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Ecological demands at a global level reinforce the need to use clean and renewable energy sources, such as hydrogen (H 2), which is a promising energy source obtained mainly through water electrolysis. Although this process is environmentally friendly, it requires high energy value and electrocatalysts are needed to become viable. Thus, efforts have been made to develop electrocatalysts based on Earth-abundant transition metals such as nickel, cobalt, and iron, with high performance. In this way, this work reports a green synthetic route using aqueous (H) and ethanolic (E) extract of the wild sage plant (Ipomoea asarifolia) as a stabilizing agent to obtain nickel oxide (NiO) nanoparticles at different calcination temperatures (300 °C, 500 °C and 700 °C). The materials were characterized by X-ray diffraction (XRD), vibrational spectroscopy (FTIR and Raman), solid-state UV-VIS spectroscopy, and scanning electron microscopy (SEM-EDS). The techniques confirmed the NiO cubic structure, with residual organic groups on the surface of the materials. Raman spectroscopy signals also confirmed the presence of roughness and structural defects, which contribute positively to the electrocatalytic process. Synthesis using aqueous extracts led to the formation of materials with sphere-like morphology and uniform size, while ethanolic extracts resulted in nonuniform polyhedral particles. The electrocatalytic performances in alkaline solution (KOH, 1.0 mol/L) were investigated. The experimental overpotentials at J = 10 mAcm−2 were 307 mV, 321 mV, and 312 mV and the Tafel slope was 60.35 mVdec−1, 50.29 mVdec−1 and 53.36 mVdec−1 for the NiO300H, NiO500H, and NiO700H samples, respectively; and 316 mV, 324 mV, and 318 mV and Tafel slope of 50.44 mVdec−1, 51.58 mVdec−1 and 45.65 mVdec−1 for the NiO300E, NiO500E and NiO700E samples, respectively. Experiments in acidic and neutral media showed moderate catalytic activities compared to the literature. Therefore, this green synthesis route proves to be economically viable, producing high-performance NiO electrocatalysts, compared to similar results in the literature. [Display omitted] • Six NiO nanoparticles were obtained from extracts of I. asarifolia, applied in the H2 production. • The influence of the solvent on the morphology and OER electrocatalytic activity was studied. • NiO obtained at low calcination temperature (300 °C) showed higher electrocatalytic activity. [ABSTRACT FROM AUTHOR]

Understanding the processes of different cathode materials is part of the journey toward the development of intermediate-temperature solid oxide fuel cells. In this work, the one-dimensional Ca 3 Co 2 O 6 (C326) electrode and its composite with Ce 0.9 Gd 0.1 O 2- δ (CGO) are systematically investigated as a function of sintering temperature using a symmetrical cell test. For both batches of samples, the optimum sintering temperature is 1000 °C. Interpretation of the impedance spectra reveals that this optimization is associated with improvements in the processes of surface diffusion and interfacial charge transfer due to the higher effective reaction area for ionic transfer at the electrode/electrolyte interface and better connectivity between the electrode grains. The temperature dependence of the individual processes indicates that the addition of CGO significantly improves the steps of dissociative oxygen adsorption and diffusion, which should occur first on its surface, with the charge transfer of oxygen species at the CGO/C326 boundary being the rate-limiting step. • Single-phase Ca 3 Co 2 O 6 (C326) and composite C326/CGO electrodes are proposed for SOFCs. • The lowest polarization resistance was optimized at the sintering temperature of 1000 °C. • The oxygen reduction reaction mechanism of both electrodes is outlined. • C326-based cathodes are limited by oxygen surface diffusion and charge transfer. • CGO phase accelerates the dissociative adsorption of O 2 and subsequent diffusion. [ABSTRACT FROM AUTHOR]

The development of highly efficient catalysts for oxygen (OER) and hydrogen (HER) evolution reactions are fundamental for the generation of new sources of energy. To meet the global demand of energy, non-noble metal catalysts based on transition metals, in particular, Nickel (Ni) and Copper (Cu) metals are being widely explored and studied as promising candidates for electrocatalysts. Therefore, here we present: (a) the Sol-gel synthesis that uses chitosan as a polymerizing agent and subsequent reduction in H 2 gas to synthesize carbon-coated NiCu nanoparticles, and (b) the microstructural and electrochemical evaluation of Ni 95 Cu 5 -C and Ni 85 Cu 15 -C samples. X-ray diffraction confirms the NiCu phase, while Raman spectroscopy confirms the presence of carbon. Field emission scanning electron microscopy shows nanoparticles with spherical morphology. From the electrochemical point of view, it was observed that the increase in copper content impairs the activity for oxygen evolution reaction when evaluated in 1.0 M KOH electrolyte. The Ni 95 Cu 5 -C electrocatalyst exhibited higher activity than Ni 85 Cu 15 -C, confirmed by its low overpotential value (η = 319 mV vs. RHE) and Tafel slope (73 mV dec−1), and high values of C DL (4.1 mF cm−2), ECSA (102.5 cm2), SA (2.19 mA cm−2) and TOF (1.87 x10−4 mol O 2 s−1). This work highlights a simple and inexpensive method to obtain Nickel-Copper-Carbon catalysts for application in water splitting in alkaline media. [Display omitted] • Nickel-Copper-Carbon was prepared by sol-gel methodology with subsequent reduction in H 2. • Commercial chitosan biopolymer was used as polymerizing agent. • Electrochemical performance of samples is highly dependent on Nickel content. • Ni 95 Cu 5 -C showed excellent OER performance, with an overpotential of 319 mV vs. RHE at J = 10 mA cm−2. [ABSTRACT FROM AUTHOR]

The potential of the [BaCoO 3 ] n [BaCo 8 O 11 ] family as a cathode for solid oxide fuel cells (SOFCs) or as an anode for solid oxide electrolyser cells (SOECs) is investigated via structural, microstructural and electrochemical characterisation. The crystallographic structure of the n = 1 member compound, Ba 2 Co 9 O 14 (BCO), exhibits rhombohedral symmetry and presents a microstructure consisting of large platelets. Overall, the electrochemical performance of the Ba 2 Co 9 O 14 /Ce 0.8 Gd 0.2 O 2- δ (BCO/CGO-20) composite electrode is found to be enhanced under cathodic polarisation, while becoming impaired under anodic polarisation. The latter behaviour may result from the high local oxygen partial pressures upon increasing the applied anodic polarisation that lead to a depletion of oxygen vacancies at the electrode/electrolyte interface, thus, decreasing the ionic conductivity as well as electrocatalytic activity of this interface. This work, therefore, provides the first electrochemical analysis of the performance of BCO-based electrodes under applied polarisation conditions for SOFC and SOEC applications, and highlights the higher potential of this compound as a cathode material for intermediate-temperature solid oxide fuel cells. • The n = 1 member of the [BaCoO 3 ] n [BaCo 8 O 11 ] family is studied as an oxygen electrode for SOFC/SOEC. • Impedance spectroscopy as a function of the temperature, p O2 and applied potential. • BCO-CGO performance is improved under cathodic polarisation and impaired under anodic polarisation. [ABSTRACT FROM AUTHOR]