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We report on the first fabrication of vertically oriented niobium–zirconium oxynitride nanotube arrays and their use as an attractive and robust material for visible-light-driven water oxidation. These nanotube arrays with an average diameter of ∼120 nm and very short length ∼90 nm were synthesized via one-step anodization of Nb–Zr alloy sheet in NH4F-containing electrolytes. Ammonolysis of the nanotubes resulted in narrowing the bandgap energy from 3.23 to ∼2.67 eV. The Nb–Zr oxynitride nanotube arrays showed approximately an enhancement of about 1900% over that reported for thin film electrodes made of niobium oxynitride and 3700% greater than that recorded for nitrogen-doped mesoporous Nb2O5. Mott–Schottky and the valence band XPS analyses revealed the favorable band positions of the fabricated oxynitride nanotubes with respect to the water redox potentials with very high charge carriers density. The photocurrent transient measurements revealed the remarkable stability of the fabricated oxynitride nanotubes.

Designing highly active, durable, and nonprecious metal‐based bifunctional electrocatalysts for overall water electrolysis is of urgent scientific importance to realize the sustainable hydrogen production, which remains a grand challenge. Herein, an innovative approach is demonstrated to synthesize flower‐like 3D homogenous trimetallic Mn, Ni, Co phosphide catalysts directly on nickel foam via electrodeposition followed by plasma phosphidation. The electrochemical activity of the catalysts with varying Mn:Ni:Co ratios is assessed to identify the optimal composition, demonstrating that the equimolar trimetallic phosphide yields an outstanding HER catalytic performance with a current density of 10 mA cm−2 at an ultra‐low overpotential of ~14 mV, outperforming the best reported electrocatalysts. This is asserted by the DFT calculations, revealing strong interaction of the metals and the P atom, resulting in enhanced water activation and optimized GH* values for the HER process. Moreover, this optimal composition appreciably catalyzes the OER by exposing more intrinsic active species in‐situ formed on the catalyst surface during the OER. Therefore, the Mn1‐Ni1‐Co1‐P‐(O)/NF catalyst exhibits a decreased overpotential of ~289 mV at 10 mA cm−2. More importantly, the electrocatalyst sustains perfect durability up to 48 h at a current density of 10 mA cm−2 and continued 5000 cycling stability for both HER and OER. Meanwhile, the assembled MNC‐P/NF||MNC‐P/NF full water electrolyzer system attains an extremely low cell voltage of 1.48 V at 10 mA cm−2. Significantly, the robust stability of the overall system results in a remarkable current retention of ~96% after a continuous 50‐h run. Therefore, this study provides a facile design and a scalable construction of superb bifunctional ternary MNC‐phosphide electrocatalysts for efficient electrochemical energy production systems.

Designing efficient and stable water splitting photocatalysts is an intriguing challenge for energy conversion systems. We report on the optimal fabrication of perfectly aligned nanotubes on trimetallic Ti–Mo–Fe alloy with different compositions prepared via the combination of metallurgical control and facile electrochemical anodization in organic media. The X-ray diffraction (XRD) patterns revealed the presence of composite oxides of anatase TiO2 and magnetite Fe3O4 with better stability and crystallinity. With the optimal alloy composition Ti–(5.0 atom %) Mo–(5.0 atom %) Fe anodized for 16 h, enhanced conductivity, improved photocatalytic performance, and remarkable stability were achieved in comparison with Ti–(3.0 atom %) Mo–(1.0 atom %) Fe samples. Such optimized nanotube films attained an enhanced photocatalytic activity of ∼0.272 mA/cm2 at 0.9 VSCE, which is approximately 4 times compared to the bare TiO2 nanotubes fabricated under the same conditions (∼0.041 mA/cm2 at 0.9 VSCE). That was mainly correlated with the emergence of Mo and Fe impurities within the lattice, providing excess charge carriers. Meanwhile, the nanotubes showed outstanding stability with a longer electron lifetime. Moreover, carrier density variations, lower charge transfer resistance, and charge carriers dynamics features were demonstrated via the Mott–Schottky and electrochemical impedance analyses.

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The direct growth of sub-100 nm thin-film metal oxides has witnessed a sustained interest as a superlative approach for the fabrication of smart energy storage platforms. Herein, sub-100 nm Zr-doped orthorhombic Nb 2 O 5 nanotube films are synthesized directly on the Nb-Zr substrate and tested as negative supercapacitor electrode materials. To boost the pseudocapacitive performance of the fabricated films, supplement Nb 4+ active sites (defects) are subtly induced into the metal oxide lattice, resulting in 13% improvement in the diffusion current at 100 m V/s over that of the defect-free counterpart. The defective sub-100 nm film (H-NbZr) exhibits areal and volumetric capacitances of 6.8 mF/cm 2 and 758.3 F/cm 3 , respectively. The presence of oxygen-deficient states enhances the intrinsic conductivity of the thin film, resulting in a reduction in the band gap energy from 3.25 to 2.5 eV. The assembled supercapacitor device made of nitrogen-doped activated carbon (N-AC) and H-NbZr (N-AC//H-NbZr) is able to retain 93, 83, 78, and 66% of its first cycle capacitance after 1000, 2000, 3000, and 4500 successive charge/discharge cycles, respectively. An eminent energy record of approximately 0.77 μW h/cm 2 at a power of 0.9 mW/cm 2 is achieved at 1 mA/cm 2 with superb capability.

In the recent few decades, the demand for green sources of energy that are clean and sustainable became very essential to reduce the greenhouse and global warming problems. Consequently, there is an increasing demand to identify nonprecious, cheap bifunctional electrocatalysts for water splitting. Herein, nanosheets of different earth-abundant Ni, Co, Mn, and Fe combinations are electrodeposited over commercial Ti mesh and tested for the overall water splitting. The bare Ti mesh requires overpotentials of −486.6 mV at −10 mA cm −2 and 534.5 mV at 10 mA cm −2 for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. However, the electrodeposited catalysts show much higher catalytic activity for both HER and OER with overpotentials of −300 and 279 mV at −10 and 10 mA cm −2 , respectively, lowering the overpotential needed to drive the OER by 50%. Nevertheless, to enhance the electrocatalytic performance of the fabricated catalysts, they are phosphidized using different phosphorous precursors. The resulted NiCoMnFe−P catalysts exhibit much lower HER overpotential (−200 mV at −10 mA cm −2 ), which is 40% lower than that needed by the bare Ti mesh. For the overall water splitting, a cell voltage of 1.71 V is recorded to achieve a current density of 10 mA cm −2 . Lastly, the stability test of the overall device reveals very high stability with current retention of 90% over 22 h of continuous electrolysis. Furthermore, the synergy between the metallic components in the absence and presence of P is elucidated using density functional theory calculations, revealing optimized G H* and G Hd 2 O* for the HER reaction over the P-top site of the MnFeCoNiP catalyst. In addition, the calculations explain the superiority of the NiCoMnFe catalyst over the NiCoMnFeP counterpart for the OER.

Point defects play important and crucial roles in the design of high performance photocatalysts. We report on the electrochemical fabrication of black TiÀ Mo-NiÀ O nanotubes as a promising electrode material for solar-assisted water splitting. The ternary TiÀ Mo-NiÀ O catalyst was annealed in hydrogen atmosphere to induce point defects in the material to enhance its conductivity, charge carriers density, and performance. The effect of annealing duration on the performance of ternary TiÀ Mo-NiÀ O nanotube films was investigated. The hydrogenannealed nanotubes showed enhanced optical characteristics in the visible spectrum, which can be related to the formation of defect states upon hydrogen annealing. The 10 h-annealed sample showed an exceptionally enhanced photocurrent density of ∼ 10 mA/cm 2 with a remarkable open-circuit voltage of ∼ À 1.0 V Ag/AgCl under AM 1.5G illumination. This improved photocurrent is in agreement with the obtained 75 % incidentphoton-to-current-conversion-efficiency (IPCE), confirming the improved photoactivity of the hydrogen-treated mixed oxide nanotubes.