Electroless Deposited IrOx Nanoparticles for Ni Foam Functionalization with Low Iridium Loading

Abstract

Hydrogen is potentially a reliable, affordable and sustainable energy carrier. Unfortunately hydrogen gas must be usually be generated from other compounds: 96% of Hydrogen is still extracted from fossil fuels, such as natural gas, oil and coal. The generation by electrolysis using the solar energy is a promising carbon free approach but it needs to be improved in terms of efficiency and durability to become economically appealing. A crucial factor is represented by the electrode and catalyst materials. The catalysts with highest activity and stability are noble or rare materials. In particular, IrO2 represents the best compromise for an active and stable oxygen evolution reaction catalyst. In order to minimize the amount of rare material, an affordable technique consists in depositing the catalyst on the surface of a less noble metal, such as nickel or steel. The deposition of IrO2 usually requires high temperature (200-1000°C), high purity precursors and complex experimental set up. In this paper we propose the use of electroless deposition through spontaneous galvanic displacement as a promising and simple deposition technique to produce interconnected IrO2 nanoparticles on nickel foam electrodes (figure 1). The deposition process of Ir on Ni has been studied as a function of time, pH and temperature. The formation of the iridium oxide has been achieved by thermal annealing in air. The experimental conditions have been optimized in order to improve the catalytic activity during water splitting measurements in 1M KOH, using a saturated calomel electrode (SCE) as reference and a Pt wire as counter. Scanning Electron Microscopy (SEM) analyses, coupled with X-ray Energy Dispersive Spectroscopy (EDX) and X-Rays Photoelectron Spectroscopy (XPS) show that, under proper deposition conditions, we can overcome the limits of other deposition techniques, achieving a uniform IrO2 coverage throughout the 3D structure of the Ni foam. Such a condition is crucial for the long term stability of the electrodes under constant current stress. The amount of Ir on the Ni foam has been experimentally evaluated, obtaining optimal results with 8 mg cm−2 of noble metal in a 0.16 cm thick electrode. Such a value is more than two orders of magnitude lower than typical values employed in the PEM electrolyzers, therefore making the present approach very promising. Figure 1