Investigation of Various Calcium-Based Transition Metal Oxides Compounds for the Oxygen Evolution Reaction in Alkaline Media

Electrocatalysts for oxygen evolution reaction in alkaline medium are critical for the successful realization of various kinds of aqueous metal-air rechargeable batteries as well as in the electrolysis of water. The seminal work of Trasatti, Buckris, and other researchers has unfolded the importance of metal oxides for the evolution of oxygen in alkaline medium. [1,2] 

 Many of the state-of-the-art electrocatalysts based on metal oxides for the oxygen evolution reaction contain rare earth element as for example in the perovskites. [3]  However, use of materials containing rare earth elements will be constraint for large scale deployment of systems due to its relatively high price and limited availability.  Apart from rare earth element, some of the transition metals like cobalt also add significantly to the cost. Therefore, it would be very useful to maximize the activity of cobalt based complex metal oxide catalysts without the use of rare earth metals.  In this work, we have explored calcium-based oxides of various complex transition metals such as cobalt and manganese for oxygen evolution reaction, with the goal of avoiding rare earth metals and minimizing the use of transition metal in the electrocatalyst.

The sol-gel method using the citrate route was used to synthesize all calcium-based transition metal oxides catalysts.[3] The  compounds of calcium and cobalt as well as calcium and manganese with the following formulae have been synthesized: CaCo₂O₄ ; Ca₂Co₂O₅ ; Ca₃Co₄O₉ ; Ca₉Co₁₂O₂₈ ; CaMn₂O₄ ; Ca₂Mn₃O₈ ; A simple two-step based sintering method   was used to complete the synthesis of all these compounds.[3]. All catalysts samples were prepared by this method were sintered at temperatures in the range of 750 to 900 degrees Celsius. Powder X-ray diffraction method was used to identify the crystalline phases.

The kinetics of oxygen evolution reaction was studied using thin film catalyst layers of the synthesized powder catalysts on a glassy carbon electrode. The steady state polarization method was used to evaluate oxygen evolution activity   in 1 M potassium hydroxide solution for all the catalysts. Figure 1 shows the results of steady state current from  polarization measurements for oxygen evolution experiments at 615 mV (vs. Hg/HgO 20% KOH) for all catalysts. The activity data has been normalized to the total mass of the catalyst.

Fig. 1: Mass-normalized steady-state current for oxygen evolution reaction on various complex oxides of calcium, cobalt, and manganese at 615 mV (vs. reference Hg/HgO 20 % KOH) in 1M potassium hydroxide solution).  The normalization is for unit mass of the transition metal.

We find that the activity per milligram of cobalt varies considerably among the various types of cobalt-containing catalysts. Further, the manganese based catalysts have about one-fifth the activity of the best cobalt based catalysts.   

The presentation will discuss these findings in more elaborately and offer some insights for the design of new electrocatalysts.  

Acknowledgement 

 

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

References

1. S. Trassati, J. Electroanal. Chem.,  111, 125 (1980).
2. J. O’M Bockris and  T. Otagawa,  J. Electrochem. Soc.,131, 290 (1984).
3. S. Malkhandi, B. Yang, A. K. Manohar, A. Manivannan,  G. K. S. Prakash and S. R. Narayanan,  J. Phys. Chem. Lett., 3, 967 (2012).