Structure–Property Relationship of Bifunctional MnO₂ Nanostructures: Highly Efficient, Ultra-Stable Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media

Abstract

Manganese oxides of various structures (α-, β-, and δ-MnO2 and amorphous) were synthesized by facile methods. The electrocatalytic properties of these materials were systematically investigated for catalyzing both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media. Extensive characterization was correlated with the activity study by investigating the crystal structures (XRD, HRTEM), morphologies (SEM), porosities (BET), surfaces (XPS, O2-TPD/MS), and electrochemical properties (Tafel analysis, Koutechy-Levich plots, and constant-current electrolysis). These combined results show that the electrocatalytic activities are strongly dependent on the crystallographic structures, and follow an order of α-MnO2 > AMO > β-MnO2 > δ-MnO2. Both OER studies and ORR studies reveal similar structure-determined activity trends in alkaline media. In the OER studies, α-MnO2 displays an overpotential of 490 mV compared to 380 mV shown by an Ir/C catalyst in reaching 10 mA cm(-2). Meanwhile, α-MnO2 also exhibits stability for 3 h when supplying a constant current density of 5 mA cm(-2). This was further improved by adding Ni(2+) dopants (ca. 8 h). The superior OER activity was attributed to several factors, including abundant di-μ-oxo bridges existing in α-MnO2 as the protonation sites, analogous to the OEC in PS-II of the natural water oxidation system; the mixed valencies (AOS = 3.7); and the lowest charge transfer resistances (91.8 Ω, η = 430 mV) as revealed from in situ electrochemical impedance spectroscopy (EIS). In the ORR studies, when reaching 3 mA cm(-2), α-MnO2 shows 760 mV close to 860 mV for the best ORR catalyst (20% Pt/C). The outstanding ORR activity was due to the strongest O2 adsorption capability of α-MnO2 suggested by temperature-programmed desorption. As a result, this discovery of the structure-related electrocatalytic activities could provide guidance in the further development of easily prepared, scalable, and low-cost catalysts based on metal oxides and their derivatives.

Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis

“…The LiMn 2 O 4 –carbon composite electrode reached 10 A g −1 at 1.705(3) V versus RHE (0.476(3) V overpotential) at pH 13.4 and 14.0, which is similar to Ca 2 Mn 2 O 5 (1.70 V versus RHE; 5th cycle; “sub‐micron”)65 and β‐MnO 2 (1.71 V versus RHE; 5 m 2  g −1 ) 66. Yet, the overpotential is higher than that of Mn oxides with higher surface area, for example, solvent‐free α‐MnO 2 ‐SF (1.64 V versus RHE; 112 m 2  g −1 )66 and oxides with similar surface area that exhibit higher activity per oxide surface, for example, ball‐milled Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3‐ δ (1.53 V versus RHE; 3.9 m 2  g −1 ) 7…”

One‐Dimensional Earth‐Abundant Nanomaterials for Water‐Splitting Electrocatalysts

“…For instance, manganese oxides with different structures, including α‐, β‐, δ‐MnO 2 and amorphous) were synthesized and compared 77. The structures of different MnO 2 depend on the connectivity between the [MnO 6 ] units via sharing corners or edges, with δ‐MnO 2 as layered structures and α‐, β‐MnO 2 as 1D structures.…”

Mn3O4@CoMn2O4–CoxOyNanoparticles: Partial Cation Exchange Synthesis and Electrocatalytic Properties toward the Oxygen Reduction and Evolution Reactions

“…(7)(8)(9)(10)(11)(12)(13)(14) Among transition metal oxides, Mn-based oxides and particularly spinels have shown outstanding performance for ORR and, to a minor extent, for OER. (7,(15)(16)(17)(18)(19)(20)(21) Besides, Co-based oxides are excellent electrocatalysts for OER, (22)(23)(24)(25) However, the multiple valence and related structural variability of transition metal oxides, which is at the origin of their exceptional electrocatalytic performance, is also behind the difficulty to produce these compounds in a reproducible and controlled manner. This is a particularly important limitation when taking into account the strong dependence of the physicochemical properties of transition metal oxides on composition, structural parameters, and distribution and oxidation state of cations.…”