Nitrogen‐Rich Manganese Oxynitrides with Enhanced Catalytic Activity in the Oxygen Reduction Reaction

The catalytic activity of manganese oxynitrides in the oxygen reduction reaction (ORR) was investigated in alkaline solutions to clarify the effect of the incorporated nitrogen atoms on the ORR activity. These oxynitrides, with rock‐salt‐like structures with different nitrogen contents, were synthesized by reacting MnO, Mn₂O₃, or MnO₂ with molten NaNH₂ at 240–280 °C. The anion contents and the Mn valence states were determined by combustion analysis, powder X‐ray diffraction, and X‐ray absorption near‐edge structure analysis. An increase in the nitrogen content of rock‐salt‐based manganese oxynitrides increases the valence of the manganese ions and reinforces the catalytic activity for the ORR in 1 m KOH solution. Nearly single‐electron occupancy of the antibonding e_g states and highly covalent Mn?N bonding thus enhance the ORR activity of nitrogen‐rich manganese oxynitrides.     

Mass‐Production of Mesoporous MnCo2O4 Spinels with Manganese(IV)‐ and Cobalt(II)‐Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis

A mesoporous MnCo₂O₄ electrode material is made for bifunctional oxygen electrocatalysis. The MnCo₂O₄ exhibits both Co₃O₄‐like activity for oxygen evolution reaction (OER) and Mn₂O₃‐like performance for oxygen reduction reaction (ORR). The potential difference between the ORR and OER of MnCo₂O₄ is as low as 0.83 V. By XANES and XPS investigation, the notable activity results from the preferred Mn^IV‐ and Co^II‐rich surface. The electrode material can be obtained on large‐scale with the precise chemical control of the components at relatively low temperature. The surface state engineering may open a new avenue to optimize the electrocatalysis performance of electrode materials. The prominent bifunctional activity shows that MnCo₂O₄ could be used in metal–air batteries and/or other energy devices.     

Three‐Dimensional Macroporous NiCo2O4 Sheets as a Non‐Noble Catalyst for Efficient Oxygen Reduction Reactions

A facile and low‐cost strategy is developed to prepare three‐dimensional (3D) macroporous NiCo₂O₄ sheets, which can be used as a highly efficient non‐noble metal electrocatalyst for the oxygen reduction reaction (ORR) in alkaline conditions. The as‐obtained sheets have a thickness of about 150 nm and feature a typical 3D macroporous structure with pore volumes of up to 0.23 cm³ g^?1, which could decrease the mass transport resistance and allow easier access of the reactants to the active surface sites. The as‐prepared macroporous NiCo₂O₄ sheets exhibit high electrocatalytic activity for ORR with a four‐electron pathway, good long‐term stability and high tolerance against methanol. The unique 3D macroporous structure and intrinsic properties may be responsible for their high performance.     

A Porphyrin‐Based Porous rtl Metal–Organic Framework as an Efficient Catalyst for the Cycloaddition of CO2 to Epoxides

A porous rtl metal–organic framework (MOF) [Mn₅L(H₂O)₆?(DMA)₂]?5DMA?4C₂H₅OH ( 1? Mn) (H₁₀L=5,10,15,20‐tetra(4‐(3,5‐dicarboxylphenoxy)phenyl)porphyrin; DMA=N,N′‐dimethylacetamide) was synthesized by employing a new porphyrin‐based octacarboxylic acid ligand. 1? Mn exhibits high Mn^II density in the porous framework, providing it great Lewis‐acid heterogeneous catalytic capability for the cycloaddition of CO₂ with epoxides. Strikingly, 1? Mn features excellent catalytic activity to the cycloaddition of CO₂ to epoxides, with a remarkable initial turnover frequency 400 per mole of catalyst per hour at 20 atm. As‐synthesized 1? Mn also exhibits size selectivity to different epoxide substrates on account of their steric hindrance. The high catalytic activity, size selectivity, and stability toward the epoxides on catalytic cycloaddition of CO₂ make 1? Mn a promising heterogeneous catalyst for fixation and utilization of CO₂.     

Electrochemical Preparation of N‐Doped Cobalt Oxide Nanoparticles with High Electrocatalytic Activity for the Oxygen‐Reduction Reaction

Nitrogen‐doped CoO (N‐CoO) nanoparticles with high electrocatalytic activity for the oxygen‐reduction reaction (ORR) were fabricated by electrochemical reduction of CoCl₂ in acetonitrile solution at cathodic potentials. The initially generated, highly reactive nitrogen‐doped Co nanoparticles were readily oxidized to N‐CoO nanoparticles in air. In contrast to their N‐free counterparts (CoO or Co₃O₄), N‐CoO nanoparticles with a N content of about 4.6 % exhibit remarkable ORR electrocatalytic activity, stability, and immunity to methanol crossover in an alkaline medium. The Co?N_x active sites in the CoO nanoparticles are held responsible for the high ORR activity. This work opens a new path for the preparation of nitrogen‐doped transition metal oxide nanomaterials, which are promising electrocatalysts for fuel cells.     

Synergistic Enhancement of Electrocatalytic Activity toward Oxygen Reduction Reaction in Alkaline Electrolytes with Pentabasic (Fe,B, N,S, P)‐Doped Reduced Graphene Oxide

As alternatives to Pt‐based electrocatalysts, the development of nonprecious metal catalysts with high performance in the cathodic oxygen reduction reaction (ORR) is highly desirable for widespread use in fuel cells. Here we report a simple approach for preparing pentabasic (Fe, B, N, S, P)‐doped reduced graphene oxide (rGO) via a two‐step doping method of adding boric acid and ferric chloride to ternary (N, S, P)‐doped rGO (NSPG). Electrochemical investigation of the composites for the ORR revealed that simultaneously doping appropriate amounts of Fe and B into the NSPG produced a synergistic effect that endowed the prepared catalyst with both a positively shifted ORR half‐wave potential and high selectivity for the 4e^? reduction of O₂. The optimized Fe₂B‐NSPG catalyst approached a 4e^? process for the ORR with a half‐wave potential (E_1/2=0.90 V vs. RHE) even 30 mV higher than that of the commercial Pt/C catalyst in alkaline solution. Furthermore, relative to the Pt/C catalyst, the Fe₂B‐NSPG demonstrated superior stability and excellent tolerance of the methanol cross‐over effect. This simple method afforded pentabasic (Fe, B, N, S, P)‐doped rGO as a promising nonprecious metal catalyst used for alkaline fuel cells.     

A candidate for a single‐chain magnet: [Mn3(OAc)6(py)2(H2O)2]n (OAc is acetate and py is pyridine)

The title complex, catena‐poly[di‐μ₃‐acetato‐κ⁶O:O:O′‐tetra‐μ₂‐acetato‐κ⁴O:O;κ⁴O:O′‐diaquabis(pyridine‐κN)trimanganese(II)], [Mn₃(CH₃COO)₆(C₆H₅N)₂(H₂O)₂]_n, is a true one‐dimensional coordination polymer, in which the Mn^II centres form a zigzag chain along [010]. The asymmetric unit contains two metal centres, one of which (Mn1) lies on an inversion centre, while the other (Mn2) is placed close to an inversion centre on a general position. Since all the acetates behave as bridging ligands, although with different μ₂‐ and μ₃‐coordination modes, a one‐dimensional polymeric structure is formed, based on trinuclear repeat units (Mn1...Mn2...Mn2′), in which the Mn2 and Mn2′ sites are related by an inversion centre. Within this monomeric block, the metal–metal separations are Mn1...Mn2 = 3.36180 (18) Å and Mn2...Mn2′ = 4.4804 (3) Å. Cation Mn1, located on an inversion centre, displays an [MnO₆] coordination sphere, while Mn2, on a general position, has a slightly stronger [MnO₅N] ligand field, as the sixth coordination site is occupied by a pyridine molecule. Both centres approximate an octahedral ligand field. The chains are parallel in the crystal structure and interact via hydrogen bonds involving coordinated water molecules. However, the shortest metal–metal separation between two chains [5.3752 (3) Å] is large compared with the intrachain interactions. These structural features are compatible with a single‐chain magnet behaviour, as confirmed by preliminary magnetic studies.     

Graphitic C<Subscript>3</Subscript>N<Subscript>4</Subscript>@MWCNTs supported Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> as a novel electrocatalyst for the oxygen reduction reaction in zinc–air batteries

A series of catalysts (g-C₃N₄@MWCNTs/Mn₃O₄) were prepared from g-C₃N₄, MWCNTs, and Mn₃O₄ for oxygen reduction reaction (ORR) in zinc–air batteries. From the half-cell tests, the loading of 35 % Mn₃O₄ (sample GMM35) presents an excellent activity toward ORR in alkaline condition. Rotating ring-disk electrode (RRDE) studies reveal that 3.6～3.8 electrons are transferred with a H₂O₂ yield of 11.4 % at ?0.4 V. Meanwhile, the GMM35 nanocomposite exhibits the same durability as commercial 20 wt% Pt/C in alkaline condition, but it shows lower peak power density (192.4 mW cm^?2 at 229.1 mA cm^?2) and cell voltage than those with a commercial Pt/C catalyst (260.9 mW cm^?2 at 285.4 mA cm^?2).     

Synthesis of Highly Active and Stable Spinel‐Type Oxygen Evolution Electrocatalysts by a Rapid Inorganic Self‐Templating Method

Composition‐adjustable spinel‐type metal oxides, Mn_xCo_3?xO_4?δ (x=0.8–1.4), were synthesized in ethanol solutions by a rapid inorganic self‐templating mechanism using KCl nanocrystals as the structure‐directing agent. The Mn_xCo_3?xO_4?δ materials showed ultrahigh oxygen evolution activity and strong durability in alkaline solutions, and are capable of delivering a current density of 10 mA cm^?2 at 1.58 V versus the reversible hydrogen electrode in 0.1 M KOH solution, which is superior in comparison to IrO₂ catalysts under identical experimental conditions, and comparable to the most active noble‐metal and transition‐metal oxygen evolution electrocatalysts reported so far. The high performance for catalytic oxygen evolution originates from both compositional and structural features of the synthesized materials. The moderate content of Mn doping into the spinel framework led to their improved electronic conductivity and strong oxidizing ability, and the well‐developed porosity, accompanied with the high affinity between OH^? reactants and catalyst surface, contributed to the smooth mass transport, thus endowing them with superior oxygen evolution activity.     

Silver supported on Co3O4 modified carbon as electrocatalyst for oxygen reduction reaction in alkaline media

The electroreduction of oxygen was firstly studied on Ag/Co₃O₄–C in alkaline media prepared by depositing Ag on Co₃O₄ modified carbon (Co₃O₄–C). The Ag/Co₃O₄–C composite not only displayed relatively large electrochemical active surface area (ESA), high catalytic activity towards oxygen reduction reaction (ORR), but also exhibited good methanol tolerance and stability in alkaline media. Ag/Co₃O₄–C could be a valuable catalyst for ORR and be applied to alkaline fuel cells and metal–air batteries.     

Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework

Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E_1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E_1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies.     

Electrocatalytic activity of dispersed platinum and silver alloys and manganese oxides for the oxygen reduction in alkaline electrolyte

This work reviews the studies conducted in this laboratory of the oxygen reduction reaction (ORR) on electrocatalysts formed by Pt-M/C (M = V, Cr, Co) and Ag-Pt/C alloys and on different Mn oxides (MnO/C, Mn₃O₄/C, MnO₂/C) in KOH electrolyte. The physical and electronic properties of the materials are investigated by in situ XAS (x-ray absorption spectroscopy) in the XANES (x-ray absorption near edge structure) region. The electrocatalytic activity for the ORR on the different catalysts is compared through mass-transport-corrected Tafel plots. The XANES results for the Pt-M/C and Ag-Pt/C composites at high electrode potentials show lower vacancy of the Pt 5d band compared to pure Pt/C, while for the results indicate a chance of the Mn oxidation state as a function of the electrode potential. The electrochemical measurements evidence increased electrocatalytic activity of the Pt alloys compared to pure Pt and this is attributed to a lowering of the adsorption strength of adsorbed oxygen species caused by the reduced Pt reactivity. An activity enhancement of the Ag atoms on the Ag-Pt/C alloys compared to pure Ag is ascribed to an electronic effect induced by the presence of Pt, increasing the Ag-O adsorption strength. In the case of the Mn_yO_x/C materials, the electrochemical results show low activity for MnO/C and higher activity for MnO₂/C and Mn₃O₄/C. This is explained based on the activation for the ORR, which is higher for the material with higher MnO₂ contents and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 12, pp. 1417–1426. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Valence Distributions of Iron and Manganese in Pure,Manganese‐doped and Calcium‐doped Yttrium Orthoferrite

The valence distributions of Fe and Mn in yttrium orthoferrite (YFeO,), YFe_0.6Mn0.4O3 and Y_0.9Ca_0.1Fe_0.6Mn_0.4O_3.8 were studied by the measurement of thermal power. Pure YFeO₃ shows the n‐type electrical conductivity associated with small polaron hopping between Fe²⁺ and Fe³⁺. Both YFe_0.4Mn_0.4‐O₃ and Y_0–9Ca_0–1.Fe_0–6Mn_0–4O_3–8 show n‐type and p‐type conductivities at high and low temperatures respectively associated with small polaron hopping between Mn³⁺ and Mn⁴⁺, iron has only oxidation state of Fe³⁺ and does not have contribution to the electrical conductivity.     

Ni3FeN‐Supported Fe3Pt Intermetallic Nanoalloy as a High‐Performance Bifunctional Catalyst for Metal–Air Batteries

Electrocatalysts for both the oxygen reduction and evolution reactions (ORR and OER) are vital for the performances of rechargeable metal–air batteries. Herein, we report an advanced bifunctional oxygen electrocatalyst consisting of porous metallic nickel‐iron nitride (Ni₃FeN) supporting ordered Fe₃Pt intermetallic nanoalloy. In this hybrid catalyst, the bimetallic nitride Ni₃FeN mainly contributes to the high activity for the OER while the ordered Fe₃Pt nanoalloy contributes to the excellent activity for the ORR. Robust Ni₃FeN‐supported Fe₃Pt catalysts show superior catalytic performance to the state‐of‐the‐art ORR catalyst (Pt/C) and OER catalyst (Ir/C). The Fe₃Pt/Ni₃FeN bifunctional catalyst enables Zn–air batteries to achieve a long‐term cycling performance of over 480 h at 10 mA cm⁻² with high efficiency. The extraordinarily high performance of the Fe₃Pt/Ni₃FeN bifunctional catalyst makes it a very promising air cathode in alkaline electrolyte.     

Surface and Structural Investigation of a MnOx Birnessite‐Type Water Oxidation Catalyst Formed under Photocatalytic Conditions

Catalytically active MnO_x species have been reported to form in situ from various Mn‐complexes during electrocatalytic and solution‐based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnO_x catalysts form without supports in situ under photocatalytic conditions. Our most active ⁴MnO_x catalyst (～0.84 mmol O₂ mol Mn^?1 s^?1) forms from a Mn₄O₄ bearing a metal–organic framework. ⁴MnO_x is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR‐TEM as a disordered, layered Mn‐oxide with high surface area (216 m²g^?1) and small regions of crystallinity and layer flexibility. In contrast, the ^SMnO_x formed from Mn²⁺ salt gives an amorphous species of lower surface area (80 m²g^?1) and lower activity (～0.15 mmol O₂ mol Mn^?1 s^?1). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p_3/2 core levels detects enriched Mn³⁺ and Mn²⁺ content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.     

Titanium Dioxide‐Grafted Copper Complexes: High‐Performance Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathodes of fuel cells significantly hampers fuel cell performance. Therefore, the development of high‐performance, non‐precious‐metal catalysts as alternatives to noble metal Pt‐based ORR electrocatalysts is highly desirable for the large‐scale commercialization of fuel cells. TiO₂‐grafted copper complexes deposited on multiwalled carbon nanotubes (CNTs) form stable and efficient electrocatalysts for the ORR. The optimized catalyst composite CNTs@TiO₂–ZA–[Cu(phen)(BTC)] shows surprisingly high selectivity for the 4 e^? reduction of O₂ to water (approximately 97 %) in alkaline solution with an onset potential of 0.988 V vs. RHE, and demonstrates superior stability and excellent tolerance for the methanol crossover effect in comparison to a commercial Pt/C catalyst. The copper complexes were grafted onto the surface of TiO₂ through coordination of an imidazole‐containing ligand, zoledronic acid (ZA), which binds to TiO₂ through its bis‐phosphoric acid anchoring group. Rational optimization of the copper catalyst’s ORR performance was achieved by using an electron‐deficient ligand, 5‐nitro‐1,10‐phenanthroline (phen), and bridging benzene‐1,3,5‐tricarboxylate (BTC). This facile approach to the assembly of copper catalysts on TiO₂ with rationally tuned ORR activity will have significant implications for the development of high‐performance, non‐precious‐metal ORR catalysts.     

Highly dispersed Mn2O3−Co3O4 nanostructures on carbon matrix as heterogeneous Fenton-like catalyst

This work reports the synthesis of various carbon (Vulcan XC-72 R) supported metal oxide nanostructures, such as Mn₂O₃, Co₃O₄ and Mn₂O₃−Co₃O₄ as heterogeneous Fenton-like catalysts for the degradation of organic dye pollutants, namely Rhodamine B (RB) and Congo Red (CR) in wastewater. The activity results showed that the bimetallic Mn₂O₃−Co₃O₄/C catalyst exhibits much higher activity than the monometallic Mn₂O₃/C and Co₃O₄/C catalysts for the degradation of both RB and CR pollutants, due to the synergistic properties induced by the Mn−Co and/or Mn (Co)−support interactions. The degradation efficiency of RB and CR was considerably increased with an increase of reaction temperature from 25 to 45°C. Importantly, the bimetallic Mn₂O₃−Co₃O₄/C catalyst could maintain its catalytic activity up to five successive cycles, revealing its catalytic durability for wastewater purification. The structure–activity correlations demonstrated a probable mechanism for the degradation of organic dye pollutants in wastewater, involving •OH radical as well as Mn²⁺/Mn³⁺ or Co²⁺/Co³⁺ redox couple of the Mn₂O₃−Co₃O₄/C catalyst.     

Hydration preferences for Mn4Ca cluster models of photosystem II: location of potential substrate-water binding sites

Density functional theory calculations are reported on a set of three model structures of the Mn₄Ca cluster in the water‐oxidizing complex of Photosystem II (PSII), which share the structural formula [CaMn₄C₉H₁₀N₂O₁₆]^q+ ? (H₂O)_n (q=?1, 0, 1, 2, 3; n=0–7). In these calculations we have explored the preferred hydration sites of the Mn₄Ca cluster across five overall oxidation states (S₀ to S₄) and all feasible magnetic‐coupling arrangements to identify the most likely substrate–water binding sites. We have also explored charge‐compensated structures in which the overall charge on the cluster is maintained at q=0 or +1, which is consistent with the experimental data on sequential proton loss in the real system. The three model structures have skeletal arrangements that are strongly reminiscent, in their relative metal‐atom positions, of the 2.9‐, 3.7‐, and 3.5 Å‐resolution crystal structures, respectively, whereas the charge states encompassed in our study correspond to an assignment of (Mn^III)₃Mn^II for S₀ and up to (Mn^IV)₃Mn^III for S₄. The three models differ principally in terms of the spatial relationship between one Mn (Mn(4)) and a generally robust Mn₃Ca tetrahedron that contains Mn(1), Mn(2), and Mn(3). Oxidation‐state distributions across the four manganese atoms, in most of the explored charge states, are dependent on details of the cluster geometry, on the extent of assumed hydration of the clusters, and in some instances on the imposed magnetic‐coupling between adjacent Mn atoms. The strongest water‐binding sites are generally those on Mn(4) and Ca. However, one structure type displays a high‐affinity binding site between Ca and Mn(3), the S‐state‐dependent binding‐energy pattern of which is most consistent with the substrate water‐exchange kinetics observed in functional PSII. This structure type also permits another water molecule to access the cluster in a manner consistent with the substrate–water interaction with the Mn cluster, seen in electron spin‐echo envelope modulation (ESEEM) studies of the functional enzyme in the S₀ and S₂ states. It also rationalizes the significant differences in hydrogen‐bonding interactions of the substrate water observed in the FTIR measurements of the S₁ and S₂ states. We suggest that these two water‐binding sites, which are molecularly close, model the actual substrate‐binding sites in the enzyme.     

The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions

Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn_0.5Fe_0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn_0.5Fe_0.5 exhibits high stability for both reactions.

     

Mn−O Covalency Governs the Intrinsic Activity of Co‐Mn Spinel Oxides for Boosted Peroxymonosulfate Activation

Transition metal (TM)‐based bimetallic spinel oxides can efficiently activate peroxymonosulfate (PMS) presumably attributed to enhanced electron transfer between TMs, but the existing model cannot fully explain the efficient TM redox cycling. Here, we discover a critical role of TM?O covalency in governing the intrinsic catalytic activity of Co_3?xMn_xO₄ spinel oxides. Experimental and theoretical analysis reveals that the Co sites significantly raises the Mn valence and enlarges Mn?O covalency in octahedral configuration, thereby lowering the charge transfer energy to favor Mn_Oh–PMS interaction. With appropriate Mn^IV/Mn^III ratio to balance PMS adsorption and Mn^IV reduction, the Co_1.1Mn_1.9O₄ exhibits remarkable catalytic activities for PMS activation and pollutant degradation, outperforming all the reported TM spinel oxides. The improved understandings on the origins of spinel oxides activity for PMS activation may inspire the development of more active and robust metal oxide catalysts.