NiFe2O4 hollow spheres electrocatalysts for anion exchange membrane electrolysers

Among various energy systems that work through electrochemical reactions, electrochemical water splitting is today one of the most efficient and reliable methods. It is one of the greenest ways to produce pure hydrogen without any greenhouse gases which makes it a sustainable, pollution-free, eco-friendly method when powered by renewable sources such as wind or solar. Among the different electrolysis types, lowtemperature systems (< 100 °C) are more promising since they allow flexibility, high current density, and longterm durability, depending on the employed technology. Liquid alkaline electrolysis is the most developed and mature technology; the advantages are related to the cost since the high pH allows the use of platinum group metal (PGM)-free catalysts whereas the disadvantages include a low current density due to the more considerable physical distance between electrodes, and maintenance costs associated with the caustic electrolyte. For proton exchange membrane (PEM)-based systems, the electrodes are physically in contact with a polymeric membrane to form a membrane-electrode assembly (MEA). The zero-gap approach can improve performance by minimizing the electrode distance and ohmic loss. Compared with liquid alkaline systems, the PEM electrolysis is more recent. Still, it allows for a higher operating current density, better efficiency and dynamic behaviour in terms of rapid response, fast start-up and broader operating range. PEM electrolysis technology is favourite in terms of safety and high gas purity with the possibility to work under high differential pressure conditions. However, the need for PGM catalysts and component coatings can increase costs. Compared with the other technologies, AEM electrolysis systems have less commercial history and are developmental, but they share similarities with liquid alkaline and PEM electrolysis. As with PEM systems, AEM electrolyzers use a zero-gap approach, but the membrane conducts hydroxide. AEM electrolyzers should combine the advantages of both the liquid alkaline and PEM technologies in terms of the higher purity of generated H2, better efficiency and dynamic behaviour. Furthermore, using an alkaline solution of lower concentration, they are less prone to corrosion, and, finally, they can use inexpensive catalysts. For the water splitting process, two reactions are fundamental: the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). To satisfy the efficiency of both reactions, the research to develop new and func-tional catalysts is mandatory. In particular, the OER is the rate determing step and requires very active electrocata-lysts to decrease the overpotential for this reaction. Several proposals emerged in recent years through which various combinations of electrocatalysts were evaluated in a half-cell configuration to investigate the catalytic activities for the reactions occurring at the electrodes. Even noble metal-based catalysts deposited on polymeric membranes were tested in the electrolyzers. Abundant availability, strong ac-tivity, cost-effectiveness, and substantial electrocatalytic ac-tivities of transition metals make them an alternative to re-place the noble-metal electrocatalysts. The use of transition metals in an oxide form have been widely developed in recent years. Simple structures such as Co3O4, NiO QDs and MnO with overpotentials of 339, 320, 540 mV, respectively, prove to be acceptable competitors in the electro-oxidation of water. The window of opportunity opened by these variants of transition metals in the OER has led to incorporate these into nanostructures such as NiCo nanosheets, which yielded 332 mV results com-pared to their Ni-Co oxide bulk structure with an overpotencial close to 340 mV. Nanostructured morphologies of-fer the advantage of larger amounts of active sites for OER; this advantage has increasingly led to the development of 2D and 3D nanostructured materials which in theory provide a greater number of active sites exposed to the OER. This is the case of NiCo2O4 hollow microcuboids with 290 mV overpotential and a tafel slope of 53 mV dec-1 very close to IrO2. Some of these promising mixed oxides such as 3D NiFe2O4 hollow spheres have been brought to real-life conditions in water electrolysis micro-fuelcell tests where sig-nificant micro-scale hydrogen production of 2.5 x10-5 mg s-1. This work explores the activity in an anion exchange membrane electrolyzer of a 3D NiFe2O4 with hollow spheres morphology.