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The application of conventional electro-Fenton (EF) is limited by the narrow pH range and iron-containing sludge production, while the bifunctional cathodes in the heterogeneous electro-Fenton (HEF) can realize the synchronous generation and activation of H 2 O 2 , broaden the adaptable pH range and render catalyst easy to recycle, which has gained wide attention in electrochemical advanced oxidation processes (EAOPs). In this review, we summarized the classification of bifunctional metallic composite cathodes in HEF, the preparation and loading methods of catalyst on the cathode, as well as the advantages and the corresponding characterization of the bifunctional cathodes. The approaches to enhance the catalysts performance for H 2 O 2 generation and activation and electrode stability were addressed. The origin, identification and transformation of reactive species in degradation mechanism were explored. Finally, the challenge and trend of bifunctional cathode for HEF was prospected. We hope this review can provide a useful guideline for the design and application of bifunctional cathodes in HEF. [ABSTRACT FROM AUTHOR]

[Display omitted] • S, N self-doped electrocatalyst was derived from waste ginkgo leaves. • A Janus electrode was first constructed in flow-through electrochemical oxidation system. • The system could enhance oxygen mass transfer and enrich pollutant simultaneously. • This system had higher efficiency, lower energy consumption, and wider application pH. • It was promising and stable for removing various pollutants in water/wastewater. Facing low treatment efficiency, narrow adaptive pH and high energy consumption in electro-Fenton (EF), we proposed a novel flow-through metal-free electrochemical advanced oxidation processes (EAOPs) using biomass derived S, N self-doped catalytic Janus cathode named SNJC. The SNJC was composed of a hydrophobic gas diffusion layer in the middle and hydrophilic catalytic membrane at both ends, while the catalytic membrane of S, N self-doped biomass carbon named SN-BC was derived from waste ginkgo leaves without additional supporting templates or activation processes. The conversion of graphite N, pyridinic N and thiophene S in SN-BC played a significant role in efficient oxygen reduction reaction (ORR) for H 2 O 2 generation and in-situ active species generation. The efficient enrichment and rapid degradation of pollutants in catalytic membrane achieved almost complete removal of tetracycline within 120 min with a low energy consumption of 16.8 kWh/kg TOC. This flow-through system exhibited superior catalytic performance in wide pH ranges (3–11) due to the collective effect of radicals (•OH and O 2 •−) and non-radical (1O 2). This work provides a new insight towards the design of S, N self-doped Janus electrode and activation mechanism of in-situ generation and metal-free catalysis of H 2 O 2 in flow-through EAOPs. [ABSTRACT FROM AUTHOR]

• A flow-through system based on the bifunctional cathode of Fe@BN-C/GF was developed. • The acceleration of the O 2 mass transfer promoted the stable production of 1O 2. • The SMT degradation was found not affected in the presence of inorganic salts and organic matter. • TOC removal in flow-through was 1.4 ∼ 2.4-folds that of the batch system under the pH 3 ∼ 11. • The TP, NH 3 -N and TOC in reverse osmosis concentrate could be simultaneously removed. To improve the mineralization efficiency of pollutants under wide pH ranges by self-sustained electrochemical advanced oxidation processes (EAOPs) without any reagent addition, a flow-through system based on the bifunctional cathode of B, N-doped carbon nanotubes embedded with nano-Fe0 particles that loaded on graphite felt (Fe@BN-C/GF) was developed. Dominant by 1O 2 oxidation, the sulfamethazine (SMT) degradation rate and TOC removal in this recirculated flow-through system were 2.6 ∼ 3.3-folds and 1.4 ∼ 2.4-folds that of the batch system under the pH ranges of 3 ∼ 11, respectively. The SMT degradation performance was found not affected in the presence of HCO 3 –, NO 3 –, Cl-, K+, or humic acid (HA). The Fe@BN-C/GF cathode had more potential to treat enlarged wastewater by 5-folds during 10 h operation, and the SMT and TOC removal was 99% and 70.31%, respectively. The total phosphorus (TP), ammonia nitrogen (NH 3 -N) and TOC in reverse osmosis concentrate and secondary wastewater effluent could be simultaneously removed to <0.01 mg L-1 and 8 mg L-1 by this flow-through system with a low electric energy consumption (EEC) of 0.022 and 0.18 kWh gTOC-1, respectively. This EAOPs based on the flow-through Fe@BN-C/GF cathode was self-sustained, and had very promising application prospect of high mineralization of pollutants even for low-conductivity wastewater. [ABSTRACT FROM AUTHOR]