Understanding the role of graphene and mildly reduced graphene oxide in cerium dioxide, zinc oxide and bismuth oxide composite electrocatalyst electrodes using cyclic voltammetry and electrochemical impedance spectroscopy

Metal oxides used in conjunction with graphene species offer a route to improved electrocatalytic activity for a variety of applications. In this work CeO₂, ZnO, and Bi₂O₃ were drop-cast on to screen-printed carbon electrodes (SPCEs) to which graphene (Gr) and mildly reduced graphene oxide (mrGrO) were added, forming modified composite electrodes. Energy-dispersive X-ray spectroscopy confirmed the successful deposition of each metal oxide and scanning electron microscopy (SEM), revealed distinctly different surface morphologies with particle sizes ranging from ~ 4 μm (ZnO) up to ~ 21.9 μm (Bi₂O₃). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) performed with a ferri/ferrocyanide redox probe demonstrated enhanced electron-transfer kinetics of the mrGrO modified electrodes. Modification with mrGrO doubled the electroactive surface area compared with a bare SPCE, while incorporation of CeO₂, ZnO and Bi₂O₃ further increased the surface area by factors of 5.07, 2.45 and 4.16 respectively. EIS fitting of the experimental data using a variety of equivalent circuit models including Randles, two time constant and nested varieties confirmed that Bi₂O₃–mrGrO exhibited the lowest interfacial resistance, thus highlighting the ability of mrGrO to overcome the intrinsically poor conductivity of Bi₂O₃. This confirmed the CV results which found that Bi₂O₃-mrGrO had the largest standard heterogeneous rate constant of 5.5 × 10^{− 3} cms^{− 1} This was accounted for by increased electroactive surface area, together with enhanced surface adsorption effects. The metal oxide-mrGrO modified electrodes also displayed significantly enhanced voltammetric responses toward diclofenac and mefenamic acid producing approximately twelve-fold and ten-fold increases in anodic peak currents respectively in the presence of an electroactive film formed in-situ. Excellent reproducibility was found. As a demonstration, differential pulse voltammetry performed on a Bi₂O₃-mrGrO modified SPCE with an electroactive film formed in-situ enabled diclofenac quantification achieving a low limit of detection of 0.0614 µM (19.5 ppb) and excellent linearity. There is considerable potential for this approach to be used in future development of single-use SPCE sensors for detection of pharmaceutical compounds in clinical and environmental samples.