A Mechanistic Study of NiCo2O4 Spinel As Cathode Material in Alkaline Media
Authors: Velraj Samgopiraj , Zhu Jiahong , Salazar-Gastelum Moises Israel , Sandoval-Corona Erick , Beltrán-Gastélum Mara , Pérez-Sicairos Sergio , Félix-Navarro Rosa María 
Published: February 2020

Carbon materials are predominantly used as the support material in air electrodes due to its notable merits as support, including low cost, wide availability, wettability, large surface areas, high electrical conductivity and good stability in harsh environments. However, the carbon support in the bifunctional electrode is severely attacked by the highly reactive oxygen generated from the oxygen evolution reaction (OER) during charging [1]. Recognizing the corrosion/oxidation issues with carbon-based materials, both metallic and oxide supports have been proposed and evaluated as alternate support materials. Co- and Mn- containing spinels such as Co3O4, Li-doped Co3O4, NiCo2O4, and (Co,Mn)3O4 have been studied and some of them are also excellent candidates as electrocatalyst for bifunctional air electrodes because of their high catalytic activity and good corrosion stability in alkaline solutions [2,3]. Among these, NiCo2O4is the ideal candidate as a potential stand-alone cathode material due to its high electrical conductivity, electrochemical activity, and stability for both OER and ORR.
Even though NiCo2O4 has been shown as a good bifunctional catalyst material [4-6], a detailed study on their reaction kinetics and mechanism is lacking. Also, to the best of our knowledge, no work has been done to study the effect of carbon support on the activity of catalyst, number of electrons transfer, and generation of hydrogen peroxide (H2O2) during ORR which plays a detrimental role towards catalyst degradation and efficiency loss.
The objectives of this study are as follows: (i) synthesis and evaluation of electrochemical performance and stability of NiCo2O4 catalyst for ORR and OER using RDE (ii) evaluation of the effect of Vulcan XC 72R carbon on the catalytic activity of the catalyst, and, (iii) identification of the number of electrons transfer and amount of H2O2generation for different carbon-catalyst ratios during ORR.
Fig. 1. shows the SEM and XRD of the single phase NiCo2O4 catalyst synthesized using the glycine nitrate method. Fig. 2 shows the RDE/RRDE results of different carbon-catalyst weight ratios during ORR. The Koutecky-Levich analysis confirmed that all catalyst-carbon composition were closer to 4-electron transfer process for ORR. In addition, the H2O2 generation were significantly higher (~19%) when only Vulcan carbon (no catalyst). However, with the addition of even 20 wt.% NiCo2O4 catalyst, the amount of H2O2 generation dropped dramatically for a wide range of potential.

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Figure 1