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1. Charge distributions and oxidation states in Mn-based oxidants from ab initio molecular dynamics simulation

   Colin Crago, Shifa Zhong§ (August 2021, Chemosphere)

   null (Ed.)

   We used ab initio molecular dynamics simulations to address Mn oxidation states in different Mn systems. We first develop a correlation between Mn partial atomic charge and the oxidation state, based on results of 31 simulations on known Mn aqueous complexes. The results collapse to a master curve; the dependence of partial atomic charge on oxidation state weakens with increasing oxidation state, which concurs with a previously proposed feedback effect. This correlation is then used to address oxidation states in Mn systems used as oxidants. Simulations of MnO2 polymorphs immersed in water give average oxidation states (AOS) in excellent agreement with experimental results, in that b-MnO2 has the highest AOS, a-MnO2 has an intermediate AOS, and d-MnO2 has the lowest AOS. Furthermore, the oxidation state varies substantially with the atom’s environment, and these structures include Mn(III) and Mn(V) species that are expected to be active. In regard to the MnO4−/HSO3−/O2 system that has been shown to be a highly effective oxidant, we propose a novel Mn complex that could give rise to the oxidative activity, where Mn(III) is stabilized by sulfite and dissolved O2 ligands. Our simulations also show that the O2 would be activated to O22- in this complex under acidic conditions, and could lead to the formation of OH radicals that serve as oxidants. 

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2. Green growth of mixed valence manganese oxides on quasi-freestanding bilayer epitaxial graphene-silicon carbide substrates

   https://doi.org/10.1016/j.mtadv.2024.100467  

   Pedowitz, Michael; Lewis, Daniel; DeMell, Jennifer; Pennachio, Daniel J.; Hajzus, Jenifer R.; Myers-Ward, Rachael; Kim, Soaram; Daniels, Kevin M. (March 2024, Materials Today Advances)

   Nanostructured manganese oxides (MnOx) have shown incredible promise in constructing next-generation energy storage and catalytic systems. However, it has proven challenging to integrate with other low-dimensional materials due to harsh deposition conditions and poor structural stability. Here, we report the deposition of layered manganese dioxide (δ-MnO2) on bilayer epitaxial graphene (QEG) using a simple three-step electrochemical process involving no harsh chemicals. Using this process we can synthesize a 50 nm thick H–MnO2 film in 1.25s. This synthetic birnessite is inherently water-stabilized, the first reported in the literature. We also confirm that this process does not cause structural damage to the QEG, as evidenced by the lack of D peak formation. This QEG heterostructure enhanced MnO2's redox active gas sensing, enabling room temperature detection of NH3 and NO2. We also report on transforming this δ-MnO2 to other MnOx compounds, Mn2O3 and Mn3O4, via mild annealing. This is confirmed by Raman spectroscopy of the films, which also confirms limited damage to the QEG substrate. To our knowledge, this is the first synthesis of Mn2O3 and Mn3O4 on pristine graphene substrates. Both methods demonstrate the potential of depositing and transforming multifunctional oxides on single-crystal graphene using QEG substrates, allowing for the formation of nanostructured heterostructures previously unseen. Additionally, the electrochemical nature of the deposition presents the ability to scale the process to the QEG wafer and adjust the solution to produce other powerful multifunctional oxides. 

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3. Tunnel structured manganese oxide nanowires as redox active electrodes for hybrid capacitive deionization

   https://doi.org/https://doi.org/10.1016/j.nanoen.2017.12.015  

   Byles, Bryan W.; Cullen, David A.; More, Karren L.; Pomerantseva, Ekaterina (January 2018, Nano energy)

   Hybrid capacitive deionization (HCDI), which combines a capacitive carbon electrode and a redox active electrode in a single device, has emerged as a promising method for water desalination, enabling higher ion removal capacity than devices containing two carbon electrodes. However, to date, the desalination performance of few redox active materials has been reported. For the first time, we present the electrochemical behavior of manganese oxide nanowires with four different tunnel crystal structures as faradaic electrodes in HCDI cells. Two of these phases are square tunnel structured manganese oxides, α-MnO2 and todorokite-MnO2. The other two phases have novel structures that cross-sectional scanning transmission electron microscopy analysis revealed to have ordered and disordered combinations of structural tunnels with different dimensions. The ion removal performance of the nanowires was evaluated not only in NaCl solution, which is traditionally used in laboratory experiments, but also in KCl and MgCl2 solutions, providing better understanding of the behavior of these materials for desalination of brackish water that contains multiple cation species. High ion removal capacities (as large as 27.8 mg g−1, 44.4 mg g−1, and 43.1 mg g−1 in NaCl, KCl, and MgCl2 solutions, respectively) and high ion removal rates (as large as 0.112 mg g−1 s−1, 0.165 mg g−1 s−1, and 0.164 mg g−1 s−1 in NaCl, KCl, and MgCl2 solutions, respectively) were achieved. By comparing ion removal capacity to structural tunnel size, it was found that smaller tunnels do not favor the removal of cations with larger hydrated radii, and more efficient removal of larger hydrated cations can be achieved by utilizing manganese oxides with larger structural tunnels. Extended HCDI cycling and ex situ X-ray diffraction analysis revealed the excellent stability of the manganese oxide electrodes in repeated ion removal/ion release cycles, and compositional analysis of the electrodes indicated that ion removal is achieved through both surface redox reactions and intercalation of ions into the structural tunnels. This work contributes to the understanding of the behavior of faradaic materials in electrochemical water desalination and elucidates the relationship between the electrode material crystal structure and the ion removal capacity/ion removal rate in various salt solutions. 

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4. Bound manganese oxides capable of reducing the bacteriochlorophyll dimer of modified reaction centers from Rhodobacter sphaeroides

   https://doi.org/https://doi.org/10.1007/s11120-019--00680-3  

   E. Espiritu, K. Chamberlain (January 2020, Photosynthesis research)

   A biohybrid model system is described that interfaces synthetic Mn-oxides with bacterial reaction centers to gain knowledge concerning redox reactions by metal clusters in proteins, in particular the Mn4CaO5 cluster of photosystem II. The ability of Mn-oxides to bind to modified bacterial reaction centers and transfer an electron to the light-induced oxidized bacteri- ochlorophyll dimer, P+, was characterized using optical spectroscopy. The environment of P was altered to obtain a high P/ P+ midpoint potential. In addition, different metal-binding sites were introduced by substitution of amino acid residues as well as extension of the C-terminus of the M subunit with the C-terminal region of the D1 subunit of photosystem II. The Mn-compounds MnO2, αMn2O3, Mn3O4, CaMn2O4, and Mn3(PO4)2 were tested and compared to MnCl2. In general, addition of the Mn-compounds resulted in a decrease in the amount of P+ while the reduced quinone was still present, demonstrating that the Mn-compounds can serve as secondary electron donors. The extent of P+ reduction for the Mn-oxides was largest for αMn2O3 and CaMn2O4 and smallest for Mn3O4 and MnO2. The addition of Mn3(PO4)2 resulted in nearly complete P+ reduc- tion, similar to MnCl2. Overall, the activity was correlated with the initial oxidation state of the Mn-compound. Transient optical measurements showed a fast kinetic component, assigned to reduction of P+ by the Mn-oxide, in addition to a slow component due to charge recombination. The results support the conjecture that the incorporation of Mn-oxides by ancient anoxygenic phototrophs was a step in the evolution of oxygenic photosynthesis. 

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5. Structural identification of disordered γ-MnOOH in the alkaline MnO2 discharge mechanism

   https://doi.org/10.1557/s43579-025-00743-8  

   Zimmerer, Eric K; Gallaway, Joshua W (August 2025, MRS Communications)

   Abstract The alkaline battery MnO₂discharge mechanism progresses from tunneled γ-MnO₂to tunneled α-MnOOH to layered Mn(OH)₂as the reduction proceeds from charged Mn(IV) to discharged Mn(II). However, the existence of a disordered intermediate has recently been suggested in the “second electron” region between Mn(III) and Mn(II). The authors useoperandoextended X-ray absorption fine structure (EXAFS) to observe the structural evolution. A disordered γ-MnOOH was identified, which was the majority material for a substantial period before the sudden appearance of Mn(OH)₂in the final moments of discharge. In rechargeable Bi-modified electrodes, the disordered γ-MnOOH was more efficiently converted to Mn(OH)₂. Graphical abstract 

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