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The electrochemical promotion of ammonia formation on Fe-based electrode catalysts is investigated using proton-conducting-electrolyte-supported cells of H2–Ar, Pt|BaCe0.9Y0.1O3 (BCY)| Fe-based catalysts, H2–N2 at temperatures between 550 °C and 600 °C, and ambient pressure. To clarify the reaction mechanism, the ammonia formation rate is examined using two cathodes: (I) a porous pure Fe electrode with a shorter triple phase boundary (TPB) length and (II) a cermet electrode consisting of Fe–BCY (or W–Fe–BCY) with a longer TPB length. Using the different electrode structures, we investigate the effects of cathodic polarization, hydrogen partial pressure, and electrode materials. The porous pure Fe electrode shows better performance than the Fe–BCY cermet electrode, which suggests that the ammonia formation is accelerated by the electrochemical promotion of catalysis (EPOC) effect on the Fe surface rather than the charge-transfer reaction at the TPB. The electrochemical promotion is governed by a dissociative mechanism, i.e., acceleration of direct N2 bond dissociation with cathodic polarization on the Fe surface, with a smaller contribution by a proton-assisted associative mechanism at the TPB. These findings indicate that the porous pure Fe electrode is more effective for ammonia formation than the (W–)Fe–BCY cermet electrode. Despite the relatively short TPB length, the porous pure Fe cathode achieves a very high ammonia formation rate of 1.4 × 10−8 mol cm−2 s−1 (450 μg h−1 mg−1) under appropriate conditions. This significant result suggests that the effective double layer spreads widely on the Fe electrode surface. Using the identified reaction mechanism, we discuss key processes for improving ammonia formation.

The performance of an anode-supported proton-conducting solid oxide fuel cell (p-SOFC) was investigated by controlling transport properties of thin electrolyte membranes. Hole conductivity in electrolyte, which induces to degrade the cell performance, was sufficiently suppressed by using lanthanum tungstate La28-xW4+xO54+3x/2v2-3x/2 (LWO) with high proton conductivity and low hole conductivity. SrFe0.95Nb0.05O3-δ (SFN) and Ni-Ce0.9Gd0.1O2-δ (Ni-GDC10) cermet were used as the cathode and anode, respectively. GDC10 was also used as an interlayer to prevent chemical reactions between the electrolyte and electrodes. SEM images and electrochemical impedance spectroscopy showed that the boundaries between the electrolyte and electrodes were stable because GDC10 blocked chemical reactions and cation diffusion at the electrolyte-electrode interfaces. Fuel cell tests were also performed with SFN/GDC10/LWO/GDC10/Ni-GDC anode-supported single cells. The thickness of the LWO thin electrolyte was approximately 3 μm. The OCV values were measured to be 0.88 V and 0.94 V at 800 °C and 600 °C, respectively. Moreover, the evaluation of the leakage current indicated that thin LWO could achieve the low leakage current due to transport properties such as high proton conductivity and low hole and electron conductivities.

Highly efficient carriers are developed for hydrogen production by methane cracking with carbon-dioxide activation in a chemical looping system. The carrier materials comprise BaZr0.9Y0.1O3−δ (BZY) with a high proton conductivity and Fe as a base metal, and they are prepared by doping Fe into BZY. Thermal gravimetric analysis (TGA) showed that Fe-doped BZY (BaZr0.9−xFexY0.1O3−δ, BZFY) with x = 0.4 (BZFY40) exhibited a high activity for carbon deposition with methane thermal decomposition. Surface observation and kinetic analysis suggested that the high activity for carbon deposition results from the promotion of tubular carbon growth with growth-mode transition from horizontal to vertical direction for the tubular carbon on reduced BZFY40. The rapid growth of carbon tubes proceeded with the base-growth mechanism, which can provide an advantage in the stable cycle reactions. The carbon removal reaction with carbon dioxide was also investigated. In BZFY, the removal reaction was completed in ~10 min. BZFY reacted with carbon dioxide under carbon dioxide flow to generate barium carbonate. From the BZFY surface observation before and after the reactions, the BZFY plate that was mechanically broken during the carbon deposition and removal cycles could be repaired by filling cracks with carbonate in the BZFY that was generated via the reduction reaction with methane. Therefore, carbon removal from the BZFY surface with carbon dioxide contributes to maintaining the BZFY particle shape during cycles with the self-healing scheme. The BZFY40 carrier has a high activity and durability for the carbon deposition and removal cycle and provides an effective system design for carbon dioxide-free hydrogen generation.

Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization., Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.

A single crystal of Fe (0.3%)-doped BaTiO3 was grown by a top-seeded solution growth method, and the photovoltaic (PV) properties (at 3.1 eV) in a multi-domain state with a 90° domain structure are investigated. We show that the overall behavior can be well understood by an analytical expression of the domain wall (DW)-PV effect superimposed on the bulk-PV effect. The fitting of photocurrents as a function of light polarization reveals that the offset component of the DW-PV effect is much larger than that of the bulk-PV effect, even though the volume fraction of the DW region is extremely small. A local electric field in the DW region is estimated to be several hundred kV cm−1, which is three orders of magnitude higher than the effective field inside the domains.

Domain structures in polar materials provide an additional degree of freedom to tune ferroelectric photovoltaic (PV) effects. One of the approaches to control domain structures is to form a solid solution with analogs in different symmetries. In this study, we investigate the influence of domain structures on the PV properties of ferroelectric thin films in the xBaTiO3–(1−x)BiFeO3 (BT–BFO) solid-solution system (x = 0.1 and 0.2). We found that a substitution of BT for BFO substantially decreases the domain size down to several tens of nanometers, leading to an enhanced PV response owing to a marked contribution of the domain-wall PV effect.

We have investigated the local polarization switching behaviors of epitaxial (Bi1/2Na1/2)TiO3 (BNT) thin films obtained by a pulsed laser deposition (PLD). Using ozone gas as a deposition atmosphere of PLD, epitaxial growth of BNT films was achieved on SrRuO3/SrTiO3(100) electrode substrates. Piezoresponse force microscopy (PFM) revealed that BNT grains in the films have a single-domain structure without domain walls. These domain structures and the local polarization switching measured by PFM indicate that the oxidizing atmosphere of ozone gas is considered to suppress the generation of bismuth and oxygen vacancies in the deposited BNT layers. We propose that the PLD method using ozone is effective in obtaining high-quality single-phase BNT films with a less concentration of lattice vacancies.

SnNb2O6 is one of the promising photoanode materials for photoelectrochemical water splitting. We prepared the SnNb2O6 thin-film photoanode with various Sn/Nb ratios by a pulsed-laser deposition method and investigated impacts of lattice defects in the SnNb2O6 photoanode on the water oxidation property. Photoelectrochemical measurements and Raman spectroscopy analyses for the SnNb2O6 photoanodes presented that the water oxidation property can be improved by suppressing the formation of the acceptor site caused by the substitution of Sn4+ for the Nb5+ site in the crystal lattice of SnNb2O6. This study provides material’s design strategy based on defect chemistry to realize an efficient SnNb2O6 photoanode used for the photoelectrochemical cell, which can operate without an external bias.

We investigate the photovoltaic (PV) effects in ferroelectrics based on the polar structure in domains and the intrinsic symmetry breaking of ferroelastic domain walls (DWs). A comprehensive analysis for $\mathrm{BiFe}{\mathrm{O}}_{3}$ films with the single-domain and 71\ifmmode^\circ\else\textdegree\fi{} domain structures reveals a major contribution of the DWs superimposed on the bulk PV effect, which can explain the enhanced PV response in the multidomain film exhibiting photovoltages greater than the band gap. Our methodology can reveal the contributions of the bulk PV and the DW-PV effects quantitatively and uncover a design strategy for enhancing ferroelectric photovoltaics.

We investigate ferroelectric BaTiO3 capacitors with SrRuO3-based electrodes employing a (BaxSr1−x)TiO3 (BST) buffer layer on (100) SrTiO3 substrates prepared by pulsed laser deposition. Structural analysis by high-resolution X-ray diffraction reciprocal space mapping shows that the BST (x = 0.5 and 0.7) layers are relaxed and have their bulk in-plane lattices in the upper part owing to strain relief. The bottom electrodes on the buffer layers grow heteroepitaxially but cannot withstand a tensile stress and then show a diminution of the in-plane lattice. On the BST (x = 0.7) layer, compared with the capacitor with an SrRuO3 electrode, that with a Ba0.1Sr0.9RuO3 one has a reduced in-plane lattice relaxation of the BaTiO3 film and then exhibits a larger remanent polarization (Pr) of 34 μC/cm2 associated with a smaller shift of the hysteresis loop. The polarization hysteresis shift is attributed to a flexoelectric effect stemming from the coupling between out-of-plane polarization and a strain gradient in the BaTiO3 film. We conclude that a reduced misfit strain relaxation in the ferroelectric film achieved with the Ba0.1Sr0.9RuO3 electrode on the thicker BST (x = 0.7) buffer layer is the origin of an enhanced Pr with a smaller hysteresis shift.

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1− x Sr x )TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

Electrical conduction properties of SrRuO 3 (SRO)/BiFeO 3 (BFO)/SRO and SRO/10% Mn-doped BFO(BFMO)/SRO ferroelectric thin-film capacitors are investigated. The BFO capacitors exhibit a switchable diode effect accompanied by a conduction change from ohmic to space-charge-limited current with increasing external field. In contrast, the BFMO capacitors show only an ohmic conduction, arising from a considerable reduction in depletion layer width at the SRO/BFMO interfaces. These results suggest that the diode property can be tuned by Mn content in the BFO film. Our study opens the possibility of controlling the diode effect in BFO-based devices by a dilute Mn doping.