Electrochemically reconstructed copper-polypyrrole modified graphite felt electrodes for efficiently electrocatalytic reduction of nitrate

In this work, a simple two-step electrodeposition was employed to prepare PPy-Cu/GF (PPy, polypyrrole; GF, graphite felt) composite cathodes for the nitrate reduction. Characterized results revealed that the introduction of PPy as an intermediate layer resulted in the transformation of both granular and dendritic Cu in form of Cu⁺/Cu⁰, Cu²⁺ on the electrode surface. The ${{\text{NO}}_3}^{-}$-N of 50 mg·L⁻¹ was almost completely removed (99.01%) using the PPy-Cu/GF cathode under the optimum condition, which is obviously higher than the GF, PPy/GF and Cu/GF electrodes. ${{\text{NO}}_3}^{-}$-N removal was slightly affected over the pH scale of 3.0–11.0, whereas increasing the current density from 10 to 25 mA·cm⁻² boosted the reduction of ${{\text{NO}}_3}^{-}$-N. At a Cl⁻ concentration of 2000 mg·L⁻¹, the removal of ${{\text{NO}}_3}^{-}$-N was slightly reduced, while the selectivity for N₂ increased dramatically due to the active chlorine could oxidize ${{\text{NH}}_4}^{+}$-N to N₂. Meanwhile, PPy-Cu/GF cathode exhibits an average removal rate of 97.83% within 12 cycles, highlighting its potential for application in actual water bodies. The EPR analysis and the active species trapping experiment confirmed that the nitrate reduction on the PPy-Cu/GF cathode mainly relies on direct reduction mediated by electron transfer, while ^∗H influences the reduction of nitrite to ammonia.