Crystal-facet engineering of the earth-abundant oxide α-Mn2O3 was shown in 2020 to enhance bifunctional oxygen-electrode catalysis: cubic particles predominantly exposing the (100) facet beat octahedral, (111)-dominant particles for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in alkaline electrolyte, plausibly because the {100} surface forms oxygen vacancies more readily. The same study revealed a severe unresolved weakness: under OER both morphologies lose roughly 40–60% of their current within hours, with post-test formation of Mn3O4 and MnO2, a rising average Mn oxidation state, and inferred Mn dissolution. In the literature since that study, operando dissolution and reconstruction behavior of manganese oxides is now broadly documented, facet-dependent restructuring has been demonstrated on other oxides, and constant-potential DFT methods exist — but no published work establishes facet-resolved degradation pathways of α-Mn2O3 under electrochemical OER bias, facet-specific vacancy formation energetics as a function of electrode potential, or a stabilization route that preserves facet-dependent activity. The problem of closing this activity–stability gap therefore remains open.

Background

Rechargeable metal–air batteries and alkaline water electrolyzers need an inexpensive electrode catalyst that accelerates both the oxygen reduction reaction (ORR, O2+2H2O+4e−→4OH−\mathrm{O_2+2H_2O+4e^-\rightarrow4OH^-}) and its reverse, the oxygen evolution reaction (OER). α-Mn2O3 — the cubic bixbyite polymorph of manganese(III) oxide — is a candidate bifunctional catalyst because manganese is earth-abundant and its 3d shell supports several accessible oxidation states (Mn2+^{2+}/Mn3+^{3+}/Mn4+^{4+}). One established lever on transition-metal-oxide activity is crystal-facet engineering: particles of the same phase but different shape expose different crystallographic planes, and surface atoms on different planes sit in different coordination environments, hence differ in adsorption energies and redox behavior. Facet-dependent ORR/OER activity was demonstrated for MnO by Kuo et al. (2015), and for α-Mn2O3 facet-dependent reactivity was established in gas-phase soot combustion by Li et al. (2017), whose density-functional theory (DFT) calculations on dry, vacuum-referenced slab models (periodic supercell simulations of a crystal surface) computed an oxygen-vacancy formation energy (the energy change on removing a surface oxygen atom, referenced to gas-phase O2\mathrm{O_2}) of −1.03 eV on the (001) termination versus +1.68 eV on (111); {100} and {001} denote the same cube-face family of bixbyite. An oxygen vacancy — a missing lattice oxygen — is relevant because it creates low-coordinated, electron-rich surface sites that can speed charge transfer and tune the adsorption energies of oxygen-containing intermediates.

Wang et al. (2020) extended the facet question to oxygen electrocatalysis. They synthesized phase-pure cubic particles predominantly exposing (100) (α-Mn2O3–C) and octahedral particles predominantly exposing (111) (α-Mn2O3–O), assigning facets by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) lattice-fringe spacings. X-ray photoelectron spectroscopy (XPS) showed higher fitted Mn2+^{2+}/Mn3+^{3+} (0.61 vs 0.39) and adsorbed-to-lattice-oxygen ratios (0.67 vs 0.58) on the cubic sample, read as greater surface oxygen-vacancy character. In O2-saturated 0.1 M KOH with a rotating-disk electrode (an electrode spun at a controlled rate so that mass transport to the surface is known), the cubic sample gave better ORR metrics (half-wave potential E1/2E_{1/2}, the potential at half the diffusion-limited current, 0.74 vs 0.72 V; kinetic current density, the current with mass-transport limitation removed, 18.9 vs 9.5 mA cm−2^{-2} at 0.7 V) and OER metrics (potential at 10 mA cm−2^{-2}, Ej=10E_{j=10} = 1.78 V; oxygen-electrode gap ΔE=Ej=10−Ej=−3\Delta E=E_{j=10}-E_{j=-3} = 1.03 V, better than the octahedral sample and a Pt/C benchmark), consistent with a vacancy-mediated facet mechanism built on the dry-DFT prior art.

The same measurements, however, exposed the unresolved side of the facet effect. In chronoamperometric holds (current monitored at fixed potential), ORR retention after 4 h was 88%/85%, but under OER both samples degraded severely — roughly 40% (cubic) and 60% (octahedral) current loss within about 3 h — and post-test XRD/XPS revealed new Mn3O4 and MnO2 phases and an increased average Mn oxidation state (more Mn4+^{4+}), attributed to manganese dissolution and irreversible surface chemical evolution. This sits inside a broader, actively evolving picture. On-line inductively coupled plasma mass spectrometry (ICP-MS, which continuously measures dissolved metal reaching the electrolyte) and operando spectroscopy (measurements performed on the catalyst while it is working under bias) show that manganese oxides dissolve continuously under OER potentials in alkaline media and develop disordered surface layers of steady-state thickness (Speck et al. 2019; Rabe et al. 2019); operando Raman spectroscopy shows MnOx converting to birnessite-type MnO2 motifs (birnessite is a layered, alkali- and water-containing Mn(IV) oxide) under anodic bias (Radinger et al. 2021); and in situ liquid-cell transmission electron microscopy of chemically driven OER on Mn2O3 nanoparticles directly visualized a dynamic, oxygen-deficient, Mn3O4-like amorphous surface layer whose thickness oscillates with a period of seconds and self-limits at 3–4 nm (Zhao et al. 2021). Reviews increasingly treat oxide OER catalysts as 'precatalysts' whose true active surface forms by reconstruction (Zhao et al. 2023; Suresh and Bang 2025), and facet-dependent restructuring has been demonstrated or deliberately exploited on nickel (oxy)hydroxides and spinel cobalt oxides (Yao et al. 2024; Sheng et al. 2023; Yeom et al. 2024). None of this work is facet-resolved for α-Mn2O3, so whether the (100)-facet activity advantage can survive — or must be protected against — OER-induced restructuring and dissolution remains open.

Problem Statement

The open problem is to close the activity–stability gap of facet-engineered α-Mn2O3 as a bifunctional ORR/OER catalyst in alkaline electrolyte: to determine whether, how, and at what energetic cost the facet-dependent activity advantage of the (100)-exposed surface can be maintained under prolonged oxygen-evolution conditions. Concretely, on shape-controlled α-Mn2O3 samples predominantly exposing {100} versus {111} facets, the question centers on facet-resolved durability and comprises the following linked aspects.

(1) Facet-resolved degradation pathways under electrochemical OER bias: the time-resolved onset and rate of manganese dissolution, the sequence and facet dependence of the observed Mn3O4/MnO2 phase evolution and Mn oxidation-state increase, and their causal connection to the measured current loss — which the 2020 study could only infer from post-test characterization.

(2) Facet-specific oxygen-vacancy formation energetics under electrochemical bias: how the dry, vacuum-referenced DFT contrast between the (001)/(100) and (111) terminations of bixbyite α-Mn2O3 behaves under solvated conditions representative of the OER electrode — by any appropriate computational or experimental approach — including how the vacancy formation energies and the relative stability of the terminations and their vacancy content shift with electrode potential.

(3) Stabilization of the facet-dependent activity: whether a surface, compositional, or structural modification can retain the {100}-facet activity advantage under prolonged OER operation, over durations comparable to or beyond the source's ~3 h chronoamperometric holds and at OER current densities such as the source's 10 mA cm−2^{-2} benchmark, with facet and phase identity characterized after testing.

These aspects address one scientific question — whether facet engineering of α-Mn2O3 has a durable benefit under OER, and why. The mechanistic picture (1) and the energetic understanding (2) explain and predict the degradation that any stabilization direction (3) must contend with; aspects (2) and (3) are directions in which the durability question may be explained and acted upon, not separately mandated deliverables.

References

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