Development of non-PGM bifunctional electrocatalysts with high electrocatalytic activity and durability for both oxygen reduction and evolution reactions (ORR and OER) is of outmost importance to realize the full potential of rechargeable metal-air batteries (e.g., Zn-air, Al-air, Mg-air, Li-air) and regenerative H₂-O₂ fuel cells. Manganese oxides have been in the spotlight especially as alkaline ORR electrocatalysts, due to low cost and natural abundance. Regarding the bifunctional ORR and OER electrocatalytic performance of MnO_x, further improvements in activity and durability are required for implementation in commercial energy storage and conversion systems. The aim of this study is to enhance the bifunctional activity and durability by investigating the role of MnO_x morphology, co-catalyst addition, potassium ion doping and support effect (e.g., graphene and graphitized carbon). The experiments were performed in 6 M KOH, 293 K and 1 atm(abs) over a wide potential window encompassing the ORR and OER polarization regions. Results obtained with both flooded dissolved O₂ and gas (O₂ or air)-diffusion cells are presented. The combination of MnO₂ with a structurally different oxide co-catalyst such as perovskite (LaCoO₃) or fluorite-type oxide (Nd₃IrO₇) produces a synergistic catalytic effect improving both the activity and durability compared to the individual oxides. Doping of the oxide catalyst with potassium ions, either by long-term exposure to 6 M KOH or potential driven insertion (PDI), increases further the activity and durability as revealed in accelerated degradation experiments.^1,2 The effect of MnO₂ morphology on the bifunctional performance was investigated by carrying out a statistically designed MnO₂ electrodeposition study. Four main variables were studied through factorial experiments: a) anodic MnO₂ electrodeposition potential, b) surfactant type (non-ionic, cationic and anionic) and concentration, c) Mn(II) precursor salt concentration and d) temperature. Fig. 1 shows the surface plots for three different responses: ORR and OER mass activities, and ORR/OER onset potential window.³

Fig. 1. Effect of MnO₂ electrodeposition conditions on the ORR and OER bifunctional electrocatalytic performance. Surface plots for the 2⁴-1+3 factorial design showing the three responses: A) ORR mass activity, B) OER mass activity and C) OER/ORR onset potential window. Legend: E - anodic MnO₂electrodeposition potential (mV vs. Hg/HgO/0.1 M KOH), S - surfactant (Triton X-100) concentration in the deposition bath (%vol.), T - electrodeposition temperature (K).

As shown by Fig. 1, optimizing the MnO_x electrodeposition conditions can produce nanostructured morphologies that are favorable for bifunctional activity. The activity metrics compare favorably to either commercially obtained MnO₂ sampled or literature reported activities for various catalysts including CoMn₂O₄ and core-corona structured bifunctional catalyst. The electrochemical results are discussed in conjunction with extensive surface analysis (SEM, TEM, XPS, EDX, EELS) and the modern prevailing theory of ORR and OER electrocatalytic activity based on the scaling relationship between the binding energies of HO^* and HOO^*.

References:

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

2. P. Hosseini-Benhangi, M. A. Garcia-Contreras, A. Alfantazi and E. L. Gyenge, JES, 162, F1356 (2015).

3. P. Hosseini-Benhangi, C.H. Kun, A. Alfantazi and E.L. Gyenge, manuscript in preparation (2016).