Structural Transformations Leading to the Oxygen Evolution Active Phase for Fe Containing Layered Double Hydroxides

Abstract

NiFe based (oxy)hydroxides have been identified among the most active catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes.1 Their crystal structure is known as layered double hydroxide (LDH) and is composed of layers of edge sharing metal oxygen octahedra that are intercalated with water molecules and charge balancing anions (Figure 1, insert).2 The incorporation of Fe3+ ions in Ni(OH)2 layers is considered to be responsible for the formation of this structure and its activity enhancement.3 Similarly, incorporation of Fe3+ into Co(OH)2 leads to CoFe LDH. This catalyst has also superior OER activity respect to Fe-free Co(OH)2.4 Despite their structure is well characterized, there are evidences that the OER active structure of LDH catalysts during OER is different than the one of the as prepared materials.3 In particular, oxidative deprotonation and the electrocatalytic reaction might lead to reversible structural differences that are only observable with in operando or in situ methods. More investigations are necessary, since the identification of the OER active structure of a catalyst is important for designing more active and stable catalysts. Theoretical predictions based on density functional theory (DFT) can guide this process, however so far different atomic models have been used in DFT calculations due the lack of clear structural information concerning the crystalline phase of Fe containing LDH under OER active conditions. In this presentation we will discuss the results of an in operando X-ray absorption spectroscopy and diffraction study on crystalline NiFe and CoFe LDH nanostructured catalysts. We identified the structure of the catalytically silent and active catalyst state and observed that for both catalysts the interlayer distance contracts under operating conditions to a similar value (figure 1). Our results show structural modifications under operating conditions which correlate with the electrochemical activity in alkaline electrolyte. Thus they provide new insights into the structure of the active catalysts state which will help to understand the processes under catalytic conditions. Figure 1. Shift of the (003) diffraction peak associated to the interlayer distance for NiFe LDH catalysts between the resting state (black) and OER active state (red) (unpublished). 3D structural model of the as prepared NiFe LDH with intercalated water and carbonate ions shown in the insert. Figure 1