Exsolution of Ni Nanoparticles from A-Site-Deficient Layered Double Perovskites for Dry Reforming of Methane and as an Anode Material for a Solid Oxide Fuel Cell.

Exsolution is a promising technique to design metal nanoparticles for electrocatalysis and renewable energy. In this work, Ni-doped perovskites, (Pr0.5Ba0.5)1-x/2Mn1-x/2Nix/2O3-δ with x = 0, 0.05, 0.1, and 0.2 (S-PBMNx), were prepared to design exsolution systems as solid oxide fuel cell anodes and for catalysis applications. X-ray diffraction and transmission electron microscopy (TEM) analyses demonstrated that correlating A-site deficiency with Ni content can effectively induce exsolution of all Ni under H2 atmosphere at T ∼ 875 °C, yielding the reduced (exsolved) R-PBMNx materials. On heating the exsolution systems in air, metal incorporation in the oxide lattice did not occur; instead, the Ni nanoparticles oxidized to NiO on the layered perovskite surface. The lowest area-specific resistance (ASR) under wet 5% H2/N2 in symmetrical cells was observed for R-PBMN0.2 anode (ASR ∼ 0.64 Ω cm2 at 850 °C) due to the highest Ni particle density in the R-PBMNx series. The best performance for dry reforming of methane (DRM) was also obtained for R-PBMN0.2, with CH4 and CO2 conversion rates at 11 and 32%, respectively, and the highest production of H2 (37%). The DRM activity of R-PBMN0.2 starts at 800 °C and is sustained for up to at least 5 h operation with little carbon deposition (0.017 g·gcat-1·h-1). These results clearly demonstrate that varying Ni-doping in layered double perovskite oxides is an effective strategy to manipulate the electrochemical performance and catalytic activity for energy conversion purposes.

Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation.

Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe(2+) species which become oxidized toward Fe(3+) during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.