2 /g for GAL-Fe-A and GAL-Fe-N, respectively. Pore size distribution curves for GAL-Fe-A ( Figure S4B) were calculated from the adsorption branch of the isotherm shows three different pores with mean pore sizes of 5.36 Å, 8.04 Å and 12.68 Å. The results show that the pores associated on the surface of the carbon microspheres contributed to the high surface area in GAL-Fe-A. However, the N-doping process disrupted the surface and interfered with the pores responsible for the higher adsorption and exposed the larger pores, therefore reducing the adsorption in GAL-Fe-N.

FTIR analysis of GAL-Fe-HT and GAL-Fe-A.
FTIR analysis was conducted to analyse the functional groups present in the hydrothermally treated GAL-Fe-HT and N-doped GAL-Fe-N as presented in Fig. S5. The results show that GAL-Fe-HT had characteristic peaks at 1213, 1590, 1706 and 3390 cm -1 [10][11][12] which can be attributed to C-O and C-H stretching, stretching vibration of C=O, carbonyl vibrations and stretching vibration of O-H, respectively. The reduction of the intensities of these peaks in GAL-Fe-N ( Figure S5b) can be attributed to the reduction of oxygen functional groups due to dehydration during the annealing process.

Kinetics of electron transfer reaction of ORR.
Equation 1 and 2 were employed to further elaborate the dependance of number of electrons (n)and percentage of H2O2 with the applied potential.

=
(1) where ID and IR are the disc and ring currents, respectively and N is the collection efficiency [13].
To further study the kinetics of the ORR using RRDE voltammetry, Scheme 2 developed by Damjanovic et al. and Hsueh et al. was used for this study [14][15]. Damjanovic et al. [14] suggested that the mechanism of ORR should follow one of the two different pathways or a combination of both of them [14]. In other words, the reduction mechanism could follow a direct four electron pathway via k1 kinetics and directly reduce oxygen to water or it could be driven through a two-electron parhway via k2 kinetics producing peroxide intermediates and then be further reduced to water through another two-electron pathway through k3 kinetics. A series of Equations (3, 4 and 5) suggested by Hsueh et al. [15] was used to calculate the rate constants k1, k2 and k3. where Id, Ir, IdL and ω are the disc current, ring current, limiting disc current and the rotation speed, respectively.
, and ν are the diffusion coefficient of oxygen, diffusion co efficient of H2O2 and kinematic viscosity, respectively. N is the collection efficiency.

Mechanism of carbon sphere formation.
The water-soluble galactose can be attributed to the hydrophilicity of the resin solution due to the presence of oxygen containing functional groups formed from the sugars present in apricot sap. FeMNPs has the affinity to electrostatically bind to the negatively charged oxygen functional groups carbonaceous colloids [17]. During the polymerisation and carbonisation process of galactose, metal oxide nanoparticles can electrostatically attach to the oxygen functional groups and scattered as metal nanoparticles or metal clusters within the spheres. By collectively analysing the FTIR and Raman results of the annealed products, we can conclude the presence of hydrophilic oxygen groups and hydrophobic graphitic carbons present in the same product. It can be assumed that the mechanism follows a similar pattern suggested by Sun et.al [18] using LaMer model [19] where the spheres were constructed with a hydrophilic shell and a hydrophobic core as shown in Scheme1.