Development of novel bifunctional electrocatalysts with high electrocatalytic activity and durability for both oxygen reduction and evolution reactions (ORR and OER) is of outmost importance to grasp the full potential of the regenerative H₂-O₂ fuel cells and metal-air batteries (e.g., Zn-air, Al-air, Mg-air, Li-air) as beneficial energy conversion and storage devices. Manganese oxides have been on the spotlight as an attractive catalyst material due to cost-competitiveness, environmentally friendliness, natural abundance as well as excellent reactivity for ORR and to some extent for OER (1, 2). The physicochemical and electrochemical properties of MnO_x are highly dependent on its morphology and crystallographic nature (3). The aim of this work is to provide a systematic study on finding an active nanostructured manganese oxide for both ORR and OER via anodic electrodeposition method. A comprehensive study have been performed to investigate the main and interaction effects of key electrodeposition factors that significantly influence the electrosynthesis of manganese oxides, i.e. Mn²⁺ concentration (C), applied anodic potential (E), temperature (T), surfactant type and concentration (S), on the bifunctional activity of MnO_x using a two-level half-fraction factorial design. Sodium dodecyl sulfate (SDS) as anioinc, hexadecyl-trimethyl-ammonium bromide (CTAB) as cataionic and Triton X-100 as non-ionic surfactants were used in this study to electro-synthesize the nanostructured MnO_x. Surface characterization methods including XPS and SEM has been employed to analyze morphology and Mn valance of the synthesized electrocatalysts. Fig. 1 shows the surface plots of three different responses studied here for the electrodeposited manganese oxides in presence of Triton X-100, correlating them to the most important factors and two-factor interactions based on the Pareto plots of estimates. The highest ORR mass activity can be achieved at high surfactant concentration and low temperature (Fig. 1-A). Moreover, low applied anodic potential was found to further improve the ORR mass activity of the electrodeposited samples. The same trend was observed for highest OER mass activity as it appeared at low applied anodic potential, high surfactant concentration and low temperature (Fig. 1-B). The lowest ORR/OER potential window of below 600 mV can be obtained at high surfactant concentration, low applied anodic potential but high temperature (Fig. 1-C). The temperature seems to be a defining factor for the bifunctional characteristics of electrodeposited manganese oxides with high temperatures providing low ORR/OER potential window while low temperatures lead to high ORR/OER electrocatalytic activities.

References:

1.             P. H. Benhangi, A. Alfantazi and E. Gyenge, Electrochimica Acta, 123, 42 (2014).

2.             P. Hosseini-Benhangi, M. A. Garcia-Contreras, A. Alfantazi and E. L. Gyenge, Journal of The Electrochemical Society, 162, F1356 (2015).

3.             Y. Chabre and J. Pannetier, Prog. Solid State Ch., 23, 1 (1995).