Speaker
Description
Electrocatalytic CO$_{2}$ conversion and electrochemical energy storage systems are urgently pursued to tackle CO$_{2}$ - driven climate change, and a common characteristic of such systems is the electrode-electrolyte interface which is crucial for their performance as well as longevity. The optimisation of (electro)catalytic interfaces, e.g. by engineering of nanoscale structures exposing low-coordination sites with a high catalytic activity$^{1}$, is critically important for high system performance$^{2}$.
By combining functional imaging with the high spatial resolution afforded by atomic force microscopy (AFM), we are addressing the challenge of resolving nanoscale heterogeneities of electrode surfaces and associated spatial variations in the interfacial electron transfer. As AFM relies on the measurement of tip-sample interaction forces, both electrically conductive and insulating areas can be imaged in a stable manner. In this work, we demonstrated conductive AFM (c-AFM) for electric current imaging$^{3}$ in relevant aqueous electrolytes, beyond the common operation in air$^{4}$ or nonpolar liquids$^{5}$.
With a view in its application to the electrochemical CO$_{2}$ reduction reaction (CO$_{2}$RR), we showed that in situ c-AFM allows laterally resolved analysis of variations in the electric current occurring across the electrode/electrolyte/probe interface, where the electrode was exemplified by polycrystalline Au surfaces with or without nanofabricated Cu islands. Further to imaging such mono- and bimetal electrodes in air, water and bicarbonate aqueous electrolytes, the versatility of the approach was demonstrated by using both, solid Pt and TiN coated AFM probes. For the bimetallic electrocatalyst, distinct current contrasts were observed between the Au surface and the Cu islands which showed a larger electric resistance. For the polycrystalline Au surface, clear current contrasts were found between convex intragranular and concave intergranular regions. Our approach enables spatially resolved detection of passive electrode areas as well as analysis of current-voltage (I-V) curves relating to selected electrode-electrolyte interface sites.
Ongoing work is concerned with the development of tailored probes, including an insulating top coating, to eliminate leak currents potentially occurring between the tip shank and the sample surface. Insulating hafnium oxide top coatings were deposited by atomic layer deposition and subsequently removed from the tip apex region by focused ion beam milling. Employing a purpose-built electrochemical cell, the efficacy of the insulating top coatings is being verified by leak current measurements.
References
1. D. Gao, R. M. Arán-Ais, H. S. Jeon, B. Roldan Cuenya, Nat. Catal. 2, 198 (2019).
2. V. R. Stamenkovic, D. Strmcnik, P. P. Lopes, N. M. Markovic, Nat. Mater. 16, 57 (2017).
3. M. A. Lantz, S. J. O’Shea, M. E. Welland, Phys. Rev. B 56, 15345 (1997).
4. M. Munz, B. Cappella, H. Sturm, M. Geuss, E. Schulz, Adv. Polym. Sci. 164, 87 (2003).
5. N. Gosvami, K. H. A. Lau, S. K. Sinha, S. J. O’Shea, Appl. Surf. Sci. 252, 3956 (2006).
