(485g) Non-Noble Metal Catalysts for the Oxygen Reduction Reaction (ORR) in Proton Exchange Membrane (PEM) Fuel Cells

One
of the major roadblocks in the commercialization of Proton Exchange Membrane
(PEM) fuel cells is the cost associated with platinum electrodes. While the
hydrogen oxidation reaction on the anode is relatively fast, requiring low
platinum loadings, the oxygen reduction reaction (ORR) on the cathode exhibits
slow kinetics, necessitating higher platinum loadings (up to 0.5 mg/cm²).

Transition
metal containing nitrogen and carbon catalysts have been found to be a viable
replacement of platinum for ORR. The first generation of these catalysts
consisted of Fe- or Co-containing macrocycles, which were heat treated between
400-1000^oC, to provide more stability. Later, it was found that macrocycles
were not necessary as starting materials: separate sources of carbon, nitrogen
and a transition metal, when heat treated together could yield catalysts with
significantly high activity and stability.

Previous
work in our research group led to the development of non-noble metal ORR
catalysts which consisted of carbon nanostructures containing nitrogen (CN_x)
which were grown on an Fe or Co-impregnated oxide support by pyrolysis at 900^oC in the presence of an organic
nitrogen precursor:
acetonitrile. The catalyst thus obtained was then acid washed to leach out any exposed
surface metal and non-conductive species. More recently, our efforts have been
directed towards resolving the active site debate of these non-noble metal
catalysts. We have shown that CN_x is not sensitive to poisoning by
known metal poisons such as H₂S, CO and cyanide [1, 2], which would
not have been possible if there existed a single active site comprising of a
metal center. ORR activity of CN_x showed an improvement after H₂S
treatment (Fig. 1a), in contrast with Pt/VC catalyst, which showed a marked
decrease in activity after treatment (Fig. 1b).

 Figure 1. Oxygen reduction activity of
a. CN_x b. Pt/VC catalyst without treatment and after H₂S
or H₂ treatment. (Inset: Onset potential for ORR) [1]

X-ray
Absorption Near Edge Structure (XANES) obtained from X-ray absorption
spectroscopy experiments also showed no difference between untreated and H₂S-treated
CN_x (Fig. 2).  These
findings led us to believe that the acid washing step leaches out any exposed
iron in CN_x, and the remaining iron present is encased within the
carbon structure, not accessible by oxygen molecules to contribute to ORR.

Figure 2. XANES Fe K-edge spectra for
untreated, H₂S-, and H₂-treated CN_x [1]

Our
more recent efforts focused on synthesizing ORR catalysts, which consist of
iron impregnated on a high surface area carbon support along with an organic
porefiller. These catalysts are ball milled and subjected to different heat
treatments, such as NH₃, Ar, or N₂. These catalysts have
been thought to have a metal-N₂ or metal-N₄ type active
site and are commonly known as Fe/N/C catalysts. Though they have a high
initial performance, their activity diminishes significantly over time in a
fuel cell environment. The activity losses could be attributed to leaching of
iron under the acidic conditions prevalent on the cathode of the fuel cell.

Our current studies involve synthesizing and tailoring
both metal-free CN_x as well as Fe/N/C catalysts while optimizing
their stability under half cell and fuel cell conditions. Comparisons between
CN_x and Fe/N/C catalysts based on spectroscopic techniques such as
extended X-ray absorption fine structure (EXAFS), transmission electron
microscopy (TEM) and X-ray photon spectroscopy (XPS) are aimed at providing
additional insight for the differences pertaining to their active sites.

[1]       D. von Deak, D. Singh, E.J. Biddinger, J.C. King, B.
Bayram, J.T. Miller, U.S. Ozkan, J. Catal. 285 (2012) 145-151.

[2]       D.
von Deak, D. Singh, J.C. King, U.S. Ozkan, Appl. Catal. B-Environ. 113-114
(2012) 126-133