NSF-DFG Echem: Operando Electronic Structure Determination of Iron and its Time-Dependent Dynamics in FexNi100-x(OH)y Electrooxidation Catalysts

• Applicant: Dr. Lothar Weinhardt
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 460479428

The question of Fe electronic structure in FexNi100-x(OH)y and sister FexM100-x(OH)y materials, and in particular, the dynamics of metal oxidation state in the electrochemical environment has been a long-standing question across disparate technology applications, such as battery electrodes and alkaline water electrolysis catalysts, which depend on similar electrochemically-induced metal redox states. Fe only undergoes these transitions under applied voltage, and oxidation state changes belie complex shifts in electronic structure. In electrocatalysts, the Fe atoms in these materials [i.e., FexM100-x(OH)y, M = Ni, Co, Mn] were previously thought to simply enhance M catalytic activity; now, Fe is considered to be directly involved in the active catalyst site. Few spectroscopic techniques exist that can unambiguously delineate a higher Fe oxidation state (e.g., Fe(IV)) from resonant structures or fully describe the occupied and unoccupied electronic density of states around the Fermi energy. We thus aim to understand the operando structure of Fe valence and conduction bands with highly sophisticated spectroscopy techniques. The overarching research goals of this proposal are to combine the material expertise of Dr. Lauren Greenlee’s group at the U of Arkansas with the x-ray spectroscopy expertise of Dr. Lothar Weinhardt’s group at the Karlsruhe Institute of Technology. Collaboratively conducted operando experiments will allow us to unequivocally describe electronic state(s) of Fe and to understand the time dependency and hysteresis of Fe structural changes in FexNi100-x(OH)y nanocatalysts.The challenge to unambiguously determine the existence of an Fe(IV) oxidation state lies in the difficulty in characterizing the Fe(III)-O• vs the Fe(IV)=O electronic state, where the 3+ and 4+ species are resonance structures. Most characterization methods, including hard x-ray XAS and Mössbauer spectroscopy, cannot unambiguously distinguish one resonance form from the other, and in an (Fe)NiOOH phase, either (or both) electronic structures could exist. To answer the controversy about the Fe oxidation state, dedicated operando setups and soft x-ray spectroscopy, which gives the require sensitivity to oxidation state and the Fe 3d derived valence structure. In tandem, the spectroscopic approaches and experimental setups developed in this collaborative effort will set the stage for new explorations of electrocatalytic materials and electrochemical systems and establish soft XES and RIXS for the identification of Fe4+.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professorin Lauren Greenlee, Ph.D.