Me (Fe, Co, Ni)-Porous N Doped Graphene Catalysts Derived from One Step Synthesis Method for ORR in Alkaline Electrolyte
Authors: Qiao Jinli 
Published: February 2020

Graphene based catalysts have been regarded as one of the most promising candidates of replacing commercial Pt/C for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells for its low cost and rich resource. It has been reported that doping some heteroatoms, such as N, S, P and B, into graphene structure is an effective way to tailor the electron-donor properties of non-precious metal catalysts thus effectively weakening the O−O bonding1 and leading to a high ORR catalytic activity. Up to now, extensive research efforts have been made to explore the N doped graphene oxygen reduction catalysts2 since nitrogen has electronegativity higher than other elements3. Furthermore, it has been demonstrated that introducing transition metal, such as Fe, Co, Ni and Mn, into catalysts is an efficient method to enhance the activity of catalyst for ORR4. It was reported that transition metal play a key role in facilitating to form active sites rather than being a component of catalyst.5 Thus pyrolyzing graphene after addition of transition metal and then removing the added metal by washing with HCl is a feasible way to improve both ORR activity and stability of graphene based catalyst. However, there were few literatures about the influence of different transition metals in facilitating combination of nitrogen atoms and carbon atoms. Therefore, it is necessary to investigate the different abilities of transition metals to form active sites and the mechanism. Besides heteroatoms and transition metals, the specific surface area and porous structure, which determine the accessible part of active sites and the transport properties of ORR-relevant species (H+, e−, O2, H2O), are believed to play the important role in the performance of catalysts.6,7

Based on the above conceptions, in this work, we report a novel kind of Me (Fe, Co, Ni)-porous N doped graphene catalyst from graphene oxide (GO), N-containing alkylate (3-aminopropyltriethoxysilane) and transition metals by one step synthesis method and acid leaching. 3-aminopropyltriethoxysilane (AMPTS) was employed as sources of nitrogen and “template”, Fe2+, Co2+ and Ni2+ as precursor of metals. 

Me (Fe, Co, Ni)-porous N doped graphene catalysts were prepared by homogeneously dispersion of AMPTS, metal precursors and GO. After dried overnight, the powder was pyrolyzed at 900oC for 1h in nitrogen atmosphere. The excess amount of hydrofluoric acid was added to remove the silica released from AMPTS and metal. After that the obtained catalyst was reheat at 900oC for 1h and then the electrocatalytic activity and kinetics of the catalysts were measured using cyclic voltammograms (CV) and linear sweep voltammetry (LSV), respectively. All the tests were carried out in a glass cell consisting of a three-electrode system both in 0.1M KOH at the room temperature. Until now we have prepared Fe-porous N doped graphene (Fe-PNG) and Co-porous N doped graphene (Co-PNG), Ni-porous N doped graphen (Ni-PNG) will be synthesized in our next work.

Figure1 shows the cyclic voltammograms of Fe-PNG and Co-PNG catalysts, respectively. It can be seen that Fe-PNG catalysts showed much better ORR activities than that of Co-PNG. Although the ORR peak potentials of Fe-PNG (-0.07V) and Co-PNG(-0.01V) electrodes were similar, the peak current density on Fe-PNG electrode is 200% higher than that on Co-PNG electrode. Since almost all of the transition metals can be removed, the difference in ORR performances on these catalysts was probably due to fact that the transition metal played in promoting formation of active sites, instead of forming active sites themselves. 

Figure 2 shows the polarization curves of Fe-PNG and Co-PNG electrodes. Clearly, Fe-PNG was found to show the higher ORR activity among the catalysts studied.  The onset potential at 0.261V and the half-wave potential at 0.071Vwere obtained for Fe-PNG catalyst.

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Figure 1