Mn₃O₄@C Nanoparticles Supported on Porous Carbon as Bifunctional Oxygen Electrodes and their Electrocatalytic Mechanism

Abstract

Mn3O4 nanoparticles encapsulated inside carbon nanospheres supported on a carbon plate (Mn3O4@CS/CP) were designed, their bifunctional electrocatalytic performance for both, the oxygen evolution reaction (OER) at the anode and the oxygen reduction reaction (ORR) at the cathode were studied in alkaline solution. The prepared Mn3O4@CS/CP catalyst exhibits excellent electrocatalytic performance towards ORR, with less negative onset potential and long durability. Combining the results of in‐situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations, it is revealed that Mn3O4 acts as active site for ORR to promote the transformation of OH* into OH−. In addition, Mn3O4@CS/CP possesses an excellent electrochemical OER performances with the lower overpotential along with prosper Tafel slope and robust durability. DFT calculations show the transformation from O* to OOH* is the rate determining step for OER, and Mn3O4@CS/CP shows the minimum barrier, indicating that graphene sheet on the outer of Mn3O4 improves the transformation from O* to OOH*. The oxygen electrode activity parameter (potential difference between OER at current density of 10 mA/cm2 and ORR at current density of −2 mA/cm2) on Mn3O4@CS/CP is 0.72 V, which is superior to those of most bifunctional electrocatalysts reported to date, predicting a promising application in metal‐air batteries and fuel cells.

Effect of Experimental Operations on the Limiting Current Density of Oxygen Reduction Reaction Evaluated by Rotating‐Disk Electrode

“…The steady‐state limiting current density I lim , which indicates the maximum diffusion current in the ORR, is easily influenced by experimental conditions. The theoretical limiting current density I lim can be calculated by the following equation: , where ω is the rotating speed (rad ⋅ s −1 ), υ is the kinematic viscosity of the electrolyte (m 2 ⋅ s −1 ), C is the bulk concentration of O 2 (mol L −1 ), D is the diffusion coefficient (m 2 ⋅ s −1 ) of the electroactive species, F is the Faraday constant (96485.34 C mol −1 ), and n is the total number of electrons transferred during the electrochemical reaction . The limiting current density I lim is independent on the kinetics of the reaction.…”

“…The universal RDE measurements of ORR are derived from Pt/C catalysts, [1][2][3] and have been extended to non-noble metal catalysts. [4][5][6][7] Typically, a certain amount of catalyst adheres to the glassy carbon electrode, and then, the ORR activity is investigated by LSV with different rotating speeds in oxygen-saturated electrolyte solution. The LSV curve with 1600 rpm is usually chosen to compare the ORR activity of different catalysts.…”

“…where ω is the rotating speed (rad · s À 1 ), υ is the kinematic viscosity of the electrolyte (m 2 · s À 1 ), C is the bulk concentration of O 2 (mol L À 1 ), D is the diffusion coefficient (m 2 · s À 1 ) of the electroactive species, F is the Faraday constant (96485.34 C mol À 1 ), and n is the total number of electrons transferred during the electrochemical reaction. [6,[14][15][16][17] The limiting current density I lim is independent on the kinetics of the reaction. For a 4e ORR, I lim should be a fixed value in a particular concentration solution at 1600 rpm on a 5 mm working electrode, such as 5.277 mA cm À 2 for 0.5 M H 2 SO 4 , 6.332 mA cm À 2 for 0.1 M HClO 4 , and 5.968 mA cm À 2 for 0.1 M KOH (The parameters were listed in support information).…”

Manganese Oxide Derived from a Spent Zn–C Battery as a Catalyst for the Oxygen Evolution Reaction

“…This overpotential value is less than that reported previously at α‐, β‐, and δ‐MnO 2 materials in the same electrolyte where η was found to be 490, 600 and 740 mV, respectively . It is also less than that reported for Mn 3 O 4 nanoparticles encapsulated inside carbon nanospheres supported on a carbon plate (Mn 3 O 4 @CS/CP) which exhibited an overpotential of 390 mV . Upon further increase of the potential reasonable current densities of 45 mA cm −2 can be achieved.…”

“…The current density passed at the MnO 2 at higher potentials was also lower than Mn 3 O 4 . The Tafel slope for Mn 3 O 4 and MnO 2 was determined to be 64 and 71 mV dec −1 respectively (Figure b) indicating better electron transfer kinetics which is lower than that reported for α‐MnO 2 of 77.5 mV dec −1 , Mn 3 O 4 @CS/CP electrodes of 98 mV dec −1 and comparable to many other transition metal (Fe, Co, Ni) oxide based electrocatalysts operated under alkaline conditions . This was confirmed by electrochemical impedance spectroscopy (EIS) where the resistance to charge transfer for Mn 3 O 4 was less than MnO 2 (Figure c).…”