Concentration dependence of resistance components in solutions containing dissolved Fe2+/Fe3.

Electrolyte solutions containing Fe2+/Fe3+ are suitable for liquid thermoelectric conversion devices (LTEs), because they are inexpensive materials and exhibit a high electrochemical Seebeck coefficient α. Here, we investigated the concentration (c) dependence of resistance components, i.e., solvent (Rs), charge-transfer (Rct), and diffusion (Rdif) resistances, of dissolved-Fe2+/Fe3+-containing aqueous, methanol (MeOH), acetone, and propylene carbonate (PC) solutions. We found that the c dependence of Rs and Rdif are well reproduced by empirical formulas, and , where η(c) is viscosity at c. We further found that the magnitudes of Cs and Cdif are nearly independent of solvent, suggesting that η is one of the significant solution parameters that determine Rs and Rdif.

Temperature switch of electrochemical Seebeck coefficient of Fe2+/Fe3+ via formation of [FeCl4]2-/[FeCl4].

The electrochemical Seebeck coefficient (α = dV/dT; V and T are the redox potential and temperature, respectively) is important parameter for thermoelectric conversion. Here, we found that α of Fe2+/Fe3+ in dimethyl sulfoxide (DMSO) with small amount of 19 M LiCl aqueous solution exhibits a crossover behavior from 1.4 mV K-1 (20 °C ≤ T ≤ 40 °C) to ≈0 mV K-1 (50 °C ≤ T ≤ 80 °C). The molar absorption (ε) spectra revealed that the crossover is ascribed to the transformation of the dominant Fe3+ complex from [FeL6]3+ (L is solvent molecule) to [FeCl4]-.

Control of Fe3+ coordination by excess Cl- in alcohol solutions.

We spectroscopically investigated coordination state of Fe3+ in methanol (MeOH) and ethanol (EtOH) solutions against Cl- concentration ([Cl-]). In both the system, we observed characteristic absorption bands due to the FeCl4 complex at high-[Cl-] region. In the MeOH system, the proportion (r) of [FeCl4]- exhibits a stationary value of 0.2-0.3 in the intermediate region of 10 mM < [Cl-] < 50 mM, which is interpretted in terms of [FeCl n L6-n ]3-n (n = 1 and 2). In the EtOH system, r steeply increases from 0.1 at [Cl-] = 1.5 mM to 0.7 at [Cl-] = 3.5 mM, indicating direct transformation from [FeL6]3+ to [FeCl4]-. We further found that the coordination change significantly decreases the redox potential of Fe2+/Fe3+.

Electron transfer phase transition and oxidization process in NaxCo0.44Mn0.56[Fe(CN)6]0.90 (0.00 ≤ x ≤ 1.60).

We investigated the phase diagram of NaxCo0.44Mn0.56[Fe(CN)6]0.90 in the entire Na concentration range of 0.00 ≤ x ≤ 1.60. We found that the compound shows an electron transfer (ET) phase transition in a wide x range of 0.19 ≤ x ≤ 1.38. The extended ET model well reproduces the variation of the [Fe2+(CN)6]4- and [Fe3+(CN)6]3- concentration at the phase transition. The ET phase transition reverses the oxidation process of the compound; the process is in the order of Co, Mn, and Fe (Fe, Mn, and Co) in the low (high) temperature phase.

Chemical passivation of the under coordinated Pb2+ defects in inverted planar perovskite solar cells via ß-diketone Lewis base additives.

Hybrid organic-inorganic perovskite solar cells (PSCs) are promising new generations of solar cells, which is low in cost with high power conversion efficiency (PCE). However, PSCs suffer from structural defects generated from the under coordinated ions at the surface, which limits their photovoltaic performances. Herein we report, two ß-diketone Lewis base additives 2,4-pentanedione and 3-methyl-2,4-nonanedione within the chlorobenzene anti-solvent to passivate the surface defects generated from the under coordinated Pb2+ ions in CH3NH3PbI3 perovskite films. The incorporation of the two ß-diketone passivators could successfully enhance the open-circuit voltage of the PSCs by 52 mV and 17 mV for 3-methyl-2,4-nonanedione and 2,4-pentanedione, respectively, with improved PCE by 45% for 3-methyl-2,4-nonanedione compared to the pristine PSC. This enhancement in the photovoltaic performance of the PSCs can be attributed to passivation of the defects through the interaction between two carbonyl groups of the ß-diketone Lewis base additives and the under coordinated Pb2+ defects in the perovskite film, which improved the PSCs PCE and stability.

In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues.

Cobalt Prussian blue analogues (Co-PBA; NaxCo[Fe(CN)6]y), consisting of cyano-bridged transition metal network, -Fe-CN-Co-NC-Fe-, are promising cathode materials for Na-ion secondary batteries. In the oxidation process, oxidization of Fe and/or Co are compensated by Na+ deintercalation. Here, we investigated the oxidization process of three Co-PBAs by means of in situ infrared absorption (IR) spectroscopy. With use of an empirical rule of the frequencies of the CN- stretching mode in ferrocyanide ([FeII(CN)6]4-) and ferricyanide ([FeIII(CN)6]3-), the oxidation processes of Co-PBAs were determined against the Fe concentration (y) and temperature (T). We will discuss the interrelation between the oxidation processes and Fe concentration (y).

Controllable Magnetic Proximity Effect and Charge Transfer in 2D Semiconductor and Double-Layered Perovskite Manganese Oxide van der Waals Heterostructure.

Optically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic properties of the materials underneath. Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe2 on double-layered perovskite Mn oxide ((La0.8 Nd0.2 )1.2 Sr1.8 Mn2 O7 ) is demonstrated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photoluminescence intensity via dielectric screening is observed. Further, a relatively high trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polarization of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic proximity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins.

Aggregation tendency of guest Fe in NaCo1-xFexO2 (x < 0.1) as investigated by systematic EXAFS analysis.

In transition metal (M) compounds, the partial substitution of the host transition metal (Mh) to guest one (Mg) is effective to improve the functionality. To microscopically comprehend the substitution effect, degree of distribution of Mg is crucial. Here, we propose that a systematic EXAFS analysis against the Mg concentration can reveal the spatial distribution of Mg. We chose NaCo1-xFexO2 as a prototypical M compound and investigated the local intermetal distance around the guest Fe [dFe-M(x)] against Fe concentration (x). dFe-M(x) steeply increased with x, reflecting the larger ionic radius of high-spin Fe3+. The x-dependence of dFe-M(x) was analyzed by an empirical equation, [Formula: see text], where dFe-Fe and dFe-Co are the Fe-Fe and Co-Fe distances, respectively. The parameter s represents degree of distribution of Fe; s = 1, > 1, < 1 are for random, attractive, and repulsive distribution, respectively. The obtained s value (= 4.8) indicates aggregation tendency of guest Fe.

Energy harvesting thermocell with use of phase transition.

A thermocell that consists of cathode and anode materials with different temperature coefficients (α = dV/dT) of the redox potential (V) can convert environmental thermal energy to electric energy via the so-called thermal charging effect. The output voltage Vcell of the current thermocell, however, is still low (several tens mV) and depends on temperature, which are serious drawbacks for practical use of the device as an independent power supply. Here, we report that usage of phase transition material as electrode qualitatively improve the device performance. We set the critical temperature (Tc) for the phase transition in cobalt Prussian blue analogue (Co-PBA; NaxCo[Fe(CN)6]y) to just above room temperature, by finely adjusting the Fe concentration (y = 0.82). With increase in the cell temperature (Tcell), Vcell of the NaxCo[Fe(CN)6]0.82 (NCF82)/NaxCo[Fe(CN)6]0.9 (NCF90) cell steeply increases from 0 mV to ~120 mV around 320 K. Our observation indicates that the thermocell with use of phase transition is a promising energy harvesting device.

The effect of 3d-electron configuration entropy on the temperature coefficient of redox potential in Co1-zMnz Prussian blue analogues.

Recently, it was reported that a thermocell can convert temperature into electrical energy by using the difference in the thermal coefficient (α ≡ dV/dT) of the redox potential (V) between the cathode and anode materials. Here, we systematically investigated α of NaxCo1-zMnz[Fe(CN)6]y (Co1-zMnz-PBA) against Mn concentration (z). The z-dependence of α is interpreted in terms of the 3d-electron configuration entropy (ΔS3d) of the redox site. Utilization of S3d is a strategy effective for the design of high-|α| materials for energy harvesting thermocells.

Thermal efficiency of a thermocell made of Prussian blue analogues.

Recently, it was reported that a thermocell can convert temperature into electric energy by using the difference in the thermal coefficient (α = dV/dT) of the redox potential (V) between the cathode and anode materials. Among battery materials, Prussian blue analogues (PBAs) are promising materials for thermocell, because α changes from approximately -0.3 mV/K in NaxMn[Fe(CN)6]0.83 3.5 H2O (NMF83) to approximately 1.3 mV/K in NaxCo[Fe(CN)6]0.92,9H2O (NCF90). In this work, we systematically investigated the thermal efficiency (η) of the NMF83/NCF90 thermocell relative to the difference (ΔT) between low (TL = 282 K) and high (TH = 292-338 K) temperatures. We found that the thermal efficiency (η) increased proportionally with ΔT. The linear increase in η is ascribed to the linear increase in the cell voltage (Vcell) and the charge (QNCF90) extracted from NCF90. Moreover, η reached 3.19% at ΔT = 56 K, which corresponds to 19% of the Carnot efficiency (ηcarnot = 17.0%). We further confirmed that the magnitude of QNCF90 is quantitatively reproduced by the slopes of the discharge curves of NMF83 and NCF90.

Thermal Expansion in Layered Na x MO2.

Layered oxide Na x MO2 (M: transition metal) is a promising cathode material for sodium-ion secondary battery. Crystal structure of O3- and P2-type Na x MO2 with various M against temperature (T) was systematically investigated by synchrotron x-ray diffraction mainly focusing on the T-dependences of a- and c-axis lattice constants (a and c) and z coordinate (z) of oxygen. Using a hard-sphere model with minimum Madelung energy, we confirmed that c/a and z values in O3-type Na x MO2 were reproduced. We further evaluated the thermal expansion coefficients (α a and α c ) along a- and c-axis at 300 K. The anisotropy of the thermal expansion was quantitatively reproduced without adjustable parameters for O3-type Na x MO2. Deviations of z from the model for P2-type Na x MO2 are ascribed to Na vacancies characteristic to the structure.

Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy.

In order to obtain a fundamental understanding of the interplay between charge, spin, orbital and lattice degrees of freedom in magnetic materials and to predict and control their physical properties1-3, experimental techniques are required that are capable of accessing local magnetic information with atomic-scale spatial resolution. Here, we show that a combination of electron energy-loss magnetic chiral dichroism 4 and chromatic-aberration-corrected transmission electron microscopy, which reduces the focal spread of inelastically scattered electrons by orders of magnitude when compared with the use of spherical aberration correction alone, can achieve atomic-scale imaging of magnetic circular dichroism and provide element-selective orbital and spin magnetic moments atomic plane by atomic plane. This unique capability, which we demonstrate for Sr2FeMoO6, opens the door to local atomic-level studies of spin configurations in a multitude of materials that exhibit different types of magnetic coupling, thereby contributing to a detailed understanding of the physical origins of magnetic properties of materials at the highest spatial resolution.

Publisher Correction: Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy.

In Fig. 1 of the version of this Letter originally published, the word 'Subtract' was missing from the green box to the left of panel f. This has now been corrected in all versions of the Letter.

Strong localization of oxidized Co3+ state in cobalt-hexacyanoferrate.

Secondary batteries are important energy storage devices for a mobile equipment, an electric car, and a large-scale energy storage. Nevertheless, variation of the local electronic state of the battery materials in the charge (or oxidization) process are still unclear. Here, we investigated the local electronic state of cobalt-hexacyanoferrate (Na x Co[Fe(CN)6]0.9), by means of resonant inelastic X-ray scattering (RIXS) with high energy resolution (~100 meV). The L-edge RIXS is one of the most powerful spectroscopic technique with element- and valence-selectivity. We found that the local electronic state around Co2+ in the partially-charged Na1.1Co2+0.5Co3+0.5[Fe2+(CN)6]0.9 film (x = 1.1) is the same as that of the discharged Na1.6Co2+[Fe2+(CN)6]0.9 film (x = 1.6) within the energy resolution, indicating that the local electronic state around Co2+ is invariant against the partial oxidization. In addition, the local electronic state around the oxidized Co3+ is essentially the same as that of the fully-charged film Co3+[Fe2+(CN)6]0.3[Fe3+(CN)6]0.6 (x = 0.0) film. Such a strong localization of the oxidized Co3+ state is advantageous for the reversibility of the redox process, since the localization reduces extra reaction within the materials and resultant deterioration.

Invariant nature of substituted element in metal-hexacyanoferrate.

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of materials. In order to design the highly functional materials, we first have to know the local structure and electronic state around the substituted element. Here, we systematically investigated the local structure and electronic state of the host (M h) and guest (M g) transition metals in metal-hexacyanoferrate (M-HCF), Na x (M h, M g)[Fe(CN)6] y (1.40 < x < 1.60 and 0.85 < y < 0.90), by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analyses. The EXAFS and XANES analyses revealed that the local structure and electronic state around M g are essentially the same as those in the pure compound, i.e, M g-HCF. Such an invariant nature of M g in M-HCF is in sharp contrast with that in layered oxide, in which the M g valence changes so that local M g-O distance (d M-Og) approaches the M h-O distance (d M-Oh).

Local structures around the substituted elements in mixed layered oxides.

The chemical substitution of a transition metal (M) is an effective method to improve the functionality of a material, such as its electrochemical, magnetic, and dielectric properties. The substitution, however, causes local lattice distortion because the difference in the ionic radius (r) modifies the local interatomic distances. Here, we systematically investigated the local structures in the pure (x = 0.0) and mixed (x = 0.05 or 0.1) layered oxides, Na(M1-xM'x)O2 (M and M' are the majority and minority transition metals, respectively), by means of extended X-ray absorption fine structure (EXAFS) analysis. We found that the local interatomic distance (dM-O) around the minority element approaches that around the majority element to reduces the local lattice distortion. We further found that the valence of the minority Mn changes so that its ionic radius approaches that of the majority M.

In situ observation of macroscopic phase separation in cobalt hexacyanoferrate film.

Lithium-ion secondary batteries (LIBs) store electric energy via Li+ deintercalation from cathode materials. The Li+ deintercalation frequently drives a first-order phase transition of the cathode material as a result of the Li-ordering or Li-concentration effect and causes a phase separation (PS) into the Li-rich and Li-poor phases. Here, we performed an in situ microscopic investigation of the PS dynamics in thin films of cobalt hexacyanoferrate, LixCo[Fe(CN)6]0.9, against Li+ deintercalation. The thick film (d = 1.5 µm) shows a characteristic macroscopic PS of several tens of µm into the green (Li1.6Co[Fe(CN)6]0.9) and black (Li.6Co[Fe(CN)6]0.9) phases in the x range of 1.0 < x < 1.6. Reflecting the substrate strain, the thin film (d = 0.5 µm) shows no trace of the PS in the entire x region. Our observation suggests that the macroscopic PS plays a significant role in the charge/discharge dynamics of the cathode.

Na(+) diffusion kinetics in nanoporous metal-hexacyanoferrates.

Metal-hexacyanoferrates (metal-HCFs) are promising candidates for cathode materials of sodium-ion secondary batteries (SIBs). Here, we systematically investigated Na(+) diffusion constants (D) and the activation energies (Ea) of metal-HCFs against the framework size (= a/2). We found that the magnitude of D (Ea) systematically increases (decreases) with increases in a, indicating that steric hindrance plays a dominant role in Na(+) diffusion.

Carrier density effect on recombination in PTB7-based solar cell.

Organic solar cells (OSCs) are promising alternatives to the conventional inorganic solar cells due to their low-cost processing and compatibility with flexible substrates. The development of low band-gap polymer, e.g., poly-[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3]thiophenediyl]] (PTB7), increases the power conversion efficiency (PCE) in the last decade. Here, we investigated the interrelation between the instantaneous carrier density (n) per donor (D)/acceptor (A) interface area and the carrier density (ncollected) collected as photocurrent in PTB7/C70 heterojunction (HJ) device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device. We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm(-2)). We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device. Our quantitative investigation based on the HJ device demonstrates that the fast carrier escape from the D/A interface region is indispensable for high PCE, because the carrier accumulation nonlinearly accelerates the carrier recombination process.