Zn[superscript 2+] Induced Phase Transformation of K[subscript 2]MnFe(CN)[subscript 6] Boosts Highly
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Deng, Wenjun↗
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Breaking the Linear Scaling Relations for the Oxygen Reduction Reaction with a Dual‐Atom Catalyst Composed of a MnFe‐Porphyrrole Aerogel
Bimetallic catalysts offer enhanced catalytic performance through synergistic interactions between the two metals, allowing them to break the linear scaling relations and reach high electrocatalytic activity. This study presents bimetallic aerogel-based catalyst synthesized as a covalent, three-dimensional framework containing neighboring iron and manganese sites. The aerogel structure provides a high surface area and porosity, facilitating an ultra-high active site density and efficient mass transport. The MnFe porphyrrole's unique structure is obtained by alternately linking Mn-porphyrin and Fe-corrole complexes. It exhibited outstanding performance with an onset potential of 0.99 V RHE . Comparative studies with a free-base Fe porphyrrole catalyst (E onset 0.97 V RHE ) revealed that while Mn incorporation led to only a slight improvement in half-cell performance, it resulted in significantly enhanced performance in anion exchange membrane fuel cell. The MnFe catalyst achieved an OCV of 0.97 V and a peak power density of 0.27 W cm −2 , outperforming the free-base Fe counterpart. Using density functional theory calculations, we show that the higher ORR activity of MnFe-porphyrrole is due to charge transfer between Mn and Fe atoms, which is absent in the reference free-base Fe-porphyrrole. These findings underscore the advantages of bimetallic catalysts in improving ORR activity and fuel cell efficiency by leveraging synergistic effects.
Aerogel↗
Materials Data on MnFe(PO4)2 by Materials Project
MnFe(PO4)2 crystallizes in the trigonal R3 space group. The structure is three-dimensional. there are three inequivalent Mn3+ sites. In the first Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six PO4 tetrahedra. There is three shorter (1.91 Å) and three longer (2.01 Å) Mn–O bond length. In the second Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six PO4 tetrahedra and a faceface with one FeO6 octahedra. There are three shorter (1.94 Å) and three longer (2.10 Å) Mn–O bond lengths. In the third Mn3+ site, Mn3+ is bonded to six O2- atoms to form MnO6 octahedra that share corners with six PO4 tetrahedra. There is three shorter (1.94 Å) and three longer (2.04 Å) Mn–O bond length. There are three inequivalent Fe3+ sites. In the first Fe3+ site, Fe3+ is bonded to six O2- atoms to form distorted FeO6 octahedra that share corners with six PO4 tetrahedra, a faceface with one MnO6 octahedra, and a faceface with one FeO6 octahedra. All Fe–O bond lengths are 2.11 Å. In the second Fe3+ site, Fe3+ is bonded to six O2- atoms to form distorted FeO6 octahedra that share corners with six PO4 tetrahedra and a faceface with one FeO6 octahedra. There are three shorter (1.93 Å) and three longer (2.13 Å) Fe–O bond lengths. In the third Fe3+ site, Fe3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are three shorter (2.18 Å) and three longer (2.19 Å) Fe–O bond lengths. There are two inequivalent P5+ sites. In the first P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two FeO6 octahedra and corners with three MnO6 octahedra. The corner-sharing octahedra tilt angles range from 29–52°. There are a spread of P–O bond distances ranging from 1.52–1.58 Å. In the second P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two FeO6 octahedra and corners with three MnO6 octahedra. The corner-sharing octahedra tilt angles range from 27–53°. There are a spread of P–O bond distances ranging from 1.52–1.58 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a bent 150 degrees geometry to one Mn3+ and one P5+ atom. In the second O2- site, O2- is bonded in a 3-coordinate geometry to one Mn3+, one Fe3+, and one P5+ atom. In the third O2- site, O2- is bonded in a bent 150 degrees geometry to one Fe3+ and one P5+ atom. In the fourth O2- site, O2- is bonded in a 3-coordinate geometry to one Mn3+, one Fe3+, and one P5+ atom. In the fifth O2- site, O2- is bonded in a bent 150 degrees geometry to one Mn3+ and one P5+ atom. In the sixth O2- site, O2- is bonded in a 3-coordinate geometry to two Fe3+ and one P5+ atom. In the seventh O2- site, O2- is bonded in a 3-coordinate geometry to one Mn3+, one Fe3+, and one P5+ atom. In the eighth O2- site, O2- is bonded in a bent 150 degrees geometry to one Mn3+ and one P5+ atom.
36 MATERIALS SCIENCE↗
Materials Data on MnFe(PO4)2 by Materials Project
MnFe(PO4)2 crystallizes in the orthorhombic Pmc2_1 space group. The structure is three-dimensional. Mn3+ is bonded to six O2- atoms to form distorted MnO6 pentagonal pyramids that share corners with four equivalent FeO6 octahedra, corners with four PO4 tetrahedra, and an edgeedge with one PO4 tetrahedra. The corner-sharing octahedra tilt angles range from 48–50°. There are a spread of Mn–O bond distances ranging from 1.90–2.36 Å. Fe3+ is bonded to six O2- atoms to form distorted FeO6 octahedra that share corners with four equivalent MnO6 pentagonal pyramids, corners with four PO4 tetrahedra, and an edgeedge with one PO4 tetrahedra. There are a spread of Fe–O bond distances ranging from 1.94–2.17 Å. There are two inequivalent P5+ sites. In the first P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two equivalent FeO6 octahedra, corners with two equivalent MnO6 pentagonal pyramids, and an edgeedge with one FeO6 octahedra. The corner-sharing octahedra tilt angles range from 42–44°. There is two shorter (1.52 Å) and two longer (1.58 Å) P–O bond length. In the second P5+ site, P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with two equivalent FeO6 octahedra, corners with two equivalent MnO6 pentagonal pyramids, and an edgeedge with one MnO6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 53°. There are a spread of P–O bond distances ranging from 1.54–1.56 Å. There are six inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Mn3+ and one P5+ atom. In the second O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Fe3+ and one P5+ atom. In the third O2- site, O2- is bonded in a 3-coordinate geometry to one Mn3+, one Fe3+, and one P5+ atom. In the fourth O2- site, O2- is bonded in a 3-coordinate geometry to one Mn3+, one Fe3+, and one P5+ atom. In the fifth O2- site, O2- is bonded in a bent 120 degrees geometry to one Mn3+ and one P5+ atom. In the sixth O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Fe3+ and one P5+ atom.
36 MATERIALS SCIENCE↗
Materials Data on MnFe by Materials Project
FeMn is Tetraauricupride structured and crystallizes in the tetragonal P4/mmm space group. The structure is three-dimensional. Mn is bonded to four equivalent Mn and eight equivalent Fe atoms to form MnMn4Fe8 cuboctahedra that share corners with twelve equivalent MnMn4Fe8 cuboctahedra, edges with eight equivalent MnMn4Fe8 cuboctahedra, edges with sixteen equivalent FeMn8Fe4 cuboctahedra, faces with eight equivalent FeMn8Fe4 cuboctahedra, and faces with ten equivalent MnMn4Fe8 cuboctahedra. All Mn–Mn bond lengths are 2.46 Å. All Mn–Fe bond lengths are 2.46 Å. Fe is bonded to eight equivalent Mn and four equivalent Fe atoms to form FeMn8Fe4 cuboctahedra that share corners with twelve equivalent FeMn8Fe4 cuboctahedra, edges with eight equivalent FeMn8Fe4 cuboctahedra, edges with sixteen equivalent MnMn4Fe8 cuboctahedra, faces with eight equivalent MnMn4Fe8 cuboctahedra, and faces with ten equivalent FeMn8Fe4 cuboctahedra. All Fe–Fe bond lengths are 2.46 Å.
36 MATERIALS SCIENCE↗
High-efficiency, air-stable manganese–iron oxide nanoparticle-pigmented solar selective absorber coatings toward concentrating solar power systems operating at 750 °C
Solar selective absorber coating with long-term thermal stability at high temperatures ≥750 °C in air is an important component to reduce the levelized cost of energy (LCOE) of concentrating solar power (CSP) systems toward 50% power efficiency and dispatchable solar electricity. Conventionally, solar spectral selectivity requires multilayer-interference coatings implemented by stringently controlled vacuum deposition, and these coatings degrade significantly at >700 °C in air. Herein, we established a quantitative design approach and demonstrated a proof-of-concept air-stable, manganese–iron oxide nanoparticle (NP)-pigmented solar selective coatings with a high solar absorptance of ~93%, a relatively low thermal emittance of ~52%, and an optical-to-thermal energy conversion efficiency >89% under 1,000× solar concentration at 750 °C toward Generation 3 CSP systems. These coatings demonstrate spectral selectivity using cost-effective spray coating approach, a notable improvement over conventional vacuum-deposited, multilayer solar selective coatings for low-cost, high-efficiency solar thermal receivers. In contrast to the thermal degradation of spectrally non-selective benchmark Pyromark 2500 coatings at 750 °C in air, the solar absorptance of the MnFe 2 O 4 -pigmented coatings on stainless steel 310 (SS 310) substrates is increased to ~92.9% and the optical-to-thermal energy conversion efficiency is improved to 89.7% after serving at 750 °C in air for 700 h. X-ray diffraction results reveal that this improvement is due to the transformation of MnFe 2 O 4 NPs into more thermodynamically stable, non-stoichiometric manganese-rich manganese ferrite and iron-rich manganese–iron oxide phases after 500 h aging at 750 °C. For >1,000 h-endurance testing at 750 °C in air and the subsequent 19 day-night thermal cycling between 750 °C (12 h/cycle) and 25 °C (12 h/cycle) on SS 310 substrates, the thermal degradation is mainly due to the CrO x microflake formation from SS 310 substrates rather than the coatings, which can be suppressed by preoxidizing the surface of SS 310. With lower emittance matrix material and further optimization of pigment NP stoichiometry, concentration, and coating thickness, it is promising to achieve an optimized thermal efficiency ≥92.5% with long-term thermal stability at 750 °C for Generation 3 CSP systems.
14 SOLAR ENERGY↗
Temperature dependence of corrosion behavior of a dual-phase Fe 50 Mn 30 Co 10 Cr 10 high entropy alloy in supercritical water at 380–650 °C
Corrosion behavior of a dual-phase Fe 50 Mn 30 Co 10 Cr 10 HEA subjected to supercritical water at different temperatures was investigated. Both weight gain and oxide film thickness increased with increasing temperatures. An outer layer with polyhedral MnFe 2 O 4 particles and an inner layer containing a mixture of CrMn 1.5 O 4 and Fe 3 O 4 were formed at 380 °C. Mn 2 O 3 outer and CrMn 1.5 O 4 inner layers developed at 550 and 650 °C, but the loose granular Mn 2 O 3 particles at 550 °C transformed into a compact oxide film at 650 °C. Besides, Cr 2 O 3 oxide particles were observed along the grain boundaries. Finally, the corrosion mechanism at different temperatures was also discussed.
36 MATERIALS SCIENCE↗
On the synthesis of bi-magnetic manganese ferrite-based core–shell nanoparticles
Dissolution phenomena in seed-mediated growth approaches of MnFe 2 O 4 -based bi-magnetic core–shell nanoheterostructures with a cobalt ferrite or spinel iron oxide shell.
36 MATERIALS SCIENCE↗
Self-Limitations of Heat Release in Coupled Core-Shell Spinel Ferrite Nanoparticles: Frequency, Time, and Temperature Dependencies
We explored a series of highly uniform magnetic nanoparticles (MNPs) with a core-shell nanoarchitecture prepared by an efficient solvothermal approach. In our study, we focused on the water dispersion of MNPs based on two different CoFe 2 O 4 core sizes and the chemical nature of the shell (MnFe 2 O 4 and spinel iron oxide). We performed an uncommon systematic investigation of the time and temperature evolution of the adiabatic heat release at different frequencies of the alternating magnetic field (AMF). Our systematic study elucidates the nontrivial variations in the heating efficiency of core-shell MNPs concerning their structural, magnetic, and morphological properties. In addition, we identified anomalies in the temperature and frequency dependencies of the specific power absorption (SPA). We conclude that after the initial heating phase, the heat release is governed by the competition of the Brown and Néel mechanism. In addition, we demonstrated that a rational parameter sufficiently mirroring the heating ability is the mean magnetic moment per MNP. Our study, thus, paves the road to fine control of the AMF-induced heating by MNPs with fine-tuned structural, chemical, and magnetic parameters. Importantly, we claim that the nontrivial variations of the SPA with the temperature must be considered, e.g., in the emerging concept of MF-assisted catalysis, where the temperature profile influences the undergoing chemical reactions.
36 MATERIALS SCIENCE↗
Hard–Soft Core–Shell Architecture Formation from Cubic Cobalt Ferrite Nanoparticles
Cubic bi-magnetic hard–soft core–shell nanoarchitectures were prepared starting from cobalt ferrite nanoparticles, prevalently with cubic shape, as seeds to grow a manganese ferrite shell. The combined use of direct (nanoscale chemical mapping via STEM-EDX) and indirect (DC magnetometry) tools was adopted to verify the formation of the heterostructures at the nanoscale and bulk level, respectively. The results showed the obtainment of core–shell NPs (CoFe 2 O 4 @MnFe 2 O 4 ) with a thin shell (heterogenous nucleation). In addition, manganese ferrite was found to homogeneously nucleate to form a secondary nanoparticle population (homogenous nucleation). This study shed light on the competitive formation mechanism of homogenous and heterogenous nucleation, suggesting the existence of a critical size, beyond which, phase separation occurs and seeds are no longer available in the reaction medium for heterogenous nucleation. These findings may allow one to tailor the synthesis process in order to achieve better control of the materials’ features affecting the magnetic behaviour, and consequently, the performances as heat mediators or components for data storage devices.
36 MATERIALS SCIENCE↗