Electrocatalytic degradation of naphthenic acids using reduced graphene oxide modified nickel foam cathode: Role of surface structure and superoxide radical

Abstract

In this work, the reduced graphene oxide (RGO) modified nickel foam (Ni-foam) was prepared via the hydrothermal reduction self-assembly method and was applied as the cathode for the effective electrocatalytic (EC) degradation of recalcitrant naphthenic acids (NAs) that are abundantly present in the petroleum industrial wastewater. The Ni@RGO cathode has greatly enhanced the production of H2O2 in the system to boost the degradation of 1-adamantanecarboxylic acid (ACA) that was a model NA compound. Results showed that the EC/Fe(III)/Ni@RGO system achieved complete ACA degradation within 15 min due to its elevated H2O2 generation and Fe(III)/Fe(II) circulation, compared to EC/Fe(III)/Ni-foam and EC/Fe(III)/Graphite systems. Interestingly, abundant ·O2– radicals, which were produced in EC/Fe(III)/Ni@RGO system as a key intermediate for 2e- oxygen reduction reaction (ORR), could effectively accelerate the circulation of Fe(III)/Fe(II) to activate H2O2 to generate ·OH for ACA degradation. Results of material characterization, electrochemical analysis and density functional theory (DFT) calculations showed that Ni@RGO cathode has obtained elevated electron transfer efficiency compared to that of Ni-foam cathode. Surface structure of RGO, such as carbon edge and vacancy with specific configuration, residual –COOH oxygenation groups on RGO surface are main catalytic sites to enhance the capacity of 2e- ORR and electron capture processes in the system. This study provides new insights regarding the synthesis process of catalytic electrodes, the influence of carbon-based material structure on catalytic activity, and the critical role of free radicals in the electro-Fenton system with Ni@RGO cathode