High-Performance Electrocatalytic Reduction and Sensing of Hazardous Hexavalent Chromium Using a Redox-Active Binol Species-Impregnated Carbon Nanofiber-Modified Electrode

Electrochemical reduction of the hexavalent chromium compounds, a serious environmental pollutant and carcinogen, to a nontoxic Cr(III) species is of significant research interest in interdisciplinary areas. Due to its high oxidation potential (1.36 V vs RHE) and associated surface fouling problems with organic-electrocatalyst systems, expensive precious metal-based electrodes like Au, Ag, Pt, and Pd have been widely used for this purpose. Herein we report, an in situ electrochemically prepared highly redox-active Binol species (Binol-Redox)-impregnated carbon nanofiber-modified electrode (CNF@Binol-Redox), Eo′ = 0.50 V versus Ag/AgCl and surface excess value (ΓBinol-Redox) = 32.5 × 10–9 mol·cm–2, as a low-cost, stable, and high-performance molecular electrocatalyst for selective electrochemical reduction of Cr(VI) species in pH 2 HCL/KCl. The new electrode material was characterized using SEM, FTIR, Raman, GC-MS, 1H NMR (active site isolated as an ethanolic extract), and scanning electrochemical microscopy (SECM) techniques. It has been revealed that an intermediate electrochemical oxygen reduction reaction to H2O2 occurred at CNF@Binolads, which is a key step for the surface-adsorbed Binol-precursor oxidation to redox-active molecular electrocatalyst. CNF@Binol-Redox showed a high-performance reduction reaction at a reduction potential of 0.5 V versus Ag/AgCl (low overpotential, η = 0.5 V), which is much better than that of several conducting polymers and Pd-based electrocatalysts (high η = 1.5–0.6 V) and closer to the performance of a bulk gold electrode system. Further, this new electrode does not show any dissolved oxygen interference or surface-fouling problem. The Andrieux-Savent electrochemical kinetic model was adopted to calculate the heterogeneous rate constant (kchem) as 4.29 × 105 mol–1·dm3·s–1. As practical applications, a prototype electrochemical Cr(VI) sensor operatable by one-drop analysis and bulk electrochemical detoxification of Cr(VI) to Cr(III) have been successfully demonstrated.