Synthesis of Non-PGM ORR Catalysts: Control of Chemical, Morphological and Catalytic Properties

Development of non-platinum catalysts for both the anode [1] and cathode [2] attracts significant attention from researchers around the globe. Catalysts consisting of transition metal, nitrogen and carbon (M-N-C) where M=Fe, Co, Ni etc)framework possess reasonable stability and activity in both acidic and alkaline media. The use of inexpensive organic precursors doped with low molecular weight metal salts is proposed in the present research to provide a high surface area framework consisting of C-N decorated with M species.

All catalysts presented in this work were synthesized using a modified sacrificial silica support method which has been widely adopted in our research group [4]. Appropriate amounts of metal precursor (iron nitrate or cobalt nitrate) and nitrogen-containing, precursors (e.g. polyethyleneimine (PEI)) were deposited on the surface of fumed silica (surface area ~380 m^-2 g^-1) by Mechanochemical approach. The composite material was pyrolyzed in a nitrogen atmosphere at various temperatures, to form the M-N-C-SiO₂framework. The pyrolysis conditions were determined from a MS-TGA analysis, where MS was used to identify the precursor’s decomposition products as a function of thermal treatment.  After heat treatment, the fumed silica was removed by etching in HF acid and washed appropriately, revealing a high surface area M-N-C matrix

The catalysts were comprehensively characterized by BET, SEM, TEM, XRD and XPS. Catalytic activity for ORR was measured in acidic media (0.5M H₂SO₄) using the RRDE method and fuel cell test.

Figure 1.  Mechanochemical synthesis

The broad XRD peak for Fe indicates small metallic iron particles exist on the order of several nanometers (Fig. 1). It can be assumed that the homogeneous distribution of precursors on the high surface of fumed silica prevents metal particles from agglomeration.

Analysis of material morphology by SEM reveals that the Fe-DAAPyr catalyst yields a highly porous surface structure. BET analysis of the adsorption isotherm reveals a surface area of ~700m^-2 g^-1.

Synthesized materials were found to be active in ORR in both RRDE and MEA fuel cell tests conditions.

We would like to acknowledge funding support from DOE-EERE Fuel Cell Technology Program: “Development of Novel Non Pt Group Metal Electrocatalysts for PEM Fuel Cell Applications” (S. Mukerjee, NEU, PI).

References:

1. M. Min, C. Park, H. Kim, C. Kwak, A. A. Serov, H. Kweon, S. Lee, Electrochimica Acta, 52, (2006) 1670-1675.
2. A. Serov, K. Artyushkova, P. Atanassov, Adv. Energy Mater., 4: 1301735 (2014). doi: 10.1002/aenm.201301735..
3. U. Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C, 118 (17) (2014) pp 8999–9008.
4. A. Serov, A. Aziznia, P. H. Benhangi, K. Artyushkova, P. Atanassov, E. Gyenge  J. Mater. Chem. A 1 (2013) 14384-14391.