Structurally stabilized Na₃V₂(PO₄)₃/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na₃V₂(PO₄)₃ structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na₃V₂(PO₄)₃ cathode materials for Na-ion batteries.

The effects of <mml:math display="inline"><mml:mrow><mml:msup><mml:mi>Cl</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math>Cl− on the electronic properties of the n-type passive film on Fe and the p-type passive film on Ni in pH 8.5 buffer solution were investigated using photocurrent spectroscopy and Mott–Schottky analysis in order to elucidate the mechanism of passivity breakdown on metals. Metastable pitting events were observed in the potentiostatic current transients (chronoamperograms) for Fe measured at <mml:math display="inline"><mml:mrow><mml:mn>0.7</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:msub><mml:mi mathvariant="normal">V</mml:mi><mml:mi>SCE</mml:mi></mml:msub></mml:mrow></mml:math>0.7VSCE when 0.1 M of NaCl was added to the solution, demonstrating that <mml:math display="inline"><mml:mrow><mml:msup><mml:mi>Cl</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math>Cl− induces passivity breakdown. However, the Mott–Schottky analysis revealed chloride ion, present at a concentration of 0.10 M, did not change the concentration of oxygen vacancies in the passive film on Fe, indicating that the oxygen vacancy is not responsible for the passivity breakdown. It was observed by Mott–Schottky analysis that as the concentration of <mml:math display="inline"><mml:mrow><mml:msup><mml:mi>Cl</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math>Cl− in the solution increased, the concentration of cation vacancy in the passive film on Ni also increased, which is in concert with the view that metal vacancies are responsible for the breakdown of passivity on nickel. Finally, by using electrochemical impedance spectroscopy to interrogate the point defect generation and annihilation reactions that occur at the metal/film and film/solution interfaces, we show that the observed increase in cation vacancy concentration in the passive film on Ni is due to chloride-catalyzed ejection of cations from the film/solution interface. These findings are inconsistent with chloride-catalyzed film dissolution and chloride penetration mechanisms for passivity breakdown, but they are entirely consistent with cation vacancy generation at the barrier layer/solution interface and subsequent cation vacancy condensation at the metal/barrier layer interface, as postulated in the point defect model.

A high-performance bifunctional Co–P foam catalyst was successfully synthesized by facile one-step electrodeposition at a high cathodic current density. The synthetic approach includes fast generation of hydrogen bubbles as well as fast deposition of Co–P, which played a key role in forming a porous Co–P foam structure. The Co–P foam exhibits remarkable electrocatalytic activity and stability in both acidic and alkaline solution. Its HER activity was recorded with an overpotential of 50 mV in 0.5 M H₂SO₄ and 131 mV in 1 M KOH at 10 mA cm⁻², which is comparable to that of commercial Pt/C (η@10 mA cm⁻² 0.5 M H₂SO₄ : 33 mV, η_{10 mA, 1 M KOH}: 80 mV). The Co–P foam (η@10 mA cm⁻²: 300 mV) exhibits better OER activities than Ir/C (η@10 mA cm⁻²: 345 mV) and RuO₂ (η@10 mA cm⁻²: 359 mV) in 1 M KOH solution. The excellent performance of the Co–P foam as an HER and OER catalyst can be attributed to the charge separation between Co and P in Co–P foam as well as the porous foam structure providing a large electrochemically active surface area (ECSA). The ECSA of the Co–P foam was calculated to be 118 cm², which was 2.4 times higher than that of a Co–P film (49 cm²).

Recently, major efforts have been devoted to exploring cheap and active non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells for large-scale applications. Herein, we report electrospun cobalt-carbon nanofibers (Co-CNFs) as an efficient catalyst for the ORR, together with a systematic study of the active site formation. The ORR activity of the Co-CNFs increases with increasing Co content up to approximately 30 wt%, at which high ORR activity is exhibited, comparable with a commercial Pt/C catalyst in alkaline media. XPS and structural analysis reveals a Co–pyridinic N_x bond at the edge plane, and more Co nanoparticles were found in the Co-CNFs as the Co content was increased. These sites can behave as the primary and the secondary active sites for the ORR, according to a dual-site mechanism. The ORR activity of the Co-CNFs may deteriorate even if only one of these sites is limited. The high ORR activity of the Co-CNF catalysts results from the synergetic effect of dual site formation for the ORR.

Black TiO_2−x nanotube arrays, which are synthesized by an electrochemical method and subsequent thermal conversion in a hydrogen atmosphere, are employed as binder-free, free-standing electrodes for high rate Li-ion microbatteries.