(Invted) The Use of Redox Nano-Titrations for Elucidating Reactive Heterogeneity on Electrodes for Energy Conversion and Charge Storage
Authors: Simpson Burton H , Hui Jingshu , Lichtenstein Timothy , Zhou Xuan , Rodríguez-López Joaquín 
Published: February 2020

We present on the use of the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM)[1] for the quantification of reactive intermediates at heterogeneous reactive electrode structures. In this mode, an SECM probe is approached to the electrode under study and a pulse of reactive redox mediator is directed to the substrate while it rests at open circuit. Since the tip operates in the feedback mode, if this pulse induces a chemical reaction on the surface species, an increased flux of the redox mediator sensed by the tip can be used to obtain the local titrated charge and the kinetics of reaction between the mediator and the surface species. Here, we will present how SI-SECM can be used to quantify by titration the reactive species present on an extended surface of a relatively inert electrode. This technique was used for interfaces of interest for energy conversion and storage: 1) for the detection of photogenerated hydroxyl radical on a semiconducting surface, and 2) for the detection of redox active polymer particles [2] on single layer graphene (SLG). Both semiconducting and SLG interfaces permit only the reactivity of reactive species on their surface to be detected.
Studies using finite element simulations indicate that the efficiency and extent of titration of a surface are strongly dependent on the SECM tip electrode size and on the tip-substrate distance. As the SECM tip size is made smaller, the apparent titrated charge increases. Likewise, the distance between the SECM tip and substrate determines the titrated charge – very small tip-substrate gaps lead to a constrained area, while large tip-substrate gaps increase the area titrated at the expense of a decreased signal-to-background. An optimum distance can be obtained. We will present on a system that validates these findings, as well as examples of interest to our laboratory, which include the titration of photogenerated intermediates on semiconductors for water splitting, and the titration of redox active polymer particles for charge storage in redox flow batteries. In the first case, we were able to decouple the reactivity of hydroxyl radicals and obtained their coverage and reaction kinetics. In the second case, the relaxed tip-substrate geometry suggested by our simulated findings allowed to evaluate the charge stored in polydisperse polymer nanoparticles for flow batteries. SI-SECM redox nano-titrations will allow to evaluate the role and importance of reactive intermediates in charge storage and energy conversion.
[1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24.
Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352
[2] Gavvalapalli, N.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J.S.; Rodríguez-López, J. J. Am. Chem. Soc. 2014, 136, 16309-16316