Efficient Fe, Co-Containing, Nitrogen-Doped Carbon Nanomaterials for Oxygen Reduction Reaction

The sluggish kinetics of the oxygen reduction reaction (ORR) is generally considered to be the main obstacle in producing viable polymer electrolyte membrane fuel cells. As a solution, noble metal-containing catalysts are traditionally used. However, the higher activity of Pt-based catalyst materials comes at a cost – they are unstable, limited in supply and expensive, which severely inhibits their usage in commercial applications. In recent years, nitrogen-doping of carbon materials has emerged as a way of achieving highly active materials that are without these limitations.^1,2 There is a large body of evidence that N-doped nanocarbons are active catalysts for ORR in alkaline solution, however, in acid media these materials show only modest activity.³ N-containing carbon materials can be used as cathode catalysts for alkaline membrane fuel cells.⁴ It has been reported that the presence of transition metals such as Co or Fe further enhances the electrocatalytic properties of N-doped carbons.⁵

In this work, a simple synthesis method for attaining transition metal and nitrogen-doped carbon nanomaterials with high activity towards the oxygen reduction reaction is demonstrated. Both multi-walled carbon nanotubes (MWCNT) treated in a concentrated HNO₃-H₂SO₄ mixture and graphene oxide (GO) synthesized via the modified Hummers’ method were used as carbon supports. These carbon nanomaterials were first dispersed in ethanol by sonication, after which dicyandiamide as the nitrogen source and a Fe or Co salt were added along with a dispersing agent. The mixture was then heat-treated at 800 °C for 2 h in a tube furnace. Polished glassy carbon electrodes were modified with a dispersion containing the resulting materials and either Nafion or Tokuyama OH⁻ ionomer AS-04. The rotating disk electrode (RDE) measurements were performed in 0.1 M KOH and 0.5 M H₂SO₄ solutions. The stability and methanol tolerance of the catalysts were also tested using RDE. X-ray photoelectron spectroscopy was used to characterize the surface composition of the materials.

In alkaline conditions, all of the transition metal and N-doped nanocarbons showed excellent activity for oxygen electroreduction, in particular those containing Co. Among the carbon supports, MWCNTs were shown to produce the catalysts of higher activity. The number of electrons transferred per O₂ molecule (n) calculated from the Koutecky–Levich equation was close to 4 for all the Co-containing catalysts studied. In acid solution, iron-containing materials exhibited medium activity while cobalt-containing materials were determined to be highly active. Methanol tolerance tests were carried out by measuring activities in 0.1 M KOH solution containing 3 M MeOH, with all catalysts displaying exceptional immunity toward methanol crossover and the stability of all catalysts was monitored over 1000 potential cycles in alkaline conditions, with a minimal loss in activity.

Fe- and Co-containing nitrogen-doped carbon nanomaterials can be considered as promising candidates for polymer electrolyte fuel cell cathode catalysts as they are stable and highly active toward oxygen reduction. Furthermore, their excellent methanol tolerance renders them immune from crossover effect and they could thus also be used in direct methanol fuel cells. Work is in progress to test these materials in fuel cell conditions.

References:

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2. M. Vikkisk, I. Kruusenberg, U. Joost, E. Shulga, I. Kink, and K. Tammeveski, Appl. Catal. B, 147, 369 (2014).

3. N. Alexeyeva, E. Shulga, V. Kisand, I. Kink, and K. Tammeveski,  J. Electroanal. Chem., 648, 169 (2010).

4. I. Kruusenberg, S. Ratso, M. Vikkisk, P. Kanninen, T. Kallio, A. M. Kannan, and K. Tammeveski, J. Power Sources, 281, 94 (2015).

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