(Invited, Digital Presentation) In-Situ Prussian Blue Formation on Intrinsic Iron-Containing MWCNT As a Template and Its Scanning Electrochemical Microscopic Interrogation and Hydrogen Peroxide Electrocatalysis

Prussian blue (PB; iron (III) hexacyanoferrate), is a mixed-valent polynuclear complex widely used in electrochromic display, biosensors, ion-selective electrodes, electrocatalysis and energy storage systems like batteries and fuel cells [1,2]. For the PB preparation, an electrochemical deposition using an equimolar concentration of Fe 2+ and Fe(CN) 6 3- in 0.1 M KCl-electrolyte condition was reported. In another case, a template approach, wherein either Fe 2+ (or ironic species) or Fe[(CN) 6 ] 3- immobilized on the electrode surface followed by interaction with counter ion were reported [3]. For instance, a deliberately added ironic species (>20 wt.%) into a multiwalled carbon nanotube (MWCNT) modified electrode has been utilized as a template for the PB functionalized MWCNT preparation by potential cycling of the precursor electrode with Fe[(CN) 6 ] 3- [3]. Meanwhile, an intrinsic iron species that originated from its catalytic source has been referred an influencing parameter in altering the electronic conductivity of the MWCNT (MWCNT-Fe*, Fe*-Instrinsic iron) [4]. Our group has utilized the pristine MWCNT as a template for in-situ preparation MWCNT@Fe*(bpy) 3 2+ and as an electro-Fenton catalyst for electroanalytical applications [5]. However, the in-situ derivation of iron impurity (2-4 wt.%) into the Prussian blue complex has not been explored yet. The inaccessibility of the iron and its low concentrations are the likely reason for the limitation. This report demonstrates an in-situ electrochemical PB formation using MWCNT-Fe* modified glassy carbon electrode as a template and Fe(CN) 6 3- as a counter ionic species. This modified electrode (GCE/MWCNT-Fe@PB) showed a characteristic redox peak for the high-spin ironic state of PB at about 0.2 V vs Ag/AgCl in addition with a post-peak at 0.3 V vs Ag/AgCl [Figure 1A]. The nickel impurity in the pristine MWCNT is suspected to be involved in the complexation with Fe(CN) 6 3- ion along with the ironic species. In-situ formation of PB on MWCNT-Fe* was probed using the scanning electrochemical microscope technique (SECM). In a typical experiment, MWCNT-Fe* modified GCE was taken as a base-electrode along with 5 mM Fe(CN) 6 3- in pH 2 KCl-HCl electrolyte and the potentials were set as 0.1 V and 0.5 V vs Ag/AgCl accordingly to reduce Fe III (CN) 6 3- + e - ® Fe II (CN) 6 4- and parallely reoxidize it as Fe II (CN) 6 4- ® Fe III (CN) 6 3- + e - on the Pt-tip surface (Figure 1B). The Fe II (CN) 6 4- ion formed as