Progress on catalytic electrodes and fuel cell systems for industrial wastewater treatment

Industrial wastewater containing dyes, antibiotics, heavy metal ions and other refractory organic pollutants has complicated components and poor biodegradability. Comparing to traditional bio-treatment processes, electrochemical technologies have significant advantage in treating such kinds of wastewater due to the electrochemically generated reactive species. Modifying the electrodes with catalytic components can enhance pollutant removal capacity in electro-catalytic (EC) or photo-electro-catalytic (PEC) integrated systems, but high energy consumption of electricity still restricts their implement. Self-biased fuel cells, including photocatalytic fuel cells (PFCs) and microbial fuel cells (MFCs), are more sustainable in industrial wastewater/pollutants treatment. Highly active catalytic electrodes are essential to promote pollutant removal and energy conservation. This article reviews the recent development of novel catalytic electrodes in preparation, optimization and sustainable application for industrial wastewater treatment. Technical advantages and optimization spaces of fuel-cell integrated systems (based on PFCs and MFCs) are introduced, and their challenges in large-scale application are pointed out. Catalytic electrodes have broader application fields than powder-form catalysts due to the easy-recyclability and the synergy of catalysis and electrochemistry. An ideal catalytic electrode should be conductive, highly (photo-)electro-active, physically and chemically stable, easy to prepare and low-cost. By optimizing the preparation/loading of novel catalytic materials (heterojunctions, single-atom catalysts etc.), various catalytic electrodes, in forms of self-standing (metal-based, carbon-based and others), film/membrane and particles, etc., can be obtained with extraordinary (photo-)electro-catalytic activity. Innovative design of catalytic electrodes in structure and component may in-situ integrate multiple technologies such as (photo-)electro-catalysis, advanced oxidation processes (AOPs) and membrane filtration, etc., which provides more possibility for enhancing electrochemical systems. Novel catalytic electrode of low-cost and high efficiency is still desirable for further optimization of integrated electrochemical systems. PFCs convert solar energy into electricity from the wastes (fuels). Various PFCs (single or dual photo-electrode(s)) have been developed for the degradation of dyes, antibiotics and other refractory pollutants. The electricity generation of novel PFC has been improved to 1 V (open circuit voltage) or more in some cases. Enhanced pollutant degradation and energy recovery by optimizing the function of electrode and structure of PFCs are still desirable for complex wastewater treatment. By using exoelectrogens, highly-active catalytic electrodes (anode and/or cathode) can be applied in MFC integrated systems to promote the degradation of refractory pollutants. The synergy of bacteria and catalytic electrodes broadens the application of bio-processes in industrial wastewater treatment, and also benefits the energy recovery (electricity, hydrogen, heavy metals, etc.) of systems. Innovative designs of systems are expected to further reduce the operating costs from ion-exchange membrane, light irradiation and aeration. Although pollutant removal and energy conservation of fuel cells can be further improved by integrating AOPs (Fenton process, sulfate or chlorine radical advanced oxidation, etc.) or combining other processes (EC, PEC, desalination, etc.), less chemical consumption and simpler system configuration/operation are the main trends of sustainable and cleaner production. In the future, efforts need to be done for large-scale industrial wastewater treatment by fuel cell systems: (1) Develop novel catalytic electrodes of low-cost with enhanced performance (visible light response) and integrated functions (photo- and/or electro-catalysis, filtration, AOPs, etc.). (2) Construct novel integrated fuel cell systems with highly active catalytic electrodes for multiple pollutant control and energy recovery. (3) Optimize large-scale preparation of catalytic electrodes, and simplify the operations of fuel cell systems, for energy-efficient and cost-effective treatment of real industrial wastewater, during long-term run.