We investigated the effects of stoichiometry on photovoltages and photocurrents in CuBi2O4 thin-film photocathodes grown by pulsed laser deposition under different oxygen partial pressures to manipulate their stoichiometry. While the X-ray diffraction patterns show crystalline phases in the CuBi2O4 thin films, it is found that the Cu/Bi ratio of the CuBi2O4 thin films varied from ~0.3 to ~0.5 which are analyzed by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. The slightly off-stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.44 shows the highest photocurrent density in the CuBi2O4 thin films. More interestingly, the off-stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.44 exhibited a stable open-circuit voltage difference of ~0.2 V RHE without severe degradation over time. On the other hand, the photovoltage of the stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.5 gradually decreased as a function of time. Our results suggest that stoichiometry manipulation can be one of the promising strategies to achieve long-term stable Cu-based oxide photocathodes with the maintenance of a stable photovoltage.

Boosting dielectric permittivity representing electrical polarizability of dielectric materials has been considered a keystone for achieving scientific breakthroughs as well as technological advances in various multifunctional devices. Here, we demonstrate sizable enhancements of low-frequency dielectric responses in oxygen-deficient oxide ceramics through specific treatments under humid environments. Ultrahigh dielectric permittivity (~5.2 × 10 6 at 1 Hz) is achieved by hydrogenation, when Ni-substituted BaTiO 3 ceramics are exposed to high humidity. Intriguingly, thermal annealing can restore the dielectric on-state (exhibiting huge polarizability in the treated ceramics) to the initial dielectric off-state (displaying low polarizability of ~10 3 in the pristine ceramics after sintering). The conversion between these two dielectric states via the ambient environment–mediated treatments and the successive application of external stimuli allows us to realize reversible control of dielectric relaxation characteristics in oxide ceramics. Conceptually, our findings are of practical interest for applications to highly efficient dielectric-based humidity sensors.

Water electrolysis at high temperatures (> 600 ˚C) provides both kinetic (no precious metals) and thermodynamic (lower voltage requirement) benefits for hydrogen generation. However, recently steam electrolysis at intermediate temperatures (300 - 600 ˚C) has attracted attention, given potentially faster startup times, ready access to waste heat, and mitigation of some degradation modes, while maintaining some of the high temperature benefits. Intermediate-temperature electrolysis can be accomplished with protonic ceramic electrolysis cells (PCECs), for which a primary challenge is the need for efficient, chemically stable, and long-term durable oxygen/steam electrodes. Electrode material candidates tend to be perovskite-based oxides that must be triple conductors, enabling transfer of oxide ions, protons, and electronic species for PCECs. While Barium (Ba) containing perovskite (ABO3) oxygen electrodes are an active focus due to their low hydration enthalpies enabled by the basicity of Ba, their poor long-term stability and non-optimized catalytic activity hinder the practical deployment of PCECs. Additionally, segregation of large cations (e.g., Ba, Sr) often takes place on electrode surfaces, which further hinders their catalytic performance and long-term stability. Thus, we have been investigating synthesis and performance of lower-Ba or Ba-free oxygen/steam electrodes and relating these to the evolution of the surface chemistry during operation. In this work, we fabricated Pr-based perovskite oxides, including Pr(Ga,Mg)O3-x, Ba(Pr,Y)O3-x, and others, as thin films with well-defined geometries and surface areas using pulsed laser deposition and compared their performance to the standard Ba(Co,Zr,Fe,Y)O3-x. Considering the multiple valence states (3+, 4+) of Pr and its large size, which may lead to surface segregation, the inclusion of Pr, or even replacement of Ba with Pr, is hypothesized to benefit catalytic activity and stability. We examined the surface water splitting/ hydrogenation coefficients (k-values) using electrical conductivity relaxation with pH2O steps, as a function of time and gas atmosphere. Further investigations of the role of crystalline quality, composition, and thickness have been made to understand the factors controlling the thin film electrode activity and stability. Our results of quantitative k-values on well-defined surfaces can guide design of high performance steam/oxygen electrodes and understanding of degradation mechanisms impacting performance of PCECs.