Fachbeirat 2022

Tracking the Incorporation of Fe Impurities into NiO Electrocatalysts during Reaction using Electrochemical Cell Transmission Electron Microscopy and Operando Spectroscopy

Not scheduled

20m

Speaker

Fengli Yang

Description

The electrolysis of water is a promising method to store renewable electricity generated by solar and wind power in the form of useful products. The efficiency of water splitting, however, is limited by the sluggish half-reaction of anodic oxygen evolution (OER), which has, in turn, led to immense research efforts to find better catalysts. Electrocatalysts based on nickel oxides/hydroxides¹ are widely explored as non-precious metal catalyst alternatives because of their excellent performance for OER in alkaline electrolytes. In particular, it is known that trace amounts of Fe can generate highly active catalytic sites on these electrocatalysts^2-5. While the impact of added Fe has been widely reported in previous work, we still lack a definitive description for the role of Fe partly due to insufficient insight into the morphological changes induced by Fe incorporation.
In this work, we tracked the structural and chemical changes induced in model octahedral NiO electrocatalysts when Fe³⁺ ions were added into the electrolyte under reaction conditions using a combination of operando electrochemical cell transmission electron microscopy (EC-TEM) and energy dispersive X-ray spectroscopy (EDX)⁶. Our results indicate that Fe was initially incorporated uniformly into the surface of NiO, turning it into a NiFe layered double (oxy)hydroxide (LDH). During extended reaction, this layer did not grow, instead, subsequent Fe was deposited on the surface of the octahedra in the form of FeO_x aggregates. The transformation into NiFe LDH also resulted in a shift towards a lower OER onset potential, while the subsequent formation of FeO_x aggregates gradually reduced the measured current, presumably due to the aggregates blocking the accessible active surface area. These results were complemented with X-ray photoelectron spectroscopy, operando Raman and operando extended X-ray absorption fine structure spectroscopy. These results show how Fe impurities alter the surface of nickel-based catalysts during OER and their corresponding impact on their electrocatalytic behavior.

References
1. Dionigi, F. & Strasser, P.: NiFe-based (oxy)hydroxide catalysts for oxygen evolution reaction in non-acidic electrolytes. Adv. Energy Mater. 6, 1600621 (2016).
2. Trotochaud, L., Young, S. L., Ranney, J. K. & Boettcher, S. W.: Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 136, 6744-6653 (2014).
3. Corrigan, D. A.: The catalysis of the oxygen evolution reaction by iron impurities in thin film nickel oxide electrodes. J. Electrochem. Soc. 134, 377-384 (1987).
4. Klaus, S., Cai, Y., Louie, M. W., Trotochaud, L. & Bell, A. T.: Effects of Fe electrolyte impurities on Ni(OH)₂/NiOOH structure and oxygen evolution activity. J. Phys. Chem. C 119, 7243-7254 (2015).
5. Spanos, I., Masa, J., Zeradjanin, A. & Schlögl, R.: The effect of iron impurities on transition metal catalysts for the oxygen evolution reaction in alkaline environment: Activity mediators or active sites? Catal. Lett. 151, 1-14 (2020).
6. Yang, F.L. et al.: Spatially- and Chemically-Resolved Visualization of Fe Incorporation into NiO Octahedra during the Oxygen Evolution Reaction. Submitted (2022).