• Energy Research

Bioelectrochemical system (BES) is promising technology to simultaneously treat wastewater and recover energy, and electrode material is important for the system performance. Microbial fuel cell (MFC) is one of typical BES to be applied in wastewater treatment. How to improve the electrode material is significant to improve wastewater treatment, energy recovery and cost effectiveness. In this study, 3D-weaved carbon electrode entity, assembled by multiple pieces of carbon mesh (CM), was proposed to combine all electrode components as entity to facilitate electron conduction and ionic migration, compared with carbon brush (CB) and granular activated carbon (GAC). The result showed that current density and internal resistance of MFC using 3D-weaved CM as horizontally extended inside anode (CM(T)) were 30.9 A m-3 and 4.5 Ω, respectively, with higher output than traditional GAC (22.6 A m-3 and 6.2 Ω). Though GAC had greater electrode filling and surface area for biomass growth, the electron transfer efficiency per unit electrode biomass was only at 0.0019 ± 0.0002 mol g-1 d-1, much lower than CM(T) at 0.0077 ± 0.0009 mol g-1 day-1. Higher ionic migration rate of CM(T) suggested the assisting effect of composite electrode to enhance ionic transportation towards the cathode. Microbial analysis further indicated that 3D-CM electrode network could simultaneously enhance Geobacter abundance and methanogen activity, suggesting the importance of electrode network on electricigens. Furthermore, CM(T) could obtain 10 times higher energy output efficiency than traditional GAC when applied inside anode chamber. This study proved that network construction of anode electrode could promote the electrode performance and cost effectiveness, suggesting the future development of reactor design of bioelectrochemical system.

Abstract A binder-free, pseudocapacitive anode was fabricated by coating reduced graphene oxide (rGO) and manganese oxide (MnO2) nanoparticles on stainless steel fibre felt (SS). Microbial fuel cell (MFC) equipped with this novel anode yielded a maximum power density of 1045 mW m−2, 20 times higher than that of a similar MFC with a bare SS anode (46 mW m−2). Transient-state regulation (TSR) was implemented to further improve the MFC’s power generation. The optimal TSR duty cycle ranged from 67% to 95%, and the MFC’s power density increased with TSR frequency. A maximum power density output of 1238 mW m−2 was achieved at the TSR duty cycle of 75% and the frequency of 1 Hz, 18.4% greater than that obtained from the steady state operation. The TSR mode delivered better MFC performance especially when the external resistance was small. Long-term operation tests revealed that the current density and power density yielded in the TSR mode were on average 15.0% and 32.7% greater than those in the steady state mode, respectively. The TSR mode was believed to reduce the internal resistance of the MFC while enhance substrate mass transfer and electron transfer within the anode matrix, thereby improving the MFC performance.

Waste heat can be captured as electrical energy to drive hydrogen evolution in microbial reverse-electrodialysis electrolysis cells (MRECs) by using thermolytic solutions such as ammonium bicarbonate. To determine the optimal membrane stack configuration for efficient hydrogen production in MRECs using ammonium bicarbonate solutions, different numbers of cell pairs and stack arrangements were tested. The optimum number of cell pairs was determined to be five based on MREC performance and a desire to minimize capital costs. The stack arrangement was altered by placing an extra low concentration chamber adjacent to anode chamber to reduce ammonia crossover. This additional chamber decreased ammonia nitrogen losses into anolyte by 60%, increased the coulombic efficiency to 83%, and improved the hydrogen yield to a maximum of 3.5 mol H2/mol acetate, with an overall energy efficiency of 27%. These results improve the MREC process, making it a more efficient method for renewable hydrogen gas production.

Biohythane is alternative fuel to replace fossil fuel for car combustion, and biohythane generation could be potential pathway for energy recovery from wastewater treatment. Microbial electrolysis cell (MEC) is electrochemical technique to convert waste to methane and hydrogen gas for biohythane generation, but the feasibility and stability of MEC needs further investigation to assure sustainable energy recovery. System configuration is paramount factor for electrochemical reaction and mass transfer, and this study was to investigate the configuration impact (single vs dual chamber) of MEC for biohythane generation rate and stability. This study showed that dual-chamber MEC could separate methane and hydrogen gas production in the anode and cathode, and combined both together to produce biohythane. To reduce ohmic resistance for higher current, cation exchange membrane (CEM) was removed from dual-chamber to single-chamber MEC. However, free hydrogen diffusion was allowed in the single chamber since CEM was removed. The diffused hydrogen and substrate towards the cathode would favor the methanogen growth, and thus the hydrogen was consumed to reduce the biohythane generation and energy recovery efficiency (i.e., 7.5 × 10-3 reduced to 5.7 × 10-3 kWh kg-1 degraded COD day-1 after converting dual-chamber to single-chamber MEC). Absolute abundance of methanogen in single-chamber MEC was greatly boosted, as Methanosarcina and Methanobacteriale on the anode surface, increased by 132% and 243%, respectively, while the original dual-chamber MEC could maintain Geobacter growth for high current generation. This is the keystone study to demonstrate the importance of dual-chamber MEC for the feasibility and stability for the biohythane generation, building up the foundation to use electrochemical device to convert the organic waste to the alternative biohythane.

Typical microbial fuel cells (MFCs) rely on precious metals for reduction of oxygen at the cathode, but recently MFCs have been developed that use biocathodes for power generation with alternate electron acceptors. It is shown here that with illumination it is possible to develop a biocathode that uses dissolved carbon dioxide (bicarbonate) as the acceptor. During acclimation, the cathode was set at a potential of 0.242 V (vs.SHE) using a potentiostat. After approximately one month of acclimation, a current of 1 mA was sustained. Bicarbonate was reduced in stoichiometric agreement with current generation, with 0.28 ± 0.02 moles of bicarbonate reduced per mole of electrons. When this biocathode was used in a two-bottle MFC, a power density of 750 mW m−2 was produced. These results demonstrate that MFCs can be used to fix carbon dioxide as well as produce electricity.

The combined use of brush anodes and glass fiber (GF1) separators, and plastic mesh supporters were used here for the first time to create a scalable microbial fuel cell architecture. Separators prevented short circuiting of closely-spaced electrodes, and cathode supporters were used to avoid water gaps between the separator and cathode that can reduce power production. The maximum power density with a separator and supporter and a single cathode was 75 ± 1 W/m(3). Removing the separator decreased power by 8%. Adding a second cathode increased power to 154 ± 1 W/m(3). Current was increased by connecting two MFCs connected in parallel. These results show that brush anodes, combined with a glass fiber separator and a plastic mesh supporter, produce a useful MFC architecture that is inherently scalable due to good insulation between the electrodes and a compact architecture.

Bioelectrochemical systems (BESs) hold prospects in wastewater energy and resource recovery. Anode optimization is important for simultaneous enhancement of wastewater energy conversion and effluent quality in BESs. In this study, a multi-physics model coupling fluid flow, organic degradation and electrochemical process was constructed to guide the design and optimization of BES anodes. Based on the multi-physics simulation, spatially-assembled binary carbon anodes composed of three-dimensional carbon mesh skeleton and granular activated carbon were proposed and established. The granular activated carbon conducive to microbe accommodation played a vital role in improving effluent water quality, while the carbon mesh skeleton favoring electron collection and transfer could enhance the bioelectricity output. With an average chemical oxygen demand (COD) removal rate of 0.442 kg m-3 d-1, a maximum power density of 20.6 W m-3 was achieved in the optimized composite anode BES, which was 25% and 154% higher than carbon mesh skeleton BES and granular activated carbon BES. Electroactive bacteria were enriched in composite anodes and performed important functions related to microbial metabolism and energy production. The spatially-assembled binary carbon anode with low carbon mesh packing density was more cost-effective with a daily energy output per anode cost of 221 J d-1 RMB-1. This study not only provides a cost-efficient alternative anode to simultaneously improve organic degradation and power generation performance, but also demonstrates the potential of multi-physics simulation in offering theoretical support and prediction for BES configuration design as well as optimization.

Separators are used to protect cathodes from biofouling and to avoid electrode short-circuiting, but they can adversely affect microbial fuel cell (MFC) performance. A spray method was used to apply a polyvinyl alcohol (PVA) separator to the cathode. Power densities were unaffected by the PVA separator (339±29mW/m(2)), compared to a control lacking a separator in a low conductivity solution (1mS/cm) similar to wastewater. Power was reduced with separators in solutions typical of laboratory tests (7-13mS/cm), compared to separatorless controls. The PVA separator produced more power in a separator assembly (SEA) configuration (444±8mW/m(2)) in the 1mS/cm solution, but power was reduced if a PVA or wipe separator was used in higher conductivity solutions with either Pt or activated carbon catalysts. Spray and cast PVA separators performed similarly, but the spray method is preferred as it was easier to apply and use.

Cation (CEMs) and anion exchange membrane (AEMs) are commonly used in microbial fuel cells (MFCs) to enhance Coulombic efficiencies (CEs) by reducing the flux of oxygen through the cathode to bacteria on the anode. AEMs typically work better than CEMs, but in initial experiments we observed the opposite using a membrane electrode assembly MFC. The reason was identified to be membrane deformation, which resulted in water and gas trapped between the membrane and cathode. To correct this, stainless steel mesh was used to press the membrane flat against the cathode. With the steel mesh, AEM performance increased to 46+/-4 W/m(3) in a single cathode MFC, and 98+/-14 W/m(3) in a double-cathode MFC. These power densities were higher than those using a CEM of 32+/-2 W/m(3) (single cathode) and 63+/-6 W/m(3) (double cathode). Higher pH gradients across the membrane and salt precipitation on the cathode were responsible for the reduced performance of the CEM compared to the AEM. CEs reached over 90% for both membranes at >2A/m(2). These results demonstrate the importance of avoiding water accumulation in thin films between membranes and electrodes, and explain additional reasons for poorer performance of CEMs compared to AEMs.