Preparing high-performance oxygen evolution reaction (OER) catalysts with low precious metal loadings for water electrolysis applications (e.g., for green hydrogen production) is challenging and requires electrically conductive, high-surface-area, and stable support materials. Combining the properties of stable TiO₂ with those of active iridium oxide, we synthesized highly active electrodes for OER in acidic media. TiO₂ powders (both commercially available Degussa P-25^® and hydrothermally prepared in the laboratory from TiOSO₄, either as received/prepared or following ammonolysis to be converted to titania black), were decorated with IrO₂ by UV photodeposition from Ir(III) aqueous solutions of varied methanol scavenger concentrations. TEM, EDS, FESEM, XPS, and XRD measurements demonstrate that the optimized version of the photodeposition preparation method (i.e., with no added methanol) leads to direct deposition of well-dispersed IrO₂ nanoparticles. The electroactive surface area and electrocatalytic performance towards OER of these catalysts have been evaluated by cyclic voltammetry (CV), Linear Sweep Voltammetry (LSV), and Electrochemical Impedance Spectroscopy (EIS) in 0.1 M HClO₄ solutions. All TiO₂-based catalysts exhibited better mass-specific (as well as intrinsic) OER activity than commercial unsupported IrO₂, with the best of them (IrO₂ on Degussa P-25^® ΤiO₂ and laboratory-made TiO₂ black) showing 100 mAmg_Ir⁻¹ at an overpotential of η = 243 mV. Chronoamperometry (CA) experiments also proved good medium-term stability of the optimum IrO₂/TiO₂ electrodes during OER.

Transparent supercapacitors find a large number of applications as components of many electronic devices and circuits. Mixed Ni–Co oxides (NCOs) are among the most promising supercapacitor electrode materials exhibiting high pseudo-capacitance and good electronic conductivity, while inkjet printing is a low cost and versatile technique for electrode printing. Surprisingly, although there have been many studies of NCO supercapacitor films on ITO glass substrates, these have not been prepared by the inkjet technique, and their optical properties were not fully characterized. Hereby, we report the fabrication and characterization of thin (295 and 477 nm thick; 0.017 and 0.035 mg cm⁻² NCO loading) semi-transparent NiCo₂O₄/ITO supercapacitor electrodes, showing transparency to visible light (60–30%, from the thinner to the thicker electrode layers tested), typical mass specific capacitance for NCO-based supercapacitor electrodes (1294–829 Fg⁻¹ at 1 mA cm⁻² discharge current density) and high volumetric capacitance (746–608 F cm⁻³ at 1 mA cm⁻²). The NCO nanoparticles were prepared by hydrothermal synthesis followed by thermal treatment and ball milling (ZrO₂ balls, 0.5 mm diameter), resulting in a cubic nickel–cobalt oxide structure and particle size in the 30–150 nm range, whereas the electrode layers were printed from water-propylene glycol solutions using a Dimatix DMP-2850 drop-on-demand (DoD) inkjet printer. Constant current charge–discharge experiments of the supercapacitor electrode (at ca 0.5 mA cm⁻²) for 1000 cycles confirmed stability of performance.

This study explores the fabrication and electrochemical performance of flexible NiCo₂O₄-based pseudo-capacitor electrodes, inkjet-printed onto flexible Kapton substrates. To circumvent the insulating nature of Kapton, a thin Au interlayer (20 nm) was introduced, significantly enhancing electrical conductivity. The effect of NiCo₂O₄ mass loading, ranging from 0.1 to 0.5 mg cm⁻², was investigated. Optimal performance was achieved at a loading of 0.3 mg cm⁻² on Au/Kapton substrates, yielding a specific capacitance of 520 F g⁻¹ at 3.3 A g⁻¹ and 90% capacitance retention after 1000 charge–discharge cycles. These results confirm that inkjet-printed NiCo₂O₄ electrodes, when combined with a conductive interlayer, exhibit excellent pseudo-capacitive behavior on flexible, non-conductive substrates. This approach demonstrates the feasibility of scalable, low-temperature fabrication techniques for high-performance flexible energy storage devices, suitable for emerging wearable technologies.