Ion Immobilized Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reaction

Development of nonprecious metal based bifunctional catalyst with superior oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity in alkaline solution is of great importance for aqueous metal-air batteries. Herein, we discuss the ion immobilized transition metal oxide nanoparticles embedded into the sulfur-doped carbon framework M-S-C (M = Mn, Co, and Mn-Co) via carbonization strategy, with high catalytic activity and stability. Due to ion exchange, the position of the metal ion is limited to the ion exchange site limiting the particle size. The synergistic effect between S-doped carbon and transitionmetal nanoparticles in the catalyst led to high bifuctionality for all three catalysts (Mn-S-C, Co-S-C, and Mn-Co-S-C). The overall oxygen electrode activities (Delta E = E-j=10 - E-1/2) for Mn-S-C, Co-S-C, Mn-Co-S-C, S-C, and Pt/C are 0.85, 0.81, 0.83, 1.1, and 1.03 V, respectively, for bifunctionality and are smaller than most of the precious and nonprecious metal catalysts reported in literature. OER (10 mA cm(-2)) and ORR (from onset potential) overpotentials for Mn-S-C, Co-S-C, Mn-Co-S-C, and S-C are (460 mV, 290 mV), (360 mV, 350 mV), (410 mV, 330 mV), and (510 mV, 420 mV), respectively. Among the synthesized catalysts, Mn-S-C exhibited higher ORR activity (Mn-S-C > Mn-Co-S-C > Co-S-C) and Co-S-C exhibited better OER activity (Co-S-C > Mn-Co-S-C > Mn-S-C). Hence, optimized actvity for both ORR and OER is achieved by Co-S-C catalyst. This potential bifunctional catalyst, when employed as an electrode material for rechargeable zinc-air battery, exhibited effective bifunctionality for catalyzing both ORR and OER with significantly higher energy efficiency, reduced voltage polarization, and improved cyclic stability up to 100 cycles at discharge current density of 10 mA cm(-2). The improved catalytic activity arises from favorable factors such as particle size, homogeneous dispersion of metal nanoparticles on carbon framework, and synergistic effect of sulfur and metal oxides.