Detection of Intermediate in Ammonia Oxidation Reaction Using Aggregation-Induced Emission Phenomenon

Abstract

Ammonia electro-oxidation reaction (AOR) is a vital reaction for solving energy and environment problems as it is the anodic reaction in direct ammonia fuel cells (DAFCs), ammonia electrolyzer and in the remediation of wastewater containing ammonia.1-2 Designing high-performance electrocatalysts for AOR requires thorough understanding of mechanism and intermediates produced. Techniques like in-situ FTIR have provided information about intermediates produced during AOR in aqueous alkaline media.3-4 However, the mechanism is still not well understood and more proof of exact intermediates are needed. Furthermore, these in-situ methods are complex with multiple procedures requiring extreme precision.N2Hx is an important intermediate in the dimerization step according to Gerischer and Mauerer mechanism. In this work, aggregation-induced emission (AIE) phenomenon was used for the first time to target and detect hydrazine (N2H4) as an intermediate during AOR using commercial Pt/C as electrocatalyst. Tetraphenylethene-CHO (TPE-CHO) molecule as AIEgen was used for this purpose. Combination of photoluminescence (PL) spectra showing turn-off fluorescence along with the electrochemistry results illustrated the presence of hydrazine as an intermediate. In Figure 1, fluorescence spectra in THF/water mixture with 90% of water fractions for different electrocatalyst loading keeping the AIE concentration same is depicted. It was observed that with increase in catalyst concentration, PL spectra intensity decreased considerably indicating towards the desired reaction of intermediate produced during reaction with the already present AIE on GC electrode. Control experiments using graphene and TPE were performed to elucidate the results further. Mass spectra of the product formed after reaction between hydrazine hydrate and TPE-CHO was obtained and compared with the one obtained during electrochemical reaction. A very simple and novel method for detection of an intermediate, hydrazine in AOR using AIE phenomenon is demonstrated opening new strategy for mechanism understanding of an electrochemical reaction. Figure 1. Fluorescence spectra in THF/water mixture with 90% of water fractions for different electrocatalyst loading keeping the AIE concentration same.