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Temperature change switches the electrochemical Seebeck coefficient (α) of Fe2+/Fe3+via transformation of the electrochemically-active redox pair.

Fe3+ coordination in alcohol solution can be controlled by the Cl− concentration ([Cl−]). The coordination state changes from FeL6 (L: solvent molecule) to FeCl4 type via FeClnL6−n with increases in [Cl−].

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 β-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 β-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 β-diketone Lewis base additives and the under coordinated Pb2+ defects in the perovskite film, which improved the PSCs PCE and stability.

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.

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, $${d}_{\mathrm{F}\mathrm{e}-M}(x)=sxd_{\mathrm{F}\mathrm{e}-\mathrm{F}\mathrm{e}}+(1-sx)d_{\mathrm{F}\mathrm{e}-\mathrm{C}\mathrm{o}}$$dFe-M(x)=sxdFe-Fe+(1-sx)dFe-Co, 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, s value (= 4.8) indicates aggregation tendency of guest Fe.

Tin perovskite solar cells (TPSCs) as one of the most promising candidates for lead-free PSCs have large potential to be industrialized in the future. However, TPSCs have often been fabricated at an extremely low oxygen concentration to avoid oxidation from Sn2+ to Sn4+, which is not applicable to their large scale production. In this work, we construct a carbonylate antioxidant capping layer atop the perovskite surface via a solvent engineering method to prevent oxygen permeation into the perovskite layer during the fabrication process of the device to enlarge the oxygen region from 0.1 ppm to 100 ppm. Meanwhile, a pinhole-free perovskite film with a large grain size, reduced Sn4+ defects and long carrier recombination lifetime was fabricated. As a result, high-performance TPSCs could be achieved in a wide range of oxygen concentration with a power conversion efficiency (PCE) of 9.47% at 0.1 ppm oxygen and 9.03% at 100 ppm oxygen, respectively; the device was stable, maintaining over 93% of its initial PCE after operating at the maximum power point under continuous light soaking (AM 1.5G, 100 mW cm−2) for 600 hours.

We spectroscopically investigated coordination state of Fe

We fabricated eight tertiary batteries made of transition metal hexacyanoferrates and evaluated thermally-induced voltage (V cell) and discharge capacity (Q cell) per unit weight of the total active material contained in the cathode and anode. V cell and Q cell are reproduced by the values ( V cell calc and Q cell calc ) calculated from temperature (α) and charge (β) coefficients of the redox potential (V) of cathode and anode materials. Reflecting small −β of Na x Zn3[Fe(CN)6]1.967.2H2O (ZnHCF), Q cell for Na x Co[Fe(CN)6]0.873.4H2O/ZnHCF cell reaches 4.0 mAh g−1 between 20 °C and 40 °C.

We evaluated the performances of liquid thermoelectric conversion devices (LTEs) composed of nine organic solvents containing an Fe2+/Fe3+ redox pair against the electrolyte concentration (m). In protic solvents, i.e. methanol (Me), ethanol (Et), 1-propanol (Pr), and ethylene glycol (EG), the electrochemical Seebeck coefficient (α = d V/d T, where V and T are the electrode potential and temperature, respectively) increases with m in the small m region. Based on the ultraviolet—visible absorption spectroscopy, we interpreted the enhancement of α in terms of the OH− and/or H2O coordination to Fe3+, which was introduced by the crystal water of the solute. At a temperature difference (ΔT) of 30 K, the maximum value of the power factor of the Me LTE reaches 11.5 μW K−2m.

Diffusion of alkali metal ions plays an important role in the rate property of the battery. Here, we investigated macroscopic Na+ diffusion in a thin film of Na x Co[Fe(CN)6]0.90 (NCF90) by means of transmitted light intensity image at 540 nm. The distribution of the Na+ concentration (x) was determined against the elapsed time (t). We found that the diffusion constant (D) for Na+ shows x-dependence as D = 6.0 × 10−8 − (1.6 − x) 6.0 × 10−8 cm2 s−1. The average D value (=3.0 × 10−8 cm2 s−1) is much larger than the reported value (=0.5 × 10−10 cm2 s−1 M. Takachi, Y. Fukuzumi, and Y. Moritomo, Dalton Trans. 45, 458 (2016)) of Na x Co[Fe(CN)6]0.88 (x = 0.8) determined by the electrochemical impedance spectroscopy. The large D value evaluated in the present investigation is ascribed to the fast intergrain Na+ diffusion process.

We systematically investigated the local structure of Fe2+ and Fe3+ in 11 solvents by extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge structure (XANES). The EXAFS analyses with a one-shell model indicate that Fe2+ and Fe3+ are coordinated by six oxygen of the ligand molecules (L) and form a FeL 6-type complex in all solvents. The XANES suggests that the [FeL 6]3+ complex shows noncentrosymmetric deformation in several solvents. In protic solvents, the EXAFS analyses with a two-shell model revealed that the coordination number (n C) of the second nearest carbon around Fe3+ significantly depends on L. We interpreted the observation in terms of the distribution of the Fe-C distance.

Prussian blue analogues (PBA), which composed of cyano-bridged jungle-gym type framework and alkali metal ions, are promising material for electrode of batteries, because its framework is robust against electrochemical intercalation/deintercalation of alkali metal ions. In addition, the material consists of ubiquitous elements and is easily synthesized from aqueous solution. Thus, PBAs are promising electrode material for sodium ion secondary battery (SIB). We will introduce the overall features of PBAs as SIB electrode. Furthermore, PBAs are promising material for energy harvesting. We proposed a new type of battery (tertiary battery) that can be charged by the environmental heat using the difference in the thermal coefficient (α= dV/dT) of the redox potential (V) between the anode (αanode) and cathode (αcathode) materials. The tertiary battery can convert the environmental heat energy into electric energy during the thermal cycle, i.e., (i) heating to TH, (ii) discharge at TH, (iii) cooling to TL, and (iv) discharge at TL. In the (i) heating process, the V values of the anode and cathode change by αanodeΔTand αcathodeΔT, respectively. We expect a thermally induced change in the cell voltage (V cell) as large as (αcathode - αanode)ΔT. The stored electric energy can be extracted by the (ii) discharge process at TH. We fabricated several tertiary batteries made of PBA electrodes with different a and evaluated the thermal voltage (Vcell) and discharge capacity (Qcell). For example, NaxCo[Fe(CN)6]0.87/NaxNi[Fe(CN)6]0.94 tertiary battery exhibits Vcell = 24 mV and Qcell = 2.4 mAh/g per unit weight of total active material contains in the cathode and anode. We will correlate the observed Qcell values with the electrode parameters of the tertiary batteries. Y. Shimaura, T. Shibata, and Y. Moritomo, "Interrelation between discharge capacity and charge coefficient of redox potential in tertiary batteries made of transition metal hexacyanoferrate", Jpn. J. Appl. Phys. 61, 044004 (2022) I. Nagai, Y. Shimaura, T. Shibata, and Y. Moritomo, "Performance of tertiary battery made of Prussian blue analogues", Appl, Phys. Express. 14, 094004 (2021). Y. Moritomo, Y. Yoshida, D. Inoue, H. Iwaizumi, S. Kobayashi, S. Kawaguchi, and T. Shibata "Origin of the material dependence of the temperature coefficient of the redox potential in coordination polymers", J. Phys. Soc. Jpn. 90, 063801 (2021). T. Shibata, H. Iwaizumi, Y. Fukuzumi, and Y. Moritomo, "Energy harvesting thermocell with use of phase transition” Sci. Res., 10 1813 (2020) I. Takahara, T. Shibata, Y. Fukuzumi, and Y. Moritomo, " Improved thermal cyclability of tertiary battery made of Prussian blue analogues ", Chem. Select 4, 8558-8563 (2019) Y. Fukuzumi, K. Amaha, W. Kobayashi, H. Niwa, and Y. Moritomo, “Prussian blue analogues as promising thermal power generation materials”, Energy Technology, 6 (2018) 1 – 7. T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, “Thermal efficiency of a thermocell made of Prussian blue analogues”, Sci. Reps. Appl. Phys. Express. 8 (2018) 14784. T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, Thermal power generation during heat cycle near room temperature, Appl. Phys. Express. 11, 018101 (2018). M. Takachi, T. Matsuda, and Y. Moritomo, “Cobalt hexacyanoferrate as cathode material of Na+ secondary battery”, Appl. Phys. Express 6, 025802 (2013). M. Takachi, T. Matsuda, and Y. Moritomo, “Redox reaction in Prussian blue analogues with fast Na+ intercalation”, Jpn. J. Appl. Phys. 52, 092202 (2013).

Tertiary battery is charged by the environmental heat, not by the electric energy, by using the difference (Δα) in the thermal coefficient (α =dV /dT ) of redox potential (V ) between the anode and cathode materials. The thermal cyclability is not good in the prototypical Nax Co[Fe(CN)6]0.71 (NCF71)/Nax Co[Fe(CN)6]0.90 (NCF90) tertiary battery. Here, we significantly improved the thermal cyclability of the tertiary battery with using Ni‐substituted Nax Ni[Fe(CN)6]0.68 (NNF68). The Ni‐substituted NNF68/NCF90 tertiary battery shows good thermal cyclability: both the cell voltage and capacity essentially unchanged up to the 10th thermal cycles.

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.

The electrochemical Seebeck coefficient (α) of Fe2+/Fe3+ in acetone–methanol (MeOH) mixed solution was systematically investigated against the acetone molar ratio (n) and molar ratio of Fe3+ to Fe (n Fe3+). At n Fe3+ = 0.75, α monotonously increases with increase in n from 0.3 mV K−1 at n = 0.00 to 3.4 mV K−1 at n = 1.00. At n Fe3+ = 0.50 and 0.25, we found anomalous dip structures in the n−α plot at n ≈ 0.9 and 0.4, respectively. Based on a spectroscopic investigation, we interpreted the observed dip structures in terms of the transition state between the FeCl4-active and FeL 6-active regions.

The liquid thermoelectric device (LTE) exhibits a very simple structure that consists of hot/cold electrodes of identical type and a solution with a redox couple. Here, we evaluated the performances of LTE composed of acetone (or acetonitrile) solutions of Fe(ClO4)2/Fe(ClO4)3 against their concentration (m). The electrochemical Seebeck coefficients (α) of organic LTE range from 1.5 to 2.5 mV K−1 and are higher than α (=1.4 mV K−1) of the corresponding aqueous LTE. The dimensionless figure of merit (ZT) of organic LTE is ≈0.015 at maximum and is higher than the maximum ZT (=0.009) of the corresponding aqueous LTE. Thus, the organic LTE is promising as an energy harvesting device.

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 β-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 Pb

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.

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.

Layered cobalt oxides are promising cathode materials for sodium ion secondary batteries (SIBs). By combined study of the X-ray absorption spectroscopy (XAS) around the O K-edge and ab initio calculation, we investigated the electronic state of the NaxCoO2 with different oxidization state, i.e, in O3-Na0.91CoO2 (CoO2−0.91) and P2-Na0.66CoO2 (CoO2−0.66). The O K-edge spectra in the pre-edge (529–536 eV) region shows significant change with oxidization of NaxCoO2. In O3-Na0.91CoO2, the spectra shows an intense band (B band) at 531 eV. In P2-Na0.66CoO2, the spectral weight of the B band increases and a new band (A band) appears at 530 eV. These spectral changes are qualitatively reproduced by the calculated partial density of states (pDOSs) of O3-NaCoO2 and P2-Na1/2CoO2. These results indicate that the electrons are partially removed from the O 2p state with oxidization of NaxCoO2.

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.

The thermal coefficient α (= dV/dT) of the redox potential (V) is an important physical parameter for thermal energy harvesting. We systematically investigated α of six Prussian blue analogues (M-P...

Cobalt Prussian blue analogues (Na x Co[Fe(CN)6] y ; x ∼ 4 y − 3 ) whose valences are close to Co 3 + and Fe 2 + show characteristic phase transitions triggered by cooperative charge transfer (CT) from Fe 2 + to Co 3 + . Here, we extended the cooperative CT to the entire region of sodium concentration ( 0 ≤ x ≤ 4 y − 2 ). We systematically investigated (1) upper ( T c u ) and lower ( T c l ) critical temperature, (2) variation of the [ Fe 2 + (CN)6] 4 − ([ Fe 3 + (CN)6] 3 − ) concentration, and (3) spectral weight ( I ET ) due to electron transfer from Fe 2 + to Co 3 + of thin films of Na x Co[Fe(CN)6]0.82 against x. In a wide x region from 0.00 to 0.57, we observed first-order phase transition triggered by the cooperative CT. The extended CT model reproduces the x-dependent behavior of the [FeII(CN)6] 4 − ([FeIII(CN)6] 3 − ) concentration and I ET , except for the high-x region above x ∼ 0.9 .

The temperature coefficient α (= dV/dT) of the redox potential (V) in a redox system, or so-called electrochemical Seebeck coefficient, is a key parameter for thermal energy harvesting. We systemat...

The thermal coefficient α (=d V/d T) of the redox potential (V) is an important physical parameter for thermal energy harvesting. To clarify the microscopic origin of α in conjugated polymers, we compared α and the physical quantity obtained by a quantum chemistry calculation in typical polymers with small monomer molecular weight. We observed a strong correlation between α and the number (N active) of the active atom, which is defined as the atom whose variation of the Mulliken charge (Δq) at the oxidation process is beyond a threshold (Δq th). We interpreted the material dependence of α in terms of N active, because active atoms have a significant effect on the configuration entropy of the surrounding solvent molecules.

The X-ray microbeam diffraction from each particle leads to a deeper structural characterization, i.e. inter-particle structural fluctuation, of powdery compound. Here, we performed X-ray microbeam diffractions of Na x Co1−z Ni z [Fe(CN)6] y at BL40XU beamline of SPring-8. In the whole region of z, the inter-particle structural fluctuation is negligible in the as-grown powder. We found that the average ( d 012 ¯ ) of the spacing (d 012) of the (012) plane decreases with X-ray irradiation time (t). The standard deviation (σ) of d 012 steeply increases with t from 0.006 Å at t = 1 s to 0.031 Å at t = 60 s, indicating that inter-particle structural fluctuation is induced by the X-ray irradiation.

Prussian blue analogues (PBAs) are nanoporous materials that consist of a cyano-bridged three-dimensional (3D) metal framework, i.e., –M–NC–Fe–CN–M– (M = Cr, Mn, Fe, Co, Ni, Cu, and Cd), and a gues...

Manganese hexacyanoferrate (Mn-HCF) is a promising cathode material for sodium-ion secondary batteries (SIBs). We investigated the effects of partial substitution of Fe, Co, and Ni for Mn in Mn-HCF on the SIB rate and cycle performance. The discharge capacity is discernible at 50C in the substituted samples, while it disappears at 20C in the non-substituted samples. We ascribed the enhancement of the rate performance to the suppression of the Jahn-Teller distortion of Mn3+ and the resultant activation of the Mn reduction reaction.

Subpicosecond transient absorption spectroscopy is a powerful tool used to clarify the exciton and carrier dynamics within the organic solar cells (OSCs). In this review article, we introduce a method to determine the absolute numbers of the excitons and carriers against delay time (t) only from the photoinduced absorption (PIA) and electrochemically induced absorption (EIA) spectra. Application of this method to rr-P3HT-, PTB7-, and SMDPPEH-based OSCs revealed common aspects of the carrier formation dynamics. First, the temporal evolution of the numbers of the excitons and carriers indicates that the late decay component of exciton does not contribute to the carrier formation process. This is probably because the late component has not enough excess energy to separate into the electron and hole across the donor/acceptor (D/A) interface. Secondly, the spectroscopy revealed that the exciton-to-carrier conversion process is insensitive to temperature. This observation, together with the fast carrier formation time in OSCs, is consistent with the hot exciton picture.

Prussian blue analogues (PBAs) shows a cyano-bridged three-dimensional (3D) transition metal network, –M–NC–Fe–CN–M–NC–Fe–CN–M– (M = Co and Mn). Here, by in situ infrared (IR) spectroscopy, we inve...