Nitrate ions are well-known water toxic inorganic pollutants and their high concentrations can cause a serious problem for both aquatic systems and human health. In this sense, it is of great importance to monitor nitrate ions in various kind of source [1]. Among several analytical technologies that been used for its detection, electrochemical technique offers a simple, rapid, selective, sensitive, inexpensive, safety method for this purpose, not to mention its considered green technology [2]. To improve the nitrate process performance and selectivity, novel electrode materials have been widely investigated. This work aims to develop a sensitive and reliable three dimensional electrode consisting of Cu particles/reduced oxide graphene (rGO)/carbon fiber (CF) for monitoring nitrate concentration. CF produced at heat treatment temperature of 1000 ºC was used in this study. The rGO electrodeposition on CF (1 cm²) was performed at a fixed potential of -1.25 V vs. Ag/AgCl/KCl(sat) for 15 min in a 20 mL of the graphene oxide (3mg mL^-1) + 0.1 mol L^-1 LiClO₄ aqueous solution. The Cu electrodeposition on rGO/CF was carried out at a fixed potential of -0.7 V for 3, 6, 10, 20 min in a 10 mmol L^-1 CuSO₄ + 100 mmol L^-1 H₂SO₄ aqueous solution. Cu/rGO/CF composites morphologies and structures were analyzed by field emission gun scanning electron microscopy (FEG-SEM), X-ray diffraction, and Raman spectroscopy. Nitrate electro-reduction was evaluated using cyclic voltammetry at 50 mV s^-1 in 0.1 mol L^-1 K₂SO₄ solution (pH = 3.0) with and without the presence of 10^-2 mol L^-1 KNO₃. For rGO/CF binary composite, FEG-SEM images showed the characteristic aggregates with irregular and flakes-like shape with high particle density on CF substrate. The Cu deposits depicted small grain morphologies distributed throughout the rGO/CF surface. As expected, Cu deposit rate increased with increasing the deposition time. From X-ray diffraction analyses of the Cu particles, a mixture of phases containing cuprous oxide and metallic copper was identified. Under optimal experimental conditions, the Cu/rGO/CF electrode showed good catalytic activity for nitrate electro-reduction. This behavior can be associated to the presence of rGO and CF with high specific surface area in addition to Cu particle with high electron transfer capability. These results showed that Cu/rGO/CF composite has a great potential as new promising electrode for nitrate electroreduction application.

Acknowledgements: This work was supported by FAPESP, Process 2016/13393-9, CAPES, and CNPq Brazilian Agencies.

References

[1]R. Gürkan, N. Altunay, J. Food Composition Anal., 2016.

[2] J. Martínez, A. Ortiz, I. Ortiz, Applied Catalysis B: Environmental, 207, 2017.