CoMn-LDH@carbon nanotube composites: Bifunctional electrocatalysts for oxygen reactions

In the present work, a set of five Layered Double Hydroxides, LDH, containing Co and Mn derived electrocatalysts was prepared for the oxygen electrode, evaluating their performances in ORR and OER. A LDH with a Co/Mn ratio of 4, CoMn4, was the reference electrocatalyst of the set, presenting the typical hydrotalcite structure with a trace of MnCo2O4, with a surface area of 76 m(2) g(-1). HNO3 oxidized carbon nanotubes (CNT_HNO3) and CNT_HNO3 with a low content of carboxylic acids (CNT_HNO(3)tt350), were incorporated into the CoMn4 synthesis, obtaining CoMn4@CNT_HNO3 and CoMn4@CNT_HNO(3)tt350 with surface areas of 111 and 167 m(2) g(-1), respectively. According to X-ray diffraction (XRD), the oxide phase of CoMn4@CNT_HNO3 is composed by a mixture of LDH with MnCo2O4, while the low% COOH on the carbon surface promoted the synthesis of LDHs as the only oxide structure at CoMn4@ CNT_HNO(3)tt350. Moreover, after a thermal treatment of the latter electrocatalyst, the LDHs were totally converted into MnCo2O4. All electrocatalysts showed to have activity over both oxygen reactions when the KOH solution was saturated with O-2. A detailed ORR study showed that the LDH structures mixed with CoMn oxides, present at CoMn4@CNT_HNO3, play a relevant role in ORR, exhibiting an onset potential, E-ORR, of -0.274 V. On the other hand, CoMn4@CNT_HNO(3)tt350 led to a four-electron mechanism for ORR, similar to the Pt/C standard. The best OER potential (0.636 V) was also obtained for CoMn4@CNT_HNO(3)tt350. Regarding the oxygen electrode bifunctionality, a good balance between ORR and OER was observed for the CoMn4@CNT_HNO(3)tt350_N-2 composite.