Phase and composition controllable synthesis of cobalt manganese spinel nanoparticles towards efficient oxygen electrocatalysis

Spinel-type oxides are technologically important in many fields, including electronics, magnetism, catalysis and electrochemical energy storage and conversion. Typically, these materials are prepared by conventional ceramic routes that are energy consuming and offer limited control over shape and size. Moreover, for mixed-metal oxide spinels (for example, CoxMn3−xO4), the crystallographic phase sensitively correlates with the metal ratio, posing great challenges to synthesize active product with simultaneously tuned phase and composition. Here we report a general synthesis of ultrasmall cobalt manganese spinels with tailored structural symmetry and composition through facile solution-based oxidation–precipitation and insertion–crystallization process at modest condition. As an example application, the nanocrystalline spinels catalyse the oxygen reduction/evolution reactions, showing phase and composition co-dependent performance. Furthermore, the mild synthetic strategy allows the formation of homogeneous and strongly coupled spinel/carbon nanocomposites, which exhibit comparable activity but superior durability to Pt/C and serve as efficient catalysts to build rechargeable Zn–air and Li–air batteries.

and tetragonal (c, d) spinels with three compositions upon heating from 25 C to 1260 C and cooling from 1260 C to 100 C in air flow. Heating/cooling rate: 10 C min -1 . The slow mass loss before 250 C is caused by the removal of water and hydroxy groups probably adsorbed on the surface of the oxide particles. The continuous mass loss in TG curves from 250 C to 800 C is ascribed to the gradual removal of oxygen, while the progressive reduction from Mn 4+ and Co 3+ to Mn 3+ and Co 2+ occurs 6,7 . Of particulate note, the mass loss of cubic series oxides within this temperature range is larger than that of the corresponding tetragonal phase with same composition, suggesting more Mn 4+ contained in cubic spinels and thus confirming their higher Mn valence values than those of tetragonal oxides. From 800 C up to the limit temperature (1260 C), the rapid mass loss around 1000 C was assigned to the formation of rock-salt-type manganese cobalt oxides along with O 2 evolution 7 . Upon cooling, a partly reversibly mass gain takes place, which is considerably smaller than the mass loss due to the formation of CoO.  whereas that of tetragonal intermediate is between Mn(III) and Mn(II).

Supplementary Figure 11 | Nitrogen adsorption and desorption isotherms measured at 77 K. The
Brunauer-Emmett-Teller (BET) surface areas of current spinel samples are much larger than those of cubic and tetragonal spinel particles prepared through conventional high-temperature method 9 . In addition, the corresponding pore size distribution (inset) determined by the Barrett-Joyner-Halenda (BJH) shows the pore diameter centered below 20 nm, verifying the porous structure.

Supplementary
where I k , I and I d are the kinetic, measured and diffusion-limiting current densities, respectively.

Supplementary Note 6. Calculation of turnover frequency (TOF).
The TOF value is determined from equations 6 and 7 (Ref. 32):  Zn-air battery assembly. The zinc-air battery was assembled by the zinc plate anode, porous air cathode and 25-30 mL aqueous electrolyte consisting of 6.0 M KOH without the use of separator. The air cathode was prepared by casting homogenous ink of c-CoMn 2 /C (10 mg catalyst and 35 L Nafion solution (5 wt.%, Sigma-Aldrich) in 1 mL mixed solvent of water/isopropyl alcohol (volume ratio: 3:1) on a 0.8 cm 2 carbon paper (Fuel Cell Store) and dried at 100 C for 3 h to obtain a catalyst loading of ~2.0 mg/cm 2 . Battery testing and cycling were performed at room temperature using LAND-CT2001A testing system.
Synthesis of other spinels. Four representative Co and Mn-based spinel oxides have been successfully synthesized using the similar oxidation-precipitation and crystallization route described in this study.