Facile synthesis of nanostructured perovskites by precursor accumulation on nanocarbons.

Perovskite-type oxides have impacted various research fields, including materials and energy science. Despite their vast potential in various applications, general and simple synthesis methods for nano-perovskites remain limited. Herein, various nano-perovskites were synthesized by a facile approach involving the use of nanocarbons. The calcination of the nanocarbon deposited with metal salts yielded nano-perovskites, emulating the morphology of nanocarbons. The accumulation of precursors (i.e., metal salts) on the surface of the nanocarbon during the evaporation of the solvent is the key step in which the precursors are homogeneously mixed prior to calcination. The homogeneity of the precursors facilitated low-temperature calcination that resulted in the formation of nano-perovskites. Various nano-perovskites, including LaMnO3, LaCoO3, LaFeO3, LaNiO3, LaAlO3, LaGaO3, CaMnO3, BaMnO3, SrMnO3, La0.7Sr0.3FeO3, La2CuO4, and Ca2Fe2O5, were successfully synthesized, demonstrating the simplicity and novelty of the method for the general synthesis of nano-perovskites.

One-Step Synthesis of Highly Active NiFe Electrocatalysts for the Oxygen Evolution Reaction.

Electrochemical water splitting is a key technology for the conversion of renewable energy into chemical resources such as hydrogen. However, the oxygen evolution reaction (OER), a half-reaction of water splitting, is so slow that various effective catalysts for the OER have been explored. In this study, we demonstrate a simple and direct process for the synthesis of OER-active NiFe catalysts over electrodes. A NiFe/C catalyst layer was formed on a glassy carbon electrode by simply dropping the catalyst ink containing only metal nitrates and carbon black. The catalyst layer exhibited higher OER performance than the state-of-the-art Ir/C catalyst. The presence of carbon black is essential to enhance the OER activity of NiFe because carbon black helps to disperse the NiFe active sites. Cyclic voltammetry indicated that Ni and Fe are adjacent to each other on the surface of carbon black, resulting in significantly higher activity of NiFe/C compared to those of Ni/C and Fe/C. The effects of the Ni/Fe ratio, amount of carbon black, and type of carbon black on the OER activity of NiFe/C were examined in detail. Furthermore, we discuss the factors that determine the OER performance of NiFe/C.

Precursor accumulation on nanocarbons for the synthesis of LaCoO3 nanoparticles as electrocatalysts for oxygen evolution reaction.

Oxygen evolution reaction (OER) is a key step in energy storage devices. Lanthanum cobaltite (LaCoO3) perovskite is an active catalyst for OER in alkaline solutions, and it is expected to be a low-cost alternative to the state-of-the-art catalysts (IrO2 and RuO2) because transition metals are abundant and inexpensive. For efficient catalysis with LaCoO3, nanosized LaCoO3 with a high surface area is desirable for increasing the number of catalytically active sites. In this study, we developed a novel synthetic route for LaCoO3 nanoparticles by accumulating the precursor molecules over nanocarbons. This precursor accumulation (PA) method for LaCoO3 nanoparticle synthesis is simple and involves the following steps: (1) a commercially available carbon powder is soaked in a solution of the nitrate salts of lanthanum and cobalt and (2) the sample is dried and calcined in air. The LaCoO3 nanoparticles prepared by the PA method have a high specific surface area (12 m2 g-1), comparable to that of conventional LaCoO3 nanoparticles. The morphology of the LaCoO3 nanoparticles is affected by the nanocarbon type, and LaCoO3 nanoparticles with diameters of less than 100 nm were obtained when carbon black (Ketjen black) was used as the support. Further, the sulfur impurities in nanocarbons significantly influence the formation of the perovskite structure. The prepared LaCoO3 nanoparticles show excellent OER activity owing to their high surface area and perovskite structure. The Tafel slope of these LaCoO3 nanoparticles is as low as that of the previously reported active LaCoO3 catalyst. The results strongly suggest that the PA method provides nanosized LaCoO3 without requiring the precise control of chemical reactions, harsh conditions, and/or special apparatus, indicating that it is promising for producing active OER catalysts at a large scale.

Coating of Silica Nanolayers on Carbon Nanofibers via the Precursor Accumulation Method.

Surface modification of nanocarbons, for example, by coating with oxide nanolayers, is a research topic of significant interest because of the drastic changes in the physicochemical properties of the modified nanocarbons. One simple method of creating these oxide nanolayer coatings on nanocarbons is the precursor accumulation (PA) technique, which entails the following: (1) a precursor solution is added dropwise onto nanocarbon powder; (2) the solvent is dried, leaving the accumulated precursor on the nanocarbon surface; and (3) hydrolysis or decomposition of the precursor in air leads to the formation of oxide nanolayers on the nanocarbons. In this study, tetraethoxysilane (TEOS) was used as a precursor for coating silica nanolayers onto carbon nanofibers (CNFs). TEOS is so stable that it hardly undergoes hydrolysis on the surface of pristine CNFs. By treating CNFs with H2SO4/HNO3, acidic functional groups were introduced onto the CNF surfaces. Silica nanolayers were successfully synthesized on these acid-treated CNFs via PA coating because the acidic functional groups catalyzed the hydrolysis of TEOS accumulated on the CNF surfaces. Scanning transmission electron microscopy indicated that the thickness of silica layer is approximately several nanometers. Pore size distribution analysis for the silica nanolayer suggested the presence of nanopores with 3-5 nm. The TEOS molecules could have accessed the functional groups through the nanopore; therefore, the number of silica nanolayers formed increased with the number of PA coatings. Finally, we compared the PA coating with conventional sol-gel and atomic layer deposition techniques.

Synthesis and investigation of sulfonated poly(p-phenylene)-based ionomers with precisely controlled ion exchange capacity for use as polymer electrolyte membranes.

To achieve precise control of sulfonated polymer structures, a series of poly(p-phenylene)-based ionomers with well-controlled ion exchange capacities (IECs) were synthesised via a three-step technique: (1) preceding sulfonation of the monomer with a protecting group, (2) nickel(0) catalysed coupling polymerisation, and (3) cleavage of the protecting group of the polymers. 2,2-Dimethylpropyl-4-[4-(2,5-dichlorobenzoyl)phenoxy]benzenesulfonate (NS-DPBP) was synthesised as the preceding sulfonated monomer by treatment with chlorosulfuric acid and neopentyl alcohol. NS-DPBP was readily soluble in various organic solvents and stable during the nickel(0) catalysed coupling reaction. Sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (S-PPBP) homopolymer and seven types of random copolymers (S-PPBP-co-PPBP) with different IECs were obtained by varying the stoichiometry of NS-DPBP. The IECs and weight average molecular weights (M ws) of ionomers were in the range of 0.41-2.84 meq. g-1 and 143 000-465 000 g mol-1, respectively. The water uptake, proton conductivities, and water diffusion properties of ionomers exhibited a strong IEC dependence. Upon increasing the IEC of S-PPBP-co-PPBPs from 0.86 to 2.40 meq. g-1, the conductivities increased from 6.9 × 10-6 S cm-1 to 1.8 × 10-1 S cm-1 at 90% RH. S-PPBP and S-PPBP-co-PPBP (4 : 1) with IEC values >2.40 meq. g-1 exhibited fast water diffusion (1.6 × 10-11 to 8.0 × 10-10 m2 s-1), and were comparable to commercial perfluorosulfuric acid polymers. When fully hydrated, the maximum power density and the limiting current density of membrane electrode assemblies (MEAs) prepared with S-PPBP-co-PPBP (4 : 1) were 712 mW cm-2 and 1840 mA cm-2, respectively.