Comparison of Pd catalysts supported on both carbon supports and titanium suboxides for the air electrode of iron-air batteries

Batteries are the key components of the electric vehicles (EVs) and at the same time their most expensive parts. Lithium ion batteries are currently considered as the best option for transportation to be used in plug-in hybrid and full EVs [1]. However, to fully implement EVs in the market, a new generation of battery system will be required providing higher energy density, lower cost and use of abundant and safe materials [2]. In this regard, one of the main fields being explored for a possible breakthrough is metal-air batteries. This devices generate electricity through a redox reaction between a metal (such as Li, Zn, Al, Mg, Na or Fe) and oxygen in air [3]. In comparison to Li, metals such as Zn, Mg, Fe, etc, are much more abundant, present lower costs and are more environmentally safe [1-6]. Among them, the iron-based system is one of the most promising given that combines the high energy density typical of this type of batteries with other specific aspects like reversibility of the iron electrochemistry, simple cell design (no need of membrane), use of abundant low cost manufacturing materials and safety [5, 7]. In this system, the air electrode is a key component. Molecular oxygen takes electrons from the counter metal electrode to complete the electrochemical reaction generating electrical energy, being necessary not only a good performance but a proper stability. In the present work several Pd-based catalysts have been studied for both the Oxygen Reduction Reaction (ORR), taking place in the discharge process of the battery, and the Oxygen Evolution Reaction, taking place during charging. Pd-nanoparticles (3-7 nm) were supported on several supports by means of a sulphite complex route: (i) carbonaceous supports such as Vulcan-XC-72-R (from Cabot) and (ii) non-carbonaceous supports, Ti-suboxides. The electrochemical behavior of these catalysts for both ORR and OER was investigated in half-cell configuration in an alkaline solution (6M KOH) at room temperature. Stability tests were carried out in order to properly select the most appropriate catalyst to be used in a complete cell.