Ultrahigh discharge energy density (W_dis = 10.5 J cm⁻³) and efficiency (η = 87%) have been obtained in doped BiFeO₃–BaTiO₃ ceramic multilayers by achieving an electrically rather than chemically homogeneous microstructure. Back scattered scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy mapping of (0.7 − x)BiFeO₃–0.3BaTiO₃–xNd(Zn_0.5Zr_0.5)O₃ (0.05 ≤ x ≤ 0.10) ceramics revealed a core–shell grain structure which switched from a bright to dark contrast as x increased. Compositions with x = 0.08 were at the point of cross over between these two manifestations of core–shell contrast. Dielectric measurements together with the absence of macrodomains in diffraction contrast TEM images suggested that compositions with x = 0.08 exhibited relaxor behaviour within both the core and shell regions. Impedance spectroscopy demonstrated that, despite being chemical dissimilar, the grains were electrically homogeneous and insulating with little evidence of conductive cores. Multilayers of x = 0.08 had enhanced breakdown strength, E_BDS > 700 kV cm⁻¹ and a slim hysteresis loop which resulted in large W_dis and high η which were temperature stable to <15% from 25 to 150 °C.

The ferroelectric perovskite Na_0.5Bi_0.5TiO₃, NBT, can exhibit three types of electrical behaviour, i.e. oxide-ion conduction (type I), mixed ionic–electronic conduction (type II) and insulating/dielectric (type III) based on various defect mechanisms. Here we review how to tune the electrical properties of NBT via several mechanisms, including A-site Na or Bi non-stoichiometry, isovalent substitution, and acceptor- and donor-doping. The diversity of the electrical behaviour in the NBT lattice is attributed to the high level of oxide-ion conductivity originating from highly mobile oxygen ions which can be fine-tuned to optimise or suppress ionic conduction. High oxide-ion conductivity can be obtained by manipulating the starting Na/Bi ≥1 and by acceptor-doping to make NBT a potential electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs). In contrast, the oxide-ion conduction can be partially or fully suppressed by having a starting (nominal) composition with Na/Bi <1, donor-doping, or utilising the trapping effect between oxygen vacancies and some B-site acceptor dopants. This significantly reduces the dielectric loss and makes NBT-based materials excellent candidates as high-temperature dielectrics for capacitor applications.

Green synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) shows great potential as an alternative to the Haber-Bosch process but is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. However, further improvement of the system is restricted by limited recognition of its mechanism. Herein, we demonstrate that lithium-mediated NRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we quantified the extent to which the freshly deposited active lithium lost its activity toward NRR due to a parasitic reaction between lithium and electrolyte. A high ammonia yield of 0.410 ± 0.038 μg s⁻¹ cm⁻² geo and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm⁻² geo and 10 mA cm⁻² geo, respectively, which can be attributed to fresher lithium obtained at high current density.

σ_b of NBT is optimised by acceptor-doping, which is restrained by oxygen-vacancy diffusivity limit in perovskites.

(Na_0.5Bi_0.5TiO₃)_1−x(BiAlO₃)_x (0 ≤ x ≤ 0.08) solid solutions were prepared by a solid state reaction and their electrical properties were established by ac impedance spectroscopy and electromotive force transport number measurements. Incorporation of BiAlO₃ (BA) decreases the electrical conductivity of Na_0.5Bi_0.5TiO₃ (NBT) and sequentially changes the conduction mechanism with increasing x from predominant oxide-ion conduction to mixed ionic–electronic conduction and finally to predominant electronic conduction. The suppressed oxide-ion conduction by BA incorporation significantly reduces the dielectric loss at elevated temperatures and produces excellent high-temperature dielectric materials for high BA contents. The possible reasons for the suppressed oxide-ion conduction in the NBT–BA solid solutions have been discussed and we propose that the local structure, especially trapping of oxygen vacancies by Al³⁺ on the B-site, plays a key role in oxide-ion conduction in these apparently ‘stoichiometric’ NBT-based solid-solution perovskite materials.