The Effect of Dispersion of Metal Oxides on Carbon on the Electrocatalytic Activity for Oxygen Reduction Reaction in Alkaline Media

The electrocatalysis of the oxygen reduction reaction in alkaline media is of great interest for the development of metal-air batteries and alkaline fuel cells. Recent studies have demonstrated that oxygen reduction reaction on perovskite, and other metal oxides based carbon composite catalysts is a result of a synergetic interaction of carbon and metal oxides [1-3]. The primary catalysts for oxygen reduction is carbon, and complex transition metal oxides act as peroxide decomposer resulting in a series pathway shown below.

O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻

HO₂⁻ → OH⁻ + 1/2O₂

To carry out the second step in the above scheme efficiently, hydroperoxide ions have to be transported rapidly to the metal oxide surface. Therefore, degree of dispersion of the metal oxides on carbon is expected to influence the electrocatalytic activity of the overall composite catalysts significantly.

Although transition metals such as nickel and cobalt are much cheaper than noble metal catalysts, since oxide-carbon catalysts are not as active as platinum, and thus a significant amount of oxide is required in practical applications. Therefore, maximizing the mass activity of nickel and cobalt-based transition metal oxides catalysts is also important from the standpoint of economics. To address this issue of mass activity of the catalysts, we have examined a simple method of preparing these catalysts with the goal of effective utilization of the oxide materials when dispersed on carbon.

Usually the transition metal oxides powder are synthesized separately, by a multi-step process that may take several hours to days of preparation time depending on the method. Afterwards, transition metal oxides are mixed with carbon by hand-grinding in mortar and pestle, followed by ultrasonic dispersion. In our method, we coated the carbon powder (Acetylene black as well as Vulcan XC-72) with catalysts precursors directly. Subsequently the coated mixture was heat treated to obtain the final composite catalysts. With this method, we avoid complicated and long-drawn synthesis of the transition metal oxides powder, and obtain the final composite catalysts in a rapid single-step process.

The various composite catalysts synthesized by this method have been tested for their oxygen reduction reaction activity by polarization studies on rotating disk electrodes coated with the different catalysts. The polarization study was conducted with slow linear sweep voltammetry ( 2 mV/s)  between 0.1 V and -0.5 V vs. the mercury mercuric oxide reference in 1 M potassium hydroxide.  The working electrode was rotated at 400, 900, 1600  and 2500 rotations per minute. The data was analyzed by the standard Koutecky-Levich method to obtain the kinetic parameters.

We have shown that the composite catalysts prepared by new method have a significantly higher mass specific oxygen reduction activity compared to the conventionally prepared materials. These findings emphasize the paramount importance of dispersion of transition metal oxide on carbon and suggest a method to lower the use of expensive transition metal on electrode using a simple process for catalyst synthesis. The benefit of the dispersion most likely results from the improved access to the oxide for the decomposition of the peroxide produced on the carbon. We have also examined the effect of different carbon materials and different catalyst loadings in addition to various oxide compositions.

Fig. 1:   Kinetic current  for  oxygen reduction reaction at -100 mV (vs. Hg/HgO 20% KOH) in 1M potassium hydroxide saturated with oxygen, 1) 16 mg Acetylene Black coated with 20 % of NiCo₂O₄ 2) 16 mg Vulcan XC-72 coated with 5 % of NiCo₂O₄ 3) 16 mg Vulcan XC-72 coated with 10 %  of NiCo₂O₄ 4)  16 mg Acetylene Black mixed with 80 mg NiCo₂O₄ .

Acknowledgement 

 

The work presented here was funded by ARPA-E Grids Program, the University of Southern California and the Loker Hydrocarbon Research Institute.

References

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